Emerging Materials for Environment Protection and Renewable Energy 2018026973, 2018028512, 9781536138511, 9781536138504

Table of contents :
Contents
Preface
Part I. Fundamentals of Functional Materials: Applications for Sensors
Section 1. Metal Oxides based Gas Sensors
Chapter 1
Science and Technology of Metal Oxide Semiconductor Gas Sensor
Abstract
1. Introduction
1.1. Electrochemical Gas Sensors
1.2. Metal Oxide Semiconductors (MOS) Gas Sensor
1.3. Catalytic Gas Sensors
1.4. Infrared (IR) Gas Sensors
2. Metal Oxide Semiconductor Gas Sensors
2.1. Basic Working Principles
2.1.1. Bulk Conductivity Changes in MOS
2.1.2. Surface Conductive Changes in MOS
2.2. Parameters for MOS Gas Sensor Performance
2.2.1. Response
2.2.2. Selectivity
2.2.3. Response and Recovery Time
2.2.4. Robustness
2.3. Materials Properties for MOS Gas Sensors Performance
2.3.1. Receptor Function
2.3.2. Transducer Function
2.3.3. Utility
2.4. Instability of MOS Gas Sensor
3. Non-Metal Oxide Gas Sensors
3.1. Working Principle
3.2. Working Environment
Conclusion
Future Aspects of MOS Gas Sensors as the Diagnostic Breath Analyzer
Acknowledgments
References
Section 2. Zinc Oxide (ZnO) based Chemical Sensors
Chapter 2
Application of ZnO Nanowhiskers for the Detection of p-Hydroquinone
Abstract
Introduction
Morphological and Crystalline Properties of ZnO Nanowhiskers
The Field Emission Scanning Electron Microscopy, the Energy Dispersive X-Rays Spectroscopy and X-Rays Diffraction Patterns
Optical and Structural Studies of ZnO Nanowhiskers
The UV–Vis Absorption, Photoluminescence and the Raman Scattering Spectroscopy
The Sensitivity Parameters of ZnO Nanowhiskers Modified GCE through Electrochemical System
Conclusion
References
Chapter 3
Selective Monitoring of Piperidine by Spindles Shaped ZnO Modified Glassy Carbon Electrode
Abstract
Introduction
Morphological, Elemental Composition and Crystalline Properties of Spindles Shaped ZnO
The Field Emission Scanning Electron Microscopy, the Energy Dispersive X-Rays and X-Rays Diffraction Patterns
Optical and Structural Studies of Spindles Shaped ZnO
Ultra-Violet Diffused Reflectance, Photoluminescence and the Raman Scattering Spectroscopy
Sensing Studies and Schematic Illustration of Spindles Shaped ZnO Based Piperidine Chemical Sensor
Conclusion
References
Chapter 4
ZnO Nanotubes as Efficient Electrodes for the Detection of Ethanolamine Chemical
Abstract
Introduction
Morphological, Structural and Crystalline Properties of Aligned ZnO Nanotubes
The Field Emission Scanning Electron Microscopy, X-Rays Diffraction Patterns and Fourier Transform Infrared Spectroscopy
Optical Properties of Aligned ZnO Nanotubes
Ultra-Violet Diffused Reflectance and Photoluminescence Spectra
Structural Properties of Aligned ZnO Nanotubes
The Raman Scattering Spectroscopy and Raman Mapping
Sensitivity Measurements of Aligned ZnO Nanotubes Based Ethanolamine Chemical Sensor
Conclusion
References
Chapter 5
Cabbage-Like ZnO Nanostructures for the Electrochemical Detection of Resorcinol
Abstract
Introduction
Morphological Properties of Cabbages like Zinc Oxide Nanostructures
The Field Emission Scanning Electron Microscopy and the Transmission Electron Studies
The Elemental Compositions, Structural, Crystalline and Optical Properties of Cabbages like Zinc Oxide Nanostructures
The Energy Dispersive X-Rays Spectroscopy, X-Rays Diffraction Patterns, Fourier Transform Infrared, UV–Vis Absorption Spectrum
The X-Rays Photoelectron Spectroscopy Studies of Cabbages Like Zinc Oxide Nanostructures
Sensitivity Measurements of Cabbages like Zinc Oxide Nanostructures
Conclusion
References
Section 3. Titanium Oxide (TiO2) based Chemical Sensors
Chapter 6
TiO2 Nanotube Arrays for the Sensing of Phenyl Hydrazine
Abstract
Introduction
Morphological and Structural Properties| of TiO2 Nanotube Arrays
The Field Emission Scanning Electron Microscopy, the Transmission Electron Microscopy and the Raman Scattering Spectroscopy
The Sensing Measurements of TiO2 Nanotubes Arrays Based Chemical Sensor for the Detection of Phenyl Hydrazine
Conclusion
References
Section 4. Conducting Polymers based Chemical Sensors
Chapter 7
Application of Polypyrrole Nanobelts as Electrode Material for the Detection of Aliphatic Alcohols
Abstract
Introduction
Morphological Studies of Polypyrrole Nanobelts
The Field Emission Scanning Electron Microscopic and the Transmission Electron Microscopic Studies
The Atomic Force Spectroscopy
The Line Scan Element Mapping
Structural Properties of Polypyrrole Nanobelts
Fourier Transform Infrared Spectroscopy
The Raman Scattering Spectroscopy
The Optical Characterizations of Polypyrrole Nanobelts
The UV–Vis Absorption and the Photoluminescence Spectra
The Electrocatalytic Activity and Conductivity Measurements of Polypyrrole Nanobelts Electrode
The Electrochemical Impedance Spectroscopy Measurements
DC Conductivity of PPy Nanobelts Based Electrode
The Sensing Properties of Polypyrrole Nanobelts Electrode
The I–V Characteristics of Polypyrrole Nanobelts Based Chemical Sensor
Schematic Illustration of Electrochemical System and Interference Tests of the Fabricated Aliphatic Alcohol Sensors
FTIR and 1H NMR Spectra of PPy Nanobelts Electrode
Conclusion
References
Chapter 8
The Fabrication of the Schottky Junction Diode Using Aligned Polypyrrole Nanofibers for the Broad Range Detection of M-Dihydroxybenzene
Abstract
Introduction
Morphological Properties of Aligned Polypyrrole Nanofibers
The Field Emission Scanning Electron Microscopy
Structural and Optical Studies of Aligned Polypyrrole Nanofibers
Fourier Transform Infrared Spectroscopy
The Raman Scattering and Raman Mapping Spectroscopy
The UV–Vis Absorption and Photoluminescence Spectra
The Sensing Properties of Aligned Polypyrrole Nanofibers Electrode
The I–V Characteristics of Pt/p-Aligned PPy NFs/n-Si Schottky Junction Diode Based Chemical Sensor
Cyclovoltammetry Studies
Amperometric Response and Interference Test of the Fabricated Sensor
1H NMR Spectra of PPy Nanobelts Electrode and Proposed Mechanism
Conclusion
References
Chapter 9
Using the p-NiO/n-Polyaniline/n-Si Schottky Diode to Detect Hydrazinobenzene
Abstract
Introduction
Morphological, and Crystalline Characterizations of p-NiO, n-PANI and p-NiO/n-PANI Thin Films
The Field Emission Scanning Electron Microscopy and X-Rays Diffraction Patterns
Atomic Force Microscopy
Structural and Optical of PANI EB and n-PANI
Fourier Transform Infrared Spectroscopy
The UV–vis Absorption and Photoluminescence Spectra
Electrochemical Characterizations of Pt/p-NiO/n-PANI/n-Si Schottly Barrier Diode
Cyclicvoltammetry (CV) Measurements of Pt/p-NiO/n-PANI/n-Si Schottky Barrier Diode
Pt/p-NiO/n-PANI/n-Si Schottky Barrier Diode Based Hydrazinobenzene Chemosensor
The Stability and the Reproducibility of the Fabricated Hydrazinobenzene Chemical Sensor
The Electrochemical Impedance Spectroscopy (EIS) of Pt/p-NiO/n-PANI/n-Si Schottky Barrier Diode
Electrochemical Mechanism of Hydrazinobenzene Chemical Over Pt/p-NiO/ n-PANI/n-Si Schottky Barrier Diode
Illustration of Chemical Sensors over the Surface of Pt/p-NiO/n-PANI/n-Si Schottky Barrier Diode and Its Schottky Barrier Heights without and with Hydrazinobenzene Chemical
1H NMR Spectra of the Fabricated Pt/p-NiO/n-PANI/n-Si Schottky Barrier Diode
Conclusion
References
Chapter 10
Aligned Polyaniline Nanowires as Electrode Material for Glucose Biosensor
Abstract
Introduction
Morphological Studies of Aligned Polyaniline Nanowires
The Field Emission Scanning Electron Microscopy and the Line Scanning Element Mapping
Atomic Force Microscopy
Structural and Optical Properties of Aligned Polyaniline Nanowires
The Raman Scattering Spectroscopy and Raman Mapping
UV–Vis and Photoluminescence Spectroscopies
Non-Enzymatic Biosensor Based on Aligned Polyaniline Nanowires Electrode
Investigations of Electroctalytic Activity through Cyclovoltametry
The I–V Responses of the Fabricated Sensor
The Amperometric Response and Interference Tests of the Fabricated Electrode Based Non-Enzymatic Glucose Biosensor
Proposed Mechanism of the Fabricated Electrode Based Non-Enzymatic Glucose Biosensor
1H NMR Spectra of NCa-PANI NWs
Conclusion
References
Part II. Fundamentals of Functional Materials: Applications for Photocatalyst
Section 1. Zinc Oxide (ZnO) based Photocatalysts
Chapter 11
Degradation of Bromophenol Dye over ZnO Nanoflowers
Abstract
Introduction
Morphological and Crystalline Studies of ZnO Nanoflowers
The Field Emission Scanning Electron Microscopy, the Energy-Dispersive X-Ray Spectroscopy and X-Rays Diffraction Patterns
Optical and Structural Properties of ZnO Nanoflowers
The UV-Vis Absorption Spectrum, Photoluminescence Spectrum and Raman Scattering Spectroscopy
The Photocatalytic Activities of ZnO Nanoflowers
UV-Vis Absorbance Spectra of Decomposed Bromophenol Dye Solution over ZnO Nanoflowers under UV Light Irradiation and the Mass Spectra of Bromophenol Dye Sluotions
Conclusion
References
Chapter 12
ZnO Flower Nanomaterials as Photocatalyst for the Degradation of Crystal Violet Dye
Abstract
Introduction
Morphological and Crystalline Studies of ZnO Nanoflowers
The Field Emission Scanning Electron Microscopy, the Energy-Dispersive X-Ray Spectroscopy and X-Rays Diffraction Patterns
Optical and Structural Properties of ZnO Nanoflowers
The Ultraviolet-Diffused Reflectance Spectroscopy, Fourier Transform Infrared Spectroscopy, Raman Scattering Spectroscopy and the Photoluminescence Spectrum
The Photocatalytic Degradation of Crystal Violet Dye over the Surface of ZnO Flowers under UV Illumination
UV-Vis Absorbance Spectra of Decomposed Crystal Violet Dye Solution over ZnO Flowers under UV Light Irradiation and the Mass Spectra of Bromophenol Dye Solutions
Conclusion
References
Chapter 13
The Mineralization of Cationic Dye Using ZnO Hollow Nano-Baskets
Abstract
Introduction
Morphological and Crystalline Studies of ZnO Hollow Nano-Baskets
The Field Emission Scanning Electron Microscopy, the Energy-Dispersive X-Ray Spectroscopy and X-Ray Diffraction Patterns
Optical and Structural Properties of ZnO Hollow Nano-Baskets
The Ultraviolet-Diffused Reflectance Spectroscopy, Raman Scattering and Photoluminescence Spectrum
The Photocatalytic Degradation of Rhodamine 6G (Rh6G) Dye over the Surface of ZnO Hollow Nano-Baskets under UV Illumination
UV-Vis Absorbance Spectra of Decomposed Rhodamine 6G (Rh6G) Dye Solution over ZnO Hollow Nano-Baskets and the Mass Spectra of Rhodamine 6G (Rh6G) Dye Solutions
Conclusion
References
Chapter 14
The Facile Synthesis of ZnO–Graphene Oxide Nanohybrid and its Photocatalytic Application
Abstract
Introduction
Morphological and Crystalline Studies of ZnO–Graphene Oxide Nanohybrid
The Field Emission Scanning Electron Microscopy, the Element Line Scanning and X-Rays Diffraction Patterns
Structural and Optical Properties of ZnO–Graphene Oxide Nanohybrid
The Raman Scattering and Photoluminescence Spectrum
The Photocatalytic Degradation of Crystal Violet (Cv) Dye Over the Surface of ZnO–Graphene Oxide Nanohybrid
UV-Vis Absorbance Spectra of Decomposed Crystal Violet (Cv) Dye Solution Over ZnO–Graphene Oxide Nanohybrid Under Light Illumination
Mass Spectra of Crystal Violet (Cv) Solutions Before and After the Photocatalytic Reaction
Conclusion
References
Section 2. Titanium Oxide (TiO2) based Photocatalysts
Chapter 15
Visible Light Driven Photocatalytic Degradation of Bromophenol Dye over CeO2/TiO2 Nanocomposite
Abstract
Introduction
Morphological Studies of CeO2-TiO2 Nanocomposite
The Field Emission Scanning Electron and Transmission Electron Microscopy
Crystalline and Optical Properties of CeO2-TiO2 Nanocomposite
X-Rays Diffraction Patterns and UV-Vis Absorption Spectrum
The Photocatalytic Activity of CeO2-TiO2 Nanocomposite by the Degradation of Bromophenol Dye
UV-Vis Absorbance Spectra of Decomposed Bromophenol Dye Solution over CeO2-TiO2 Nanocomposite under Light Illumination
Mass Spectra of Bromophenol Dye Solutions over CeO2-TiO2 Nanocomposite
The Schematic Illustration of the Photocatalytic Activity of CeO2-TiO2 Nanocomposite
Conclusion
References
Chapter 16
The Effect of Fe Doping on TiO2 Nanoparticles for the Photocatalytic Degradation of Toxic Organic Compounds
Abstract
Introduction
Crystalline Properties of Fe-doped TiO2 Nanoparticles
X-Rays Diffraction Patterns
Morphological Studies of Fe-doped TiO2 Nanoparticles
The Field Emission Scanning Electron Microscopy
The Transmission Electron Microscopy
Structural Properties of Fe-doped TiO2 Nanoparticles
Fourier Transform Infrared Spectroscopy
Thermogravimetric and Differential Scanning Calorimetry of Fe-Doped TiO2 Nanoparticles
Optical Studies of Fe-doped TiO2 Nanoparticles
The UV–vis Absorption Spectrum
Photoluminescence Spectrum
XPS Spectra of Fe-doped TiO2 Nanoparticles
Photocatalytic Degradation of Paranitrophenol over the Surface of Fe-doped TiO2 Nanoparticles
UV–vis Absorbance Spectra of Visible Light Induced Degradation of Paranitrophenol Aqueous Solution
Conclusion
References
Chapter 17
The Application of Sr-Doped TiO2 Nanoparticles for the Effective Photocatalytic Degradation of “Brilliant Green” Dye
Abstract
Introduction
Crystalline Studies
X-Ray Diffraction Patterns
Structural Properties, Surface Area, Thermogravimetric Analysis and the Optical Characterizations of Sr-Doped TiO2 Sample
Transmission Electron Microscopy Studies
BET Surface Area and Pore Size Distribution
Fourier Transform Infrared Spectroscopy and Thermogravimetric Analysis
X-Ray Photoelectron Spectroscopy Analysis
Photoluminescence Studies
Photocatalytic Activity
Conclusion
References
Section 3. Conducting Polymers based Photocatalysts
Chapter 18
The Utilization of Graphene/Polyaniline Nanocomposites for the Degradation of Rose Bengal Dye
Abstract
Introduction
Morphological Studies of Graphene/Polyaniline Nanocomposites
The Field Emission Scanning Electron Microscopy
Optical Properties of Graphene/ Polyaniline Nanocomposites
UV-Vis Absorbance Spectra and Photoluminescence Characterizations
Structural Characterizations of Graphene/Polyaniline Nanocomposites
Fourier Transform Infrared Spectroscopy and Raman Scattering Spectroscopy
X-Ray Photoelectron Spectroscopy Analysis of Graphene/Polyaniline Nanocomposites
The Photocatalytic Activity of PANI–Gr Nanocomposites
UV–Vis Spectra of Decomposed Rose Bengal Dye Solution under Light Illumination over the Surface of PANI–Gr
Schematic Illustrations of Photocatalytic RB Dye Degradation over the Surface of PANI–Gr Nanocomposite
Mass Spectra of Rose Bengal Dye Solution over PANI–Gr Nanocomposite and the Possible Reaction Intermediates after the Photocatalytic Reaction
Conclusion
References
Part III. Fundamentals of Functional Materials: Applications for Renewable Energy
Section 1. Small Organic Molecules based organic Solar Cells
Chapter 19
The Performance of Organic Solar Cells: Small Molecules Based on Thiazolothiazole
Abstract
Introduction
Scheme for the Synthesis of Thiazolothiazole Based Linear Chromophore (RTzR)
Optical Properties of RTzR
UV–Vis and Photoluminescence Spectra
The Cyclic Voltammetry (CV) of RTzR Thin Film
The Current (J)–Voltage (V) Curves of Organic Solar Cells Device of RTzR:PCBM Active Layers
Atomic Force Microscopy Spectroscopy of RTzR:PCBM Active Layer
Conclusion
References
Chapter 20
Solution-Processed Bulk-Heterojunction Organic Solar Cell Based on a Furan-Bridged Thiazolo [5,4-d]thiazole Based D–π–A–π–D Type Linear Chromophore
Abstract
Introduction
Synthetic Route of the Furan-Bridged Organic Chromophore (RFTzR)
Synthesis of 2,5-Di(Furan-2-Yl)Thiazolo[5,4-D]Thiazole, 2
Synthesis of 2,5-Bis(5-Bromofuran-2-Yl)Thiazolo[5,4-D]Thiazole, 3
Synthesis of 2,5-Bis(5-(5-(5-Hexylthiophen-2-Yl)-Thiophen-2-Yl)Furan-2-Yl) Thiazolo[5,4-D]Thiazole (RFTzR)
Thermogravimetric and Differential Scanning Calorimetry Thermograms of Furan-Based Linear RFTzR Chromophore
Optical Characterizations of RFTzR
UV–Vis Spectroscopy and Photoluminescence Spectra
Cyclic Voltammetry of the Furan-Bridged Organic Chromophore
The Photovoltaic Parameters of the Fabricated Organic Solar Cell
The Current (J)–Voltage (V) Curves and the Incident Photon-to-Current Conversion Efficiency Spectra of the Fabricated Organic Solar Cells with the Active Layer of RFTzR: PC60BM
Atomic Force Microscopy (AFM) Spectroscopy of RFTzR:PCBM Active Layer
Conclusion
References
Chapter 21
Fumaronitrile-Core and Terminal Alkylated Bithiophene for Solution Processed Small Molecule Organic Solar Cells
Abstract
Introduction
Synthetic Route of Fumaronitrile Based Organic Chromophore (RCNR)
1-(5-(Thiophen-2-yl) Thiophen-2-yl) Hexan-1-One (2)
2-Decyl-5-(Thiophen-2-yl) Thiophene (3)
5-Bromo-5’-Decyl-2,2’-Bithiophene (4)
2-{5-(5-Decylthiophen-2-yl) Thiophen-2-yl}-4,4,5,5-Tetramethyl-1,3,2-Dioxaborolane (5)
Bis (4-Bromophenyl) Fumaronitrile (7)
2,3-Bis(4-(5-(5-Hexylthiophen-2-yl)Thiophen-2-yl)Phenyl)Fumaronitrile (RCNR)
Thermogravimetric Analysis and Differential Scanning Colorimetry Plots of the Organic Chromophore
Optical Characterizations
Ultraviolet-Visible and Photoluminescence Spectra of RCNR
Cyclic Voltammetry of RCNR Thin Film
The Current Density (J)-Voltage (V) Curves of Fabricated Small Molecule Organic Solar Cells with the Different RCNR:PC60BM Active Layers
Atomic Force Microscopy Spectroscopy of RCNR:PCBM Active Layer
Conclusion
References
Chapter 22
Spirobifluorene-Core Electron-Donor Material for Bulk-Heterojunction Solar Cells
Abstract
Introduction
Synthetic Route of Spirobifluorene-Based Small Molecule
1-(5-(Thiophen-2-yl)Thiophen-2-yl)Hexan-1-One (2)
5-Bromo-50-Decyl-2,20-Bithiophene (3)
2-(5-(5-Decylthiophen-2-yl) Thiophen-2-yl) -4,4,5,5-Tetramethyl-1,3,2-Dioxaborolane (4)
2-(7-Bromo-9,90-Spirobifluorene-2-yl)-5-(5-Hexylthiophen-2-yl) Thiophene (6)
2-(7-(3,5-Bis(Trifluoromethyl)Phenyl)-9,90-Spirobifluorene-2-yl)-5-(5-Hexylthiophen-2-yl)Thiophene (7)
Thermogravimetric and Differential Scanning Colorimetry Thermo Gram of Spirobifluorene-Based Small Molecule
Optical Properties of RTh-Sp-CF3
UV–Vis and Photoluminescence Spectra of RTh-Sp-CF3
Cyclic Voltammetry of the Spirobifluorene-Based Thin Film
The Current Density (J)-Voltage (V) Curves of Fabricated Organic Solar Cells with the Active Layer of RTh-Sp-CF3:PC61BM
Atomic Force Microscopy Spectroscopy of RTh-Sp-CF3:PC61BM Active Layer
Conclusion
References
Section 2. Zinc Oxide (ZnO) Photoanode based Dye Sensitized Solar Cells (DSSCs)
Chapter 23
Hydrothermal Synthesis of ZnO Materials for a Dye-Sensitized Solar Cell
Abstract
Introduction
Morphological Studies of Synthesized ZnO Nanostructures
The Field Emission Scanning Electron Microscopy
The Transmission Electron Microscopy
Crystalline, Structural and Optical Properties of ZnO Nanostructures
X-Rays Diffraction Patterns
The Raman Scattering Spectroscopy
UV–Vis Spectrum
Photovoltaic Performance of ZnO Nanostructures Based Dsscs
The Current Density (I)-Voltage (V) of the Fabricated DSSC
Conclusion
References
Chapter 24
Low Temperature Grown ZnO Nanotubes for Dye Sensitized Solar Cells
Abstract
Introduction
Morphological Studies of ZnO Nanotubes
The Field Emission Scanning Electron Microscopy
The Transmission Electron Microscopy
Crystalline, Optical and Structural Properties of ZnO Nanotubes
X-ray Diffraction Patterns and UV–vis Spectra
The Raman Scattering Spectroscopy and Photoluminescence Spectra
Possible Growth Mechanism and the Formation of ZnO Nanotubes
Photovoltaic Performances of ZnO Nanotubes Based DSSCs
The Current (I)-Voltage (V) Cureves of DSSCs Fabricated with ZnO Nanotubes
The Incident Photon-to-Current Conversion Efficiencycurves of the DSSCs Fabricated with ZnO Nanotubes
Conclusion
References
Chapter 25
Nanospikes Decorated ZnO Sheets for Solar Cell Application
Abstract
Introduction
Morphological Studies of Nanospikes Decorated ZnO Sheets
The Field Emission Scanning Electron Microscopy
The Transmission Electron Microscopy
Crystalline and Optical Characterization of Nanospikes Decorated ZnO Sheets
X-Ray Diffraction Patterns and UV–Vis Spectrum
Structural Properties of Nanospikes Decorated ZnO Sheets
Fourier-Transform Infrared and Raman Scattering Spectroscopy
X-Ray Photoelectron Spectroscopy of Nanospikes Decorated ZnO Sheets
Possible Growth Mechanism and the Formation of Nanospikes Decorated ZnO Sheets
Photovoltaic Performances of DSSCs Fabricated with Nanospikes Decorated ZnO Sheets Photoanode
The Current Density-Voltage (J-V) Curve and the Incident Photon to Current Conversion Efficiency of the Fabricated DSSC
Conclusion
References
Chapter 26
Tin (Sn) Doped ZnO Nanostructures for the Application of Dye Sensitized Solar Cells
Abstract
Introduction
Morphological Studies of Sn Doped ZnO Nanostructures
The Field Emission Scanning Electron Microscopy
The Transmission Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy
Crystalline and Structural Properties of Sn Doped ZnO Nanostructures
X-Rays Diffraction Patterns and Raman Scattering Spectroscopy
Optical Characterizations of Sn Doped ZnO Nanostructures
Ultra-Violet Diffused Reflectance and Photoluminescence Spectrum
X-Ray Photoelectron Spectroscopy of Sn Doped ZnO Nanostructures
Photovoltaic Performance of DSSCs Fabricated with ZnO and Sn-ZnO Thin Film Electrodes
The Current Density-Voltage (J-V) Characteristics of DSSCs
Conclusion
References
Section 3. Titanium Oxide (TiO2) Photoanode based Dye Sensitized Solar Cells (DSSCs)
Chapter 27
Crystalline-TiO2 Flowers for Dye Sensitized Solar Cells
Abstract
Introduction
Morphological Studies of TiO2 Nanoflowers
The Field Emission Scanning Electron Microscopy
The Transmission Electron Microscopy
Atomic Force Microscopy
Crystalline and Optical Properties of TiO2 Nanoflowers
X-Rays Diffraction Patterns, Ultra Violet-Diffused Reflectance, and Photoluminescence Spectroscopy
Structural Studies of TiO2 Nanoflowers
The Raman Spectrum and Raman Mapping
X-Ray Photoelectron Spectroscopy
Schematic Illustration for the Proposed Growth Mechanism of TiO2 Nanoflowers
Charge Transportation and Charge Collection Properties of TiO2 Nanoflowers Based DSSCs
Electrochemical Impedance (EIS), Intensity Modulated Photocurrent Spectroscopy (IMPS) and Intensity-Modulated Photovoltage Spectroscopy (IMVS) Measurements
Photovoltaic Performance of TiO2 Nanoflowers Thin Film Photoanode-Based DSSC
The Current Density–Voltage (J–V) Characteristics and the Incident Photon-to-Current Conversion Efficiency (IPCE) of the TiO2 NF Thin Film Photoanode Based DSSC
Conclusion
References
Chapter 28
Graphene Oxide (GO) Incorporation in TiO2 Nanofibers for Dye-Sensitized Solar Cells
Abstract
Introduction
Morphological Studies of GO Incorporated TiO2 Nanofibers
The Field Emission Scanning Electron Microscopy
Structural and Crystalline Characterizations of GO Incorporated TiO2 Nanofibers
X-Ray Photoelectron Spectroscopy
The Raman Spectrum
Photovoltaic Performance of GO Incorporated TiO2 Nanofibers Photoanode-based DSSC
Electrochemical Impedance Measurements
The Incident Photon-To-Current Conversion Efficiency of the TiO2 NF Thin Film Photoanode Based DSSC
The Current Density–Voltage (J–V) Characteristics
Conclusion
References
Chapter 29
Gel Electrolytes with Titania Nanotube Fillers for Solid-State Dye-Sensitized Solar Cell
Abstract
Introduction
Morphological Studies of TiO2 Thin Film
The Field Emission Scanning Electron Microscopy
Structural Characterizations of TiO2 Thin Film
X-Ray Photoelectron Spectroscopy
Photovoltaic Performance of TiO2 Thin Film Photoanode-Based DSSC
The Current Density–Voltage (J–V) Characteristics
Conclusion
References
Section 4. Perovskite Solar Cells (PSCs)
Chapter 30
ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cells
Abstract
Introduction
The Investigation of the Morphology and the Surface Modifications
Atomic Force Spectroscopy and Contact Angle Measurements
The Transmittances and Sheet Resistance of ITO-PET and Thin Films Substrates
Structural Characterizations of TiO2 Thin Film
X-Ray Photoelectron Spectroscopy
The Raman Spectrum
Schematic Illustration of the Fabricated ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/Spiro-MeOTAD/ Ag Flexible Perovskite Solar Cell
Photovoltaic Performances of the Fabricated Flexible Perovskite Solar Cells
The Current Density–Voltage (J–V) Characteristics and the Incident Photon-to-Current Conversion Efficiency (IPCE) of Fabricated Flexible Perovskite Solar Cells
Electrochemical Impedance Measurements
The Charge Collection Efficiency and Photoelectron Density Analysis
The Intensity-Modulated Photocurrent Spectroscopy (IMPS) and the Intensity-Modulated Photovoltage Spectroscopy (IMVS)
Conclusion
References
Chapter 31
RF Sputtered Ti Layer for Flexible Perovskite Solar Cells
Abstract
Introduction
Morphological Studies of Ti Thin Film
The Field Emission Scanning Electron Microscopy
Atomic Force Microscopy
The Transmittance of Thin Film Substrates
Structural Characterizations of Thin Film Substrate
X-Ray Photoelectron Spectroscopy
The Performance of the Flexible Perovskite Solar Cell
Current Density (J)–Voltage (V) Measurements and the Incident Photon-to-Current Conversion Efficiency
Electrochemical Impedance Measurements
The Charge Collection Efficiency and Photoelectron Density Analysis
The Intensity-Modulated Photocurrent Spectroscopy (IMPS) and Intensity-Modulated Photovoltage Spectroscopy (IMVS)
Conclusion
References
Chapter 32
Conducting Channels of the Hole Transporting Layer to Adjacent Photoactive Perovskite Sensitized TiO2 Thin Films for a Solar Cell
Abstract
Introduction
Morphological Studies of CH3NH3PbI3/ITO-PET and PPy/PC61BM/CH3NH3PbI3/ITO-PET Thin Film
The Field Emission Scanning Electron Microscopy
Atomic Force Microscopy
The Line Scan Element Mapping Spectroscopy
Crystalline and Optical Properties of CH3NH3PbI3/FTO and CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO Thin Films
X-ray Powder Diffraction
UV and Photoluminescence Spectra
Schematic Representation of the Fabricated Ag/PPy/PC61BM/ CH3NH3PbI3/PEDOT:PSS/ ITO-PET Flexible Perovskite Solar Cell
The Performance of the Perovskite Solar Cell
Current Density (J)–Voltage (V) Measurements, Electrochemical Impedance and the Incident Photon-to-Current Conversion Efficiency
The Charge Collection Efficiency and Photoelectron Density Analysis
The Intensity-Modulated Photocurrent Spectroscopy (IMPS) and Intensity-Modulated Photovoltage Spectroscopy (IMVS)
Conclusion
References
Chapter 33
Metal Oxide Free Perovskite Solar Cell
Abstract
Introduction
Morphological Studies of CH3NH3PbI3/ITO-PET and PPy/PC61BM/CH3NH3PbI3/ITO-PET Thin Films
Atomic Force Microscopy
The Confocal Laser Scanning Microscopy
Crystalline and Optical Properties of CH3NH3PbI3/ITO-PET and PPy/PC61BM/CH3NH3PbI3/ITOPET Thin Films
X-Ray Powder Diffraction and UV−Vis Spectra
The Performance of the Perovskite Solar Cell
Schematic Representation and the Nyquist Plots of the Fabricated Flexible Perovskite Solar
The Current (J)–Voltage (V) Curve and the Incident Photon-to-Current Conversion Efficiency of the Fabricated Flexible Perovskite Solar Cell
The Charge Collection Efficiency and Photoelectron Density Analysis
The Intensity-Modulated Photocurrent Spectroscopy (IMPS) and Intensity-Modulated Photovoltage Spectroscopy (IMVS)
Conclusion
References
About the Editors
Index
Blank Page

Citation preview

ENVIRONMENTAL RESEARCH ADVANCES

EMERGING MATERIALS FOR ENVIRONMENT PROTECTION AND RENEWABLE ENERGY

No part of this digital document may be reproduced, stored in a retrieval system or transmitted in any form or by any means. The publisher has taken reasonable care in the preparation of this digital document, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained herein. This digital document is sold with the clear understanding that the publisher is not engaged in rendering legal, medical or any other professional services.

ENVIRONMENTAL RESEARCH ADVANCES Additional books in this series can be found on Nova’s website under the Series tab.

Additional e-books in this series can be found on Nova’s website under the eBooks tab.

ENVIRONMENTAL RESEARCH ADVANCES

EMERGING MATERIALS FOR ENVIRONMENT PROTECTION AND RENEWABLE ENERGY

M. SHAHEER AKHTAR SADIA AMEEN AND

HYUNG-SHIK SHIN EDITORS

Copyright © 2018 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. We have partnered with Copyright Clearance Center to make it easy for you to obtain permissions to reuse content from this publication. Simply navigate to this publication’s page on Nova’s website and locate the “Get Permission” button below the title description. This button is linked directly to the title’s permission page on copyright.com. Alternatively, you can visit copyright.com and search by title, ISBN, or ISSN. For further questions about using the service on copyright.com, please contact: Copyright Clearance Center Phone: +1-(978) 750-8400 Fax: +1-(978) 750-4470 E-mail: [email protected].

NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book. Library of Congress Cataloging-in-Publication Data Names: Akhtar, M. Shaheer, editor. | Shin, Sadia Ameen, editor. | Shin, Hyung-Shik, 1955- editor. Title: Emerging materials for environment protection and renewable energy / M. Shaheer Akhtar, Sadia Ameen and Hyung-Shik Shin, Chonbuk National University, Jeonbuk, Republic of Korea, editors. Description: Hauppauge, New York : Nova Science Publishers Inc., [2018] | Series: Environmental research advances | Includes bibliographical references and index. Identifiers: LCCN 2018026973 (print) | LCCN 2018028512 (ebook) | ISBN 9781536138511 (ebook) | ISBN 9781536138504 (hardcover) | ISBN 9781536138511 (ebook) Subjects: LCSH: Pollution control equipment--Materials. | Chemical detectors. | Pollutants. | Catalysts. | Renewable energy sources. Classification: LCC TD192 (ebook) | LCC TD192 .E43 2018 (print) | DDC 621.042028/4--dc23 LC record available at https://lccn.loc.gov/2018026973z

Published by Nova Science Publishers, Inc. † New York

CONTENTS Preface Part I.

xi Fundamentals of Functional Materials: Applications for Sensors

1

Section 1.

Metal Oxides based Gas Sensors

3

Chapter 1

Science and Technology of Metal Oxide Semiconductor Gas Sensor Prabhakar Rai and Praveen Kumar Sekhar

5

Section 2.

Zinc Oxide (ZnO) based Chemical Sensors

27

Chapter 2

Application of ZnO Nanowhiskers for the Detection of p-Hydroquinone Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

29

Selective Monitoring of Piperidine by Spindles Shaped ZnO Modified Glassy Carbon Electrode Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

39

ZnO Nanotubes as Efficient Electrodes for the Detection of Ethanolamine Chemical Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

49

Chapter 3

Chapter 4

Chapter 5

Cabbage-Like ZnO Nanostructures for the Electrochemical Detection of Resorcinol Sadia Ameen, Eun-Bi Kim, M. Shaheer Akhtar and Hyung Shik Shin

59

vi

Contents

Section 3.

Titanium Oxide (TiO2) based Chemical Sensors

69

Chapter 6

TiO2 Nanotube Arrays for the Sensing of Phenyl Hydrazine Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo and Hyung Shik Shin

71

Section 4.

Conducting Polymers based Chemical Sensors

81

Chapter 7

Application of Polypyrrole Nanobelts as Electrode Material for the Detection of Aliphatic Alcohols Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo and Hyung Shik Shin

Chapter 8

Chapter 9

Chapter 10

Part II.

83

The Fabrication of the Schottky Junction Diode Using Aligned Polypyrrole Nanofibers for the Broad Range Detection of M-Dihydroxybenzene Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo and Hyung Shik Shin

103

Using the p-NiO/n-Polyaniline/n-Si Schottky Diode to Detect Hydrazinobenzene Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

123

Aligned Polyaniline Nanowires as Electrode Material for Glucose Biosensor Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

145

Fundamentals of Functional Materials: Applications for Photocatalyst

165

Section 1.

Zinc Oxide (ZnO) based Photocatalysts

167

Chapter 11

Degradation of Bromophenol Dye over ZnO Nanoflowers Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

169

Chapter 12

ZnO Flower Nanomaterials as Photocatalyst for the Degradation of Crystal Violet Dye Sadia Ameen, M. Shaheer Akhtar, M. Nazim and Hyung Shik Shin

Chapter 13

The Mineralization of Cationic Dye Using ZnO Hollow Nano-Baskets Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

179

189

Contents Chapter 14

The Facile Synthesis of ZnO–Graphene Oxide Nanohybrid and its Photocatalytic Application Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo and Hyung Shik Shin

Section 2.

Titanium Oxide (TiO2) based Photocatalysts

Chapter 15

Visible Light Driven Photocatalytic Degradation of Bromophenol Dye over CeO2/TiO2 Nanocomposite Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo and Hyung Shik Shin

Chapter 16

Chapter 17

The Effect of Fe Doping on TiO2 Nanoparticles for the Photocatalytic Degradation of Toxic Organic Compounds Swati Sood, Ahmad Umar, Surinder Kumar Mehta and Sushil Kumar Kansal The Application of Sr-Doped TiO2 Nanoparticles for the Effective Photocatalytic Degradation of “Brilliant Green” Dye Swati Sood, Ahmad Umar, Surinder Kumar Mehta, A. S. K. Sinha and Sushil Kumar Kansal

Section 3.

Conducting Polymers based Photocatalysts

Chapter 18

The Utilization of Graphene/Polyaniline Nanocomposites for the Degradation of Rose Bengal Dye Sadia Ameen, Hyung-Kee Seo, M. Shaheer Akhtar and Hyung Shik Shin

Part III.

vii

199

209 211

221

235

249 251

Fundamentals of Functional Materials: Applications for Renewable Energy

267

Section 1.

Small Organic Molecules based organic Solar Cells

269

Chapter 19

The Performance of Organic Solar Cells: Small Molecules Based on Thiazolothiazole M. Nazim, Sadia Ameen, M. Shaheer Akhta, Youn-Sik Lee and Hyung Shik Shin

Chapter 20

Solution-Processed Bulk-Heterojunction Organic Solar Cell Based on a Furan-Bridged Thiazolo [5,4-d]thiazole Based D–π–A–π–D Type Linear Chromophore M. Nazim, Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo and Hyung Shik Shin

271

281

viii Chapter 21

Chapter 22

Section 2. Chapter 23

Chapter 24

Contents Fumaronitrile-Core and Terminal Alkylated Bithiophene for Solution Processed Small Molecule Organic Solar Cells M. Nazim, Sadia Ameen, Hyung-Kee Seo and Hyung Shik Shin Spirobifluorene-Core Electron-Donor Material for Bulk-Heterojunction Solar Cells M. Nazim, Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin Zinc Oxide (ZnO) Photoanode based Dye Sensitized Solar Cells (DSSCs) Hydrothermal Synthesis of ZnO Materials for a Dye-Sensitized Solar Cell M. Shaheer Akhtar, M. Alam Khan, Myung Seok Jeon and O-Bong Yang Low Temperature Grown ZnO Nanotubes for Dye Sensitized Solar Cells Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim, O-Bong Yang and Hyung Shik Shin

Chapter 25

Nanospikes Decorated ZnO Sheets for Solar Cell Application Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim and Hyung Shik Shin

Chapter 26

Tin (Sn) Doped ZnO Nanostructures for the Application of Dye Sensitized Solar Cells Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim and Hyung Shik Shin

Section 3.

Titanium Oxide (TiO2) Photoanode based Dye Sensitized Solar Cells (DSSCs)

Chapter 27

Crystalline-TiO2 Flowers for Dye Sensitized Solar Cells Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo and Hyung Shik Shin

Chapter 28

Graphene Oxide (GO) Incorporation in TiO2 Nanofibers for Dye-Sensitized Solar Cells Moaaed Motlak, Nasser A. M. Barakat, M. Shaheer Akhtar, A. M. Hamza, Ayman Yousef, H. Fouad and O-Bong Yang

Chapter 29

Gel Electrolytes with Titania Nanotube Fillers for Solid-State Dye-Sensitized Solar Cell M. Shaheer Akhtar, Ji-Min Chun and O-Bong Yang

297

313

329 331

341

355

369

381 383

399

411

Contents Section 4.

Perovskite Solar Cells (PSCs)

Chapter 30

ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cells Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo, Mohammad Khaja Nazeeruddin and Hyung Shik Shin

Chapter 31

RF Sputtered Ti Layer for Flexible Perovskite Solar Cells Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo, Mohammad Khaja Nazeeruddin and Hyung Shik Shin

Chapter 32

Conducting Channels of the Hole Transporting Layer to Adjacent Photoactive Perovskite Sensitized TiO2 Thin Films for a Solar Cell Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo and Hyung Shik Shin

Chapter 33

Metal Oxide Free Perovskite Solar Cell Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo and Hyung Shik Shin

ix 419 421

441

457

473

About the Editors

487

Index

489

PREFACE The earth has been supported by nature with most fundamental requirements of life such as food, water, clean air and energy. These natural features are the subjects of many fields of scientific research for deliberate improvement of life on the earth. Particularly, for the past few decades, environment pollution and global energy demand have been more persistent due to the increased rate of human population. Hence much attention has been devoted for the development of new technologies for both environmental protection and renewable energy in an economically viable way. Among the many technologies for the environmentally clean energy production, sensors, photocatalytic degradation and solar energy utilization techniques have been attracting the greatest attention because of their low cost and easy fabrication techniques. The sensing of toxic chemicals and harmful dyes through metal oxides/conducting polymers based electrode is under extensive research all over the world, and are finding increasing application in industry, environmental monitoring, medicine, military, agriculture, transportation, and chemical analysis. This is evidenced by the annual growth in the number of published articles in which advances in the field of electrochemical sensors are reported. The recent rapid growth of the industrial sector has led to environmental problems and to high levels of pollution worldwide. Additionally, there is an increase in demand for water in the industrial, agricultural, and domestic sectors, which generate large amounts of polluted wastewater. Among major pollutants, dyes are a serious contributor to pollution and are often difficult to decompose in water as they have composite molecular structures that cause them to be more stable toward light and resistant to biodegradation. This causes damage to the environment as dyes are toxic to aquatic life. The primary methods of water treatment such as coagulation, flocculation, filtration, electro‐flocculation, reverse osmosis, and adsorption do not degrade pollutants but instead decrease their levels by converting the pollutants from one form to another, thereby creating secondary pollution. However, heterogeneous semiconductor

xii

M. Shaheer Akhtar, Sadia Ameen and Hyung-Shik Shin

photocatalysis has been widely explored over the last few decades for various environmental applications. Photocatalysts are a class of compound that produces electron‐hole pairs upon the absorption of light quanta and they induce chemical transformations in reaction substrates that come into contact with them and then undergo regeneration to their original electronic composition. Out of several metal oxides, TiO2 is widely used as a photocatalyst because it is inexpensive, stable in biological and chemical environments, and is stable to photocorrosion. TiO2 has a unique property in that natural (solar) UV light generates electron‐hole pairs for redox reactions. This is because TiO2 has a suitably sized bandgap of 3.2 eV allowing energy of near‐UV light with a wavelength greater than 387 nm to generate electron‐hole pairs. Likewise, ZnO has characteristics similar to TiO2 and is a suitable alternative for the photocatalytic degradation of pollutants. On the other hand, solar energy is a key technology obtained from sunlight which includes utilization of sun energy in different manner. Sunlight is inexpensive, non-polluting, abundant, unique natural resource of clean energy. This fascinating fact of sunlight is pinching researcher worldwide regarding its utilization for mankind by making interfaces which convert sunlight energy into the electrical energy. The commercially existing solar cells are currently based on the inorganic silicon semiconductors which will result proliferation of silicon demand in next decade and price of silicon will rise dramatically. Due to this, Organic Solar Cells (OSCs) also known as Organic Photovoltaics, Dye Sensitized Solar Cells (DSSCs) and a newly emerging class of solar cells called as perovskite solar cells (PSCs) are widely adopted for solar-toelectric power conversions. Emerging Materials for Environment Protection and Renewable Energy includes most of our published research articles along with the work of other authors. This book considers three major parts of (1) Sensors, (2) Photocatalyst and (3) Renewable Energy and provides an in-depth knowledge of synthesis of nanomaterials, characterization of nanomaterials, and the possibilities for full-scale applications of these nanomaterials for environmental protection and renewable energy. The major three parts of this book are further sub sectioned into 33 chapters covering the topics of metal oxides based gas sensors (Part-I, section-1), zinc oxide based chemical sensors (Part-I, section-2), titanium oxide based chemical sensors (Part-I, section-3), conducting polymers based chemical sensors (Part-I, section-4), zinc oxide based photocatalysts (Part-II, section-1), titanium oxide based photocatalysts (Part-II, section-2), conducting polymers based photocatalysts (Part-II, section-3), organic solar cells (Part-III, section-1), zinc oxide based DSSCs (Part-III, section-2), titanium oxide based DSSCs (Part-III, section-3) and perovskite solar cells (Part-III, section-4) The central theme of this book is interrelatedness. Each part of this book highlights a work contributing towards environmental protection and renewable energy with the presentation of tables, graphs, and figures. Of course, not all the three major topics are covered equally and in many cases, the level of detailed elaboration is determined by

Preface

xiii

their significance and interest shown in that part. We believe that topics of sensors, photocatalysts and solar cells will be of great value to scientists, engineers and students. During our work on this book we tried to cover the field more or less completely. We are thankful to contributors and copyright owners as without their support it would not have been possible for us to publish this book. We wish to express our gratitude to the staff of Nova Publishers for their patience during the development of this project and for encouraging us during the various stages of preparation. Editors Professor M. Shaheer Akhtar, PhD Dr Sadia Ameen, PhD Professor Hyung-Shik Shin, PhD

PART I. FUNDAMENTALS OF FUNCTIONAL MATERIALS: APPLICATIONS FOR SENSORS

SECTION 1. METAL OXIDES BASED GAS SENSORS

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 1

SCIENCE AND TECHNOLOGY OF METAL OXIDE SEMICONDUCTOR GAS SENSOR Prabhakar Rai1,* and Praveen Kumar Sekhar2 1

Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, India 2 Nanomaterials and Sensor Laboratory, School of Engineering and Computer Science, Washington State University Vancouver, Vancouver, WA, US

ABSTRACT Gas sensors become one of the indispensable technologies for modern society due to growing concern about environmental pollution and related health problem. Gas sensor technology has boosted remarkably during past few decades due to recent advances in nanoscience and nanotechnology. Much effort is being made to develop gas sensors having very small size with very low power consumption. Varieties of gas sensors working on different principles are commercially available and using innovative ideas for further improvements. However, this chapter provides a review of recent progress in gas sensor based on metal oxide semiconductors (MOS). Semiconductor gas sensors have a wide range of applications in safety, process control, environmental monitoring, indoor or cabin air quality and medical diagnosis. This chapter summarises recent research on basic principles, new materials and emerging technologies in this essential field. These include an understanding of principles and sensing mechanisms for n- and p-type oxide semiconductors, development of new materials and methods to synthesize them into selective sensors, measurement methods, microfabrication of sensors, and describes the role of electrode-semiconductor interfaces. Future aspect of chemiresitive gas sensors in breath analysis for disease diagnosis has also been discussed.

*

Corresponding Author Email: [email protected].

6

Prabhakar Rai and Praveen Kumar Sekhar

1. INTRODUCTION Gas sensors detect the interaction of gas molecules with the sensing device, which is used as a signal and translated to an instrument for displaying the measurements. Gas sensors are used in the detection of toxic and combustible gases. Gas sensors measure a specified gas concentration (reference point or scale) and when the response of the sensor surpasses the specified gas concentration then an alarm gets activated to warn. There are various types of gas sensors serving the same function, however, they can be categorized by the type of gas they detect: combustible or toxic. They are further categorized within this broad categorization on the basis of technology they use: The major gas sensor types are:

1.1. Electrochemical Gas Sensors Electrochemical gas sensors function via electrodes signals when a toxic, like carbon monoxide, chlorine, and nitrogen oxides gas, is detected. An electrochemical cell consists of two/three electrodes immersed in an electrolyte medium. The interaction of gas molecules with electrochemical cell results in the generation of electric current. This current is proportional to gas concentration. The advantage of these electrochemical cells is that they are very accurate and tend to be used principally for toxic gases, where high levels of accuracy are needed. However, they are relatively expensive with a short lifespan which is a critical disadvantage.

1.2. Metal Oxide Semiconductors (MOS) Gas Sensor The MOS gas sensors function by measuring the resistance change (proportional to the concentration) as gas is absorbed on to the surface of MOS. MOS gas sensors can be used for both combustible and toxic gases. The advantage of MOS gas sensors is led by their low cost, easy maintenance, portability, high sensitivity, long life, resistant to poisoning gases, short response and recovery time. However, they are not selective as they are not suitable for the detection of a single gas in a mixture. Furthermore, they are not useful where high concentrations of interfering gases are likely to be present.

1.3. Catalytic Gas Sensors Catalytic sensors are mainly used for combustible gases. They are also called as a bead or pellistor type sensor. It is the most popular sensor and comprises a large number

Science and Technology of Metal Oxide Semiconductor Gas Sensor

7

of gas detector devices that are manufactured today. Catalytic sensors are made of the platinum treated wire coil, which detects the gas by oxidization of combustible gas at the surface of the bead. This results in the change in resistance of beads which is measured to detect the gas. This change in resistance is proportional to gas concentration. The advantage of these gas sensors is that they are relatively low cost and long lifespan. They are also selective as they respond at different rates to each and so can be calibrated for particular gases.

1.4. Infrared (IR) Gas Sensors Majority of gases have a characteristic absorption band in the infrared region of the spectrum which can be used to detect them by using Infrared technology. In functioning, it involves transmitters (light sources) and receivers (light detector) to detect combustible gases, specifically hydrocarbon vapors. The presence of gas in the optical path interferes with the power of the light transmission between the transmitter and receiver. This interaction of the gas with the light result in the alteration of transmitted light which determines what type of gas is present. This is the reason why IR sensors are very selective and provide a very accurate reading. Therefore, IR sensors are used where a high level of accuracy and specificity is required. However, the very high precision in performance has resulted in high price. Table 1. Comparison of various types of gas sensors [1] Parameters Sensitivity Accuracy Selectivity Response time Stability Durability Maintenance Cost Suitability to Portable instrument

Types of gas sensors Semiconductor Catalytic Excellent Good Good Good Poor Bad Excellent Good Good Good Good Good Excellent Excellent Excellent Excellent Excellent Good

Electro-chemical Good Good Good Poor Bad Poor Good Good Poor

Infrared Excellent Excellent Excellent Poor Good Excellent Poor Poor Bad

To simply understand the advantage and disadvantage of these gas sensors a comparison has been made and summarized in Table 1, on the basis of various sensing parameters. It shows that metal oxide gas sensor is a good candidate for development of sensing device.

8

Prabhakar Rai and Praveen Kumar Sekhar

2. METAL OXIDE SEMICONDUCTOR GAS SENSORS The use of MOS as the gas sensor was first proposed by Seiyama et al. using ZnO thin films [2]. In the same year, Taguchi et al. proposed SnO2 as a gas sensor [3]. Since 1968, MOS based gas sensors have been commercially available and Figaro is the leader of this group [4]. In principle, MOS gas sensors are a partially sintered metal oxide bulk device and their resistance in the air is very high. The resistance in the air of these MOS gas sensor decreases sharply when exposed to reducing gases such as combustibles (H 2, CO, CH4) or volatile organic vapors and increases when exposed to oxidizing gases (NO2). Due to simplicity in its operation, low fabrication cost and stability relative to other competitive systems, such as polymers and organic films, these MOS gas sensors became very popular. Other metal oxides such as Fe2O3, TiO2, WO3, and Co3O4 have also been tried to use as gas sensors but they could not achieve the same success as achieved by SnO2.

2.1. Basic Working Principles MOS gas sensor simply involves adsorption and desorption of gas which produce an electrical change. In brief, the process involved in sensing is diffusion and adsorption of reactants to the active region followed by surface reaction and finally desorption and diffusion of product away from the active region. The adsorption is of two types namely, physisorption and chemisorption. In the case of physisorption, the species are bonded only by weak physical forces (van der Waals-type forces) to the surface. Chemisorption bonds have a re-arrangement of the electron density between the adsorbed gas and the surface. This type of solid gas reaction results in the change in conductance which can be used for the sensing. There are following two mechanisms to explain the sensing;

2.1.1. Bulk Conductivity Changes in MOS In bulk conduction based sensors the interaction of the gas with sensing materials results in the change in the stoichiometry of the grains. Thus, the change in the bulk conductivity (σs) is simply the equilibration between the oxygen activity in the oxide and the oxygen content (oxygen partial pressure, Po2) in the surrounding atmosphere, which can be described by the following equation:

(1)

Science and Technology of Metal Oxide Semiconductor Gas Sensor

9

where σo is a constant, Ea is the activation energy for conduction, and the magnitude and sign of 1/n are determined by the type of dominant bulk defect involved in the equilibration process. The positive and negative signs of 1/n correspond to p-type and ntype conduction, respectively. The sensitivity of these gas sensors is determined by the value of 1/n, where higher the value of 1/n, the greater is the sensitivity of the sensor. Since Eq. 1 describes a conductivity characteristic common to oxides, virtually all oxides are oxygen sensors. Many semiconducting oxide systems have been investigated for oxygen sensor applications, such as SrTiO3 and Nb2O5, notably the automotive exhaust gas oxygen (EGO) sensors for air-to-fuel ratio control. The development of bulk conduction based gas sensors are less attractive despite their automotive applications, mainly because they are limited to direct monitoring of oxygen, lack of reliability and their response times are often slower. Finally, since the sensing behavior is mostly explainable and predictable based on well-established thermodynamic principles [5], there is less incentive to study them for further details.

2.1.2. Surface Conductive Changes in MOS In surface layer controlled gas sensors, the interaction of the gas with MOS surface and subsequent chemical reaction results in the change in the concentration of conduction electrons. Figure 1 illustrates this arrangement and corresponding energy band diagram, using an n-type semiconducting oxide as an example. When exposed to oxidizing gas species such as oxygen, adsorption of oxygen on the surface results in the removal of electrons from the conduction band by transfer to the oxygen (trapping the electron) thereby forming a Schottky barrier at the intergranular contact, resulting in a decrease in the conductance. The conduction is then determined by the height of the barrier (qVs) at the intergranular contacts: σs = σo exp(-qVs/Kt)

(2)

where, σs and σo is the conductivity and pre-exponential constant respectively, whereas k is Boltzmann constant and t is temperature. When exposed to a reducing gas such as CO, the adsorbed CO reacts with the adsorbed ionized oxygen anions (i.e., O2−, O−, and O2−), releasing the trapped electrons back to the conduction band, subsequently lowering the barrier height and the resistance as shown in Figure 2.

Prabhakar Rai and Praveen Kumar Sekhar

10

Figure 1. Compressed powder model for gas sensor showing the (a) depletion region and (b) energy barrier (qVs) at the intergranular contact.

Air

Air and CO

O¯ O¯ O¯

O¯ O¯ O¯ O¯ O¯ O¯ O¯

O¯O¯ O¯

O¯

O¯

O¯

CO O¯

O¯ O¯

eVs Grain Boundary

CO2

O¯ eVs

Grain Boundary

Figure 2. Schottky barrier formations due to oxygen chemisorption and CO sensing mechanism of ntype semiconductors.

Thus, equivalent circuits of n-type semiconductor gas sensors involve serial connections between semiconducting cores (Rcore) and resistive interparticle contacts (Rshell) (Figure 3a). However, the conduction in p-type oxide semiconductors involves competition between parallel paths across the wide, resistive core (Rcore) and along the narrow, p-semiconducting shell (Rshell) regions. Thus, both n- and p-type oxide semiconductors exhibit significantly different conduction behaviors, although establish electrical core–shell layers by adsorbing oxygen. These gas sensors based on surface layer controlled principle are mainly used for monitoring of gas species other than oxygen. In addition, the reliable detection of gases using these sensors requires an atmosphere of fixed oxygen partial pressure. Therefore, a surface layer controlled gas sensors are heat treatment at elevated temperatures to reduce or eliminate oxygen interference and freeze the bulk defect concentrations because oxygen may affect the chemisorption process by competing with oxidizing gases, such as NOx, SOx, etc. However, these sensors must be used at low temperatures to avoid interdiffusion or release of the frozen bulk defects.

Science and Technology of Metal Oxide Semiconductor Gas Sensor

11

O CO O

Air (O2) O

(a)

CO

Electron depletion Layer (high resistance) CO

O O

O

O O

ee-

e-

Semiconducting core layer (low resistance)

e-

e-

e-

ee-

O

Gas (CO)

O CO

O

Air (O2) O

(b)

Hole accumulation Layer (low resistance) CO

O

CO

O O

O O

e-

e- + h+= null

Insulating core Layer (low resistance)

Gas (CO)

O

Figure 3. Gas sensing mechanism and the equivalent circuit of (a) n-type and (b) p-type oxide semiconductors [6].

2.2. Parameters for MOS Gas Sensor Performance The basic principles behind the operation of all the gas sensors are the sensitivity, selectivity, and reversibility of their sensing response. These terms can be defined in accordance with measurable parameters.

Gas off Ra

Response

Response (Rs)

Gas on Ra

Response time

Rg

Recovery time

Time (sec.) Figure 4. Response transient of typical MOS based gas sensors.

Prabhakar Rai and Praveen Kumar Sekhar

12

2.2.1. Response The response of a sensor is a measure of the lowest concentration of an analyte gas that can be detected. The response MOS gas sensor is calculated from the measured resistance values.

(3) where Ra the resistance of the sensor in the air; and Rg is the resistance of the sensor in analyte gas. The sensitivity of semiconductor gas sensors can also be empirically represented as [8]. Sg = A× Pgβ

(4)

where, Pg is partial pressure of target gas, which is directly proportional to its concentration of the target gas. The sensitivity is characterized by the pre-exponential factor A and the exponent ß may have some rational fraction value (usually 1 or 1/2), depending on the charge of the surface species and the stoichiometry of the elementary reactions on the surface.

2.2.2. Selectivity Selectivity is the most important parameter of any gas sensor which is related to sensor’s ability to recognize single gas amongst many gases. The selectivity of MOS gas sensors is defined as the ratio of the sensitivity of one analyte gas (Rs1) relative to another analyte gas (Rs2) under same conditions [9].

(5)

2.2.3. Response and Recovery Time The response time and recovery time are defined as the time taken to achieve 90% of the final change in resistance (T90) [10]. The response and recovery time of a sensor is also a very important parameter for its commercial usage and a gas sensor that has a short response and recovery time will have greater applications in the commercial market. 2.2.4. Robustness Robustness of a sensor level is related to it ability to perform over a range of ambient condition e.g., humidity, temperature etc. and over a range of times in the presence of

Science and Technology of Metal Oxide Semiconductor Gas Sensor

13

drift and stability variations. It is interesting to note that the parameters which are used to improve robustness are often the same parameters that result in a decrease in sensitivity and selectivity [11].

2.3. Materials Properties for MOS Gas Sensors Performance 2.3.1. Receptor Function Receptor function of a MOS sensor is related to its ability to interact with the target gas. In bare MOS gas sensors, surface oxygen (adsorbed or lattice oxygen) participates in the oxidation reaction, which results in the decrease of surface oxygen at steady state. This induces a change in surface space charge layer, which in turn is transduced into a change in electrical resistance as mentioned above. Thus, pure metal oxides itself can sensitively detect the gas through their chemical properties. However, the sensitivity of bare MOS can be further boosted by the addition of an additive (noble metals, acidic or basic oxides) on the oxide surface because in many cases, the bare oxide surface is not active enough. Thus, MOS gas sensors need a catalyst deposited on the surface of the film to accelerate the reaction and to increase the sensitivity. Mostly, small amounts of noble metal additives, such as Pd or Pt are commonly dispersed on the MOS surface as activators or sensitizers to improve the gas selectivity, sensitivity and to lower the operating temperature [12]. There are two ways i.e., electronic and chemical sensitization by which the catalysts can affect the film resistance and hence sensor performance (Figure 5). The film resistance is controlled by Fermi energy control or electronic sensitization. In the electronic mechanism, noble metal acts as an electron acceptor on semiconductor oxide surfaces, which contributes to the increase of the depletion layer through Schottky barrier formation due to the difference in Fermi energy level of metal oxides and noble metals [13]. Therefore, the change in resistance in noble metal deposited metal oxides is larger as compared with the pristine oxide, leading to the increase in response. In the chemical mechanism, noble metals catalytically activate the dissociation of molecular oxygen. The atomic products of dissociated oxygen then adsorb on the metal oxide support resulting in a greater degree of electron withdrawal from the metal oxide than for the pristine metal oxide. This again results in the higher change in air resistance for noble metals loaded metal oxide as compared with the bare metal oxides, which results in the increase in gas response. Thus, noble metal helps in the improvement of sensing performance. However, the catalytic theory proposed, as spillover and Fermi energy control, have not led to a widely accepted catalyst mechanism that predicts or explains sensor behavior in different environments. Traditionally, surface modification of metal oxides involves the deposition of noble metals on the metal oxide surface. However, this exercise has many limitations such as; (a) it passivates the effective surface area of metal oxides involved in gas sensing. (b)

Prabhakar Rai and Praveen Kumar Sekhar

14

Metal oxide generally sintered at 500oC and operated at 200-400oC and at these temperatures noble metal NPs are highly unstable because of the increased mobility of the metal NPs on the support and loss their catalytic activity [14]. This mobility of metal NPs on oxide support affect the sensing performance by the formation of either a shunting layer or an active membrane filter, which effectively obstruct the penetration of the targeting gas into the surface of the gas sensing matrix. (c) Many gases, such as H2S, SO2, thiols etc. poisoned the noble metal NPs which is another problem for their application [15]. Therefore, many researchers are using core-shell type structure, where noble metal is used as core and metal oxide as the shell to protect the noble metals from agglomeration and poisoning [16-24]. This type of structures has shown excellent thermal stability shown potential applications in the gas sensor.

(a)

O2

(b) e¯

O¯

O2

O¯

O¯

O¯

CB Conduction channel

Au VB ZnO

O¯

O2

O2 O¯ O¯ O¯O¯

Au

Depletion layer

O¯

Spillover zone

Figure 5. Mechanism of sensitization by noble metal additives in MOS gas sensors.

It has been found that the performance of noble metal@metal oxide core-shell NPs based gas sensor can be further enhanced in light by using plasmonic NPs (Au or Ag) as core materials. For example, Xu et al. [25] have recently designed and fabricated Agx@(2D-WO3) core@shell NPs for localized surface Plasmon-enhanced chemical sensor. They have synthesized pure WO3, Agx-WO3 mixture, and Agx@(2D-WO3) core@shell NPs and investigated their sensing properties (Figures 6a-d). It has been found that the performance of sensor was enhanced by the combination of the Ag core and the 2D layered structure of WO3 shell due to effective localized surface Plasmon generation and propagation. The sensor response (Rs = Ra/Rg) was increased approximately 4 times for Agx@(2D-WO3) core@shell structure towards 500 ppm alcohol as compared to pure WO3. More important the performance of core-shell time geometry was still 3 times higher as compared to the normal mixture of Agx-WO3.

Science and Technology of Metal Oxide Semiconductor Gas Sensor

15

Response and recovery time are also shortened for the Agx@(2D-WO3) core@shell nanostructure as compared to bare WO3 and the mixture of Agx-WO3. Furthermore, optimum sensor working temperature was also lowered from 370 oC to 340 oC. Thus, the sensors made of Agx@(2D-WO3) core@shell NPs show significantly better performance compared to those from pure WO3 and Agx-WO3 mixture. The sensing mechanism was explained on the basis electronic as well as chemical sensitization of Ag NPs. However, the lower response of Ag–WO3 mixture as compared to Agx@(2D-WO3) core@shell was explained on the basis of poor Schottky junction formation. It has been suggested that Ag–WO3 mixture was formed due to the mixing of WO3 powder and Ag NPs, which resulted in the long distance between Ag and WO3 NPs and/or the agglomeration of Ag and WO3 NPs. Furthermore, the effect of surface Plasmon resonance of Ag NPs on gas sensing has been investigated by light irradiation. The sensor response as a function of illumination wavelength using LEDs emitting at 405 nm, 530 nm, and 680 nm was also measured as shown in Figure 6e. The response of Agx@(2D-WO3) sensor was increased after light irradiation especially at the blue wavelength. The sensor response increases approximately twice when irradiated using blue LED, whereas only 10% improvement was observed in the red LEDs irradiation. This is mainly because Ag NPs absorbs light in this region due to surface Plasmon resonance, therefore it resonates well in this region with the absorption of Ag NPs. Furthermore, it has been found that with increasing light intensity from darkness to 87 mW/cm2 resulted in 308% response of Ag(25nm)@(2D-WO3) core@shell NPs as compared to 40% and 60% increase in response of the pure WO3 and Agx-WO3 mixture, respectively (Figure 6f). The highest increase in response at 405 nm wavelengths clearly suggests that the sensor enhancement is due to the surface Plasmon effect of Ag NPs, as surface Plasmon wavelength for 25 nm Ag NPs is closer to 405 nm (Figure 6g).126 It clearly demonstrates that surface plasmon effect of core metal NPs can be effectively used to boost sensor performance of a core@shell nanostructure based gas sensors. These core-shell NPs were further improved by making core-void-shell type structure where the presence of void helps in the access of noble metal to the gas molecules, which improve the sensitivity [26-28]. For example, Li et al. [28] has synthesized Au@ZnO yolk@shell nanospheres for acetone sensor applications and found that response of the Au@ZnO nanospheres was about 2 and 3 times higher than that of ZnO hollow and solid nanostructures, respectively (Figure 7a). The Au@ZnO yolk@shell nanospheres exhibited enhanced responses for other gases also as compared ZnO hollow nanospheres (Figure 7b). The enhanced performance of Au@ZnO hollow nanospheres as compared to bare hollow ZnO is due to the catalytic activity of Au NPs. However, better sensing performance of hollow structure as compared to solid one was related to its distinctive configuration (hollow interiors and porous shells), which has facilitated the in-diffusion of the test gas (utility factor) and improved the kinetics of the reaction between the test gas and surface adsorbed oxygen species as shown in Figure 7c.

Prabhakar Rai and Praveen Kumar Sekhar

16

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Reprinted with permission from ref. 25. Copyright 2014 Nature. Figure 6. (a-d) TEM images of Agx@(2D-WO3) CSNS (insets in a and b show the FESEM image of the corresponding Ag NPs with expected diameters). (e) Sensor response of Ag(25nm)@(2D-WO3) CSNS at the different wavelength of LED irradiation. (f) Normalized sensor response for Ag(25nm)@(2D-WO3) CSNS, Agx-WO3 mixture and pure WO3 vs. light illumination intensity using a xenon arc light source (150 W) attenuated using neutral density filters. (g) An illustration of the Schottky junction and LSP enhanced mechanism in Agx@(2D-WO3) based sensors.

Science and Technology of Metal Oxide Semiconductor Gas Sensor

(a)

17

(b)

(c)

Reprinted with permission from ref. 28, Copyright 2014 American Chemical Society. Figure 7. (a) Responses of the sensor devices upon exposure to 100 ppm acetone at different working temperatures, (b) Responses of sensors based on ZnO hollow nanospheres and Au@ZnO yolk−@hell nanospheres to various gases (100 ppm), and (c) Gas sensing principles of solid ZnO nanospheres, hollow ZnO nanospheres, and Au@ZnO nanospheres.

Apart from this, the acid–base properties of oxide surfaces are also important for semiconductor gas sensors. For example, if the target gas is acidic like H2S then gas– solid interaction would be more important when the metal oxide surface is more basic. This tendency has been confirmed and it has been found that ethanol gas is greatly promoted when SnO2 or In2O3 is loaded with a basic oxide like CaO [29] or La2O3 [30]. In my recent study, the modulation of the ZnO NRs electronic properties through CuO NPs was carried out for CO sensing applications [31]. It is found that the response of CuO/ZnO NRs is 5 times higher than that of bare ZnO NRs. It has been found that Cu in +2 oxidation state preferred to adsorb CO gas through Cu–CO bonding consisted of the donation of CO 5σ electrons to the metal and the reverse donation of π-electrons from d orbitals of Cu to CO. However, a complete understanding of role of additives in the gas sensing mechanism is lacking. Therefore, a deep analysis of the material-gas interaction and its influence on the sensor electrical response needs to be thoroughly investigated.

18

Prabhakar Rai and Praveen Kumar Sekhar

2.3.2. Transducer Function Receptor function creates electronic change on metal oxide surface (work function change) and the function which translates this signal into the electrical signal is called as transducer function. Transducer function is related to grains and boundary between grains. The resistance of electrical signal depends on the barrier height and concentration of the target gas. In general, transducer function is affected by following factors. Transduce factor mainly depends on carrier mobility and grain size of sensing layer. The carrier mobility is related to mobility of main carriers (electrons or holes) present in the conduction band. It provides the proportionality constant of the change of electrical conductivity. The change in electrical conductivity is related to the change in the number of main carriers caused by gas–solid interactions. The mobility of conduction electrons of n-type oxides is reported in the literature, such as 160 (SnO2); 200 (ZnO), 100-15 (In2O3); and 10 cm2/V.s (WO3) [9]. In comparison to n-type semiconductor, the mobility of p-type oxide is usually much less, such as 0.2 cm2/V.s of NiO. This is basically the reason why p-type oxides are not utilized in MOS gas sensors. Some n-type, such as TiO2 is also not suited for gas sensor application due to its low mobility of conduction electrons (4 cm2/V.s). The most important factor which affects the transducer function and thus the sensing property of MOS is the grain size of polycrystalline materials. It is well known that the adsorption of the oxygen molecules on metal oxide surface results in the transfer of the electron from metal oxide to oxygen molecules. These oxygen molecules get dissociated and chemisorb on the metal oxide surface, which results in electron depletion layer formation on MOS surface, also called as space charge layer. The depth of electron depletion layer (L) in the air is determined by Debye length (dm) and the strength of chemisorptions [9]. It is found that electrical resistance of the sensor under exposure to air (Ra) and a target gas (Rg) undergo very characteristic changes as d m changes. Furthermore, the thickness (L) of the space charge layer mostly remained constant, hence the proportion of space charge region in each particle changes relative to a change in dm. The critical dm value corresponds to a point where dm becomes equal to twice of L. therefore, the whole region of a particle gets depleted of electrons, when dm < 2L; while the depletion takes place on the surface region only for dm > 2L. It has been found when particle size reduced to 6 nm then the whole particle gets depleted of electrons. Thus, controlling dm smaller than 2L is very important for the design of high sensitivity sensors. However, the drawback of MOS gas sensor is their high operating temperature and these temperatures these particles either start growing or get agglomerated. Therefore, it becomes practically difficult to achieve. It is worth to mention that transducer function also depends on the morphology of the sensitive layer. Furthermore, gas sensing property depends not only on sensing material characteristics but also contact resistance between sensing material and electrodes [9].

Science and Technology of Metal Oxide Semiconductor Gas Sensor

E bias Pt

e¯ e¯

19

Gas Pt

ZnO NRs E bias Pt

Pt

e¯ ZnO NPs

Figure 8. (a) Inter-electrode gap and particle size relationship and (b) its effect on gas sensing.

In my recent study, the sensing capabilities of different types of ZnO sensing layers (NRs and NPs) have been investigated taking an inter-electrode gap into account. It has been found that the phase, structural defects, diameter of ZnO NPs and NRs were almost similar and they differ in their morphology and surface area (surface area of NPs was much higher than NRs). The response of ZnO NPs was poor than NRs for NO2. Meanwhile, the response of NRs was poor than NPs for CO gas. The response of ZnO NRs was 30 times higher than those of NPs for NO2 gas, while 4 times lower for CO gas. A relationship between morphology and inter-electrode gap suggest that the number of grains present between inter-electrode gaps has significantly affected the response. NPs having a large number of grain boundary in between two electrodes as compared to NRs have shown higher air resistance as compared to NRs. Since the response to reducing gas (CO) is measured by Ra/Rg, hence higher the Ra for NPs as compared to NRs resulted in the higher response. In contrast response to oxidizing gas (NO2) is measured by Rg/Ra, hence higher the Ra for NPs as compared to NRs resulted in the lower response. This study suggests that materials having a large number of grain boundaries are suitable for reducing gas sensing, whereas materials with the lower number of grain boundaries are suitable for oxidizing gas sensing.

2.3.3. Utility In the gas sensor, mostly upper surface involved in sensing whereas particles present in bulk generally remain unutilized. Therefore, the concern related to accessibility of inner oxide grains to the target gas is called as utility factor. The factor becomes very important when the rate of reaction is too large compared with that of diffusion. In this situation, the gas molecules cannot access the grains located at inner sites, leaving them un-utilized for gas sensing and thus resulting in a loss in sensor response. Utility factor depends on porosity and thickness of the film. It has been found that mesoporosity plays a vital role in the application of MOS gas sensors. It is not only because it increases the

20

Prabhakar Rai and Praveen Kumar Sekhar

surface area for high sensitivity but also the presence of well-defined porosity offer powerful opportunities with respect to selectivity [34, 35]. For example, in mesopores (pore radius 410 μM) in PBS. The fabricated phenyl hydrazine chemical sensor based on TiO2 NT arrays electrode exhibits significantly high and reproducible sensitivity of ∼40.9 μA mM−1 cm−2 and the detection limit of ∼0.22 μM with correlation coefficient (R) of ∼0.98601 and short response time of 10 s. Importantly, the fabricated phenyl hydrazine chemical sensor based on TiO2 NT arrays electrode displays a good linearity in the range of 0.25 μM-1μM. The phenyl hydrazine sensing mechanism over the surface of TiO2 NT arrays electrode is explained by using the surface-depletion model [26]. Figure 2(e) shows the illustration of sensing response of phenyl hydrazine chemical over the surface of TiO2 NT arrays electrode. Primarily, the phenyl hydrazine is chemisorbed on the surface of TiO2 NT arrays. TiO2 NT arrays interact easily with atmospheric oxygen by transferring electrons from the conduction band to the adsorbed oxygen atoms and presents in the form of O−, O2−, etc., [26], as shown in Figure 2(e). Second, these oxygenated species interact with phenyl hydrazine and oxidizes phenyl hydrazine into less harmful diazenyl benzene on the surface of TiO2 NT arrays. Thus, TiO2 NT arrays electrode provides suitable surface for the oxidation of phenyl hydrazine and determines the sensing responses by increases the current values. The reusability and reproducibility of the fabricated phenyl hydrazine chemical sensor based on TiO2 NT arrays electrode were elucidated by measuring the sensing responses with the I-V characteristics for three consecutive weeks. The sensing parameters or properties showed the negligible drops in the fabricated phenyl hydrazine chemical sensor based on TiO2 NT arrays electrode, which deduces the long term stability of the fabricated phenyl hydrazine chemical sensor. Herein, the achieved sensitivity, detection limit, and the correlation coefficient of the fabricated phenol chemical sensor are superior to the reported literatures on hydrazine sensor based on TiO2 electrodes, as summarized in Table 1 [27-30]. Thus, TiO2 NT arrays with anatase phase and good crystal quality are promising and effective working electrode for the detection of phenyl hydrazine chemical or other hazardous chemicals.

TiO2 Nanotube Arrays for the Sensing of Phenyl Hydrazine

77

Reprinted with permission from [S. Ameen, 2013], Appl. Phy. Lett., 103 (2013) 061602©2013 AIP Publishing LLC. Figure 2. (a) Typical amperometric plot and (b) linear plot of current versus concentration of phenyl hydrazine of TiO2 NT arrays based chemical sensor. (c) The I-V characteristics and (d) the calibration curve of current versus phenyl hydrazine concentration of TiO 2 NT arrays electrode based chemical sensor at different phenyl hydrazine concentrations (0.25 μM-0.10 mM) in 10 ml of 0.1 M PBS, and (e) schematic illustration of proposed mechanism of phenyl hydrazine chemical sensors over the surface of TiO2 NT arrays based electrode.

78

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al. Table 1. Sensing response characteristics of TiO2 nanomaterials based electrode for hydrazine and its derivatives

Working electrodes TiO2-Pt hybrid nanofibers C0HCF@TNT Nano-Au/porousTiO2/GCE Nano-Au/Ti TiO2 NT arrays

Chemicals Hydrazine

Phenyl hydrazine

Limit of detection 0.142 μM

Sensitivity 44.4 μA mM−1 cm−2

Reference 27

1 × 10−3 0.5 μM

72.8 μA mM−1 0.172 μA mM−1

28 29

42 μM 0.22 μM

1.117 μA mM−1 40.9 μA mM−1 cm−2

30 This work

Reprinted with permission from [S. Ameen, 2013], Appl. Phy. Lett., 103 (2013) 061602©2013 AIP Publishing LLC.

CONCLUSION The TiO2 NT arrays working electrode exhibits reasonably high electron transfer process via high electrocatalytic activity towards the phenyl hydrazine chemical. The fabricated phenyl hydrazine chemical sensor based on TiO2 NT arrays working electrode demonstrates significantly high sensitivity of ∼40.9 μA mM−1 cm−2 and the detection limit of ∼0.22 μM with correlation coefficient (R) of ∼0.98601 and short response time (10 s). The enhanced sensing properties are attributed to the presence of the depleted oxygen layer on the surface of TiO2 NT arrays and its good electrocatalytic activity towards phenyl hydrazine chemical.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]

Ibrahim A. A., Dar G. N., Zaidi S. A., Umar A., Abaker M., Bouzid H., S. Baskoutas, Talanta. 93 (2012) 257. Umar A., Rahman M. M., Kim S. H., Hahn Y. B., Chem. Commun. (Cambridge). 2 (2008) 166. Ameen S., Akhtar M. S., Shin H. S., Talanta. 100 (2012) 377. Tang L., Zeng G. M., Yang Y. H., Shen G. L., Huang G. H., Niu C. G., Sun W., Li J. B., Int. J. Environ. Anal. Chem. 85 (2005) 111. Yang Z. S., Wu W. L., Chen X., Liu Y. C., Anal. Sci. 24 (2008) 895. Ameen S., Akhtar M. S., Kim Y. S., Shin H. S., Appl. Catal. B. 103 (2011) 136. Ameen S., Akhtar M. S., Song M., Shin H. S., ACS Appl. Mater. Interfaces. 4 (2012) 4405. Lee S. M., Cho S. N., Cheon J., Adv. Mater. 15 (2003) 441. Limmer S. J., Chou T. P., Cao G. Z., J. Mater. Sci. 39 (2004) 895.

TiO2 Nanotube Arrays for the Sensing of Phenyl Hydrazine

79

[10] Sun L., Li J., Wang C. L., Li S. F., Lai Y. K., Chen H. B., Lin C. J., J. Hazard. Mater. 171 (2009) 1045. [11] Perillo P. M., Rodriguez D. F., Sens. Actuators B. 171 (2012) 639. [12] Xu H., Zhang Q., Zheng C. L., Yan W., Chu W., Appl. Surf. Sci. 257 (2011) 8478. [13] Wang Q., Pan Y. Z., Huang S. S., Ren S. T., Li P., Li J. J., Nanotechnology. 22 (2011) 025501. [14] Shankar K., Mor G. K., Prakasam H. E., Yoriya S., Paulose M., Varghese O. K., Grimes C. A., Nanotechnology. 18 (2007) 065707. [15] Mohapatra S. K., Misra M., Mahajan V. K., Raja K. S., J. Phys. Chem. C. 111 (2007) 8677. [16] Wang Q., Wen Z. H., Li J. H., J. Nanosci. Nanotechnol. 7 (2007) 3328. [17] Chopra N. G., Luyken R. J., Cherrey K., Crespi V. H., Cohen M. L., Louie S. G., Zettl A., Science. 269 (1995) 966. [18] Tenne R., Margulis L., Genut M., Hodes G., Nature (London). 360 (1992) 444. [19] Fujikawa S., Takaki R., Kunitake T., Langmuir. 21 (2005) 8899. [20] Mor G. K., Varghese O. K., Paulose M., Shankar K., Grimes C. A., Sol. Energy Mater. Sol. Cells. 90 (2006) 2011. [21] Li L. J., Zhou Z. Q., Lei J. L., He J. X., Zhang S. T., Pan F. S., Appl. Surf. Sci. 258 (2012) 3647. [22] Kwon Y., Kim H., Lee S., Chin I. J., Seong T. Y., Lee W. I., Lee C., Sens. Actuators B. 173 (2012) 441. [23] Chen K. S., Xie K., Feng X. R., Wang S. F., Hu R., Gu H. S., Li Y., Int. J. Hydrogen Energy. 37 (2012) 13602. [24] Wang J., Lin Z. Q., Chem. Mater. 20 (2008) 1257. [25] Zhao J. L., Wang X. H., Chen R. Z., Li L. T., Solid State Commun. 134 (2005) 705. [26] Feng P., Wan Q., Wang T. H., Appl. Phys. Lett. 87 (2005) 213111. [27] Ding Y., Wang Y., Zhang L. C., Zhang H., Li C. M., Lei Y., Nanoscale. 3 (2011) 1149. [28] Nm S. J. Sophia, Devi S., Pandian K., Int. J. Electrochem. Sci. 7 (2012) 6580. [29] Wang G. F., Zhang C. H., He X. P., Li Z. J., Zhang X. J., Wang L., Fang B., Electrochim. Acta. 55(24) (2010) 7204. [30] Yi Q. F., Yu W. Q., J. Electroanal. Chem. 633 (2009) 159.

SECTION 4. CONDUCTING POLYMERS BASED CHEMICAL SENSORS

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 7

APPLICATION OF POLYPYRROLE NANOBELTS AS ELECTRODE MATERIAL FOR THE DETECTION OF ALIPHATIC ALCOHOLS Sadia Ameen1, M. Shaheer Akhtar2, Hyung-Kee Seo1 and Hyung Shik Shin1,* a

Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea b New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT The effective working electrode based on polypyrrole (PPy) nanobelts is employed for the fabrication of highly sensitive and reproducible aliphatic alcohols chemical sensor. The unique PPy nanobelts are simply synthesized by in situ chemical polymerization of pyrrole monomer in the presence of ferric chloride as oxidant and methylene blue as reactive self-degraded template. The morphological and elemental mapping properties reveal that the synthesized PPy nanobelts possess the uniform dimension with a high aspect of chemical compositions. PPy nanobelts based electrode toward the detection of aliphatic alcohols are elucidated by measuring the electrochemical impedance spectroscopy (EIS) measurements to define the electrochemical and electrocatalytic behavior of PPy nanobelts. Furthermore, the current (I)–

*

Corresponding Author Email: [email protected].

84

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al. voltage (V) characteristics are performed to evaluate the sensing performance of PPy nanobelts electrode toward the detection of aliphatic alcohols. Among different aliphatic alcohols, the methanol chemical sensor based on PPy nanobelts electrode displays the highest sensitivity of ∼205.64 μA.mM−1cm−2, good detection limit of∼6.92 μM with correlation coefficient (R) of ∼0.98271 and short response time (10 s).

INTRODUCTION The major environment pollutants like CO2, CO, SO2 and volatile organic compounds (VOCs) are usually produced by the consumption of commonly used chemicals such as ammonia (NH3), ethanol (C2H5OH), methanol (CH3OH) and other aliphatic alcohols [1]. Moreover, the excessive use of VOCs such as aliphatic alcohols and ketones are continuously polluting the environment and causes the health problems [2]. Methanol is highly used as automotive fuel in motor vehicles and in making dyes and perfumes [3]. The surplus exposure of methanol to human could cause blindness, metabolic acidosis and might lead to death [4]. The combustion of other alcohol like propanol releases the toxic materials which are harmful to human beings [5]. The sensor technology is well known tool for the detection of VOCs such as, alcohols, ethers, esters, halocarbons, ammonia, NO2 and warfare agent stimulants [6] and is highly demanding in industries, medicines and for domestic applications to detect the pollutants, toxic and harmful chemicals [7]. The electrochemical sensors have received much attentions as promising chemical sensing tools to detect the harmful chemicals [8]. In this regards, the designing of effective working electrode with high electron transfer rate is still a challenge. The chemical sensor based on working electrodes of different nanomaterials such as polymers and metal oxide semiconductors are promising for the reliable detection of harmful chemicals owing to their reliability, high surface-to-bulk ratio, good adsorption characteristics and high selectivity [9]. Conjugated polymers are known as p-type semiconductors with unique electronic properties due to their reasonable electrical conductivity, low energy optical transitions, low ionization potential and high electron affinity [10]. These polymers could be easily synthesized through simple chemical or electrochemical processes and their conductivities could be altered by modifying the electronic structures through doping or de-doping procedures [11]. Therefore, conducting polymers could suitably work as an effective working electrode and might offer the fast response toward the detection of various harmful chemicals [7]. In general, the good selectivity, wide linear range, rapid response, portability and the room temperature working abilities are the basic requirements for the efficient working of chemical sensors [12].

Application of Polypyrrole Nanobelts as Electrode Material …

85

Polypyrrole (PPy), a conducting polymer, is much explored material because it shows high electrical conductivity, high stability in air and aqueous media and thus, an extremely useful material for actuators, electric devices and for the efficient detection of the harmful chemicals [13, 14]. Few literatures are reported on the sensor performances of PPy nanostructure based electrodes for the detection of aliphatic alcohols S. J. Hong et al. studied nitro vinyl substituted PPy as a unique reaction-based chemosensor for cyanide anion [15]. Lin et al. prepared the composite electrode of PPy-poly vinyl alcohol (PVA) by electrochemical method for the detection of methanol and ethanol vapor [16, 17]. Jiang et al. prepared the composite films of PPy–PVA by in situ vapor state polymerization method and demonstrated the methanol sensing behavior based on the thickness of PPy–PVA film electrodes [18]. Recently, Babaei et al. determined the residual methanol content in the biodiesel samples by developing new PPy–ClO4 electrodes via electrodeposition on interdigital electrodes [19]. The roughness and morphology of the PPy greatly influence the responses for the detection of harmful chemicals [20]. Herein, a unique and effective working electrode based on PPy nanobelts are utilized for the fabrication of highly sensitive and reproducible aliphatic alcohols chemical sensor. The unique PPy nanobelts are simply synthesized by the in situ chemical polymerization of pyrrole monomer in the presence of ferric chloride as oxidant and methylene blue as reactive self-degraded template. Bulk samples in the form of pellets are prepared by compressing the finely grounded PPy nanobelts powder under a pressure of ∼6.6 Ton. The prepared PPy electrode shows the surface area of ∼39.8 m2/g. The contacts are made by attaching the thin Cu wire on the pellet through the silver paste. Thereafter, the electrode is subjected to drying at 60 ± 5◦C for 2 h in an electric oven. The sensing performances of aliphatic alcohols are studied by a simple two electrode I–V characteristics using PPy nanobelts electrode as working and Pt wire as a cathode. The I–V characteristics are measured by the electrometer. A fixed amount of 0.1 M phosphate buffer solution (PBS, 10.0 ml) of pH 7 and the wide concentration range of aliphatic alcohols (methanol, propanol and butanol) from 20 μM–1 mM are used for the experiments. The sensitivity of the fabricated alcohol chemical sensor is estimated by the slope of the current vs. concentration from the calibration plot, divided by the value of active area of sensor/electrode. The current response was measured from 0–2.5 V and the response time is measured as 10 s. The fabricated PPy nanobelts based electrode shows the rapid detection of methanol with the high sensitivity of ∼205.64 μA.mM−1cm−2, detection limit of ∼6.92 μM with a correlation coefficient (R) of ∼0.98271 and short response time (10 s).

86

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

MORPHOLOGICAL STUDIES OF POLYPYRROLE NANOBELTS The Field Emission Scanning Electron Microscopic and the Transmission Electron Microscopic Studies The morphology of the synthesized PPy nanomaterials is analyzed by FESEM and TEM images, as shown in Figure 1. From FESEM images (Figure 1(a) and (b)), the synthesized PPy nanomaterials possess smooth and the uniform belt like morphology. Each PPy nanobelt presents the average thickness of ∼100 nm and width of ∼400 nm, as shown in Figure 1(b). The morphology of the synthesized PPy has been further characterized by the TEM analysis (Figure 1(c)). Similar morphology and the dimensions are observed in TEM image, which is consistent with the FESEM results. Interestingly, the morphology of PPy nanobelts has not changed under high energy electron beam, indicating the stability of PPy nanobelts.

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 1. FESEM images at (a) low and (b) high resolution and (c) TEM image of PPy nanobelts.

Application of Polypyrrole Nanobelts as Electrode Material …

87

The Atomic Force Spectroscopy Figure 2 shows the topographic and three dimensional (3D) AFM images of synthesized PPy nanobelts. As observed in FESEM and TEM analysis, the nanobelts morphology is visibly seen in the AFM images. The synthesized PPy nanobelts show the reasonable root mean roughness (Rms) of ∼18.1 nm. It is reported that the high electrochemical behavior and electrocatalytic activity of electrode are related to the large roughness factor of the electrode materials [23]. In our case, the PPy nanobelts with reasonable Rms value might deliver the electrochemical behavior toward the detection of aliphatic alcohols.

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 2. (a) Topographic and (b) three dimensional AFM images of PPy nanobelts.

The Line Scan Element Mapping The elemental compositions of the synthesized PPy nanobelts are estimated by the element line scan image through EDS, as depicted in Figure 3. The line scan image and pie profile (Figure 3(a) and (b)) display that the synthesized PPy nanobelts are largely composed of carbon and nitrogen elements. Few traces of oxygen elements are also recorded which might due to surface moisture or atmospheric oxygen on the surface of nanobelts. Figure 3(c) summarizes the existing elements of PPy nanobelts in weight percentage (wt%) and atomic percentage (at%). The synthesized PPy nanobelts are consisted of uniformly distributed carbon and nitrogen with at% of ∼60.09 and ∼39.41 respectively. The detection of C, N and O elements confirm the formation of synthesized PPy nanobelts.

88

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 3. (a) Line scanning elemental mapping image, (b) corresponding pie bar graph and (c) summary of elemental mapping of PPy nanobelts.

STRUCTURAL PROPERTIES OF POLYPYRROLE NANOBELTS Fourier Transform Infrared Spectroscopy FTIR spectroscopy analysis of synthesized PPy nanobelts is examined to elucidate the structures of PPy, as shown in Figure 4. The IR peaks at ∼1553 and ∼1483 cm−1 present the characteristic peaks of PPy, corresponding to the C antisymmetric and symmetric stretching vibration in PPy ring respectively [24]. The IR peaks at ∼1308 cm−1 and ∼1204 cm−1 are ascribed to C-N stretching vibration and the C-C stretching in PPy nanobelts [25]. The appearance of two IR peaks at ∼1192 and ∼798 cm−1 are assigned to C-C stretching of PPy backbone. Additionally, the C-H deformation and C-H rocking vibration are confirmed by the appearance of IR peaks at ∼924 cm−1 and ∼651 cm−1 respectively [26]. A broad IR peak at ∼3224 cm−1 and small peak at ∼1958 cm−1 are attributed to N-H stretching and the stretching vibration of the CN group in PPy nanobelts respectively [27, 28].

Application of Polypyrrole Nanobelts as Electrode Material …

89

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 4. FTIR spectrum of PPy nanobelts.

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 5. (a) Raman spectrum and (b) corresponding Raman mapping images in 950–1050 cm−1and (c) 1550–1600 cm−1of PPy nanobelts.

90

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

The Raman Scattering Spectroscopy The Raman scattering spectroscopy has been used to analyze the structural properties of PPy nanobelts. Figure 5(a) shows the Raman spectrum of synthesized PPy nanobelts. The strong Raman band at ∼1558 cm−1 corresponds to the characteristics C-C back-bone stretching of PPy [29]. The Raman bands at ∼1317 cm−1 and ∼1044 cm−1 are attributed to the ring-stretching mode and the C H in-plane of PPy respectively [30]. However, the peaks at ∼972 cm−1 confirms the ring deformation, associated with the radical cation (polaron) in PPy [31]. The detailed structure of PPy nanobelts are analyzed by the Raman mapping. The corresponding Raman mapping images in the two ranges of ∼950–1050 cm−1 and ∼1550–1600 cm−1 are depicted in Figure 5(b) and (c). In the Raman mapping, the uniform scattered light green color in the major dark green area is seen in the range of 950–1050 cm−1 (Figure 5(b)), which is assigned to the ring deformation associated with the radical cation (polaron) in PPy (Raman shift at ∼972 cm−1). (For interpretation of the references to color in the text, the reader is referred to the web version of the article.) On the other hand, the Raman mapping in the range of 1550–1600 cm−1 exhibits the uniform distribution of C-C stretching in PPy nanobelts, as shown in Figure 5(c). Thus, the uniformly distributed C-C bonding along with ring deformation, associated with the radical cation (polaron) in Raman mapping confirms the formation of high quality of PPy nanobelts. These results are in excellent agreement with the FTIR results.

THE OPTICAL CHARACTERIZATIONS OF POLYPYRROLE NANOBELTS The UV–Vis Absorption and the Photoluminescence Spectra Figure 6(a) exhibits the UV–Vis absorption spectrum of PPy nanobelts. The absorption band at ∼277 nm is attributed to –* transition in PPy nanobelts [32]. Another broad absorption band at ∼521 nm presents the characteristic of an oxidized PPy [32]. The UV–Vis spectrum of PPy is similar to the reported literature [33]. The room temperature photoluminescence (PL) spectrum of PPy nanobelts is shown in Figure 6(b) which is obtained with an excitation wavelength of ∼360 nm. A single broad band in the blue-green region at ∼448 nm is observed, corresponding to the non-radiative quenching in the aggregated phase which might improve the charge carrier diffusion through the extended PPy chains [34]. Additionally, the generation of singlet excitons with the highest intensity might represent the arrangements of PPy nanoparticles in nanobelt morphology.

Application of Polypyrrole Nanobelts as Electrode Material …

91

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 6. (a) UV–vis and (b) PL spectrum of PPy nanobelts.

THE ELECTROCATALYTIC ACTIVITY AND CONDUCTIVITY MEASUREMENTS OF POLYPYRROLE NANOBELTS ELECTRODE The Electrochemical Impedance Spectroscopy Measurements The electrochemical impedance spectroscopy (EIS) measurements have been performed for the fabricated aliphatic alcohols chemical sensors based on novel PPy nanobelts electrode to explain the electrocatalytic activity of the electrodes. Figure 7 shows the EIS plots of the fabricated aliphatic alcohols chemical sensors based on novel PPy nanobelts electrode using 0.1 M phosphate sulfate solution (PBS) with methanol, propanol and butanol at similar concentration of 20 μM. All EIS measurements are carried out at a frequency range from 100 kHz–1 Hz. From Figure 7, the fabricated

92

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

aliphatic alcohols chemical sensor displays two semicircles, in which the large semicircle in the high frequency region is attributed to the parallel combination of the charge transfer resistance (RCT) of the electrochemical reaction and the double layer capacitance (Cdl) at the interface of the PPy electrode/PBS electrolyte [35]. RCT of sensor device defines the electron-transfer kinetics of the redox probe at the electrode interface [35]. In general, the signal response for sensing device is determined by the values of RCT at the interfaces of the PPy electrode and different concentrations of alcohol in PBS [36]. Herein, all electrochemical alcohol chemical sensors present similar nature of EIS plots in 10 ml of PBS (0.1 M) with different alcohols at the same concentration of 20 μM. A relatively low RCT value (∼353 Ω) is obtained by the fabricated methanol chemical sensor based on PPy nanobelts electrode, however the propanol and butanol chemical sensors show the high RCT values of ∼507 Ω and ∼597 Ω respectively. Generally, the low charge transfer rate at the interface of electrode/electrolyte in the electrochemical system is originated from high RCT value [37]. This result suggests that the fabricated methanol chemicalsensor based on PPy nanobelts electrode presents the better charge transfer rate and the electrocatalytic activity toward the methanol chemical, resulting to the high sensing response on the surface of PPy nanobelts electrode. Whereas, other aliphatic alcohols based chemical sensors are quite inferior to methanol chemical sensor.

DC Conductivity of PPy Nanobelts Based Electrode Additionally, Figure 8 shows the plots of ln σdc vs. 1000/T in the temperature range of 300 – 400 K for the prepared PPy nanobelts electrode.

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 7. EIS plots of fabricated different aliphatic alcohol sensors based on PPynanobelts electrode.

Application of Polypyrrole Nanobelts as Electrode Material …

93

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 8. Plots of DC conductivity vs 1000/T for PPy nanobelts based electrode.

A reasonable high electrical conductivity of∼1.80 × 10−4 Ω−1cm−1 is recorded by the prepared PPy nanobelts electrode at room temperature (298 K). The electrical conductivity gradually increases with the increase of temperature from 298 to 423 K, indicating the stability and the maintained conductivity at high temperature. The reason of enhanced conductivity might arise due to the increased charged carrier concentration with the improved mobility of the charged carriers [38]. Like EIS results, the prepared PPy nanobelts electrode with high electrical conductivity might show high ions transport or charge transfer in the electro-chemical system or sensors. Thus, the unique morphology of PPy nanobelt and methanol chemical significantly favor the high ions transport or charge transfer at the interface of PPy electrode and PBS.

THE SENSING PROPERTIES OF POLYPYRROLE NANOBELTS ELECTRODE The I–V Characteristics of Polypyrrole Nanobelts Based Chemical Sensor The detailed sensing properties including the detection limit, linearity, correlation coefficient and sensitivity are extensively evaluated by the two electrodes I–V characteristics measurements where PPy nanobelts electrodes are used as working electrode while the Pt wire is applied as cathode. All I–V characteristics are measured with the applied voltage ranging from 0–2.5 V. The typical fabricated alcohol chemical sensor depicts the aliphatic alcohol sensing mechanism over the surface of PPy nanobelts. The fabricated aliphatic alcohol chemical sensor is performed in PBS with and without alcohols and the sensing behavior is simply explained by the I–V characteristics. It is

94

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

noticed that the drastic increase in current is observed after the addition of aliphatic alcohols (20 μM) in all the fabricated chemical sensors as compared to chemical sensor without alcohol, as shown in inset of Figure 9. The addition of methanol chemical (20 μM) in PBS displays the highest current of ∼60.4 μA while the lower currents of ∼58.7 μA and ∼55.4 μA are obtained with the addition of propanol and butanol chemicals in PBS respectively. This gradual increase in the current indicates the rapid sensing response of PPy nanobelts electrode toward the detection of methanol, propanol and butanol chemicals, which might result from the better electrocatalytic or electrochemical behavior and the fast electron exchange of PPy nanobelts electrode. The I–V responses of PPy nanobelts electrode with various concentrations of aliphatic alcohols ranging from 20 μM- 1 mM in10 ml of 0.1 M PBS have been measured to investigate the detailed sensing behavior of PPy nanobelt electrode. Figure 9 (a) shows the I–V characteristics of the fabricated methanol chemical sensor with various concentrations of methanol chemical (20 μM – 1mM) in0.1 M PBS solution of pH 7. When PPy nanobelts based electrode is exposed to methanol, the current drastically increases with the increase of the methanol concentrations, showing the good sensing response to methanol chemical. From the typical calibration curve (Figure 9(b)), the fabricated methanol chemical sensor based on PPy nanobelts electrode exhibits the reproducible, reliable and the highest sensitivity of ∼205.64 μA mM−1cm−2 with the linearity of 20 μM–0.16 mM, detection limit of ∼6.92 μM and correlation coefficient (R) of ∼0.98271.

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 9. (a) The I–V characteristics of PPy nanobelts based methanol chemical sensor at different methanol concentrations (20 μM–1 mM) in 10 ml of 0.1 M PBS and (b) thecalibration curve of current versus methanol concentration of the fabricated chemical sensor.

Application of Polypyrrole Nanobelts as Electrode Material …

95

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 10. (a) The I–V characteristics of PPy nanobelts based propanol chemical sensor at different propanol concentrations (20 μM–1 mM) in 10 ml of 0.1 M PBS and (b) the calibration curve of current versus methanol concentration of the fabricated chemical sensor.

Table 1. Sensing response characteristics of various conducting polymers based electrode for different chemicals Electrode

Materials

Detection limit

Response time (s)

Sensitivity

Refs.

-

Correlation coefficient (R) 0.999

aPPy-CFs aPPy-CFs aPPy-CFs bPAnPAN/PPO cGRPANI/GCE Layered PANI sheet electrode PPy nanobelts PPy nanobelts PPy nanobelts

Methanol Propanol Hexanol Phenol

400 ng 2 µg 7.5 µg -

1.5 10-4(1/µg) 3.9 10-5(1/µg) 1 10-5(1/µg) 960 (µA mM-1 cm-2)

[42] [42] [42] [43]

Phenol

0.065 µM

-

0.9987

[44]

Phenol

4.43 µM

10

0.9981

177.6;604.2 (µAmM1) 1485.3 (µA mM-1 cm-2)

Methanol

6.92 µM

10

0.9827

Propanol

13.7 µM

10

0.9640

Butanol

12.06 µM

10

0.95931

205.64 (µA mM-1 cm-2) 90.76 (µA mM-1 cm2) 146.34 (µA mM-1 cm-2)

This work This work This work

[21]

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd a PPy-CFs = Polypyrrole-commercial polystyrene fibres electrode. b PAn-PAN = Polyaniline-polyacrilonitrile composite electrode. c GR-PANI = Graphene-polyaniline composite electrode.

96

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 11. (a) The I–V characteristics of PPy nanobelts based butanol chemical sensor at different butanol concentrations (20 μM–1 mM) in 10 ml of 0.1 M PBS and (b) the calibration curve of current versus methanol concentration of the fabricated chemical sensor.

However, the fabricated propanol and butanol chemical sensors based on PPy nanobelts electrode (Figures 10 and 11) show the low sensitivities of ∼190.76 and ∼146.34 μA mM−1cm−2 and moderate detection limits of ∼13.7 and ∼12.06 μM respectively. All aliphatic alcohol sensors display the same response time of 10 s. The sensing parameters of all sensors are summarized in Table 1. As compared to other aliphatic alcohols, the highest current response and sensitivity have been observed for methanol chemical sensing which might suggest the high electron mobility and electrochemical activity over the surface of PPy nanobelts electrode, as described in EIS results.

Schematic Illustration of Electrochemical System and Interference Tests of the Fabricated Aliphatic Alcohol Sensors Figure 12 depicts the schematic illustration of the proposed mechanism of aliphatic alcohol chemical sensors over the surface of PPy nanobelts electrode. The detection of aliphatic alcohol chemicals in liquid phase is generally obtained by the adsorption of oxygenated species in PBS to the surface of PPy nanobelts electrode. These adsorbed oxygenated species change the concentration of oxygen species due to surface reactions and thus, develops a potential barrier and enhances the resistance of the material [39] Further, the chemisorbed oxygen reacts with alcohols over the PPy nanobelts electrode and forms a weak hydrogen bonding, which is due to the generation of an ion-dipole between -HN+• of PPy and neutral molecule (alcohols) [40]. A hydroxyl terminated PPy is generated by the substitution of adsorbed alcohol on PPy nanobelts electrode [41]. Thus, the terminated hydroxyl might have electrons from the surface of PPy nanobelts to produce the highly reactive oxygenated species, resulting in the decrease of conductance.

Application of Polypyrrole Nanobelts as Electrode Material …

97

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 12. (a) Schematic illustration of electrochemical system and (b) proposed mechanism of aliphatic alcohol chemical sensors over the surface of PPy nanobelts electrode.(c) Interference tests of the fabricated aliphatic alcohol sensors upon addition of methanol (5 μM), propanol (5 μM), and butanol (5 μM) in PBS (pH 7) and (d) plot of sensitivityversus time interval (days) of the fabricated aliphatic alcohol sensor.

The releasing of electrons (ē) might significantly dissociate the adsorbed alcohols into CO2 and H2O by the electrocatalytic behavior of PPy nanobelts electrode. These electrons are back to the surfaces of PPy nanobelts and again increase the conductance of electrode. In our case, the differences in the sensing response of PPy nanobelts based electrode for different aliphatic alcohols could attribute to the variations in the concentration of adsorbed oxygen species which might considerably affect their sensing reaction and conductance of PPy nanobelts electrode. Additionally, the interference study has been performed to understand the high sensing response of methanol chemical through the fabricated chemical sensor based on PPy nanobelts electrode, as shown in Figure 12 (c). The methanol, propanol and butanol are consecutively added into PBS to

98

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

evaluate the sensing response. From the amperometic results (Figure 12 (c)), a rapid response current is seen after the addition of methanol while propanol and butanol have shown weak response current. This phenomenon clearly indicates that the fabricated chemical sensor based PPy nanobelts electrode is sensitive to methanol chemical as compared to other aliphatic alcohols.

FTIR and 1H NMR Spectra of PPy Nanobelts Electrode Figure 13 (A) shows the FTIR spectra of PPy nanobelts electrode before and after the sensing measurements. Before the sensing measurements, the main IR peaks at ∼1560 and ∼1480 cm−1 are observed for PPy nanobelts electrode corresponding to C=C antisymmetric and symmetric stretching vibration in PPy ring respectively [24]. Another main IR peak at ∼3206 cm−1 presents the N-H stretching in the PPy ring. After the sensing measurements, PPy nanobelts electrode shows almost similar IR spectrum with the peaks of high intensities, indicating that no structural changes have occurred after the sensing measurements. The increase in IR peak’s intensity might attribute to the partial interaction between the OH group of alcohols and NH group of PPy nanobelts electrode. Noticeably, no effects or damages have been observed in the structural properties of PPy nanobelts after the sensing measurements in PBS electrolyte with various concentrations of aliphatic alcohols. Therefore, the prepared PPy nanobelts electrode is highly stable in aqueous medium and could be reused for other sensing measurements. The 1H NMR spectra of PPy nanobelts electrode before and after the sensing measurements as shown in Figure 13(B), are recorded by 600 MHz FT-NMR spectrometer (JNM-ECA600) in DMSO solvent. The1H NMR spectra of PPy nanobelts display the peaks (a, b, c) in the range of 7.0–8.5 ppm, which are assigned to proton of NH group and the aromatic protons on pyrrole ring. After the sensing measurements, two NMR peaks are seen at 5.0–4.5 ppm, indicating the interaction of aliphatic alcohols on PPy nanobelts surface. The NMR peaks at 5.0–4.5 ppm correspond to the protons of carbon, attached with OH group (alcoholic group). The existence of these peaks might suggest that the aliphatic alcohol first interact with the surface of PPy nanobelts through NH group during the sensing measurement, which is proposed in the illustrated mechanism. This result is fully consistent with FTIR results of PPy nanobelts electrode. The PPy nanobelts electrode based aliphatic alcohols chemical sensor shows the higher sensing performance compared to the reported literatures based on conducting polymers modified electrodes, as shown in Table 1 [21, 42–44]. The superior sensitivity and other sensing parameters of PPy nanobelts electrode toward aliphatic alcohol chemical might impute to the excellent adsorption ability and high electrocatalytic/ electrochemical activities of PPy nanobelts. Moreover, the fabricated PPy nanobelts

Application of Polypyrrole Nanobelts as Electrode Material …

99

electrode based aliphatic alcohol chemical sensor also exhibits good stability and PPy nanobelts electrode retains its sensing performances for more than four weeks, as shown in Figure 12(d). Therefore, PPy nanobelts are the excellent and novel working electrode materials for the fabrication of aliphatic alcohol chemical sensors.

Reprinted with permission from [S. Ameen, 2014], Appl. Catal. B: Environ., 144 (2014) 665 © 2014 Elsevier Ltd. Figure 13. (A) FTIR and (B) 1H NMR spectra of PPy nanobelts electrode before (a) andafter (b) the sensing measurements.

CONCLUSION The unique PPy nanobelts are simply synthesized by in-situ chemical polymerization of pyrrole monomer in the presence of ferric chloride as oxidant and methylene blue as reactive self-degraded template and applied as an effective working electrode for the fabrication of highly sensitive and reproducible aliphatic alcohols chemical sensor. The morphological properties and the elemental mapping demonstrate the uniform

100

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

morphology of nanobelts and the high aspect of chemical compositions. The EIS measurements of the fabricated methanol chemical sensor based on PPy nanobelts electrode clearly exhibits low RCT value, indicating the better charge transfer rate and electrocatalytic activity toward the methanol chemical. Whereas, other aliphatic alcohols based chemical sensors are quite inferior to methanol chemical sensor. Furthermore, the current (I)–voltage (V) characteristics have been performed to evaluate the sensing performance of PPy nanobelts electrode toward the detection of aliphatic alcohols. The fabricated PPy nanobelts based electrode shows the rapid detection of methanol with the high sensitivity of ∼205.64 μA mM−1cm−2, detection limit of ∼6.92 μM with a correlation coefficient (R) of ∼0.98271 and a short response time (10 s).

REFERENCES [1]

[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

[14]

(a) M. Consales, A. Cutolo, M. Penza, P. Aversa, M. Giordano, A. Cusano, J. Sens., (2008) 1; (b) S. C. Kim, W. G. Shim, Appl. Catal. B: Environ., 98 (2010) 180. P. C. Lekha, M. Balaji, S. Subramanian, D. P. Padiyan, Curr. Appl. Phy., 10 (2010) 457. P. Pandey, J. K. Srivastava, V. N. Mishra, R. Dwivedi, J. Nat. Gas Chem., 20 (2011) 123. M. A. Medinsky, D. C. Dorman, Tox. Lett., 707 (1995) 82. E. Segal, R. Tchoudakov, M. Narkis, A. Siegmann, Y. Wei, Sens. Actuators B Chem.,104 (2005) 140. C. E. Collins, L. Buckley, Synth. Met., 78 (1996) 93. C. L. Zhu, Y. J. Chen, R. X. Wang, L. J. Wang, M.S. Cao, X. L. Shi, Sens. Actuators B Chem., 140 (2009) 185. S. Ameen, M. S. Akhtar, H. S. Shin, Talanta, 100 (2012) 377. Y. Lin, H. S. Nalwa, Handbook of Electrochemical Nanotechnology, American Scientific Publishers, USA, 2009. (a) S. Ameen, M. S. Akhtar, M. Husain, Sci. Adv. Mater., 2 (2010) 441; (b) R. K. Pandey, V. Lakshminarayanan, Appl. Catal. B: Environ., 125 (2012) 271. L. A. Mashat, H. D. Tran, W. Wlodarski, R. B. Kaner, K. K. Zadeh, Sens. Actuators B Chem., 134 (2008) 826. S. Pirsa, N. Alizadeh, Sens. Actuators B Chem., 147 (2010) 461. (a) S. Pirsa, N. Alizadeh, IEEE J. Sens., 11 (2011) 3400; (b) R. Aydın, H. Ö. Dogan, F. Köleli, Appl. Catal. B: Environ., 478 (2013) 140; (c) W. I. Son, J. M. Hong, B. S. Kim, Korean J. Chem. Eng., 22 (2005) 285. S. J. Hong, C. H. Lee, Tetrahedron Lett., 53 (2012) 3119.

Application of Polypyrrole Nanobelts as Electrode Material …

[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44]

101

M. Şenel, C. Nergiz, Curr. Appl. Phy., 12 (2012) 1118. C. W. Lin, B. J. Hwang, C. R. Lee, Mater. Chem. Phys., 55 (1998) 139. C. W. Lin, B. J. Hwang, C. R. Lee, J. Appl. Polym. Sci., 73 (1999) 2079. L. Jiang, H. K. Jun, Y. S. Hoh, J. O. Lim, D. D. Lee, J. S. Huh, Sens. Actuators B Chem., 105 (2005) 132. M. Babaei, N. Alizadeh, Sens. Actuators B Chem., 183 (2013) 617. F. Quadrifoglio, V. Crescenzi, J. Colloid Interface Sci., 35 (1971) 447. S. Ameen, M. S. Akhtar, H. S. Shin, Talanta, 104 (2013) 219. S. M. Lin, X. Cao, J. H. Ma, J. J. Wu, H. F. Yin, W. F. Zhu, Dev. Appl. Mater., 2 (2009) 39. Y. Jia, P. Xiao, H. He, J. Yao, F. Liu, Z. Wang, Y. Li, Appl. Surf. Sci., 258 (2012) 6627. L. J. Bellamy, The Infrared Spectra of Complex Molecules, 2, 2nd ed., Chapman and Hall, London, 1980. J. B. Ayers, W. H. Piggs, J. Inorg. Nucl. Chem., 33 (1971) 721. S. Deivanayaki, V. Ponnuswamy, R. Mariappan, P. Jayamurugan, Optik, 124 (2013) 1089. G. Han, G. Shi, Sens. Actuators B Chem., 113 (2006) 259. F. E. Chen, G. Q. Shi, M. X. Fu, L. T. Qu, X. Y. Hong, Synth. Met., 132 (2003) 125. Y. Fukukawa, S. Tazawa, Y. Fujii, I. Harada, Synth. Met., 24 (1988) 329. J. Duchet, R. Legras, S. D. Champagne, Synth. Met., 98 (1988) 113. A. B. Gongcalves, A. S. Mangrich, A. J. H. Zarbin, Synth. Met., 114 (2000) 119. M. R. Nabid, A. A. Entezami, J. Appl. Polym. Sci., 94 (2004) 254. K. G. Neoh, K. K. S. Lau, V. V. T. Wong, E. T. Kang, Chem. Mater., 8 (1996) 167. A. J. R. Son, H. Lee, B. Moon, Synth. Met., 157 (2007) 597. P. Fiordiponti, G. Pistoia, Electrochim. Acta, 34 (1989) 215. C. M. Li, C. Q. Sun, W. Chen, L. Pan, Surf. Coat. Technol., 198 (2005) 474. M. A. Vorotyntsev, J. P. Badiali, G. Inzelt, J. Electroanal. Chem., 472 (1999) 7. S. Ameen, M. S. Akhtar, G. S. Kim, Y. S. Kim, O. B. Yang, H. S. Shin, J. Alloys Compd., 487 (2009) 382. P. Feng, Q. Wan, T. H. Wang, Appl. Phys. Lett., 87 (2005) 213111. M. Hara, A. Eisenberg, Macromolecules, 17 (1984) 1335. L. T. Sein, J. Phys. Chem. A, 112 (2008) 598. S. Pirsa, N. Alizadeh, Sens. Actuators B Chem., 147 (2010) 461. H. Xue, Z. Shen, Talanta, 57 (2002) 289. Y. Fan, J. H. Liu, C. P. Yang, M. Yu, P. Liu, Sens. Actuators B Chem., 157 (2011) 669.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 8

THE FABRICATION OF THE SCHOTTKY JUNCTION DIODE USING ALIGNED POLYPYRROLE NANOFIBERS FOR THE BROAD RANGE DETECTION OF M-DIHYDROXYBENZENE Sadia Ameen1, M. Shaheer Akhtar2, Hyung-Kee Seo1 and Hyung Shik Shin1,* 1

Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT Aligned p-type polypyrrole (PPy) nanofibers (NFs) thin film is grown on n-type silicon (100) substrate by an electrochemical technique to fabricate Schottky junction diode for the efficient detection of mdihydroxybenzene chemical. The highly dense and well aligned PPy NFs with the average diameter (~150-200 nm) are grown on n-type Si substrate. The formation of aligned PPy NFs is confirmed by elucidating the structural, compositional and the optical properties. The electrochemical behavior of the fabricated Pt/p-aligned PPy NFs/n-silicon Schottky junction diode is evaluated by cyclovoltametry (CV) and current (I)-voltage (V) measurements with the variation of m-dihydroxybenzene concentration in the phosphate buffer solution (PBS). The fabricated Pt/paligned PPy NFs/n-silicon Schottky junction diode exhibits the rectifying behavior of I-V curve with the addition of m-dihydroxybenzene chemical, while a weak rectifying I-V *

Corresponding Author Email: [email protected].

104

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al. behavior is observed without m-dihydroxybenzene chemical. This nonlinear I-V behavior suggests the formation of Schottky barrier at the interface of Pt layer and aligned PPy NFs/n-silicon thin film layer. By analyzing the I-V characteristics, the fabricated Pt/paligned PPy NFs/n-silicon Schottky junction diode displays reasonably high sensitivity ~23.67 μAmM-1cm-2, good detection limit of ~1.51 mM with correlation coefficient (R) of ~0.9966 and short response time (10 s).

INTRODUCTION The effluents of aromatic compounds especially the phenolic compounds from the industries like resins and plastics, wood preservation, petroleum refining, dyes, chemicals and textiles have adversely affected the aquatic ecosystem [1, 2]. m-dihydroxybenzene is a toxic and corrosive phenolic compound [3, 4] which is often used in cosmetics, tanning, pesticides, flavoring agents, medicines and photography chemicals [5]. It could be voluntarily absorbed from the gastrointestinal tract and get metabolized which might cause injuries to eyes, skin, oral and gastrointestinal functions [6]. These days, mdihydroxybenzene is considered as a priority pollutant under the high consideration of international agencies like the US Environmental Protection Agency (EPA) and the European Union (EU) [7]. A rapid and ecofriendly technology is needed to detect this chemical at very low concentration. The numerous techniques like spectrophotometric analysis [8], quartz crystal microbalance [9] and surface plasmon resonance [10] are the specialized methods for the detection of phenols and its derivatives. However, the analytical techniques such as liquid chromatography [11], fluorescence [12, 13], electrochemical [14], microchip capillary electrophoresis [15] and flow injection chemiluminescence methodologies [16] are also promising for the specific quantification of m-dihydroxybenzene. These analysis methods are facing the drawbacks of timeconsumption, complex performance and difficult operation under in-situ monitoring. Recently, an electrochemical technique is promising for detecting the harmful chemicals and organic compounds due to the advantages of high sensitivity, greater selectivity, time efficiency, and reproducibility [17]. Organics conducting polymers have shown good sensing properties owing to their low energy optical transitions, low ionization potential and high electron affinity [18, 19]. Apart from other conducting polymers, polypyrrole (PPy) shows highly tunable conductivity and an environmental stability [20] which could be easily synthesized through chemical or the electrochemical processes [21]. PPy could be easily deposited on various substrates from aqueous and non-aqueous media due to its high adherence, high electrical conductivity and good stability in air and aqueous media. The chemical synthesis of PPy usually involves polar or nonpolar solvents followed by its deposition on a substrate by drop casting, air brushing, dip coating and spin coating. These techniques are often limited to the deposition on a smaller substrate area whereas, an electrochemical deposition technique

The Fabrication of the Schottky Junction Diode …

105

allows a controllable polymer deposition on various conductive substrates [22]. PPy is extremely promised material for actuators, electric devices and the efficient detection of harmful chemicals [23-26]. In particularly, PPy electrode in bulk considerably limits both the efficiency and the sensitivity of gas and chemical sensors [27]. In order to improve the sensing performances, the one dimensional (1D) polymer nanostructure ensures the sensitivity by providing a high surface-to volume ratio, high porous structure and responds faster to detect chemicals due to its easier diffusion and the higher number of active centers for the production of donor-acceptor complex between the sensing materials and the targeted molecules [28]. Recently, efforts have been made for the development of chemical sensors utilizing silicon substrates for the fabrication of Schottky junction diode based sensors [29]. In general, Schottky contacts are formed by depositing a metal of large work function over the p-type deposited materials on n-type semiconducting substrates [30-32]. Nowadays, the metal-organic semiconductor Schottky junctions are utilized as an alternate to the metal/inorganic semiconductor junction thus, supports the new possibility of replacing conventional inorganic devices by organic semiconductors based devices [33-36]. In the organic semiconductors based devices, the charge carrier transport is mainly governed by choosing organic materials in terms of the morphology, sizes and surface-to-volume ratio [37]. A large number of Schottky junction diodes have been fabricated and characterized using organic semiconducting polymers with metals [38]. In Schottky junction diodes, the interfacial properties between the organic semiconductors and metal layers have a great impact on the sensitivity, reliability and stability of the sensors. So far, the nanofibrous semiconducting polymers based electrodes have been already utilized for the detection of various volatile organics, gases, and aromatic compounds [39-41] owing to their high surface area, fast diffusion of gas or chemical species and the high porosity. In this regards, W. Li et al. prepared polyaniline (PANI) nanofibers (NFs) through insitu oxidative polymerization method and used for the detection of aromatic organic compounds (AOCs) in the concentration range of 200e1000 ppm [42]. Likewise, Y. Liang et al. studied the imprinting of 1,3-dinitrobenzene (DNB) molecules on the surface of functional PANI NFs and demonstrated a linear response of DNB concentration between 2.20 x 10-8 and 3.08 x 10-6 M with a detection limit of ~7.33 x 10-9 M [43]. Thus, the fibrous structures of semiconducting polymers are responsible for the faster response time and the higher sensing response towards various aliphatic and aromatic organics gases and chemicals. In this chapter, the aligned p-type PPy NFs thin film on n-type silicon (100) substrate by an electrochemical technique is grown to fabricate Schottky junction diode for the efficient detection of m-dihydroxybenzene chemical. The electrochemical behaviors of the fabricated Pt/p-aligned PPy NFs/n-silicon Schottky junction diode are explained by cyclovoltametry (CV) and current (I)-Voltage (V) measurements with the variation of mdihydroxybenzene concentration in PBS. The obtained Pt/p-aligned PPy NFs/n-Si based Schottky junction diode displays non-linear I-

106

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

V curves, confirming the formation of Schottky contact. The fabricated mdihydroxybenzene chemical sensor based on Pt/p-aligned PPy NFs/n-Si Schottky junction diode exhibits a high and the reproducible sensitivity of ~23.67 μAmM-1cm-2 with a response time 10 s. The p-aligned PPy NFs deposited/n-type Si is used to fabricate Schottky junction diode by depositing Pt electrode with a Schottky contact in a vacuum coating unit. The fabricated Pt/p-aligned PPy NFs/n-type Si Schottky junction diode is directly employed for the sensing performance of m-dihydroxybenzene through the I-V characteristics by using an Electrometer. A fixed amount of 0.1 M phosphate buffer solution (PBS, 10.0 ml) and the wide concentration range of m-dihydroxybenzene from 0.01 mM to 10 mM are used for measuring the sensing properties of the fabricated Schottky junction diode. From I-V characteristics, the sensitivity of the fabricated m-dihydroxybenzene chemical sensor is estimated by the slope of the current versus concentration from the calibration plot divided by the value of active area of sensor/electrode. The current response is measured from -2 V to +2 V and the response time is measured as ~10 s.

Reprinted with permission from [S. Ameen, 2015], Anal. Chim. Acta, 886 (2015) 165 © 2015 Elsevier Ltd. Figure 1. (a, b) Surface and (c, d) tilted view of FESEM images of p-aligned PPy NFs.

The Fabrication of the Schottky Junction Diode …

107

MORPHOLOGICAL PROPERTIES OF ALIGNED POLYPYRROLE NANOFIBERS The Field Emission Scanning Electron Microscopy The FESEM images of p-aligned PPy NFs on n-Si electrode are shown in Figure 1. The surface view of grown PPy NFs display dots all across n-Si substrate which is associated to the tips of NFs and is perpendicularly oriented to the substrate, as shown in Figure 1a, b. However, when the grownp-aligned PPy NFs/n-Si substrate is tilted at 30o, the fibrous morphology in a vertical manner could be clearly seen, as shown in Figure 1c, d. From the high magnification image, the grown p-aligned PPy NFs are standing upright with a high density on n-Si substrate. Moreover, the grown p-aligned PPy NFs are uniformly distributed on n-Si substrate with an average diameter of ~150-200 nm.

STRUCTURAL AND OPTICAL STUDIES OF ALIGNED POLYPYRROLE NANOFIBERS Fourier Transform Infrared Spectroscopy The FTIR spectrum of p-aligned PPy NFs is shown in Figure 2 to describe the structural properties. Typical FTIR spectrum exhibits the peaks at ~787 cm-1, ~929 cm-1, and ~1035 cm-1, assigning to C-H bonding and =C-H in plane deformation vibration, respectively [44-46]. Additionally, the IR peaks at ~1203 cm-1 and ~1315 cm-1 correspond to N-C stretching band and =C-H in plane vibration, respectively [44-46]. The presence of PPy ring and the formation of PPy is confirmed by the existence of IR peaks at ~1396 cm-1and ~1553 cm-1 which are assigned to the vibration of PPy ring and the ring stretching mode of PPy ring respectively [44-46]. The IR peaks at ~1035 and ~1730 cm-1 substantiate the characteristic peak of -SO3- and C = O groups and confirms the presence of L-CSA, respectively [47, 48]. The existences of CH3 and CH2 groups of L-CSA are confirmed by the existence of the peaks at ~2839 and ~2961 cm-1 [49].

The Raman Scattering and Raman Mapping Spectroscopy The Raman spectrum of p-aligned PPy NFs has been analyzed to further deduce the structural properties of PPy NFs.

108

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Reprinted with permission from [S. Ameen, 2015], Anal. Chim. Acta, 886 (2015) 165 © 2015 Elsevier Ltd. Figure 2. FTIR spectrum of p-aligned PPy NFs.

Reprinted with permission from [S. Ameen, 2015], Anal. Chim. Acta, 886 (2015) 165 © 2015 Elsevier Ltd. Figure 3. Raman spectrum (a) and corresponding Raman mapping images in 900-1050 cm-1 (b) and 1500-1600 cm-1 (c) of p-aligned PPy NFs.

The Fabrication of the Schottky Junction Diode …

109

Reprinted with permission from [S. Ameen, 2015], Anal. Chim. Acta, 886 (2015) 165 © 2015 Elsevier Ltd. Figure 4. UV-DRS (a) and PL spectrum (b) of p-aligned PPy NFs.

From Figure 3a, the strong Raman bands at ~1402 cm-1 and ~1586 cm-1 represent the C = C backbone stretching and antisymmetrical C-N stretching of aligned PPy NFs, respectively [49, 50]. The Raman band at ~1057 cm-1 is associated with the bipolaron and the ring deformation vibrations [50] whereas, the polaron structure is also represented by the peak at ~984 cm-1 and corresponds to the symmetrical C-H in-plane bending [50, 51]. Figure 3b, c show the corresponding Raman mapping images in the two ranges of ~9001050 cm-1 and ~1500-1600 cm-1 of grown p-aligned PPy NFs. Figure 3b depicts the scattered bright brown color in the range of 900-1050 cm-1 which is assigned to the ring deformation associated with the radical cation (polaron) in PPy (Raman shift at ~984 cm-1). The other Raman mapping in the range of 1500-1600 cm-1 (Figure 3c) displays the dark brown color distribution which might correspond to the uniform distribution of C = C stretching in p-aligned PPy NFs. The obtained Raman mapping images again confirm the formation of high quality of PPy NFs by the existence of C = C bonding along with ring deformation and the radical cations (polarons).

The UV–Vis Absorption and Photoluminescence Spectra The UV-DRS spectrum of p-aligned PPy NFs has been examined to explain the optical properties, as shown in Figure 4a. The broad intense absorption edge at ~588 nm is associated to the -* transition in the polymer chain of PPy [52]. The optical band gap (Eg) is estimated as ~2.1 eV from the absorption edge wavelength obtained by grown p-aligned PPy NFs thin film which is lower to the reported PPy band gap due to the interstitially embedded L-CSA into p-aligned PPy NFs [53]. Furthermore, the room temperature photoluminescence (PL) spectrum of p-aligned PPy NFs is shown in Figure

110

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

4b, obtained with an excitation wavelength of ~340 nm. The grown p-aligned PPy NFs possess a sharp emission peak at ~443 nm in the blue-green region, which has slightly red shifted compared to the reported PL peak at ~448 nm of PPy [54]. This red shifting might occur due to the presence of L-CSA in the grown p-aligned PPy NFs, corresponding to the non-radiative quenching for improving the charge carrier diffusion through the extended PPy chains [55]. Moreover, the sharpness of PL peak at ~443 nm is associated to the good arrangements of p-aligned PPy NFs morphology on n-Si substrate [54].

THE SENSING PROPERTIES OF ALIGNED POLYPYRROLE NANOFIBERS ELECTRODE The I–V Characteristics of Pt/p-Aligned PPy NFs/n-Si Schottky Junction Diode Based Chemical Sensor The fabricated Pt/p-aligned PPy NFs/n-Si Schottky junction diode has been designed as chemical sensor for the detection of m- dihydroxybenzene. The sensing properties of Pt/p-aligned PPy NFs/n-Si Schottky junction diode is elucidated through a series of IeV characteristics by varying the concentration of m-dihydroxybenzene chemical. Figure 5 shows the forward and reverse bias I-V characteristics of p-aligned PPy NFs/n-Si heterostructures device without the deposition of Pt and with Pt Schottky contact, carried out by two probes system at the room temperature (~298 K) with an applied voltage ranges from -2 V to +2 V. The p-aligned PPy NFs/n-Si heterostructures device exhibits almost the symmetrical behavior in both the reverse and forward bias, as shown in Figure 5a. In other words, the current linearly increases with the increase of an applied voltage and reaches the maximum current value of ~10.3 mA at the maximum voltage of ~2V, which occurs due to the absence of barrier between p-aligned PPy NFs and n-Si electrode. However with Pt contact, the typical rectifying behavior is observed in I-V characteristics of Pt/p-aligned PPy NFs/n-Si Schottky junction diode, as shown in Figure 5b. The rectifying behavior is seen due to the formation of Schottky barrier between paligned PPy NFs/n-Si and Pt layer and thus, results to non-linear curve of IeV characteristics. From Figure 5b, the fabricated Pt/p-aligned PPy NFs/n-Si Schottky diode presents the turn-on voltage of ~0.604 V with low leakage current of ~0.31 mA and slightly high break down voltage (~1.19 V). Herein, the architecture of p-aligned PPy NFs on n-Si substrate might possess a large surface area and is expected to enhance the molecular contact and the transport kinetics. To understand the detailed diode behavior of the fabricated Pt/p-aligned PPy NFs/nSi Schottky junction device without and with m-dihydroxybenzene chemical, the diode

The Fabrication of the Schottky Junction Diode …

111

parameters like ideality factor (h), saturation current (Io) and rectification ratio (R) are calculated and summarized in Table 1. Table 1. Summary of rectification ratios (R), ideality factors (η), and saturation currents (µA) of the fabricated Pt/p-aligned PPy NFs/n-Si Schottky junction diode Concentration of mdihydroxybenzene Without Low High

Rectification ratio (R) 3.16 2.15

Ideality factor (η) 3.71 2.76 1.37

Saturation current (µA) 2.66 2.81 2.76

Reprinted with permission from [S. Ameen, 2015], Anal. Chim. Acta, 886 (2015) 165 © 2015 Elsevier Ltd

Reprinted with permission from [S. Ameen, 2015], Anal. Chim. Acta, 886 (2015) 165 © 2015 Elsevier Ltd. Figure 5. The I-V characteristics of (a) p-aligned PPy NFs/n-Si thin film electrode, (b) Pt/p-aligned PPy NFs/n-Si Schottky junction diode, and (c) I-V characteristics of Pt/p-aligned PPy NFs/n-Si Schottky junction diode in 0.1 M PBS with different dihydroxybenzene isomers at the concentration of 0.01 mM.

112

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

The above parameters of the fabricated Schottky junction is evaluated by the ideal diode equation [56]. I=Ioexp(qV/ηkT)-1 where Io is the saturation current, q is the electronic charge, V is the junction voltage, h is the diode ideality factor, k is the Boltzmann constant and T is the temperature in K. The values of η and Io could be determined from the slope and the intercept of semilogarithmic forward bias IeV plot of Figure 5. Herein without m-dihydroxybenzene, the fabricated diode shows low Io of ~2.66 µA whereas upon the addition of low and high concentration of m-dihydroxybenzene, Io values have increased. It is known that the work function of Pt is larger than PPy and thus, forms a potential barrier at Pt/p-aligned PPy NFs [57]. The diode with m-dihydroxybenzene shows the chemical diffusion into Pt thin film layer, which might lower the work function of Pt metal at Pt/p-aligned PPy NFs junction due the reaction of m-dihydroxybenzene with Pt metal layer near metal-polymer interface [58]. The decrease in the work function of Pt metal at Pt/p-aligned PPy NFs junction clearly helps in the increase of the forward bias current and enhances Io. On the other hand, the fabricated Pt/p-aligned PPy NFs/n-Si Schottky junction device presents the large ideality factor than unity which might due to the possibility of barrier inhomogenity [59]. Additionally, the large ideality factor of Schottky diode is attributed to the aligned morphology of PPy NFs on n-Si substrate, resulting to certain voltage drops across paligned PPy NFs/n-Si layer. Moreover, the ratio of forward (If) and reverse current (Ir), called as rectification ratio (R) of the fabricated Schottky junction diode with different concentration of m-dihydroxybenzene have been calculated. The fabricated Schottky barrier diode expresses moderately good R value without and with m-dihydroxybenzene chemical. It is reported that the generation of minority charge carriers in Schottky junction diode are dependent on the nature of p-type materials [60]. In our case, these minority charge carriers move fast and freely along the aligned morphology of PPy NFs through the conjugation of the bonding [61-63] and improves an overall rectification of the fabricated Schottky junction diode. The selectivity of sensor based on Schottky junction diode towards isomers of dihydroxybenzene has been examined by performing a series of experiments using various concentrations of dihydroxybenzene isomers and the sensing performance of each system is compared. Figure 5c displays the I-V characteristics of Schottky junction diode towards different dihydroxybenzene chemicals of ~0.01 mM in 10 ml PBS electrolyte. The low forward current is observed when odihydroxybenzene and p-dihydroxybenzene are inserted in PBS, indicating less active surface of diode for o- and p-dihydroxybenzene chemicals. However, relatively the high forward current is attained with m-dihydroxybenzene chemical, showing the good sensing response of the fabricated Schottky junction diode based on aligned PPy NFs.

The Fabrication of the Schottky Junction Diode …

113

Reprinted with permission from [S. Ameen, 2015], Anal. Chim. Acta, 886 (2015) 165 © 2015 Elsevier Ltd. Figure 6. I-V characteristics of (a) Pt/p-aligned PPy NFs/n-Si Schottky junction diode based mdihydroxybenzene chemical sensor at different m-dihydroxybenzene concentration (0.01 mM - 10 mM) in 10 ml of 0.1 M PBS and (b) the calibration curve of current versus m-dihydroxybenzene concentration for the fabricated chemical sensor and (c) the sensitivity plot of different dihydroxybenzene chemicals.

The sensing behavior of the fabricated Pt/p-aligned PPy NFs/n-Si Schottky junction diode towards dihydroxybenzene chemical is elucidated by measuring a series of IeV plots using different concentrations of dihydroxybenzene ranging from 0.01 mM to 10 mM in 0.1 M PBS. Figure 6a shows that with the increase in the concentration of mdihydroxybenzene, the current of Schottky junction diode increases, which explains the generation of large number of ions with the addition of m-dihydroxybenzene and enhances the ionic strength of the solution. The other sensing parameters of Pt/paligned PPy NFs/n-Si Schottky junction diode are evaluated from the calibration curve of current versus concentration, as shown in Figure 6b. The current increases linearly with the increase in mdihydroxybenzene concentration due to the availability of large number of active sites over the surface of p-aligned PPy NFs/n-Si thin film. From the calibration plot, the reproducible and high sensitivity of ~23.67 μAmM-1 cm-2 with a correlation

114

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

coefficient (R) of ~0.9966 towards m-dihydroxybenzene are estimated from the calibration curve by measuring the slope and dividing it by the active area of the electrode (0.15 cm2). The fabricated Pt/p-aligned PPy NFs/n-Si Schottky junction diode for m-dihydroxybenzene chemical attains a reasonable detection limit of ~1.51 mM, a short response time (10 s) and a good linearity in the range of 0.01mM-10 mM. The sensitivity of the fabricated Pt/p-aligned PPy NFs/n-Si Schottky junction diode towards other isomers such as odihydroxybenzene and p-dihydroxybenzene is compared, as shown in the Figure 6c. The low sensitivities of ~2.13 μAmM-1 cm-2 and ~10.2 μAmM1 cm-2 are estimated for o-dihydroxybenzene and p-dihydroxybenzene respectively by the fabricated Pt/p-aligned PPy NFs/n-Si Schottky junction diode. The remarkably high sensitivity of m-dihydroxybenzene chemical might be due to the unique aligned morphology of PPy NFs, good optical/electronic behavior, strong electrocatalytic activity and the effective adsorptive properties of the fabricated Schottky junction diode towards m-dihydroxybenzene. The observed sensing parameters are comparable to the reported literatures on gold (Au)/carbon electrode based sensors [64, 65]. The stability and reproducibility or reversibility of the fabricated m-dihydroxybenzene chemical sensor is examined by measuring the I-V characteristics for three consecutive weeks. No significant drop is detected in the sensing parameters or properties, suggesting a long term stability or durability of the fabricated m-dihydroxybenzene chemical sensor based on Pt/p-aligned PPy NFs/n-Si Schottky junction diode.

Cyclovoltammetry Studies To elucidate the electrochemical properties of Pt/p-aligned PPy NFs/n-Si Schottky junction diode, the cyclovoltametry (CV) analysis is performed with different concentration (0.01 mM-10 mM) of m-dihydroxybenzene chemical in 0.1MPBS (pH = 7.0) at the scan rate of 50 mV s-1, as depicted in Figure 7. The CV shows a prominent oxidation peak or anodic current with weak reduction peak or cathodic current. As the concentration of m-dihydroxybenzene increases, the anodic current also increases gradually. Significantly, at the highest concentration of m-dihydroxybenzene (10 mM), the highest anodic current value of ~0.21 mA is obtained, which is higher than the anodic current at the lowest concentration of mdihydroxybenzene. It suggests the favorable electro-oxidation of m-dihydroxybenzene during the potential sweep and the dynamic response might generate due to the interaction between the p-aligned PPy NFs/n-Si thin film and m-dihydroxybenzene chemical. The high anodic current is generally associated to the high electrocatalytic behavior and the faster electronetransfer reaction in an electrochemical system [66]. Herein, the grown aligned PPy NFs on Si substrate has considerably improved the electron transfer process in Pt/p-aligned PPy NFs/n-Si Schottky junction diode, which might increase the sensing performances.

The Fabrication of the Schottky Junction Diode …

115

Reprinted with permission from [S. Ameen, 2015], Anal. Chim. Acta, 886 (2015) 165 © 2015 Elsevier Ltd. Figure 7. Cyclovoltammetry curves with different concentration of m-dihydroxybenzene in 10 ml of 0.1 M PBS solution.

Amperometric Response and Interference Test of the Fabricated Sensor The sensing response of m-dihydroxybenzene chemical is further determined using steady state current-time measurements of Pt/p-aligned PPy NFs/n-Si Schottky junction diode. To stabilize the background current, the amperometric measurement of Pt/paligned PPy NFs/n-Si Schottky junction diode is performed without m-dihydroxybenzene in PBS (10 ml) solution. Afterward, m-dihydroxybenzene solution of ~0.001 mM concentration is added successively drop by drop using a peristaltic pump into the 10 ml of 0.1 M PBS. Figure 8a shows the steady state current-time responses of Pt/p-aligned PPy NFs/n-Si Schottky junction diode towards m-dihydroxybenzene chemical. The step by step increment in the current has been observed with the addition of mdihydroxybenzene chemical, indicating a linear relationship between the current and m-dihydroxybenzene concentrations. Herein, Pt/p-aligned PPy NFs/n-Si Schottky junction diode displays rapid response and the continuous increase in the current upon the addition of mdihydroxybenzene might attribute to the fast and direct electron transfer through the active sites on the surface of p-aligned PPy NFs/n-Si thin film. Moreover, the fibrous morphology of PPy NFs might provide high surface area and generates the large number of active sites over the surface of p-aligned PPy NFs/n-Si thin film [67]. Figure 8b shows the interference study by introducing m-dihydroxybenzene (0.01 mM) and other interfering electroactive species (p-dihydrobenzene (0.01 mM)) and o-dihydroxybenzene

116

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

(0.01 mM)) into 0.1 M PBS to understand the sensing response of the fabricated chemical sensor based on Pt/p-aligned PPy NFs/n-Si Schottky junction diode. The intense current response is observed just after the addition of m-dihydroxybenzene however, weak current response is seen upon the addition of the other p-dihydrobenzene and odihydroxybenzene into PBS. This experiment clearly suggests that the fabricated chemical sensor based Pt/paligned PPy NFs/n-Si Schottky barrier diode is sensitive for mdihydroxybenzene chemical even in the presence of other interfering chemicals in the electrolyte. The fabricated chemical sensor based Pt/p-aligned PPy NFs/n-Si Schottky junction diode exhibits almost stagnant stability by retaining its original response after being checked for our consecutive weeks, as shown in Figure 8c. Thus, Pt/p-aligned PPy NFs/n-Si Schottky junction diode is an excellent chemical sensing behavior for the efficient detection of m-dihydroxybenzene.

Reprinted with permission from [S. Ameen, 2015], Anal. Chim. Acta, 886 (2015) 165 © 2015 Elsevier Ltd Figure 8. (a) Amperometric response of Pt/p-aligned PPy NFs/n-Si Schottky junction diode with successive addition of m-dihydroxybenzene into a 0.1 M PBS buffer solution (pH = 7.0), (b) Interference test of the fabricated m-dihydroxybenzene sensors upon addition of p-dihydrobenzene and o-dihydroxybenzene in PBS (pH 7) and (c) plot of sensitivity versus time interval (days) of the fabricated chemical sensor.

The Fabrication of the Schottky Junction Diode … 1H

117

NMR Spectra of PPy Nanobelts Electrode and Proposed Mechanism

The after sensing effects on Pt/p-aligned PPy NFs/n-Si Schottky junction diode is studied by 1H NMR (Figure 9) spectra before and after the sensing measurements, which are recorded by 600 MHz FTNMR spectrometer in DMSO solvent. The 1H NMR spectra of Pt/p-aligned PPy NFs/n-Si Schottky junction diode exhibits three peaks (a, b, c) in the range of 7.0-8.5 ppm, corresponding to proton of NH group and the aromatic protons on pyrrole ring, respectively. Two NMR peaks are seen in the range of 5.0-4.5 ppm after the sensing measurements of the device, indicating the interaction of phenolic group in mdihydroxybenzene on Pt/p-aligned PPy NFs/n-Si Schottky junction diode. The origin of these 1HNMR peaks might deduce the m-dihydroxybenzene interaction with the surface of Pt/p-aligned PPy NFs/n-Si Schottky junction diode through NH group during the sensing measurement, as proposed in the illustrated mechanism (Figure 10).

Reprinted with permission from [S. Ameen, 2015], Anal. Chim. Acta, 886 (2015) 165 © 2015 Elsevier Ltd. Figure 9. 1H NMR spectra of PPy nanobelts electrode before (a) and after (b) the sensing measurements.

118

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Reprinted with permission from [S. Ameen, 2015], Anal. Chim. Acta, 886 (2015) 165 © 2015 Elsevier Ltd. Figure 10. Proposed mechanism for m-dihydroxybenzene chemical sensor over the surface of Pt/paligned PPy NFs/n-Si Schottky junction diode.

CONCLUSION The highly p-aligned PPy NFs thin film is grown on n-Si substrate by an electrochemical technique and utilizes for the fabrication of the Schottky junction diode to detect mdihydroxybenzene chemical. The morphological analysis reveals the highly dense and well aligned PPy NFs grown on n-type Si substrate. The introduction of Pt layer on p-aligned PPy NFs/n-Si thin film exhibits the non-linear or rectifying I-V behavior due to the formation of Schottky barrier between p-aligned PPy NFs/n-Si and Pt layer. The CV and I-V measurements with the variations of m-dihydroxybenzene concentration in PBS are performed to evaluate an electrochemical behavior of the fabricated Pt/p-aligned PPy NFs/n-silicon Schottky junction diode. By analyzing the I-V characteristics, the fabricated Pt/p-aligned PPy NFs/n-silicon Schottky junction diode displays the reasonably high sensitivity ~23.67 μAmM-1cm-2, good detection limit of ~1.51 mM with correlation coefficient (R) of ~0.9966 and short response time (10 s). The rapid and enhanced sensing is related to the highly dense p-aligned PPy NFs on Si substrate with high surface-to-volume ratio, which considerably improves the electron transfer process in Pt/p-aligned PPy NFs/n-Si Schottky junction diode. Thus, the fabricated Pt/p-aligned PPy NFs/n-Si Schottky barrier diode is highly promising for the detection of harmful chemicals at the room temperature.

The Fabrication of the Schottky Junction Diode …

119

REFERENCES [1] [2] [3] [4]

[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

G. W. Ware, Rev. Environ. Contamin. Toxicol., 126 (1992) 100. A. M. Klibanov, T. M. Tu, K. P. Scott, Science, 221 (1983) 259. W. H. Wei, C. M. Lan, S. Dan, Z. Yan, Chinese J. Anal. Chem., 40 (2012) 1747. S. J. Li, C. Qian, K. Wang, B. Y. Hua, F. B. Wang, Z. H. Sheng, X. H. Xia, Sens. Actuators B Chem., 174 (2012) 441. H. S. Yin, Q. M. Zhang, Y. L. Zhou, Q. Ma, T. Liu, L. S. Zhu, S. Y. Ai, Electrochim. Acta, 56 (2011) 2748. Y. C. Kim, H. B. Matthews, Fund. Appl. Tox., 9 (1987) 409. T. Xie, Q. Liu, Y. Shi, Q. Liu, J. Chromat. A, 1109 (2006) 317. C. Kang, Y. Wang, R. Li, Y. Du, J. Li, B. Zhang, L. Zhou, Y. Du, Microchem. J., 6 (2000) 161. A. Mirmohseni, A. Oladegaragoze, Sens. Actuators B Chem., 98 (2004) 28. J. D. Wright, J. V. Oliver, R. J. M. Nolte, S. J. Holder, N. A. J. M. Sommerdijk, P. I. Nikitin, Sens. Actuators B Chem., 51 (1998) 305. H. Cui, J. Zhou, F. Xu, C. Z. Lai, G. H. Wan, Anal. Chim. Acta, 511 (2004) 273. W. Xiao, D. Xiao, Talanta, 72 (2007) 1288. J. Fan, T. Zhang, J. Sun, M. Fan, J. Fluor., 17 (2007) 113. S. M. Ghoreishi, M. Behpour, E. Hajisadeghian, M. Golestaneh, Arab. J. Chem., 9 (2016) S1563. M. R. Gomez, R. A. Olsina, L. D. Martínez, M. F. Silva, Talanta, 61 (2003) 233. J. Du, Y. Li, J. Lu, Talanta, 55 (2001) 1055. V. Pino, A. M. Afonso, V. Gonzalez, W. L. Hinze, J. Liq. Chrom. Rel. Tech., 26 (2003)1. S. Ameen, M. S. Akhtar, M. Husain, Sci. Adv. Mater., 2 (2010) 441. R. K. Pandey, V. Lakshminarayanan, Appl. Catal. B Environ., 125 (2012) 271. L. X. Wang, X. G. Li, Y. L. Yang, React. Fun. Pol., 47 (2001) 125. L. A. Mashat, H. D. Tran, W. Wlodarski, R. B. Kaner, K. K. Zadeh, Sens. Actuators B Chem., 134 (2008) 826. L. A. Mashat, C. D. Chouvy, S. Borensztajn, W. Wlodarski, J. Phys. Chem. C, 116 (2012) 13388. S. Pirsa, N. Alizadeh, IEEE Sens. J., 11 (2011) 3400. R. Aydın, H. Ö. Dogan, F. Köleli, Appl. Catal. B Environ., 140 (2013) 478. W. I. Son, J. M. Hong, B.S. Kim, Korean J. Chem. Eng., 22 (2005) 285. S. J. Hong, C. H. Lee, Tetrahedron Lett., 53 (2012) 3119. L. A. Mashat, H. D. Tran, W. Wlodarski, R. B. Kaner, K. K. Zadeh, IEEE Sens. J., 8 (2008) 365.

120

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

[28] J. Huang, S. Virji, B. H. Weiller, R. B. Kaner, Chem. Euro. J., 10 (2004) 1314. [29] K. S. Kim, G. S. Chung, Sens. Actuators B Chem., 160 (2011) 1232. [30] M. Shafiei, J. Yu, R. Arsat, K. Kalantarzadeh, E. Comini, M. Ferroni, G. Sberveglieri, W. Wlodarski, Sens. Actuators B Chem., 146 (2010) 507. [31] J. Yu, S. J. Ippolito, M. Shafiei, D. Dhawan, W. Wlodarski, K. Kalantarzadehn, Appl. Phys. Lett., 94 (2009), 013504-1. [32] J. Yu, S. J. Ippolito, W. Wlodarski, M. Strano, K. Kalantarzadeh, Nanotechnology, 2 (2010), 265502-1. [33] A. Assadi, C. Svensson, M. Willander, O. Inganas, J. Appl. Phy., 72 (1992) 2900. [34] S. A. Chen, Y. Fang, H. T. Lee, Synth. Met., 57 (1993) 4082. [35] H. Lin, H. Liu, X. Qian, S. W. Lai, Y. Li, N. Chen, C. Ouyang, C. M. Che, Y. Li, Inorg.Chem., 50 (2011) 7749. [36] K. Wang, X. Qian, L. Zhang, Y. Li, H. Liu, ACS Appl. Mater. Interf., 5 (2013) 5825. [37] S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, J. Nanosci. Nanotech., 11 (2011) 3306. [38] R. K. Gupta, R. A. Singh, Compos. Sci. Tech., 65 (2005) 677. [39] M. A. Chougule, S. G. Pawar, P. R. Godse, R. N. Mulik, S. Sen, V. B. Patil, Soft Nanosci. Lett., 1 (2011) 6. [40] M. A. Chougule, S. G. Pawar, S. L. Patil, B. T. Raut, V. B. Patil, IEEE Sen. J., 11 (2011)2137. [41] E. Pretsch, P. Buhlmann, M. Badertscher, Structure Determination of Organic Compounds Tables of Spectral Data, fourth ed., 1993, p. 283. [42] A. Z. Sadek, C. O. Baker, D. A. Powell, W. Wlodarski, R. B. Kaner, K. Kalantarzadeh, IEEE Sens. J., 7 (2007) 213. [43] S. Virji, J. Huang, R. B. Kaner, B. H. Weiller, Nano Lett., 4 (2004) 491. [44] K. Wang, H. Yang, X. Qian, Z. Xue, Y. Li, H. Liu, Y. Li, Dalton Trans., 43 (2014) 11542. [45] B. T. Raut, M.A. Chougule, A. A. Ghanwat, R. C. Pawar, C. S. Lee, V. B. Patil, J. Mater.Sci. Mater. Electr., 23 (2012) 2104. [46] W. Li, N. D. Hoa, Y. Cho, D. Kim, J. S. Kim, Sens. Actuators B Chem., 143 (2009) 132. [47] C. He, Y. Tan, Y. Li, J. Appl. Poly. Sci., 87 (2003) 1537. [48] Y. Liang, L. Gu, X. Liu, Q. Yang, H. Kajiura, Y. Li, T. Zhou, G. Shi, Chem. Eur. J., 17 (2011) 5989. [49] L. Qu, G. Shi, J. Yuan, G. Han, F. Chen, J. Electroanal. Chem., 561 (2004) 149. [50] M. Li, Z. Wei, L. Jiang, J. Mater. Chem., 18 (2008) 2276. [51] A. B. Gongcalves, A. S. Mangrich, A. J. H. Zarbin, Synth. Met., 114 (2000) 119. [52] V. G. Veselago, Phys. Usp., 10 (1968) 509.

The Fabrication of the Schottky Junction Diode …

121

[53] S. T. Navale, A. T. Mane, A. A. Ghanwat, A. R. Mulik, V. B. Patil, Measurement, 50 (2014) 363. [54] S. Ameen, M. S. Akhtar, H. K. Seo, H. S. Shin, Appl. Catal. B Environ., 144 (2014) 665. [55] A. J. R. Son, H. Lee, B. Moon, Synth. Met., 157 (2007) 597. [56] S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, J. Nanosci. Nanotech., 11 (2011) 3306. [57] S. M. Sze, Physics of Semiconductors Devices, Wiley, New York, 1969, p. 370. [58] P. S. Abthagir, R. Saraswathi, J. Appl. Poly. Sci., 81 (2001) 2127. [59] A. B. Yadav, C. Periasamy, S. Bhaumik, S. Jit, Ind. J. Pure Appl. Phys., 51 (2013) 792. [60] W. Mtangi, F. D. Auret, C. Nyamhere, R. P. J. Jansevan, M. Diale, A. Chawanda, Phys. B Cond. Matt., 404 (2009) 1092. [61] B. Xu, J. Choi, A. N. Caruso, P. A. Dowben, Appl. Phys. Lett., 80 (2002) 4342. [62] R. P. McCall, J. M. Ginder, J. M. Lang, H.J. Ye, S. K. Manohar, J. G. Masters, G. E. Asturias, A. G. MacDiarmid, A. J. Epstein, Phys. Rev. B, 41 (1990) 5202. [63] Y. Cao, Synth. Met., 35 (1990) 263. [64] X. Wang, M. Wu, H. Li, Q. Wang, P. He, Y. Fang, Sens. Actuators B Chem., 192 (2014) 452. [65] C. Wang, R. Yuan, Y. Chai, F. Hu, Anal. Methods, 4 (2012) 1626. [66] Y. Cao, P. Smith, A. J. Heeger, Synth. Met., 32 (1989) 263. [67] S. Ameen, M. S. Akhtar, H. S. Shin, Sens. Actuators B Chem., 173 (2012) 177.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 9

USING THE P-NIO/N-POLYANILINE/N-SI SCHOTTKY DIODE TO DETECT HYDRAZINOBENZENE Sadia Ameen1, M. Shaheer Akhtar2 and Hyung Shik Shin1,* 1

Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New and Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT New and effective chemosensor is fabricated using p-nickel oxide (NiO)/ nconducting polyaniline (PANI) based Schottky barrier diode for the detection of hydrazinobenzene chemical. The n-PANI is synthesized through in-situ chemical doping of PANI EB by using calcium hydride as dopant and subjected to elemental analysis, optical, structural and morphological properties. The appearance of a non-linear I–V behavior at the interface of Pt and p-NiO/n-PANI layer confirms the formation of Schottky junction of the fabricated Pt/p-NiO/n-PANI/n-Si Schottky barrier diode. The electrochemical properties of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode towards the detection of hydrazinobenzene are elucidated by cyclovoltametry (CV) measurements. The sensing results reveal that the Pt/p-NiO/n-PANI/n-Si Schottky barrier diode exhibited a stable, reliable high sensitivity ~90.5 μA mM-1cm-2, good detection limit of ~5.11 μM with correlation coefficient (R) of ~0.99417 and short response time (10 s). Herein, n-type chemical doping of PANI and the formation of Schottky barrier elicits the sensing parameters such as sensitivity, detection limit and correlation coefficient.

*

Corresponding Author Email: [email protected].

124

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

INTRODUCTION Hydrazine and their derivatives are vastly used chemicals in pesticides, pharmaceuticals, strong reducing agents, chemical blowing agents, dyes, as antioxidants, plant growth regulators, and rocket propellants [1]. Hydrazinobenzene is one of the hydrazine derivatives which is basically used in pharmaceutical, agrochemical, and chemical industries [2, 3]. It is highly toxic in nature and hazardous to an environment and human beings. The small assays of hydrazinobenzene chemical could readily enter through inhalation, oral and dermal routes in humans which might cause skin irritation, dermatitis, hemolytic anemia, liver and kidney injuries [4]. Therefore, an immediate action is required to detect hydrazinobenzene chemical at low concentration. So far, the technologies like chromatography [5, 6] spectrophotometry [7, 8] and capillary electrophoresis [[9, 10] (a)] are applied for the determination of hydrazinobenzene. Apart from the existing technologies, the electrochemical method is one of the simple and feasible processes for the detection of toxic chemical owing to its rapid response time, high sensitivity, wide linear range, economical and highly portable [[4, 10] (b)]. However, the major drawback of an electrochemical method is related to the oxidation of toxic chemicals which demands high potential (≥1 V) at the working electrode. In order to surplus the over potential required for the oxidation of such toxic chemicals, the electrode surface is required to be modified by various materials, electroanalysts, etc. Polyaniline (PANI) is one of the popular conjugated semiconducting polymers which is used in the varieties of application like, optoelectronics, bio-sensors, gas sensors, electrochemical sensors, microelectronics etc., due to its high chemical stability, simple polymerization, and the high conductivity [11, 12]. Generally, PANI behaves as p-type semiconductor in nature which could be modified by the protonation using protonic acid or by charge transfer with an oxidation agent [13]. The doping of PANI with strong reductants such as calcium hydride, sodium hydride and potassium hydride produces ntype PANI, which enhances the transition, electrical conductivity and the electronic properties. On the other hand, nickel oxide (NiO) as p-type semiconductor with a band gap of ~3.6–4.0 eV drags a lot of attention as active material for various applications such as electrochemical sensors, gas sensors, supercapacitors, photoelectrochemical cells and hydrogen storage etc. [14, 15]. In particular, NiO nanomaterials have been used in gas sensors, biosensors and electrochemical sensors due to its high physical and chemical stability [16]. Although, NiO materials show an excellent stability and outstanding electrocatalytic activities but still, thrust exists for the exploration to achieve highly efficient and economical sensors [17]. At present, the electrochemical method for the determination of toxic chemicals is attracting much attention by the researchers to develop high performance chemical sensors. S. Ameen et al. has reported the chemical sensor based on

Using the p-NiO/n-Polyaniline/n-Si Schottky Diode to Detect Hydrazinobenzene 125 PANI electrode for the detection of phenol chemical which showed a stable and high sensitivity ~1485.3 μA mM-1cm-2 and the detection limit of ~4.43 μM with correlation coefficient (R) of ~0.9981 [18]. The high and reproducible sensitivity of ~1.66 AmM-1cm-2 with correlation coefficient of R = ~0.965 were accounted for the phenol chemical sensor based on PANI nanoglobules electrode [19]. In continuation, PANI electrode with sea-cucumber-like hollow spheres showed the high sensitivity of ~426.5 mAmM-1cm-2 and a detection limit of ~515.7 mM with R of ~0.90157 for the ethanol chemical sensor [20]. The Schottky junction of PANI with metal has shown a great technological and practical importance in several electrochemical and electronic devices [21]. In particular, Schottky junction diodes based on PANI are reported for the development of chemical sensors to detect the concentration of toxic chemicals [22]. The interfacial properties between PANI and metal layers in Schottky junction diodes have shown great impacts on the sensitivity, reliability and stability of the sensors. Moreover, the choices of organic and inorganic materials in terms of the morphology, uniformity, and interconnection of p-type and n-type semiconductors could affect the charge separation and charge carrier transport in Schottky junction diodes [23]. Paul et al. fabricated the Schottky diode using layer-by-layer assembly of ZnO/PANI and demonstrated high and rapid photoelectric response [24]. Xu et al. reported the heterostructure of PANI/Bi2Te3 nanowires based Schottky diode for gas sensing [25]. The heterojunction of n-CdSe/p-PANI and n-CdTe/p- PANI was prepared by Joshi et al. for a room temperature LPG sensor [26, 27]. PANI acts as a p-type semiconductor when it is in emeraldine state (ES) whereas, pernigraniline structure of PANI is associated with n-type semiconductor properties [28]. In the present study, Schottky barrier diode is fabricated by depositing p-NiO on n-PANI/n-Si substrate with Pt top layer for the effective detection of hydrazinobenzene chemical. Herein, n-PANI is obtained through in-situ chemical doping of PANI EB, using calcium hydride as a strong reducing agent. The fabricated hydrazinobenzene chemical sensor based on Pt/p-NiO/n-PANI/n-Si Schottky barrier diode exhibits a high and the reproducible sensitivity of ~90.5μA mM1 cm-2 with a response time 10 s. For diode fabrication, the synthesized n-PANI is dissolved in 10 ml DMSO and spin coated on n-Si substrate at ~2500 rpm for ~30 s using ~0.45 mm pore PVDF membrane syringe filter (Jet Biofil) at an ambient atmosphere. After air drying, the suspension of p-NiO (~10 ml, ~0.2 g NiO in nafion solution) particles is spun at ~2000 rpm for 45 s on n-doped PANI/n-Si thin film and thereafter, keep in an oven for ~20 min at ~120oC. Finally, the top platinum (Pt) contact with thickness of ~100 nm is deposited through ion sputtering to accomplish Pt/p-NiO/n- PANI/n-Si Schottky barrier diode. In this work, we have used the optimized device for detecting the sensing behavior of a diode. Moreover, the analytical parameters of the sensors are typically based on the uniform deposition of the thin film for the fabrication of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode. The fabricated diode displays a rectifying and nonlinear current-voltage (I–V) behavior which

126

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

might due to the formation of Schottky contact/ barrier between Pt layer and p-NiO/ n-PANI/n-Si. The sensing of hydrazinobenzene chemical based on the fabricated Pt/p-NiO/ n-PANI/n-Si Schottky barrier diode is performed through the I–V characteristics by two electrodes system, using Electrometer. For the measurements, one Ag contact by silver paste is made on the top of Pt layer and second Ag contact is on the back side of Si wafer of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode (Figure 1(a)) and the measurements are carried out in 10 ml PBS with different concentrations of hydrazinobenzene. The wide concentration range of hydrazinobenzene from 25 μM -1 mM is prepared in 10 ml of 0.1 M phosphate buffer solution (PBS, 10.0 ml). The sensitivity of the fabricated hydrazinobenzene chemical sensor is estimated from the slope of the current versus concentration from the calibration plot divided by the value of active area of sensor/electrode. The following equations are used to evaluate LOD and QD: LOD = 3.3 x standard deviation (SD)/slope of the calibration plot QD = 10 x SD/slope of the calibration plot. The current responses from -3.5 V to 3.5 V with the response time of ~10 s are measured.

Morphological, and Crystalline Characterizations of p-NiO, n-PANI and p-NiO/n-PANI Thin Films The Field Emission Scanning Electron Microscopy and X-Rays Diffraction Patterns The morphologies of the synthesized p-NiO, n-PANI and p-NiO/ n-PANI thin films are examined by FESEM measurements. FESEM images of p-NiO, n-PANI and p-NiO/n-PANI thin films are shown in Figure 1(b–d). The synthesized p-NiO exhibits small spherical particles (Figure 1(b)) with the average particle size of ~20–30 nm. From Figure 1(c), the spherical porous particles with the average diameter of ~30–50 nm are seen in the micrograph of n-PANI (Figure 1(c)). The deposition of p-NiO creates several aggregates, as shown in p-NiO/n-PANI thin film (Figure 1(d)), suggesting the influence of p-NiO deposition for the formation of p-n junction. In order to study the crystalline properties of the synthesized p-NiO nanoparticles, X-Rays diffraction (XRD) analysis has been performed. Figure 1(e) shows the characteristic diffraction patterns of p-NiO nanomaterials (JCPDS No. 89-5881), corresponding to the typical face centered cubic p-NiO nanostructure with the space group of Fm-3m. Furthermore, the absence of other diffraction peak confirms the excellent crystal and high purity of the synthesized p- NiO.

Using the p-NiO/n-Polyaniline/n-Si Schottky Diode to Detect Hydrazinobenzene 127

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd. Figure 1. Schematic representation of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode (a), FESEM images of (b) p-NiO, (c) n-PANI (d) p-NiO/n-PANI and (e) XRD of p-NiO thin film.

Atomic Force Microscopy The topographical AFM images (Figure 2) support the FESEM observations. The n-PANI possesses the spherical nanoparticles, as shown in Figure 2(a). The p-NiO/ n-PANI thin film (Figure 2(b)) displays several aggregates with evident voids generation, which might helpful for the oxidation of hydrazinobenzene over the surface of p-n junction. The presence of free voids is also seen in 3D AFM images of p-NiO/n-PANI thin film, as shown in Figure 2(d). The roughness factor of the thin films is examined by analyzing the AFM images. The n-PANI thin film (Figure 2(c)) presents the high root mean roughness (Rrms) of ~152 nm, while p-NiO/n-PANI thin film obtains the lower Rrms value (~55.6 nm). Herein, the changes in roughness might originate from the presence of voids over the surface of thin film, which likely helpful for the dissemination of hydrazinobenzene chemical on the surface of p-NiO/n-PANI thin film electrode.

128

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd. Figure 2. Topographic (a, b) and three dimensional (c, d) atomic force microscopy (AFM) images of n-PANI and p-NiO/n-PANI thin films.

Structural and Optical of PANI EB and n-PANI Fourier Transform Infrared Spectroscopy Figure 3 shows the FTIR spectra of PANI EB, n-PANI, and p-NiO/n- PANI. The synthesized PANI EB presents almost similar IR bands, as reported in the literature [30, 31]. Two IR band at ~1492 and ~1590 cm-1 are associated to C-C aromatic ring-stretching vibration and quinonoid rings of PANI, respectively [32]. The peak at ~1301 cm-1 corresponds to C-N stretching in the neighborhood of quinonoid ring [33]. The other IR bands at ~519, ~832 and ~1142 cm-1 represent the out-of-plane C-H wagging vibrations,

Using the p-NiO/n-Polyaniline/n-Si Schottky Diode to Detect Hydrazinobenzene 129 out-of-plane deformation of C-H and aromatic C-H in-plane bending, respectively [34, 35]. In particular, the protonation form C-N+ stretching vibration in the polaron structure occurs due to the existence of IR band at ~1254 cm-1 [36]. However, the appearance of IR band at ~1142 cm-1 is assigned to a vibration mode of -NH+= structure and the intensity of this particular peak signifies the degree of doping of the polymer backbone [37].

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd. Figure 3. FTIR spectra of PANI EB, n-PANI and p-NiO/n-PANI thin films.

From Figure 3(b), the obtained IR bands in n-PANI shows a slight shift as compared to PANI EB, suggesting the n-type doping in PANI. The FTIR spectrum of p-NiO/ n-PANI is almost identical to n-PANI and exhibits broad IR band near ~700–500 cm-1. The existence of this band is attributed the Ni-O stretching, confirming the presence of p-NiO on n-PANI [38]. The IR band at 3600 cm-1 could be assigned to the protonation of amines functional group at the polymer backbone. This observation clearly supports a strong interaction between n- PANI and p-NiO nanomaterial.

The UV–vis Absorption and Photoluminescence Spectra Optical characterizations of PANI EB and n-PANI the UV–vis absorption and photoluminescence (PL) studies of PANI EB and n-PANI are examined to analyze the optical properties. Figure 4(A) presents the UV–vis spectra of PANI EB and n-PANI which shows two absorption bands at ~326 nm and ~625 nm. The characteristic absorption band at ~326 nm corresponds to π-π* transition within the benzenoid segment,

130

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

related to the extent of conjugation between adjacent phenyl rings in PANI [39]. However, the existence of broad absorption at ~625 nm describes n-π* transition [40].

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd. Figure 4. (A) UV–vis absorption and (B) photoluminescence (PL) of PANI EB and n-PANI.

The prominent red shift in the absorption bands are seen in UV–vis spectrum of nPANI, indicating the successful n-type chemical doping into PANI. Figure 4(B) depicts

Using the p-NiO/n-Polyaniline/n-Si Schottky Diode to Detect Hydrazinobenzene 131 the room temperature photoluminescence (PL) spectra of PANI EB and n- PANI with an excitation wavelength of ~380 nm. A large amplitude PL emission in the blue green region at ~444 nm is seen in PL spectra of PANI EB, corresponding to π-π* transition of the benzenoid unit of PANI [41]. In addition, a single PL emission originates probably due to the benzenoid or amine groups and subsequently quenches to a quinoid or imine group of PANI therefore, eliminating the quinoid unit of PANI. However, the n-type chemical doping into PANI causes a significant shift which might happen due to the chemical interaction between -NH groups of PANI chains and the dopants. Moreover, the change in the PL intensity of n-PANI might occur due to the decrease of defect states in the polaronic band which generates the large transitions from the polaronic band to the p band (HOMO) [42].

Electrochemical Characterizations of Pt/p-NiO/n-PANI/n-Si Schottly Barrier Diode Cyclicvoltammetry (CV) Measurements of Pt/p-NiO/n-PANI/n-Si Schottky Barrier Diode Cyclicvoltammetry (CV) measurements, as shown in Figure 5 have been used to elucidate the electrochemical behavior of Pt/p-NiO/n- PANI/n-Si Schottky barrier diode in the phosphate buffer solution (PBS, pH 7.0) electrolyte with different concentration of hydrazinobenzene chemical. The electrochemical activities of active p- NiO/n-PANI thin film electrode is governed by the oxidation and reduction peaks in CV plot. Herein, without hydrazinobenzene chemical, a very low redox current is obtained by p-NiO/n-PANI thin film electrode. However, the addition of hydrazinobenzene chemical in PBS displays a sudden increment in the redox current, suggesting a quick and rapid response of p-NiO/n-PANI thin film electrode towards hydrazinobenzene chemical. In other words, p-NiO/n-PANI thin film electrode shows good electrocatalytic activity towards hydrazinobenzene chemical at low concentration. A series of CV measurements have been performed by varying the hydrazinobenzene concentration ranging from 25 μM–1 mM in 0.1 M PBS to examine the detailed electrochemical properties of p- NiO/n-PANI thin film electrode. The continuous increase in the redox current upon the increase of hydrazinobenzene concentration might result from the high electrocatalytic behavior and the fast electron-transfer reaction in the electrochemical system via p-NiO/n-PANI thin film electrode [43]. At high hydrazinobenzene concentration, the highest current response occurs due to the fast kinetics of electron transfer [44], via advanced and good electrocatalytic surface of p-NiO/n-PANI thin film electrode. From the calibration plot (inset of Figure 5), the limit of detection (LOD) of ~5.11 μM and R of ~0.9955 are estimated for p-NiO/n-PANI thin film electrode.

132

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd. Figure 5. Cyclicvoltammetry (CV) measurements of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode in the phosphate buffer solution (PBS, pH 7.0) electrolyte with different concentration of hydrazinobenzene chemical. Inset shows the calibration plot of oxidation peak current vs. hydrazinobenzene concentrations.

Pt/p-NiO/n-PANI/n-Si Schottky Barrier Diode Based Hydrazinobenzene Chemosensor The Pt thin layer was deposited on p-NiO/n-PANI thin film via ion sputtering to create a Schottky barrier contact between Pt and p-NiO/n-PANI layer and finally, the Pt/p-NiO/n-PANI/Si Schottky barrier diode was obtained. Figure 6(a, b) shows the typical I–V characteristics of Pt/p-NiO/n-Si and Pt/n-PANI/n-Si devices dis-playing a straight line or linear I–V behavior. The existence of linear behavior is evident that these devices possess Ohmic contact. However, Pt/p-NiO/n-PANI/n-Si Schottky barrier diode displays a rectifying and nonlinear I–V behavior. The origin of non-linear behavior might due to the formation of Schottky contact/barrier between Pt layer and p-NiO/n-PANI/nSi. The device shows high rectifying ratio of ~8.1 at the bias voltage of ~3.5 V, indicating a weak rectifying I–V nature. In addition, I–V characteristics in terms of electronic transport of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode is explained by Schottky/thermionic emission model with fixed potential barrier at the interface where the electrons are thermionically emitted [45] and described by the forward bias I–V characteristic. These emissions are explained by an Eq. (1); I = Is [exp (qV/hkT)-1]

(1)

Using the p-NiO/n-Polyaniline/n-Si Schottky Diode to Detect Hydrazinobenzene 133 where Is is the saturation current, q denotes the electronic charge, V represents the junction voltage, h refers to diode ideality factor, k shows the Boltzmann constant and T is the temperature. From Figure 6(c), the fabricated Pt/p-NiO/n-PANI/n-Si Schottky barrier diode displays high ideality factor value of ~7.4 and low saturation current of ~2.5 μA. The large h refers to weak diode behavior and poor I–V properties of the fabricated Schottky barrier diode. Interestingly, these electrical parameters have sufficiently increased upon the utilization of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode as chemosensor for the detection of hydrazinobenzene chemical.

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd. Figure 6. I-V characteristics of (a) Pt/p-NiO/n-Si, (b) Pt/n-PANI/n-Si devices, (c) I–V measurements at different hydrazinobenzene concentration (0 mM–1 mM) in 10 ml of 0.1 M PBS, (d) saturation current vs. concentration, and (e) ideality factor Vs. hydrazinobenzene concentration of the fabricated Pt/p-NiO/n-PANI/n-Si Schottky barrier diode based chemical sensor.

134

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

The detailed sensing behavior of hydrazinobenzene chemical by Pt/p-NiO/n-PANI/pn-Si Schottky barrier diode is elucidated by measuring a series of I–V characteristics, as shown in Figure 6(c). It is notable that the forward current constantly increases with the increase of the hydrazinobenzene concentrations and the prominent decrease in reverse bias mode leads to a better rectifying behavior. The electrical parameters such as ideality factor, rectifying ratio, saturation current and Schottky barrier height of the fabricated diode based chemical sensor are summarized in Table 1. In the electrochemical system, the addition of hydrazinobenzene chemical has slightly increased Is values as compared to Is value of the fabricated diode (as shown in Figure 6(d)), which might due to the formation of a potential barrier at Pt/p- NiO/n-PANI [46]. It could be described by the lowering in work function of Pt metal at p-NiO/n-PANI junction which probably related to the reaction of hydrazinobenzene with Pt metal layer near to the polymer interface [47]. Thus, the lowering in work function of Pt metal at Pt/p-NiO/n-PANI considerably increases the forward bias current and Is. It is known that the values of ideality factor are inversely proportional to hydrazinobenzene concentrations (Figure 6 (e)). Herein, the values of ideality factor for a diode are still high due to the morphology of the deposited polymer on n-Si substrate which might drop certain voltage across p-NiO/n-PANI layer [48, 49]. In addition, the high ideality might be due to the aggregation of particles and the nature of n-type doping of PANI which might give rise to two different transport mechanisms [50]. Moreover, the current is mainly governed by the Schottky barrier height (SBH). The high work function of Pt metal contact leads to a high SBH and consequently a small number of free charge carriers are available, leading to low forward current (as evidenced in Figure 6(c)). However in the presence of hydrazinobenzene, the chemical is diffused into Pt thin film layer and might react with Pt metal layer near to metal-polymer interface which lowers the work function of Pt metal and thus, decreases SBH [51]. The decrease in SBH might favor the forward bias current and also enhances Is. Table 1. Sensing parameters of the fabricated Pt/p-NiO/n-PANI/n-Si Schottky barrier diode. Concentration of hydrazinobenzene 0 µM 25 µM 50 µM 100 µM 200 µM 500 µM 1 mM

Ideality factor (η) 7.425 6.234 6.124 5.796 5.633 5.363 4.843

Saturation current (µA) 2.5031 2.8285 2.7705 2.7155 2.6808 2.6228 2.5638

Rectifying ratio (R) 8.10 7.92 6.69 6.62 5.52 4.76 4.21

Voltage barrier (eV) 0.7021 0.7086 0.7091 0.7096 0.7100 0.7105 0.7111

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd

Using the p-NiO/n-Polyaniline/n-Si Schottky Diode to Detect Hydrazinobenzene 135 The other sensing parameters of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode are evaluated from the calibration curve versus concentration, as shown in Figure 7. The current linearly increases with respect to hydrazinobenzene concentration. It suggests well-defined current response during the successive addition of hydrazinobenzene due to the generation of large number of carriers which move freely on n-PANI chain over the surface of Pt/ p-NiO/n-PANI Schottky barrier diode. From the calibration plot (Figure 7(a)), the Pt/p-NiO/n-PANI/n-SiSchottky barrier diode based chemical sensor exhibits a reproducible and high sensitivity of ~90.5 μA mM-1cm-2, limit of detection (LOD) of ~5.11 μM with a correlation coefficient (R) of ~0.99417 towards hydrazinobenzene chemical. Additionally, the hydrazinobenzene chemical sensor based on Pt/p-NiO/nPANI/n-Si Schottky barrier diode shows good linearity in a wide range of 25 μM–1 mM, limit of quantification (LOQ) of ~15.5 μM with a short response time (10 s). It has been seen that LOD and R values are similar to the estimated LOD and R values from CV results. For comparison, n-PANI/n-Si and p-NiO/n- Si electrodes based chemical sensors are studied to explain the sensitivity of the hydrazinobenzene chemical. From Figure 7(b), the low sensitivities of ~10.6 μAmM-1cm-2 and ~12.21 μAmM-1.cm-2 are respectively observed for n-PANI/n-Si and p-NiO/n-Si electrodes which are inferior to hydrazinobenzene chemical sensor based on Pt/p-NiO/n-PANI/n-Si Schottky barrier diode. Additionally, the observed sensitivity and other sensing parameters of the fabricated Pt/p-NiO/n-PANI/n-Si Schottky barrier diode for the detection of hydrazinobenzene chemical are superior to the reported literature on chemical sensors based on PANI modified electrodes, as presented in Table 2 [52–56, 43]. The improved in the sensitivity and other sensing parameters could be attributed to the excellent electrocatalytic behavior, electrochemical activities, and enhanced electrical properties such as ideality factors, saturation current, and increased forward bias current of Pt/p-NiO/n-PANI/n- Si Schottky barrier diode. Table 2. Comparison of sensing responses of various electrodes Electrode GR-PANI/GCE HMWCNT/GCE PdNPs-PANI/GCE Fe2O3/CPANI Pt/P-NiO/n-PANI/n-Si n-PANI/n-Si p-NiO/n-Si

Limit of detection 0.065 0.68 µM 0.06 0.153 µM ~5.11 µM -

Linearity (µA) 0.2-20; 20-100 µM 2.0-122.8 µM 10-300 µM 0.2-40 µM 25-200 µM -

Sensitivity (R) 177.6;6042(mA mM-1) 0.0208 (µA µM-1) 0.5 (µA/µmolcm-2) 1.93 (µA µM-1cm-2) ~90.5 (µA mM-1cm-2) 10.6 (µA mM-1cm-2) ~12.21 (µA mM-1cm-2)

Refs. [53] [54] [55] [56] This work This work This work

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd

136

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd. Figure 7. (a) The calibration curve of current versus hydrazinobenzene concentrations for the fabricated Pt/p-NiO/n-PANI/n-Si Schottky barrier diode based chemical sensor, (b) the sensitivity of different devices (Pt/p-NiO/n-Si Pt/n-PANI/n-Si, and Pt/p-NiO/n-PANI/n-Si) and (c) the plot of sensitivity versus time interval (days) of the fabricated chemical sensor.

The Stability and the Reproducibility of the Fabricated Hydrazinobenzene Chemical Sensor The stability and the reproducibility of the fabricated hydrazinobenzene chemical sensor (Figure 7(c)) were monitored by measuring the I–V characteristics for three consecutive weeks and significantly, no fall was detected in the sensing parameters or properties, suggesting the long term stability or durability of the fabricated hydrazinobenzene chemical sensor based on Pt/p-NiO/n-PANI/n-Si Schottky barrier diode. The remarkably high sensitivity might due to n-type chemical doping of PANI, good optical/electronic behavior, strong electrocatalytic activity and strong adsorptive properties of the fabricated Schottky barrier diode towards hydrazinobenzene.

Using the p-NiO/n-Polyaniline/n-Si Schottky Diode to Detect Hydrazinobenzene 137

The Electrochemical Impedance Spectroscopy (EIS) of Pt/p-NiO/n-PANI/n-Si Schottky Barrier Diode The electrochemical impedance spectroscopy (EIS) has been carried out to further study the electrochemical properties of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode in the wide range concentration of hydrazinobenzene 25 μM–1 mM in 0.1 M PBS with the frequency range 100 kHz–1 Hz. Figure 8 shows the EIS plots and depicts the inclined straight line or lee perfect semicircles at the high frequency region which basically explains the parallel combination of the charge transfer resistance (RCT) of the electrochemical reaction and a constant phase element (CPE) at the interface of diode/electrolyte [57], as represented in an equivalent circuit diagram (inset of Figure 8). CPE is initially imputed to a dispersion of capacity with frequency and usually originates from the distribution of potential and the current which are due to non-homogeneous interface like surface disorders, defects and high roughness [58, 59]. The less perfect semicircles at the high-frequency region could be interpreted by using a modified Ershler- Randles equivalent circuit, as shown in inset of Figure 8. In general, the typical EIS plot shows well-defined semicircles but, practically the semicircles are deformed and wide (as observed in this case). The impedance of CPE could be explained by the following equation. ZCPE = 1/Q(jω)n where, n (0 < n < 1) is a CPE exponent and Q (Fcm-2 Sn-1) as the CPE parameter. When n = 1, CPE is considered as a capacitor, n = 0 for a resistor and behaves as an inductor for n = -1. The exponent ‘n’ of CPE is a highly relevant quantity which is basically regarded as an indicator of the heterogeneity degree. In particular, CPE exponent n in the field of the electrodes is believed as inverse relation to the surface roughness of the electrode [60]. Thus, the effective capacitance (Ceff) related to CPE could be denoted as: Cef f = Q1/nR(1-n) /n CPEorCT where, RCPE or RCT is the charge transfer resistance of the diode in parallel with CPE. From the above equation, it is clear that low RCT or RCPE value is responsible for the high charge storage as obtaining the high Ceff value. From Figure 8, the high RCT at the interface of diode/electrolyte is associated to the high barrier height and the depletion layer moves to the interface of device and electrolyte. Moreover, Pt/p-NiO/n-PANI/n-Si Schottky barrier diode exhibits low RCT value at the highest hydrazinobenzene concentration (1 mM), suggesting the good sensing response towards hydrazi- nobenzene chemical.

138

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd. Figure 8. EIS plots of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode based chemical sensor in 10 ml of 0.1 M PBS with different concentrations of hydrazinobenzene chemical. Inset shows the equivalent circuit of EIS.

On the other hand, the linear portion at low frequency region is associated to the diffusion process. With the increase of hydrazinobenzene concentrations, the linear plot becomes slightly curvy, indicating the increase in ions diffusion which significantly results in the fast electron transfer. Thus, it might be concluded that high hydrazinobenzene concentration might generate large number of ions which significantly results to the increase of ionic conductivity of the system and enhances the sensing responses. The sensing mechanism of hydrazinobenzene chemical based on p-NiO/ n-PANI thin film electrode is governed by the surface properties of the materials and the nature of oxidation-reduction reactions which occurs during the sensing process. The adsorbed oxygen on the surface of sensing electrode is responsible for determining the electrical properties of the sensing material.

Electrochemical Mechanism of Hydrazinobenzene Chemical Over Pt/p-NiO/ n-PANI/n-Si Schottky Barrier Diode In general, the reactive oxygen species (O2- and O-) are chemisorbed over the surface of p-NiO/n-PANI [61] wherein, the amount of chemisorbed reactive oxygen species is counted on the interface between p-NiO/n-PANI. In our case, the hydrazinobenzene

Using the p-NiO/n-Polyaniline/n-Si Schottky Diode to Detect Hydrazinobenzene 139 sensing is regularized by the amount of adsorbed oxygen on sensing electrode surface. First the analyte ‘hydrazinobenzene’ reacts with the adsorbed oxygen (O2-) and forms diazenyl benzene by losing two electrons and a proton.

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd. Figure 9. Electrochemical mechanism of hydrazinobenzene chemical over Pt/p-NiO/ n-PANI/n-Si Schottky barrier diode.

The electron simultaneously introduces into p-NiO/n-PANI electrode, resulting to the increase of conductance and enhances the current response with the variation of voltage. Afterwards, the diazenyl benzene receives a proton in the presence of oxygen to produce benzenediazonium, which could be seen as the prominent anodic peak in CV results. The following reactions are presented in Figure 9. The oxidizing power for the oxidation of hydrazinobenzene has basically increased due to the origin of good interfacial contact between p-NiO and n-PANI in Schottky barrier diode.

Illustration of Chemical Sensors over the Surface of Pt/p-NiO/n-PANI/n-Si Schottky Barrier Diode and Its Schottky Barrier Heights without and with Hydrazinobenzene Chemical Further the sensing performance of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode based chemical sensor is explained by the proposed illustration on the basis of depletion region. Figure 10 shows the physical mechanism models of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode without and with hydrazinobenzene chemical. In general, the Schottky barrier height (SBH) of the diode is proportional to the depletion layer [62]. Herein, without the presence of hydrazinobenzene chemical, low SBH is obtained by Pt/p-NiO/n-PANI/n-Si Schottky barrier diode. However, Pt/p-NiO/n-PANI/n-Si Schottky barrier diode with hydrazinobenzene chemical exhibits high SBH, indicating the

140

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

enlargement of the depletion region and the resistance of the p-NiO/n-PANI. Thus, the addition of hydrazinobenzene chemical results to the increase in the current response and width of the depletion region increases likely due to the decrease in the conductivity of the polyaniline channel, as shown in CV results.

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd. Figure 10. Illustration of chemical sensors over the surface of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode and its Schottky barrier heights without and with hydrazinobenzene chemical. 1

H NMR Spectra of the Fabricated Pt/p-NiO/n-PANI/n-Si Schottky Barrier Diode To check the interaction of hydrazinobeneze with the fabricated electrode, 1H NMR spectra of p-NiO/n-PANI before and after the sensing measurements in DMSO solvent, as shown in Figure 11. p-NiO/n-PANI depicts the peaks (a, c) near 8.0 ppm and the peaks (b, d) in the range of 7.0–7.5 ppm correspond to proton of NH group and the aromatic protons on PANI, respectively. After the hydrazinobeneze chemical sensing measurements, the proton of hydrazine is detected near the 4.0 ppm along with aromatic protons in the range of 6–7.5 ppm. The existence of hydrazine proton after the sensing suggests the interaction of hydrazino- beneze with p-NiO/n-doped PANI electrode. Thus, p-NiO/n-doped PANI based Schottky barrier diode might have good surface for the adsorption of hydrazinobeneze chemical through NH group of PANI and active oxygen of NiO during the sensing measurements.

Using the p-NiO/n-Polyaniline/n-Si Schottky Diode to Detect Hydrazinobenzene 141

Reprinted with permission from [S. Ameen, 2016], Electrochim. Acta, 215 (2016) 200 © 2016 Elsevier Ltd. Figure 11. 1H NMR spectra of the fabricated Pt/p-NiO/n-PANI/n-Si Schottky barrier diode based chemical sensor before (a) and after (b) the sensing measurements.

142

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

CONCLUSION The facile method for the fabrication of Schottky barrier diode is adopted and the diode characteristics are analyzed by the standard thermionic emission model of a Schottky junction. The present work provides in-depth studies of the Schottky barrier diode fabricated with Pt metal and conducting PANI. Herein, PANI has been modified as n-type semiconductor through in-situ chemical doping. The I–V characteristics are performed to evaluate the sensing performance of Pt/p-NiO/n-PANI/n-Si Schottky barrier diode towards the detection of hydrazinobenzene chemical. The interaction of hydrazinobenzene chemical with Pt metal might reduce the SBH and enhance the forward bias and the saturation current. The rapid detection of hydrazinobenzene chemical with the high sensitivity of ~90.5 μA mM-1cm-2, detection limit of ~5.11 μM, LOQ of ~15.5 μM with correlation coefficient (R) of ~0.99417 and short response time (10 s) are shown by Pt/p-NiO/n- PANI/n-Si Schottky barrier diode.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

[11] [12] [13]

fourth ed., H. W. Schess, K. Othmer (Eds.), Encyclopedia of Chemical Technology, vol. 13, Wiley/Interscience, New York, 1995, pp. 560. A. A. Ibrahim, G. N. Dar, S. A. Zaidi, A. Umar, M. Abaker, H. Bouzid, S. Baskoutas, Talanta, 93 (2012) 257. Umar, M. M. Rahman, S. H. Kim, Y. B. Hahn, Chem. Commun., 2 (2008) 166. S. Ameen, M. S. Akhtar, H. S. Shin, Talanta, 100 (2012) 377. M. Mori, K. Tanaka, Q. Xu, M. Ikedo, H. Taoda, W. Hu, J. Chrom. A, 1039 (2004)135. I. M. P. L. V. O. Ferreira, S. Silva, Talanta, 74 (2008) 1598. Afkhami, A. R. Zarei, Talanta, 62 (2004) 559. M. Bru, M. I. Burguete, F. Galindo, S. V. Luis, M. J. Marin, L. Vigara, Tetrahedron Lett., 47 (2006) 1787. W. Siangproh, O. Chailapakul, R. Laocharoensuk, J. Wang, Talanta, 67 (2005) 903. (a) E. Szoko, T. Tabi, A. S. Halasz, M. Palfi, K. Magyar, J. Chromat. A, 1051 (2004) 177; (b) W. Zhang, X. Zhang, L. Zhang, G. Chen, Sens. Actuators B Chem., 192 (2014) 459. P. N. Bartlett, K. Sim, L. Chung, Sens. Actuators B Chem., 19 (1989) 141. R. Nohria, R. K. Khillan, Y. Su, R. Dikshit, Y. Lvov, K. Varahramya, Sens. Actuators B Chem., 114 (2006) 218. M. Y. Hua, G. W. Hwang, Y. H. Chuang, S. A. Chen, Macromolecules, 33 (2000) 6235.

Using the p-NiO/n-Polyaniline/n-Si Schottky Diode to Detect Hydrazinobenzene 143 [14] H. Steinebach, S. Kannan, L. Rieth, F. Solzbacher, Sens. Actuators B Chem., 151(2010) 162. [15] C. Y. Cao, W. Guo, Z. M. Cui, W. G. Song, W. Cai, J. Mater. Chem., 21 (2011) 3204. [16] L. Wang, Z. Lou, R. Wang, T. Fei, T. Zhang, Sens. Actuators B Chem., 171 (2012)1180. [17] J. Y. Kim, S. Y. Jo, G. J. Sun, A. Katoch, S. W. Choi, S. S. Kim, Sens. Actuators B Chem., 192 (2014) 216. [18] H. K. Seo, S. Ameen, M. S. Akhtar, H. S. Shin, Talanta, 104 (2013) 219. [19] S. Ameen, M. S. Akhtar, A. Umar, H. S. Shin, Chem. Eng. J., 229 (2013) 267. [20] S. Ameen, M. S. Akhtar, H. S. Shin, RSC Adv., 3 (2013) 10460. [21] P. Assadi, C. Svensson, M. Willander, O. Inganas, J. Appl. Phy., 72 (1992) 2900. [22] R. Rivera, N. Pinto, J. Physica E, 41 (2009) 423. [23] S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, J. Nanosci. Nanotech., 11 (2011) 3306. [24] G. K. Paul, A. Bhaumika, A. S. Patra, S. K. Bera, Mater. Chem. Phys., 106 (2007) 360. [25] X. Xu, L. Chen, C. Wang, Q. Yao, C. Feng, J. Solid State Chem., 178 (2005) 2163. [26] S. S. Joshi, C. D. Lokhande, S. H. Han, Sens. Actuators B Chem., 123 (2007) 240. [27] S. S. Joshi, T. P. Gujar, V. R. Shinde, C. D. Lokhande, Sens. Actuators B Chem., 132(2008) 349. [28] E. M. Genies, M. Lapkowski, Synth. Met., 24 (1988) 69. [29] Ameen, M. S. Akhtar, S. G. Ansari, O. B. Yang, H. S. Shin, Superlattices Microstruct., 46 (2009) 872. [30] S. Quillard, G. Louarn, S. Lefrant, A. G. MacDiarmid, Phys. Rev. B, 50 (1994) 12496. [31] M. I. Boyer, S. Quillard, E. Rebourt, G. Louarn, J. P. Buisson, A. Monkman, J. Phys. Chem. B, 102 (1998) 7382. [32] S. Ameen, M. S. Akhtar, S. G. Ansari, O. B. Yang, H. S. Shin, Superlattices Microstruct., 46 (2009) 872. [33] S. Ameen, S. G. Ansari, M. Song, Y. S. Kim, H. S. Shin, Superlattices Microstruct., 46 (2009) 745. [34] Ping, J. Chem. Soc. Faraday Trans., 92 (1996) 3063. [35] M. Cochet, G. Louarn, S. Quillard, M. I. Boyer, J. P. Buisson, S. Lefrant, J. Raman Spectrosc., 31 (2000) 1029. [36] M. Trchova, I. Sedenkova, E. Tobolkova, J. Stejskal, Polym. Degrad. Stab., 86 (2004) 179. [37] J. C. Chiang, A. G. MacDiarmid, Synth. Met., 13 (1986) 193. [38] A. C. Sonavane, A. I. Inamdar, D. S. Dalavi, H. P. Deshmukh, P. S. Patil, Electrochim, Acta, 55 (2010) 2344.

144

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

[39] P. S. Khiew, N. M. Huang, S. Radiman, M. S. Ahmad, Mater. Lett., 58 (2004) 516. [40] S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, J. Nanosci. Nanotechnol., 11 (2011) 1559. [41] J. Y. Shimano, A. G. MacDiarmid, Synth. Met., 123 (2001) 251. [42] J. Huang, Pure Appl. Chem., 78 (2006) 15. [43] S. Ameen, M. S. Akhtar, H. S. Shin, Sens. Actuators B Chem., 173 (2012) 177. [44] K. Radhapyari, P. Kotoky, M. R. Das, R. Khan, Talanta, 111 (2013) 47. [45] E. H. Rhoderick, Metal Semiconductor Contacts, Oxford University Press, 1988,pp. 121. [46] S. M. Sze, Physics of Semiconductors Devices, Wiley, New York, 1969, pp. 370. [47] P. S. Abthagir, R. Saraswathi, J. Appl. Poly. Sci., 81 (2001) 2127. [48] W. Mtangi, F. D. Auret, C. Nyamhere, R. P. J. Jansevan, M. Diale, A. Chawanda, Physica B, 404 (2009) 1092. [49] Fabre, G. P. Lopinski, D. D. M. Wayner, J. Phys. Chem. B, 107 (2003) 14326. [50] S. Angappane, N. R. Kimi, T. S. Natarajan, G. Rangarajan, B. Wessling, Thin Solid Films, 417 (2002) 202. [51] A. B. Yadav, C. Periasamy, S. Bhaumik, S. Jit, Ind. J. Pure Appl. Phy., 51 (2013) 792. [52] J. Wang, S. Chan, R. R. Carlson, Y. Luo, G. Ge, R. S. Ries, J. R. Heath, H. R. Tseng, Nano Lett., 4 (2004) 1693. [53] Y. Fan, J. H. Liu, C. P. Yang, M. Yu, P. Liu, Sens. Actuators B Chem., 157 (2011) 669. [54] H. R. Zare, N. Nasirizadeh, Electrochim. Acta, 52 (2007) 4153. [55] S. Ivanov, U. Lange, V. Tsakova, V. M. Mirsky, Sens. Actuators B Chem., 150 (2010) 271. [56] F. A. Harraz, A. A. Ismail, S. A. Al-Sayari, A. Al-Hajrya, M. S. Al-Assiri, Sens. Actuators B Chem., 234 (2016) 573. [57] P. Fiordiponti, G. Pistoia, Electrochim. Acta, 34 (1989) 215. [58] J. Tymoczko, W. Schuhmann, A. S. Bandarenka, Electrochem. Commun., 27 (2013) 42. [59] M. R. S. Abouzari, F. Berkemeier, G. Schmitz, D. Wilmer, Solid State Ion., 180 (2009) 922. [60] P. C. Torres, T. J. Mesquita, R. P. Nogueira, Electrochim. Acta, 92 (2013) 323. [61] Hierlemann, R. G. Osuna, Chem. Rev., 108 (2008) 563. [62] G. D. Khuspe, S. T. Navale, D. K. Bandgar, R. D. Sakhare, M. A. Chougule, V. B. Patil, Electron Mater. Lett., 10 (2014) 191.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 10

ALIGNED POLYANILINE NANOWIRES AS ELECTRODE MATERIAL FOR GLUCOSE BIOSENSOR Sadia Ameen1, M. Shaheer Akhtar2 and Hyung Shik Shin1,* 1

Energy Materials and Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New and Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT Non-enzymatic glucose biosensor is developed by utilizing the electrochemically grown nanocages-augmented polyaniline nanowires (NCa-PANI NWs) on silicon (Si) substrate. The NCa-PANI NWs are comprehensively analyzed in terms of the composition, optical, structural and the morphological properties. The grown NCa-PANI NWs are distributed uniformly on the entire surface of Si substrate, which confirms the formation of highly dense NCa-PANI NWs networks during the electrochemical oxidation. A series of sensing performances for NCa-PANI NWs electrode are investigated by current (I)–voltage (V), cyclic voltammetry (CV) and amperometry measurements. The sensing results reveal that fabricated non-enzymatic sensor shows an excellent response to glucose with a stable, reliable, and high sensitivity of ∼156.4 mAmM−1cm−2, good detection limit of ∼0.657 μM with correlation coefficient (R) of ∼ 0.99493. The fabricated glucose sensor based on NCa-PANI NWs electrode exhibits significant electrochemical stability, good reproducibility and the selectivity.

*

Corresponding Author Email: [email protected].

146

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

INTRODUCTION The diabetes mellitus is a severe disease and a big threat to human health. The detection of glucose is significant to diagnose and control diabetes by developing quick, efficient and inexpensive sensing technologies [1]. The glucose sensors are the excellent tool for the quality control in the food industry, biotechnology, and are clinical indicator for diabetes [2–4]. The glucose biosensors with enzymatic reactions are facing several drawbacks such as complicated immobilization techniques, rigorous operating conditions, and inherent instability [5, 6]. Hence, it is vital to develop a highly sensitive and selective non-enzymatic biosensor for the detection of glucose. The continuous increasing research demands have improved the detection strategies for the vital application of glucose biosensor. In regards, the analytical techniques like gas chromatography [7], high performance liquid chromatography [8], capillary electrophoresis [9] and spectrophotometry [10] have been used as sensing technologies to determine glucose. However, the major drawbacks associated with these techniques are time-consumption, complexity in the performance and difficult control under in-situ monitoring which limits the practical utilizations [11]. The development of fast, simple and low cost sensor with high sensitivity, greater selectivity, and reproducibility are fascinating researchers to devote their attention towards the electrochemical analysis methods [12, 13].The nano-sized conducting polymers have gained a great deal of attention in the field of biosensors due to their excellent electrical conductivity and large surface-to-volume ratio [14–16]. Especially, the conducting polymers such as polypyrrole, polyaniline, polythiophene, etc. are commonly used materials for the fabrication of working electrode in the sensor technology. Apart from the other conducting polymers, polyaniline (PANI) owns numerous attentions due to its outstanding chemical and physical properties including reversible doping-dedoping chemistry, good electrical conductivity, high environmental stability and ease of synthesis [17]. PANI could be directly and easily deposited on the conducting substrates, the charge-transfer doping and protonation in PANI could alter the electrical and conduction properties [18]. Recently, Zhong et al. demonstrated the glucose amperometric biosensor based on the electrodeposited platinum nanoparticles onto the sur-face of multi-wall carbon nanotube (MWNT)-PANI nanocomposites and reported the sensitivity of ∼16.1 μA mM−1, detection limit of ∼1.0 μM with linearity in the range from ∼3.0 μM to ∼8.2 mM [19]. In other report, the glucose amperometric biosensors were fabricated with two nanocomposite structures of glucose oxidase (GOx) entrapped within PANI layer (GOx/PANI) and GOx/gold nanoparticles (Au-NPs) entrapped within PANI layer (GOx/Au-NPs/PANI) which showed the good catalytic activity of glucose oxidase [20]. Chen et al. synthesized PANI/Prussian blue (3DOMSPAN/PB) bi-component film via an inverted crystal template technique using step-by-step electrodeposition and the fabricated biosensor showed a wide linear range over three orders of mag-nitude in glucose concentrations

Aligned Polyaniline Nanowires as Electrode Material for Glucose Biosensor

147

(from 2 to 1600 μM) with a low detection limit of ∼0.4 μM [21]. In general, silicon (Si) has been applied in various aspects, particularly as the working electrode in electrochemical research. Si substrates could be available from the industrial production at affordable cost, and shows the advantages of high thermal conductivity, wide optical adsorption range, mobility of electrons and holes, and stable electrochemical and physical properties [22]. In this work, the nanocages-augmented PANI NWs (NCa-PANI NWs) are grown on Si substrate through the electrochemical oxidation and and utilized as an effective non-enzymatic biosensor towards the efficient detection of glucose. The fabricated non-enzymatic glucose biosensor based on NCa-PANI NWs exhibits a high sensitivity of ∼156.4 mA mM−1cm−2, good detection limit of ∼0.657 μM and correlation coefficient (R) of ∼0.99493. The sensing performances are studied by a simple two electrode I–V characteristics using NCa-PANI NWs thin film grown on Si wafer as working electrode and Pt wire as counter electrode. The I–V characteristics are measured by the Electrometer in a fixed amount of 0.1 M phosphate buffer solution (PBS, 10.0 ml, pH 7) and the wide concentration range of glucose from 10 μM to 2 mM is used for the experiments. The sensitivity of the fabricated non-enzymatic glucose biosensor is estimated from the slope of the current versus concentrations from the calibration plot divided by the value of active area of sensor/electrode. The current response is measured from 0 V to 1.5 V and the response time is measured as 10 s. The amperometry analysis of the fabricated NCaPANI NWs electrode based non-enzymatic glucose biosensor is performed by using the potentiostat through the successive addition of ∼2 μM glucose at an applied potential of ∼0.3 V (vs. AgCl reference electrode) in 10 ml PBS (0.1 M pH 7) under constant stirring. The anti-interference study of the fabricated NCa-PANI NWs electrode based non-enzymatic biosensor is analyzed by the sequential addition of several electro-active species such as glucose, fructose, uric acid, lactose, citric acid, and ascorbic acid of similar concentration of∼10 μM in 10 ml PBS (0.1 M pH 7) at constant applied potential of ∼0.3 V (vs. AgCl reference electrode). For the NMR measurements, the prepared NCa-PANI NWs are scratched from the Si substrate and dissolved in DMSO solvent. 1H NMR spectra of the scratched NCa-PANI NWs from the electrode is taken before and after the sensing measurements.

MORPHOLOGICAL STUDIES OF ALIGNED POLYANILINE NANOWIRES The Field Emission Scanning Electron Microscopy and the Line Scanning Element Mapping The grown PANI nanostructures are morphologically analyzed by FESEM observation, as shown in Figure 1 (a and b). For viewing the FESEM images, the sample

148

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

was tilted by 30◦to obtain the clear view of NCa-PANI NWs. Figure 1(a) reveals that the grown PANI thin film possesses the networks of PANI NWs, which are oriented perpendicular to Si substrate. Noticeably, the grown NCa-PANI NWs are distributed uniformly on the entire surface of Si substrate, confirming the growth of highly dense NCa-PANI NWs structures. From Figure 1(b), the average diameter of NCa-PANI NWs is estimated in the range from ∼40–50 nm. Importantly, the obtained morphology is reproducible, indicating that the growth of NCa-PANI NWs networks thin film electrode could be scaled up for the commercial purposes. The compositional examination of grown NCa-PANI NWs is evaluated by line scanning elemental analysis. Figure 1(c and d) shows the line scanning mapping and its corresponding profile of NCa-PANI NWs. Line scanning image (Figure 1(c)) exhibits the majorly distributed carbon (C), along with nitrogen (N), and oxygen (O) elements, confirming that NCa-PANI NWs is composed of aforementioned elements. However, the appearance of strong silicon (Si) arises from the Si substrate, indicating that the deposited material is consisted of C, and N with few traces of O elements.

Reprinted with permission from [S. Ameen, 2016], Appl. Catal. A: General, 517 (2016) 21 © 2016 Elsevier Ltd. Figure 1. FESEM images at low (a), high (b) resolution, line scanning mapping (c) and its corresponding profile (d) of NCa-PANI NWs electrode.

Aligned Polyaniline Nanowires as Electrode Material for Glucose Biosensor

149

Atomic Force Microscopy The morphology of grown NCa-PANI NWs is further characterized by the topographic and three dimensional (3D) AFM images as shown in Figure 2. The vertically oriented networks of PANI NWs are seen in Figure 2(a), which are similar to the FESEM observations.

Reprinted with permission from [S. Ameen, 2016], Appl. Catal. A: General, 517 (2016) 21 © 2016 Elsevier Ltd. Figure 2. Topographic (a) and corresponding 3D (b) AFM image of NCa-PANI NWs electrode.

3D AFM image (Figure 2(b)) clearly supports the existence of vertically oriented PANI NWs on Si substrate. The roughness analysis reveals that the grown NCa-PANI NWs thin film exhibits quite high root mean roughness (Rrms) of ∼31.2 nm. In general, the rough-ness of electrode is usually related to the electrochemical behavior and electrocatalytic activity of the electrode. It is reported that the large roughness factor of the electrode is responsible for the good electrochemical properties and catalytic activity toward redox electrolytes [23]. In our case, the unique NCa-PANI NWs with high Rrms might show good electrochemical behavior towards the detection of glucose in a nonenzymatic system.

STRUCTURAL AND OPTICAL PROPERTIES OF ALIGNED POLYANILINE NANOWIRES The Raman Scattering Spectroscopy and Raman Mapping Figure 3 shows Raman scattering spectrum and its corresponding Raman mapping of NCa-PANI NWs for examining the structural properties. The Raman band at ∼1178 cm−1 corresponds to the C-H bending vibration of semi quinonoid rings (cation-radical

150

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

segments) [24, 25]. The appearance of Raman band at ∼1368 cm−1 explains C N+• vibration of delocalized polaronic structures in NCa-PANI NWs [26]. The other two Raman bands at ∼1471 cm−1 and ∼1596 cm−1 are associated to N H deformation vibration, representing the semi-quinonoid structures and C=C stretching vibration in the quinonoid ring, respectively [27]. Thus, the presence of these significant Raman bands confirms the typical PANI nanostructure in the electrodeposited PANI thin film. Furthermore, the Raman mapping images at the two Raman ranges of ∼1300–1400 cm−1 and ∼ 1540–1640 cm−1 are analyzed, as shown in Figure 3 (b and c). From Figure 3(b), the major bright color along with few part of dark color are observed in Raman mapping at ∼1300–1400 cm−1 which represents the C-N+• vibration of delocalized polaronic structures in NCa-PANI NWs, as also observed by a Raman band at ∼1368 cm−1. On the other hand, the Raman mapping image in the range of ∼1540–1640 cm−1 (Figure 3(c)) significantly favors the N H deformation vibration, which rep-resents the semiquinonoid structures in NCa-PANI NWs. The corresponding Raman mapping of Raman spectrum reveals that the grown NCa-PANI NWs are composed of uniformly distributed of C-N+• vibration of delocalized polaronic structures and N-H deformation vibrations of the semiquinonoid rings.

Reprinted with permission from [S. Ameen, 2016], Appl. Catal. A: General, 517 (2016) 21 © 2016 Elsevier Ltd. Figure 3. Raman scattering spectrum (a) and its corresponding Raman mapping in ∼1300–1400 cm−1(b) and ∼1540–1640 cm−1(c) of NCa-PANI NWs electrode.

Aligned Polyaniline Nanowires as Electrode Material for Glucose Biosensor

151

UV–Vis and Photoluminescence Spectroscopies The optical properties for studying the absorption and emission properties of the grown NCa-PANI NWs are studied by UV–vis and photoluminescence (PL) spectroscopies. UV–vis spectrum is shown in Figure 4(a), which observes an absorbance band at ∼328 nm and broad band in the range of ∼450–700 nm. The former absorbance band is originated from the π-π* transition of PANI [28–30] and the latter ascribes to nπ* transition, suggesting the formation of polarons [31].

Reprinted with permission from [S. Ameen, 2016], Appl. Catal. A: General, 517 (2016) 21 © 2016 Elsevier Ltd. Figure 4. UV–vis (a) and PL spectrum (b) of NCa-PANI NWs electrode.

The presence of aforementioned transitions in NCa-PANI NWs favors the generation of the electrical conductivity and electronic properties [32]. With addition, Figure 4(b) shows the room temperature PL spectrum of NCa-PANI NWs with an excitation

152

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

wavelength of ∼330 nm. The grown NCa-PANI NWs exhibits the blue-green emission peak at ∼441 nm, which usually assign to π– π* transition of the benzenoid unit of PANI [33]. The emission peak at ∼583 nm confirms the benzenoid and quinoid units in PANI where are specifically arranged in a proper order, and shows the formation of excitons and increase in the delocalization length of a singlet exciton [33]. Moreover, the higher intensity of PL emission could be attributed to the larger oscillator strength in the synthesized NCa-PANI NWs [34].

NON-ENZYMATIC BIOSENSOR BASED ON ALIGNED POLYANILINE NANOWIRES ELECTRODE Investigations of Electroctalytic Activity through Cyclovoltametry The NCa-PANI NWs electrode has been used for the detection of glucose through the electrochemical sensor system. The electrocatalytic activity of NCa-PANI NWs electrode is investigated by the cyclovoltametry (CV) with a series of glucose concentrations (10 μ M–2 mM) in 0.1 M phosphate buffer (pH 7.0) at the scan rate of ∼50 mVs−1, as shown in Figure 5. The CV measurement with-out glucose expresses relatively low redox current (Figure 5(a)), as compared to redox current of NCa-PANI NWs electrode with very low concentration of glucose (10 μM). It is noticed that significant increment in the redox current after the addition of lowest glucose concentration (10 μM) indicates the quick sensing response and good electrocatalytic activity of NCa-PANI NWs electrode. The detail electrocatalytic activity of NCa-PANI NWs electrode are further confirmed by taking a series of CV plots with a series of glucose concentrations ranging from 10 μM to 2 mM in 0.1 M PBS. The typical quasi-reversible redox peaks are observed in all the concentrations of glucose which favors the electrochemical reaction towards glucose in PBS [35]. The magnitude of the redox peak increases rapidly with the increase of glucose concentration from 10 μM to 2 mM, as shown in Figure 5(b). The high redox current with respect to glucose concentration represents the faster electron-transfer reaction through high electrocatalytic behavior of the NCa-PANI NWs electrode as working electrode [36]. In our case, it could be seen that the anodic potential shifts towards positive side and the cathodic peak potential shifts in the reverse direction with the increase of glucose concentration. This phenomenon might be occurred due to biocompatible nature of NCa-PANI NWs which acts as an electron mediator and thus,

Aligned Polyaniline Nanowires as Electrode Material for Glucose Biosensor

153

might result in an accelerated electron transfer via high electrocatalytic activity of the electrode.

Reprinted with permission from [S. Ameen, 2016], Appl. Catal. A: General, 517 (2016) 21 © 2016 Elsevier Ltd. Figure 5. (a) Cyclovoltametry (CV) curves of the fabricated NCa-PANI NWs based non-enzymatic glucose biosensor without and with glucose (10 M), and (b) CV curves of a series of glucose concentrations (10 μM–2 mM) in 0.1 M phosphate buffer (pH7.0) at the scan rate of ∼50 mV/s.

The I–V Responses of the Fabricated Sensor The detailed investigation of sensing performance has been carried out by measuring the current (I)–voltage (V) responses of the fabricated non-enzymatic glucose biosensor based on NCa-PANI NWs electrode. Figure 6 shows the I–V responses of NCa-PANI NWs as working electrode and Pt wire as counter electrode in 0.1 MPBS (10 ml) solution

154

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

with and without the presence of glucose. Figure 6(a) shows a rapid increase in the current after the addition of lowest glucose concentration (10 μM), which might happen dueto the improvement in the surface resistance properties of NCa-PANI NWs electrode through the increase in ionic strength and fast communication of electrons during the reaction. A series of I–V responses according to various concentrations of glucose from10 μM to 2 mM has been demonstrated to elucidate the sensing behavior of the fabricated non-enzymatic biosensor based on NCa-PANI NWs electrode, as depicted in Figure 6(b). It is seen that the current increases continuously with the increase of glucose concentration from 10 μM to 2 mM, suggesting the proportional increment in the ionic strengths and the faster electron transportation. To evaluate the sensing parameters, a calibration current versus glucose concentrations is plotted, as shown in Figure 6(c). It is clear from the calibration curve, the current increases linearly up to ∼120 μM and thereafter, reaches to the saturation point at the specific concentration range. The fabricated NCa-PANI NWs electrode based biosensor exhibits the good linear dynamic range (LDR) from 10 μM to 120 μM for glucose with the correlation coefficient (R) of∼ 0.99493. The relatively high, reliable and reproducible sensitivity of ∼156.4 mAmM−1cm−2 with low limit of detection (LOD) of∼0.657 μM are achieved by the fabricated NCa-PANI NWs electrode based non-enzymatic glucose biosensor. In our case, the recorded sensitivity is higher than the previously reported literatures based on the glucose biosensor modified electrodes, as summarized in Table -1. Herein, the grown NCa-PANI NWs electrode has essentially built the unique platform for the excellent adsorption ability, high electrocatalytic and electrochemical activities to glucose which might result to the high sensitivity and the good linearity.

The Amperometric Response and Interference Tests of the Fabricated Electrode Based Non-Enzymatic Glucose Biosensor Additionally, the amperometric response of the fabricated NCa-PANI NWs electrode based non-enzymatic glucose biosensor is obtained by the successive addition of ∼2 µM glucose at an applied potential of ∼0.3 V (vs. SCE) in 10 ml PBS (0.1 M pH 7), as shown in Figure 7. Before starting the measurements, the electrochemical experiment was performed in PBS without glucose to stabilize the background current. Thereafter, the glucose (∼2 µM) was injected successively drop by drop in 10 ml of PBS using the peristaltic pump.

Aligned Polyaniline Nanowires as Electrode Material for Glucose Biosensor

155

Table 1. Comparison of non-enzymatic glucose biosensor based on different nanomaterials modified electrodes Electrode Nafion-silica/MWCNTg-PANI/GOx GOx/Au-(SH)PANI-gMWNT}n PANI nanofiber dendrites GOx/Pt/MWCNT/PAN I GOx/nTiO2/PANI/GCE Pt/PANI/BDD

Sensitivity (R)

Linear range

Detection limit

Response time

Refs.

5.01 µA mM-1

1-10 mM

-

6s

[40]

0.06 µA mM-1

1-9 mM

3.97 µM

-

[41]

-

50 mM-12 mM

100 µM

5s

[42]

16.10 µA mM-1cm-2

0.003-8.2 mM

1.0 µM

5s

[43]

6.31 µA mM-1cm-2

0.02-6.0 mM

18 µM

-

[44]

5.54 µA mM-1cm-2

0.102 µM

-

[45] [46]

0.5 µM 0.5 µM 0.5 µM

5-10 s 5s 5s

GOx/Pani-MCPE

-

GOx/nano-PANI/GC PANI/CNT composite Pt/GOD-PANI/PB Au-nano/PANI/GOx Nf-silica/MWCNT-gPANI/GOx PANI PANI-NW/GOx/CC Pt/Au-PAni/GOx Au-g-PANI-c-(CSCNTs)-GOD

42.0 µA mM-1cm-2 1.90 µA mM-1cm-2 2300 µA mM-1cm-2

0.0059-0.514 mM 5.0×10-7-1.0×10-5 M 0.01-1 mM 1 µM-12 mM 8 mM 0.001-0.8 mM

5.01 µA mM-1cm-2

1-10 mM

0.1 µM

~6 s

39.63 µA mM-1cm-2 2500 µA mM-1cm-2 14.63 µA mM-1cm-2

0.01-4.5 mM 0-8 mM 1 µM-20 mM

5 µM 0.05 µM 1 µM

-

16.50 µA mM-1cm-2

1-20 mM

0.1 µM

5-10 s

97.18±4.62 µA mM1cm-2

0.01-5.5 mM

0.3-0.1 µM 3 s

42 mA M-1cm-2

-

0.5 µM

5s

128 mA M-1cm-2

-

1 µM

-

67.1 mA M-1cm-2 73.25 mA M-1cm-2 96.1 mA M-1cm-2 156.4 mA mM-1cm-2

10-120 µM

2 µM 0.5 µM 0.7 µM ~0.66 µM

3s 10 s

Pani-NTs GOx/PtDENs/PANI/CNT/Pt GOx/Pt/MWNTPANI/GCE GOx/PANI/PAN/Pt GOx/AuNPs/PANI/GC GOx/PtNP/PANI/Pt NCa-PANI NWs

5.0×10-8 M 200 s

[47] [48] [49] [50] [51] [48] [52] [53] [54] [55] [56] [57] [58] [50] [59] This work

Reprinted with permission from [S. Ameen, 2016], Appl. Catal. A: General, 517 (2016) 21 © 2016 Elsevier Ltd

156

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

Reprinted with permission from [S. Ameen, 2016], Appl. Catal. A: General, 517 (2016) 21 © 2016 Elsevier Ltd. Figure 6. (a) I–V responses of the fabricated NCa-PANI NWs based non-enzymatic glucose biosensor without and with glucose (10 M), (b) I–V responses of a series of glucoseconcentrations (10 μM–2 mM) in 0.1 M phosphate buffer (pH 7.0), and (c) derived calibration current versus glucose concentrations.

Figure 7(a) presents the plot of steady state current value versus the glucose concentrations from 0 to 30 µM. The successive addition of glucose has gradually increased the current, indicating the excellent electrocatalytic surface of NCa-PANI NWs electrode for glucose. Moreover, it is also visible that the biosensor shows amperometric response upon the injection of glucose and reaches the steady state current within 5s which could be attributed to superior electron ability and the synergic effect of NCaPANI NWs networks. The corresponding calibration curves at different applied voltages (Figure 7(b)) display a linear relationship between the current response and the glucose concentrations. Noticeably, the current response considerably increases with the increase of applied voltage from 0.1 to 0.3 V. From Figure 7(b), at 0.3 V, a good linearity at low glucose concentration and similar correlation coefficient (R) of ∼0.99353 are detected, which fairly infers that the fabricated NCa-PANI NWs electrode based biosensor is adequate to detect glucose by linear sweep voltametry (i.e., I–V measurement) and

Aligned Polyaniline Nanowires as Electrode Material for Glucose Biosensor

157

amperometric methods. The anti-interference study of the fabricated NCa-PANI NWs electrode based non-enzymatic biosensor is examined by the collective addition of several electroactive species such as glucose, fructose, uric acid, lactose, citric acid, and ascorbic acid in 10 ml PBS (0.1 M pH 7) at an applied potential of ∼0.3 V (vs. SCE), as shown in Figure 7(c). This study signifies the high sensing response of the fabricated NCa-PANI NWs electrode based non-enzymatic biosensor to glucose. In general, these coexist species disturbs the sensing response due to the overlapping of the peak i.e., proportional to the concentration of the analytes. From the interference results, a rapid current increase is seen for glucose whereas, the current response of the other interfering species has not revealed any obvious signal. This study indicates that the NCa-PANI NWs electrode has shown good selectivity towards glucose in-spite the presence of other electro-active species. The superior sensitivity and other sensing parameters of NCaPANI NWs networks electrode might be credited to the excellent adsorption ability and high electrocatalytic/electrochemical activities.

Reprinted with permission from [S. Ameen, 2016], Appl. Catal. A: General, 517 (2016) 21 © 2016 Elsevier Ltd. Figure 7. The amperometric response (a) and the corresponding calibration curve (b) of the fabricated NCa-PANI NWs electrode based non-enzymatic glucose biosensor at different applied voltages of 0.1, 0.2 and 0.3 V. (c) Interference tests of the fabricated NCa-PANI NWs electrode based non-enzymatic glucose biosensor upon the addition of glucose, fructose, uric acid, lactose, citric acid, and ascorbic acid in 10 ml PBS (0.1 M pH 7) at constant applied voltage of 0.3 V.

158

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

Proposed Mechanism of the Fabricated Electrode Based Non-Enzymatic Glucose Biosensor The glucose sensing mechanism has been explained by using proposed schematic illustration of biosensor based on NCa-PANINWs electrode, as shown in Figure 8(a).

Reprinted with permission from [S. Ameen, 2016], Appl. Catal. A: General, 517 (2016) 21 © 2016 Elsevier Ltd. Figure 8. (a) Proposed mechanism of the fabricated NCa-PANI NWs electrode based non-enzymatic glucose biosensor and (b) plot of sensitivity versus time interval (days) of fabricated NCa-PANI NWs electrode based non-enzymatic glucose biosensor.

The detection of glucose in the liquid phase is generally governed by the chemisorption of glucose layer over the surface of the PANI electrode. As seen in CV results, the fabricated NCa-PANI NWs electrode presents the excellent redox current which basically resulted from the conversion of PANI form emeraldine state to leucoemeraldine state [37].This state facilitates the generation of an ion-dipole between HN+(ES+) of PANI and neutral molecule like glucose. After the introduction of glucose, leucoemeraldine state (insulating state) of PANI accepts an electron to change into emeraldine state (conducting state) and behaves as electron transporting system [38,39], then the electrons travel from glucose to the electrode, which considerably increases the peak current, as seen in CV results. Here, the fabricated NCa-PANI NWs electrode might adsorb large amount of glucose molecules due to its unique nanocages formation in NW networks and the surface area (42.4 m2/g). Moreover, PBS supplies the hydroxyl ions which might take electrons from the surface of PANI electrode and generates the highly reactive oxygenated sites, which is responsible for the oxidation of glucose to gluconolactone. This process considerably decreases the conductance of the electrolyte, as observed in the I–V measurements. Importantly, the fabricated NCa-PANI NWs electrode based biosensor shows the good stability and reproducibility, as observed by the sensing responses through the I–V characteristics for four consecutive weeks, as observed in Figure 8(b). It is evident that the fabricated NCa-PANI NWs electrode based

Aligned Polyaniline Nanowires as Electrode Material for Glucose Biosensor

159

non-enzymatic biosensor retains ∼95% of its original sensitivity after 30 days by obtaining several repetitive I–V measurements for glucose and thus, deduces the long term stability of the fabricated NCa-PANI NWs electrode based biosensor.

1H

NMR Spectra of NCa-PANI NWs

To check the interaction of glucose with the fabricated electrode, 1HNMR spectra of NCa-PANI NWs before and after the sensing measurements are measured in DMSO solvent. Figure 9 illustrates the 1HNMR spectra of NCa-PANI NWs electrode before and after the sensing measurements.

Reprinted with permission from [S. Ameen, 2016], Appl. Catal. A: General, 517 (2016) 21 © 2016 Elsevier Ltd. Figure 9. 1H NMR spectra of the fabricated NCa-PANI NWs electrode before (a) andafter (b) the sensing measurements.

160

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

The NCa-PANI NWs electrode possesses the characteristics peaks (a and c) near 8.0 ppm and peaks (b and d) in the range of 7.0–7.5 ppm, which usually represent the protons of NH group and the aromatic ring of PANI, respectively. The protons of glucose are recorded in between 3 and 4.0 ppm with the protons of PANI in the range of 6–7.5 ppm after the sensing measurements, suggesting the adsorption of glucose on the surface of NCa-PANI NWs electrode. This result again deduces that the grown NCa-PANI NWs electrode might provide the unique platform and highly adsorptive surface for the adsorption of glucose through NH group of PANI during the sensing measurements.

CONCLUSION The electrochemically grown NCa-PANI NWs have been utilized as an effective electrode for the fabrication of a highly sensitive, reliable and reproducible nonenzymatic glucose biosensor. The grown NCa-PANI NWs are distributed uniformly on the entire surface of Si substrate, which confirms the formation of highly dense NCaPANI NWs networks during the electrochemical oxidation. The fabricated non-enzymatic sensor based on NCa-PANI NWs electrode shows an excellent response and the selectivity to glucose with wide linearity in the range of 10-120 µM. A stable, reliable high sensitivity of ∼156.4 mAmM−1cm−2, with good detection limit of ∼0.657 µM and correlation coefficient (R) of ∼0.99493 are recorded by the fabricated non-enzymatic glucose biosensor. The interference study reveals that NCa-PANI NWs electrode shows good electrochemical properties and selectivity towards glucose as compared to other electro-active species. Thus, the unique morphology of electrochemically grown NCaPANI NWs electrode is promising and effective as working electrode for the detection of glucose with significant electrochemical stability, good reproducibility and selectivity.

REFERENCES [1] [2] [3] [4] [5] [6]

J. Wang, Chem. Rev. 108 (2008) 814. Y. Mu, D. L. Jia, Y. Y. He, Y. Q. Miao, H. L. Wu, Biosens. Bioelectron., 26 (2011) 2948. X. Li, A. Hu, J. Jiang, R. Ding, J. Liu, X. Huang, J. Solid State Chem., 184 (2011) 2738. J. M. Qian, A. L. Suo, Y. Yao, Z. H. Jin, Clin. Biochem., 37 (2004) 155. J. P. Wang, D. F. Thomas, A. C. Chen, Nonenzymatic Anal. Chem., 80 (2008) 997. Z. Yuting, L. Shuai, L. Yu, D. Dongmei, S. Xiaojing, D. Yaping, H. Haibo, L. Liqiang, W. Zhenxi, Biosen. Bioelectron., 66 (2015) 308.

Aligned Polyaniline Nanowires as Electrode Material for Glucose Biosensor [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

161

A. Kovacs, A. Kende, M. Mortl, G. Volk, T. Rikker, K. Torkos, J. Chromatogr. A, 1194 (2008) 139. X. Ye, L. J. Tao, L. L. Needham, A. M. Calafat, Talanta, 76 (2008) 865. S. Morales, R. Cela, J. Chromatogr. A, 896 (2000) 95. M. Hasani, M. J. Moloudi, Hazard. Mater., 157 (2008) 161. H. K. Seo, S. Ameen, M. S. Akhtar, H. S. Shin, Talanta, 104 (2013) 219. W. C. Yang, A. M. Yu, Y. Q. Dai, H. Y. Chen, Anal. Lett., 33 (2000) 3343. M. Abaker, G. N. Dar, A. Umar, S. A. Zaidi, A. A. Ibrahim, S. Baskoutas, A. AlHajry, Adv. Mater., 4 (2012) 893. S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, Electrochim. Acta, 56 (2011) 1111. S. Ameen, M. S. Akhtar, A. Umar, H. S. Shin, Chem. Eng. J., 229 (2013) 267. S. Ameen, M. S. Akhtar, H. K. Seo, H. S. Shin, Appl. Cata. B: Environ., 144 (2014) 665. S. Ameen, M. S. Akhtar, Y. S. Kim, H. S. Shin, Chem. Eng. J., 181–182 (2012) 806. S. Ameen, M. S. Akhtar, H. S. Shin, RSC Adv., 3 (2013) 10460. H. Zhong, R. Yuan, Y. Chai, W. Li, X. Zhong, Y. Zhang, Talanta, 85 (2011) 104. V. Mazeiko, A. K. Minkstimiene, A. Ramanaviciene, Z. Balevicius, A. Ramanavicius, Sens. Actuators B Chem, 189 (2013) 187. X. Chen, Z. Chen, R. Tian, W. Yan, C. Yao, Anal. Chim. Acta, 723 (2012) 94. N. Megouda, Y. Cofininier, S. Szunerits, T. Hadjersi, O. ElKechai, R. Boukherroub, Chem. Commun., 47 (2011) 991. Y. Jia, P. Xiao, H. He, J. Yao, F. Liu, Z. Wang, Y. Li, Appl. Surf. Sci., 258 (2012) 6627. M. I. Boyer, S. Quillard, G. Louarn, G. Froyer, S. Lefrant, J. Phys. Chem. B, 104 (2000) 8952. M. Cochet, G. Louarn, S. Quillard, M. I. Boyer, J. P. Buissont, S. Lefran, J. Raman Spectrosc., 31 (2000) 1029. R. Mazeikiene, A. Statino, Z. Kuodis, G. Niauras, A. Malinauska, Electrochem. Commun., 8 (2006) 1082. S. Ameen, M. S. Akhtar, Y. S. Kim, H. S. Shin, Appl. Catal. B: Environ., 103 (2011) 136. X. Zhang, G. Yan, H. Ding, Y. Shan, Mater. Chem. Phys., 102 (2007) 249. S. Ameen, S. G. Ansari, M. Song, Y. S. Kim, H. S. Shin, Superlattice Microstruct., 46 (2009) 745. S. Ameen, M.S. Akhtar, S. G. Ansari, O. B. Yang, H. S. Shin, Superlattices Microstruct., 46 (2009) 872. D. Han, Y. Chu, L. Yang, Y. Liu, Z. Lv, Colloids. Surf. A: Physicochem. Eng. Asp., 259 (2005) 179.

162

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

[32] S. Ameen, M. Song, D. G. Kim, Y. B. Im, H. K. Seo, Y. S. Kim, H. S. Shin, Macromol. Res., 20 (2012) 30. [33] J. Y. Shimano, A. G. MacDiarmid, Synth. Met., 123 (2001) 251. [34] A. Nabok, Organic and Inorganic Nano-structures, 120, Artech House Inc., London, 2005. [35] F. Xu, K. Cui, Y. Sun, C. Guo, Z. Liu, Y. Zhang, Y. Shi, Z. Li, Talanta, 82 (2010) 1845. [36] V. Rumbau, J. A. Pomposo, J. A. Alduncin, H. Grande, D. Mecerreyes, E. Ochoteco, Microb. Technol., 40 (2007) 1412. [37] A. Ray, A. F. Richter, A. G. MacDiarmid, A. J. Epstein, Synth. Met., 29 (1989) 151. [38] A. J. Heeger, J. Phys. Chem. B, 105 (2001) 8475. [39] E. M. Genies, C. Tsintavis, J. Electroanal. Chem. Interfacial Electrochem., 195 (1985) 109. [40] I. G. Anantha, P. L. Kwang, R. Dhanusuraman, H. L. Se, W. L. Jong, Biomaterials, 30 (2009) 5999. [41] S. Komathi, A. I. Gopalan, K. P. Lee, Biosens. Bioelectron., 24 (2009) 3131. [42] R. Ramya, M. V. Sangaranarayanan, J. Appl. polym. Sci., 129 (2013) 735. [43] H. Zhang, H. Zhong, R. Yuan, Y. Chai, W. Li, X. Zhong, Y. Zhang, Talanta, 85 (2011) 104. [44] W. Tang, L. Li, X. Zeng, Talanta, 131 (2015) 417. [45] M. J. Song, J. H. Kim, S. K. Lee, J. H. Lee, D. S. Lim, S. W. Hwang, D. Whang, Microchim. Acta, 171 (2010) 249. [46] O. Ҫolak, H. Arslan, H. Zengin, G. Zengin, Int. J. Electrochem. Sci., 7 (2012) 6988. [47] M. Zhao, X. Wu, C. Cai, J. Phys. Chem. C, 113 (2009) 4987. [48] L. Xu, Y. Zhu, X. Yang, C. Li, Mater. Sci. Eng. C, 29 (2009) 1306. [49] R. Garjonyte, A. Malinauskas, Biosens. Bioelectron., 15 (2000) 445. [50] Y. Xian, Y. Hu, F. Liu, Y. Xian, H. Wang, L. Jin, Biosens. Bioelectron., 21 (2006) 1996. [51] A. I. Gopalan, K. P. Lee, D. Ragupathy, S. H. Lee, J. W. Lee, Biomaterials, 30 (2009) 5999. [52] Y. Y. Horng, Y. K. Hsu, A. Ganguly, C. C. Chen, L. C. Chen, K. H. Chen, Electroch Commun., 11 (2009) 850. [53] A. D. Chowdhury, R. Gangopadhyay, A. De, Sens. Actuators B Chem, 190 (2014) 348. [54] D. Wan, S. Yuan, G. L. Li, K. G. Neoh, E. T. Kang, ACS. Appl. Mater. Interfaces, 2 (2010) 3083. [55] Z. Wang, S. Liu, P. Wu, C. Cai, Anal. Chem., 81 (2009) 1638. [56] L. Xu, Y. Zhu, X. Yang, C. Li, Mater. Sci. Eng., C 29 (2009) 1306. [57] H. Zhong, R. Yuan, Y. Chai, W. Li, X. Zhong, Y. Zhang, Talanta, 85 (2011) 104.

Aligned Polyaniline Nanowires as Electrode Material for Glucose Biosensor

163

[58] H. G. Xue, Z. Q. Shen, C. M. Li, Biosens. Bioelectron., 20 (2005) 2330. [59] D. Zhai, B. Liu, Y. Shi, L. Pan, Y. Wang, W. Li, R. Zhang, G. Yu, ACS Nano, 7 (2013) 3540.

PART II. FUNDAMENTALS OF FUNCTIONAL MATERIALS: APPLICATIONS FOR PHOTOCATALYST

SECTION 1. ZINC OXIDE (ZNO) BASED PHOTOCATALYSTS

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 11

DEGRADATION OF BROMOPHENOL DYE OVER ZNO NANOFLOWERS Sadia Ameen1, M. Shaheer Akhtar2 and Hyung Shik Shin1,* 1

Energy Materials & Surface Science Laboratory, Solar Energy Research Center,School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT A simple, low temperature solution method is employed to synthesize ZnO nanoflowers (NFs) using zinc nitrate as precursor for the photocatalytic degradation of bromophenol (Bph) dye. The synthesized material exhibits well-defined flower like morphology comprised of several defined nanorods. The crystalline and optical observations manifest the high crystallinity of ZnO-NFs with wide band gap of ~3.21 eV. The synthesized ZnO NFs as catalyst presents a rapid degradation of Bph-dye with the degradation rate of ~96% within 120 min under the UV light irradiation. The fragmentations of Bph-dye after the photocatalytic reaction over ZnO-NFs are analyzed by interpreting the mass spectroscopy of degraded Bph-dye.

*

Corresponding Author Email: [email protected]

170

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

INTRODUCTION Much research has been devoted to find cost-effective and efficient treatment technologies for contaminated water especially from colored dyes. The wastes of coloring and textiles factories are the major sources of water contamination or water pollution because these industries use non-biodegradable organic color molecules [1-2]. For instance, bromophenol (Bph) dye is an organic dye, non-biodegradable and widely used as color marker to monitor the process of agarose gel electrophoresis, drugs, cosmetics, textiles, printing inks, and as an acid-base indicator [3]. Bph-dye is highly water soluble and could slowly contaminate the soil and fresh water [4]. Thus, adequate treatment techniques are demanded for wastewater treatment to clean the environment and sustainable methods are required for remediating the broad diversity of dye wastes like Bph-dye. A remediation process called the photocatalytic process is adopted broadly and received a great deal of attention due to its ability to mineralize organic into less toxic chemicals or transfer to another environment in the presence of semiconducting materials as catalysts [5, 6]. Recently, zinc oxide (ZnO) semiconducting materials are testified as the photocatalyst for dye remediation, and wastewater treatments because of their non-toxic and environment friendly nature [7, 8]. ZnO is wide band gap (3.37 eV) material which is known for the excellent electron mobility, dielectric, ferroelectric, piezoelectric, pyroelectric properties and shows the promising applications in solar cells, catalysts, chemical sensors, memory resistors, gas sensors, etc. [9-12]. ZnO nanomaterials have been utilized as photocatalyst for refining and protecting the environment such as the degradation of organic pollutants in water [13] and an elimination of odor as disinfectants [14]. ZnO nanomaterials with different morphologies show good photocatalytic properties towards the degradation of harmful organic dyes [15]. Recently, hollow nano-baskets of ZnO were synthesized for the degradation of rhodamine 6G dye by 97% within 90 min [16]. In other report, the synthesized ZnO flowers as photocatalyst degraded the crystal violet dye by 96% within the time interval of 80 min [15]. Herein, a simple solution method is adopted for the synthesis of ZnO-NFs comprised of small nanorods with the average size of 1-2 μm and utilized for the degradation of bromophenol (Bph)-dye under UV visible light irradiation. To check the photocatalytic activity of synthesized ZnO-NFs, the degradation of Bph-dye is carried out under light illumination using the Xenon arc lamp [17]. 10 ppm of Bph-dye solution is prepared in DI water and then 100 mg ZnO-NFs photocatalyst is added. The suspension is stirred for 1 h to obtain the adsorption-desorption equilibrium between Bph-dye and the ZnO-NF catalyst. Under continuous stirring, the light illumination is exposed to an aqueous dye suspension. For measuring the degradation

Degradation of Bromophenol Dye over ZnO Nanoflowers

171

rate, the decomposed dye is taken out after every 10 min, and centrifuge at 10,000 rpm to separate out the catalyst. The UV-visible spectrophotometer is used to measure the absorption spectrum of the decomposed dye. Degradation rate (%) = (Co - C/Co) x 100 = (Ao - A/Ao) x 100

(1)

where Co denotes the initial concentration, C denotes variable concentration, Ao shows the initial absorbance, and A corresponds to variable absorbance.

MORPHOLOGICAL AND CRYSTALLINE STUDIES OF ZNO NANOFLOWERS The Field Emission Scanning Electron Microscopy, the Energy-Dispersive X-Ray Spectroscopy and X-Rays Diffraction Patterns The synthesized ZnO nanomaterials are morphologically characterized by the field emission scanning electron microscopy, as shown in Figure 1(a, b). The highly dense flowers like structures are seen in Figure 1(a). The high magnification mode of ZnO-NFs (Figure 1(b)) clearly displays that each flower comprises of small nanorods with the average size of 1-2 μm. The synthesized ZnO-NFs are indeed nanograined and contain quite developed surfaces and interfaces [18]. These surfaces and interfaces in ZnO-NFs could influence the physical and optical properties. Figure 1(c) shows the energydispersive X-ray spectroscopy (EDS) of the synthesized ZnO-NFs. Only zinc (Zn) and oxygen (O) peaks are observed, indicating that the synthesized ZnO-NFs possess good stoichiometric ratio of both the elements. The EDS result confirms that the synthesized ZnO-NFs are pure ZnO nanomaterials. In support, X-Rays diffraction patterns of the synthesized ZnO-NFs is investigated to describe the crystalline phases, as shown in Figure 1(d). The observed diffraction peaks at 31.8o (1 0 0), 34.4o (0 0 2), 36.3o (1 0 1), 47.6o (1 0 2), 56.6o (1 1 0), 62.8o (1 0 3), 66.2o (2 0 0), 67.9o (1 1 2), 69.0o (2 0 1), 72.6o (0 0 4), and 75.9o (2 0 2) are perfectly indexed with JCPDS No. 36-1451 [19]. The appearance of sharp diffraction peaks manifests a good crystalline nature of synthesized ZnO-NFs. Importantly, no other diffraction peaks are seen, suggesting the high purity of ZnO-NFs. Moreover, ZnO nanograins demonstrate the presence of amorphous surficial, interfacial and intergranular layers (invisible for XRD result), which results to surface defects like interphase boundaries and grain boundaries [20]. The presence of these defects might alter the physical properties of ZnO nanograins [21].

172

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

Reprinted with permission from [S. Ameen, 2017], Mater. Lett., 209 (2013) 150 ©2017 Elsevier Ltd. Figure 1. FESEM images at (a) low and (b) high magnification, (c) EDX spectrum and (d) XRD pattern of ZnO-NFs.

OPTICAL AND STRUCTURAL PROPERTIES OF ZNO NANOFLOWERS The UV-Vis Absorption Spectrum, Photoluminescence Spectrum and Raman Scattering Spectroscopy UV-visible absorbance spectroscopy and the room temperature photoluminescence have been examined to elucidate the optical properties of synthesized ZnO-NFs. From Figure 2(a), the synthesized ZnO-NFs depicts the absorbance at 386.7 nm, which is typically associated to the charge transfer from the valence band (VB) to the conduction band (CB) (O2p → Zn3d) [22]. As analyzed by UV-absorbance peak, the synthesized ZnONFs exhibits the estimated band gap of 3.21 eV, and the value is very close to the band gap of bulk ZnO [23]. The PL spectroscopy of the synthesized ZnO-NFs is shown in Figure 2(b). A sharp near-band edge (NBE) UV emission at 392.4 nm is detected,

Degradation of Bromophenol Dye over ZnO Nanoflowers

173

corresponding to the recombination of the free excitons of ZnO [19]. Moreover, ZnONFs possess a broader green emission peak at 572.2 nm, which is usually originated from the singly ionized oxygen deficiencies (VO+)/structural defects [24]. The structural orientation of ZnO-NFs are characterized by Raman scattering spectroscopy (Raman microscope, Renishaw), as depicted in Figure 2(c). The synthesized ZnO-NFs present a prominent E2 Raman band at 437.2 cm-1, representing the characteristics hexagonal wurtzite ZnO [25]. Three weak Raman bands at 332.2, 385.1 and 580.1cm-1 are associated to the multiple phonon ~580.1 cm-1 are associated to the multiple phonon scattering [E2 (high)-E2 (low)], A1g and E1(LO) modes, respectively [26], which usually appear due to the formation of defects such as oxygen vacancy, zinc interstitial, or their complexes [23]. Importantly, a strong E2 mode confirms a good crystal structure, quality along with few impurities of ZnO-NFs.

Reprinted with permission from [S. Ameen, 2017], Mater. Lett., 209 (2013) 150©2017 Elsevier Ltd. Figure 2. (a) UV-Vis, (b) PL, and (c) Raman spectra of ZnO-NFs.

174

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

THE PHOTOCATALYTIC ACTIVITIES OF ZNO NANOFLOWERS UV-Vis Absorbance Spectra of Decomposed Bromophenol Dye Solution over ZnO Nanoflowers under UV Light Irradiation and the Mass Spectra of Bromophenol Dye Sluotions The photocatalytic activities of the synthesized ZnO-NFs have been evaluated for the light induced photocatalytic degradation of Bph-dye. Figure 3(a) displays the UV-Vis spectra of degraded Bph-dye within the time interval of 0-120 min in the presence of ZnO-NFs photocatalyst. In order to determine Bph-dye degradation, a maximum absorption peak of Bph-dye at 590 nm is picked and the intensity of absorption peak as a function of light exposure time is monitored. It is notable that the absorbance intensity decreases continuously with the increase of light exposing time, suggesting the lowering of the concentration of Bph-dye due to the degradation of dye under UV light irradiation. From above Equation (1), the major degradation of 85% is carried out in 90 min and the complete degradation occurs within 120 min over the surface of ZnO-NFs photocatalyst. From Figure 3(b), a very low degradation rate for Bph-dye has been detected when photocatalytic reaction is performed in the absence of ZnO-NFs under UV light irradiation. The plot of A/Ao versus time interval (Figure 3 (b)) presents a gradual decrease with the increase of time interval in the presence of ZnO-NFs, indicating a rapid degradation of Bph-dye under UV light irradiation. In support, the kinetic of Bph-dye degradation has been studied to elucidate the order of the photocatalytic reaction. Generally, Langmuir–Hinshelwood mechanism is used for apparent first order kinetics. A relation is represented as: r = dC/dt = kKC/l +KC

(2)

where r is the degradation rate of reactant dye (mg/l min), C is concentration of dye (mg/l), t is illumination time, K is the adsorption coefficient of dye (l/mg) and k is the reaction rate constant (mg/l min). Assuming, C is very small, the Equation (2) could be written as: ln(C0/C) = kKt ≈ kappt

(3)

The rapid and first order photocatalytic degradation could be accounted for the numerous generations of charge carriers. Using the Equation (2), a plot of ln(C0/C) versus exposure time could be illustrated to determine the reaction rate constant (Figure 3(c) which shows a straight line, and the fitting of linear plot is adopted to estimate the first

Degradation of Bromophenol Dye over ZnO Nanoflowers

175

order rate constant (kapp) i.e., 0.0245 min/t. The estimated kapp value is consistent with the reported work [27].

Reprinted with permission from [S. Ameen, 2017], Mater. Lett., 209 (2013) 150©2017 Elsevier Ltd. Figure 3. (a) UV-Vis absorbance spectra of decomposed Bph-dye solution over ZnO-NFs under UV light irradiation, (b) the curve of A/Ao versus time interval, (c) a plot of ln(C 0/C) as a function of exposure time and (d) mass spectra of Bph-dye solutions before and after the photocatalytic reaction over ZnO-NFs.

When the light exposes to the photocatalytic reactor, it excites ē from valance band (VB) of ZnO to the conduction band (CB) and leaves a hole (h+) near VB. This phenomenon is called the creation of ē-h+ pairs and then it separates out, which might result to the formation of active radicals for the dye degradation. In this work, the synthesized ZnO-NFs absorb large photons due to its unique structures and originate excited ē. These photogenerated ē interacts with absorbed oxygen to give superoxide radicals (O2●, HOO●), while the holes in the VB reacts with surface hydroxyl groups to form hydroxyl (OH●). The highly reactive hydroxyl radicals (OH) and superoxide radicals (O2●, HOO●) produced over ZnO-NFs might attract with the Bph-dye molecules, resulting to the fast degradation of Bph-dye into less harmful minerals [28, 29]. Figure 3(d) shows the mass spectroscopy of Bph-dye before and after the photocatalytic reaction to investigate the possible fragmentations/intermediates of Bph-

176

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

dye degradation. A strong mass signal at m/z = 671.1 is recorded for Bph-dye at initial state, which is similar to the mass of Bph-dye. After 120 min, the multiple mass signals at m/z = 516.8, m/z = 426.9, m/z = 258.6 and m/z = 168.9 are detected, indicating the degradation of Bph-dye over the surface of ZnO-NFs under UV light irradiation. The mass results clearly explain that Bphdye is completely degraded into less harmful organic minerals on the surface of ZnO-NFs under UV light irradiation.

CONCLUSION A simple, low temperature solution method is adopted to synthesize ZnO-NFs using Zinc nitrate as precursor and are characterized in details which reveals well-crystalline, high purity, and excellent optical properties of ZnO-NFs. The FESEM image displays that each ZnO-NFs is comprised of small nanorods with the average size of 1-2 μm. The mass spectroscopy confirms the complete mineralization of Bph-dye after 120 min as main m/z = 671.1 signal disappears and splits into small masses signals. The assynthesized ZnO-NFs are further used as potential photocatalyst for the photocatalytic degradation of Bph-dye and displays about 96% degradation of Bph-dye within 120 min of UV light irradiation. The observed results summarize that the synthesized ZnO-NFs could be effectively utilized for the photocatalytic degradation of hazardous organic pollutants present in wastewater.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]

Arbuj S. S., Hawaldar R. R., Mulik U. P., Wani B. N., Amalnerkar D. P., Waghmode S. B., Mater. Sci. Eng. B. 168 (2010) 90. Zhang S., Chen L., Liu H., Guo W., Yang Y., Guo Y., Huo M., Chem. Eng. J. 200202 (2012) 300. Bouanimba N., Zouaghi R., Laid N., Sehili T., Desalination. 275 (2011) 224. Gao B., Liu L., Liu J., Yang F., Appl. Catal. B: Environ. 129 (2013) 89. Sampaio M. J., Silva C. G., Marques R. R. N., Silva A. M. T., Catal. Today. 161 (2011) 91. Ballari M. M., Alfano O. M., Cassano A. E., Chem. Eng. Sci. 65 (2010) 4931. Ameen S., Akhtar M. S., Seo H. K., Kim Y. S., Shin H. S., Chem. Eng. J. 187 (2012) 351. Ong B. S., Li C. S., Li Y. N., Wu Y. L., Loutfy R., J. Am. Chem. Soc. 129 (2007) 2750. Jia B., Jia W., Ma Y., Wu X., Qu F., Sci. Tech. Adv. Mater. 4 (2012) 702.

Degradation of Bromophenol Dye over ZnO Nanoflowers

177

[10] Ameen S., Akhtar M. S., Kim Y. S., Yang O. B., Shin H. S., Coll. Poly. Sci. 288 (2010) 633. [11] Straumal B. B., Mazilkin A. A., Protasova S. G., Straumal P. B., Myatiev A. A., Schütz G., Goering E., Tietze T., Baretzky B., Philos. Mag. 93 (2013) 1371. [12] Protasova S. G., Straumal B. B., Mazilkin A. A., Stakhanova S. V., Straumal P. B., Baretzky B., J. Mater. Sci. 49 (2014) 4490. [13] Curridal M. L., Comparelli R., Cozzli P. D., Mascolo G., Agostiano A., Mater. Sci. Eng., CBiomim. Mater. Sens. Syst. 23 (2003) 285. [14] Tian J. T., Chen L. J., Yin Y. S., Wang X., Dai J. H., Zhu Z. B., Liu X. Y., Wu P. W., Surf. Coat Tech. 204 (2009) 205. [15] Ameen S., Akhtar M. S., Nazim M., Shin H. S., Mater. Lett. 96 (2013) 228. [16] Ameen S., Akhtar M. S., Shin H. S., Mater. Lett. 183 (2016) 329. [17] Ameen S., Akhtar M. S., Kim Y. S., Shin H. S., Appl. Catal. B: Environ. 103 (2011) 136. [18] Straumal B. B., Protasova S. G., Mazilkin A. A., Goering E., Schütz G., Straumal P. B., Baretzky B., J. Nanotechnol. 7 (2016) 1936. [19] Ameen S., Akhtar M. S., Shin H. S., Mater. Lett. 148 (2015) 188. [20] Straumal B. B., Mazilkin A. A., Protasova S. G., Stakhanova S. V., Straumal P. B., Bulatov M. F., Tietze Th., Goering E., Baretzky B., Rev. Adv. Mater. Sci. 41 (2015) 61. [21] Straumal B. B., Baretzky B., Mazilkin A. A., Protasova S. G., Myatiev A. A., Straumal P. B., J. Eur. Cer. Soc. 29 (2009) 1963. [22] Sun J. H., Dong S. Y., Feng J. L., Yin X. J., Zhao X. C., J. Mol. Catal. A: Chem. 335 (2011) 145. [23] Ameen S., Akhtar M. S., Shin H. S., Talanta. 100 (2012) 377. [24] Jang M. S., Ryu M. K., Yoon M. H., Lee S. H., Kim H. K., Onodera A., Curr. Appl. Phys. 9 (2009) 651. [25] Zhang H., Shen L., Guo S. W., J. Phys. Chem. C. 111 (2007) 12939. [26] Fauteux C., Longtin R., Pegna J., Therriault D., Inorg. Chem. 46 (2007) 11036. [27] Li Y. F., Xu D., Oh J. I., Shen W., Li X., Yu Y., ACS Catal. 2 (2012) 391. [28] Jing L. Q., Qu Y. C., Wang B. Q., Li S. D., Jiang B. J., Yang L. B., Fu W., Fu H. G., Sun J. Z., Sol. Energy Mater. Sol. Cells. 90 (2006) 1773. [29] Lin X. P., Huang T., Huang F. Q., Wang W. D., Shi J. L., J. Phys. Chem. B. 110 (2006) 24629.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 12

ZNO FLOWER NANOMATERIALS AS PHOTOCATALYST FOR THE DEGRADATION OF CRYSTAL VIOLET DYE Sadia Ameen1, M. Shaheer Akhtar2, M. Nazim1 and Hyung Shik Shin1,* 1

Energy Materials and Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New and Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT A simple solution method is employed for the direct synthesis of ZnO-flowers using zinc acetate precursor in basic medium and utilized as photocatalyst for the degradation of crystal violet (Cv) dye. Each ZnO-flower is comprised of well-defined petals with the average length of ~300nm. The photocatalytic activity of ZnO-flowers is elucidated towards the degradation of Cv-dye under light illumination. The as-synthesized ZnOflowers show very fast degradation of Cv-dye with the degradation rate of ~96% within the time interval of 80min. Mineralization of the dye is extensively described by investigating the mass spectroscopy of the dye before and after the photocatalytic reaction.

*

Corresponding Author Email: [email protected].

180

Sadia Ameen, M. Shaheer Akhtar, M. Nazim et al.

INTRODUCTION The residues of textile paper and other coloring industries are the major pollutants to contaminate water and affect the aquatic life as well as human life [1]. Colored organic dyes from industries are mostly non-biodegradable in nature, creating major problems to the aquatic creatures and drastically disturb the water ecosystem [2]. The crystal violet (Cv) is known a cationic dye which is generally used as ink for pen and the coloration of textile products [3]. The Cv-dye easily interacts with the negatively charged cell membrane surfaces in mammals and enters into cells [4, 5]. Excess inhalation of Cv-dye causes irritation of the respiratory tracts, vomiting, diarrhea, headache, dizziness and its long term exposure might damage the mucous membrane and the gastrointestinal tract [6]. In general, the remediation of organic dyes is performed by the conventional treatments like chemical precipitation/separation of pollutants, coagulation by a chemical agent, ozone oxidation, hypochlorite oxidation, electrochemical method and elimination by adsorption [7]. Photocatalytic degradation is an economical and easy method for the decomposition of harmful organic pollutants into less dangerous minerals [8]. Recently, the semiconductor nanomaterials have been applied as promising photocatalyst for the effective degradation of contaminants (organic dye) for purifying water [9]. Zinc oxide (ZnO) nanomaterials in the form of flowers and nanorods have shown numerous applications in the electromechanical transducer materials for sensors, actuators in microelectromechanical systems etc. due to their excellent dielectric, ferroelectric, piezoelectric, and pyroelectric properties [10]. In particular, ZnO is a promising material for spintronics as it possesses ferromagnetic properties [10]. ZnO semiconducting materials also show promising photocatalytic behavior owing to their non-toxic nature, inexpensive, and excellent chemical and mechanical stability with a wide band gap of 3.4 eV [11]. Particularly, ZnO nanomaterials with different morphologies have presented great impacts on the performance of photocatalytic process [12]. In the previous report, Ameen et al. prepared an effective nanocomposite of polyaniline and ZnO for the photocatalytic degradation of methylene blue dye [13]. Recently, Rahman et al. reported rapid and effective photocatalytic degradation of rhodamine B dye by ZnO nanoparticles [14]. Kansal et al. studied the decoloration of pararosaniline chloride dye using ZnO nanostructure as a photocatalyst under UV irradiation [15]. In this chapter, we report the synthesis of ZnO- in basic medium by simple chemical route and its application as photocatalyst for the degradation of Cv-dye under light irradiation.

ZnO Flower Nanomaterials as Photocatalyst …

181

The photocatalytic degradation of Cv-dye is performed under UV light using the illumination of Xenon arc lamp. The photocatalytic activity of ZnO-flower towards Cvdye is performed as reported elsewhere [16]. In brief, 100mg of as-synthesized ZnOflowers is added in 10ppm solution of Cv-dye under continuous stirring. The suspension is continuously stirred for 1h to obtain the adsorption–desorption equilibrium between Cv-dye and ZnO photocatalyst under dark condition. Finally, UV light illumination is used to stable the aqueous dye suspension under constant stirring. The decomposed dye is taken out after every 10min and subjected to centrifugation at 10,000rpm to separate out the ZnO-powder. By UV–vis spectrophotometer, the absorption spectrum of decomposed dye is recorded. The degradation rate of Cv-dye over ZnO-flower is estimated by the following equation: Degradation rate (%) = (Co-C/Co)100 = (Ao-A/Ao)100 where Co is the initial concentration, C denotes the variable concentration, Ao is the initial absorbance, and A corresponds to variable absorbance.

Reprinted with permission from [S. Ameen, 2013], Mater. Lett., 96(2013) 228 © 2013 Elsevier Ltd. Figure 1. FESEM images of ZnO-flowers at low (a) and high (b) magnification. EDX spectrum (c) and XRD patterns (d) of ZnO-flowers.

182

Sadia Ameen, M. Shaheer Akhtar, M. Nazim et al.

MORPHOLOGICAL AND CRYSTALLINE STUDIES OF ZNO NANOFLOWERS The Field Emission Scanning Electron Microscopy, the Energy-Dispersive XRay Spectroscopy and X-Rays Diffraction Patterns The morphology of ZnO-flowers is characterized by field emission scanning electron microscopy, as shown in Figure 1(a and b). The synthesized ZnO nanomaterial displays the uniform morphology of flower structures with an average size of 1–2μm. Each ZnO-flower comprises the petals of average size of ~300nm, as depicted in Figure 1(b). It is reported that the optical and physical properties of nanograined ZnO strongly depend on the presence of defects like grain boundaries [10]. From the FESEM images, it could be seen that the synthesized ZnO flowers are nanograined and contain developed free surfaces as well as grain boundaries and interfaces. The elemental composition has been confirmed by the Energy dispersive X-ray spectroscopy (EDX) analysis coupled with FESEM. Figure 1(c) shows major peaks of zinc and oxygen with small peak of carbon element, indicating the formation of ZnOflowers. Figure 1(d) presents the X-ray diffraction pattern of the as-synthesized ZnOflowers. The XRD patterns of ZnO-flowers are perfectly indexed in (JCPDS No. 361451) [17], confirming the existence of typical ZnO wurtzite crystal structure. The existence of grain boundaries in ZnO flowers is explained by the presence of amorphous superficial and intergranular layers which are invisible in XRD results [18]. Noticeably, no other diffraction peaks related to any impurity are detected in the XRD pattern which confirms well-crystallinity and purity of the as-synthesized ZnO-flowers with few grain boundaries or defects.

OPTICAL AND STRUCTURAL PROPERTIES OF ZNO NANOFLOWERS The Ultraviolet-Diffused Reflectance Spectroscopy, Fourier Transform Infrared Spectroscopy, Raman Scattering Spectroscopy and the Photoluminescence Spectrum The ultraviolet-diffused reflectance spectroscopy of ZnO-flowers is shown in Figure 2(a). ZnO-flowers display the absorbance near 400nm, corresponding to the direct band gap of ZnO due to the electron transitions from the valence band to the conduction band (O2p→Zn3d) [19]. The band gap of 3.24 eV is estimated from UV-DRS which is close to

ZnO Flower Nanomaterials as Photocatalyst …

183

the band gap of bulk ZnO. The determination of the structural orientation in ZnO-flowers is characterized by FTIR spectroscopy, as shown in Figure 2(b). The sharp peak at 547cm-1 is associated with the Zn–O stretching mode [20] and a short peak at 887cm-1 is due to carbonate moieties [21]. The broad band at 3377 cm-1 and the small band at 1387 cm-1 are assigned to O–H bending vibrations and C = O stretching mode from the absorption of atmospheric moisture and CO2 on the surface of ZnO-flower respectively [22]. Figure 2(c) shows the Raman scattering spectroscopy of ZnO-flowers to investigate the structural disorders and the defects of the materials. The strong Raman shifts at 437.4cm-1 presents the E2 mode which is the characteristic peaks of wurtzite ZnO and is consistent with the XRD results [23]. The Raman shifts at 331.6 cm-1 and 383.2cm-1 correspond to the multiple phonon scattering processes E2 (high)–E2 (low) and E1 (LO) modes respectively [24, 25]. Furthermore, Figure 2(d) depicts the PL spectra of ZnOflowers at room temperature which are consisted of prominent UV emission at 383.4nm and broader green emission peak at 544nm. The UV emission corresponds to the nearband edge (NBE) emission originates from the recombination of the free excitons of ZnO [24, 25] and the latter is associated with the singly ionized oxygen vacancies (VO-) in ZnO [26].

Reprinted with permission from [S. Ameen, 2013], Mater. Lett., 96(2013) 228 © 2013 Elsevier Ltd. Figure 2. UV-DRS (a), FTIR(b), Raman(c) and PL (d) spectra of ZnO-flowers.

184

Sadia Ameen, M. Shaheer Akhtar, M. Nazim et al.

Appearance of the strong and prominent UV emission peaks of ZnO-flowers clearly indicate the good optical properties from the high crystal quality of ZnO-flowers.

THE PHOTOCATALYTIC DEGRADATION OF CRYSTAL VIOLET DYE OVER THE SURFACE OF ZNO FLOWERS UNDER UV ILLUMINATION UV-Vis Absorbance Spectra of Decomposed Crystal Violet Dye Solution over ZnO Flowers under UV Light Irradiation and the Mass Spectra of Bromophenol Dye Solutions The Cv-dye degradation is examined by the photocatalytic reaction over the surface of ZnO-flowers under UV illumination. Figure 3(a) shows UV–vis spectra of the decomposed Cv-dye within the time interval from 0 to 80min over the surface of ZnOflowers as a photocatalyst. Cv-dye exhibits the maximum absorption wavelength at 590nm. Noticeably, with the increase of the exposed time, the intensity of Cv-dye absorbance continuously decreases which shows the decrease in Cv-dye concentration. The Cv-dye within a short exposed time of 80min substantially degrades by 96%. The rate of Cv-dye degradation with and without ZnO-flowers under UV illumination is depicted in the plot of degradation rates versus time interval (inset of Figure 3(a)). The degradation rate of Cv-dye gradually decreases with the increase of the exposed time. However, the color or concentration of dye is unchanged under dark condition for 2h, indicating no self-degradation of Cv-dye. Figure 3(b) shows the pie chart of Cv-dye degradation and demonstrates that the major dye degradation is observed during the first 50min over the surface of ZnO-flowers. This result confirms the rapid Cv-dye degradation under UV illumination. The rapid degradation by ZnO-flowers could be explained by the fast generation of ē–h+ pairs between the conduction (CB) and valence band (VB) of ZnO under UV illumination. Firstly, the photogenerated ē in CB of ZnO travels to the surface and scavenges the ubiquitous O2 to form superoxide anion O2 and further protonation produces HOO● radicals. Whereas, h+ at VB migrates to the back side of the ZnO surface and produces active species such as OH● by the reaction of either H2O or OH. The generation of these active oxygen species {O2, O2●-, HOO●, or ●

OH} are significantly initiated by the rapid degradation of Cv-dye into less harmful

organic or minerals. The mineralization of Cv-dye is extensively discussed by mass spectroscopy (Figure 3(c)) of Cv-dye before and after the photocatalytic reaction. The strong mass signal at

ZnO Flower Nanomaterials as Photocatalyst …

185

m/z = 372.2 is related to the formula mass of Cv-dye solution. After 10min, this mass signal splits into multiple mass signals, indicating the formation of reaction intermediates and revealing the dye removal or mineralization by adsorption via ZnO-flowers.

Reprinted with permission from [S. Ameen, 2013], Mater. Lett., 96(2013) 228 © 2013 Elsevier Ltd. Figure 3. UV–vis absorbance spectra of decomposed Cv-dye solution over ZnO-flowers under light illumination (a) and Cv-dye degradation pie chart as a function of time (b). Inset shows the degradation rate (%) versus time interval. Mass spectra of Cv-dye solutions over ZnO flowers with the scan 200– 400 m/z (c) and the possible reaction intermediates after the photocatalytic reaction (d) under light illumination.

These intermediates are illustrated in Figure 3(d). After 80 min, m/z = 372.2 signal disappears and small masses signals are detected as shown in Figure 3(c and d). The mass results clearly deduce the complete and rapid degradation of Cv-dye over the surface of ZnO-flowers under light illumination. Thus, the as- synthesized ZnO-flowers with good crystallinity and optical properties have sufficiently increased the reactive species on the surface of the photocatalyst and result in rapid Cv-dye degradation under illumination.

186

Sadia Ameen, M. Shaheer Akhtar, M. Nazim et al.

CONCLUSION The direct synthesis of ZnO-flowers is performed by a simple solution method using zinc acetate as precursor under basic medium and utilized as photocatalyst for the degradation of Cv dye. Each ZnO-flower is comprised of well-defined petals with an average length of 300nm. The photocatalytic activity shows that as-synthesized ZnOflowers display very rapid degradation of Cv-dye with the degradation rate of 96% within the time interval of 80min. The generation of active oxygenated species on the surface of advanced ZnO-flowers significantly initiates the rapid degradation of Cv-dye into less harmful organic or minerals. Mass spectroscopy confirms the complete mineralization of Cv-dye after 80min as main m/z = 372.2 signal disappears and splits into small masses signals.

REFERENCES [1]

(a) O. Mekasuwandumrong, P. Pawinrat, P. Praserthdam, J. Panpranot, Chem. Eng. J., 164 (2010) 77; (b) L. S. Roselin, R. Selvin, Sci. Adv. Mater., 3 (2011) 113. [2] (a) S. Ameen, H. K. Seo, M. S. Akhtar, H. S. Shin, Chem. Eng. J., 210 (2012) 220; (b) D. Tassalit, A. N. Laoufi, F. Bentahar, Sci. Adv. Mater., 3 (2011) 944; (c) L. S. Roselin, R. Selvin, Sci. Adv. Mater., 3 (2011) 251. [3] K. P. Singh, S. Gupta, A. K. Singh, S. Sinha, J. Hazard. Mater., 186 (2011) 1462. [4] S. Li, Bioresour. Technol., 101 (2010) 2197. [5] H. He, S. Yang, K. Yu, Y. Ju, C. Sun, L. Wang, J. Hazard. Mater., 173 (2010) 393. [6] D. Ghosh, K. G. Bhattacharyya, Appl. Clay. Sci., 20(2002) 295. [7] N. Daneshvar, A. R. Khataee, A. R. Ghadim, M. H. Rasoulifard, J. Hazard. Mater., 148 (2007) 566. [8] S. Ameen, M. S. Akhtar, Kim. Y. S., O. B. Yang, H. S. Shin, Colloid. Polym. Sci., 288 (2010) 1633. [9] (a) C. L. Ren, B. F. Yang, M. Wu, J. Xu, Z. P. Fu, Y. Lv, et al., J. Hazard. Mater., 182 (2010) 123; (b) B. Jia, W. Jia, Y. Ma, X. Wu, F. Qu, Sci. Adv. Mater., 4 (2012) 702. [10] (a) S. Singh, G. M. Ali, P. Chakrabarti, Sci. Adv. Mater., 3 (2011) 926; (b) B. B. Straumal, A. A. Mazilkin, S. G. Protasova, A. A. Myatiev, P. B. Straumal, E. Goering, et al., Thin Solid Films, 520 (2011) 1192. [11] (a) A. Umar, M. M. Rahman, Y. B. Hahn, J. Nanosci. Nanotechnol., 9 (2009) 4686; (b) S. Singh, P. Chakrabarti, Sci. Adv. Mater., 4 (2012) 199; (c) S. Ameen, M. S. Akhtar, H. S. Shin, Chem. Eng. J., 195-196 (2012) 307. [12] S. Chakrabarti, B. K. Dutta, J. Hazard. Mater., 112 (2004) 269.

ZnO Flower Nanomaterials as Photocatalyst …

187

[13] S. Ameen, M. S. Akhtar, Y. S. Kim, Yang. Yang, H. S. Shin, Colloid. Polym. Sci., 289 (2011) 415. [14] Q. I. Rahman, M. Ahmad, S. K. Misra, M. Lohani, Mater. Lett., 91 (2013) 170. [15] S. K. Kansal, A. H. Ali, S. Kapoor, D. W. Bahnemann, Sep. Purif. Technol., 80 (2011) 125. [16] S. Ameen, M. S. Akhtar, Y. S. Kim, H. S. Shin, Appl. Catal. B: Environ., 103 (2011) 136. [17] A. Al-Hajry, A. Umar, Y. B. Hahn, D. H. Kim, Superlatt. Microstruct., 45 (2009) 529. [18] B. B. Straumal, S. G. Protasova, A. A. Mazilkin, B. Baretzky, A. A. Myatiev, P. B. Straumal, et al., Mater. Lett., 71 (2012) 21. [19] J. H. Sun, S. Y. Dong, J. L. Feng, X. J. Yin, X. C. Zhao, J. Mol. Catal. A. Chem., 335 (2011) 145. [20] G. Xiong, U. Pal, J. G. Serrano, K. Ucer, B. R. T. Williams, Phys. Status. Solidi., 3 (2006) 3577. [21] A. Umar, M. M. Rahman, A. Al-Hajry, Y. B. Hahn, Talanta, 78 (2009) 284. [22] S. Ameen, M. S. Akhtar, H. S. Shin, Chem. Eng. J., 195 (2012) 307. [23] H. Zhang, L. Shen, S. W. Guo, J. Phys. Chem. C., 111 (2007) 12939. [24] M. S. Jang, M. K. Ryu, M. H. Yoon, S. H. Lee, H. K. Kim, A. Onodera, et al., Curr. Appl. Phys., 9 (2009) 651. [25] C. Fauteux, R. Longtin, J. Pegna, D. Therriault, Inorg. Chem., 46 (2007) 11036. [26] K. Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. A. Voigt, B. E. Gnade, J. Appl. Phys., 79 (1996) 7983.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 13

THE MINERALIZATION OF CATIONIC DYE USING ZNO HOLLOW NANO-BASKETS Sadia Ameen1, M. Shaheer Akhtar2 and Hyung Shik Shin1,* 1

Energy Materials and Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New and Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT The hollow nano-baskets (HNBs) of zinc oxide (ZnO) are synthesized by a simple low temperature solution method and used for the degradation of rhodamine 6G (Rh6G) dye. Well-defined hollow nano-baskets morphology of ZnO is observed which displays a band gap of ~3.26 eV. The result of photocatalytic degradation reveals that the synthesized ZnO-HNBs exhibits a rapid degradation rate of ~97% of Rh6G-dye within ~90min. The mineralization of Rh6G-dye is examined before and after the photocatalytic reaction by a mass spectroscopy of the dye. The rapid and high degradation of Rh6G-dye might be attributed to ZnO hollow nanostructures which significantly improves the catalyst contact with dye by the inner and external surface of ZnO.

INTRODUCTION The paint, textiles, metallurgy and tannery industries release the wastes to rivers, ponds, and canals which causes severe environmental problems especially to the aquatic *

Corresponding Author Email: [email protected].

190

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

ecosystems. In particular, the residues from papers, textiles and other coloring industries are the major sources of water pollutants which pose a serious hazard to human beings and the aquatic livings [1]. An organic dye, Rh6G-dye is highly water soluble and widely used for coloring wool, cotton, silk, and in research laboratories as a diagnostic tool to detect the antigens and for fluorescence microscopy and fluorescence correlation spectroscopy [2, 3]. Rh6G-dye contaminated water usually causes several problems to human being such as skin irritation, reddening of eyes and blockage of respiratory system [4]. The consumption of Rh6G fouled water is highly carcinogenic and poisonous to the living organisms [5]. The remediation of organic dye pollutants like Rh6G by the processes such as chlorination and ozonation could not avoid the toxic nature of dyes due to the presence of stable aromatic structures [6, 7]. On the other hand, the decolorization or mineralization of organic dyes from water is of great significance which could be performed through the physical adsorption, catalytic and photocatalytic processes. The photocatalytic degradation of dye is the simplest, easy and cost effective method for the degradation of harmful organic pollutants to less harmful minerals [8]. Among various metal oxides nanomaterials, ZnO with its special wurtzite crystal structure and the wide band gap of 3.37 eV is accredited as one of the important semiconducting materials for several applications such as UV light-emitting devices [9], electroluminescent [10], fieldemission devices [11] and photocatalyst [12]. ZnO nanomaterials are non-toxic semiconductors with excellent chemical and mechanical stability [13]. In addition, ZnO is also a potential material for spintronics as it could (depending upon the manufacturing conditions) possess the ferromagnetic properties [14, 15]. Furthermore, the properties of ZnO such as sizes, shapes, and dimensions [16, 17] show great impact on the catalytic performances. Hollow ZnO nanostructures have recently explicated a wide range of applications due to their unique structures associated by the two surfaces of external and internal shells, high surface-to-volume ratio, and good surface permeability [18, 19]. Uniquely, the hollow morphology could be helpful for capturing large numbers of ultra violet light which might reduce the combination of the photo-generated electron-hole pairs [20-22]. Ameen et al. reported the synthesis of hollow mesoporous ZnO nanoglobules via hydrothermal method and demonstrated the electrocatalytic activity towards piperidine [23]. In other report, ZnO-graphene oxide (GO) nano-hybrid was synthesized by a chemical route and utilized as an effective photocatalyst for the photodegradation of crystal violet dye which showed enormously high degradation of ∼95% within 80min [24]. In this work, low temperature solution process is adopted to synthesize hollow nano-baskets of ZnO-HNBs and characterizes in terms of structures, crystallinity and morphology. The photodegradation of Rh6G-dye has been performed to investigate the photocatalytic activity of the synthesized ZnO-HNBs. The photodegradation of Rh6G-dye over the surface of the synthesized ZnO-HNBs is carried out as reported elsewhere [25]. In brief, the freshly prepared 10 ppm of Rh6G-dye solution is taken and added into 100mg of the synthesized ZnO-HNBs under stirring. To

The Mineralization of Cationic Dye …

191

achieve the adsorption-desorption equilibrium of Rh6G-dye solution to ZnO-HNBs catalyst, the suspension is gradually stirred for 1h under dark condition. Afterward, the light illumination using Xenon arc lamp is exposed to an aqueous dye suspension under constant stirring. The decomposed Rh6G-dye is monitored by taking the relative intensity of UV–vis spectroscopy and degradation rate is estimated by the following equation: Degradation rate (%) = (Co – C/Co) x 100 = (Ao –A/Ao) x 100 where Co represents the initial concentration, C denotes variable concentration, Ao shows the initial absorbance, and A corresponds to a variable absorbance.

MORPHOLOGICAL AND CRYSTALLINE STUDIES OF ZNO HOLLOW NANO-BASKETS The Field Emission Scanning Electron Microscopy, the Energy-Dispersive X-Ray Spectroscopy and X-Ray Diffraction Patterns The synthesized ZnO nanostructures are morphologically characterized by field emission scanning electron microscopy, as shown in Figure 1(a,b). The low magnification mode (Figure 1(a)) reveals that the synthesized nanomaterials possess hollow nano-baskets like morphology in a high aspect ratio. As shown in Figure 1(b), the average inner and outer diameters of HNBs are estimated as 80nm and 100nm, respectively. From FESEM results, it is witnessed that the synthesized ZnOHNBs are surely nanograins which contain well developed grain boundaries and free surfaces [26]. The element composition and the purity of the synthesized ZnO-HNBs are demonstrated by FESEM coupled with electron dispersive X-Rays spectroscopy (EDS), as shown in Figure 1(c). Zn and O elements are detected in the EDX spectrum, illustrating the existence of only Zn and O in the synthesized ZnO-HNBs, which clearly depicts the high purity of the synthesized ZnO-HNBs. The synthesized ZnO-HNBs are further characterized by X-ray diffraction patterns to explain the crystalline phase of HNBs. Figure 1(d) displays the diffraction peaks at 31.8°(100), 34.5°(002), 36.3°(101), 47.8°(102), 56.7°(110), 63.6°(103), 69.8°(112) and 76.9°(202), which are in accordance with JCPDS no: 80-0075. The observed diffraction peaks clearly explain the typical hexagonal wurtzite structure of ZnO-HNBs [27]. The absence of impurity phase in the spectra confirms well-crystalline and the purity of ZnOHNBs.

192

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

Reprinted with permission from [S. Ameen, 2013], Mater. Lett., 183 (2016) 329 © 2016 Elsevier Ltd. Figure 1. FESEM image of ZnO-HNBs at low (a) and high (b) magnifications, EDS spectrum (c) and XRD patterns (d) of ZnO-HNBs.

Recently, it has been demonstrated that the physical properties of nanograined ZnO like ZnO-HNBs count strongly on the appearances of structural defects such as interphase boundaries and grain boundaries, resulting from the presence of the amorphous surficial, interfacial and intergranular layers in ZnO-HNBs (invisible in XRD) [28, 29].

OPTICAL AND STRUCTURAL PROPERTIES OF ZNO HOLLOW NANO-BASKETS The Ultraviolet-Diffused Reflectance Spectroscopy, Raman Scattering and Photoluminescence Spectrum The ultra violet-diffused reflectance (UV-DRS) spectroscopy and photoluminescence (PL) spectroscopes are used to analyze the optical properties of the synthesized ZnO HNBs. Figure 2(a) shows the UV-DRS spectrum of ZnO-HNBs, exhibiting a strong absorption edge in the UV-region due to the excitation of electrons from the valance band (VB) to the conduction band (CB) of ZnO [30].

The Mineralization of Cationic Dye …

193

Reprinted with permission from [S. Ameen, 2013], Mater. Lett., 183 (2016) 329 © 2016 Elsevier Ltd. Figure 2. (a) UV-DRS spectrum (Inset shows corresponding Eg plot), (b) PL spectrum and (c) Raman spectrum of ZnO-HNBs.

The estimated band gap (Eg) from UV-DRS spectrum for the synthesized ZnO-HNBs is 3.26 eV, which is very close to Eg value of the bulk ZnO [31]. Furthermore, the room temperature PL spectrum of ZnO-HNBs (Figure 2(b)) displays prominent emission peak at 387nm, corresponding to near-band edge (NBE) emission which usually originates from the recombination of the free excitons of ZnO [32]. The origin of broad green emission peak at 557 nm signifies the radiative transition from interstitials donors (Zinc interstitial (Zni), Oxygen vacancy to acceptors (Zinc vacancy (Vzn), oxygen interstitial (Oi)) [33]. Thus, a strong UV emission for the synthesized ZnO-HNBs distinctly reveals good optical properties of the synthesized ZnO. The synthesized ZnO-HNBs are further characterized in terms of the structural properties by the Raman scattering spectroscopy. Figure 2(c) shows a prominent Raman band at ∼436.8cm-1 which is assigned to E2 mode and matches with Raman peak of bulk ZnO crystals [34]. The other peaks at ∼332.8cm-1, 382.3cm-1 and ∼582.5cm-1 are subjected

194

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

to the second order Raman spectrum arising from zone-boundary phonons 3E2H–E2L and E1(LO) mode of ZnO respectively [34]. Additionally, the appearance of sharp Raman peak, E2 Raman Mode, displays the improved optical and crystalline properties of ZnO HNBs [35].

THE PHOTOCATALYTIC DEGRADATION OF RHODAMINE 6G (RH6G) DYE OVER THE SURFACE OF ZNO HOLLOW NANO-BASKETS UNDER UV ILLUMINATION UV-Vis Absorbance Spectra of Decomposed Rhodamine 6G (Rh6G) Dye Solution over ZnO Hollow Nano-Baskets and the Mass Spectra of Rhodamine 6G (Rh6G) Dye Solutions The photocatalytic degradation of Rh6G-dye over the surface of ZnO-HNBs has been investigated by UV–vis spectroscopy after certain illumination intervals, as presented in Figure 3(a). It can be seen that the intensity of absorbance at 544nm decreases continuously with the variations of an exposed time, suggesting the degradation of Rh6Gdye upon the light illumination. After 90min, Rh6G-dye is almost vanished with the degradation rate of 97%, which clearly represents a complete mineralization of Rh6Gdye. In order to examine the role of ZnO-HNBs as a catalyst, the degradation of Rh6Gdye is carried out with and without ZnO-HNBs under the light illumination, as depicted in the plot of A/Ao versus time interval of Figure 3(b). Herein, without ZnO-HNBs catalysts, a very less amount of Rh6G-dye are degraded under the light illumination, whereas, a rapid Rh6G-dyedegradation is recorded in the presence of ZnO-HNBs. In support, Figure 3(c) shows the pie chart of Rh6G-dye degradation, indicating that Rh6Gdye has majorly degraded in the first 60min over the surface of ZnO-HNBs. The rapid degradation by ZnO-HNBs could be explained on the basis of the availability of more photocatalytic sites which are provided by the interior walls of ZnO hollow structure. In the beginning of the photocatalytic reaction, the excitation of ē from VB to CB of ZnO is occurred upon the light illumination. This photogenerated ē in CB could travel to the surface and scavenges by surface oxygen (O2) molecule to form superoxide anion O2- and simultaneously its protonation produces HOO● radicals. Whereas, the generated h+ at VB migrates to the back side of the ZnO surface and produces active species such as OH● by the reaction of either H2O or OH‾.The production of these active oxygenated species {O2- O2●-, HOO●, or OH} has

The Mineralization of Cationic Dye …

195

significantly facilitated the rapid degradation of Rh6G-dye into less harmful organics or minerals.

Reprinted with permission from [S. Ameen, 2013], Mater. Lett., 183 (2016) 329 © 2016 Elsevier Ltd. Figure 3. (a) UV–vis absorbance spectra of decomposed Rh6G-dye solution over ZnO-HNBs under light illumination, (b) the A/Ao versus exposed time, (c) Rh6G-dye degradation pie chart as a function of exposed time and (d) mass spectra of Rh6G-dye solutions before and after the photocatalytic reaction.

The mineralization efficiency of the ZnO-HNBs photocatalyst towards Rh6G-dye degradation is determined by measuring the mass spectroscopy of initial concentration of Rh6G-dye and degraded Rh6G-dye, as depicted in Figure 3(d). It has been detected that the initial Rh6G-dye shows the main signal at m/z = 443.3, which is very close to the molecular weight of Rh6G-dye. After 90min, the main signal is disappeared and fragmented into various small mass signals, indicating the mineralization of Rh6G-dye over the surface of ZnO HNBs under the light illumination. Therefore, the synthesized ZnO-HNBs provide a strong inhibition of ē-h+ via excellent physicochemical properties, such as the hollow nanostructures, optical and structural properties, resulting to the enhanced the photocatalytic efficiency towards Rh6G-dye degradation.

196

Sadia Ameen, M. Shaheer Akhtar and Hyung Shik Shin

CONCLUSION ZnO-HNBs are synthesized by a low temperature solution method and successfully applied as photocatalyst for the Rh6Gdye degradation. The morphological, crystalline and the structural characterizations have shown well defined hollow nano-baskets like morphology of high quality and the crystalline nature. The synthesized ZnO-HNBs present a band gap of 3.26 eV. From the photocatalytic results, a rapid degradation of Rh6G-dye by 97% within 90min has been recorded over the surface of ZnO-HNBs under the light illumination. Mass spectroscopy is used to demonstrate the mineralization of Rh6G-dye before and after the photocatalytic reaction. The hollow nanostructures might considerably improve the opportunity of the inner and external surface of ZnO contacting with Rh6G-dye molecules to facilitate the efficient degradation of Rh6G-dye into less harmful organics or minerals.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

T. Robinson, G. McMullan, R. Marchant, P. Nigam, Bioresou. Technol., 77 (2001) 247. L. S. Roselin, R. Selvin, Sci. Adv. Mater., 3 (2011) 251. M. N. Ghazzala, H. Kebaili, M. Joseph, Appl. Cat. B: Environ., 115–116 (2012) 276. R. Jain, M. Mathur, S. Sikarwar, A. Mittal, J. Environ. Manag., 85 (2007). T. F. Robinson, G. McMullan, R. Marchant, P. Nigam, Bioresour. Technol., 77 (2001) 247. F. Atmani, A. Bensmaili, N. Y. Mezenner, J. Environ. Sci. Technol., 2 (2009) 153. N. Pasukphun, S. Vinitnantharat, S. Gheewala, J. Pak, Biol. Sci., 13 (2010) 316. S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, Colloid Polym. Sci., 288 (2010) 1633. D. K. Wang, F. Wang, Y. P. Wang, Y. Fan, B. Zhao, D. X. Zhao, J. Phys. Chem. C., 119 (2015) 2798. W. I. Park, G. C. Yi, Adv. Mater., 16 (2004) 87. J. Zhou, L. Gong, S. Z. Deng, J. Chen, J. C. She, N. S. Xu, R. Yang, Z. L. Wang, Appl. Phys. Lett., 87 (2005) 223108. M. S. Akhtar, S. Ameen, H. K. Seo, H. S. Shin, Mater. Lett., 13 (2013) 20, (b) S. Ameen, M. S. Akhtar, H. S. Shin, Mater. Lett., 15 (2015) 82. M. L. Curri, R. Comparelli, P. D. Cozzli, G. Mascolo, A. Agostiano, Mater. Sci. Eng. C., 23 (2003) 285.

The Mineralization of Cationic Dye …

197

[14] B. Straumal, A. Mazilkin, S. Protasova, A. Myatiev, P. Straumal, E. Goering, B. Baretzky, Phys. Status Solidi B., 248 (2011) 1581. [15] B. B. Straumal, A. A. Myatiev, P. B. Straumal, A. A. Mazilkin, S. G. Protasova, E. Goering, B. Baretzky, JETP Lett., 92 (2010) 433. [16] S. Chakrabarti, B. K. Dutta, J. Hazard. Mater., 112 (2004) 269. [17] B. Yan, C. Wang, M. Jianzhong, Ceram. Int., 42 (2016) 1746. [18] F. Q. Chang, X. B. Huang, H. Wei, Mater. Lett., 125 (2014) 128. [19] M. Chen, C. Y. Ye, S. X. Zhou, Adv. Mater., 25 (2013) 5343. [20] S. H. Wei, M. H. Zhoua, W. P. Du, Sens. Actuators B: Chem., 160 (2011) 753. [21] J. P. Kar, S. N. Das, J. H. Choi, J. M. Myoung, Mater. Lett., 63 (2009) 2327. [22] S. H. Yue, L. Zhang, J. J. Lu, J. Y. Zhang, Mater. Lett., 63 (2009) 1217. [23] S. Ameen, M. S. Akhtar, H. K. Seo, H. S. Shin, Chem. Eng. J., 270 (2015) 564. [24] B. B. Straumal, S. G. Protasova, A. A. Mazilkin, A. A. Myatiev, P. B. Straumal, G. Schü tz, E. Goering, B. Baretzky, J. Appl. Phys., 108 (2010) 073923. [25] S. Ameen, M. S. Akhtar, H. K. Seo, H. S. Shin, Mater. Lett., 100 (2013) 261. [26] S. Ameen, M. S. Akhtar, H. K. Seo, H. S. Shin, Chem. Eng. J., 210 (2012) 220. [27] S. Ameen, M. S. Akhtar, H. K. Seo, H. S. Shin, Chem. Eng. J., 270 (2015) 564. [28] B. B. Straumal, A. A. Mazilkin, S. G. Protasova, S. V. Stakhanova, P. B. Straumal, M. F. Bulatov, G. Schutz, Th. Tietze, E. Goering, B. Baretzky, Rev. Adv. Mater. Sci., 41 (2015) 61. [29] B. B. Straumal, A. A. Mazilkin, S. G. Protasova, P. B. Straumal, A. A. Myatiev, G. Schütz, E. Goering, B. Baretzky, Phys. Metal Metall., 113 (2012) 1244. [30] S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, Electrochim. Acta, 56 (2011) 1111. [31] B. Li, Y. Wang, J. Phys. Chem. C., 114 (2010) 890. [32] T. Matsumoto, H. Kato, K. Miyamoto, M. Sano, E. A. Zhukov, T. Yao, Appl. Phys. Lett., 81 (2002) 1231. [33] A. B. Djurisic, Y. H. Leung, K. H. Tam, Y. F. Hsu, L. Ding, W. K. Ge, Y. C. Zhong, K. S. Wong, W. K. Chan, H. L. Tam, K. W. Cheah, W. M. Kwok, D. L. Phillips, Nanotechnology, 18 (2007) 095702. [34] G. J. Exarhos, S. K. Sharma, Thin Solid Films, 270 (1995) 27. [35] Warren W. L. Vanheusden, C. H. Seager, D. R. Tallant, J. A. Voigt, B. E. Gnade, J. Appl. Phys., 79 (1996) 7983.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 14

THE FACILE SYNTHESIS OF ZNO–GRAPHENE OXIDE NANOHYBRID AND ITS PHOTOCATALYTIC APPLICATION Sadia Ameen1, M. Shaheer Akhtar2, Hyung-Kee Seo1 and Hyung Shik Shin1,* 1

Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT A nanohybrid of Zinc oxide (ZnO)–graphene oxide (GO) is synthesized by chemical route and utilized as an effective photocatalyst for the photodegradation of crystal violet (Cv) dye. The substantially mixed ZnO–GO nanohybrid is formed by the introduction of GO during the synthesis of ZnO. The prepared ZnO–GO nanohybrid shows the better optical properties and exhibits the good interaction between GO and ZnO. The photocatalytic activity of ZnO–GO nanohybrid is analyzed by performing the photodegradation of Cv-dye under light illumination. Enormously high degradation of Cv-dye by ∼95% within 80min is observed over the surface of advanced ZnO–GO nanohybrid, which might attribute to the presence of GO sheets as a supporting material and high ē -h+ pair separation under light illumination.

*

Corresponding Author Email: [email protected].

200

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

INTRODUCTION Continuous discharge of synthetic dyes and pigments from industries severely disturb the aquatic ecosystem [1]. Reports have revealed that the synthetic dyes and pigments are annually produced worldwide, but ∼5–15% of the synthetic dyes are generally lost during the synthesis and processing [2–3]. Particularly, the triphenylmethane dye such as crystal violet (Cv) is being used for the applications of biological stains, dermatological agents, veterinary medicine and commercially for dyeing nylon, wool, waxes, ink for pens, temporary coloring of textiles and plastics [4–5]. Cv-dye in water causes coloration of water and destroys the aquatic life [6]. So far, approaches like biological decoloration [7], adsorption [8], advanced oxidation processes (AOPs) [9] and chemical oxidation methods [10] have been used to mineralize the harmful Cv-dye. Recently, the photocatalytic degradation has gained much popularity due to its ease of decomposition of harmful organic dye into less dangerous organics under light illumination [11]. Generally, the photocatalytic degradation requires the semiconductor nanomaterials with the band gap of ∼1–4 eV for the effective degradation of contaminants for the purification of water [12]. Zinc oxide (ZnO), n-type semiconductor shows promising photocatalytic activities due to its direct wide bang-gap of ∼3.37 eV, excellent chemical and mechanical stability, non-toxic, inexpensive and is highly photo sensitive [13–14]. Additionally, ZnO nanomaterials have presented a large number of applications in the electromechanical transducer materials for sensors, actuators in micro-electromechanical systems etc., due to their excellent dielectric, ferroelectric, piezoelectric, pyroelectric properties [14]. Particularly, ZnO is a promising material for spintronics as it possesses the ferromagnetic properties [15]. An emerging carbon nanomaterial called graphene is extensively explored in various applications such as photocatalysis, electrochemical, electronics and solar devices owing to its good conductivity, superior chemical stability, mechanical flexibility, high mobility of charge carriers (200,000 cm 2 V−1 s −1) and high specific surface area [16–17]. The composites/hybrids of metal oxide semiconductors or conducting polymers and graphene have shown good photocatalytic activity because it reduces the recombination of charge carriers on the surface of catalysts. Ameen et al. reported the synthesis of graphene-polyaniline nanocomposites for the effective photocatalytic degradation of rose bengal dye [18]. Further, Liu et al. prepared GO-CdS nanocomposites for the degradation of Rhodamine B dye under visible-light [19]. Particularly, the ZnO–GO composites exhibit the better photocatalysts for various photocatalytic reactions because the introduction of GO causes the structural changes such as lattice constants and band gap energy of ZnO [20–21]. In this chapter, the synthesis of ZnO–GO nanohybrid by simple chemical route and applied as efficient photocatalyst for the photodegradation of Cv-dye under light illumination. The

The Facile Synthesis of ZnO–Graphene Oxide …

201

synthesized ZnO–GO nanohybrid considerably degrades Cv-dye by ∼95% within 80 min under light illumination. The prepared ZnO–GO nanohybrid is applied as photocatalyst for the degradation of Cv-dye. The photocatlaytic experiment is performed under the light illumination of Xenon arc lamp as reported elsewhere [14]. The amount of Cv-dye degradation over the surface of ZnO–GO catalyst is evaluated by measuring the relative intensity of UV–vis spectroscopy. The degradation rate of Cv-dye over ZnO–GO nanohybrid is estimated by the following equation: Degradation rate (%) = (C0-C/C0) × 100 = (A0-A/A0) × 100 where C0 represents the initial concentration, C denotes variable concentration, Ao shows the initial absorbance, and A corresponds to variable absorbance.

Morphological and Crystalline Studies of ZnO–Graphene Oxide Nanohybrid The Field Emission Scanning Electron Microscopy, the Element Line Scanning and X-Rays Diffraction Patterns The morphological characterization of the synthesized ZnO–GO nanohybrid is analysed by field emission scanning electron microscopy (FESEM), as shown in Figure 1(a,b). Bare ZnO nanoparticles presents the average diameter of ∼20–40 nm. The addition of GO considerably increases the size of ZnO particles in ZnO–GO nanohybrid and achieves well mixed morphology of ZnO–GO nanohybrid. Moreover, the nanohybrid shows the visible GO sheets which are covered by the agglomerated ZnO nanoparticles (Figure 1(b)). Straumal et al. demonstrated that the defects like grain boundaries in ZnO nanograins significantly affect the optical and physical properties of nanograined ZnO [15]. In our case, the synthesized ZnO nanoparticles are nanograined and contain developed free surfaces as well as grain boundaries and interfaces. However, ZnO nanograin increases after the addition of GO which might create more free surfaces or grain boundaries and improve the surface interfaces. The large free surfaces or grain boundaries in ZnO–GO nanohybrid might provide the suitable surface for the fast photocatalytic reaction. The element line scanning image (Figure 1(c)) reveals the presence of C element which confirms the formation of well dispersed ZnO–GO nanohybrid. Moreover, the element profile also records the patterns of carbon, oxygen and zinc elements, confirming the formatting of ZnO-GO nanohybrid. Figure 1(d) shows the XRD patterns of bare ZnO and ZnO–GO nanohybrid. The diffraction peaks in ZnO and ZnO–GO nanohybrid are well indexed with JPCDS: 36–1451 of bulk ZnO, indicating the typical wurtzite hexagonal structure. The presence of amorphous

202

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

superficial and intergranular layers in ZnO and ZnO–GO nanohybrid (invisible in XRD results) has again confirmed the existence of grain boundaries in ZnO and ZnO–GO nanohybrid [23]. No other diffraction peaks related to any impurity is detected in the XRD pattern which confirms well-crystalline and the purity of as synthesized ZnO. Noticeably, the small diffraction peak at ∼20– 30o which is assigned to GO structure is obtained along with ZnO hexagonal structure in ZnO–GO nanohybrid.

Reprinted with permission from [S. Ameen, 2013], Mater. Lett., 100 (2013) 261 © 2013 Elsevier Ltd. Figure 1. FESEM images of (a) ZnO nanoparticles (b) ZnO–GO nanohybrid, (c) Line scanning elemental mapping with the corresponding pie bar graph of ZnO–GO nanohybrid and (d) XRD patterns of ZnO and ZnO–GO nanohybrid.

Structural and Optical Properties of ZnO–Graphene Oxide Nanohybrid The Raman Scattering and Photoluminescence Spectrum The Raman spectra of bare ZnO and ZnO–GO nanohybrid are shown in Figure 2(a). The bare ZnO and ZnO–GO exhibit a strong Raman peak at ∼437.6 cm−1 corresponds to E2 mode of ZnO crystal however, a small Raman peaks at ∼330.8 cm−1 is assigned to zone boundary phonons 3E2H–E2L for wurtzite hexagonal ZnO single crystals which is

The Facile Synthesis of ZnO–Graphene Oxide …

203

matched up with Raman peak of bulk ZnO crystals [24]. The appearance of Raman peaks at ∼1351.2 cm−1 and ∼1586.4 cm−1 in ZnO–GO nanohybrid are related to D and G bands of GO which present the breathing mode of κ-point phonons of A1g symmetry and the E2g phonon of sp2-C atoms, respectively [25]. The presence of GO Raman peaks along with ZnO Raman peaks suggests the interaction between ZnO and GO moieties. Furthermore, Figure 2(b) shows the room photoluminescence spectra of ZnO and ZnO– GO nanohybrid. A sharp UV emission at ∼383 nm and a broader green emission at ∼ 536 nm are recorded in ZnO and ZnO–GO nanohybrid which are attributed to free exciton emission from the wide band gap of ZnO and the recombination of electrons in single occupied oxygen vacancies in ZnO nanomaterials respectively [26]. It is interesting to note that the PL peak of GO is not visible in ZnO–GO nanohybrid which indicates the transfer of electron from ZnO to the outermost surface of GO due to quenching [27]. The PL peaks intensity at ∼383 nm has drastically decreased after the addition of GO, which again confirms the formation of ZnO–GO nanohybrid.

Reprinted with permission from [S. Ameen, 2013], Mater. Lett., 100 (2013) 261 © 2013 Elsevier Ltd. Figure 2. (a) Raman scattering and (b) PL spectra of ZnO and ZnO–GO nanohybrid.

The Photocatalytic Degradation of Crystal Violet (Cv) Dye Over the Surface of ZnO–Graphene Oxide Nanohybrid UV-Vis Absorbance Spectra of Decomposed Crystal Violet (Cv) Dye Solution Over ZnO–Graphene Oxide Nanohybrid Under Light Illumination The photocatalytic activity of ZnO–GO nanohybrid has been examined by performing the degradation of Cv-dye under UV illumination. Figure 3(a) shows UV–vis spectra of the decomposed Cv-dye with respect to light exposed time from 0 min to 80 min over the surface of ZnO–Gr nanohybrid photocatalyst. The maximum absorption

204

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

wavelength at ∼588 nm is obtained by Cv-dye. The intensity of Cv-dye absorbance regularly decreases with the increase of the exposed time, indicating the continuous decrease in Cv-dye concentration. Interestingly, ZnO–GO nanohybrid catalyst displays significantly high degradation of Cv-dye by ∼95% within very short exposed time of 80 min. From Figure 3(b), Cv-dye shows no significant degradation without ZnO–GO nanohybrid catalyst under UV illumination. However, the degradation rate of Cv-dye has continuously increased with the increase of the exposed time, suggesting the fast photodegradation of Cv-dye on the surface of ZnO–Gr nanohybrid. Additionally, the pie chart of Cv-dye degradation (Figure 3(c)) exhibits that most of Cv-dye degrades within 60 min over the surface of ZnO–GO nanohybrid and therefore, deduces the fast Cv-dye degradation under UV illumination. This phenomenon could be seen in the photographs of the decomposed Cv-dye solutions (Figure 3d). Compared to the initial color of Cv-dye, the color of Cv-dye becomes lighter with the increase of time duration from 0 min to 80 min. The fast Cv-dye degradation is explained by an illustration as shown in Figure 4(a). The exposure of ZnO–GO nanohybrid to light illumination causes the photoexcitation of ē from VB to CB of ZnO by the effective electronic interaction between GO and ZnO. This phenomenon initiates the ē–h+ pair charge separation in ZnO with the support of GO. Significantly, GO is an electron acceptor with 2D π-conjugation structure which effectively suppresses the recombination of photo-generated charge carriers [28]. The photoexcited ē in CB travels to the surface of ZnO–GO and produces the large amount of reactive oxyradicals such as superoxide radical ion O2●‾, and hydroxyl radical HOO●/●OH over the surface of ZnO–GO nanohybrid. These oxy radicals readily degrade the Cv-dye into less harmful minerals.

Mass Spectra of Crystal Violet (Cv) Solutions Before and After the Photocatalytic Reaction The mineralization of Cv-dye over the surface of ZnO–GO nanohybrid is studied by the mass spectra of Cv-dye (Figure 4(b)) before and after the photocatalytic reaction. Cvdye exhibits the strong mass signal at m/z = 372.2 at initial state which is similar to the formula mass of Cv-dye. The multiple mass signals along with m/z = 372.2 have seen after 10 min of photocatalytic reaction, indicating the beginning of Cv-dye degradation. After 80 min, the mass signal at m/z = 372.2 is vanished and the other small masses signals are recorded at m/z = 269, m/z = ∼255, m/ z = ∼241.2, m/z = ∼227, confirming the complete mineralization or decoloration of Cv-dye over the surface of ZnO–GO nanohybrid under light illumination. Thus, the ZnO–GO nanohybrid has significantly increased the adsorption of Cv molecules and photoinduced charge transfer along the GO sheet over

The Facile Synthesis of ZnO–Graphene Oxide …

205

the surface of the photocatalyst and results to the fast Cv-dye degradation under light illumination.

Reprinted with permission from [S. Ameen, 2013], Mater. Lett., 100 (2013) 261 © 2013 Elsevier Ltd. Figure 3. (a) UV–vis absorbance spectra of decomposed Cv-dye solution over ZnO–GO nanohybrid under light illumination, (b) the degradation rate (%) versus exposed time, (c) Cv-dye degradation pie chart as a function of exposed time and (d) the photographs of decomposed Cv-dye solution before and after the photocatalytic reaction.

Reprinted with permission from [S. Ameen, 2013], Mater. Lett., 100 (2013) 261 © 2013 Elsevier Ltd. Figure 4. (a) Schematic illustrations for photocatalytic degradation of Cv-dye over the surface of ZnO– GO nanohybrid and (b) mass spectra of Cv-dye solutions with the scan 200–400 m/z before and after the photocatalytic reaction.

206

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

CONCLUSION Highly effective photocatalyst of ZnO–GO nanohybrid is synthesized by a chemical route using GO and ZnO for the photodegradation of Cv-dye. The morphological properties depict the advanced ZnO–GO nanohybrid with well mixed morphology of ZnO and GO nanomaterials. The Raman and PL results demonstrate the better optical properties and exhibit the good interaction between GO and ZnO nanoparticles. The synthesized ZnO–GO nanohybrid shows the good photocatalytic activity towards the degradation of Cv-dye under light illumination. Enormously high degradation of Cv-dye by ∼95% within 80 min has been observed over the surface of advanced ZnO– GO nanohybrid. The fast Cv-dye degradation over ZnO–GO nanohybrid photocatalyst is attributed to GO as a supporting material which enhances the absorption capacity and suppresses the recombination of photo-generated charge carriers.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

S. Ameen, M. S. Akhtar, M. Nazim, H. S. Shin, Mater. Lett. 96 (2013) 228. F. A. Alshamsi, A. S. Albadwawi, M. M. Alnuaimi, M. A. Rauf, S. S. Ashraf, Dyes. Pigm. 74 (2007) 283. C. C. Chen, H. J. Fan, C. Y. Jang, J. L. Jan, H. D. Lin, C. S. J. Lu, Photochem. Photobiol A. 184 (2006) 147. C. C. Chen, W. C. Chen, M. R. Chiou, S. W. Chen, Y. Y. Chen, H. J. Fan, Hazard. Mater. 196 (2011) 420. K. P. Singh, S. Gupta, A. K. Singh, S. J. Sinha, Hazard. Mater. 186 (2011) 1462. M. M. Nassar, Y. H. Magdy, Chem. Eng. J. 66 (1997) 223. (a) S. M. Thomas, D. G. MacPhee, Mutat. Res. Lett. 140 (1984) 165; (b) Yesilada. O, Tur. J. Biol. 20 (1996) 129. (a) O. Gezici, M. Kucukosmanoglu, A. J. Ayar, Colloid. Interface. Sci. 304 (2006) 307; (b) L. S. Roselin, R. Selvin, Sci. Adv. Mater. 3 (2011) 251. (a) D. Tassalit, A. N. Laoufi, F. Bentahar, Sci. Adv. Mater. 3 (2011) 944; (b) I. Siminiceanu, C. I. Alexandru, E. Brillas, Environ. Eng. Manage. J. 7 (2008) 9. Y. M. Slokar, A. M. Le. Marechal, Dyes. Pigm. 37 (1998) 335. F. Chen, P. Fang, Y. Gao, Z. Liu, Y. Liu, Y. Dai, Chem. Eng. J. 204-206 (2012) 107. S. Ameen, H. K. Seo, M.S. Akhtar, H. S. Shin, Chem. Eng. J. 210 (2012) 220. (a) A. Umar, M. M. Rahman, Y. J. Hahn, Nanosci. Nanotechnol. 9 (2009) 4686; (b) J. Wu, D. Xue, Sci. Adv. Mater. 3 (2011) 127.

The Facile Synthesis of ZnO–Graphene Oxide …

207

[14] S. Ameen, M.S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, Colloid. Polym. Sci. 288 (2010) 1633. [15] B. B. Straumal, A. A. Mazilkin, S. G. Protasova, A. A. Myatiev, P. B. Straumal, E. Goering, B. Baretzky, Thin Solid Films. 520 (2011) 1192. [16] S. J. Park, R. S. Ruoff, Nat. Nanotechnol. 4 (2009) 217. [17] L. L. Zhang, R. Zhou, X. S. J. Zhao, Mater. Chem. 20 (2010) 5983. [18] S. Ameen, H. K. Seo, M. S. Akhtar, H. S. Shin, Chem. Eng. J. 210 (2012) 220. [19] F. Liu, X. Shao, J. Wang, S. Yang, H. Li, X. Meng, X. Liu, M. Wang, J. Alloys. Compd. 551 (2013) 327. [20] D. Fu, G. Han, Y. Chang, J. Dong, Mater. Chem. Phys. 132 (2012) 673. [21] Y. Liu, Y. Hu, M. Zhou, H. Qian, X. Hu, Appl. Catal. B: Environ. 125 (2012) 425. [22] S. Ameen, M. S. Akhtar, H. S. Shin, Sens. Actu. B: Chem. 173 (2012) 177. [23] B. Straumal, A. Mazilkin, S. Protasova, A. Myatiev, P. Straumal, E. Goering, B. Baretzky, Phys. Stat. Sol. B. 248 (2011) 1581. [24] C. Roy, S. Byrne, E. McGlynn, J. P. Mosnier, E. de. Posada, D. O. Mahony, J. G. Lunney, M. O. Henry, B. Ryan, A. A. Cafolla, Thin Solid Films. 436 (2003) 273. [25] M. Guo, P. Diao, X. Wang, S. J. Cai, Solid State Chem. 178 (2005) 3210. [26] P. K. Samanta, P. R. Chaudhuri, Sci. Adv. Mater. 3 (2011) 919. [27] (a) Y. Zhang, H. Li, L. Pan, T. Lu, Z. J. Sun, Electroanal. Chem. 634 (2009) 68; (b) F. Vietmeyer, B. Seger, P. V. Kamat, Adv. Mater. 19 (2007) 2935. [28] Z. Liu, Q. Liu, Y. Huang, Y. Ma, S. Yin, X. Zhang, W. Sun, Y. Chen, Adv. Mater. 20 (2008) 3924.

SECTION 2. TITANIUM OXIDE (TIO2) BASED PHOTOCATALYSTS

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 15

VISIBLE LIGHT DRIVEN PHOTOCATALYTIC DEGRADATION OF BROMOPHENOL DYE OVER CEO2/TIO2 NANOCOMPOSITE Sadia Ameen1, M. Shaheer Akhtar2, Hyung-Kee Seo1 and Hyung Shik Shin1,* 1

Energy Materials and Surface Science Laboratory, Solar Energy Research Center,School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New and Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT A solution-processed cerium oxide (CeO2)-titanium oxide (TiO2) nanocomposite is utilized as an effective visible-light driven photocatalyst for the photodegradation of bromophenol (Bph) dye. The structural and the morphological properties reveal the good interaction between CeO2 and TiO2 nanomaterials in CeO2-TiO2 nanocomposite. The CeO2-TiO2 nanocomposite shows the enhanced optical properties with a significant red shift after the addition of CeO2 nanoparticles. The CeO2-TiO2 nanocomposite as photocatalyst accomplishes enormously high degradation of Bph-dye by ~72% within 3 h under visible-light illumination. The improved degradation might attribute to the higher adsorption capacity and the better ē-h+ pair separation under light illumination.

*

Corresponding Author Email: [email protected]

212

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

INTRODUCTION The effluent of coloring and textile industry causes the severe damage to the aquatic ecosystem and human being [1, 2]. These colors and dyes are non-biodegradable organic compounds. Among brominated dyes, bromophenol (Bph) dye is usual effluent of the textile and chemical industries. Bph dye is commonly used as a color marker to monitor the process of agarose gel electrophoresis and as an acid-base indicator [3]. The excess of Bph dye contaminates the soil and fresh water due to its high water solubility [4]. So far, several remediation or treatment technologies such as chemical precipitation/separation of pollutants, coagulation by a chemical agent, ozone oxidation, hypochlorite oxidation, electrochemical method and elimination by adsorption have been used to degrade the organic dyes into less harmful chemicals [5-7]. A remediation process called photocatalytic process has received a great deal of attention due to its ease of degrading harmful organic dyes in the presence of semiconducting materials as catalyst [8]. Titanium dioxide (TiO2) is well known and important n-type semiconducting material for the photocatalytic degradation of organic pollutants due to its high chemical and thermal stability [9, 10]. The low quantum efficiency and broad band gap of TiO2 material limits its photocatalytic applications in visible light illumination [11]. The transition metal or nonmetal doping [12, 13], co-deposition of metals [14], and dye sensitization [15], have already been executed to improve the band gap and the photocatalytic properties of TiO2 under the visible illumination. The combinations of TiO2 with other metal oxides such as SnO2-TiO2, ZnO-TiO2, Bi2O3-TiO2, and CeO2-TiO2 have recently shown the higher photocatalytic degradation rate of harmful organic chemicals [16-20]. The nanocomposites of TiO2 with cerium oxide (CeO2) have received a great deal of interest because of special f and d electron orbital structures and the unique UV absorbing ability, high thermal stability, high electrical conductivity and large oxygen storage capacity of CeO2 which significantly improves the photocatalytic efficiency of TiO2 [19, 21]. Ghasemia et al. reported the preparation of TiO2-CeO2 nanoparticles on graphene nanosheets for the photocatalytic degradation of Reactive Red 195 and 2,4-Dichlorophenoxyacetic acid [22]. Li et al. synthesized thermally stable mesoporous ZrO2-CeO2-TiO2 nanocomposite and demonstrated the photodegradation of rhodamine B dye by ~90% within 160 min under visible light [23]. In this work, CeO2TiO2 nanocomposite is simply synthesized by a facile solution process and successfully applied as efficient photocatalyst for the photodegradation of Bph-dye under visible-light illumination. The photocatalytic results reveal that the synthesized CeO2-TiO2 nanocomposite has considerably degraded Bph-dye by ~72% within 3 h under visiblelight illumination. The synthesized CeO2-TiO2 nanocomposite is utilized as photocatalyst for the degradation of Bph-dye. The photocatalytic experiment is performed under the visible light illumination using Xenon arc lamp with cut filter (400 nm) as reported elsewhere

Visible Light Driven Photocatalytic Degradation of Bromophenol Dye ...

213

[24]. The degradation rate of Bph-dye over the surface of CeO2-TiO2 photocatalyst is evaluated by measuring the relative intensity of UV-Vis spectroscopy. The degradation rate is usually calculated by the following equation: Degradation rate (%) = (Co - C/Co) × 100 = (Ao - A/Ao) × 100 where Co represents the initial concentration, C denotes variable concentration, Ao shows the initial absorbance, and A corresponds to variable absorbance.

MORPHOLOGICAL STUDIES OF CEO2-TIO2 NANOCOMPOSITE The Field Emission Scanning Electron and Transmission Electron Microscopy The field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) are used to investigate the morphological characterizations of the synthesized CeO2-TiO2 nanocomposite.

Reprinted with permission from [S. Ameen, 2014], Chem. Eng. J., 247 (2014) 193©2014 Elsevier Ltd. Figure 1. FESEM image of (a) TiO2 nanoparticles, (b) low and high (c) magnifications of CeO2-TiO2 nanocomposite and (d) TEM image of CeO2-TiO2 nanocomposite.

Figure 1(a) shows bare TiO2 nanoparticles of spherical morphology with the average particle size of ~20 nm. CeO2-TiO2 nanocomposite (Figure 1(b)) displays two distinctive

214

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

morphologies of large CeO2 particles and small TiO2 particles. The large CeO2 particles are uniformly embedded into TiO2 nanoparticles, indicating well mixing of CeO2 into TiO2 nanoparticles. At high resolution FESEM image (Figure 1(c)), the embedded large hexagonal CeO2 nanoparticles along with small TiO2 nanoparticles are visibly seen. The TEM image is shown in Figure 1(d) which exhibits the similar morphology as seen in FESEM images.

CRYSTALLINE AND OPTICAL PROPERTIES OF CEO2-TIO2 NANOCOMPOSITE X-Rays Diffraction Patterns and UV-Vis Absorption Spectrum Figure 2(a) shows the X-Rays diffraction patterns (XRD) of synthesized CeO2-TiO2 nanocomposite. The diffraction peaks at 28.3o, 32.8o, 47.2o, 56.1 o and 69.3o are assigned to the cubic fluorite structure of CeO2 [25, 26]. The diffraction peaks at ~25.3o corresponds to TiO2 phase in CeO2-TiO2 nanocomposite. Figure 2(b) shows the ultraviolet-diffused reflectance spectroscopy (UV-DRS) of bare TiO2 and CeO2-TiO2 nanocomposite. The characteristic absorption band at ~390 nm corresponds to O2→Ti4+ charge transfer and related to electron excitation from valence band to the conduction band in TiO2 [27]. The synthesized CeO2-TiO2 nanocomposite presents red shift to higher wavelength at ~465 nm, indicating the incorporation of Ce cations into the lattice of TiO2. Further, the band gap (Eg) value of ~2.67 eV for CeO2-TiO2 nanocomposite is lower than bare TiO2 (Eg = ~3.18 eV) which again confirms the incorporation of CeO2 into TiO2 nanoparticles. The lowering in Eg value of CeO2-TiO2 nanocomposite is an indication of the red shifting from UV to the visible region due to the substitution of Ti4+ cations by Ce4+ cations in TiO2 network as well as by Ti4+ titanium deficiency created per unit cell [V+Ti4+] [28].

THE PHOTOCATALYTIC ACTIVITY OF CEO2-TIO2 NANOCOMPOSITE BY THE DEGRADATION OF BROMOPHENOL DYE UV-Vis Absorbance Spectra of Decomposed Bromophenol Dye Solution over CeO2-TiO2 Nanocomposite under Light Illumination The photocatalytic activity of CeO2-TiO2 nanocomposite is examined by the degradation of Bph-dye under visible-light illumination. Figure 3(a) shows UV-Vis absorbance of decomposed Bph-dye with the exposed time of 0-180 min over the surface

Visible Light Driven Photocatalytic Degradation of Bromophenol Dye ...

215

of CeO2-TiO2 nanocomposite photocatalyst. At initial, Bph-dye exhibits the maximum absorption wavelength at ~590 nm.

Reprinted with permission from [S. Ameen, 2014], Chem. Eng. J., 247 (2014) 193©2014 Elsevier Ltd. Figure 2. XRD patterns (a) and UV-Vis spectra (b) of CeO2-TiO2 nanocomposite.

Reprinted with permission from [S. Ameen, 2014], Chem. Eng. J., 247 (2014) 193©2014 Elsevier Ltd. Figure 3. (a) UV-Vis absorbance spectra of decomposed Bph dye solution over CeO2-TiO2 nanocomposite under light illumination, (b) the degradation rate (%) over CeO2-TiO2 nanocomposite and TiO2 versus time interval and (c) pie chart for Bph dye degradation as a function of time.

The absorbance intensity of Bph-dye gradually decreases with the increase of exposed time from 0 to 180 min, indicating the drastic decrease in the concentration of Bph-dye. A reasonably high degradation rate by ~72% of Bph-dye within 3 h is detected

216

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

over the surface of CeO2-TiO2 nanocomposite catalyst whereas, very low degradation rate (6%) is obtained when Bph-dye degradation takes place over the surface of bare TiO2 (P25) catalyst under visible light illumination. From Figure 3(b), the increase in exposed time has continuously decreased the concentration of Bph-dye, suggesting the fast photodegradation of Bph-dye on the surface of CeO2-TiO2 nanocomposite. The pie chart of degraded Bph-dye is presented in Figure 3(c), which displays that the major Bph-dye has been degraded within first 100 min. Thus, the synthesized CeO2-TiO2 nanocomposite could be a good visible-light driven photocatalyst for the degradation of Bph-dye.

Mass Spectra of Bromophenol Dye Solutions over CeO2-TiO2 Nanocomposite In order to investigate the intermediates and mineralization of Bph-dye during the photocatalytic degradation, the mass spectroscopy has been used, as presented in Figure 4(a).

Reprinted with permission from [S. Ameen, 2014], Chem. Eng. J., 247 (2014) 193©2014 Elsevier Ltd. Figure 4. (a) Mass spectra of Bph dye solutions over CeO 2-TiO2 nanocomposite with the scan 100-800 m/z and (b) the possible reaction intermediates after the photocatalytic reaction.

Visible Light Driven Photocatalytic Degradation of Bromophenol Dye ...

217

The initial Bph-dye displays the strong mass signal at m/z = 669.9, corresponding to the formula weight of Bph-dye. After 20 min of the photocatalytic reaction, the main mass signal at m/z = 669.9 splits into multiple mass signals along with the main signal, which suggests an indication of Bph-dye degradation under light illumination. The multiple small mass signals are recorded at m/z = 684.5, m/z = 439.4, m/z = 271.2, m/z = 159.2, m/z = 127.6 and m/z = 111.2 along with small main mass signals, establishes the mineralization or degradation of Bph-dye over the surface of CeO2-TiO2 nanocomposite as catalyst under light illumination. The formation of possible intermediates is illustrated in Figure 4(b). The intermediates of Bph-dye clearly reveal that the multiple fragmentation of Bph-dye (macromolecule) might lead the complete mineralization with the ending products of CO2 and H2O.

Reprinted with permission from [S. Ameen, 2014], Chem. Eng. J., 247 (2014) 193©2014 Elsevier Ltd. Figure 5. The schematic illustration of the photocatalytic activity of CeO2-TiO2 nanocomposite.

The Schematic Illustration of the Photocatalytic Activity of CeO2-TiO2 Nanocomposite The mechanism of Bph-dye degradation is illustrated in Figure 5. Upon light illumination, CeO2 firstly absorbs light and the photoexcited electron moves to the conduction band (CB) of CeO2 where CB level is higher than the CB level of TiO2 nanoparticles. The photoexcited electrons inject into CB of TiO2 which easily scavenges the surface oxygen to produce the large amount of reactive oxyradicals such as superoxide radical ion O2●-, and hydroxyl radical HOO●/●OH over the surface of CeO2TiO2 nanocomposite. These oxyradicals react with Bph-dye molecules and get converted into CO2/H2O by passing through the number of intermediates. On the other hand, the

218

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

photoexcitation leaves photoinduced holes at VB of CeO2 which are easily trapped by the electronic donors and produces the active species such as OH● by the reaction of either H2O or OH- [29]. Herein, the synthesized CeO2-TiO2 nanocomposite catalyst might produce large number of oxygenated species on the surface of catalyst under lightillumination which might significantly initiate the photodegradation of Bph-dye into less harmful minerals.

CONCLUSION The effective visible-light driven photocatalyst of CeO2-TiO2 nanocomposite is synthesized by the facile chemical route for the photodegradation of Bph-dye. The interacted and well mixed CeO2 and TiO2 nanoparticles are visibly seen by the morphological properties. The crystalline studies reveal the formation of well-crystalline CeO2-TiO2 nanocomposite. CeO2-TiO2 nanocomposite shows the enhanced optical properties with significant red shift after the addition of CeO2. The CeO2-TiO2 nanocomposite as photocatalyst presents enormously high degradation of Bph-dye by ~72% within 3 h under the visible-light illumination. The reasonable Bph-dye degradation might attribute to the higher adsorption capacity and the better ē-h+ pair separation under light illumination. The mass result successfully demonstrates the formation of intermediates during the photodegradation of Bph-dye into less harmful chemical under light illumination.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]

Roselin L. S., Selvin R., Sci. Adv. Mater. 3 (2011) 113. Ameen S., Seo H. K., Akhtar M. S., Shin H. S., Chem. Eng. J. 210 (2012) 220. Bouanimba N., Zouaghi R., Laid N., Sehili T., Desalination. 275 (2011) 224. Gao B., Liu L., Liu J., Yang F., Appl. Catal. B: Environ. 129 (2013) 89. Ameen S., Akhtar M. S., Seo H. K., Shin H. S., Mater. Lett. 100 (2013) 261. Cao S., Yeung K. L., Yue P. L., Appl. Catal. B: Environ. 68 (2006) 99. Cao S., Yeung K. L., Kwan K. C., et al., Appl. Catal. B: Environ. 86 (2009) 127. Ameen S., Akhtar M. S., Kim Y. S., Shin H. S., Appl. Catal. B: Environ. 103 (2011) 136. [9] Fujishima A., Hashimoto K., Watanabe T., TiO2 photocatalysis: Fundamentals and Applications, BKC Inc., Tokyo, 1999. [10] Dwivedi C., Raje N., Nuwad J., Kumar M., Bajaj P. N., Chem. Eng. J. 193-194 (2012) 178.

Visible Light Driven Photocatalytic Degradation of Bromophenol Dye ...

219

[11] Yanqing Z., Erwei S., Zhizhan C., Wenjun L., Xingfang H., J. Mater. Chem. 11 (2001) 1547. [12] Yamashita H., Harada H., Misaka J., Takeushi M., Ikeue K., Anpo M., J. Photochem. Photobiol. A: Chem. 148 (2002) 257. [13] Yu J. C., Zhang L., Zheng Z., Zhao J., Chem. Mater. 15 (2003) 2280. [14] Wang W., Zhang J., Chen F., He D., Anpo M., J. Colloid Interface Sci. 323 (2008) 182. [15] Chen F., Zou W., Qu W., Zhang J., Catal. Commun. 10 (2009) 1510. [16] Bessekhouad Y., Robert D., Weber J. V., Catal. Today. 101 (2005) 315. [17] Ohsaki H., Kanai N., Fukunaga Y., Suzuki M., Watanabe T., Hashimoto K., Thin Solid Films. 502 (2006) 138. [18] Kim D. W., Lee S., Jung H. S., Kim J. Y., Shin H., Hong K. S., J. Hydrogen Energy. 32 (2007) 3137. [19] Li G., Zhang D., Yu J. C., Phys. Chem. Chem. Phys. 11 (2009) 3775. [20] Dar G. N., Umar A., Zaidi S. A., Ibrahim A. A., Abaker M., Baskoutas S., Al-Assiri M. S., Sens. Actuators B - Chem. 173 (2012) 72. [21] Goharshadi E. K., Samiee S., Nancarrow P., J. Colloid Interface Sci. 356 (2011) 473. [22] Li M., Zhang S., Lv L., Wang M., Zhang W., Pan B., Chem. Eng. J. 229 (2013) 118. [23] Ameen S., Akhtar M. S., Kim Y. S., Yang O. B., Shin H. S., Colloid Polym. Sci. 288 (2010) 1633. [24] Kingondu C. K., Opembe N. N., Genuino H. C., Garces H. F., Njagi E. C., Iyer A., Huang H., Dharmarathna S., Suib S. L., J. Phys. Chem. C. 115 (2011) 23273. [25] Bonelli R., Albonetti S., Morandi V., Ortolani L., Riccobene P. M., Scire S., Zacchini S., Appl. Catal. A. 395 (2011) 10. [26] Ghasemi S., Rahimnejad S., Setayesh S. R., Rohani S., Gholami M. R., J. Hazard. Mater. 172 (2009) 1573. [27] Galindo F., Gomez R., Aguilar M., J. Mol. Catal A: Chem. 281 (2008) 119. [28] Ameen S., Akhtar M. S., Nazim M., Shin H. S., Mater. Lett. 96 (2013) 228.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 16

THE EFFECT OF FE DOPING ON TIO2 NANOPARTICLES FOR THE PHOTOCATALYTIC DEGRADATION OF TOXIC ORGANIC COMPOUNDS Swati Sood1, Ahmad Umar2,3,*, Surinder Kumar Mehta1 and Sushil Kumar Kansal4 1

Department of Chemistry, Panjab University, Chandigarh, India 2 Department of Chemistry, College of Science and Arts, Najran University, Najran, Saudi Arabia 3 Promising Centre for Sensors and Electronic Devices (PCSED), Najran University, Najran, Saudi Arabia 4 Dr. S.S.B University Institute of Chemical Engineering and Technology, Panjab University, Chandigarh, India

ABSTRACT In this work, various molar concentrations of iron (Fe)-doped TiO2 nanoparticles are used as potential photocatalysts for photocatalytic degradation of toxic and harmful chemical, para-nitrophenol. The nanoparticles are synthesized by a novel and facile ultrasonic assisted hydrothermal method and characterized in detail by various analytical techniques in terms of their morphological, structural, compositional, thermal, optical, pore size distribution, etc. properties. The photocatalytic activities of the as-prepared Fe-doped TiO2 nanoparticles are examined under visible light illumination using paranitrophenol as target pollutant. By detailed experimental findings, the Fe dopant content crucially determines the catalytic activity of TiO2 nanoparticles. The maximum degradation rate of para-nitrophenol observes 92% in 5 h when the Fe3+ molar concentration is 0.05 mol%, without addition of any oxidizing reagents. The prepared *

Corresponding Author Email: [email protected].

222

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al. nanoparticles demonstrate excellent photocatalytic response because of their small size, excellent crystalline structure, increase in threshold wavelength response and maximum separation of photogenerated charge carriers. Further, the determination of reaction intermediates has also been carried out and proposes a plausible mechanism for the photocatalytic degradation of para-nitrophenol.

INTRODUCTION Nitrophenols are the one among the highly persistent organic contaminants found in industrial effluents, agricultural and urban wastewater, which are extensively used in production of various drugs, chemicals, pesticides, dyes and pigments [1–5]. Nitrophenols impose dreadful health hazards to living organisms like cancer and disorders in various vital organs, blood and the central nervous system. Substituted phenols such as para-nitrophenol, ortho-nitrophenol, and dinitrophenols have been enlisted as “Priority Pollutants” by the US E.P.A. [6–8]. Dyes constitute another class of harmful and visibly recognized pollutants present in wastewaters. Various industries such as textile, printing, pulp and paper, food, leather tanning, hair coloring and photographic industries are responsible for their massive usage and indiscriminate disposal into the water bodies [9–13]. It thus becomes a matter of prime concern to completely degrade these organic pollutants before discharging them into the environment. Nowadays, one of the most advanced and globally accepted techniques is the photocatalysed oxidation processes using semiconducting metal oxide nanoparticles like TiO2 which has relatively wide band gap of 3.2 eV. Thus, TiO2 is photoactive only under ultraviolet (UV) light region of the solar spectrum. While, the major portion of the freely available solar energy comprises of visible light [13–16]. Because of the massive energy crisis, fabrication of new visible light responsive nanomaterials for the photocatalytic decomposition of organic pollutants is attaining a major concern. One of the strategies is to introduce a metal ion dopant into the matrix of photocatalyst, which yields following benefits: (a) Inhibits recombination of photogenerated ē/h+ pairs (b) increases the threshold wavelength response range into visible region. Amongst various metals, Fe is considered to be a suitable transition metal candidate as dopant because: (i) the ionic radius of Fe3+ (0.69 Å) is nearly same as that of Ti4+ (0.745 Å) as a result, Fe3+ can be conveniently integrated into TiO2 matrix; (ii) its stable half-filled d5 configuration and (iii) Furthermore, Fe3+ act as charge carrier trap and inhibits the recombination of photogenerated electron–hole pair, and logically adds to enhanced photoactivity. But, the role of Fe3+ ions in TiO2 photocatalysis is somewhat controversial. In literature, many authors have reported enhancement in photocatalytic activity upon introduction of Fe dopant [17–23]. And, there also have been few reports citing the detrimental behavior of Fe ions in photocatalysis, as they behave as charge carrier traps and promote electron–

The Effect of Fe Doping on TiO2 Nanoparticles …

223

hole recombination [23–25]. So, it is interesting to study the effect of Fe3+ ions on the catalytic behavior of TiO2. Till now, TiO2-based photocatalysts have been synthesized globally by a number of techniques, especially the solvothermal processes because these chemical solution synthesis routes offer certain advantages [26–28]. Uniform doping of metal ions into the lattices of metal oxide can be achieved with high dispersion ability. Additionally, by monitoring experimental conditions, one can easily synthesize nano-architectured materials with variable sizes and tunable properties. Since last decade, significant research has been dedicated to the heterogeneous semiconductor photocatalytic technology for degrading 4-nitrophenol (4-NP) with Fenton’s, UV with H2O2, UV with Photo-Fenton reagents and UV, TiO2 and H2O2 [29–36]. But, these suffer through some shortcomings like incomplete removal of the pollutant and addition of chemicals further enhances the problem of environmental pollution. In this chapter, Fe (III) doped TiO2 photocatalysts have been synthesized by a hydrothermal method assisted by ultrasonic radiations. The synthesized nanomaterials have been further characterized using various techniques to study their crystalline phase, morphology, surface area, composition, optical and luminescent properties. The photocatalytic activity of the catalysts and the effect of dopant concentration on photodegradation of 4-NP and methylene blue (MB) dye under visible light have been investigated. The intermediates produced during the course of degradation and the plausible mechanism have also been studied. The photocatalytic experiments are performed in a specially designed double walled batch photo reactor. The experimental procedure for the photodegradation is similar to our previous reports [37]. The catalytic activity of all photocatalysts is studied for degradation of 4-NP (10 mg/L aqueous solution, pH 4) and MB (10 mg/L aqueous solution, pH 7) with catalyst loading of 0.5 g/L. The pH of the solution is adjusted using 0.1 N HCl (or) 0.1 N NaOH. In order to attain adsorption–desorption equilibrium, the reaction mixture is magnetically stirred in dark for half hour. A 150 W Philips CFL bulb is used as a visible light source (the average light intensity was 1475 lux, wavelength range of 400 nm–520 nm). At certain intervals, the aliquot is drawn and filtered through a 0.45 μm Millipore filter. Obtained filtrates are then analyzed by studying the maximum absorbance values at different time intervals.

CRYSTALLINE PROPERTIES OF FE-DOPED TIO2 NANOPARTICLES X-Rays Diffraction Patterns The XRD patterns of prepared TiO2 and Fe doped TiO2 nano-powders at different doping concentrations are shown in Figure 1. XRD studies show that the TiO2 and Fe

224

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al.

(III) doped catalysts annealed at a temperature of 450°C have both anatase and rutile structures with a dominance of anatase structure. In Figure 1(a), the 2θ peaks observed at 25.61°, 38.10°, 48.47°, 54.24°, 55.36°, 62.99°, 69.20°, 70.59°, 75.47° in the XRD pattern of TiO2 nanopowders are consistent with anatase (1 0 1), (1 0 3), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), (2 2 0) and (1 0 7) lattice planes (JCPDS No. 21-1272).

Reprinted with permission from [A. Umar, 2015], J. Colloid Inter. Sci., 450(2015) 213 © 2015 Elsevier Ltd. Figure 1. XRD pattern of (a) synthesized TiO2 nanoparticles; (b) different Fe doped x% TiO2 nanoparticles.

The diffraction peaks corresponding to rutile phase also appeared at 2θ = 27.5° and 41.54° corresponding to (1 2 1) and (1 1 1) planes (JCPDS No. 21-1276). The peaks for pristine and doped TiO2 appear similar, but in case of Fe doped TiO2 samples, there was some noticeable reduction in peak intensity. This suggests that some perturbation takes place in anatase structure after the introduction of Fe (III) ions [Figure 1(b)] [25, 38–40]. The diffractograms of all the samples do not show any diffraction peaks of iron or iron compounds, which shows that there is dispersion of metal ions on TiO2, which can be

The Effect of Fe Doping on TiO2 Nanoparticles …

225

attributed to very low dopant amount in these samples or replacement of Ti (IV) ions by Fe (III) ions into TiO2 matrix. For the coordination number 6, the ionic radius of Ti4+ is 0.745 Å and that of Fe3+ is 0.69 Å, it is easily assumed that Ti4+ is substituted by Fe3+ [41–43]. Because of low iron content no Fe2O3 phase was formed [25–27]. Scherrer formula was used to find the crystallite size of the synthesized products and was found to be ∼25 nm. These observations show that Fe3+ ions have been successfully introduced into TiO2.

Morphological Studies of Fe-doped TiO2 Nanoparticles The Field Emission Scanning Electron Microscopy The detailed morphological characterization of Fe doped TiO2 were examined by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM).

Reprinted with permission from [A. Umar, 2015], J. Colloid Inter. Sci., 450(2015) 213 © 2015 Elsevier Ltd. Figure 2. Typical FESEM images of (a) and (b) 0.025 FeT; (c) and (d) 0.05 FeT; (e) and (f) 0.075 FeT; (g) and (h) 0.1 FeT samples at different magnifications.

226

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al.

It can be seen from FESEM images (Figure 2) that the prepared nanoparticles are of about 20 nm in size, possess spherical shapes and are grown in high-density. Slight agglomeration was also seen.

The Transmission Electron Microscopy A detailed TEM analysis was carried out to obtain more information about the nature of Fe ions doped on TiO2 matrix and it was observed as shown in Figure 3 that the crystals included both TiO2 (bigger) and Fe dopant (smaller) particle. The grey particles in TEM images are TiO2 nanoparticles and the dark particles are Fe dopants on the grey surface of TiO2 particles [28, 42]. Nearly spherical shaped particles ranging from ∼10 to 20 nm are observed. The particle sizes obtained from FESEM and TEM analysis are nearly in agreement with each other.

Reprinted with permission from [A. Umar, 2015], J. Colloid Inter. Sci., 450(2015) 213 © 2015 Elsevier Ltd. Figure 3. Typical TEM images of as-prepared (a) and (b) 0.05 FeT and (c) and (d) 0.075 FeT nanoparticles.

The Effect of Fe Doping on TiO2 Nanoparticles …

227

Structural Properties of Fe-doped TiO2 Nanoparticles Fourier Transform Infrared Spectroscopy FTIR spectra were also recorded between the wave number of 400 and 4000 cm−1 for all the samples. The FTIR spectrum of all the FeT samples is shown in Figure 4 for which nearly similar patterns were observed. Various well-defined peaks were seen. The peak at 3330 cm−1 is assigned to the absorption of surface OH bonds. The peak at 1640 cm−1 corresponds to H O–H bending vibration of adsorbed water [11, 43]. It was deduced that, the introduction of Fe (III) ions into TiO2 matrix results in such changes that lead to the absorption of more amounts of OH groups. This is further an added advantage in the photocatalytic properties of the catalyst. Ti–O stretching peaks are also observed around 400–500 cm−1 [11].

Reprinted with permission from [A. Umar, 2015], J. Colloid Inter. Sci., 450(2015) 213 © 2015 Elsevier Ltd. Figure 4. Typical FTIR spectra of as-prepared different Fe doped x% TiO2 nanoparticles.

Thermogravimetric and Differential Scanning Calorimetry of Fe-Doped TiO2 Nanoparticles Thermal analysis of the doped TiO2 catalyst was performed. Figure 5 shows the TGA and DSC curves for the 0.05 FeT catalyst. It is seen that catalyst possesses excellent thermal stability and high purity. The final weight loss (up to a temperature of 1000°C) of the sample is only about 3%. The weight loss of about 1% seen till 100˚C is because of loss of adsorbed water on the surface of the catalyst. Further, the region of weight loss from 200 to 600°C can be observed because of decomposition of organic solvents, also

228

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al.

reflected in the DSC curve’s exothermic peaks localized at mainly 360°C. After 600°C, no major weight loss is seen, which confirms the stability and purity of prepared catalyst.

Reprinted with permission from [A. Umar, 2015], J. Colloid Inter. Sci., 450(2015) 213 © 2015 Elsevier Ltd. Figure 5. TGA/DSC curve of as-prepared 0.05 mol% Fe doped TiO2 nanoparticles.

Optical Studies of Fe-doped TiO2 Nanoparticles The UV–vis Absorption Spectrum In order to study the optical absorption characteristics of the prepared samples, UV– vis diffuse reflectance spectroscopy was performed. Figure 6 shows the UV–vis DRS spectra of TiO2 and FeT samples. It was observed that absorption was significantly enhanced with increasing content of Fe dopant. The introduction of dopant (Fe 3+ ions) into TiO2 matrix, leads to enhancement in absorption of light, extending the spectral response of photocatalyst to 426 nm. By increasing the Fe content, the Ebg are hence, decreased. The Ebg of 0.1 FeT catalyst is 2.9 eV, which is lower than that of bare TiO2 catalyst prepared by this method (3.2 eV). The changes in color of the samples observed, from white (TiO2), cream (0.025 FeT, 0.05 FeT) to crème yellow (0.1 FeT) can be attributed to the increase in visible light absorption upon increasing the Fe (III) dopant concentration. It has been suggested that this visible light absorption can arise because of: (i) the formation of a Fe3+/Fe4+ dopant energy level within the band gap of TiO2. (ii) d–d transition of Fe3+ (2T2g → 2A2g, 2T1g) or the charge transfer (CT) transition between the Fe ions [26, 27].

The Effect of Fe Doping on TiO2 Nanoparticles …

229

Reprinted with permission from [A. Umar, 2015], J. Colloid Inter. Sci., 450(2015) 213 © 2015 Elsevier Ltd. Figure 6. Typical UV–vis spectra of as-prepared different Fe doped x% TiO2 nanoparticles.

Photoluminescence Spectrum To investigate the role played by Fe3+ ions on fate of charge carriers, room temperature PL was recorded for all the samples from 320 to 550 nm wavelength range at an excitation wavelength of 300 nm at a PMT operating voltage of 600 V.

Reprinted with permission from [A. Umar, 2015], J. Colloid Inter. Sci., 450(2015) 213 © 2015 Elsevier Ltd. Figure 7. Typical room-temperature photoluminescence spectra of as-prepared different Fe doped x% TiO2 nanoparticles.

It has been reported that, bare TiO2 nanoparticles exhibit the inter-band recombination emission peak at around 360 nm [44–46]. As compared to sole TiO2, Fe

230

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al.

doped samples revealed a decrease in the PL intensity, which indicates that they inhibit the recombination of photo electron and whole pairs (Figure 7). It can also be seen that 0.075 and 0.050 mol% Fe samples show a maximum decrease in intensity which is because of optimum dopant amount which acts as trap for photo-electron and hole pairs, thereby improving the separation of charge carriers [17–23]. But at higher concentration of 0.1% molar Fe (III) ions, the emission spectrum is different. At higher concentrations Fe ions act as recombination centers instead [24, 25]. Therefore, leading to an expected increase in intensity as compared to other Fe doped TiO2 samples of lesser dopant concentration.

XPS Spectra of Fe-doped TiO2 Nanoparticles X-ray photoelectron spectroscopy (XPS) analysis was also performed in order to investigate its electronic environment and oxidation state. Figure 8(a) shows the XPS survey spectrum of 0.05 FeT and Figure 8(b)–(d) depicts core level spectra of characteristic elements. From Figure 8(a) it is clear that 0.05 FeT sample contains Ti, O and Fe. The peak for C 1s at binding energy of 284.8 eV was observed due to the adventitious hydrocarbon from XPS instrument.

Reprinted with permission from [A. Umar, 2015], J. Colloid Inter. Sci., 450(2015) 213 © 2015 Elsevier Ltd. Figure 8. (a) XPS spectrum of as-prepared 0.05 mol% Fe doped TiO2 nanoparticles; (b) core level spectra of characteristic Ti 2p; (c) Fe 2p; and (d) O 1s.

The Effect of Fe Doping on TiO2 Nanoparticles …

231

Figure 8(b) shows the Ti 2p binding energy region. The Ti 2p3/2 and Ti 2p1/2 spin– orbital splitting photoelectrons for the sample is located at binding energies of 458.7 eV and 464.3 eV, respectively. These values are all in good agreement with values of Ti4+ [28, 47]. Fe 2p spectrum shown in Figure 8(c) depicts peak at 707.6 and 722 eV, which belong to the binding energy of Fe 2p3/2 and Fe 2p1/2 respectively [23, 28]. This suggests that dopant of Fe in TiO2 lattice shows a chemical state of Fe3+. In the O (1s) spectrum (Figure 8(d)) peak seen at binding energy 529.9 eV corresponds to the lattice oxygen of TiO2 [17].

Photocatalytic Degradation of Paranitrophenol over the Surface of Fe-doped TiO2 Nanoparticles UV–vis Absorbance Spectra of Visible Light Induced Degradation of Paranitrophenol Aqueous Solution As has been mentioned above, owing to its highly stable and stubborn nature, the photocatalytic degradation of 4-NP is generally carried out by using oxidizing reagents such as Fenton’s, H2O2, photo-Fenton reagents under UV light. But, in this work we are trying to obtain better photocatalytic results without addition of any oxidizing reagent and by monitoring the concentration of Fe (III) ions doped in TiO2. Figure 10(a) shows the absorption spectra of 4-NP (pH 4) at different time intervals using 0.05 mol% FeT catalyst under visible light irradiation. It is seen that absorbance at λmax 315 nm progressively decreases as the irradiation time increases. This indicates that the pollutant is successfully degrading on the surface of photocatalyst with time. However, without addition of any photocatalyst no considerable degradation was observed (Figure 10(b)). The experiments were also conducted without illumination, in order to check the physical adsorption of pollutant over the catalyst. A very slight decrease in concentration of the pollutant was observed, showing that the decrease in absorbance is primarily due to photodegradation process, as illustrated in Figure 10(b). It has always been interesting to study the effect of Fe 3+ ions on the catalytic activity of titania. So, in order to evaluate the photocatalytic activity of FeT catalyst and determine the optimum content of Fe (III) ion dopants, a set of experiments were performed for photocatalytic degradation of aqueous solutions of 4-NP under visible light illumination. As shown in Figure 10(c) it was found that, as compared to the bare TiO2, the 4-NP photocatalytic degradation rate of Fe doped samples is higher. The maximum degradation rate of 92% in 5 h is observed when the Fe doping content is 0.050 mol% because this causes maximum separation of photogenerated charge carriers, hence resulting in excellent photocatalytic activity. If Fe content is further increased, it

232

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al.

leads to a decrease in the photocatalytic behavior of the catalyst, also well supported by the PL spectra.

Reprinted with permission from [A. Umar, 2015], J. Colloid Inter. Sci., 450(2015) 213 © 2015 Elsevier Ltd. Figure 10. (a) UV–vis absorbance spectra of visible light induced degradation of 4-NP aqueous solution (10 mg/L), (0.05 mol% Fe-doped TiO2 nanoparticles, Catalyst dose: 0.05 gm/L, pH: 4); (b) Comparison of photocatalysis, adsorption and photolysis; (c) Effect of Fe doping on % degradation rate of 4-NP; (d) Comparison of % photodegradation of synthesized catalysts with those of commercial catalysts.

CONCLUSION To conclude, TiO2 nanoparticles doped with different molar concentrations of Fe (III) ions were successfully fabricated by an ultrasonic assisted hydrothermal method followed by calcination. Fe (III) doped TiO2 nanoparticles possess small size, hence larger surface areas, more adsorbed OH groups and high visible light response. From PL studies it was confirmed that optimal doping of Fe (III) ions into TiO2 matrix leads to the inhibition of recombination of charge carriers thereby, enhancing photochemical quantum efficiency. All this contribute to their excellent photocatalytic activity for the degradation of para nitrophenol and Methylene Blue dye under visible irradiation. Also, the effect of Fe dopant on photocatalytic behavior of TiO2 nanoparticles was critically examined. The

The Effect of Fe Doping on TiO2 Nanoparticles …

233

maximum degradation rate of para-nitrophenol was 92% in 5 h when the Fe molar concentration was 0.05 mol%, without addition of any oxidizing agents. So, this work is presented as a promising and easy work in the field of environmental remediation in treatment of highly stable and toxic molecules such as nitrophenols and dyes.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

S. Weihua, Z. Zheng, A. Rami, Z. Tao, H. Desheng, Radiat. Phys. Chem. 65 (2002) 559. K. Parida, D. Prakasini Das, J. Photochem. Photobiol. A 163 (2004) 561. M. Khatamian, Z. Alaji, Desalination 286 (2012) 248. S. Yi, W. Q. Zhuang, S. T. Tay, J. H. Tay, Environ. Sci. Technol. 40 (2006) 2396. V. Kavitha, K. Palanivelu, J. Photochem. Photobiol. A 170 (2005) 83. Z. She, M. Gao, C. Jin, Y. Chen, J. Yu, Process Biochem. 40 (2005) 3017. D. T. Sponza, O. S. Kuscu, Process Biochem. 40 (2005) 1679. V. L. Gemini, A. Gallego, V. M. de Oliveira, C. E. Gomez, G. P. Manfio, S. E. Korol, Int. Biodeteriot. Biodegrad. 55 (2005) 103. G. Crini, Bioresour. Technol. 97 (2006) 1061. S. P. Alves, D. M. Brum, É. C. B. Andrade, A. D. P. Netto, Food Chem. 107 (2008) 489. S. K. Kansal, S. Sood, A. Umar, S. K. Mehta, J. Alloys Compd. 581 (2013) 392. F. Hueber-Becker, G. J. Nohynek, E. K. Dufour, W. J. A. Meuling, A. T. H. J. de Bie, H. Toutain, H. M. Bolt, Food Chem. Toxicol. 45 (2007) 160. R. Lamba, A. Umar, S. K. Mehta, S.K. Kansal, J. Alloys Compd. 620 (2015) 67. S. Sood, A. Umar, S. K. Mehta, S. K. Kansal, New J. Chem. 38 (2014) 3127. L. Gu, Z. Chen, C. Sun, B. Wei, X. Yu, Desalination 263 (2010) 107. D. G. Huang, S. J. Liao, W. B. Zhou, S. Q. Quan, L. Liu, Z. J. He, J. B. Wan, J. Phys. Chem. Solids 70 (2009) 853. J. F. Zhu, W. Zheng, B. He, J. L. Zhang, M. Anpo, J. Mol. Catal. A Chem. 216 (216) (2004) 35. Y. Zhang, Y. Shen, F. Gu, M. Wu, Y. Xie, J. Zhang, Appl. Surf. Sci. 256 (2009) 85. H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, B. Neppolian, M. Anpo, Catal. Today 84 (2003) 191. K. L. Yeung, A. J. Maira, J. Stolz, E. Hung, N. K. C. Hu, A. C. Wei, J. Soria, K. J. Cho, J. Phys. Chem. B 106 (2002) 4608. A. Fuerte, M. D. Hernandez-Alonso, A. J. Maira, A. Martınez-Arias, M. Fernandez Garcia, J. C. Conesa, J. Soria, Chem. Commun. 24 (2001) 2178. M. F. Garcia, A. M. Arias, J. C. Hanson, J. A. Rodriguez, Chem. Rev. 104 (2004) 4063.

234 [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47]

[48] [49]

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al. M. Zhou, J. Yu, B. Cheng, J. Hazard Mater. B 137 (2006) 1838. Z. Zhang, C. C. Wang, R. Zakaria, J. Y. Ying, J. Phys. Chem. B 102 (1998) 10871. W. Choi, A. Termin, M. R. Hoffmann, J. Phys. Chem. 98 (1994) 13669. J. Zhua, F. Chen, J. Zhang, H. Chen, M. Anpo, J. Photochem. Photobiol. A Chem. 180 (2006) 196. Y. Niu, M. Xing, J. Zhang, B. Tian, Catal. Today 201 (2013) 159. T. Sun, E. Liu, J. Fan, X. Hu, F. Wu, W. Hou, Y. Yang, L. Kang, Chem. Eng. J. 228 (2013) 896. N. Daneshvar, M. A. Behnajady, Y. Z. Asghar, J. Hazard. Mater. 139 (2007) 275. B. X. Zhao, G. Mele, I. Pio, J. Li, L. Palmisano, G. Vasapollo, J. Hazard. Mater. 176 (2010) 569. A. Kotronarou, G. Mills, M. R. Hoffmann, J. Phys. Chem. 95 (1991) 3630. F. J. Beltrán, V. Gómez-Serrano, A. Durán, Water Res. 26 (1992) 9. E. Lipczynska-Kochany, Chemosphere 24 (1992) 1369. D. W. Chen, A. K. Ray, Water Res. 32 (1998) 3223. M. A. Oturan, J. Peiroten, P. Chartrain, A. J. Acher, Environ. Sci. Technol. 34 (2000) 3474. W. B. Zhang, X. M. Xiao, T. C. An, Z. G. Song, J. M. Fu, G. Y. Sheng, M. C. Cui, J. Chem. Technol. Biotechnol. 78 (2003) 788. S. K. Kansal, A. H. Ali, S. Kapoor, Desalination 259 (2010) 147. M. S. Nahar, K. Hasegawa, S. Kagaya, Chemosphere 65 (2006) 1976. K. Hasegawa, T. Ito, W. Nakamura, M. Nagai, S. Kagaya, Chem. Lett. 32 (2003) 596. J. A. Wang, R. Limas-Ballesteros, T. Lopez, A. Moreno, R. Gomez, O. Novaro, X. Bokhimi, J. Phys. Chem. B 105 (2001) 9692. L. H. Ahrens, Geochim. Cosmochim. Acta 2 (1952) 155. P. Sathishkumara, S. Anandana, P. Maruthamuthub, T. Swaminathanc, M. Zhoud, M. Ashokkumard, Colloids Surf. A 375 (2011) 231. J. L. Ropero-Vega, A. Aldana-Perez, R. Gomez, M. E. Nino-Gomez, Appl. Catal. A 379 (2010) 24. B. Liu, X. Zhao, L. Wen, Mater. Sci. Eng. B 134 (2006) 27. D. H. Zhang, Q. P. Wang, Z. Y. Xue, Appl. Surf. Sci. 207 (2003) 20. W. F. Zhang, M. S. Zhang, Z. Yin, Q. Chen, Appl. Phys. B. 70 (2000) 261. C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder, G. E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Physical Electronics Division, USA, 1979. K. H. Wang, Y. H. Hsieh, L. J. Chen, J. Hazard. Mater. 59 (1998) 251. L. Yang, S. Luo, Y. Li, Y. Xiao, Q. Kang, Q. Cai, Environ. Sci. Technol. 44 (2010) 7641.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 17

THE APPLICATION OF SR-DOPED TIO2 NANOPARTICLES FOR THE EFFECTIVE PHOTOCATALYTIC DEGRADATION OF “BRILLIANT GREEN” DYE Swati Sood1, Ahmad Umar2,3,*, Surinder Kumar Mehta1, A. S. K. Sinha4 and Sushil Kumar Kansal5 1

Department of Chemistry, Panjab University, Chandigarh, India 2 Department of Chemistry, College of Science and Arts, Najran University, Najran, Kingdom of Saudi Arabia 3 Promising Centre for Sensors and Electronic Devices (PCSED), Najran University, Najran, Kingdom of Saudi Arabia 4 Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi, India 5 Institute of Chemical Engineering and Technology, Panjab University, Chandigarh, India

ABSTRACT Strontium doped TiO2 nanoparticles have been successfully synthesized by an ultrasonic-hydrothermal method and post calcination treatments. The synthesized catalyst has been characterized in details using X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer Emmett–Teller (BET) surface area analyzer, thermosgravimetric analysis (TGA), Fourier transform infrared (FT-IR), XPS (X-ray

*

Corresponding Author Email: [email protected]

236

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al. photoelectron spectroscopy) and photoluminescence (PL) spectroscopy. Sr-doped TiO2 catalyst has been further investigated as an efficient photocatalyst for the degradation of a triaryl methane dye, Brilliant Green. Excellent degradation efficiency of 96% is achieved within 60 min of photocatalytic reaction using the synthesized nanoparticles. In conclusion, the doping TiO2 with strontium ions causes the inhibition of recombination of photogenerated charge carriers, leading to high quantum efficiency.

INTRODUCTION A large quantity of wastewater composed of dyes discharged from industries into the environment is a matter of grave concern. Dyes are being used massively in various textile, food, beverage, hair color and leather tanning industries [1-3]. Most of the dyes used nowadays are of synthetic origin and are of different types such as acidic, basic, azo, acridine, triphenylmethane, quinine, thiazole, anthroquinone-based dyes [4]. Owing to their color, stability and the toxic raw materials used for their manufacture, the presence of dyes in wastewater has attracted major attention worldwide [5, 6]. Among them, triphenylmethane dyes such as Brilliant Green (BG) find wide applications in biological staining, modern textile and leather industries, etc. These dyes have been reported to cause hypersensitivity, carcinogenicity and toxicity to living organisms [7, 8]. Several techniques have been used for the treatment of effluents containing dyes but semiconductor photocatalysis has proved to be an effective and environmental friendly technology [9-11]. TiO2 has emerged as a promising semiconductor for the detoxification of the harmful environmental pollutants in aqueous as well as in gaseous phase [12-16]. Owing to its desirable properties such as stability, ease of availability, cost effectiveness and nontoxicity, TiO2 has been widely researched since the discovery of its photocatalytic water splitting ability [16]. But loss of photonic energy caused by the recombination of photogenerated electron-hole pairs poses a limitation on its efficiency. So, researchers have tried to overcome this limitation by modifying TiO2 by selective treatments such as dye photosensitization [17-19], metal ion doping [20, 21], metal loading [22-24] or coupling with metal oxides [25-28]. It has been observed that doping metal oxides such as TiO2 with noble and transition metals has greatly influenced the efficiency of photocatalysis and that doping causes formation of electron trap-centers which suppress the electronhole recombinations [29, 30]. Alkaline earth metals are widely available in nature and find many applications in various fields such as in military, industrial and biological applications. But, doping of metal oxide nanoparticles with alkaline earth metals for photocatalytic treatment of hazardous molecules has seldom been reported [31]. This work reports the simple and facile hydrothermal synthesis and characterizations of Sr-doped TiO2 nanoparticles. Further, the prepared nanoparticles are used as efficient

The Application of Sr-Doped TiO2 Nanoparticles ...

237

photocatalysts for photocatalytic degradation of brilliant blue dye which show excellent degradation efficiency of 96% within 60 min. The photocatalytic activity of synthesized Sr2+ doped TiO2 catalyst is evaluated by degradation of an aqueous solution of BG dye (25 mg/L) in a cylindrical slurry batch reactor under solar light irradiation. The required amount of catalyst (0.5 g/L) is added into the reaction solution. Prior to illumination, the reaction mixture is stirred in dark for 30 min to maintain adsorption-desorption equilibrium. The photocatalytic experiments are performed under solar light for 60 min. For the photocatalytic experiments, all the analytical samples are drawn from the reaction mixture timely and filtered through the Millipore syringe filter (size = 0.45 µm). The absorbance of filtrates is then analyzed to determine the time dependent decrease in the concentration of BG dye in presence of photocatalyst and under UV-light irradiation.

CRYSTALLINE STUDIES X-Ray Diffraction Patterns In order to study the crystalline structure of Sr-doped TiO2 catalyst, XRD analysis was performed. The results are depicted in Figure 1. The major diffraction peaks at 2θ = 25.5°, 38.2°, 48.2°, 54.24°, 55.46°, 62.99°, 69.20°, 70.59°, 75.47° are consistent with anatase (101), (103), (004), (200), (105), (211), (204), (116), (220) and (107) lattice planes (JCPDS no. 21-1272) [14]. The crystallite size of the catalyst was calculated using Scherrer‫׳‬s formula with the most intense peak observed at 2θ = 25.5° which is attributed to the anatase (101) phase. The crystallite size was found to be 48 nm. It is difficult for Sr2+ ions to substitute Ti4+ ions from the lattice because the ionic radius of Sr2+ ion is quite high (1.12 Å) as compared to Ti4+ (0.74 Å). So, the lattice parameters were found to see if there is any effect in the crystal lattice of TiO2 because of Sr ions. First, dhkl (the spacing between the planes in the atomic lattice spacing) was calculated using Bragg‫׳‬s law: 2dhld sin θ = n λ

(1)

where, n is an integer, λ is the wavelength of incident X ray and θ is the angle between the incident ray and the scattering planes. For tetragonal system following was equation used to calculate lattice parameters a and c: 1/d2hld = (h2 + k2)/ a2 + l2/c2

(2)

238

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al.

where, h, k, l refer to the miller indices. Upon doping, there was almost no change in lattice parameter a, while a slight increase in the lattice parameter c was observed. Lattice parameters calculated for doped sample were a = 3.764 Å and c = 9.420 Å, as compared to undoped a = 3.783 Å and c = 9.475 Å. This suggests that doping TiO2 lattice with Sr2+ ions causes only a slight disruption in the crystal lattice structure of TiO2. That can be because of low dopant amount and also large ionic radii of Sr 2+ ions as compared to Ti4+ ions. Also, no diffraction peaks for SrTiO3 at 2θ = 23, 34, 40, and 48° (JCPDS card no. 35-734), or SrO were observed due to low amount of Sr doping [31, 32].

Reprinted with permission from [A. Umar, 2015], Ceram. Inter., 41 (2015) 3533©2015 Elsevier Ltd. Figure 1. XRD pattern of synthesized Sr-doped TiO2 nanoparticles.

Reprinted with permission from [A. Umar, 2015], Ceram. Inter., 41 (2015) 3533©2015 Elsevier Ltd. Figure 2. TEM images of synthesized Sr-doped TiO2 nanoparticles.

The Application of Sr-Doped TiO2 Nanoparticles ...

239

STRUCTURAL PROPERTIES, SURFACE AREA, THERMOGRAVIMETRIC ANALYSIS AND THE OPTICAL CHARACTERIZATIONS OF SR-DOPED TIO2 SAMPLE Transmission Electron Microscopy Studies The morphology of prepared Sr-doped TiO2 sample was studied by TEM, as shown in Figure 2. It was found that TiO2 exist as particles of size ranging from 40 to 50 nm which is in close agreement with the XRD results. The grey particles in TEM images are TiO2 nanoparticles and the smaller dark particles are Sr-doped on TiO2. This is because Sr is a higher molecular mass dopant in the lattice so it appears as dark particles in the image while the TiO2 particles appear lighter.

BET Surface Area and Pore Size Distribution The surface area and the pore structure of the synthesized catalyst were investigated by performing N2 physical adsorption–desorption studies at 77 K. N2 adsorption/ desorption isotherm and pore size distributions using the Barret-Joyner-Halender (BJH) method obtained from the adsorption branch of the isotherms of Sr-doped TiO2 catalyst are depicted in Figure 3. It was observed that the isotherms are of Type IV, which is typical for mesoporous material according to the IUPAC classification (Figure 3(a)). The Sr-doped TiO2 sample has an average pore diameter of 11.4 nm (Figure 3(b)). The multipoint BET specific surface area obtained from adsorption data in relative pressure (P/P0) range 0.05-0.3 was found to be 100.78 m2/g. Table 1 shows the data for surface area, pore volume and pore radius obtained from different methods (Barrett–Joyner–Halenda (BJH), Dollimore Heal (DH) and density functional theory (DFT)) for the catalyst. From the surface area analysis it was confirmed that nanoparticles with high surface area and mesoporosity were fabricated. Table 1. Surface area, pore volume and pore radius obtained from different methods for synthesized Sr-doped TiO2 nanoparticles Method BJH DH DFT

Surface area (m2/g) 71.186 72.465 89.405

Pore volume (cc/g) 0.215 0.210 0.194

Pore radius (nm) 3.594 3.594 2.719

Reprinted with permission from [A. Umar, 2015], Ceram. Inter., 41 (2015) 3533©2015 Elsevier Ltd

240

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al.

Reprinted with permission from [A. Umar, 2015], Ceram. Inter., 41 (2015) 3533©2015 Elsevier Ltd. Figure 3. (a) N2 adsorption-desorption BET isotherm and (b) BJH pore size distributions (from the adsorption branch of the isotherms) of synthesized Sr-doped TiO2 nanoparticles.

FOURIER TRANSFORM INFRARED SPECTROSCOPY AND THERMOGRAVIMETRIC ANALYSIS Figure 4(a) shows FTIR spectrum of the prepared catalyst. The band observed at 3283 cm-1 is due to the stretching vibrations of the -OH group present in catalyst. The sharp peaks at 1633 cm-1 and 1353 cm-1 are because of C = O and C-O stretching, respectively, that can be due to presence of precursor and solvents used [10]. The peak observed at 588 cm-1 arises due to Ti-O stretching present in TiO2 [33]. From TGA results (Figure 4b), a total of 14% weight loss up to a temperature of 1000°C was observed. The thermogravimetric loss of 10% up to 100°C is because of evaporation of water present in the catalyst. The remaining loss of weight till 400°C is because of the loss of organic solvents and their residues that might be present in the sample.

The Application of Sr-Doped TiO2 Nanoparticles ...

241

Reprinted with permission from [A. Umar, 2015], Ceram. Inter., 41 (2015) 3533©2015 Elsevier Ltd. Figure 4. (a) FTIR spectrum and (b) TGA of synthesized Sr-doped TiO2 nanoparticles.

X-Ray Photoelectron Spectroscopy Analysis X-ray photoelectron spectroscopy (XPS) analysis was also performed on the catalyst. Figure 5 shows the XPS spectrum of Sr-doped TiO2 with core level spectra of characteristic elements. Figure 5(a) shows that the sample contains Ti, O and Sr. The peak for C 1s at binding energy of 285.6 eV was used as internal standard. Figure 5(b) shows the Ti 2p binding energy region. The Ti 2p3/2 and Ti 2p1/2 spin-orbital splitting photoelectrons for the sample is located at binding energies of 459.6 eV and 465.1 eV, respectively [34]. The Sr 3d spectrum shown in Figure 5(c) depicts peak at binding energy of 133.7 eV and 135.5 eV, which confirms the presence of Sr ions [31, 32]. In the O (1s) spectrum (Figure 5(d)) peak seen at binding energy 531.2 eV corresponds to the lattice oxygen of TiO2.

242

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al.

Reprinted with permission from [A. Umar, 2015], Ceram. Inter., 41 (2015) 3533©2015 Elsevier Ltd. Figure 5. (a) XPS spectrum of synthesized Sr-doped TiO2 nanoparticles; (b) core level spectra of characteristic Ti 2p; (c) Sr 3d; and (d) O 1s.

Photoluminescence Studies The photocatalytic efficiency of a catalyst is largely governed by the separation of charge carriers, since a large amount of photonic energy is wasted if the electron hole pairs recombine during the photocatalytic process. So, in order to increase the photocatalytic efficiency of a catalyst, it becomes vital to suppress the recombination of charge carriers which is largely determined by the phenomenon of photoluminescence. Thus, room temperature PL was recorded for the samples from 320 to 550 nm wavelength range (λex290 nm, PMT operating voltage 600 V), to study the efficiency of charge carrier trapping. Figure 6 shows the photoluminescence obtained for bare TiO2 and Sr-doped TiO2nanoparticles. However, metal dopant may act as recombination centre beyond a particular molar concentration, thereby causing detrimental effects on the photocatalytic efficiency of the catalyst. In this study it was observed that Sr doping in TiO2 lattice leads to an efficient charge separation, hence saving the wastage of photonic energy caused due to annihilation of electrons and holes. Since, the PL emission peak is greatly quenched, suggesting a significant separation of photogenerated electrons and

The Application of Sr-Doped TiO2 Nanoparticles ...

243

holes takes place. The PL emission peak corresponding to the inter-band recombination of TiO2 for bare TiO2 nanoparticles was observed at 340 nm. And this emission peak was found to be significantly reduced with Sr-doped TiO2 catalyst [14]. This suggests that Sr doping in TiO2 has markedly improved the separation of charge carriers, resulting in higher photocatalytic efficiency.

PHOTOCATALYTIC ACTIVITY To study the catalytic activity of Sr-doped TiO2 catalyst, a set of experiments for the degradation of BG dye under solar light were carried out. The results are shown in Figure 7. It was observed that during the photocatalytic reaction, the absorbance of dye solution decreases with increasing the illumination time. The absorption maxima corresponding to the aromatic hydrocarbon part were also decreasing; showing complete destruction of dye takes place in presence of the catalyst (Figure 7(a)). Almost complete dye degradation (96%) was observed within 60 min of photocatalytic reaction. Two control reactions under similar conditions were carried out in the absence of catalyst so as to monitor photolysis, and in the absence of light to study the adsorption phenomenon. It was observed that during photolysis, dye remained as such in aqueous phase without any marked degradation. A slight decrease in concentration of the pollutant was observed during adsorption process (Figure 7(b)). From the results, it was concluded that catalyst in presence of light plays an important role in photo-degradation of the organic moiety.

Reprinted with permission from [A. Umar, 2015], Ceram. Inter., 41 (2015) 3533©2015 Elsevier Ltd. Figure 6. Room temperature PL spectra of bare TiO2 and synthesized Sr-doped TiO2 nanoparticles. The PL spectra were obtained at excitation wavelength of 290 nm.

244

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al.

Reprinted with permission from [A. Umar, 2015], Ceram. Inter., 41 (2015) 3533©2015 Elsevier Ltd. Figure 7. (a) UV-vis absorbance spectra of solar light induced degradation of BG aqueous solution (25 mg/L), (catalyst: synthesized Sr-doped TiO2 nanoparticles, catalyst dose: 0.5 g/L); (b) comparison of extent of photocatalysis, adsorption and photolysis; (c) comparison of extent of degradation of BG dye using different photocatalysts; and (d) comparison of degradation efficiencies of different photocatalysts.

Reprinted with permission from [A. Umar, 2015], Ceram. Inter., 41 (2015) 3533©2015 Elsevier Ltd. Figure 8. Mechanism of photocatalytic reaction.

The Application of Sr-Doped TiO2 Nanoparticles ...

245

Photocatalytic experiments were also carried out by using bare TiO2 nanoparticles prepared by the similar method and some photocatalysts which are available in market and being used widely such as ZnO (Merck), TiO2 PC-50 and TiO2PC-500. Figure 7(c) shows the comparison of extent of photocatalytic degradation efficiency for BG dye obtained with different photocatalysts under similar experimental conditions. A decrease in the relative concentration of BG with increase in time of exposure was seen. The percentage degradation rates have also been compared for the catalysts (Figure 7(d)). It is clear that the prepared photocatalyst exhibit much higher degradation efficiency as compared to other photocatalysts. This can be credited to the inhibition of electron-hole recombinations due to Sr-doped in TiO2 lattice and high surface area of the catalyst. Figure 8 shows the mechanism of photocatalytic reaction. It can be inferred that Sr doping in TiO2 leads to a marked increase in catalytic efficiency, prominently because Sr acts as an electron trap and leads to suppression of electron and hole recombination.

CONCLUSION TiO2 nanoparticles doped with alkaline earth metal ions (Sr2+) have been synthesized by a hydrothermal cum ultrasonic method. It is concluded that the prepared catalyst possesses excellent crystalline, thermal, morphological and photoluminescence properties. In addition, the sunlight assisted catalytic experiments have been performed for the degradation of BG dye. Excellent photo-degradation rate of 96% within 60 min is observed. It is found that Sr doping tremendously enhances the photocatalytic efficiency of TiO2 nanoparticles, credits to its high mesoporosity, surface area and inhibition of recombination of electron hole pairs. This suggests that solar light can be utilized for the photocatalytic degradation of harmful organic contaminants present in wastewater using alkaline earth metal doped TiO2 nanoparticles.

REFERENCES [1]

[2]

(a) Huang G. L., Jani M., Huang Y. L., Vincent Y. W. C., Sci. Adv. Mater. 5 (2013) 1444; (b) Wróbel D., Boguta A., Ion R. M., J. Photochem. Photobiol. A, 138 (2001) 7; (c) Li Q., Zong L., Xing Y., Wang X., Yu L., Yang J., Sci. Adv. Mater. 5 (2013) 1316. (a) Bensalah N., Alfaro M. A. Q., Martínez-Huitle C. A., Chem. Eng. J. 149 (2009) 348; (b) Shao M., Xu X., Huang J., Zhang Q., Ma L., Sci. Adv. Mater. 5 (2013) 962; (c) Liu M., Wang H., Yan C., Bell J., Mater. Focus 1 (2012) 136.

246 [3]

[4] [5] [6]

[7] [8] [9] [10] [11] [12]

[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

Swati Sood, Ahmad Umar, Surinder Kumar Mehta et al. (a) Sponza D. T., J. Hazard. Mater. 138 (2006) 438; (b) Shi J., Jing D., Shen S., Guo L., Sci. Adv. Mater. 5 (2013) 982; (c) Khan A., Mir Niyaz A., Haque M. M., Muneer M., Mater. Focus 2 (2013) 335. Shaul G. M., Holdsworth T. J., Dempsey C. R., Dostal K. A., Chemosphere 22 (1991) 107. Yang X., Zou R., Huo F., Cai D., Xiao D., J. Hazard. Mater. 164 (2009) 367. (a) Zainudin N. F., Abdullah A. Z., Mohamed A. R., J. Hazard. Mater. 174 (2009) 299; (b) Oliveira Haroldo G., Fitzmorris Bob C., Longo Claudia, Zhang Jin Z., Sci. Adv. Mater. 4 (2012) 673. Vachálková A., Novotný L., Blesová M., Neoplasma 43 (1996) 113. Munusamy S., Aparna R. S. L., Prasad R. G. S., Sustain. Chem. Processes 1 (2013) 4. Kansal S. K., Ali A. H., Kapoor S., Bahnemann D. W., Sci. Adv. Mater. 5 (2013) 630. Kansal S. K., Lamba R., Mehta S. K., A. Umar, Mater. Lett. 106 (2013) 385. Hoffmann M. R., Martin S. T., Choi W., Bahnemann D. W., Chem. Rev. 95 (1995) 69. (a) Ali A. H., Kapoor S., Kansal S. K., Desalin. Water Treat. 25 (2011) 268; (b) Cao Chunlan, Hu Chenguo, Shen Weidong, Shuxia Wang, Jianli Wang, Hong Liu, Yi Xi, Sci. Adv. Mater. 5 (2013) 796. Tang G., Liu S., Tang H., Zhang D., Li C., Yang X., Ceram. Int. 39 (2013) 4969. (a) Kansal S. K., Sood S., Umar A., J. Alloys Compd. 581 (2013) 392; (b) Jia Chu Jing Ma, Qiang Liangsheng, Jiang Zaixing, Sci. Adv. Mater. 4 (2012) 1185. Fujishima A., Rao T. N., Tryk A. D., J. Photochem. Photobiol. C1 (2000) 1. (a) Fujishima A., Honda K., Nature 238 (1972)37; (b) Huang Jianhui, Ho Wingkei, Lee Frank S. C., Sci. Adv. Mater. 4 (2012) 863. Hara K., Dan-Oh K., Kasada C., Ohga Y., Shinpo A., Suga S., Sayama K., Arakawa H., Langmuir 20 (2004) 4205. Pelet S., Moser J. E., Gratzel M., J. Phys. Chem. B 104 (2000) 1791. Sood S., Mehta S. K., Umar A., Kansal S. K., New J. Chem. 38 (2014) 3127. Choi W., Termin A., Hoffmann M. R., J. Phys. Chem. 98 (1994) 13669; (b). Teh C. M., Mohamed A. R., J. Alloys Compd. 509 (2011) 1648. Li X., Zhuang Z., Li W., Pan H., Appl. Catal. A Gen. 429 (2012) 31. Liu L., Ji Z., Zou W., Gu X., Deng Y., Gao F., Tang C., Dong L., ACS Catal. 3 (2013) 2052. Madarasz D., Potari G., Sapi A., Laszlo B., Csudai C., Oszko A., Kukovecz A., Erdohelyi A., Konya Z., Kiss J., Phys. Chem. Chem. Phys. 15 (2013) 15917. Anderson C., Bard A., J. Phys. Chem. B. 101 (1997) 2611. Liu Z., Sun D. D., Guo P., Leckie J. O., Nano Lett. 7 (2007) 1081.

The Application of Sr-Doped TiO2 Nanoparticles ...

247

[27] (a) Liu G., Li G., Qiu X., Li L., J. Alloys Compd. 481 (2009) 492; (b) Lamba R., Umar A., Mehta S. K., Kansal S. K., J. Alloys Compd. 620 (2015) 67. [28] Zhu H., Jiang R., Fu Y., Guan Y., Yao J., Xiao L., Zeng G., Desalination 286 (2012) 41. [29] Wu J. C. S., Chen C. H., J. Photochem. Photobiol. A: Chem. 163 (2004) 509. [30] He Z., Hong T., Chen J., Song S., Sep. Purif. Technol. 96 (2012) 50. [31] Kumaresan L., Mahalakshmi M., Palanichamy M., Murugesan V., Ind. Eng. Chem. Res. 49 (2010) 1480. [32] Hamedani H. A., Allam N. K., Garmestani H., El-Sayed M. A., J. Phys. Chem. C 115 (2011) 13480. [33] Zhang D. H., Wang Q. P., Xue Z. Y., Appl. Surf. Sci. 207 (2003) 20. [34] Wagner C. D., Riggs W. M., Davis L. E., Moulder J. F., Muilenberg G. E., Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Physical Electronics Division, USA, 1979.

SECTION 3. CONDUCTING POLYMERS BASED PHOTOCATALYSTS

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 18

THE UTILIZATION OF GRAPHENE/POLYANILINE NANOCOMPOSITES FOR THE DEGRADATION OF ROSE BENGAL DYE Sadia Ameen1, Hyung-Kee Seo1, M. Shaheer Akhtar2 and Hyung Shik Shin1,* 1

Energy Materials and Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New and Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT The polyaniline/graphene (PANI–Gr) nanocomposites are prepared by the polymerization of aniline monomer with graphene (Gr) under in situ condition. The nanocomposites are utilized as the effective photocatalyst towards the photocatalytic degradation of Rose Bengal (RB) dye. The uniformity and the miscibility of PANI nanomaterials are improved with the increase of Gr contents which forms the advanced PANI–Gr nanocomposites. The absorption properties reveals the presence of Gr in PANI–Gr nanocomposites with significant interaction/bonding between PANI and Gr. The PANI–Gr nanocomposites possess the partial hydrogen bonding between imine of PANI and the carboxylic group on the surface of Gr sheets. The prepared PANI–Gr nanocomposites delivers a significant degradation of RB dye by ~56% within 3 h under light illumination. As compared to PANI, the considerable degradation of RB dye is attributed to the presence of Gr sheets in PANI–Gr nanocomposites which might result the high photogenerated electron–hole pairs charge separation under light illumination. *

Corresponding Author Email: [email protected].

252

Sadia Ameen, Hyung-Kee Seo, M. Shaheer Akhtar et al.

INTRODUCTION The discharges of textile and the photographic industries basically, nonbiodegradable organic dyes are one of the main contaminants in waste water [1] which disturbs the ecosystem and environment. Generally, the Rose Bengal (RB) dye is extensively used in insecticides, dyeing material and in printing industries. The dissipation of RB dye causes several harmful diseases to the stomach and liver of humans [2]. So far, it is an important issue to maintain a friendly ecosystem by removing RB dye from the polluted water. From last few decades, several methods like coagulation, reverse osmosis and the adsorbents have been studied to remove organic dye such as RB, methylene blue dye from the waste water [3]. Among them, the heterogeneous photocatalytic degradation is an economical and easy method to decompose these organic pollutants into less dangerous matter and therefore, assists in environment cleaning as waste water purification [4]. The organic/inorganic nanocomposites have been recently known as promising photocatalysts for the degradation of harmful organic dye under light illumination [5]. A new kid of carbon nanomaterials named as graphene (Gr) with a two dimensional (2D) covalent bonded lattice has attracted much attentions due to its high surface area, good transmittance, unique optical, mechanical, excellent transport and electronic properties [6–8]. Its superior electronic and transport properties promise various applications in the catalytic activities, solar cells, hydrogen storage, batteries and sensors [9, 10]. On the other hand, the unique category of organic conducting polymers, especially polyaniline (PANI) is widely explored conducting polymer on account of its promising electrical and photoelectric properties with high environmental stability and high process feasibility [11]. The conducting properties could be easily changed either by the oxidation of PANI chain or by the protonation of imine nitrogen in PANI backbone [12]. Recently, PANI integrated carbon nanotubes (CNTs) nanocomposites display significantly enhanced conductivity and electrocatalytic activity towards various electrical and electrochemical systems or devices [13–16]. It is expected that the combination of Gr and PANI nanomaterials could improve the structural, optical, electrical and thermal properties, which results to numerous applications in nanoelectronics, catalytic properties, rechargeable batteries and electrochemical systems. To the best of our knowledge, the photocatalytic degradation of RB dye over the surface of PANI–Gr nanocomposites as photocatalyst has not been reported elsewhere. The present work deals the synthesis of PANI–Gr nanocomposites by in situ polymerization of aniline monomer with different wt.% of Gr, using APS as initiator. The photocatalytic degradation of RB dye has been studied over the surface of the prepared PANI–Gr nanocomposites under light illumination. RB dye has efficiently degraded by 56% within 3 h. These results demonstrate that the PANI–Gr nanocomposite has a potential viability

The Utilization of Graphene/Polyaniline Nanocomposites …

253

for using as an effective photocatalyst and its photocatalytic efficiency is enhanced under light irradiation. The photocatalytic degradation of RB dye is performed under the illumination of Xenon arc lamp. The photocatalytic activities are performed as reported elsewhere [19], using PANI–Gr nanocomposites as efficient photocatalysts. The amount of dye degradation over the surface of nanocomposites catalyst is measured by the relative intensity of the UV–Vis spectroscopy. The degradation rate of RB dye over PANI–Gr nanocomposites is estimated by the following equation: Degradation rate (%) = (Co-C/Co) × 100 = (Ao-A/Ao) × 100

where Co represents the initial concentration, C denotes variable concentration, Ao shows the initial absorbance, and A corresponds to variable absorbance.

MORPHOLOGICAL STUDIES OF GRAPHENE/POLYANILINE NANOCOMPOSITES The Field Emission Scanning Electron Microscopy

Reprinted with permission from [S. Ameen, 2012], Chem. Eng. J., 210 (2012) 220 © 2012 Elsevier Ltd. Figure 1. FESEM images of (a) Gr and PANI–Gr nanocomposites with (b) 0.5 wt.%, (c) 1 wt.% and (d) 3 wt.% of Gr.

254

Sadia Ameen, Hyung-Kee Seo, M. Shaheer Akhtar et al.

Figure 1 shows the FESEM images of Gr and PANI–Gr nanocomposites. The Gr displays the typical layered sheets morphology with the average thickness of several hundred nm, as shown in Figure 1a. The mixed morphologies (Figure 1b–d) are observed in PANI–Gr nanocomposites. The miscibility of Gr and PANI nanomaterials increases with the increase of Gr contents in the nanocomposites and at higher Gr contents of 3 wt.%, both Gr and PANI nanomaterials are uniformly mixed with each other. However, the nanocomposites with lower Gr wt.% show non-uniform mixing of Gr and PANI nanomaterials. Thus, the high Gr wt.% is required to form the uniform PANI–Gr nanocomposites, which might result from the largely adsorbed aniline monomer on the surface of Gr sheets during the in situ polymerization.

OPTICAL PROPERTIES OF GRAPHENE/ POLYANILINE NANOCOMPOSITES UV-Vis Absorbance Spectra and Photoluminescence Characterizations The UV–Vis spectra of PANI, Gr and PANI–Gr nanocomposites have been examined to explain the optical properties and the interaction between Gr and PANI, as presented in Figure 2A. The typical spectrum of Gr (inset of Figure 2A) shows a single absorbance peak at 243 nm which attributes to –* transitions of the C-C bonds in hexagonal ring [20]. However, the two absorption bands at 328, and 630 nm are observed in the spectrum of PANI (Figure 2Aa, corresponding to –* and n–* transitions of PANI respectively [21]. The PANI–Gr nanocomposites exhibit all the characteristic absorption peaks of PANI and the Gr with higher peak intensities than that of PANI, as shown in (Figure 2Ab–d. Moreover, the absorption peaks in PANI–Gr nanocomposites are slightly shifted to higher absorption edge, which might attribute to the molecule conjugation like –*interaction between PANI and Gr in the nanocomposites [22]. Room temperature PL spectra of PANI and PANI–Gr nanocomposites are carried out with an excitation wavelength of 340 nm, as shown in Figure 2B. In case of PANI (Figure 2Ba, a single broad absorption emission at 410 nm is obtained in the blue–green region due to the benzenoid/amine groups of the oxidized/reduced PANI [23]. The PANI–Gr nanocomposites evince a considerable shift in the blue–green emission peak from 410 nm to 440 nm, shown in (Figure 2Bb–d. The presence of Gr in the PANI–Gr nanocomposite exhibits the prominent quenching of the PL peak. In other words, the quenching of the PL spectra causes the generation of the singlet excitons, and the reduced quenching occurs due to the interaction of Gr moieties and the interchain PANI species. In general, the change in the interchain polymer species is crucial to commute the emission process of conjugated polymers like PANI. Therefore, in situ polymerization of

The Utilization of Graphene/Polyaniline Nanocomposites …

255

aniline monomer with Gr produces highly advanced PANI–Gr nanocomposites with improved morphological, structural and the optical properties.

Reprinted with permission from [S. Ameen, 2012], Chem. Eng. J., 210 (2012) 220 © 2012 Elsevier Ltd. Figure 2. (A) UV–Vis spectra and (B) Photoluminescence spectra of (a) PANI and PANI– Gr nanocomposites with (b) 0.5 wt.%, (c) 1 wt.% and (d) 3 wt.% of Gr. Inset show the UV–Vis spectrum of Gr.

STRUCTURAL CHARACTERIZATIONS OF GRAPHENE/POLYANILINE NANOCOMPOSITES Fourier Transform Infrared Spectroscopy and Raman Scattering Spectroscopy The PANI, Gr and PANI–Gr nanocomposites are characterized by FTIR spectra, shown in Figure 3A. The Gr exhibits the IR bands at 1721 cm-1, 1592 cm-1, 1231 cm1 /1048 cm-1 which are the characteristics of C=O, C=C, and C-O in C-O-C/C-O-H groups, respectively [24]. The pristine PANI obtains the IR bands at ~1566 cm-1,

256

Sadia Ameen, Hyung-Kee Seo, M. Shaheer Akhtar et al.

~1474 cm-1, ~1296 cm-1, and ~1229 cm-1, correspond to the C=N and C=C stretching modes in quinoid and benzenoid units, C-N-C stretching modes in the benzenoid and quinoid imine units, respectively [25]. As compared with PANI, the IR bands in the nanocomposites have shifted positively to ~1590 cm-1, ~1501 cm-1, ~1305 cm-1 and ~1236 v, indicating the clear interaction between Gr and PANI. Importantly, the shifting in the IR band at ~1590 cm-1 indicates that the two components such as Gr and PANI are mainly interacted through the π–π* interactions and hydrogen bonding. This result is consistent with the UV–Vis results. Thus, PANI–Gr nanocomposites possess the hydrogen bonding between O=C-O- of Gr and the active PANI backbone during the in situ polymerization.

Reprinted with permission from [S. Ameen, 2012], Chem. Eng. J., 210 (2012) 220 © 2012 Elsevier Ltd. Figure 3. (A) FTIR spectra and (B) Raman spectra of Gr, PANI and PANI–Gr nanocomposites.

The Utilization of Graphene/Polyaniline Nanocomposites …

257

The Raman spectra of the PANI, Gr and PANI–Gr nanocomposites are shown in Figure 3B. The Raman spectra of Gr exhibits two bands at ~1356 cm-1 and ~1595 cm-1, corresponding to the Raman active D band or K-point phonons of A1g symmetry and Raman active G band presenting the vibration of sp2 hybridized carbon (the E2g phonons) respectively [26, 27]. The pristine PANI displays characteristic bands at ~1163, ~1212, ~1468 and ~1590 cm-1 which are ascribed to C-H bending of the quinoid ring, C-N+● stretching of the bipolaron structure, C=N of the quinoid non protonated di-imine units and C=C of the quinoid rings respectively [28]. As compared with PANI and Gr, the positions of Raman bands have alerted in the Raman spectrum of PANI–Gr nanocomposites which obtain both PANI and Gr Raman bands at ~1174 cm-1, ~1228 cm1 , ~1372 cm-1 (Gr), ~1502 cm-1 and ~1596 cm-1 (Gr/PANI, C=C skeleton), indicating the interaction between PANI and Gr moieties. In particular, the shifting of Raman bands at ~1212 cm-1 to ~1228 cm-1 and ~1590 cm-1 to ~1596 cm-1 indicate the presence of bonding between C-N+● of PANI and carboxylate group of Gr through the π – π* electron interaction of PANI and Gr sheets. These results are in excellent agreement with the FTIR results.

X-Ray Photoelectron Spectroscopy Analysis of Graphene/Polyaniline Nanocomposites The surface interaction and bonding in PANI–Gr nanocomposite have been investigated by XPS analysis, as shown in Figure 4. The survey XPS spectrum (Figure 4a) exhibits the three binding energies peaks at 282 eV, 397 eV and 529 eV, representing the of C 1s, N 1s and O 1s peaks respectively. These binding energies peaks are deconvoluted to define the interaction and bonding between PANI and Gr moieties. Figure 4b shows C 1s XPS spectra of PANI–Gr nanocomposites with five deconvoluted peaks at 288.4, 287.5, 286.3 eV, 285.3 and 284.4 eV which are assigned to -O-C=O, C=O, C-O, C-N+ /C=N+, neutral C-C/C-H bond in PANI–Gr nanocomposites [29]. However, the deconvoluted C 1s peak at 284.4 eV also represent the sp2 carbon network by C-C bond in Gr and CAC of the benzonoid ring showing a combination of protonation of imine and amine sites via shake-up processes [30]. The appearance of single binding energy peak at 284.4 eV also indicates the intermixing of C-C aromatic bond of Gr and PANI. Interestingly, the binding energies peaks at 286.3 eV and 288.4 eV deduce the carbon atoms attached with polaronic-type and the bipolaronic-type nitrogen atoms and with oxygen atoms in the form of carboxylate group respectively. The deconvoluted O 1s spectrum of PANI–Gr nanocomposite decomposes into three binding energies peaks with one center binding energy peak, as shown in Figure 4c. The center binding energy peak at 530.2 eV usually

258

Sadia Ameen, Hyung-Kee Seo, M. Shaheer Akhtar et al.

assigned to C-O or C-OH group in carbon based nanomaterials [31]. The binding energy at 529.9 eV could be attributed to the C=O and O=C-O- functional groups in PANI–Gr nanocomposite, while the higher binding energy peak at 532.3 eV corresponds to the bound water molecules [32]. Figure 4d shows the N 1 s XPS spectrum of PANI–Gr nanocomposite which decomposes into three deconvoluted peaks at 398.6 eV, 399.8 eV and 400.8 eV. The lowest binding energy at 398.6 eV represents the quinoid amine in the backbone of PANI. While, the peak at 399.8 eV ascribes to the benzenoid di-amine nitrogen of PANI [33]. The higher binding energy at 400.8 eV addresses the cationic radical nitrogen i.e., positively charged nitrogen (-N+●) and the protonated imine (=N+●) [33].

Reprinted with permission from [S. Ameen, 2012], Chem. Eng. J., 210 (2012) 220 © 2012 Elsevier Ltd. Figure 4. (a) Survey and deconvoluted (b) C 1s, (c) O 1s and (d) N 1s XPS spectra of PANI–Gr nanocomposites.

In general, this peak is originated at 401.2 eV in case of PANI [34], which slightly moves to 400.8 eV. It suggests that the cationic nitrogen radical might interact with the carboxylate group of Gr and might form a partial hydrogen bonding with each other. Thus, the in situ polymerization process is an effective method to achieve advanced

The Utilization of Graphene/Polyaniline Nanocomposites …

259

PANI–Gr nanocomposite in which PANI and Gr are interconnected with the partial hydrogen bonding between the cationic nitrogen species (=N+● and =N+●) of PANI and O=C-O- functional group of Gr moieties.

THE PHOTOCATALYTIC ACTIVITY OF PANI–GR NANOCOMPOSITES UV–Vis Spectra of Decomposed Rose Bengal Dye Solution under Light Illumination over the Surface of PANI–Gr The degradation of RB dye has been studied to investigate the photocatalytic activity of PANI–Gr nanocomposites under light irradiation and compared with the photocatalytic activity performed without catalyst and with pristine PANI. Figure 5a shows the UV–Vis absorption spectra of the decomposed RB dye solution over the surface of PANI–Gr (3 wt.%) nanocomposite under light illumination. The amount of RB dye degradation over the surface PANI–Gr (3 wt.%) nanocomposite is measured by the relative intensity of the maximum absorbance at 550 nm in UV–Vis. spectra. The intensity of UV absorbance of RB dye molecules decreases continuously with the increase of time interval, suggesting the degradation of RB dye over the surface of PANI–Gr (3 wt.%) nanocomposite. From the photographs of decomposed RB solutions (Figure 5b), the initial color of RB dye become lighter with the increase of time interval from 0 min to 180 min. It is seen that the PANI–Gr (3 wt.%) nanocomposite catalyst degrades the RB dye by 56% within 180 min under the light illumination.Figure 5c shows the variation in the relative concentration (A/Ao) of RB dye solution versus time intervals for RB dye without using catalysts and with pristine PANI and PANI–Gr nanocomposite. In order to confirm any external effect on the RB degradation, a separate catalytic experiment is performed under dark condition for 3–4 h, which shows the negligible decrease in the concentration of RB dye. Moreover, the RB degradation without catalyst shows very low degradation rate (4%) within time interval of 180 min, indicating that the RB dye cannot degrade by itself under light illumination. On comparison with pristine PANI (9%), the degradation rate increases with the increase of Gr contents in the nanocomposites. The order of degradation rate are as follows: PANI (9%) < PANI–Gr (0.5 wt.%, 19%) < PANI–Gr (1 wt.%, 23%) < PANI–Gr (5 wt.%, 45%) < PANI–Gr (3 wt.%, 56%). It is noticed that the lower degradation rate at higher amount of Gr (5 wt.%) in PANI–Gr nanocomposites is occurred which might due to the significant decrease of light harvesting efficiency or cut down light penetration [35]. While, the RB degradation rate over PANI–Gr (3 wt.%) nanocomposites catalyst has been significantly improved by eight times as compared to pristine PANI. The

260

Sadia Ameen, Hyung-Kee Seo, M. Shaheer Akhtar et al.

amount of Gr in nanocomposites displays considerable effects on the surface areas of the nanocomposites as well as the degradation rates, as shown in Figure 5d.

Reprinted with permission from [S. Ameen, 2012], Chem. Eng. J., 210 (2012) 220 © 2012 Elsevier Ltd. Figure 5. (a) UV–Vis spectra of decomposed RB dye solution under light illumination over the surface of PANI–Gr (3 wt.%) nanocomposites, (b) the photographs of decomposed RB dye solution over the surface of PANI–Gr (3 wt.%) nanocomposites with respect to time intervals and (c) the extent of RB dye degradation (A/Ao) versus time intervals by pristine PANI, PANI–Gr nanocomposites and RB degradation without catalyst. (d) RB dye degradation rate and surface areas versus different wt.% of Gr in PANI–Gr nanocomposites and (e) the adsorption/desorption analysis over the surface of PANI–Gr (3 wt.%) nanocomposite.

The Utilization of Graphene/Polyaniline Nanocomposites …

261

It is known that the large surface area of catalyst is associated high dye adsorbability over the surface of catalyst, which might originate from the formation of π–π* stacking between dye molecules and aromatic regions of the non-covalent Gr [36]. The higher amount of Gr (PANI–Gr (3 wt.%)) has substantially increased the surface area (58.7 m2 /g) of PANI–Gr nanocomposite, resulting in the high RB degradation over the surface of PANI–Gr nanocomposite. Furthermore, the adsorption/desorption analysis (Figure 5e) shows the high adsorption and low desorption of RB dye on the surface of PANI–Gr nanocomposites within 180 min, which again confirms the moderate dye degradation (56%) within 180 min.

Schematic Illustrations of Photocatalytic RB Dye Degradation over the Surface of PANI–Gr Nanocomposite A schematic illustration (Figure 6) shows the interaction between RB dye and PANI– Gr under light illumination to understand the enhanced photocatalytic RB degradation.

Reprinted with permission from [S. Ameen, 2012], Chem. Eng. J., 210 (2012) 220 © 2012 Elsevier Ltd. Figure 6. Schematic illustrations of photocatalytic RB dye degradation over the surface of PANI–Gr nanocomposites.

262

Sadia Ameen, Hyung-Kee Seo, M. Shaheer Akhtar et al.

Firstly, most of RB dye molecules are adsorbed on the surface of the PANI–Gr catalyst due to the large surface area of the nanocomposites. It is observed that the higher adsorption of RB dye molecules on PANI–Gr nanocomposite is attributed to formation of π–π* stacking between RB molecules and aromatic regions of the Gr in the nanocomposites. Under light illumination, the photoexcitation of electron occurs in PANI by the electronic interaction between Gr and PANI. This phenomenon has considerably increased the electron/hole or charge separation with the addition of Gr in PANI. It is reported that the electron transfer from PANI to Gr under light illumination generates the reactive radicals oxyradicals such as superoxide radical ion O2●-, and hydroxyl radical HO●, on the surface of Gr sheets [3].

Mass Spectra of Rose Bengal Dye Solution over PANI–Gr Nanocomposite and the Possible Reaction Intermediates after the Photocatalytic Reaction The mass spectroscopy has been studied to identify the possible reaction intermediates after 180 min of reaction, as shown in Figure 7. The RB dye solution displays a prominent mass signal at m/z = 1022 in Figure 7a i.e., very close to the formula mass of RB dye. Noticeably, no mass signals are detected about the formation of the reaction intermediates, which clearly reveal the removal by adsorption. The m/z = 1022 signal is weaken after 180 min of photocatalytic reaction over the PANI–Gr nanocomposites and multiple mass signals have appeared (Figure 7b, indicates the formation of reaction intermediates during the photocatalytic degradation. Figure 7c depicts the molecular structures of possible reaction intermediates from fragmentations of the main skeleton of RB dye which have the oxy groups in their rings. It is believed that the formations of these reaction intermediates are crucial to determine the degree of degradation of the organic compounds to complete mineralization [37]. Importantly, the structure of PANI–Gr nanocomposites might crucial to achieve high surface area of the catalysts which might have significantly increased the adsorption of RB molecules and photo-induced charge transfer along the Gr sheets in nanocomposites. In our case, the superior photocatalytic activity of PANI–Gr (3 wt.%) nanocomposites catalyst might result from the increased charge separation and the formation of oxyradicals (O2●, HO2●, OH●) by the large surface of nanocomposites provided by Gr sheets.

The Utilization of Graphene/Polyaniline Nanocomposites …

263

Reprinted with permission from [S. Ameen, 2012], Chem. Eng. J., 210 (2012) 220 © 2012 Elsevier Ltd. Figure 7. Mass spectra of RB dye solutions over PANI–Gr nanocomposite after (a) 0 min and (b) 180 min with the scan 200–1200 m/z and (c) the possible reaction intermediates after the photocatalytic reaction.

264

Sadia Ameen, Hyung-Kee Seo, M. Shaheer Akhtar et al.

CONCLUSION The PANI–Gr nanocomposites have been synthesized by the in situ polymerization of aniline monomer and Gr sheets and applied as an effective photocatalyst for the degradation of RB dye. The uniformity and the miscibility of PANI nanomaterials have improved with the increase of Gr contents in the nanocomposites. The presence of Gr in PANI–Gr nanocomposites is confirmed by the absorption studies, revealing a significant interaction/ bonding between PANI and Gr. The surface analysis studies shows the existence of the partial hydrogen bonding between imine (ANH) of PANI and the carboxylic group present on the surface of Gr sheets. The prepared PANI–Gr (3 wt.%) nanocomposite delivers a significant degradation of RB dye by 56% within 3 h under light illumination, resulting from the increased surface area of the nanocomposites due to high wt.% of Gr. The higher adsorption of RB dye molecules on PANI–Gr nanocomposite is attributed to the formation of π-π stacking between RB molecules and the aromatic regions of Gr in nanocomposites, which might result to the high photogenerated electron–hole pairs charge separation under light illumination.

REFERENCES [1] [2] [3]

X. Li, S. Ouyang, N. Kikugawa, J. Ye, Appl. Catal. A: Gen. 334 (2008) 51. N. Mittal, A. Shah, P. B. Punjabi, V. K. Sharma, J. Chem. 2 (2009) 516. (a) S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, Colloid Polym. Sci. 288 (2010) 1633; (b) B. Jia, W. Jia, Y. Ma, X. Wu, F. Qu, Sci. Adv. Mater. 4 (2012) 702. [4] (a) H. X. Li, X. Y. Zhang, Y. N. Huo, J. Zhu, Environ. Sci. Technol. 41 (2007) 4410; (b) U. G. Akpan, B. H. Hameed, Chem. Eng. J. 169 (2011) 91; (c) Q. Sun, Y. Lu, Y. Liu, J. Nanoeng. Nanomanuf. 2 (2012) 46. [5] S. Ameen, M. S. Akhtar, Y. S. Kim, H. S. Shin, Appl. Catal. B: Environ. 103 (2011) 136. [6] D. W. Wang, F. Li, J. Zhao, W. Ren, Z. G. Chen, J. Tan, Z. S. Wu, I. Gentle, G. Q. Lu, H. M. Cheng, ACS Nano. 3 (2009) 1745. [7] G. C. Wang, Z. Y. Yang, X. W. Li, C. Z. Li, Carbon. 43 (2005) 2564. [8] D. Li, M. B. Müller, S. Gilje, R. B. Kaner, G. G. Wallace, Nat. Nanotechnol. 3 (2008) 101. [9] X. Wang, L. Zhi, K. Mullen, Nano Lett. 8 (2008) 323. [10] J. D. Fowler, M. J. Allen, V. C. Tung, Y. Yang, R. B. Kaner, B. H. Weiller, ACS Nano. 3 (2009) 301.

The Utilization of Graphene/Polyaniline Nanocomposites …

265

[11] (a) Q. Hao, W. Lei, X. Xia, Z. Yan, X. Yang, L. Lu, X. Wang, Electrochim. Acta. 55 (2010) 632; (b) A. Katoch, M. Burkhart, T. Hwang, S. S. Kim, Chem. Eng. J. 192 (2012) 262. [12] (a) A. A. Khan, M. Khalid, R. Niwas, Sci. Adv. Mater. 2 (2010) 474; (b) S. Ameen, M. S. Akhtar, Y. S. Kim, H. S. Shin, Chem. Eng. J. 181–182 (2012) 806. [13] (a) E. Granot, B. Basnar, Z. Cheglakov, E. Katz, I. Willner, Electroanalysis. 18 (2006) 26; (b) R. Chauhan, Deepshikha, T. Basu, Sci. Adv. Mater. 4 (2012) 96. [14] T. Yang, N. Zhou, Y. Zhang, W. Zhang, K. Jiao, G. Li, Biosens. Bioelectron. 24 (2009) 2165. [15] Y. Li, Y. Umasankar, S. M. Chen, Talanta. 79 (2009) 486. [16] M. Li, L. Jing, Electrochim. Acta. 52 (2007) 3250. [17] W. S. Hummers Jr., R. E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [18] S. Ameen, M. S. Akhtar, S. G. Ansari, O. B. Yang, H. S. Shin, Superlatt. Microstruc. 46 (2009) 872. [19] S. Ameen, Y. B. Im, C. G. Jo, Y. S. Kim, H. S. Shin, J. Nanosci. Nanotech. 11 (2011) 541. [20] S. Goswami, U. N. Maiti, S. Maiti, S. Nandy, M. K. Mitra, K. K. Chattopadhyay, Carbon 49 (2011) 2245. [21] D. Han, Y. Chu, L. Yang, Y. Liu, Z. Lv, Coll. Surf. A: Physicochem. Eng. Aspects. 259 (2005) 179. [22] H. Wang, Q. Hao, X. Yang, L. Lu, X. Wang, ACS Appl. Mater. Inter. 2 (2010) 821. [23] S. A. Chen, K. R. Chuang, C. I. Chao, H. T. Lee, Synth. Met. 82 (1996) 207. [24] H. Wang, Q. Hao, X. Yang, L. Lu, X. Wang, Electrochem. Commun. 11 (2009) 1158. [25] S. Ameen, M. S. Akhtar, M. Husain, Sci. Adv. Mater. 2 (2010) 441. [26] A. C. Ferrari, J. Robertson, Phys. Rev. B. 61 (2000) 14095. [27] L. G. Cancado, M. A. Pimenta, B. R. A. Neves, M. S. S. Dantas, A. Jorio, Phys. Rev. Lett. 93 (2004) 247401. [28] S. H. Domingues, R. V. Salvatierr, M. M. Oliveira, A. J. G. Zarbin, Chem. Commun. 47 (2011) 2592. [29] L. Tang, Y. Wang, Y. Li, H. Feng, J. Lu, J. Li, Adv. Funct. Mater. 19 (2009) 2782. [30] A. P. Monkman, G. C. Stevens, D. Bloor, J. Phys. D Appl. Phys. 24 (1991) 738. [31] O. Akhavan, Carbon. 48 (2010) 509. [32] D. Yang, A. Velamakanni, G. Bozoklu, S. Park, M. Stoller, R. D. Piner, S. Stankovich, I. Jung, D. A. Field, C. A. Ventrice, R. S. Ruoff, Carbon. 47 (2009) 145. [33] K. Zhang, L. L. Zhang, X. S. Zhao, J. Wu, Chem. Mater. 22 (2010) 1392. [34] S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, Microchim. Acta. 172 (2011) 471. [35] K. Woan, G. Pyrgiotakis, W. Sigmund, Adv. Mater. 21 (2009) 2233.

266

Sadia Ameen, Hyung-Kee Seo, M. Shaheer Akhtar et al.

[36] Z. Liu, J. T. Robinson, X. M. Sun, H. J. Dai, J. Am. Chem. Soc. 130 (2008) 10876. [37] L. Xueyan, W. Desong, C. Guoxiang, L. Qingzhi, A. Jing, W. Yanhong, Appl. Catal. B: Environ. 81 (2008) 267.

PART III. FUNDAMENTALS OF FUNCTIONAL MATERIALS: APPLICATIONS FOR RENEWABLE ENERGY

SECTION 1. SMALL ORGANIC MOLECULES BASED ORGANIC SOLAR CELLS

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 19

THE PERFORMANCE OF ORGANIC SOLAR CELLS: SMALL MOLECULES BASED ON THIAZOLOTHIAZOLE M. Nazim1, Sadia Ameen1, M. Shaheer Akhta2, Youn-Sik Lee3 and Hyung Shik Shin1,* 1

Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea 3 School of Chemical Engineering, Chonbuk National University, Jeonju, Chonbuk, Republic of Korea

ABSTRACT Novel π-conjugated linear organic chromophore (RTzR) based on a thiazolothiazolecore (acceptor-unit) and terminal alkylated-thiophenes (donor-unit) is designed as an active material for organic solar cells. Terminal alkyl units at both ends of chromophore significantly improves its solubility and also induces liquid crystalline property which is responsible for the self-assembly behavior of the chromophore. The fabricated bulkheterojunction (BHJ) solar cell shows reasonable power conversion efficiency of ~1.57% with high photocurrent density of ~7.85 mA/cm2

*

Corresponding Author Email: [email protected].

272

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al.

INTRODUCTION Small organic conjugated chromophores are considered as an alternative low-bandgap conjugated polymers in solution processed bulk heterojunction (BHJ) organic solar cells (OSC) [1–3]. Small organic molecules are highly explored active layer material in solution processed solar cells due to their numerous advantages such as the reproducibility of synthesis and ease of purification process [4]. Other component such as fullerenes (PCBM) and its derivatives have contributed the important role to achieve the high performance BHJ solar cells [5–8]. Solution processed small-molecule organic solar cells (SMOSCs) with p-conjugated oligomer of donor–acceptor–donor (D–A–D) type have received great deal of interests owing to their ease of fabrication, good solution processability and high power conversion efficiency (~6%) [9–10]. Recently, Zhou et al. fabricated SMOSCs with a linear small molecules (DR3TBDT) which exhibited the highest power conversion efficiency (PCE) of ~7.38% than other SMOSCs [11]. To achieve the high performance SMOSCs, the molecular design and processing conditions are the crucial factors to get the uniform and homogeneous BHJ thin film [12, 13]. For high performance SMOSCs, various parameters such as synthesis characteristic of organic molecules as well as the device parameters like composition ratio, film thickness and annealing temperatures are significant [14–16]. In recent studies, small molecules with alternating donor–acceptor–donor (D–A–D) structures have gained much attention because these molecules could increase the absorption band through intramolecular charge transfer (ICT) for better match-up the solar spectrum and attains high power conversion efficiency in solar cells devices [4]. So far, many researchers have reported the D–A system for SMOSCs [17, 18]. Mikroyannidis et al. and Shen et al. developed the triphenylamine (TPA) containing organic molecules and star-shaped BHJ solar cells and demonstrated the good photovoltaic performances [19–21]. Furthermore, Li et al. synthesized a series of D–A small molecules based on a TPA donor moiety linked to different acceptor moieties with linear or star-shaped molecular architectures and their fabricated SMOSCs showed the PCE improvement from 0.35% to 1.33% in terms of molecular geometry [22, 23]. Dutta et al. described a thiazolothiazole-based conjugated copolymer with a didecyloxynaphthalene donor unit with PCE of approximately 1% [24]. The thiazolothiazole unit is fused heterocycles ensuring a rigid, coplanar structure and electron-accepting tendency because of the presence of imine (–C=N–) backbone which has been proved an effective way of lowering the LUMO level of the small molecule [25–27]. Thiazolothiazole as acceptor unit has aroused much interest for synthesizing efficient small organic molecules of D–A–D or A–D–A type and accomplishes good photovoltaic materials [28–31]. In this work, a novel thiazolothiazolebased liquid crystalline, linear chromophore, 2,5-bis (5-(5-(5-hexylthiophen-2-yl) thiophen-2-yl) thiophen-2-yl) thiazolo[5,4-d]thiazole (RTzR), has been designed,

The Performance of Organic Solar Cells

273

synthesized, characterized and applied for the fabrication of solution processed BHJ– SMOSCs. The presence of terminal alkyl units have considerably improved the solubility of the resultant chromophore in common organic solvents and therefore, facilitates the charge conduction in the photovoltaic device.

Scheme for the Synthesis of Thiazolothiazole Based Linear Chromophore (RTzR) The linear chromophore (RTzR) is synthesized by three steps chemical route (Scheme 1) starting from the reaction of thiophene-carboxaldehyde (1) with dithiooxamide followed by bromination with N-bromosuccinimide. The final product (RTzR) is then obtained by the Suzuki cross-coupling reaction between dibromo-thiazolothiazole monomer (3) and n-hexylbithiophene boronic-acid pinacole ester (4) using Pd(PPh3)4 (10 mol%) as catalyst and potassium carbonate as a base in anhydrous toluene solvent. The product RTzR is purified by flash column chromatography followed by recrystallization in the mixed solvent of dichloromethane/methanol (2:1, v/v) as red solid (yield: 71%). It is found that RTzR is readily soluble in common organic solvents, such as dichloromethane, chloroform, THF, and chlorinated benzenes at room temperature. The small molecule based solar cells are fabricated using ITO substrates.

Reprinted with permission from [M. Nazim, 2013], Chem. Phys. Lett., 574 (2013) 89 © 2013 Elsevier Ltd. Scheme 1. The three step synthetic route of the linear chromophore (RTzR).

274

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al.

The ITO glass substrate is first cleaned with detergent, ultrasonicated in water, acetone and isopropyl alcohol, and subsequently dried overnight in an oven. PEDOT:PSS thin film is coated on ITO substrates by solution spin-coating with a speed of ~4000 rpm for 60 s. The deposited layer is then annealed at 130 oC for 10 min in vacuum oven. The active layer of RTzR:PC60BM blend ratio (1:1 and 1:2 w/w) in 10 mg/ml chlorobenzene is then deposited on PEDOT:PSS thin films coated ITO glass substrates by spin-coating the blend solution at a scan rate of ~700 rpm for 60 s. The active layer is heated at 80 oC for 10 min to evaporate the residual solvent. Finally, the silver cathode (~100 nm) is deposited through a shadow mask by thermal evaporation under a vacuum of about 3 x10–6 torr. The active area of device is 1.5 cm2. In PC60BM concentration studies, the observed increase in the performance with 1:1 weight ratios compared to 1:2 weight ratio might attribute to the increased short circuit current density (Jsc) and open circuit voltage (Voc) at this ratio. Current density–voltage (J–V) curves are measured by using computerized digital multimeter and a variable load. A 1000W metal halide lamp (Phillips) served as a simulated sun light source, and its light intensity (or radiant power) is adjusted to simulated AM 1.5 radiation at 100 mW/cm2 with a Si photodetector fitted with a KG-5 filter (Schott) as a reference calibrated at NREL (USA).

Optical Properties of RTzR UV–Vis and Photoluminescence Spectra UV–vis absorption spectra of RTzR in dilute chloroform (1×10-4 M) and thin film state are shown in Figure 1 A. RTzR exhibits a broad absorption band at λmax ~447 nm with the significant red shift and the molar absorption coefficient in solution (e) is calculated as ~1.78×104 M-1 cm-1. It indicates a good intramolecular interaction between donor and acceptor moieties in the linear chromophore in chloroform solution [33]. However, a slight red shift is seen in the thin film of RTzR as compared to chloroform solution which might due to formation of aggregates from the solution to the solid thin film. It is also deduced that RTzR thin film has exact linear and more planar structure due to the absence of any side chain or groups, resulting in the enhanced intermolecular electronic delocalization and self-assembly behavior within the organic chromophore. Additionally, the absorption edge (konset) of RTzR has been used to estimate the optical band gap (Eg). RTzR film records the reasonably good Eg of ~2.21 eV (Table 1). To examine the donor–acceptor charge transfer process from RTzR to PCBM, the photoluminescence (PL) spectroscopy (Figure 1B) has been analyzed in chloroform solution and for thin film. A single broad emission band at ~547 nm is recorded in RTzR in chloroform solution, whereas RTzR thin film displays red shift at ~562 nm. Moreover, a large red shift (λ = 682 nm) occurs in thin film of RTzR:PCBM active layer (1:1, w/w),

The Performance of Organic Solar Cells

275

indicates that RTzR has substantially quenched after blending with PCBM. This phenomenon confirms the fast charge transfer from RTzR (donor) to PCBM (acceptor). Table 1. Optical and Electrochemical properties of RTzR Small molecule RTzR

λmaxa (nm) 447

λmaxb (nm) 452

HOMOc (eV) -5.49

LUMOd (eV)

Ege (eV)

Egf (eV)

-3.63

1.86

2.21

Reprinted with permission from [M. Nazim, 2013], Chem. Phys. Lett., 574 (2013) 89 © 2013 Elsevier Ltd aAbsorption in chloroform solution b Absorption of thin film on ITO. cEstimated from the onset of oxidation wave of cyclic voltammogram, dEstimated from the onset of reduction wave of cyclic voltammogram, eElectrochemical band gap calculated from cyclic voltammogram, fOptical band gap calculated from the onset of the UV-vis spectra of the film.

Reprinted with permission from [M. Nazim, 2013], Chem. Phys. Lett., 574 (2013) 89 © 2013 Elsevier Ltd. Figure 1. (A) UV–vis spectra of RTzR in chloroform solution and thin film deposited on ITO substrate; (B) Room temperature photoluminescence spectra of RTzR in chloroform solution (black), thin film on ITO (red) and blend thin film of RTzR:PCBM active layer (blue) (1:1, w/w).

The Cyclic Voltammetry (CV) of RTzR Thin Film The cyclic voltammetry (CV) of RTzR thin film (Figure 2) in 0.1 M CH3CN solution of tetrabutyl ammonium hexa flouro phosphate [nBu4N]+[PF6]- at a potential scan rate of 50 mV/s is carried out to investigate the electrochemical properties. The reversible oxidation and reversible reduction peaks are situated at onset value Eox = 1.09 ± 0.02 and Ered = 0.77 ± 0.02 eV, respectively. Noticeably, RTzR thin film exhibits the good energy levels of HOMO value -5.49 eV and LUMO value ~3.63 eV. The difference of HOMO

276

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al.

and LUMO is quite lower than that of the optical band gap, indicating the fast delocalization of charges or electrons in the solid film [34–36]. It is also suggests that terminal alkyl units at both ends might substantially enhance the intermolecular interaction and self-assembly behavior in RTzR:PCBM active layer thin film. The synthesized linear chromophore has been utilized as an effective active material for the fabrication of solution processed BHJ–SMOSCs. The fabrication of SMOSCs is followed the layer pattern of ITO/PEDOT:PSS/RTzR:PCBM/Ag. The thicknesses estimated for PEDOT:PSS layer as ~50 nm, the RTzR active layer of ~100 nm, and Ag layer of 80–100 nm.

Reprinted with permission from [M. Nazim, 2013], Chem. Phys. Lett., 574 (2013) 89 © 2013 Elsevier Ltd. Figure 2. Cyclic Voltammogram of RTzR thin film in 0.1 mol/L acetonitrile solution with a scan rate of 50 mV/s of [nBu4N]+[PF6]-.

The Current (J)–Voltage (V) Curves of Organic Solar Cells Device of RTzR:PCBM Active Layers Figure 3 shows the current density-voltage (J-V) curves of the fabricated SMOSCs, recorded under 100 mW/cm2 (1.5 AM). SMOSC fabricated with RTzR:PCBM active layer (1:1, w/w) achieves reasonably good power conversion efficiency (PCE) of ~1.57% with high short circuit current (JSC) of ~7.85 mA/cm2 and open circuit voltage (VOC) of

The Performance of Organic Solar Cells

277

~0.650 V. However, low PCE of ~1.20% with low VOC of ~0.424 V are obtained in SMOSCs fabricated with RTzR:PCBM (1:2, w/w) active layer (Table 2). Table 2. Summary of J-V curves of the RTzR based fabricated devices in SMOSCs RTzR:PC60BM 1:1, w/w 1:2, w/w

Photovoltaic parameters JSC (mA/cm2) VOC (V) 7.85 0.650 7.59 0.424

FF 0.31 0.37

PCE (%) 1.57 1.20

Reprinted with permission from [M. Nazim, 2013], Chem. Phys. Lett., (2013) © 2013 Elsevier Ltd

The higher JSC is generally related to the better absorption properties of RTzR:PCBM active layer (1:1, w/w) which might result to the large generation of excitons and simultaneous fast dissociation into free charge carriers at the interface of RTzR:PCBM active layer (1:1, w/w) [37]. Additionally, the presence of terminal n-hexyl units in RTzR-chromophore significantly benefits the ultrafast and complete intermolecular charge transfer between RTzR and PCBM, resulting to the suppression of charge recombination which has greatly improved JSC and performance of the solar cells. The increased amount of PCBM causes the lowering in VOC and shows slight increase in the fill factor of cell because some of PCBM domains come in direct contact with ITO glass, and then induces the reverse electron flow from top layer to the active layer [38].

Reprinted with permission from [M. Nazim, 2013], Chem. Phys. Lett., 574 (2013) 89 © 2013 Elsevier Ltd. Figure 3. J–V curves of fabricated small molecule organic solar cells device of RTzR:PCBM active layers.

278

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al.

Atomic Force Microscopy Spectroscopy of RTzR:PCBM Active Layer The surface morphology of the photoactive layers is crucial factor to achieve the high performance of solar cell devices. In this regard, the atomic force microscopic (AFM) images are analysed to explain the morphological structures of the blend thin film of RTzR:PCBM active layer (1:1, w/w) before and after annealing, as shown in Figure 4. It is seen in Figure 4a that RTzR:PCBM active layer (1:1, w/w) exhibits very rough thin film which is clearly visible in the AFM image of RTzR:PCBM (1:1, w/w) film. Whereas, the roughness of the film has decreased after annealing at 80 C (Figure 4b), indicating good blends of both RTzR and PCBM. Importantly, a homogenous nature of thin film is formed in nanoscale phase separation, revealing the good blending or miscibility of RTzR with PCBM. Usually the nanoscale phase separation in the blend film is responsible for the large interface area for exciton dissociation [39]. Therefore, the terminal n-hexyl units in RTzR and homogenous blend thin film of RTzR:PCBM active layer (1:1, w/w) have significantly enhanced the JSC and power conversion efficiency of SMOSCs. Contrary, the low FF could be obtained from the high rough surface of blend film which might increase the series resistance of blend layer/ITO and results in the recombination [40]. The photovoltaic performance of SMOSC with RTzR:PCBM active layer (1:1, w/w) is better than the reported literatures on solution processed SMOSCs [41–44]. The further optimization is required in terms of the molecular structure of organics, blend composition, thickness and physical treatment to improve the power conversion efficiency of solar cells.

Reprinted with permission from [M. Nazim, 2013], Chem. Phys. Lett., 574 (2013) 89 © 2013 Elsevier Ltd. Figure 4. Topographic AFM images of RTzR:PCBM active layer (1:1, w/w) thin film on PEDOT:PSS spin-coated ITO substrate from chlorobenzene before (a) and after annealing (b).

The Performance of Organic Solar Cells

279

CONCLUSION In conclusion, a novel D–A–D architecture linear chromophore, RTzR with a thiazolothiazole-core as acceptor and the terminal n-hexylthiophene units as donor is synthesized by Suzuki cross-coupling reaction by three steps synthetic route. The synthesized linear chromophore is efficiently utilized as a donor material for solution processed BHJ–SMOSCs. RTzR shows a strong red shift (λ = 120 nm) in RTzR:PCBM active layer (1:1, w/w) as compared to RTzR thin film in emission spectra which reveals strong intermolecular interactions between donor and acceptor units. The existence of the terminal n-hexyl side chains in RTzR leads to good solubility in common organic solvents, enhances HOMO energy levels and induces liquid crystalline property and therefore, self-assembly behavior of the chromophore. The fabricated SMOSCs with RTzR:PCBM active layer (1:1, w/w) shows reasonably high PCE of ~1.57% with high JSC of ~7.85 mA/cm2 under 100 mW/cm2 (1.5 AM). The enhanced JSC and PCE are attributed to the presence of terminal alkyl units and the homogenous miscibility of the blend thin film which benefices the ultrafast and complete intermolecular charge transfer from RTzR as donor to PCBM as acceptor.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

A. A. Mishra, P. Bauerle, Angew. Chem., Int. Ed. 51 (2012) 2020. Y. Li et al, Energy Environ. Sci. 3 (2010) 1427. M. A. Green, K. Emery, Y. Hishikawa, W. Warta, Prog. Photovoltaics Res. Appl. 19 (2011) 84. S. Paek et al, J. Phys. Chem. C. 116 (2012) 23205. C. Z. Li, H. L. Yip, A. K. Y. Jen, J. Mater. Chem. 22 (2012) 4161. J. L. Delgado, F. Oswald, F. Cardinali, F. Langa, N. Martin, J. Org. Chem. 73 (2008) 3184. J. L. Delgado, P. A. Bouit, S. Filippone, M. A. Herranz, N. Martin, Chem. Commun. 46 (2010) 4853. J. L. Delgado, N. Martin, P. de la Cruz, F. Langa, Chem. Soc. Rev. 40 (2011) 5232. Y. Sun et al, Nat. Mater. 11 (2012) 44. X. Liu et al, J. Am. Chem. Soc. 134 (2012) 20609. J. Zhou et al, J. Am. Chem. Soc. 134 (2012) 16345. J. Peet et al, Adv. Mater. 21 (2009) 1521. P. M. Beaujuge, J. M. J. Frechet, J. Am. Chem. Soc. 133 (2011) 20009. N. Zhou et al, Adv. Mater. 24 (2012) 2242. Z. Li et al, Adv. Energy Mater. 2 (2012) 74.

280 [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44]

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al. S. Wang et al, Chem. Mater. 23 (2011) 4789. P. Dutta et al, Chem. Commun. 48 (2012) 573. Q. Peng et al, Chem. Commun. 48 (2012) 11452. M. M. Stylianakis, J. A. Mikroyannidis, Q. Dong, J. Pei, Z. Liu, W. Tian, Sol. Energy Mater. Sol. Cells 93 (2009) 1952. J. A. Mikroyannidis, M. M. Stylianakis, P. Suresh, P. Balraju, G. D. Sharma, Org. Electron. 10 (2009) 1320. S. Shen, L. Gao, C. He, Z. Zhang, Q. Sun, Y. Li, Org. Electron. 14 (2013) 875. Z. Li et al, J. Phys. Chem. C. 114 (2010) 18270. Z. Li et al, J. Mater. Chem. 21 (2011) 2159. P. Dutta et al, Synth. Met. 161 (2011) 1582. Q. Sun, K. Park, L. Dai, J. Phys. Chem. C 113 (2009) 7892. Y. Lin et al, Adv. Mater. 24 (2012) 3087. M. Zhang, X. Guo, Y. Li, Adv. Energy Mater. 1 (2011) 557. X. Wang et al, Polym. Chem. 2 (2011) 2872. Q. Shi, P. Cheng, Y. F. Li, X. Zhan, Adv, Energy Mater. 2 (2012) 63. I. H. Jung et al, Chem. Eur. J. 16 (2010) 3743. S. K. Lee et al., Chem. Commun. 47 (2011) 1791. M. Melucci et al, Adv. Funct. Mater. 20 (2010) 445. B. Walker et al, Adv. Funct. Mater. 19 (2009) 3063. N. Crivillers et al., Chem. Commun. 48 (2012) 12162. C. M. Cardona, W. Li, A. E. Kaifer, D. Stockdale, G. C. Bazan, Adv. Mater. 23 (2011) 2367. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti, A. B. Holmes, Nature. 376 (1995) 498. Z. Li, et al., Sol. Energy Mater. Sol. Cells. 95 (2011) 2272. L. Xue, L. Liu, Q. Gao, S. Wen, J. He, W. Tian, Sol. Energy Mater. Sol. Cells. 93 (2009) 501. B. Walker, et al., Adv. Energy Mater. 1 (2011) 221. M. Ali, et al., Org. Electron. 13 (2012) 2130. S. W. Shelton, et al., ACS Appl. Mater. Interfaces. 4 (2012) 2534. G. Zhao, et al., J. Phys. Chem. C. 113 (2009) 2636. K. A. Mazzio, M. Yuan, K. Okamoto, C. K. Luscombe, ACS Appl. Mater. Interfaces. 3 (2011) 271. B. Walker, et al., ACS Appl. Mater. Interfaces. 4 (2012) 244.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 20

SOLUTION-PROCESSED BULK-HETEROJUNCTION ORGANIC SOLAR CELL BASED ON A FURANBRIDGED THIAZOLO [5,4-D]THIAZOLE BASED D–Π–A–Π–D TYPE LINEAR CHROMOPHORE M. Nazim1, Sadia Ameen1, M. Shaheer Akhtar2, Hyung-Kee Seo1 and Hyung Shik Shin1,* 1

Energy Materials and Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New and Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT Furan-bridged thiazolo[5,4-d]thiazole based π-conjugated organic chromophore (RFTzR) is formulated and utilizes for small molecule organic solar cells (SMOSCs). The presence of furan spacer along with two terminal alkyl units significantly improves its absorption and solubility in the common organic solvents. RFTzR exhibits the reasonable HOMO and LUMO energy levels of -5.36 eV and -3.14 eV, respectively. The fabricated SMOSCs with RFTzR (donor) and PC60BM (acceptor) as photoactive materials presents relatively high power conversion efficiency of ~2.72% (RFTzR: PC60BM, 2:1, w/w) along with good open-circuit voltage of ~0.756 V and high photocurrent density of ~10.13 mA cm-2, which might attribute to its improved

*

Corresponding Author Email: [email protected].

282

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al. absorption, electrochemical properties and the presence of strong electron-withdrawing furan moieties.

INTRODUCTION Solution-processed small molecule organic solar cells (SMOSCs) have recently grown as an alternative photovoltaic device to the conventional photovoltaics due to various advantages over their polymer counterparts, such as well-defined molecular structures, definite molecular weight, easy synthetic processes and also easy purification techniques [1–7]. Small p-conjugated organic chromophores are considered as a good alternative for low band-gap organic conjugated polymers in solution-processed bulkheterojunction (BHJ) small molecule organic solar cells (SMOSCs) [8–11]. Zhou et al. recently synthesized a linear chromophore of A–D–A type (DR3TBDT) for the SMOSCs and demonstrated the high power conversion efficiency (PCE) of ~7%. [12]. The performance of SMOSCs is usually influenced by various fabrication parameters, such as composition ratio, film-thickness, homogeneity of materials and annealing temperatures [13–16]. In particular, the size of the donor (D) and acceptor (A) unit in the organic chromophore is a crucial factor for the exciton-diffusion towards the D–A interface and then charge-separation for the effective charge-transport to the electrodes [17–19]. Small organic molecules with a thiazolo[5,4-d]thiazole unit present a rigid and coplanar structure with good electron accepting tendency owing to the presence of imine (C=N–) groups and fused-heterocyclic rings [20–23]. It is known that the electronegativity of the oxygen atom is stronger than that of the sulfur atom, which might help to reduce the highest occupied molecular orbital (HOMO) level of the chromophore by introducing oxygen based heterocyclic moieties. In BHJ organic solar cells, the high open circuit voltage (VOC) is generally the difference between the HOMO of the electron donor and lowest unoccupied molecular orbital (LUMO) of the electron acceptor [24, 25]. Furan, a heterocyclic unit, has been proved a good alternative for the thiophene unit because of its good electron withdrawing ability and better planarity, and it tunes the HOMO–LUMO level appropriately in organic conjugated chromophores and dyes for solar cells [26–29]. In furan-based organic materials, the smaller size of the oxygen atom compared with the sulfur atom leads to less steric demand in oligofurans than in oligothiophenes, which might also contribute to the significant difference in the rigidity [30, 31]. Importantly, furan-based materials are biodegradable and could be obtained directly from the natural sources. Furan based materials are significantly more electronrich, show higher fluorescence, better molecular-packing, and greater rigidity with better processability (due to their greater solubility) than their thiophenecounterparts.

Solution-Processed Bulk-Heterojunction Organic Solar Cell …

283

Recently, organic solar cells fabricated with furan-containing polymers and PC71BM showed quite high power conversion efficiencies [32, 33]. Hitherto, various D–A–D or A–D–A type thiazolo[5,4-d]thiazole small organic molecules have aroused a lot of interest for the efficient solution-processed organic photovoltaic materials [34–37]. In this chapter, a new and novel furan-bridged thiazolo[5,4-d]thiazole based D– π –A– π –D type linear chromophore, 2,5-bis (5-(5-(5-hexylthiophen-2-yl)thiophen-2-yl) furan-2-yl) thiazolo[5,4-d]thiazole (RFTzR), has been synthesized, characterized and applied for the fabrication of solution processed SMOSCs. The terminal alkyl units at both ends of the chromophore have considerably improved its solubility in common organic solvents, which is an essential criterion for solution-processed fabrication devices to facilitate the charge conduction on the donor–acceptor interface.

SYNTHETIC ROUTE OF THE FURAN-BRIDGED ORGANIC CHROMOPHORE (RFTZR) The synthetic route of p-conjugated linear organic chromophore, RFTzR, is shown in Scheme 1. The monomeric precursors of the thiazolo[5,4-d]thiazole-based chromophores were synthesized by previously reported procedures.38 Compound, 5-di(furan-2yl)thiazolo[5,4-d]thiazole (2) was synthesized by a ring-closing reaction of furfural (1) and dithiooxamide, which on bromination with N-bromosuccinimide gives 2,5-bis(5bromofuran-2-yl)thiazolo[5,4-d]thiazole (3). Suzuki cross-coupling reactions of 3 and 4 yielded the furan-based linear organic chromophore in good yield under inert atmosphere.

Synthesis of 2,5-Di(Furan-2-Yl)Thiazolo[5,4-D]Thiazole, 2 A solution of furfural (1) (1.2 g, 12 mmol) and dithiooxamide (0.60 g, 5 mmol) in nitrobenzene (~20 ml) was heated to reflux at 130oC for 24 h under inert atmosphere. The reaction changes were monitored by TLC. After completion of the reaction, the reaction mixture was cooled to room temperature and diethyl ether was added (50 ml) to obtain the precipitates. The obtained precipitates were washed by ether, ethanol and hexane several times. The residue was purified by column chromatography on silica gel with hexane: DCM (1:1, v/v) as eluents. The final product was dissolved in chloroform and heated for 15 min. The obtained solution was filtered and allowed to cool. The desired product was finally obtained by recrystallization in diethyl ether as greenish needle-like crystals with a bitter almond smell (0.98 g, 72% yield). 1H NMR (600 MHz, CDCl3, ppm) d: 6.45 (d, 2H), 7.00 (d, 2H), 7.10 (d, 2H). FTIR (KBr, cm-1): 3144, 3106, 3069, 1648, 1596, 1500, 1455, 1384, 1313, 1241, 1214, 1192, 1147, 1032, 1014, 925, 880, 845, 815, 755, 660, 595, 560.

284

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al.

Synthesis of 2,5-Bis(5-Bromofuran-2-Yl)Thiazolo[5,4-D]Thiazole, 3 2,5-Di(furan-2-yl)thiazolo[5,4-d]thiazole, 2 (0.48 g, 1.8 mmol) was dissolved in anhydrous dimethylformamide (~20 ml). A solution of N-bromosuccinimide (0.82 g, 4.5 mmol) in anhydrous dimethylformamide (~8 ml) was added, and the reaction mixture was refluxed for 2 h under inert atmosphere. The precipitates were collected and dissolved in dichloromethane (DCM). After solvent removal by the reduced pressure, the residue was washed several times with diethyl ether, hexane and ethanol. The desired product was obtained by recrystallization from hexane as yellow crystals (0.65 g, 83.3% yield). 1H NMR (600 MHz, CDCl3, ppm) d: 6.52 (d, 2H), 7.03 (d, 2H) FT-IR (KBr, cm1 ): 3118, 3106, 3097, 2985, 2927, 2853, 1732, 1584, 1497, 1466, 1312, 1228, 1198, 1080, 1017, 941, 918, 845, 797, 700, 641.

Synthesis of 2,5-Bis(5-(5-(5-Hexylthiophen-2-Yl)-Thiophen-2-Yl)Furan-2-Yl) Thiazolo[5,4-D]Thiazole (RFTzR) 2,5-Bis(5-bromofuran-2-yl)thiazolo[5,4-d]thiazole (3) (0.20 g, 0.43 mmol) and nhexyl-bithiophene boronic acid pinacol ester (4) (2.1 eq.) (0.25 g, 0.95 mmol) were dissolved in ~15 ml anhydrous toluene into a two-neck round bottom flask. The reaction mixture was mixed with N2 for 20 min, and then Pd(PPh3)4 (15 mg, 5 mol%) was added. The reaction mixture was flushed again with N2 for another 10 min followed by slow addition of degassed aqueous K2CO3 solution (2 M) (4 ml) by syringe, and the reaction mixture was stirred at 110oC for 24 h under inert atmosphere. The reaction mixture was cooled to room temperature and extracted with dichloromethane (DCM). The organic layer was collected and washed with water and brine and then dried over anhydrous MgSO4. After solvent removal by reduced pressure, the residue was purified by column chromatography on silica gel with hexane:DCM (2:1, v/v) and then recrystallized from dichloromethane:methanol (2:1) to obtain the target product, RFTzR (0.14 g, 71% yield) as red crystals. 1H NMR (600 MHz, CDCl3, ppm) d: 7.31–7.33 (d, 2H), 7.16–7.18. (d, 2H), 7.08–7.10 (d, 2H), 7.05–7.07 (d, 2H), 6.72–6.75 (d, 2H), 6.67–6.69 (d, 2H), 2.80–2.84 (t, 4H), 1.69–1.73 (m, 4H), 1.29–1.40 (m, 12H), 0.86–0.92 (t, 6H). 13C NMR (400 MHz, CDCl3, ppm) d: 152.61, 152.40, 151.20, 150.98, 147.71, 147.57, 145.19, 145.07, 139.40, 139.18, 138.61, 138.45, 137.10, 136.91, 135.46, 135.27, 128.14, 127.92, 127.11, 126.90, 125.66, 125.46, 104.32, 104.22, 43.30, 43.19, 32.37, 31.13, 30.92, 29.31, 29.18, 22.88, 22.95, 13.98, 13.90. FT-IR (KBr, cm-1): 3113, 3068, 2954, 2917, 2848, 1836, 1741, 1643, 1541, 1464, 1398, 1325, 1203, 1161, 1129, 1047, 990, 863, 802, 786, 717, 667, 575. MS (ESI): m/z calc. for C40H38N2F2S6 +: 770.23; found: 770.81 [M+].

Solution-Processed Bulk-Heterojunction Organic Solar Cell …

285

Reprinted with permission from [M. Nazim, 2015], RSC Adv., 5 (2015) 6286 © 2015 Royal Society of Chemistry Ltd. Scheme 1. Synthetic route of the furan-bridged organic chromophore.

Thermogravimetric and Differential Scanning Calorimetry Thermograms of Furan-Based Linear RFTzR Chromophore The thermal behaviour of RFTzR is analyzed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under the N2 atmosphere. The synthesized chromophore, RFTzR, shows a good thermal stability with ~5% weight loss at ~394oC, as shown in the TGA plot (Figure 1). The DSC analysis (Figure 1 inset) reveals that the synthesized chromophore exhibits a strong melting peak (Tm) at ~205oC and a weak melting peak at ~225oC, suggesting the crystalline property of the chromophore. This chromophore also displays a weak isotropic-to crystalline peak at ~222oC with a strong peak at ~187oC during the cooling cycle. The alkyl chains of organic molecules induce the solubility and the liquid-crystalline (LC) nature of the chromophore via self-assembly behavior. Generally, self-assembly behavior is the electrostatic interactions, which might be due to the result of π–π staking and the hydrogen bonding ability of the organic

286

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al.

molecules. The present chromophore has shown the possibility of hydrogen bonding due to oxygen (furan spacer) as well as strong π – π staking ability [39, 40].

Reprinted with permission from [M. Nazim, 2015], RSC Adv., 5 (2015) 6286 © 2015 Royal Society of Chemistry Ltd. Figure 1. TGA and DSC (inset) thermograms of furan-based linear RFTzR chromophore.

OPTICAL CHARACTERIZATIONS OF RFTZR UV–Vis Spectroscopy and Photoluminescence Spectra The UV-vis absorption spectra of RFTzR in dilute (1 x 10-4 M) chloroform (Figure 2a) shows a strong absorption band at λmax = 473 nm due to the π–π* transition, which is red shifted to ~484 nm in thin film state (Table 1), and hence RFTzR obtains a higher absorption edge and lower optical band gap than that of previously reported RTzR [20]. The organic chromophore shows a reasonably high molar absorption coefficient (ε) ~2.0 x 104 M-1 cm-1 in solution at the absorption maxima (λmax = 473 nm), suggesting a better intramolecular charge transfer (ICT) transition between thiazolo [5,4-d]thiazolecore and thiophene rings due to the rigid molecular geometry in the furan-bridged linear chromophore [20, 41]. The red shifting occurs due to its strong intermolecular ordered packing through a long planar molecular structure in solid state thin film on glass [42, 43]. Moreover, the replacement of thiophene by furan spacer unit in the RFTzR molecule might also cause the higher shifting in the absorption band because furan has strong electro-negativity and a small sized oxygen atom compared to the thiophene sulphur atom. In this case, the optimized RFTzR film thickness is estimated as ~60 nm, which delivers the uniform and homogenous thin film on glass substrate. Therefore, the

Solution-Processed Bulk-Heterojunction Organic Solar Cell …

287

molecular geometry of the RFTzR molecule permits the good electronic interaction between acceptor and donor moieties, which might result in the improved internal charge transfer. It is noticed that the chromophore, RFTzR, exhibits a relatively small optical band gap of ~2.18 eV, which is calculated using the wavelength of the absorption edge of the thin film of the small organic molecule. Hence, this value could be considered as a reasonable optical band gap for small organic molecules [20].

Reprinted with permission from [M. Nazim, 2015], RSC Adv., 5 (2015) 6286 © 2015 Royal Society of Chemistry Ltd. Figure 2. (a) UV-vis spectra of RFTzR in chloroform solution and RFTzR thin film deposited on glass substrate by spin coating; (b) photoluminescence spectra of RFTzR thin film on ITO substrate and RFTzR:PC60BM (2:1 w/w) blend layer thin film.

To examine the donor–acceptor charge transfer process, the photoluminescence spectroscopy (Figure 2b) of the synthesized chromophore has been analyzed in thin film on glass substrate and blend with the PC60BM layer. The RFTzR thin film displays a strong emission band at ~625 nm, which might reduce the possibility of charge recombination in film state, and hence induces exciton-diffusion towards the donor–

288

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al.

acceptor interface, which is ultimately beneficial for exciton-dissociation [44, 45]. Moreover, the RFTzR:PC60BM blend active layer thin film (2:1, w/w) displays the complete quenching in PL emission, suggesting a better blend formation and charge transport for the organic chromophore, which might enhance the photovoltaic properties of the conjugated chromophore during solution processed fabrication [46]. Table 1. Optical and Electrochemical properties of the furan-bridged small molecule, RFTzR. Reprinted with permission from [M. Nazim, 2015], RSC Adv., 5 (2015) 6286 © 2015 Royal Society of Chemistry Ltd Small molecule λmaxa (nm) λmaxb (nm) HOMOc (eV) LUMOd (eV) Ege (eV) RFTzR 473 484 -5.36 -3.14 2.18 a Absorption in chloroform solution bAbsorption of thin film on ITO cEstimated from the onset of oxidation potential from the formula: HOMO = ‒(4.4 + Eoxonset) (eV), dEstimated from the formula: LUMO = Egopt + HOMO, eOptical band gap calculated from the onset of the UV-vis spectra of the film from the formula: Egopt = 240/(λonset)film.

Cyclic Voltammetry of the Furan-Bridged Organic Chromophore The cyclic voltammetry (CV) studies of RFTzR thin film in 0.1 M acetonitrile solution of tetrabutylammonium hexa-fluorophosphate [nBu4N]+[PF6]- at a potential scan rate of 100 mV s-1 has been carried out, as shown in Figure 3. The furan bridged chromophore exhibits relatively low electrochemical stability because the oxidation potential peak is not completely reversible, which might affect the photovoltaic properties of the organic chromophore. The oxidation potential peak is situated at the onset values of Eoxonset =+0.96 ± 0.02 V. From the CV observations, HOMO value of ~5.36 eV is obtained using the equation: HOMO = -(4.4 + Eox onset) (eV). The LUMO energy level of -3.14 eV is estimated using the formula: LUMO = Eg opt + HOMO [47–49]. In general, the proper electron transfer from donor to the acceptor molecule requires a higher LUMO level of donor by at least ~0.3 eV to the LUMO energy level of the acceptor molecule [48]. The LUMO energy level of PC60BM has values that range between ~4.0 and ~4.3 eV [49]. In support, a LUMO–LUMO offset of 0.3–0.4 eV is necessary for an exciton-dissociation and the efficient electron transfer from donor to PC60BM [50–52]. In our case, the LUMO–LUMO offset between RFTzR and PC60BM is larger than ~0.3 eV; therefore, it could be expected that the exciton might easily dissociate at the donor–acceptor interface. The substitution of thiophene from the furan unit and terminal alkyl units at both ends in RFTzR might improve the intermolecular charge transfer (ICT) transition and hence, induces higher absorption and self-assembly

Solution-Processed Bulk-Heterojunction Organic Solar Cell …

289

behavior via liquid crystal properties of RFTzR and therefore, displays better efficiency [53, 54].

Reprinted with permission from [M. Nazim, 2015], RSC Adv., 5 (2015) 6286 © 2015 Royal Society of Chemistry Ltd. Figure 3. Cyclic voltammetry of the furan-bridged organic chromophore with 0.1 M TBAPF6 as supporting electrolyte in anhydrous acetonitrile solution at a scan rate of 100 mV s-1.

THE PHOTOVOLTAIC PARAMETERS OF THE FABRICATED ORGANIC SOLAR CELL The Current (J)–Voltage (V) Curves and the Incident Photon-to-Current Conversion Efficiency Spectra of the Fabricated Organic Solar Cells with the Active Layer of RFTzR: PC60BM The photovoltaic parameters of the fabricated SMOSCs are evaluated by the current density (J)–voltage (V) measurements (Figure 4a) under the 1 sun light (100 mW cm-2, 1.5 AM). Among the fabricated SMOSCs, a high power conversion efficiency (PCE) of ~2.72% is achieved by SMOSC fabricated with RFTzR:PC60BM (2:1, w/w), whereas other fabricated SMOSCs exhibit inferior PECs of ~1.72% for RFTzR:PC60BM (1:1, w/w) and ~1.94% for RFTzR:PC60BM (3:1, w/w). SMOSC fabricated RFTzR:PC60BM (2:1, w/w) presents the high short-circuit current density (JSC) of ~10.13 mAcm-2 due to higher wavelength and scattering behavior along with high open circuit voltage (VOC) of ~0.756. The lowering in the VOC value at low and high concentrations of RFTzR in the blend layer might relate to their morphological features. As shown in AFM analysis, RFTzR:PC60BM (1:1, w/w) and RFTzR:PC60BM (1:1, w/w) blend thin films exhibit high

290

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al.

roughness, indicating less homogeneous film between RFTzR and PC60BM molecules, which might result in the low VOC. It is believed that the weight ratio 2:1 (RFTzR:PC60BM) might optimize the sample containing the highly homogeneous thin film of RFTzR and PC60BM. The improved JSC and VOC might be explained by the ultrafast and complete intermolecular charge transfer (ICT) between RFTzR and PC60BM due to the introduction of the furan unit with the thiazolo[5,4-d]thiazole backbone. Furthermore, the introduction of the furan unit as spacer in RFTzR chromophore might be an effective supplier of holes owing to its smaller resonance energy (16 kcal mol-1) than the thiophene unit as spacer (29 kcal mol-1) [55, 56]. Compared to the thiophene unit, the small size of the oxygen atom in the furan unit might induce the planarity of the molecule and improve the morphology of blend thin which significantly enhances the photocurrent density and performances in solar cell devices [57].

Reprinted with permission from [M. Nazim, 2015], RSC Adv., 5 (2015) 6286 © 2015 Royal Society of Chemistry Ltd. Figure 4. (a) J–V curves of fabricated SMOSCs with the active layer of RFTzR:PC 60BM at various ratios of 1:1, 2:1 and 3:1 (w/w). (b) IPCE spectra of the fabricated SMOSCs with the active layer of RFTzR:PC60BM at various ratios of 2:1 and 3:1 (w/w).

Solution-Processed Bulk-Heterojunction Organic Solar Cell …

291

Table 2. Photovoltaic parameters of RFTzR based fabricated devices. Reprinted with permission from [M. Nazim, 2015], RSC Adv., 5 (2015) 6286 © 2015 Royal Society of Chemistry Ltd RFTzR: PC60BM 1:1, w/w 2:1, w/w 3:1, w/w

Photovoltaic parameters JSC (mA/cm2) 6.38 10.13 8.29

VOC (V) 0.667 0.756 0.723

FF 0.40 0.34 0.32

PCE (%) 1.72 2.72 1.94

In support, the presence of two terminal alkyl chains in the molecules might also induce aggregation and hence, facilitate the charge transfer rate and increase the photocurrent density of the devices. At large amounts of RFTzR, the low VOC and fill factor is related to the fast electron transfer and the recombination rate at the interface of TCO and the active organic layer of the device [58–60]. The incident photon-to-current efficiency (IPCE) measurements have been conducted to examine the contribution of incident photons to the photocurrent for the fabricated SMOSCs. Figure 4b shows the IPCE plots of the fabricated SMOSCs with different active layer ratios. SMOSC with RFTzR:PC60BM (2:1, w/w) records the maximum IPCE of 37% in the broad absorption wavelengths between 450 and 650 nm, whereas SMOSC with RFTzR:PC60BM (3:1, w/w) shows low IPCE of 25% in the same absorption range. The obtained IPCE results are in good accordance with the photovoltaic parameters, especially JSC values.

Atomic Force Microscopy (AFM) Spectroscopy of RFTzR:PCBM Active Layer The morphological analysis is investigated to explain the homogeneity of the blend RFTzR:PC60BM active layer (1:1, 2:1, 3:1, w/w) using the atomic force microscopy (AFM), as depicted in Figure 5. The AFM samples were prepared via spin coating of the active layer on ITO/PEDOT:PSS coated ITO substrate for a better understanding and comparison of film morphology, as presented elsewhere [61]. From Figure 5(c and d), The blended thin film of RFTzR:PC60BM (2:1, w/w) exhibits homogeneous and smooth morphology of low surface roughness (Rrms = 1.94 nm) with nanoscale phase separation, suggesting a good mixing between RFTzR and PC60BM in chlorobenzene solvent, whereas other blended thin films of RFTzR:PC60BM (1:1 and 3:1 w/w) record the high surface roughness of Rrms = 2.73 nm and Rrms = 2.02 nm, respectively. These results show that the active layer of RFTzR:PC60BM (2:1, w/w) is at an optimized ratio for achieving the best performance of organic solar cell devices. It is known that the smooth morphology and homogeneous blend with the nanoscale phase separation are responsible

292

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al.

for the large donor–acceptor interface area needed for exciton dissociation [62]. Herein, the morphological analysis reveals that RFTzR:PC60BM (2:1, w/w) blend thin film depicts the low surface roughness value (1.94 nm) compared to other RFTzR:PC60BM ratios (1:1 and 3:1, w/w), which suggests the homogeneity of RFTzR and PC60BM molecules in the blend layer and provides enough surface area for exciton-dissociation. This significantly improves the JSC and power conversion efficiency of SMOSCs. The fill factor value is low for all SMOSCs devices due to a number of reasons, such as domain size, film morphology, misalignment of energy levels and series resistance. The series resistance is composed of the resistance of the different semiconductor layers of the cell in addition to the metal semiconductor contacts.

Reprinted with permission from [M. Nazim, 2015], RSC Adv., 5 (2015) 6286 © 2015 Royal Society of Chemistry Ltd. Figure 5. Topographical and 3D AFM images of different active layer blend thin films of (a and b) RFTzR:PC60BM (1:1, w/w), (c and d) RFTzR:PC60BM (2:1, w/w), and (e and f) RFTzR:PC60BM (3:1, w/w).

Solution-Processed Bulk-Heterojunction Organic Solar Cell …

293

The low FF is related to the increase in the series resistance of RFTzR:PC60-BM/ITO, resulting in the high recombination rate [63,64]. The high FF in RFTzR:PCBM (1:1, w/w) based SMOSC is related to its low series resistance of 6.16 Ωcm2 as compared to other SMOSCs with high series resistances of ~9.18 Ωcm2 (RFTzR:PC60BM (2:1, w/w)) and ~11.42 Ωcm2 (RFTzR:PC60BM (3:1, w/w)). The AFM images of the active layer RFTZR:PC60BM (2:1, w/w) suggest that RFTzR has a good miscibility with PC60BM in the blended films, and hence a spontaneous phase-segregation process in the blend layers could form a bicontinuous network structure, which acts as percolation channels for the efficient carrier collection within the active layer of BHJ solar cells [65]. Therefore, the presence of the furan spacers in linear chromophore with the homogeneous blended thin film of RFTzR:PC60BM (2:1, w/w) substantially improves the interface area for exciton dissociation, resulting in the high photocurrent density and the power conversion efficiency of the devices.

CONCLUSION A new and novel furan-bridged thiazolo[5,4-d]thiazole based p-conjugated organic (RFTzR) chromophore is synthesized and applied as an active material for the fabrication of SMOSCs. The synthesized chromophore RFTzR is highly soluble in common organic solvents due to the presence of two terminal alkyl units at both ends of the molecule. The RFTzR chromophore substantiates the reasonable HOMO and LUMO energy levels of ~5.36 eV and ~3.14 eV, respectively. The fabricated SMOSCs with RFTzR:PC60BM (2:1, w/w) exhibits relatively high power conversion efficiency of ~2.72% with high photocurrent density of ~10.13 mAcm-2 and high VOC of 0.756V. The improvement in the efficiency for the furan-bridged chromophore might attribute to the better solubility, good miscibility with PC60BM and uniform film morphology of the devices, which serves an ultrafast and complete intermolecular charge transfer (ICT) between RFTzR and PC60BM. The introduction of the furan unit in place of thiophene in the thiazolo[5,4d]thiazole core organic chromophore significantly increases absorption, solubility and better thin film morphology of solar cell devices and hence, this study has shown a promising way for the furan based organic chromophores in future.

REFERENCES [1] [2]

J. S. Moon, C. J. Takacs, Y. Sun, A. J. Heeger, Nano Lett. 11 (2011) 1036. B. Walker, C. Kim, T. Q. Nguyen, Chem. Mater. 23 (2011) 470.

294 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al. Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan, A. J. Heeger, Nat. Mater. 11 (2012) 44. J. Roncali, Acc. Chem. Res. 42 (2009) 1719. X. Liu, Y. Sun, L. A. Perez, W. Wen, M. F. Toney, A. J. Heeger, G. C. Bazan, J. Am. Chem. Soc. 134 (2012) 20609. J. Peet, M. L. Senatore, A. J. Heeger, G. C. Bazan, Adv. Mater. 21 (2009) 1521. J. Zhou, X. Wan, Y. Liu, Y. Zuo, Z. Li, G. He, G. Long, W. Ni, C. Li, X. Su, Y. Chen, J. Am. Chem. Soc., 134 (2012) 16345. M. A. Green, K. Emery, Y. Hishikawa, W. Warta, Prog. Photovoltaics, 19 (2011) 84. A. Mishra, P. Bauerle, Angew. Chem., Int. Ed., 51 (2012) 2020. S. Paek, N. Cho, K. Song, M. J. Jun, J. K. Lee, J. K. Ko, J. Phys. Chem. C. 116 (2012) 23205. P. M. Beaujuge, J. M. J. Frechet, J. Am. Chem. Soc. 133 (2011) 20009. J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li, G. He, C. Li, B. Kan, M. Li, Y. Chen, J. Am. Chem. Soc. 135 (2013) 8484. S. Wang, L. Hall, V. V. Diev, R. Haiges, G. Wei, X. Xiao, P. I. Djurovich, S. R. Forrest, M. E. Thompson, Chem. Mater. 23 (2011) 4789. P. Dutta, W. Yang, S. H. Eom, W. H. Lee, I. N. Kang, S. H. Lee, Chem. Commun. 48 (2012) 573. Z. Li, J. Pei, Y. Li, B. Xu, M. Deng, Z. Liu, H. Li, H. Lu, Q. Li, W. Tian, J. Phys. Chem. C. 114(2010)18270. F. Yang, M. Shtein, S. R. Forrest, Nat. Mater. 4 (2005) 37. Q. Peng, Q. Huang, X. Hou, P. Chang, J. Xu, S. Deng, Chem. Commun. 48 (2012) 11452. G. Wei, R. R. Lunt, K. Sun, S. Wang, M. E. Thompson, S. R. Forrest, Nano Lett. 10 (2010) 3555. F. Yang, K. Sun, S. R. Forrest, Adv. Mater. 19 (2007) 4166. M. Nazim, S. Ameen, M. S. Akhtar, Y. S. Lee, H. S. Shin, Chem. Phys. Lett. 574 (2013) 89. P. Cheng, Q. Shi, Y. Lin, Y. F. Li, X. Zhan, Org. Electron. 14 (2013) 599. M. Melucci, L. Favaretto, A. Zanelli, M. Cavallini, A. Bongini, P. Maccagnani, P. Ostoja, G. Derue, R. Lazzaroni, G. Barbarella, Adv. Funct. Mater. 20 (2010) 445. B. Walker, A. B. Tamayo, X. D. Dang, P. Zalar, J. H. Seo, A. Garcia, M. Tantiwiwat, T. Q. Nguyen, Adv. Funct. Mater. 19 (2009) 3063. M. C. Scharber, D. Muhlbacher, M. Koppe, P. Denk, C. Waldauf, A. J. Heeger, C. J. Brabec, Adv. Mater. 18 (2006) 789. X. Wang, S. Chen, Y. Sun, M. Zhang, Y. F. Li, X. Li, H. Wang, Polym. Chem. 2 (2011) 2872.

Solution-Processed Bulk-Heterojunction Organic Solar Cell …

295

[26] J. T. Lin, P. C. Chen, Y. S. Yen, Y. C. Hsu, H. H. Chou, M. C. P. Yeh, Org. Lett. 11(2009) 97. [27] J. Y. Mao, F. L. Guo, W. J. Ying, W. J. Wu, J. Li, J. L. Hua, Chem. Asian J. 7 (2012) 982. [28] O. Gidron, Y. D. Posner, M. Bendikov, J. Am. Chem. Soc. 132 (2010) 2148. [29] S. S. Zade, N. Zamoshchik, M. Bendikov, Chem. Eur. J. 15 (2009) 8613. [30] C. H. Woo, P. M. Beaujuge, T. W. Holcombe, O. P. Lee, J. M. J. Frechet, J. Am. Chem. Soc. 132 (2010) 15547. [31] A. T. Yiu, P. M. Beaujuge, O. P. Lee, C. H. Woo, M. F. Toney, J. M. J. Frechet, J. Am. Chem. Soc. 134 (2012) 2180. [32] A. Gandini, Macromolecules. 41 (2008) 9491. [33] J. B. Binder, R. T. Ronald, J. Am. Chem. Soc. 131 (2009) 1979. [34] N. Crivillers, L. Favaretto, A. Zanelli, I. Manet, M. Treier, V. Morandi, M. Gazzano, P. Samor`ı, M. Melucci, Chem. Commun. 48 (2012) 12162. [35] C. M. Cardona, W. Li, A. E. Kaifer, D. Stockdale, G. C. Bazan, Adv. Mater. 23 (2011) 2367. [36] Z. Li, Q. Dong, B. Xu, H. Li, S. Wen, J. Pei, S. Yao, H. Lu, P. Li, W. Tian, Sol. Energy Mater. Sol. Cells. 95 (2011) 2272. [37] L. Xue, L. Liu, Q. Gao, S. Wen, J. He, W. Tian, Sol. Energy Mater. Sol. Cells. 93 (2009) 501. [38] I. H. Jung, J. Yu, J. Jeong, J. Kim, S. Kwon, H. Kong, K. Lee, H. Y. Woo, H. K. Shim, Chem. Eur. J. 16 (2010) 3743. [39] P. Deshmukh, S. k. Ahn, L. G. de Merxem, R. M. Kasi, Macromolecules. 46 (2013) 8245. [40] C. J. Takacs, Y. Sun, G. C. Welch, L. A. Perez, X. Liu, W. Wen, G. C. Bazan, A. J. Heeger, J. Am. Chem. Soc. 134 (2012) 16597. [41] D. H. Wang, J. S. Moon, J. Seifter, J. Jo, J. H. Park, O. O. Park, A. J. Heeger, Nano Lett. 11 (2011) 3163. [42] J. J. A. Chen, T. L. Chen, B. S. Kim, D. A. Poulsen, J. L. Mynar, J. M. J. Frechet, B. Ma, ACS Appl. Mater. Interfaces. 2 (2010) 2679. [43] L. Biniek, C. L. Chochos, N. Leclerc, G. Hadziioannou, J. K. Kalitsis, R. Bechara, P. Leveque, T. Heiser, J. Mater. Chem. 19 (2009) 4946. [44] L. Huo, X. Guo, Y. Li, J. Hou, Chem. Commun. 47 (2011) 8850. [45] P. Dutta, W. Yang, S. H. Eom, S. H. Lee, Org. Electron. 13 (2012) 273. [46] S. W. Shelton, T. L. Chen, D. E. Barclay, B. Ma, ACS Appl. Mater. Interfaces. 4 (2012) 2534. [47] J. K. Lee, B. S. Jeong, J. Kim, C. Kim, J. Ko, J. Photochem. Photobiol. A, 251 (2013) 25. [48] H. Zhou, L. Yang, W. You, Macromolecules. 45 (2012) 607.

296

M. Nazim, Sadia Ameen, M. Shaheer Akhta et al.

[49] R. Zhou, Q. D. Li, X. C. Li, S. M. Lu, L. P. Wang, C. H. Zhang, J. Huang, P. Chen, F. Li, X. H. Zhu, W. C. H. Choy, J. Peng, Y. Cao and X. Gong, Dyes Pigm. 101 (2014) 51. [50] M. D. Irwin, D. B. Buchholz, A. W. Hains, R. P. H. Chang, T. J. Marks, Proc. Natl. Acad. Sci. 105 (2008) 2783. [51] A. P. Zoombelt, M. Fonrodona, M. G. R. Turbiez, M. M. Wienk, R. A. J. Janssen, J. Mater. Chem. 19 (2009) 5336. [52] G. Zhao, G. Wu, C. He, F. Q. Bai, H. Xi, H. X. Zhang, Y. Li, J. Phys. Chem. C. 113 (2009) 2636. [53] R. Katoh, A. Furube, S. Mori, M. Miyashita, K. Sunahara, N. Koumuraa, K. Hara, Energy Environ. Sci. 2 (2009) 542. [54] U. F. Bunz, Angew. Chem., Int. Ed. 49 (2010) 5037. [55] J. Yang, F. Guo, J. Hua, X. Li, W. Wu, Y. Qu, H. Tian, J. Mater. Chem. 22 (2012) 24356. [56] J. Preat, C. Michaux, D. Jacquemin, E. A. Perpete, J. Phys. Chem. C. 113 (2009) 16821. [57] K. A. Mazzio, M. Yuan, K. Okamoto, C. K. Luscombe, ACS Appl. Mater. Interfaces, 3 (2011) 271. [58] B. Walker, X. Han, C. Kim, A. Sellinger, T. Q. Nguyen, ACS Appl. Mater. Interfaces, 4 (2012) 244. [59] Q. Shi, P. Cheng, Y. F. Li, X. Zhan, Adv. Energy Mater. 2 (2012) 63. [60] M. Nazim, S. Ameen, M. S. Akhtar, H. K. Seo, H. S. Shin, Synth. Met. 187 (2014) 178. [61] B. Walker, A. B. Tamayo, D. T. Duong, X. D. Dang, C. Kim, J. Granstrom, T. Q. Nguyen, Adv. Energy Mater. 1 (2011) 221. [62] K. Takemoto, M. Karasawa, M. Kimura, ACS Appl. Mater. Interfaces. 4 (2012) 6289. [63] Y. Li, Q. Guo, Z. Li, J. Pei, W. Tian, Energy Environ. Sci. 3 (2010) 1427. [64] J. S. Moon, C. J. Takacs, S. Cho, R. C. Coffin, H. Kim, G. C. Bazan, A. J. Heeger, Nano Lett. 10 (2010)4005. [65] T. W. Holcombe, C. H. Woo, D. F. J. Kavulak, B. C. Thompson, J. M. J. Frechet, J. Am. Chem. Soc. 131 (2009) 14160.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 21

FUMARONITRILE-CORE AND TERMINAL ALKYLATED BITHIOPHENE FOR SOLUTION PROCESSED SMALL MOLECULE ORGANIC SOLAR CELLS M. Nazim, Sadia Ameen, Hyung-Kee Seo and Hyung Shik Shin* Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea

ABSTRACT A new and novel organic π-conjugated chromophore (named as RCNR) based on fumaronitrile-core acceptor and terminal alkylated bithiophene is designed, synthesized and utilized as an electrondonor material for the solution-processed fabrication of bulkheterojunction (BHJ) small molecule organic solar cells (SMOSCs). The synthesized organic chromophore exhibits a broad absorption peak near the green region and the strong emission peak due to the presence of strong electron-withdrawing nature of two nitrile (–CN) groups of fumaronitrile acceptor. The highest occupied molecular orbital (HOMO) energy level of –5.82 eV and the lowest unoccupied molecular orbital (LUMO) energy level of –3.54 eV are estimated for RCNR due to the strong electron-accepting tendency of –CN groups. The fabricated SMOSC devices with RCNR:PC60BM (1:3, w/w) active layer exhibits the reasonable power conversion efficiency (PCE) of ~2.69% with high short-circuit current density (JSC) of ~9.68 mA/cm2 and open circuit voltage (VOC) of ~0.79 V.

*

Email: [email protected].

298

M. Nazim, Sadia Ameen, Hyung-Kee Seo et al.

INTRODUCTION In organic solar cells (OSCs), organic π-conjugated chromophores have shown a great potential as an alternative to organic π-conjugated polymers in solution-processed bulk-heterojunction (BHJ) organic photovoltaic devices due to their various advantages such as light weight, flexibility, low cost, and the ease of synthesis and fabricationprocessing [1–5]. Organic chromophores with aromatic fumaronitrile-core have attracted a significant attention in electroluminescent (EL) devices due to their efficient emission properties in the solid state [6–8]. The presence of diphenylfumaronitrile-core greatly reduces the fluorescence quenching in the solid state because of the interaction of antiparallel dipoles [9, 10]. In few decades, a lot of substantial efforts have been performed for improving the device performances of solution-processed small molecule organic solar cells (SMOSCs) and attained the high PCE through the development of organic photoactive electron-donor materials [11, 12]. The achievement of PCEs of over ~8% in SMOSCs has made them a serious candidates for the next generation of solar cells, polymer solar cells (PSCs), thin film solar cells and dye-sensitized solar cells (DSSCs) [13, 14]. The high-efficiency solar cell devices have been reported for a solutionprocessed bulk-heterojunction (BHJ) OSCs containing low-band gap semiconducting polymers and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), or [6,6]-phenylC71-butyric acid methyl ester (PC71BM) [15, 16]. Oligothiophenes have been employed as organic electron-donors owing to their high electron-density, well-defined and planar structure, and good solubility for the design and construction of optical and organic electronic materials. The development of organic conjugated donor–acceptor–donor (D–A–D) systems composed of oligothiophenes (donor) and electron-deficient molecule (acceptor) provides an efficient approach since these molecules could increase an absorption band through intramolecular charge transfer (ICT) for better match-up of the solar spectrum and thus, attain the high PCE in organic solar cells devices [17–19]. In general, the presence of –CN group in organic polymers lowers HOMO and LUMO values compared to without –CN-groups analogues [20–26]. However, only a few –CN group-modified polymers have been reported to function in photovoltaic devices [27]. To achieve organic compounds with –CN group for PSCs is still a challenge and more efforts are needed to explore the fundamental aspect of –CN group based D−A systems for high performance devices [28]. In this regard, a new, symmetrical D–A–D organic semiconductor framework is designed with the fumaronitrile (FN) as an electron-withdrawing moiety and utilized in solution-processed SMOSCs. In this work, we report the synthesis and organic photovoltaic characteristics of a novel and efficient D–A–D type (Figure 1) fumaronitrile-based organic π -conjugated chromophore, 2,3-bis(4-(5-(5-hexylthiophen-

Fumaronitrile-Core and Terminal Alkylated Bithiophene …

299

2-yl)thiophen-2-yl)phenyl) fumaronitrile (named as RCNR). The photovoltaic characteristics are significantly influenced by the self-assembly behavior of RCNR due to the existence of different liquid-crystal phases.

SYNTHETIC ROUTE OF FUMARONITRILE BASED ORGANIC CHROMOPHORE (RCNR) The reaction intermediates 2, 3, 4, 5, and 7 were synthesized as reported elsewhere [6, 44]. In brief, the target product, RCNR was finally obtained by the Suzuki crosscoupling reaction between intermediate 7 and intermediate 5 using Pd(PPh3)4 (2.5 mol %) as catalyst and potassium carbonate, K2CO3 as a base in anhydrous toluene solvent under inert atmosphere. The synthesized red-colored chromophore was then purified by repeated crystallization in the mixed solvent of dichloromethane/methanol (2:1, v/v) with a decent yield of 86.4%.

1-(5-(Thiophen-2-yl) Thiophen-2-yl) Hexan-1-One (2) Hexanoyl chloride (4.07 mL, 20.0 mmol) was added to a solution of 2,2’-bithiophene 1 (3.17 g, 19.1 mmol) in anhydrous benzene (20 mL) at the room temperature. Then TiCl4 (2.25 mL, 20.5 mmol) was added slowly to the reaction mixture at 0°C and was stirred for 15 min at 0°C. After completion of the reaction, cold water was added into the reaction mixture to quench the reaction. The resulting mixture was diluted with CH2Cl2 (50 mL), washed successively with water (200 mL) and saturated aqueous solution of NaHCO3 (100 mL), then dried over MgSO4 followed by an evaporation under vacuum to afford a yellow solid (5.00 g, 85%), anticipated as the desired ketone intermediate 2 which was used directly for next step of the reaction.

2-Decyl-5-(Thiophen-2-yl) Thiophene (3) Under nitrogen atmosphere, the solution of intermediate 2 (5.00 g, 18.9 mmol) in anhydrous toluene (40 mL) and a suspension of LiAlH4 (4.6 g, 121 mmol) and AlCl3 (4.03 g, 30.3 mmol) in anhydrous Et2O (100 mL) were mixed slowly at 0°C with extreme care. The reaction mixture was then stirred for 1 h at the room temperature, and again cooled at 0°C, then ethyl acetate (20 mL) and HCl (6 M) solution (50 mL) were added to

300

M. Nazim, Sadia Ameen, Hyung-Kee Seo et al.

the reaction mixture. The resulting mixture was extracted with diethyl ether (300 mL), washed with NaCl solution and distilled water (50 mL) thereafter, dried over MgSO4. Afterward, solvent of the organic phase was evaporated by using rotatory evaporator followed by drying in the vacuum. The yellow residue was then purified by flash column chromatography on silica gel (hexane) to produce compound 3 (4.65 g, 98%) as a colorless oil. 1H NMR (400 MHz, CDCl3, δ, ppm): 7.15 (d, 1H), 7.10 (d, 1H), 6.96 (d, 1H), 6.95 (d, 1H), 6.65 (d, 1H), 2.75 (t, 2H), 1.69 (m, 2H), 1.35 (m, 14H), 0.90 (t, 3H); 13C NMR (100 MHz, CDCl3, δ, ppm): 146.4, 145.2, 138.0, 134.8, 127.7, 125.7, 124.3, 123.8, 123.5, 123.0, 32.0, 31.7, 30.2, 29.5, 29.4, 29.2, 22.8, 14.2.

5-Bromo-5’-Decyl-2,2’-Bithiophene (4) N-bromo succinimide (1.22 g, 6.86 mmol) was added to a solution of compound 3 (2.00 g, 8.0 mmol) in dimethylformamide (30 mL) and the reaction mixture was stirred for 30 min in the absence of light then, diluted with hexane (50 mL), washed with saturated aqueous solution of NH4Cl (50 mL), dried over MgSO4, and evaporated under vacuum. The residue was purified by column chromatography on silica gel (hexane) to give a white solid compound 4 (2.36 g, 89.9%). 1H NMR (400 MHz, CDCl3, δ, ppm): 6.95 (d, 1H), 6.90 (d, 1H), 6.85 (d, 1H), 6.68 (d, 1H), 2.78 (t, 2H), 1.66 (m, 2H), 1.35 (m, 14H), 0.90 (t, 3H); 13C NMR (100 MHz, CDCl3, δ, ppm): 145.6, 139.9, 133.5, 130.5, 130.1, 124.8, 123.7, 123.1, 110.2, 32.2, 31.7, 30.3, 29.9, 29.6, 29.4, 22.9, 14.4.

2-{5-(5-Decylthiophen-2-yl) Thiophen-2-yl}-4,4,5,5-Tetramethyl-1,3,2Dioxaborolane (5) Under nitrogen atmosphere, a solution of compound 4 (1.0 g, 3.06 mmol) and tetrahydrofuran (20 mL) was added to n-BuLi (1.6 M, 3.17 mmol) at –78°C. The temperature was increased slowly up to –50°C within 20 min. 2-Isopropoxy-4,4,5,5tetramethyl-1,3,2-dioxaborolane (0.58 mL, 5.27 mmol) was added and the temperature was increased slowly to the room temperature. The reaction mixture was then stirred for 3 h at room temperature and 2N HCl (20 mL) was added. The resulting mixture was extracted with diethylether (30 mL), washed with NaCl solution followed by distilled water (500 mL), dried over MgSO4, and evaporated under vacuum. The obtained residue was recrystallized from hexane (10 mL) to yield a white solid compound, 5 (0.85 g, 73.3%). 1H NMR (400 MHz, CDCl3, δ, ppm): 7.55 (d, 1H), 7.20 (d, 1H), 7.05 (d, 1H), 6.70 (d, 1H), 2.81 (t, 2H), 1.72 (m, 2H), 1.38 (m, 14H), 0.91 (t, 3H); 13C NMR (100 MHz, CDCl3, δ, ppm): 146.1, 144.8, 137.9, 134.7, 124.9, 124.5, 124.1, 84.1, 31.9, 31.5, 30.2, 29.6, 29.4, 29.3, 24.8, 22.6, 14.3.

Fumaronitrile-Core and Terminal Alkylated Bithiophene …

301

Reprinted with permission from [M. Nazim, 2015], Sci. Rep., 5 (2015) 11143 © 2015 Macmillan Publishers Ltd (Nature-Springer group). Figure 1. Synthetic route of fumaronitrile based organic chromophore (RCNR).

Bis (4-Bromophenyl) Fumaronitrile (7) A mixture of 4-bromophenylacetonitrile, 6 (4.86 g, 24.8 mmol) and iodine (6.35 g, 25 mmol) was purged with N2 and subsequently anhydrous diethyl ether (100 ml) was injected via syringe. A solution was cooled to –78°C. Sodium methoxide (NaOCH3, 2.84 g, 52.6 mmol) and methanol (40 ml) was added slowly over a period of 30 min and then

302

M. Nazim, Sadia Ameen, Hyung-Kee Seo et al.

stirred for 40 min. Then the reaction solution was put to ice-water bath at 0°C with stirring for further 4 h. Hydrochloric acid (3–6%) was added dropwise to the reaction mixture and the solution was filtered to isolate the precipitate, which was then rinsed with cold methanol-water solution. Filtrate was concentrated further and a second crop of target product was obtained as a pale yellow solid, 7 (5.87 g, 61.3%). FT-IR (KBr pellet, cm−1): 3096, 2220, 1585, 1488, 1396, 1245, 1074, 1007, 845, 816, 710, 665, 627, 573, 514. 1H NMR (400 MHz, CDCl3, δ, ppm):7.67-7.72 (m, 8H).

2,3-Bis(4-(5-(5-Hexylthiophen-2-yl)Thiophen-2-yl)Phenyl)Fumaronitrile (RCNR) In a 50 mL round bottom flask, monomer 5 (0.46 g, 1.22 mmol) and monomer 7 (0.198 g, 0.51 mmol) with triphenylphosphine (0.034 g, 0.03 mmol) were mixed and then subjected to three cycles of evacuation and nitrogen purging in anhydrous toluene (~10 mL) solvent. Aqueous solution of potassium carbonate (2 M, ~5 mL) was added by syringe to the reaction mixture and was stirred at 110°C for 12 h. The reaction mixture was cooled down to the room temperature followed by the addition of water. Subsequently, an organic phase was extracted with dichloromethane (~20 mL) and the reaction mixture was washed with brine and distilled water and dried over magnesium sulfate. The solution was filtered and evaporated in vacuum to achieve a red colored residue, which was then recrystallized several times in dichloromethane and methanol (2:1 v/v) mixture to get organic chromophore as a dark red solid (0.32 g, 86.4%). FT-IR (KBr pellet, cm-1): 3067, 2955, 2926, 2853, 2219, 1631, 1581, 1488, 1396, 1245, 1084, 1007, 845, 816, 710, 665, 627. 1H NMR (600 MHz, CDCl3, δ, ppm): 7.89–7.87 (d, 4H, Ar H), 7.75-7.71 (d, 4H, ArH), 7.55–7.52 (d, 2H), 7.36-7.34 (d, 2H) 7.11-7.02 (d, 2H), 6.72-6.71 (d, 2H), 2.84-2.78 (d, 4H), 1.70-1.68 (m, 4H), 1.39-1.31 (m, 12H), 0.92-0.88 (d, 6H). 13C NMR (100 MHz, CDCl3, δ, ppm): 145.2, 139.2, 138.2, 136.3, 133.2, 129.1, 128.6, 128.4, 124.8, 124.6, 124.1, 122.9, 116.2, 115.9, 30.6, 30.4, 29.2, 29.0, 27.8, 27.5, 21.5, 21.3, 12.9; MS: m/z 726 (M+).

THERMOGRAVIMETRIC ANALYSIS AND DIFFERENTIAL SCANNING COLORIMETRY PLOTS OF THE ORGANIC CHROMOPHORE The thermal stability of the synthesized organic chromophore, RCNR has been analyzed by the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under N2 atmosphere. The TGA plot (Figure 2) reveals that RCNR starts to decompose over ~300°C. The decomposition temperature (Td) of the RCNR is found as

Fumaronitrile-Core and Terminal Alkylated Bithiophene …

303

~368°C, indicating a relatively high thermal-stability of the organic chromophore which is expedient for the solution-processed device fabrication and the operation of organic solar cells [29]. From differential scanning calorimetry (DSC) measurement (Figure 2 inset), the RCNR show numbers of melting phase transitions (Tm) at ~69°C, ~161°C, and ~172°C, with no signs of a glass-transition temperature (Tg), while an isotropic transition phase is observed after ~270°C [30]. The increase in the thermal transition temperatures is an indication of enhanced intermolecular connectivity and thin film crystallinity in RCNR, which is attributed to the presence of induced π–π stacking [31]. The difference in film crystallinity is an important factor for solution-processed organic solar cells, as it shows a direct effect on the surface roughness of the thin film morphology and consequently, the solar cell device performance [32]. The presence of terminal alkyl chains of organic chromophore induces the solubility in common organic solvents. Additionally, different melting transitions suggest the occurrence of various liquid-crystalline (LC) phases of RCNR via selfassembly behavior [33]. Generally, self-assembly behavior is the result of electrostatic interactions which might be due to the result of π−π staking and hydrogen-bonding ability of the organic conjugated molecules [34, 35]. This clearly indicates the interconversion of different LC phases from smectic C to smectic A to nematic phase as a function of temperature [36, 37].

Reprinted with permission from [M. Nazim, 2015], Sci. Rep., 5 (2015) 11143 © 2015 Macmillan Publishers Ltd (Nature-Springer group). Figure 2. Thermogravimetric analysis (TGA) and Differential scanning colorimetry (DSC) plots of the organic chromophore.

M. Nazim, Sadia Ameen, Hyung-Kee Seo et al.

304

Reprinted with permission from [M. Nazim, 2015], Sci. Rep., 5 (2015) 11143 © 2015 Macmillan Publishers Ltd (Nature-Springer group). Figure 3. (a) Ultraviolet-visible (UV-Vis) spectra of RCNR in chloroform solution (Black line) and thin film (Red line) deposited on ITO substrate, and (b) Photoluminescence spectra of RCNR in chloroform solution (Black line), thin film (Red line) and RCNR:PC 60BM (1:3, w/w) active layer thin film (Blue line).

OPTICAL CHARACTERIZATIONS Ultraviolet-Visible and Photoluminescence Spectra of RCNR UV-Vis absorption spectra (Figure 3a) of RCNR have displayed a good absorption in dilute chloroform solution (1 × 10−5 M) and thin film state. In chloroform solution, the two distinct peaks are observed. The spectra shows a relatively small absorption peak at λ max ≈ 368 nm and another broad absorption peak at λ max ≈ 465 nm. The molar absorption coefficient (ε) in solution is calculated as ~1.58 × 104 M−1cm−1 which indicates a strong intramolecular charge transfer (ICT) interaction behavior between thiophene donor and fumaronitrile-acceptor [38, 39]. However, a slight red shift with broad absorption spectrum is observed for the chromophore in the solid thin film as compared to chloroform solution which might be due to an aggregation in the solid thin film state [40]. RCNR indicates an ordered and planar structure due to the presence of alkyl side chains, resulting in a good intermolecular electron-delocalization and hence, evolves the self-assembly behavior [41, 42]. Moreover, an optical band gap (Eg opt) of ~2.03 eV is calculated by the absorption edge (λ edge) from solid thin film absorption by the formula: Eg = 1240/λedge

(1)

Fumaronitrile-Core and Terminal Alkylated Bithiophene …

305

Reprinted with permission from [M. Nazim, 2015], Sci. Rep., 5 (2015) 11143 © 2015 Macmillan Publishers Ltd (Nature-Springer group). Figure 3. (a) Ultraviolet-visible (UV-Vis) spectra of RCNR in chloroform solution (Black line) and thin film (Red line) deposited on ITO substrate, and (b) Photoluminescence spectra of RCNR in chloroform solution (Black line), thin film (Red line) and RCNR:PC 60BM (1:3, w/w) active layer thin film (Blue line).

The photoluminescensce spectra (Figure 3b) of the synthesized organic chromophore has shown a good potential of light emitting properties in solution as well as solid thin film state. A single strong green emission peak at ~649 nm is recorded in chloroform solution at the room temperature which shows a slight red-shift in thin film spectra. This strong emission of RCNR is due to the intramolecular planarization or aggregation of organic chromophore [43]. It clearly indicates the fluorescence quenching after mixing with PC60BM acceptor, suggesting the electron transfer from donor to acceptor and the fast charge-transfer which is enough to compete with the radiative recombination of the excitons [44, 45].

CYCLIC VOLTAMMETRY OF RCNR THIN FILM The redox properties of the organic chromophore are measured by cyclic voltammetry (CV) studies of RCNR thin film (Figure 4) in 0.1 M CH 3CN solution of tetrabutyl ammonium hexa flouro phosphate [nBu4N]+[PF6]− at a potential scan rate of 100 mV/s. The oxidation and reduction peaks are situated at the onset value of Eox = + 1.42 ± 0.02 eV and Ered = –0.86 ± 0.02 eV. Hence, the RCNR solid thin film exhibits HOMO and LUMO of –5.82 eV and –3.54 eV, respectively. The observed electrochemical band gap is found to be Egel = 2.28 eV. The difference of HOMO and LUMO energy level is a crucial factor for determining the energy band gap which indicates the electrons delocalization in the solid thin films [46, 47]. Solutionprocessed BHJ small molecule organic solar cells are fabricated using RCNR as an

306

M. Nazim, Sadia Ameen, Hyung-Kee Seo et al.

electron-donor and [6,6]-phenyl C61-butyric acid methyl ester (PC60BM) as an electronacceptor with a standard device structure of ITO/PEDOT:PSS (~80 nm)/RCNR:PC60BM blend (~60 nm)/Ag (~100 nm).

Reprinted with permission from [M. Nazim, 2015], Sci. Rep., 5 (2015) 11143 © 2015 Macmillan Publishers Ltd (Nature-Springer group). Figure 4. Cyclic Voltammogram of RCNR thin film in 0.1 M acetonitrile solution containing [nBu4N]+[PF6]− as supporting electrolyte with a scan rate of 100 mV/s.

Reprinted with permission from [M. Nazim, 2015], Sci. Rep., 5 (2015) 11143 © 2015 Macmillan Publishers Ltd (Nature-Springer group). Figure 5. J-V curves of fabricated small molecule organic solar cells with the different RCNR:PC60BM active layers.

Fumaronitrile-Core and Terminal Alkylated Bithiophene …

307

The blended active layers of the solar cell devices are developed by spin-casting the various (1:1, 1:2, 1:3, 1:4, w/w) mixtures of the RCNR with PC60BM. The photovoltaic properties (Table 2) of the fabricated solar cell devices of RCNR have been examined by the current density (J)-voltage (V) measurements (Figure 5) under the 1 sun light (100 mW/cm2, 1.5 AM).

THE CURRENT DENSITY (J)-VOLTAGE (V) CURVES OF FABRICATED SMALL MOLECULE ORGANIC SOLAR CELLS WITH THE DIFFERENT RCNR:PC60BM ACTIVE LAYERS The PCE of ~2.69% is achieved by the SMOSC devices fabricated with RCNR:PC60BM (1:3, w/w) active layer ratio, whereas the other fabricated SMOSC devices exhibit inferior PCEs of ~1.50% for RCNR:PC60BM (1:1, w/w), ~2.0% for RCNR:PC60BM (1:2, w/w) and ~2.23% for RCNR:PC60BM (1:4, w/w) active layer ratios, as shown in Figure 5. The SMOSC fabricated with RCNR:PC60BM (1:3, w/w) active layer presents the JSC of ~9.68 mA/cm2, and high VOC of ~0.792 V. Herein, the presence of –CN groups connecting with vinyl double bond enhances the conjugation length of chromophore and hence, better electron-delocalization which might affect the open-circuit voltage and short-circuit density of the solar cell devices [8, 48]. Moreover, the presence of the terminal side chains has a strong impact on the aggregation and selforganizing behavior of the electron-donor molecules in BHJ thin films and hence, increases the photocurrent-density of the devices due to better charge transport [49]. The thin film morphology of the devices might be related to the lowering of the V OC value at low concentration ratios (1:1, 1:2, w/w) of RCNR in the blended active layers.

ATOMIC FORCE MICROSCOPY SPECTROSCOPY OF RCNR:PCBM ACTIVE LAYER The atomic force microscopy (AFM) analysis is used to investigate the morphological behavior of the blended active layer RCNR:PC60BM (1:1, 1:2, 1:3, 1:4, w/w) thin films, as shown in Figure 6. The RCNR:PC60BM (1:3, w/w) blended active layer (Figure 6(e,f)) clearly exhibits a homogeneous and smooth morphology of low rootmean-square surface roughness (Rrms = 2.06 nm) in nanoscale phase separation which contributes to good miscibility of donor-acceptor, high exciton-dissociation rate and better charge transport. On the other hand, other blended active layers of RCNR:PC60BM (1:1, 1:2 and 1:4, w/w) record high Rrms values of 9.20 nm, 2.63 nm, 3.29 nm, respectively.

308

M. Nazim, Sadia Ameen, Hyung-Kee Seo et al.

Reprinted with permission from [M. Nazim, 2015], Sci. Rep., 5 (2015) 11143 © 2015 Macmillan Publishers Ltd (Nature-Springer group). Figure 6. Topographic and three dimensional AFM images of the fabricated small molecule organic solar cells device of various ratios with RCNR:PC60BM (a,b) 1:1 w/w, (c,d) 1:2 w/w, (e,f) 1:3 w/w and (g,h) 1:4 w/w active layers.

Fumaronitrile-Core and Terminal Alkylated Bithiophene …

309

These results show that the RCNR:PC60BM (1:3, w/w) active layer is the optimized one for homogeneous miscibility between donor and acceptor yielding a smooth thin film morphology and a large donor-acceptor (D−A) interface area with nanoscale phase separation [33] and high exciton-dissociation rate, which eventually assists to achieve the best performance of organic solar cell devices. In addition, the morphological analysis reveals that RCNR:PC60BM (1:3, w/w) film depicts the lowest Rrms of ~2.06 nm as compared to other blended RCNR:PC60BM (1:1, 1:2 and 1:4, w/w) active layers, suggesting the homogenous nature of RCNR and PC60BM molecules in the blended layer which provides enough surface area for exciton-dissociation 40. For all the fabricated SMOSCs devices, the fill factor (FF) value is rather low due to a number of factors like unfavorable domain size, film morphology, misalignment of energy levels, large seriesresistance, etc. [44]. The active layer RCNR:PC60BM (1:3, w/w) device shows a minimum series-resistance and hence, a maximum FF of ~0.35. Furthermore, the lower values of FF are related to increase in the series-resistance of RCNR:PC60BM/ITO in SMOSCs, resulting in a higher recombination rate over the surface of RCNR:PC60BM blended active layers [6]. Due to spontaneous phase-segregation process in the blended active layers of RCNR and PC60BM, a bicontinuous network structure might form which creates the percolation channels for the efficient charge carrier collection within the active layer of BHJ solar cells [50]. On the other hand, the improvement in the VOC value might be due to the presence of two –CN groups which induce the better film morphology and strong intermolecular charge-transfer (ICT) between RCNR and PC60BM [9, 51]. Thus, the presence of two strong electron-withdrawing –CN groups might have electrostatic-attractions with PC60BM which improves the film-morphology of the blended active layers and ultimately increases the photocurrent-density for the better performance of solar cell devices [52].

CONCLUSION A novel, symmetric D-A-D type fumaronitrile-acceptor based organic π -conjugated chromophore (RCNR) is synthesized and applied as an electron-donor material for the solution-processed fabrication of SMOSCs. The synthesized organic chromophore presents a broad absorption peak near green region and strong emission peak due to the presence of two strong electron-withdrawing −CN groups. The cyclic voltammetry study of RCNR shows relatively deep HOMO of −5.82 eV and LUMO of −3.54 eV, which suggests a strong electron-accepting tendency of –CN groups. The fabricated SMOSC device of active layer RCNR:PC60BM (1:3, w/w) achieves a reasonable PCE of ~2.69% with JSC of ~9.68 mA/cm2 and VOC of ~0.79 V. The variation in the concentration of PC60BM acceptor in blended active layers has considerably affected the thin film morphology and hence, the performance of the fabricated solar devices.

310

M. Nazim, Sadia Ameen, Hyung-Kee Seo et al.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]

B. Walker, C. Kim, T. Q. Nguyen, Chem. Mater. 23 (2011) 470. Y. Li, Q. Guo, Z. Li. Pei, J. W. Tian, Energy Environ. Sci. 3 (2010) 1427. Y. Lin, H. Fan, Y. Li, X. Zhan, Adv. Mater. 24 (2012) 3087. A. Mishra, P. Bauerle, Angew. Chem. Int. Ed. 51 (2012) 2020. B. Walker, X. Han, C. Kim, A. Sellinger, T. Q. Nguyen, ACS Appl. Mater. Interfaces. 4 (2012) 244. H. C. Yeh, et al., J. Org. Chem. 69 (2004) 6455. A. Leliege, P. Blanchard, T. Rousseau, J. Roncali, Org. Lett. 13 (2011) 3098. H. C. Yeh, S. J. Yeh, C. T. Chen, Chem. Commun. (2003) 2632. D. Khuseynov, A. R. Dixon, D. J. Dokuchitz, A. Sanov, J. Phys. Chem. A. 118 (2014) 4510. X. Y. Shen, et al., J. Phys. Chem. C. 116 (2012) 10541. S. S. Palayangoda, X. Cai, R. M. Adhikari, D. C. Neckers, Sci. Rep. 3 (2013) 3356. Y. Sun, et al., S Nat. Mater. 11 (2012) 44. X. Liu, et al., J. Am. Chem. Soc. 134 (2012) 20609. J. Zhou, et al., J. Am. Chem. Soc. 134 (2012) 16345. Q. Wang, M. et al., J. Am. Chem. Soc. 133 (2011) 9638. Peng, et al., Chem. Commun. 48 (2012) 11452. Y. J. Cheng, S. H. Yang, C. S. Hsu, Chem. Rev. 109 (2009) 5868. F. Zhang, D. Wu, Y. Xu, X. Feng, J. Mater. Chem. 21 (2011) 17590. R. Fitzner, et al., Adv. Mater. 24 (2012) 675. A. Gupta, et al., Chem. Commun. 48 (2012) 1889. Y. Z. Lin, et al., Chem. Commun. 48 (2012) 9655. Y. Lin, Y. Li, X. Zhan, Chem. Soc. Rev. 41 (2012) 4245. S. H. Zeng, et al., Chem. Commun. 48 (2012) 10627. B. C. Thompson, Y. G. Kim,, T. D. McCarley, J. R. Reynolds, J. Amer. Chem. Soc. 128 (2006) 12714. Y. Nicolas, et al., Org. Lett. 6 (2004) 273. C. H. Duan, et al., Chem. Mater. 22 (2010) 6444. S. Mukhopadhyay, R. B. Kanth, S. Ramasesha, S. Patil, J. Phys. Chem. A. 114 (2010) 4647. X. Liu, et al., J. Am. Chem. Soc. 136 (2014) 5697. S. C. Lan, et al., ACS Appl. Mater. Interfaces. 6 (2014) 9298. M. C. Um, et al., J. Mater. Chem. 18 (2008) 2234. W. Shin, T. Yasuda, G. Watanabe, S. Yang, C. Adachi, Chem. Mater. 25 (2013) 2549. C. Kim, et al., Chem. Mater.24 (2012) 1699. G. K. Dutta, S. Guha, S. Patil, Org. Electron. 11 (2010) 1.

Fumaronitrile-Core and Terminal Alkylated Bithiophene …

[35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52]

311

X. Li, et al., Org. Lett. 10 (2008) 3785. J. A. Hur, et al., Synth. Met. 161(2012) 2776. M. Melucci, et al., Adv. Funct. Mater. 20 (2010) 445. K. Takemoto, M. Karasawa, M. Kimura, ACS Applied Mater. Interfaces. 12 (2012) 6289. N. Crivillers, et al., Chem. Commun. 48 (2012) 12162. K. A. Mazzio, M. Yuan, K. Okamoto, C. K. Luscombe, ACS Appl. Mater. Interfaces. 3 (2011) 271. Y. Yang, et al., J. Phys. Chem. C. 114 (2010) 3701. Q. Shi, P. Cheng, Y. F. Li, X. Zhan, Adv. Energy Mater. 2 (2012) 63. B. Walker, et al., Adv. Funct. Mater. 19 (2009) 3063. M. Wana, et al., Synth. Met. 178 (2013) 22. K.Takemoto, M. Karasawa, M. Kimura, ACS Appl. Mater. Interfaces. 4 (2012) 6289. J. Mei, K. R. Graham, R. Stadler, J. R. Reynolds, Org. Lett. 12 (2010) 660. C. M. Cardona, W. Li, A. E. Kaifer, D. Stockdale, G. C. Bazan, Adv. Mater. 23 (2011) 2367. T. Johansson, W. Mammo, M. Svensson, M. R. Andersson, O. Inganas, J. Mater. Chem. 13 (2003) 1316. H. G. Jeong, et al., J. Polym. Sci. Part A: Poly. Chem. 51 (2013) 1029. H. Y. Chen, J. L. Wu, C. T. Chen, Chem. Commun. 48 (2012) 1012. N. Leclerc, et al., Chem. Mater. 17 (2005) 502. L. Xue, et al., Sol. Energy Mater. Sol. Cells. 93 (2009) 501.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 22

SPIROBIFLUORENE-CORE ELECTRON-DONOR MATERIAL FOR BULK-HETEROJUNCTION SOLAR CELLS M. Nazim1, Sadia Ameen1, M. Shaheer Akhtar2 and Hyung Shik Shin1,* 1

Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT An efficient spirobifluorene-based organic small molecule (RTh-Sp-CF3) is synthesized in a simple manner via Suzuki coupling reaction containing an alkyl bithiophene as donor and 3,5-bis (trifluoromethyl) benzene as acceptor unit. The spirobifluorene-based small molecule is utilized as an electron-donor materials with wellknown electron-acceptor material, phenyl-C61-butyric acid methyl ester (PC61BM) in the solution-processed small molecule organic solar cells (SMOSCs). The incorporation of 3,5-bis (trifluoromethyl) benzene unit as electron-acceptor has significantly tuned the energy levels of small molecule and obtained the HOMO and LUMO energy levels of ~5.35 eV and ~3.92 eV, respectively. SMOSCs fabricated with RTh-Sp-CF3 accomplishes an overall power conversion efficiency (PCE) of ~2.12% with short circuit current (JSC) of ~8.42 mA/cm2 and the open-circuit voltage (VOC) of ~0.66 V. The reasonable JSC and VOC of devices might be attributed to the presence of strong electron-

*

Corresponding Author Email: [email protected].

314

M. Nazim, Sadia Ameen, M. Shaheer Akhtar et al. withdrawing fluorine units in RTh-Sp-CF3, which results from the improved absorption and electrochemical properties.

INTRODUCTION Organic solar cells especially, bulk-heterojunction organic solar cells have owned considerable interest, both academically and industrially due to their versatile merits of low cost, large-scale fabrication, flexibility and diversity of electron-donors [1–4]. Apart from other organic solar cells, the small molecule organic solar cells (SMOSCs) have been growing as an alternative to polymer counterparts in solution-processed bulkheterojunction (BHJ) owing to their simple molecular structures, easy synthesis and purification techniques [5–7]. Recently, SMOSCs have encountered as highly promising materials for solar cell application, after reaching the highest power conversion efficiencies (PCEs) over 9% [8,9]. For the further improvement in PCEs, it is necessary to design and synthesize new effective small organic molecules for SMOSCs. In this regards, the incorporation of the electron-donor (D) and electron-acceptor (A) units in an organic small molecule plays a crucial role for the exciton-formation and its diffusion toward D-A interface which affects charge-transport properties [10–13]. In general, the fabrication of OSCs is carried out by utilizing fullerene derivatives especially, [6,6]phenyl-C61 butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) as an electron-acceptor materials because of their good electron accepting and electron-transporting ability. PCBM molecules are easily blended with electron-donor materials and produce the excellent film morphologies for excellent charge dissociation and transport. In spites of these good properties, it is difficult to tune the optical or absorption properties and energy level of PCBM molecules by simple chemical modification. Recently, Zhan and Yao described the utilization of non-fullerene organic acceptors in place of PCBM derivatives for the development of high efficiency OSCs [14]. Even though, non-fullerene derivatives show excellent optoelectronic properties, but they do not blend properly with donor molecules and create aggregates of up to hundreds of nanometer over photoactive film which is larger in size to the excitondiffusion length. As a result, the excitons might be quenched before they reach at the donor-acceptor (D-A) interface and fails to dissociate the excitons [15, 16]. On the other hand, PC61BM or PC71BM is easily blended with donor materials, and forms nanoscale aggregates which are similar in size to the exciton-diffusion length [17]. However, the performances of SMOSCs depend on various fabrication parameters such as thickness of films, the composition ratio of donor/acceptor, and annealing time as well as annealing temperature of devices [18–20].

Spirobifluorene-Core Electron-Donor Material for Bulk-Heterojunction …

315

Oligothiophenes have recently been employed as organic electron-donors (D) unit owing to well-defined and planar structure, good solubility, and high electron-density for the designing and constructing the optical and organic electronic materials [21]. On the other hand, the trifluoromethyl group (ACF3) shows a significant structural motif for organic molecules with a wide range of interesting applications in medicinal and agricultural chemistry [22–24]. The fluorinated organic materials have shown the high hydrophobicity and electro negativity which are responsible for the strong polarization behavior and high bond energy (ca. 480 kJ mol-1) of the carbon–fluorine bond [25, 26]. The fluorinated organic compounds contain a strong tendency to have hydrogen bonds FHAC interactions with much lower energy that might play an important role in the solid state organization. Moreover, the conversion of CAH bonds to CAF bonds might have several potential advantages as, the CAF bond is a very effective promoter for radiationless decay which could reduce the rate of radiationless deactivation and enhance the photoluminescence efficiency [27]. In addition, the morphology of film with CF3-based material and PC61BM blend is fibrous because the presence of fluorine atom in CF3 strengthens the intermolecular interactions between two active materials. Thus, the fluorine-termination of organic molecules could induce the morphology of blend film and increase the π- π interactions between both organic molecules [28]. Recently, the organic small molecules containing trifluoromethylbenzene as the endgroup presented high PCE (~6.0%) in BHJ fabricated solar cell devices [29]. It is realized that the subtle changes in the end-groups like fluorinated organic unit in small molecules could significantly influence the photovoltaic parameters of SMOSCs [30]. However, the spiro-based organic molecules with inherent nonlinear and rigid structures have attracted great attentions as the organic functional materials owing to their physical properties, high glass transition temperatures, good solubility and amorphous nature which make them very promising for optoelectronic materials [31, 32]. The spirobifluorene-based derivatives have expressed an excellent thermal and chemical stabilities with high quantum efficiencies as well as non-dispersive ambipolar carrier transporting properties [33, 34]. Most of the spirobifluorene-based small molecules are synthesized from the central spirobifluorene, but it requires expensive tools and chemicals [35–37]. The introduction of D and A groups in two biphenyl branches of the spirobifluorene core affords a class of spiro compounds with an asymmetric 2,7-substitution pattern, resulting a good candidate for the construction of highly transparent nonlinear organic materials due to the spiro-conjugation effects between the two fluorene units [38, 39]. These substitutions induce stability and electron transport or ambipolar transport in organic materials by lowering the energy levels (both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In this work, a new and effective spirobifluorene-based D-D-A type organic small molecule, RTh-Sp-CF3 with

316

M. Nazim, Sadia Ameen, M. Shaheer Akhtar et al.

alkyl bithiophene donor and 3,5-bis(trifluoromethyl) benzene as acceptor, has been synthesized and utilized for SMOSCs. The terminal alkyl group in bithiophene units of the small molecule have considerably improved its solubility in common organic solvents and 3,5-bis (trifluoromethyl) benzene as acceptor unit improves the generation of charge carriers.

SYNTHETIC ROUTE OF SPIROBIFLUORENE-BASED SMALL MOLECULE The synthetic route of spirobifluorene-based organic small molecule, RTh-Sp-CF3 is shown in Scheme 1. The monomeric precursors 2, 3, of the n-hexyl bithiophene boronic acid pinacole ester, 4 and other related intermediates were synthesized as previously reported procedures [40–42]. The intermediate product, 6 (RTh-Sp-Br) was obtained by the successive Suzuki cross-coupling reactions between 2,7-dibromo-9,90spirobifluorene, 5 and n-hexyl bithiophene boronic-acid pinacole ester, 4 using Pd(PPh3)4 (2.5 mol%) as catalyst and potassium carbonate as a base in anhydrous toluene solvent and then it finally coupled again by Suzuki coupling with the 3,5-bis(trifluoromethyl) phenyl boronic acid to give the final product, 7 named as RTh-Sp-CF3. The synthesized small molecule was purified by flash column chromatography and repeated recrystallization in the mixed solvent of dichloromethane/methanol (2:1, v/v) as pale yellow solid (Yield: 69.5%).

1-(5-(Thiophen-2-yl)Thiophen-2-yl)Hexan-1-One (2) In a solution of 2,20-bithiophene 1 (3.17 g, 19.1 mmol) in anhydrous benzene (20 mL), add hexanoyl chloride (4.07 mL, 20.0 mmol) was added at room temperature. The TiCl4 (2.25 mL, 20.5 mmol) was added slowly to the reaction mixture at 0oC and then stirred for 15 min at 0oC. After completion of the reaction, ice water was added to quench the reaction and the resulting mixture was diluted with CH2Cl2 (50 mL), washed successively with water (200 mL) and saturated aqueous solution of NaHCO3 (100 mL), then dried over MgSO4 and evaporated under reduced pressure evaporator to afford 5.00 g (85%) of yellow solid expected as the desired ketone intermediate which was used for next step without purification. Under inert atmosphere, a suspension of LiAlH4 (4.6 g, 121 mmol) and AlCl3 (4.03 g, 30.3 mmol) in anhydrous Et2O (100 mL) was added to the toluene (40 mL) solution of ketone intermediate at 0oC. Then the reaction was stirred for 1 h at the room temperature. After completion, the reaction mixture was cooled to 0oC. Afterward, ethyl acetate (20 mL) and HCl (6 M) solution (50 mL) were added to the reaction mixture. The resulting

Spirobifluorene-Core Electron-Donor Material for Bulk-Heterojunction …

317

mixture was then extracted with diethyl ether (300 mL), washed with brine solution and water (50 mL), dried over MgSO4 and evaporated in the vacuum oven. The obtained yellow residue was purified by flash column chromatography on silica gel (hexane) to give a colourless oil (6.00 g, 93%). 1H NMR (400 MHz, CDCl3, d, ppm): 7.17 (dd, 1H), 7.13 (dd, 1H), 6.95 (dd, 1H), 6.97 (d, 1H), 6.66 (d, 1H), 2.74 (t, 2H), 1.67 (m, 2H), 1.37 (m, 14H), 0.92 (t, 3H); 13C NMR (100 MHz, CDCl3, d, ppm): 146.5, 145.3, 138.1, 134.7, 127.4, 125.5, 124.4, 123.9, 123.7, 123.2, 32.1, 31.6, 30.3, 29.7, 29.5, 29.1, 22.7, 14.1.

5-Bromo-50-Decyl-2,20-Bithiophene (3) In solution of compound 2 (2.00 g, 6.53 mmol) in dimethylformamide (30 mL), Nbromo succinimide (NBS) (1.22 g, 6.86 mmol) was added slowly and the obtained reaction mixture was stirred for 30 min in dark. The reaction mixture was diluted with hexane (50 mL), and washed with saturated aqueous solution of NH4Cl (50 mL), dried over MgSO4 and evaporated under reduced pressure. The obtained residue was purified by flash column chromatography on silica gel (hexane) to give a white solid (2.36 g, 94%). Mp 35–38oC; 1H NMR (400 MHz, CDCl3, d, ppm): 6.96 (d, 1H), 6.91 (d, 1H), 6.87 (d, 1H), 6.69 (d, 1H), 2.75 (t, 2H), 1.65 (m, 2H), 1.37 (m, 14H), 0.93 (t, 3H); 13C NMR (100 MHz, CDCl3, d, ppm): 145.3, 139.7, 133.6, 130.1, 130.7, 124.5, 123.5, 123.3, 110.5, 32.3, 31.5, 30.6, 29.7, 29.5, 29.1, 22.7, 14.2.

2-(5-(5-Decylthiophen-2-yl) Thiophen-2-yl) -4,4,5,5-Tetramethyl-1,3,2Dioxaborolane (4) n-BuLi (1.6 M, 3.17 mmol) was added to a solution of 3 (1 g, 2.6 mmol) in tetrahydrofuran (20 mL) at ~78oC under nitrogen atmosphere. Then temperature was slowly increased up to ~50oC within 20 min. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2dioxaborolane (0.58 mL, 5.27 mmol) solution was added by syringe at ~50oC and temperature was increased slowly up to the room temperature. The reaction mixture was then stirred for 3 h at room temperature and 2 N HCl (20 mL) was added. The obtained reaction mixture was extracted with diethyl ether (30 mL), washed with brine and water (500 mL), dried over MgSO4, and then evaporated under reduced pressure. The obtained residue was then recrystallized in hexane (10 mL) to get a white solid (0.84 g, 93%). 1H NMR (400 MHz, CDCl3, d, ppm): 7.57 (d, 1H), 7.24 (d, 1H), 7.07 (d, 1H), 6.73 (d, 1H), 2.86 (t, 2H), 1.75 (m, 2H), 1.35 (m, 14H), 0.93 (t, 3H); 13C NMR (100 MHz, CDCl 3, d, ppm): 146.3, 144.8, 137.7, 134.5, 124.7, 124.2, 84.4, 31.8, 31.3, 30.3, 29.8, 29.5, 29.2, 24.6, 22.5, 14.4.

318

M. Nazim, Sadia Ameen, M. Shaheer Akhtar et al.

2-(7-Bromo-9,90-Spirobifluorene-2-yl)-5-(5-Hexylthiophen-2-yl) Thiophene (6) 2-(5-(5-Decylthiophen-2-yl) thiophen-2-yl) -4,4,5,5-tetramethyl-1,3,2-dioxaborolane, 4 (0.752 g, 2 mmol), 2,7-Dibromo-9,90-spirobifluorene, 5 (0.948 g, 2 mmol) and Pd(PPh3)4 (15 mg, 5 mol %) were refluxed under inert atmosphere in degassed toluene (~10 mL) with an aqueous K2CO3 solution (2 M) (4 mL) for 24 h. The reaction was cool and deionized water was added to quench the reaction and then diluted with hexane (10 mL). The organic layer was extracting with dichloromethane (20 mL), washed with deionized water, brine solution and dried over anhydrous MgSO4. The solvent evaporated by rotary evaporator and the obtained compound was dried in vacuo. The crude product was purified by flash column chromatography by using hexane:DCM (2:1, v:v) followed by recrystallization from DCM:methanol (5:1, v:v) to get pale yellow solid (0.641 g, 68%). 1H NMR (600 MHz, CDCl3, ppm) d: 0.82–0.88 (t, 3H), 1.17–1.26 (m, 6H), 1.85 (t, 2H), 3.45 (t, 2H), 6.70 (d, 1H), 6.76 (d, 1H), 6.78 (d, 1H), 7.06–7.12 (t, 4H), 7.18–7.20 (d, 2H), 7.36 (d, 2H), 7.41–7.51 (d, 4H), 7.63 (s, 2H), 7.76 (d, 2H). 13C NMR (100 MHz, CDCl3, ppm) d: 150.62, 147.43, 147.13, 141.77, 139.72, 131.20, 128.36, 128.16, 127.41, 126.96, 126.67, 123.45, 121.96, 121.45, 120.88, 120.46, 120.35, 119.37, 31.65, 30.69, 30.52, 30.27, 28.83, 28.46, 27.62, 27.34, 22.77, 22.69, 14.22, 14.16. FTIR (KBr pellets, cm-1): 3439, 3028, 2962, 2855, 1731, 1653, 1587, 1485, 1404, 1323, 1256, 1228, 1099, 1022, 976, 948, 836, 792, 691.

2-(7-(3,5-Bis(Trifluoromethyl)Phenyl)-9,90-Spirobifluorene-2-yl)-5-(5Hexylthiophen-2-yl)Thiophene (7) A solution of 3,5-bis(trifluoromethyl)phenyl boronic acid (0.361 gm, 1.4 mmol) and intermediate bromide, 6 (0.642, 1 mmol) were added with aqueous solution (2 M) of K2CO3 (5 mL) and Pd (PPh3)4 catalyst (10 mg, 0.01 mmol) in anhydrous toluene (~10 mL) solvent. The reaction mixture was degassed several times, heated to reflux under inert atmosphere for 24 h. After completion, the reaction (as monitored by TLC) was cooled to room temperature and diluted with the hexane (10 mL). To ensure the completion of the coupling reaction, the boronic acid was used in access. The organic layer was extracted with dichloromethane (20 mL) and washed with DI water, brine and dried over anhydrous MgSO4. The crude product was purified by flash column chromatography using hexane:DCM (5:1, v:v) followed by recrystallization from DCM:methanol (2:1, v:v) to get the yellow solid (0.46 g, 71.4%). 1H NMR (600 MHz, CDCl3, ppm) d: 0.84–0.88 (t, 3H), 1.18–1.28 (m, 6H), 1.82 (t, 2H), 3.42 (t, 2H), 6.64 (d, 1H), 6.70 (d, 1H), 6.76 (d, 1H), 6.78 (d, 1H), 7.06–7.12 (t, 4H), 7.18–7.20 (d, 2H), 7.36 (d, 2H), 7.43–7.52 (d, 4H), 7.62 (s, 3H), 7.79 (d, 2H). 13C NMR (100 MHz, CDCl 3,

Spirobifluorene-Core Electron-Donor Material for Bulk-Heterojunction …

319

ppm) d: 150.62, 147.43, 147.13, 141.77, 139.72, 131.20, 128.36, 128.16, 127.41, 126.96, 126.67, 123.45, 121.96, 121.45, 120.88, 120.46, 120.35, 119.37, 119.26, 36.09, 30.69, 30.56, 30.42, 28.70, 28.46, 27.62, 27.34, 22.19, 21.69, 13.61, 13.07. FT-IR (KBr pellets, cm_1): 3439, 3062, 3026, 2954, 2922, 2850, 1590, 1486, 1463, 1423, 1378, 1302, 1278, 1234, 1072, 972, 828, 786, 747, 693, 612, 522. MS (ESI): m/z calc. for [C47H34F6S2]+: 776.62; found: 776.98 [M+].

Reprinted with permission from [M. Nazim, 2016], Chem. Phys. Lett., 663 (2016) 137 © 2016 Elsevier Ltd. Scheme 1. Synthetic route of spirobifluorene-based small molecule.

320

M. Nazim, Sadia Ameen, M. Shaheer Akhtar et al.

THERMOGRAVIMETRIC AND DIFFERENTIAL SCANNING COLORIMETRY THERMO GRAM OF SPIROBIFLUORENE-BASED SMALL MOLECULE The thermal properties of RTh-Sp-CF3 are investigated by means of differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) under inert atmosphere, and the corresponding data are displayed in Figure 1. To obtain an organic electron-donor with high thermal stability, the RTh-Sp-CF3 is designed to possess a high glass transition temperature (Tg) value with attachment of alkyl chain and phenyl rings which induce the morphological stability of the small molecule. The RTh-Sp-CF3 molecule shows a good thermal stability with ~3% weight loss at ~299oC. The organic small molecule shows a glass transition temperature (Tg) at ~58oC. Owing to the existence of various liquid-crystal phases, the synthesized organic small molecule displays various crystallization (Tc) and melting transitions (Tm) as a function of temperature [43]. The liquid-crystalline (LC) properties of the organic small molecule are detected by observing three melting transitions (Tm) at ~90oC, ~368oC and ~424oC along with the three crystallization transitions (Tc) at ~298oC and ~403oC and ~430oC which might be accountable for the self-organization of the small molecule [44]. These results revealed the good morphology and thermal stability of RTh-Sp-CF3 molecule.

Reprinted with permission from [M. Nazim, 2016], Chem. Phys. Lett., 663 (2016) 137 © 2016 Elsevier Ltd. Figure 1. TGA (inset-DSC) thermo gram of spirobifluorene-based small molecule.

Spirobifluorene-Core Electron-Donor Material for Bulk-Heterojunction …

321

Reprinted with permission from [M. Nazim, 2016], Chem. Phys. Lett., 663 (2016) 137 © 2016 Elsevier Ltd. Figure 2. (a) UV–Vis spectra of RTh-Sp-CF3 in chlorobenzene solution; (b) RTh-Sp-CF3 thin film deposited by solution spin coating; (c) Photoluminescence spectra of RTh-Sp-CF3 thin film and RThSp-CF3:PC61BM thin film (1:2) ratio deposited by solution spin coating.

OPTICAL PROPERTIES OF RTH-SP-CF3 UV–Vis and Photoluminescence Spectra of RTh-Sp-CF3 The UV–Vis absorption spectra results of RTh-Sp-CF3 are depicted in Figure 2a and summarized in Table 1. The organic small molecule exhibits the absorption peaks at ~312 and ~388 nm in chlorobenzene solution. The former absorption peak belongs to the π-π* transition while later absorption peak corresponds to n-π transition of small molecules. The wavelength maximum peak (~388 nm) corresponds to intramolecular charge transfer (ICT) transitions for the small molecule [45]. However, the RTh-Sp-CF3 thin film presents the slightly shifted visible region absorption with ~λonset = 478 nm as compared to solution, resulting from the formation of aggregates from the solution to the solid thin film. This phenomenon improves the intermolecular electronic delocalization in the donor to acceptor units. Photoluminescence (PL) spectra (Figure 2b) show a strong emission band at ~459 nm in thin solid film which shows a strong fluorescence quenching behavior in thin solid film blending with PC61BM due to the occurrence of weak intermolecular charge transfer interactions. The dual nature of electronic effect of fluorine atom of -CF3 group assists the geometrical and structural interactions in thin film state because it is most electron-deficient as a -CF3 group in the meta-positions on the benzene ring of the organic small molecules. Hence, incorporation of the strong electron accepting functional groups (-CF3) in the small molecule might enhance the charge transfer from donor to acceptor which is responsible for the strong quenching behavior. It

322

M. Nazim, Sadia Ameen, M. Shaheer Akhtar et al.

is also notable that the organic molecule, RTh-Sp-CF3 displays a strong Stokes shift suggesting the significant changes in geometrical configuration of the organic molecules upon the excitation of wavelength. The loss mechanisms involve the exciton splitting at the heterojunction, and subsequently, the recombination and charge collection process [46]. The exciton transfer is not a loss mechanism because the exciton has a tendency to ‘bounce’ from donor to acceptor which guides the exciton toward the D-A interface and hence, allows to find an efficient D-A interface area for exciton diffusion and consequently, the exciton-dissociation [47].

CYCLIC VOLTAMMETRY OF THE SPIROBIFLUORENE-BASED THIN FILM The cyclic voltammetry (CV) spectrum of RTh-Sp-CF3 thin film has been carried out in 0.1 M CH3CN solution of tetra butyl ammonium hexafluoro phosphate [nBu4N]+[PF6]at a potential scan rate of 100 mV/s. CV plot is presented in Figure 3 and summarized in Table 1. Typically, the oxidation potential and reduction potential peaks for RTh-Sp-CF3 thin film are situated at onset value Eox = +0.95 ± 0.02 eV and Ered = -0.48 ± 0.02 eV, respectively. From the CV observations, HOMO (~5.35 eV) and LUMO (~3.92 eV) energy level are estimated for the RTh-Sp-CF3 thin film. The electrochemical band gap (Eg) is found to be ~1.43 eV, which is smaller than the reported spirobifluorene-based nonlinear small molecules [48].

Reprinted with permission from [M. Nazim, 2016], Chem. Phys. Lett., 663 (2016) 137 © 2016 Elsevier Ltd. Figure 3. Cyclic voltammetry of the spirobifluorene-based thin film of organic small molecule with cathodic and anodic cycles at the scan rate of 100 mV/s in 0.1 M TBAPF6/anhydrous acetonitrile electrolyte in acetonitrile solution.

Spirobifluorene-Core Electron-Donor Material for Bulk-Heterojunction …

323

Reprinted with permission from [M. Nazim, 2016], Chem. Phys. Lett., 663 (2016) 137 © 2016 Elsevier Ltd. Figure 4. J-V curves of fabricated SMOSCs with the active layer of RTh-Sp-CF3:PC61BM at various ratios of 1:1, 1:2 and 1:3 (w/w).

The presence of 3,5-bis(trifluoromethyl)benzene acceptor unit in RTh-Sp-CF3 might responsible for the strong fluorescence emission due to the strong electron-withdrawing groups of -CF3, resulting in the reduction in the HOMO energy levels. Moreover, the CH…..F interactions like partial hydrogen bonds, play an important role in the solid state organization of such fluorine compounds bearing both C-F and C-H bonds, and forms a typical p stack arrangement which enhances the charge carrier mobility of organic materials [49, 50].

THE CURRENT DENSITY (J)-VOLTAGE (V) CURVES OF FABRICATED ORGANIC SOLAR CELLS WITH THE ACTIVE LAYER OF RTH-SP-CF3:PC61BM The photovoltaic parameters of the fabricated SMOSC devices are evaluated by the current density (J)-voltage (V) measurements (Figure 4) under the 1 sun light (100 mW/ cm2, 1.5 AM). Among the fabricated SMOSC devices, a moderate efficiency of ~2.12% is achieved by the RTh-Sp-CF3:PC61BM (1:2, w/w) ratio, whereas other fabricated SMOSC devices exhibit inferior PECs of ~1.63% for RTh-Sp-CF3:PC61BM (1:1, w/w) ratio and ~1.82% for RTh-Sp-CF3:PC61BM (1:3, w/w) ratio. The fabricated devices of RTh-Sp-CF3:PC61BM (1:2, w/w) ratio presents the high short-circuit current density (Jsc) of ~8.42 mA/cm2 due to the scattering behavior along with open circuit voltage (Voc) of ~0.66 V. The improved JSC and performance of the RTh-SpCF3:PC61BM (1:2, w/w) active layer might explain by the smoother film morphology and fast intermolecular charge transfer (ICT) between RTh-Sp-CF3 and PC61BM due to the

324

M. Nazim, Sadia Ameen, M. Shaheer Akhtar et al.

introduction of trifluoromethyl-acceptor unit with spirobifluorene backbone [51]. Moreover, the nonlinear spirobifluorene based molecules and the terminal alkyl chain in bithiophene unit might significantly facilitate the charge carriers mobility, resulting in the high photocurrent density and the performance of solar cell devices [52, 53]. The low fill factors of SMOSCs are related to the series resistance (Rs) of the devices. Generally, Rs governs by four ways (i) the bulk resistance of the active layer and functional layers in the film, (ii) bulk resistance of the electrodes, (iii) contact resistance of every interface in the device and, (iv) the probe resistance. During the fabrication of device, the creation of less large barriers at the interfaces between the organic layers results the large Rs. It is believed that interfacial contact between RTh-Sp-CF3:PC61BM and conducting layer might not properly created, which considerable results in high Rs of devices. In RTh-SpCF3:PC61BM (1:1 and 1:3, w/w) ratios based SMOSCs, the low JSC and less FF are related to the fast electron transfer and the recombination rate at the D-A interface [54].

Reprinted with permission from [M. Nazim, 2016], Chem. Phys. Lett., 663 (2016) 137 © 2016 Elsevier Ltd. Figure 5. Topographical and 3D AFM images of different active layer blend thin films of (a and b) RTh-Sp-CF3:PC61BM (1:1, w/w) and (c and d) RTh-Sp-CF3:PC61BM (1:2, w/w).

Spirobifluorene-Core Electron-Donor Material for Bulk-Heterojunction …

325

ATOMIC FORCE MICROSCOPY SPECTROSCOPY OF RTH-SP-CF3:PC61BM ACTIVE LAYER Atomic force microscopy (AFM) is used to analyze the surface morphology of the active layers to explain the homogeneity of various blend RTh-Sp-CF3:PC61BM active layer (w/w) ratios, as shown in Figure 5. Thin film deposition by solution spin-coating techniques with different weight ratios of RTh-Sp-CF3:PC61BM (1:1 and 1:2, w/w) has been used in AFM measurement. The blend thin film of RTh-Sp-CF3:PC61BM (1:2, w/w) exhibits a homogeneous and smooth film morphology with low surface roughness (Rrms = 6.85 nm) as compared to blend thin film of RTh-Sp-CF3:PC61BM (1:1, w/w) ratio with high surface roughness (Rrms = 13.10 nm). The thin film of RTh-SpCF3:PC61BM (1:1, w/w) ratio exhibits the nanoscale phase separation and suggests a good blend properties of RTh-Sp-CF3 and PC61BM in chlorobenzene solvent [55, 56]. These results show that the active layer of RTh-Sp-CF3:PC61BM (1:2, w/w) ratio is the optimized ratio for achieving the best performance of solar cell devices. The smoother film morphology and homogeneous active layer with the nanoscale phase separation are responsible for the large donor-acceptor interface area needed for efficient exciton dissociation [57]. Herein, AFM results reveal that RTh-Sp-CF3:PC61BM (1:2, w/w) ratio blend thin film depicts the low surface roughness and hence, provide the enough surface area for easy dissociation of excitons at the D–A interface. The low fill factor for devices might be due to the domain size, surface morphology, and series resistance etc. The low FF is related to the increase in the series resistance of RTh-Sp-CF3:PC61BM/ITO, resulting in the high recombination rate. The AFM images of active layer RTHSPCF3:PC61BM (1:2, w/w) ratio suggests that RTh-Sp-CF3 has good miscibility with PC61BM, and hence, a spontaneous phase segregation could form a bicontinuous network structure, which facilitate the efficient carrier collection for BHJ solar cell devices [58].

CONCLUSION A novel spirobifluorene-based organic small molecule (RTh-Sp-CF3) is synthesized and applied as photoactive material for the solution-processed fabrication of SMOSCs. The synthesized small molecule, RTh-Sp-CF3 is highly soluble in common organic solvents due to the presence of terminal alkyl chain. RTh-Sp-CF3 substantiates the reasonable HOMO and LUMO energy levels of ~5.35 eV and ~3.92 eV, respectively. The fabricated SMOSCs with RTh-Sp-CF3:PC61BM (1:2, w/w) exhibits moderate PCEs of ~2.12% with high photocurrent density of ~8.42 mA/cm2 and high VOC of ~0.67 V.

326

M. Nazim, Sadia Ameen, M. Shaheer Akhtar et al.

These improvements might attribute to the enhanced intermolecular charge transfer between RTh-Sp-CF3 and PC61BM.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

B. Walker, C. Kim, T. Q. Nguyen, Chem. Mater. 23 (2011) 470. J. S. Moon, C. J. Takacs, Y. Sun, A. J. Heeger, Nano Lett. 11 (2011) 1036. J. Roncali, Acc. Chem. Res. 42 (2009) 1719. Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan, A. J. Heeger, Nat. Mater. 11 (2012) 44. X. Liu, Y. Sun, L.A. Perez, W. Wen, M. F. Toney, A. J. Heeger, G. C. Bazan, J. Am. Chem. Soc. 134 (2012) 20609. S. Paek, N. Cho, K. Song, M. J. Jun, J. K. Lee, J. K. Ko, J. Phys. Chem. C 116 (2012) 23205. J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li, G. He, C. Li, B. Kan, M. Li, Y. Chen, J. Am. Chem. Soc. 135 (2013) 8484. Y. S. Liu, C. C. Chen, Z. R. Hong, J. Gao, Y. M. Yang, H. P. Zhou, L. T. Dou, G. Li, Y. Yang, Sci. Rep. 3 (2013) 3356. S. P. Wen, C. Wang, P. F. Ma, Y. X. Zhao, C. Li, S. P. Ruan, J. Mater. Chem. A 3 (2015) 13794. G. Wei, R. R. Lunt, K. Sun, S. Wang, M. E. Thompson, S. R. Forrest, Nano Lett. 10 (2010) 3555. F. Yang, K. Sun, S. R. Forrest, Adv. Mater. 19 (2007) 4166. P. Zhou, D. F. Dang, M. J. Xiao, Q. Wang, J. Zhong, H. Tan, Y. Pei, R. Q. Yang, W. G. Zhu, J. Mater. Chem. A. 3 (2015) 10883. P. Zhou, D. F. Dang, Q. Wang, X. W. Duan, M. J. Xiao, Q. Tao, H. Tan, R. Q. Yang, W. G. Zhu, J. Mater. Chem. A. 3 (2015) 13568. C. Zhan, J. Yao, Chem. Mater. 28 (2016) 1948. Z. G. Zhang, X. Zhang, C. L. Zhan, Z. H. Lu, X. L. Ding, S. G. He, J. N. Yao, Soft Matter. 9 (2013) 3089. I. A. Howard, F. Laquai, P. E. Keivanidis, R. H. Friend, N. C. Greenham, Phys. Chem. C. 113 (2009) 21225. A. L. Tang, Z. H. Lu, S. M. Bai, J. H. Huang, Y. X. Chen, Q. Shi, C. L. Zhan, J. N. Yao, Chem. Asian J. 9 (2014) 883. S. Wang, L. Hall, V. V. Diev, R. Haiges, G. Wei, X. Xiao, P. I. Djurovich, S. R. Forrest, M. E. Thompson, Chem. Mater. 23 (2011) 4789. Z. Li, J. Pei, Y. Li, B. Xu, M. Deng, Z. Liu, H. Li, H. Lu, Q. Li, W. Tian, J. Phys. Chem. C. 114 (2010) 18270.

Spirobifluorene-Core Electron-Donor Material for Bulk-Heterojunction …

327

[20] Q. Peng, Q. Huang, X. Hou, P. Chang, J. Xu, S. Deng, Chem. Commun. 48 (2012) 11452. [21] W. K. Chow, C. M. So, C. P. Lau, F. Y. Kwong, J. Org. Chem. 75 (2010) 5109. [22] M. Schlosser, Angew. Chem. Int. Ed. 45 (2006) 5432. [23] P. Wipf, T. Mo, S. J. Geib, D. Caridha, G. S. Dow, L. Gerena, N. Roncal, E. E. Milner, Org. Biomol. Chem. 7 (2009) 4163. [24] S. Jayaprakash, Y. Iso, B. Wan, S. G. Franzblau, A. P. Kozikowski, Chem. Med. Chem. 1 (2006) 593. [25] B. Zhang, G. Chen, J. Xu, L. Hu, W. Yang, J. Chem. 40 (2016) 402. [26] S. Nagamatsu, S. Oku, K. Kuramoto, T. Moriguchi, W. Takashima, T. Okauchi, S. Hayase, ACS Appl. Mater. Interfaces. 6 (2014) 3847. [27] Q. Shi, W. Q. Chen, J. Xiang, X. M. Duan, X. Zhan, Macromolecules. 44 (2011) 3759. [28] C. Shim, M. Kim, S. G. Ihn, Y. S. Choi, Y. Kim, K. Cho, Chem. Commun. 48 (2012) 7206. [29] J. H. Kim, J. B. Park, H. Yang, I. H. Jung, S. C. Yoon, D. Kim, D. H. Hwang, ACS Appl. Mater. Interfaces. 7 (2015) 23866. [30] Y. Kim, T. M. Swager, Chem. Commun. 46 (2005) 372. [31] C. Fan, Y. H. Chen, P. Gan, C. L. Yang, C. Zhong, J. G. Qin, D. G. Ma, Org. Lett. 12 (2010) 5648. [32] Y. Y. Chien, K. T. Wong, P. T. Chou, Y. M. Cheng, Chem. Commun. (2002) 2874. [33] Y. L. Liao, C. Y. Lin, Y. H. Liu, K. T. Wong, W. Y. Hung, W. J. Chen, Chem. Commun. (2007) 1831. [34] T. P. I. Saragi, T. Spehr, A. Siebert, T. Fuhrmann Lieker, J. Salbeck, Chem. Rev. 107 (2007) 1011. [35] D. F. Dang, P. Zhou, M. J. Xiao, R. Q. Yang, W. G. Zhu, Dyes Pigm. 133 (2016) 1. [36] D. F. Dang, P. Zhou, Y. Zhi, X. C. Bao, R. Q. Yang, L. J. Meng, W. G. Zhu, J. Mater. Sci. 51 (2016) 8018. [37] H. Xiao, H. Yin, L. Wang, L. Ding, S. Guo, X. Zhang, D. Ma, Org. Electron. 13 (2012) 1553. [38] Z. Li, Z. Wu, B. Jiao, P. Liu, D. Wang, X. Hou, Chem. Phys. Lett. 527 (2012) 36. [39] S. I. Hintschich, C. Rothe, S. M. King, S. J. Clark, A. P. Monkman, J. Phys. Chem. B. 112 (2008) 16300. [40] M. Melucci, L. Favaretto, A. Zanelli, M. Cavallini, A. Bongini, P. Maccagnani, P. Ostoja, G. Derue, R. Lazzaroni, G. Barbarella, Adv. Funct. Mater. 20 (2010) 445. [41] N. Crivillers, L. Favaretto, A. Zanelli, I. Manet, M. Treier, V. Morandi, M. Gazzano, P. Samorı, M. Melucci, Chem. Commun. 48 (2012) 12162. [42] M. Nazim, S. Ameen, H. K. Seo, H. S. Shin, Sci. Rep. 5 (2015) 11143. [43] N. Zhou, X. Guo, R. Ortiz, S. Li, S. Zhang, R. Chang, A. Facchetti, T. J. Marks, Adv. Mater. 24 (2012) 2242.

328

M. Nazim, Sadia Ameen, M. Shaheer Akhtar et al.

[44] Z. Li, G. He, X. Wan, Y. Liu, J. Zhou, G. Long, Y. Zuo, M. Zhang, Y. Chen, Adv. Energy Mater. 2 (2012) 74. [45] J. J. A. Chen, T. L. Chen, B. S. Kim, D. A. Poulsen, J. L. Mynar, J. M. J. Frechet, B. Ma, ACS Appl. Mater. Interfaces. 2 (2010) 2679. [46] L. Biniek, C. L. Chochos, N. Leclerc, G. Hadziioannou, J. K. Kalitsis, R. Bechara, P. Leveque, T. Heiser, J. Mater. Chem. 19 (2009) 4946. [47] S.W. Shelton, T. L. Chen, D. E. Barclay, B. Ma, ACS Appl. Mater. Interfaces. 4 (2012) 2534. [48] O. Usluer, J. Mater. Chem. C. 2 (2014) 8098. [49] Q. Shi, P. Cheng, Y. F. Li, X. Zhan, Adv. Energy Mater. 2 (2012) 63. [50] J. S. Reddy, T. Kale, G. Balaji, A. Chandrasekaran, S. Thayumanavan, J. Phys. Chem. Lett. 2 (2011) 648. [51] B. K. An, D. S. Lee, J. S. Lee, Y. S. Park, H. S. Song, S. Y. Park, J. Am. Chem. Soc. 126 (2004) 10232. [52] K. A. Mazzio, M. Yuan, K. Okamoto, C. K. Luscombe, ACS Appl. Mater. Interfaces. 3 (2011) 271. [53] F. Polo, F. Rizzo, M. V. Gutierrez, L. D. Cola, S. Quici, J. Am. Chem. Soc. 134 (2012) 15402. [54] Z. Li, B. Jiao, Z. Wu, P. Liu, L. Ma, X. Lei, D. Wang, G. Zhou, H. Hud, X. Hou, J. Mater. Chem. C. 1 (2013) 2183. [55] M. Nazim, S. Ameen, M. S. Akhtar, H. K. Seo, H. S. Shin, Synth. Met. 187 (2014) 178. [56] M. L. Tang, Z. Bao, Chem. Mater. 23 (2011) 446. [57] F. Babudri, G. M. Farinola, F. Naso, R. Ragni, Chem. Commun. (2007) 1003. [58] Z. Li, Z. Wu, W. Fu, P. Liu, B. Jiao, D. Wang, G. Zhou, X. Hou, J. Phys. Chem. C. 16 (2012) 20504.

SECTION 2. ZINC OXIDE (ZNO) PHOTOANODE BASED DYE SENSITIZED SOLAR CELLS (DSSCS)

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 23

HYDROTHERMAL SYNTHESIS OF ZNO MATERIALS FOR A DYE-SENSITIZED SOLAR CELL M. Shaheer Akhtar1,*, M. Alam Khan1, Myung Seok Jeon2 and O-Bong Yang1 1

School of Semiconductor and Chemical Engineering, Center for Advanced Radiation Technology & The New and Renewable Energy Center, Chonbuk National University, Jeonju, Republic of Korea 2 Photocatalysis and Photoelectrochemistry Research Center, Korea Institute of Energy Research (KIER), Daejon, Republic of Korea

ABSTRACT In this work, the morphology of ZnO materials is controlled by changing the capping agents at a constant alkali solution in hydrothermal process. ZnO nanomaterials with the structure of flowers, sheet-spheres and plates are obtained with the capping agent of ammonia, citric acid and oxalic acid, respectively. Thus prepared ZnO nanomaterials are characterized and applied as the photoanode materials for dye-sensitized solar cell. All synthesized ZnO nanomaterials possess high crystalline wurtzite structures grown in the (0 0 1) direction with the size of 2–4μ m, which consist of ZnO units around 20–400 nm. Among them, sheet-sphere ZnO shows the highest crystallinity, surface area and uniform film morphology, resulting in the significantly improved PV performance with the overall conversion efficiency of ~2.61% in dye-sensitized solar cell (DSSC) fabricated with sheet-sphere ZnO. It is notable that the ZnO materials with sphere structure might be the optimal photo-anode material among various ZnO nanomaterials for DSSC.

Keywords: ZnO, nanostructures, Working Electrode, Dye sensitized solar cells.

*

Corresponding Author Email: [email protected].

332

M. Shaheer Akhtar, M. Alam Khan, Myung Seok Jeon et al.

INTRODUCTION A dye-sensitized solar cell (DSSC) with the TiO2 electrode layers has been extensively studied due to the reasonably high efficiency in a low-cost [1–4]. A typical DSSC consists of a dye-sensitized TiO2 layer as the working electrode, an electrolyte containing a redox couple (I−/I3−) and a Pt layer counter electrode. Recently, considerable research advances and studies on DSSCs have focused on the improvement of electron transport and reducing the recombination rate by the use of alternative semiconductor materials or core–shell structures [5, 6]. At present, the electron transport and photovoltaic (PV) performance of DSSC has been improved mainly by controlling the morphology, particle size and thickness of the TiO2 layer [7], which provides the better interfacial contact between the TiO2 layer and TCO. To date, many efforts have been expended in controlling the sizes, shapes and crystal structures of inorganic oxide such as TiO2, ZnO, SiO2, etc., for the improvement of optoelectrical properties. ZnO is a unique electronic and photonic semiconductor with band gap of 3.37 eV and appeared as a promising semiconductors candidate for biosensor [8], field-effect transistor (FETs) [9] and electro-optical devices [10, 11]. ZnO has been expected to compatible with TiO2 nanomaterials because of its higher electronic mobility, similar electron affinity and energy level of the conduction band. It has been chosen as an alternative of TiO2 which is well known in DSSCs as working electrode. Several efforts have been made to prepare different nanotextured ZnO films which are promising as photoanodes in DSSCs to improve the light harvesting and electron injection properties [12, 13]. Rensmo et al., [14] obtained high overall solar energy conversion efficiency of 2.0% with the DSSC based on nanostructured ZnO electrode and Ru(II) complex-dye (N3). Recently, high overall conversion efficiency of 1.3% has been obtained bymicrostructured ZnO electrode based photo-electrochemical solar cell [15]. In this chapter, we demonstrate the synthesis of ZnO nanomaterials by adjusting the capping agent at constant alkali solution in hydrothermal process. Thus prepared ZnO nanomaterials are characterized and applied as the photo-anode materials for DSSCs. cell. For DSSCs fabrication [16], ZnO paste is prepared by the incremental addition of 2ml of aqueous polyethylene glycol solution to 0.5 g of ZnO materials powder in a mortar under vigorous grinding with pestle. Thus, the prepared uniform ZnO paste is coated on fluorinated tin oxide glass (FTO) by a doctor blade technique. After natural drying at room temperature, the thin film is calcined in static air at 450 ◦C for 30 min. The prepared ZnO electrode is immersed in dye solution consisting 0.3mM ruthenium 535 bis-TBA (N719, Solaronix), in spectrograde ethanol at room temperature for 24 h. The dye-adsorbed ZnO electrodes are then rinsed with ethanol and dried under a nitrogen stream. Pt counter electrodes are prepared by electron beam deposition of a thin layer of Pt on the top of TCO glass. The Pt electrode is placed over the dye-adsorbed ZnO electrode and the edges of the cell were sealed with 60-μm thick sealing sheet. Sealing is

Hydrothermal Synthesis of ZnO Materials …

333

accomplished by pressing the two electrodes together on a double hot-plate at a temperature of about 70 ◦C. The electrolyte, consisting of 0.5M LiI, 0.05mM I2, and 0.2M tert-butyl pyridine in acetonitrile is introduced into the cell through one of two small holes drilled in the counter electrode. The holes are then covered and sealed with a small square of sealing sheet and microscope objective glass. An active area of the resulting cell is about 0.25cm2.

Morphological Studies of Synthesized ZnO Nanostructures The Field Emission Scanning Electron Microscopy Figure 1 shows the FESEM image of various ZnO nanomaterials prepared as a function of capping agents in constant pH 10–11. In a capping agent of ammonia solution (Figure 1a), flower shape ZnO (ZnO–NH3) are obtained, which consists of sharp sword ends rods and laterally initiating from the central rod. The central rod tip is composed of multiple hexagonal layers as shown in inset of Figure 1a. These flowers with rods petals could not be detached and separated by the treatment of ultrasonication for longer time, indicating strong stability of this morphology. In a capping agent of citric acid, sheetsphere shaped ZnO (ZnO–citric) were obtained, as shown in Figure 1b, a typical sphere morphology are consist of 20 nm thickness sheets interconnected to each other and formed 4μm diameter spheres. While, plates shape of ZnO (ZnO–oxalic) was obtained in a capping agent of oxalic acid. ZnO plates are regularly arranged to the central vertical plates with the thickness of 100 nm (Figure 1c). It is notable that the capping agent is essential parameters to determine the morphology of ZnO nanomaterials by the hydrothermal method at constant pH.

Reprinted with permission from [M. S. Akhtar, 2008], Electrochim. Acta, 53 (2008) 7869 © 2008 Elsevier Ltd. Figure 1. FE-SEM images of (a) ZnO–NH3, (b) ZnO–citric and (c) ZnO–oxalic materials prepared by hydrothermal method. Inset shows the high magnification FESEM images of a, b and c.

334

M. Shaheer Akhtar, M. Alam Khan, Myung Seok Jeon et al.

The Transmission Electron Microscopy Figure 2 shows the TEM and high-resolution TEM (HRTEM) images of the synthesized ZnO materials, which again confirm the detailed morphology and structure of FE-SEM images (Figure 1).

Reprinted with permission from [M. S. Akhtar, 2008], Electrochim. Acta, 53 (2008) 7869 © 2008 Elsevier Ltd. Figure 2. TEM (left) and HRTEM images (right) of (a and d) ZnO–NH3, (b and e) ZnO–citric and (c and f) ZnO–oxalic. Insets are SEAD patterns.

Figure 2a shows the flower structure ZnO–NH3, which exhibits with pointed nanorods of 600 nm length and 200 nm diameter at a central multiple hexagonal notches.

Hydrothermal Synthesis of ZnO Materials …

335

In the ZnO–citric, the sheets with thickness 20–30 nm are assembled into a sphere like structures as shown in Figure 2b. ZnO–oxalic shows the regularly arranged ZnO nanoplates with the thickness of 20 nm and size of about 1μm. Crystalline properties of ZnO nanomaterials are characterized by HRTEM (Figure 2d–f) and selected area electron diffraction (SAED, the inset of Figure 2d–f). The SAED patterns clearly show that all synthesized ZnO nanomaterials are single crystalline with hexagonal structure grown to the (0 0 1) direction [18]. It is revealed that the crystallinity of ZnO–citric is highest and ZnO–NH3 is lowest by determining from the relative clear spot of SAED patterns. HRTEM images indicate that all of the ZnO shows very clear fringe spacing between the two adjacent lattice fringes of 0.265 nm. It confirms the highly crystalline nature with preferred orientation (0 0 1) phase, which coincide with the (0 0 2) plane of the wurtzite hexagonal-shaped ZnO [19].

Crystalline, Structural and Optical Properties of ZnO Nanostructures X-Rays Diffraction Patterns The SAED patterns and HR-TEM observations are fully consistent with the XRD results as shown in Figure 3. Figure 3 shows the XRD patterns of synthesized ZnO nanomaterials. XRD patterns can be indexed as the hexagonal ZnO, consistent with the values in the standard card (JCPDS card no. 36-1451).

Reprinted with permission from [M. S. Akhtar, 2008], Electrochim. Acta, 53 (2008) 7869 © 2008 Elsevier Ltd. Figure 3. XRD patterns of (a) ZnO–NH3, (b) ZnO–citric and (c) ZnO–oxalic materials prepared by hydrothermal method.

336

M. Shaheer Akhtar, M. Alam Khan, Myung Seok Jeon et al.

Compared with the standard diffraction patterns, (0 0 2) peak intensities of ZnO–NH3 and ZnO–citric are stronger than that of bulk ZnO, indicating the preferential growth of the ZnO nanostructures in the (0 0 1) orientation. Relative intensity of ZnO–citric is highest, indicating the highest crystallinity of ZnO–citric. It is known that the full width at half maximum (FWHM) is directly related to the crystal quality and crystallinity of the sample, in which a narrower line width is regarded as a clear evidence for high crystal quality and crystallinity [20]. The fitted value of the FWHM of XRD peak is obtained from the (1 0 1) peak of ZnO materials. The FWHM value of ZnO–citric is 0.20◦, which is lower than the 0.32◦ in ZnO–NH3 and 0.38◦ in ZnO–oxalic, indicating the higher crystallinity of ZnO–citric than the others.

The Raman Scattering Spectroscopy

Reprinted with permission from [M. S. Akhtar, 2008], Electrochim. Acta, 53 (2008) 7869 © 2008 Elsevier Ltd. Figure 4. Raman spectra of (a) ZnO–NH3, (b) ZnO–citric and (c) ZnO–oxalic materials prepared by hydrothermal method.

Figure 4 shows the Raman spectra of the ZnO nanomaterials. The spectra show that the three Raman peaks at 437, 378 and 331 cm−1 are ascribed to ZnO E2, A1T mode and E2H–E2L multiphonon process for wurtzite hexagonal ZnO single crystals, respectively [21]. ZnO–oxalic shows the very weak E1L peak at 579 cm−1, resulting from the oxygen vacancies and impurities in ZnO materials. The intensity of main peak at 437 cm−1 are decreased in the order of ZnO–citric (arbitrary intensity: 2463) > ZnO–oxalic (arbitrary intensity: 2394) >ZnO–NH3 (arbitrary intensity: 1529), indicating the highest crystallinity of ZnO–citric. Generally, the higher intensity and narrower spectral width of the Raman active E2 mode is attributed to the high optical and crystalline properties of the materials.

Hydrothermal Synthesis of ZnO Materials …

337

UV–Vis Spectrum Figure 5 shows the UV–vis absorption spectra of desorbed dye from the ZnO thin films by using 0.1mM NaOH solution. The amount of absorbed dye is calculated by the integration of spectra area: ZnO–citric (3.8×10−7 mol/cm2) > ZnO–oxalic (3.0×10−7 mol/cm2) >ZnO–NH3 (1.9×10−7 mol/cm2). Hence, the highest dye absorption of ZnO– citric is ascribed to its high surface area (13.11m2/g).

Reprinted with permission from [M. S. Akhtar, 2008], Electrochim. Acta, 53 (2008) 7869 © 2008 Elsevier Ltd. Figure 5. UV–vis absorption of dye (N-719) extracted with 2M NaOH from the electrodes of (a) ZnO– NH3, (b) ZnO–citric and (c) ZnO–oxalic.

Photovoltaic Performance of ZnO Nanostructures Based Dsscs The Current Density (I)-Voltage (V) of the Fabricated DSSC Figure 6 and Table 1 show the I–V curve and PV performance data of DSSCs fabricated with the synthesized ZnO nanomaterials. DSSCs with ZnO–citric achieved a maximum conversion efficiency of 2.6% with short circuit current (ISC) of 12.28 mA/cm2, open circuit voltage (VOC) of 0.557 V and a fill factor of 48.3%. This superior PV performance of DSSCs with ZnO–citric may be attributed to the uniform morphology with the high and surface area and crystallinity, which lead to high dye absorption and less resistance to electron transfer. The highest amount of absorbed dye generates the highest number of excitons on the electrode of ZnO–citric. Many of the excitons should be alive to make high current, which significantly depend on the shape of the ZnO materials and the morphology of the electrode film. However, the very low conversion

338

M. Shaheer Akhtar, M. Alam Khan, Myung Seok Jeon et al.

efficiency and photocurrent density of DSSC with ZnO–NH3 may be ascribing to the lower amount of absorbed dye.

Reprinted with permission from [M. S. Akhtar, 2008], Electrochim. Acta, 53 (2008) 7869 © 2008 Elsevier Ltd. Figure 6. Current–voltage curve of DSSC fabricated with (a) ZnO–NH3, (b) ZnO–citric and (c) ZnO– oxalic at 1.5AM.

Table 1. Physicochemical property and PV performance data

Reprinted with permission from [M. S. Akhtar, 2008], Electrochim. Acta, 53 (2008) 7869 © 2008 Elsevier Ltd

CONCLUSION The morphology of nanocrystalline ZnO materials was well controlled by changing the capping agents in hydrothermal method at constant pH, flower structured ZnO in liquid ammonia, sheet sphere structured ZnO in citric acid and nanoplates structured ZnO in oxalic acid. All of the synthesized ZnO nanomaterials possess high crystalline wurtzite structures grown in the (0 0 1) direction with the size of 2–4 μm, which consist of ZnO units around 20–400 nm. The DSSC with sheet-sphere ZnO showed maximum conversion efficiency of 2.61% due to the uniform morphology with the high surface area and crystallinity, which lead to high dye absorption and less resistance to electron

Hydrothermal Synthesis of ZnO Materials …

339

transfer. The morphological property of ZnO significantly affects the photoelectrode properties and the photovoltaic performances of DSSC. It is notable that the ZnO materials with sphere structure may be the optimal photo-anode material among various ZnO nanomaterials for DSSC.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

B. O’Regan, M. Gratzel, Nature 353 (1991) 737. M. K. Nazeeruddin, A. Kay, I. Podicio, R. Humphy-Baker, E. Muller, P. Liska, N. Vlachopoulos, M. Gratzel, J. Am. Chem. Soc. 115 (1993) 6382. M. S. Kang, Y. J. Kim, J.Won, Y. S. Kang, Chem. Commun. 21 (2005) 2686. M. S. Kang, J. H. Kim, Y. J. Kim, J. Won, N. G. Park, Y. S. Kang, Chem. Commun. 7 (2005) 889. A. Kay, M. Gratzel, Chem. Mater. 14 (2002) 2930. E. Palomares, J. N. Clifford, S. A. Haque, T. Lutz, J. R. Durrant, J. Am. Chem. Soc. 125, (2003) 475. S. Ito, T. Kitamura, Y.Wada, S. Yanagida, Sol. Energy Mater. Sol. Cells 76 (2003) 3. X. Xu, X.W. Sun, Appl. Phys. Lett. 83 (2003) 3806. X. Wang, X. W. Sun, A. Wei, Y. Lei, X. P. Cai, C. M. Li, Z. L. Dong, Appl. Phys. Lett. 88, (2006) 233106. W. Pan, Z.R. Dai, Z.L.Wang, Science 291 (2001) 1947. M. Bangall, Y.G. Chen, Z. Zhu, T. Yao, Appl. Phys. Lett. 70 (1997) 2230. C. Bauer, G. Boschloo, E. Mukhtar, A. Hagfeldt, J. Phys. Chem. B 105 (2001) 5585. M. Law, L. E. Greene, J. C. Johnson, R. Saykally, P. Yang, Nat. Mater. 4 (2005) 455. H. Rensmo, K. Keis, H. Lindstorm, S. Sodergren, A. Solbrand, A. Hagfeldt, E. Lindquist, L. N.Wang, M. Muhammed, J. Phys. Chem. B 101 (1997) 2598. Y. Hao, M. Yang, W. Li, X. Qiao, L. Zhang, S. Cai, Sol. Mater. Energy Sol. Cells 60, (2000) 349. T. V. Nguyen, H. C. Lee, M. A. Khan, O. B. Yang, Sol. Energy 81 (2007) 529. K. A. Emery, C. R. Osterwald, H. Aharoni, Solid-State Electron. 30 (1987) 213. L. Margulis, G. Salitra, R. Tenne, M. Talianker, Nature 365 (1993) 113. B. Liu, H. C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430. R. Hong, J. Huang, H. He, Z. Fan, J. Shao, Appl. Surf. Sci. 242 (2005) 346. G. J. Exharhos, S. K. Sharma, Thin Solid Films 270 (1995) 27.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 24

LOW TEMPERATURE GROWN ZNO NANOTUBES FOR DYE SENSITIZED SOLAR CELLS Sadia Ameen1, M. Shaheer Akhtar2, Young Soon Kim1, O-Bong Yang2 and Hyung Shik Shin1,* 1

Energy Materials and Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 School of Semiconductor and Chemical Engineering and Solar Energy Research Center, Chonbuk National University, Jeonju, Republic of Korea 3 New and Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT Non-aligned and aligned ZnO nanotube (NTs) are synthesized by low temperature solution method are applied as photoanode materials for the fabrication of efficient dyesensitized solar cells (DSSCs). The crystalline and the morphological analysis reveals that the grown aligned ZnO NTs possesses a typical hexagonal crystal structure of outer and inner diameter ∼250 nm and ∼100 nm, respectively. ZnO seeding on FTO substrates is an essential step to achieve the aligned ZnO NTs. A DSSC fabricated with aligned ZnO NTs photoanode achieves high solar-to-electricity conversion efficiency of ∼2.2% with short circuit current (JSC) of 5.5 mA/cm2, open circuit voltage (VOC) of 0.65V and fill factor (FF) of 0.61. Significantly, the aligned ZnO NTs photoanode shows three times improved solar-to-electricity conversion efficiency than DSSC fabricated with nonaligned ZnO NTs. The enhanced performances are credited to the aligned morphology of ZnO NTs which executes the high charge collection and the transfer of electrons at the interfaces of ZnO NTs and electrolyte layer. *

Corresponding Author Email: [email protected].

342

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

INTRODUCTION The nanoscale semiconducting materials are widely used for photovoltaic and other electronic devices owing to their unique charge transport and absorption of photons from light. The photovoltaic device, a dye-sensitized solar cell (DSSC) [1-2] comprises a dye modified nanocrystalline titanium dioxide (TiO2) electrode, platinum (Pt) counter electrode and redox electrolyte solution of iodide ion/tri-iodide ions redox coupled between the electrodes. The dynamic and the interfacial properties of DSSCs are mainly improved by controlling the morphology and the particle size of the photoanode layer [3] which could enhance the solar-to-electricity conversion efficiency and reduces the recombination rate at the interface of semiconducting and electrolyte layer [4]. So far, several semiconducting metal oxide films like SnO2 [5], Nb2O5 [6] and CeO2 [7] have been applied as photoanode materials in DSSCs. Amongst these, Zinc Oxide (ZnO) has received a great interest in photovoltaic devices due to its wide band gap (3.4 eV), high exciton binding energy (60 meV), high electronic mobility, and environmental benign [89]. Efforts are attempted to improve solar-to-electricity conversion efficiency of ZnO based DSSCs using branched networks and different mixed morphologies of ZnO photoanodes [10-12]. One dimensional (1D) ZnO nanostructures, such as nanowires, nanobelts and nanotubes are promising candidates to fabricate efficient DSSCs owing to their high surface-to-volume ratio, which allows more adsorption of dye molecules [13]. Recently, a high surface area ZnO nanotube (NT) photoanode prepared by anodic aluminum oxide template for DSSCs was designed by Martinson et al. showed that NT’s morphology promoted the charge separation and charge transport in the cell and generates high open circuit voltage (VOC) [14]. The utilization of ZnO NTs array for the efficient performances of DSSCs is still a challenge. Recently, it is found that the aligned ZnO NTs possess sufficiently high surface-to-volume ratio with good electrical characteristics [15-16]. Methods like solvothermal, hydrothermal and electrochemical process [17-18] have been largely adopted to grow these aligned structures but the effective, simple and easy routes are still demanded to synthesize the highly dense aligned ZnO NTs on conducting substrates. The low temperature solution method is an effective and easy controlled technique for the growth of aligned ZnO NTs. In this work, we report the growth, structural, optical and the subsequent photovoltaic properties of aligned ZnO NTs thin film photoanode for the fabrication of DSSCs. The ZnO seeded FTO glass substrate supports the synthesis of highly dense aligned ZnO NTs arrays whereas, nonseeded FTO substrates generate non-aligned ZnO NTs. The prepared ZnO NTs electrodes are used as photoanodes for the fabrication of DSSCs. The non-aligned ZnO NTs photoanode based fabricated DSSC exhibits low solar-to-electricity conversion efficiency

Low Temperature Grown ZnO Nanotubes for Dye Sensitized Solar Cells

343

of ∼0.78%. However, DSSC fabricated with aligned ZnO NTs photoanode shows three times improved solar-to-electricity conversion efficiency than DSSC fabricated with non-aligned ZnO NTs. For the fabrication of DSSC, the grown ZnO NTs thin film substrates are immersed in the ethanolic solution of ruthenium (II) 535 bis-TBA (0.3 mM, N-719, Solaronix) dye for 12 h at room temperature under dark condition. The dye adsorbed ZnO NTs thin film electrodes are rinsed with absolute ethanol and dried under a nitrogen stream at 40oC. The platinum (Pt) coated FTO substrate, as counter electrode is placed over the dye adsorbed ZnO NTs electrode and the edges of the cell were sealed with 60 μm thick Surlyn sheet by hot-pressing of the two electrodes at 80°C. An electrolyte comprising of 0.5 M LiI, 0.05 mM I2, and 0.2 M tert-butyl pyridine in acetonitrile, is introduced through holes in the counter electrode using a syringe on the dye-immobilized ZnO NTs thin film working electrode, and holes are sealed with small microscopic glass and Surlyn sheet.

Morphological Studies of ZnO Nanotubes The Field Emission Scanning Electron Microscopy Figure 1 shows the surface FESEM images of ZnO NTs deposited on non-seeded and ZnO seeded FTO substrates. Figure 1 (a, b) exhibits the highly dense aligned ZnO NTs, substantially grown on ZnO seeded FTO substrates. Importantly, the ZnO NTs possess a hexagonal hollow structure, as shown in Figure 1 (c, d) with average inner and outer diameter of ~150 nm and ~300 nm respectively. However, non-seeded FTO substrates obtain random and non-aligned morphology of NTs with the average diameter of 800 nm, as shown in Figure 1 (e). The high resolution image clearly displays the typical hexagonal hollow and round end of the NTs (Figure 1 (f)). Thus, the orientations of aligned and ordered ZnO NTs are greatly improved by the controlled ZnO seeding on FTO substrates. The Transmission Electron Microscopy Figure 2 shows the transmission electron microscopy (TEM), high resolution (HR) TEM and selected area electron patterns (SAED) of grown ZnO NTs. Figure 2 (a) reveals hollow NT morphology with the outer and inner diameter of ~250 nm and ~100 nm respectively. SAED patterns (Figure 2 (c)) exhibits a single crystal with a wurtzite hexagonal phase which is preferentially grown in the [0001] direction. It is further confirmed from the HRTEM image of the grown ZnO NTs, presented in Figure 2 (b). HRTEM image shows well-resolved lattice fringes of crystalline ZnO NTs with the interplanar spacing of ~0.52 nm which is consistent to the lattice constant in the reference

344

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

(JCPDS No. 36–1451) for ZnO. Additionally, this value corresponds to the d-spacing of [0001] crystal planes of wurtzite ZnO. Thus, the synthesized ZnO NTs is a single crystal and preferentially grown along the c-axis [0001].

Reprinted with the permission from S. Ameen et al, Electrochimica Acta 56 (2011) 1111–1116. © 2011, lsevier Ltd. Figure 1. FESEM images of aligned ZnO NTs (a) at low magnification and (b, c, d) at high magnification and non-aligned ZnO NTs (e) at low magnification (f) at high magnification.

Low Temperature Grown ZnO Nanotubes for Dye Sensitized Solar Cells

345

Reprinted with the permission from S. Ameen et al, Electrochimica Acta 56 (2011) 1111–1116. © 2011, Elsevier Ltd. Figure 2. (a) TEM, (b) HR-TEM and (c) corresponding SAED of grown ZnO NTs.

Crystalline, Optical and Structural Properties of ZnO Nanotubes X-ray Diffraction Patterns and UV–vis Spectra The XRD pattern of non-aligned and aligned ZnO NTs thin film, shown in Figure 3 (a), exhibit same pattern at 320, 34.60, 36.50, 47.70, 63.30 and 68.30 which are perfectly indexed in (JCPDS No. 36-1451) [19] and attribute to the typical wurtzite structure of ZnO crystals. Noticeably, the intensity of (002) diffraction is much higher compared to other peaks and exits along the c-axis [20]. It is obvious that the c-axis is the preferential growth direction due to the instability of polar (002) plane [21]. The FTO peaks at 26.7o, 34.0o, 51.6o, 55.0o, 62.0o and 66.0o are observed in both the XRD patterns of ZnO NTs. XRD peaks for ZnO NTs grown on seeded substrates appear at the same position but with high intensity might due to high crystalline properties of aligned morphology of ZnO NTs. Furthermore, to understand the absorption properties, UV-Vis spectroscopy of grown ZnO NTs are examined, as depicted in Figure 3 (b). ZnO NTs show a strong narrow absorption peak in the UV region at ~362 nm, corresponds to the characteristic band of the wurtzite hexagonal structure. The appearance of single peak indicates that the grown ZnO NTs do not contain impurities. Moreover, the aligned morphology of ZnO

346

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

NTs attains high absorption, indicating higher crystalline properties than non-aligned ZnO NTs.

Reprinted with the permission from S. Ameen et al, Electrochimica Acta 56 (2011) 1111–1116. © 2011, Elsevier Ltd. Figure 3. (a) XRD pattern and (b) UV-Vis spectra of aligned and non-aligned ZnO NTs.

The Raman Scattering Spectroscopy and Photoluminescence Spectra Figure 4 (a) shows the Raman spectra of non-aligned and aligned ZnO NTs thin film substrates. The grown ZnO NTs exhibits a strong Raman peak at ~437 cm-1 corresponds to E2 mode of ZnO crystal and matches with Raman peak of bulk ZnO crystals [22]. Additionally, two small peaks at ~330 cm-1 and ~578 cm-1 are assigned to the second

Low Temperature Grown ZnO Nanotubes for Dye Sensitized Solar Cells

347

order Raman spectrum arising from zone-boundary phonons 3E2H–E2L for wurtzite hexagonal ZnO single crystals and E1 (LO) mode of ZnO associated with oxygen deficiency in ZnO nanomaterials respectively [23]. Compared to non-aligned ZnO NTs, the stronger E2 mode and much lower E1 (LO) mode indicates the presence of lower oxygen vacancy.

Reprinted with the permission from S. Ameen et al, Electrochimica Acta 56 (2011) 1111–1116. © 2011, Elsevier Ltd. Figure 4. (a) Raman spectra and (b) Photoluminescence spectra of aligned and non-aligned ZnO NTs.

348

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

The Raman active E2 mode with high intensity and narrower spectral width is generally ascribed to the better optical and crystalline properties of the materials [24]. Thus, the grown aligned ZnO NTs results high crystallinity of ZnO crystals with less oxygen vacancies. Figure 4 (b) depicts the PL spectra of grown non-aligned and aligned ZnO NTs substrates. An intensive sharp UV emission at ~378 nm and a broader green emission at ~581 nm are attributed to the free exciton emission from the wide band gap of ZnO NTs and the recombination of electrons in single occupied oxygen vacancies in ZnO nanomaterials [25]. The lower and broader green emission of the aligned ZnO NTs reveals the existence of defects such as surface oxygen vacancies [26-27] due to the high surface-volume-ratio of hollow structure of ZnO nanomaterials. Moreover, it has been reported that the broadened green emission intensity is originated from the increased concentration of singly ionized oxygen vacancies and non-stoichiometric phase structure formation. Herein, the low intensity and less broaden green emission indicates that the aligned ZnO NTs exhibits less oxygen vacancies and considerable stoichiometric phase structure formation. Thus, the photoluminescence indeed suggests that ZnO seeding on FTO substrates might improve surface-to-volume ratio and optical properties of ZnO NTs. These results are in consistent with the Raman studies.

Possible Growth Mechanism and the Formation of ZnO Nanotubes In solution method, the growth of ZnO NTs is greatly affected by pH and the concentration of Zn(NO3)2. Moreover, Zn (NO3)2 and HMT are chosen as zinc source and OH− source in the solution, respectively. First, HMT molecule decomposes into HCHO and NH3 afterwards, NH3 interacts with H2O to provide OH- ions slowly upon heating [28]. The generated OH- reacts with Zn2+ cations to form Zn(OH)2 at elevating temperature, and the subsequent dehydration reaction generates ZnO nuclei on the substrate [29]. The involved reactions are expressed by the following reaction formulas: (CH2)6N4 + 6H2O → 6HCHO + 4NH3

(1)

NH3 + H2O → NH4+ +OH−

(2)

2OH− + Zn2+ → Zn(OH)2

(3)

Zn(OH)2 → ZnO + H2O

(4)

Low Temperature Grown ZnO Nanotubes for Dye Sensitized Solar Cells

349

The growth of ZnO NTs could be explained on the basis of the schematic representation, as shown in Figure 5. In general, the ZnO nuclei are grown to form a hexagonal wurtzite planar nucleus on the surface of the seeded substrates. As the ideal growth rates of the wurtzite hexagonal ZnO crystals in different directions are found in the order of [0001] > [01ī ī] > [01 ī 0] > [01 ī1] > [000 ī] under hydrothermal conditions. Therefore, the (0001) faces are the most rapid-growth-rate planes as compared to other growth facets [30, 31]. In our case, the synthesized nanotubes are fully consistent with the typical growth habit of ZnO crystals, in which the nanotubes are grown along the c-axis direction which are bounded with the six crystallographic non-polar [01ī 0] planes. In contrast to non-polar surfaces, the polar surfaces are metastable in nature which could be easily etched at high pH conditions [30, 31]. It is reported that well defined side facets are highly stable as compared to the central portion of the hexagonal structure and possess more defect prone. Hence, slightly higher pH and heating temperature might enhance the etching in the central zone of the hexagonal disks, and create hollow structure. Subsequently, the one-dimensional growth produces the tubular structures at the same time [32]. Finally, a hexagonal tubular structure is formed and vertically aligned on the seeded FTO substrates. It is noteworthy that the well aligned morphology is obtained with seeded FTO substrates but same morphology is not seen with non-seeded FTO substrates. Generally, the seed layer decreases the interface energy between ZnO crystal and the conducting substrate and thus, provides sufficient pathway for the aligned structures [33]. Hence, to obtain aligned ZnO NTs, the ZnO seeding is a necessary step during the preparation procedure.

Reprinted with the permission from S. Ameen et al, Electrochimica Acta 56 (2011) 1111–1116. © 2011, Elsevier Ltd. Figure 5. Possible growth mechanism and the formation of ZnO NTs.

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

350

Photovoltaic Performances of ZnO Nanotubes Based DSSCs The Current (I)-Voltage (V) Cureves of DSSCs Fabricated with ZnO Nanotubes Figure 6 compares the current density–voltage (J–V) characteristics of DSSCs fabricated with non-aligned and aligned ZnO NTs photoanodes. The corresponding J–V characteristics are measured under 100 mW/cm2 light intensity and the J-V curves of both the cells are summarized in Table 1. Table 1. Summary of photovoltaic performance of fabricated DSSCs with aligned and non-aligned ZnO NTs photoanodes Photoanode Vertically aligned ZnO NTs Non-aligned ZnO NTs

Photovoltaic performance IPCE (%) VOC (V) JSC (mA/cm2) 31.5 0.65 5.5 21 0.60 2.2

FF 0.61 0.57

η (%) 2.2 0.78

Reprinted with the permission from S. Ameen et al, Electrochimica Acta 56 (2011) 1111–1116. © 2011, Elsevier Ltd

Reprinted with the permission from S. Ameen et al, Electrochimica Acta 56 (2011) 1111–1116. © 2011, Elsevier Ltd. Figure 6. J-V curve of the DSSCs fabricated with aligned (red line) and non-aligned (black line) ZnO NTs photoanode.

DSSCs fabricated with aligned ZnO NTs photoanode achieve high solar-to-electricity conversion efficiency (η) of 2.2% with a high short circuit current (J SC) of 5.5 mA/cm2, open circuit voltage (VOC) of 0.65 V, and fill factor (FF) of 0.61. However, DSSC fabricated with non-aligned ZnO NTs photoanode executes relatively low η of 0.78%

Low Temperature Grown ZnO Nanotubes for Dye Sensitized Solar Cells

351

with JSC 2.22 mA/cm2, VOC 0.60 V and FF 0.57. Compared with non-aligned ZnO NTs photoanode based DSSC, the aligned ZnO NTs photoanode has appreciably enhanced the conversion efficiency by three times with significantly improved J SC, VOC and FF. The enhanced photovoltaic performances and the improved J SC are mainly related to the highly dense morphology of aligned ZnO NTs and also, high dye absorption which leads to improved light harvesting efficiency. The aligned morphology might result from the sufficiently high surface area of ZnO NTs. It is reported that the aligned morphology of ZnO nanomaterials improve the charge collection and transfer properties [14]. Herein, the improved photovoltaic properties of the aligned ZnO NTs are attributed to the aligned morphology which might execute reasonably high charge collection and the transfer of electrons at the interface of ZnO NTs and electrolyte layer. While, low efficiency of non– aligned ZnO NTs might due to low surface area of ZnO NTs and non-uniform surface which might result to low light harvesting efficiency and increases the recombination rate between the electrolyte and the FTO substrate. It is noteworthy that the performance of DSSCs with grown aligned ZnO NTs photoanode is significantly higher than the reported DSSCs with aligned ZnO nanorods, nanowires and nanotubes based photoanode [34, 11].

The Incident Photon-to-Current Conversion Efficiencycurves of the DSSCs Fabricated with ZnO Nanotubes The performance of DSSCs could be quantified on a macroscopic level in terms of incidence photon to current conversion (IPCE) efficiency. The IPCE gives the ratio between the number of generated charge carriers contributing to the photocurrent and the number of incident photons, as expressed by equation 1. IPCE (%) = 1241 JSC (μAcm-2)/λ (nm)Pin (mWcm-2) x 100

(5)

where JSC is the short-circuit photocurrent density for monochromatic incident light and λ and Pin are the wavelength and the intensity of the monochromatic light, respectively. Figure 7 shows the IPCE curves plotted as a function of excitation wavelength of DSSCs fabricated with non-aligned and aligned ZnO NTs photoanode. The aligned ZnO NTs based DSSC achieves a maximum IPCE value of ∼31.5% at ~520 nm whereas, the considerably low IPCE (~21%) is obtained with non-aligned ZnO NTs photoanode based DSSC. It is noticed that aligned ZnO NTs photoanode presents approximately two times improved IPCE compared to non-aligned ZnO NTs photoanode. The enhancement in IPCE imputes the influence of highly ordered aligned ZnO NTs morphology with high surface area which might improve the light scattering capacities and provides the better interaction between the photons and the dye molecules [35]. Thus, the enhanced IPCE results of the aligned ZnO NTs photoanode based DSSC are resulted to the high JSC, VOC, and the improved photovoltaic performances.

352

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

Reprinted with the permission from S. Ameen et al, Electrochimica Acta 56 (2011) 1111–1116. © 2011, Elsevier Ltd. Figure 7. IPCE curves of the DSSCs fabricated with aligned (hallow circle) and non-aligned ZnO NTs (hallow triangle) photoanode.

CONCLUSION The non-aligned and aligned ZnO NTs are synthesized by low temperature solution method and applied as new photoanode for the fabrication of DSSCs. A possible growth mechanism and the formation of non-aligned and aligned morphology of ZnO NTs have been investigated. The crystalline and optical properties reveal that the aligned ZnO NTs attain higher crystalline properties with good crystal quality than non-aligned ZnO NTs. The photovoltaic performances of aligned ZnO NTs photoanode based DSSCs achieve considerably high solar-to-electricity conversion efficiency with reasonable J SC, VOC and FF. The aligned ZnO NTs photoanode attains three times improved solar-to-electricity conversion efficiency compared to DSSC fabricated with non-aligned ZnO NTs. The superior photovoltaic performances might attribute to highly dense aligned structure of ZnO NTs which substantially provide large surface for high dye absorption, light harvesting efficiency and increases the electron transportation at the interfaces of photoanode layer and electrolyte layer. Thus, ZnO seeding on FTO substrate is beneficial to achieve aligned ZnO NTs and for the better photovoltaic performance of fabricated DSSCs.

Low Temperature Grown ZnO Nanotubes for Dye Sensitized Solar Cells

353

REFERENCES [1] [2]

[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

B. O’Regan, M. Grätzel, Nature 335 (1991) 737. M. K. Nazeeruddin, P. Pechy, T. Renouard, S. M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G. B. Deacon, C. A. Bignozzi, M. Grätzel, J. Am. Chem. Soc. 123 (2001) 1613. S. Ito, T. Kitamura, Y. Wada, S. Yanagida, Sol. Energy Mater. Sol. Cells 76 (2003) 3. S. S. Kim, J. H. Yum, Y. E. Sung, J. Photochem. Photobiol. Chem. 171 (2005) 269. I. Bedja, S. Hotchandani, P. V. Kamat, J. Phys. Chem. 98 (1994) 4133. K. Sayama, H. Sugihara, H. Arakawa, Chem. Mater. 10 (1998) 3825. A. Turkovic, Z. C. Orel, Sol. Energy Mater. Sol. Cells 45 (1997) 275. Y. Wu, H. Yan, M. Huang, B. Messer, J. Song, P. Yang, Chem. Eur. J. 8 (2002) 1260. Z. G. Chen, Y. W. Tang, L. S. Zhang, L. J. Luo, Electrochim. Acta 51 (2006) 5870. C. Y. Jiang, X. W. Sun, G. Q. Lo, D. L. Kwong, J. X. Wang, Appl. Phys. Lett. 90 (2007) 263501. D. I. Suh, S. Y. Lee, T. H. Kim, J. M. Chun, E. K. Suh, O. B. Yang, S. K. Lee, Chem. Phys. Lett. 442 (2007) 348. M. S. Akhtar, K. K. Cheralathan, J. M. Chun, O. B. Yang, Electrochim. Acta 53 (2008) 6623. E. Galoppini, J. Rochford, H. Chen, G. Saraf, Y. Lu, A. Hagfeldt, G. Boschloo, J. Phys. Chem. B 110 (2006) 16159. A. B. F. Martinson, J. W. Elam, J. T. Hupp, M. J. Pellin: Nano Lett. 7 (2007) 2183. Y. F. Gao, M. Nagai, T. C. Chang, J. J. Shyue, Cryst. Growth. & Des. 7 (2007) 2467. E. Galoppini, J. Rochford, H. H. Chen, G. Saraf, Y. C. Lu, A. Hagfeldt, G. Boschloo, J. Phys. Chem. C 110 (2006) 16159. X. L. Yuan, B. Dierre, J. B. Wang, B. P. Zhang, T. Sekiguchi, J. Nanosci. Nanotechnol. 7 (2007) 3323. Y. Sun, G. M. Fuge, N. A. Fox, D. J. Riley, M. N. R. Ashfold, Adv. Mater. 17 (2005) 2477. A. Al-Hajry, A. Umar, Y. B. Hahn, D. H. Kim, Superlatt. Microstruc. 45 (2009) 529. L. Vayssieres, Adv. Mater. 15 (2003) 464. R. A. Laudise, E. D. Kolb, A. J. J. Caporason, Am. Ceram. Soc. 47 (1964) 9. C. Roy, S. Byrne, E. McGlynn, J. P. Mosnier, E. de Posada, D. O. Mahony, J. G. Lunney, M. O. Henry, B. Ryan, A. A. Cafolla, Thin Solid Films 436 (2003) 273. G. J. Exarhos, S. K. Sharma, Thin Solid Films 270 (1995) 27.

354

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

[24] J. Serrano, F. J. Manjon, A. H. Romero, F. Widulle, R. Lauck, M. Cardona, Phys. Rev. Lett. 90 (2003) 055510. [25] K. Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. A. Voigt, B. E. Gnade, J. Appl. Phys. 79 (1996) 7983. [26] Y. Li, G. Cheng, L. Zhang, J. Mater. Res. 15 (2000) 2305. [27] K. Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. A. Voigt, B. E. Gnade, J. Appl. Phys. 79 (1996) 7983. [28] K. Govender, D. S. Boyle, P. B. Kenway, P. O. Brien, J. Mater. Chem. 14 (2004) 2575. [29] X. X. Liu, Z. G. Jin, S. J. Bu, J. Zhao, Z. F. Liu, J. Am. Ceram. Soc. 89 (2006) 1226. [30] B. C. Bunker, P. C. Rieke, B. J. Tarasevich, A. A. Campbell, G. E. Fryxell, G. L. Graff, L. Song, J. Liu, J. W. Virden, Science 264 (1994) 48. [31] (a) F. Li, Y. Ding, P. X. X. Gao, X. Q. Xin, and Z. L. Wang, Angew. Chem. Int. Ed. 43 (2004) 5238. (b) A. Umar, Y. B. Hahn, Appl. Phys. Lett. 88 (2006) 173120. [32] S. Kar, S. Santra, J. Phys. Chem. C 112 (2008) 8144. [33] M. Guo, P. Diao, S. M. Cai, J. Solid State Chem. 178 (2005) 1864. [34] A. D. Pasquier, H. Chen, Y. Lu, Appl. Phys. Lett. 89 (2006) 253513. [35] Y. Tachibana, K. Hara, K. Sayama, H. Arakawa, Chem. Mater. 14 (2002) 2527.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 25

NANOSPIKES DECORATED ZNO SHEETS FOR SOLAR CELL APPLICATION Sadia Ameen1, M. Shaheer Akhtar2, Young Soon Kim1 and Hyung Shik Shin1,* 1

Energy Materials and Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New and Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT This work reports the synthesis of nanospikes decorated ZnO sheets on the electrodeposited ZnO seeded fluorine doped tin oxide (FTO) substrates through the hydrothermal method at 90oC and applied as photoanode for the efficient dye sensitized solar cells (DSSCs). The unique nanospikes decorated ZnO sheets are comprised of nanospikes with the average diameter of ~80-100 nm and the sheets of several nanometers. The ZnO nanospikes are decorated on either one side or the both side of the single ZnO sheets (thickness ~50-60 nm). The X-Rays diffraction and UV-Vis spectroscopy results reveal that the grown nanospikes decorated ZnO sheets exhibit well crystalline with typical wurtzite hexagonal phase of ZnO nanostructures. A growth mechanism is proposed to investigate the formation of the grown nanospikes decorated ZnO sheets. The solar-to-electricity conversion efficiency of ∼2.51% with the high short circuit current (JSC) of 6.07 mA/cm2 is attained by DSSC fabricated with nanospikes decorated ZnO sheets photoanode. The enhanced performance might be associated to the high charge collection and the fast electrons transfer at the interfaces of ZnO and the electrolyte layer due to the high dye absorption over the surface of ZnO leading to high light harvesting. *

Corresponding Author Email: [email protected].

356

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

INTRODUCTION An emerging photovoltaic device known as dye-sensitized solar cells (DSSCs) has gained much attention due to its easy manufacturing, low cost and reasonable high overall light to electricity conversion [1-2]. The typical DSSC comprises of dye sensitized semiconductor metal oxides (TiO2) electrode, metal coated conducting glass electrode and a redox electrolyte between the two electrodes [3]. DSSCs are facing the major problems such as the back recombination reactions of the photoinjected electrons with the holes of tri-iodide ions in the electrolyte and the presence of oxygen/iodine species on photoanode, which hinders the performance and photocurrent of the device [4]. Moreover, some photogenerated electrons drop during the charge transport process at the interface of electrolyte and nanostructured semiconducting interface which influences the functioning of the effective DSSCs. To improve the performance of DSSCs, the effective photogenerated charge separation and the light harvesting efficiency are required for the high light to energy conversion efficiency of DSSCs [5]. These parameters could be intensified by the properties of semiconducting materials such as the band gap, particle size, surface morphology, porosity, surface area and thickness of the photoanode [6]. Zinc oxide (ZnO) nanomaterials are recently dealing with the versatile applications in field-effect transistors, lasers, photodiodes, sensors and photovoltaics owing to their unique photoelectric properties, optical transparency, electric conductivity and piezoelectricity properties [7-8]. As compared to traditional TiO2, ZnO materials possess similar wide band gap (3.37 eV) with large exciton binding energy (60 meV) and higher electron mobility [9]. Recently, M. S. Akhtar et al. demonstrated that the performance of DSSCs could be effectively altered by varying the morphologies of ZnO nanomaterials [10]. Furthermore, the parameters like morphology, physical and the crystalline properties of ZnO nanomaterials are required for improving the performance of DSSCs [10-11]. So far, the photoanodes constructed from the different morphologies of ZnO nanostructures such as nanorods [12], nanotetrapods, nanosheet [13] and nanobelts [14] have been studied for the fabrication of efficient DSSCs [15]. The different morphologies of ZnO nanomaterials are grown by various methods like sonochemical, hydrothermal, electrochemical and thermal evaporation processes [10, 16-17]. Among these, hydrothermal process is largely adopted to control the structures, sizes and the properties of ZnO nanomaterials [10]. In this work, the unique nanospikes decorated ZnO sheets are grown on the electrodeposited ZnO seed layer coated FTO substrates by the generalized hydrothermal method and applied as photoanode for DSSC. An overall energy conversion efficiency of ~2.51% has achieved at 100 mW/cm2 light intensity (1.5AM). The fabrication of DSSC is performed as reported in the previous work [11]. In brief, the prepared photoanode of nanospikes decorated ZnO sheets thin film substrates are immersed in an ethanolic solution of ruthenium (II) 535 bis-TBA dye (0.3 mM, N-719,

Nanospikes Decorated ZnO Sheets for Solar Cell Application

357

Solaronix) for 12 h at room temperature under dark condition. The platinum (Pt) counter electrode is placed over dye-immobilized nanospikes decorated ZnO sheets thin film photoanode and the edges of the cell are sealed with 60 μm thick Surlyn sheet. Sealing is accomplished by hot-pressing of the two electrodes together at 80oC. An electrolyte of specified composition (0.5M LiI, 0.05mM I2, and 0.2M tert-butyl pyridine in acetonitrile) is introduced through the holes of counter electrode on the dye-immobilized nanospikes decorated ZnO sheets thin film photoanode by syringe for the fabrication of DSSC.

MORPHOLOGICAL STUDIES OF NANOSPIKES DECORATED ZNO SHEETS The Field Emission Scanning Electron Microscopy Figure 1 shows the surface FESEM images of nanospikes decorated ZnO sheets thin film deposited FTO substrates. The low magnification image (Figure 1 (a)) reveals that the nanospikes decorated ZnO sheets morphology is deposited uniformly and densely on the FTO substrate. Each nanospikes decorated ZnO sheets is comprised of a sheet with the average thickness of ~50-60 nm and the aligned nanospikes with the average diameter of ~80-100 nm and length of ~150-200 nm. It has been observed that the nanospikes are consisted of the bundles of small nanorods. The nanospikes are aligned either on one side or other side of ZnO sheet, but in some cases, these nanospikes are found on the both sides of ZnO sheets, as shown in Figure 1 (b).

Reprinted with the permission from S. Ameen et al., Chem. Eng. J., 196 (2012) 307-313. © 2012, Elsevier Ltd. Figure 1. FESEM images of nanospikes decorated ZnO sheets (a) at low magnification and (b) at high magnification.

358

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

The Transmission Electron Microscopy Figure 2 shows the analysis of nanospikes decorated ZnO sheets morphology by the transmission electron microscopy (TEM), high resolution (HR) TEM and selected area electron patterns (SAED). Similar to FESEM images, the nanospikes are decorated on both the sides of ZnO sheet. From Figure 2 (a), the average thickness of the sheet is ~5060 nm and the decorated nanospikes possess the average diameter of ~30 nm (single rods) and the length of ~150-200 nm. Interestingly, the nanospikes are composed of small nanorods and each nanorod has the average diameter of ~20-40 nm, as seen in Figure 2 (a). From the HRTEM image (Figure 2(b)), the well-resolved lattice indicates that the grown ZnO nanomaterials exhibit the good crystallinity. The inter-planar spacing of ∼ 0.52 nm is observed which is consistent to the lattice constant in the reference (JCPDS No. 36–1451) for ZnO nanomaterials. This inter planar spacing value of the lattice fringes correspond to the [0001] crystal plane of the wurtzite ZnO confirming that the grown ZnO nanomaterials are almost defect free [18]. Moreover, the corresponding selected area electron diffraction (SAED) presented in Figure 2 (c), also indicates the typical wurtzite single crystalline structure and the ZnO nanomaterials are grown along caxis direction [0001].

Reprinted with the permission from S. Ameen et al., Chem. Eng. J., 196 (2012) 307-313. © 2012, Elsevier Ltd. Figure 2. TEM image (a), HR-TEM image (b) and corresponding SAED patterns (c) of grown nanospikes decorated ZnO sheets.

Nanospikes Decorated ZnO Sheets for Solar Cell Application

359

CRYSTALLINE AND OPTICAL CHARACTERIZATION OF NANOSPIKES DECORATED ZNO SHEETS X-Ray Diffraction Patterns and UV–Vis Spectrum Figure 3 (a) illustrates the X-ray diffraction (XRD) analysis to determine the crystalline structure of the nanospikes decorated ZnO sheets morphology. All the diffraction peaks appeared at 32.3o (100), 35.2o (002), 36.8o (101), 48.2o (102), 57.2o (110), 63.5o (103) and 66.2o (200) are well matched with the JCPDS card no. 36-1451, confirming that the ZnO nanomaterials possess the hexagonal wurtzite phase with the lattice parameters: a-3.246 and c-5.206Å.

Reprinted with the permission from S. Ameen et al., Chem. Eng. J., 196 (2012) 307-313. © 2012, Elsevier Ltd. Figure 3. XRD patterns (a) and UV-Vis spectra (b) of nanospikes decorated ZnO sheets.

360

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

However, the other XRD peaks at 27.1o, 34.5o, 52.2o and 68.6o are observed which correspond to the FTO layer of the glass substrates. In the spectrum, the intensity of (101) diffraction peak is much higher compared to other peaks, indicating the preferential growth direction due to the instability of polar (101) plane [19]. Figure 3 (b) shows the UV-Vis absorbance spectrum of nanospikes decorated ZnO sheets structures. A single narrow absorption peak is observed near the UV region at ∼376 nm, corresponds to the characteristic band of the wurtzite hexagonal structure in bulk ZnO [11]. Further, the single peak suggests that the grown nanospikes decorated sheets morphology is the pure ZnO materials. This result is consistent with the XRD studies.

STRUCTURAL PROPERTIES OF NANOSPIKES DECORATED ZNO SHEETS Fourier-Transform Infrared and Raman Scattering Spectroscopy The nanospikes decorated ZnO sheets morphology are further characterized by FTIR spectroscopy to determine the structural orientation, as shown in Figure 4 (a). The broad absorption band at ∼3410 cm−1 and a small shoulder at ∼1648 cm−1 are assigned to O–H bending vibrations and stretching vibrations of the water molecules or moisture adsorbed on ZnO or on KBr pellets, respectively. The band at ~884cm−1 attributes to the bending vibration of nitrate. A sharp band at ~523 cm-1 is ascribed to the typical Zn-O group of bulk ZnO [20] indicating that the grown ZnO nanomaterials possess pure Zn-O groups. The Raman scattering spectroscopy of nanospikes decorated ZnO sheets has been performed to investigate the crystallization, structural disorder and the defects of materials, as shown in Figure 4 (b). The grown nanospikes decorated ZnO sheets exhibits four Raman shifts including one very strong peak at ~436.4 cm-1 and two weak peaks at ~331.3 and ~580.7 cm-1. According to the group theory, the typical hexagonal wurtzite ZnO crystal belongs to the C6v4 space group and the optical phonons at the Γ point of the Brillouin zone are A1 +2B1 + E1 +2E2. The A1 and E1 modes are known as Raman and infrared active, which split into longitudinal optical (LO) and transverse optical (TO) components [21]. The E2 modes represent the nonpolar nature having two frequencies such as E2 (high) associated with the oxygen displacement and E2 (low) associated with Zn sub-lattice which is Raman active. While the B1 modes are Raman silent modes [21, 22-23]. In our case, the grown nanospikes decorated ZnO sheets exhibits a dominated and strong intensity Raman peak at ~436.4 cm−1, corresponds to E2 mode which is the characteristic peaks of wurtzite ZnO [22, 24]. The weak Raman shifts at ~331.3 cm−1 and ~580.7 cm−1 are due to the multiple phonon scattering processes (E2 (high)-E2 (low)) [2526] and the E1 (LO) mode, respectively. The E1 (LO) mode is associated with the

Nanospikes Decorated ZnO Sheets for Solar Cell Application

361

structural defects such as oxygen vacancies and zinc interstitials in ZnO [27-28]. Therefore, the appearance of dominant E2 (high) mode and the suppressed weak E1 (LO) mode suggests that the grown nanospikes decorated ZnO sheets possess highly crystalline structure with the hexagonal wurzite phase.

Reprinted with the permission from S. Ameen et al., Chem. Eng. J., 196 (2012) 307-313. © 2012, Elsevier Ltd. Figure 4. FTIR spectra (a) and Raman spectra (b) of nanospikes decorated ZnO sheets.

X-Ray Photoelectron Spectroscopy of Nanospikes Decorated ZnO Sheets The chemical composition of the grown nanospikes decorated ZnO sheets is further analyzed by the XPS spectroscopy, as shown in Figure 5. The survey XPS spectrum (Figure 5 (a)) shows the three strong binding energies of Zn 2p3/2, Zn 2p1/2 and O 1s along with small C 1s binding energy. The other binding energies peaks are not detected in the Figure 5 (a), indicating the presence of Zn and O with very small traces of carbon impurities. However, the C1s binding energy at ~ 284.6 eV is usually used as calibration for other binding energies in the spectrum to avoid the specimen charging [29]. Figure 5 (b) shows the Zn 2p spectrum of the doublet peaks with the binding energies of ~1021 eV and ~1045 eV which are assigned to Zn 2p3/2 and Zn 2p1/2 in better symmetry, respectively. These binding energies are attributed to the typical lattice Zinc in ZnO [30-31]. The binding energy difference between the two peaks is estimated to ~24 eV, which matches well to the standard reference value of ZnO [29]. The peak at ~1021 eV is associated with the Zn2+ in ZnO wurzite structure [32]. Moreover, Zn 2p binding energy and the binding energy difference values confirm that Zn atoms are in +2 oxidation state in ZnO. Figure 5 (c) presents the deconvolution of O 1s XPS spectrum which exhibits the main peak at ~528.3 V along with three resolved peaks at ~529.2V, ~530.1V and ~531.1V respectively. The higher and lower binding energy component at ~528.3 eV and

362

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

~529.2 eV are attributed to O2− ions on the wurtzite structure of the hexagonal Zn2+ ions [33]. Every O2− ions are surrounded by Zn atoms with the full appreciation of nearest neighbour O2− ions. The other binding energies at ~530.1 eV and ~531.1 eV are ascribed to the oxygen deficiency or oxygen vacancies within the ZnO materials. Therefore, highest binding energy of Zn 2p and O 1s spectra are associated with Zn +2 and O-2 ions which form Zn–O bonds in ZnO crystals.

Reprinted with the permission from S. Ameen et al., Chem. Eng. J., 196 (2012) 307-313. © 2012, Elsevier Ltd. Figure 5. Survey (a), Zn 2p (b) and O 1s (c) XPS spectra of nanospikes decorated ZnO sheets.

POSSIBLE GROWTH MECHANISM AND THE FORMATION OF NANOSPIKES DECORATED ZNO SHEETS The formation of nanospikes decorated ZnO sheets is illustrated in Figure 6. Firstly, ZnO seeding on the FTO substrates is carried out by the electrodeposition at the applied voltage of 25V using the ZnCl2 as source material and H2O2 as oxidant or electrolyte

Nanospikes Decorated ZnO Sheets for Solar Cell Application

363

which reacts immediately and forms ZnO seed layer on the FTO substrate. For nanospikes decorated ZnO sheets growth, Zn (NO3)2.6H2O was mixed with NaOH, where NaOH is used to maintain the pH of the reaction mixture and gets easily dissociated and supplies the OH− ions to the reaction. Zn (NO3)2.6H2O reacts with OH- to form Zn(OH)2 and the soluble NaNO3 as shown in reaction (1). Under heating at 90oC, the formed Zn(OH)2 subsequently splits into Zn2+ and OH− ions, which sequentially forms Zn(OH)42− ions in accordance to the following chemical reaction (2). Finally, the Zn(OH)42− ions dissociate to form the ZnO nuclei in accordance to the reaction (3). The involved chemical reactions are as follows: Zn (NO3)2.6H2O + 2NaOH → Zn (OH)2 + 2NaNO3 + 6H2O

(1)

Zn (OH)2 + H2O → Zn2+ + 2OH− + 2H2O → Zn (OH)42−

(2)

Zn (OH)42− → ZnO + H2O + 2OH−

(3)

Reprinted with the permission from S. Ameen et al., Chem. Eng. J., 196 (2012) 307-313. © 2012, Elsevier Ltd. Figure 6. Possible growth mechanism and the formation of nanospikes decorated ZnO sheets.

Initially, the formed ZnO nuclei exploit the basic structure of the final ZnO nanostructures. Under the prolonged heating, the ZnO nuclei grow to larger sizes via the self-catalytic growth process which tends to form the sheets on FTO substrates [34]. It is known that the [0001] faces are the most rapid-growth-rate planes of hexagonal ZnO crystals as compared to other growth facets [35]. In the second step, the growth of some

364

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

ZnO nuclei is very rapid at high pH and temperature and decorated on the chemically stable ZnO sheets. It is reported that the wurtzite hexagonal phase ZnO possesses the catalytically active Zn-terminated [0001] surfaces and the chemically inert O- [0001] surfaces, which could provide an active site to grow the vertically aligned rods along [0001] direction [36]. Herein, the small nanorods forms collectively a spike like structures, these are decorated on the surface of ZnO sheets. Hence, the nanospikes are grown in the [0001] direction on the ZnO sheets, as evident from the SAED pattern.

PHOTOVOLTAIC PERFORMANCES OF DSSCS FABRICATED WITH NANOSPIKES DECORATED ZNO SHEETS PHOTOANODE The Current Density-Voltage (J-V) Curve and the Incident Photon to Current Conversion Efficiency of the Fabricated DSSC The current density-voltage (J-V) curve of DSSC fabricated with nanospikes decorated ZnO sheets photoanodes is shown in Figure 7 (a) which is measured under the light intensity of 100 mW/cm2 (1.5AM). The overall conversion efficiency of ~2.51% is achieved by the fabricated DSSC with the photoanode of nanospikes decorated ZnO sheets. The reasonably high short circuit current (JSC) of 6.07 mA/cm2, open circuit voltage (VOC) of 0.68 V and fill factor (FF) of 0.60 are observed. The relatively high J SC is related to high dye absorption through nanospikes decorated ZnO sheets morphology. Further the morphology and the crystal quality of ZnO nanostructures have profound impact on the photovoltaic properties of DSSC [10, 11]. The unique morphology of the prepared nanospikes decorated ZnO sheets might improve the charge collection and transfer properties of electrode due to the presence of standing spikes on the ZnO sheets [37]. The improved VOC and FF of DSSC might result from the reduced charge recombination and improves the series resistance by the photoanode of nanospikes decorated ZnO sheets. Importantly, in this case, the sheets morphology of ZnO display highly uniform and the standing nanospikes which create an aligned structure and might collectively facilitates the electrons transfer at the interface of the conduction and the electrolyte layer. As compared to the reported DSSCs based on ZnO nanostructures photoanodes, the nanospikes decorated ZnO sheets photoanode based DSSC shows the significantly higher conversion efficiency with improved J SC, VOC and FF [38-39]. In order to explain the high photocurrent density, Figure 7 (b) shows the incident photon to current conversion (IPCE) efficiency of the fabricated DSSC with photoanode of nanospikes decorated ZnO sheets. In general, the IPCE curve is plotted as a function of the excitation wavelength of the fabricated DSSCs. The IPCE gives the ratio between the

Nanospikes Decorated ZnO Sheets for Solar Cell Application

365

number of the generated charge carriers contributing to the photocurrent and the number of the incident photons, as expressed by Eq. (4) IPCE (%) = 1240. JSC (µA cm− 2)/λ (nm) Pin (mW cm−2) ×100

(4)

where JSC is the short-circuit photocurrent density for the monochromatic incident light and λ and Pin are the wavelength and the intensity of the monochromatic light respectively. The fabricated DSSC with photoanode of nanospikes decorated ZnO sheets attains the moderate IPCE of ~31.8% This value is probably obtained by the larger amount of dye-loading through large surface area of sheet and the standing spikes of photoanode, resulting in the high photocurrent density i.e., JSC. The presence of nanospikes on ZnO sheets might efficiently enhance the electron transport and reduces the recombination rate to high IPCE value [40].

Reprinted with the permission from S. Ameen et al., Chem. Eng. J., 196 (2012) 307-313. © 2012, Elsevier Ltd. Figure 7. J-V curve (a) and IPCE (b) curve of the DSSC fabricated with nanospikes decorated ZnO sheets photoanode.

CONCLUSION The nanospikes decorated ZnO sheets has been synthesized on the electrodeposited ZnO seeded fluorine doped tin oxide (FTO) substrates through the hydrothermal method at 90oC and applied as photoanode for the efficient DSSCs. The morphological characterizations confirm that the unique nanospikes of average diameter ~80-100 nm are decorated one or both the sides of ZnO sheets. The grown nanospikes decorated ZnO sheets exhibited well crystalline typical wurtzite hexagonal phase of ZnO nanostructures with the good optical quality. The possible growth mechanism and the involved chemical reaction have been investigated. The fabricated DSSC with the photoanode of nanospikes

366

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

decorated ZnO sheets showed relatively the high solar-to-electricity conversion efficiency of ∼2.51% with the high short circuit current (J SC) of 6.07mA/cm2. The enhanced performance and photocurrent might be related to the high charge collection and the fast electrons transfer at the interfaces of ZnO and the electrolyte layer due to substantially large surface for the high dye absorption leading to the light harvesting efficiency. The fabricated DSSC with photoanode of nanospikes decorated ZnO sheets accomplishes the moderate IPCE of ~31.8%. Thus, this relatively low cost hydrothermal synthesis method could be scaled up for the industrial production.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

M. Gratzel, Nature, 414 (2001) 338. Z. Chen, Y. Tang, L. Zhang, L. Luo, Electrochim. Acta, 51 (2006) 5870. S. Ameen, M. S. Akhtar, M. Husain, Sci. Adv. Mater., 2 (2010) 441. M. Law, L. E. Greene, J. C. Johnson, R. Saykally, P. Yang, Nat. Mater., 4 (2005) 455. S. Barazzouk, S. Hotchandani, J. Appl. Phys., 96 (2004) 7744. W. S. Chae, S. W. Lee, Y. R. Kim, Chem. Mater., 17 (2005) 3072. H. Rensmo, K. Keis, H. Lindstorm, S. Sodergren, A. Solbrand, A. Hagfeldt, E. Lindquist, L. N. Wang, M. Muhammed, J. Phys. Chem. B, 101(1997) 2598. R. Chakravarty, C. Periasamym, Sci. Adv. Mater., 3 (2011) 276. C. H. Chao, C. H. Chan, J. J. Huang, L. S. Chang, H. C. Shih, Curr. Appl. Phys., 11 (2011) S136. M. S. Akhtar, M. A. Khan, M. S. Jeon, O. B. Yang, Electrochim. Acta, 53 (2008) 7869. S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, Electrochim. Acta, 56 (2011) 1111. Y. K. Tseng, H. C. Hsu, W. F. Hsieh, K. S. Liu, I. C. Chen, J. Mater. Res., 18 (2003) 2837. W. D. Yu, X. M. Li, X. D. Gao, Appl. Phys. Lett., 84 (2004) 2658. Z. W. Pan, Z. R. Dai, Z. L. Wang, Science, 291 (2001) 1947. P. Uthirakumar, J. H. Kang, S. Senthilarasu, C. H. Hong, Physica E, 43 (2011) 1746. P. Mishra, R. S. Yadav, A. C. Pandey, Ultrason. Sonochem., 17 (2010) 560. A. Umar, Y. B. Hahn, Appl. Phys. Lett., 88 (2006) 173120. A. Umar, S. H. Kim, J. H. Kim, A. Al-Hajry, Y. B. Hahn, J. Alloys Comp., 463 (2008) 516. R. A. Laudise, E. D. Kolb, A. J. Caporason, J. Am. Ceram. Soc., 47 (1964) 9.

Nanospikes Decorated ZnO Sheets for Solar Cell Application

367

[20] G. Xiong, U. Pal, J. G. Serrano, K. Ucer, B. R. T. Williams, Phys. Stat. Sol. (c), 3 (2006) 3577. [21] H. C. Zhu, J. Iqbal, H. J. Xu, J. Yu, J. Chem. Phys., 129 (2008) 124713. [22] H. Zhang, L. Shen, S. W. Guo, J. Phys. Chem. C, 111(2007) 12939. [23] M. Millot, J. Gonzalez, I. Molina, B. Salas, Z. Golacki, J. M. Broto, H. Rakoto, M. Goiran, J. Alloys Compd., 423(2006) 224. [24] T. C. Damen, S. P. S. Porto, B. Tell, Phys. Rev., 142 (1966) 570. [25] C. Fauteux, R. Longtin, J. Pegna, D. Therriault, Inorg. Chem., 46 (2007) 11036. [26] R. P. Wang, G. Xu, P. Jin, Phys. Rev. B, 69 (2004) 113303. [27] A. Umar, Y. B. Hahn, Appl. Surf. Sci., 254 (2008) 3339. [28] S. J. Chen, G. R. Wang. Y. C. Liu, J. Lumin., 129 (2009) 340. [29] C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder, G. E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer, Eden Prairie (1979). [30] N. S. Ramgir, D. J. Late, A. B. Bhise, M. A. More, I. S. Mulla, D. S. Joag, K. Vijayamohanan, J. Phys. Chem. B, 110 (2006) 18236. [31] C. Y. Leunga, A. B. Djurisic, Y. H. Leung, L. Ding, C. L. Yang, W. K. Ge. J. Cryst. Growth, 290 (2006) 131. [32] W. J. Li, E. W. Shi, W. Z. Zhong, Z. W. Yin, J. Cryst. Growth, 203 (1999) 186. [33] J. C. C. Fan, J. B. Goodenough, J. Appl. Phys., 48 (1977) 3524. [34] A. Umar, Y. B. Hahn, Nanotechnology, 17 (2006) 2174. [35] S. Kar, S. Santra, J. Phys. Chem. C, 112 (2008) 8144. [36] Z. L. Wang, X. Y. Kong, J. M. Zuo, Phys. Rev. Lett., 91(2003) 185502. [37] A. B. F. Martinson, J. W. Elam, J. T. Hupp, M. J. Pellin, Nano. Lett., 7 (2007) 2183. [38] N. Karst, G. Rey, B. Doisneau, H. Roussel, R. Deshayes, Consonni, C. Ternon, D. Bellet, Mater. Sci. Eng. B, 176 (2011) 653. [39] (a) A. Cheng, Y. Tzeng, Y. Zhou, M. Park, T. H. Wu, C. Shannon, D. Wang, W. Lee, Appl. Phys. Lett., 92 (2008) 92113; (b) J. H. Xiang, P. X. Zhu, Y. Masuda, M. Okuya, S. Kaneko, K. Koumoto, J. Nanosci. Nanotech., 6 (2006) 1797. [40] A. Al-Hajry, A. Umar, Y. B. Hahn, D. H. Kim, Superlatt. Microstruct., 45 (2009) 529.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 26

TIN (SN) DOPED ZNO NANOSTRUCTURES FOR THE APPLICATION OF DYE SENSITIZED SOLAR CELLS Sadia Ameen1, M. Shaheer Akhtar2, Young Soon Kim1 and Hyung Shik Shin1,* 1

Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT The tin (Sn) doped zinc oxide (ZnO) nanostructures are synthesized by the simple hydrothermal method and the photovoltaic performances of Sn doped ZnO derived photoanodes were studied. The doping of Sn significantly alters the morphology of ZnO into spindle shape by the arrangement of small ZnO nanoparticles. The crystalline and structural properties deduce the clear decrement in the crystallite sizes from ~143.9 nm to ~82.2 nm, which might be suggested the doping of Sn-ions into ZnO nanostructures. Xray photoelectron spectroscopy (XPS) study shows Sn-O and Zn-O bonding in the synthesized spindle shaped Sn-ion doped ZnO nanostructures, which confirms Zn substitution by the Sn-ions. Dye sensitized solar cell (DSSC) fabricated with Sn-ZnO photoanode achieves a solar-to-electricity conversion efficiency of ~1.82% with short circuit current (JSC) of 5.1 mA/cm2, open circuit voltage (VOC) of 0.786 V and fill factor (FF) of 0.45, which are higher than that of DSSC with ZnO photoanode. The increased conversion efficiency and the photocurrent density are attributed to the significant Sn-ion *

Corresponding Author Email: [email protected].

370

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al. doping into ZnO nanostructures and considerable arrangement of ZnO nanoparticles into spindle shaped morphology which might improve the high charge collection and the transfer of electrons at the interfaces of doped ZnO layer and the electrolyte layer.

INTRODUCTION The wide bandgap (>3 eV) metal oxides, especially zinc oxide (ZnO) have been extensively studied as the encouraging materials for several optical-electrical and the photovoltaic devices [1-3]. ZnO nanomaterials have shown the unique multifunctional properties with wide conductivity from metallic to the insulating range [4], and also exhibit the high exciton binding energy (60 meV), low resistivity, non-toxic, highly transparent in the visible range and high light trapping characteristics [5]. It has been seen that the various morphologies and the sizes of ZnO nanomaterials significantly influence the chemical, stability, conductivity and the electrical properties [6, 7]. On comparison with TiO2 nanomaterials, ZnO nanomaterials are popularly used as alternative semiconducting materials for photocatalysis and solar devices owing to its high electronic mobility, stability against photo-corrosion and similar photochemical properties [8-10]. One of the methods still in the developing stage, called doping by metals like F, Cu, Ag, Ga, Al, In, Sn and Sb is utilized to tailor the chemical, conductive and the electrical properties of ZnO nanomaterials [11-13]. The metal doping is the effective procedure to modify the grain size, orientation and the conductivity and could greatly influence the crystalline, optical and the electrical properties of the ZnO nanostructures. Recently, Snion is the known promising dopant to ZnO nanomaterials for enhancing the electrical and optical properties [14]. Tsay et al. [15] prepared the Sn doped ZnO thin films coated glass substrates and investigated the effects of Sn doping on the crystallinity, microstructures and the optical properties of ZnO thin film. Several reports are available on the preparation of the Sn doped ZnO thin films and the effects of Sn doping on grain size, vibrational structure, optical and the structural properties of ZnO thin film substrates [1617]. So far, the doping of Sn-ion into the ZnO nanostructures has been achieved by the methods like thermal evaporation [18], successive ionic layer adsorption [19], pulsed laser [20], chemical route [21], and spray pyrolysis [22]. The chemical deposition is the simplest, safe, and inexpensive technique which facilitates easy in-situ Sn-ion doping into ZnO nanomaterials. The dye absorbed nanocrystalline TiO2 thin film based solar cells are well known alternative to the commercial silicon solar cells owing to its reasonable solar to electric energy conversion with ease of manufacture and low cost [23-24]. Principally, a typical DSSC comprises dye absorbed nanocrystalline titanium dioxide (TiO2) thin film on a transparent conducting oxide (TCO), a platinum (Pt) counter-electrode, and an electrolyte solution with a dissolved iodide ion/tri-iodide ion redox couple between the electrodes

Tin (Sn) Doped ZnO Nanostructures …

371

[25]. To date, much work has been performed on DSSCs fabricated with different nanocrystalline inorganic oxide such as TiO2, ZnO, SnO2, TiO2-SiO2, etc., to understand the electron transport mechanisms [26]. The ZnO nanomaterials are an alternative to TiO2 nanomaterials because they show almost similar electron affinity, energy level of the conduction band and higher electronic mobility [27]. The metal ion doped ZnO nanomaterials could be expected as an effective photoanode for fabricating the advanced DSSCs with improved photovoltaic parameters. In this work, spindle shaped Sn doped ZnO nanomaterials are synthesized by the simple hydrothermal method and utilized as photoanode material for DSSCs. Compared to photoanode of ZnO nanoparticles, the DSSC with Sn-ion doped ZnO nanostructures exhibits the improved solar-to-electricity conversion efficiency of ~1.82% with short circuit current (JSC) of ~5.1 mA/cm2, open circuit voltage (VOC) of ~0.786 V and fill factor (FF) of ~0.45. For thin film photoanodes, the slurry of the synthesized ZnO nanostructures (0.5 g) is prepared with the addition of 2 ml polyethylene glycol solution and deposited on the fluorinated tin oxide glass (FTO) substrate through doctor blade technique. The average active area of ZnO thin film photoanode is 0.25 cm2. Finally, the ZnO thin film substrates are calcined at 450 °C for 30 min. The fabrication of DSSC is performed as reported in the previous work [7]. In brief, the prepared photoanodes of ZnO and Sn-ZnO thin film substrates are immersed in an ethanolic solution of ruthenium (II) 535 bis-TBA dye (0.3 mM, N-719, Solaronix) for 12 h at the room temperature under dark condition. The platinum (Pt) counter electrodes are placed over dye-sensitized pristine ZnO and Sn-ZnO thin film photoanodes and the edges of the cells are sealed with 60 μm thick Surlyn sheet. Sealing is accomplished by hotpressing the two electrodes together at 80 oC. The fabrication is accomplished by introducing an electrolyte of specified composition (0.5 M LiI, 0.05mM I2, and 0.2 M tert-butyl pyridine in acetonitrile) by syringe through the holes of counter electrode on the dye-sensitized ZnO thin film photoanode.

Morphological Studies of Sn Doped ZnO Nanostructures The Field Emission Scanning Electron Microscopy The synthesized ZnO and Sn-ZnO nanostructures are morphologically characterized by the FESEM images, as shown in Figure 1. The irregular, non-uniform and highly aggregated nanoparticles are observed in ZnO nanostructures, as seen in Figure 1 (a). The average size of ZnO nanoparticles is in the range of ~150-200 nm. The ZnO nanostructures have dramatically arranged into the spindle shaped morphology after Snion doping, as shown in Figure 1 (b-d). It has been observed that each Sn-ZnO spindle with average size of 350±50 nm is comprised of small aggregated nanoparticles (Figure 1 (d)).

372

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

Reprinted with the permission from S. Ameen et al, Chem. Eng. J., 187 (2012) 351-356. © 2012, Elsevier Ltd. Figure 1. FESEM images of ZnO (a) and Sn-ZnO (b) nanostructures at low resolution, and FESEM images of Sn-ZnO nanostructures at high resolution (c, d).

The Transmission Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy The Sn-ZnO nanostructures are further investigated by TEM analysis. Figure 2 shows the TEM and high resolution TEM images of Sn-ZnO nanostructures. The morphology and the size of Sn-ZnO are completely consistent with FESEM observations. Figure 2 (a, b) clearly exhibits that the aggregated ZnO nanoparticles form a spindle shaped morphology, in which some black spots or particles might suggest the presence of the Snions. Moreover, Sn-ions are also seen in the HRTEM image of Sn-ZnO which is expressed by the circles in Figure 2 (c). The morphological changes in Sn-ZnO nanostructures might due to the substantive influence of Sn-ion into ZnO nanostructures. The energy dispersive spectroscopy (EDS) analysis (Figure 2 (d)) is performed to investigate the elemental composition of Sn-ZnO nanostructures. The EDS spectrum exhibits two high intense peaks and single small peak which are associated with O, Zn and C atoms respectively. In spite of these, the presence of Sn peaks is again confirmed the Sn-ion doping into the ZnO nanostructures.

Tin (Sn) Doped ZnO Nanostructures …

373

Reprinted with the permission from S. Ameen et al, Chem. Eng. J., 187 (2012) 351-356. © 2012, Elsevier Ltd. Figure 2. TEM images of Sn-ZnO nanostructures at low resolution (a, b), HRTEM image (c), and EDS spectrum of Sn-ZnO nanostructures (d).

Crystalline and Structural Properties of Sn Doped ZnO Nanostructures X-Rays Diffraction Patterns and Raman Scattering Spectroscopy Figure 3 (a) shows the XRD patterns of ZnO and Sn-ZnO nanostructures. The obtained XRD patterns at ~31.6o, ~34.2o, ~36.1o, ~47.5o, ~56.5o, ~62.7o, ~67.8o and ~69.1o are well indexed with JCPDS 36-1451, correspond to a typical hexagonal wurtzite structure of ZnO. After Sn-ion doping, XRD patterns attain similar spectra to undoped ZnO, except decreased peak intensities which occur due to internalization of Sn-ion into ZnO nanostructures. Moreover, the Sn ion doping effects have been confirmed by calculating the crystallite sizes of ZnO and Sn-ZnO using the Scherrer formula (D = Kαλ/βcosθ). The Sn-ZnO exhibits the crystallite size of ~82.2 nm with FWHM of 0.7o while ZnO exhibits large crystallite size of ~143.9 nm with high FWHM of 0.4 o. The large FWHM values of Sn-ZnO as compared with ZnO nanostructures might due to the lesser radii of Sn than Zn. Thus, during the hydrothermal synthesis, some of Zn ions are easily substituted by Sn-ions and exhibits the successful doping of Sn-ion into ZnO

374

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

nanostructures. Figure 3 (b) shows the Raman spectra of the ZnO and Sn-ZnO nanostructures. A strong Raman peak at ~437 cm-1 along with two weak Raman peaks at ~331 cm-1 and ~583 cm-1 is observed in ZnO and Sn-ZnO nanostructures. The former peak corresponds to E2 mode of ZnO crystal and matches with Raman peak of bulk ZnO crystal. This peak ascribes the main characteristic of the ZnO wurtzite hexagonal structure, which is in good agreement with the results of XRD. The latter two peaks are assigned to the second order Raman spectrum arising from zone-boundary phonons 3E2H–E2L for wurtzite hexagonal ZnO single crystals and E1 (LO) mode of ZnO associated with oxygen deficiency in ZnO nanomaterials respectively [28]. The intensity of Raman peak at 437 cm-1 (E2 mode) in Sn-ZnO is lowered that ZnO nanostructures, which might due to the doping effects of Sn-ion. It is known that the high intensity of Raman active E2 mode is usually attributed to the better optical and crystalline properties of the materials [29]. Herein, the low intensity of E2 mode in Sn-ZnO indicates the decreased crystallinity of ZnO after Sn-ion doping.

Reprinted with the permission from S. Ameen et al, Chem. Eng. J., 187 (2012) 351-356. © 2012, Elsevier Ltd. Figure 3. XRD patterns (a) and Raman spectra (b) of ZnO and Sn-ZnO nanostructures.

Tin (Sn) Doped ZnO Nanostructures …

375

Optical Characterizations of Sn Doped ZnO Nanostructures Ultra-Violet Diffused Reflectance and Photoluminescence Spectrum Figure 4 (a) shows the UV-DRS spectra of ZnO and Sn-ZnO nanostructures. The broad intense absorption edge from ~400 nm to lower wavelengths region is associated with a charge-transfer process from the valence band to conduction band of ZnO [30]. Besides, the absorption wavelength of ZnO red shifts from ~389 nm to ~406 nm after Snion doping and its band gap has changed from ~3.18 eV to ~3.05 eV, which is due to the presence of interstitially embedded Sn-ion into ZnO nanomaterials. This small variation in band gaps again confirms the Sn-ion doping into ZnO nanomaterials, as also indicated by the XRD results.

Reprinted with the permission from S. Ameen et al, Chem. Eng. J., 187 (2012) 351-356. © 2012, Elsevier Ltd. Figure 4. UV-DRS spectra (a) and PL spectra (b) of ZnO and Sn-ZnO nanostructures.

376

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

The synthesized ZnO and Sn-ZnO nanostructures are further analyzed by room temperature PL spectroscopy, as shown in Figure 4 (b). The as-synthesized ZnO nanostructures exhibit a prominent UV emission at ~387 nm and the green emission at ~584 nm which correspond to the near-band-edge (NBE) emissions originates from the recombination of the free excitons of ZnO [31] and the singly ionized oxygen vacancies (VO+) respectively [32]. Interestingly, Sn-ZnO nanostructures present the significant red shift in PL peaks which might influence by Sn-ion doping into ZnO nanomaterials. In other words, Sn-ion might take part in the substitution of Zn ion and shares the oxygen with Zn atoms.

X-Ray Photoelectron Spectroscopy of Sn Doped ZnO Nanostructures

Reprinted with the permission from S. Ameen et al, Chem. Eng. J., 187 (2012) 351-356. © 2012, Elsevier Ltd. Figure 5. Survey (a), Zn 2p (b), O 1s (c), and Sn 3d (d) XPS spectra of Sn-ZnO nanostructures.

The survey and high resolution XPS spectra of Zn 2p, O 1s, and Sn 3d of Sn-ZnO nanostructures are shown in Figure 5. The survey spectrum displays the Zn 2p, O 1s and Sn 3d binding energy peaks with very small C 1s binding energy, as shown in Figure 5

Tin (Sn) Doped ZnO Nanostructures …

377

(a). Typically, Zn 2p XPS spectra (Figure 5(b)) shows the doublet binding energies at ~1022.1 eV and ~1046.1 eV which correspond to Zn 2p3/2 and Zn 2p1/2 respectively [33]. It has been calculated that the energy difference between doublet binding energies is ~24 eV, which is in excellent agreement with the standard value of ~22.97 eV [34]. The O1s spectra presents four deconvoluted binding energies peaks at ~533.6 eV, ~532.4 eV, ~530.8 eV and ~529.1 eV, as shown in Figure 5 (c). The highest binding energy at ~533.6 eV is originated from the oxygen atoms chemisorbed at the surface of synthesized materials [35] whereas, the binding energy at ~532.4 e V is ascribed to O2- ions (surface hydroxyl (-OH) group) on the synthesized Sn-ZnO (in the oxygen deficient region) and the lowest binding energy at ~529.1 eV is attributed to O2- ions in the Zn-O structures [35]. The binding energy at ~530.8 eV is attributed to oxidized metal ions in the synthesized Sn-ZnO such as, O-Sn and O-Zn in the ZnO lattice [35]. Sn 3d spectra (Figure 5 (d)) presents the doublet binding energies at ~487.2 eV and ~496.7 eV, correspond to Sn 3d5/2 and Sn 3d3/2 respectively. The appearance of these peaks indicates the incorporation of Sn dopant in the form of O-Sn in the ZnO lattice [36], as deduced by O 1s XPS results. Moreover, the energy gap of ~9.5 eV is observed between these two peaks which resembles to the reported value [37]. It is observed that since no diffraction peaks corresponding to the SnO and SnO2 are observed in the XRD spectra therefore, the O-Sn bonding could be considered as the substitutional doping of Sn-ions into the ZnO lattice.

Photovoltaic Performance of DSSCs Fabricated with ZnO and Sn-ZnO Thin Film Electrodes The Current Density-Voltage (J-V) Characteristics of DSSCs Figure 6 shows the current density-voltage (J-V) characteristics of DSSCs fabricated with ZnO and Sn-ZnO thin film electrodes measured under 100mW/cm2 light intensity (1.5AM) for the photovoltaic performances. DSSC fabricated with Sn-ZnO photoanode achieves a solar-to-electricity conversion efficiency of ~1.82% with short circuit current (JSC) of 5.1 mA/cm2, open circuit voltage (VOC) of 0.786 V and fill factor (FF) of 0.45. While, the relatively low conversion efficiency of ~1.49% with J SC of 4.05 mA/cm2, VOC (0.761 V) and FF of 0.48 is delivered by DSSC with ZnO photoanode. Noticeably, the conversion efficiency and JSC are considerably enhanced by ~20% and ~21% respectively upon Sn-ion doping into ZnO nanostructures. These enhancements might be due to the increase of high charge collection and the transfer of electrons at the interface of Sn-ZnO and the electrolyte layer. In general, the dopants like Sn, is known to enhance the electrons transport capacity and electron mobility of ZnO nanomaterials [38]. Moreover, the Sn-ion doping into ZnO nanostructures might increase the specific surface area by lowering the particle size and arranging into spindle shaped morphology, which might

378

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

contribute to high dye absorption. Sn-ZnO with high dye absorption and the improved electron transport might lead the enhancement in light harvesting efficiency and photoexcited electron transportation under sun light, resulting in the increased photocurrent density and the improved photovoltaic performance. Therefore, the arrangement of ZnO nanoparticles into Sn-ZnO spindle shaped, and good optical properties of Sn-ZnO are crucial to improve the conversion efficiency and the photocurrent density of the fabricated DSSCs.

Reprinted with the permission from S. Ameen et al, Chem. Eng. J., 187 (2012) 351-356. © 2012, Elsevier Ltd. Figure 6. J-V curve of the DSSC fabricated with ZnO and Sn-ZnO nanostructures based photoanodes.

CONCLUSION The simple hydrothermal method is used for synthesizing ZnO and Sn-ZnO photoanodes for the photovoltaic performances of DSSCs. The doping of Sn-ion significantly alters the morphology of ZnO into spindle shaped by the arrangement of small ZnO nanoparticles. The decrement in the crystallite sizes reveals the incorporation of Sn-ion into ZnO nanomaterials, suggesting the doping of Sn-ions into ZnO nanostructures. The absorbance and XPS studies deduce Sn-O and Zn-O bonding in the synthesized spindle shaped Sn-ZnO nanostructures, which is clearly evidenced the Zn substitution by Sn-ions. A solar-to-electricity conversion efficiency of ~1.82% is achieved by DSSC fabricated with Sn-ZnO photoanode, while the relatively low conversion efficiency of ~1.49% is attained by DSSC with ZnO photoanode. The superior photovoltaic performance might attribute to the significant Sn-ion doping into

Tin (Sn) Doped ZnO Nanostructures …

379

ZnO nanostructures which might improve the high charge collection and the transfer of electrons at the interfaces of doped ZnO layer and the electrolyte layer.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

J. Y. Son, S. J. Lim, J. H. Cho, W. K. Seong, H. Kim, Appl. Phys. Lett., 93 (2008) 053109. S. Jager, B. Szyszka, J. Szczyrbowski, G. Brauer, Surf. Coat. Technol., 98 (1998) 1304. R. W. Birkmire, E. Eser, Annu. Rev. Mater. Sci., 27 (1997) 625. C. Y. Tsay, H. C. Cheng, Y. T. Tung, W. H. Tuan, C. K. Lin, Thin Solid Films, 517 (2008) 1032. Y. Caglar, S. Aksoy, S. Ilican, M. Caglar, Superlatt. Microstr., 46 (2009) 469. A. Umar, A. Al-Hajry, Y. B. Hahn, D. H. Kim, Electrochim. Acta, 54 (2009) 5358. S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, Electrochim. Acta, 56 (2011) 1111. A. Khan, S. N. Khan, W. M. Jadwisienczak, Sci. Adv. Mater., 2 (2010) 572. S. Li, S. Meierott, J. M. Kohler, Chem. Eng. J., 165 (2010) 958. L. Tang, B. Zhou, Y. Tian, F. Sun, Y. Li, Z. Wang, Chem. Eng. J., 139 (2008) 642. S. Ilican, Y. Caglar, M. Caglar, F. Yakuphanoglu, Appl. Surf. Sci., 255 (2008) 2353. F. Yakuphanoglu, Y. Caglar, S. Ilican, M. Caglar, Physica B, 394 (2007) 86. X. B. Wang, C. Song, K. W. Geng, F. Zeng, F. Pan, Appl. Surf. Sci., 253 (2007) 6905. M. Peiteado, Y. Iglesias, J. F. Fernandez, J. D. Frutos, A. C. Caballero, Mater. Chem. Phys., 101 (2007) 1. C. Y. Tsay, H. C. Cheng, Y. T. Tung, W. H. Tuan, C. K. Lin, Thin Solid Films, 517 (2008) 1032. H. Benelmadjat, B. Boudine, O. Halimi, M. Sebais, Opt. Laser Technol., 41 (2009) 630. K. C. Yung, H. Liem, H. S. Choy, J. Phys. D Appl. Phys., 42 (2009) 185002. S. C. Navale, I. S. Mulla, Mat. Sci. Eng. C, 29 (2009) 1317. S. T. Shishiyanu, T. S. Shishiyanu, O. I. Lupan, Sens. Actu. B, 107 (2005) 379. E. Holmelund, J. Schou, S. Tougaard, N. B. Larsen, Appl. Surf. Sci., 197-198 (2002) 467. M. R. Vaezi, S. K. Sadrnezhaad, Mat. Sci. Eng. B, 141 (2007) 23. S. Aksoy, Y. Caglar, S. Ilican, M. Caglar, Opt. Appl., 40 (2010) 7. B. O. Regan, M. Grätzel, Nature, 353 (1991) 737. M. K. Nazeeruddin, A. Kay, I. Podicio, R. H. Baker, E. Müller, P. Liska, N. Vlachopoulos, M. Grätzel, J. Am. Chem. Soc., 115 (1993) 6382.

380

Sadia Ameen, M. Shaheer Akhtar, Young Soon Kim et al.

[25] S. Ameen, M. S. Akhtar, M. Husain, Sci. Adv. Mater., 2 (2010) 441. [26] M. Law, L. E. Greene, J. C. Johnson, R. Saykally, P. Yang, Nat. Mater., 4 (2005) 455. [27] M. S. Akhtar, J. H. Hyung, O. B. Yang, N. K. Cho, H. I. Hwang, S. K. Lee, J. Nanotech. Nanosci., 10 (2010) 3654. [28] G. J. Exarhos, S. K. Sharma, Thin Solid Films, 270 (1995) 27. [29] J. Serrano, F. J. Manjon, A. H. Romero, F. Widulle, R. Lauck, M. Cardona, Phys. Rev. Lett., 90 (2003) 055510. [30] J. H. Sun, S. Y. Dong, J. L. Feng, X. J. Yin, X. C. Zhao, J. Mol. Catal. A: Chem., 335 (2011) 145. [31] T. Matsumoto, H. Kato, K. Miyamoto, M. Sano, E. A. Zhukov, T. Yao, Appl. Phys. Lett., 81 (2002) 1231. [32] X. L. Wu, G. G. Siu, C. L. Fu, Appl. Phys. Lett., 78 (2001) 2285. [33] S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, Microchim. Acta, 172 (2011) 471. [34] G. E. Muilenbenger, In Handbook of X-ray Photoelectron Spectroscopy; Ed.; Perkin-Elmer Corporation: Eden Prarie, MN, (1979). [35] J. Yang, J. Lee, K. Im, S. Lim, Physica E, 42 (2009) 51. [36] F. J. Sheini, M. A. More, S. R. Jadkar, K. R. Patil, V. K. Pillai, D. S. Joag, J. Phys. Chem. C, 114 (2010) 3843. [37] R. Dolbec, M. A. El-Khakani, A. M. Serventi, R. G. S. Jacques, Sens. Actu. B, 93 (2003) 566. [38] N. Ye, J. Qi, Z. Qi, X. Zhang, Y. Yang, J. Liu, Y. Zhang, J. Power Sources, 195 (2010) 5806.

SECTION 3. TITANIUM OXIDE (TIO2) PHOTOANODE BASED DYE SENSITIZED SOLAR CELLS (DSSCS)

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 27

CRYSTALLINE-TIO2 FLOWERS FOR DYE SENSITIZED SOLAR CELLS Sadia Ameen1, M. Shaheer Akhtar2, Hyung-Kee Seo1 and Hyung Shik Shin1,* 1

Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT Highly dense and well-defined TiO2 nanoflowers (NFs) are grown by the hydrothermal process on a titanium (Ti) coated FTO substrate. The Ti layer with a thickness of ~500–600 nm is deposited on FTO at room temperature with a pressure of ~5 mTorr using a Ti-source through RF magnetic sputtering. The unique TiO2 NF thin film substrate is applied as a photoanode for the fabrication of a dye sensitized solar cell (DSSCs). Each NF is made of uniform clover leaf-like petals of an average diameter of ~80–100 nm. The synthesized TiO2 NFs possesses a pure anatase phase with good crystal quality. The fabricated DSSC with TiO2 NF thin film photoanode accomplishes a reasonably good overall solar-to-electricity conversion efficiency (η) of ~3.64% with a high short circuit photocurrent density (J SC) of ~9.6 mA cm−2. The improved performance and photocurrent density are explained by the charge transport time, diffusion coefficient, diffusion length and charge collection efficiency of the fabricated DSSC.

*

Corresponding Author Email: [email protected].

384

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

INTRODUCTION The growing demands for a clean environment and energy require alternative renewable energy sources at low costs, such as solar conversion energy. Among conventional photovoltaic devices, DSSCs are gaining attention as promising alternative photovoltaic devices to p-n junction solar cells [1, 2] owing to their low manufacturing cost with a reasonably high solar-to-electricity conversion efficiency. Traditionally, a DSSC is a sandwiched structure of a Ru-dye absorbed semiconducting metal oxide based photoelectrode, a platinum (Pt) layer based counter electrode and a redox electrolyte as a charge transport layer [3]. DSSCs fabricated with a modified porphyrin as a sensitizer and an electrolyte based on cobalt (II/III) complexes have reached efficiencies of ~12.3%, as reported in a pioneer work of Gratzel’s group [4]. However, a high performance DSSC is still needed to overcome several issues, such as stability [5], price [6], electron collection efficiency [7] and optical properties. Furthermore, the back recombination reactions of the photo-injected electrons on the photoanode with the holes of the tri-iodide ions in the redox electrolyte is the main issue to resolve in order to achieve a high performance DSSC with a high photocurrent [8]. Several efforts have been made to advance the conversion efficiency of DSSCs by controlling the structural architectures of the photoanode [9]. The nanostructures of titania (TiO2) are highly explored metal oxide semiconductors, exhibiting exotic structural, optical and inert surface properties [10]. Ordered TiO2 nanostructures, such as nanorods, nanowires or nanotubes, have recently received a great deal of attention due to their unique morphology, high surface-to-volume ratio and optical properties [11–13]. These TiO2 nanostructures could be potential photoanodes to improve the collection of charges and minimize the recombination at the grain boundaries. By adopting these nanomaterials, faster transport and slower recombination could be achieved to minimize the charge losses at the external circuit. In this regard, intensive research is devoted to controllable TiO2 nanostructures using various methods like sol–gel, hydrothermal, sonochemical and thermal vapor deposition [14–16]. TiO2 nanostructures with a flower-like morphology have the advantage of a large number of active sites, unique morphology, and the combination of micro- and nanoscales [17]. Therefore, it might be assumed that unique TiO2 flower-like nanostructures possessing a high surface area could be a suitable photoanode material to achieve high performance DSSCs. Feng et al. reported the synthesis of pure spheres, flowers and mixed (flower/sphere) TiO2 nanostructure based thin films for DSSCs with both ionic liquids and gelled ionic liquid electrolytes and demonstrated a considerably enhanced overall conversion efficiency [18]. In another report, Jiang et al. studied a photoanode based on a self-assembled composite nanostructure of TiO2 nanoflower

Crystalline-TiO2 Flowers for Dye Sensitized Solar Cells

385

clusters (NFC) and P25 (TiO2 particles) for the fabrication of DSSCs. However, the reported conversion efficiency was quite low [19]. In general, the growth of ordered TiO2 nanostructures, such as those with a flower-like morphology, requires a seed layer of a Ti precursor to achieve the desired morphology. In the present work, unique and highly dense TiO2 NF thin films have been grown on Ti coated FTO substrates by a modified HF supported hydrothermal method. Herein, HF serves as an etching agent for the Ti coated FTO substrate, which also provides a Ti source for the formation of TiO2 NFs. Importantly, a Ti seed layer has not been used for the growth and the Ti coated FTO substrate is directly used for the growth of TiO2 NFs. The as-grown TiO2 NFs are applied as an effective photoanode for the fabrication of DSSCs. An improved solar to electric conversion efficiency of ~3.64% is achieved under ~100 mW cm−2 (1.5 AM) by the highly dense TiO2 NF thin film. The fabrication of the DSSC was performed accordingly, as reported elsewhere [20]. In brief, the prepared TiO2 NF thin film substrates are immersed in an ethanolic solution of ruthenium (II) 535 bis-TBA dye (0.3 mM, N-719, Solaronix) for 20 h at room temperature under dark conditions. After dye absorption, the dye adsorbed TiO2 NF thin film substrates are rinsed with absolute ethanol and dried under a nitrogen stream at 40°C. On the other hand, the Pt deposited FTO substrate counter electrode is placed over the dye absorbed TiO2 NF thin film photoanode and the edges of the cell are sealed with a 60 μm thick Surlyn sheet (SX 1170-60, Solaronix) by hot-pressing the two electrodes at 80 °C. Later, the redox electrolyte (0.5 M LiI, 0.05 mM I2, and 0.2 M tert-butyl pyridine in acetonitrile) is introduced through the holes in the counter electrode on the dye absorbed TiO2 NF thin film photoanode and lastly, the holes are sealed with small microscopic glass and a Surlyn sheet.

MORPHOLOGICAL STUDIES OF TIO2 NANOFLOWERS The Field Emission Scanning Electron Microscopy The morphology of the as-grown TiO2 thin film has been investigated by FESEM measurements, as presented in Figure 1. The low magnification images (Figure 1(a, b)) reveal highly dense and uniform TiO2 NFs grown on the surface of the FTO substrate by the hydrothermal process. Noticeably, the Ti layer on the FTO substrate is completely changed to a flower-like morphology in the presence of the HF solution. Figure 1(c, d) are the high magnification images of the TiO2 NFs, which display a consistent flowerlike morphology. Each TiO2 NF is comprised of well-defined and uniform clover-like petals with the average diameter of ~80–100 nm, as shown in Figure 1(d), whereas the average diameter of the as-grown TiO2 NFs is ~400–500 nm.

386

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Reprinted with permission from [S. Ameen, 2014], Cryst. Eng. Comm., 16 (2014) 3020 © 2014 The Royal Society of Chemistry. Figure 1. FESEM images of TiO2 NF thin film at low (a, b), and high (c) magnifications and (d) cross sectional view of the Ti coated FTO substrate.

The Transmission Electron Microscopy The morphological properties of the as-grown TiO2 NFs are further characterized by TEM, HRTEM and the selected area electron patterns (SAED), as shown in Figure 2. According to the FESEM results, well defined and uniform TiO2 NFs with an average diameter of ~400–500 nm are observed, as shown in Figure 2(a). Moreover, each flower is made up of a bundle of clover-leaf petals with an average diameter of ~80–100 nm. Figure 2(b) depicts the corresponding SAED patterns, exhibiting clear bright spots or phases towards the [100] growth direction. Likewise, the HRTEM images (Figure 2(c, d)) show well-resolved lattice fringes of the as-grown TiO2 NFs with an average interplanar distance between the two fringes of ~0.35 nm. The obtained interplanar distance is close to the typical interplanar distance of anatase TiO2 nanomaterials [21].

Crystalline-TiO2 Flowers for Dye Sensitized Solar Cells

387

Reprinted with permission from [S. Ameen, 2014], Cryst. Eng. Comm., 16 (2014) 3020 © 2014 The Royal Society of Chemistry. Figure 2. TEM (a), SAED pattern (b) HRTEM (c) and corresponding FFT (d) images of TiO2 NFs thin film.

Reprinted with permission from [S. Ameen, 2014], Cryst. Eng. Comm., 16 (2014) 3020 © 2014 The Royal Society of Chemistry. Figure 3. Topographic (a) and 3D (b) AFM of TiO2 NFs thin film.

388

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Atomic Force Microscopy Figure 3 depicts the topographic and 3D atomic force microscopic (AFM) images of the as-grown TiF thin film. Figure 3(a) exhibits the aggregated petals or flower-like morphology of the as-grown TiO2 thin film on the FTO substrate, which is consistent with the FESEM and TEM results. The average size of the petals is estimated as ~100 nm and each flower presents an average size of ~500 nm. The 3D AFM image (Figure 3(b)) displays a uniform surface and low roughness of ~33.7 nm (Rrms). Thus, the AFM results again deduce the growth of a uniform and smooth thin film of TiO2 flowers on the FTO substrate by the hydrothermal process.

CRYSTALLINE AND OPTICAL PROPERTIES OF TIO2 NANOFLOWERS X-Rays Diffraction Patterns, Ultra Violet-Diffused Reflectance, and Photoluminescence Spectroscopy The X-ray diffraction (XRD) pattern of the TiO2 NFs thin film is presented in Figure 4(a) in order to define the crystal structure of the as-grown TiO2 NF thin film. The appearance of diffraction peaks at ~25.2°, ~37.5°, ~47.7°, ~53.8°, ~62.4°, and ~68.1° are associated with typical anatase TiO2 nanomaterials (JCPDS no. 89-4203). The other diffraction peaks at ~26.4°, ~33.8°, ~51.3°, ~61.3° and ~65.3° represent the FTO substrate (JCPDS no. 88-0287), 10 suggesting the deposition of TiO2 NFs on the FTO substrate. No pure Ti diffraction peak is seen, which confirms the complete conversion of Ti into TiO2 NFs. The dominant anatase phase supports that the as-grown TiO2 NF thin film exhibits only anatase TiO2. In order to investigate the optical properties, the UVDRS and room temperature PL spectra of the as-grown TiO2 NF thin film are examined. Figure 4(b) shows the UV-DRS of the as-grown TiO2 NF thin film. A broad absorption edge in the ultraviolet region near ~393 nm is seen, which corresponds to a typical anatase TiO2 absorption edge in the UV region [22]. This peak usually occurs due to the charge-transfer from the valence band to the conduction band of TiO2 [22]. The corresponding band gap energy of ~3.16 eV is much closer to anatase TiO2 in bulk. Figure 4(c) represents the room temperature PL spectrum of the TiO2 NFs. Two emission peaks at ~470 nm and ~580 nm are obtained in the PL spectrum of the as-grown TiO2 NFs. The emission peak at ~470 nm is assigned to the charge-transfer transition from Ti3+ to the oxygen anion in a TiO68− complex, which is related to the oxygen vacancies at the surface [23]. The existence of an emission peak at ~580 nm is related to green emission and might arise due to some surface defects. However, the exact origin is unknown at this time. Thus, most of the excited electrons turn back to the shallow donor level created by

Crystalline-TiO2 Flowers for Dye Sensitized Solar Cells

389

the oxygen vacancies in the valence band and simultaneously, at room temperature, most of the electrons in the shallow level are thermally ionized due to the transition in the conduction band, which might recombine through non-radiative transitions, resulting in the dominance of the emission peak at ~470 nm [24].

Reprinted with permission from [S. Ameen, 2014], Cryst. Eng. Comm., 16 (2014) 3020 © 2014 The Royal Society of Chemistry. Figure 4. XRD pattern (a), UV-DRS spectra (b), and photoluminescence spectra (c) of TiO2 NFs thin film.

STRUCTURAL STUDIES OF TIO2 NANOFLOWERS The Raman Spectrum and Raman Mapping The Raman scattering spectrum is studied to investigate the structural composition of the as-grown TiO2 NF thin film. Figure 5(a) shows the Raman spectrum of the as-grown TiO2 NF thin film in the range of 100–800 cm−1. The as-grown TiO2 NF thin film

390

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

exhibits a strong Raman band at ~147.8 cm−1with three other Raman bands at ~396.4, ~513.6 and ~634.2 cm−1, corresponding to Eg, B1g, A1g and Eg3 respectively [25]. The origin of the strongest Raman band at ~147.8 cm−1 is due to the anatase phase and consistent with the reported literature [26]. The Raman spectrum does not exhibit any Raman mode near ~445 cm−1, indicating the absence of a rutile phase in the as-grown TiO2 NFs. Thus, the as-grown TiO2 NFs exhibit a pure anatase phase with good crystal quality. Figure 5(b, d) show the corresponding Raman mapping in the range of ~125– 198, ~479–535 and ~585–660 cm−1. Figure 5(b) reveals a highly bright and visible surface in the mapping, which is obviously due to the Raman band at ~146.4 cm−1. However, the Raman mapping in the ranges of ~479–535 and ~585–660 cm−1depict less visible regions with few bright parts, suggesting the existence of few defects or oxygen deficiencies.

Reprinted with permission from [S. Ameen, 2014], Cryst. Eng. Comm., 16 (2014) 3020 © 2014 The Royal Society of Chemistry. Figure 5. Raman spectra (a), and corresponding Raman mapping in the range of ~125–198 cm−1 (b), 479–535 cm−1 (c), and ~585–660 cm−1 (d) for TiO2 NF thin film.

Crystalline-TiO2 Flowers for Dye Sensitized Solar Cells

391

Reprinted with permission from [S. Ameen, 2014], Cryst. Eng. Comm., 16 (2014) 3020 © 2014 The Royal Society of Chemistry. Figure 6. (a) Survey, (b) Ti 2p and (c) O 1s XPS spectra of TiO2 NF thin film.

X-Ray Photoelectron Spectroscopy The X-ray photoelectron spectrum (XPS) has been analyzed to explain the existence of chemical species on the surface of the as-grown TiO2 NFs, as shown in Figure 6(a). The survey XPS exhibits four major binding energy peaks of Ti 2p, Sn 3d, O 1s and F 1s. The binding energies of Sn 3d and F 1s are associated with the FTO substrate. Figure 6(b) shows the fitted Ti 2p XPS spectrum of the as-grown TiO2 NFs, exhibiting doublet binding energy peaks at ~456.8 eV and ~462.4 eV, which are assigned to Ti 2p3/2 and Ti 2p1/2 respectively [27]. The observation of these binding energies establishes the +4 oxidation state of Ti, i.e., the Ti+4 oxidation state. The appearance of resolved binding energies at ~461.5 and ~463.7 eV confirm the existence of Ti+3 states, as also seen in the PL results. Importantly, the difference between the Ti 2p3/2 and Ti 2p1/2 binding energies is estimated as ~5.6 eV, which is in excellent agreement with the reported pure anatase TiO2 values.28 Figure 6(c) depicts the fitted O 1s XPS spectra and displays two fitted peaks at ~529.9 and ~531.1 eV with the main center peak at ~528.8 eV. The main central binding energy at ~528.8 eV corresponds to the oxygen atom bonded with the Ti atom as

392

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Ti–O–Ti (lattice O). Additionally, the nature of the oxygen is confirmed as an oxide form. The other fitted binding energies at ~529.9 and ~531.1 eV are mainly ascribed to moisture and a few contaminants which are present on the surface of the TiO2 NFs [28]. Thus, the as-grown TiO2 NFs might present nearly stoichiometric ratios of Ti :O (2 : 1).

SCHEMATIC ILLUSTRATION FOR THE PROPOSED GROWTH MECHANISM OF TIO2 NANOFLOWERS The formation of the TiO2 NFs is explained by the proposed mechanism, as illustrated in Figure 7. Firstly, a uniform and smooth thin Ti layer is deposited on the FTO substrate by RF magnetic sputtering with an RF power of 150 W. Secondly, the prepared Ti coated FTO substrate is directly used as Ti precursor for TiO 2 growth. Prior to the hydrothermal treatment, the Ti coated FTO substrate is horizontally dipped into a 20 mM HF solution. At the beginning of the hydrothermal process, HF etches the Ti layer on the FTO substrate and creates a roughened surface. Following the chemical etching, the Ti layer reacts with HF to form H2TiF6 under the hydrothermal condition: Ti + 6HF → H2TiF6 + 2H2 As the reaction time increases, the continuously produced H2TiF6 further reacts with H2O and forms Ti(OH)4: H2TiF6 + 4H2O → Ti(OH)4 + 6HF The formed Ti(OH)4 initially produces TiO2 particles under the hydrothermal conditions as: Ti(OH)4 → TiO2 + 2H2O The formed TiO2 nanoparticles are distributed uniformly on the surface of the FTO substrate at first nucleation. Under prolonged heating, the formed TiO2 starts aggregating and increases in size to form leaf petals [29]. After 8 h of the hydrothermal reaction, the leaf petals are rearranged into flowerlike TiO2 nanostructures on the FTO substrate, as shown in Figure 7. In this synthesis, the Ti layer and HF provide a Ti source for the formation of flower-like TiO2 nanostructures on the FTO substrate and an etch agent respectively.

Crystalline-TiO2 Flowers for Dye Sensitized Solar Cells

393

Reprinted with permission from [S. Ameen, 2014], Cryst. Eng. Comm., 16 (2014) 3020 © 2014 The Royal Society of Chemistry Figure 7. Schematic illustration for the proposed growth mechanism of TiO2 NFs.

Reprinted with permission from [S. Ameen, 2014], Cryst. Eng. Comm., 16 (2014) 3020 © 2014 The Royal Society of Chemistry. Figure 8. Typical EIS plot for the frequency range 100 KHz–1 Hz (a), and corresponding bode phase plots (b), intensity modulated photocurrent spectroscopy (IMPS) plot (c) and intensity-modulated photovoltage spectroscopy (IMVS) plot (d) of the DSSC fabricated with TiO 2 NF thin film photoanode.

394

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

CHARGE TRANSPORTATION AND CHARGE COLLECTION PROPERTIES OF TIO2 NANOFLOWERS BASED DSSCS Electrochemical Impedance (EIS), Intensity Modulated Photocurrent Spectroscopy (IMPS) and Intensity-Modulated Photovoltage Spectroscopy (IMVS) Measurements Charge transportation and charge collection in DSSCs are very complex systems constituting various interfaces in which a high accumulation of electrons usually occur due to the transportation of photo-generated electrons under light-illumination. In this regard, electrochemical impedance (EIS) measurements of the TiO2 NFs thin film photoanode-based DSSC were carried out in a frequency range from 100 kHz–1 Hz by applying 10 mV and the alternating current (ac) signal is examined, as shown in Figure 8(a). In general, an EIS plot is composed of two semicircles which represent the resistance of the redox electrolyte solution (RS) at high frequency, the charge transfer resistance (RCT) at the interface of the electrolyte and TiO2, the charge transfer resistance at the interface of TiO2 and TCO (RTIO2/TCO) and the charge transfer resistance at the interface of the counter electrode and TCO (RCE) [30]. In an EIS plot, the diameter of each semicircle defines the charge transfer resistance of the fabricated DSSCs. Herein, the fabricated DSSC with the TiO2 NF thin film photoanode presents a reasonable RTIO2/TCO of ~15.6 Ω and RCT of ~33.2 Ω. In particular, the RCT value is quite low compared to reported DSSCs fabricated with TiO2 nanostructure photoanodes [31]. A low RCT value usually favors a fast charge transfer rate in the electrochemical system at the TiO2/dye/electrolyte layer interface [32]. Therefore, the TiO2 NF thin film photoanode provides a good surface for high charge transfer, which might result in a high conversion efficiency. Figure 8(b) shows the corresponding Nyquist curve in Bode imaginary plots of imaginary parts of the impedance versus frequency. The presence of a phase peak at higher frequencies is related to the recombination sites at the TiO2/electrolyte interface, suggesting a decrease in the photovoltage and photocurrent. In order to explain the charge transfer and the recombination processes, IMPS and IMVS measurements have been performed. The charge-transport time (τCT) and the electron recombination time (τR) of the fabricated device could be calculated from the IMPS and IMVS plots respectively. The minimum frequencies of the IMPS and IMVS plots are used to calculate the τCT and τR values respectively, as presented in Figure 8(c, d). The TiO2 NF thin film photoanode-based DSSC shows a good τCT of ~20 ms and τR of ~289 ms, indicating that the flower-like morphology could facilitate the charge transfer and shorten the recombination rate. It is seen that the distribution of flowers on the FTO substrates is uniform and they are closely connected to each other, which might reduce the grain boundaries and improve the charge transfer and the collection rate. The

Crystalline-TiO2 Flowers for Dye Sensitized Solar Cells

395

processes called electron–hole recombination and electron diffusion are explained by observing the value of the electron diffusion length (DL) i.e., DL = (Dn · τR)1/2 [33]. The Dn value is estimated as ~3.06 × 103 μm2 s−1 from the IMPS plot of the TiO2 NF thin film photoanode-based DSSC. The DL value of the TiO2 NF thin film photoanode-based DSSC is ~29.7 μm, suggesting a higher probability of electrons entering from the TiO2 photoanode layers to the counter electrode. By taking the values of τCT and τR, the charge collection efficiency of the fabricated DSSC could be determined by the relation ηCC =1 − (τCT/τR). The obtained ηCC value for the TiO2 NFs thin film photoanode-based DSSC is reasonably high due to the better life time of the charge recombination and the high electrontransport time, as shown in Figure 8(c, d). These factors have promptly enhanced the photocurrent density and the conversion efficiency of TiO2 NF thin film photoanodebased DSSC.

PHOTOVOLTAIC PERFORMANCE OF TIO2 NANOFLOWERS THIN FILM PHOTOANODE-BASED DSSC The Current Density–Voltage (J–V) Characteristics and the Incident Photonto-Current Conversion Efficiency (IPCE) of the TiO2 NF Thin Film Photoanode Based DSSC The current density–voltage (J–V) characteristics of the TiO2 NF thin film photoanode-based DSSC are obtained under a light intensity of 100 mW cm−2 (1.5 AM), as shown in Figure 9. Compared to DSSCs fabricated with a P-25 photoanode (5.9%), an overall conversion efficiency of ~3.64% with a high J SC value of ~9.6 mA cm−2, VOC of ~0.671 V, and FF of ~0.57 are achieved by the TiO2 NF thin film photoanode-based DSSC. The high photocurrent density and performance are explained by the high charge collection and the lower numbers of recombination sites towards the photoanode. As shown in Figure 8(c, d), the TiO2 NF thin film photoanode-based DSSC exhibits enhanced τCT, τR and DL values, which have significantly upgraded the photocurrent and performances of the fabricated DSSC. In support, the charge collection efficiency is reasonably high to define high photocurrent density and photovoltage. On the other hand, the unique and the closely connected NFs might be helpful for increasing the dye loading through the high surface area of the TiO2 NFs, resulting in the high light harvesting efficiency and photocurrent density. Furthermore, Figure 9(b) shows the IPCE of the TiO2 NF thin film photoanodebased DSSC to elucidate the origin of the photocurrent under the range of wavelengths. The TiO2 NF thin film photoanodebased DSSC acquires broad IPCE curves with a maximum IPCE of ~39.4% at the highest absorption edge of ~515 nm. This result

396

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

is consistent with the J–V and IMPS/IMVS results. It is reported that high dye loading through the photoanode gives a high light harvesting efficiency, leading to photoelectron injection from the dye to the CB of TiO2 and producing the photocurrent. Thus, the TiO2 NF thin film photoanodebased DSSC accomplishes better photovoltaic properties, such as conversion efficiency, JSC, VOC, and FF, owing to its improved charge transfer/ collection time, lower recombination time, good charge collection efficiency and low RCT.

Reprinted with permission from [S. Ameen, 2014], Cryst. Eng. Comm., 16 (2014) 3020 © 2014 The Royal Society of Chemistry. Figure 9. J–V curve (a) and IPCE curve (b) of the DSSC fabricated with TiO2 NF thin film photoanode.

CONCLUSION Highly dense and well-defined TiO2 NFs are directly grown on Ti coated FTO substrates by the hydrothermal process. Herein, Ti coated FTO substrates with a thickness of ~500–600 nm are obtained by RF magnetic sputtering at room temperature with a pressure of ~5 mTorr. The unique TiO2 NF thin film substrates are successfully

Crystalline-TiO2 Flowers for Dye Sensitized Solar Cells

397

applied as a photoanode for the fabrication of DSSCs. The morphological and crystalline properties reveal that the TiO2 NFs are composed of highly crystalline and uniform clover-like petals with an average diameter of ~80–100 nm. The Raman results deduce that the synthesized TiO2 NFs possess a pure anatase phase with a good crystal quality. The TiO2 NF thin film photoanode-based DSSC accomplishes a reasonably good overall conversion of ~3.64% with a high short circuit photocurrent density (J SC) of ~9.6 mA cm−2. The TiO2 NF photoanode-based DSSC exhibits a reasonably high ηCC due to the better life time of the charge recombination and high electron-transport time.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev., 110 (2010) 6595. M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E. D. Dunlop, Prog. Photovoltaics, 19 (2011) 565. S. Ameen, M. S. Akhtar, M. Husain, Sci. Adv. Mater., 2 (2010) 44. A. Yella, H. W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W. G. Diau, C. Y. Yeh, S. M. Zakeeruddin, M. Gratzel, Science, 334 (2011) 629. Y. Bai, Y. M. Cao, J. Zhang, M. Wang, R. Z. Li, P. Wang, Nat. Mater., 7 (2008) 626. B. A. Andersson, Prog. Photovoltaics, 8 (2000) 61. E. Ghadiri, N. Taghavinia, S. M. Zakeeruddin, M. Gratzel, J. E. Moser, Nano Lett., 10 (2010) 1632. M. Law, L. E. Greene, J. C. Johnson, R. Saykally, P. Yang, Nat. Mater., 4 (2005) 455. S. H. Han, S. Lee, H. Shin, H. S. Jung, Adv. Energy Mater., 1 (2011) 546. S. Ameen, M. S. Akhtar, Y. S. Kim, H. S. Shin, RSC Adv., 2 (2012) 4807. J. E. Boercker, E. E. Pommer, E. S. Aydil, Nanotechnology, 19 (2008) 095604. C. K. Xu, P. H. Shin, L. L. Cao, J. Wu, D. Gao, Chem. Mater., 22 (2010) 143. L. G. García, I. G. Valls, M. L. Cantu, A. Barranco, A. R. G. Elipe, Energy Environ. Sci., 4 (2011) 3426. K. Asagoe, Y. Suzuki, S. Ngamsinlapasathian, S. Yoshikawa, J. Phys. Conf. Ser., 61 (2007) 1112. J. B. Baxter, E. S. Aydil, Appl. Phys. Lett., 86 (2005) 053114. S. Pavasupree, S. Ngamsinlapasathian, M. Nakajima, Y. Suzuki, S. Yoshikawa, J. Photochem. Photobiol. A, 184 (2006) 163. X. L. Hu, J. C. Yu, J. M. Gong, J. Phys. Chem. C, 111 (2007) 11180. L. Feng, J. Jia, Y. Fang, X. Zhou, Y. Lin, Electrochim. Acta, 87 (2013) 629.

398

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

[19] Y. Jiang, M. Li, R. Ding, D. Song, M. Trevor, Z. Chen, Mater. Lett., 107 (2013) 210. [20] S. Ameen, M. S. Akhtar, Y. S. Kim, O. B. Yang, H. S. Shin, Electrochim. Acta, 56 (2011) 1111. [21] K. Shin, S. I. Seok, S. H. Im, J. H. Park, Chem. Commun., 46 (2010) 2385. [22] N. Venkatachalam, M. Palanichamy, B. Arabindoo, V. Murugesan, Catal. Commun., 8 (2007) 1088. [23] J. C. Yu, J. G. Yu, W. K. Ho, L. Z. Zhang, Chem. Mater., 14 (2002) 3808. [24] J. Preclıkova, P. Galar, F. Trojanek, B. Rezek, Y. Nemcova, P. Maly, J. Appl. Phys., 109 (2011) 083528. [25] T. Ohsaka, J. Phys. Soc. Jpn., 48 (1980) 1661. [26] M. Vishwas, K. N. Rao, R. P. S. Chakradhar, Spectrochim. Acta, Part A, 99 (2012) 33. [27] Z. Song, J. Hrbek, R. Osgood, Nano Lett., 5 (2005) 1327. [28] E. Mccafferty, J. P. Wightman, Surf. Interface Anal., 26 (1998) 549. [29] J. M. Macak, H. Tsuchiya, P. Schmuki, Angew. Chem. Int. Ed., 44 (2005) 2100. [30] J. Bisquert, J. Phys. Chem. B, 106 (2001) 325. [31] L. G. García, J. Idígoras, A. R. G. Elipe, A. Barranco, J. A. Anta, J. Photochem. Photobiol. A, 241 (2012) 58. [32] M. A. Vorotyntsev, J. P. Badiali, G. Inzelt, J. Electroanal. Chem., 472 (1999) 7. [33] Z. Z. Bandic, P. M. Bridger, E. C. Piquette, T. C. McGill, Appl. Phys. Lett., 73 (1998) 3276.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 28

GRAPHENE OXIDE (GO) INCORPORATION IN TIO2 NANOFIBERS FOR DYE-SENSITIZED SOLAR CELLS Moaaed Motlak1, Nasser A. M. Barakat2,3, M. Shaheer Akhtar4,*, A. M. Hamza5, Ayman Yousef2, H. Fouad6,7 and O-Bong Yang8 1

Department of Physics, College of Science, Anbar University, Anbar, Iraq 2 Bionanosystem Engineering Department, Chonbuk National University, Jeonju, Republic of Korea 3

Chemical Engineering Department, Faculty of Engineering, Minia University, El-Minia, Egypt 4 New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea 5 Center of Nanotechnology and Advanced Materials, University of Technology, Alsenaa Street, Baghdad, Iraq 6 Applied Medical Science Department, RCC, King Saud University, Riyadh, Saudi Arabia 7 Biomedical Engineering Department, Faculty of Engineering, Helwan University, Helwan, Egypt 8 School of Semiconductor and Chemical Engineering & Solar Energy Research Center, Chonbuk National University, Jeonju, Republic of Korea

ABSTRACT Graphene oxide (GO)-incorporated TiO2 nanofibers is successfully synthesized via a simple and effective technique, electrospinning and applied as a working electrode for

*

Corresponding Author Email: [email protected].

400

Moaaed Motlak, Nasser A.M. Barakat, M. Shaheer Akhtar et al. dye sensitized solar cells (DSSCs). The effects of graphene oxide on the structural and photoelectric conversion performance of the DSSCs are inspected by various analytical techniques. The results suggest that the presence of graphene oxide increases the amount of dye absorption, leading to high migration of photoinduced electrons to the conduction band of the collection electrode and inhibition of electron recombination. Furthermore, the presence of graphene oxide improves the electron transport from the films to the fluorine doped tin oxide (FTO) substrates. Accordingly, remarkably enhanced power conversion efficiency of 4.52% is observed in case of utilizing 0.5 wt% graphene oxideincorporated TiO2 nanofibers as working electrode based DSSC which is higher than that of the conversion efficiency in case of pristine TiO2 nanofibers (i.e., 1.54%). The high amount of graphene oxide content results in low power conversion efficiency. Therefore, the graphene oxide-incorporated TiO2 nanofibers as working electrode is a promising candidate for improving the performance of the DSSCs.

Keywords: Graphene Oxide, Working electrode, TiO2 nanofibers, solar cells.

INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted much attention due to the directly converting sunlight into electrical power, so they are promising candidates for clean and renew- able future source of energy [1, 2]. The basic structure of the DSSC is based on four elements, namely the nanostructured wide band gap semiconducting layer, the organometallic dye as sensitizer, the electrolyte, and counter electrode (typically platinum metal). One dimensional (1-D) nanostructured TiO2 was announced as one of the most promising materials used as a working electrode to replace the nanoparticles (NPs) because it provides a larger surface area for the rapid charge percolation pathway, high absorption of the dye and more favorable for electron transport [3–5]. Moreover, in case the TiO2 nanopar- ticles, the rate of the recombination between the injected electrons and the oxidized dye or ions in the electrolyte at working electrode/dye/electrolyte interface is relatively high due to the grain boundaries among TiO2nanoparticles (NPs) that leads to an evident decrease in the conversion efficiency. Therefore, 1-D nanostructures TiO2 are expecting to become important materials to overcome the recombination and enhance the conversion efficiency of DSSCs. Although, compared to nanoparticles and other 1-D nanostructures (i.e., nanorods and nanowires), nanofibers (NFs) have lower surface area, the corresponding large axial ratio of the nanofibrous morphology provides distinct advantage especially in the electrons-transfer processes [6, 7]. Moreover, the conductivity of pristine TiO2 nanofibers is limited due to the low number of electrons in the conduction band. Up to date, many methods have been developed for the fabrication of TiO2 NFs [8–11]. However, some of those methods are not capable of large-scale production and others are not suitable for control- ling the shape and morphology. On the other hand, the electrospinning is simple and cost-effective technique which offers new opportunities to produce TiO2 NFs with

Graphene Oxide (GO) Incorporation in TiO2 Nanofibers …

401

different morphologies, aligned nanostructures, chemical composition control, and wide range of nanofibers average diameter (50– 500 nm) in a wise manner [12–15]. Specifically for inorganic nanofibers, electrospinning is most widely used because, up to now, it is the unique process can produce large axial ratio nanofibers this besides the simplicity, low cost and high yield. Therefore, in this study electrospinning was utilized because the axial ratio strongly affects the performance. Beside the morphology control, the researchers are focusing on doping of TiO2 matrix by foreign elements; metal or non-metal. Doping of TiO2 nanostructure can lead to gain many advantages including reducing the band gap, originating new states in the energy forbidden region, increasing the carriers concentration in the conduction band, and enhance the absorption of the solar spectrum [16–27]. Subsequently, the charge carriers increase and then cause additional transfer to the external circuit as electric current. Subsequently, these modifications on TiO2 nanostructures could enhance the efficiency of the DSSCs. Recently, carbonaceous materials are quite attractive to increase the efficiency of TiO2 such as graphite, activated carbon, graphene and graphene oxide [28–33]. These materials have different interesting properties such as large surface area, fine thermal/chemical stability, and the potential to control these properties through structural and compositional modification. In this regard, recently some researchers employed graphene–TiO2 composite nanofibers to improve the conversion efficiency of the DSSCs [34]. Compared to graphene, graphene oxide (GO) is easy to prepare and possesses a hydrophilic nature for the oxygenated graphene layers. As a result, GO readily forms stable colloidal suspensions of thin sheets [35] which makes the mixing process with the electro- spun solution easier task compared to graphene [36]. In the present work, GO-incorporated TiO2 nanofibers have been synthesized by electrospinning of sol–gel composing of GO, titanium isopropoxide and poly(vinyl acetate); calcination of the obtained electrospun NFs leads to produce GO-incorporated TiO2 NFs. The final product is used as working electrode in the DSSCs. The results indicate that the GO incorporation strongly enhances the performance of DSSCs. For the fabrication of DSSC, first pristine and GO-incorporated TiO2 nanofibers thinfilm electrode are prepared by coating the nanofibers paste onto fluorinated tin oxide glass (FTO) with the help of a doctor blade. Typically, the original paste is prepared by mixing of 2 ml of polyethylene glycol (PEG) aqueous solution (Fluka, average MW of 20,000 g/mol) with 0.5 g of functional TiO2 NFs under vigorous grinding with a pestle in a mortar. By a simple doctor blade technique, the prepared paste is coated onto FTO glass. After air drying of TiO2 NFs- based thin film electrodes, the electrodes are further calcined at 450 1C for 30 min in a static air furnace to remove the polymer binder. The prepared TiO2 thin-film electrodes are dipped into a dye solution consisting 0.3 mM ruthenium 535 bis-TBA (N719, Solaronix) in ethanol (spectroscopy grade) at room temperature for 20 h under dark conditions. After completion of the dye-loading, dye

402

Moaaed Motlak, Nasser A.M. Barakat, M. Shaheer Akhtar et al.

absorbed TiO2 electrodes are rinsed with ethanol and dried under a nitrogen stream. Secondly, Pt counter electrode is prepared by the deposition of Pt layer on FTO glass through the electrode beam deposition. Both working and counter electrodes are sealed with a sealing surlyn sheet (SX 1170-60, Solaronix, thickness 60 μm) at ~70 oC using double hot plate. A redox electrolyte composed of 0.5 M LiI, 0.05 mM I2, and 0.2 M tertbutyl pyridine in acetonitrile is inserted into the cell trough a small hole in the counter electrode, and lastly the holes are sealed with a small piece of microscopic glass with surlyn sheet. The obtained active area of DSSCs is estimated to ~0.25 cm2

Morphological Studies of GO Incorporated TiO2 Nanofibers The Field Emission Scanning Electron Microscopy Figure 1A and B displays the FESEM images of the obtained powder after calcination of pristine and GO-containing electrospun NFs at 680 1C for 1 h in air, respectively. It can be seen that both formulations composed of continuous, randomly oriented and good morphology nanofibers. It can be also observed that incorporation of GO in the TiO2 nanofibers did not have negative influence on the nanofibrous morphology.

Reprinted with the permission from M. Shaheer Akhtar et al, Ceramics Intern. 41 (2015) 1205–1212. © 2015, Elsevier Ltd. Figure 1. FESEM images of the sintered GO-free; (A) and GO-incorporated (GO= 1%); (B) nanofibers. The pies display the diameter distribution for pristine (left) and GO-incorporated (right) TiO2 nanofibers.

Graphene Oxide (GO) Incorporation in TiO2 Nanofibers …

403

The two pies in the bottom of Figure 1 display the diameter distribution for the pristine and GO-incorporated TiO2 nanofibers. As shown in the figure, addition of GO led to observable increase in the nanofibers diameters. Typically, the average diameters of the pristine and GO-incorporated TiO2 nanofibers were 340 and 600 nm, respectively.

Structural and Crystalline Characterizations of GO Incorporated TiO2 Nanofibers X-Ray Photoelectron Spectroscopy Figure 2 displays the XPS analysis of the obtained powders. The survey scan revealed the presence of Ti, O and C in GO-incorporated and pristine TiO2 NFs samples. XPS spectra displayed C 1s peak in both samples but amount of carbon on the surface of GO-incorporated was greater than of pristine TiO2 nanofibers which evidenced the incorporation of graphene oxide into the lattice of TiO2nanofibers.

Reprinted with the permission from M. Shaheer Akhtar et al, Ceramics Intern. 41 (2015) 1205–1212. © 2015, Elsevier Ltd. Figure 2. XPS survey spectrum of the GO-incorporated (GO= 0.5 wt%) and pristine TiO2 nanofibers.

The Raman Spectrum To observe the vibrational, rotational, and other low-frequency modes in compounds, Raman spectroscopy is the optimum spectroscopic technique. Especially for the carbonac- eous nanostructural materials, the researchers exploit Raman spectroscopy to confirm their conclusions because of the distinct difference in the spectra of the various

404

Moaaed Motlak, Nasser A.M. Barakat, M. Shaheer Akhtar et al.

forms of the carbonaceous materials. For instance, the Raman spectrum of the pristine graphite displays a prominent G peak at 1581 cm-1, which corresponds to the first-order scattering of the E2g mode [41]. However, in the Raman spectrum of GO, the G band is broadened and shifted to 1594 cm-1. In addition, the D band at 1363 cm-1 becomes relatively prominent, which indicates that the in-plane sp2 domains become smaller because of the extensive oxidation. Figure 3 displays the Raman spectra of the introduced pristine and GO- incorporated TiO2 NFs, as shown in the figure the G band can be observed. It is noteworthy mentioning that the low intensity of the D band indicates that the utilized GO was not thermally reduced to graphene during the utilized calcination process. Actually, the Raman spectrum of the graphene produced from the GO reduction also contains both G and D bands (at ~1580 and ~1350 cm-1, respectively); however graphene has a higher D/G intensity ratio than GO [42]

Reprinted with the permission from M. Shaheer Akhtar et al, Ceramics Intern. 41 (2015) 1205–1212. © 2015, Elsevier Ltd. Figure 3. Raman shift within the characterized D and G peaks of graphene for the pristine and GOincorporated (GO= 1 wt%) TiO2 nanofibers; the inset displays the full survey spectra.

Photovoltaic Performance of GO Incorporated TiO2 Nanofibers Photoanodebased DSSC Electrochemical Impedance Measurements Electrochemical impedance spectroscopy (EIS) is used as a major tool for investigating the properties and quality of DSSC devices. Particularly, EIS was invoked

Graphene Oxide (GO) Incorporation in TiO2 Nanofibers …

405

to investigate the interfacial charge transfer between the working electrode and the electrolyte, and describe the charge recombination in the TiO2 nanofibers film. Figure 4 shows an analysis of the EIS of the fabricated DSSCs which was done with a voltage applied between the working electrode and counter electrode. The measurement was carried out in dark under a forward bias of -0.75 V. The impedance plots ordinarily show three semi-circles. The high frequency semicircle is due to the counter electrode charge transfer resistance. The second semicircle at the middle frequency is the result of the charge transfer resistance at the TiO2/electrolyte interface (RCT). Finally, the third semicircle, at the low frequency, is due to the impedance of diffusion in the electrolyte [24]. The obtained Nyquist plots (Figure 6) display two semicircles including a large semicircle at middle-frequency (1–103 Hz) and a small one at high frequency. Compared to the pristine TiO2 nanofibers based working electrode, it is noted that incorporation of GO into the TiO2 nanofibers increases the resistance at the TiO2/dye/ electrolyte interface. A larger RCT value in theory means lower charge recombination between electrons in TiO2 and electron acceptors in the electrolyte which leads to produce high Voc. From Figure 6, the fitted diameter of RCT semicircle evidently increased after GO incorporation to be 233 Ω which is larger than pristine TiO2 nanofibers working electrodes (152 Ω), this finding is consistent with high Voc values measured in the devices (Figure 9). It is noteworthy mentioning that there is no conflict between high value of RCT and the corresponding Voc values as the EIS measurement was carried out in dark, this finding was already reported before [43,44].

Reprinted with the permission from M. Shaheer Akhtar et al, Ceramics Intern. 41 (2015) 1205–1212. © 2015, Elsevier Ltd. Figure 4. Nyquist plots for DSSC based on pristine and 0.5% GO incorporated TiO2 nanofibers photoanodes, measured at a forward bias of - 0.75 V in the dark.

406

Moaaed Motlak, Nasser A.M. Barakat, M. Shaheer Akhtar et al.

The Incident Photon-To-Current Conversion Efficiency of the TiO2 NF Thin Film Photoanode Based DSSC The incident photon-to-current-efficiency (IPCE) of the various DSSCs devices are shown in Figure 5. The IPCE of the pristine TiO2 NFs was approximately 26.5%, but this value increased to approximately 30% in the case of 0.5% GO- incorporated TiO2 NFs device. This shows that larger amounts of dye molecules are adsorbed, and more light is transmitted through and scattered in the 0.5% GO-incorporated TiO2 than that in the pristine TiO2 nanofibers working electrode, while the IPCE value of the 1% GOincorporated TiO2 NFs was more than that of 0.5% GO-incorporated and pristine TiO2 NFs in the wavelength range of (390–435 nm) and then the value became less than others. It can be claimed that addition of GO into the TiO2 increases the flatband potential (Vfb), and reduces the difference between the higher quasi-Fermi level and the lowest unoccupied molecular orbital (LUMO) of the utilized dye, which results in the weakening of the driving force for the photoelectron injecting from the dye excited state into the conduction band of TiO2, and the increase of the titania crystallite size also influences the dye absorption of TiO2 film [45].

Reprinted with the permission from M. Shaheer Akhtar et al, Ceramics Intern. 41 (2015) 1205–1212. © 2015, Elsevier Ltd. Figure 5. IPCE of DSSCs based on the pristine, 0.5%, and 1% GO-incorporated TiO2 nanofibers film electrodes.

Graphene Oxide (GO) Incorporation in TiO2 Nanofibers …

407

The Current Density–Voltage (J–V) Characteristics The photocurrent density–voltage (J–V) characteristics of DSSCs using GOincorporated TiO2 and pristine TiO2 nanofibers as working electrodes are shown in Figure 6. 0.5% GO- incorporated TiO2 NFs based working electrode shows the highest conversion efficiency (η = 4.52%) with a significantly higher open circuit voltage (Voc =0.784 V), short-circuit current density (Jsc = 9.41 mA/cm2) and fill factor (FF = 0.61) as compared to the pristine TiO2 NFs which shows conversion efficiency of 1.54%, open circuit voltage (Voc) of 0.635 V, short-circuit current density (Jsc) of 6.75 mA/cm2 and fill factor (FF) of 0.35.

Reprinted with the permission from M. Shaheer Akhtar et al, Ceramics Intern. 41 (2015) 1205–1212. © 2015, Elsevier Ltd. Figure 6. The J–V characteristics in the illumination states of the pristine, 1% GO and 0.5% GO incorporated TiO2 nanofibers electrode-based DSSCs (light intensity of 100 mW cm-2, AM 1.5 filter, and illumination area of 0.25 cm2).

These DSSC characteristic values indicate that the incorporation of GO into the TiO2 NFs films can increase the interfacial contact, electron life time and retard the charge recombination rate by providing rapid electron trans- port paths from the films to the FTO substrates. Although, GO incorporation leads only to enhance the open circuit potential but the cell performance in converting the solar energy to electrical energy has been distinctly increased as the efficiency depends on both of the open circuit potential and the current density. Actually, the incorporation of GO provides more active sites for the absorption of dye molecules which leads to more charge carriers

408

Moaaed Motlak, Nasser A.M. Barakat, M. Shaheer Akhtar et al.

injected from the excited state of the dye into the conduction band of the collection electrode and enhances the photocurrent in the external circuit which increases the overall efficiency of the DSSCs [47,48]. However, higher GO content (1% GOincorporated TiO2 nanofibers) causes higher surface resistance of the TiO2 electrode and decrease of light harvest for the dye molecules due to competition between the dye and GO (graphene oxide can absorb visible light at wavelengths above 400 nm, Figure 8) which leads to low efficient electron transfer at TiO2/electrolyte layer interfaces [47–50]. Therefore, the short-circuit current density (Jsc) drastically drops in comparison to the pristine and 0.5% GO-incorporated TiO2 nanofibers based devices. It has a short-circuit current density (Jsc) of 5.65mA/cm2, open-circuit voltage (Voc) of 0.804 V and fill factor (FF) of 0.63 with a power conversion efficiency (η) of 2.84%.

CONCLUSION Graphene oxide-incorporated titanium oxide nanofibers with good morphology can be synthesized by the electrospinning of a sol–gel composed of GO, titanium isopropoxide and poly (vinyl acetate) sol–gel. Addition of GO strongly enhances the dye absorption and annihilates electrons/holes recombination which subsequently enhances the DSSCs performance. However, the optimum graphene oxide content was estimated to be 0.5 wt% with respect to the TiO2 nanofibers. Thus, the nanofibrous morphology and GO incorporation can be recommended strategies to enhance the performance of TiO2 working electrodes in the DSSCs.

REFERENCES [1] [2] [3] [4] [5] [6] [7]

S. Ahmad, E. Guillén, L. Kavan, M. Grätzel, M. K. Nazeeruddin, Energy Environ. Sci., 6 (2013) 3439. C. Cheng, H. J. Fan, Nano Today, 7 (2012) 327. C. Chen, J. Wang, Z. Ren, G. Qian, Z. Wang, Cryst. Eng. Comm., 16 (2013) 1681. A. I. Hochbaum, P. Yang, Chem. Rev., 110 (2009) 527. B. H. Lee, M. Y. Song, S. Y. Jang, S. M. Jo, S. Y. Kwak, D. Y. Kim, J. Phys. Chem. C, 113 (2009) 21453. N. A. Barakat, M. A. Abdelkareem, M. El-Newehy, H. Y. Kim, Nanoscale Res. Lett., 8 (2013) 1. N. A. Barakat, M. A. Kanjawal, I. S. Chronakis, H. Y. Kim, J. Mol. Catal. A: Chem., 336 (2012) 333.

Graphene Oxide (GO) Incorporation in TiO2 Nanofibers … [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

409

W.-Q. Wu, B.-X. Lei, H.-S. Rao, Y.-F. Xu, Y.-F. Wang, C.-Y. Su, D.-B. Kuang, Sci. Rep., 3 (2013) 1352. S. J. Limmer, S. Seraji, Y. Wu, T. P. Chou, C. Nguyen, G. Z. Cao, Adv. Funct. Mater., 12 (2002) 59. M. F. Casula, Y.-W. Jun, D. J. Zaziski, E. M. Chan, A. Corrias, A. P. Alivisatos, J. Am. Chem. Soc., 128 (2006) 1675. M. Adachi, Y. Murata, J. Takao, J. Jiu, M. Sakamoto, F. Wang, J. Am. Chem. Soc., 126 (2004) 14943. W. Teo, S. Ramakrishna, Nanotechnology, 17 (2006) R89. D. Li, Y. Wang, Y. Xia, Nano Lett., 3 (2003) 1167. N. A. Barakat, M. A. Abdelkareem, A. Yousef, S. S. Al-Deyab, M. El-Newehy, H. Y. Kim, Int. J. Hydrog. Energy, 38 (2013) 183. N. A. M. Barakat, A. M. Hamza, S. S. Al-Deyab, A. Qurashi, H. Y. Kim, Mater. Sci. Eng. B, 177 (2012) 34. C.-S. Chou, Y.-J. Lin, R.-Y. Yang, K.-H. Liu, Adv. Powder Technol., 22 (2011) 31. C.-S. Chou, R.-Y. Yang, C.-K. Yeh, Y.-J. Lin, Powder Technol., 194 (2009) 95. Y. Duan, N. Fu, Q. Zhang, Y. Fang, X. Zhou, Y. Lin, Electrochim. Acta, 107 (2013) 473. Y. Horie, T. Watanabe, M. Deguchi, D. Asakura, T. Nomiyama, Electrochim. Acta, 105 (2013) 394. J. Liu, H. Yang, W. Tan, X. Zhou, Y. Lin, Electrochim. Acta, 56 (2010) 396. Q. Liu, Y. Zhou, Y. Duan, M. Wang, Y. Lin, Electrochim. Acta, 95 (2013) 48. N. Massihi, M. R. Mohammadi, A. M. Bakhshayesh, M. A. Jalebi, Electrochim. Acta, 111, (2013) 921. H. Melhem, P. Simon, J. Wang, C. D. Bin, B. Ratier, Y. Leconte, N. H. Boime, M. M. Janusik, A. Kassiba, J. Bouclé, Sol. Energy Mater. Sol. Cells, 117 (2013) 624. M. Motlak, M. S. Akhtar, N. A. M. Barakat, A. M. Hamza, O. B. Yang, H. Y. Kim, Electrochim. Acta, 115 (2014) 493. M. Wang, S. Bai, A. Chen, Y. Duan, Q. Liu, D. Li, Y. Lin, Electrochim. Acta, 77 (2012) 54. J. Yu, Y. Yang, R. Fan, H. Zhang, L. Li, L. Wei, Y. Shi, K. Pan, H. Fu, J. Power Sources, 243 (2013) 436. Y.-Q. Zhang, D.-K. Ma, Y.-G. Zhang, W. Chen, S.-M. Huang, Nano Energy, 2 (2013) 545. X. Hu, K. Huang, D. Fang, S. Liu, Mater. Sci. Eng. B, 176 (2011) 431. J. Zhang, X. Li, W. Guo, T. Hreid, J. Hou, H. Su, Z. Yuan, Electrochim. Acta, 56 (2011) 3147. C.-T. Hsieh, B.-H. Yang, J.-Y. Lin, Carbon, 49 (2011) 3092. M.-Y. Yen, M.-C. Hsiao, S.-H. Liao, P.-I. Liu, H.-M. Tsai, C.-C.M. Ma, N.-W. Pu, M.-D. Ger, Carbon, 49 (2011) 3597.

410

Moaaed Motlak, Nasser A.M. Barakat, M. Shaheer Akhtar et al.

[32] W. Hong, Y. Xu, G. Lu, C. Li, G. Shi, Electrochem. Commun., 10 (2008) 1555. [33] L. Wan, S. Wang, X. Wang, B. Dong, Z. Xu, X. Zhang, B. Yang, S. Peng, J. Wang, C. Xu, Solid State Sci., 13 (2011) 468. [34] A. A. Madhavan, S. Kalluri, D. K. Chacko, T. Arun, S. Nagarajan, K. R. Subramanian, A. S. Nair, S. V. Nair, A. Balakrishnan, RSC Adv., 2 (2012) 13032. [35] S. Stankovich, R. D. Piner, X. Chen, N. Wu, S. T. Nguyen, R. S. Ruoff, J. Mater. Chem., 16 (2006) 155. [36] S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. B. T. Nguyen, R. S. Ruoff, Carbon, 45 (2007) 1558. [37] Y. Xu, H. Bai, G. Lu, C. Li, G. Shi, J. Am. Chem. Soc., 130 (2008) 5856. [38] W. S. Hummers Jr., R. E. Offeman, J. Am. Chem. Soc., 80 (1958) 1339. [39] M. A. Kanjwal, N. A. Barakat, F. A. Sheikh, M. S. Khil, H. Y. Kim, Int. J. Appl. Ceram. Technol., 7 (2010) E54. [40] N. A. M. Barakat, M. A. Kanjwal, S. S. Al-Deyab, I. S. Chronakis, H. Y. Kim, Mater. Sci. Appl., 2 (2011) 1188. [41] F. Tuinstra, J. Koenig, J. Compos. Mater. 4 (1970) 492. [42] A. M. B. Nasser, K. K. Abdelrazek, G. E.-D. Ahmad, K. H. Yong, Nano-Micro Lett., 5 (2013) 303. [43] X. Luan, L. Chen, J. Zhang, G. Qu, J. C. Flake, Y. Wang, Electrochim. Acta, 111 (2013) 216. [44] G. Sharma, R. Kurchania, R. Ball, M. Roy, J. Mikroyannidis, Int. J. Photoenergy, 2012 (2012) 1. [45] H. Tian, L. Hu, C. Zhang, W. Liu, Y. Huang, L. Mo, L. Guo, J. Sheng, S. Dai, J. Phys. Chem. C, 114 (2010) 1627. [46] M. Motlak, M. S. Akhtar, N. A. M. Barakat, A. M. Hamza, O. B. Yang, H. Y. Kim, Electrochim. Acta, 115 (2014) 493. [47] J. Fan, S. Liu, J. Yu, J. Mater. Chem., 22 (2012) 17027. [48] C.-Y. Yen, Y.-F. Lin, S.-H. Liao, C.-C. Weng, C.-C. Huang, Y.-H. Hsiao, C.-C. M. Ma, M.-C. Chang, H. Shao, M.-C. Tsai, Nanotechnology, 19 (2008) 375305. [49] T.-H. Tsai, S.-C. Chiou, S.-M. Chen, Int. J. Electrochem. Sci., 6 (2011) 3333. [50] H. Wang, S. L. Leonard, Y. H. Hu, Ind. Eng. Chem. Res., 51 (2012) 10613.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 29

GEL ELECTROLYTES WITH TITANIA NANOTUBE FILLERS FOR SOLID-STATE DYE-SENSITIZED SOLAR CELL M. Shaheer Akhtar*, Ji-Min Chun and O-Bong Yang Department of Semiconductor and Chemical Engineering, Chonbuk National University, Duckjin-Dong, Jeonju, Chonbuk, Republic of Korea

ABSTRACT A novel inorganic–organic composite solid electrolyte is prepared by using TiO2 nanotubes (TiNTs) as filler in polyethylene glycol (PEG) and eff ectively used for the fabrication of solid-state dye-sensitized solar cells (DSSCs). Comparably high conversion effi ciency ~4.43% has been observed by using the newly designed inorganic–organic (PEG–TiNTs) composite solid electrolyte. By performing several experiments by using PEG–TiNTs composite solid electrolytes, it is observed that the appropriate ratios of TiNTs and PEG are important to obtain higher conversion effi ciency. Moreover, the morphologies, chemical interactions of PEG and TiNTs and their performance to the DSSCs are studied extensively by FESEM, DSC, and XPS measurements.

Keywords: Gel electrolyte, Ionic conductivity, Dye sensitized solar cells, TiO2 nanotubes

*

Corresponding Author Email: [email protected].

412

M. Shaheer Akhtar, Ji-Min Chun and O-Bong Yang

INTRODUCTION From the pioneering work of Grätzel on dye-sensitized solar cell (DSSC), it has received a significant attention from last decades due to its high conversion efficiency and low cost fabrication [1, 2]. However, the utilization of liquid electrolytes in DSSCs usually has some severe problems such as leakage and evaporation, which are critically aff ected the long term stability for practical applications [3]. Therefore, many eff orts have been made to replace the liquid electrolytes with solid or quasi-solid chargetransporting materials, such as room-temperature molten ionic salts [4], polymer based solids [5] and gel electrolytes [6, 7]. Nevertheless, DSSCs containing solid electrolytes have limited applications due to their low conversion efficiencies and poor electric contact between photo-electrode and electrolyte. Thus, various efforts have been done to enhance the low ionic conductivity (σ = 10-8– 10-7Scm-1) of solid polymer electrolytes (SPEs) at ambient temperatures [8, 9]. It is known that the transport properties, ionic conductivity and mechanical properties of polymer electrolytes can be enhanced by the addition of inorganic fillers such as nanoparticulate or ceramic materials which includes SiO2, TiO2, Al2O3, etc., [10–12]. Moreover, these properties are depend upon the particle size, shape, crystallinity of the inorganic nano-filler materials and bonding between the surface functional groups of polymer and the nano-fillers [13–16]. In this work, we present the preparation and DSSCs application of new composite electrolytes by using TiNTs as the inorganic fillers into PEG matrix for application to DSSCs. The DSSCs fabricated with optimized composite electrolyte, PEG–TiNT10 containing 10 wt.% TiNTs in PEG, show significantly improved conversion efficiency (4.43%) in comparison to the DSSC fabricated with PEGonly electrolyte. The penetration of the composite electrolyte into TiO2 films provides the proper interfacial contact between the dye-absorbed TiO2 layer and electrolyte, which is known to be crucial for the photovoltaic (PV) performances [19].

Morphological Studies of TiO2 Thin Film The Field Emission Scanning Electron Microscopy Figure 1 shows the top view and cross-section field emission scanning electron microscope (FESEM) images of working electrode before and after introducing the composite electrolyte, which present the degree of penetration of the composite electrolyte into the porous TiO2 film. PEG–TiNT10 showed high penetration and complete filling of the electrolyte into the pores of TiO2 film (Figure 1c). However, the lower concentration of TiNTs (PEG–TiNT5) exhibits the low penetration and partially filling of the electrolytes into the porous TiO2 films which is most probably due to the

Gel Electrolytes with Titania Nanotube Fillers …

413

poor gelation properties with the PEG molecules. With a high concentration of TiNTs, as in PEG–TiNT20, the pore filling and penetration could potentially be plugged due to the high viscosity and bulkiness. Hence, penetration and pore filling of the composite electrolytes into the TiO2 of the PEG–TiNT5 and PEG–TiNT20 films did not fully proceed. These penetration properties can be confirmed by the top view images of TiO2 thin films before and after introducing electrolytes (inset). Bare TiO2 film (Figure 1a) shows the uniform surface and distinguishable particles with many pores. However, PEG–TiNT10 composite electrolyte fully covered the film surface and well penetrated through the pores of TiO2 particles in contrast to the case of PEG–TiNT5 (Figure 1b) and PEG–TiNT20 (Figure 1d), which showed the partially segregated electrolytes even on the film surface. This improved penetration of the PEG–TiNT10 composite electrolyte into the pores of TiO2 film leads to the advanced interfacial contact between electrolyte and electrode due to the homogenous gelation and mixing of PEG and TiNTs.

Reprinted with the permission from M. S. Akhtar et al., Electrochem. Comm. 9 (2007) 2833–2837. © 2007, Elsevier Ltd. Figure 1. Cross-section and top view (inset) FE-SEM images of the TiO2 thin film (a) before filling and after introducing composite electrolytes of (b) PEG– TiNT5, (c) PEG–TiNT10 and (d) PEG– TiNT20.

414

M. Shaheer Akhtar, Ji-Min Chun and O-Bong Yang

Structural Characterizations of TiO2 Thin Film X-Ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy (XPS) is used to determine the bonding properties of the as-prepared composite electrolyte films. Figure 2 shows the XPS spectra of composite electrolyte films. Three major peaks, at around 285, 457 and 531 eV, are ascribed to carbon (C 1s, Figure 2a–c), titanium (Ti 2p, inset) and oxygen (O 1s, Figure 2d), respectively. No Ti 2p peak at 457 eV was observed in PEG–TiNT5, but is significantly detected in PEG–TiNT10. There are two general C 1s peaks at 284.6 eV (very strong) and at 286.7 eV (shoulder), corresponding to CO and CH2, respectively.

Reprinted with permission from M. S. Akhtar et al., 2007, Electrochem. Commun. 9 (2007) 2833© 2007, Elsevier Ltd. Figure 2. XPS spectra of the composite electrolytes. (a) PEG-TiNT5, (b) PEG-TiNT10, (c) PEGTiNT20, and (d) O 1 s of PEG (—), PEG-TiNT5 (- - - -) PEG-TiNT10 (…..) and PEG-TiNT20 (-.-.-).

Another significant new C 1s peak at 283.0 eV appears in the spectra of PEG– TiNT10 and PEG–TiNT20 due to the formation of C–O– Ti bonding between the C in the polymer and the Ti in the TiNTs [20]. It is evident that a significant C–O–Ti bonding between PEG and TiNTs was formed in both PEG–TiNT10 and PEG–TiNT20.

Gel Electrolytes with Titania Nanotube Fillers …

415

A slight shift in the O 1s XPS spectra (Figure 2d) of PEG–TiNT10 and PEG– TiNT20 at 531.2 eV has been observed, compare to pure PEG (530.9 eV), which reveals an interaction between the titanium atoms with the polymer network. The C1s peak intensity at 283.0 eV of the C–O–Ti bonding is highly shifted in PEG–TiNT10, indicating the most prominent linkage formation compared to PEG–TiNT5 and PEG– TiNT20. Thus, it could be confirmed that the PEG–TiNT10 contained the optimal nano-filler to polymer ratio in terms of cross-linking and attachment. Moreover, it is evident that the strong bonding is formed between the PEG and TiNTs in the composite gel electrolyte.

Photovoltaic Performance of TiO2 Thin Film Photoanode-Based DSSC The Current Density–Voltage (J–V) Characteristics Figure 3 exhibits the current–voltage characteristic data of the DSSC fabricated with the composite electrolytes in the simulated sunlight irradiance of 100 mW/cm2. DSSC fabricated with PEG–TiNT10 showed the maxi- mum overall conversion efficiency of 4.43%, short circuit current density (JSC) of 9.36 mA/cm2, an open circuit voltage (VOC) of 0.725 V and fill factor (FF) of 65.3%. No significant decreasing of the conversion efficiency for 30 days was observed in DSSC fabricated with PEG–TiNT10 (inset of Figure 3), indicating the high stability of the composite electrolytes. The conversion efficiency and fill factor of DSSCs with composite electrolytes have shown higher than of those fabricated with PEG-only electrolyte, as expected from the enhanced ion conductivity and enlargement of amorphous phase of polymer upon the addition of TiNTs into PEG matrix. The lower cur- rent density in PEG–TiNT20 is due to its lower ion conductivity, lower penetration and weak interaction between PEG to TiNTs. Generally it is believed that the excellent interfacial contact between electrolyte and TiO2 layer is due to high penetration of composite electrolyte into the dye-adsorbed TiO2 particles electrodes. In our case, it is observed that the better penetration into the pores of TiO2 layer was obtained at a particular ratio of TiNT and PEG in composite electrolyte (PEG–TiNT10). Thus, due to better interfacial contact between electrolyte and TiO2 layer, high ion conductivity is obtained which enhance the photocurrent density. Therefore, the high ion conductivity and high penetration of the composite electrolyte are essential factors for the high current density and high PV performance. High penetration of electrolyte into the porous film provides good interfacial contacts between electrolytes to TiO2, which leads to improved photovoltaic performance. The PEG–TiNT composite electrolytes might be facilitated the fast electron transfer in redox (I-/I3-) couple due to the fast electron transfer characteristics of nanotubes with less grain boundary in comparison with nanoparticles.

416

M. Shaheer Akhtar, Ji-Min Chun and O-Bong Yang

Reprinted with permission from [M. S. Akhtar, 2007], Electrochem. Commun. 9 (2007) 2833.© 2007, Elsevier Ltd. Figure 3. Current–voltage characteristics of DSSC fabricated with composite electrolytes of (a) PEGTiNT5, (b) PEG-TiNT10, and (c) PEG-TiNT20. Inset shows the stability test of DSSC fabricated with composite electrolyte of PEG-TiNT10.

CONCLUSION Advanced composite gel electrolytes for DSSC are prepared by introducing titania nanotubes (TiNTs) as the nanofillers into polyethylene glycol (PEG). The optimized composite electrolyte, PEG–TiNT10, shows the advanced morphological properties by the formation of strong interaction between TiNTs and PEG, resulting in excellent penetration, improved conductivity and high stability. The DSSCs fabricated with the optimal composite electrolyte achieves a high conversion efficiency of 4.43%, maintained for 30 days without a significant decreasing. This is the first report on DSSC with the composite electrolytes using TiNTs as the fillers, which might be useful nanofiller materials in electrolytes.

REFERENCES [1] [2] [3]

B. O’Regan, M. Grätzel, Nature, 353 (1991) 737. M. Grätzel, Nature, 414 (2001) 338. N. Papageorgiou, Y. Athanassov, M. Armand, P. Bonhote, H. Pettersson, A. Azam, M. Grätzel, J. Electrochem. Soc., 143 (1996) 3099.

Gel Electrolytes with Titania Nanotube Fillers … [4]

417

U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissörtel, J. Salbeck, H. Spreitzer, M. Grätzel, Nature, 395 (1998) 583. [5] A. F. Nogueira, J. R. Durrant, M. A. D. Paoli, Adv. Mater., 13 (2001) 826. [6] T. Stergiopoulos, I. M. Arabatzis, G. Katsaros, P. Falaras, Nano Lett., 2 (2002) 1259. [7] E. Stathatos, P. Lianos, U. L. Stangar, B. Orel, Adv. Mater., 14 (2002) 354. [8] G. Mao, R. F. Perea, W. S. Howells, D. L. Price, M. L. Saboungi, Nature, 405 (2000) 163. [9] Z. Gadjourova, Y. G. Andreev, D.P. Tunstall, P. G. Bruce, Nature, 412 (2001) 520. [10] F. Croce, G. B. Appetechi, L. Persi, B. Scrosati, Nature, 394 (1998) 456. [11] F. Croce, R. Curini, A. Martinelli, L. Persi, F. Ronci, B. Scrosati, J. Phys. Chem. B, 103 (1999) 10632. [12] G. Jiang, S. Maeda, H. Yang, Y. Saito, S. Tanase, T. Sakai, J. Power Sources, 141 (2005) 143. [13] M. Wagemaker, G. J. Kearley, A. A. V. Well, H. Mutka, F. M. Mulder, J. Am. Chem. Soc., 125 (2003) 840. [14] S. H. Chung, Y. Wang, L. Persi, F. Croce, S. G. Greenbaum, B. Scrosati, E. Plichta, J. Power Sources, 97-98 (2001) 644. [15] F. Croce, L. Persi, B. Scrosati, Electrochim. Acta, 46 (2001) 2457. [16] A. Stashans, S. Lunell, R. Bergstrom, A. Hagfeldt, L. S. Eric, Phys. Rev. B, 53 (1996) 159. [17] M. A. Khan, H. T. Jung, O. B. Yang, J. Phys. Chem. B, 110 (2006) 6626. [18] T. V. Nguyen, H. C. Lee, M. A. Khan, O. B. Yang, Solar Energy, 81 (2007) 529. [19] J. H. Kim, M. S. Kang, Y. J. Kim, J. G. Won, N. G. Park, Y. S. Kang, Chem. Commun., (2004) 1662. [20] A. A. Galuska, J. C. Uht, N. Marquez, J. Vac. Sci. Technol. A, 6 (1988) 110.

SECTION 4. PEROVSKITE SOLAR CELLS (PSCS)

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 30

ZNO QUANTUM DOTS THIN FILM FOR FLEXIBLE PEROVSKITE SOLAR CELLS Sadia Ameen1, M. Shaheer Akhtar2, Hyung-Kee Seo1, Mohammad Khaja Nazeeruddin3,* and Hyung Shik Shin1,† 1

Energy Materials and Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New and Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea 3 Group for Molecular Engineering of Functional Materials (GMF), Institute of Chemical Science and Engineering, Faculty of Basic Science, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland

ABSTRACT A new flexible perovskite solar cell is based on graphene (Gr) as the barrier layer and the atmospheric plasma jet (APjet)-treated ZnO quantum dots (QDs) as the mesoscopic metal oxide layer on ITO-PET substrates. The ITO-PET flexible substrate is treated with oxygen (O2) plasma before creating an efficient barrier layer of Gr, and thereafter as-synthesized ZnO QDs are deposited by spin coating on ITO-PET/Gr thin film. ITO-PET/Gr/ZnO-QDs thin film substrates are finally subjected to APjet treatment using RF power of ∼40 W with frequency of ∼13.56 MHz, which substantially improves the interfacial properties of the deposited layers. The fabricated ITOPET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell obtains the high conversion efficiency of ∼9.73% along with high short circuit current

* †

Corresponding Author Email: [email protected]. Corresponding Author Email: [email protected].

422

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al. (JSC) of ∼16.8 mA/cm2, open circuit voltage (VOC) of ∼0.935 V, and high fill factor (FF) of ∼0.62. The APjet treatment on ITO-PET/Gr/ZnO QDs thin film enhances the performances and the photocurrent density as compared to other solar cells fabricated without APjet treated ITO-PET/Gr/ZnO QDs thin film. By analyzing the intensitymodulated photocurrent (IMPS)/photovoltage spectroscopy (IMVS), the fabricated flexible perovskite solar cell exhibits a good charge transfer rate and a reduction in the recombination rate. The APjet treatment and the introduction of low-cost Gr barrier layer are promising prospects to approach low cost photovoltaic devices.

INTRODUCTION To find cost-effective photovoltaics, thin film photovoltaic technology has emerged as the most promising technology compared to conventional silicon solar cells. A thin film photovoltaic based on organo-metal halide perovskites has gained attention due to its high solar-to-electricity energy conversion efficiency [1−4]. In the perovskite solar cells, the effective photogenerated charge separation and the light harvesting efficiency are greatly influenced by the properties like band gap, particle size, surface morphology, porosity, surface area, thickness of semiconducting nanomaterials, and the nature of organo-metal halide perovskites [5]. Among perovskite absorbers, the organo-metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) possesses a direct band gap (∼1.5 eV) [6] with a large absorption coefficient (∼1.5 × 104 cm−1 at 550 nm) and the high charge carrier mobility [7]. The perovskite thin film solar cells have reached a high power conversion efficiency of over ∼17% along with broad light absorption, and high open-circuit voltages (VOC) of over ∼1.0 V.8 At present, lightweight photovoltaic devices such as flexible solar cells generate a great deal of interest due to their low production cost, variable shapes, and large-scale roll to-roll processing.9,10 So far, various flexible substrates such as poly(ethylene terephthalate) (PET)/indium tin oxide (ITO), metal foils, and metal sheets have already been employed for the fabrication of different flexible solar cells [11, 12]. Generally, mesoporous TiO2 or other metal oxide thin layers are achieved by the sintering process at high temperature [13], which substantially limits the usage of conducting plastic-film and malleable metal foil substrates for the perovskite solar cells [14]. Hitherto, the configuration and the choice of a mesoporous layer are essential factors to understand the physical electronic mechanisms in these solar cells, which regulate processes such as carrier separation, transport, extraction, and the recombination. Thus, the device structure configuration is essential to modify different mesoporous materials, electrode contacts, and the barrier layer for understanding these processes and mechanisms. Several mesoporous thin film nanomaterials such as TiO2, ZnO, Al2O3, fullerene derivatives, etc. [15−17] have been used for perovskite sensitized solar cells. Among them, zinc oxide (ZnO) nanomaterials could be promising semiconducting metal

ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cells

423

oxides because of their wide band gap (∼3.37 eV), large exciton binding energy (∼60 meV), higher electron mobility [18], unique photoelectric properties, optical transparency, electric conductivity, and piezoelectricity properties [19, 20], Apart from other ZnO nanostructures, ZnO quantum dots (QDs) possess a higher stability and resistivity toward oxygen and water with tunable band gaps [21]. ZnO QDs due to threedimensional confinement of carriers and phonons are anticipated to improve the device performances by changing the optoelectronic properties [22]. Additionally, 2D sp2bonded carbon materials, like graphene (Gr), have shown high electrical conductivity, high charge mobility [23−25], and large specific surface area of ∼2630 m2/g [26−28]. The incorporation of Gr into polymers [29, 30], ceramic materials [31], and metal oxides have shown remarkable improvements in the optoelectrical and electrochemical properties of the host materials. Recently, the combination of Gr and ZnO nanomaterials seems to be the most promising material, as it could improve the carrier transport and the collection efficiency of ZnO-based UV photodetectors, sensors, and electrochemical devices [32, 33]. Hwang and Kim et al. [34, 35] have recently grown vertically aligned ZnO nanowires on reduced graphene/PDMS substrates and fabricated a transparent and flexible optoelectronic material. Chang et al. 36 reported the heterostructures of ZnO nanorods/Gr via a facile in situ solution growth method and demonstrated a highly sensitive visible-blind ultraviolet (UV) sensor. In the present work, Gr thin film as a barrier layer is deposited on O2 plasma treated ITO-PET substrate, and the as synthesized ZnO QDs is coated on ITOPET-Gr substrates by spin coating. The deposited ZnO QDs thin film is further subjected to APjet plasma treatment to enhance the interfacial contacts and modifying the surface properties of ITO-PET-Gr/ZnO QDs thin film substrate. An atmospheric plasma technology, also called non-thermal or low-temperature plasma technology requires no vacuum systems and provides higher plasma density due to the large surface-to-volume ratio and low-discharge current of the plasma. ITO-PET-Gr/ZnO QDs thin films substrate is used for the fabrication of flexible perovskite solar cell of the composition ITOPET/Gr/ZnOQDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag. The flexible perovskite solar cell presents a reasonably high solar-to-electric conversion efficiency of ∼9.73% with high photocurrent density and open-circuit potential. The incident-photon-to-current efficiency (IPCE) of ∼59.2% in the wavelength range of ∼400−700 nm is achieved by the ITO-PET/Gr/ZnO-QDs-(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell. In the beginning, the perovskite solution of methyl lead ammonium iodide (CH3NH3PbI3) is synthesized as reported elsewhere [39] and was spin coated at a speed of ∼2000 rpm for 40s on ITO-PET/Gr/ZnO-QDs(APjet) flexible substrate using a ∼ 0.45 μm pore PVDF membrane syringe filter (Jet Biofil) at ambient temperature. The perovskite deposited thin films are annealed at ∼100°C for 30 min to achieve ITO-

424

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3 thin films. A separate spiro-MeOTAD solution in chlorobenzene (∼15 mg/1 mL) with ∼13.6 μL Li-bis (tri fluoro methane sulfonyl) imide (CF3SO2NLiSO2CF3, Li-TFSI, ∼28.3 mg/1 mL, TCI, >98%) and ∼6.8 μL TBP (C9H13N, Aldrich, 96%) as additives is further spin-coated on ITO-PET/Gr/ZnOQDs (APjet)/CH3NH3PbI3 thin films at ∼3000 rpm for 30 s and annealed at 100°C for 15 min to get ITO-PET/Gr/ZnOQDs(APjet)/CH3NH3PbI3/spiro-MeOTAD thin films. Lastly, silver (Ag) contacts (thickness ∼100 nm) are made by the thermal evaporation and achieves the final flexible device structure of ITO-PET/Gr/ZnO-QDs (APjet)/CH3 NH3PbI3/spiro-MeOTAD/Ag, as illustrated in Figure 6b.

THE INVESTIGATION OF THE MORPHOLOGY AND THE SURFACE MODIFICATIONS Atomic Force Spectroscopy and Contact Angle Measurements The surface modifications are analyzed by the contact angles and three-dimensional (3D) AFM images of bare ITO-PET substrate and modified ITO-PET substrates by O2 plasma and Apjet treatments. Figure 1a,b shows the contact angles and 3D AFM images (Figure 1a1,b1) of bare and O2 plasma-treated ITO-PET substrates. The contact angle of ∼89.6° is obtained for bare ITO-PET, while a substantive decrease in the contact angle of ∼75.2° is recorded after O2 plasma treatment of ITOPET substrate. The change in the contact angle imputes the increment of hydrophilicity and the surface energy of ITO-PET substrate due to the production of polar group moieties through the oxidation plasma process. The plasma treatment in the presence of O2 usually changes the structure of ITO-PET substrate by the incorporation of oxygencontaining functionalities due to the strong surface oxidation and therefore, improves the hydrophilicity of ITO-PET surface [40]. Moreover, the molecular oxygen in the plasma ionization becomes active and dissociates into extremely reactive oxygen species, which could react immediately with the polymer surface to produce oxygen-containing polar groups such as C−O, O−C−O and OH [41, 42]. The enhancement in the contact angle of ∼81.2° is further observed after the deposition of Gr thin film on O2 plasma treated ITO-PET substrates, as shown in Figure 1c and c1. This increment might referable to the amphiphilic Gr thin film possessing both hydrophilic groups (−COOH, −OH, and C−O) and hydrophobic (C−C, C−H) groups. From Figure 1d, the further increase in the contact angle of ∼96.2° of Apjet treated ITOPET/Gr/ZnO-QDs thin film indicates the deformation of surfaces caused by the O2 plasma-treated hydrophilic ITO-PET surface and the amphiphilic Gr sheets. Importantly,

ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cells

425

the Apjet treatment might also improve the surface-to-volume ratio and shows good contact between hydrophilic ITO and amphiphilic Gr surfaces of thin film (As shown in (Figure 1d1). Thus, it is believed that the enhanced hydrophilicity and amphiphilicity of the flexible substrates by the O2 plasma and Apjet treatments might deliver a suitable surface for the good deposition of new functionalities, i.e., spiro-MeOTAD and CH3NH3PbI3.

1 2 3 4

1 2 3 4

0.5 1.0 1.5

Reprinted with permission from [S. Ameen, 2015], J. Phys. Chem. C, 119 (2015) 10379 © 2015 American Chemical Society. Figure 1. Contact angles and the corresponding 3D AFM images of (a, a1) ITO-PET, (b, b1) O2 plasmatreated ITO-PET, (c, c1) ITO-PET/Gr, and (d, d1) ITO-PET/Gr/ZnO-QDs(APjet) thin films.

426

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Reprinted with permission from [S. Ameen, 2015], J. Phys. Chem. C, 119 (2015) 10379 © 2015 American Chemical Society. Figure 2. Topographic AFM images of (a) ITO-PET, (b) O2 plasma-treated ITO-PET, (c) ITO-PET/Gr, and (d) ITO-PET/Gr/ZnO-QDs(APjet) thin films.

The topographic AFM images of bare ITO-PET, O2 plasma treated ITO-PET, Gr/O2 plasma-treated ITO-PET and Apjet treated ITO-PET/Gr/ZnO-QDs thin films are analyzed to understand the morphological and roughness factor of the thin films. From Figure 2a, bare ITO-PET presents uniform and regular morphology; however, O2 plasmatreated ITO-PET substrate exhibits non regular morphology (Figure 2b). On comparison to bare ITO-PET substrate (root-mean-square roughness, Rrms = ∼7.7 nm), the low Rrms value of ∼4.3 nm is recorded for O2 plasma-treated ITO-PET substrate. In support, the surface analysis (AFM and contact angle results) reveals that both roughness and contact angle decrease upon the O2 plasma treatment of ITO-PET. The diminution in contact angle might be due to the increment of hydrophilicity and the surface energy of ITO-PET substrate, which occur due to the production of polar group moieties through the incorporation of oxygen-containing functionalities on ITO-PET substrate [42]. In addition, the deposition of Gr and ZnO QDs reduce the roughness and contact angle, which might suggest the case of photoinduce hydrophilicity in the obtained thin film. Interestingly, as shown in Figure 2c,d, the low roughness (∼1.1 nm) is obtained after the

ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cells

427

deposition of Gr and Apjet treatment of ZnO QDs, which indicates the better interaction of O2 plasma-treated ITO-PET substrate, Gr and ZnO QDs. These results are also consistent with the contact angles measurements. To check the porosity of ITOPET/Gr/ZnOQDs thin film, the surface area has been analyzed by a BET surface analysis. The ITO-PET/Gr/ZnO-QDs thin film records the high specific surface area of ∼268.4 m2/g; however, the APjet treatment on ITO-PET/Gr/ZnO-QDs thin film enhances the surface area from ∼268.4 to ∼334.2 m2/g. This significant increment in surface area suggests the heightening in the porosity of the ITO-PET/Gr/ZnO-QDs thin film after APjet treatment.

THE TRANSMITTANCES AND SHEET RESISTANCE OF ITO-PET AND THIN FILMS SUBSTRATES Figure 3 shows the transmittance spectra and sheet resistances of bare ITO-PET and modified ITO-PET substrates. A transmittance of maximum ∼82% in the absorbance range of ∼400−1000 nm is obtained by bare ITO-PET substrate, as shown in Figure 3a, which is consistent to the reported transparency of ITO-PET substrate [43]. After O2 plasma treatment, the optical transmittance decreases to ∼79.2% and further decreases to ∼74.5% and ∼62.2% after depositing Gr thin film on O2 plasma-treated ITO-PET and Apjet treated ZnO-QDs deposition on Gr/O2 plasma treated ITO-PET substrates, respectively. These observations suggest the slight change in the transparent nature of ITO-PET substrate.

Reprinted with permission from [S. Ameen, 2015], J. Phys. Chem. C, 119 (2015) 10379 © 2015 American Chemical Society. Figure 3. Transmittance spectra (a) and variation plot of sheet resistance and transmittance (b) of ITOPET, F-1(O2 plasma treated ITO-PET), F-2 (ITO-PET/Gr), F-3 (ITO-PET/Gr/ZnO-QDs), and F-4 (ITOPET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD thin films.

428

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Furthermore, the ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD on ITO-PET records the lowest transmittance of ∼47.2%, indicating the interaction of the deposited layers on ITO-PET/Gr layer substrate. The transmittance properties are further supported by the measurement of sheet resistances of bare ITO-PET and the modified ITO-PET substrates. From Figure 3b, the sheet resistance decreases with the increase of the transparency of bare ITO-PET and the modified ITO-PET substrates. It is visible that the low sheet resistance favors the strong interactions between Gr, ZnO-QDs and O2 plasma-treated ITO-PET substrate, which helps in restoring the inherently high electrical conductivity of Gr. In general, the low sheet resistance represents good electrical conductivity and charge transportation [44]. In our case, the low resistance of ITOPET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD thin film substrate supports an appropriate conductivity and enhanced charge carriers and transportation, which might result in achieving high performance of the flexible perovskite solar cell.

STRUCTURAL CHARACTERIZATIONS OF TIO2 THIN FILM X-Ray Photoelectron Spectroscopy XPS analysis is employed to evaluate the interfacial interactions and the bonding between ITO-PET/Gr and ITO-PET/Gr/ZnO-QDs(APjet) substrates. Figure 4 shows C 1s, O 1s and Zn 2p XPS plots of ITO-PET/Gr and ITO-PET/Gr/ZnO-QDs(APjet) substrates. Figure 4a exhibits four resolved binding energies at ∼285.2, ∼286.3, ∼ 287.5 and ∼288.6 eV (weak) for Gr/ITO-PET substrate, which are assigned to C−H/ C−C, C−O, C−O−C/C=O, and −O−C=O bonds [45, 46]. The existence of these bonds in Gr/ITO-PET confirms the uniform deposition of Gr on O2 plasma-treated ITO-PET substrate. The weak −O−C=O bond occurs due to the chain-cutting of some ITO-PET molecules, especially the weakest bonds −O−C=O groups by O2 plasma treatment [47]. After the Apjet treatment of ITO-PET/Gr/ZnO-QDs substrate, C 1s XPS (Figure 4b) presents four similar resolved binding energies. Importantly, the binding energy at ∼ 288.6 eV is prominent as compared to ITO-PET/Gr substrate, indicating the increased − O−C=O bond strength through the Apjet treatment. The high −O−C=O bond strength might be due to the strong partial hydrogen bond formation of −O−C=O in Gr and Zn−O in ZnO via an ester linkage. The deconvoluted O 1s XPS spectra of ITO-PET/Gr substrate and ITO-PET/Gr/ZnO-QDs(APjet) substrates are shown in Figure 4c,d. ITOPET/Gr substrate obtains the center peak at ∼531.4 eV along with three resolved peaks at ∼531.1 eV, ∼532.1 eV, and ∼533.0 eV, correspond to an O atom attached to an sp2/sp3 C atom and moisture or hydroxyl contaminants, respectively.

ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cells

429

Reprinted with permission from [S. Ameen, 2015], J. Phys. Chem. C, 119 (2015) 10379 © 2015 American Chemical Society. Figure 4. C 1s XPS spectra of (a) ITO-PET/Gr, (b) ITO-PET/Gr/ZnO-QDs(APjet), and O 1s XPS spectra of (c) ITO-PET/Gr, (d) ITO-PET/ Gr/ZnO-QDs(APjet), and (e) Zn 2p XPS of ITOPET/Gr/ZnO-QDs(APjet) thin films.

The binding energies are shifted to lower binding energy in O 1s XPS of ITOPET/Gr/ZnO-QDs(APjet) substrate, indicating the interaction between metal (Zn) and O atom. The binding energy at ∼530.6 eV is ascribed to the presence of O2− ions on the hexagonal Zn2+ ion in wurtzite ZnO structure [48]. The other resolved binding energies represent the oxygen deficiency or oxygen vacancies over the surface of ZnO and hydroxyl contaminants. Furthermore, the doublet peaks of Zn 2p3/2 at ∼1022.1 eV and Zn 2p1/2 at ∼1045 eV in Zn 2p (Figure 4e) have again confirmed the existence of Zn−O species, and the binding energy difference of ∼22.9 eV deduces the typical hexagonal ZnO wurzite structure [49]. Thus, O2 plasma and Apjet treatment have considerably improved the bond strength of Gr with ITO-PET and increase the interaction between Gr

430

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

and ZnO QDs in ITO-PET/Gr/ZnO-QDs(APjet) substrate, which might be beneficial for achieving high performance of the device.

The Raman Spectrum Raman scattering spectroscopy is obtained for explaining the nature of Gr and the interaction with ZnO-QDs thin film on ITO-PET substrate, as shown in Figure 5. For ITO-PET/Gr thin film, typically two Raman bands at ∼1349.2 and ∼1583.4 cm−1 are ascribed to D and G bands, which are due to the defects/out-of-plane breathing mode of sp2 atoms of carbon thin film and the E2g vibrational mode/double degenerate phonon mode at the Brillouin zone center, respectively [50].

Reprinted with permission from [S. Ameen, 2015], J. Phys. Chem. C, 119 (2015) 10379 © 2015 American Chemical Society. Figure 5. Raman spectra of ITO-PET/Gr and ITO-PET/Gr/ZnOQDs(APjet) thin films.

Another Raman band at ∼2708.2 cm−1 represents the second order peak due to fourth-order phonon momentum exchange double resonance process [51−53]. It is reported that the width of the 2D peak and the I2D/IG ratio in the Raman analysis of Gr is used to estimate the number of layers and quality of Gr [54]. From Figure 5, the intensity ratios of ID/IG (∼0.24)/I2D/IG (∼0.64) and the large full width at half maximum (FWHM) of the 2D (≈ 77 cm−1) peak are estimated, which indicate few (non-interacting) layer graphene in non-AB stacking arrangement. However, the ITO-PET/Gr/ZnO-QDs-(APjet) thin film obtained a large I2D/IG (∼0.93) and (fwhm) of 2D (≈ 77 cm−1), which again

ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cells

431

confirm few interlayers of graphene on the deposited thin film. Moreover, in the case of ITO-PET/Gr/ZnO-QDs(APjet) thin film, a strong Raman band at ∼436.7 cm−1 along with weak Raman bands of Gr are present. The Raman band at ∼436.7 cm−1 elucidates the main characteristic E2 mode of ZnO wurtzite hexagonal structure in ITOPET/Gr/ZnO-QDs(APjet) thin film. Additionally, the Raman band at ∼1110 cm−1 is assigned to second-order signals in the 2xLO phonon, which is usually due to the presence of band disorder in ZnO nanomaterials [55]. The weakening of Gr Raman band might due to the bonding formation between −O−C=O of Gr and Zn−O of ZnO-QDs, resulting to well disperse, regular and smooth thin film on ITO-PET substrate.

SCHEMATIC ILLUSTRATION OF THE FABRICATED ITOPET/GR/ZNO-QDS(APJET)/CH3NH3PBI3/SPIRO-MEOTAD/ AG FLEXIBLE PEROVSKITE SOLAR CELL Figure 6 shows the photograph of ITO-PET/Gr thin film substrate and the schematic illustration of the fabricated flexible perovskite solar cells. Figure 6a explains the good flexibility with inherent transparency of ITO-PET/Gr substrate. The structure of the fabricated flexible perovskite solar cell of composition ITO-PET/Gr/ZnO-QDs(APjet)/ CH3NH3PbI3/spiro-MeOTAD/Ag is illustrated in Figure 6b. Herein, the introduction of Gr layer on ITO-PET prevents the direct contact of ITO-PET substrate with perovskite and hole transporting layer (HTL). The performances of the fabricated flexible perovskite solar cells have been evaluated by measuring the current density (J)−voltage (V) curves under the light illumination of 100 mW/cm2 (1.5 AM). Figure 7A shows the J−V curves of the fabricated flexible perovskite solar cells with different thin films such as ITOPET/Gr, ITO-PET/Gr/ZnO-QDs and ITO-PET/Gr/ZnO-QDs(APjet). The highest conversion efficiency of ∼9.73% along with high short circuit current (J SC) of ∼16.8 mA/cm2, open circuit voltage (VOC) of ∼0.935 V and high fill factor (FF) of ∼0.62 are accounted by the fabricated ITO-PET/Gr/ZnO QDs(APjet)/CH3NH3PbI3/spiroMeOTAD/Ag flexible perovskite solar cell. However, the flexible perovskite solar cells with ITO-PET/Gr/ZnO-QDs and ITO-PET/Gr thin films present the low conversion efficiencies of ∼5.28% and ∼2.89%, respectively along with inferior JSC and VOC. The brilliant performance with ITO-PET/Gr/ZnO-QDs(APjet) thin film might be rationalized with respect to an effective interaction of the perovskite with Apjet treated ZnO-QDs. In support, the Apjet treatment has significantly increased the porosity of ITOPET/Gr/ZnOQDs(APjet) thin film, which might favor a good adsorption and the penetration of perovskite (CH3NH3PbI3) into ZnO-QDs and results in low roughness of the ITOPET/Gr/ZnO-QDs(APjet) thin film, as evidenced from AFM results.

432

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Reprinted with permission from [S. Ameen, 2015], J. Phys. Chem. C, 119 (2015) 10379 © 2015 American Chemical Society. Figure 6. (a) Photograph of ITO-PET/Gr and (b) a schematic illustration of the fabricated ITOPET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell.

PHOTOVOLTAIC PERFORMANCES OF THE FABRICATED FLEXIBLE PEROVSKITE SOLAR CELLS The Current Density–Voltage (J–V) Characteristics and the Incident Photonto-Current Conversion Efficiency (IPCE) of Fabricated Flexible Perovskite Solar Cells Moreover, APjet-treated ZnO QDs are aggregated randomly to form a loose matrix, which might beneficiary for the reflection and the refraction of incident light, leading to a multiple scattering effects which might capture huge amount of incident light and improve the generation of photoelectrons [56, 57]. The high J SC with ZnO-QDs(APjet) might due to the inhibition of the leakage current between the electrodes through the uniform and high penetration of perovskite and spiro-MeOTAD on ITO-PET/Gr/ZnOQDs(APjet) thin film substrate, resulting in the enhancement in the charge transfer rate through less generation of pin-holes and shunting paths [58]. The considerable increase in VOC and FF with ITO-PET/Gr/ZnO-QDs(APjet) thin film is related to the improved interfacial contact conducting layer, ZnO-QDs and Gr layer via APjet treatment followed by O2 plasma treatment. The performances of the fabricated flexible perovskite solar cell are further investigated in terms of the IPCE efficiency to explain the light harvesting, charge collection, and the photocurrent. Figure 7B shows the IPCE graphs of the fabricated flexible perovskite solar cells with ITO-PET/Gr, ITO-PET/Gr/ZnO-QDs, and ITO-PET/Gr/ZnO-QDs(APjet) thin films. The highest IPCE of ∼59.2% in the wavelength range of ∼400−700 nm is observed by the fabricated flexible perovskite solar cell with ITO-PET/Gr/ZnO-QDs(APjet) thin film, and correlated with the JSC value, whereas the other flexible perovskite solar cells with ITO-PET/Gr and ITO-PET/Gr/ZnO-QDs

ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cells

433

thin film present the low IPCE values. In ITO-PET/Gr/ZnO-QDs(APjet)/ CH3NH3PbI3/spiro-MeOTAD/Ag solar cell, APjet-treated ZnO-QDs with improved porosity and surface area might significantly increase the light scattering capacities and the interaction between the CH3NH3PbI3 sensitizer and ZnO-QDs interfaces. The O2 plasma treatment, Gr layer, and APjet treatment during the preparation of desired thin films are essential for charge transfer, charge collection, and charge recombination.

Electrochemical Impedance Measurements The Nyquist plots, as shown in Figure 8a, investigate the series and charge transfer resistances of the fabricated flexible perovskite solar cells with ITO-PET/Gr, ITOPET/Gr/ZnOQDs, and ITO-PET/Gr/ZnO-QDs(APjet) thin film under a frequency range from 100 kHz to 1 Hz. Figure 8b depicts the Nyquist plots of the fabricated flexible perovskite solar cells with ITO-PET/Gr/ZnO-QDs(APjet) thin film at different applied voltages under a frequency range from 100 kHz to 1 Hz. The corresponding equivalent circuit (Figure 8c) illustrates several resistances, i.e., the resistance related to diffusion of holes through hole transporting material (HTM) (R1) with HTM capacitance (C1), and a recombination resistance (Rrec) at lower frequency with a chemical capacitance, (Cμ) which is related to the interface of HTM (spiro-MeOTAD) and ITOPET/Gr/ZnOQDs(APjet). It is clear that the appearance of the main arc occurs due to the existence of the recombination resistance (Rrec) at the interface of HTM/perovskite and ZnOQDs(APjet) layer [59].

Reprinted with permission from [S. Ameen, 2015], J. Phys. Chem. C, 119 (2015) 10379 © 2015 American Chemical Society. Figure 7. J−V curves (A) and IPCE spectra (B) of the fabricated flexible perovskite solar cells with (a) ITO-PET/Gr, (b) ITO-PET/Gr/ZnO-QDs, and (c) ITO-PET/Gr/ZnO-QDs(APjet) thin films.

434

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Reprinted with permission from [S. Ameen, 2015], J. Phys. Chem. C, 119 (2015) 10379 © 2015 American Chemical Society. Figure 8. (a) Nyquist plots of the fabricated flexible perovskite solar cells with different thin films and (b) Nyquist plots of an ITO-PET/Gr/ZnOQDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell at different voltages.

THE CHARGE COLLECTION EFFICIENCY AND PHOTOELECTRON DENSITY ANALYSIS The Intensity-Modulated Photocurrent Spectroscopy (IMPS) and the Intensity-Modulated Photovoltage Spectroscopy (IMVS) The fabricated flexible perovskite solar cells with ITO-PET/Gr/ZnO-QDs(APjet) thin film exhibits a large Rrec as compared to other fabricated solar cells. The observed large Rrec might govern few surface recombination sites by the generation of oxygenated species on ZnO-QDs via APjet treatment, resulting in high VOC. From Figure 8b, the Rrec value continuously decreases with the increase of the applied voltage, suggesting the retardation of the recombination sites. In other words, the APjet treatment of ITOPET/Gr/ZnOQDs thin film improves the interfacial contact between CH3NH3PbI3/spiroMeOTAD layer and ZnO-QDs layer which might create surface charge recombination, resulting in increased Rrec and high VOC of the device. In support, O2 plasma treatment of ITO-PET substrate and Gr coating might reduce the charge recombination rate and surplus the charge collection efficiency because the Fermi level of Gr is more positive than the conduction band energy of ZnO-QDs. The IMPS and IMVS are further studied for the characterization of diffusion transit time (τtr), the electron recombination time (τR), charge collection efficiency, and diffusion coefficient under the fixed light intensity at different voltages of light. Figure 9a,b

ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cells

435

presents the gradual decrease in IMPS and IMVS of the fabricated flexible perovskite solar cells with the increase of voltage of light from 1.0 to 2.5 V. The minimum frequency from IMPS and IMVS plots of the fabricated flexible perovskite solar cell at 2.5 V is selected to evaluate τtr and τR values [61]. The fabricated ITO-PET/Gr/ZnOQDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag solar cell demonstrates the faster τtr and smaller τR, which might attribute to less trapping sites and the recombination centers along with enhanced charge transport rate during the operation of the device, leading to high JSC and high VOC respectively. τtr and τR as a function of the incident light intensity is shown in Figure 10(a, b). The τtr and τR values gradually decrease with the increase of the photon fluxes, corresponding to the high penetration of perovskite and HTM to the closely connected ZnO-QDs(APjet) thin film. The charge collection efficiency of the fabricated flexible perovskite solar cell could be calculated by the following relation (1): [62] ηCC = 1 − τtr/τR

(1)

where τtr and τR values are estimated from IMPS and IMVS plots of the fabricated flexible perovskite solar cell [63]. From Figure 10c, the high ηCC value is appraised for the fabricated ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag solar cell, representing the high charge generation and the collection under the illumination, tending to the fast electron-transport rate and the high photocurrent density. Furthermore, the diffusion coefficient and diffusion length in the flexible perovskite solar cell could be estimated by using the following expressions: Dn = d2/2.35·τtr DL = (DnτR)1/2 where Dn is the diffusion coefficient obtained by IMPS plot, and d is the film thickness. A Dn value of ∼9.18 × 10−8 cm2·s−1 is obtained by the fabricated ITO-PET/Gr/ZnOQDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag solar cell, as presented in Figure 10d. In general, the DL defines the average distance of an electron travels before it recombines with either the absorber (perovskite) or the hole conductor [64]. In our case, the fabricated ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag solar cell exhibits a good DL value of ∼1.46 μm, which is attributed to the probability of large electrons to enter from the ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI33/spiro-MeOTAD thin film layers to the top Ag layer electrode and significantly improves the charge collection efficiency, as also evidenced in EIS and IPCE results. Therefore, the fabricated ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell presents the improved electron transport rate, high charge collection, and low DL

436

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

value, which considerably leads to achieving high J SC, VOC, and high photovoltaic performance.

Reprinted with permission from [S. Ameen, 2015], J. Phys. Chem. C, 119 (2015) 10379 © 2015 American Chemical Society. Figure 9. IMVS (a) and IMPS (b) measurement plots of the fabricated ITO-PET/Gr/ZnOQDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell.

Reprinted with permission from [S. Ameen, 2015], J. Phys. Chem. C, 119 (2015) 10379 © 2015 American Chemical Society. Figure 10. Electron transport (a), the recombination lifetime of electrons (b), charge collection efficiency (c), and the diffusion coefficient (d) of the fabricated ITO-PET/Gr/ZnOQDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell with respect to different incident photon fluxes.

ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cells

437

In spite of all the above factors, the improved surface-to-volume ratio, porosity, and the structure of ZnO-QDs by APjet treatment might intensify the adsorption capability of the perovskite molecules and facilitate high light-harvesting efficiency, resulting in high charge carrier generation and collections.

CONCLUSION For the first time, the modification of ZnO QDs in terms of surface area, pore size, and porosity has been performed using highly advanced APjet technology for the high infiltration of perovskite and HTL for the fabrication of flexible perovskite solar cells. O2 plasma treatment of ITO-PET substrate considerably enhances the hydrophilicity and surface energy of ITO-PET substrate due to the production of polar group moieties through the oxidation plasma process. The increased contact angle in ITO-PET/Gr/ZnOQDs(APjet) thin film results in improvement in the surface-to-volume ratio and the contact between hydrophilic ITO and amphiphilic Gr surfaces of thin film. The ITOPET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell attains the high conversion efficiency of ∼9.73% along with high short circuit current (JSC) of ∼16.8 mA/cm2, open circuit voltage (VOC) of ∼0.935 V, and high fill factor (FF) of ∼0.62, which is higher than other flexible perovskite solar cells fabricated with ITO-PET/Gr and ZnO-QDs/Gr/ITO-PET thin films. IMPS and IMVS studies reveal that the fabricated ITO-PET/Gr/ZnO-QDs(APjet)/CH3NH3PbI3/spiro-MeOTAD/Ag flexible perovskite solar cell shows good charge transfer rate and a reasonable recombination rate. The APjet treatment and O2 plasma treatment on ITO-PET substrate are promising approaches to enhance the surface-to-volume ratio, porosity, and structure of ZnO-QDs, which substantially intensifies the adsorption capability of the perovskite/HTM molecules and facilitates high light-harvesting efficiency. The simple fabrication process and the long-term stability open up a new avenue for the future development of low-cost photovoltaic cells.

REFERENCES [1] [2] [3] [4]

K. Liang, D. B. Mitzi, M. T. Prikas, Chem. Mater., 10 (1998) 403. H. J. Snaith, J. Phys. Chem. Lett., 4 (2013) 3623. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science, 338 (2012) 643. L. Etgar, P.Gao, Z. Xue, Q. Peng, A. K. Chandiran, B. Liu, M. K. Nazeeruddin, M. Gratzel, J. Am. Chem. Soc., 134 (2012) 17396.

438 [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al. J. Liu, Y. Wu, C. Qin, X. Yang, T. Yasuda, A. Islam, K. Zhang, W. Peng, W. Chen, L. Han, Energy Environ. Sci., 7 (2014) 2963. H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J. Moon, R. Humphry-Baker, J. H. Yum,; J. E. Moser, et al. Sci. Rep., 2 (2012) 591. C. C. Stoumpos, C. D. Malliakas, M. G. Kanatzidis, Inorg. Chem., 52 (2013) 9019. J. H. Im, I. H. Jang, N. Pellet, M. Grätzel, N. G. Park, Nat. Nanotechnol., 9 (2014) 927. D. Angmo, T. Larsen-Olsen, M. Jørgensen, R. Søndergaard, F Krebs, Adv. Energy Mater., 3 (2013) 172. F. Krebs, M. Hösel, M. Corazza, B. Roth, M. Madsen, S. Gevorgyan, R. Søndergaard, D. Karg, M. Jørgensen, Energy Technol., 1 (2013) 378. M. Durr, A. Schmid, M. Obermaier, S. Rosselli, A. Yasuda, G. Nelles, Nat. Mater., 4 (2005) 607. M. G. Kang, N. G. Park, K. S. Ryu, S. H. Chang, K. J. Kim, Sol. Energy Mater. Sol. Cells, 90 (2006) 574. B. S. Ong, C. S. Li, Y. N. Li, Y. L. Wu, R. Loutfy, J. Am. Chem. Soc., 129 (2007) 2750. Z. M. Beiley, M. D. McGehee, Energy Environ. Sci., 5 (2012) 9173. J. Burschka, Nature, 499 (2013) 316. M. Liu, M. B. Johnston, H. J. Snaith, Nature, 501 (2013) 395. D. Liu, T. L. Kelly, Nat. Photonics, 8 (2014) 133. C. H. Chao, C. H. Chan, J. J. Huang, L. S. Chang, H. C. Shih, Curr. Appl. Phys., 11 (2011) S 136. H. Rensmo, K. Keis, H. Lindstorm, S. Sodergren, A. Solbrand, A. Hagfeldt, E. Lindquist, L. N. Wang, M. Muhammed, J. Phys. Chem. B, 101 (1997) 2598. R. Chakravarty, C. Periasamym, Sci. Adv. Mater., 3 (2011) 276. I. Robel, V. Subramanian, M. Kuno, P. V. Kamat, J. Am. Chem. Soc., 128 (2006) 2385. G. Rani, P. D. Sahare, J. Mater. Sci. Technol., 29 (2013) 1035. J. H. Chen, C. Jang, S. Xiao, M. Ishigami, M. S. Fuhrer, Nat. Nanotechnol., 3 (2008) 206. A. K. Geim, K. S. Novoselov, Nat. Mater., 6 (2007) 183. S. V Morozov, K. S. Novoselov, M. I. Katsnelson, F. Schedin, D. C. Elias, J. A. Jaszczak, A. K. Geim, Phys. Rev. Lett., 100 (2008) 016602. X. Du, I. Skachko, A. Barker, E. Y. Andrei, Nat. Nanotechnol., 3 (2008) 491. O. Akhavan, Carbon, 48 (2010) 509. H. K. Seo, M. Song, S. Ameen, M. S. Akhtar, H. S. Shin, Chem. Eng. J., 222 (2013) 464. F. Liu, H. Luo, Y. Wu, C. Liu, J. Zhang, Adv. Funct. Mater., 18 (2008) 1518. G. Eda, M. Chhowalla, Nano Lett., 9 (2009) 814.

ZnO Quantum Dots Thin Film for Flexible Perovskite Solar Cells

439

[31] S. Watcharotone, D. A. Dikin, S. Stankovich, R. Piner, I. Jung, G. H. B. Dommett, G. Evmenenko, S. E. Wu, S. Chen, C. P. Liu, et al, Nano Lett., 7 (2007) 1888. [32] D. Shao, M. Yu, H. Sun, T. Hu, J. Lian, S. Sawyer, Nanoscale, 5 (2013) 3664. [33] D. Shao, M. Yu, J. Lian, S. Sawyer, Nanotechnology, 24 (2013) 295701. [34] J. O. Hwang, D. H. Lee, J. Y. T. Kim, H. Han, B. H. Kim, M. Park, K. No, S. O. Kim, J. Mater. Chem., 21 (2011) 3432. [35] Y. Yang, T. Liu, Appl. Surf. Sci., 257 (2011) 8950. [36] H. X. Chang, Z. H. Sun, K. Y. F. Ho, X. M. Tao, F. Yan, W. M. Kwok, Z. J. A. Zheng, Nanoscale, 3 (2011) 258. [37] S. Ameen, M. S. Akhtar, H. K. Seo, H. S. Shin, Mater. Lett., 136 (2014) 379. [38] S. Ameen, M. S. Akhtar, H. S. Shin, Actuators B: Chem., 173 (2012) 177. [39] S. Ameen, M. S. Akhtar, H. K. Seo, H. S. Shin, Langmuir, 30 (2014) 12786. [40] G. Borcia, C. A. Anderson, N. M. D. Brown, Plasma Sources Sci. Technol., 12 (2003) 335. [41] R. R. Deshmukh, N. V. Bhat, Mater. Res. Innovat., 7 (2003) 283. [42] R. Morent, N. D. Geyter, C. Leys, Nucl. Instru. Meth. Phys. Res. B, 266 (2008) 308. [43] J. M. Park, Z. J. Wang, D. J. Kwon, G. Y. Gu, K. L. DeVries, Solid-State Electron., 79 (2013) 147. [44] L. Wan, S. Wang, X. Wang, B. Dong, Z. Xu, X. Zhang, B. Yang, S. Peng, J. Wang, C. Xu, Solid State Sci., 13 (2011) 468. [45] S. J. Yumitori, J. Mater. Sci., 35 (2000) 139. [46] D. Fu, G. Han, Y. Chang, J. Dong, Mater. Chem. Phys., 132 (2012) 673. [47] X. Lin, J. Jia, N. Yousefi, X. Shen, J. K. Kim, J. Mater. Chem. C, 1 (2013) 6869. [48] J. C. C. Fan, J. B. Goodenough, J. Appl. Phys., 48 (1977) 3524. [49] W. J. Li, E. W. Shi, W. Z. Zhong, Z. W. Yin, J. Cryst. Growth, 203 (1999) 186. [50] Z. G. Wang, Y. F Chen, P. J. Li, X. Hao, Y. Fu, K. Chen, L. X. Huang, D. Liu, Vacuum, 86 (2012) 895. [51] H. An, W. J. Lee, J. Jung, Curr. Appl. Phys., 11 (2011) S81. [52] A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, A. K. Geim, Phys. Rev. Lett., 97 (2006) 187401. [53] L. M. Malard, M. A. Pimenta, G. Dresselhaus, M. S. Dresselhaus, Phys. Rep., 473 (2009) 51. [54] J. M. Velasco, N. Kelaidis, E. Xenogiannopoulou, Y. S. Raptis, D. Tsoutsou, P. Tsipas, T. Speliotis, G. Pilatos, V. Likodimos, P. Falaras, A. Dimoulas, Appl. Phys. Lett., 103 (2013) 213108. [55] F. Friedrich, N. H. Nickel, Appl. Phys. Lett., 91 (2007) 111903. [56] A.Usami, Chem. Phys. Lett., 277 (1997) 105. [57] N. P. Apageorgiou, P. Liska, A. Kay, M. Gratzel, J. Electrochem. Soc., 146 (1999) 898.

440

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

[58] H. J. Snaith, N. C. Greenham, R. H. Friend, Adv. Mater., 2004, 16, 1640−1645. [59] I.Mora-SerÓ, J. Bisquert, F. Fabregat-Santiago, G. Garcia-Belmonte, Nano Lett., 6 (2006) 640. [60] D. Shao, X. Sun, M. Xie, H. Sun, F. Lu, S. M. George, J. Lian, S. Sawyer, Mater. Lett., 112 (2013) 165. [61] K. Zhu, N. R. Neale, A. Miedaner, A. J. Frank, Nano Lett., 7 (2006) 69. [62] L. Bertoluzzi, S. Ma, Phys. Chem. Chem. Phys., 15 (2013) 4283. [63] R. Katoh, M. Kasuya, S. Kodate, A. Furube, N. Fuke, N. Koide, J. Phys. Chem. C, 113 (2009) 20738. [64] A. J. Frank, N. Kopidakis, J. van de Lagemaat, Coord. Chem. Rev., 248 (2004) 1165.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 31

RF SPUTTERED TI LAYER FOR FLEXIBLE PEROVSKITE SOLAR CELLS Sadia Ameen1, M. Shaheer Akhtar2, Hyung-Kee Seo1, Mohammad Khaja Nazeeruddin3 * and Hyung Shik Shin1,† 1

Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New and Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea 3 Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland

ABSTRACT In this work, the effects of a titanium (Ti) layer on the charge transport and recombination rates of flexible perovskite solar cells are studied. Ti as an efficient barrier layer is deposited directly on PET-ITO flexible substrates through RF magnetic sputtering using a Ti-source and a pressure of ∼5 mTorr. A Ti coated PET-ITO is used for the fabrication of a flexible perovskite solar cell without using any metal oxide layer. The fabricated flexible perovskite solar cell is composed of a PET-ITO/Ti/perovskite (CH3NH3PbI3)/organic hole transport layer of 2,2’,7,7’-tetrakis [N,N’-di-pmethoxyphenylamine]-9,9’-spirobifluorene (spiro-OMeTAD)-Li-TFSI/Ag. A high conversion efficiency of ∼8.39% along with a high short circuit current (JSC) of ∼15.24 mA cm−2, an open circuit voltage (VOC) of ∼0.830 V and a high fill factor (FF) of ∼0.66 * †

Corresponding Author Email: [email protected]. Corresponding Author Email: [email protected]

442

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al. are accomplished by the fabricated flexible perovskite solar cell under a light illumination of ∼100 mWcm−2 (1.5 AM). Intensity-modulated photocurrent (IMPS)/photovoltage spectroscopy (IMVS) studies demonstrates that the fabricated flexible perovskite solar cell considerably reduces the recombination rate.

INTRODUCTION The emerging thin film photovoltaics based on organometallic lead perovskite materials are attracting tremendous attention owing to their high conversion efficiency and simple fabrication procedure [1, 2]. Several groups have focused on improving the light absorption and charge transporting properties of the perovskite sensitized mesoporous network [1, 3]. Organometallic perovskite materials are ambipolar and exhibit a long charge carrier lifetime with high charge carrier mobilities (∼50 cm2 V−1 s−1) [4–11]. The opto-electronic properties of organometallic perovskites can be easily tuned by changing the chemical compositions in terms of using different alkyl groups, metal atoms and halides [3, 12, 13] Light weight photovoltaic devices like flexible solar cells have received a great deal of attention due to their low cost production, variable shapes and large-scale roll-to-roll processing which are required for the industrial production of OPV materials [14, 15]. Several flexible substrates such as poly(ethylene) terephthalate (PET)/indium tin oxide (ITO) and metal sheets are commonly used for the fabrication of flexible solar cells [16, 17]. Moreover, titanium (Ti) foils and Ti meshes have already been utilized to manufacture large-area flexible solar cells because of their high flexibility, relatively low sheet resistance and superior corrosion resistance [18, 19]. Recently, Ti thin film has received much attention for excellent applications in microelectronics, machinery, aerospace, and medical industry owing to its remarkable photoelectric performances [20]. In general, a Ti thin film has been extensively grown on various substrates using the deposition techniques of atomic layer deposition (ALD), chemical vapor deposition (CVD) and electrochemical techniques like chemical bath deposition [21]. The deposition of a Ti thin film through the sputtering method is highly dense and shows good homogeneity with great reproducibility in a large area because of the high-energy bombarding particles [20] therefore, the sputtering method is a promising low temperature deposition technique for depositing a Ti layer on various substrates. In particular, a Ti thin film as a barrier layer offers the fabrication of light weight highperformance perovskite solar cells using different hole transport materials (HTMs). Conducting polymers such as polypyrrole [22, 23], polyaniline [24, 25], and poly(3,4ethylenedioxythiophene) (PEDOT) [26, 27] are substantially used as HTMs in perovskite solar cells owing to their low cost, good stability, and simple easy preparation of designable structures. Nevertheless, the efficiencies of perovskite solar cells using the

RF Sputtered Ti Layer for Flexible Perovskite Solar Cells

443

above HTMs are significantly lower when compared to the efficiency achieved with spiro-OMeTAD as the HTM. In the present work, we report the fabrication of a metal oxide free flexible perovskite solar cell using RF sputtered Ti as the barrier layer, CH3NH3PbI3 perovskite as the light absorber and sipro-OMeTAD as the HTM. The metal oxide free flexible perovskite solar cell of the structure PET-ITO/Ti/CH3NH3PbI3/spiroOMeTAD/Ag presents a reasonably high solar-to-electric conversion efficiency of ∼ 8.39% with high photocurrent density and open-circuit potential. An incident-photon-tocurrent efficiency (IPCE) of ∼66% in the wavelength range of ∼450–700 nm is achieved. The charge transport and reduced recombination processes of the fabricated flexible perovskite solar cells have been studied by intensity-modulated photocurrent/ photovoltage spectroscopy (IMPS/IMVS).

MORPHOLOGICAL STUDIES OF TI THIN FILM The Field Emission Scanning Electron Microscopy The surface properties of the deposited Ti thin films on PET-ITO substrates have been examined by analyzing the field emission scanning electron microscopy (FESEM) images, as shown in Figure 1(b–d).

Reprinted with permission from [S. Ameen, 2015], Dalton Trans., 44 (2015) 6439 © 2015 The Royal Society of Chemistry. Figure 1. (a) Schematic representation of the fabricated PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag flexible perovskite solar cell. FESEM images of Ti thin films with a deposition time of (b) 15 min, (c) 21 min and (d) 30 min.

444

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

From FESEM observations, a uniform and porous morphology is observed for all Ti deposited thin film substrates. It is noticed that the grain size of Ti slightly increases with the increase in deposition time. In particular, the deposition of a Ti thin film for 30 min over an ITO-PET substrate, as shown in Figure 1(d) displays a highly uniform, porous and well organized grain morphology.

Atomic Force Microscopy The topographic and three dimensional (3D) atomic force microscopy (AFM) images of PET-ITO/Ti and PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD with Ti thickness of ∼ 100 nm are analyzed to understand the morphological features of a thin film, as shown in Figure 2.

Reprinted with permission from [S. Ameen, 2015], Dalton Trans., 44 (2015) 6439 © 2015 The Royal Society of Chemistry. Figure 2. Topographic (a, c) and three dimensional (b, d) atomic force microscopy (AFM) images of PET-ITO/Ti and PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD with a Ti thickness of ∼100 nm.

The topographic AFM image (Figure 2(a)) of a PET-ITO/Ti thin film shows highly dense and uniform small particles over PET-ITO, indicating that Ti atoms are well adhered to the flexible PET-ITO substrate. The highly rough surface of PET-ITO/Ti is seen in a 3D AFM image (Figure 2(b)). From Figure 2(c), the interconnected homogeneously mixed thin film of nanosized CH3NH3PbI3 grains/spiro-OMeTAD and Ti grains over a flexible PET-ITO substrate is visibly seen, which suggests that the

RF Sputtered Ti Layer for Flexible Perovskite Solar Cells

445

CH3NH3PbI3 grains/spiro-OMeTAD are well mixed with Ti grains in the deposited Ti layer, as depicted in the 3D AFM image (Figure 2(d). From a roughness analysis, a PETITO/Ti/CH3NH3PbI3/spiro-OMeTAD thin film presents a lower root mean roughness (Rrms) of ∼8.9 nm than that of the Rrms (∼23.4 nm) of a PET-ITO/Ti thin film. It could be seen that the lower roughness might arise due to the good covering of CH3NH3 PbI3/spiro-OMeTAD over the barrier layer.

THE TRANSMITTANCE OF THIN FILM SUBSTRATES Transmittance studies are conducted to investigate the transparency of a deposited Ti layer and a Ti/CH3NH3PbI3 layer on the PET-ITO substrates. Figure 3 depicts the transmittance spectra of bare PET-ITO, PET-ITO/Ti and PET-ITO/Ti/CH3NH3PbI3 thin films. The bare PET-ITO substrate displays ∼82% transmittance in the absorbance range of the visible region, which is consistent with the reported transparency of the PET-ITO substrate. The transmittance decreases to half as compared to bare PET-ITO and shows a slight edge shift after the deposition of a Ti layer by RF magnetic sputtering which suggests quite a transparent nature of the PET-ITO/Ti thin film. Furthermore, the deposition of CH3NH3PbI3 on PET-ITO/Ti records a significant fall in transmittance to ∼11% in the visible region, indicating the interaction of perovskite with the Ti layer on the PET-ITO substrate.

Reprinted with permission from [S. Ameen, 2015], Dalton Trans., 44 (2015) 6439 © 2015 The Royal Society of Chemistry. Figure 3. UV-vis spectra at the transmittance mode of bare PET-ITO, PET-ITO/Ti and PETITO/Ti/CH3NH3PbI3 substrates with a Ti thickness of ∼100 nm.

446

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

STRUCTURAL CHARACTERIZATIONS OF THIN FILM SUBSTRATE X-Ray Photoelectron Spectroscopy The X-ray photoelectron spectrum (XPS) has been analyzed to explain the existence of chemical species in the PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD thin film, as shown in Figure 4. Figure 4(a) shows the doublet Ti 2p XPS spectrum of the PET-ITO/ Ti/CH3NH3PbI3/spiro-OMeTAD thin film. The doublet binding energies at ∼458.2 eV and ∼463.8 eV are ascribed to Ti 2p3/2 and Ti 2p1/2 respectively, confirming the existence of Ti species over the surface of the thin film.28 The resolved C 1s XPS (Figure 4(b)) is composed of four resolved binding energies with a center binding energy. The resolved binding energies at ∼284.9 eV and ∼285.9 eV are assigned to the C–C/C–H and C– N+/C=N+ bonds bonding in spiro-OMeTAD and perovskite respectively.29 The higher binding energy at ∼287.2 eV might represent the spiro carbon or the oxidized form of spiro-OMeTAD in the presence of TFSI. Moreover, the carbon to metal bonding is seen due to the appearance of binding energy at ∼288.7 eV, confirming the interaction of carbon species with Pb species. N 1s XPS explains the nature of N element in the PETITO/Ti/CH3NH3PbI3/spiro-OMeTAD thin film, as shown in Figure 4(c). The binding energy at ∼400.1 eV originates from pure spiro-OMeTAD which is similar to the measurements of multilayers of another triphenylamine-based hole conducting molecule [30]. The occurrence of binding energy at ∼398.4 eV is due to N–H/amino bonding in the perovskite layer of the thin film. The higher binding energy at ∼404.1 eV might express the existence of a bond formation between NH3 and Pb salt. Figure 4(d) shows Pb 4f XPS of the PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD thin film and records two binding energies at ∼138.1 eV and ∼143.1 eV, representing Pb 4f7/2 and Pb 4f5/2 levels respectively. A spin–orbit split between Pb 4f7/2 and Pb 4f5/2 is recorded at ∼5.00 eV, which is similar to the perovskite as reported in the literature [31]. The metallic (Pb 2+) binding energy peak has been confirmed in the perovskite from Pb 4f spectrum, indicating the non-existence of Pb0 linked to iodide (I). Furthermore, iodine nature in the thin film has been analyzed by I 3d XPS, as shown in Figure 4(e). Doublet binding energies at ∼619.2 eV and ∼630.5 eV are obtained, corresponding to I 3d5/2 and I 3d3/2 core levels respectively. A spin–orbit split of ∼11.3 eV is estimated from I 3d XPS, similar to the reported literature [32]. The spin–orbit split binding energies of Pb 4f and I 3d core levels are almost similar to the previously reported literature on perovskite materials [32], confirming the Pb2+ chemical state along with I− in the prepared thin film.

RF Sputtered Ti Layer for Flexible Perovskite Solar Cells

447

Reprinted with permission from [S. Ameen, 2015], Dalton Trans., 44 (2015) 6439 © 2015 The Royal Society of Chemistry. Figure 4. Ti 2p (a), C 1s (b), N 1s (c), Pb 4f (d) and I 3d (e) X-ray photoelectron spectra of a PETITO/Ti/CH3NH3PbI3/spiro-OMeTAD thin film.

THE PERFORMANCE OF THE FLEXIBLE PEROVSKITE SOLAR CELL Current Density (J)–Voltage (V) Measurements and the Incident Photon-toCurrent Conversion Efficiency The photovoltaic properties of the fabricated flexible perovskite solar cells (PETITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag) are analyzed by taking the current density (J)–

448

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

voltage (V) characteristics under a light intensity of ∼100 mW cm−2 (1.5 AM). From Figure 5(A), the fabricated flexible PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag attains a reasonably high solar-to-electric conversion efficiency of ∼8.39% with a high short circuit current density (JSC) of ∼15.24 mA cm−2, a high open circuit voltage (VOC) of ∼ 0.830 V and a fill factor (FF) of ∼0.66. Table 1 shows the photovoltaic parameters of the flexible perovskite solar cells fabricated with different thicknesses of the Ti layer on the ITO-PET substrate. It is seen that low conversion efficiencies are observed when solar cells are fabricated with ∼50 and ∼75 nm thicknesses of the Ti layer.

Reprinted with permission from [S. Ameen, 2015], Dalton Trans., 44 (2015) 6439 © 2015 The Royal Society of Chemistry. Figure 5. (A) J–V curves of (a) 50 nm, (b) 75 nm and (c) 100 nm Ti thickness based flexible perovskite solar cells. (B) IPCE curve of the fabricated PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag flexible perovskite solar cell with a Ti thickness of ∼100 nm.

As seen in the AFM images, the low roughness of the PET-ITO/Ti/CH3NH3PbI3/ spiro-OMeTAD thin film and good penetration of CH3NH3PbI3/spiro-OMeTAD to the Ti layer results in the high performance of the flexible perovskite solar cell. Moreover, the

RF Sputtered Ti Layer for Flexible Perovskite Solar Cells

449

direct contact of PET-ITO substrate with the perovskite/HTM might be suppressed due to the introduction of Ti layer on ITO-PET, which might increase the charge recombination rate and enhance the charge collection efficiency [33]. It is believed that high charge carriers might be generated and collected over the CH3NH3PbI3 layer through the interface of the compact blocking Ti layer, and may result in high photocurrent density. The incident photon to current conversion efficiency (IPCE) of a PETITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag flexible perovskite solar cell has been examined to deduce the high JSC and light harvesting efficiency. Figure 5(B) shows a high IPCE of ∼66% in a broad adsorption wavelength in the range of ∼450–700 nm obtained by the fabricated PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag flexible perovskite solar cell. The high IPCE is attributed to the high JSC of the solar cell, as presented in Figure 5(A). In this case, the introduction of a Ti layer might significantly improve the interfacial contact between CH3NH3PbI3/spiro-OMeTAD PET-ITO substrates, which significantly magnifies the light scattering capacities, photon absorption, and produces large photocurrent.

Electrochemical Impedance Measurements Figure 6(a) shows the Nyquist plot measurements of the fabricated flexible PETITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag with a Ti thickness of ∼100 nm elucidating the characteristics of charge transfer and the recombination rate. Impedance measurements were carried out at different applied voltages with a frequency range from 100 kHz to 1 Hz under dark conditions. The fabricated flexible PET-ITO/Ti/CH3NH3PbI3/spiroOMeTAD/Ag shows a single semicircular arc at a lower frequency, corresponding to the combination of the recombination resistance (Rrec) and the chemical capacitance of the film (Cμ). The appearance of a single semicircular arc is due to the low electron transport through ITO-PET and the Ti layer, which results in the absence of a small semicircular arc at a higher frequency [34]. The inset of Figure 6(a) describes the corresponding equivalent circuit, which is composed of resistance related to the diffusion of holes through HTM (R1), in parallel with HTM capacitance (C1), and a recombination resistance (Rrec) at a lower frequency with a chemical capacitance, (Cμ) related to the electron Fermi level in spiro-OMeTAD [35]. From Figure 6(a), a single semicircle originates due to the dominance of the recombination resistance (Rrec) at the interface of HTM and the perovskite layer with the chemical capacitance of the film (Cμ) [36]. Rrec is usually reciprocal to the recombination rate [34]. The high Rrec could be explained in two possible ways: (i) bad charge extraction enhances the charge carrier density at the bulk material and (ii) recombination sites are generated at the interfacial contacts. It has been reported that low Rrec at high applied voltages is explicitly associated with the low VOC of solar cells [37]. In our case, the

450

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

deposition of a Ti layer on a flexible perovskite solar cell exhibits high Rrec which might arise from the charge recombination of the injected electron and an electron acceptor at the interface of the top contact and the CH3NH3PbI3/spiro-OMeTAD layer, resulting in an enhanced VOC of the device. Moreover, Figure 6(b) depicts the corresponding Nyquist plot in the phase mode of imaginary parts versus frequency. The appearance of a broad phase towards the higher frequencies represents a reduction in the recombination sites at the interface of the Ti/perovskite and the HTM layer, indicating a significant improvement in the photovoltage.

Reprinted with permission from [S. Ameen, 2015], Dalton Trans., 44 (2015) 6439 © 2015 The Royal Society of Chemistry. Figure 6. (a) Nyquist plots, (b) corresponding bode phase plots and inset shows the equivalent circuit diagram of the fabricated PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag flexible perovskite solar cell with a Ti thickness of ∼100 nm.

The measurements of impedance with respect to bias voltage have been performed to explain the recombination rates in a PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag flexible perovskite solar cell with a Ti thickness of ∼100 nm. Figure 7(a) shows the plot of change in Rrec with respect to bias voltage. It is seen that the values of Rrec are decreasing with increasing voltage, similar to previous literature [38, 39]. Noticeably, in a PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag solar cell, the high Rrec at a lower voltage clearly supports the high VOC. The conductivity of spiro-OMeTAD (HTM) has been evaluated from the following expression: σHTM = L/RHTM

where L is the half of the sum of HTM thickness and Ti thickness [40]. The conductivity of HTM increases with the increase in bias voltage (Figure 7(b)), indicating the good conductive nature of HTM in the fabricated device. At a low voltage, the loss of conductivity might due to the presence of few recombination sites during the hole

RF Sputtered Ti Layer for Flexible Perovskite Solar Cells

451

transport process. In our case, a good DL value of ∼1.169 μm is ascribed to the high recombination resistance or lower recombination rate, as evidenced by Figure 7(a). The obtained DL value clearly depicts a great chance for a large number of electrons to enter from the PET-ITO/Ti/CH3NH3PbI3thin film layers to the top Ag layer by improving the charge collection at a lower light intensity. Herein, the high efficiency of the PETITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag perovskite solar cell might be credited to high JSC and VOC which are attributed to high transport and low recombination rates.

Reprinted with permission from [S. Ameen, 2015], Dalton Trans., 44 (2015) 6439 © 2015 The Royal Society of Chemistry. Figure 7. Recombination resistance, Rrec (a) and hole transport material conductivity, σHTM (b) of the fabricated PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag flexible perovskite solar cell with a Ti thickness of ∼100 nm at different bias potentials.

THE CHARGE COLLECTION EFFICIENCY AND PHOTOELECTRON DENSITY ANALYSIS The Intensity-Modulated Photocurrent Spectroscopy (IMPS) and IntensityModulated Photovoltage Spectroscopy (IMVS) Intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) have been studied to further explore the electron transport and recombination properties of the fabricated flexible perovskite solar cell at different voltages of light and different photon fluxes. Figure 8 shows the IMVS and IMPS of the fabricated flexible perovskite solar cell at different voltages of light. The IMPS and IMVS of the device decreases with an increase in the voltage of light from 0.5 V to 2.0 V. At the maximum voltage of light (2.0 V), the charge-transport time (τCT) and electron recombination time (τR) have been estimated by selecting the minimum

452

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

frequencies from the IMPS and IMVS plots of the fabricated flexible perovskite solar cell. The τCT and τR values are calculated by the following equations: [40] τCT = 1/2πfmin(IMPS)

τR = 1/2πfmin(IMVS)

where fmin is the characteristic minimum frequency of the IMPS and IMVS plots [41] The fabricated flexible perovskite solar cell presents a better τCT of ∼15.9 ms and τR of ∼204 ms, suggesting that the flexible perovskite solar cell presents an enhanced charge transport rate and lower the recombination rate during the operation. In general, longer DL could lead to higher charge collection and light-harvesting efficiencies for achieving a high conversion efficiency. The charge collection efficiency of the fabricated flexible perovskite solar cell is determined by the following relation: [42] ηCC = (1-τCT/τR)

Reprinted with permission from [S. Ameen, 2015], Dalton Trans., 44 (2015) 6439 © 2015 The Royal Society of Chemistry. Figure 8. IMVS (a) and IMPS (b) measurement plots of the fabricated PET-ITO/Ti/CH3NH3PbI3/spiroOMeTAD/Ag flexible perovskite solar cell with a Ti thickness of ∼100 nm.

RF Sputtered Ti Layer for Flexible Perovskite Solar Cells

453

Reprinted with permission from [S. Ameen, 2015], Dalton Trans., 44 (2015) 6439 © 2015 The Royal Society of Chemistry. Figure 9. The electron transport (a), the recombination lifetime of electrons (b) and charge collection efficiencies (c) of the fabricated PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag flexible perovskite solar cell with a Ti thickness of ∼100 nm versus different incident photon fluxes.

Furthermore, the order of τCT and τR values with respect to photon fluxes is presented in Figure 9. The increase in photon flux results in the decrease in the τCT and τR values, as shown in Figure 9(a and b). The low τCT and τR values originate from the uniform deposition of CH3NH3PbI3/spiro-OMeTAD on PET-ITO/Ti and good interfacial contact between Ti and CH3NH3PbI3/spiro-OMeTAD layers which might increase the electron transport by making it difficult for electrons to recombine with holes and hence, decrease the recombination. On the other hand, the electron diffusion length in the flexible perovskite solar cell could be determined by using the expression: DL = (Dn .τR)1/2 where Dn is the diffusion coefficient which is obtained by the IMPS plot [42]. A Dn value of ∼6.7 × 10−8 cm2 s−1 has been estimated for the fabricated flexible perovskite solar cells. It is known that DL represents the average distance an electron travels before it recombines with either the absorber (perovskite) or the hole conductor (spiro-OMeTAD)

454

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

[43]. Figure 9(c) shows that the fabricated PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag flexible perovskite solar cell obtains a high ηCC value which considerably supports the high charge carrier generation and collection during the operation of the device, resulting in a fast electron-transport rate and high photocurrent density. Literature has shown that the Ti metal acts as a barrier layer at the interface of TCO/metal due to its high work function properties and high electrical conductivity, as observed in the Ti/TiO2 electrode in DSSCs [44]. In support, the introduction of a metal with good conductivity is basically responsible for the fast transport of the injected electrons to the external circuit of the device, which efficiently improves the charge collection [45]. Similarly in our case, the deposition of Ti as a barrier layer with a thickness of ∼100 nm has improved the electron transport rate, exhibits high charge collection and depicts a reasonable DL value, resulting in the high performance of the flexible PET-ITO/Ti/CH3NH3PbI3/spiro-OMeTAD/Ag perovskite solar cell.

CONCLUSION The effect of a titanium (Ti) layer on the charge transport and recombination rates in flexible perovskite solar cells has been studied, in which the Ti layer as an efficient barrier layer is deposited on PET-ITO flexible substrates through RF magnetic sputtering using a Ti-source and a pressure of ∼5 mTorr. The deposited Ti coated PET-ITO is directly used for the fabrication of a flexible perovskite solar cell without using any metal oxide layer. The fabricated perovskite solar cell with a Ti thickness of ∼100 nm shows a high conversion efficiency of ∼8.39% along with a high short circuit current (J SC) of ∼ 15.24 mA cm−2, an open circuit voltage (VOC) of ∼0.830 V and a high fill factor (FF) of ∼0.66 under a light illumination of ∼100 mW cm−2 (1.5 AM). However, low conversion efficiencies are observed for the fabricated flexible solar cells with ∼50 nm and ∼75 nm Ti thicknesses. The enhanced VOC and FF might be attributed to the improvement in the interfacial contact between the Ti layer and the spiro-OMeTAD-Li-TFSI layer. IMPS and IMVS studies reveal a better charge transport time, efficient diffusion coefficient, diffusion length and a high charge collection efficiency of the fabricated flexible perovskite solar cell, resulting from the improvement in the interfacial contact between the Ti conduction layer and the hole transporting layer. Thus, thickness of the Ti layer is crucial to achieve high performance of the flexible PET-ITO/Ti/CH3NH3PbI3/spiroOMeTAD/Ag perovskite solar cell along with an improved electron transport rate, high charge collection, and quite a high DL value.

RF Sputtered Ti Layer for Flexible Perovskite Solar Cells

455

REFERENCES [1] [2] [3] [4] [5] [6]

[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

I. Chung, B. Lee, J. He, R. P. H. Chang, M. G. Kanatzidis, Nature, 485 (2012) 486. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science, 338 (2012) 643. J. M. Ball, M. M. Lee, A. Hey, H. J. Snaith, Energy Environ. Sci., (2013) 1739. W. A. Laban L. Etgar, Energy Environ. Sci., 6 (2013) 3249. J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Gratzel, Nature, 499 (2013) 316. J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C. S. Lim, J. A. Chang, Y. H. Lee, H. J. Kim, A. Sarkar, M. K. Nazeeruddin, M. Grätzel, S. I. Seok, Nat. Photonics, 7 (2013) 486. J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, S. I. Seok, Nano Lett., 13 (2013) 1764. E. Edri, S. Kirmayer, D. Cahen, G. Hodes, J. Phys. Chem. Lett., 4 (2013) 897. L. Etgar, P. Gao, Z. Xue, Q. Peng, A. K. Chandiran, B. Liu, M. K. Nazeeruddin, M. Grätzel, J. Am. Chem. Soc., 134 (2012) 17396. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami H. J. Snaith, Science, 338 (2012) 643. H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J. Moon, R. Humphry-Baker, J. H. Yum J. E. Moser, Sci. Rep., 2 (2012) 591. H. S. Jung, J. K. Lee, J. Phys. Chem. Lett., 4 (2013) 1682. L. Chaorong, W. Jing, Z. Yingying, D. Wenjun, Mater. Lett., 76 (2012) 187. D. Angmo, T. Larsen-Olsen, M. Jørgensen, R. Søndergaard, F. Krebs, Adv. Energy Mater., 3 (2013) 172. F. Krebs, M. Hösel, M. Corazza, B. Roth, M. Madsen, S. Gevorgyan, R. Søndergaard, D. Karg, M. Jørgensen, Energy Technol., 1 (2013) 378. M. Durr, A. Schmid, M. Obermaier, S. Rosselli, A. Yasuda, G. Nelles, Nat. Mater., 4 (2005) 607. M. G. Kang, N. G. Park, K. S. Ryu, S. H. Chang, K. J. Kim, Sol. Energy Mater. Sol. Cells, 90 (2006) 574. Y. Xiao, J. Wu, G. Yue, J. Lin, M. Huang, L. Fan Z. Lan, Electrochim. Acta, 58 (2011) 621. Y. Xiao, J. Wu, G. Yue, J. Lin, M. Huang, L. Fan, Z. Lan, J. Power Sources, 208 (2012) 197. Y. Tang, J. Tao, Y. Zhang, T. Wu, H. Tao, Z. Bao, Acta Phys. Chim., 4 (2008) 2191. H. U. Igwe, O. E. Ekpe, E. I. Ugwu, J. Appl. Sci. Eng. Technol., 2 (2010) 447. J. Wu, Q. Li, L. Fan, Z. Lan, P. Li, J. Lin S. Hao, J. Power Sources, 181 (2008) 172.

456

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

[23] Y. Guo, Y. Zhang, H. Liu, S. Lai, Y. Li, Y. Li, W. Hu, S. Wang, C. Che, D. Zhu, J. Phys. Chem. Lett., 1 (2010) 327. [24] J. Gong, Y. Li, Z. Hu, Z. Zhou, Y. Deng, J. Phys. Chem. C, 114 (2010) 9970. [25] K. Huang, J. Huang, C. Wu, C. Liu, H. Chen, C. Chu, J. Lin, C. Lin, K. Ho, J. Mater. Chem., 21 (2011) 10384. [26] Y. Xiao, J. Wu, J. Lin, G. Yue, J. Lin, M. Huang, Z. Lan, L. Fan, J. Power Sources, 241 (2013) 373. [27] J. Koh, J. Kim, B. Kim, J. Kim, E. Kim, Adv. Mater., 23 (2011) 1641. [28] Z. Song, J. Hrbek, R. Osgood, Nano Lett., 5 (2005) 1327. [29] R. Scholin, M. H. Karlsson, S. K. Eriksson, H. Siegbahn, E. M. J. Johansson, H. Rensmo, J. Phys. Chem. C, 116 (2012) 26300. [30] E. M. J. Johansson, M. Odelius, P. G. Karlsson, H. Siegbahn, A. Sandell, H. Rensmo, J. Chem. Phys., 128 (2008) 184709. [31] J. Moulder, W. Stickle, P. Sobol and K. Bomben, Handbook of Xray Photoelectron Spectroscopy, Physical Electronics Division, Eden Prairie, MN, 1995. [32] A. Roy, D. D. Sarma, A. K. Sood, Spectrochim. Acta, Part A, 48 (1992) 1779. [33] F. Y. Ouyang, W. L. Tai, Appl. Surf. Sci., 276 (2013) 563. [34] H. S. Kim, I. Mora-Sero, V. Gonzalez-Pedro, F. Fabregat-Santiago, E. J. JuarezPerez, N. G. Park, J. Bisquert, Nat. Commun., 4 (2013) 2242. [35] J. A. Christians, R. C. M. Fung, P. V. Kamat, J. Am. Chem. Soc., 136 (2014) 758. [36] I. Mora-Seró, J. Bisquert, F. Fabregat-Santiago, G. Garcia-Belmonte, K. Y. D. Zoppi, Y. Proskuryakov, I. Oja, A. Belaidi, T. Dittrich, R. Tena-Zaera, A. Katty, C. Lévy-Clement, V. Barrioz, S. J. C. Irvine, Nano Lett., 6 (2006) 640. [37] W. A. Laban, L. Etgar, Energy Environ. Sci., 6 (2013) 3249. [38] F. Fabregat-Santiago, G. Garcia-Belmonte, I. Mora-Seró J. Bisquert, Phys. Chem. Chem. Phys., 13 (2011) 9083. [39] P. P. Boix, G. Larramona, A. Jacob, B. Delatouche, I. Mora-Seró, J. Bisquert, J. Phys. Chem. C, 116 (2012) 1579. [40] K. Zhu, N. R. Neale, A. Miedaner, A. J. Frank, Nano Lett., 7 (2006) 69. [41] H. X. Wang, P. G. Nicholson, L. Peter, S. M. Zakeeruddin, M. Gratzel, J. Phys. Chem. C, 114 (2010) 14300. [42] R. Katoh, M. Kasuya, S. Kodate, A. Furube, N. Fuke, N. Koide, J. Phys. Chem. C, 113 (2009) 20738. [43] A. J. Frank, N. Kopidakis J. van de Lagemaat, Coord. Chem. Rev., 248 (2004) 1165. [44] N. Fuke, A. Fukui, Y. Chiba, R. Komiya, R. Yamanaka, L. Han, Jpn. J. Appl. Phys., 46 (2007) L420. [45] S. Chappel, L. Grinis, A. Ofir, A. Zaban, J. Phys. Chem. B, 109 (2005) 1643.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 32

CONDUCTING CHANNELS OF THE HOLE TRANSPORTING LAYER TO ADJACENT PHOTOACTIVE PEROVSKITE SENSITIZED TIO2 THIN FILMS FOR A SOLAR CELL Sadia Ameen1, M. Shaheer Akhtar2, Hyung-Kee Seo1 and Hyung Shik Shin1,* 1

Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT A high performance perovskite solar cell is fabricated using the distinguished morphology of polyaniline nanoparticles (PANI-NPs) as an efficient hole transporting layer (HTL) with methylammonium lead iodide perovskite (CH3NH3PbI3) as sensitizer. PANI-NPs are simply synthesized by the oxidative chemical polymerization of aniline monomer at 0−5°C. A reasonable solar-to-electricity conversion efficiency of ∼6.29% with a high short circuit current (JSC) of ∼17.97 mA/cm2 and open circuit voltage (VOC) of ∼0.877 V are accomplished by Ag/PANI-NPs/CH3NH3PbI3/mp-anatase-TiO2/blTiO2/FTO perovskite solar cell. The transient photocurrent and photovoltage studies reveal that the fabricated solar cell shows better charge transport time, diffusion coefficient, diffusion length, and charge collection efficiency. Herein, the use of PANI-

*

Corresponding Author Email: [email protected].

458

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al. NPs as the HTL improves the charge carrier generation and the charge collection efficiency of the fabricated solar cell.

INTRODUCTION A new kind of inorganic−organic solar cell based on organic halide perovskite materials has recently gained a great deal of attention due to good electron and hole conductivity [1−3], high carrier mobility (50 cm2/V·s) [4], direct band gap (∼1.55 eV), and high stability [5]. Interestingly, the perovskites as sensitizers display very strong absorption in the visible region to the nearinfrared region [6, 7] and their optical and electronic properties could be tuned by changing the chemical compositions of the perovskites [2, 8]. The light absorption in perovskite solar cells could be improved by using light-absorbing polymers as hole transporting layers (HTLs), which could improve the optical density of the mesoporic thin films due to the advancement in the light harvesting by the hole conductors with the pores of thin films [9, 10]. The perovskites are highly versatile materials with spectral tunability which efficiently improves the photoinduced electron transfer from the p-type polymers to n-type metal oxide thin films [11, 12]. In general, the hole transporting material 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) is commonly used in various dye sensitized solar cells and perovskite solar cells because it shows efficient charge transport, shows low recombination rates, and improves the pore filling of the TiO2 layer. Hitherto, the synthetic process of spiro-MeOTAD has been excessively costly at the laboratory level, and therefore, it is important to find cheap and effective alternatives to spiro-MeOTAD for the fabrication of low cost perovskite solar cells. So far, several conducting polymers such as poly[2,1,3-benzothiadiazole-4,7-diyl [4,4-bis(2-ethylhexyl)4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]] (PCPDTBT), poly[[9-(1-octylnonyl)9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT), and poly-triarylamine (PTAA) have been used as HTLs or electronblocking layers for the fabrication of perovskite solar cells [13]. Apart from various conducting polymers, polyaniline (PANI) is one of the most extensively studied polymers owing to its ease of synthesis, high conductivity, and good environmental stability [14]. PANI shows versatility in nanostructures of nanofibers, nanorods, nanowires, nanotubes, and nanoflakes with a high surface to volume ratio and low diffusional resistance [15]. PANI nanomaterials are widely used for the fabrication of efficient electronic and nanodevices such as field-effect transistors (FETs), sensors, catalysts, photovoltaics, etc. due to their improved optical, structural, electronic, and electrical properties [16]. Recently, Xiao et al. fabricated a solid-state perovskite-sensitized solar cell using the dual function of PANI with a brachyplast structure, in which PANI structure was deposited by

Conducting Channels of the Hole Transporting Layer …

459

a two-step cyclic voltammetry (CV) approach [17]. In this work, a highly efficient solar cell is fabricated using organic halide perovskite (CH3NH3PbI3) as the light harvester which is coated on mesoporous TiO2 and organic polyaniline nanoparticles (PANI-NPs) are applied as the HTL. The fabricated perovskite solar cell achieves a reasonably high incident-photon-to-current efficiency (IPCE) of ∼51% in the wavelength range of ∼ 450−700 nm and a maximum overall solar-to-electricity conversion efficiency of ∼ 6.29% under AM 1.5 illumination at an intensity of 100 mW cm−2. The use of PANI-NPs as the HTL has substantially improved the charge carrier generation and the charge collection efficiency of the fabricated perovskite solar cell. The fluorinated tin oxide glass (FTO) substrates are partially etched by using zinc powder and 2 M hydrochloric acid. The etched FTO substrates are cleaned with an ultrasonic bath using acetone, isopropyl alcohol, and DI water. Thereafter, the etched FTO substrates are coated with a blocking layer (bl) using 0.1 M Ti(IV) bis(ethyl acetoacetato)-diisopropoxide (CH3CH2OCOCH=C(O-)CH3]2Ti(OCH-(CH3)2)2 in 1butanol solution by a spin-coating method, and then the substrates are heated at 500°C for 30 min. The mesoporous (mp) anatase TiO2 layer is screen-printed on the pretreated FTO glass substrates using a diluted TiO2 paste and again sintered at 500°C for 30 min. The synthesized perovskite solution is coated on the annealed TiO2 thin film through spincoating at the speed of ∼2000 rpm for 40 s using a 0.45 μm pore PVDF membrane syringe filter (Jet Biofil) at ambient atmosphere. Thereafter, the thin films are annealed at 100°C for 30 min to achieve CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO. PANI-NPs as the HTL are spin-coated on the CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO substrate at ∼3000 rpm for 30 s using PANI solution in choloroform (15 mg/1 mL) with 13.6 μL of lithium bis-(trifluoromethanesulfonyl) imide (CF3SO2NLiSO2CF3, Li-TFSI)/acetonitrile (28.3 mg/1 mL,) and ∼6.8 μL of TBP (C9H13N) as additives in the ambient condition and dried at 100°C for 15 min to obtain PANI-NPs/CH3NH3PbI3/mpanatase-TiO2/blTiO2/FTO. Finally, Ag contact (thickness ∼100 nm) is made by thermal evaporation to achieve Ag/PANI-NPs/CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO as the fabricated perovskite solar cell.

MORPHOLOGICAL STUDIES OF CH3NH3PBI3/ITO-PET AND PPY/PC61BM/CH3NH3PBI3/ITO-PET THIN FILM The Field Emission Scanning Electron Microscopy The morphological investigations are examined by taking the surface-view and crosssectional FESEM images of the fabricated perovskite solar cell, as shown in Figure 1. The surface view (Figure 1a) of TiO2 exhibits the uniformly distributed mesoporous thin

460

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

film of TiO2 nanoparticles. After the perovskite coating, the thin film possesses a uniform grain structure of microscale size which is completely penetrated into the mesoporous TiO2 thin film deposited on the FTO substrate (Figure 1b). The cross-sectional FESEM image (Figure 1c) reveals the layered structure of the perovskite solar cell. From Figure 1c, the thickness of the overall fabricated perovskite solar cell is estimated as ∼1.5 μm, consisting of a CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2 mesoporous layer (∼600−700 nm) which is uniformly capped by the PANINPs HTL. A thin Ag contact layer of ∼100 nm could be clearly seen on top of the active layer of the fabricated perovskite solar cell.

Reprinted with permission from [S. Ameen, 2014], Langmuir, 30 (2014) 12786 © 2014 American Chemical Society Figure 1. Surface FESEM images of (a) mp-anatase-TiO2/bl-TiO2/FTO, (b) CH3NH3PbI3/mp-anataseTiO2/bl-TiO2/FTO, and (c) cross section of Ag/PANI-NPs/CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO thin film.

Atomic Force Microscopy Figure 2 shows the topographic and three-dimensional (3D) AFM images of the synthesized PANI-NPs and PANI-NPs/CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2 thin films. Figure 2a displays spherical aggregated nanoparticles of PANI with an average size of ∼ 20 nm. The well-mixed structure of perovskite and PANI-NPs is observed in Figure 2 c, indicating the good penetration of PANI-NPs into the perovskite thin film. From 3D AFM images (Figure 2b,d), PANI-NPs/CH3NH3PbI3/mpanatase-TiO2/bl-TiO2 thin film

Conducting Channels of the Hole Transporting Layer …

461

shows a less rough surface compared to PANI-NPs thin film. The root mean roughness (Rrms) of films is calculated by the AFM images of the thin films. PANI-NPs thin film obtains the higher Rrms value (∼46.5 nm) than PANI-NPs/CH3NH3PbI3/mp-anataseTiO2/bl-TiO2 thin film (∼28.2 nm), indicating the good pore filling of mesoporous TiO2 thin film along with the perovskite sensitizer.

The Line Scan Element Mapping Spectroscopy The elemental compositions of each layer in PANI-NPs/CH3NH3PbI3/mp-anataseTiO2/bl-TiO2 thin film are estimated by the elemental mapping and line scanning of a cross-sectional view through electron X-ray dispersive spectroscopy (EDS), as depicted in Figure 3. The mapping image (Figure 3a) displays that the thin film is mainly comprised of C, Ti, O, N, I, and Pb with values in atomic percent of ∼9.25, ∼28.76, ∼ 56.26, ∼3.74, 0.51, and 1.48, respectively. Similar patterns are seen in the line scanning profile, as shown in Figure 3b. It has been observed that the atomic percent ratio of Pb to I is 1:3, which deduces the formation of PbI3 in the perovskite sensitizer.

Reprinted with permission from [S. Ameen, 2014], Langmuir, 30 (2014) 12786 © 2014 American Chemical Society. Figure 2. (a, c) Topographic and (b, d) three-dimensional AFM images of synthesized PANI-NPs and PANI-NPs/CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2 thin film.

462

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Reprinted with permission from [S. Ameen, 2014], Langmuir, 30 (2014) 12786 © 2014 American Chemical Society. Figure 3. (a) Elemental mapping and (b) line scanning profile of PANI-NPs/CH3NH3PbI3/mp-anataseTiO2/bl-TiO2 thin film.

Reprinted with permission from [S. Ameen, 2014], Langmuir, 30 (2014) 12786 © 2014 American Chemical Society. Figure 4. XRD patterns of (a) CH3NH3PbI3/FTO and (b) CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO thin films.

CRYSTALLINE AND OPTICAL PROPERTIES OF CH3NH3PBI3/FTO AND CH3NH3PBI3/MP-ANATASE-TIO2/BL-TIO2/FTO THIN FILMS X-ray Powder Diffraction The X-ray diffraction patterns of CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO and CH3NH3PbI3/FTO thin films are examined to investigate the crystalline nature of the perovskite sensitizer deposited on TiO2 thin film. Figure 4a shows the diffraction peaks at

Conducting Channels of the Hole Transporting Layer …

463

∼14.2, ∼19.6, ∼28.6, ∼32.1, ∼40.6, and ∼43.3°, which correspond to the (110), (112), (220), (310), (224), and (314) crystal planes of the tetragonal perovskite structure [19, 20]. The other peaks at ∼26.6, ∼33.8, ∼37.7, ∼51.4, and ∼54.6° belong to the FTO layer of the glass substrate [16]. The XRD pattern of CH3NH3PbI3/mp-anataseTiO2/bl-TiO2/ FTO exhibits the visible TiO2 diffraction peaks along with significant perovskite and FTO diffraction peaks. Importantly, no impurity peaks other than the deposited materials are observed, suggesting that CH3NH3PbI3/mp-anatase-TiO2/blTiO2/FTO thin films possess pure phases of CH3NH3PbI3 and TiO2 materials.

Reprinted with permission from [S. Ameen, 2014], Langmuir, 30 (2014) 12786 © 2014 American Chemical Society. Figure 5. (a) UV−vis spectra and (b) room temperature photoluminescence spectra of CH3NH3PbI3/FTO and PANI-NPs/ CH3NH3PbI3/FTO thin film. Inset shows the UV−vis spectrum of PANI-NPs.

464

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

UV and Photoluminescence Spectra The ultraviolet−visible (UV−vis) and room temperature photoluminescence (PL) spectroscopies of CH3NH3PbI3/FTO and PANI-NPs/CH3NH3PbI3/FTO thin films are analyzed to investigate the optical properties. Figure 5a shows a typical UV−vis absorption spectrum of CH3NH3PbI3/FTO thin film. From the derived Tauc plot, CH3NH3PbI3/FTO thin film exhibits an optical band gap (Eg) of ∼1.69 eV, which is consistent with other reports for CH3NH3PbI3 [20, 21]. The deposited CH3NH3PbI3/FTO thin film absorbs a higher wavelength edge near ∼700 nm. However, after the deposition of PANI-NPs on CH3NH3PbI3/FTO thin film, a red shift occurs and decreases the optical band gap. The increase in the peak intensity of UV−vis spectra of the PANINPs/CH3NH3PbI3/FTO thin film might be ascribed to the selective interactions between CH3NH3PbI3 and the quinoid ring of PANI-NPs, which might facilitate the charge transfer from the quinoid unit to CH3NH3PbI3 via highly reactive imine groups. The room temperature PL spectra of CH3NH3PbI3/FTO and PANI/ CH3NH3PbI3/FTO thin films are carried out with an excitation wavelength of ∼320 nm. Figure 5b shows that CH3NH3PbI3/FTO and PANI-NPs/CH3NH3PbI3/FTO thin films display similar emission bands in the blue and green regions. Noticeably, the PANI-NPs/CH3NH3PbI3/FTO thin film presents a lower PL intensity than the CH3NH3PbI3/FTO thin film due to the significant PL quenching which clearly indicates a contact between the perovskite layer and the PANI layer. The occurrence of PL quenching might facilitate the charge carrier generation in CH3NH3PbI3 at PANI-NPs/CH3NH3PbI3/FTO interface [22].

SCHEMATIC REPRESENTATION OF THE FABRICATED AG/PPY/PC61BM/ CH3NH3PBI3/PEDOT:PSS/ ITO-PET FLEXIBLE PEROVSKITE SOLAR CELL A schematic representation of the fabricated perovskite solar cell is depicted in Figure 6a. The first layer of the fabricated solar cell is formed by a TiO2 blocking layer followed by the crystalline meosporous (mp) TiO2 layer on the FTO substrate. The crystalline CH3NH3PbI3 is deposited on top of the mp anatase TiO2 by solvent-drying during spin-coating, followed by evaporation at 80°C under vacuum. The hole transport layer (PANI-NPs) is coated on the layered structure of the CH3NH3PbI3/mp-anataseTiO2/bl-TiO2 thin film in which HTL assists in the hole extraction to the top deposited Ag electrode. Figure 6b shows the energy level diagram of the perovskite solar cell. Under light illumination, the photons are absorbed by CH3NH3PbI3 and create the electron−hole pairs. Thereafter at the CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2 interface, the generated excitons dissociate and the photoexcited electron injects into the conduction band of

Conducting Channels of the Hole Transporting Layer …

465

TiO2, and simultaneously the hole reaches to the HTL. During the operation of the solar cell, some of the remaining holes run across the perovskite layer before reaching to the HTL. The exciton dissociates and the charge transfers at CH3NH3PbI3/mp-anataseTiO2/bl-TiO2 and PANI/CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2 interfaces to generate the energy. The bottom section of the perovskite solar cell acts as an electron collector which is composed of a ∼100 nm thick hole-blocking compact TiO2 film deposited on the FTO followed by the deposition of a ∼600−700 nm thick layer of PANI/CH3NH3PbI3/mpanatase-TiO2/bl-TiO2.

Reprinted with permission from [S. Ameen, 2014], Langmuir, 30 (2014) 12786 © 2014 American Chemical Society. Figure 6. (a) Schematic representation and (b) energy level diagram of Ag/PANI-NPs/ CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO perovskite solar cell.

THE PERFORMANCE OF THE PEROVSKITE SOLAR CELL Current Density (J)–Voltage (V) Measurements, Electrochemical Impedance and the Incident Photon-to-Current Conversion Efficiency The photovoltaic parameters of the fabricated Ag/PANINPs/CH3NH3PbI3/mpanatase-TiO2/bl-TiO2/FTO perovskite solar cell are obtained by measuring the current density−voltage (J−V) characteristics under a light intensity of 100 mW cm−2 (1.5 AM). The J−V curves are shown in Figure 7a for the fabricated perovskite solar cells with and without PANI-NPs. The overall solar-to-electric conversion efficiency of ∼6.29% with high short circuit current density (JSC) of ∼17.97 mA/cm2, open circuit voltage (VOC) of ∼0.877 V, and low fill factor (0.40) are accounted by the fabricated Ag/PANINPs/CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO perovskite solar cell. However, the inferior JSC of ∼9.67 mA/cm2 and solar-to-electric conversion efficiency of ∼1.95% are observed for the fabricated solar cell without using PANI-NPs HTL, indicating that the

466

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

introduction of PANI-NPs as the HTL is a crucial step to improving the photovoltaic parameters. Importantly, the photocurrent and the photovoltage are much superior to those of dye-sensitized solar cells based on PANI HTLs [23]. The uniform covering of the HTL over the CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO thin film has reduced the recombination, and the extraction of holes improves the charge transfer rate [24]. The small spherical nanoparticles of PANI might help in the achievement of a uniform surface coverage over CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO thin film, which might reduce the occurrence of pinholes (due to extraction of holes) and avoid the direct contact of the HTL with the mp-TiO2 layer, and enhance the dark current leakages [4, 25].

Reprinted with permission from [S. Ameen, 2014], Langmuir, 30 (2014) 12786 © 2014 American Chemical Society. Figure 7. (a) J−V curves and (b) Nyquist plots of fabricated perovskite solar cells with and without PANI-NPs and (c) IPCE curve of the fabricated Ag/PANI-NPs/CH3NH3PbI3/mp-anatase-TiO2/blTiO2/FTO perovskite solar cell.

The performance of the Ag/PANI-NPs/CH3NH3PbI3/mpanatase-TiO2/bl-TiO2/FTO perovskite solar cell has been quantified on a macroscopic level in terms of the incident photon-to-current conversion (IPCE) efficiency, as shown in Figure 7c. The fabricated

Conducting Channels of the Hole Transporting Layer …

467

Ag/PANI-NPs/CH3NH3PbI3/mpanatase-TiO2/bl-TiO2/FTO perovskite solar cell presents a maximum IPCE value of ∼51% in the wavelength range of ∼450−700 nm and drops at longer wavelengths. The obtained IPCE is consistent with the J SC of the perovskite solar cells. Herein, PANI-NPs with high surface area might significantly improve the light scattering capacities and provide a better interaction between the photons and CH3NH3PbI3 sensitizer. In order to investigate series and charge transfer resistances of the solar cells, Nyquist plots are obtained from the fabricated perovskite solar cells with and without PANI-NPs under a frequency range from 100 kHz−1 Hz, as shown in Figure 7b. From impedance plots, the intercept of Zre at high frequency belongs to the ohmic series resistance (RS) and the diameter of the first semicircle at high frequency represents the charge transfer resistance (RCT) at PANI/ CH3NH3PbI3 interface [26]. The fabricated perovskite solar cells with and without PANI show large RS values of ∼20.5 and ∼41.2 Ω, respectively. It is reported that the high RS effectively results in a low fill factor (FF) and increases the recombination sites [27]. In our case, the fabricated perovskite solar cell with PANI obtains a large RS which might considerably result in a low FF. However, the high JSC is related to the improved transporting ability for the electron and hole, as observed in the low RCT values.

The Charge Collection Efficiency and Photoelectron Density Analysis The Intensity-Modulated Photocurrent Spectroscopy (IMPS) and IntensityModulated Photovoltage Spectroscopy (IMVS) The intensity modulation photocurrent spectroscopy (IMPS) and the intensity modulation photovoltage spectroscopy (IMVS) measurements have been examined to elucidate the charge transfer and the recombination processes in the fabricated perovskite solar cells. IMPS and IMVS measurements are performed under a fixed light intensity at different voltages of light, as shown in Figure 8. From Figure 8, the charge transport time (τCT) and the electron recombination time (τR) of the fabricated thin film solar cell are estimated using the minimum frequencies of IMPS and IMVS plots, respectively [28]. The fabricated Ag/PANI-NPs/CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO perovskite solar cell records the reasonable τCT of ∼25.2 ms and τR of ∼146 ms, suggesting a high charge transport rate and lesser recombination rate. Figure 9 shows the plots of τCT and τR versus different photon fluxes of the fabricated perovskite solar cell which are derived by IMPS and IMVS. The order of τCT and τR values decreases with the increase of photon flux. The generation of electron−hole recombination and the electron diffusion in the perovskite solar cell are examined by evaluating the electron diffusion length (DL) parameter; i.e., DL = (Dn.τR)1/2 [29], where Dn is the diffusion coefficient obtained by the IMPS plot. For the fabricated Ag/PANI-

468

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

NPs/CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO perovskite solar cell, the Dn value is estimated as ∼3.83 × 10−6 cm2 s−1. The relatively low DL value of ∼2.36 μm is observed by the fabricated perovskite solar cell which represents the probability of large electrons to enter from CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO thin film layers to the top Ag layer electrode. The low FF might result from the low DL value.

Reprinted with permission from [S. Ameen, 2014], Langmuir, 30 (2014) 12786 © 2014 American Chemical Society. Figure 8. (a) IMVS and (b) IMPS plots of fabricated Ag/PANI/ CH 3NH3PbI3/mp-anatase-TiO2/blTiO2/FTO perovskite solar cell.

Moreover, the charge collection efficiency could be calculated by the relation ηCC = 1 − (τCT/τR) using τCT and τR values of the fabricated perovskite solar cell [29]. The high ηCC value is obtained for the fabricated Ag/PANI-NPs/CH3NH3PbI3/mp-anataseTiO2/bl-TiO2/FTO perovskite solar cell, suggesting the high charge collection during the

Conducting Channels of the Hole Transporting Layer …

469

illumination and results in the high electron transport rate and the photocurrent density. Thus, the introduction of PANI-NPs as the HTL might enhance the charge transfer time and the charge collection of the fabricated perovskite solar cells and produces the high performance of the perovskite solar cell.

Reprinted with permission from [S. Ameen, 2014], Langmuir, 30 (2014) 12786 © 2014 American Chemical Society. Figure 9. (a) Electron transport and (b) recombination lifetime of electrons of fabricated Ag/PANINPs/CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO perovskite solar cell with respect to different incident photon fluxes.

CONCLUSION In summary, a unique and well-defined morphology of PANINPs as an efficient HTL is employed to fabricate a high performance perovskite solar cell using CH3NH3PbI3 as

470

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

sensitizer. The reasonable solar-to-electricity conversion efficiency of ∼6.29% with short circuit current (JSC) of ∼17.97 mA/cm2 and open circuit voltage (VOC) of ∼0.877 V are obtained by the Ag/PANI-NPs/CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO perovskite solar cell. The IPCE measurement reveals that the fabricated Ag/PANI-NPs/ CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO perovskite solar cell presents the maximum value of ∼51% in the wavelength range of ∼450−700 nm and drops at longer wavelengths. The transient photocurrent and photovoltage studies elucidate that the fabricated solar cell exhibits a high ηCC value, suggesting the high charge collection during the illumination due to good transport time and the low recombination time which significantly results in a high electron transport rate and photocurrent density. Hence, the use of PANI-NPs as the HTL improves the charge carrier generation and the charge collection efficiency of the fabricated perovskite solar cell.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

E. Mosconi, A. Amat, M. K. Nazeeruddin, M. Grätzel, F. DeAngelis, J. Phys. Chem. C, 117 (2013) 13902. M. Liu, M. Johnston, H. Snaith, Nature, 501 (2013) 395. C. R. Kagan, D. B. Mitzi, C. D. Dimitrakopoulos, Science, 286 (1999) 945. E. Edri, S. Kirmayer, D. Cahen, G. Hodes, J. Phys. Chem. Lett., 4 (2013) 897. J. Ball, M. Lee, A. Hey, H. Snaith, Energy Environ. Sci., 6 (2013) 1739. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science, 338 (2012) 643. I. Chung, B. Lee, J. Q. He, R. P. H. Chang, M. G. Kanatzidis, Nature, 485 (2012) 486. D. B. Mitzi, Prog. Inorg. Chem., 48 (2007) 1. H. J. Lee, H. C. Leventis, S. A. Haque, T. Torres, M. Gratzel, M. K. Nazeeruddin, J. Power Sources, 196 (2011) 596. G. K. Mor, S. Kim, M. Paulose, O. K. Varghese, K. Shankar, J. Basham, C. A. Grimes, Nano Lett., 9 (2009) 4250. G. Grancini, R. S. S. Kumar, A. Abrusci, H. L. Yip, C. Z. Li, A. K. Y. Jen, G. Lanzani, H. J. Snaith, Adv. Funct. Mater., 22 (2012) 2160. A. Abrusci, I. K. Ding, M. Al-Hashimi, T. Segal-Peretz, M. D. McGehee, M. Heeney, G. L. Frey, H. J. Snaith, Energy Environ. Sci., 4 (2011) 3051. J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C. S. Lim, J. A. Chang, Y. H. Lee, H. J. Kim, A. Sarkar, M. K. Nazeeruddin, M. Gratzel, S. Seok, Nat. Photonics, 7 (2013) 486.

Conducting Channels of the Hole Transporting Layer …

471

[14] S. Ameen, M.S. Akhtar, Y. S. Kim, O-B. Yang, H. S. Shin, J. Phys. Chem. C, 114 (2010) 4760. [15] H. K. Seo, S. Ameen, M. S. Akhtar, H. S. Shin, Talanta, 104 (2013) 219. [16] S. Ameen, M. S. Akhtar, S. G. Ansari, O. B. Yang, H. S. Shin, Superlattices Microstructures, 46 (2009) 872. [17] Y. Xiao, G. Han, Y. Chang, H. Zhou, M. Li, Y. Li, J. Power Sources, 267 (2014) 1. [18] J. H. Im, C. R. Lee, J. W. Lee, S. W. Park, N. G. Park, Nanoscale, 3 (2011) 4088. [19] J. Qiu, Y. Qiu, K. Yan, M. Zhong, C. Mu, H. Yan, S. Yang, Nanoscale, 5 (2013) 3245. [20] T. Baikie, Y. Fang, J. M. Kadro, M. Schreyer, F. Wei, S. G. Mhaisalkar, M. Gratzel, T. J. White, J. Mater. Chem. A, 1 (2013) 5628. [21] H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J. Moon, R. Humphry-Baker, J. H. Yum, J. E. Moser, M. Gratzel, N. G. Park, Sci. Rep., 2 (2012) 591. [22] A. Abrusci, S. D. Stranks, P. Docampo, H. L. Yip, A. K.-Y. Jen, H. J. Snaith, Nano Lett., 13 (2013) 3124. [23] S. Ameen, M. S. Akhtar, G. S. Kim, Y. S. Kim, O. B. Yang, H. S. Shin, J. Alloys Compd., 487 (2009) 382. [24] L. Etgar, P. Gao, Z. Xue, Q. Peng, A. K. Chandiran, B. Liu, M. K. Nazeeruddin, M. Grätzel, J. Am. Chem. Soc., 134 (2012) 17396. [25] T. M. Koh, K. Fu, Y. Fang, S. Chen, T. C. Sum, N. Mathews, S. G. Mhaisalkar, P. P. Boix, T. Baikie, J. Phys. Chem. C, 118 (2014) 16458. [26] S. Ameen, M. S. Akhtar, G.-S. Kim, Y. S. Kim, O. B. Yang, H.-S. Shin, J. Alloys Compd. 487 (2009) 382. [27] Li, J. Li, L. Wang, G. Niu, R. Gao, Y. Qiu, J. Mater. Chem. A, 1 (2013) 11735. [28] K. Zhu, N. R. Neale, A. Miedaner, A. J. Frank, Nano Lett., 7 (2006) 69. [29] R. Katoh, M. Kasuya, S. Kodate, A. Furube, N. Fuke, N. Koide, J. Phys. Chem. C, 113 (2009) 20738.

In: Emerging Materials for Environment Protection … ISBN: 978-1-53613-850-4 Editors: M. Shaheer Akhtar, Sadia Ameen et al. © 2018 Nova Science Publishers, Inc.

Chapter 33

METAL OXIDE FREE PEROVSKITE SOLAR CELL Sadia Ameen1, M. Shaheer Akhtar2, Hyung-Kee Seo1 and Hyung Shik Shin1,* 1

Energy Materials and Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea 2 New and Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

ABSTRACT The flexible perovskite thin film solar cell is fabricated using polypyrrole (PPy) as a binding agent on ITO-PET flexible substrates. PPy as a binding agent and PC61BM as electron transporting material on perovskite (CH3NH3PbI3)-sensitized PEDOT:PSS/ITOPET substrates forms an efficient flexible perovskites solar cell, which demonstrates the reasonably high solar-to-electricity conversion efficiency (g) of ~6.38% with high short circuit current (JSC) of ~15.59 mA/cm2 and open circuit voltage (VOC) of ~0.852 V under the light illumination of ~100 mW/cm2 (1.5 AM). The transient photocurrent and photovoltage studies explain the improved charge transport time, efficient diffusion coefficient, diffusion length and high charge collection efficiency of the fabricated flexible perovskite solar cells. These enhancements might attribute to active PPy as binding agent along with electron transporting material of PC61BM. Thus, a reduced recombination and the improved charge transfer properties might render PPy as a costeffective competitor in the flexible perovskites solar cell.

*

Corresponding Author Email: [email protected].

474

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

INTRODUCTION New and novel light harvesting materials based on organometallic halide perovskites (CH3NH3PbI3) and its derivative are good photo-sensitizer for the photo-to-electricity conversion device [1, 2]. The applicability of organometallic halide perovskites is usually driven by their high charge carrier mobility of 50 cm 2 V1s, and their optical and electronic properties which could be easily improved by changing the chemical compositions in terms of using different alkyl groups, metals atom, and halides [3–5]. These perovskites materials are promising light absorbers in the photovoltaic cells [6, 2]. Generally, these perovskites are applied as sensitizers for titania to fabricate liquid junction solar cell which demonstrates high open-circuit voltage (VOC) in the range of 0.8–1.0 V and efficiency of 3.1–3.8% [7]. A wide variety of organic hole conductors have been employed as hole transporting layers (HTLs) in perovskite solar cells. In this regards, the small organic HTL, spiro-OMeTAD (2,2`,7,7`-tetrakis-(N,N-di-p-methoxy phenylamine) 9,9`-spirobifluorene) has appeared as highly promising and effective material in the perovskite solar cells [8, 9]. Recently, Burschka et al. and Liu et al. fabricated perovskite solar cells of highest efficiency of 15% using spiro-OMeTAD as HTL [10, 11]. The high performance perovskite solar cells based on organic HTLs could be potential alternative photovoltaic devices for the implementation of commercial solar cells. Even though, the perovskite solar cells based on spiro-OMeTAD as HTL exhibits high performance, but the high cost is the main hurdle in their commercialization [6]. At present, the commercial price of high purity spiro-OMeTAD is much high and is roughly over ten times higher than gold and platinum thus, the cost of HTLs like spiroOMeTAD needs to lower down by searching other potential and cost effective HTLs for the high performance of perovskite solar cells. Several HTLs such as poly (3hexylthiophene) (P3HT), poly[N-9-heptadecanyl-2,7-carbazole-alt-3,6-bis(thiophen-5yl)-2,5-dioctyl-2,5-dihydropyrrolo-[3,4] pyrrole-1,4-dione](PCBTDPP), poly [2,1,3-benzothiadiazole-4,7-diyl [4,4-bis (2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0] dithiophene2,6-diyl] (PCPDTBT), poly [9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT), and poly-(triarylamine) (PTAA) have been utilized for the fabrication of perovskite solar cells [1, 12, 13]. On the other hand, a well-known conducting polymer, polypyrrole (PPy) has received tremendous attention for the wide applications in optoelectronics devices, solar cells, etc. due to its high catalytic activity, the remarkable environmental stability, and the low cost facile synthesis [14–16]. PPy has already been used in the dye sensitized solar cells because of its unique electronic and structural properties. In this work, flexible perovskite solar cell has been fabricated using CH3NH3PbI3 as light harvesters and the chemically synthesized PPy as binding agent. The fabricated flexible perovskite solar

Metal Oxide Free Perovskite Solar Cell

475

cell, Ag/PPy/PC61BM/CH3NH3PbI3/PEDOT:PSS/ITO-PET presents reasonably high solar-to-electric conversion efficiency of 6.38% and an incident photon-to-current efficiency (IPCE) of 46% in the wavelength range of 450–700 nm. The intensitymodulated photocurrent/photovoltage spectroscopies (IMPS/IMVS) have been studied to elucidate the improved charge transport properties and the reduced recombination processes. For the fabrication, PEDOT:PSS is first spin coated at 2000 rpm for 40 s on cleaned ITO-PET substrate, and annealed at 120 C for 10 min. Afterward, the synthesized perovskite (CH3NH3PbI3) solution is coated on annealed PEDOT:PSS/ITO-PET thin film through spin coating at the speed of 2000 rpm for 40 s with 0.45 lm pore PVDF membrane syringe filter (Jet Biofil) at an ambient atmosphere. The obtained thin films are annealed at 100 C for 30 min to achieve CH3NH3PbI3/PEDOT:PSS/ITO-PET. Phenyl-C61-butyric acid methyl ester (PC61BM, 2 wt.%) solution in chlorobenzene is coated at 1000 rpm to obtain PC61BM/ CH3NH3PbI3/PEDOT:PSS/ITO-PET thin film. PPy solution in m-cresol (15 mg/1 ml) with 13.6 ll Li-bis (trifluoromethanesulfonyl) imide (CF3SO2NLiSO2CF3, Li-TFSI, 28.3 mg/1 ml, TCI, >98%) and 6.8 ll TBP (C9H13N, Aldrich, 96%) as additives is again spin-coated on PC61BM/CH3NH3PbI3/ PEDOT:PSS/ITO-PET substrate at 3000 rpm for 30 s, and dried at 100 C for 15 min. Finally, Ag contacts (thickness 100 nm) are made by the thermal evaporation to achieve the flexible perovskite solar cell as Ag/PPy/PC61BM/CH3NH3PbI3/PEDOT:PSS/ITOPET.

MORPHOLOGICAL STUDIES OF CH3NH3PBI3/ITO-PET AND PPY/PC61BM/CH3NH3PBI3/ITO-PET THIN FILMS Atomic Force Microscopy The morphology and roughness of CH3NH3PbI3/ITO-PET and PPy/PC61BM/CH3NH3 PbI3/ITO-PET thin films are analyzed by atomic force microscopy (AFM), as shown in Figure 1. The topographical AFM image (Figure 1(a)) reveals the interconnected nanosized CH3NH3PbI3 over the ITO-PET substrate. From Figure 1(b), PPy/PC61 BM/CH3NH3PbI3/ITO-PET thin film displays homogeneously mixed thin film of nanosized CH3NH3PbI3 grains and PPy. It is seen that well interconnected PPy /PC61BM/CH3NH3PbI3 layer is achieved on ITO-PET substrate. Moreover, PPy/ PC61BM/CH3NH3PbI3/ITO-PET thin film obtains lower root mean roughness (Rrms) of 36.1 nm than CH3NH3PbI3/ITO-PET thin film (45.6 nm) might be due to the uniform penetration of PPy into CH3NH3PbI3 thin film.

476

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Reprinted with permission from [S. Ameen, 2015], Chem. Eng. J., 281 (2015) 599 © 2015 Elsevier Ltd. Figure 1. Topographical AFM images of (a) CH3NH3PbI3/ITO-PET and (b) PPy/PC61BM/CH3NH3PbI3/ITO-PET thin film.

The Confocal Laser Scanning Microscopy The presence of PPy distribution in perovskites is studied by the confocal laser scanning microscopy as shown in Figure 2 (a and b). From the confocal images of green fluorescent, the pristine perovskites and PPy distributed perovskites samples are separately examined. The fluorescence images verify that PPy is well-distributed in perovskites solution with the maximum emission at 550 nm.

Reprinted with permission from [S. Ameen, 2015], Chem. Eng. J., 281 (2015) 599 © 2015 Elsevier Ltd. Figure 2. Confocal laser microscopy images of (a) CH3NH3PbI3/ITO-PET and (b) PPy/PC61BM/CH3NH3PbI3/ITO-PET thin film.

Metal Oxide Free Perovskite Solar Cell

477

CRYSTALLINE AND OPTICAL PROPERTIES OF CH3NH3PBI3/ITOPET AND PPY/PC61BM/CH3NH3PBI3/ITOPET THIN FILMS X-Ray Powder Diffraction and UV−Vis Spectra Figure 3(a) compares the X-ray diffraction patterns of CH3NH3PbI3/ITO-PET and PPy/PC61BM/CH3NH3PbI3/ITO-PET thin films. The diffraction peaks at 14.1, 19.9, 28.5, 31.8, 40.6, and 43.2o are well indexed to (110), (112), (220), (310), (224), and (314) crystal planes of the tetragonal perovskite structure [19, 20], suggesting the good crystalline nature of the perovskite materials. The other diffraction peaks at 26.4, 33.3, 37.7, 51.5, and 54.6o correspond to ITO layer of PET substrate [21]. However, almost similar diffraction patterns are observed in PPy/PC61BM/CH3NH3PbI3/ITO-PET thin film, indicating that PPy has not affected the crystallinity of the perovskite materials. It is noticed that the intensities of the diffraction peaks in PPy/PC61BM/CH3NH3PbI3/ITOPET thin film are slightly lower as compared to CH3NH3PbI3/ITO-PET thin film, confirming the intermixing of PPy and CH3NH3PbI3 perovskite. Importantly, no impurity peaks are observed in the XRD patterns, suggesting pure phases of each deposited materials. The ultraviolet-visible (UV–Vis) spectroscopies of PPy/ITO-PET and PPy/PC61BM/ CH3NH3PbI3/ITO-PET thin films are analyzed to investigate the optical properties. From Figure 3(b), PPy/ITO-PET thin film displays the two absorption peak at 380 nm and 500 nm attributing to the transition of electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital LUMO) which is related to π-π⁄ electronic transition [22]. The absorption peaks have slightly shifted to the higher wavelength indicating the interaction between PPy and CH3NH3PbI3 materials over ITO-PET substrate. The shifting reveals that NH groups of PPy and amino group of CH3NH3PbI3 have interacted and improved the generation of the charge carriers and the charge mobility. Figure 4 shows the schematic presentation of the fabricated flexible perovskites solar cell. From the schematic representation (Figure 4(a)), PEDOT:PSS solution is deposited on ITO-PET substrate using spin coating and thereafter, the crystalline CH3NH3PbI3 is spin coated on top of the buffer layer and then a thin layer PC61BM is deposited on CH3NH3PbI3/PEDOT:PSS thin film where PC61BM acts as electron transport layer. Thereafter, PPy as binding agent is spin coated to achieve PPy/PC61BM/CH3NH3 PbI3/PEDOT:PSS thin film structure. Finally, the top contact has been made by Ag through the thermal evaporation method.

478

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

Reprinted with permission from [S. Ameen, 2015], Chem. Eng. J., 281 (2015) 599 © 2015 Elsevier Ltd. Figure 3. (a) XRD patterns of CH3NH3PbI3/ITO-PET and PPy/PC61BM/CH3NH3PbI3/ITOPET thin films and (b) UV-Vis spectra PPy/ITO-PET and PPy/PC61BM/CH3NH3PbI3/ITO-PET thin films.

THE PERFORMANCE OF THE PEROVSKITE SOLAR CELL Schematic Representation and the Nyquist Plots of the Fabricated Flexible Perovskite Solar Under illumination, the generated exciton dissociates into the photoexcited electrons and holes in which holes reach to HTL, and the photoexcited electrons conduct the charge transfer at CH3NH3PbI3/ITO-PET and PPy/PC61BM/CH3NH3PbI3/ITO-PET interfaces to generate the energy. Figure 4(b) shows the Nyquist plots obtained from the fabricated flexible perovskite solar cells at different applied voltages under a frequency range from 100 kHz to 1 Hz to understand the charge transfer properties at different interfaces. Whereas, Figure 4(c) displays the Nyquist plots obtained from the fabricated

Metal Oxide Free Perovskite Solar Cell

479

flexible perovskite solar cells with and without PPy layer. The fabricated flexible perovskite solar cell records one semicircle arc in the impedance plot at lower frequency. In contrary, Nonexistence of small semicircle arc at higher frequency is due to low electron transport resistance through ITO and PEDOT-PSS layer [23]. The corresponding equivalent circuit (Figure 4(d)) describes the resistance related to the diffusion of holes through HTL (R1), in parallel with HTL capacitance (C1), and a recombination resistance (Rrec) at lower frequency with a chemical capacitance, (Cl) which is related to the electron Fermi level in PPy [24]. In this case, the origin of main arc is due to the coupling of the recombination resistance (Rrec) with the chemical capacitance of the film (Cl) [25]. Rrec is associated to CH3NH3PbI3/PEDOT: PSS/ITO-PET thin film, where the charge transfer might basically due to the recombination of electrons from the PEDOT:PSS/ITO-PET thin film with holes in the transporting layer [25](b). The charge transport recombination behavior could be explained by the classical spectral feature of a transmission line (TL), as used in the dye sensitized solar cells [26]. In order to correlate the charge transport and Rrec, a simplified relation of the carrier diffusion length (DL) and Rrec is as follows [26]: DL = (Rrec/Rtr)1/2 L where L is layer thickness L, and Rtr is the transport resistance. DL is related to the diffusion coefficient of the solar cell which depends on the charge transport properties of the solar cell, as later explained by IMPS and IMVS results. From Figure 4(b), at different voltages, Rrec values decrease with the increase of voltage during the measurements, suggesting the retardation of the recombination sites. Herein, the binding agent (PPy) might substantially avoid the charge recombination between the injected electron and electron acceptor at the top contact layer, and increases V OC of the device. Further, the introduction of PC61BM provides the most efficient pathway for the charge movements and increases the charge transfer rate, resulting to high J SC.

The Current (J)–Voltage (V) Curve and the Incident Photon-to-Current Conversion Efficiency of the Fabricated Flexible Perovskite Solar Cell The current density (J)–voltage (V) characteristics have been carried out to evaluate the photovoltaic parameters of the fabricated flexible perovskites solar cell (Ag/PPy/PC61BM/CH3NH3PbI3/PEDOT:PSS/ITO-PET) which are measured under a light intensity of 100 mW/cm2 (1.5 AM). Figure 5(a) shows the J–V curve of the fabricated flexible perovskite solar cell with and without PPy as a binding agent. A reasonably high solar-to-electric conversion efficiency of 6.38% is attained by the fabricated Ag/PPy/ PC61BM/CH3NH3PbI3/PEDOT:PSS/ITO-PET flexible perovskite solar cells, which is comparable to the reported flexible perovskite solar cells [27, 28]. The flexible device

480

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

also records high short circuit current density (J SC) of 15.59 mA/cm2, open circuit voltage (VOC) of 0.852 V and low fill factor of 0.48. However, a low conversion efficiency of 2.27% with low JSC of 8.59 mA/cm2 is observed by the fabricated flexible perovskite solar cell without PPy as binding agent. The high J SC with PPy binding agent might occur due to the inhibition of the leakage current between the Ag electrode through the uniform coating of PPy on PC61BM/CH3NH3PbI3/PEDOT:PSS/ITO-PET thin film, in which the presence of PPy as binding agent (as also evidenced from confocal images) might improve the connectivity to perovskite absorber and reduce the generation of pin-holes and shunting paths [29]. On the other hand, the uniform PPy layer over PC61BM/ CH3NH3PbI3/PEDOT:PSS/ITO-PET thin film might inhibit the recombination and the extraction of hole improves with the help of PC61BM layer which results to the enhancement in the charge transfer rate. Herein, the improved charge transfer and reduction in the recombination rate significantly account the high J SC and VOC of the fabricated Ag/PPy/PC61BM/CH3NH3PbI3/PEDOT:PSS/ITO-PET flexible solar cell, as evidenced in dark current (Figure 5(b)). In this case, PPy along with PC61BM might sufficiently improve the driving force for hole injection owing to a high offset in the energy level between perovskites/PPy and increases the electron transfer rate, which results to the high JSC and performance of the flexible perovskite solar cell.

Reprinted with permission from [S. Ameen, 2015], Chem. Eng. J., 281 (2015) 599 © 2015 Elsevier Ltd. Figure 4. (a) Schematic representation of the fabricated Ag/PPy/PC61BM/CH3NH3PbI3PEDOT:PSS/ ITO-PET flexible perovskite solar cell, (b) Nyquist plots the fabricated Ag/PPy/PC 61BM/CH3NH3PbI3/ PEDOT:PSS/ITO-PET flexible perovskite solar cell at different applied voltages under a frequency range from 100 kHz to 1 Hz, and (c) Nyquist plots of flexible perovskite solar cell with and without PPy binding agent. (d) The equivalent circuit diagram of the fabricated flexible perovskite solar cell.

Metal Oxide Free Perovskite Solar Cell

481

Reprinted with permission from [S. Ameen, 2015], Chem. Eng. J., 281 (2015) 599 © 2015 Elsevier Ltd. Figure 5. J–V curves (a) and dark current plots (b) of flexible perovskite solar cell with and without PPy binding agent and IPCE curve (c) of the fabricated flexible perovskite solar cell. J–V curve was measured at the scan rate of 50 mV/s.

The performance of Ag/PPy/PC61BM/CH3NH3PbI3/PEDOT:PSS/ITO-PET flexible perovskite solar cell has been quantified on a macroscopic level by measuring the incident photon-to-current conversion (IPCE) efficiency, as shown in Figure 5(c). The high IPCE of 46% is obtained in a broad adsorption wavelength in the range 450–700 nm by the fabricated Ag/PPy/PC61BM/CH3NH3PbI3/PEDOT:PSS/ITO-PET flexible perovskite solar cell. The improved IPCE is attributed to enhance the connectivity to perovskite absorber which increases the electron transfer rate from PC61BM layer in solar cells. Additionally, high surface area of PPy along with PC61BM might also improve the light scattering capacities and absorb large amount of photons via CH3NH3PbI3 sensitizer. The obtained IPCE is consistent with the high photocurrent density of the fabricated flexible perovskite solar cells. Moreover, a bending test has been performed over a roll with a diameter of 3 mm to investigate the effects of mechanical bending on device performance. The results (not shown here) reveal that the flexible solar device sustains its photovoltaic parameters through mechanical bending up to 20 times. Therefore, no

482

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

significant changes in the photovoltaic parameters clearly suggest the toleration of repetitive mechanical deformation of flexible device.

THE CHARGE COLLECTION EFFICIENCY AND PHOTOELECTRON DENSITY ANALYSIS The Intensity-Modulated Photocurrent Spectroscopy (IMPS) and IntensityModulated Photovoltage Spectroscopy (IMVS) Intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) under the fixed light intensity at different voltages of light. Figure 6(a and b) depicts the IMVS and IMPS of the fabricated flexible perovskite solar cell at different voltages. By selecting the minimum frequencies from IMPS and IMVS plots, the diffusion transit time (τtr) and the electron recombination time (τR) of the fabricated flexible perovskite solar cell are estimated respectively [30]. It is seen that the fabricated flexible perovskite solar cell with PPy binding agent presents improved τtr (14 ms) and low Rs (0.825 s) values, suggesting the enhanced charge transport and reduced recombination rate during the operation of the device which results to high J SC and VOC respectively. On contrary, the fabricated flexible perovskite solar cell without PPy presents the low str and high τR values. Moreover, the charge collection efficiency of flexible perovskite solar cell could be calculated by the following relation [31]: ηCC = 1 – τtr/τR where τtr and τR values are estimated from IMPS and IMVS plots of fabricated flexible perovskite solar cell [32]. The high ηCC value (0.98) is estimated for the fabricated flexible Ag/PPy/PC61BM/CH3NH3PbI3/PEDOT:PSS/ITO-PET flexible perovskite solar cell, which again supports the high charge generation and collection under the illumination, resulting to the fast electron-transport rate and the high photocurrent density. Furthermore, the order of τtr and τR value decreases with the increase of photon flux, as presented in the Figure 7(a and b). The electron diffusion length of the flexible perovskite solar cell could be determined by using the expression, DL = (Dn.τtr)1/2, where Dn is diffusion coefficient obtained by IMPS plot [32]. In this case, the Dn is estimated as 1.49 x10-6 cm2 s-1. In general, the electron diffusion length defines the average distance of an electron travels before it recombines with either the absorber or the hole conductor [33].

Metal Oxide Free Perovskite Solar Cell

483

Reprinted with permission from [S. Ameen, 2015], Chem. Eng. J., 281 (2015) 599 © 2015 Elsevier Ltd. Figure 6. (a) IMVS and (b) IMPS measurement plots of the fabricated Ag/PPy/PC61BM/CH3NH3PbI3/ PEDOT:PSS/ITO-PET flexible perovskite solar cell.

However, a longer DL could usually leads to the higher charge collection and the light-harvesting efficiencies for high solar conversion efficiency. The fabricated flexible perovskite solar cell with PPy binding agent exhibits moderately good DL value which is associated to the probability of large electrons to enter from CH3NH3PbI3/ PEDOT:PSS/ITO-PET thin film layers to the top Ag layer electrode and improves the charge collection at lower light intensity. Moreover, it is reported that low DL value of perovskite solar cells presents the larger recombination rate [33]. Herein, the low DL value has considerably resulted to higher VOC and JSC in the fabricated solar cell, as observed by the high recombination resistance and recombination rate, as shown in Figures 4 and 7. Therefore, the fabricated flexible Ag/PPy/PC61BM/CH3NH3PbI3/ PEDOT:PSS/ITO-PET perovskite solar cell presents the improved electron transport rate,

484

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

high charge collection, and moderately good DL value, which significantly results to high JSC, VOC and high photovoltaic performance.

Reprinted with permission from [S. Ameen, 2015], Chem. Eng. J., 281 (2015) 599 © 2015 Elsevier Ltd. Figure 7. The electron transport (a) and the recombination lifetime of electrons (b) with respect to different incident photon flux for of the fabricated flexible perovskite solar cells with and without PPy binding agent.

CONCLUSION The flexible perovskite thin film solar cell is fabricated using polypyrrole (PPy) as a binding agent with PC61BM as electron transport layer on ITO-PET flexible substrates. The fabricated flexible Ag/PPy/PC61BM/CH3NH3PbI3/PEDOT:PSS/ITO-PET solar cell demonstrates the reasonably high solar-to-electricity conversion efficiency of 6.38% with high short circuit current (JSC) of 15.59 mA/cm2 and open circuit voltage (VOC) of 0.852 V under the light illumination of 100 mW/cm2 (1.5 AM). The IPCE measurement reveals that the fabricated flexible perovskite solar cell shows the maximum quantum efficiency

Metal Oxide Free Perovskite Solar Cell

485

of 47% in the wavelength range of 450–700 nm. The IMPS and IMVS studies record the better charge transport time, efficient diffusion coefficient, diffusion length, and the high charge collection efficiency of the fabricated flexible perovskite solar cell. The use of active PPy as binding agent along with PC61BM as electron transporting materials might considerably enhance charge transfer and charge collection process. The fabricated flexible Ag/PPy/PC61BM/ CH3NH3PbI3/PEDOT:PSS/ITO-PET solar cell exhibits the improved electron transport rate, high charge collection, and moderately good D L value, which significantly results to high J SC, VOC and high photovoltaic performance. Thus, PPy is a promising HTL for highly efficient flexible perovskite solar cells.

REFERENCES [1]

[2]

[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C. S. Lim, J. A. Chang, Y. H. Lee, H. J. Kim, A. Sarkar, M. K. Nazeeruddin, M. Gratzel, S. I. Seok, Nat. Photonics, 7 (2013) 486. H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J. Moon, R. Humphry Baker, J. H. Yum, J. E. Moser, M. Grätzel, N. G. Park, Sci. Rep., 2 (2012) 591. J. M. Ball, M. M. Lee, A. Hey, H. Snaith, J. Energy Environ. Sci., 6 (2013) 1739. H. S. Jung, J. K. Lee, J. Phys. Chem. Lett., 4 (2013) 1682. L. Chaorong, W. Jing, Z. Yingying, D. Wenjun, Mater. Lett., 76 (2012) 187. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science, 338 (2012) 643. A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc., 131 (2009) 6050. J. H. Noh, N. J. Jeon, Y. C. Choi, M. K. Nazeeruddin, M. Gratzel, S. I. Seok, J. Mater. Chem. A, 1 (2013) 11842. N. J. Jeon, J. Lee, J. H. Noh, M. K. Nazeeruddin, M. Gratzel, S. I. Seo, J. Am. Chem. Soc., 135 (2013) 19087. J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Gratzel, Nature, 499 (2013) 316. M. Lui, M. B. Johnston, H. J. Snaith, Nature, 501 (2013) 395. J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, S. I. Seok, Nano Lett., 13 (2013) 1764–1769. B. Cai, Y. Xing, Z. Yang, W. H. Zhang, J. Qiu, Energy Environ. Sci., 6 (2013) 1480. C. Bu, Q. Tai, Y. Liu, S. Guo, X. Zhao, J. Power Sources, 221 (2013) 78. B. He, Q. Tang, J. Luo, Q. Li, X. Chen, H. Cai, J. Power Sources, 15 (2014) 170. X. Zhang, S. Wang, S. Lu, J. Su, T. He, J. Power Sources, 15 (2014) 491.

486

Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo et al.

[17] S. Deivanayaki, V. Ponnuswamy, R. Mariappan, P. Jayamurugan, Optik, 124 (2013) 1089. [18] J. H. Im, C. R. Lee, J. W. Lee, S. W. Park, N. G. Park, Nanoscale, 3 (2011) 4088. [19] J. Qiu, Y. Qiu, K. Yan, M. Zhong, C. Mu, H. Yan, S. Yang, Nanoscale, 5 (2013) 3245. [20] T. Baikie, Y. Fang, J., M. Kadro, M. Schreyer, F. Wei, S. G. Mhaisalkar, M. Gratzel, T. J. White, J. Mater. Chem. A, 1 (2013) 5628. [21] S. Ameen, M. S. Akhtar, H. K. Seo, H. S. Shin, Cryst. Eng. Comm., 16 (2014) 3020. [22] H. S. Abdulla, A. I. Abbo, Int. J. Electrochem. Sci., 7 (2012) 10666. [23] H. S. Kim, I. Mora-Sero, V. Gonzalez-Pedro, F. Fabregat-Santiago, E.J. JuarezPerez, N.G. Park, J. Bisquert, Nat. Commun., 4 (2013) 2242. [24] J. A. Christians, R. C. M. Fung, P. V. Kamat, J. Am. Chem. Soc. 136 (2014) 758. [25] (a) I. Mora-Seró, J. Bisquert, F. Fabregat-Santiago, G. Garcia-Belmonte, K. Y. D. Zoppi, Y. Proskuryakov, I. Oja, A. Belaidi, T. Dittrich, R. Tena-Zaera, A. Katty, C. Lévy-Clement, V. Barrioz, S. J. C. Irvine, Nano Lett., 6 (2006) 640; (b) J. Bisquert, Phys. Chem. Chem. Phys., 5 (2003) 5360. [26] (a) J. Bisquert, G. Garcia-Belmonte, F. Fabregat-Santiago, N. S. Ferriols, P. Bogdanoff, E. C. Pereira, J. Phys. Chem. B, 104 (2000) 2287; (b) J. J. Bisquert, Phys. Chem. B, 106 (2002) 325. [27] M. H. Kumar, N. Yantara, S. Dharani, M. Graetzel, S. Mhaisalkar, P. P. Boix, N. Mathews, Chem. Commun., 49 (2013) 11089. [28] J. You, Z. Hong, Y. Yang, Q. Chen, M. Cai, T. B. Song, C. C. Chen, S. Lu, Y. Liu, H. Zhou, Y. Yang, ACS Nano, 8 (2014) 1674. [29] H. J. Snaith, N. C. Greenham, R. H. Friend, Adv. Mater., 16 (2004) 1640. [30] K. Zhu, N. R. Neale, A. Miedaner, A. J. Frank, Nano Lett., 7 (2006) 69. [31] L. Bertoluzzi, S. Ma, Phys. Chem. Chem. Phys., 15 (2013) 4283. [32] R. Katoh, M. Kasuya, S. Kodate, A. Furube, N. Fuke, N. Koide, J. Phys. Chem. C, 113 (2009) 20738. [33] A. J. Frank, N. Kopidakis, J. Lagemaat van de, Coord. Chem. Rev., 248 (2004) 1165–1179.

ABOUT THE EDITORS

M. Shaheer Akhtar completed his PhD in Chemical Engineering, 2008, from Chonbuk National University, South Korea. Presently, he is working as full time Associate Professor at Chonbuk National University, South Korea. His research interests constitute the photoelectrochemical characterizations of thin film semiconductor nanomaterials, composite materials, polymer based solid-state films, solid polymer electrolytes and electrode materials for dye-sensitized solar cells (DSSCs), hybrid organic-inorganic solar cells, small molecules based organic solar cells, and photocatalytic reactions.

488

About the Editors

Sadia Ameen obtained her PhD in Chemistry in 2008, and then moved to Chonbuk National University to work with Professor Hyung-Shik Shin on solar cell devices and chemical sensors. Presently, she is working as Research Professor at the School of Chemical Engineering, Chonbuk National University. She has achieved a Gold Medal in academics and is a holder of the Merit Scholarship and Merit Certificate for the best academic performances. Her research is focused on the synthesis of conducting polymers, nanocomposites, polymer blends, small organic molecules, for the fabrication of bulk heterojunction organic solar cells, perovskite based solar cells, hybrid organic-inorganic solar cells, dye sensitized solar cells (DSSCs), heterostructure devices and diodes, and chemical sensors.

Hyung-Shik Shin received his PhD in the kinetics of initial oxidation Al (1 1 1) surface from the Cornell University, USA, in 1984. Presently, he is a Professor in the School of Chemical Engineering, Chonbuk National University, Korea. He has been a promising researcher, has visited several universities as a visiting professor, and is an invited speaker worldwide. He is an active executive member of various renowned programs/committee such as KiChE, copyright protection, KAERI, etc. He has extensive experience in electrochemistry, renewable energy sources, solar cells, organic solar cells, charge transport properties of organic semiconductors, inorganic-organic solar cells, biosensors, chemical sensors, nanopatterning of thin film materials, and photocatalytic degradation.

INDEX A absorption spectra, 231, 259, 274, 286, 304, 321, 337 academic performance, 488 acetone, 15, 17, 274, 459 acetonitrile, 276, 288, 289, 306, 322, 333, 343, 357, 371, 385, 402, 459 acid, 17, 124, 147, 157, 170, 212, 273, 284, 298, 302, 306, 313, 314, 316, 318, 331, 333, 338, 459, 475 activated carbon, 401 active oxygen, 35, 44, 65, 140, 184, 186, 194 active radicals, 175 active site, 22, 34, 36, 44, 55, 65, 76, 113, 115, 364, 384, 407 actuators, 85, 105, 180, 200 additives, 13, 14, 17, 20, 22, 424, 459, 475 adsorption, xi, 8, 9, 18, 20, 22, 34, 35, 44, 55, 65, 84, 96, 98, 140, 147, 154, 157, 160, 170, 174, 180, 181, 185, 190, 191, 200, 204, 211, 212, 218, 223, 231, 232, 237, 239, 240, 243, 244, 260, 261, 262, 264, 342, 370, 431, 437, 449, 481 aggregation, 134, 291, 304, 305, 307 agricultural chemistry, 315 alcohols, 83, 84, 85, 87, 91, 93, 96, 98, 99 alkaline earth metals, 236 amine, 31, 40, 49, 131, 254, 257 ammonia, 21, 23, 84, 331, 333, 338 ammonium, 275, 305, 322, 423 anatase, 74, 75, 76, 224, 237, 383, 386, 388, 390, 391, 397, 457, 459, 460, 461, 462, 464, 465, 466, 467, 468, 469, 470 aniline, 251, 252, 254, 255, 264, 457

aquatic life, xi, 180, 200 aqueous solutions, 231 aromatic compounds, 104, 105 atmosphere, 8, 10, 22, 125, 283, 284, 285, 299, 300, 302, 316, 317, 318, 320, 459, 475 atomic force, 128, 278, 291, 307, 388, 444, 475 atoms, 42, 44, 76, 203, 257, 361, 372, 376, 377, 415, 430, 442, 444 automotive applications, 9

B bacterial infection, 23 band gap, 33, 49, 52, 56, 63, 109, 124, 172, 182, 189, 193, 196, 200, 212, 214, 228, 274, 275, 276, 286, 288, 298, 304, 305, 322, 332, 356, 375, 388, 401, 422, 423, 458, 464 benzene, 76, 139, 299, 313, 316, 321, 323 bias, 110, 112, 132, 134, 135, 142, 405, 450, 451 binding energy, 30, 60, 63, 230, 241, 257, 342, 356, 361, 370, 376, 391, 423, 428, 429, 446 biodegradation, xi biodiesel, 85 biomarkers, 23 biomaterials, 50 biomolecules, 30, 40, 50, 60 biosensors, 50, 72, 124, 146, 488 biotechnology, 146 blowing agent, 124 Boltzmann constant, 9, 112, 133 bonding, 17, 72, 90, 96, 107, 109, 112, 251, 256, 257, 264, 285, 303, 369, 377, 378, 412, 414, 428, 431, 446

Index

490 breath analysis, 5, 23 breathing, 203, 430 bromination, 273, 283

C calcium, 123, 124, 125 calibration, 20, 34, 35, 45, 55, 65, 73, 76, 77, 85, 94, 106, 113, 126, 131, 132, 135, 136, 147, 154, 156, 157, 361 capillary, 30, 104, 124, 146 carbon, 6, 29, 39, 60, 87, 98, 114, 146, 148, 182, 200, 201, 252, 257, 315, 361, 403, 414, 423, 430, 446 carbon atoms, 257 carbon materials, 423 carbon monoxide, 6 carbon nanotubes, 252 carcinogenicity, 236 catalyst, 13, 22, 75, 169, 170, 189, 191, 194, 201, 204, 212, 216, 217, 218, 223, 227, 228, 231, 235, 237, 239, 240, 241, 242, 243, 244, 245, 253, 259, 260, 262, 273, 299, 316, 318 catalytic activity, 14, 15, 71, 146, 149, 221, 223, 231, 243, 474 catalytic properties, 72, 252 C-C, 88, 90, 128, 254, 257 central nervous system, 50, 222 ceramic materials, 412, 423 chemical, xi, xii, 7, 9, 13, 14, 21, 22, 29, 30, 31, 34, 35, 39, 40, 44, 45, 49, 50, 54, 55, 56, 60, 61, 63, 64, 65, 71, 72, 73, 75, 77, 78, 83, 84, 85, 91, 93, 94, 95, 96, 97, 98, 99, 103, 104, 105, 106, 110, 112, 113, 114, 115, 116, 118, 123, 124, 126, 127, 130,131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 146, 170, 180, 190, 199, 200, 206, 212, 218, 221, 223, 231, 273, 314, 315, 361, 363, 365, 370, 391, 392, 401, 411, 433, 442, 446, 449, 457, 458, 474, 479, 488 chemical deposition, 370 chemical etching, 392 chemical interaction, 131, 411 chemical properties, 13 chemical reactions, 363 chemical stability, 30, 60, 72, 124, 200, 401 chemical vapor deposition, 442 chemiluminescence, 30, 104 chemisorption, 8, 10, 44, 158

chlorobenzene, 274, 278, 291, 321, 325, 424, 475 chloroform, 273, 274, 275, 283, 286, 287, 288, 304, 305 chromatography, 23, 30, 124, 146, 273, 283, 284, 300, 316, 317, 318 C-N, 88, 109, 128, 150, 256, 257 composition, xii, 32, 75, 145, 182, 191, 223, 272, 278, 282, 314, 357, 361, 371, 372, 389, 401, 423, 431 concentration ratios, 307 conductance, 8, 9, 20, 21, 22, 96, 139, 158 conduction, 8, 9, 10, 18, 33, 35, 42, 44, 52, 76, 146, 172, 175, 182, 184, 192, 214, 217, 273, 283, 332, 364, 371, 375, 388, 400, 401, 406, 408, 434, 454, 464 conductivity, 8, 9, 18, 20, 21, 85, 93, 104, 124, 138, 140, 147, 200, 252, 370, 400, 411, 412, 415, 416, 428, 450, 451, 454, 458 configuration, 15, 222, 322, 422 conjugation, 21, 112, 130, 204, 254, 307, 315 consumption, 5, 40, 84, 104, 146, 190 contaminated water, 170, 190 correlation, 29, 31, 34, 36, 40, 44, 49, 55, 56, 59, 61, 65, 66, 75, 78, 84, 85, 93, 94, 100, 104, 113, 118, 123, 125, 135, 142, 145, 147, 154, 156, 160, 190 correlation coefficient, 29, 31, 34, 36, 40, 44, 49, 55, 56, 59, 61, 65, 66, 75, 78, 84, 85, 93, 94, 100, 104, 114, 118, 123, 125, 135, 142, 145, 147, 154, 156, 160 corrosion, 40, 370, 442 cosmetics, 30, 60, 104, 170 cost, xi, 6, 7, 8, 23, 30, 40, 60, 146, 147, 170, 190, 236, 298, 314, 332, 356, 366, 370, 384, 400, 412, 422, 437, 442, 458, 473, 474 crystal quality, 34, 36, 42, 43, 45, 53, 59, 62, 66, 76, 184, 336, 352, 364, 383, 390, 397 crystal structure, 60, 173, 182, 190, 332, 341, 388 crystalline, 29, 36, 39, 44, 45, 49, 51, 53, 56, 59, 61, 66, 74, 126, 169, 171, 176, 191, 194, 196, 202, 218, 222, 223, 237, 245, 271, 272, 279, 285, 303, 320, 331, 335, 336, 338, 341, 343, 345, 348, 352, 355, 356, 358, 359, 361, 365, 369, 370, 374, 397, 462, 464, 477 crystallinity, 22, 74, 169, 182, 185, 190, 303, 331, 335, 336, 337, 338, 348, 358, 370, 374, 412, 477 crystallization, 299, 320, 360 crystals, 43, 51, 53, 63, 193, 202, 226, 283, 284, 345, 346, 349, 362, 363

Index D decomposition, 180, 200, 222, 227, 302 decomposition temperature, 302 defects, 10, 31, 42, 53, 59, 62, 66, 137, 171, 173, 182, 183, 201, 348, 360, 388, 390, 430 deficiency, 34, 43, 53, 63, 173, 214, 347, 362, 374, 390, 429 deformation, 88, 90, 107, 109, 129, 150, 424, 482 degradation, xi, xii, 30, 169, 170, 174, 176, 179, 180, 181, 184, 185, 186, 189, 190, 191, 194, 195, 196, 199, 200, 201, 203, 204, 205, 206, 211, 212, 214, 215, 216, 217, 218, 221, 223, 231, 232, 236, 237, 243, 244, 245, 251, 252, 253, 259, 260, 261, 262, 264, 488 degradation rate, 169, 171, 174, 179, 181, 184, 185, 186, 189, 191, 194, 201, 204, 205, 212, 213, 215, 221, 231, 232, 233, 245, 253, 259, 260 density functional theory, 239 deposition, 13, 30, 72, 104, 110, 125, 126, 212, 325, 332, 384, 388, 402, 424, 426, 427, 428, 442, 443, 444, 445, 450, 453, 454, 464, 465 derivatives, 21, 30, 60, 72, 78, 104, 124, 272, 314, 315, 422 desorption, 8, 170, 181, 191, 223, 237, 239, 240, 260, 261 detection, 6, 10, 23, 29, 31, 34, 35, 39, 40, 44, 45, 49, 50, 55, 56, 59, 60, 65, 66, 71, 72, 75, 78, 83, 84, 85, 87, 93, 94, 96, 100, 103, 104, 105, 110, 114, 116, 118, 123, 124, 125, 131, 133, 135, 142, 145, 146, 149, 152, 154, 158, 160 differential scanning calorimetry, 285, 302, 320 diffraction, 32, 42, 51, 61, 126, 171, 182, 191, 201, 214, 224, 237, 238, 336, 345, 355, 359, 377, 388, 462, 477 diffusion, 8, 15, 19, 20, 90, 105, 110, 112, 138, 282, 287, 314, 322, 383, 395, 405, 433, 434, 435, 436, 449, 453, 454, 457, 467, 473, 479, 482, 485 diffusion process, 138 dimethylformamide, 284, 300, 317 diodes, 105, 125, 488 dispersion, 137, 223, 224 dissociation, 13, 277, 278, 288, 292, 293, 307, 314, 322, 325 distribution, 53, 60, 73, 90, 109, 137, 221, 394, 402, 403, 476 diversity, 170, 314 dopants, 131, 226, 231, 377

491

doping, 21, 84, 123, 124, 125, 129, 130, 134, 136, 142, 146, 212, 223, 231, 232, 236, 238, 242, 245, 369, 370, 371, 372, 373, 375, 377, 378, 401 drugs, 170, 222 drying, 20, 85, 125, 300, 332, 401, 464 dye sensitized solar cells, viii, xii, 72, 329, 331, 341, 355, 381, 383, 400, 411, 458, 474, 479, 488

E ecosystem, 72, 104, 180, 200, 212, 252 electric conductivity, 356, 423 electric current, 6, 401 electrical conductivity, 18, 30, 84, 93, 104, 124, 146, 151, 212, 423, 428, 454 electrical properties, 72, 135, 138, 332, 370, 458 electrical resistance, 13, 18 electricity, 341, 342, 350, 352, 355, 356, 366, 369, 371, 377, 378, 383, 384, 422, 457, 459, 470, 473, 474, 484 electrochemical behavior, 87, 94, 103, 105, 118, 131, 149 electrochemical deposition, 104 electrochemical impedance, 83, 91, 137, 394 electrochemistry, 488 electrode surface, 124, 139 electrodeposition, 85, 146, 362 electrodes, 6, 18, 50, 61, 76, 84, 85, 91, 93, 98, 105, 126, 135, 137, 154, 155, 282, 324, 332, 337, 342, 343, 356, 357, 370, 371, 377, 385, 401, 405, 406, 407, 408, 415, 432 electrolyte, 6, 44, 54, 64, 92, 98, 112, 116, 131, 132, 137, 158, 289, 306, 322, 332, 333, 341, 342, 343, 351, 352, 355, 356, 357, 362, 364, 366, 370, 371, 377, 379, 384, 385, 394, 400, 402, 405, 408, 411, 412, 414, 415, 416 electron, xii, 8, 9, 13, 18, 21, 22, 39, 40, 42, 45, 46, 50, 56, 60, 71, 73, 75, 78, 84, 86, 92, 94, 96, 104, 114, 115, 118, 131, 138, 139, 152, 154, 156, 158, 170, 182, 190, 191, 203, 204, 212, 214, 217, 222, 230, 236, 242, 245, 251, 257, 262, 264, 272, 277, 282,288, 291, 297, 298, 304, 305, 306, 307, 309, 313, 314, 315, 320, 321, 323, 324, 332, 337, 338, 343, 352, 356, 358, 365, 371, 377, 384, 386, 394, 397, 400, 405, 407, 412, 415, 423, 434, 435, 449, 450, 451, 453, 454, 458, 461, 464, 467, 469, 470, 473, 477, 479, 480, 481, 482, 483, 484 electronic materials, 298, 315

Index

492

electronic structure, 84 electrophoresis, 30, 104, 124, 146, 170, 212 electrospinning, 399, 400, 401, 408 emission, 33, 34, 42, 43, 52, 110, 131, 132, 142, 151, 172, 183, 184, 190, 193, 203, 229, 230, 242, 254, 274, 279, 287, 297, 298, 305, 309, 321, 323, 348, 376, 388, 412, 464, 476 energy, xi, xii, 9, 10, 13, 21, 32, 42, 63, 84, 86, 104, 171, 200, 222, 228, 231, 236, 241, 242, 257, 275, 279, 281, 288, 290, 292, 293, 297, 305, 309, 313, 314, 315, 322, 325, 332, 356, 361, 370, 372, 376, 384, 388, 391, 400, 401, 407, 422, 429, 434, 442, 446, 464, 465, 478, 480 environment, xi, 30, 50, 72, 84, 124, 170, 222, 230, 236, 252, 384 environmental protection, xi, xii ester, 273, 284, 298, 306, 313, 314, 316, 428, 475 ethanol, 17, 40, 49, 50, 72, 84, 85, 125, 283, 284, 332, 343, 385, 401 ethyl acetate, 299, 316 excitation, 33, 90, 110, 131, 151, 192, 194, 214, 229, 243, 254, 322, 351, 364, 464 exciton, 30, 60, 152, 203, 278, 282, 287, 288, 292, 293, 307, 309, 314, 322, 325, 342, 348, 356, 370, 423, 465, 478 experimental condition, 223, 245 exposure, 17, 18, 84, 174, 175, 180, 204, 245

F fabrication, xi, 8, 21, 40, 44, 49, 56, 72, 83, 85, 99, 105, 118, 125, 142, 146, 160, 222, 272, 273, 276, 282, 283, 288, 293, 297, 298, 303, 309, 314, 324, 325, 332, 341, 342, 343, 352, 356, 371, 383, 385, 397, 400, 401, 411, 412, 422, 423, 437, 441, 442, 454, 458, 474, 475, 488 Fermi level, 406, 434, 449, 479 ferromagnetic, 60, 180, 190, 200 field emission scanning electron microscopy, 51, 61, 73, 171, 182, 191, 201, 213, 225, 443 film thickness, 22, 272, 286, 435 films, 8, 20, 21, 85, 126, 293, 314, 332, 342, 370, 385, 424, 426, 431, 433, 458, 461, 463, 464, 478, 487 flexibility, 200, 298, 314, 431, 442 flowers, 170, 171, 179, 180, 181, 182, 183, 184, 185, 186, 331, 333, 384, 388, 394

fluorescence, 30, 104, 190, 282, 298, 305, 321, 323, 476 fluorine, 49, 314, 315, 321, 323, 355, 365 food additive, 40 food industry, 146 fructose, 147, 157 FTIR, 52, 53, 59, 62, 63, 88, 89, 90, 98, 99, 107, 108, 128, 129, 183, 227, 240, 241, 255, 256, 257, 283, 318, 360, 361 FTIR spectroscopy, 88, 183, 360 fullerene, 314, 422 furan, 281, 282, 283, 284, 285, 286, 288, 289, 290, 293

G gas sensors, v, xii, 3, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 17, 18, 19, 20, 21, 23, 25, 72, 124, 170 gastrointestinal tract, 40, 60, 104, 180 gel, 170, 212, 283, 284, 300, 317, 384, 401, 408, 412, 415, 416 gel electrolyte, viii, 411, 412, 415, 416 glass transition, 315, 320 glass transition temperature, 315, 320 glucose, 145, 146, 147, 149, 152, 153, 154, 155, 156, 157, 158, 159, 160 glucose oxidase, 146 glycol, 332, 371, 401, 411, 416 gold nanoparticles, 146 grain boundaries, 19, 31, 42, 44, 51, 61, 62, 171, 182, 191, 192, 201, 384, 394, 400 grain size, 18, 20, 22, 370, 444 graphene oxide, viii, 190, 199, 201, 202, 203, 399, 400, 401, 403, 408 graphite, 401, 404 gravimetric analysis, 320 growth, xi, 20, 51, 72, 148, 336, 342, 345, 348, 349, 352, 355, 360, 363, 365, 385, 386, 388, 392, 393, 423 growth mechanism, 349, 352, 355, 363, 365, 393

H harvesting, 259, 332, 351, 352, 355, 356, 366, 378, 395, 422, 432, 437, 449, 452, 458, 474, 483 height, 9, 18, 134, 137, 139, 427 hemolytic anemia, 72, 124 hexane, 283, 284, 300, 317, 318

Index homogeneity, 282, 291, 325, 442 human, xi, 23, 30, 40, 72, 84, 124, 146, 180, 190, 212 human body, 40 human health, 146 hydrazine, 50, 71, 72, 73, 75, 77, 78, 124, 140 hydrogen, 72, 96, 124, 251, 252, 256, 258, 264, 285, 303, 315, 323, 428 hydrogen bonds, 315, 323 hydrophilicity, 424, 425, 426, 437 hydroquinone, 29, 30, 31, 60 hydrothermal process, 331, 332, 356, 383, 385, 388, 392, 396 hydrothermal synthesis, 236, 366, 373 hydrothermal synthesis method, 366 hydroxyl, 96, 158, 175, 204, 217, 262, 377, 428, 429

493

irradiation, 15, 16, 169, 170, 174, 175, 176, 180, 231, 232, 237, 253, 259 isomers, 60, 111, 112, 114 isotherms, 239, 240 I-V curves, 64, 106

K KBr, 283, 284, 302, 318, 319, 360 kinetics, 15, 92, 110, 131, 174, 488 Korea, 29, 39, 49, 59, 71, 83, 103, 123, 145, 169, 179, 189, 199, 211, 251, 271, 281, 297, 313, 331, 341, 355, 369, 383, 399, 411, 421, 441, 457, 473, 487, 488

L I illumination, 15, 16, 170, 174, 179, 181, 184, 185, 191, 194, 195, 196, 199, 200, 201, 203, 204, 205, 206, 211, 212, 214, 215, 217, 218, 221, 231, 237, 243, 251, 252, 253, 259, 260, 261, 264, 394, 407, 431, 435, 442, 454, 459, 464, 469, 470, 473, 478, 482, 484 improvements, 5, 326, 423 impurities, 34, 51, 173, 336, 345, 361 industries, 30, 84, 104, 124, 170, 180, 189, 200, 212, 222, 236, 252 industry, xi, 23, 212, 442 inhibition, 195, 232, 236, 245, 400, 432, 480 initial state, 176, 204 injuries, 60, 104, 124 interface, 92, 93, 104, 112, 123, 132, 134, 137, 138, 277, 278, 282, 283, 288, 291, 292, 293, 309, 314, 322, 324, 325, 342, 349, 351, 356, 364, 377, 394, 400, 405, 433, 449, 450, 454, 464, 467 interference, 10, 20, 97, 115, 147, 157, 160 intermolecular interactions, 279, 315 interphase, 42, 44, 62, 171, 192 iodine, 301, 356, 446 ionic conduction, 22 ionic conductivity, 138, 411, 412 ionization, 84, 104, 424 ions, 22, 34, 44, 54, 63, 64, 65, 76, 93, 113, 138, 158, 222, 224, 226, 227, 228, 229, 230, 231, 232, 236, 237, 238, 241, 342, 348, 356, 362, 363, 369, 372, 373, 377, 378, 384, 400, 429

lactose, 147, 157 lasers, 50, 356 lattice parameters, 237, 359 lifetime, 436, 442, 453, 469, 484 light, xi, 7, 14, 16, 30, 60, 90, 170, 174, 175, 179, 180, 181, 185, 190, 191, 194, 195, 196, 199, 200, 201, 203, 204, 205, 206, 211, 212, 214, 215, 217, 218, 221, 222, 223, 228, 231, 232, 237, 243, 244, 245, 251, 252, 259, 260, 261, 264, 274, 289, 298, 300, 305, 307, 323, 332, 342, 350, 351, 352, 355, 356, 364, 365, 366, 370, 377, 394, 395, 406, 407, 408, 422, 431, 432, 434, 435, 437, 442, 448, 451, 452, 454, 458, 459, 464, 465, 467, 473, 474, 479, 481, 482, 483, 484 light scattering, 351, 433, 449, 467, 481 light transmission, 7 liquid chromatography, 104, 146 liquid phase, 96, 158 liver, 30, 72, 124, 252 low temperatures, 10 Luo, 25, 144, 234, 353, 366, 438, 485

M manufacturing, 30, 60, 190, 356, 384 mechanical properties, 21, 412 melting, 285, 303, 320 memory, 30, 40, 50, 60, 170 mesoporous materials, 422 metabolic acidosis, 84 metal ion, 222, 223, 224, 236, 245, 371, 377

Index

494

metal organic chemical vapor deposition, 30 metal oxide semiconductors, 5, 6, 84, 200, 384 metal oxides, xi, xii, 8, 13, 20, 21, 30, 72, 190, 212, 236, 356, 370, 423 metals, 13, 14, 105, 212, 222, 236, 370, 474 methanol, 84, 85, 91, 93, 94, 95, 96, 97, 100, 273, 284, 299, 301, 302, 316, 318 methylene blue, 83, 85, 99, 180, 223, 252 microelectronics, 124, 442 microscope, 53, 173, 333, 412 microscopy, 128, 190, 291, 307, 325, 444, 475, 476 microstructures, 370 mineralization, 176, 184, 186, 189, 190, 194, 195, 196, 204, 216, 262 moisture, 87, 183, 360, 392, 428 molecular mass, 239 molecular orbital, 282, 297, 315, 406, 477 molecular oxygen, 13, 35, 424 molecular structure, xi, 262, 278, 282, 286, 314 molecular weight, 20, 195, 282 molecules, 6, 15, 18, 19, 20, 21, 22, 23, 35, 65, 105, 158, 170, 175, 196, 204, 217, 233, 236, 258, 259, 261, 262, 264, 272, 282, 283, 285, 287, 290, 291, 292, 298, 303, 307, 309, 314, 315, 321, 322, 324, 342, 351, 360, 406, 407, 413, 428, 437, 487, 488 morphology, 18, 29, 30, 31, 34, 35, 36, 40, 42, 44, 45, 49, 51, 56, 59, 60, 61, 66, 72, 73, 85, 86, 87, 90, 93, 100, 105, 107, 110, 112, 114, 115, 125, 134, 148, 149, 160, 169, 182, 189, 190, 191, 196, 201, 206, 213, 223, 239, 254, 278, 290, 291, 293, 303, 307, 309,315, 320, 323, 325, 331, 332, 333, 334, 337, 338, 341, 342, 343, 345, 349, 351, 352, 356, 357, 358, 359, 360, 364, 369, 371, 372, 377, 378, 384, 385, 388, 394, 400, 401, 402, 408, 422, 426, 444, 457, 469, 475 mucous membrane, 30, 180

N nanobelts, 50, 83, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 117, 342, 356 nanocomposites, 146, 200, 212, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 264, 488 nanodevices, 458 nanofibers, 103, 105, 400, 458 nanomaterials, xii, 29, 30, 31, 40, 42, 43, 49, 50, 51, 53, 56, 60, 61, 62, 63, 72, 78, 84, 86, 124, 126, 155, 170, 171, 180, 190, 191, 200, 203, 206, 211,

222, 223, 251, 252, 254, 258, 264, 331, 332, 333, 335, 336, 337, 338, 347, 351, 356, 358, 359, 360, 370, 371, 374, 375, 377, 378, 384, 386, 388, 422, 423, 431, 458, 487 nanoparticles, 90, 126, 127, 146, 180, 201, 202, 206, 211, 212, 213, 214, 217, 218, 221, 224, 226, 227, 228, 229, 230, 232, 235, 236, 238, 239, 240, 241, 242, 243, 244, 245, 369, 371, 372, 378, 392, 400, 415, 457, 459, 460, 466 nanorods, 50, 60, 72, 169, 170, 171, 176, 180, 334, 356, 357, 358, 364, 384, 400, 458 nanostructures, v, viii, 15, 30, 39, 42, 50, 59, 60, 61, 63, 64, 66, 72, 147, 189, 190, 191, 195, 196, 331, 333, 335, 336, 337, 342, 355, 356, 363, 364, 365, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 384, 392, 400, 401, 423, 458 nanotube, 60, 71, 146, 341, 342 nanowires, 50, 60, 72, 125, 145, 342, 351, 384, 400, 423, 458 nitrogen, 6, 21, 87, 148, 252, 257, 299, 300, 302, 317, 332, 343, 385, 402 nuclei, 348, 349, 363

O one dimension, 50, 72, 105 optical properties, 30, 33, 34, 43, 53, 72, 103, 109, 129, 151, 171, 172, 176, 184, 185, 192, 193, 199, 206, 211, 218, 254, 255, 348, 352, 370, 378, 384, 388, 464, 477 optoelectronic properties, 314, 423 optoelectronics, 124, 474 organ, 400, 442, 474 organic chemicals, 212 organic compounds, 104, 105, 212, 262, 298, 315 organic polymers, 298 organic solvents, 227, 240, 273, 279, 281, 283, 293, 303, 316, 325 oxidation, 13, 17, 31, 36, 46, 71, 72, 76, 114, 124, 127, 131, 132, 138, 139, 145, 147, 158, 160, 180, 200, 212, 222, 230, 252, 275, 288, 305, 322, 361, 391, 404, 424, 437, 488 oxide nanoparticles, 222, 236 oxygen, 8, 9, 10, 13, 15, 18, 33, 34, 43, 44, 45, 52, 53, 63, 65, 71, 76, 78, 87, 96, 138, 148, 171, 173, 175, 182, 183, 193, 194, 201, 203, 212, 217, 231, 241, 257, 282, 286, 290, 336, 347, 356, 360, 362,

Index 374, 376, 377, 388, 390, 391, 414, 421, 423, 424, 426, 429

P pH, 54, 85, 94, 97, 114, 116, 131, 132, 147, 152, 154, 156, 157, 223, 231, 232, 333, 338, 348, 349, 363, 364 phenol, 30, 50, 76, 125 phonons, 34, 43, 53, 194, 202, 257, 347, 360, 374, 423 phosphate, 31, 41, 51, 54, 61, 64, 73, 85, 91, 103, 106, 126, 131, 132, 147, 152, 153, 156, 275, 305, 322 photocatalysis, xii, 30, 40, 50, 60, 200, 218, 222, 232, 236, 244, 370 photodegradation, 190, 199, 200, 204, 206, 211, 212, 216, 218, 223, 231, 232 photoluminescence, 33, 43, 52, 90, 109, 129, 130, 151, 172, 192, 203, 229, 236, 242, 245, 274, 275, 287, 315, 348, 389, 463, 464 photolysis, 232, 243, 244 photons, 175, 291, 342, 351, 365, 464, 467, 481 photovoltaic cells, 437, 474 photovoltaic devices, 298, 342, 370, 384, 422, 442, 474 physical properties, 32, 42, 51, 62, 146, 171, 182, 192, 201, 315 physicochemical properties, 195 PL spectrum, 33, 43, 91, 109, 151, 193, 388 platinum, 7, 31, 41, 125, 146, 342, 343, 357, 370, 371, 384, 400, 474 polar, 51, 104, 345, 349, 360, 424, 426, 437 poly(ethylene terephthalate), 422 polymer, 21, 22, 85, 105, 109, 112, 129, 134, 252, 254, 282, 298, 314, 401, 412, 414, 415, 424, 474, 487, 488 polymer blends, 488 polymer chain, 22, 109 polymer electrolytes, 412, 487 polymer films, 21, 22 polymerization, 83, 85, 99, 105, 124, 251, 252, 254, 256, 258, 264, 457 polymerization process, 258 polymers, xi, xii, 8, 21, 22, 84, 95, 98, 104, 105, 124, 146, 200, 252, 254, 272, 282, 283, 298, 423, 442, 458, 488 porosity, 19, 22, 105, 356, 422, 427, 431, 433, 437

495

portability, 6, 23, 50, 84 potassium, 124, 273, 299, 302, 316 probability, 395, 435, 468, 483 protons, 98, 117, 140, 160 purification, 200, 272, 282, 314, 316 purity, 32, 42, 51, 62, 126, 171, 176, 182, 191, 202, 227, 474

R radiation, 274, 315 radicals, 175, 184, 194, 204, 262 radius, 20, 222, 225, 237, 239 Raman spectra, 53, 173, 202, 256, 257, 336, 346, 347, 361, 374, 390, 404, 430 Raman spectroscopy, 403 reactions, xii, 12, 21, 138, 139, 146, 200, 243, 283, 316, 348, 356, 384, 487 reactive oxygen, 96, 138, 158, 424 reagents, 222, 223, 231 recombination, 43, 50, 52, 173, 183, 193, 200, 203, 204, 206, 222, 229, 230, 232, 236, 242, 245, 277, 278, 287, 291, 293, 305, 309, 322, 324, 325, 332, 342, 348, 351, 356, 364, 365, 376, 384, 394, 395, 397, 400, 405, 407, 408, 422, 433, 434, 436, 437, 441, 443, 449, 450, 451, 452, 453, 454, 458, 466, 467, 469, 470, 473, 475, 479, 480, 482, 483, 484 recombination processes, 394, 443, 467, 475 recovery, 6, 12, 15, 23 recrystallization, 273, 283, 284, 316, 318 rectification, 111, 112 red shift, 110, 130, 211, 214, 218, 274, 279, 286, 304, 375, 464 renewable energy, xi, xii, 384, 488 researchers, 14, 124, 146, 236, 272, 401, 403 residue, 283, 284, 300, 302, 317 resolution, 62, 63, 64, 73, 86, 148, 214, 334, 343, 358, 372, 373, 376 resorcinol, 59, 60, 61, 64, 65, 66 response, 6, 9, 11, 12, 13, 14, 15, 16, 17, 19, 20, 21, 22, 23, 30, 31, 34, 40, 41, 44, 46, 50, 51, 54, 55, 56, 60, 61, 64, 65, 71, 73, 75, 78, 84, 85, 92, 94, 95, 96, 97, 100, 104, 105, 106, 112, 114, 115, 116, 118, 123, 124, 125, 126, 131, 135, 137, 139, 140, 142, 145, 147, 152, 154, 156, 157, 160, 222, 228, 232 response time, 9, 12, 21, 31, 34, 40, 41, 44, 46, 51, 55, 56, 61, 65, 71, 73, 75, 78, 84, 85, 96, 100,

Index

496

104, 105, 106, 114, 118, 123, 124, 125, 126, 135, 142, 147 room temperature, 21, 33, 43, 52, 60, 84, 90, 93, 109, 110, 118, 125, 131, 151, 172, 183, 193, 229, 242, 273, 283, 284, 299, 300, 302, 305, 316, 317, 318, 332, 343, 357, 371, 376, 383, 385, 388, 396, 401, 463, 464 root-mean-square, 307, 426 roughness, 85, 87, 127, 137, 149, 278, 290, 291, 303, 307, 325, 388, 426, 431, 445, 448, 461, 475 ruthenium, 332, 343, 356, 371, 385, 401 rutile, 75, 224, 390

S selected area electron diffraction, 335, 358 self-assembly, 271, 274, 276, 279, 285, 288, 299, 303, 304 self-organization, 320 semicircle, 92, 394, 405, 449, 467, 479 semiconductor, xi, 5, 10, 12, 13, 17, 18, 50, 60, 105, 124, 125, 142, 180, 200, 223, 236, 292, 298, 332, 356, 487 sensitivity, 6, 9, 11, 12, 13, 15, 18, 20, 22, 23, 29, 30, 31, 34, 36, 39, 40, 41, 44, 45, 46, 49, 50, 51, 54, 56, 59, 60, 61, 65, 66, 71, 73, 75, 78, 84, 85, 93, 94, 96, 98, 100, 104, 105, 106, 113, 114, 116, 118, 123, 124, 125, 126, 135, 136, 142, 145, 146, 147, 154, 157, 158, 159, 160 side chain, 274, 279, 304, 307 signals, 6, 176, 185, 186, 195, 204, 217, 262, 431 silica, 155, 283, 284, 300, 317 silicon, xii, 103, 105, 118, 145, 147, 370, 422 single crystals, 43, 202, 336, 347, 374 skin, 30, 50, 60, 72, 104, 124, 190 solar cells, vii, viii, ix, xii, xiii, 72, 170, 252, 269, 271, 272, 273, 276, 277, 278, 281, 282, 283, 289, 293, 297, 298, 303, 305, 306, 307, 308, 309, 313, 314, 323, 331, 341, 355, 356, 369, 370, 384, 399, 400, 411, 419, 421, 422, 431, 432, 433, 434, 435, 437, 441,442, 447, 448, 449, 453, 454, 458, 465, 466, 467, 469, 473, 474, 478, 479, 481, 483, 484, 485, 487, 488 solid state, 286, 298, 315, 323 species, 8, 9, 10, 12, 15, 20, 31, 35, 36, 44, 55, 65, 76, 96, 105, 115, 138, 147, 157, 160, 184, 185, 186, 194, 218, 254, 259, 356, 391, 424, 429, 434, 446

specific surface, 200, 239, 377, 423, 427 spectrophotometry, 124, 146 spectroscopy, 23, 32, 34, 42, 43, 52, 63, 75, 83, 90, 91, 137, 169, 171, 172, 175, 176, 179, 182, 183, 184, 186, 189, 190, 191, 192, 193, 194, 195, 196, 201, 213, 214, 216, 228, 236, 253, 262, 274, 287, 345, 355, 360, 361, 372, 376, 393, 401, 403, 404, 422, 430, 442, 443, 451, 461, 467, 482 spin, 30, 104, 125, 231, 241, 274, 278, 287, 291, 307, 321, 325, 421, 423, 446, 459, 464, 475, 477 spindle, 39, 42, 369, 371, 372, 377, 378 stretching, 51, 62, 88, 90, 98, 107, 109, 128, 129, 150, 183, 227, 240, 256, 257, 360 strong interaction, 129, 416, 428 structural changes, 98, 200 structural defects, 19, 32, 33, 34, 52, 173, 192, 361 substitution, 96, 214, 288, 315, 369, 376, 378 surface area, 13, 19, 20, 30, 31, 40, 72, 85, 105, 110, 115, 158, 223, 232, 235, 239, 245, 252, 260, 262, 264, 292, 309, 325, 331, 337, 338, 342, 351, 356, 365, 384, 395, 400, 401, 422, 427, 433, 437, 467, 481 surface energy, 424, 426, 437 surface layer, 9, 10 surface modification, 13, 424 surface properties, 138, 384, 423, 443 surface reaction, 8, 21, 96 surface region, 18 synthesis, xii, 23, 31, 40, 104, 146, 170, 179, 180, 186, 190, 199, 200, 223, 252, 272, 298, 314, 332, 342, 355, 384, 392, 458, 474, 488

T target, 12, 13, 17, 18, 19, 22, 23, 221, 284, 299, 302 temperature, 9, 12, 13, 15, 18, 20, 22, 39, 40, 49, 50, 56, 59, 60, 72, 73, 92, 93, 112, 133, 169, 176, 189, 190, 196, 224, 227, 229, 240, 243, 254, 275, 300, 303, 314, 317, 320, 332, 341, 342, 348, 349, 352, 364, 388, 412, 422, 423, 442, 464 thermal evaporation, 30, 274, 356, 370, 424, 459, 475, 477 thermal oxidation, 30 thermal properties, 252, 320 thermal stability, 14, 212, 227, 285, 302, 320 thermogravimetric analysis, 235, 285, 302 thin film layers, 435, 468, 483

Index thin films, 8, 20, 50, 126, 127, 128, 129, 274, 289, 291, 292, 305, 307, 324, 337, 370, 384, 423, 425, 426, 427, 429, 430, 431, 432, 433, 434, 437, 443, 445, 458, 459, 460, 462, 464, 475, 477, 478 tin oxide, 49, 332, 355, 365, 371, 400, 401, 422, 442, 459 TiO2 nanofibers, 399, 400 TiO2 nanotubes, 75, 411 titania, 72, 231, 384, 406, 416, 474 titanium, xii, 211, 214, 342, 370, 383, 401, 408, 414, 415, 441, 442, 454 titanium isopropoxide, 401, 408 toluene, 273, 284, 299, 302, 316, 318 transition metal, 212, 222, 236 transition temperature, 303, 320 transmission electron microscopy, 61, 73, 213, 225, 235, 343, 358 transmittance spectra, 427, 445 treatment, xi, 10, 72, 170, 212, 233, 236, 278, 333, 392, 421, 423, 424, 425, 426, 427, 428, 429, 431, 432, 434, 437

U uniform, 40, 42, 53, 72, 73, 75, 83, 86, 90, 99, 109, 125, 182, 254, 272, 286, 293, 331, 332, 337, 338, 351, 364, 371, 383, 385, 386, 388, 392, 394, 397, 413, 426, 428, 432, 444, 453, 460, 466, 475, 480 universal gas constant, 20 uric acid, 147, 157 UV irradiation, 180 UV light, xii, 169, 174, 175, 176, 181, 190, 231

V valence, 33, 42, 52, 172, 182, 184, 214, 375, 388 vapor, 22, 85, 384 variations, 13, 20, 97, 118, 194 vibration, 51, 62, 88, 98, 107, 128, 149, 227, 257, 360

497 W

wastewater, xi, 170, 176, 222, 236, 245 water, xi, 20, 22, 40, 51, 62, 170, 180, 190, 200, 212, 222, 227, 236, 240, 252, 258, 274, 284, 299, 302, 316, 317, 318, 360, 423, 459 water purification, 252 wavelengths, 15, 42, 291, 375, 395, 408, 467, 470 weight loss, 227, 240, 285, 320 weight ratio, 274, 290, 325 wide band gap, 30, 50, 60, 169, 170, 180, 190, 203, 222, 342, 348, 356, 400, 423 working electrode, 29, 31, 40, 41, 44, 45, 49, 50, 51, 54, 56, 59, 61, 71, 72, 76, 78, 83, 84, 85, 93, 99, 124, 146, 147, 152, 153, 160, 331, 332, 343, 399, 400, 401, 405, 406, 407, 412

X X-ray photoelectron spectroscopy (XPS), 59, 63, 64, 230, 235, 241, 242, 257, 258, 361, 362, 369, 376, 378, 391, 403, 411, 414, 415, 428, 429, 446 X-ray diffraction (XRD), 32, 36, 41, 42, 43, 52, 62, 63, 126, 127, 171, 172, 181, 182, 183, 191, 192, 201, 202, 214, 215, 223, 224, 235, 237, 238, 239, 335, 345, 346, 359, 360, 373, 374, 375, 377, 388, 389, 462, 463, 477, 478

Z zinc, xii, 39, 49, 59, 60, 66, 169, 170, 171, 173, 179, 182, 186, 189, 201, 348, 361, 369, 370, 422, 459 zinc oxide, xii, 39, 49, 59, 60, 66, 170, 189, 369, 370, 422 ZnO nanorods, 31, 40, 351, 423 ZnO nanostructures, 50, 60, 190, 191, 336, 342, 355, 356, 363, 364, 365, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 423