Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication [1 ed.] 9781617618062, 9781616683443

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Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova Science

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

NANOTECHNOLOGY SCIENCE AND TECHNOLOGY

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NANOPARTICLES: PROPERTIES, CLASSIFICATION, CHARACTERIZATION, AND FABRICATION

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Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

NANOTECHNOLOGY SCIENCE AND TECHNOLOGY

NANOPARTICLES: PROPERTIES, CLASSIFICATION, CHARACTERIZATION, AND FABRICATION

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

AIDEN E. KESTELL AND

GABRIEL T. DELOREY EDITORS

Nova Science Publishers, Inc. New York

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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1. Nanoparticles. I. Kestell, Aiden E. II. DeLorey, Gabriel T. TP156.P3N358 2010 660--dc22 2009052737

Published by Nova Science Publishers, Inc.    New York Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

CONTENTS   Preface Chapter 1

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Chapter 2

vii  Nanostructured Materials: Classification, Properties, Fabrication, Characterization and their Applications in Biomedical Sciences Anees A. Ansari, M. Naziruddin Khan, M. Alhoshan,   A. S. Aldwayyan and M. S. Alsalhi Semiconductor Nanoparticles in Photocatalysis: The Present Status and Perspectives Olexander L. Stroyuk, Stepan Y. Kuchmiy, Anatoliy I. Kryukov   and Vitaliy D. Pokhodenko

1 

79 

Chapter 3

Nanoparticle Synthesis by Thermal Plasmas Takayuki Watanabe and Masaya Shigeta 

Chapter 4

TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic Paint Formulations Giuseppe Cappelletti  

213 

Morphology Changes in Carbon Nanoparticles Due to Different Atom Arrangements N. Koprinarov and M. Konstantinova  

255 

Gold Nanoparticle Labelled DNA Hairpin Grafting on Transparent and Conductive Oxide (TCO) Films: Characterization of Grafting and Hybridization V. Stambouli, V. Lavalley, A. Bionaz, P. Chaudouët, L. Rapenne, H. Roussel, A. Laurent, R. Jones and P. J. Pigram  

287 

Synthesis and Optical Properties of Polymer Functionalized Inorganic Nanoparticles Zhiguo Wang, Xiaotao Zu and Jingbo Li  

323 

Chapter 5

Chapter 6

Chapter 7

Index

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

149 

339 

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

PREFACE In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. It is further classified according to size: in terms of diameter, fine particles cover a range between 100 and 2500 nanometers, while ultrafine particles, on the other hand, are sized between 1 and 100 nanometers. Similar to ultrafine particles, nanoparticles are sized between 1 and 100 nanometers. Nanoparticles may or may not exhibit size-related properties that differ significantly from those observed in fine particles or bulk materials. This new book covers all aspects of nanoparticles. Chapter 1- In the rapidly changing scientific world, nanotechnology is one of the most emerging fields especially in material and biomedical sciences for technology applications in the past two decades [1-9]. Recent surge of interest in the nanotechnology has significant expanded the breadth and depth of nanostructured materials [10]. The nanoscale grain size can facilitate important advances in the development of nanodevices and nanomedicine to reduce the problems of disease to problems of molecular science, and are creating new opportunities for treating and curing disease [11-26]. Such advances have led to a new discipline of nanotechnology and lead to a variety of approaches for relieving suffering and prolonging life. Nanotechnology defines as the creation of functional materials, devices and systems through control of matter at the range of 1-100 nm scale [27]. The chemical and physical properties of nanostructured materials can be significantly different from those of atomic, molecular and bulk materials of the same composition [1-4, 27-30]. The uniqueness of chemistry, structure, response and dynamics of the nanostructures is the indispensable motivation for the study of this class of materials. Suitable control of the properties of nanometer scale structures can lead to new science as well as new products, devices and technologies [28-30]. The development of new sophisticated tools like STM, HRTEM, SESEM, AFM, XRD etc., to observe, measure and manipulate processes at the nano-scale level gave a breakthrough to the nanotechnology. The term nanotechnology was first used by the Japanese researcher Taniguchi in 1974 when he referred to the ability to engineer materials at the nanometer scale [31]. The main idea behind this terminology was the miniaturization in the electronics industry. Even as early as 1970s a huge number of nanostructures were created as small as 40-70 nm using electron beam lithography. The term nanomaterials covers materials in one dimension (quantum dots and thin films etc.) [27], two dimension (nanofibers, nanowires, nanotubes,etc.), and three dimension (nanoparticles/nanopowders, nanocapsules, fullerenes, dendrimers, molecular electronics, nanostructured materials, nanoporous materials etc.). Due to their small grain size, these

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viii

Aiden E. Kestell and Gabriel T. DeLorey

materials exhibit unique mechanical, chemical, physical, thermal, electrical, optical, magnetic, biological and also specific high surface area properties, which cannot be found in their bulk counterpart turn define them as nanostructures, nanoelectronics, nanophotonics, nanobiomaterials, nanobioactivators, nanobiolables, etc. Therefore, many researchers have been devoted to developed new synthesis routes, novel characterization methodologies and their application in bionanotechnology. Until now, a wide variety of nanostructured materials and devices with new capabilities have been generated employed in the literature some of them are polymeric materials such as (polyaniline, polypyrrole, polythiophene, polyethylene glycol, chitosan, polysaccharides etc.) [32-43], CNT(multi-walled and single walled) etc.) [44-49], metallic nanoaprticles (Ag, Au, Ge, Mo, Si, Pt, Pd, and Ru etc.) [50-74], semiconductor metal oxide (Al2O3,[75,76] CeO2[77-80], CdO[81,82], CoO,Co3O4[83], CuO,Cu2O [84,85], FeO, Fe2O3, Fe3O4[86-112], Ga2O3 [113], Hf2O3[114], In2O3[115-120], Ln2O3[121] MgO [122,123], MoO3[124,125], MnO2, Mn2O3, Mn3O4[126-131] Nb2O5[132,133] NiO[134,135], PbO2[136,137] Pr2O3[138-140] Sb2O3[141,142], SiO2[143147], SnO2[148-155] V2O5[156-158], WO3[159-161] TiO2[162-175] ZnO[176-186], ZrO2[187-194], Ln2O3[195-205], YVO4[206-214] (Ln = Y, La-Lu)), fluorescent metal nanoparticles (LnF3, LnF3:Ln, YF3, YF3:Ln [215-226], NaYF4, NaYF4:Ln NaLnF4, NaLnF4:Ln[227-248], LnPO4, LnPO4:Ln[249-258], (Ln = La-Lu)), semiconductor quantum dots (AgS, AlN, CdS, CdSe, CdTe, CuS, GaN, GaP, PbS, PbTe, ZnS, ZnSe, ZnTe etc. [259280]) and their hybrid derivative nanoparticles with different properties for wide applications in the fields of biomedical sciences. These size and structured dependent nanostructured materials offer excellent prospects in a wide variety of other practical applications, such as transparent electrodes, catalysis, solar energy conversion, field emission, photonic devices, drug delivery, biosensors(DNA, optical, chemical, gas and immunosensors) bio-labeling, biomarker, magnetic resonance imaging (MRI), laser technology, tissue engineering and in forensic sciences[281-295]. Chapter 2- A background and principles of semiconductor nanophotocatalysis – a new trend in photochemistry dealing with the photocatalytic redox-reactions with the participation of semiconductor nanoparticles – is discussed. The origins of various size-dependent phenomena in the semiconductor photocatalysis are highlighted with the special attention paid to the quantum size effects originating from a spatial confinement of the photogenerated charge carriers (excitons) in ultra-small semiconductor nanoparticles (quantum dots). The specifics of quantum-confined semiconductor nanoparticles is discussed, including the sizedependent optical properties (the position and shape of absorption and photoluminescence bands, the oscillator strength of interband electron transitions, etc.), thermodynamic characteristics (the band gap, the potentials of conduction and valence band edges, the nature, number and depth of charge trapping sites, etc.), as well as the dynamics of photogenerated charge carriers (charge migration in semiconductor nanoparticles, its localization on the structure defects, interfacial charge transfer, etc.). The most important consequences of spatial exciton confinement in semiconductor nanoparticles affecting their photocatalytic behavior are discussed. Particular attention is paid to the development of the photocatalytic properties in narrow-band-gap semiconductors at the nanoscale, the increase of a number of photocatalytic reactions for a given nanocrystalline semiconductor as compared with the bulk material, and the acceleration of photocatalytic reactions with the participation of semiconductor nanoparticles due to size-dependent growth of the energy of charge carriers. The photoinduced polarization caused by the accumulation of

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Preface

ix

excessive charge by semiconductor nanoparticles and resulting in a remarkable increase of their photocatalytic activity is discussed. The specific features of the photocatalytic behavior of semiconductor nanoparticles are illustrated in the chapter by a number of examples. In particular, the size-related phenomena in the photocatalytic metal reduction, photocatalytic formation of binary semiconductor nanoheterostructures, photoinduced polymerization of acrylic monomers, photocatalytic reduction of sulfur compounds, and photocatalytic water reduction with the participation of semiconductor nanoparticles are discussed. Special attention is paid to the photochemical behavior of quantum-confined semiconductor nanoparticles under powerful pulse illumination. The examples of simultaneous and additive influence of different size effects upon the photocatalytic properties of semiconductor nanoparticles are demonstrated. In conclusion, perspectives of future development of the photocatalytic systems based on nanostructured semiconductors are given, outlining the ways for their further perfection, broadening of the number of known nanophotocatalysts and photocatalytic processes, as well as the benefits of the utilization of such systems in modern nanotechnologies. Chapter 3- Thermal plasmas, source of very high enthalpy, high chemical reactivity offer rapid evaporation rate, steep temperature gradients, wide area and high deposition/growth rate, and an attractive and chemically nonspecific route for the synthesis of fine powders down to the nanometer size range. Among various types of thermal plasma reactors, radio frequency induction thermal plasma (ITP) reactors offer several advantages such as high purity due to absence of electrode, large volume of plasma for processing, low plasma velocity, simple power supply unit and wide pressure range. ITP reactors, which basically convert the electrical energy into heat energy, have been widely used as a clean heat source for synthesis of nanoparticles. The high-temperature of ITP leads to short evaporation time which translates into relatively small torch with high throughput. Powder synthesis needs sharp temperature gradient for rapid condensation in the reaction chamber. ITP offers the distinct advantage of providing essentially one-step processes, avoiding the multitude of steps required in the creation of particles with conventional methods. The feasibility of producing nanoparticles of various intermetallic compounds, alloys, oxides, and nitrides by vapor-phase synthesis in reactive thermal plasma systems will be demonstrated in this chapter. To investigate the complicated phenomena in the synthesis processes of nanoparticles by ITP, theoretical and numerical studies are powerful approaches. The precursory powders of the raw materials are injected into the plasma and vaporized due to the heat transfer from the plasma. The vapor is transported downstream with the plasma flow. Since the saturation pressure drastically decreases with the rapid temperature drop at the tail of the plasma, the vapor becomes supersaturated. As a result, nuclei are generated by homogeneous nucleation, and the supersaturated vapor easily condenses on the nuclei by heterogeneous condensation. This process is the fundamental formation mechanism of nanoparticles by ITP. Simultaneously nanoparticles collide and coagulate among themselves. Coagulation also plays an important role for the nanoparticle growth. The mathematical models are introduced to simulate the processes. The fields in the plasma are expressed on the basis of the electromagnetic fluid dynamics. The trajectory and temperature history of the precursory powders are examined by Lagrangian approach taking into account the rarefied gas effects. The nanoparticle formation can be modeled by the aerosol dynamics, basically with Eulerian approach, taking into account not only nucleation, condensation, and coagulation but also convection, diffusion, and thermophoresis. Through the computation with these models, the

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particle size distributions or compositions of the produced nanoparticles are obtained and the formation mechanisms are clarified. Chapter 4- This review chapter is focused on the physico-chemical features and the related semiconductor activity of nanometric TiO2 both pure and formulated as photocatalytic paint. First the synthesis of the particles, with controlled structural and morphological aspects, is discussed on the grounds of both literature and original research data. Different synthetic approaches are presented: combustion, precipitation and solvothermal routes, sol-gel synthesis, and also more innovative methods like those assisted by ultrasound and microwaves. The photocatalytic activity with respect to the degradation of both water (organic dyes, chlorophenols, etc.) and air pollutants (nitrogen oxides, NOx, volatile organic compounds, VOC, etc.) is analyzed. Literature, commercial and home made samples are considered both in the case of pure and doped TiO2. The second part of the chapter concerns the formulation of photoactive paints. Liquid coating paint formulations are complex, multicomponent systems including resins as film forming agents, binders, additives, solvents and extenders. The possibility to incorporate nanoactive semiconductor materials (such as nano-TiO2) in order to obtain photocatalytic paints leads to several problems also due to the possible simultaneous photodegradation of the desired air pollutants and of the organic matrix of the coating. The few data, present in the literature concerning this latter points, are compared also with original laboratory results. Chapter 5- The intensive investigations of carbon materials stimulated remarkably by the discovery of fullerenes and carbon nanotubes resulted in the conclusion that the character of the carbon atom connection in the carbon network has crucial importance for the structure and the properties of carbon nanoparticles. Carbon materials are in the most stable energetic situation if their atoms are connected via sp3 bonds and build 6 atom rings (n-gons with n=6), which, on their side, construct a honeycomb-like network. In nature, it is normal and frequently observed how variations in local conditions hinder the building of this perfect structure via self organization of carbon atoms. Because of this, rings of atoms other than 6 often emerge in the growing carbon nanoparticle network, transforming the last to unique ones in regard to shape and properties. In this chapter, such rings are considered n-gons with defects. Nevertheless, from the fact that only one or few atoms or one or few n-gons emerge in the whole network, these defects change it, resulting in a significant modification in nanoparticle structure and properties. In this chapter, more attention is paid to the effects caused by the emergence of n-gons with n = 5 and 7, because their incorporation explains very well the structure of the most of the observed carbon nanoparticles. The simultaneous emerging of 5/7 defects such as pentaheptite, defects oppositely situated in the nanoparticle structure and the repetitive pair and carbon network deformation caused by them, are also discussed. The theoretically proposed carbon networks constructed from repetitive combinations of 5/7 and 5/6/7 n-gons are exposed. Those can be sources for the construction of a new kind of nanostructure in the same manner as nanostructures built from a honeycomb-like network. It is shown, too, how the connection between two carbon particles can be realized through a row of atom bond transformation, in the course of which different n-gons are created and destroyed. The examples given for the correlation between theoretically predicted carbon nanostructures and their actually observed analogs confirm the ability of theory to predict and to tailor the properties of a new type of carbon nanostructure.

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Chapter 6- Biosensors and biochips can be hybrid nanobiosystems involving different kinds of components, i.e. solid surface, bio-molecules and nanoparticles. These components are confined in a very small area (nanometre range). It is expected that interactions are produced between both components due to their close proximity. So to optimize the performance of these biosensors, it is very important to get a deeper insight of their surface characteristics. In this context, nanoparticles linked to DNA strands (in a ratio of 1:1) immobilized on a solid surface give the opportunity to combine complementary techniques to characterize the hybrid system. A typical example will be illustrated in this study. The authors have grafted DNA hairpins at their 3’-end via a silanisation process using aminopropyltriethoxysilane (APTES) on different transparent and conductive (TCO) oxide film surfaces. DNA hairpins comprise a stem in which both strands are complementary and a loop. These molecules exhibit a particularly high sensitivity for the detection of mismatches compared to the corresponding linear strands. They have been monolabelled at their 5’-end by a 1.4 nm gold nanoparticle. Because of the hairpin conformation, the label is close to the surface. Upon hybridization with a complementary target, the formation of a linear duplex structure with relative rigidity forces the label away from the surface. Due to their conductive properties, TCO films are attractive materials for biochips. They can advantageously replace the classical gold electrodes as working electrodes for direct electrochemical detection of DNA hybridization. As for silica, their surface chemistry allows the covalent and strong binding of DNA. Here, the authors used different TCO films: ITO films, doped SnO2 films as well as insulating SiO2 films. Thanks to the presence of gold nanoparticles bounded to DNA probes, the effects of grafting and hybridization of DNA could be studied on both conductive oxide surfaces. Particularly, the authors studied the modifications of surface morphology and chemistry as well as fluorescence results. By coupling AFM with SEM-FEG analyses, dispersed and well-resolved groups of gold nanoparticles linked to DNA were emphasized on the SnO2 films. Their surface density is 2.1 ± 0.3 x 1011 groups.cm-2. TEM images obtained after silver enhancement of gold nanoparticles on ITO films revealed round spheres corresponding to silver coated gold nanoparticles. Their density was in agreement with the data obtained by AFM on SnO2 films. The evolution of the chemical state of the modified oxide surfaces was monitored using XPS and ToF-SIMS. As expected, the XPS N 1s peak intensity increased after grafting and hybridization of DNA. The Au 4d peak was detected only on samples modified with Au labelled hairpin probes. Its intensity decreased with probe concentration. From the ratio Au/Si (Si belonging to APTES), the surface DNA density was estimated to be 9.6x1011 cm-2 and 3.7x1011 cm-2 for SnO2 and ITO films respectively. The P 2p peak was observed only after hybridization with a weak intensity. Its presence was essentially correlated to phosphate residues originating from the hybridization solution. Positive and negative fragments of sugar, bases and phosphates from DNA probes were identified by TOF-SIMS. Positive and negative ions from Au nanoparticles were detected only in the case of Au labelled hairpin probes before and after hybridization. After hybridization of Au labelled hairpin probes with complementary Cy3 targets, quenching of the Cy3 fluorescence by gold nanoparticles was evidenced using fluorescence microscopy. This phenomenon was obtained for both oxides and is in agreement with the Nanometal Surface Energy Transfer (NSET) theory.

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Chapter 7- Nanoparticles possess many special characteristics that can be used in many fields such as optics, electricity, magnetism and catalysis. However, it is difficult to obtain a uniform dispersion system of nanoparticles owing to their strong tendency of agglomeration for their high surface energy, so surface modification is urgently needed. Using polymer to modify the surface performance of nanoparticles is an interesting method. One major topical areas on preparing polymer nanoparticles are summarized in this chapter. Poly (methyl methacrylate) functionalized nanoparticles (anatase, γ-Al2O3, SiO2, and ZnO) are prepared using γ and electron radiation. A blue luminescence peak (at ~430 nm) can be observed for the nanocomposite. Annealing in air atmosphere destroys the carboxylate bonds and induces the decrease of luminescence density.

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In: Nanoparticles: Properties, Classification,… Editor: A. E. Kestell and G. T. DeLorey, pp. 1-78

ISBN: 978-1-61668-344-3 © 2010 Nova Science Publishers, Inc.

Chapter 1

NANOSTRUCTURED MATERIALS: CLASSIFICATION, PROPERTIES, FABRICATION, CHARACTERIZATION AND THEIR APPLICATIONS IN BIOMEDICAL SCIENCES Anees A. Ansari*, M. Naziruddin Khan, M. Alhoshan, A. S. Aldwayyan and M. S. Alsalhi King Abdullah Institute for Nanotechnology, King Saud University, Riyadh-11451, P.O Box-2454, Saudi Arabia.

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INTRODUCTION In the rapidly changing scientific world, nanotechnology is one of the most emerging fields especially in material and biomedical sciences for technology applications in the past two decades [1-9]. Recent surge of interest in the nanotechnology has significant expanded the breadth and depth of nanostructured materials [10]. The nanoscale grain size can facilitate important advances in the development of nanodevices and nanomedicine to reduce the problems of disease to problems of molecular science, and are creating new opportunities for treating and curing disease [11-26]. Such advances have led to a new discipline of nanotechnology and lead to a variety of approaches for relieving suffering and prolonging life. Nanotechnology defines as the creation of functional materials, devices and systems through control of matter at the range of 1-100 nm scale [27]. The chemical and physical properties of nanostructured materials can be significantly different from those of atomic, molecular and bulk materials of the same composition [1-4, 27-30]. The uniqueness of chemistry, structure, response and dynamics of the nanostructures is the indispensable motivation for the study of this class of materials. Suitable control of the properties of nanometer scale structures can lead to new science as well as new products, devices and technologies [28-30]. The development of new sophisticated tools like STM, HRTEM, SESEM, AFM, XRD etc., to observe, measure and manipulate processes at the nano-scale *

Corresponding author: E-mail: [email protected]

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Anees A. Ansari, M. Naziruddin Khan, M. Alhoshan, et al.

level gave a breakthrough to the nanotechnology. The term nanotechnology was first used by the Japanese researcher Taniguchi in 1974 when he referred to the ability to engineer materials at the nanometer scale [31]. The main idea behind this terminology was the miniaturization in the electronics industry. Even as early as 1970s a huge number of nanostructures were created as small as 40-70 nm using electron beam lithography. The term nanomaterials covers materials in one dimension (quantum dots and thin films etc.) [27], two dimension (nanofibers, nanowires, nanotubes,etc.), and three dimension (nanoparticles/nanopowders, nanocapsules, fullerenes, dendrimers, molecular electronics, nanostructured materials, nanoporous materials etc.). Due to their small grain size, these materials exhibit unique mechanical, chemical, physical, thermal, electrical, optical, magnetic, biological and also specific high surface area properties, which cannot be found in their bulk counterpart turn define them as nanostructures, nanoelectronics, nanophotonics, nanobiomaterials, nanobioactivators, nanobiolables, etc. Therefore, many researchers have been devoted to developed new synthesis routes, novel characterization methodologies and their application in bionanotechnology. Until now, a wide variety of nanostructured materials and devices with new capabilities have been generated employed in the literature some of them are polymeric materials such as (polyaniline, polypyrrole, polythiophene, polyethylene glycol, chitosan, polysaccharides etc.) [32-43], CNT(multi-walled and single walled) etc.) [44-49], metallic nanoaprticles (Ag, Au, Ge, Mo, Si, Pt, Pd, and Ru etc.) [50-74], semiconductor metal oxide (Al2O3,[75,76] CeO2[77-80], CdO[81,82], CoO,Co3O4[83], CuO,Cu2O [84,85], FeO, Fe2O3, Fe3O4[86-112], Ga2O3 [113], Hf2O3[114], In2O3[115-120], Ln2O3[121] MgO [122,123], MoO3[124,125], MnO2, Mn2O3, Mn3O4[126-131] Nb2O5[132,133] NiO[134,135], PbO2[136,137] Pr2O3[138-140] Sb2O3[141,142], SiO2[143147], SnO2[148-155] V2O5[156-158], WO3[159-161] TiO2[162-175] ZnO[176-186], ZrO2[187-194], Ln2O3[195-205], YVO4[206-214] (Ln = Y, La-Lu)), fluorescent metal nanoparticles (LnF3, LnF3:Ln, YF3, YF3:Ln [215-226], NaYF4, NaYF4:Ln NaLnF4, NaLnF4:Ln[227-248], LnPO4, LnPO4:Ln[249-258], (Ln = La-Lu)), semiconductor quantum dots (AgS, AlN, CdS, CdSe, CdTe, CuS, GaN, GaP, PbS, PbTe, ZnS, ZnSe, ZnTe etc. [259280]) and their hybrid derivative nanoparticles with different properties for wide applications in the fields of biomedical sciences. These size and structured dependent nanostructured materials offer excellent prospects in a wide variety of other practical applications, such as transparent electrodes, catalysis, solar energy conversion, field emission, photonic devices, drug delivery, biosensors(DNA, optical, chemical, gas and immunosensors) bio-labeling, biomarker, magnetic resonance imaging (MRI), laser technology, tissue engineering and in forensic sciences[281-295]. Due to their specific surface properties of different nanostructured materials, specific interests in biomedical applications. Noticed that the diameter of nano-structured materials, surface condition, crystal structure and its quality i.e., chemical composition, crystallographic orientation are key important parameters that influence the properties of the materials [296]. Moreover, the conductance of nanostructured materials strongly depends on their crystalline structure. For instance, in the case of perfect crystalline Si nanowires having four atoms per unit cell, generally three conductance channels are found. One-or two-atom defect, either by addition or removal of one or two atoms may disrupt the number of such conductance channel and may cause variation in the conductance. Observation results revealed that change in the surface conditions of the nanowires can cause remarkable change in the transport, electrical, mechanical and catalytic behavior.

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Nanostructured Materials: Classification, Properties, Fabrication…

3

The manipulation of nanometer-scale structures can lead to new sciences as well as new devices and technologies. In the last two decade, some new synthesis methodologies have been employed for the preparation of nanocrystals of metals, semiconductors, quantum dots and magnetic nanomaterials using various chemical and physical methods. Detail on various preparation techniques are being discussed with their classification, novel properties, synthesis methodologies, characterization and applications that are likely to fuel research into the coming decades. And other related nanoparticle syntheses by various physical and chemical synthesis process is discussed in this chapter. The goal of this chapter is to summaries the classification, properties, fabrication process, characterization technique of nanostructured materials and their advancement in the development of biomedical sciences.

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2. CLASSIFICATION OF NANOSTRUCTURED Nanostructured describe the intermediate size between molecular and microscopic (micrometer-sized) structures. They can be divided according to their crystal symmetry and dimensionality, which highly affects the functionality of the correspondence parameter. Further the dimensionalities of nanomaterials are commonly classified into zero dimensional (dots or mostly spherical nanoparticles), one dimensional (nanowires, nanotubes and nanobelts) or two-dimensional (thin films). These categories refer to the number of dimensions in which the material is outside the nano regime. A thin film, for example, consists of large expanses of material in both in-plane directions and nanosized only in its thickness; therefore it is termed as two-dimensional (2D) nanomaterial. Due to the effects of size and dimensionality of a nanomaterial can alter its properties, such that a thin film may behave very differently from a nanowire with the same chemistry. These distinctions therefore are not arbitrary, but important in distinguishing the materials according to unique characteristics. Figure 1 shows the geometrical differences between the classes of nanostructures. Nanodiscs, nanoplates, nanosheets, and nanomembranes are two-dimensional (2-D) nanostructures: the x and y directions are fully extended with only the thickness along the z direction between 1 and 100 nm of the polygon-shaped surfaces. Nanorods, nanowires, and nanotubes are examples of one-dimensional (1-D) nanostructures. They can be hollow tubes or cylinders with polygon shapes of nanoscale diameter while the length of the nanostructures increases. Nanostructures with zero dimensions (0-D) on the nanoscale are classified as nanoparticles, each spatial dimension having a scale within 100 nm. The shapes can be spheres, isotropic spheres, icosahedrons, octahedrons, cubes, or tetrahedrons. The potential of these materials use for structural, commercial, environmental, electronic, and biological applications, have lately sparked their interest and imaginations in all science disciplines, and even the general public. While certainly improved synthesis techniques have helped move 1D nanomaterial research forward, the driving force behind their rapid development is a new appreciation for their unique properties, driven by the need to utilize these properties in highly demanding applications (Table 1).

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Figure 1. Geometric shapes of basic nanostructures.

3. PROPERTIES OF NANOSTRUCTURED MATERIALS Materials in the micrometer scale usually exhibit similar physical properties as that of bulk form; however, materials in the nanometer scale may exhibit distinctively different physical properties. When the dimensions of materials go down to nanometer scale, some remarkable specific physical, chemical and biological properties due to increased surface area, smaller particle size, reduced number of free electron, and quantum confinement effect would be developed. Size effect has a dramatic impact on structural, thermodynamic, electronic, spectroscopic, electromagnetic and chemical properties of nanomaterials. Size affects the structure, melting point and the electronic absorption of nanomaterials such as that of ZnS and ZnO. For example, semiconductor nanocrystals are zero-dimensional quantum dots, in which the spatial distribution of the excited electron-hole pairs are confined within a small volume, resulting in enhanced non-linear optical properties. Due to the small grain size, the electronic properties of a semiconductor nanocrystal are significantly affected by the transport of a single electron, giving the possibility of producing single electron devices.

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Table 1. Summery of the siz of different nanomaterials.

Semiconductor crystals exhibit a broad spectrum of strong size dependent properties for particle sizes below the Bohr radius of their bulk exciton. The size dependence of the nanocrystal electronic energy level has attracted special interest. It allows one to tailor the materials optical and electronic properties by controlling particle size, and many potential new applications such as nanocrystal bio-tags, quantum dot lasers, polymer-based nanocrystal solar cells, and single electron transistors have been suggested. The concept of quantum confinement describes the effects of reduced dimensions on the electronic properties of nanostructures. If the size of the nanoparticle becomes similar to the wavelength of the charge carriers in the valence and conduction band, the particle can be

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described as a potential well where waves are reflected at the potential barrier and spatially confined. As a result, the electronic structure of the crystal is altered from continuous bands to discrete levels and the continuous optical transitions between the electronic bands become discrete and size-dependent as well. The basic idea behind quantum confinement can be visualized as a classical particle in a box problem. For a free particle with an effective mass of m* confined in a crystal with a one dimension barrier at a distance of L, the movement of the particle can be described with the Schröedinger equation:

…………………….(Eq.1)

where Ψ(x) is the wave function and E is the energy of the particle. This equation can be solved by using the boundary conditions that Ψ(x) = 0 at x = 0 and x = L:

for

n = 1,2,3…………..(Eq.2)

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where kn is the allowed wave factors and kn = n π / L When this simplified solution is extended to a semiconductor nanocrystal with a size close to the Bohr radius aB, the resulting expression for the energy shift from the ground state of the corresponding bulk materials is:

ΔEn = n2ħ2π2/2m*a2 ………………………..(Eq.3) where a is the size of the nanocrystal, m* is the reduced effective exciton mass which is defined by 1/m* = 1/me*+1/mh*.me* and mh* are the effective mass of electron and hole of the material, respectively. The Bohr radius of a semiconductor can be obtained from the dielectric constant ε and the reduced exciton mass m* : aB = εh2/m*e2. Thus, the band gap of the material can be engineered to a desired level by changing the size of the nanocrystals:

Eg, nanocrystal = Eg, bulk +ΔE =Eg,bulk + ħ 2p2/2m*a2 ………….(Eq.4) The band gap energy increases with a decrease in the size of the nanocrystals. Since the optical transition is directly related to the band gap, it is possible to tune the absorption and photoluminescence wavelength of the nanocrystals just by manipulating their size.

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Figure 2. Illustration of a semiconductor band structure showing shift of energy band gap.

There are numerous properties of metal oxides that are affected by decreasing the grain size within the material. Some of the most important properties of nanophase metal oxides are discussed here. Mechanical, electronic, magnetic properties and catalytic of nanophase materials have important applications. Also, the transport properties play an important role in technological development. These properties make nanophase materials attractive to study both for fundamental aspects as well as for potential novel applications (Figure 2).

3.1. Mechanical Properties In metal oxides formation, grain size reduction can yield improvements in strength and hardness. Grain size reduction increases the strength and hardness, due to new grain boundaries, which act as effective barriers to dislocation motion. However, it may affect other mechanical properties negatively, such as creep rate and ductility. On the other hand, in materials that are conventionally quite strong but very brittle, such as intermetallic compounds and ceramics (ZnO, CeO2, ZrO2, TiO2 and SnO2), enhanced ductility from grain size reduction, through the increased grain boundary sliding can be considered favorable.

3.1.1. Hardness While the hardness of materials clearly increases as their grain size is reduced into the nanophase regime (Figure 3) that is dependent in the variation of the grain size. Increased Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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hardness with decreased grain size conflicts with the softening of nanophase materials seen in samples annealed to increase their grain size. Inter metallic alloys show initial hardening with decreasing grain size, but at further reduced grain sizes two different cases with either hardening at a reduced slope or softening can be separated. Although hardening in nanophase materials is analogous to Hall-Petch strengthening it is considered to result from different mechanisms. The grain sizes in ultrafine particles are smaller than the critical bowing length for Frank-Read dislocation sources needed to operate at the stresses involved and smaller than the normal spacing between dislocations in a pile-up. The presence of sample porosity, flaws, or contamination from synthesis and processing could influence the available hardness results, as could the nature of the grain boundaries and their state of relaxation.

3.1.2. Thermal stability Thermal stability of nanocrystalline materials shows basically that nanocrystalline metals exhibit grain growth at relatively low annealing temperatures. This is to be expected because of the large energy stored in the material as a result of the large volume fraction of grain boundaries. The general observation emerging from these studies is that for metals with an equilibrium melting temperature (Tm) less than approximately 873K and with starting grain size of 10nm or less, doubling of the grain size takes place in approximately 24 hours at about ambient temperature. However, for metals with high Tm, the stability against grain growth seems to be improved. Gleiter [297] reported that nanocrystalline iron, with a starting grain size of about 10nm, is thermally stable up to 473K. The grain size increases by a factor of 5 after annealing for 10 hours at 673K. Annealing at 773K for 1 hour transformed the material into the conventional polycrystalline state. Figure 4 shows the grain size as a function of the annealing time for nanocrystalline Ni-P at various temperatures. The results indicate no grain growth for up to 10 hours at the lowest annealing temperature (473K). However, at the intermediate temperature, 573 and 623K, the grain size initially increased rapidly by a factor of about 2 to 3 before stabilizing at 15 and 22nm, respectively.

Figure 3. Microhardness (Vickers) Measurements across nanophase Cu samples ranging in grain size from 6 to 50 nm and compared with conventional 50 μm grain size Cu. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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Figure 4. Grain size as a function of the annealing time for nanocrystalline Ni-P at various temperatures [From reference 298].

3.1.3. Tensile strength and fracture Face-centered cubic metals tested in tension have exhibited similar improvements in strength as those seen in their hardness behavior, but they have shown limited ductility. The limited levels of ductility exhibited by nanophase metals may arise because of difficulties in creating, multiplying, and moving dislocations, but may as well relate to the presence of significant flaw populations in these materials. The results to date on the fracture properties of nanophase materials have been limited in scope and hindered by the presence of porosity or interfacial phases [299]. Conventionally brittle ceramics have been observed to become ductile, permitting large plastic deformations at low temperature if the ceramics are generated in the nanocrystalline form. Doping of nanocrystalline ceramics has been demonstrated to reduce grain growth dramatically. Grain boundary sliding, grain rotation and grain shape accommodation by diffusion processes seem to play a crucial role in the deformation of nanocrystalline ceramics, i.e. processes that are typical for superplasticity [300].

Figure 5. A stress-strain curve for nanophase Pd compared with that for a coarse-grained Pd sample [301]. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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It appears that the increasing hardness and strength in nanophase metals with decreasing grain size indicates a result of diminishing dislocation activity and increasing grain boundary density. Figure 5 shows, the frequency of dislocation activity decreases and that of the grain boundary sliding increases [301].

3.2. Transport Properties

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The solid oxide fuel cell is a candidate for efficient electric power generation systems. Solid electrolytes used in such cells have been increased with investigation for many years due to their suitability as ionically conductive materials at high temperatures, such as solid oxide fuel cells and oxygen sensors. A considerable amount of research is being carried out in order to develop materials suitable for applications in the medium temperature range [302]. These solid electrolytes are mostly fluorite structure. Open structure is required for the material to have high ionic conductivity. The IVB group oxides ThO2, CeO2, PrO2, UO2 and PuO2 have the fluorite structure and by doping it is possible to stabilize this structure for ZrO2 and HfO2. The addition of dopants gives rise to the creation of oxygen vacancies responsible for the ionic conductivity in these oxides [303].

3.2.1 Ionic and electronic conductivity There are two important ways in which materials can conduct electrical current. Both electrons and ions can carry electric charge. For metals, the electronic conductivity dominates while for many ceramic oxides ionic conductivity is the major contributor that may exceed electronic conductivity by several orders of magnitude. In metals the non-localized electrons carry the charge. The ionic conductivity is associated with ion motion. Lattice defects are of great importance for ionic conductivity [304]. Diffusion and ionic conductivity are related through the Nerst-Einstein equation.

Figure 6. Absorption spectrum of CdS particles in aqueous solution as a function of particle size (left). Photoluminescence spectra of nanocrystalline ZnO with different crystal sizes in comparison with the bulk material (right) [308] .

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By doping cerium oxide, it becomes an anionic conductor at high oxygen pressures. Lower valent cations in the lattice make the formation of anion vacancies energetically more favorable than the formation of electron holes. At intermediate oxygen pressures n-type conductivity interferes [303].

3.2.2. Effects of particle size and doping on electrical conductivity Transport properties of ceramics, in semi- and ionic conductors, are often limited by grain boundaries. The defect and transport properties of nanocrystalline oxides are unique in two principal aspects. Grain boundary impedance is greatly reduced due to size dependent impurity segregation [305]. In varistors as an example this gives rise to useful electrical barriers. Defect thermodynamics dominated by interfaces are obtained in a bulk material when particle size is in the nanometer regime. The unusual defect thermodynamics of the nanocrystals are attributed to interfacial reduction [306]. The heat of reduction of nanophase ceria is lowered by more than 2.4 eV per oxygen vacancy compared to coarse particles. This reduction occurs at suitably low-energy to dominate the nonstoichiometry and electrical conductivity of the material as whole. Diffusion usually takes place by the movement of ions to neighboring vacancies. Oxygen atoms are located in the oxygen interstitials on non-fluorite sites. For stoichiometric compounds, the vacancy concentration and ionic conductivity are very small, but suitable doping may increase both. Also, smaller particle size increases the non-stoichiometry of a material. The oxygen ion conductivity of doped cerium oxide with a constant doping level has been found to be dependent on the lattice constant of the compound. Low doping changes the conductivity of ceria from an ionic to an electronic conductor [307].

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3.3. Electronic Properties For small particles, the wave functions of electrons and holes are confined to the particle volume. If the particle size becomes comparable to or smaller than the de Broglie wavelength of the charge carriers, the confinement increases the energy required for creating an electron/hole pair. This increase shifts the absorption/luminescence spectra towards shorter wavelengths (so called "blue shift" [308]) as shown in Figure 6. In direct gap semiconductors, the band gap increases with decreasing particle size, and the excited electronic states become discrete with high oscillator strength. The optical absorption spectrum of γ-Fe2O3 is red shifted, which can be explained by the lattice strain in small particles [309]. In a polymer matrix γ-Fe2O3 shows optical transparency in the visible range.

3.3.1. Indirect semiconductors Nanocrystalline Si and porous Si are indirect semiconductors where the absorption involves both a photon and a phonon because the conduction and valence bands are widely separated in k space [310]. Figure 7 shows the differences in optical absorption and optical transition in direct and indirect gap semiconductors. A direct photon transition at the minimum gap cannot satisfy the requirement of conservation of energy, because photon wave vectors are negligible at the energy range of interest. But if a phonon of wave vector K and frequency W is created in the process, then the

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excitation can occur. Several vibronic interactions should increase when wave functions become more compact in nanocrystals. Thus, the vibronic luminescence rate as well as the electronic luminescence rate should increase with decreasing size in nanocrystals. For sizes larger than roughly 15 Å, the vibronic luminescence dominates. Although these nanoparticles have high luminescence yields, the oscillator strengths of nanocrystalline Si and porous Si are not markedly increased with respect to bulk Si [310-312]. In silicon the quantum size effect is primarily kinetic, while in the conventional semiconductors the effect is more spectroscopic [313]. Silicon luminescence increases because spatial confinement keeps the electron and hole superimposed and surface nonradiative rates are also extremely slows.

3.4. Magnetic Properties

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3.4.1. Magnetism of multi and single domain particles A crystal will spontaneously break up into a number of domains in order to reduce the large magnetostatic of single domain particles. Dipolar energy can be reduced by dividing a magnetic specimen into uniformly magnetized domains of macroscopic size, whose magnetization vectors point in widely different directions. Such subdivision is paid for in exchange energy, for the spins near the boundary of a domain will experience unfavorable exchange interactions with the nearby spins in the neighboring domain. Only the spins near the boundaries will have higher exchange energies but the magnetic dipolar energy drops for every spin when domains are formed [314]. When the crystal size is reduced below a critical value Lc of a few hundred Å a single crystal will become a single domain. In the single domain particles (SD) it is not energetically favorable to get magnetization reversed in the particle so no walls are formed.13

Figure 7. Optical absorption in pure insulators with direct and indirect gaps at absolute zero [310].

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The extrinsic magnetic properties of particles depend strongly upon their shape and size. This result was obtained when magnetostatic, exchange and domain wall energies were considered. Interactions between particles also affect the critical size. Among magnetic properties coercivity Hc shows a marked size effect. Saturation magnetization on the other hand is not dependent on the particle size [315].

3.4.2. Superparamagnetism Superparamagnetic particles posses a magnetic moment that may be many orders of magnitude greater than that of a paramagnetic atom μ = MsV and, if a field is applied, the field will tend to align the moments of the particles. Thermal energy on the other hand will tend to misalign them. This is just like the normal paramagnetic behavior, just with an exceptionally large magnetic moment. In very small particles random thermal forces are large enough to cause the magnetization direction to reverse spontaneously between the easy directions [316]. The average time between reversals is an exponential function of the ratio of the particle volume to absolute temperature. A particle will spontaneously reverse its magnetization even in the absence of an applied field when the energy barrier to rotation is about 25kT [317]. When the field is turned off the initial magnetization will begin to decrease due to the thermal energy. The rate of decrease is proportional to the magnetization existing at the time and to the Boltzmann factor eKV/kBT. Particles that exhibit super paramagnetic behavior have a large saturation magnetization but no remanence or coercivity. Hysteresis will appear and super-paramagnetism disappears, when particles of a certain size are cooled to a particular temperature, or when the particle size, at constant temperature, increases beyond a particular diameter. For superparamagnetic particles magnetization curves measured at different temperatures superimpose when M is plotted as a function of H/T. Since the direction of magnetization in SP particles is changing, the particles cannot be used for magnetic recording. Potential applications of nanoscale magnetic particles are in color imaging, ferro fluids and magnetic refrigeration. Nano size metallic particles may be so reactive that they undergo surface oxidation [318321]. Magnetism of these particles can be explained with a core-shell structure, where the core consists of metallic iron and the shell is composed of iron oxides. The magnetic properties depend strongly on particle size and the amount of oxidation. The saturation magnetization is determined from M vs. 1/H2 plots by extrapolating the value of magnetization to infinite fields. Ms was found to increase with increasing particle size. The decrease in magnetization with decreasing particle size is related to the higher surface to volume ratio in the smaller particles resulting in a much higher contribution from the surface oxide layer. The coercivity of the particles is found to depend strongly on particle size [318,319]. Hc decreases as a function of temperature and decreasing particle size makes the difference in coercivity larger by increasing the coercivity at low temperature and dropping to zero at lower temperatures. Larger surface to volume ratio could result in a higher effective anisotropy with large contributions, from the surface and interface anisotropy. In addition to the usual magnetocrystalline and magnetostatic anisotropies, often there are strong influences from the anisotropies due to strains, surface and magnetic interactions coming from the other particles. Experimentally it has been found that K is one or two orders of magnitude larger for Fe, Feoxides and Fe-SiO2 in the nanosize particles than in bulk [318,319]. The large effective

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anisotropy is thought to partly originate from the shell-type particle morphology with strong shell-core interactions and partly from the large surface effects expected in ultrafine particles.

3.5. Catalytic Properties Studies on catalytic properties are predominantly based on single crystal surfaces of metal in vacuum. There is very little information on catalytic processes involving oxides. Catalytic reactions involve the following general surface chemical steps [322]. 1. Adsorption of the reactant molecules at surface sites 2. Bond breaking of the adsorbates 3. Rearrangement of the adsorbed reaction intermediates 4. Desorption of the molecules

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The influence on decreasing the particle size at different stages in catalysis is discussed in the following sections. Materials become more stepped and rough as the particles become smaller in size. Doping gives rise to anisotropic surfaces.

3.5.1. Particle size dependence of surface processes Several surface processes are dependent on the surface structure and particle size. It has been shown that some catalytic reactions are structure insensitive, and only one active surface metal atom is needed. For structure-sensitive reactions several adjacent active surface atoms are needed. Reactions involving M-O bond formation or breaking are usually structure insensitive, while reactions involving O-O bond breaking/formation are usually structure sensitive. For the structure-sensitive reactions particle size and other properties can be of importance. Both catalytic activity and selectivity is dependent on particle size [323]. 3.5.2. Adsorption All catalytic reactions proceed through adsorption. The adsorbate atoms are trapped at sites where the well depth of the attractive surface potential is higher than the kinetic energy of the atoms. The sticking probability of atoms is defined as the rate of adsorption divided by the rate of collision, and it is higher for more open and rough surfaces than on flat ones. This is due to the higher heat of adsorption at a kink or step edge site. 3.5.3. Diffusion and desorption Diffusion is very fast on catalyst surfaces and requires a much lower activation energy than desorption. Diffusion is anisotropic since diffusion rates parallel to steps are greater than diffusion rates perpendicular to steps. The presence of co-adsorbates influences surface diffusion markedly. Elements that decrease the melting point of the substrate cause an increase in surface diffusion rates and a decrease in activation energies for diffusion in general. The desorption of atoms from kink and step sites requires a much higher energy than from flat surfaces. The change of surface from flat to stepped and kinked gives the material a wider range of desorption/adsorption energies.

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Figure 8. The ionization potential of iron clusters as a function of cluster size [324].

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3.5.4. Work function The work function is the ionization potential required to eject the most loosely bound electrons into vacuum at 0 K. The work function depends on both the surface roughness and the particle size of the material. It is easier to ionize metals when step density increases [324]. The effect of particle size on the work function is less studied but there is evidence that the ionization potential increases with decreasing particle size (Figure 8). It is not yet known whether these changes are due to changes in cluster structure or some other effect. 3.5.5. Bond breaking Bond breaking of the adsorbate molecules is the main function of catalysts. Bonding of adsorbate to metal generally increases from left to right in the periodic table. The d-electron back bonding depends on the degree of filling of the antibonding states. This means that early transition metals with fewer d-electrons form stronger chemical bonds. The heat of adsorption correlates with the heats of formation of the corresponding oxides. Molecules dissociate on more open and atomically rough surfaces at lower temperatures than on flat, and close-packed surfaces. The heat of adsorption is higher at defect sites on a surface. Defect sites, surface roughness and low packing density of surfaces give rise to higher charge densities near the Fermi level. The work function of these sites is lower and the density of filled electronic states is higher. They are likely to have more adsorbate-induced restructuring. These factors can give rise to enhanced reactivity and bond strength and lead to surface-structure sensitivity of the adsorbate bond. 3.5.6. Charge transfer Most of the catalytic reactions involve charge transfer of either electrons or protons. Electron transfer capability is described as a site being able to receive a pair of electrons (Lewis base) or having a free pair of electrons that can be transferred to the adsorbate (Lewis acid). Acidity of metal ions of equal radius increases with increasing charge. The binding energies of the charge-transfer complexes determine the strength of Lewis acidity. Proton transfer requires Broensted sites that can lose (acid) or accept (base) a proton. The acidity of oxides is related to metal-oxygen bond energy. The general rule is that acidity increases as a function of charge on the metal ion. Acidity of mixed oxides are based on two fundamental assumptions;

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There will be charge imbalance due to the rare coordination. Acidity will be increased. Bonds can contract due to impurity contamination and X-O-Y bonds exist [325]. The acidity of a surface has a large effect on catalytic behavior of an oxide material. The air to fuel ratio (λ) window is very important and is generally very narrow. In order to widen this window, cerium oxide is added to the support. Cerium oxide works as an oxygen buffer releasing oxygen under reducing atmosphere thus transforming to Ce2O3. Under oxidizing conditions cerium can be re-oxidized to CeO2.

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3.6. Oxides in Three-Way Catalysts Oxide materials used in three way catalysts can be divided to two groups. The first groups being the very stable oxides like silicon dioxide and aluminum oxide that are stable under the reaction conditions. These oxides are using as catalyst supports to disperse catalyst and give it a high surface to weight ratio. The second groups of oxide materials are the transition metal oxides that can catalyze redox reactions by actively taking part in the reactions through transfer of the lattice oxygen of the oxides. These oxides require high oxygen vacancy mobility [326,327]. Nanophase materials have a high vacancy concentration combined with a large surface area and high diffusivity which increases the catalytic activity of the oxide materials. The spillover phenomenon refers to migration of mobile species across two phases. In three way catalysts, oxygen in particular can be transported between the noble metal islands and the oxide support. Ions can move in both directions depending on the ambient atmosphere. Spillover is strongly dependent on the basicity of the oxide phase as well as the size and morphology of the noble metal particles. For cerium oxide (CeO2) O spillover is very fast while hydrogen spillover is totally suppressed [328]. Studies of the relation between particle morphology and the structure of the metal support interface, and catalytic properties have been performed on several different metal-support systems for different reactions. Vaarkamp et al. [329] found that high temperature aging of Pt/g-Al2O3 changed the Pt particle morphology from three- to two-dimensional, and the particle support interface. Hydrogen in the interface was desorbed. These changes in the structure affected the catalytic, chemisorption and electronic properties. Aging decreased the selectivity of this catalyst for hydrogenolysis of neopentane and methylcyclopentane to methane. At the same time, the activity of the isomerization of neopentane and ring opening of methylcyclopentane increased. On the electronic level, the number of holes in the d-band increased. All these changes in properties are related to the change in structure of the metal support interface.

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4. PREPARATION OF NANOSTRUCTURED MATERIALS Syntheses of zero dimensional (or so-called quantum dots or nanoparticles), one dimensional (including rods, wires, belts, and tubes), two dimensional and three dimensional nanostructured materials have attracted for the past many years. Because of their size dependent catalytic, electronic, optical, mechanical and optoelectronic properties, these can be broadly tuned through size variation. They play an important role in both interconnects and functional units in fabricating electronic, optoelectronic, electrochemical, and electromechanical devices with nanoscale dimensions. The importance of nano-structured materials in catalysis, electrochemistry, functional ceramics, and sensors, their fabrication in nano-size form with anisotropic morphology are considered. When developing a synthetic method for generating nanostructures, the most important issue that one need to address is the simultaneous control over dimensions, morphology (or shape), and monodispersity (or uniformity) [27,29,296]. Literatures survey indicates that a large number of techniques are available for the development of nanostructured materials of varying shape and size. Those techniques which are capable of producing fine crystals are potentially suitable to synthesis nano-structured materials. For consolidation, these methods have been categorized as either physical vapor deposition (PVD), chemical vapor deposition (CVD) or solution-based chemistry (SBC). Each respective method can been subdivided into the individual techniques that are summarized in Scheme as given below in Table 2.

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4.1. Physical Vapor Deposition Physical vapor deposition (PVD) occurs when a material is physically released from a source and transferred to a substrate. The three most important technologies for deposition of nanostructured materials are thermal evaporation, sputtering, and pulsed laser deposition; each of which will be described below. Table 2. Preparation methods of nanostructured materials. Physical Vapor deposition Chemical Vapor deposition

Solution-based Chemistry

Miscellaneous Methods

Thermal evaporation Rf sputtering method Pulsed laser deposition Thermal (CVD) Low Pressure (LPCVD) Plasma enhanced(PECVD) Metal-organic(MOCVD) Molecular beam epitaxy (MBE) Atomic layer deposition(ALD) Sol-gel chemical rout Sonochemical method Hydrothermal Synthesis Solvothermal method Homogeneous/heterogeneous precipitation Coprecipitation method Microemulsion method TemplateAssisted Synthesis Methods Electrochemical synthesis Electrophoretic deposition (EPD) technique High-energy ball milling

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4.1.1. Thermal evaporation During thermal evaporation, the substrate, crucible, and source materials placed inside a vacuum chamber at room temperature. A vacuum is required to increase the vapor pressure during sublimation and often ranges between 10-2 and 10-9 Torr (ultra high vacuum). Once the vacuum chamber has stabilized at the appropriate pressure, a heating source is used to heat the source material within the crucible to its vapor point. Upon evaporation, the material will re-deposit along the cooler surfaces of the vacuum chambers, as well as the collection substrate. Typical heating sources include electron-beam, radio-frequency (RF) inductive, and resistive heating. During electron-beam evaporation, an electron source is aimed at the source material causing localized heating. In comparison, RF induction uses an AC power supply to produce an alternating current through an induction coil. The alternating current generates a magnetic field within the coil. When the source material is placed inside the coil, the magnetic field induces eddy currents within the source material providing localized heat. Although higher frequencies equate to higher heat rates, lower frequencies are better suited for thicker samples. Finally, resistive heating provides heat by sending a high current source through a resistive coil, such as tungsten, and is a non-localized heat source and therefore commonly used for furnace applications. Although inductive RF and electron-beam sources have been limited to highly-oriented, thin-films of ZnO, resistive sources have produced thin-films, as well as a diversity of ZnO nanostructures with different shapes, sizes, and orientations. 4.1.2. Rf magnetron sputtering Sputtering is the removal of surface atoms via high energy ions. Sputtered films are typically polycrystalline and form at low temperatures with good adhesion properties. Common types of sputtering are focused ion-beam, direct current (DC), radio-frequency (RF), and magnetron. During focused ion-beam sputtering, gallium ions are accelerated through a vacuum towards a sample surface. Acceleration and focusing capabilities are provided by a series of capacitive plates and magnetic coils, respectively. In general, the focused beam of ions provides an exquisite tool for milling and cutting at the nanoscale and has been used within this dissertation as a tool for building prototype nanobelt devices. In comparison, during DC sputtering, the substrate and source (target) material are placed inside a vacuum chamber. Upon evacuation of foreign gases, an inert gas, such as argon, is introduced into the chamber at low pressures. Then, a DC power supply is used to ionize the inert gas in order to produce charged plasma. The ions are accelerated towards the surface of the target, causing atoms of the source material to break off from the target and condense on all surfaces including the substrate. A limitation to DC sputtering is the high voltage required to sputter insulating materials due to the build-up of positive charge on the target material. To solve this problem, the DC power source should be replaced by an RF power source (RF sputtering). In addition, a strong magnetic field (magnetron sputtering) can be used to concentrate the plasma near the target to increase the deposition rate. When applied to ZnO, sputtering has been limited to polycrystalline thin-films [330]. 4.1.3. Pulsed laser deposition During pulsed laser deposition (PLD), a laser beam focused through a vacuum onto the surface of a target material. At sufficiently high flux densities and short pulse durations, the

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target material is rapidly heated to its evaporation temperature and forms a vapor plume. Unlike thermal evaporation, where the vapor composition is dependent on the vapor pressures of the elements within the source material, laser ablation produces a plume of material with similar stoichiometry to that of the target material. Once the vapor plume has been formed, it is collected onto a cooler substrate that promotes nucleation and growth of crystalline films. It is important to note that by using a single crystal substrate, epitaxial single-crystals can be grown that are equal in quality to molecular beam epitaxy. As applied to ZnO, fabrication of (0001) epitaxial films on cubic (111) substrates have been formed using pulsed laser deposition, as well as aligned ZnO nanorod and nanodot arrays without the aid of a catalyst [331-334].

4.2. Chemical Vapor Deposition

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During chemical vapor deposition (CVD), the substrate is placed inside a reaction vessel where the pressure and gas flow are controlled. Fundamentally, the process is a chemical reaction between source gases; the product of which condenses during the formation of a solid material within the reaction vessel. The most common CVD techniques used to deposit nanostructured materials are thermal CVD, low pressure CVD (LPCVD), plasma-enhanced CVD (PECVD), metal-organic CVD (MOCVD), molecular beam epitaxy (MBE), and atomic layer deposition (ALD). For each of these methodologies, a vacuum chamber with gas flow control is required. Each methodology is described in detail below.

4.2.1. Thermal and low pressure chemical vapor deposition During thermal and low-pressure chemical vapor deposition (LPCVD), pressures range between 10-3 Torr (thermal CVD) and 0.1 Torr (LPCVD) where the reactions occur at excess of 900ºC between supplied gases. The processes typically performed simultaneously on both sides of the substrate and produce layers with excellent uniformity and material characteristics. Although LPCVD is commonly used for depositing thermal oxides within the semiconductor industry, it is limited by its high temperatures and slow deposition rates. However, the method has recently been revitalized through the synthesis of carbon nanotubes , silicon nanowires, and well-aligned ZnO nanorods[335-337]. 4.2.2. Plasma-enhanced chemical vapor deposition In comparison to LPCVD, plasma enhanced CVD (PECVD) is performed at temperatures as low as 300ºC. To compensate for the low temperatures within the reactor, plasma is generated to increase the energy available to the chemical reaction. Using PECVD, thick films of ZnO have been prepared [338]. 4.2.3. Metal-organic chemical vapor deposition Metal-organic chemical vapor deposition (MOCVD) is nearly identical to the above listed types of CVD, except a metal-organic precursor is used to catalyze the chemical reaction. The method has been extensively used for the ZnO thin films and aligned nanotips, nanowires, and nanotubes [339-342].

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4.2.4. Molecular beam epitaxy During molecular beam epitaxy (MBE), pure elements are slowly heated in individual quasi-knudsen cells. Upon evaporation of the individual materials (commonly Gallium and Arsenide), a computer-controlled shutter introduces alternate vapors (beams) into the ultra high vacuum chamber. The term “beam” is used because the mean-free-path of the vapors is sufficiently large to prevent all atomic interaction prior to substrate arrival. Upon condensation onto the single-crystal substrate, atomic layers epitaxiall-grown one layer at a time. During crystal growth, reflective high energy electron diffraction (RHEED) is used to monitor crystal quality and thickness. The exquisite control of MBE has led to the development of structures where the electrons are spatially-confined to quantum wells and quantum dots. Single-crystalline films of ZnO and quantum dots have been grown through this technique [343,344]. 4.2.5. Atomic layer deposition Atomic layer deposition (ALD) is a self-limiting, sequential, surface chemistry technique used to deposit conformal thin-films onto substrates of varying composition. Although it is similar to CVD, ALD divides the chemical reaction into two half-reactions by preventing the precursors from interacting within the reaction vessel. For example, when considering alumina (Al2O3), the binary reaction in Eq. 5 can be split into Eq. 6 and 7:

2Al(CH3)3 + 3H2O - > Al2O3 + 6CH4 (Eq.5)

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AlOH* + Al(CH3)3 - > AlOAl(CH3)2* + CH4 (Eq.6) AlCH3* + H20 - > AlOH* + CH4 (Eq.7) where the asterisks denote surface species. From Eq. 6, aluminum is deposited onto a methylated surface by reacting Al(CH3)3 with hydroxyl (OH*) species. The reaction is complete either when all of the hydroxyl groups have reacted or when the remaining precursors are evacuated from the reaction vessel. Once aluminum has been deposited, the reaction chamber is purged with an inert gas to prevent mixing of the respective precursors. Then oxygen is deposited using Eq. 7 onto a rehydroxylated surface by reacting H2O with AlCH3*. Once again, the reaction is complete either when all of the methyl species have reacted or when the remaining precursors are evacuated from the reaction vessel. By sequentially alternating Eq. 6 and 7, Al2O3 will be deposited with atomic layer control [345]. When ALD has been used for ZnO, inverted opals [346] have been grown for photonic applications, as well as thin films [347] for RF MEMS applications.

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4.3. Solution-Based Chemistry

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Solution-based chemistry (SBC) is defined by any chemical reaction that requires a liquid. As a generic method for synthesis, SBC has been vital to the production of a diversity of materials that are often difficult to make using PVD or CVD. Typically, solution-based methodologies provide materials with high yield and uniformity. However, a common drawback is an increased number of point, line, and planar defects when compared to their simpler physical counterparts. The most important and common techniques for nanostructured materials synthesis are co-precipitation, solvothermal, sonochemical, hydrothermal and sol-gel synthesis, which are discussed below.

4.3.1. Sol-gel method The sol-gel process, as the name implies, involves the evolution of inorganic networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). The sol-gel process is a process for making glass/ceramic materials. The sol-gel process involves the transition of a system from a liquid (the colloidal “sol") into a solid (the "gel") phase. The sol-gel process allows the fabrication of materials with a large variety of properties: ultrafine or spherical shaped powders, monolithic ceramics and glasses, ceramic fibers, inorganic membranes, thin film coatings and extremely porous aerogels. Sol-gel chemistry is a remarkably versatile approach for fabricating materials. An overview of the sol-gel process is presented in a simplified chart (Figure 9). The sol is a stable suspension of colloidal solid particles of a diameter of few hundred nm, usually inorganic metal salts, within a liquid phase. For a sol to exist, the solid particles, denser than the surrounding liquid, must be small enough for the forces responsible of dispersion to be greater than those of gravity. Practically, particles in a colloidal sol must have a size comprised between 2 nm and 200 nm; this corresponds to 103 to 109 atoms per particle. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, then the particles condense in a new phase, the gel, in which a solid macromolecule is immersed in a solvent. Hydrolysis is a chemical reaction or process, in which a molecule is split into two parts by reacting with a molecule of water. In inorganic chemistry, the word is often applied to solutions of salts and the reactions by which they are converted to new ionic species or to precipitates (oxides, hydroxides, or salts). Many metal ions are strong Lewis acids, and in water they may undergo hydrolysis to form basic salts. Such salts contain a hydroxyl group (OH) that is directly bound to the metal ion in place of a water ligand. At the functional group level, three reactions are generally used to describe the sol-gel process: hydrolysis, alcohol condensation, and water condensation. However, the characteristics and properties of a particular sol-gel inorganic network are related to a number of factors that affect the rate of hydrolysis and condensation reactions, such as, pH, temperature and time of reaction, reagent concentrations, catalyst nature and concentration, H2O/Si molar ratio (R), aging temperature and time, and drying. Of the factors listed above, pH, nature and concentration of catalyst, H2O/Si molar ratio (R), and temperature have been identified as most important. When applied to ZnO, nanocrystals, thin films, and nanorods have been synthesized with preferred crystallographic orientations using the sol-gel method [248-253].

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Figure 9. Simplified Chart of sol-gel processes.

4.3.2. Sonochemical method Sonochemical method is a cheap method for preparation of nanomaterials for example, in Romania scientist carried out organic reactions using cheap ultrasonic baths as their source of radiation. Sonochemistry allowed the synthesis of a variety of nanomaterials such as metal nanoparticles, metallic colloids, metallic alloys, composite of polymer and metal nanoparticles etc. In Sonochemistry, molecules undergo chemical reaction due to the application of ultrasound radiation (10 KHz-20 KHz). The phenomenon responsible for the sonochemical process is the acoustic cavitation [354]. The main event in the sonochemistry is the creation, growth, and collapse of the bubble that is formed in the liquid. Ag, Pd, Au, Pt and Rh nanoparticles have been synthesized sonochemically in the presence of surfactants like sodium dodecylsulfate (SDS), sodium dodecyl benzene sulfonate (anionic surfactant), polyethylene glycol monostearate, (PEG-MS), and dodecyl trimethyl ammonium bromide [355,356]. The preparation of Pd and Pt nanoparticles by the sonochemical reduction of solutions containing H2PtCl3 or K2PtCl4 was reported by Fujimoto et al [357]. Colloidal solutions of gold nanoparticles have been prepared from the sonochemical reduction of tetrachloroaurate (III) ions [358]. The reduction rate of auric chloride solution to gold nanoparticles is strongly dependent on the atmosphere, the bulk solution temperature, ultrasound intensity, and the reaction vessel-oscillator distance. A comprehensive study on the sonochemical synthesis of colloidal solutions of noble metals is available elsewhere [359361]. Nanoparticles have been deposited in polymeric materials, microspheres and in amorphous alumina using ultrasound irradiation [362-364]. Some metal oxide nanoparticles have also been prepared using ultrasound irradiation, Zhu and co workers discovered a novel method for preparing highly photoactive nano sized TiO2 photocatalysts. The method has been developed by hydrolysis of titanium isopropoxide in pure water or a 1:1 EtOH:H2O solution under ultrasonic irradiation [364,365].

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4.3.3. Solvothermal method Solvothermal method for synthesis of nanoparticles is almost identical to hydrothermal method but in this no aqueous solvent is used and the temperature can be elevated than in hydrothermal method. Solvothermal synthesis utilizes a solvent under pressures and temperature above its critical point to increase the solubility of a solid and to speed up the reactions between solids. In a typical procedure, a precursor and possibly a reagent (capable of regulating or templating the crystal growth) are added into solvent at the appropriate ratio. This mixture is then placed in an autoclave to allow the reaction and nanowires growth to proceed at elevated temperature and pressure. The solvothermal method normally has better control than hydrothermal methods of the size and shape distributions and crystalinity of nanomaterials. A variety of nanoparticles and nanorods such as CeO2 and CeOx, V2O5, ZnO, TiO2, SnO2, In2O3, and PbO have been synthesized using solvothermal method with/without the aid of surfactants [366-370]. 4.3.4. Hydrothermal synthesis As defined, hydrothermal synthesis is a subset of solvothermal synthesis which involves water at elevated conditions. The basic principle is that small crystals will homogeneously nucleate and grow from solution when subjected to high temperatures and pressures. During the nucleation and growth process, water is both a catalyst and occasionally a solid-state phase component. Under the extreme conditions of the synthesis vessel (autoclave or bomb), water often becomes supercritical, thereby increasing the dissolving power, diffusivity, and mass transport of the liquid by reducing its viscosity. In addition, the ability to tune the pressure of the vessel provides an avenue to tailor the density of the final product. When compared to other methodologies, hydrothermal synthesis is environmentally benign, inexpensive, and allows for the reduction of free energies for various equilibria. Materials that are made hydrothermally are generally high-quality, single crystals with a diversity of shapes and sizes. Although hydrothermal synthesis is an established synthesis route within the ceramics industry, it has recently been rekindled within the scientific community by synthesizing one-dimensional nanostructures, such as carbon nanotubes and oxide nanowires. Different types of nanoparticles such as ZnO nanorods, Ag, GaN, TiO2, LaCrO3, ZrO2, Sb2S3, CrN, α-SnS2, PbS, Ni2P and SnS2 nanotubes, Bi2S3 nanorods and SiC nanowires, TiO2 nanorods have been successfully synthesized in this way [371-375].

Figure 10. Schematic representation of the sequential precipitation of Al(III) over the already precipitated Ce- Me oxalate particles that serve as seeds.

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4.3.5. Homogeneous/heterogeneous precipitation A precipitation phenomenon denotes a relatively rapid formation of a solid product from a solution. Many investigators have dealt with the precipitation category and there are other reference sources for treating precipitation processes. Homogeneous and heterogeneous nucleation refers to the formation of stable nuclei with or without foreign species, respectively. Precipitation refers to more than a nucleation phenomenon. It is not depend on the presence of solute crystalline matter, and in this respect, it does not involve secondary nucleation. Co-precipitation refers to the precipitation in which several constitutes of the solution are found in the same precipitate. It does not result ‘A study on nanosized cerium oxides systems for environmental catalysis’- 10 - from homogeneous or heterogeneous nucleation process. Precipitation implies a dynamic system, composed of two or more phases. If carried out by rapid mixing of reactant, it is mostly subject to homogeneous nucleationbased phase change. When the catalytic converter operates at high temperatures for a long time, CeO2-based catalyst is subject to severe temperature conditions, which lead to its particles growth and sintering. Therefore, there is a strong demand for maintaining the thermal stability of the CeO2-based catalysts. The schematic presentation of this process is presented in Figure 10. 4.3.6. Co-precipitation methods for metal oxide synthesis One of the conventional methods to prepare nanoparticles of metal oxide ceramics is the precipitation method. This process involves dissolving a salt precursor, usually a chloride, oxychloride or nitrate, such as AlCl3 to make Al2O3, Y(NO3)3 to make Y2O3, and ZrOCl2 to make ZrO2. The corresponding metal hydroxides usually form and precipitate in water by adding a base solution such as sodium hydroxide or ammonium hydroxide to the solution. The resulting chloride salts, i.e. NaCl or NH4Cl, are then washed away and the hydroxide is calcined after filtration and washing to obtain the final oxide powder. This method is useful in preparing composites of different oxides by co-precipitation of the corresponding hydroxides in the same solution. One of the disadvantages of this method is the difficulty to control the particle size and size distribution. Very often, fast (uncontrolled) precipitation takes place resulting in large particles. Nanophase powders of YxZr1-xO2-x/2 have been prepared from a mixture of commercially available ZrO2 and Y2O3 powders [376]. It was found that depending on the starting powder mixture composition, the yttrium content in the nanophases can be controlled and the tetragonal or cubic phases can be obtained. Tetragonal or a mixture of tetragonal and cubic were observed for low yttria content (3.5 mol% yttria), and cubic for higher yttria contents (19, 54, 76 mol% yttria). These powders were found to have a most probable grain radius of about 10 to 12 nm and the grains appear as isolated unstrained single crystals with polyhedral shapes. The grain shapes appeared to be polyhedral and not very anisotropic. Lattice fringes were parallel to the surfaces demonstrating that (100) and (111) faces dominate. 4.3.7. Micro-emulsion method The term “micro-emulsion” was introduced by Schulman et al., [377] micro-emulsion is made up of water, oil and surfactant. This method has been successfully applied to synthesize metal, alloy, metal suphides and metal oxide nanoparticles. A literature survey depicts that the ultrafine nanoparticles in the size range between 5-50 nm can be easily prepared by this

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method. This technique uses an inorganic phase in water-in-oil microemulsions which are isotropic liquid media with nanosized water droplets that are dispersed in a continuous oil phase. Synthesis of mixed metal nanoparticles by the micro-emulsion method allows simultaneous control of size and composition. Micro-emulsion is a liquid droplet containing the noble metal precursor being engulfed by surfactant molecules and uniformly dispersed in an immiscibly continuous organic phase which serves as a micro or nano-scaled reactor in which the chemical reaction takes place. The size of the micro-emulsions is of the order of a few to hundred nanometers and is determined by the balance of the surface free energy mediated by the surfactant molecules and the difference in free energy caused by the immiscibility of the two liquid phases. Reduction of the metal can be carried out by adding the reducing agent directly into the micro-emulsion system, or by the introduction of another reducing-agent containing micro-emulsion. The reducing agent must be stable in an aqueous environment and must not react with the other components of the system. Thus, all nonaqueous reducing agents are excluded. The most commonly employed reducing agents are borohydride and hydrazine. The surfactant molecules serve the role as a protective agent, thus preventing the agglomeration of nanoparticles. The main attractiveness of the micro-emulsion method is its ease in controlling the size distribution and composition of the metal particles within a narrow distribution by varying the synthetic conditions. The particle sizes depend on the precursor concentration and the amounts of surfactant. Metal alloys can be synthesized if the salts of different metals are dissolved in the solution before the reduction is carried out, provided that the metal salts are miscible in the metallic state. The final composition of the mixed metal nanoparticles can be easily controlled by the ratio of the metal precursor solutions. However, the micro-emulsion method employs costly surfactant molecules and substantial number of separation and washing steps are needed before usage. Thus, this method may not be economical and suitable for large-scale production (Figure 11). However, one problem associated with this method is the extraction of nanoparticles from the micro-emulsion system. The commonly used method is deposition of the capped nanoparticles on a solid support. An example is the deposition of platinum and palladium nanoparticles synthesized by w/o micro-emulsion on γ-Al2O3 alumina support. A common solid support is silica and this support is not directly added but prepared in-situ by hydrolysis and polycondensation of tetraethylorthosilicate (TEOS) with dilute ammonium solution. Deposition of nanoparticles on solid support could have the following two disadvantages. Firstly, the nanoparticles are firmly held by and will not be able to detach from the solid support. Secondly, the reversed micelle is spherical in shape, hence, only nanoparticle but not other 1D nanomaterials can be synthesized. A diverse range of metal nanoparticles have been prepared by this method including Fe, Fe/Au, Pt, Ag, CdS, Pd, Cu, Ni and Au [378-390].

4.3.8. Template-assisted synthesis methods Among all the synthetic methods for nanomaterials, the template methods are widely used. Most template methods, which confined the growth of materials within a template (such as pores), followed by removal of the template, provide a flexible synthetic route for a variety of nanostructured materials. Both hard-templates (e.g. porous materials, mesoporous silica, track-etched polycarbonate film and carbon nanotubes) and soft-templates such as polymer, self-assembly of surfactant molecules and solvents could be used. Recently, there are few

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reports on the use of biomolecules such as DNA, tripeptide glutathione (GSH) and lysozyme as template for the synthesis of nanoparticles. When porous materials are used as templates, the materials should have channels with nanometer scale. Figure 12 illustrates how nanotubes or nanowires can be formed using porous materials template. Firstly, the substrates are loaded into these nano-channels by various methods such as vapor phase sputtering, liquid phase injection, solution-phase deposition or electrochemical deposition. Nucleation, growth and crystallization are confined in these nanometer sized channels. Selective removal of the porous template results in the formation of nanorods, nanowires or nanotubes. Incomplete filling of the channels in the initial step will lead to the formation of nanotubes while complete filling will lead to the formation of nanowires or nanorods. The most commonly used porous materials are porous polymer and alumina films. One typical example is the use of porous anodic alumina (PAA) templates to prepare ZnO nanowires and nanotubes. The zinc-based sol particles were loaded into the PAA nano-channel due to the electrostatic attraction. The ZnO nanowires and nanotubes can be fabricated when the sol particles sintered inside the channels. Although this method is good at obtaining uniform and well-segregated 1D nanomaterials, it is difficult to get nanomaterials with good crystallinity. Outstanding examples of arrays that has been generated by this route are those of oxides nanotubes like TiO2, In2O3, Ga2O3, BaTiO3, PbTiO3, In2O3 and Fe2O3, as well as nanorods of MnO2, WO3, Co3O4 , V2O5, and ZnO. Another widely applicable route to inorganic nanotubes and nanorods is to use CNTs as templates. CNTs have been coated with a thin film of secondary materials that builds up the tube wall of the desire inorganic nanotube followed by removal of the carbon nanotube. Most oxide nanotubes and nanorods, such as V2O5, Al2O3, WO3, MoO2, Sb2O3 and MoO3, ZrO2, RuO2, SiO2, and TiO2 have been prepared using CNTs as templates [391-401]. Self-assembly of surfactant molecules is another important class of templates. Surfactant molecules can be classified into cationic, anionic, non-ionic and zwitterionic according to their polar head groups. When the concentration of surfactant molecules is below its critical micelle concentration (CMC), the surfactant molecules will only lie up at the liquid-vapor interface or dissolve into the liquid. However, once the concentration is over CMC, these surfactant molecules can self-assemble into spherical micelle, cylindrical micelle or lamellar structure depending on the concentration and types of surfactant molecules. Figure 12 depicts the structure of these micelles. These self-assembled structures can be used as templates for the growth of nanomaterials as shown in Figure 14. Reactants can load into or grow outside the cylindrical template. Subsequently removal of surfactant molecules gives nanorods, nanowires or nanotubes. The uses of surfactant as a template for nanomaterials synthesis have been widely explored. Existing 1D nanomaterials such as nanowires and nanotubes can also be used as a template for the synthesis of other 1D nanomaterials. The substrates can coat onto the nanowire or nanotubes for growth. Removal of the template results in the formation of nanotubes. The substrates can also react with the nanowires so that growth of the new materials is directed by the existing nanowires. For example, single-walled carbon nanotubes (SWNTs) were used as a template for the synthesis of one-dimensional SiC, BN nanostructures, ZnO nanorods, CoFe2O4 nanowires, β-zeolite nanowires.

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Figure 11. Schematic diagrams on the synthesis of nanoparticles by water-in oil (w/o) microemulsion.

Figure 12. Schematic diagram showing the formation of 1D nanomaterials using porous materials as templates. (a) Porous materials such as porous anodic alumina with nanometer scale channels. (b) Incomplete filling of the substrates in the channel. (c) Formation of nanotube due to incomplete filling. (d) Complete filling of the substrates in the channel. (e) Formation of nanorod or nanowire due to complete filling.

Figure 13. Schematic diagram showing the structures of self-assembled surfactant molecules. Arrows stand for the change of structure as the concentration of surfactant molecules increases. (a) Structure of spherical micelle; (b) Structure of cylindrical micelle; (c) Lamellar structure; (d) Structure of spherical reverse micelle; (e) Structure of cylindrical reverse micelle.

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Figure 14. Schematic diagram showing the formation of nanorods, nanowires and nanotubes by cylindrical template. (a) Cylindrical micelle formed by self-assembly of surfactant molecules. (b) Substrate molecules attach onto the outer wall of the cylindrical micelle. (c) Sintering of the substrate molecules form a wall outside the cylindrical template. (d) Selective removal of the surfactant molecules gives the desired nanotubes. (e) Cylindrical reverse micelle formed by self-assembly of surfactant molecules. (f) Substrate molecules load into the cylindrical reverse micelle. (g) Nucleation, growth and crystallization occurs inside the cylindrical template. (h) Selective removal of the surfactant molecules gives the desired nanorods or nanowires.

4.3.9. Electrochemical synthesis Electrochemical synthetic technique was established by Reetz and co-workers in 1994 [402-406]. In this method, an anode is made up of the metal whose nanoparticles one interested in and a cathode is of any other metal. Under the suitable applied current density the anode sacrificially dissolves in the electrolyte, the metal ions migrate towards the cathode and reduction occurs. The nucleation and growth of reduced metal atoms occurs at the electrode surface. The stabilizing agent in the reaction vessel arrests the growth of the nanoparticles and facilitates the formation of stable nanostructures. The final step is diffusion of nanoparticles from the electrode surface into the bulk of the solution. Equation 1.1 gives the reactions taking place at the electrodes surfaces. Anode : Mbulk → Mn+ + neCathode : Mn+ + ne- + stabilizer → Mcoll/stabilizer

(Eq.8)

Sum :Mbulk + stabilizer → Mcoll/stabilizer The advantages of the electrochemical pathway are that the contamination with byproducts resulting from chemical reducing agents is avoided and that the products are easily isolated from the precipitate. Further, the electrochemical preparation allows for size-selective particle formation. The particle size obtained by the electrochemical route depends on many factors, the distance between the electrodes, reaction time, temperature, and polarity of the solvent contribute to the particle size. Experiments have also shown that the applied current density also has a major influence on the particle size. Electro-deposition of TiO2 film from TiOSO4+H2O2+HNO3+KNO3 (pH 1.4, Eq. 9) solutions involves indirect deposition of a gel

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of hydrous titanium oxo-hydrides (Eq.11), resulting from the reaction of titanium peroxosulfate (Eq.10) with hydroxide ions produced by nitrate electrochemical reduction. NO3− + H2O + 2e− → NO2− + 2OH−

(Eq.9)

TiOSO4 + H2O2 → Ti (O2)SO4 + H2O

(Eq. 10)

Ti(O2)SO4 + 2OH− + (x + 1)H2O → TiO(OH)2·xH2O2 + SO42−

(Eq. 11)

A number of investigators have been employed electrochemical method for the development of nanocomposites of Au/polypyrrole, polypyrrole, polyaniline, ZnO, CeO2, TiO2, SnO2 and ZrO2 [406-410]. The composites system can be prepared by simultaneous reduction of metal ion and autopolymerization of monomers of polypyrrole, polyaniline [411].

4.3.10. Electrophoretic deposition (EPD) The phenomenon of electrophoresis has been known since the beginning of the 19th century and it has found application in traditional ceramics technology. EPD is essentially a two-step process: in the first step, charged particles suspended in a liquid migrate towards an electrode under the effect of an electric field (electrophoresis). In the second step, the particles deposit on the electrode forming a relatively dense and homogeneous compact or film. In general, EPD can be applied to any solid that is available in the form of a fine powder ( Eg light absorbance becomes continuous, giving a solid spectral band rising toward the shorter wavelengths. The semiconductor NPs with perfect crystal structure and low concentration of surface defects at direct electron transitions do not absorb light with hv < Eg. Absorbance in this area can arise from the NP size distribution or the surface defect states capable of ionization under the sub-band gap photoexcitation.

0

260

300 λ, nm

а

3.0

4.0

340

hv, eV

5.0

b

Figure 1. (а) Absorption spectrum of the aqueous colloidal solution of ZnS (1) and its transformation as ln(α·hv) vs ln(hv-Eg) (2). [ZnS] = 2.0×10-3 M, optical path l = 10.0 mm; (b) Curve 1 presented as d{ln(α×hv)}/d{hv} vs hv. Dashed line corresponds to the discontinuity point. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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Olexander L. Stroyuk, Stepan Y. Kuchmiy, Anatoliy I. Kryukov, et al.

The Bohr radius of exciton аВ can be calculated from the expression (3), which is similar to the Bohr equation for hydrogen atom:

aB =

h 2ε e2

⎡ 1 1 ⎤ ⎢ * + *⎥ ⎣ må mh ⎦

(3)

where ћ is the reduced Planck constant, m*e, m*h is the effective mass of electron and hole, respectively, ε is the dielectric constant of the semiconductor, е is the electron charge. The value of аВ may be considered as an upper limit of the NP diameter for the quantum size effects to be detectable. The ratio between R and aB defines regime of weak (aB < R 2-5 nm). For example, for PbSe NPs (m*e = 0.05me [88], aB = 46 nm [89]) the ΔE achieves a record value of 2.8 eV as the NP size decreases to 2-3 nm [88]. A substantial, up to 2.0 eV, increment of the exciton energy can be achieved for PbS NPs (m*e= 0.1me, aB = 18 nm [23]) by the NP size reduction from 20 to 2 nm [23]. Comparatively small increments in Eg (0.1-0.2 eV) are typical for the semiconductors with small exciton diameter, for example, CuCl (aB = 0.7 nm [2]), PbCl2 (aB = 1.9 nm [2]), TiO2 (aB = 0.8-1.9 nm [90]), and ZnS (aB = 1.5 nm [73]). In the photocatalytic and photovoltaic systems based on nanostructured semiconductor materials, the size-dependent absorption band edge shift was observed for CdS [91-121], ZnS [98, 122, 123], In2S3 [94, 124-126], PbS [42, 110, 127], Bi2S3 [110, 128-130], Sb2S3 [110, 130], MoS2 [131-134], WS2 [131, 135, 136], CdSe [118, 125, 137-145], PbSe [146], ZnSe [147], the ultra-small TiO2 NPs (2R kT) and the hole migrating among "shallow" traps (Th v = ⎜⎜ ⎝ π M0 ⎠ 3

(55)

The nanoparticle formation process is inherently accompanied with the consumption process of the material vapor. Thus, the concentration of the material vapor must be solved simultaneously. The conservation equation of the vapor is written as

⎛N u ⋅ ∇⎜⎜ vapor ⎝ ρ

⎡ ⎞ ⎛N ⎟⎟ = ∇ ⋅ ⎢ Dvapor ∇⎜⎜ vapor ⎠ ⎝ ρ ⎣

⎞ ⎤ N& vapor . ⎟⎟ ⎥ + ρ ⎠⎦

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The net production rate of the vapor N& vapor is estimated with the contributions from evaporation, nucleation and condensation:

[

N& vapor = N& vapor

]

−I⋅ evaporation

v*p vmonomer

−

[M& ]

1 condensation

vmonomer

.

(57)

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The production by evaporation is obtained from the computation of the precursory powder behavior written in the previous section. As the boundary conditions, the Neumann conditions are given to each of the moments and the vapor concentration on the central axis and at the exit. The Neumann condition is also imposed to the vapor concentration at the wall. The value of zero is set for the moments at the wall for simplicity.

Figure 25. Heat transfer characteristics: (a) Particle temperature. (b) Particle diameter.

The properties of the materials are obtained from Ref. [52] and the diffusion coefficient of the vapor can be calculated from the formula proposed in Ref. [62].The finer grid system is required in order to capture the homogeneous nucleation precisely. The information of the thermofluid fields and the vapor concentration obtained from the prior computations is interpolated. The equations of the moments and the vapor concentration can be solved using SIMPLE-like algorithm [64].

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Figure 26. Concentrations of vapors.

Figure 27. Concentrations of total nanoparticles.

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Numerical investigation is carried out for four cases. Case (A) is set as a base case where molybdenum nanoparticles are synthesized without a counterflow, while the counterflow is utilized in Case (B) of molybdenum, Case (C) of iron and Case (D) of silicon. Figure 26 portrays the two-dimensional distributions of the vapor concentration. In all the cases, the vapor consumption by nanoparticle formation is observed near the wall because the saturation pressure is small owing to the low temperature. However, only a little vapor of molybdenum is consumed in Case (A) in r250 mm. Figure 27 shows the concentrations of the total nanoparticles with the whole size range formed in the cooling region. In general, contour plots of the nanoparticle concentration include the embryos such as nuclei or clusters which cannot be considered as “particles”. Even if the embryos can be regarded as particles, particles with dp5 nm are presented in Figure. 28 as the two-dimensional distributions of “nanoparticles”. The comparison between Figure. 27 and Figure. 28 gives the information of the generation of the embryos and their growth to nanoparticles. The volume mean diameter distributions of the nanoparticles are also shown in Figure. 29. Case (B) indicates that the counterflow cooling effectively promotes the production of the molybdenum nanoparticles. As shown in Figure. 28 (A) and (B), the nanoparticles are generated only near the wall in Case (A), while the nanoparticles are produced in the whole region downstream from the counterflow nozzle in Case (B). The counterflow cooling in Case (B) also results in the more uniform distribution of the volume mean diameter than that in Case (A) as shown in Figure. 29 (A) and (B). The significant differences depending on the materials are also presented in Case (B), Case (C) and Case (D). In Case (B) of molybdenum, the largest number of nanoparticles are produced with the smallest mean volume diameter of all the cases. Molybdenum has the remarkably small saturation pressure, which results in the generation of large number of small nuclei by homogeneous nucleation [80]. The large amount of vapor are subsequently consumed by heterogeneous condensation around z=250 mm. This corresponds to the drastic decrease of the vapor as shown in Figure. 26 (B). However, because the large number of nuclei must share the finite amount of vapor, each nucleus cannot grow very much. Neither the decrease of the nanoparticles nor the increase of the volume mean diameter is observed in z>300 mm, which indicates that coagulation is not effective in Case (B). Therefore, homogeneous nucleation is considered to determine the formation of molybdenum nanoparticles. In Case (C) of iron, the smallest number of nanoparticles including the embryos are produced, since iron nucleates in a highly supersaturated state with the small nucleation rate compared with other metals [29]. As shown in Figure. 26 (C), the vapor of iron is not consumed as much as the other cases. Thus, the effect of heterogeneous condensation is small in Case (C). Figures 28 (C) and 29 (C) depict the considerable decrease of the nanoparticles and increase of the volume mean diameter in the region of z>280 mm, which indicates the

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iron nanoparticles grow dominantly by coagulation between nanoparticles formed near the central axis and those formed near the wall.

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Figure 28. Concentrations of nanoparticles with dp > 5 (nm).

Figure 29. Volume mean diameter distributions. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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The volume mean diameter of the silicon nanoparticles in Case (D) increases drastically around z=250 mm and gradually in z>250 mm, even though the number of nanoparticles does not decrease as much as Case (C) of iron. The large amount of silicon vapor is consumed in this region as shown in Figure. 26 (D). Neither the number of nanoparticles nor the volume mean diameter changes very much in the region of z>350 mm where a little amount of the vapor remains. These indicate that coagulation is not effective in Case (D). Therefore, the dominant mechanism in the formation of silicon nanoparticles is considered to be heterogeneous condensation. Compared with Case (C) of iron, the growth rate by heterogeneous condensation in Case (D) of silicon is larger, since silicon has smaller saturation pressure than iron (Si/Fe = 10-1).

(II) Multicomponent cocondensation model - fundamental Nanoparticles of intermetallic compounds such as silicides have also been demanded for a variety of applications. The synthesis of them, however, is arduous in contrast with that of pure metal nanoparticles because the processing involves cocondensation of multicomponent materials. Hence, a model that can analyze this problem is introduced. As mentioned above, because the fundamental processes of nanoparticle synthesis are homogeneous nucleation and heterogeneous condensation, the model is first constructed on the basis of these two processes. When vapors of several materials are supersaturated, nuclei of each material can be generated by homogeneous nucleation. Extending Eq. (32) with Eq. (37), the nucleation rate of the material m can be written as

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I m = N S2 m S m

⎛ ⎞ 2σ st m 4Θm3 ⎟ exp⎜⎜ Θm − 2 ⎟ π mm 27(ln S m ) ⎠ ⎝

(58)

The saturation ratio, the normalized surface tension, and the critical volume can also be extended to

Sm =

Θm =

v *p m

pvapor m

(59)

pS m

σ st m s monomer m

(60)

k BT

π ⎛ 4σ st m v monomer m ⎞

= ⎜⎜ 6⎝

k BT ln S m

3

⎟⎟ . ⎠

(61)

Once stable nuclei are generated by homogeneous nucleation, supersaturated vapor condenses heterogeneously on the nuclei resulting in rapid growth of nanoparticles. The rate of nanoparticle growth by heterogeneous condensations, taking place among several

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materials, can be estimated from the net molecular flux between the gas phase and the condensed phase, in considering all of the range of the Knudsen numbers [84]:

( )=

d d pm

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dt

∑ l

⎤ ⎡ 1 + Kn m Dvapor l ( X monomer l − X S l ) ⎢ ⎥. 2 d p m ρl ⎣⎢1 + 1.7 Kn m + 1.333 Kn m ⎦⎥ 4ρ

(62)

In order to observe the fundamental growth mechanism of silicide nanoparticles with cocondensation processes, this model is first applied to the one-dimensional flow profile extracted from the ITP with natural cooling [80]. The temperature profile is plotted with the computational results of the cobalt-silicide nanoparticle formation in Figure. 30. It is noted that the silicon content in the precursory powders is chosen to be 66.7 at.% which is the stoichiometric composition of CoSi2. Figure 30 (a) shows that the saturation vapor pressures of cobalt and silicon decrease in tandem with the temperature decrease. The vapors of cobalt and silicon are transported with the flow decreasing the temperature. The silicon vapor becomes supersaturated ahead of the cobalt vapor and the pressure rapidly drops because of the vapor consumption by homogeneous nucleation and heterogeneous condensation as shown in Figure. 30 (b), which results in silicon nanoparticle formation. At the more downstream position, the cobalt vapor pressure reaches its saturation pressure and then is decreased by heterogeneous condensation on silicon nanoparticles. Since condensation is more dominant than homogeneous nucleation in a low-supersaturated state, the homogeneous nucleation of cobalt does not take place. During the condensation process of the cobalt vapor on silicon nanoparticles, the cobalt is considered to be well-mixed in the liquid silicon nanoparticles since the cobalt-silicide nanoparticles are promptly formed under the temperature greater than the silicon melting point. It has been reported that cobalt-silicide nanoparticles are successfully synthesized in the experiment [29]. Figure 30 (c) indicates that approximately 70% of silicon vapor is converted to particles in only 1 mm from the nucleation position, and in the very short period of 0.7 ms. Although the cobalt vapor is converted at a more moderate pace than silicon vapor, the majority of the cocondensation process for the Co-Si system is considered to be complete at the axial position of 239 mm. Figure 31 depicts the homogeneous nucleation rate, the critical diameter, and the supersaturation ratio. When the supersaturation ratio exceeds 1.6, the homogeneous nucleation rate begins to be distinguished. Both profiles reach their maximum values and then decrease, because a large number of nuclei start to consume the vapor by heterogeneous condensation. The critical diameter has its minimum value of 0.8 nm when the nucleation rate reaches its maximum value. Figure 32 shows the particle size distribution function (PSDF) and the silicon content finally obtained by the present model with the experimental result [29]. The results show good agreement in their size distributions. The experimental result presents a somewhat broader distribution. This is attributed to experimental errors such as plasma instability or size non-uniformity in the precursory powders. In addition, because this fundamental model does not take into account coagulation and the two-dimensionality of the flow field, its result can be ideal and can deviate from the experimental result. The cobalt-silicide nanoparticles present a wide-ranged silicon content 49-79 at.%. The larger nanoparticles show a larger

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content of silicon. Even though larger silicon nanoparticles capture a larger amount of cobalt vapor, the fraction of silicon is estimated to be large.

Figure 30. Growth mechanism of nanoparticles for Co-Si system: (a) Vapor pressure and temperature, (b) Vapor consumption rate, (c) Conversion ratio.

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Figure 31. Nucleation characteristics of silicon nuclei.

Figure 32. Particle size distribution functions and silicon content for Co-Si system. (Experimental result is obtained from Ref. [29])

(III) Multicomponent cocondensation model – improved The model mentioned above takes into account only homogeneous nucleation and heterogeneous condensation to comprehend the fundamental mechanism of silicide Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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197

nanoparticle formation. However, the contribution of coagulation should not be neglected particularly at the early stage of formation. In addition, the nanoparticles can show a multimodal size distribution as well as a log-normal one. Hence, the multicomponent cocondensation model is improved with the nodal discretization of a size distribution, which makes it possible to analyze the comprehensive processes of nanoparticle formation including coagulation even if the nanoparticles have any size distribution.

Nodal discretization of a particle size distribution The particle size space is discretized into nodes and the formed particles are supposed to reside only at the nodes. To cover the wide range of particle sizes, the nodes are spaced linearly on a logarithmic scale.

v p k +1 = f q v p k (k = 1, 2, …, k max )

(63)

where the subscript k represents the node number. The geometric spacing factor fq and the number of nodes kmax are verified in detail by Ref. [85]. fq and kmax are here chosen to be 1.6 and 42, respectively. These values cover with the sufficient accuracy for the size range of the nanoparticles synthesized by an ITP. The particle volume at the first node is set to be 10-mer of silicon atom.

v p1 = 10vmonomer Silicon

(64)

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The governing equation for the particle concentration Np at the node k is obtainable from the general dynamic equation for aerosol Eq. (31):

d ⎛ N pk ⎜ dt ⎜⎝ ρ

⎞ 1 ⎟⎟ = ⎠ ρ

{[N& ]

p k nucleation

[ ]

+ N& p k

coagulation

[ ]

+ N& p k

condensation

}.

(65)

[ N& p k ] signifies the net production rate of the nanoparticles at the k-th node. In order to

investigate the essential growth mechanism, the effects of convection, diffusion and thermophoresis are removed from Eq. (65) at this stage.

Homogeneous nucleation The net production rate at the node k by homogeneous nucleation is written as

[N& ]

p k nucleation

= ∑ I mξ k( nucleation ) m . m

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Since the nuclei have a critical size at which growth and evaporation are balanced, only at the next higher node will the particles be embryos for nanoparticle growth as shown in Figure. 33 (a). For conservation of the particle volume, the size operator for homogeneous nucleation of the material m is introduced:

; if (v p k −1 < v *p < v p k ) ;

if (v *p < v1 )

;

otherwise

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ξ k( nucleation ) m

⎧ v *p ⎪ m ⎪ vpk ⎪ * ⎪vp =⎨ m ⎪ v p1 ⎪ ⎪ 0 ⎪ ⎩

Figure 33. Basic algorithms of nanoparticle growth under nodal discretization. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

(67)

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Coagulation between nanoparticles The net production rate at the node k attributable to coagulation between nanoparticles is expressed by the Smoluchowski’s equation:

[N& ]

p k coagulation

=

1 ξijk( coagulation ) β ij N p i N p j − N p k ∑ β ik N p i . ∑∑ 2 i j i

(68)

The size-splitting operator for coagulation is given as

( coagulation ) ξ ijk

⎧ v p k +1 − (v p i + v p j ) ⎪ v p k +1 − v p k ⎪ ⎪ (v + v ) − v pj p k −1 ⎪ pi =⎨ v p k − v p k −1 ⎪ ⎪ ⎪ 0 ⎪ ⎩

; if (v p k < v p i + v p j < v p k +1 ) ; if (v p k −1 < v p i + v p j < v p k ) . ;

(69)

otherwise

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The first term and the second term of the right hand side in Eq. (68) express the gain of the node k by the collision between the particles at the other nodes and the loss of the node k by the collision between the particles at the node k and the particles at the other node, respectively. The particles with the volume vpi and vpj collide and coagulate, resulting in the formation of a new particle with the volume vpi+vpj. The new particles are split into the adjacent nodes under mass conservation as shown in Figure. 33 (b). Even though the collision frequency function β ij is often used with the form for free molecular regime, particles with large diameters comparable to the mean free path of the gas phase are also considered. To cover the wide size range, Fuchs form of the collision frequency function is adopted [86]:

β ij = 2π ( D p i + D p j )(d p i

gi =

⎡ d pi + d p j 8( D p i + D p j ) + + d p j ) ⎢⎢ 2 2 2 2 ⎢⎣ d p i + d p j + 2 g i + g j (d p i + d p j ) ci + c j

πci 24d p i D p i

⎡ 8D p i 3 8D p i 2 3 ⎤ 2 ⎢( d p + ) ( d ( ) ) 2 ⎥ − d pi − + pi πci πci ⎢⎣ i ⎥⎦

⎤ ⎥ ⎥ ⎥⎦

−1

(70)

(71)

The diffusion coefficient of each particle is given in Ref. [71] as

Dpi =

k BT 3πηd p i

⎧⎪ ⎡ ⎛ 1.1 ⎨1 + Kn i ⎢1.257 + 0.4 exp⎜⎜ − ⎪⎩ ⎝ Kn i ⎣⎢

⎞⎤ ⎫⎪ ⎟⎥ ⎬ . ⎟ ⎠⎦⎥ ⎪⎭

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Heterogeneous cocondensation In the synthesis of silicide nanoparticles, simultaneous cocondensation of vapors of silicon and the metal should be evaluated precisely. The net production rate of condensation with the concept of size-splitting is analogous to that of coagulation as shown in Figure. 33 (c). All the particles with the volume vpi at the node i grow to the new particles with the volume vpi+Δvpi by gaining vapors during the time increment Δt. It is noted that the node i becomes empty through the process. Thus, the net production rate of condensation is given as

[N& ]

p k condensation

ξ ik(condensation )

=∑ i

(ξik( condensation ) − δ ik ) N p i Δt

.

⎧ v p − (v p + Δv p ) i i ; if (v p k < v p i + Δv p i < v p k +1 ) ⎪ k +1 v p k +1 − v p k ⎪ ⎪ (v + Δv ) − v pi k −1 ⎪ pi ; if (v p k −1 < v p i + Δv p i < v p k ) . =⎨ v v − pk p k −1 ⎪ ⎪ ⎪ 0 ; otherwise ⎪⎩

(73)

(74)

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δ ik represents the Kronecker delta written as

⎧1 ; if (i = k ) . ⎩0 ; if (i ≠ k )

δ ik = ⎨

(75)

The volume change of each particle Δvp is obtained from the growth rate by heterogeneous condensations of several materials. The growth rate is estimated from the net molecular flux in more general form than Eq. (62) [86]:

⎡ ⎤ 0.75α m (1 + Kn i ) = ∑ 2πd p i Dvapor m vmonomer m ( N monomer m − N ' S i m ) ⎢ ⎥ . (76) 2 dt m ⎣⎢ 0.75α m + 0.283α m Kn i + Kn i + Kn i ⎦⎥

dv p i

In Eq. (76), the concentration in the saturated state is modified by considering Kelvin effect which is considerable for particularly small particles as the following [71].

⎛ 4σ v N ' S i m = N S m exp⎜ st m monomer m ⎜ d p i k BT ⎝

⎞ ⎟. ⎟ ⎠

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It is relatively easy to estimate the amounts of materials in the nanoparticle of interest from vapor consumption by homogeneous nucleation and heterogeneous cocondensation. However, when coagulation takes place, the estimation of the composition in nanoparticles becomes complicated because a node has nanoparticles with the same size but different compositions. Hence, it is assumed that all nanoparticles in a node have the same composition. Under this assumption, statistical calculation becomes possible by expanding the nodal model. In the coagulation process, a single particle with the volume vpi and a single particle with the volume vpj unite so that the fraction of the material m in the new particle is given as

v pi X im + v p j X j m v pi + v p j

.

Monomer balance The concentration of the monomers of the material vapor is determined simultaneously. The monomers in the gas phase are consumed by homogeneous nucleation and heterogeneous condensation:

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d ⎛ N monomer m ⎞ 1 ⎟⎟ = ⎜ ρ dt ⎜⎝ ⎠ ρ

{ [N&

]

monomer m nucleaction

[

+ N& monomer m

]

condensation

}.

(78)

Since the number of the monomers is decreased by the consumption, both terms of the right hand side in this equation have negative values. Figures 34 and 35 portray the characteristics of the vapor conversion for Mo-Si system and Ti-Si system, respectively. In Mo-Si system, as the temperature decreases, molybdenum vapor becomes supersaturated earlier than silicon vapor because the saturation pressure of molybdenum is much smaller than that of silicon. As shown in Figure. 34 (a), molybdenum shows the characteristic profiles of the vapor consumption rate by homogeneous nucleation and heterogeneous condensation at the upstream position, where the molybdenum nanoparticles are generated in advance. Although the heterogeneous condensation of molybdenum appears to take place ahead of the homogeneous nucleation of molybdenum because Figure. 34 (a) represents the main scale of the processes, it is confirmed that the homogeneous nucleation actually starts in advance of the heterogeneous nucleation in the computation. During the growth of the molybdenum nanoparticles, the silicon vapor reaches its supersaturated state and heterogeneously cocondenses on the molybdenum nanoparticles with the molybdenum vapor. It should be noted that the homogeneous nucleation of silicon is not observed in this system. Figure 34 (b) additionally shows that 99.0% of the molybdenum vapor is consumed in only 4.9 mm corresponding to 4.1 ms. The vapor consumption process in this system is completed in approximately 12.6 mm corresponding to 12.6 ms. On the other hand in Ti-Si system, the silicon vapor nucleates, and the vapors of silicon and titanium immediately cocondense on the silicon nuclei simultaneously as shown in Figure. 35 (a). This formation process of the titanium-silicide nanoparticles is completed with 99.0% of the vapor consumption in 5.2 mm corresponding to 5.0 ms. Figures 36 (a) and 37 (a) present the evolution of the particle concentration for Mo-Si system and Ti-Si system respectively, while Figures. 36 (b) and 37 (b) show the evolution of the silicon content in the nanoparticles synthesized for Mo-Si system and Ti-Si system

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respectively. In Mo-Si system, the curve of z=222.0 mm in Figure. 36 (a) indicates that the smaller nanoparticles including the newborn nuclei provide the higher concentration. Corresponding to Figure. 34 (a), the molybdenum is nucleating effectively at this position. In the heterogeneous cocondensation process, less vapors can condense on smaller nanoparticles because of the rarefied gas effect with larger Knudsen numbers as indicated in Eq. (76). The profile, therefore, passes into the bimodal distribution with two peaks around z=230.0 mm. However, the peak of the smaller nanoparticles gradually disappears because the smaller nanoparticles are consumed by coagulation. Figure 36 (b) shows that a larger amount of the silicon vapor condenses on the larger nanoparticles. After all the vapors are consumed, the silicon content does not change very much although the nanoparticles slowly grow by coagulation. Figures 34 (a) and (b) tell that the smaller nanoparticles obtain less silicon but that the nanoparticles with the diameters larger than 10 nm approach the stoichiometric composition. On the other hand in Ti-Si system in Figure. 37 (a), numerous nuclei are generated around z=228.0 mm, and immediately coagulation process becomes more effective attributable to the high concentration. Therefore, the smaller nanoparticles are easily consumed for the growth so that the profile does not show a remarkable bimodal distribution. Since the vapors of silicon and titanium cocondense simultaneously, the profiles of the silicon content stay identical as shown in Figure. 37 (b). Additionally, Figures. 37 (a) and (b) show that the generated nuclei are only silicon.

Figure 34. Characteristics of vapor conversion for Mo-Si system: (a) Vapor consumption rate, (b) Conversion ratio.

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Figure 35. Characteristics of vapor conversion for Ti-Si system: (a) Vapor consumption rate, (b) Conversion ratio.

Figure 36. Evolution of particle profiles for Mo–Si system: (a) Size distribution, (b) Si content distribution. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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Figure 37. Evolution of particle profiles for Ti–Si system: (a) Size distribution, (b) Si content distribution.

Figure 38. Particle size distribution functions. (Experimental result is obtained from Ref. [29])

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Figure 39. Relative content of the compounds for the silicon content in the precursory powders. (Experimental result is obtained from Ref. [29])

Figure 38 portrays the particle size distribution functions obtained from the present model in comparison with those obtained from the experiment [29]. Because the nodal model provides the concentration of the nanoparticles at each node, the size distribution based on the concentration should be converted into the particle size distribution function. Hence, the size bin for the node k is defined as

Δv p k = v p k +1v p k − v p k v p k −1

(79)

which is based on a logarithmic size space expressed by Eq. (63). The particle size distribution function is determined by considering that this size bin includes all the nanoparticles at the node k. The computational results and the experimental results show good agreements in both Mo-Si system and Ti-Si system. Figure 39 presents the relative contents of the compounds for the different silicon content in the precursory powders. Compared with the phase diagrams of silicides in Ref. [87], the quantitative information of the composition of the final products is obtainable. For MoSi2, the result by the present model shows good agreement with that by the experiment [29]. It is guaranteed that the nanoparticles of the disilicide MoSi2 are successfully synthesized when the silicon content in the precursory powders is set to be 66.7% which is the stoichiometric composition of MoSi2. It is also predicted that the nanoparticles of Mo5Si3 are mainly synthesized in the case of the initial silicon content of 40.0%. In Ti-Si system as shown in Figure. 39 (b), the similar tendencies are obtained for Ti5Si3 between the result by the model and that by the experiment. Particularly when the initial silicon content is 33.0%, the relative

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content obtained from the model quantitatively agrees with that obtained from the experiment very well.

CONCLUSION

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Thermal plasma processing may become a useful tool in production of new materials through chemical conversion processes. Especially, ITP reactor has been used for mass scale synthesis of nanoparticles from various types of precursors. New and interesting researches have been described in this section with respect to nanoparticle formation. The theoretical/numerical approach provides the beneficial information about the formation mechanism of nanoparticles, which helps us to design the synthesis systems well-controlled for required nanoparticles. The model can be used to optimize the process parameters to achieve the maximum efficiency during nanoparticle formation by ITP. Efficient synthesis of nanoparticles depends not only on the physical properties of the precursor, but also on the plasma discharge conditions and quenching conditions. A: vector potential (= ( Ar , Aθ , Az ) ) a: thermal accommodation coefficient B: magnetic flux density vector (= ( Br , Bθ , Bz ) 　 )　 c: specific heat c : mean thermal speed CD: drag force coefficient Cp: specific heat at constant pressure D: diffusion coefficient d: diameter < d > v : volume mean diameter E: electric field vector (= ( E r , Eθ , E z ) ) E: electric field strength EI: ionization energy e: electric charge F: Lorentz force vector (= ( Fr , Fθ , Fz ) ) f: correction coefficient fq: geometric spacing factor G: growth rate by heterogeneous condensation g: gravitational force vector (= (0,0, g z ) ) H: latent heat h: enthalpy of fluid ht: heat transfer coefficient I: homogeneous nucleation rate K: coefficient Kn: Knudsen number kB: Boltzmann constant L: length l: mean free path

X: molar fraction x: molar fraction of liquid phase z: axial coordinate β : collision frequency function γ : specific heat ratio ε : emissivity φ F : floating potential η : viscosity Θ : normalized surface tension θ : azimuthal coordinate λ : thermal conductivity μ 0 : permeability in vacuum μ : mobility ρ : density of fluid σ e : electrical conductivity σ g : geometric standard deviation σ SB : Stefan-Boltzmann coefficient σ st : surface tension ω : angular frequency Superscript *: critical value Subscripts Ar: argon atom Ar+: argon ion c: counterflow Coil: coil boil: boiling

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Nanoparticle Synthesis by Thermal Plasmas M: moment m: mass N: concentration Nu: Nusselt number n: particle size distribution function (PSDF) p: pressure of fluid Pr: Prandtl number Q: flow rate Q& : heat flux R: radius Re: Reynolds number r: radial coordinate S: supersaturation ratio s: surface area T: temperature Ta: ambient temperature t: time u: velocity vector (= (ur , uθ , u z ) ) v: volume vg: geometric mean volume

207

electron: electron f: film Joule: Joule heating melt: melting monomer: monomer p: particle r: radial component rad: radiation S: saturation s: surface Silicon: silicon th: thermophoresis vapor: vapor Wall: wall z: axial component θ : azimuthal component ∞ : bulk

REFERENCES

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

Girshick, S. L. & Yu, W. (1990). Radio-frequency induction plasmas at atmospheric pressure: Mixtures of hydrogen, nitrogen, and oxygen with argon. Plasma Chem. Plasma Process., 10, 515-529. Girshick, S. L., Li, C., Yu, W. & Han, H. (1993). Fluid boundary layer effects in atmospheric-pressure plasma diamond film deposition. Plasma Chem. Plasma Process., 13, 169-187. Watanabe, T., Tonoike, N., Honda, T. & Kanzawa, A. (1991). Flow, temperature and concentration fields in reactive plasmas in an inductively coupled RF discharge. Characteristics in argon-oxygen and argon-nitrogen thermal plasmas. J. Chem. Eng. Japan, 24, 25-32. Watanabe, T., Kanzawa, A., Ishigaki, T. & Moriyoshi, Y. (1996). Thermal plasma treatment of titanium carbide powders: Part I. Numerical analysis of powder behavior in argon-hydrogen and argon-nitrogen radio frequency plasmas. J. Mater. Res., 11, 25982610. Desilets, M., Davies, B., Soucy, G. & Proulx, P. (1998). Mixing study in an inductive plasma reactor: comparison between model calculations and experimental results. Can. J. Chem. Eng., 76, 707-716. Sakano, M., Watanabe, T. & Tanaka, M. (1999). Numerical and experimental comparison of induction thermal plasma characteristics between 0.5 MHz and 4 MHz. J. Chem. Eng. Japan, 32, 619-625. Tanaka, Y. & Sakuta, T. (2002). Chemically non-equilibrium modeling of N2 thermal ICP at atmospheric pressure using reaction kinetics. J. Phys. D, 35, 468-476.

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

208 [8]

[9]

[10]

[11]

[12]

[13]

[14] [15] [16]

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

[17]

[18]

[19]

[20]

[21]

[22]

[23]

Takayuki Watanabe and Masaya Shigeta Watanabe, T. & Sugimoto, N. (2004). Numerical analysis of oxygen induction thermal plasmas with chemically non-equilibrium assumption for dissociation and ionization. Thin Solid Films, 457, 201-208. Atsuchi, N., Shigeta, M. & Watanabe, T. (2006). Modeling of non-equilibrium argonoxygen Induction plasmas under atmospheric pressure. Int. J. Heat Mass Transfer, 49, 1073-1082. Mostaghimi, J. & Boulos, M. I. (1990). Effect of frequency on local thermodynamic equilibrium conditions in an inductively coupled argon plasma at atmospheric pressure. J. Appl. Phys., 68, 2643-2648. Tanaka, Y. (2006). Two-temperature chemically non-equilibrium modeling of highpower Ar-N2 inductively coupled plasmas at atmospheric pressure. J. Phys. D, 37, 1190-1205. Tanaka, Y. (2006). Time-dependent two-temperature chemically non-equilibrium modelling of high-power Ar-N2 pulse-modulated inductively coupled plasmas at atmospheric pressure. J. Phys. D, 39, 307-319. Watanabe, T., Atsuchi, N. & Shigeta, M. (2006). Two-temperature chemically nonequilibrium modeling of argon induction plasmas with diatomic gases, Inter. J. Heat Mass Transfer, 49, 4867-4876. Ohno, S. & Uda, M. (1984). Generation rate of ultrafine metal particles in hydrogen plasma-metal reaction. J. Japan Inst. Metals, 48, 640-646. Anekawa, Y., Koseki, T., Yoshida, T. & Akashi, K. (1985). The co-condensation process of high temperature metallic vapors. J. Japan Inst. Metals, 49, 451-456. Harada, T., Yoshida, T., Koseki, T. & Akashi, K. (1981). Co-condensation process of high temperature metallic vapors. J. Japan Inst. Metals, 45, 1138-1145. Turgut, Z., Scott, J. H. J., Huang, M. Q., Majetich, S. A. & McHenry M. E. (1998). Magnetic properties and ordering in C-coated FexCo1-x alloy nanocrystals. J. Appl. Phys., 83, 6488-6470. Turgut, Z., Nuhfer, N. T., Piehler, H. R. & McHenry, M. E. (1999). Magnetic properties and microstructural observations of oxide coated FeCo nanocrystals before and after compaction. J. Appl. Phys., 85, 4406-4408. Scott, J. H. J., Majetich, S. A., Turgut, Z., Mchenry, M. E. & Boulos, M. (1997). Carbon coated nanoparticle composites synthesized in an RF plasma torch. Mat. Res. Soc. Symp. Proc., 457, 219-224. Scott, J. H. J., Turgut, Z., Chowdary, K., McHenry, M. E. & Majetich, S. A. (1998). Thermal plasma synthesis of Fe-Co alloy nanoparticles. Mat. Res. Soc. Symp. Proc., 501, 121-126. Son, S., Taheri, M., Carpenter, E., Harris, V. G. & McHenry, M. E. (2002). Synthesis of ferrite and nickel ferrite nanoparticles using radio-frequency thermal plasma torch. J. Appl. Phys., 91, 7589-7591. Son, S., Swaminathan, R. & McHenry, M. E. (2003). Structure and magnetic properties of RF thermally plasma synthesized Mn and Mn-Zn ferrite nanoparticles. J. Appl. Phys., 93, 7495-7497. Swaminathan, R., McHenry, M., E., Poddar, P. & Srikanth, H. (2005). Magnetic properties of polydisperse and monodisperse NiZn ferrite nanoparticles interpreted in a surface structure model. J. Appl. Phys., 97, 10G104.

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Nanoparticle Synthesis by Thermal Plasmas

209

[24] Mohai, I., Gai L., Szepvolgyi, J., Gubicza, J. & Farkas, Z. (2007). Synthesis of nanosized zinc ferrites from liquid precursors in RF thermal plasma reactor. J. European Ceramic Soc., 27, 941-945. [25] Swaminathan, R., Willard, M. A. & McHenry, M. E. (2006). Experimental observations and nucleation and growth theory of polyhedral magnetic ferrite nanoparticles synthesized using an RF plasma torch. Acta Materialia, 54, 807-816. [26] Katsuda, K., Watanabe, T., Abe, Y. & Ishii, Y. (2002). Preparation of nanoparticle mixture of nitride and boride by induction thermal plasmas. Trans. Mater. Res. Soc. Japan, 27, 137-140. [27] Watanabe, T., Ibe, T., Abe, Y., Ishii, Y. & Adachi, K. (2004). Nucleation mechanism of boride nanoparticles in induction thermal plasmas. Trans. Mater. Res. Soc. Japan, 29, 3407-3410. [28] Watanabe, T., Nezu, A., Abe, Y. & Ishii, Y. (2003). Formation mechanism of electrically conductive nanoparticles by induction thermal plasmas, Thin Solid Films, 435, 27-32. [29] Watanabe, T. & Okumiya, H. (2004). Formation mechanism of silicide nanoparticles by induction thermal plasmas. Sci. Technol. Adv. Mater., 5, 639-646. [30] Eguchi, K., Ko, I. Y., Sugawara, T., Lee, H. J. & Yoshida, T. (1989). Process Control for the Formation of fine SiC powders in thermal plasma frame. J. Japan Inst. Metals, 53, 1236-1241. [31] Kameyama, T., Sakanaka, K., Motoe, A., Tsunoda, T., Nakanaga, T., Wakayama, N. I., Takeo, H. & Fukuda, K. (1990). Highly efficient and stable radio-frequency-thermal plasma system for the production of ultrafine and ultrapure β-SiC powder. J. Mater. Sci., 25, 1058-1065. [32] Guo, J. Y., Gitzhofer, F. & Boulos, M. I. (1995). Induction plasma synthesis of ultrafine SiC powders from silicon and CH4. J. Mater. Sci., 30, 5589-5599. [33] Guo, J. Y., Gitzhofer, F. & Boulos, M. I. (1997). Effects of process parameters ultrafine SiC synthesis using induction plasmas. Plasma Chem. Plasma Process, 17, 219-249. [34] Leparoux, M., Schreuders, C., Shin, J. W. & Siegman, S. (2005). Induction plasma synthesis of carbide nanopowders. Adv. Eng. Mater, 7, 349-353. [35] Buchner, P., Lutzenkirchen-Hecht, D., Strehblow, H. H. & Uhlenbusch, J. (1999). Production and characterization of nanosized Cu/O/SiC composite particles in a thermal r.f. plasma reactor. J. Mater. Sci., 34, 925-931. [36] Lee, H. J., Eguchi, K. & Yoshida, T. (1990). Preparation of ultrafine silicon nitride, and silicon nitride and silicon carbide mixed powders in a hybrid plasma. J. Am. Ceram. Soc., 73, 3356-3362. [37] Bouyer, E., Muller, M., Henne, R. H. & Schiller, G. (2001). Thermal plasma processing of nanostructured Si-based ceramic materials. J. Nanoparticle Res., 3, 373-378. [38] Ishigaki, T., Oh, S. M., Li, J. G. & Park, D. W. (2005). Controlling the synthesis of TaC nanopowders by injecting liquid precursor into RF induction plasma. Sci. Tech. Adv. Mater., 6, 111-118. [39] Kameyama, T., Tsunoda, T., Motoe A., Uematsu, T. & Fukuda, K. (1991). The preparation of ultrafine WC1-x powder in a R.F. thermal plasma and its properties. J. Jpn. Soc. Powder and Powder Metallurgy, 38, 109-113. [40] Yoshida, T., Kawasaki, A, Nakagawa, K. & Akashi K. (1979). The synthesis of ultrafine titanium nitride in an r.f. plasma. J. Mater. Sci., 14, 1624-1630.

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210

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[41] Soucy, G., Jurewicz, J. W. & Boulos, M. I. (1995). Parametric study of the plasma synthesis of ultrafine silicon nitride powders. J. Mater. Sci., 30, 2008-2018. [42] Baba, K., Shohata, N. & Yonezawa, M. (1989). Synthesis and properties of ultrafine AIN powder by rf plasma. Appl. Phys. Lett., 54, 2309-2311. [43] Okuyama, H., Honma, K. & Ohno, S. (1999). Photocatalytic Activity of Ultrafine TiO2 Particles Synthesized by an RF Plasma CVD. J. Japan Inst. Metals, 63, 74-81. [44] Oh, S. M. & Ishigaki, T. (2004). Preparation of pure rutile and anatase TiO2 nanopowders using RF thermal plasma. Thin Solid Films, 457, 186-191. [45] Oh, S. M., Li, J. G. & Ishigaki, T. (2005). Nanocrystalline TiO2 powders synthesized by in-flight oxidation of TiN in thermal plasma: Mechanisms of phase selection and particle morphology evolution. J. Mater. Res., 20, 529-537. [46] Li, Y. L. & Ishigaki, T. (2004). Controlled one-step synthesis of nanocrystalline anatase and rutile TiO2 powders by in-flight thermal plasma oxidation. J. Phys. Chem. B, 108, 15536-15542. [47] Li, J. G., Kamiyama, H., Wang, X. H., Moriyoshi, Y. & Ishigaki, T. (2006). TiO2 nanopowders via radio-frequency thermal plasma oxidation of organic liquid precursors: Synthesis and characterization. J. European Ceramic Soc., 26, 423-428. [48] Li, J. G., Wang, X. H., Kamiyama, H., Ishigaki, T. & Sekiguchi, T. (2006). RF plasma processing of Er-doped TiO2 luminescent nanoparticles. Thin Solid Films, 506-507, 292-296. [49] Sato, T., Tanigaki, T., Suzuki, H., Saito, Y., Kido, O., Kimura, Y., Kaito, C., Takeda, A. & Kaneko, S. (2003). Structure and optical spectrum of ZnO nanoparticles produced in RF plasma. J. Crystal Growth, 255, 313-316. [50] Shin, J. W., Miyazoe, H., Leparoux, M., Siegmann, S., Dorier, J. L. & Hollenstein, C. (2006). The influence of process parameters on precursor evaporation for alumina nanopowder synthesis in an inductively coupled rf thermal plasma. Plasma Sources Sci. Technol., 15, 441-449. [51] Watanabe, T. & Fujiwara, K. (2004). Nucleation and growth of oxide nanoparticles prepared by induction thermal plasmas, Chemical Engineering Communication, 191, 1343-1361. [52] Japan Institute of Metals, Metal Data Book, (Maruzen, Tokyo, Japan), 1993. [53] Bilodeau, J. F. & Proulx, P. (1996). A mathematical model for ultrafine iron powder growth in thermal plasma. Aerosol Science and Technology, 24, 175-189. [54] Desilets, M., Bilodeau, J. F. & Proulx, P. (1997). Modelling of the reactive synthesis of ultra-fine powders in a thermal plasma reactor. Journal of Physics D., 30, 1951-1960. [55] Shigeta, M., Watanabe, T. & Nishiyama, H. (2004). Numerical investigation for nanoparticle synthesis in an RF inductively coupled plasma. Thin Solid Films, 457, 192-200. [56] Rao, N., Girshick, S. L., Heberlein, J., McMurry, P., Jones, S., Hansen, D. & Micheel, B. (1995). Nanoparticle formation using a plasma expansion process. Plasma Chemistry and Plasma Processing, 15(4), 581-606. [57] Hafiz, J., Wang, X., Mukherjee, R., Mook, W., Perrey, C. R., Deneen, J., Heberlein, J. V. R., McMurry, P. H., Gerberich, W. W., Carter, C. B. & Girshick, S. L. (2004). Hypersonic plasma particle deposition of Si–Ti–N nanostructured coatings. Surface and Coatings Technology, 188-189, 364-370. [58] Kong, P. C. & Lau, Y. C. (1990). Plasma synthesis of ceramic powders. Pure and Applied Chemistry, 62(9), 1809-1816.

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Nanoparticle Synthesis by Thermal Plasmas

211

[59] Ikeda, M., Li, JG., Ye, R., Kobayashi, N., Moriyoshi, Y. & Ishigaki, T., (2007) Size control of TiO2 nanoparticles through quench gas injections into RF induction thermal plasma, in: K., Tachibana, O., Takai, K. Ono, & T. Shirafuji, (Ed.), Full-Papers CD of 18th International Symposium on Plasma Chemistry, International Plasma Chemistry Society, Kyoto, Japan, 4 pages. [60] Shigeta, M. & Watanabe, T. (2008). Two-dimensional analysis of nanoparticle formation in induction thermal plasmas with counterflow cooling. Thin Solid Films, 516, 4415-4422. [61] Mostaghimi, J. & Boulos, M. I. (1989). Two-dimensional electromagnetic field effects in induction plasma modelling. Plasma Chemistry and Plasma Processing, 9, 25-44. [62] Hirschfelder, J. O., Curtiss, C. F. & Bird, R. B. (1964). Molecular Theory of Gases and Liquids, (John Willy, NY, USA). [63] Miller, R. C. & Ayen, R. J. (1969). Temperature profiles and energy balances for an inductively coupled plasma torch. Journal of Applied Physics, 40(13), 5260-5273. [64] Patankar, S. V. (1980). Numerical Fluid Flow and Heat Transfer, (Hemisphere, NY, USA). [65] Chen, X. & Pfender, E. (1982). Unsteady heating and radiation effects of small particles in a thermal plasma. Plasma Chemistry and Plasma Processing, 2(3), 293-316. [66] Boulos, M. I. (1978). Heating of powders in the fire ball of an induction plasma. IEEE Transactions on Plasma Science, PS-6, 93-106. [67] Pfender, E. & Lee, Y. C. (1985). Particle dynamics and particle heat and mass transfer in thermal plasmas. Part I: The motion of a single particle without thermal effects. Plasma Chemistry and Plasma Processing, 5(3), 211-237. [68] Lee, Y. C., Chyou, Y. P. & Pfender, E. (1985). Particle dynamics and particle heat and mass transfer in thermal plasmas. Part II. Particle heat and mass transfer in thermal plasmas. Plasma Chemistry and Plasma Processing, 5(4), 391-414. [69] Honda, T., Hayashi, T. & Kanzawa, A. (1981). Heat transfer from rarefied ionized argon gas to a biased tungsten fine wire. International Journal of Heat and Mass Transfer, 24, 1247-1255. [70] Crowe, C. T., Sharma, M. P. & Stock, D. E. (1977). The particle-source-in cell (PSICell) method for gas-droplet flows. Transactions of the ASME, Journal of Fluids Engineering, 99, 325-332. [71] Friedlander, S. K. (2000). Smoke, Dust and Haze, Fundamentals of Aerosol Dynamics 2nd Ed., (Oxford University Press, NY, USA). [72] Girshick, S. L., Chiu, C. P. & McMurry, P. H. (1990). Time-dependent aerosol models and homogeneous nucleation rates. Aerosol Science and Technology, 13, 465-477. [73] Phanse, G. M. & Pratsinis, S. E. (1989). Theory for Aerosol Generation in Laminar Flow Condensers. Aerosol Science and Technology, 11, 100-119. [74] Talbot, L., Cheng, R. K., Schefer, R. W. & Willis, D. R. (1980). Thermophoresis of particles in a heated boundary layer. Journal of Fluid Mechanics, 101(4), 737-758. [75] Girshick, S. L., Chiu, C. P., Muno, R., Wu, C. Y., Yang, L., Singh, S. K. & McMurry, P. H. (1993). Thermal plasma synthesis of ultrafine iron particles. Journal of Aerosol Science, 24(3), 367-382. [76] Shigeta, M. & Watanabe, T. (2005). Numerical analysis for preparation of silicon-based intermetallic nano-particles in induction thermal plasma flow systems. JSME International Journal, Series B, 48(3), 425-431.

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212

Takayuki Watanabe and Masaya Shigeta

[77] Shigeta, M. & Nishiyama, H. (2005). Numerical analysis of metallic nanoparticle synthesis using RF inductively coupled plasma flows. Transactions of the ASME, Journal of Heat Transfer, 127, 1222-1230. [78] Shigeta, M. & Watanabe, T. (2007). Growth mechanism of silicon-based functional nanoparticles fabricated by inductively coupled thermal plasmas. Journal of Physics D: Applied Physics, 40, 2407-2419. [79] Shigeta, M. & Watanabe, T. (2007). Multi-component co-condensation model of Tibased boride/silicide nanoparticle growth in induction thermal plasmas. Thin Solid Films, 515, 4217-4227. [80] Shigeta, M. & Watanabe, T. (2005). Numerical analysis for co-condensation processes in silicide nanoparticle synthesis using induction thermal plasma at atmospheric pressure conditions. Journal of Materials Research, 20(10), 2801-2811. [81] Da Cruz, A. C. & Munz, R. J. (1997). Vapor phase synthesis of fine particles. IEEE Transactions on Plasma Science, 25(5), 1008-1016. [82] Shigeta, M. & Watanabe, T. (2008). Numerical investigation of cooling effect on platinum nanoparticle formation in inductively coupled thermal plasmas. Journal of Applied Physics, 103(7), 074903 (15 pages). [83] Pratsinis, S. E. & Kim, K. S. (1989). Particle coagulation, diffusion and thermophoresis in laminar tube flows. Journal of Aerosol Science, 20(1), 101-111. [84] Joshi, S. V., Liang, Q., Park, J. Y. & Batdorf, J. A. (1990). Effect of quenching conditions on particle formation and growth in thermal plasma synthesis of fine powders. Plasma Chemistry and Plasma Processing, 10(2), 339-358. [85] Prakash, A., Bapat, A. P. & Zachariah, M. R. (2003). A simple numerical algorithm and software for solution of nucleation, surface growth, and coagulation problems. Aerosol Science and Technology, 37, 892-898. [86] Seinfeld, J. H. & Pandis S. N. (1998). Atmospheric Chemistry and Physics, From Air Pollution to Climate Change, (John Wiley & Sons, Inc., New York, USA). [87] Massalski, T. B. (1990). Binary Alloy Phase Diagrams. 2nd Ed. 3, (American Society for Metals, Materials Park, OH, USA).

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ISBN: 978-1-61668-344-3 © 2010 Nova Science Publishers, Inc.

Chapter 4

TIO2 NANOPARTICLES: TRADITIONAL AND NOVEL SYNTHETIC METHODS FOR PHOTOCATALYTIC PAINT FORMULATIONS Giuseppe Cappelletti* Università degli Studi di Milano, Dipartimento di Chimica Fisica ed Elettrochimica, Via Golgi, 19 20133 Milano (ITALY).

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This review chapter is focused on the physico-chemical features and the related semiconductor activity of nanometric TiO2 both pure and formulated as photocatalytic paint. First the synthesis of the particles, with controlled structural and morphological aspects, is discussed on the grounds of both literature and original research data. Different synthetic approaches are presented: combustion, precipitation and solvothermal routes, sol-gel synthesis, and also more innovative methods like those assisted by ultrasound and microwaves. The photocatalytic activity with respect to the degradation of both water (organic dyes, chlorophenols, etc.) and air pollutants (nitrogen oxides, NOx, volatile organic compounds, VOC, etc.) is analyzed. Literature, commercial and home made samples are considered both in the case of pure and doped TiO2. The second part of the chapter concerns the formulation of photoactive paints. Liquid coating paint formulations are complex, multicomponent systems including resins as film forming agents, binders, additives, solvents and extenders. The possibility to incorporate nanoactive semiconductor materials (such as nano-TiO2) in order to obtain photocatalytic paints leads to several problems also due to the possible simultaneous photodegradation of the desired air pollutants and of the organic matrix of the coating. The few data, present in the literature concerning this latter points, are compared also with original laboratory results. *

Corresponding author: E-mail: [email protected], Phone: +390250314228, Fax: +390250314228.

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1. INTRODUCTION The synthesis of nanomaterials and the suitable control of the physico-chemical properties of synthesis with nanometric dimensions are, nowadays, crucial aspects of nanoscience. In the last few years more attention has been focused on nanoparticles, because of both new preparation strategies and advanced tools for fine characterization and investigation, such as scanning probe microscopies (SEM, TEM and AFM), diffraction and spectroscopic methods. Oxides, particularly those of transition metals, display the widest and most attractive research field of any class of materials. Among different available semiconductor oxides, TiO2 is the most frequently employed semiconductor given its cheapness, non-toxicity, and structural stability. It can be produced relatively easily and used not only as a gas sensor, as a corrosion protective coating, and in electric devices such as varistors, but especially as a photocatalyst for environmental remediation, photo-electrochemical splitting of water and conversion of solar energy to electric power. These processes are based on reactive electrons and holes generated at the surface of the semiconductor when it is illuminated by light with energy larger than its band gap (Figure 1). Once electrons are created in the conduction band and holes in the valence band by optical excitation, both carriers diffuse to the surface to initiate reduction/oxidation reactions with adsorbed molecules. However, the competing e−−h+ recombination process tends to prevent these carriers for reaching the surface.

band gap

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A + e− → A− Reduction

conduction band e−

hν e− valence band

h+

B + h+ → B + Oxidation

Figure 1. The activation mechanism of TiO2 semiconductor.

The nanometric scale of the crystals plays a key role in affecting the performance of the material. It is well known that small crystallite size can lead to quantum confinement effects in semiconductors and to a sharp increase in activity when the grain size becomes smaller than the space-charge depth [1,2]. Consequently a big effort can be seen to prepare nanomaterials with controlled size/morphology. In particular, TiO2 nanoparticles have approximately 15% of their surface atoms at step edges; high density of surface undercoordinated Ti atoms and the very strained configuration of the surface atoms are responsible for a considerable fraction of the reactivity of TiO2 nanoparticles [3]. TiO2 is a crystalline solid with a dominant ionic character, present in nature in three different forms:

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 215 rutile, anatase and brookite. Besides, nanocrystalline anatase particles (one of the TiO2 polymorphs) are transparent to visible light but are highly effective in the absorption of UV– light. The UV absorption of nano–sized anatase particles is blue–shifted (i.e. wavelength absorption occurs at 70–80 nm lower than bulk anatase), so that higher energy UV–light is absorbed by this type of nanoparticles. Hence, they are used in sunscreens to protect against UV induced skin damage. For these reasons, the pace of scientific research has doubled and redoubled in the last 5 years [4-7]. So the first part of this chapter is devoted to collect several traditional and novel synthetic methods adopted to prepare nanometric TiO2 with tailored physico-chemical features. Combustion, precipitation and solvothermal routes, sol-gel synthesis, ultrasound- and microwaves-assisted innovative preparations are finely discussed. In recent years TiO2 has also played a major role in fine photocatalytic applications. Possible uses of TiO2 as photocatalyzer are innumerable. Among these purification of wastewaters [8,9], protective coatings of ancient marbles and materials [10], degradation of organic pollutants (some species can be completely mineralized to CO2 and ions generally harmless for humans and environment) [11,12], sterilization [12-14] and self–cleaning materials. These range from paints to cements, from glasses to ceramics and asphalts. Thus, the last part of the chapter concerns the formulation of photoactive paints by adding commercial and home-made nano-TiO2. The photodegradation of the desired air pollutants (NOx, VOC) and the unwanted organic matrix of the coating are discussed.

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2. NANOTITANIA SYNTHETIC ROUTES Notwithstanding the extensive know-how on oxide particles achieved in the last decades, it is only in recent years that several methods for preparation of monodisperse nanoparticles have been optimized [15]. Up-to-date developments in the preparation of highly dispersed nanotitania powders, with special reference to their final photocatalytic applications, are reported in the following.

2.1 Combustion Synthesis Combustion synthesis leads to highly crystalline fine/large area particles [16-20]. The synthetic process involves a rapid heating of a solution/compound containing redox mixtures/redox groups. In general, combustion synthesis offers many benefits for minimizing energy requirement, simplifying equipment, and shortening operation time through the use of a sustainable exothermic solid–solid reaction among raw materials. During combustion, the temperature reaches about 650°C for a short period of time (1–2 min) making the material crystalline. Solution combustion synthesis is a single step process that can produce pure anatase phase titania [16]. Madras et al. [17] synthetized pure and doped anatase titania by a solution combustion method starting from titanyl nitrate TiO(NO3)2 and fuel glycine H2N–CH2–COOH. The formation of TiO2 by combustion reaction, assuming complete combustion of the titanyl nitrate–glycine redox mixture, follows this equation:

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9TiO(NO3)2(aq) + 10C2H5O2N(aq)→ 9TiO2(s) + 14N2(g) + 20CO2(g) + 25H2O(g) Also doped TiO2 samples, (e.g. 1 at.% Pt/TiO2, 2 and 5 at.% Cu/TiO2, and 2 at.% Mn/TiO2) were prepared by this route. The authors tested the synthetized samples towards the photocatalytic degradation of various organic dyes such as Methylene blue (MB), Alizarin S (AS), Methyl red (MR), Congo red (CR), and Orange G (OG). They also studied the effects of catalyst loading (from 0.5 to 2 kg/m3) and the initial concentrations of the dyes (from 50 to 200 ppm). The initial rate of the reaction reached saturation for catalyst loading above 1 kg/m3 and increased with increasing the initial concentration. They found that the photoactivity of the combustion synthesized titania powder was better than that of the commercial Degussa P25. Transition metal ion (Pt, Cu and Mn) substitution on TiO2 has negative effects on the activity. In particular, the photocatalytic degradation of methylene blue varied in the order: TiO2 > 2% Mn/TiO2 > 2% Cu/TiO2 > 1% Pt/TiO2 > 5% Cu/TiO2. The authors attributed this effect to the metals being in ionic state in combustion synthesized materials as evidenced by XPS studies. The same authors [18] successively showed that, under solar irradiation, the combustion synthesized nano TiO2 samples were deactivated, in time, much slower than P25 Degussa. Xu et al. [19] obtained carbon modified titania trough a solution combustion method by adopting as starting raw materials tetrabutyl titanate and glycine. They explored the photocatalytic activity of the calcined sample (350°C) with respect to the degradation of 4chlorophenol (4-CP) under various irradiation sources (both UV and solar light). The pure P25 Degussa had similar photocatalytic activity to that of carbon-modified TiO2 under UV light, but carbon-modified TiO2 showed a much higher level of activity under irradiation at wavelengths longer than 400 nm. Moreover the superior photocatalytic activities of the asprepared catalysts were demonstrated by illumination experiments in visible light (λ ≥ 420 nm). The degradation and mineralization rate of the synthesized TiO2 were 2.6 and 4.5 times higher than that of Degussa P25, respectively. The authors affirmed that the high visible light activity of the synthesized TiO2 was partially due to the crystal structure (anatase), large surface area for higher adsorption of the organic substrate, intense absorption in the visible light range (400-700 nm) of the synthesized TiO2, related to its carbon content. Kitamura et al. [20] described a new type of combustion synthesis, in which metallic titanium particles of different sizes (10 and 25 μm) and sodium perchlorate were intensively mixed by ball milling and ignited by an electrical heating foil to produce titania through the following reaction:

Ti + 0.5NaClO4 → TiO2 (s) + 0.5NaCl(s) The shape and crystal structure of the obtained nanoparticles depended significantly on the particle size of the titanium raw material: the smaller titanium particles (10 μm) resulted in rutile with an irregular shape, whereas the larger particles (25 μm) resulted in spheres of anatase phase. The products, especially that of the rutile-rich sample, exhibited a relatively higher photocatalytic activity towards methanol dehydrogenation and acetic acid decomposition under UV irradiation in deaerated and aerated solutions. Nevertheless the two samples were less active as compared with P25.

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 217

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2.2 Precipitation Methods These routes consist in the precipitation of titanium hydroxide by the addition of a basic solution (NaOH, NH3/NH4OH, urea) starting from TiCl3, TiCl4, TiOSO4 precursors [21-32]; a final annealing step is necessary to impart to the samples a certain degree of crystallinity. The main drawback of this synthetic route is often the loss of control of both particle size and relative surface area, due to fast and uncontrollable precipitation and strong synterization processes. Just to control these latter effects several authors adopted urea as the precipitating agent. In particular Bakardjieva et al. [21] prepared pure porous spherical anatase TiO2 particles by hydrolysis of TiOSO4 with urea. Photoactive titania powders with variable amount of anatase and rutile phases were prepared by heating the pure anatase in the temperatutre range 800–1150°C. Significant sintering effects between these spherical particles were observed only over 1000°C, while the size and shape remained unchanged for T < 1000°C. These latter materials showed significantly high photocatalytic activity in the decomposition of 4-chlorophenol aqueous suspension by comparison with the Degussa P25 titania photocatalyst. Other authors [25] synthetized TiO2 nanocrystals by an homogeneous precipitation method using urea; the physico-chemical characteristics of the powders were investigated in comparison with those prepared by conventional precipitation with ammonia. The addition of urea led to uniform spherical anatase particles with a mean size of 4-5 nm. With the increasing of the heating temperature, the particle size of these powders was smaller and more uniform than that of particles obtained by the ammonia precipitation method. Moreover they showed good photocatalytic activity because of the good crystallinity and a large specific surface area (280 m2/g). Usually the precipitation method promotes the anatase polymorph. Nevertheless, in particular conditions, rutile may be obtained as the main phase [23,26,31]. Xu et al. [26] obtained highly active sulphate-promoted rutile by precipitating Ti(SO4)2 in NaOH solution followed by peptizing the product in HNO3. The sulphate dopant was appropriate for improving the activity as well as extending the optical response to the visible light region. Further, the authors affirmed that the presence of sulphate species, homogeneously dispersed throughout the bulk, not only retarded the growth of crystallite sizes but also induced a higher content of surface hydroxyl groups. They found that the sulphate species were sensitive to the heating temperature and were decomposed upon calcination at 600°C. The authors observed that the photoactivity of rutile-catalysts towards the oxidation of heptane under UV and solar irradiation was larger than that of P25 for samples calcined at temperatures lower than 600°C, while the activity became comparable to that of P25 for samples fired at temperatures larger than 600°C due to the removal of sulphates. In our laboratories N-doped TiO2 nanopowders were obtained through a composite route starting from strongly acid TiCl3 aqueous solutions. N-species were loaded in the samples by NH3/NH4Cl in the liquid phase. A final calcination step at mild temperatures (300°C) promoted the crystallization of the amorphous precursors. By care controlling the suspension pH during the formation of the TiO2 precursors, different phase compositions were obtained: at pH 4 (TN4) rutile was the prevailing phase, although also anatase and brookite were present as minor components. At neutral pH (TN7), anatase was the main component in the presence of traces of rutile, while only at pH 9 (TN9) the pure anatase phase could be

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obtained. The TEM images showing the different morphology of the samples obtained at the three pH values are reported in Figure 2.

a) TN4

b) TN7

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c) TN9

Figure 2. TEM images of the synthetized samples at various pH.

TN4 (Figure 2a) displays well crystallised grains with size ranging from 10 nm up to 30 nm approximately. The grains show quite different morphologies: from round shaped to prismatic, although in all cases rather isotropic. In the case of TN7 (Figure 2b) the observed microstructure shows mostly equiaxed domains with an average size close to 10 nm, while TN9 (Figure 2c) shows mostly equiaxed grains with size ranging from a few nanometers up to 30 nm or so. In this respect the grain size distribution looks comparatively broader than in the former sample (TN7). Their photocatalytic activity was tested for the degradation of toluene in the gas phase by using a solar irradiation source [22]. Highest degradations and corresponding kinetic constants were achieved in the case of N-doped samples prepared at pH 9 (80%, 6.0×10-3 min-1) with respect to those at pH 7 (65%, 4.2×10-3 min-1) and pH 4 (56%, 2.9×10-3 min-1). The sequence in photocatalytic activity seemed to correlate simply with EPR results which showed an increasing amount of paramagnetic centres with increasing the

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 219 reaction pH, the sample prepared at pH 9 containing an additional amount of paramagnetic N centres even with respect to that synthetized at pH 7.

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2.3 Sol-Gel Techniques The sol-gel process is a soft-chemistry method of producing highly dispersed materials with physico-chemical properties finely tuned by the appropriate selection of the synthetic parameters. The sol-gel route involves the evolution of inorganic networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). From the photocatalytic point of view, many properties are driven by the nature, the structure of the TiO2 phase (mainly anatase, brookite and rutile), the extension of the surface and the size of the titania particles. Consequently, it is of primary importance to have effective characterisation tools that can accurately probe the chemical species constituting the bulk and the surface of these titania based nanomaterials. The success gained by sol-gel technique is mainly due to its extremely high flexibility. Thank to the high number of modulable parameters and to the possibility of driving the synthetic path towards different directions, in terms of control at the nanoscale of shape, size and composition, the possibilities are almost endless. In addition, with respect to other synthetic technologies, solgel presents several other advantages like the high purity of the final products, their high degree of homogeneity, ease of processing, flexibility in introducing dopants in large concentrations. Two main types of sol-gel processes are described in the literature: the non-alkoxide route, which uses inorganic salts [33-35] such as nitrates, chlorides, acetates, carbonates, etc. and the most commonly employed alkoxide route based on metal alkoxides Ti(OR)4 as starting materials, such as Ti(O–Et)4, Ti(O–iPr)4 and Ti(O–nBu)4 [36-48]. The general mechanisms in the metal-organic route, based on the growth of metal oxopolymers in a solvent, could be divided into first step: hydroxylation upon the hydrolysis of alkoxy groups

Ti(OR)4 + 4H2O → Ti(OH)4 + 4ROH

(I)

second step: polycondensation process leading to the formation of branched oligomers and polymers with a metal oxo based skeleton and reactive residual hydroxo and alkoxy groups; this step occurs by two competitive mechanisms: water condensation with the formation of oxygen bridges

Ti(OH)4 + Ti(OH)4 → (OH)3TiOTi(OH)3 + H2O alcohol condensation

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(II)

220

Giuseppe Cappelletti Ti(OH)4 + Ti(OR)4 → (OH)3TiOTi(OR)3 + ROH

(III)

The hydrolysis reaction, through the addition of water, replaces alkoxide groups (OR) with hydroxyl groups (OH). Subsequent condensation reactions, involving the Ti-OH groups produce Ti-O-Ti bonds plus the by-products water or alcohol. Under most conditions, condensation commences before hydrolysis is complete. However, conditions such as, pH, H2O/M molar ratio, acid–base catalysis can force completion of hydrolysis before condensation begins. It has been demonstrated [15,49,50] that acid catalysis increases hydrolysis rates and ultimately crystalline powders are formed from fully hydrolyzed precursors. Base catalysis is thought to promote condensation with the result that amorphous powders are obtained containing unhydrolyzed alkoxide ligands. Additionally, because water and alkoxides are immiscible, a mutual solvent such as an alcohol is utilized. With the presence of this homogenizing agent, alcohol, hydrolysis is facilitated due to the miscibility of the alkoxide and water. The solvent removal step is one of the key steps of a sol-gel process. The way in which the solvent is removed greatly influences the final morphology (crystallite dimension, specific surface area, pore dimension). The fundamental parameters are in this case two: pressure and temperature. By adopting different values of the two, it is possible to generate endless combinations. However, only three methods are usually considered. The nomenclature associated to these methods is actually related to their product:

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Xerogel: by heating in oven at room pressure; Cryogel: by freeze drying; Aerogel: by reaching supercritical solvent conditions (high pressure and temperature). The supercritical conditions, adopted to remove the solvent in the case of the aerogel, provoke the growth of the crystallites and promote the formation of the anatase polymorph. Samples obtained by freeze drying seem to be the ones retaining most of the features of the amorphous hydrous gel obtained from the hydrolysis-polycondensation reaction. The removal of the solvent by freeze drying apparently provokes the formation of very small subnanometric precursor particles with exceedingly large surface area. Xerogels represent an intermediate condition while the grown crystallites of the aerogel lead to a powder with lower surface area [36,37]. Then, the sol-gel precursor, obtained by the solvent removal step, is thermally treated (300–900°C) in order to remove the organic part and to crystallize either anatase or rutile TiO2 [37]. The calcination process will inevitably cause a decline in surface area (due to sintering and crystal growth), loss of surface hydroxyl groups, and may even induce phase transformation. Obviously, the higher the calcination temperature and its duration, the higher the final crystallinity degree. At the same time, sintering phenomena occur, thus decreasing the specific surface area and increasing aggregate dimensions. One of the most important phenomena occurring during the calcination step is pore collapsing, (small pores, 400-500°C; larger pores, 700-900°C), which greatly influences the features of the final material. The introduction of a short hydrothermal step of the xerogel at different pH values appears to further modify the final features of the calcined products, reducing the negative effects

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produced by the subsequent calcination treatments [37]. Very recently templating sol-gel methods were applied to prepare very large surface area titania phases [51-53], which exhibit a mesoporous structure. Both ionic and neutral surfactants have been successfully employed as templates to prepare mesoporous TiO2 [54-56]. By modulating the operation conditions and the surfactant features, the synthesis can be projected toward liquid crystal phases with different tridimensional organization and consequently toward TiO2 having accurately controlled pore network [39]. Limthongkul et al. [28] conducted a comparative study of TiO2 powders prepared by precipitation and sol–gel methods and finally calcined in the range 400-600°C. Titanium tetrachloride and titanium tetraisopropoxide were used as the starting materials for the precipitation and sol–gel processes, respectively.

Figure 3. XRD patterns of the dried, but not calcined powders prepared by precipitation (bottom) and sol–gel (top) methods. From Ref. [28]: Limthongkul, P. et al. Effects of precipitation, sol–gel synthesis conditions, and drying methods on the properties of nano-TiO2 for photocatalysis applications. Eur. J. Inorg. Chem. 2008, 974–979. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

They studied the effects of the synthesis methods and of two different drying conditions (freeze drying at –45 °C, and normal drying at 100°C) on phase, surface area, crystallite size, and finally on the photodegradation of methylene blue. Figure 3 shows the XRD patterns of the dried but not calcined samples synthesized by the sol–gel (top) and precipitation (bottom) methods. Only short-range ordering was found for the precipitated sample, whereas peaks corresponding to the anatase and brookite phases could clearly be seen in the case of the solgel sample. This early stage of crystallization might lead to a faster increase in crystallite size in the sol–gel-derived samples with respect to the precipitated samples. Additionally the authors underlined that in the sol–gel synthesized samples, a larger amount of the brookite phase (T < 500°C) was found relative to that found in the samples produced by the precipitation method. For the powder prepared by the precipitation method, the anatase phase was found to be stable even when heated up to 600 °C.

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Table 1. Crystallite size of TiO2, determined by TEM images, surface area and photoactivity of TiO2 synthesized by the precipitation method, followed by different drying processes, and by the sol–gel method, calcined at different temperatures. From Ref. [28]: Limthongkul, P. et al. Effects of precipitation, sol–gel synthesis conditions, and drying methods on the properties of nano-TiO2 for photocatalysis applications. Eur. J. Inorg. Chem. 2008, 974–979. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

Tcalc. (°C)

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400 500 600

dTEM (nm) Prec. + SolPrec. freeze gel drying 11 12 11 18 19 19 32 26 31

SBET (m2g-1) Prec. + Prec. freeze drying 72 98 57 75 23 36

Solgel 94 72 30

Degradation MB (%) Prec. + Prec. freeze Sol-gel drying 21 15 15 34 19 32 39 45 44

Figure 4. TEM micrographs of the synthesized powder calcined at 400, 500, and 600°C for 2 h. From Ref. [28]: Limthongkul, P. et al. Effects of precipitation, sol–gel synthesis conditions, and drying methods on the properties of nano-TiO2 for photocatalysis applications. Eur. J. Inorg. Chem. 2008, 974–979. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 223 Table 1 reports the crystallite size, calculated from the TEM micrographs (Figure 4), the surface area and the photoactivity towards the degradation of methylene blue of TiO2 synthetized by the precipitation method, followed by different drying processes, and by the sol gel method in the range 400-600°C. Apart from the methods and drying processes used, the crystallite sizes of TiO2 particles, calcined at 400°C, were less than 15 nm in size, whereas the ones calcined at 500°C were slightly larger (15–20 nm). For the powder subjected to heat treatment at 600°C, large particles greater than 20 nm in size were obtained. As the calcination temperature increased, the surface area was found to decrease. The surface areas of the powders synthesized by the sol–gel method and the precipitated freeze-dried powder were in the same range: 90–100 m2g–1 for 400°C, 70 m2g–1 for 500°C, and 30 m2g–1 for 600°C. The precipitated powders dried at 100 °C had slightly smaller surface areas of 72, 57, and 23 m2g–1 when calcined 400, 500, and 600°C, respectively. By comparing the photocatalytic activities of the samples prepared by the precipitation and sol–gel methods, although the different methods yielded slightly different particle sizes and surface areas, it was found that no significant difference in photocatalytic activity between the different synthesis methods could be observed. Calcination temperature, however, seemed to have a strong effect on the degradation of methylene blue: photocatalytic activity was also found to increase with an increase in calcination temperature. The authors indicated that the degree of crystallinity had a greater effect on the photocatalytic activity than the surface area for nano-TiO2 synthesized by these methods. The dependence of the photoactivity on the crystallite size or, inversely, on the surface area of the semiconductor TiO2 is one of the most controversial and debated topics in this research field. For example Lee et al. [47] synthetized nanotitania using a sol-gel method starting form titanium isopropoxide: two samples were obtained T1 (anatase, crystal size 9.8 nm, specific surface area 96 m2g-1) and T2 (anatase, crystal size 10.2 nm, specific surface area 37 m2g-1) prepared with the addition of PEG (polyethylene glycol) 600. The UV-assisted photodegradation of gaseous pollutants (NO and BTEX - benzene, toluene, ethylbenzene, o-xylene -) at typical indoor air ppb levels was tested. The photocatalysts were immobilized on a glass fiber filter and evaluated under different humidity levels and residence time. In this case the photocatalytic conversion of NOx followed the trend of the surface area, i.e. the larger the surface area the higher the conversion, with a trend opposite to the one observed by Limthongkul et al. [28]. The different dependence of the photocatalytic activity on the oxide parameters (crystal size, surface area) can be explained by considering that the overall photocatalytic reaction is a composite processes which can be subdivided in two main parts. The first part involves what occurs in the TiO2 particles, and this includes photon absorption, electron–hole generation, and electron–hole trapping. The second part involves foreign species and reactions at the surface; this includes, surface absorption and radical formation, reactions of the surface species with pollutants, and removal of reacted species. The first part is mainly affected by parameters of the materials such as phase, degree of crystallinity, bulk, and surface defects. The second part, however, is likely to be mainly affected by parameters such as surface acidity, surface area, concentration of the pollutant, and so on. Both parts will always be in competition, as small, high surface area particles are usually associated with less ordered structures and a high concentration of defects, which favour the recombination of electrons and holes, which in turn leads to poor photoactivity [28]. The sol–gel method has been widely studied [15] particularly for multicomponent oxides where intimate mixing is required to form a homogeneous phase at the molecular level. Thus,

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metal ions such as Fe3+ [44,57-59], V5+ [59,60], Mn2+ [59], Co2+ [61], Ni2+ [59], W6+ [61], Ag+ [62], Au3+ [63], La3+ [64], and Eu3+ [65] were introduced into TiO2 powders and films by this method and the photocatalytic activity was improved in the visible range. Li et al. [66] reported that Au3+-doped TiO2 exhibited visible light reactivity for the photodegradation of methylene blue. Kim et al. [67] prepared Pt ion-doped TiO2, and examined its visible light activity for the photodegradation of chlorinated organic compounds. Sokmen et al. [68] reported a Ag–TiO2 system for killing Escherichia coli under UV light illumination. Recently some authors have studied the incorporation of rare-earth metals, such as lanthanide ions (La3+, Eu3+, Pr3+, Nd3+ and Sm3+) into the TiO2 matrix, improving the photochemical properties [64,65,69,70]. In particular Willner et al. [65] demonstrated that europium-, praseodymium- and ytterbium oxide-doped TiO2, prepared by sol-gel method, exhibited impressively higher photocatalytic activities and complete mineralization of organic pollutants, under UV source. Likewise Li et al. [64] prepared two types of lanthanide iondoped titanium dioxide catalysts including La3+ and Nd3+ ions, at increasing nominal atomic doping concentration (0.7-2%). The XRD results showed that the crystal size greatly decreased (pure TiO2 33 nm; 2% La3+-TiO2 18 nm; 2% Nd3+-TiO2 18 nm) due to lanthanide ion-doping with the corresponding increase in the specific surface area (pure TiO2 50 m2g-1; 2% La3+-TiO2 76 m2g-1; 2% Nd3+-TiO2 91 m2g-1) and the total pore volume (pure TiO2 0.152 cm3g-1; 2% La3+-TiO2 0.205 cm3g-1; 2% Nd3+-TiO2 0.234 cm3g-1). The photocatalytic activity of the catalysts was investigated in the photocatalytic degradation of BTEX in a gaseous phase, under UV light. The photoefficiency was remarkably enhanced owing to lanthanide ion doping, and 1.2% Ln3+–TiO2 had the highest photocatalytic activity, which was much higher than that shown by P25 Degussa. The authors suggested that the enhanced activity could be due to both the increase of adsorption ability and the enhancement of electron–hole pairs separation, which could be attributed to the presence of suitable amount of Ti3+ on the surface of Ln3+–TiO2 and the introduction of lanthanide crystal field states. TiO2 photocatalysts with absorption in the visible region were obtained even by substitution of a non-metal as nitrogen [71-76], sulphur [77-81], boron [82-84] carbon [8591], phosphorus [92], fluorine [93,94] for the oxygen or titanium in TiO2. Ohno et al. [78] synthetized S4+ doped TiO2 samples by a classical sol-gel route: they showed absorption for visible light and high activities for degradation of methylene blue, 2-propanol in aqueous solution and partial oxidation of adamantane under irradiation at wavelengths longer than 440 nm. Among several non-metal dopant species, the biggest advantage of nitrogen (N-doped) TiO2 samples, compared to pure TiO2, is their lower excitation energy, which not only allows the absorbance of the UV portion of solar light, but also of the visible portion, which covers > 50% of the solar energy. The number of papers on N-doped TiO2 (N/TiO2), after the initial report of Asahi et al. [71] is undergoing an exponential increase because of the great potentiality of this approach. This kind of doping induces localized nitrogen 2p states within the band gap and above the top of the O 2p valence band. Many reports indicate that N/TiO2 exhibits modified optical properties with the onset of absorption in the visible region of the spectrum while showing catalytic activity in various reactions performed under visible light irradiation. This activity is indeed higher than that observed, in similar conditions, for the bare oxide. In our laboratories pure and doped titania samples were synthetized by sol-gel route, starting from a solution of Ti(OC3H7)4 dissolved in 2–propanol (water/alkoxide molar ratio of 100 and a water/2–propanol molar ratio of 20, pH of the synthesis around 9). The sources of

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 225 N and S species were NH3 and K2SO4, respectively; in the case of N,S-codoped sample, (NH4)2SO4 is used as the dopant salt. The precursors were dried as a xerogel (80°C overnight) and thermally treated at 400°C for 6 h under an oxygen stream (9 NL/h). Table 2 reports the surface area, the relative phase enrichment, the anatase crystallite dimensions, calculated by Scherrer equation and the dopant/Ti atomic ratio by XPS determinations for nitrogen, sulphate and mixed doped samples. Both N and S dopants promote the anatase content [95] and while the sulphate species lead to larger surface area, N dopant provokes a contraction of surface area and a promotion of anatase crystallite growth. Tab. 2. Quantitative phase composition (A = anatase, B = brookite), anatase crystallite diameter by XRD (Scherrer equation), BET surface area; N/Ti e S/Ti atomic ratio by XPS analysis for synthetized samples. Sample T TN0.7 TS0.25 TS0.25N0.7

S/Ti ⎯ ⎯ 0.25 0.25

N/Ti ⎯ 0.70 ⎯ 0.70

SBET (m2g-1) 154 45 194 75

%A 63 100 94 92

%B 37 ⎯ 6 8

(nm) 9 16 6 11

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The N, S-codoped sample (TS0.25N0.7) shows intermediate features. SO42– is widely considered a promoter of the anatase–phase; the anatase–promoting behaviour could be explained considering several mechanisms by which SO42– would act; Baozhu et al. [96] reported that, depending on the pH conditions of the medium, the SO42– group is able to adsorbs at the surface of the TiO62– octahedrons and consequently orientate the growth of crystallites toward a specific (anatase) phase. TEM images (Figure 5) confirm the structural and surface properties. Figure 5a shows a typical cluster of grains observed in the undoped sample (T). The distribution of crystallite sizes looks rather homogeneous, with an average size close to 10-15 nm, in agreement with XRPD findings. The presence of very fine, round shaped pores (arrowed) is also displayed. The TEM image of the TS0.25 (Figure 5b) shows the reduced crystallite size that promoted the formation of relatively large clusters of grains. Grains, mostly round shaped and uniform in shape and size distribution, have an average size close to 5 nm. Instead, in the N-doped sample (TN0.7) blocky clusters of nearly undistinguishable grains are observed with tight bonding among them (Figure 5c). Coherent with this picture is the lack of any nano-porosity inside each grain, whereas some pores are visible between grains. The figure confirms the role of the nitrogen dopant in increasing crystallite sizes. The photocatalytic activities of the home-made samples toward the degradation of NOx under UV irradiation are shown in the Figure 6. The S doped sample (TS0.25) improves the photocatalytic performance with respect to the undoped sample (T) probably due to the combined effect of increase of the surface area and the presence of sulphur atoms at the surface of the nanocrystals; these acts as electron traps reducing the rate of recombination between electrons and holes in the band structure of the semiconductor TiO2 [97]. This is in agreement with what already reported for gas phase by Keller et al. [95] although for a different type of pollutant (toluene). The N–doped sample

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(TN0.7) shows depressed photocatalytic activity due to the strong reduction in the surface area and the corresponding increase of crystallite size. N and S added as co–dopant have a negative synergistic effect with the lowest conversion after 85 minutes.

a) T

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b) TS0.25

c) TN0.7 Figure 5. TEM images for undoped (T), S- and N-doped (TS0.25 and TN0.7, respectively) samples.

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 227 100

  % NOx degradation 

TS0.25

T

80

TN0.7 

60

TS0.25N0.7  40 20 0

0

20 

40 

60 

80 

100

120

t / min Figure 6. NOx photocatalytic degradation for undoped (T), S- and N-doped (TS0.25 and TN0.7, respectively) and mixed samples.

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Recently new types of phototitania were successfully obtained by mixing both metal and non-metal ions. Klabunde et al. [62] proposed silver, carbon, and sulfur-doped nanoparticles photocatalysts by a modified sol-gel route, characterizated by anatase crystalline phase and high surface area. The nanopowders were tested towards the photodegradation of gaseous acetaldehyde as a model indoor pollutant. Compared to P25, the commendable visible light activity of Ag/(C,S)–TiO2 nanoparticle photocatalysts was predominantly attributed to an improvement in anatase crystallinity, high surface area, low band gap and effects of the precursor material.

2.4 Solvothermal Methods Generally thermal treatments are required to improve crystallinity of synthetized materials; however the main drawback of this procedure is the increase of sintering, which leads to mesopores collapse and lack of surface area. A simple method to circumvent this problem is to use solvothermal methods, employing aqueous (hydrothermal method) or organic solvents (such as alcohol, methanol [98], propanol, butanol [99]), under different growing conditions (low temperatures < 250°C and atmospheric pressure or supercritical condition, autoclave). The solvothermal treatment represents an alternative to high temperature calcinations; it could be useful to induce crystallization of amorphous powders under milder conditions, to simultaneously control grain size, particle morphology, surface texture by regulating the solution composition, reaction temperature, pressure, solvent properties, ageing time and pH. Literature results, relative to both hydro- and solvo-thermal preparations are aimed at obtaining either titania nanoparticles or TiO2 thin films [38,100110]. In order to evaluate the enhancement of an hydrothermal treatment with respect to traditional calcination steps, Colón et al. [100] compared the photocatalytic performance for phenol UV-assisted-photooxidation reaction of calcined and hydrothermally grown TiO2 nanoparticles. The powders were prepared by amine assisted sol-gel precipitation of Ti4+

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aqueous solution at pH 1.5 (by acetic acid) and calcined at 400, 500°C for 2 h or submitted to an hydrothermal treatment in autoclave at two temperature, pressure and time conditions (120°C, 198 kPa for 24 h and 150°C, 476 kPa for 6 h). As expected the surface area values for calcined samples at 400°C (114 m2g-1) and 500°C (68 m2g-1) appeared lower than in the case of samples prepared by hydrothermal synthesis (139 m2g-1). The authors concluded that the high surface area values obtained for hydrothermally treated samples could explain the improved photoactivities of these samples. Yanagida et al. [105] prepared amorphous TiO2 by precipitation of TiCl4 at pH 10. Phasepure TiO2 nanocrystallites with narrow particle-size distributions were selectively obtained by following hydrothermal processes (varying autoclave conditions, i.e. temperature, pH and time) using different acidic catalysts (chloridric, fluoridric, nitric and citric acids). Nanosized anatase samples were obtained by autoclaving the amorphous titania in the presence of hydrofluoric acid and either hydrofluoric acid or nitric acid in different amounts as a cooperative acid catalysts. For the anatase nanotitania prepared using HF + HCl (autoclave conditions: 220°C, 4h, pH = 0.34), TEM image shows clear cube-like geometry and the nanocrystallites have regularly exposed surfaces (Figure 7a) with mean diameter of 13 nm. When nitric acid was employed as a cooperative acid catalyst with HF, the resulting TiO2 sample (TEM image (Figure 7b), obtained in similar autoclave conditions, is composed by granular nanocrystallites (mean diameter 12 nm) with irregular surfaces when compared to the previous one. The authors affirmed that the nucleophilicity of the chloride anion should be responsible for the favorable dissolution–precipitation mechanism in the anatase crystal growth from the amorphous phase.

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a)

b)

c)

Figure 7. TEM images of nanosized TiO2 samples hydrothermally grown in different acidic media: a) HCl + HF (anatase), b) HNO3 + HF (anatase), c) HNO3 + citric acid (rutile). From Ref. [105]: Yanagida, S. et al. Hydrothermal synthesis of nanosized anatase and rutile TiO2 using amorphous phase TiO2. J. Mater. Chem, 2001, 11, 1694–1703. Reproduced by permission of the Royal Society of Chemistry.

The nanorutile sample, with rod-like morphology (Figure 7c), was obtained by subjecting the amorphous powder to an hydrothermal growth in presence of citric and nitric acid (autoclave conditions: 220°C, 6h, pH = 0.8). The authors concluded that chelation of TiO6 octahedra with citric acid and acidification with nitric acid are crucial for the phase transition

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from amorphous to rutile. Anatase nanocrystals, grown in the presence of HF + HCl, characterized by regular crystal surfaces (Figure 7a) exhibit the highest photocatalytic activity (68%) for the oxidation of 2-propanol, under UV irradiation, than both HF + HNO3 grown samples (57%) and pure rutile powders (32%). Yin et al. [110] described the physico-chemical properties and photocatalytic activity of nitrogen doped titania (TiO2−xNy) with single phase anatase, rutile and brookite prepared by homogeneous precipitation-solvothermal process. The starting precursor (TiCl3) and the nitrogen source (hexamethylenetetramine, C6H12N4) were mixed with distilled water or pure alcohols (such as methanol at pH 9 and ethanol at pH 1 and 9) and placed into autoclave. Then, the solution was heated and kept at 90° C for 1 h to realize homogeneous precipitation and then heated at 190°C for 2 h for an hydrothermal step. They found that the phase composition strongly depended on the pH value and the kind of solvent. The powders prepared at pH 9 in methanol solution led to the formation of pure anatase phase, while at the same pH ethanol was used to produce pure rutile. In addition, in ethanol, at low pH value such as pH 1, brookite single phase could be obtained. For all the samples similar rod-like morphologies were observed by TEM images. Table 3 summarizes the BET specific surface areas and nitrogen doped amount of the samples by the thermal conductivity detection (TCD) method. The corresponding N-doped titania films were prepared using a novel low temperature process, i.e. a spin-coating of TiO2−xNy powder using an organic binder removed by oxygen plasma treatment. Figure 8 shows the AFM images of the surface of anatase, rutile and brookite doped thin film. The as-prepared binder-containing thin film (Figure 8d) showed no photocatalytic activity, because the organic binder covered the surface of titania particles. The films possessed good photocatalytic activity for NOx abatement also in the visible range, due to the excellent light absorption up to 510 nm.

Table 3. BET surface areas and nitrogen contents of the nitrogen doped titania prepared using hydrothermal methods. From Ref. [110]: Yin, S. et al. Preparation of anatase, rutile and brookite type anion doped titania photocatalyst nanoparticles and thin films. Phys. Scr. 2007, T129, 268–273. Reproduced by permission of IOP Publishing Ltd.

Briefly summarizing, during hydrothermal/solvothermal routes particle growth occurs via dissolution/reprecipitation mechanisms, leading to single-crystal particles with small crystallites, large surface area and tailored phase composition.

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Figure 8. AFM images: (a) anatase, (b) rutile and (c) brookite single phase TiO2−xNy thin films and (d) as prepared anatase thin film without oxygen plasma treatment. From Ref. [110]: Yin, S. et al. Preparation of anatase, rutile and brookite type anion doped titania photocatalyst nanoparticles and thin films. Phys. Scr. 2007, T129, 268–273. Reproduced by permission of IOP Publishing Ltd.

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2.5 Microwave and Ultrasound Treatments Microwave heating (MW) and sonochemistry (US) represent two unconventional synthetic methods to prepare highly photoactive TiO2 nanoparticles with different size, shape and phase compostion.

2.5.1 Microwave heating Early fundamental research into the use of microwaves in sintering processes was reported in the 1970s by Shimomura and coworkers [111], who studied the influence of microwave heating on alumina. Several research papers were reported in the 1990s. [112115]. During the last two decades the microwave technology was transferred on the synthesis/growth of heterogeneous catalyst and nanoparticle materials. The main advantages of MW treatments are: (i) extremely rapid heating rates [116-119], (ii) fast kinetics of crystallization, (iii) simplicity and (iv) high energy efficiency. Microwave irradiation furnishes energy to the reactants by means of molecular interaction with the electromagnetic field [120]: the rapid and uniform generation of heat allows the achievement of equilibrium between the bulk and the surface of the material more quickly than with a conventional thermal treatment [121]. For these reasons microwave treatment reduces the sintering degree when particle growth occurs. In the last years several studies have reported on the preparation of TiO2 powders and films by MW assisted synthesis and/or MW hydrothermal routes [121-134]. In order to investigate the differences between calcination and MW-assisted growth in agglomerating particles, Wang et al. [130] studied the rapid formation of mesoporuous TiO2

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nanoparticles via a template microwave hydrothermal process. The gels were obtained by a modified sol-gel using tetradecylamine as a self-assembly micelle and titanium tetraisopropoxide as staring material. Then the crystallinity of the samples was achieved by three different treatments: (i) ageing at 25° or 50°C for 7 days under dark condition (conventional ageing, CA), (ii) conventional hydrothermal process at 160°-180°C for 1 h in autoclave (CH) and (iii) microwave hydrothermal growth at 100°-180°C for 1 h (MH). Finally all CA, CH, and MH samples were dried at 80°C and the surfactant was removed by calcinations at 400°C for 3 h in air or by acid extraction using 1 N nitric acid. TEM micrographs in Figs. 9 show the morphology of the CA as-synthesized powder (Figure 9a), the mesoporous, worm hole-like TiO2 powders after calcination (CA calcined 400°C/3 h, Figure 9b), and acid extraction (MH, Figure 9c,d). No nanopores within CA assynthesized spheres can be observed, due to the presence of the surfactant (low surfaces area and pore volume, 25 m2g-1 and 0.08 cm3g-1); after the removal step, either by calcination or by acid extraction, nanoporosities within the spheres are observed, as shown in Figure 9b,c,d. Calcination at 400°C for 3 h easily removes the surfactant but reduces the surface area (18 m2g-1) and pore volume (no value) with respect to microwave treatment and subsequently acid extraction method (surface area in the range 243-622 m2g-1 and pore volume ∼ 0.30 cm3g-1). All the samples were tested in the photodegradation of methylene blue under UV irradiation. Mesoporous crystalline anatase TiO2 nano-powders (MH) with a high-surface area, prepared by microwave treatment exhibit photocatalytic activity superior than those of commercial P25 and the conventional hydrothermal process (CH) as shown in Figure 10.

Figure 9. Transmission electron microscope for TiO2 powders, (a) conventional aging (CA) as synthesized, (b) CA calcination at 400°C/3h, (c) microwave hydrothermal (MH), 150°C/1h, acid extracted, (d) MH, 180°C/1h, acid extracted. From Ref. [130]: Wang, H. –W. et al. Rapid formation of active mesoporous TiO2 photocatalysts via micelle in a microwave hydrothermal Process. J. Am. Ceram. Soc. 2006, 89(11), 3388–3392. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

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Figure 10. Comparison of photocatalytic degradation for methylene blue under various conditions of TiO2. From top to bottom: (a) methylene blue only, (b) conventional hydrothermal (CH) 1701C, (c) conventional aging calcined at 4001C, (d) CH 1601C, (e) microwave hydrothermal (MH) 1501C plus calcined at 4001C, (f) P25, (g) MH 1501C, (h) MH 1801C, (i) MH 1601C, and (j) MH 1701C, respectively. From Ref. [130]: Wang, H. –W. et al. Rapid formation of active mesoporous TiO2 photocatalysts via micelle in a microwave hydrothermal Process. J. Am. Ceram. Soc. 2006, 89(11), 3388–3392. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

The synthesis of mesoporous TiO2 powders via facile and rapid microwave hydrothermal process leads to excellent photocatalytic performances, due to high surface area, porous crystalline structures and good crystallinity without pronounced sinterization. Addamo et al. [131] reported the use of microwave for a very short time in order to enhance the crystallinity of nanotitania both preventing sintering processes and minimizing the decrease of surface area. The gels prepared by TiCl4 solution at different TiCl4/H2O (v/v) ratio (1, 10 and 50) were dried at room temperature under vacuum. Aliquots of the gels were calcined in muffle for 12 h at different temperatures (393-873 K) and others were subjected to a microwave treatment at increasing time at 393 and 423 K. When the gels are calcined in muffle the surface areas decrease to values between 90 and 120 m2g-1 at 393 K and between 10 and 13 m2g-1 at 873 K. The thermal treatment causes an irreversible agglomeration process of the nanocrystallites. Differently, the microwave treatment preserves the specific surface areas (150-200 m2g-1 for samples treated at 393 K and 140 m2g-1 for the samples treated at 423 K), without an increase of particle size. Moreover, the increase of MW time (from 15 to 30 min) does not significantly affect the surface area of samples, in according to the extremely rapid crystallization of MW synthesis. The authors concluded that, from structural and morphological point of view, the microwave treatment does not induce important effects of agglomeration. MW-treated samples revealed similar photoactivity or enhanced performance than that of commercial TiO2 Degussa P25 in liquid media. Instead, a relatively low efficiency for the photooxidation of gaseous 2-propanol was obtained; the authors justified these results by considering that the presence of a noticeable amount of water in the gel network reduces the relative amount of TiO2 sites present in the surface of the catalyst.

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 233 Søgaard et al. [132] obtained thin films from both commercially available samples (Degussa P25 and Hombikat UV100) and home-made anatase powders produced by using microwave-assisted sol gel technique (220°C, 60 bar) and supercritical CO2 conditions (named SC 134). The films were deposited on glass substrate by doctor blade technique [16], dried for 30 min and heated at 450°C for 1h. The crystallite size determined by XRD, unchanged passing from powders and films and lied in the interval 7-23 nm. AFM images of the films show larger particles or aggregates of particles for SC 134 film with respect to the other films. Instead the sol–gel film and the film prepared from Degussa P25 were more uniform and characterized by the presence of smaller aggregates. These results were also verified by dynamic light scattering measurements: the sample prepared by microwave step presented the lowest size of the aggregates (270 nm) while the one under supercritical conditions the highest (1274 nm), even if the trend of surface area was the opposite (SC 134, 221 m2g-1; sol-gel film, 117 m2g-1). Finally the photoactivity of the prepared films under UV irradiation was investigated using stearic acid as a model pollutant. A good linear correlation between the rate of the stearic degradation and the amount of surface OH groups, determined by XPS investigation, was observed. As reported in literature the de-polluting performances of powders/films are related not only to the crystallinity degree and surface area but also to the presence of surface hydroxyl groups. Thus for these reasons, the powders grown in the presence of MW show high photocatalytic performance, similar to those obtained by P25 Degussa. Komarmeni et al. [122] observed that the crystallization of pure rutile from TiOCl2 concentrated solutions (2M and 3M), by using MW treatments at various pressures (190, 100, 50 and 25 psi) for 2h led to pure rutile nanopowders, the yield of rutile was 95% at all pressures, which showed that the crystallization of this solutions was pratically complete. When the TiOCl2 was further diluted, a mixture of anatase and rutile nanocrystals was obtained. Mathew et al. [133] synthetized titania nano-cubes, -spheres and -rods by coupling the nature of the precipitating agent, pH and microwave irradiation. Three samples S1, S2 and S3 were obtained starting from TiCl3 by adding NH3 (pH 11), NaCl (pH 17) and NH4Cl ((pH 5.9) concentrated solutions, respectively. The precipitated sol was irradiated in a microwave oven (operating at a frequency of 2,450 MHz) in on and off mode for different times for complete precipitation (S1, t = 20 min; S2, t = 60 min; S3, t = 60 min). Anatase nanocubes with particle size around 25 nm, and high surface area (372 m2g-1), due to the highly porous surface texture are formed when NH4OH solution was used. Instead, sample S2 is characterized by rutile nanosphere with lower surface area (77 m2g-1) and mean crystallite diameter (8 nm). Finally, rutile nanorods (SBET = 34 m2g-1, = 4nm) are obtained by adopting NH4Cl as precipitating agent. Photocatalytic dye degradation studies were conducted using methylene blue (MB) under ultraviolet light irradiation. The photocatalytic performance is observed to vary in the order S1 > S2 > S3 > P25. The synthesized mesoporous anatase TiO2 exhibit the highest photocatalytic activity than both the commercial Degussa P25 and the other two samples S2, S3, probably due to due to the higher surface area (SBET = 372 m2g-1), small crystallite size and mesoporous texture. These properties can provide more active sites for dye adsorption and lesser electron-hole recombination rate.

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2.5.2 Ultrasound growth The use of ultrasound treatment (US) has become an important tool in recent years in obtaining nanoparticles with enhanced/unusual properties [135-140]. It should be mentioned that in 1990s Suslick et al. [141,142] studied the application of ultrasound in the production of nanostructured materials. The physical phenomenon responsible for the sonochemical process is acoustic cavitation, which is the formation, growth and implosive collapse of bubbles in liquid. The implosive collapse produces high temperatures and pressures with localized hot spots, characterized by transient temperatures up to about 5000 K and pressures up to 1800 atm. The involved heating and cooling rates may be larger than 108 Ks-1. Nanoparticles showing a more uniform size distribution, higher surface area, a more controlled phase composition and better thermal stability are some of the interesting features resulting from the application of US method. Generally, in almost all sonochemical reactions leading to inorganic products, nanoparticles with tailored shape, structure and size are obtained. Because of its many applications, ultrasonic waves have been applied for the synthesis of nanosized titania [143-150]. Many authors underlined the effect of the US treatment on the phase composition of the synthetized nanoparticles. Yu and coworkers [143] discovered a novel method for preparing highly photoactive anatase/brookite TiO2 by coupling the traditional sol-gel routes (starting from tetraisopropoxide in ethanol) with ultrasonic irradiation. Huang [144], by adopting a similar reaction modulating both the kind of starting precursors and ultrasound irradiation, prepared anatase and rutile, as well as their mixture. A substantial reduction in time and reaction temperature was observed, as compared to the corresponding hydrothermal process. Meskin et al. [140] reported the hydrothermal synthesis of TiO2 in an autoclave, coupled with ultrasonic activation. The results demonstrate that ultrasonic activation markedly accelerates the crystallization rates and raises the rutile content with respect to the values obtained in synthesis carried out under identical conditions, but without sonication. Similarly, Arami et al. [138] produced nanostructured rutile with 15–20 nm crystallite size, which is usually difficult to be obtained at low temperatures, by simply treating in ultrasonic bath the product of dissolution of TiO2 pellets in 10 M NaOH. Other authors, instead, observed opposite effects. By example, in the case of ultrasonic-assisted synthesis of titania, the control of particle size, surface area pore volume is possible. Gedanken et al. [137-139] reported the beneficial effects of ultrasonic irradiation on the template synthesis of wormlike TiO2 in presence of a long chain amine. The authors measured very high surface areas (853 m2g-1) for un-calcined products and a higher thermal stability of the mesoporous structures as compared to those samples that were not ultrasonically treated. Choi et al. [148] prepared nano-TiO2 photocatalysts by sol-gel and ultrasonic-assisted sol-gel methods using two different sources of ultrasonicator, i.e., a bath type, commonly used for cleaning purposes (frequency 40 kHz and power 157W) and tip type (frequency 20 kHz and power 30W at 40% amplitude). After the hydrolysis, the powders were dried and calcined at 500°C for 3 h under air stream. The authors reported that the ultrasonic irradiation was able to produce not only smaller crystallites but also uniform and spherical TiO2 particles (bath-US 60 min d = 11 nm, tip-US 90 min d = 8 nm ) with respect to those prepared by classical sol gel method (d = 14 nm). The photocatalytic activity of the prepared photocatalysts towards the degradation of 4-chlorophenol was evaluated by using a UV light irradiation (λ = 254 nm). The samples prepared by the bath-US ultrasonic-assisted sol-gel

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method showed the highest performance, probably due to their high surface area and larger pore-diameter. Kim et al. [145] studied the influence of ultrasonic waves (Branson sonifier 450, 80 W) on the titania nanoparticles synthetized by hydrolysis and condensation of TiCl4 in a mixed 2propanol/water solvent. The calcined samples previously treated with ultrasound showed better thermal stability with respect to the samples treated without US: in fact, the rutile structure in the US-assisted samples began to appear for temperatures larger than 700°C, instead of 600°C for traditional precipitation ones. Moreover the particle size decreased from 5-15 nm for samples treated without US to 3-6 nm for powders precipitated in presence of US waves. The photocatalytic activities of the samples, under UV irradiation, were evaluated by the pseudo-first-order kinetic constant kapp for methylene blue decomposition reaction. The degradation rate is always faster for samples treated with ultrasound (large specific surface areas, small particle size and good crystallization) than for samples treated without ultrasound and in any case the highest photocatalytic activity occurs for samples annealed at 500°C.

Figure 11. SEM images of the as-washed (a) and calcined (b) HPT; TEM images of the as-washed (c) and calcined (d) HPT. From Ref. [150]: Yu, J. C. et al. A sonochemical approach to hierarchical porous titania spheres with enhanced photocatalytic activity. Chem. Commun. 2003, 2078–2079. Reproduced by permission of the Royal Society of Chemistry.

Ramaswamy et al. [149] prepared anatase nanocrystalline particles by two different methods: one is the sol–gel method using ultrasound (100 V, 30 W, and 38 kHz) and conventional stirring method and the other by surfactant (cetylpyridinium chloride) assisted hydrothermal synthesis (180°C, 1-7 days). More uniform distribution/dispersion of the nanoparticles, better thermal stability and phase purity are some of the advantages of preparation of nanocrystalline titania by sol gel ultrasonication method and hydrothermal

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synthesis method. The US nanoparticles were more effective in the UV-photodegradation of methylene blue in aqueous medium than both, the sample prepared by conventional stirring method and commercial P25 Degussa, due to the uniform distribution and lesser aggregation of TiO2 crystallites. Yu et al. [150] synthetized hierarchical porous titania spheres (HPT) in the presence of a triblock copolymer (P123 Aldrich) by a sonochemical assisted sol-gel route. During the reaction the suspension was sonicated for 3 h (3 seconds on, 1 second off, amplitude 95%) by a high-intensity probe with 13 mm diameter (Sonics and Materials, VC750, 20 KHz). The powder was dried in oven at 100 °C. Then, the amorphous as-washed sample (622 m2g-1) was calcined at 400 °C for 1h and it was transformed into anatase with average crystallite size of about 8.8 nm and surface area of 145 m2g-1. The hierarchical porous structure of HPT is confirmed by SEM and TEM images (Figure 11). The aggregates of well-defined spheres with unsmooth surfaces are shown in Figure 11a. The TEM image in Figure 11c reveals that the spheres possess wormhole-like mesopores of several nanometers in size, which are produced by the ultrasound-induced agglomeration of the small primary nanoparticles. The sizes of the mesoporous spheres are about 100 nm. Large textural meso- and macro-pores are produced by the inter-aggregation of the mesoporous spheres. Meanwhile, connections between the mesoporous spheres are also observed, due to the turbulent flow and shock waves produced by acoustic cavitation, which forces the metal particles together at sufficiently high velocities. The photocatalytic activity of the calcined HPT is about 50% higher than that of commercial nonporous photocatalyst P25 (as-received sample) on the photodegradation of n-pentane in air.

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a) TP

a) TC

Figure 12. TEM images of TP (pulsed US) and TC (continuous US) samples.

In our laboratories nanocrystalline TiO2 samples were prepared by promoting the growth of a sol–gel precursor, in the presence of water, under continuous (CW, NTS Italia, Ti horn, 20 kHz, t = 30 min), or pulsed (PW, Bandelin, Ti horn, 20 kHz, t = 1h, 0.5 on−0.5 off) ultrasound. The acoustic intensity, as determined calorimetrically, is 9 Wcm-2 for continuous and 140 Wcm-2 (maximum value) for pulsed ultrasound. In the case of PW treatment, the acoustic intensity was 84 Wcm-2. All the samples turned out to be made of both anatase and

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 237 brookite polymorphs. Pulsed US treatments (TP, ) determine an increase in the sample surface area (270 m2g-1) with respect to the untreated T sample (250 m2g-1), while TC sample shows a decreased surface area (240 m2g-1). The microstructure of the pulsed sample (TP) is shown in Figure 12a. Crystallite clusters appear to be relatively dense, so that only the peripheric regions can be analysed in detail. The microstructure of the TC sample is shown in Fig.12b. In the case of this sample, the crystallite agglomerates appear to be densely packed, possibly as a consequence of their reduced size, which can be estimated to be around 1–2 nm. The crystallites appear to be rather homodisperse concerning both shape and size.  

100 

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60 

40 

T TC TP

20 

0

0 

20 

40

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TM1  TM2 

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t / min Figure 13. % NOx degradation for microwave- (TC and TP) and ultrasound- (TM1 and TM2) assisted samples with respect to that of untreated sample (T).

All sonicated samples show better performances (Figure 13) toward the degradation of NOx in air than the untreated T sample (around 45% after 2h): the pulsed and continuous ultrasound treatments led to conversion around 70% and larger than 80%, respectively. The continuous mode induces the presence of surface defects (Ti3+) and consequently yields the best photocatalyst. In addition also samples TM1 (close vessel, 53W, 30 min., predefined temperature 110°C) and TM2 (open vessel, 300W, 30 min., predefined temperature 110°C) grown in the presence of microwaves were obtained. The figure shows that the samples assisted by microwaves present the top photocatalytic conversion with respect to both untreated and US-assisted samples.

3. PHOTOCATALYTIC PAINTS Paints are just a small part of the large class of coating materials and traditionally can be divided in accordance with different requirements: by the function of the coating (clearcoat, metallic paint, solid paint), by the specific layer in the coating system (primer, topcoat), by the purpose of the paint (car, decorative, industrial paint), by its degree of environmental

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compatibility (water based paint) and by the processing conditions (baking enamel, oxidatively curable coating material). In order to understand the world of paints, especially the photocatalytic ones, a short dissertation concerning the chemical composition of coating materials is reported in the following. A formulation paint is a complex matrix [151]: resins as film forming agents (FFAs), often incorrectly termed binders, additives, solvents, pigments and extenders are the usual ingredients of liquid coating materials. Binders are the pigment free and extender free part of the dried coating; they are made up of the film forming agent and the non-volatile part of the additives. Irrespective of the applications, a film forming agent, which solidifies as a result of physical or chemical processes, is the indispensable component of a coating formulation. Its role is to enhance both the adhesion to the substrate and the mechanical strength with simultaneous elasticity and resistance to environmental effects. Classic FFAs are exclusively oligomeric/polymeric organic compounds; they can be manufactured form natural raw materials after chemical modification or by industrial synthesis. Cellulose nitrate, thermoplastic acrylates, PVC copolymers, modified-styrene, polyurethane and epoxy resins are typical film forming agents. If the film forming agents are lower molecular liquid products, it is possible to avoid partially or wholly the use of organic solvents. The most common organic solvents are aromatic (xylene) and aliphatic hydrocarbons, esters (ethyl acetate), ether (butyl glycol) and various ketones. Water soluble resins permit water to be substituted for the solvents, with a subsequent lower environmental impact. The pigments used in the coating formulations give the chromatic power; they are divided into inorganic (such as micrometric titanium and iron oxide), organic, metallic and anticorrosive pigments. There is a huge variation in the composition of organic pigments; brilliance and long-term outdoor exposure are their main properties. The only drawback is their low stability at high temperatures. A coating paint usually also contains additives (surfactants, wetting agents, UV absorbers, preservatives, etc.), extenders (talc, kaolin, carbonates, quartz powders) and plasticizers (aliphatic and phtalic esters) added in minute quantities. Despite the low concentration they have a significant role on the properties of coating materials; additives, for example, can facilitate the dispersion of the pigment during the production, increase resistance to UV irradiation, reduce bacterial degradation and enhance flowability when the paint is applied. Lately many paints described as “photocatalytic” have been launched on the market. The addition of photoactive nano-TiO2 in the paint formulation gives self cleaning [152,153], antibacterial [154], and air/water purifying and depolluting [155-157] properties. It is important that the coating exhibits high durability for a reasonable cost-effective system. Also the photo active paints must be stable to flocculation and viscosity changes, curing or drying at ambient temperature and ideally be water based to avoid further environmental problems. The addition step of nanometric titania, the limited photoefficiency when the nanoTiO2 powder is incorporated in the paint matrix, the choice of a suitable photoresistant resin, and the compatibility of the other raw materials (preservatives, additives, pigments, etc.) in the final formulation are the principal hindrances to obtain a stable and depolluting coating. For these reasons, notwithstanding the great technological interest, only few scientific articles have been published about photocatalytic paints and coatings [158-169].

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 239 In the following the discussion concerning these topics will be presented as grouped in two parts. In the first the possible un-desired effects provoked on the organic matter of the paint by the reactions promoted by TiO2 are described. In the second part the enhanced photoactivity of the paint promoted with nano-TiO2 is studied.

3.1 Photostability of Organic Components in Paint The main problem for both outdoor and indoor photocatalytic paints is the degradation of the organic coating matter, which depends on the environment (especially sunlight intensity, temperature and humidity), but primarily on the kind of adopted polymer. The highly active holes generated by the photoactivation of the nano-TiO2 semiconductor particles are responsible of the chemical and mechanical degradation of the organic matter. The organic materials in the composite are commonly carbon based and thus can be decomposed and deteriorated by the photoinduced strong oxidizing power of TiO2 nanoparticles. The result is that the formulation becomes friable, chalking occurs on the surface and in the case of white coating a yellowish/brownish colour appears. Figure 14 shows the colour change under UV irradiation for a commercial polysiloxane paint before (a) and after (b) the addition of 5% nanoactive TiO2.

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a)

b)

Figure 14. Colour change under UV for paint containing nanoactive TiO2 (b) or without photocatalyst (a).

The NOx degradation of the nano-TiO2 paint, obtained in static condition is reported in Figure 15a. The figure shows that the addition of a preservative containing N-groups to the final formulation (5%w) leads, itself, to an increase of NOx concentration (Figure 15b) during the photocatalytic reaction due to the unwanted decomposition of the preservative under irradiation. This indicates that a good photocatalytic paint must be obtained by modulating suitable well-known raw materials and not by simply mixing the photoactive TiO2 in a commercial-made formulation. The self-degrading effect of the organic part of the coating, under the influence of light, may form high quantities of undesired byproducts, like aldehydes and ketones: these compounds are quite stable air pollutants [159] and may decrease the quality of the air [160].

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To inhibit this kind of unwanted behaviour, the binders should be stable enough to endure highly active radicals. 3.0

b) 2.5

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Figure 15. NOx photodegradation of paint formulation (a) and in the case of the addition of a preservative containing N-groups (b).

Auvinen et al. [161] studied the photocatalytic degradation of six photocatalytic (by addition of nano-TiO2) interior paints, containing different commercial and self-produced binders such as polyorganic-siloxane, silica sol–gel, lime, PVAc–ethene and styrene–acrylic copolymer. They deposited fresh paints onto glass substrates (dry film thickness 80 μm) and placed them into environmental test chambers and let them dry for 48 h, then the UVA-light was switched on. The formaldehyde emission (collected in fifteen days) from different paints was an indication of the degradation of the coatings. The authors found that all binders, except for sol-gel silica, degradated and the amount of formaldehyde produced exceeded the recommended limits for indoor air, which is 100 μg m-3. In particular the paint, characterized by PVAc-ethene binder, produced not only more formaldehyde than the other photocatalytic paints, but also higher amounts of other aldehydes and ketones, such as acetaldehyde, acetone, propianalaldehyde, etc.). Salthammer et al. [162] investigated the undesired organic emissions from three different commercially photocatalytic water-based paints under sunlight simulating lamps. The percentage of anatase TiO2-catalyst and the chemical composition of binders and additives were unknown. All paints were applied on inert glass plates and the thickness of the dried layer was 100-250 μm, determined by optical microscopy. Their main results indicate that primary emissions were acetic acid, acetone, hexanal, and toluene; when the light was turned on after 120 h, a strong increase of formaldehyde and acetaldehyde, ethylacrolein, pentanal, 1-hydroxy-butanone, and hexanal could be observed. Other compounds like propenal, propanal, butanal, methacrolein, 2-butanone, and heptanal appeared in smaller amounts with peak values < 10 μg m-3. The chamber concentrations reached peak values within 1-3 h,

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 241 followed by a fast decay. This indicates a consumption of reactive components in the paint surface around TiO2.

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Table 4. % Weight loss after 567 h Atlas exposure (various polymers plus 5% anatase sol. 10–20 nm) From Ref. [158]: Allen, N. S. et al. Photocatalytic coatings for environmental applications. Photochem. Photobiol. 2005, 81,279-290. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

Figure 16. Gloss loss versus irradiation time in a QUV weatherometer for a DSM siliconised polyester resin with 20% w/w rutile pigment O plus increasing levels of 5, 10, 15, 20, 25, 30 and 35% w/w of nanoparticle anatase F. From Ref. [158]: Allen, N. S. et al. Photocatalytic coatings for environmental applications. Photochem. Photobiol. 2005, 81,279-290. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

Moreover, the paint stability with or without anatase sol particles (5% w/w) was studied by Allen et al. [158]. Their results, regarding the weight loss % of different kinds of polymer after 567 h of weathering, are shown in Tab. 4. They affirmed that only the polysiloxane BS45 (by Wacker) proved to be resistant to the photocatalytic effects of the titania particles. The styrene acrylic, poly(vinyl acetate) and acrylic copolymers all showed high degrees of chalking (weight loss). In order to reduce the photocatalytic deterioration of the resins and at the same time to preserve the physico-chemical and mechanical properties of the dry paints and the pollutant remediation activity, hydrophobic/hydrophilic surface coatings on titania particles or addition of secondary compounds have been developed. The authors [158] affirmed that a fine balance of the weight ratios between durable (pigmentary rutile) and photocatalytic (nanoparticle anatase) titanium dioxide could be a possible solution.

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Figure 17. Mass loss versus irradiation time in a QUV weatherometer for a DSM siliconised polyester resin with 20% w/w Rutile pigment O plus increasing levels of 5, 10, 15, 20, 25, 30 and 35% w/w of nanoparticle anatase F. From Ref. [158]: Allen, N. S. et al. Photocatalytic coatings for environmental applications. Photochem. Photobiol. 2005, 81,279-290. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

Figs. 16 and 17 illustrate the gloss and mass loss for a siliconised polyester coating with fixed amount of rutile pigment (O, 20%) plus increasing levels of nano-TiO2 powders (F) in the range 5-35%, exposed in a QUV weatherometer. Gloss loss gradually reduces with irradiation time in particular with increasing loading of anatase nanoparticle (F); simultaneously mass loss is also seen to increase gradually with increasing levels of the same nanoparticle. Nonami et al. [166] concluded that notwithstanding a partial coating of TiO2 with apatite or silica, insufficient prevention effects for photocatalytic deterioration have been obtained. Instead the use of TiO2 pillared clay (containing 32 wt% nano TiO2) might prevent photodegradation effect in the composites with organic materials. Ooka et al. [169] made a formulation based on polylactic acid resin containing 5% of TiO2 pillared fluoromica powder [170,171], in which most of TiO2 is intercalated in the clay layer and only a minor part of TiO2 seems to be exposed to the exterior of the aggregated particle. They reported that the test piece containing 5 wt% TiO2 pillared mica, exposed to the sunlight and the rain for 1 month, did not change its appearance instead of the sample with 5% of P25 Degussa. The surface of the test piece with 5 wt.% P25 showed a large number of pores after exposure, formed by the decomposition of the polylactic acid resin, supported by TiO2. The surface of the film prepared with 5 wt.% TiO2 pillared mica hardly showed any surface roughness after exposure. Moreover the test piece with 1–3 wt.% TiO2 pillared mica showed higher photocatalytic activity with respect to acetaldehyde/toluene UV decomposition that with 5 wt.% P25.

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 243 Aside from organic based paints there are a number of inorganic paints in the market place including complex alkali metal silicates (for examples KEIM paints). Because of their inorganic nature they tend to be significantly stable to light exposure.

3.2 Photocatalytic Activity of Paint Coatings

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The overall photocatalytic performance of titanium dioxide particles has been found to be dependent on several parameters including preparation method, annealing temperature, crystallite size, sintering degree, specific surface area, the ratio between the anatase and rutile polymorphs, light intensity, electron/hole recombination rate, and reagent/intermediate adsorption and the kind of pollutant to be degraded [165]. Furthermore the so called quantum size effect, due to quantized motion of the electron and hole in a nanoconfined space, shifts the band gap and yields higher oxidative hole potential. So the use of size-quantized semiconductor TiO2 particles may increase photoactivity [157]. Photoinduced holes can generate active hydroxyl radicals while active oxygen species are generated through electron transfer processes. All exhibit high activity with respect to inorganic/organic gaseous compounds. Nitrogen oxide (NO2), carbon monoxide (CO), formaldehyde, and several VOCs (Volatile Organic Compounds) are commonly used as target pollutants. Nevertheless few articles concerning nanoactive polymeric and organic coatings systems have been reported in the recent literature [158-169,172].

Figure 18. Percentage concentration of NOX removed versus the surface area of anatase sol–gel particles at 5% w/w in a polysiloxane Wacker BS 45 paint system.. From Ref. [158]: Allen, N. S. et al. Photocatalytic coatings for environmental applications. Photochem. Photobiol. 2005, 81,279-290. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

Allen et al. [158] studied the influence of different photocatalytic TiO2-powders in several binder systems, such as PVC-alkyd, acrylic, and polyester siloxane. They concluded that not only nanosized anatase operates as an effective photocatalyst, but also nanometric rutile shows photocatalytic features. Their result indicates that the photocatalytic efficiency of the paint film can be modulated by using different types of TiO2 nanopowders. They reported

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the NOx degradation of polysiloxane paint coatings under dynamic condition; they showed that the % NOx degradation increases significantly with both increasing particle surface area (Figure 18) and concentration of nanoanatase commercial powders (Figure 19). They also concluded that the addition of nanocalcium carbonate (Figure 20) at increasing amount enhanced the final photocatalytic activity, due to the increased amount of porosity of the paint. On one hand higher levels of CaCO3 would react with more HNO3, produced by the oxidation of NOx; on the other hand they impart (together with TiO2) lower durability to the paint matrix.

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Figure 19. Percentage concentration of NOx removed versus the concentration of anatase sol-gel particles (10-20 nm) at 5% wt/wt in a polysiloxane Wacker BS 45 paint system. From Ref. [158]: Allen, N. S. et al. Photocatalytic coatings for environmental applications. Photochem. Photobiol. 2005, 81,279-290. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

Figure 20. Percentage NOx reduction versus volume of titania (anatase 10–20 nm) for a Polysiloxane BS45 paint substrate with 0, 2.5 and 5.0% w/w of nanoparticle calcium carbonate. From Ref. [158]: Allen, N. S. et al. Photocatalytic coatings for environmental applications. Photochem. Photobiol. 2005, 81,279-290. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 245 Celiker et al. [163] prepared paints by incorporating nanosized anatase into them. They studied the performance of the paint films by using methylene blue, rhodamine, and resazurin solutions under different light sources. They concluded that even sufficient sunlight irradiation is adequate for photocatalytic activity of the paint film, even though a UVA-lamp is the most effective. Salthammer et al. [164] studied the photocatalytic efficiency of two different types of commercially available wall paints in a 1m3 test chamber with and without air exchange using artificial daylight. They spiked the test chamber with each target compound and measured the concentration vs. time profile. The results indicate that photocatalysis works well for formaldehyde under static conditions, but not under dynamic conditions. For typical VOCs under dynamic conditions, no significant photocatalytic effect could be observed. Recently they investigated the photoactivity with respect to NO2 and VOC of commercial indoor wall paints under indoor daylight or artificial light [162]. The percentage of anatase TiO2-catalyst and the chemical composition of binders and additives were unknown. The most efficient degradation effect was found for NO2: the measured decay rate of nano-TiO2 paint (κ = 2.19 h-1) was higher than that of pigmentary rutile formulation (κ = 0.93 h-1). The drawback of these systems is the production of undesired secondary emissions (see the photostability of organic components in paint section) under influence of light. Maggos et al. [167] carried out experiments concerning the photocatalytic activity of commercial paints containing TiO2 nanoparticles towards NO and NO2 under “real world setting” conditions of temperature, relative humidity, irradiation and pollutant concentrations. They used two types of TiO2-containing paints, based on a mineral silicate and a water-based styrene acrylic resins. The authors found a higher photocatalytic performance for the organic based material (91% and 71% for NO and NO2, respectively) with respect to the silicate one (74% of NO and 27% of NO2). Finally the effect of relative humidity (RH) was also investigated: an increase of RH from 20% to 50% inhibited the NOx removal for both silicate (20%) and styrene acrylic (10%) paints. In another article, published in the same year [168], the authors evaluated the de-pollution efficiency of TiO2 paints in a real system, a car park. The closed area was fed with car exhaust gases and when the steady state was reached, the UV lamps were turned on for 5 h. The difference between the final and the initial steady state concentration indicated the removal of the pollutants was due to both photocatalytic paint and car emission reduction. Their results showed a partial photocatalytic oxidation of NOx gases (19% NO and 20% NO2 photodegradation removal). In our laboratories three different water-based paints were obtained for outdoor applications. The final formulations consisted of vinyl, acrylic or silicate emulsions as starting resins, fixed amount (5%) of commercial photoactive nanotitania (by Sachtleben), preservatives, antifoaming and inorganic (CaCO3, SiO2, etc.) additives. They were optimized by varying the ratio among all the components in order to reach a good compromise between traditional paint properties (resistance, adhesivity, waterproof characteristics, facility in application, etc.) and photocatalytic efficiency. The photocatalytic determinations were carried out in static conditions, by using a pyrex glass reactor (with a volume of 20 L) irradiated with an halogenide lamp (Jelosil, model HG500) emitting in the 340-400 nm wavelength range, with a nominal power of 500 W, at room temperature. The paints were deposited on Teflon (7 cm2) sheets and the relative humidity was kept constant in all the runs (50%). Air, NOx and N2 gas streams were mixed to obtain the desired NOx concentration (400

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ppb), inside the photoreactor. The photodegradation products concentrations (NO and NO2) were continuously monitored by an on-line chemiluminescent analyzer (Teledyne Instruments M200E). The general mechanism for NOx degradation by photocatalysis implies the oxidation of the nitric monoxide to nitric or nitrous acid induced by oxygen species produced at the TiO2 surface [38]. The reaction path for NOx conversion is generally mediated by OH radicals: NO2 + OH• → HNO3 NO + OH• → HNO2

100

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40 nano TiO2 vinyl paint silicate paint acrylic paint KEIM paint chinese paint

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0

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t / min Figure 21. % NOx degradation for home-made and commercial photocatalytic paints.

Figure 21 displays the general trend of the NOx degradation curves of both home-made and commercial reference (KEIM, Chinese) photocatalytic paints, compared with pure TiO2 nanopowders (by Sachtleben), taking into account the same amount of nanotitania present in the final dried layer. The NOx concentration is the sum of the NO and NO2 concentrations. The final NOx-depolluting sequence of the home-made paints after 140 min of UV– irradiation was silicate (47%) < vinyl (55%) < acrylic (73%) based-coatings. All the paints formulated in our laboratory showed higher photocatalytic performances with respect to the reference chinese commercial paint (42%) but lower than the formulation produced by KEIM (79%). Furthermore the dried layers deposited on glass sheets displayed high adhesivity properties and strong mechanical resistance even after four months from the original deposition of the paint. Only in the case of vinyl paint the degradation of the polymeric matrix was observed; this led to a yellowish colour after the photocatalytic tests. Hence, FTIR-ATR spectra (Figure 22) were acquired to evaluate the deterioration of the organic compounds under UV irradiation (4h, blue line) and after the photocatalytic test (red line).

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 247 All the home-made paints were stable under UV irradiation for 4h. The ATR response of the paints at the end of the NOx degradation confirmed that only the vinyl paint degenerated, notwithstanding the good photocatalytic activity: either the relative intensity or the wavenumber of the peaks changed from the pristine to worked layer. Besides after ageing for one month the photoactivity of all the paints was reduced. In order to strengthen these results, other authors [165] studied the weatherability in a QUV of a silicate inorganic (KEIM) and a vinyl paint. They affirmed that the vinyl paint was clearly more unstable with the effects of the PC105 nano-titania (by Millennium) increasing weight loss with increasing concentration.

a)

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b)

c)

vinyl paint vinyl paint + UV vinyl paint+ UV + NOx

silicate paint silicate paint + UV silicate paint + UV + NOx

acrylic paint acrylic paint + UV acrylic paint + UV + NOx

Figure 22. FTIR-ATR spectra of vynil, silicate, and acrylic paints (green lines), after under UV irradiation (4h, blue lines) and after the photocatalytic test (red lines).

Moreover a UV-treatment for at least 5 hours, followed by an immersion of the test piece for 2 hours or more in deionised water and subsequently dried at room temperature, could be Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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able to partially regenerate (or increase, in case of freshly deposited paint) the photoactivity, in according to UNI ISO 22197–1:2007.

4. CONCLUSION In the first part of the chapter different methods to prepare nanotitania, with enhanced physico-chemical properties, are discussed in details. The relative photocatalytic performances of the powders with respect to depolluting effects, both in liquid or in gas media, under UV and solar irradiation are presented. In the second part photoactivity/photodegradation of nanoTiO2-paints are studied. Several components of the paint influence the final photocatalytic activity. This also means that the formulation of photoactive paints must be tailored “ad hoc”: the choice of the resins, the addition step of a suitable nanoactive titanium dioxide, the ability of the oxide to surface in the dried coating are crucial points. Actually traditional criteria usually adopted to formulate new paints (for example high resistivity and good mechanical properties) might be different with respect to those required for ideal photocatalytic properties. Hence, proper functioning of photocatalytic coatings still remains a challenge and still needs constant development. From these results it is concluded that recipes of photocatalytic paint formulations need to be optimized for better efficiency under in situ conditions.

REFERENCES

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[1]

Ardizzone, S; Cappelletti, G; Ionita, M; Minguzzi, A; Rondinini, S; Vertova, A. Electrochimica Acta, 2005, 50, 4419-4425. [2] Ardizzone, S; Bianchi, CL; Cappelletti, G; Ionita, M; Minguzzi, A; Rondinini, S; Vertova, A. J. Electroanal. Chem., 2006, 589, 160-166. [3] Gong, XQ; Selloni, A; Batzill, M; Diebold, U. Nature Mater, 2006, 5, 665-670. [4] Thompson, TL; Yates, JT. Chem. Rev., 2006, 106(10), 4428-4453. [5] Cappelletti, G; Ardizzone, S; Bianchi, CL; Naldoni, A; Pirola, C. Environ. Sci. Technol., 2008, 42, 6671-6676. [6] Ardizzone, S; Bianchi, CL; Cappelletti, G; Gialanella, S; Pirola, C; Ragaini, V. J. Phys. Chem. C, 2007, 111, 13222-13231. [7] Ganduglia-Pirovano, MV; Hofmann, A; Sauera, J. Surface Sci. Reports, 2007, 62, 219270. [8] Zhao, J; Chen, C; Ma, W. Top. Catal, 2005, 35(3-4), 269-278. [9] Prakash, NS; Puttaiah, ET; Kiran, BR; Harish Babu, K; Mahadevan, KM. Res. J. Chem. Environ., 2007, 11(4) 73-77. [10] Poulios, I; Spathis, P; Grigoriadou, A; Delidou, K; Tsoumparis, P. J. Environ. Sci. Health Part A: Environ. Sci. Eng., 1999, A34(7), 1455-1471. [11] Chatterjee, D; Dasgupta, S. J. Photochem. Photobiol. C: Photochem. Rew., 2005, 6(23), 186-205. [12] King, CH; Shotts Jr, EB; Wooley, RE; Porter, KG. Appl. Environ. Microbiol, 1988, 54(12), 3023-3033.

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 249 [13] Szewzyk, U; Szewzyk, R; Manz, W; Schleifer, KH. Ann. Rev. Microbiol, 2000, 54, 81127. [14] Venczel, LV; Arrowood, M; Hurd, M; Sobsey, MD. Appl. Environ. Microbiol, 1997, 63, 1598-1601. [15] Carp, O; Huisman, CL; Reller, A. Prog. Solid State Chem., 2004, 32, 33-177. [16] Aruna, ST; Patil, KC. J. Mater. Synth. Process, 1996, 4(3), 175-179. [17] Sivalingam, G; Nagaveni, K; Hegde, MS; Madras, G. Appl. Catal. B: Environ, 2003, 45, 23-38. [18] Nagaveni, K; Sivalingam, G; Hegde, MS; Madras, G. Appl. Catal. B: Environ, 2004, 48, 83-93. [19] Cheng, Y; Sun, H; Jin, W; Xu, N. Chem. Eng. J, 2007, 128, 127-133. [20] Kitamura, Y; Okinaka, N; Shibayama, T; Mahaney, OOP; Kusano, D; Ohtani, B; Akiyama, T. Powder Technol, 2007, 176, 93-98. [21] Bakardjieva, S; Šubrt, J; Štengl, V; Dianez, MJ; Sayagues MJ. Appl. Catal. B: Environ, 2005, 58, 193-202. [22] Bianchi, CL; Cappelletti, G; Ardizzone, S; Gialanella, S; Naldoni, A; Oliva, C; Pirola, C. Catal. Today, 2009, 144, 31-36. [23] Cappelletti, G; Bianchi, CL; Ardizzone, S. Appl. Catal. B: Environ, 2008, 78, 193-201. [24] Ha, PS; Youn, HJ; Jung, HS; Hong, KS; Park, YH; Ko, KH. J. Colloid Interface Sci., 2000, 223, 16-20. [25] Seo, DS; Kim, H; Jung, HC; Lee, JK. J. Mater. Res., 2003, 18(3), 571-577. [26] Yang, Q; Xie, C; Xu, Z; Gao, Z; Du, Y. J. Phys. Chem. B, 2005, 109, 5554-5560. [27] Yin, S; Ihara, K; Liu, B; Wang, Y; Li, R; Sato, T. Phys. Scr., 2007, T129, 268-273. [28] Junin, C; Thanachayanont, C; Euvananont, C; Inpor, K; Limthongkul, P. Eur. J. Inorg. Chem., 2008, 974-979. [29] Zhang, Y; Xiong, G; Yao, N; Yong, W; Fu, X. Catal. Today, 2001, 68, 89-95. [30] Poznyak, SK; Kokorin, AI; Kulak, AI. J. Electroanal. Chem., 1998, 442, 99-105. [31] Pedraza, F; Vasquez, A. J. Phys. Chem. Solids, 1999, 60, 445-448. [32] Xie, Y; Yuan, C. Mater. Res. Bull, 2004, 39, 533-543. [33] Bach, U; Lupo, D; Comte, P; Moster, JE; Weissortel, F; Salbeck J. Nature, 1998, 395, 583-585. [34] Sivakumar, S; Krishna Pillai, P; Mukundan, P; Warrier, K. Mater. Lett, 2002, 57, 330335. [35] Matijevic, E; Budnik, M; Meites, L. J. Colloid Interface Sci., 1977, 61, 302-311. [36] Boiadjieva, T; Cappelletti, G; Ardizzone, S; Rondinini, S; Vertova, A. Phys. Chem. Chem. Phys., 2003, 5, 1689-1694. [37] Boiadjieva, T; Cappelletti, G; Ardizzone, S; Rondinini, S; Vertova, A. Phys. Chem. Chem. Phys., 2004, 6, 3535-3539. [38] Cappelletti, G; Ricci, C; Ardizzone, S; Parola, C; Anedda, A. J. Phys. Chem. B, 2005, 109, 4448-4454. [39] Bianchi, CL; Ardizzone, S; Cappelletti G. Dekker Encyclopedia of nanoscience and nanotechnology, Dekker: New York, USA, 2006, 1-10. [40] Watson, SS; Beydoun, D; Scott, JA; Amal, R. Chem. Eng. J, 2003, 95, 213-220. [41] Jung, KJY; Park, SB. J. Photochem. Photobiol. A: Chem., 1999, 127, 117-122. [42] Okudera, H; Yokogawa, Y. Thin Solid Films, 2003, 423, 119-124. [43] Tonejc, M; Djerdj, I; Tonejc, A. Mater. Sci. Eng. B, 2001, 85, 55-63.

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

250 [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60]

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

[61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76]

Giuseppe Cappelletti Phani, R; Santucci, S. Mater. Lett, 2001, 50, 240-245. Chen, YF., Lee, CY; Yeng, MY; Chin, HT. J. Cryst. Growth, 2003, 247, 363-370. Yang, P; Lu, C; Hua, N; Du, Y. Mater. Lett, 2002, 57, 794-801. Ao, CH; Lee, SC; Yu, JC. J. Photochem. Photobiol. A: Chem., 2003, 156, 171-177. Mehrdad Keshmiri, M; Troczynski, T; Mohseni, M. J. Hazard. Mater, 2006, B128, 130-137. Livage, J; Henry, M; Sanchez, C. Prog. Solid State Chem., 1988, 18, 259-341. Barringer, EA; Bowen, HK. Langmuir, 1985, 1, 420-428. Zhang, H; Reller, A. J. Mater. Chem., 2001, 11, 2537-2541. Stone, VF; Davis, RJ. Chem. Mater, 1998, 10, 1468-1474. Antonelli, DM; Ying, JY. Angew. Chem. Int. Ed. Engl., 1995, 34, 2014-2017. Thieme, M; Schuth, F. Micropor. Mesopor. Mater, 1999, 27, 193-200. Cabrera, S; El Haskouri, J; Beltrán-Porter, A; Beltrán-Porter, D; Marcos, MD; Amóros, P. Solid State Sci., 2000, 2, 513-518. Cassiers, K; Linssen, T; Mathieu, M; Bai, YQ., Zhu, HY; Cool, P. J. Phys. Chem. B, 2004, 108, 3713-3721. Yuan, ZH; Jia, HJ; Zhang, LD. Mater. Chem. Phys., 2002, 73, 323-326. Sonowane, RS; Kale, BB; Dongare, MK. Mater. Chem. Phys., 2004, 85, 52-57. Wang, J; Una, S; Klabunde, KJ. Appl. Catal. B: Environ., 2004, 48, 151-154. Yamashita, H; Harada, M; Misaka, J; Takeuchi, M; Anpo, KM. J. Photochem. Photobiol. A: Chem., 2002, 148, 257-261. Di Paola, A; Garcìa-Lòpez, E; Ikeda, S; Marcì, G; Ohtani, B; Palmisano, L. Catal. Today, 2002, 75, 87-93. Dambar, BH; Klabunde, KJ. J. Colloid Interface Sci., 2007, 311, 514-522. Li, FB; Li, XZ. Appl. Catal. A: Gen., 2002, 228, 15-27. Li, FB; Li, XZ; Ao, CH; Lee, SC; Hou, MF. Chemosphere, 2005, 59, 787-800. Ranjit, KT; Cohen, H; Willner, I; Bossmann, S; Braun, AM. J. Mater. Sci., 1999, 34, 5273-5280. Li, XZ; Li, FB. Environ. Sci. Technol., 2001, 35, 2381-2387. Kim, S; Hwang, SJ; Choi, W. J. Phys. Chem. B, 2005, 109, 24260-24267. Sokmen, M; Candan, F; Sumer, Z. J. Photochem. Photobiol. A, 2001, 143, 241-244. Wang, YQ; Cheng, HM; Hao, YZ; Ma, JM; Li, WH; Cai, SM. Thin Solid Films, 1999, 349, 120-125. Xu, W; Gao, Y; Liu, HQ. J. Catal, 2002, 207, 151-157. Asahi, R; Morikawa, T; Ohwaki, T; Aoki, K; Taga, Y. Science, 2001, 293(5528), 269271. Livraghi, S; Chierotti, MR; Giamello, E; Magnacca, G; Paganini, MC; Cappelletti, G; Bianchi, CL. J. Phys. Chem. C, 2008, 112, 17244-17252. Di Valentin, C; Finazzi, E; Pacchioni, G; Selloni, A; Livraghi, S; Giamello, E; Paganini, MC. Chem. Phys., 2006, 339, 44-56. Chen, H; Nambu, A; Wen, W; Graciani, J; Zhong, Z; Hanson, JC; Fujita, E; Rodriguez, JA. J. Phys. Chem. C, 2007, 111, 1366-1372. Reyes-Garcia, EA; Sun, Y; Reyes-Gil, K; Raftery, D. J. Phys. Chem. C, 2007, 111, 2738-2748. Emeline, AV; Zhang, X; Jin, M; Murakami, T; Fujishima, A. J. Phys. Chem. B, 2006, 110, 7409-7413.

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 251 [77] Umebayashi, T; Yamaki, T; Yamamoto, S; Miyashita, A; Tanaka, S; Sumita, T; Asai, K. J. Appl. Phys., 2003, 93, 5156-5160. [78] Ohno, T; Akiyoshi, M; Umebayashi, T; Asai, K; Mitsui, T; Matsumura, M. Appl. Catal. A: Gen., 2004, 265, 115-121. [79] Takeshita, K; Yamakata, A; Ishibashi, TA; Onishi, H; Nishijima, K; Ohno, T. J. Photochem. Photobiol. A: Chem., 2006, 177, 269-275. [80] Tachikawa, T; Tojo, S; Kawai, K; Endo, M; Fujitsuka, M; Ohno, T; Nishijima, K; Miyamoto, Z; Majima, T. J. Phys. Chem. B, 2004, 108, 19299-19306. [81] Crişan, M; Brâileanu, A; Râileanu, M; Zaharescu, M; Crişan, D; Drâgan, N; Anastasescu, M; Ianculescu, A; Niţoi, I; Marinescu, VE; Hodorogea, SM. J. Non-Crys. Solids, 2008, 354, 705-711. [82] Zhao, W; Ma, W; Chen, C; Zhao, J; Shuai, Z. J. Am. Chem. Soc., 2004, 126, 47824783. [83] Finazzi, E; Di Valentin, C; Pacchioni, G. J. Phys. Chem. C, 2009, 113(1), 220-228. [84] In, S; Orlov, A; Berg, R; García, F; Pedrosa-Jimenez, S; Tikhov, MS; Wright, DS; Lambert, RM. J. Am. Chem. Soc., 2007, 129(45), 13790-13791. [85] Park, JH; Kim, S; Bard, AJ. Nano Lett, 2006, 6, 24-28. [86] Sakthivel, S; Kisch, H. Angew. Chem. Internat. Ed, 2003, 42, 4908-4911. [87] Irie, H; Washizuka, S; Hashimoto, K. Thin Solid Films, 2006, 510, 21-25. [88] Hsu, SW; Yang, TS; Chen, TK; Wong, MS. Thin Solid Films, 2007, 515, 3521-3526. [89] Colón, G; Hidalgo, MC; Navìo, JA. Catal. Today, 2002, 76, 91-101. [90] Choi, Y; Umebayashi, T; Yoshikawa, M. J. Mater. Sci., 2004, 39, 1837-1839. [91] Yu, G; Chen, Z; Zhang, Z; Zhang, P; Jiang, Z. Catal. Today, 2004, 90, 305-312. [92] Lin, L; Lin, W; Zhu, Y; Zhao, B; Xie, Y. Chem. Lett, 2005, 34, 284-285. [93] Nukumizu, K; Nunoshige, J; Takata, T; Kondo, JN; Hara, M; Kobayashi, H; Domen, K. Chem. Lett, 2003, 32, 196-197. [94] Li, D; Haneda, H; Hishita, S; Ohashi, N. Chem. Mater, 2005, 17, 2596-2602. [95] Keller, N; Barraud, E; Bosc, F; Edwards, D; Keller, V. Appl. Cat. B: Environ, 2007, 70, 423-430. [96] Baozhu, T; Feng, C; Jinlong, Z; Masakazu, A. J. Colloid Interface Sci., 206, 303, 142148. [97] Jung, SM; Grange, P. Catal. Lett., 2001, 76(1), 27-30. [98] Yin, S; Fujishiro, Y; Wu, J; Aki, M; Sato, T. J. Mater. Proc. Tech., 2003, 137, 45-48. [99] Kang, M. J. Mol. Catal. A: Chem., 2003, 97, 173-183. [100] Hidalgo, MC; Aguilar, M; Maicu, M; Navìo, JA; Colòn, G. Catal. Today, 2007, 129, 50-58. [101] Yanagisava, K; Yamamoto, Y; Feng, Q; Yamasaki, N. J. Mater. Res., 1998, 13, 825829. [102] Wang, CC; Ying, JY. Chem. Mater, 1999, 11, 3113-3120. [103] Yu, K; Zhao, J; Guo, Y; Ding, X; Bala, H; Liu, Y; Wang, Z. Mater. Lett, 2005, 59, 2515-2518. [104] Kolen’ko, YV; Churagulov, BR; Kunst, MJ; Mazerolles, L; Colbeau-Justin, C. Appl. Catal. B: Environ, 2004, 54, 51-58. [105] Yin, H; Wada, Y; Kitamura, T; Kambe, S; Murasawa, S; Mori, H; Sakata, T; Yanagida, S. J. Mater. Chem., 2001, 11, 1694-1703. [106] Aruna, ST; Tirosh, S; Zaban, A. J. Mater. Chem., 2000, 10, 2388-2391.

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Giuseppe Cappelletti

[107] Yin, S; Aita, Y; Komatsu, M; Wang, J; Tang, Q; Sato, T. J. Mater. Chem., 2005, 15, 674-682. [108] Yamamoto, K; Tomita, K; Fujita, K; Kobayashi, M; Petrykin, V; Kakihana, M. J. Crys. Growth, 2009, 311, 619-622. [109] Surolia, PK; Lazar, MA; Tayade, RJ; Jasra, R. V. Ind. Eng. Chem. Res., 2008, 47, 5847-5855. [110] Yin, S; Ihara, K; Liu, B; Wang, Y; Li, R; Sato, T. Phys. Scr., 2007, T129, 268-273. [111] Shimomura, K; Miyazaki, T; Tutiya, A. Annual Report, 4811 RIKEN: Japan, 1972. [112] Sutton, WH. Microwave Processing of Ceramics s An Overview. Proceedings of the Materials Research Society Symposium, Microwave Processing of Materials III, San Francisco, CA, April 1992, RL. Beatty, Ed. 1992, 269, 3-20. [113] Sutton, WH. Am. Ceram. Soc. Bull., 1989, 68, 376-386. [114] Mingos, DMP; Baghurst, DR. Chem. Soc. Rev., 1991, 20, 1-47. [115] Rao, KJ; Vaidhyanathan, B; Ganguli, M; Ramakrishnan, PA. Chem. Mater, 1999, 11, 882-895. [116] Ramakrishnan, KN. Mater. Sci. Eng. A, 1999, 259, 120-125. [117] Upadhyaya, DD; Ghosh, A; Dey, GK; Prasad, R; Suri, AK. J. Mater. Sci., 2001, 36, 4707-4710. [118] Agrawal, DK. Curr. Opin. Solid State Mater. Sci., 1998, 3, 480-485. [119] Xu, GF; Lloyd, IK; Carmel, Y; Olorunyolemi, T; Wilson, OCJ. J. Mater. Res., 2001, 16, 2850-2858. [120] Zhang, LX; Liu, P; Su, ZX. Mater. Res. Bull., 2006, 41, 1631-1637. [121] Hart, JN; Menzies, D; Cheng, YB; Simon, GP; Spiccia, L. Sol. Energy Mater. Sol. Cells, 2007, 91, 6-16. [122] Komarneni, S; Rajha, RK; Katsuki, H. Mater. Chem. Phys., 1999, 61, 50-54. [123] Hart, JN; Cervini, R; Cheng, YB; Simon, GP; Spiccia, L. Sol. Energy Mater. Sol. Cells, 2004, 84, 135-143. [124] Wu, X; Jing, QZ; Ma, ZF; Fu, M; Shangguan, WF. Solid State Commun, 2005, 136, 513-517. [125] Bonamartini Corradi, A; Bondioli, F; Focher, B; Ferrari, AM; Grippo, C; Mariani, E; C. Villa, C. J. Am. Ceram. Soc., 2005, 88, 2639-2641. [126] Murugan, AV; Samuel, V; Ravi, V. Mater. Lett, 2006, 60, 479-480. [127] Jaroenworaluck, A; Panyathanmaporn, T; Soontornworajit, B; Supothina, S. Surf. Interf. Anal, 2006, 38, 765-768. [128] Jia, X; He, W; Zhang, X; Zhao, H; Li, Z; Feng, Y. Nanotechnology, 2007, 18, 7. [129] Wilson, GJ; Matijasevich, AS; Mitchell, DRG; Schulz, JC; Will, GD. Langmuir, 2006, 22, 2016-2027. [130] Wang, HW; Kuo, CH; Lin, HC; Kuo, IT; Cheng, CF. J. Am. Ceram. Soc., 2006, 89(11), 3388-3392. [131] Addamo, M; Bellardita, M; Carriazo, D; Di Paola, A; Milito, S; Palmisano, L; Rives, V. Appl. Catal. B: Environ., 2008, 84, 742-748. [132] Simonsen, ME; Jensen, H; Li, Z; Søgaard, EG. J. Photochem. Photobiol. A: Chem., 2008, 200, 192-200. [133] Suprabha, T; Roy, HG; Thomas, J; Praveen, K; Mathew, KS. Nanoscale Res. Lett. 2009, 4, 144-152.

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TiO2 Nanoparticles: Traditional and Novel Synthetic Methods for Photocatalytic… 253 [134] Horikoshi, S; Sakai, F; Kajitani, M; Abe, M; Emeline, AV; Serpone, N. J. Phys. Chem. C, 2009, 113(14), 5649-5657. [135] Wu, L; Yu, JC; Zhang, L; Wang, X; Ho, W. J. Solid State Chem., 2004, 177, 25842590. [136] Yu, JC; Yu, J; Zhang, L; Ho, W. J. Photochem. Photobiol. A: Chem., 2002, 148, 263271. [137] Wang, Y; Chen, S; Tang, X; Palchik, O; Zaban, A; Koltypin, Y; Gedanken, A. J. Mater. Chem., 2001, 11, 521-526. [138] Arami, H; Mazloumi, M; Khalifehzadeh, R; Sadrnezhaad, SK. Mater. Lett, 2007, 61, 4559-4561. [139] Wang, Y; Tang, X; Yin, L; Huang, W; Rosenfeld Hacohen, Y; Gedanken, A. Adv. Mater, 2000, 12, 1183-1186. [140] Meskin, PE; Baranchikov, AE; Ivanov, VK; Afanasev, DR; Gavrilov, AI; Churagulov, BR; Oleinikov, NN. Inorg. Mater, 2004, 40, 1058-1065. [141] Suslick, KS. Science, 1990, 247, 1439-1445. [142] Suslick, KS; Price, GJ. Ann. Rev. Mater. Sci., 1999, 29, 295-326. [143] Yu, JC; Yu, J; Ho, W; Zhang, L. Chem. Commun., 2001, 1942-1943. [144] Huang, WP; Tang, XH; Wang, YQ; Koltypin, Y; Gedanken, A. Chem. Commun., 2000, 415-416. [145] Kim, SY; Chang, TS; Shin, CH. Catal. Lett, 2007, 118, 224-230. [146] Cappelletti, G; Ardizzone, S; Bianchi, CL; Gialanella, S; Naldoni, A; Pirola, C; Ragaini,V. Nanoscale Res. Lett, 2009, 4, 97-105. [147] Rao, CNR; Műller, A; Cheetham, K. The chemistry of nanomaterials; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, DE, 2005, Vol. 1, 113-169. [148] Neppolian, B; Wang, Q; Jung, H; Choi, H. Ultrason. Sonochem, 2008, 15, 649-658. [149] Ramaswamy, V; Jagtap, NB; Vijayanand, S; Bhange, DS; Awati PS. Mater. Res. Bull., 2008, 43, 1145-1152. [150] Zhang, L; Yu, JC. Chem. Commun, 2003, 2078-2079. [151] Goldschmidt, A; Streitberger, HJ. Basics of coating technology; 2nd revised edition; Vincentz Network: Hannover, DE, 2007, 27-60. [152] Wang, R; Hashimoto, K; Fujishima, A; Chikuni, M; Kojima, E; Kitamura, A; Shimofukigoe, M; Watanabe, T. Nature, 1997, 388, 431-432. [153] Fujishima, A; Rao, TN; Tryk, DA. J. Photochem. Photobiol. C: Photochem. Rev., 2000, 1, 1-21. [154] Kikuchi, Y; Sunada, K; Iyoda, T; Hashimoto, K; Fujishima, A. J. Photochem. Photobiol. A: Chem., 1997, 106, 51-56. [155] Ibusuki, T; Takeuchi, K. J. Mol. Catal, 1994, 88, 93-102. [156] Legrini, O; Oliveros, E; Braun, AM. Chem. Rev., 1993, 93, 671-698. [157] Hoffman, MR; Martin, ST; Choi, W; Bahnemann, DW. Chem. Rev., 1995, 95, 69-96. [158] Allen, NS; Edge, M; Sandoval, G; Verran, J; Stratton, J; Maltby, J. Photochem. Photobiol, 2005, 81, 279-290. [159] Hodgson, T; Destaillats, H; Sullivan, DP; Fisk, WJ. Indoor Air, 2007, 17, 305-316. [160] Zhang, J; Wilson, WE; Lioy, PJ. Environ. Sci. Technol, 1994, 28, 1975-1982. [161] Auvinen, J; Wirtanen, L. Atmos. Environ., 2008, 42, 4101-4112. [162] Salthammer, T; Fuhrmann, F. Environ. Sci. Technol., 2007, 41, 6573-6578.

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[163] Celiker, G; Yu, D; Uslusoy, A; Aydin, H; Cakaz, U. Chamber of Chem. Eng., 2006, 375-382. [164] Salthammer, T; Fuhrmann, F; Schulz, N; Siwinksi, N. Removal of indoor contaminants by photocatalytic reaction. In: Proceedings of Healthy Buildings, Lisbon, Portugal, 48/6/2006. [165] Allen, NS; Edge, M; Sandoval, G; Verran, J; Stratton, J; Maltby, Bygott, C. Polym. Degrad. Stabil, 2008, 93, 1632-1646. [166] Nonami, T; Hase, H; Funakoshi, K. Catal. Today, 2004, 96, 113-118. [167] Maggos, T; Bartzis, JG; Liakou, M; Gobin, C. J. Hazard. Mater, 2007, 146, 668-673. [168] Maggos, T; Bartzis, JG; Liakou, M; Leva, P; Kotzias, D. Appl. Phys. A, 2007, 89, 81-84. [169] Ooka, C; Iida, K; Harada, M; Hirano, K; Nishi, Y. Appl. Clay Sci., 2009, 42, 363-367. [170] Mogyorósi, K; Dékány, I; Fendler, JH. Langmuir, 2003, 19, 2938-2946. [171] Kun, R; Mogyorósi, K; Dékány, I. Appl. Clay Sci., 2006, 32, 99-110. [172] Minabe, T; Tryk, DA; Sawunyama, P; Kikuchi, Y; Hashimoto, K; Fujishima, A. J. Photochem. Photobiol. A: Chem., 2000, 137, 53-62.

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Chapter 5

MORPHOLOGY CHANGES IN CARBON NANOPARTICLES DUE TO DIFFERENT ATOM ARRANGEMENTS N. Koprinarov* and M. Konstantinova Central Lab on Solar Energy and New Energy Sources, 72 Tzarigradsko Shaussee, Sofia, 1784, Bulgaria

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ABSTRACT The intensive investigations of carbon materials stimulated remarkably by the discovery of fullerenes and carbon nanotubes resulted in the conclusion that the character of the carbon atom connection in the carbon network has crucial importance for the structure and the properties of carbon nanoparticles. Carbon materials are in the most stable energetic situation if their atoms are connected via sp3 bonds and build 6 atom rings (n-gons with n=6), which, on their side, construct a honeycomb-like network. In nature, it is normal and frequently observed how variations in local conditions hinder the building of this perfect structure via self organization of carbon atoms. Because of this, rings of atoms other than 6 often emerge in the growing carbon nanoparticle network, transforming the last to unique ones in regard to shape and properties. In this chapter, such rings are considered n-gons with defects. Nevertheless, from the fact that only one or few atoms or one or few n-gons emerge in the whole network, these defects change it, resulting in a significant modification in nanoparticle structure and properties. In this chapter, more attention is paid to the effects caused by the emergence of n-gons with n = 5 and 7, because their incorporation explains very well the structure of the most of the observed carbon nanoparticles. The simultaneous emerging of 5/7 defects such as pentaheptite, defects oppositely situated in the nanoparticle structure and the repetitive pair and carbon network deformation caused by them, are also discussed. The theoretically proposed carbon networks constructed from repetitive combinations of 5/7 and 5/6/7 n-gons are exposed. Those can be sources for the construction of a new kind of nanostructure in the same manner as nanostructures built from a honeycomb-like network. It is shown, too, * Corresponding author: Email [email protected]; Phone (+359 2) 778448. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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N. Koprinarov and M. Konstantinova how the connection between two carbon particles can be realized through a row of atom bond transformation, in the course of which different n-gons are created and destroyed. The examples given for the correlation between theoretically predicted carbon nanostructures and their actually observed analogs confirm the ability of theory to predict and to tailor the properties of a new type of carbon nanostructure.

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INTRODUCTION The unique role of carbon in nature has been known for a long time. The capability of carbon atoms to form complicated networks is fundamental to organic chemistry, which is also the basis for the existence of life on Earth. After the discovery of fullerenes [1-6], carbon captured the attention of scientists again, triggering an increased interest in the field of nanoscience. This was strongly accelerated by the multitude of discoveries showing that carbon is able to create a variety of nanostructures of interesting properties. In a very short time, an array of new particles such as cylindrical fullerenes [7], nanotubes [8], nanohorns [9], nanorings [10-12], coils [13], etc. were added to the carbon nanofamily. The fact that they consist only of one element but nonetheless form stabile high-ordered structures that are able drastically to change their properties only by insignificantly varying atom arrangement is fundamental for the development of models, simulation of processes, prediction of new phenomena as well as for the explanation of their own behavior and properties. The possibility to calculate their properties in detail gives carbon nanostructures a special place in nanoscience. Numerous examples exist for which particle properties had been predicted long before they were confirmed experimentally. Carbon nanoparticles can be obtained by using different methods [14-24]. However, none of those methods allow the synthesis of structures without defects. The possible defect structures are classified into three general groups [25]: topological (introduction of ring types other than hexagons), rehybridization (ability of carbon atom to hybridize between sp2 and sp3) and incomplete bonding defects (vacancies, dislocations, irradiation damages, etc.). Here the important topological defects are considered only. Those merge during the particle formation process as result of deviations in atom arrangement. Most of the misplaced atoms successfully end up in the right place due to temperature stimulations. However, this is not the case when growth takes place at low temperatures or there is not enough time for relaxation, which leads to local defect formation. The defect sets off a change in the base network and causes a new atom arrangement. The end result is a particle of a totally new morphology. Although the defect is local, it can nonetheless influence in crucial way the properties of the entire particle because of the small particle dimension. The change can even be so considerable that it can completely alter the particle properties. On one hand, this can be a problem if the goal is to obtain defect-free structures but, on the other hand, it is a great advantage if one is interested in obtaining and exploring new structures. It is well known that carbon atoms form stable rings of 3 up to 8 atoms. The most stable is the ring structure of 6 atoms. The last is a building block for the honeycomb graphite sheet, a flat structure of hexagons (6-atom rings) that is also known as graphene (see Fig. 1a). Recently, scientists have realized that it represents a new class of materials [26-39]. It consists of a two-dimensional lattice only one atom thick that is characterized by remarkably high crystal and electronic quality. It has a zero density of states at the Fermi level without an

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energy gap, and linear, rather than parabolic, energy dispersion around the Fermi level. The latter can be used to create a new kind of nanodevice [40-42]. One of the fascinating effects that can be used for developing a new branch in electronics is that charge carriers in such a defect-free lattice can travel thousands of interatomic distances without scattering and can be tuned continuously between electrons and holes (bipolar electric field effect) [43-45]. Since defects in atomic arrangement can crucially change the system electronic properties in dependence on defect nature [28, 46], studying the above system is undoubtedly of great interest. However, graphene is the building block of all the carbon materials, and the defects observed in its structure are similar to those observed elsewhere. Before we start to discuss how defects emerge and influence the growth of carbon particles and their properties, we need to introduce and explain some features of the carbon network. Let us assume for a moment that a graphene network has grown around a single ring, as indicated in Fig. 1a. From the center of this ring, the network can be divided into 6 equal 60-degree sectors (Fig.1b). The network is divided into sectors in such a way that each sector includes one atom and one carbon-carbon bond from the marked ring. The sector also includes all the atoms between the sector edges and the atoms touching the left edge (see the sheared sector in Fig. 1b). If the creation of the whole network in Fig. 1a starts with bonding between atoms from the marked ring, one could say that each sector will grow independently from the others when its first atom is included in this ring. A missing atom in the marked ring will constitute a local defect. The corresponding sector will not be created and, as a result, will be absent in the growing network. Hence, a local defect in the marked ring will transform the whole network despite the fact that all the other rings have not been altered. The rings of n atoms (n-gon), for n = 1 up to infinity, and the changes in the network due to an n-gonal defect, are widely discussed and explained in [47]. Fig. 2a illustrates the honeycomb network that corresponds to the n-gons, when n = 1, 2, 3, 4 and 5. The bird’s-eye view of transformed network due to an n-gonal defect is shown in Fig. 2b for n = 3, 4 and 5. It should be pointed out that n-gons for n = 1 and 2 do not exist geometrically, but they are considered in [47] as well.

a

b

Figure 1. Carbon honeycomb network: (a) considered area around the marked hexagon and (b) the network divided in 6 symmetrical sectors by axes through the carbon atoms in the marked ring.

The sectors grown around the n-gons (n ≠ 6) can not be more flat as those are in the honeycomb and bend. The bending, caused by introduction of a pentagonal (n = 5) or a heptagonal (n = 7) defect into the structure of planar triangular meshes has been predicted in Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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[49]. Analogically, carbon network can be considered also as an elastic material and the occurrence of sheet buckling will depend on its elastic properties (on bending rigidity, the two-dimensional Young’s modulus or stretching rigidity, and on sheet radius (size)). The calculated by different methods carbon network transition [49-51] shows that the surface around all the n-gons in Fig. 2b is cone-like (a Gaussian positively curved surface, with the ngon on the top and rotational axis, crossing perpendicular the n-gon surface in its center). The n-gonal defects, where n is an integer less than 5, are less stable than the pentagonal defects in a hexagonal network [52].

a

b

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Figure 2. (a) the part of a carbon network around the n-gons (n = 1, 2, 3, 4 and 5); (b) a bird’s-eye view of the transformed surfaces containing an n-gonal defect, where n is 3, 4, and 5.

The above defects have been discussed in the context of a growing network but, in reality, defects can appear at a later or at the end stage of network formation, as illustrated in Fig. 3. If network (a) continues to grow in the direction marked with the black arrow, it will end up in the conical structure (b) (conical structure with a defect). On the contrary, the cone in Fig. 3a could have been produced through widening of the cone (b), which makes the two results equal and reversible. The ending of growing structures with defects is frequently observed in carbon nanotubes. Regardless of when the defect is formed, at the beginning or at the end of the growth process, the result on the network is exactly the same. This allows us to consider all the defects being formed in the growth process beginning. Some surfaces formed around n-gons for n > 6, where n is equal to 7, 8, 9, 10, 11 and 12, are illustrated in Fig. 4. They have been obtained by molecular-dynamics simulations [47] and possess obligatorily the Gaussian negatively curved surfaces as the only possible surface solution.

Figure 3. Conical structures: (a) shows a conical structure in process of growth which end shape is conical structure (b). If structure (b) grows first, it will overgo into (a).

The saddlelike surfaces shown in Fig. 4 are rolling up and down twice. However, surfaces that roll around n-gons more than twice can also be created. These surfaces are called buckled surfaces. The m-times rolling around the center is described as their m periodicity

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[47]. Such surfaces with a n-gonal defect (n = 10) and periodicity m = 5 are shown in Fig. 5. They are obtained by joining 5 sectors (Fig. 5a) bended around their symmetry axis.

Figure 4. Optimized saddlelike surfaces containing an n-gonal defect, where n is 7, 8, 9, 10, 11 and 12. (Reprinted with the permission of ref. [47] Ihara S.; Itoh S.; Akagi K.; Tamura R.; Tsukada M.,

Phys.Rev. B 54, No. 20, (1996), 14 713-14 719, © American Physical Society.)

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The possibility for a helicoidal surface of hexagons obtained as an infinite number n in the form of an n-gonal defect is also discussed in [47]. The infinity of vertices is obtained as polygon vertices coil around the fiber axis, with certain periodicity keeping the vertices bond length constant (Fig. 6). The screw dislocations are created by a combination between parts of the helically coiled cusp and the nearly planar surface layers of graphite. This structure, however, is reported to be unstable.

a

b

Figure 5. (a) joining of 5 sectors; (b) buckled surfaces containing a 10-gonal defect with periodicity 5.

Figure 6. A sketch of carbon atoms and their bonds in an n-gon for n = infinity.

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Cone-like nanoparticles (see Fig. 7a), named nanohorn, have been obtained through laser ablation on carbon materials [9]. Some of them are expected to be open (Fig. 7b). Nanohorns have been intensively investigated as nanostructures of a very promising perspective for practical application. They are monolayer particles and as a result of that all their atoms are surface atoms. This property can be important when a material of large surface and low weight is demanded for certain application (for example, as an electrode for a fuel cell or as a catalyst). The carbon structures with a negative Gaussian curvature can be obtained through nanotube polymerization [53], in a negative curvature fullerene [54] and in Schwarzites (called in honour of H. A. Schwarz, a German mathematician who was the first to investigate the differential geometry of this class of surfaces) [55-59]. In [60] it is supposed that the periodic structure C168 is of same sense as C60 but carbon atoms instead in a network of pentagons and hexagons are in sevenfold and sixfold rings. The authors predict that if this material could be synthesized it would have elastic constants comparable to those of Si and a density half that of graphite. An example for surface with negative curvature is exposed in Fig. 8 [61]. The synthesis of periodic graphite structures similar to schwarzites and its characterization is reported in [62]. This carbon structure is characterized by interconnected thin layers forming a spongy structure of meso- and macroporosity. In case multiple defects are present in a network, each of them affects surface deformation. Theoretically, carbon network (and graphene) can obtain endless shapes, depending on the total number of defect rings, their kind and location. The surface becomes closed in space if the rings altogether satisfy the Euler’s rule (3N3 + 2N4 + 1N5 + 0N6 -1N7 2N8 - 3N9 - 4N10 = 12, where Nj is the number of the n-gons with j edges). The general expression of the Euler’s theorem in a three-dimensional network is V-E+F=2(1-g) [63, 64], where F, V, and E are numbers of faces, vertices and edges, respectively, and g is the genus. Space closed networks are also feasible with all the possible n-gons, satisfying the Euler’s rule, however not all of them are stabile [65]. The stability strongly depends on surface symmetry and on the angle between neighboring faces. For example, the Fullerenes given in Fig. 9, which are symmetrical, closed in space networks and contain only 5 and 6 atom rings, must have stability growing from C20 to C60 and going downwards from C60 to C70 because the angle between neighboring faces increases from C28 to C60 and C70 is no more symmetric as C60.

Figure 7. Nanohorns: (a) closed and (b) open.

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Figure 8. Schwarzite. (Reprinted with permission from ref. [61] Terrones, H.; Terrones, M. New Journal Physics 2003, 5, 126.1–126.37, © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.)

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Figure 9. Fullerenes: C20, C28, C32, C50, C60, C70.

Fullerenes can be classified taking into account the exceptions, despite of the fact that their properties are not repeated with the increasing number of their atoms [66]. In [67] one can find recommendations for describing a wide variety of fullerenes with n-gons from C20 to C120 and of various point group symmetries, including low symmetries such as Cs, Ci, and C1 as well as many fullerenes that have been isolated and well characterized as pristine carbon allotropes or as derivatives. It employs a numbering method that uses symmetry elements as reference entities. The n-gons in a closed network can be ordered in different ways, constructing isomers with diverse shapes. For example, C40 has 40 isomers [68] and the isomers of fullerenes from C20 up to C88 are given in [69]. Carbon nanotubes (CNT) form another large family of carbon nanoparticles. They are the most studied kind of carbon nanoparticles because on their base electronic devices [70, 78], computing devices [79, 80], big palette of sensors [81-91], actuators [92-94], super capacitors [95-98], field effect emitters [99-104] can be constructed. CNT can be thought of as a rectangle part of graphene rolled up into a cylinder. Because of the fact that the properties of carbon network repeat by rotation at 60 degrees, the unit vectors of the coordinate system, used for describing network properties are chosen to be at this angle. The orientation and the name of nanotubes are determined to this coordinate system, too. The nanotube name is unique and it equals the coordinate of the right point of nanotube sircumference (presented as map) when the left corner point is on the point (0,0). This is illustrated in Fig. 10 for 3 orientations (Fig. 10a) of the tube axe to the carbon-carbon bonds. The nanotubes will be (6,0), (6,2) and (6,6) and will have diverse diameters (see Fig. 10b,c). Their hexagons are arranged in a different way according to the nanotube axe (Fig. 10b). Those 3 kind of CNT are known as zigzag (when m = 0), chiral (different from its mirror image and m ≠ n), and armchair (when m = n). That means that each possible pair of coordinates (n,m) corresponds to a nanotube which will have unlike orientation as compared to all the others and, hence, of different diameter. Because of that the electronics properties of all CNT will not be similar, too [105, 106], since atom orbitals depend on bond bending (Fig. 10d).

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Figure 10. A coordinate system used for explaining carbon network and carbon structure properties. Three type of CNT (zigzag, chiral and armchair) are exhibited with their maps (Fig. 10a), their ring arrangement in accordance with tube axis (Fig. 10b), tube diameter (Fig. 10c) and a sketch of the electron orbital deformations by some bending (Fig. 10d).

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Figure 11. (a) a map of CNT with a pentagonal defect and (b) a map of CNT with a heptagonal defect.

Figure 12. A change of tube chirality by 5/7 defect nonparallel to tube axis.

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Figure 13. A change of the tube circumference by 5/7 defect parallel to tube axis.

The pentagon and the heptagon are the frequently arising defects and can margin single or simultaneously in the nanotube structure [114-116]. The single pentagon makes the circumference to decrease as the axis is extended behind the defect. On the contrary, the heptagon causes increasing of the tube circumference and the diameter. Merging of an isolated pentagon in the nanotube and transformation of the nanotube to a cone-like structure through that is illustrated in Fig. 11a. The tube is divided in two through the pentagon and the maps of the both parts are exposed right hands and left hands at the cut line DF of the pentagon. The exposition is over the carbon honeycomb to show how such a structure can be obtained via shearing the marked area and joining the line AB with A1B1 and CD with DC1. In the case of the heptagon in Fig. 11b that is made analogically. The pentagons arise frequently in pair with heptagons and they are responsible for the most of changes in the nanotube structures. These pairs also rule the electronic behavior around the Fermi level. The pentagon and the heptagon in the pair can be isolated one to other or to have one joined edge as in the aniline structure. The effect of the 5/7 pair on the growing structure depends on its place and orientation to the network direction. When the pentagon is attached to a heptagon, the pair creates only a topological change, but no net disclination, which may be treated as a single local defect. The defect can be only a small local deformation in the width of the nanotube or a small change in the helicity, depending on its orientation in the hexagonal network. On the contrary, the defects caused by an isolated pentagon and a heptagon perturb the electronic properties of the hexagonal network significantly [117]. If the symmetry axis of the aniline structure pair is nonparallel to the tube axis it changes chirality of a nanotube by one unit (Fig. 12). There the number of rows before the defect is with 1 smaller than the number of the rows after the defect. The heptagon splits the rows and as a result after the defect merging the row number 5 will be connected to number 4b instead to 5a after the map rolling in a nanotube. In [118] such a pair is shown to cause bending of the nanotube parts (the part before and the part after the pair merging in the network) to an angle of roughly ~0–15º and to produce metal/semiconductor or semiconductor/semiconductor junction depending on the particular tubes involved. The map of a nanotube structure with 5/7 pair parallel to tube axis is shown in Fig. 13. The circumference is enlarged by one unit (one ring) after the incorporation of the pair and that extends nanotube diameter. The diameter extension becomes significant if the pentagon and the heptagon in the pair are separated by hexagons because each hexagon added between the pentagon and heptagon changes the nanotube circumstance by one unit (Fig. 14). In Fig. 14a

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there is 1 hexagon while in Fig. 14b there are 5 hexagons. The pentagon and the heptagon cause opposite surface disclinations and the nanotube changes its shape from cylindrical to conical and from conical to cylindrical again. When the pentagon and the hexagon are separated and the pair is several times incorporated aligned along nanotube axis in one and the same nanotube, so that a set of nanostructures comprised of straight tubular sections of decreasing diameters arise, such a nanostructure is known as carbon taper (Fig. 15). The taper shown in this figure is “perfect” because the junctions follow one and the same periodic rules, the case, which is not valid in general. The tapers are monochiral objects and the growth process kinetics suggests that the most prevalent tapers will have either zigzag or armchair structures [119]. Their transport properties depend on structure. In the tubular sections the number of rings which are included in the tube cross-sections decreases continuously by 1. The conductivity of each cross-section corresponds to that of the cylindrical nanotube of the same diameter. From the comments about CNT made above (see also Fig. 10) if the diameter of the zigzag CNT decreases by 1 the nanotube conductivity changes from that of a semiconductor (presence of conductance gaps in the nanotube (n,m), when m-n ≠ 3q) to the one of a metal (when m-n =3q) and, in reverse, by periodicity of 3. This regular change of conductivity along the taper decreases its total conductivity. That is not the case of armchair tapers. All the armchair CNT have a metal like conductivity independent of their diameter and that makes the entire taper conductive, at low bias voltages, including. The unique morphology of tapers makes them suitable for electron field emitters, scanning-probe tips and for sensors. The poor conducting zigzag tapers are not suitable for this purpose.

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Figure 14. (a) a change of tube circumference as a result of pentagon heptagon separation via one hexagon and (b) a change of tube circumference as a result of pentagon heptagon separation via 5 hexagons.

Figure 15. A perfect taper.

The 5/7 pairs can be more than one in a junction zone. The presence of up to 3 pairs in zigzag nanotubes in different configurations is investigated in [120] in order the influence of several defects on the electronic properties of the system to be understood. The nanotube Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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systems are (12,0); (11,0); (10,0) and (9,0). Those are selected because their electronic properties are expected to differ radically (the junction (12,0)-(9,0) is expected to be a metallic one, the two others are prototypes of a metal-semiconductor junction). The obtained results show that 5/7 pair defects are more stable when aligned along the nanotube axis rather than placed around the cylindrical circumference. If more than one the defect pairs are available those work independent on one another. Another possible arrangement of the 5/7 pair is when the pentagon and the heptagon are located on the tube opposite sides. In this case the axes of the two-tubule parts bend at 30° with respect to each other [121]. The two projection maps of such a tube after cutting through the middle of the pentagon and the heptagon are given in Fig. 16. Those are left hand and right hand at the cut line through the pentagon. Because of the both map parts symmetry in the sample cases demonstrated later only one half map will be given and will be signed as a half map. In [120] the authors come to the conclusion that the pentagon shape is essentially planar with a very small distortion, although the heptagon is boat shaped in order to reduce its bond angles in the range 124–126°, and to suitably accommodate the local negative curvature of the carbon network. The purely cylindrical shape of the nanotube is also perturbed by the presence of these 5/7 pair defects. The belt of hexagons containing the pentagon is found to be elliptical, with the long axis passing through the perturbing ring; while for the belt containing the heptagon, the section of the nanotube is elongated in the perpendicular direction. The cylindrical shape of the nanotube reappears a few belts of hexagons away from the 5/7 defect. Merging of 5/7 pair occurs also by joining of two nanotubes. That is discussed in details in [121]. It is shown there that any tube can be joined to any other in a multitude of ways but energetically the most stable junction is likely to contain the fewest defects. The condition for a successful joining is the broken carbon bonds at the joining ends of both the nanotubes to be equal in number. One example is in Fig. 17. There is demonstrated how two nanotubes (zigzag and armchair, illustrated through their half maps) can be connected by plenty of pentagons and heptagons and simply with one pentagon-heptagon pair, only. Because carbon nanotubes are metals or semiconductors, depending sensitively on their geometrical structures, the possibility for realization of different connections can be used to form metalsemiconductor, semiconductor-semiconductor or metal-metal junctions. These junctions have great potential for applications because they are stable, have nanoscale dimensions and are made entirely of a single chemical element.

Figure 16. Two projection maps (both half maps) of one tube after cutting through the pentagon and heptagon middle exhibited left hand and right hand at the cut line through the pentagon.

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Figure 17. Two possible ways for connecting two nanotubes (zigzag and armchair): (a) via pentagons and heptagons and (b) via pair pentagon-heptagon.

The opposite arrangement of the pentagon and the hexagon, considered above, is a special case. The general formula which satisfy this case and is valid for connecting two carbon nanotubes (or for structure change of one nanotube due to merging of 5/7 defect) is presented in [114], specified by the chiral vectors of the two tubes (or of the two tube parts). In that work the calculated results of the influence of the defects to the tunneling conductance are also given at the different junction of two nanotubes. The Y-branched nanotubes has been proposed in [122] and afterwards identified and observed [123, 124]. Its hexagonal structure has additionally only six heptagons, which causes the branching (Fig. 18). Y-carbon nanotubes can be grown on a graphite substrate held at room temperature by fullerene decomposition under moderate heating in the presence of Ni particles [125]. It is shown in [126] that not only “Y” but “X”, and “T” junctions (Fig. 19) between single wall CNT (SWNTs) can be produced experimentally by controlled electron beam exposure of crossing tubes at elevated temperatures (the exposure is in TEM with accelerating voltage of 1.25 MV at 800ºC). The authors were also able to remove one of the “arms” of an “X” junction in order to create a “Y” or “T” junction by using carefully conditions of irradiation. The arising of X junction via coalescence is discussed theoretically in [127]. It is indicated that controlled electron irradiation can be used for tailoring of junction geometry. That is very important in view of possible applications because such kind of junction could act as a multiterminal electronic device. The tight binding molecular dynamics calculations carried out for two individual (8,8) crossed tubes show that the energy of the system is 189 eV lower than the starting configuration. This energy difference is regarded in [126] as a driving force to promote the formation of a junction.

Figure 18. The places of the 6 heptagons initiated merging of Y junction. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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Figure 19. Nanotubes with “X”, “Y” and “T” junctions. (Reprinted with permission from ref. [126] Terrones, M.; Banhart, F.; Grobert, N.; Charlier, J.-C.; Terrones H.; Ajayan P.M. Phys. Rev. Lett. 2002, 89, 75505.1-75505.4. © American Physical Society.)

Figure.20. A coiled nanostructure. (The original figure is reprinted with the permission from [135] Ihara, S.; Itoh, S.; Kitakami, J. Phys. Rev. B 1993, 48 No. 8, 5643-5647, © American Physical Society, and is fitted to the text via small change as the cage which repeats is exposed in grey.)

The single-wall, thermal stabile, coiled carbon nanotubes were predicted by Ihara et al. [128]. It originated from the regular insertion of pentagons and heptagons in the perfect hexagonal network. Coiled carbon nanotubes were produced by co-catalyzed decomposition of acetylene [129] and ethylene [130] by the decomposition of fullerene [125] and on iron coated indium tin oxide [131]. The production of double- and triple-strand coiled carbon nanotubes was also reported recently [132]. Helically coiled and supercoiled nanostructures can be considered as nanotubes with special arrangement of the 5/7 pairs [133]. There are two kinds of coils: coils which are composed of junctions of straight segments and coils which are composed of continuously curved layers. The existence of 5/7 pairs in both the cases is obligatory because those realize the network bending. In Fig. 20 one example of coil structure is given [134, 135]. The authors pointed that from graphitic carbon cages different structures could be constructed as: supercoiled structures, nested helical structures, double and triple helices, and spiral of Archimedes. The map of one helical cage can be characterized by the two unit vectors of the supercell ex and ey (determined in the honeycomb), L and M, and the vectors a and b defining the shape of the removed parts of the network.

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Figure 21. A map of the coiled unit. For bending the map to construct the unit, see the text.

Figure 22. Haeckelite sheets consisting of various units cells: (a) rectangular (R5,7); (b)hexagonal (H5,6,7), and (c) oblique (O5,6,7); (d) to (f) are the corresponding haeckelite tubes. (Reprinted with permission from Ref. [140] Terrones, H.; Terrones, M.; Terrones, H.; Banhart, F.; Charlier, J.-C.; Ajayan, P. M. Science 2000, 288, 1226-1229. © American Physical Society.)

How the coil cage in Fig 20 (exposed in grey) can be constructed from the carbon honeycomb is proposed in [134] and can be explained with Figure 21. Two pentagonal shaped parts (a pentagon ABCB1A1 and a divided in two pentagon FGDE - D1G1F1E1) are removed from the map of the perfect carbon network as a first step. The arrangement of those parts in respect to bond direction in the carbon rings can be parallel or perpendicular. Here will be given only the parallel (zig-zag) situation. After that follows seamless sticking between (i) the parts of the networks lying left hand at the lines CH and these lying right hand at the lines AB and BC with A1B1 and B1C; and (ii) the part obtained over KK1 with the part below DD1. As it was said, the helical pitch of the carbon layer forming the nanostructures governs conductivity. Because of that the helical cages can have behaviors as metal, semiconductor or

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semimetal, in dependence of the tiling of the honeycombs and the disclination centers on the helix surface. In [136-139] it is predicted, also, that carbon nanotori constructed by coalescing C60 molecules exhibit large magnetic moments when an external magnetic field is applied. The polygonal defects other than pentagons and heptagons may play certain role for more flexible connection between tubes, too, but their importance for explaining the observed nanostructures is not so important as that of the pentagonal and heptagonal defects. Because of that their role is not considered here. As it was mentioned, the local effect of the pentagon hexagon defect on the sheet flatness is small and because of that any structure with an equal number of pentagons and heptagons and threefold carbon connections has zero net curvature and remains flat. In [112] a purecarbon planar structure was proposed (carbon pentaheptite) composed of pentagons and heptagons (Fig. 22a). In difference to the other carbon modification it is with metallic behavior and is metastable. In this manner nanotubes can be made from carbon pentaheptite as from graphene and those can be written as (n,m) tubes, too. No change occurs in the carbon pentaheptite flatness when around the couple of pentagon heptagon rings hexagons are included. Such kind of layered carbon material, consisting of ordered arrangements of pentagons, hexagons, and heptagons, was proposed and studied in [140, 131]. From it a new family of nanostructures merges named Haeckelites [141-143]. The structures turn out to be more stable than C60 and can thus be regarded as energetically viable. All the structures are predicted to be metallic, exhibiting a high density of states at the Fermi energy, and to possess high stiffness like their conventional hexagonal graphite counterparts. The authors assigned the structures to three categories: (i) Rectangular (R5,7), containing only heptagons and pentagons paired symmetrically within a flat surface (as this shown in Fig. 22a); (ii) Hexagonal (H5,6,7), which exhibits repetitive units of three agglomerated heptagons, surrounded by alternating pentagons and hexagons (Fig. 22b) and (iii) Oblique (O5,6,7), containing pentalene and heptalene units bound together and surrounded by sixmembered rings (Fig. 22c). Tubular structures can be obtained by rolling of sheared networks from those sheets (Fig. 22d-f). Structures H5,6,7 and R5,7 are predicted to be flat, while O5,6,7 is corrugated. H5,6,7 is found to be the most stable structure, possessing an energy of 0.304 eV per atom with respect to graphene. O5,6,7 is the least stable (0.408 eV per atom above graphene), whereas the stability of R5,7 (0.307 eV per atom above graphene) nears that of H5,6,7. All Haeckelite nanotubes are metallic, independently of chirality and diameter. The authors concluded that the flat Haeckelites are comparatively stable (more stable than C60), which is a strong indication of the feasibility of Haeckelite nanotubes. The strain energy for these tubules follows a similar behavior to that of conventional graphitic nanotubes and, if synthesized, would have a range of remarkable electronic and mechanical properties that could make them useful in many practical applications. A new branch of the Haeckelite nanotube family can be generated by rolling up carbon network composed of pentagon-heptagon pairs and hexagons in proportion from 2:1 to 2:3 [143, 144] (Fig. 23). The stability of the fully relaxed 5/7-3x6-type sheet was found to be close to that of a graphene plane. The corresponding network is highly strained due to the presence of adjacent pentagons and heptagons. When the sheet is folded into a cylinder, part of the strain is released by having the pentagons protrude outward, and sometimes inward the tube. Due to that property, a large variety of stable structures can be constructed such as coiled, screw-like, curled, and pearl-necklace-like nanotubes (some examples are given in Fig. 24). The coiling appears naturally by rolling up a Haeckelite-like stripe and does not

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demand the insertion of additional polygons. Most frequently the structures are semiconductors with a gap of approximately 0.6 eV, and only some are found to be nearly metallic.

Figure 23. Regular tiling of the plane with hexagons and azulene units in proportion 3:2. The unit cell is defined by the primitive vectors aW1 and aW2. The bonds shared by two pentagons (“stressors”) have been highlighted by black dots. In (a), a 7x7 cell is shown to illustrate the wrapping vectors (7,0), (0,7), and (7,7) along the special directions (n,0), (0,n), and (n,n). The stressors closest to the wrapping vectors are encircled. Rows of stressors are indicated by the thin solid lines defining the angles α, β, and γ. These rows will generate helical patterns along the structure. In (b), the wrapping vectors (4,2), (5,2), (4,4), and (2,6) are represented by arrows. The asymmetrically placed stressors in the unit cell (5,2) are highlighted by encircling. In the unit cell (2,6), all stressors are symmetrically placed around the wrapping vector. The dashed-line circle corresponds to a critical nanotube radius of 0.45 nm above which some stressors can dip inward toward the structure. (Reprinted with permission from ref. [143] Lambin, Ph.; Mark G. I.; Biro L. P. Phys. Rev. B 2003, 67, 205413.1-205413. 9, © American Physical Society.)

Figure 24. Atomic structures of open-end nanotubes with indices (4,4), (4,0), and (0,4) viewed from a direction perpendicular to the axis (top) or close to the axis (bottom). (Reprinted with permission from ref. [143] Lambin, Ph.; Mark G.I.; Biro L.P. Phys. Rev. B 2003, 67, 205413.1-205413. 9, © American Physical Society.) Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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The possible rotation of the interior bond of 4 neighbour hexagons as it is shown in Fig. 25, was considered first from Stone-Wales and is known in chemistry as Stone-Wales isomerization (SW) [145]. It was recognized as a key step in mechanical relaxation, it is favorable thermodynamically, but it requires thermal activation because of its high energy barrier. After induction of SW the relaxation of the carbon network can follow two different ways in dependence of the specific conditions, which includes generally a further sequence of SW switches [146]. The first possibility is at larger stress and can be characterized by row of arising dislocation (Fig. 26a, b). The relaxation after SW results in a sequence of 7/8/7 rings, then 7/8/8/7, etc. If the stress continues to stimulate the bond breaking, arise bigger openings (big n-gons) in the network and occurs network breaking when the critical limit is overgone. The second possibility (Fig. 26c) in difference to the case described above, characterizes with merging of two oppositely directed pairs of 5/7 rings and occurs usually at high temperature [147]. Through thermal activation both the pairs can be separated and driven from elastic forces, can move away from each other, separated by a row of hexagons (Fig. 26d). That is a “plastic” response of the network which lowers its total strain energy. In Fig. 26e it is demonstrated how the process of SW switches in one nanotube changes its structure. The trajectories of the 5/7 pairs curve into the helices. It was discussed above (see Fig. 12) that the existence of 5/7 pair changes tube chirality and destroys its axial symmetry. As result of that the part of the tube between the both 5/7 pairs will be with new chirality (and with corresponding new electronic behaviors [146, 148]). The junction angles between the nanotube parts (as it was sad above) are lower than 15 degrees and depend on tube type. The movement of the 5/7 rings stops after removing the tension and because of the opposite orientation of the 5/7 pairs and the symmetry in their translation the structure freezes in Sshaped (as in Fig. 26e) or C-shaped configuration, in dependence of the 5/7 pair place on the nanotube surface. Outside of junction areas the nanotube preserves its initial structure and properties.

Figure 25. Stone-Wales isomerization (SW).

Figure 26. SW relaxation of the carbon network: (a, b) at larger stress; (c, d) at high temperature; (e) change of the nanotube structure through SW switches. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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Figure 27. Two sequences represent the two possible SW paths in which the C60-pairs coalesce into the sections of (5,5) tubes. For each path seven (2–8) intermediate structures are shown. Numbers 0 and 1 are the separate and the dimerized buckyballs. (Reprinted with permission from Ref. [153] Zhao, Y.; Lin, Y.; Yakobson, B. I. Phys. Rev. B 2003, 68, 233403.1-233403.4, © American Physical Society.)

The supramolecular forces and interactions are normally weak and the thermal influence can easily overpower and eliminate their effect. However, those can play significant stimulation role by joining of nanoobjects when acting particularly on big area. The possibility of joining nanoscale units in singular elements is very important in practical and theoretical point of view. It can give paths for building of parts for nano- and molecular electronics and mechanics, can help for understanding of fundamental mechanisms occurring on atomic level. Through coalescence processes can be explained merging of frequently observed bundles and ropes of carbon nanotubes and their transformations. Calculation through different methods [51] shows that the cap-to-cap, cap-to-wall, and wall-to-wall coalescence of fullerene cages and nanotubes [149]; between fullerene cages; between nanotubes [150, 151] can occur by means of versatile transformation of carbon bonds. Sequential atomic rearrangements leading to coalescence can be obtained by topological analysis and a search for the minimum-energy path for formation of covalent bonds between the both structures via Stone-Wales bond rotations. A connecting neck forms and grows gradually until the separate clusters are completely fused into a coherent unit. The most favorable path is determined by comparison of the calculated energies and is further supported by molecular dynamics simulations. The coalescence process was first investigated in the polymerization of C60 cages [152], where how two fullerene molecules through a series of SW rotations can complete seamless connection was demonstrated. The connecting path (even for identical carbon structures) depends on the initial docking or mutual orientation before the covalent bonding occurs. An example for coalescence of a pair of free buckyballs with another initial docking is given in [153] Fig. 27. The buckyballs have two aligned double bonds, each one in the center of a 5/6/6/5 pyracylene. This orientation conveniently permits standard polymerization as the first step. The figure exhibits the initial steps for both P)(P path and 5)(5' paths, and the energies computed for the intermediate structures. In both cases the highest energy structure is no. 2, where a purely SW sequence begins. In the 5)(5' path, the initial orientation of C60’s is identical to (5,5) SWNT caps, with a half of each C60 involved in coalescence. That is why 5)(5' path consists of only 16 SW steps, six steps shorter than the path P)(P, which requires global reorientation of the whole C60 structure [153].

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Figure 28. A sketch of nanopeapod and a coalescence of fullerene into the tube.

The nanopeapods are a family of carbon nanostructures where the condition for buckyballs coalescence is found to play significant role. Nanopeapods consist of an array of fullerene molecules (inner peas) and a single-wall carbon nanotube (SWCNT) (an outer pod), and all components are separated from the others by the van der Waals distance [154] (Example in Fig. 28a). The “peapods” concept was first proposed in [155] to explain the concomitant occurrence of double-wall carbon nanotubes [156, 157] (its inner tube had the diameter of C60). The filling fraction is over 90%. The high fraction is associated with the exothermic encapsulating process [158]. The encapsulated particles can be C60, C70, C82, and metal fullerenes [154, 159] in dependence of the preparation methods. Electron-beam irradiation [160] or heat treatment on these carbon composites induces coalescence of core fullerenes into an inside single-wall nanotube, with the diameter being 0.71 nm smaller than that of the outer “vessel” tube. In [152, 161] is expected that the coalescence is initiated via irradiation-induced vacancies under annealing conditions. The investigations through temperature treatment [154] was found that the inside fullerenes start to coalesce at 800ºC and complete transformation to a single-wall nanotube at 1200ºC. There is proposed that diameters of the synthesized nanotubes vary depending on the size of the outer vessel tubes and on the temperature during the treatment [162, 163]. The coalescence of the fullerene into the tube is similar to that of the free one (see Fig. 27) and is illustrated via the steps shown in Fig. 28b-28e. In [158] the theoretical study is presented of C60 coalescence inside SWCNTs induced by electron irradiation and/or thermal annealing. A combination of techniques (Monte Carlo and molecular dynamics) was used in conjunction with empirical potentials, tightbinding methods, and ab initio electronic structure calculations [164] to achieve a truly multiscale picture of various systems and prolonged simulation times. As a result it was obtained that irradiated carbon atoms are displaced by electrons from the cages by knock-on effects. This results in dangling bonds and vacancies creation which in turn can be annealed to minimize the total energy of the system. The annealing process results in coalescence of fullerene cages and, finally, in formation of a small-diameter corrugated nanotube. This nanotube is characterized by the presence of a number of n-gon (heptagonal or/and octagonal) rings acting as links between the original fullerene molecules. The existence of different n-gons linking the molecules and the usually various initial cage orientations to one and the same axis leads to the conclusion that it is difficult to anneal heptagons and octagons with pentagonal rings to achieve a perfect hexagonal tubular network. Therefore, this type of carbon tubules can be considered to be nongraphitizable. In the case of high temperature thermal annealing the system can reach a more stable configuration through a process driven by surface energy minimization. This stable state will consist mainly of a corrugated tubule nested inside the original SWCNT and composed of cages joined by heptagonal, octagonal or larger carbon rings characterized by local negative curvature [158].

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Figure 29. A computing devise from C60 in SWCNT.

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Figure 30. A sequence of individual SW rotations (following initial covalent linking) has been identified for an extensive gradual morphing, where a buckyball completely penetrates the wall of a nanotube. The rightmost column shows the axial views of the tube. (Reprinted with permission from ref. [153] Zhao, Y.; Lin, Y.; Yakobson, B.I. Phys. Rev. B 2003, 68, 233403.1-233403.4, © American Physical Society.)

The considered above conditions and mechanisms of coalescence between cages into SWCNT and the possibility for coalescence between the cages and the outer tube are important for the practical application of such structures as quantum memory and computing devices [165]. It is suggested that one or 3 cages (3 C60 joined in C180) could move inside SWCNT and stay in different positions serving as computing devices (Fig. 29).

Figure 31. High-resolution transmission electron micrograph showing the spontaneous coalescence of two SWCNT within a bundle of 14–15 tubes. (Reprinted with permission from ref. [57] Terrones, H.; Terrones, M.; Lopez-Urıas, F.; Rodrıguez-Manzo, J.A.; Mackay, A. Phil. Trans. R. Soc. Lond. A 2004, 362, 2039–2063, © American Physical Society.)

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In [153] it is demonstrated also how C60 can penetrate through the wall of a carbon nanotube via more than 200 SW rotations; how the tube restores its structure and C60 becomes unconnected inside the nanotube as in a “peapod” (see Fig. 30). The energy barrier for that is very high, but might be reduced by mechanical bending or other equivalent deformation or defects in the wall. Nanotubes which are located close to each other can interact by the van der Vaals forces that can be even strong enough to cause their coalescence in some cases. This was observed in a bundle of SWCNT [57, 150] (Fig.31). The process is stimulated by the presence of defects and high temperature and causes a bond conversion by undergoing through favorable ring transformations. It passes through a “zipping” mechanism and includes the creation of an interlink. The dynamic process of diameter doubling of single-walled carbon nanotubes by wall-to-wall coalescence is investigated in [151] with molecular dynamics simulations. The reaction path (the atomic rearrangement sequence) is suggested to be the armchair: some pairs of atoms of the both closely situated nanotubes (Fig. 32a) linked by a bond, perpendicular to the axis first rebond with their counterpairs of atoms in the other tube to form a four-member ring (Fig. 32b). When enough atoms necessary to accelerate the joining process are connected (Fig. 32c), the original bonds in the ring break, driven by the curvature. The two tubes will merge into one after all the original bonds in the rings break along the axis (Fig. 32d). The figures (e)–(i) show the reaction process for (5,0) + (5,0) tubes. This process is known as zipping process. For zigzag tube joining horizontal bonds parallel to the axis are necessary. Those can be produced via SW flips. The following steps are exactly the same as in the armchair case. The perfect zigzag lattice is recovered finally through defect reduction via SW rotations [151].

Figure 32. Coalescence process of the same two achiral nanotubes by colliding with each other (top and side views). The figures (a)–(d) are the snapshots of the reaction for (3,3) + (3,3), and the figures (e)–(i) are those of the reaction process for (5,0) + (5,0). The resulting structures were identified as armchair (6,6) and zigzag (10,0) nanotubes, respectively. No defects are introduced before the collision. (Reprinted with permission from ref. [151] Kawai, T.; Miyamoto, Y.; Sugino, O.; Koga, Y. Phys. Rev. Lett. 2002, 89 No 18, 085901.1-085901.4, © American Physical Society.)

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Figure 33. Coalescence of double wall nanotubes: (a, b) coalescence between the inner and the outer nanotubes and (c, d) coalescence forming “bicable”.

Double wall nanotubes consist from two concentric nanotubes obtained at special conditions [166-169]. In a bundle of double-walled carbon nanotubes [170-172] coalescence was also observed between the inner and the outer nanotubes (Fig. 33a, b) [10, 174] and between the outer nanotubes (creation of bicables [175], given in Fig. 33 c, d). One of the intensively studied processes in the last time is a bond breaking, which occurs by high tension on the nanotubes. It is known that the nanotubes are extremely flexible, resilient to severe and cyclic mechanical deformation, and very prone to mechanical buckling [176-182]. Those are able to undergo dramatic geometric transitions reversibly, without noticeable defects in their structure. The rippling deformations have been the subject of a number of theoretical investigations [183, 184], most dealing with the understanding of the morphology of the ripples and how their wavelength and amplitude scales with the imposed curvature and the nanotube diameter. In a case of a load in the axis direction, it distributes differently to the bonds according to their orientations relative to the axis. From that follows that the tube reactions will depend on the tube sort [185] because the angle α between the load and the bonds depends on it (Fig. 34a, b). If the tension is very high, the type of first lattice transformation depends qualitatively on temperature. The interdependencies of strain, symmetry, time, and temperature is investigated and summarized in [186]. Under high strain and at low temperature, thermal fluctuations appear insignificant and that result in breaking one of the highly elongated bonds. That is illustrated in Fig. 33c. This state is metastable with shallow energy minima, corresponding to the distinguishable neighbor broken bonds and the nanotube cracks (Fig. 34c, steps 1–3). At high temperature, lattice fluctuations promote bond rotation flip into almost longitudinal position (Fig. 34d—chiral nanotube and e-armchair nanotube), which lowers the energy. (The process has maximal effectiveness at armchair nanotubes). This plastic deformation through SW rotation becomes favorable under tension, but occurs rarely because of its high activation barrier. If the extension is over the critical limit the bonds break, arise big openings in the network and occurs network breaking (Fig. 34f). Different models try to explain the arising and the initial growth of carbon nanostructures [187-191]. The achievement of correct atom arrangement in the merging nanostructure is a complicated process going through long sequence of atom bonding, bond transformation and bond breaking. The defects which arise and disappear during this course of action are short

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living; they are an object of detailed consideration in those models and will not be discussed here. In further considerations it will be shown only that stable defects incorporated already in the nanostructures can also influence the growth of nanostructures in the neighbor carbon layers in ways diverse from these considered above. That is a frequent phenomenon in the multi wall carbon nanotubes (MWCNT) growth. In some cases the concentric layers of MWCNT merge together and grow simultaneously, in other each layer acts as a template for the sequent. During the simultaneous growth the atoms which achieve opened layer ends are under the influence on the attracting forces from the neighbor layers and at suitable conditions can connect both layers, seamed them in one closed surface [192, 193] (Fig. 35). That stimulates the connection of the next closely laying layers and so one, to do the same. Such development of growth is experimentally observed in [193, 174].

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Figure 34. Nanotube reaction under tension: (a, b) the load on the bonds in dependence of the angle, (d, e) the bond rotation in the cases of chiral and armchair nanotube and (f) the network breaking.

Figure 35. Sketch of nanotubes with shells seamed one to other at the end of the tube. (Reprinted with permission from [193] Iijima, S. MRS Bulletin 1994, 19 No. 12, 46, Figure 6, © American Physical Society.)

An example of sequential growth of layers in MWCNT can be found in Fig. 36a when the growth is simultaneously stopped with the termination of their production. The structure, the diameter and the electronic properties of those completed layers produced with some time discrepancy (LWTD) depend strongly on the layer before them [194], playing a role of a “template”. When LWTDs reach during their growth a place with defects in the “template”, those react differently in dependence on the connection force between both networks. Such situations are frequently observed experimentally at the ends of MWCNT where defects cause shape disclination. Examples of layers fully copying the “template” shape (Fig. 36b), partially copying (Fig, 36c-e) and independent of defect (Fig. 36f) are exposed.

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a

c

b 100 nm

100 nm

200 nm

f

d 100 nm

100 nm

e 250 nm

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Figure 36. Some specific structures of MWCNT ends, demonstrating layer interactions.

Figure 37. Bonding structure of carbon dimer defects on a sample armchair (5,5) nanotube. (a) StoneWales defect, the result of a bond rotation; (b–e) ad-dimer defects. (Reprinted with permission from ref. [196] Sternberg, M.; Curtiss, L. A.; Gruen, D. M.; Kedziora, G.; Horner, D. A.; Redfern, P. C.; Zapol, P., Phys. Rev. Lett. 2006, 96, 075506.1-075506.4. © American Physical Society.)

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Figure 38. Structure of multiple C2 adsorbates: (a) local expansion of a (5,5) CNT; (b) alternating adsorbates around the circumference of a (5,5) tube; (c) a (5,5) CNT with alternating adsorbates in an axial direction resulting in a zipper tube, and (d) a (6,6) CNT with multiple staggered “zippers.” (Reprinted with permission from ref. [196] Sternberg, M.; Curtiss, L.A.; Gruen, D.M.; Kedziora, G.; Horner, D.A.; Redfern, P.C.; Zapol, P., Phys. Rev. Lett. 2006, 96, 075506.1-075506.4. © American Physical Society.)

The adsorption of carbon dimers on carbon nanotubes leads to a rich spectrum of structures and electronic structure modifications [195] and can be considered opposite to the situation above. Barriers to the formation of carbon dimer-induced defects are calculated and found to be considerably lower than those for the Stone-Wales defect [196]. The defects and the electronic states initiated through that depend on tube type and size. So, via adsorption of dimers on nanotubes of different structures, there have been attempts to engineer particles with electronic properties interesting for nanoelectronics. The change in the rings of nanotubes (with different carbon network orientation to their axis) by adsorption of carbon dimers is illustrated in Fig. 37 and in Fig. 38 in accordance with the considerations in [196]. The dimer position on the carbon network is shown with a thick line (Fig. 37 a–c is for only one dimer). In Fig. 37d, the dimer is adsorbed perpendicular to the network, and in Fig. 37e two adsorbed dimmers are shown. Pentagons and heptagons merged in the carbon nanostructures were considered in this chapter as defects caused by irregular atom arrangement during growth. From this point of view, the induced investigations attempt to pinpoint the reasons for bad atom arrangement and how those can be governed. From another point of view, the arrangement of carbon atoms in rings different from 6 is normal in nature, and in this case the structures with incorporated n-gons are to be treated as a new kind of structure composed of parts equal in atom arrangement joined with suitable carbon connections. The interest in those structures is not focused more on n-gons but on the structural properties of the entire complex. Regardless of how the differences in carbon atom arrangement are studied or viewed, they remain an interesting and a very important subject for study because of the utility of nanoparticles as elements in nanoscale devices and composite materials.

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The contentious progress in nanosciense and nanotechnology stimulates the creation of nanostructures with new properties and functions. The level of development at the present moment allows complicated nanostructures to be constructed and their planned properties to be realized via suitable conditions by using self-assemble processes and atom-by-atom manipulation. The more the demand for specific particle properties increases, the more complicated become the structures and the more frequently the tailoring of their structure is necessary through incorporation of different n-gons and their combinations. That will inspire the further study of the n-gon role, with the exception of pentagons and heptagons, on carbon nanoparticle properties.

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

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[10] [11] [12] [13] [14] [15] [16] [17]

[18] [19] [20] [21] [22]

Kroto, H. Science, 1988, 242, 1739-1745. Curl, RF. Carbon, 1992, 30 No 8, 1149-1155. Curl, RF; Smalley, RE. Science, 1988, 242, 1017-1021. Curl, RF; Smalley, RE. Scientific American, 1991, 55-62. Kretschmer, W; Huffman, DR. Carbon, 1992, 30 No 8, 1143-1147. Wilson, MA; Pang, LSK; Willett, GD; Fisher, KJ; Dance, IG. Carbon, 1992, 30 No 4, 675-693. Fowler, PW. J. Phys. Chem. Solids, 1993, 54 No 12, 1825-1833. Iijima, S. Nature, 1991, 354, 56-58. Iijima, S; Yudasaka, M; Yamada, R; Bandow, S; Suenaga, K; Kokai, F; Takahashi, K. Chem. Phys. Lett, 1999, 309, 165-170. Colomer, JF; Henrard, L; Van Tendeloo, G; Lucas, A. Lambin, Ph. J. Mater. Chem., 2004, 14, 603-606. Sano, M; Kamino, A; Okamura, J; Shinkai, S. Science, 2001, 293, No 5533, 1299-1301. Laszlo, I; Rassat, A. Int. J. Quantum Chem., 2001, 84, 136-139. Chen, X; Motojima, S. Carbon, 1999, 37, 1817-1823. Kretschmer, W; Lamb, LD; Fostiropoulos, K; Huffman, DR. Nature, 1990, 347, 354357. Ebbesen, TW; Ajayan, PM. Nature, 1992, 358, 220-222. Parker, DH; Wurz, P; Chatterjee, K; Lykke, KR; Hunt, JE; Pellin, MJ; Hemminger, JC; Gruen, DM; Stock, LM. J. Am. Chem. Soc., 1991, 113, 7499- 7503. Colbert, DT; Zhang, J; McClure, SM; Nikolaev, P; Chen, Z; Hafner, JH; Owens, DW; Kotula, PG; Carter, CB; Waever, JH; Rinzler, AG; Smalley, RE. Science, 1994, 266, 1218-1221. Blank, VD; Polyakov, EV; Kulnitsky, BA; Nuzhdin, AA; Alshevskiy, Yu. L; Bangert, U; Harvey, AJ; Davock, HJ. Thin Solid Films, 1999, 346, 86-90. Seraphin, S; Zhou, D. Appl. Phys. Lett, 1994, 64 No 16, 2087-2089. Satishkumar, BC; Govindaraj, A; Sen, R; Rao, CNR. Chem. Phys. Lett, 1998, 293, 4752. Moisala, A; Nasibulin, AG; Brown, DP; Jiang, H; Khriachtchev, L; Kauppinen, EI. Chemical Engineering Science, 2006, 61, 4393-4402. Bhowmick, R; Clemens, BM; Cruden, BA. Carbon, 2008, 46, 907-922.

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

Morphology Changes in Carbon Nanoparticles Due to Different Atom… [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]

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[37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60]

281

Cao, P; Zhu, D; Liu, W; Ma, X. Vacuum, 2007, 81, 953-957. Cao, G; Liu, D. Advances Colloid Interface Science, 2008, 136, 45-64. Ajayan, P; Ravikumar, V; Charlier, JC. Phys. Rev. Lett, 1998, 81, 1437-1440. Abergel, DSL; Falko, VI. Phys. Rev. B, 2007, 75, 155430. 1-155430.5. Bostwick, A; Ohta, T; Seyller, T; Horn, K; Rotenberg, E. Nature Physics, 2007, 36-40. Cortijo, A; Vozmediano, MAH. Nuclear Physics B, 2007, 763, 293-308. Falkovsky, LA; Varlamov, A A. Eur. Phys. J. B, 2007, 56, 281-284. Gunlycke, D; Lawler, HM; White, CT. Phys. Rev., B, 2007, 75, 085418.1- 085418.5. Katsnelson, MI. Materials Today, 2007, 10, 20-27. Katsnelson, MI; Novoselov, KS. Solid State Communications, 2007, 143, 3-13. Katsnelson, M. Eur. Phys. J. B, 2007, 51, 157-160. Meyer, JC; Geim, AK; Katsnelson, MI; Novoselov, KS; Booth, TJ; Roth, S. Nature, 2007, 446, 60 -63. Meyer, JC; Geim, AK; Katsnelson, MI; Novoselov, KS; Obergfelle, D; Rothe, S; Girita, C; Zettl, A. Solid State Communications, 2007, 143, 101-109. Pereira, VM; Guinea, F; Lopes dos Santos, JMB; Peres, NMR. Phys. Rev. Lett, 2006, 96, 036801. 1-036801.4. Wang, WL; Meng, S; Kaxiras E. Nano Letters, 2008, 8 No1, 241-245. Bunch, JS; van der Zande, AM; Verbridge, SS; Frank, IW; Tanenbaum, DM; Parpia, J. M; Craighead, HG; McEuen, PL. Science, 2007, 315, 490-493. Rycerz, A; Twozydlo, J; Beenakker, CW. J. Nature Physics, 2007, 3, 172- 176. Wang, X; Zhi, L; Mullen, K. Nano Letters, 2008, 8 No1, 323-327. Geim, AK; Novoselov, KS. Nature, 2007, 6, 183-191. Nomura, K; MacDonald, AH. Phys. Rev. Lett, 2006, 96, 256602.1-256602.4. Ohta, T; Bostwick, A; Seylle, T; Horn, K; Rotenberg, E. Science, 2006, 313, 951-954. Kobayash, K. Phys. Rev. B, 2000, 61, 8496-8500. Ihara, S; Itoh, S; Akagi, K; Tamura, R; Tsukada, M. Phys. Rev. B, 1996 54, No 20, 14 713-14 719. Seung, HS; Nelson, DR. Phys. Rev. Lett, 1988, 38 No 2, 1005-1018. Siber, A. Phys. Rev., E 73, 2006, 061915.1- 061915.15. Siber, A. Eur. Phys. J. B, 2006, 53, 395-400. Wang, CZ; Ho, KM. J. Comput. Theor. Nanosci, 2004, 1 No. 1, 3-17. Ping, ZZ; Yong-Da, G. Carbon, 1996, 34 No 2, 173-178. Chernozatonskii, LA. Chem. Phys. Lett, 1998, 29, 257-260. Scuseria, GE. Chem. Phys. Lett, 1992, 195, 534-536. Mackay, L; Terrones, H. Nature, 1991, 352, 762-763. Lenosky, T; Xvierver, G; Tetye, M; Elser, V. Nature, 1992, 355, 333-335. Terrones, H; Terrones, M; Lopez-Urıas, F; Rodrıguez-Manzo, JA; Mackay, A. Phil. Trans. R. Soc. Lond, A, 2004, 362, 2039-2063. Phillips, R; Drabold, DA; Lenosky, T; Adams, GB; Sankey, OF. Phys. Rev. B, 1992, 46 No3, 1941-1943. Terrones, H; Mackay, AL. Carbon, 1992, 30 No 8, 1251-1260. Vanderbilt, D; Tersoff, J. Phys. Rev. Lett, 1992, 68 No 4, 511-513. Terrones, H; Terrones, M. New Journal Physics, 2003, 5, 126. 1-126.37. Benedek,G; Vahedi-Tafreshi, H; Barborini, E; Piseri, P; Milani, P; Ducati, C; Robertson, J. Diamond Relat. Mater, 2003, 12, 768-773.

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

282

N. Koprinarov and M. Konstantinova

[61] Ebbesen, TW. Physics Today, June 1996, 26-32. [62] Terrones, H; Terrones, M; Morán-López , J. L. Current Science, 2001, 81 No 8, 10111029. [63] Terrones, H; Terrones, M. J. Phys. Chem. Solids, 1997, 58, 1789-1796. [64] Torrens, FJ. Molecular Structure Theochem, 2004, 709,135-142. [65] Cozzi, F; Powell, WH; Thilgen, C. Pure Appl. Chem., 2005, 77 No 5, 843-923. [66] Albertazzi, E. Domene, C; Fowler, PW; Heine, T; Seifert G; Van Alsenoy, C; Zerbetto, F. Phys. Chem. Chem. Phys., 1999, 1, 2913-2918. [67] Fowler, PW; Manolopoulos, DE; Rya, RP. Carbon, 1992, 30 No 8, 1235-1250. [68] Hoenlein, W; Kreupl, F; Duesberg, GS; Graham, AP; Liebau, M; Seidel, R; Unger, E. Materials Science and Engineering, C 2003, 23, 663-669. [69] Liang, YX; Wang, TH. Physica, E, 2004, 23, 232-236. [70] Burke, PJ. Solid-State Electronics, 2004, 48, 1981-1986. [71] Kumar, TP; Ramesh, R; Lin, YY; Fey, GTK. Electrochemistry Communications, 2004, 6, 520-525. [72] Jung, H; Kim, J; Hahn, J; Suh, JS. Chem. Phys. Lett, 2005, 402, 535-538. [73] Postma, HW. Ch; Teepen, T; Yao, Z; Grifoni, M; Dekker, C. Science, 2001, 293 No 5527, 76-79. [74] Suzuki, M; Tsuya, D; Moriyama, S; Fuse, T; Aoyag, Y; Ishibashi, K. Physica E, 2004, 24, 10-13. [75] Haddon, RC; Perel, AS; Morris, RC; Palstra, TTM; Hebard, AF. Appl. Phys. Lett, 1995, 67 No 1, 121-123. [76] Nihey, F; Hongo, H., Ochiai, Y; Yudasaka, M; Iijima, S. Jpn. J. Appl. Phys., 2003, 42, L1288-L1291. [77] Rueckes, Th; Kim, K; Joselevich, E; Tseng, GY; Cheung, Chin-Li; Lieber, Ch. M. Science, 2000, 289 No 5476, 94-97. [78] Bachtold, A; Hadley, P; Nakanishi, T; Dekker, C. Science, 2001, 294 No 5545, 13171320. [79] Mateiu, R; Davis, ZJ; Madsen, DN; Molhave, K; Boggild, P; Rassmusen, AM; Brorson, M; Jacobsen, CJH; Boisen, A. Microelectronic Engineering, 2004, 73-74, 670-674. [80] Guo, L; Wang, R; Xu, H; Liang, J. Physica E, 2005, 27, 240-244. [81] Liu, L; Zhang, Y. Sensors Actuators A, 2004, 11, 6394-397. [82] Ionescu, R; Espinosa, EH; Sotter, E; Llobet, E; Vilanova, X; Correig, X; Felten, A; Bittencourt, C; Van Lier, G; Charlier, JC; Pireaux, J. J. Sensors Actuators B, 2006, 113, 36-46. [83] Sayago, I; Terrado, E; Lafuente, E; Horrillo, MC; Maser, WK; Benito, AM; Navarro, R; Urriolabeitia, EP; Martinez, MT; Gutierrez, J. Synthetic Metals, 2005, 14815-14819. [84] Lia, G; Liao, JM; Hu, GQ; Ma, NZ; Wu, P. J. Biosensors Bioelectronics, 2005, 20, 2140-2144. [85] Jang, YT; Moon, SI; Ahn, JH; Lee, YH; Ju, BK. Sensors Actuators, B, 2004, 99, 118122. [86] Collins, PG; Bradley, K; Ishigami, M; Zett, A. Science, 2000, 287 No 5459, 18011804. [87] Bienfaita, M; Zeppenfeld, P; Dupont-Pavlovsky, N; Murisa, M; Johnson, M; Wilsone, T; De Piese, M; Vilchese, OE. Physica B, 2004, 350, e423-e426. [88] Xu, Z; Chen, X; Qu, X; Jia, J; Dong, S. Biosensors Bioelectronics, 2004, 20, 579- 584.

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[89] Ghosh, S; Sood, AK; Kumar, N. Science, 2003, 299, 1042-1045. [90] Kim, Ph; Lieber, Ch. M. Science, 1999, 286 No 5447, 2148-2150. [91] Landi, BJ; Raffaelle, RP; Heben, MJ; Alleman, JL; VanDerveer, W; Gennett, T. Materials Science Engineering B, 2005, 116, 359-362. [92] Baughman, RH; Cui, Ch; Zakhidov, AA; Iqbal, Z; Barisci, JN; Spinks, GM; Wallace, GG; Mazzoldi, A; De Rossi, D; Rinzler, AG; Jaschinski, O; Roth, S; Kertesz, M. Science, 1999, 284, No 5418, 1340-1344. [93] Portet, C; Taberna, PL; Simon, P; Flahaut, E. Journal of Power Sources, 2005, 139, 371-378. [94] Qin, X; Durbach, S; Wu, GT. Letters to the Editor / Carbon, 2004, 42, 423-460.Ye, J.S; Liu, X; Cui, HF; Zhang, WD; Sheu, FS; Lim, TM. Electrochemistry Communications, 2005, 7249-7255. [95] Chen, QL; Xue, KH; Shen, W; Tao, FF; Yin, SY; Xu, W. Electrochimica Acta, 2004, 49, 4157-4161. [96] Park, JH; Choi, JH; Moon, JS; Kushinov, DG; Yoo, JB; Park, Ch.Y; Nam, JW; Lee, Ch. K; Park, JH; Choe, DH. Carbon, 2005, 43, 698-703. [97] Jin, Ch; Wang, J; Wang, M; Su, J; Peng, LM. Carbon, 2005, 43, 1026-1031. [98] Fursey, GN; Novikov, DV; Dyuzhev, GA; Kotcheryzhenkov, AV; Vassiliev, PO. Applied Surface Science, 2003, 215, 135-140. [99] Huha, Y; Leea, JY; Lee, Ch. J. Thin Solid Films, 2005, 475, 267-270. [100] Uha, HS; Lee, SM; Jeon, PG; Kwak, BH; Park, SS; Kwon, SJ; Cho, ES; Ko, SW; Lee, JD; Lee, Ch. G. Thin Solid Films, 2004, 462-463, 19-23. [101] de Heer, WA; Chetelain, A; Ugarte, D. Science, 1995, 270, 1179-1180. [102] Wildoer, JWG; Venema, LC; Rinzler, AG; Smalley, RE; Dekker, C. Nature, 1998, 391, 59-62. [103] Hamada, N; Sawada, S; Oshyama, A. Phys. Rev. Lett, 1992, 68 No 10, 1579- 1581. [104] Hutchison, JL; Kiselev, NA; Krinichnaya, EP; Krestinin, AV; Loutfy, RO; Morawsky, AP; Muradyan, VE; Obraztsova, ED; Sloan, J; Terekhov, SV; Zakharov, DN. Carbon, 2001, 39, 761-770. [105] Blank, VD; Gorlova, IG; Hutchison, JL; Kiselev, NA; Ormont, AB; Polyakov, EV; Sloan, J; Zakharov, DN; Zybtsev SG. Carbon, 2000, 38, 1217- 1240. [106] Grado-Caffaro, MA; Grado-Caffaro, M. Physica A, 2008, 387, 445-448. [107] Charlier, JC. Acc. Chem. Res., 2002, 35, 1063-1069. [108] Kim, H; Lee, J; Kahng, SJ; Son, YW; Lee, SB; Lee, CK; Ihm, J; Kuk, Y. Phys. Rev. Lett, 2003, 90 No 21, 216107.1-216107.4. [109] Crespi, VH; Benedict, LX; Cohen, ML; Louie, SG. Phys. Rev. B, 1996, 53 No 20, 13 303-13 305. [110] Saito, R; Dresselhaus, G; Dresselhaus, M. Phys. Rev. B, 1996, 53, 2044-2050. [111] Iijima, S; Ichihashi, T; Ando, Y. Nature, 1992, 356, 776-778. [112] Weldon, DN; Blau, WJ; Zandbergen, HW. Chem. Phys. Lett, 1995, 241, 365- 372. [113] Lauginie, P; Conard, J. J. Phys. Chem. Solids, 1997, 58, 1949-1963. [114] Lambin, Ph; Fonseca, A; Vigneron, JP; Nagy, JB; Lucas, AA. Chem. Phys. Lett, 1995, 245, 85-89. [115] Chico, L; Crespi, VH; Benedict, LX; Louie, SG; Cohen, ML. Phys. Rev. Lett, 1996, 76, 971-974.

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

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N. Koprinarov and M. Konstantinova

[116] Meunier, V; Nardelli, MB; Roland, C. Bernholc, J. Phys. Rev. B, 2001, 64, 195419.1195419.7. [117] Charlier, J; Ebbesen, T; Lambin, Ph. Phys. Rev. B, 1996, 53 No16, 11108-11113. [118] Dunlap, BI. Phys. Rev. B, 1992, 46, 1933-1936. [119] Chernozatonskii, LA. Phys. Lett. A, 1992, 28, 173-176. [120] Zhou, D; Seraphin, S. Chem. Phys. Lett, 1995, 238, 286-289. [121] Nagy, P; Ehlich, R; Biro, LP; Gyulai, J. Appl. Phys., A, 2000, 70, 481-483. [122] Biro, LP; Ehlich R; Osvath, Z; Koos, A; Horvath, ZE; Gyulai, J; Nagy, JB. Mater. Sci. Eng. C, 2002, 19, 3-7 . [123] Terrones, M; Banhart, F; Grobert, N; Charlier, JC; Terrones H; Ajayan PM. Phys. Rev. Lett, 2002, 89, 75505.1-75505.4. [124] Meng, FY; Shi, SQ; Xu, DS; Chan, CT. J. Chem. Phys., 2006, 124, 024711.1024711.4. [125] Ihara, S., Itoh, S; Kitakami, JI. Phys. Rev. B, 1993, 47 No 19, 12908-12911. [126] Chen, X; Motojima, S. Carbon, 1999, 37, 1817-1823. [127] Gao, R; Wang, ZL; Fan, S. J. Phys. Chem., B, 2000, 104, 1227-1234. [128] Zhang, M; Nakayama, Y; Pan L. Jpn. J. Appl. Phys., 2000, 39, L1242-L1244. [129] Su, CJ; Hwang, DW; Lin, SH; Jin, BY; Hwang, LP. Phys. Chem. Comm., 2002, 5, 3436. [130] Laszlo, I; Rassat, A. J. Chem. Inf. Comput. Sci., 2003, 43, 519-524. [131] Akagi, K; Tamura, R; Tsukada, M. Phys. Rev. B, 1995, 53 No 4, 2114- 2120. [132] Ihara, S; Itoh, S; Kitakami, J. Phys. Rev. B, 1993, 48 No 8, 5643-5647. [133] Rodriguez-Manzo, JA; Lopez-Urias, F; Terrones, M; Terrones, H. Nano Lett, 2004, 4, 2179-2183. [134] Terrones, H; Lopez-Urias F; Munoz-Sandova, lE; Rodriguez- Manzo, J; Zamudio, A; Elias A; Terrones, M. Solid State Sci., 2006, 8, 303-320. [135] Haddon, RC. Nature, 1997, 388, 31-32. [136] Haddon, RC. Nature, 1995, 378, 249-255. [137] Terrones, H; Terrones, M; Terrones, H; Banhart, F; Charlier, JC; Ajayan, PM. Science, 2000, 288, 1226-1229. [138] Biro, LP; Mark, GI; Koos, AA; Horvath, ZE; Szabo, A; Fonseca, A; Nagy, JB; Colomer, JF; Lambin, Ph; Meunier V; Charlier JC; Bedoya-Martınez, ON; Hernandez E. Fullerenes, Nanotubes, and Carbon Nanostructures, 2005, 13, 523-533. [139] Lambin, Ph; Biro, LP. New J. Phys., 2003, 5, 141.1-141.14. [140] Lambin, Ph; Mark, GI; Biro, LP. Phys. Rev. B, 2003, 67, 205413.1-205413. 9. [141] Biro, LP; Mark, GI; Koos, AA; Nagy, JB; Lambin, Ph. Phys. Rev.B, 2002, 66, 165405.1-165405.6. [142] Stone, AJ; Wales, D. J. Chem. Phys. Lett., 1986, 128, 501-504. [143] Yakobson, BI; Couchman, LS. Dekker Encyclopedia Nanoscience Nanotechnology, Schwarz, JA; Contescu, CI; Putyera, K. ISBN: 978-0-8247- 5055-8 hardback 978-08247-5046-6 electronic; Taylor & Francis: London, UK, 2004. [144] Chico, L; Sancho, MPL; Munoz, MC. Phys. Rev. Lett, 1998, 81 No 6, 1278- 1281. [145] Lambin, Ph.; Meunier, V. Applied Physics A Materials Science Processing, 1999, 68, 263-266. [146] Zhao, Y; Yakobson, BI; Smalley, RE. Phys. Rev. Lett, 2002, 88 No 18, 185501.1185501.4.

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Morphology Changes in Carbon Nanoparticles Due to Different Atom…

285

[147] Terrones, M; Ajayan, PM; Banhart, F; Blase, X; Carroll, DL; Charlier, JC; Czerw, R; Foley, B; Grobert, N; Kamalakaran, R; Kohler-Redlich, P; Rühle, M; Seeger, T; Terrones, H. Appl. Phys. A, 2002, 74, 355-361. [148] Kawai, T; Miyamoto, Y; Sugino, O; Koga, Y. Phys. Rev. Lett, 2002, 89 No 18, [149] 085901.1- 085901.4. [150] Strout, D. L; Murry, R. L; Xu, Ch; Eckhoff, W. C; Odom, G. K; Scuseria, G. E. [151] Chem. Phys. Lett. 1993, 214, 576-582. [152] Zhao, Y; Lin, Y; Yakobson, B. I. Phys. Rev. B 2003, 68, 233403.1- 233403.4. [153] Smith, B.W; Luzzi, D. E. Chem. Phys. Lett. 2000, 321, 169-174. [154] Smith, B. W; Monthioux, M; Luzzi, D. E. Chem. Phys. Lett. 1999, 315, 31-36. [155] Smith, B. W; Monthioux, M; Luzz, D. E. Nature 1998, 396, 323-324. [156] Burteaux, B; Claye, A; Smith, B. W; Monthioux, M; Luzzi, D. E; Fischer, J. E. [157] Chem. Phys. Lett. 1999, 310, 21-24. [158] Hernandez, E; Meunier, V; Smith, B. W; Rurali, R; Terrones, H; Nardelli, M. [159] B; Terrones, M; Luzzi, D. E; Charlierr, J.-C. Nano Lett. 2003, 3 No 8, 1037- 1042. [160] Monthioux, M. Carbon, 2002, 40, 1809-1823. [161] Sloan, J; Dunin-Borkowski, RE; Hutchison, JL; Coleman, KS; Williams, VC; Claridge, JB; York, APE; Xu, C; Bailey, SR; Brown, G; Friedrichs, S; Green MLH. Chem. Phys. Lett, 2000, 316, 191-198. [162] Zhao, Y; Smalley, RE; Yakobson, BI. Phys. Rev. B, 2002, 66, 195409.1- 195409.9. [163] Arrondo, C; Monthioux, M; Kishita, K; Lay, ML. Euroconference on Electronic Properties of Novel Materials, AIP Conference Proceedings, 2005, 786, 329-332. [164] Kang, JW; Lee, JH; Lee, HJ; Hwang, HJ. Physica E, 2005, 25, 347-355. [165] Hornbaker, DJ; Kahng, SJ; Misra, S; Smith, BW; Johnson, AT; Mele, EJ; Luzzi, DE; Yazdani, A. Science, 2002, 295 No 5556, 828-831. [166] Kim, DH; Sim, HS; Chang, K. J. Phys. Rev. B, 2001, 64, 115409.1-115409.7. [167] Ren, W; Li, F; J. Chen, S. Bai, HM. Cheng, Chem. Phys. Lett, 2002, 359, 196-202. [168] Li, WZ; Wen, JG; Sennett, M; Ren, ZF. Chem. Phys. Lett, 2003, 368, 299-306. [169] Lyu, SC; Lee, TJ; Yang CW; Lee, CJ. Chem. Commun, 2003, 1404-1405. [170] Ci, L; Rao, Z; Zhou, Z; Tang, D; Yan, X; Liang, Y; liu, D; Yuan, H; Zhou, W; Wang, G; Liu W; Xie, S. Chem. Phys. Lett, 2002, 359, 63-67. [171] Flahaut, E; Basca, R; Peigney A; Laurent, C. Chem. Commun, 2003, 1442- 1443. [172] Wei, J; Ci, L; Jiang, B; Li, Y; Zhang, X; Zhu, H; Xu, C; Wu, D. J. Mater. Chem., 2003, 13, 1340-1344. [173] Zhou, Z; Ci, L; Chen, X; Tang, D; Yan, X; Liu, D; Liang, Y; Yuan, H; Zhou, W; Wang, G; Xie, S. Carbon, 2003, 41, 337-342. [174] Zhang, YC; Chen, X; Wang X. Composites Science Technology, 2008, 68, 572- 581. [175] Rotkin, SV; Gogotsi, Y. Mat. Res. Innovat, 2002, 5, 191-200. [176] Endo, M; Hayash, T; Muramatsu, H; Kim, YA; Terrones, H; Terrones, M; Dresselhaus, MS. Nano Letters, 2004, 4 No 8, 1451-1454. [177] Fyta, MG; Kelires, PC. Fullerenes, Nanotubes, Carbon Nanostructures, 2005, 13, 1320. [178] Yakobson, BI; Brabec, CJ; Bernholc, J. Phys. Rev. Lett, 1996, 76 No 14, 2511- 2514. [179] Garg, A; Han, J; Sinnott, SB. Phys. Rev. Lett, 1998 No 11, 2260-2263. [180] Srivastava, D; Menon, M; Cho, K. Phys. Rev. Lett, 1998, 83 No 15, 2973-2976. [181] Nardelli, MB; Yakobson, BI; Bernholc, J. Phys. Rev., B 1998, 57 No 8, R4277- R4280.

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Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

[182] Nardelli, MB; Yakobson, BI; Bernholc, Phys. Rev. lett, 1998, 23, 4656-4659. [183] Rochefort, A; Avouris, P; Lesage, F; Salahub, DR. Phys. Rev. B, 1999, 60 No 19, 13 824-13 830. [184] Arroyo, M; Belytschko, T. Phys. Rev. Lett, 2003, 91 No 21, 215505.1- 215505.4. [185] Liu, JZ; Zheng, Q; Jiang, Q. Phys. Rev. Lett, 2001, 86 No 21, 4843-4846. [186] Orlikowski, D; Nardelli, MB; Bernholc, J; Roland, C. Phys. Rev. B, 2000, 61, 1419414203. [187] Dumitrica, T; Hua, M; Yakobson, BI. PNAS, 2006, 103 No 16, 6105-6109. [188] Churilov, GN; Novikov, PV; Tarabanko, VE; Lopatin, VA; Vnukova, NG; Bulina, V. Carbon, 2002, 40, 891-896. [189] Ching-Hwa-Kiang; Goddard, WA. Phys. Rev. Lett, 1996, 76 No 14, 2515-2518. [190] Amerlinckx, S; Bernaerts, D; Zhang, XB; Van Tendeloo, G; Van Landuyt, J. Science, 1995, 267, 1334-1338. [191] Robertson, DH; Brenner, DW; White, CT. J. Phys. Chem., 1992, 96, 6133- 6135. [192] Kawai, T; Miyamoto, Y; Sugino, O; Koga, Y. Phys. Rev., B, 2002, 66, 033404.1033404 .4. [193] Iijima, S; Ajayan, PM; Ichihashi, T. Phys. Rev. Lett, 1992, 69 No 21, 3100-3103. [194] Iijima, S. MRS Bulletin, 1994, 19 No 12, 46. [195] Koprinarov, N; Marinov, M; Pchelarov, G; Konstantinova, M; Stefanov R. J. Phys.Chem.,1995, 99, 2042-2047. [196] Orlikowski, D; Nardelli, MB; Bernholc, J; Roland, C. Phys. Rev. Lett, 1999, 83, 41324135. [197] Sternberg, M; Curtiss, LA; Gruen, DM; Kedziora, G; Horner, DA; Redfern, PC; Zapol, P. Phys. Rev. Lett, 2006, 96, 075506.1-075506.4.

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Chapter 6

GOLD NANOPARTICLE LABELLED DNA HAIRPIN GRAFTING ON TRANSPARENT AND CONDUCTIVE OXIDE (TCO) FILMS: CHARACTERIZATION OF GRAFTING AND HYBRIDIZATION V. Stambouli1*, V. Lavalley1, A. Bionaz1, P. Chaudouët1, L. Rapenne1, H. Roussel1, A. Laurent2, R. Jones3 and P. J. Pigram3

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1

Laboratoire des Matériaux et du Génie Physique, Grenoble INP Minatec, 3 parvis Louis Néel, BP 257, 38016 Grenoble Cédex 1, France 2 Biomérieux, 5 rue des Berges, 38000 Grenoble, France 3 Centre for Materials and Surface Science, Department of Physics, La Trobe University, Melbourne, Victoria 3086, Australia

ABSTRACT Biosensors and biochips can be hybrid nanobiosystems involving different kinds of components, i.e. solid surface, bio-molecules and nanoparticles. These components are confined in a very small area (nanometre range). It is expected that interactions are produced between both components due to their close proximity. So to optimize the performance of these biosensors, it is very important to get a deeper insight of their surface characteristics. In this context, nanoparticles linked to DNA strands (in a ratio of 1:1) immobilized on a solid surface give the opportunity to combine complementary techniques to characterize the hybrid system. A typical example will be illustrated in this study. We have grafted DNA hairpins at their 3’-end via a silanisation process using aminopropyltriethoxysilane (APTES) on different transparent and conductive (TCO) oxide film surfaces. DNA hairpins comprise a stem in which both strands are * Corresponding author: E-mail: [email protected], URL: http://w.w.w.lmgp.fr, Tel. +33-4-56-52-9333; fax: +33-4-56-52-93-00.

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V. Stambouli, V. Lavalley, A. Bionaz, et al. complementary and a loop. These molecules exhibit a particularly high sensitivity for the detection of mismatches compared to the corresponding linear strands. They have been monolabelled at their 5’-end by a 1.4 nm gold nanoparticle. Because of the hairpin conformation, the label is close to the surface. Upon hybridization with a complementary target, the formation of a linear duplex structure with relative rigidity forces the label away from the surface. Due to their conductive properties, TCO films are attractive materials for biochips. They can advantageously replace the classical gold electrodes as working electrodes for direct electrochemical detection of DNA hybridization. As for silica, their surface chemistry allows the covalent and strong binding of DNA. Here, we used different TCO films: ITO films, doped SnO2 films as well as insulating SiO2 films. Thanks to the presence of gold nanoparticles bounded to DNA probes, the effects of grafting and hybridization of DNA could be studied on both conductive oxide surfaces. Particularly, we studied the modifications of surface morphology and chemistry as well as fluorescence results. By coupling AFM with SEM-FEG analyses, dispersed and well-resolved groups of gold nanoparticles linked to DNA were emphasized on the SnO2 films. Their surface density is 2.1 ± 0.3 x 1011 groups.cm-2. TEM images obtained after silver enhancement of gold nanoparticles on ITO films revealed round spheres corresponding to silver coated gold nanoparticles. Their density was in agreement with the data obtained by AFM on SnO2 films. The evolution of the chemical state of the modified oxide surfaces was monitored using XPS and ToF-SIMS. As expected, the XPS N 1s peak intensity increased after grafting and hybridization of DNA. The Au 4d peak was detected only on samples modified with Au labelled hairpin probes. Its intensity decreased with probe concentration. From the ratio Au/Si (Si belonging to APTES), the surface DNA density was estimated to be 9.6x1011 cm-2 and 3.7x1011 cm-2 for SnO2 and ITO films respectively. The P 2p peak was observed only after hybridization with a weak intensity. Its presence was essentially correlated to phosphate residues originating from the hybridization solution. Positive and negative fragments of sugar, bases and phosphates from DNA probes were identified by TOF-SIMS. Positive and negative ions from Au nanoparticles were detected only in the case of Au labelled hairpin probes before and after hybridization. After hybridization of Au labelled hairpin probes with complementary Cy3 targets, quenching of the Cy3 fluorescence by gold nanoparticles was evidenced using fluorescence microscopy. This phenomenon was obtained for both oxides and is in agreement with the Nanometal Surface Energy Transfer (NSET) theory.

I. INTRODUCTION After some general comments on biochips, we present our hybrid nanobiosystem with its different components and their assembly, as well as the characterization techniques. The performance characteristics of our system are then detailed.

I-1. Biochips: Interest and DNA Characteristics DNA biochips are undergoing significant development since they have shown tremendous promise for medical research diagnosis, point of care testing, process monitoring

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in the food industry and environmental testing. They rely on the specific DNA hybridization between single stranded DNA (ssDNA) immobilized on a surface (DNA probes) and free complementary ssDNA (DNA target) in a solution to be identified. DNA is a long polymer held together by a backbone of sugar and phosphate groups. This negatively charged backbone supports four types of molecules called bases, and it is the sequence of these four bases that encodes genetic information. These bases can be split into two groups: adenine (A) and guanine (G) form the larger purine groups, while cytosine (C) and thymine (T) form the smaller pyrimidine groups. If two ssDNA are complementary, they can create a double helix with hydrogen bonds forming between specific base pairs, namely A to T or C to G (see Figure I-1a). This process is known as hybridization (Figure I-1b). Due to steric reasons, no other combination is possible. Hydrogen bonds between two large purine groups would be too weak and equally electrostatic repulsion prevents two smaller pyrimidine groups forming base pairs. Therefore, only purine-pyrimidine base pairs are possible, maintaining the uniform width of the double helix despite the differing sizes of base pairs. Hybridised DNA (also called double-strand DNA, dsDNA) can become denatured. Denaturation refers to the melting of dsDNA to generate two single strands. This involves the breaking of hydrogen bonds between the bases in the duplex. A nucleotide, which refers to a sugar-base-phosphate moiety, is only 0.33 nm long. The width of dsDNA molecule is typically about 2 nm wide [2]. However, despite these small dimensions a DNA molecule can consist of several million base pairs therefore forming a polymer. The specific structure and sizes of DNA molecules used in this study are contained in the experimental section of this chapter. The detection of the DNA hybridization in biosensors can be performed using several techniques. Fluorescence microscopy is the most commonly used technique because of its high degree of sensitivity [2]. Other techniques are relevant such as the surface plasmon resonance (SPR) [3-6], quartz crystal microbalance (QCM) [7, 8] and electrochemical detection [9-17]. While some of these techniques have been already commercialized such as SPR [18], they are still under intensive development. They require surfaces with specific characteristics depending on the technique.

Figure (I-1a) Chemical structure of DNA bases.(1b) Basic structure of hybridised DNA.

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DNA hybridization is strongly influenced by parameters such as DNA conformation, density and accessibility of probes on the solid surface. Consequently, in order to improve the sensitivity, the selectivity and the reliability of biochips, the interface “solid surface/ DNA” must be characterized precisely with complementary high resolution techniques. Regarding DNA conformation, DNA probes with classic “linear” conformation are used. Recently it has been shown that the use of “hairpin” DNA probes improves the detection sensitivity.

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I-2. Molecular Beacons Hairpins DNA probes are composed of a loop and a stem where both strands are selfcomplementary. The interest in such a DNA probe conformation is that these oligonucleotides exhibit a particularly high sensitivity for the detection of one or more mismatch [19–20]. Hairpin probes can be functionalized by different kinds of labels and/or nanoparticles to realize molecular beacons (MB). In solution, the main applications of MB are based on the FRET (Fluorescence Resonance Energy Transfer) phenomenon thanks to the use of a “fluorophore/quencher” couple. These couples can be either two different fluorophores [21] or the association of a fluorophore with a metallic nanoparticle or quantum dot [22, 23]. An illustration is given in Figure I.2. Molecular beacons can be immobilized on different kinds of solid surfaces, such as glass [24], optic fibre [25], polymers such as agarose [26], gold [27-29] and Si [30]. The choice of the substrate is in agreement with the detection technique. The substrate can play either a passive role providing a simple surface for DNA immobilization [24], or it can play an active role participating in the DNA detection [28, 35]. An illustration of detection using fluorescence is given in Figure I.3. Another example is given by the work of Fan et al. [29]. After grafting of DNA hairpins on a gold surface, they used a ferrocene group for an electrochemical detection of DNA hybridization.

Figure I.-2: Illustration of “molecular beacons” for DNA detection using fluorescence. Molecular beacons can be labelled either by a fluorophore or by a « quantum dot » (QD). In the closed state, due to the FRET phenomenon, the fluorophore is quenched. So the « molecular beacon » is switched off. After hybridization with a complementary target, the « molecular beacon » unfolds and emits fluorescence [23].

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Figure I-3 : Illustration of « molecular beacons » grafted on gold surface for DNA detection using fluorescence. a) In the closed state, due to the FRET phenomenon, the fluorophore is quenched by the close Au film. b) After hybridization with a complementary target, the « molecular beacon » unfolds and emits fluorescence [28].

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I-3. Solid Surface: Oxide Films In the field of oxide surfaces for DNA biochips, many studies have been carried out on SiO2 films [36-38]. SiO2 is a well-known material with regard to the physico-chemical properties and processing technology. The SiO2 surface can undergo a functionalization process including hydroxylation and silanization steps. Therefore, it is well-suited for covalent DNA grafting and fluorescence detection of DNA hybridization. Beside SiO2, other types of oxide are of interest such as metallic oxides i.e. ITO [39-40], CdIn2O4 [10] and doped SnO2 [41, 42]. These materials, deposited as thin films, present interesting properties such as visible light transmittance, excellent substrate adherence and good chemically stability. More specifically, their surface can undergo the same functionalization process as SiO2 leading to DNA covalent grafting [41, 42]. Furthermore, they exhibit a good electrical conductivity which can be modulated by varying the doping rate. So, they can be used as the DNA-modified electrodes involved in electrical detection based biochips. Recent work in the literature has demonstrated their relevance for the electric/electrochemical detection of DNA hybridization either with redox label [39, 43] or label-free [9-11, 15-17, 42]. Regarding DNA conformation, in the references identified, the authors solely report the use of linear oligonucleotide probes. To the best of our knowledge, no use of molecular beacons has been reported on metallic oxide surfaces for electrochemical detection. In this study, we focus on two types of TCO films : Sb doped SnO2 films and ITO in addition to insulating SiO2 films. The Sb doped SnO2 films were laboratory made films whereas ITO films were commercial films. On both oxide surfaces, we have grafted MB probes comprising hairpin DNA oligonucleotides that we have monolabelled with one Au nanoparticle (Figure I.4).

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Figure I.4: Grafting of gold nanoparticle labelled hairpin DNA probes on thin metallic oxide film and hybridization with complementary targets.

After having characterized the bare oxide films, we have investigated the evolution of the surface after each biomodification step on both oxide films : after silanisation, after probe grafting, after hybridization and after denaturation. Au nanoparticles linked to DNA probes (in the ratio 1:1) give the opportunity to combine complementary techniques to characterize the resulting hybrid system. The surface study included the morphology and the chemistry. Morphology and topography aspects are obtained by optical and electronic microscopies (TEM, SEM FEG) as well as by AFM. The chemical evolution of surfaces was investigated by XPS and ToFSIMS.

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II EXPERIMENTAL II-1. Thin Film Oxide Deposition and Functionalization II-1-a. Deposition II-1-a-1. SiO2 films SiO2 films were obtained from cleaned (111) Si wafers by thermal oxidization performed at 1050°C in the presence of O2 and H2 during 190 min. The SiO2 thickness obtained was 460 ± 4 nm as measured using ellipsometry taking 1.46 as the refractive index. Such a thickness value was chosen because it corresponds to one of the maxima of the fluorescence intensity when using Cy3 fluorescent target [44] on SiO2 films. Indeed, dye excitation and emission are affected by the self interferences of the incident and emitted light respectively, that are created within the oxide on top of the silicon substrate [45]. II-1-a-2. Sb doped SnO2 films Conductive Sb doped SnO2 thin films were directly deposited on glass substrates using the aerosol pyrolysis technique. This technique, which requires a simple and inexpensive experimental set-up only, is well suited for preparing large area and high-quality tin oxide layers. The deposition procedure, which is related to chemical vapour deposition (CVD) from an organometallic solution (MOCVD), has been described elsewhere [46]. The process is

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based on the pyrolysis of an aerosol obtained by ultrahigh frequency spraying of a precursor solution on a heated substrate at atmospheric pressure. The precursor solution was obtained by dissolving SnCl4·5H2O (Sigma–Aldrich) salt in pure methanol (solution 0.2 M) and by adding a 2 % volume of a 0.2 M solution of SbCl3 (Sigma–Aldrich) salt dissolved in pure methanol (methanol 99.9 %, Sigma–Aldrich). The substrate temperature was kept at 420°C. The solution consumption was about 1.6 ± 0.1 mL.min−1. Under these conditions, the deposition rate was about 45 nm.min−1. The resulting film thickness was 90±10 nm as measured by ellipsometry taking a refractive index of 1.95. Similar to the SiO2 film, the doped SnO2 film thickness was chosen in order to correspond to a maximum in the Cy3 fluorescence intensity. In a previous paper, by varying the film thickness, we have shown that such a maximum was obtained for 90–100 nm thick doped SnO2 films [41].

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II-1-a-3. ITO films Commercial 150 nm thick ITO films from Asahi Company (Japan) were used. Their electrical resistivity, measured at room temperature using a four-point probe technique, was 2×10−4 Ohm.cm. The surface roughness was low, around 2.5 ± 0.5 nm (rms). II –1-b. Functionalization After an initial cleaning of all films with absolute ethanol and deionized water to eliminate organic contamination, the process of functionalization was strictly the same for both oxides except for the hydroxylation step. On the one hand, the SiO2 and SnO2 surfaces were hydroxylated in a Piranha solution (1/3 H2O2, 2/3 H2SO4) for 15 min to create OH groups at the surface. On the other hand, the ITO film surface was hydroxylated in a Braun solution (NaOH 4 M in 40% H2O and 60% C2H5OH) for 2 hours under agitation; samples were then rinsed. The OH groups allowed covalent bonding of a functional organosilane. For both films, silanization was accomplished by a liquid phase deposition of a solution of silane in an organic solvent. The samples were placed for 12 h in a solution 0.5 M of 3-aminopropyltriethoxy-silane (APTES, Sigma–Aldrich) in 95% ethanol under shaking. After two successive rinses with ethanol (Sigma–Aldrich) and deionized water to remove unbound silane, the samples were dried and heated for 3 h at 110°C.

II-2 Molecular Beacon and Target Fabrication II-2-a molecular beacon II-2-a-1. Gold nanoparticle The gold nanoparticles were purchased from Nanoprobes (monomaleimido nanogold®, USA). These nanoparticles are a discrete gold compound rather than a colloid. There are approximately 55 atoms in one nanoparticle [47]. They present a single maleimide functionality incorporated into a ligand on the surface of the gold particle; this has a specific reactivity towards sulfhydryl groups, and may be covalently linked to reduced disulfides, as

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shown in the Figure II.1. The surface of the gold nanoparticle is passivated by phosphine groups. Particles are stable to wide ranges of pH, including pH values lower than 4, and ionic strength, above 0.3 M, but may not be stable above 50°C. They degrade upon exposure to thiols such as ß-mercaptoethanol or dithiothreitol. Thiol compounds will also react with the maleimide group and render the particle non-reactive for conjugation. So, these compounds have to be eliminated before oligonucleotides labelling. The gold nanoparticle diameter is 1.4 ± 0.1 nm according to the supplier. We evaluated this diameter by analysis of a gold nanoparticles suspension in isopropanol by TEM (Figure II.2). The nanoparticle diameter ranged from 1.5 to 3 nm.

II-2-a-2. Hairpin probes The synthesis and functionalization of the oligonucleotides (probes and targets) were carried out by BioMerieux (France). The synthesis was achieved on an EXPEDITE 8900 DNA synthesizer (Applied Biosystems) using standard phosphoramidite chemistry at a 1 μmol scale. All realised sequences are presented in Table II.1.

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Figure II.1. Attachment of a DNA molecule to 1.4 nm gold nanoparticle [47].

Figure II.2. TEM image of gold nanoparticle suspension in isopropanol.

Table II.1. Sequences of the different hairpin probes with their respective labelling.

Probe A Probe B Probe C

Sequence from 5’ to 3’ end TTTTT GCG ATG GAT AAA CCC ACT CTA CAT CGC TTTTT GCG ATG GAT AAA CCC ACT CTA CAT CGC TTTTT GCG ATG GAT AAA CCC ACT CTA CAT CGC

5’ end NH2 C6

3’ end Au nanoparticle

NH2 C6

Fluorescein

NH2 C6

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Figure II.3. a: Hairpin DNA structure and relative sizes. The black numbers refer to the number of bases from the 5’ end. (II.3.b) UV spectrum of H: hairpin probes (λ = 260.4 nm), N: gold nanoparticle (λ = 246.3 nm) and the hybrid NH: gold nanoparticle-hairpin (λ = 256.9 nm).

32-mers synthetic oligonucleotides were used as probe precursors. They were modified with a primary amine at its 5’ end and a disulfide group at its 3’ end. The chosen sequence, 5’NH2-TTTTT GCG ATG GAT AAA CCC ACT CTA CAT CGC-SSdT-3’, allowed oligonucleotides to auto-hybridize on a stem of six bases to form a probe with the following characteristics: a spacer of five bases T, a stem of six base pairs and a loop of 15 bases

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(Figure II.3.a). The 3’-modification of the probes was introduced via the 3’-end disulfide group, which, after reduction was further mono-functionalized either by a modified fluorescein, FAM, (probe B, Table II.1) or by a monomaleimide-gold nanoparticle (probe A, Table II.1). FAM (6 FAM, succinimidyl ester of carboxyfluorescein) and gold nanoparticles (nanogold® monomaleimide, 1.4 nm diameter) were purchased from Nanoprobes. Because FAM and monomaleimide-gold nanoparticle present an equivalent reactive group, the development of the mono-functionalization process of the probe was first investigated with FAM, the latter being easier to manipulate than the nanogold®. The HPLC purified 5’-NH2ODN-SSdT-3’, was dissolved in a phosphate buffer 34 μM at pH 6.6. Then, 1.5 μL of TCEP (tri carboxyl ethyl phosphine) 1 M was added to the solution of ODN to cleave the disulfide group. The solution was stirred and placed overnight at 4°C. Then, the solution was precipitated twice by centrifugation with 18 μL of LiClO4 3 M, 230 μL of deionized water and 900 μL of acetone. The cleavage of the disulfide group was monitored by Reverse phase HPLC analyses on a Waters Alliance 2795 system using a XTERRA C18 MS 2.5 µm column 4.6×50 mm (Waters), with an acetonitrile gradient from 4.5 to 8.5 % in triethylamonium acetate buffer (50 mM), for 30 min at 60°C and at 1 mL.min−1.

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V. Stambouli, V. Lavalley, A. Bionaz, et al. Table II.2 : Sequence of DNA target. Sequence from 5’ to 3’ end GCG ATG TAG AGT GGG TTT ATC CA

5’ Cy3

3’

A part of this crude product was subjected to mass analysis. Mass spectra for oligonucleotides were obtained using a MALDI-TOF system (Bruker) using 3-hydroxy picolinic acid as matrix for oligonucleotides. The same functionalization procedure was used for FAM and nanogold® maleimide: 6 nmol of the maleimide compound was dissolved in 20 μL of isopropanol and 180 μL of deionized water. This solution was added to the solution of 5’NH2-ODN-3’SH and the coupling reaction proceeded overnight at 4°C. HPLC monitoring

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demonstrated the efficiency of the conjugation between the SH moiety and the maleimide (95 % in the case of FAM and 90 % in the case of nanogold). In the latter case, three products appeared at different times (not shown here). Crude reactions were submitted to micro-spin purification (Amersham) in order to remove the non-conjugated molecules. It is worth noting that this step was very efficient in the removal of nanogold excess. After UV quantification using a 96 well Spectramax 190 (MolecularDevices) UV spectrophotometer, products were used with no further purification for grafting experiments. The Figure II.3.b shows UV spectra of, respectively, the non-labelled hairpin probes H (λ = 260.4 nm), the free gold nanoparticles N (λ = 246.3 nm) and the gold monofunctionalised hairpin NH (λ = 256.9 nm).

II-2-a-3. Au enhancement procedure HydroQuinone Silver (HQS) was purchased from Nanoprobes (USA). This colloidal solution is highly specific and nucleates and develops around the gold nanoparticles. This process is an electroless silver deposition. The size of resulting Ag particles is a function of development time. For example, 8 minutes development results in 15-20 nm silver particles (according to the manufacturer). The HQS comprised 3 component solutions: an initiator, a moderator and an activator. Preparation was followed according to manufacturer’s guidelines and the solutions were stored at -20°C and thawed before use. The solutions were light sensitive so; preparation took place in a darkroom using a red light. 70 ± 0.1 µL of each of the initiator and moderator were placed in an Eppendorf tube and then centrifuged and mixed thoroughly. It is worth noting here that the moderator was extremely viscous and therefore it was difficult to extract the solution from the tube without obtaining bubbles of air, consequently the associated error is likely to be larger than the quoted error ± 0.1 µL of the micropipette. Next, 70 ± 0.1 µL of the activator was added and then centrifuged and mixed thoroughly. 30-50 µL of this mixed solution was then added to each sample, enhancement times were 8 minutes. Samples were then rinsed thoroughly with deionised water (Millipore) for several minutes and dried with purified air (Jet-Pur). II-2-b. Synthesis of target One sequence of complementary target was synthesized (Table II.2). This sequence was labelled by a Cy3 fluorescent dye at its 5’ end.

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II-3. Molecular Beacons: Grafting, Hybridization and Denaturation For both oxides, the processes of probe grafting and hybridization were strictly the same. To facilitate strong covalent bonding between the NH2 termination of the APTES and the 5’NH2 termination of the oligonucleotide, a cross linker molecule (10% glutaraldehyde solution in H2O, glutaraldehyde 50% purchased from Sigma–Aldrich) was applied for 90 min at room temperature. Samples were then rinsed with deionized water. The hairpin probes were diluted in a sodium phosphate solution Na2HPO4 (0.3 M/H2O) to a concentration of 10 μM. 10 µL (or 5 μL for XPS and ToFSIMS experiments) of this solution were manually deposited on the surface of each sample. The samples were incubated overnight at room temperature. The probes were then reduced and stabilized using a NaBH4 solution (0.1 M, Fluka). The hybridization was performed using a complementary DNA target labelled with Cy3 fluorescent dye: 5’-Cy3-GCG ATG TAG AGT GGG TTT ATC CA-3’ (purchased from BioMerieux, France). The target solution DNA was diluted to a concentration of 1 μM in a hybridization buffer solution (NaCl: 0.5 M, phosphate buffer solution: 0.1 M, ethylediaminetetraacetic acid or EDTA: 0.01 M at pH 5.5). Droplets (V=10μL or 5 μL for XPS and ToFSIMS experiments) of this solution were spread on the sample surface, covered with a hybrislip®, and placed into a hybridization chamber at 42°C for 45 min. The samples were then rinsed with SSC (saline sodium buffer concentrate, Fluka) 2 M solution and SSC 0.2 M solution. Samples were then dried. For some XPS measurements, instead of the phosphate buffer solution, the hybridization procedure was performed using SSC solution 0.2 M with NaCl (0.1 M). The addition of NaCl provides having the same quantity of Na+ ions in the solution which is for assistance for hybridization. The advantage of this solution is that it contains no phosphorus and allows XPS to be used to characterise oligonucleotide without P interference. The denaturation of hybridized molecular beacons was performed in solution using NaOH (0.1 M, Sigma–Aldrich) for 10 min followed by a thorough rinse with deionized water.

II-4. Characterization Techniques II-4-a. Morphology II-4-a-1. TEM TEM observations were carried out by using a JEOL 2010 microscope operated at 200 kV. Samples were prepared by scratching the surface with a diamond tip. The material removed was deposited on copper grids and coated with a carbon film. II-4-a-2. Sem feg A Scanning Electron Microscope equipped with a Field Emission Gun (SEM-FEG) (ZEISS Ultra) was used to supplement AFM observations of SnO2 and ITO films. This SEMFEG, equipped with a detector in lens, collects back-scattered electrons (BSE) which allow chemical contrast images to be obtained. The higher the atomic number of the element under investigation is, the greater the number of back scattered electrons are and the brighter this

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element appears; the contrast is enhanced. This technique has been used to visualize and locate the gold nanoparticles.

II-4-b. Topography Using AFM The surface topography and the roughness (in root mean-square, rms) of both oxide films were explored using a Dimension 3100 Atomic Force Microscopy, AFM (Veeco Inc, Santa Barbara, CA). Measurements were performed in air in tapping mode with tip ATEC NC 10 from Nanosensors. These Si tips have a curvature tip less than 10 nm, a thickness of 4.6 µm, a length of 160 µm, a width of 45 µm, a resonance frequency between 210 and 490 kHz (nominal value : 335) and a force constant between 12 and 110 N/m (nominal value : 45). Images were collected with a resolution of 512 points per line at a scan rate of 1 Hz. Most of the images were flattened in order to remove the background slope, except for the image of Figure III.11. In this case, a planefit was performed to measure step height. Contrast and brightness were adjusted.

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II-4-c. Physico-Chemical Characterizations II-4-c-1. XPS X-Ray Photoelectron Spectroscopy (XPS) experiments were performed using a Kratos Axis Ultra spectrometer (UK) with monochromatised Al Kα X-rays (hν = 1486.6 eV) operating at 150 W. Survey and high resolution region spectra were recorded using 160 and 20 eV analyser pass energies, respectively. Spectra were charge corrected with reference to C 1s at EB = 285.0 eV. Surface charge neutralisation was used to effect a slight improvement in peak resolution. The analysis area was 700 x 300 µm2. II-4-c-2. ToF-SIMS Time-of-Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) spectra were obtained using a ToF-SIMS IV instrument (Ion-TOFGmbH, Germany) with a reflectron analyser, a Bi3+ ion source (25 keV) and a pulsed electron flood source for charge neutralisation. The mass resolution was typically greater than 7500 at m/z = 29. The pressure in the analysis chamber was maintained at less than 10-10 Pa. Positive and negative ion mass spectra were acquired from a 100 ×100 µm2 analysis area.

II-4-d. Fluorescence Fluorescence intensity measurements were undertaken using an Olympus microscope (BX41M), fitted with a 100 W mercury lamp, a cyanine 3 (Cy3) dichroic cube filter (excitation 550 nm, emission 580 nm) or a fluorescein dichroic cube filter (excitation 490 nm, emission 520 nm). Each image was taken using an objective of x 20. The camera was a cooled Spot RT monochrome camera (Diagnostic, Sterling Heights, MI, USA). The Image Pro Plus software was used for image analysis and for calculation of the fluorescence

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intensity (Media Cybernetics, Carlsbad, CA, USA). The background fluorescence was measured near the hybridization area where targets were spread but where no DNA probe was grafted. This background was subtracted from the intensity of the hybridization area. The values of fluorescence intensity were normalized for an exposure time of 1 s for both oxide surfaces.

III RESULTS III-1. Bare Oxide Film Characteristics III-1-a. Insulating SiO2 films The insulating SiO2 films are amorphous with a smooth and flat surface as can be seen in Figures III.1.a, showing AFM images taken respectively in topography mode (left) and phase mode (right). The surface roughness is 0.23 ± 0.01 nm in r.m.s. Due to this low roughness value, these SiO2 films were used as reference surfaces in terms of topography characterization.

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a)

b)

c)

d)

Left, images in topography mode. Right, images in phase mode. For both images : Z range = 10 nm for topographical images and Z range = 60° for phase images, scan size 1*1 µm2.

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Figure III.1. AFM images obtained on SiO2/Si film after the different steps of biomodification a) bare surface, b) after functionalization by APTES + glutaraldehyde, c) after probes A grafting and d) after hybridization with complementary target.

700

(222)

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(400)

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(440) (622)

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(211) 100

0 20

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2-Theta - Scale Survey spectra

x 10 5

In 3d

12

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Intensity (CPS)

10

8

6

Sn 3d

In MNN Sn MNN

4

In 3p In 3s

O 1s

In 4p In 4d

O KLL In 4s

2

1400

C 1s

1200

1000

800 600 Binding Energy (eV)

400

200

0

Figure III-2. Bare ITO thin film a) X Ray diffraction spectrum and electronic diffraction image ( upper corner). b) XPS survey spectrum.

III-1-b. Conductive ITO films As-received ITO films on glass substrates are transparent. They are polycrystalline as can be observed on the X-Ray diffraction diagram (Figure III.2a). The associated TEM diffraction image is indexed like the In2O3 powder (Figure III 2a) with no preferred orientation. The measurement of the electrical resistivity provides a value of 2x10-4 ohm.cm confirming a high conductivity. The surface roughness measured by AFM is relatively low, ranging between 1 and 1.5 nm (r.m.s.).

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Chemical analyses of the ITO film surface were performed using XPS. The typical survey spectrum (Figure III-2b) exhibits In peaks, particularly In 3d5/2 and In 3d3/2, located respectively at binding energies of 444.1 eV and 452.0 eV. The O 1s peak is located at a binding energy of 530,1 eV. The C 1s peak is located at 285.0 eV (calibration peak). The Sn dopant is present as confirmed by the presence of Sn 3d5/2 and Sn 3d3/2 peaks located respectively at 486.0 eV and 495.0 eV.

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III-1-c. Conductive Sb doped SnO2 films Deposited Sb doped SnO2 films are transparent and conductive with a resistivity comparable to or higher than the one of ITO films (3 -5 x 10-3 ohm.cm). We have previously shown that these films are polycrystalline with a preferred (101) orientation [41].

a)

b)

c)

d)

e)

f)

g)

h)

Figure III-3. AFM images of SnO2/glass after the different steps of biomofidification. a) bare film, b) after functionalization by APTES + glutaraldehyde, c) after probes A grafting, and d) after hybridization with complementary target. Left : images in topography mode. Right : image in phase mode .For both images : Z range = 100 (topography images) and Z range = 90° ( images in phase.). Scan size = 1 µm * 1 µm apart images c where scan size = 500 nm *500 nm..

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The surface morphology was analyzed by AFM. Figure III-3 a,b shows the morphology aspect of the SnO2 surface, in (a) topography mode and (b) phase mode. Compared to SiO2 and ITO film surfaces, SnO2 exhibited larger roughness values, for example, 8.2 ± 2.0 nm for a 100 nm thick film. This roughness is correlated to the high deposition rate and to the polycrystalline structure of the films [41]. The roughness results in a grain agglomeration of 110 ± 20 nm (Figure III-3a). Chemical analyses of the film surface were performed with XPS. A typical survey spectrum (Figure III-4) exhibits Sn peaks, particularly Sn 3d5/2 and Sn 3d3/2, located respectively at binding energies of 487.1 eV and 496.0 eV. The O 1s peak is located at a binding energy of 530.9 eV. The C 1s peak is located at 285.0 eV (calibration peak). The Sb dopant is not detected which signifies that its concentration is lower than the detection limit of the XPS technique: (~0.5% at). However its presence is confirmed from ToFSIMS analyses. Indeed, both 121Sb and 123Sb isotopes are detected as expected and as can be seen in the ToFSIMS spectrum presented in Figure III-5. Survey spectra

x 10 5 14

Sn 3d

In ten sity (C P S )

12 10

O KLL Sn MNN

O 1s

Sn 3s

8

Si 2p Sn 3p

6

Sn 4s

Sn 4p

Si 2s

Sn 4d

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4 2

1400

C 1s

1200

1000

800

600

400

200

0

Binding Energy (eV)

Figure III.4. XPS survey spectrum of bare surface of doped SnO2/Sb film.

Figure III.5. ToF-SIMS positive spectrum obtained on a Sb doped SnO2 bare surface :Sb region of interest Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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III-2. Oxide Films after Biomodification Steps The biomodification procedure of the oxide films involves successive steps: functionalization, grafting, hybridization and denaturation. We report here the surface evolution for both films after each of these steps. In this aim, we have used microscopy techniques and spectroscopy techniques. Results are systematically presented as followings. Firstly, results from microscopy techniques including optics, fluorescence and Atomic Force Microscopy are reported. Note that AFM results are only reported for flat SiO2 and rough SnO2 films. Secondly, results from spectroscopy techniques including XPS and ToFSIMS are reported. Note that these techniques have been used only for conductive SnO2 and ITO films to avoid charging effects.

III-2-a. Functionalization step Following surface hydroxylation, the functionalization of both oxide films involves two successive steps: a silanization step (APTES) followed by a step involving glutaraldehyde coverage.

Si 2p

x 10 2

N 1s

x 10 1

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CPS

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III-2-a-1. Microscopy techniques The influence of silane (APTES) covered by glutaraldehyde was studied in terms of modifications of morphology and roughness on SiO2 and SnO2 film surfaces using AFM. First, the addition of APTES and then glutaraldehyde on SiO2 films modifies slightly the topography, as can be seen on Figure III.1.b (left). Roughness increases from 0.23 ± 0.01 nm before functionalization to 0.28 ± 0.01 nm after functionalization. This process induces a smooth and homogeneous layer. On the other hand, for naturally rough doped SnO2 films, no significant change can be observed on topography (Figure III.3.b, left).

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210 412

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Figure III.6. XPS high resolution spectra of a) Si 2p and b) N 1s region of ITO film after silanisation.

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III-2-a-2. Spectroscopy techniques The effects of first silanisation and then glutaraldehyde were studied using XPS and ToFSIMS. These analyses were achieved only on both conductive films (ITO and doped SnO2) as previously mentioned. a)

b)

80

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Atomic fraction (%)

Atomic fraction (%)

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1,5 Si 2p N 1s Au 4d P 2p

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n io at ur at n en io D at iz rid yb H ng e fti yd ra eh G ld ra ta n lu G atio n z ni io la at Si xyl ro ce yd H u rf a s e

re Ba

n io at ur at n en io D at iz rid yb H ng e fti yd ra eh G ld ra ta n lu G atio n z ni io la at Si xyl ro ce yd H u rf a s

r Ba

Figure III.7. Evolution of atomic fractions (%) of the different elements as a function of the biomodificiation step in case of ITO films a) general graph, b) zoom: atomic fractions less than 2 %.

b) 2

80

Atomic fraction (%)

Atomic fraction (%)

70 60 50 40

Sn 3d C 1s O 1s

30 20

1,5

Si 2p N 1s Au 4d P 2p

1

0,5

10

0

n

Figure III.8. Evolution of atomic fractions (in %) of the different elements as a function of the biomodificiation step in case of doped SnO2 films a) general graph, b) zoom: atomic fractions less than 2 %.

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n

ce

io at iz

ng

io at

r id

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rfa su

tio za ni

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fti ra

H

G

la Si

Si

0

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a)

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Table III.1 : Position, FWHM and attribution of fitted peaks corresponding to XPS high resolution spectra of Si 2p and N 1s obtained on ITO film after silanisation.. Main peak Si 2p N 1s

Fitted peak A B A

Position (eV) 102.38 103.28 400.19

FWHM (eV) 1.28 1.41 1.80

B

401.63

1.98

Attributions SiO2C2 SiO3C CH2-NH2 CH2NH3+HCO3-

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Regarding the silanisation, the general XPS surveys of both films are modified. Particularly, we observe the appearance of Si 2p and N 1s peaks. The corresponding high resolution peaks with their deconvolutions are presented in Fig III.6. The deconvolution parameters and the attribution of fitted peaks are listed in Table III.1. These attributions are characteristic of APTES and confirm its presence on the ITO films. Similar results are obtained for Sb doped SnO2 films. Regarding the peak intensity of other elements such as C, O, In and Sn, some modifications are observed. We have reported the evolution of the respective atomic fraction (in %) as a function of the bio-modification step for ITO films (Figure III.7.a and b) and for SnO2 films (Figure III.8.a and b). After the silanisation step, the intensity of C 1s and O 1s peaks slightly increases whereas the intensity of the metal element peaks slightly decreases. These changes are in agreement with the addition of a thin organic APTES layer including oxygen and carbon elements. The presence of this layer reduces the contribution of the metallic oxide. The addition of glutaraldehyde does not change significantly the intensities of the photoelectrons peaks derived from the ITO film, as can be seen in Figure III.7 (a) and (b).

III-2-b. Grafting Step for DNA Molecular Beacons The grafting of molecular beacons on both functionalized oxide surfaces corresponds to their covalent surface immobilization. The probes are in a closed configuration as shown in Figure I.4. To study the effects of the grafting procedure on both surfaces, we have used several kinds of techniques.

III-2-b.1 Microscopy techniques Fluorescence microscopy is very useful to validate the grafting procedure. In this aim, we have grafted probes B (table II.1) on both oxide surfaces. Probes B are fluorescein labelled instead of Au nanoparticle labelled. In Figure III.9, a part of one fluorescent spot corresponding to DNA drop obtained on doped SnO2 film can be observed. The bright and homogeneous area corresponds to grafted DNA, confirming the validity of grafting. Similar results are obtained for SiO2 and for ITO films. By AFM scanning, the near-edge of the fluorescent DNA drop (probes B), we have attempted to measure the height or apparent thickness of grafted hairpins on an oxide surface. Taking into account for the nanometer dimensions of the DNA probes, such experiments

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could only be achieved on a very flat surface such as a SiO2 film surface (Figure III.1.a and b). A typical scan is shown in Figure III.10. From this Figure, several observations can be made. On the left side of the image, the SiO2 film covered by the functionalization layers (APTES and glutaraldehyde) can be easily identified due to the low roughness and homogeneous aspect. In contrast, the upper area with higher roughness located on the right side corresponds to grafted probes.

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Figure III.9. Fluorescence image (magnification x 20) of fluorescein monolabelled probes B grafted on doped SnO2/Si. Thickness of the film was 50 nm.

Figure III.10. AFM image of near-edge of grafted fluorescein monolabelled hairpin probes (probes B), scan size 2 µm * 2 µm, obtained on SiO2 film (Z range 20 nm). A, B, C profiles gave step height respectively of 3.8, 4.9 and 4.2 nm.

We measured the thickness of the layer of interest, i.e. covalently bound probes. Thickness values range from 0.7 nm to 4.9 nm (Fig III.10, profiles A, B and C). The average height is 3.7 ± 1.0 nm. To confirm that the step height corresponds to DNA material, a counter experiment was performed. A drop containing only the grafting buffer (sodium phosphate solution) without DNA probe was deposited. AFM scans performed all along the

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drop edge show no height difference between external and internal areas of the drop. Only some residual salts contribute to roughen the surface locally.

oxide Si

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Figure III.11. Scheme of tilted hairpin probes B on the surface of oxide films. Tilt angle should be between 38 and 59°.

The theoretical value of the whole height of the hairpin probe in the closed configuration is about 6.24 nm (Fig II.3.a). This calculated value is approximately twice the experimental value measured by AFM (Figure III.10). Even if these measurements would have rather had been performed in liquid and not in air, it seems probable that these hairpins are tilted towards the surface (see schema on Figure III.11) with a tilt angle ranging from 38° to 59° as deduced from experimental heights. The effect of Au nanoparticle labelled probe (probe A) grafting on both SiO2 and SnO2 surfaces was also studied in terms of change in morphology and roughness by AFM. Scans were performed inside DNA drop surfaces. On SiO2 surfaces, the grafting step induces circular structures which roughened the surfaces, as can be seen from the Figure III.1.c left and right. The roughness (r.m.s.) increases from 0.28 ± 0.01 to 0.66 ± 0.01 nm. From statistical counts, we can estimate the density of structures to be 2.6 ±0.2 x 1011 structures.cm-2 with lateral dimensions of 20-24 nm. This density is comparable to that reported by Rouillat et al. who have grafted linear 25 base ssDNA on SiO2 films [48. On the other hand, on SnO2 surfaces, the grafting of probes A does not induce significant differences when considering the topography image (Figure III.3c left). The roughness slightly changes from 6.5 ± 2.0 nm to 6.2 ± 2.0 nm. However and surprisingly, islands, or nanodots, appear on SnO2 grains when considering the corresponding phase image (Figure III.3c right). Their distribution is not uniform. We estimate their density to be 2.1 ± 0.3 x 1011 islands.cm-2 with a lateral dimension ranging between 6 and 40 nm. To confirm that these nanodots correspond to DNA strands with gold nanoparticles, SEM-FEG analyses were performed. On the secondary electron image (Fig III.12.a), the surface of SnO2 with large grain structure is easily identified with small white dots of different sizes. To confirm if these dots can be attributed to nanogold® particles, a chemical contrast image with Back Scattered Electrons (BSE) was performed (Fig III.12.b). Small and bright islands are obtained which perfectly correspond to the white dots of Figure III.12.a. These dots, characterized by an atomic number superior to elemental Sn, are attributed to groups of gold nanoparticules. It should be noted that the resolution limit of the microscope is poorer when using BSE mode. For this reason, dots are less resolved than in secondary electron mode, so the smallest dots can not be observed. This confirms that groups including

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several DNA strands are “dispersed” on the rough SnO2 surface and do not behave like a dense carpet as it seems on a smooth SiO2 surface. The common result between these observations is that the dimensions of structures found on both SiO2 and SnO2 are over-sized in comparison with the expected lateral dimension of the DNA hairpin loop (2.05 nm) labelled by a Au nanoparticle (1.4 nm). This shift is explained by the convolution effect of the AFM tip taking into account for its own curvature radius (around 10 nm). A)

B)

Figure III.12.a. SEM-FEG images obtained with secondary electrons after hybridization of nanogold® labelled probes covalently grafted on SnO2 film. The brightest islands are gold nanoparticles.(III.12.b) Corresponding SEM-FEG image of Figure III.12.a, obtained in Back Scattered Electron mode (BSE). The brightest islands are gold nanoparticles

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In conclusion, thanks to combination of AFM with SEM-FEG the grafting procedure is confirmed on both oxide surfaces.

III-2-b-2. Spectroscopy techniques Grafting of DNA molecular beacon probes is characterized by the addition of DNA material on both oxide surfaces. Chemically this corresponds to the addition of groups such as sugar, bases and phosphates which include H, C, O, N and P (Figure I.1.a and b). Furthermore, in the case of Au labelled probes (probe A), the signature of metallic Au element is expected. We will first consider results concerning Au detection, then the results for other compounds. A typical XPS survey for ITO films after probe A grafting exhibits new peaks such as Au 4p and Au 4d (Figure III-13). Generally, the Au photoelectron peak used to study Au is Au 4f located at 87 eV. But in the case of ITO films, the Au 4f peak is very close to the In 4p peak located at 83 eV and so, Au 4f peak can not be properly studied. Consequently, we select the Au 4d peak. This doublet peak is located at binding energies of 335.03 eV and 353.28 eV for 4d5/2 and 4d3/2 transitions respectively. Figures III.14 a and b show the high resolution spectra for Au 4d obtained for probes A and fluorescein labelled probes (probe C) respectively. Au is only detected in the first case.

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Survey spectra

x 104 90

In 3d

80 70 Intensity (CPS)

S n 3d 60 C KLL

O 1s

O KLL

50

In 3s

In MNN

40

In 3p

In 4s Si 2p Au 4p

Au 4d

30

N 1s

C 1s Si 2s

In 4d

20 10

1400

1200

1000

800

600

400

200

0

Binding Energy (eV)

Figure III.13. XPS survey spectra of gold nanoparticle labelled oligonucleotide probes grafted on an ITO thin film.

The Au atomic fraction is 1.3 % in the case of ITO films and 1.8 % in the case of SnO2 films (Figure III.7.b and Figure III.8.b). These data are similar taking into account for the limit uncertainty.

350

Au 4d5/2

Intensity (CPS)

Intensity (CPS)

Au 4d

300

340 330

1 x 10 310 305

Au 4d

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b)

Au 4d

x 101

Au 4d3/2

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295 290 285 280

310

275

300

270 265

290

365

365

360

355

350 345 340 335 Binding Energy (eV)

330 325

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350 345 340 335 Binding Energy (eV)

330

325

Figure. III.14. a: XPS high resolution spectrum of Au 4d peak in the case of gold nanoparticle labelled hairpin probes (probes A) grafted on ITO films.(III.14.b ): XPS high resolution spectrum of Au 4d peak in the case of non-labelled hairpin probes (probes C) grafted on ITO films.

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FigureII.15. Evolution of Au atomic fraction depending on oligonucleotide probes concentration, ; values obtained on ITO thin film

We have studied the correlation between the concentration of DNA probes in solution and the evolution of the Au atomic fraction as well as the N atomic fraction on ITO surface. In this aim, we have divided the DNA probe concentration by 10 or by 100 compared to our standard conditions (standard concentration: 10 μM). As a result, the Au and N atomic fractions decrease. The Au decreases to 0.3 % for 1 μM and to almost 0 for 0.1 μM (Fig III.15). This strong correlation emphasises a lower density of grafted probes when decreasing their concentration in solution. Moreover we observe the decrease of N atomic fraction down to a constant value which corresponds to the N atomic fraction obtained after the silanisation step. The presence of Au nanoparticles can be detected complementary using ToF-SIMS on both ITO and SnO2 films after grafting with Au labelled probes (probe A). Indeed, if XPS is useful to study the evolution of atomic elements, ToF-SIMS is useful to detect whole or parts of molecule fragments. The Au detection is characterized by the presence of numerous fragments which are essentially obtained on the negative spectrum. These fragments are respectively Au- located at a mass of 196.967 Da, AuH- at 197. 978 Da, AuS- at 228.940 Da, C3H4Au- at 236.984 Da and AuN3- at 238.972 Da. In Figure III.16, corresponding spectra of these fragments are presented. They are present only after the grafting step with Au labelled probes (probes A) on both ITO or SnO2 films whereas no detection was possible in the case of grafting with non-labelled probes (probe C). In addition to the XPS study which evidenced Au 4d peaks, these ToF-SIMS results confirm the presence of Au nanoparticles on both ITO and SnO2 films. Let us now consider the evolution of other atomic elements obtained by XPS on ITO films. The investigation of respective high resolution spectra of C1s, N 1s, O 1s and P 2p emphasises several results. First, in comparison with the atomic fractions obtained after the functionalization step, there is a general and significant increase of their relative intensity on both oxide films as can be seen in Figure III.7 and Figure III.8. This is in agreement with DNA probe grafting on both oxide surfaces. Second, regarding the shape of peaks, the most significant change is the broadening of the N 1s peak. Indeed, it can be deconvoluted with 3 peaks: A, B and C (Figure III.17). They

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are located respectively at 399.30 eV (peak A), 400.40 (peak B) and 402.00 eV (peak C). Their respective FWHMs are 1.58, 1.74 and 1.88 eV. According to the study of Mateo-Marti et al. [49], they can be attributed respectively to the following chemical states of N: –N= (peak A), -NH- (peak B) and –NH2 (peak C). These states are characteristic of the N forms in DNA. The atomic fraction of N is 1.1 % for ITO films and 1.7 % for SnO2 films. Its intensity does not depend on the type of probes (A or C). The presence of Au nanoparticle labels does not modify the grafted probe density. Regarding phosphorous, it is strongly characteristic of the DNA molecule. But, surprisingly, we do not observe a significant intensity for the P 2p peak located at 133 eV. This observation is reproducible whatever the type of DNA probe (A or C) and whatever the type of biomodified oxide film (ITO and Sb doped SnO2 films).

a)

b)

Bare Hydroxylation Silanisation Glutaraldehyde

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Probe A Probe C c)

d)

Figure III.16. ToF-SIMS spectra obtained on ITO films of fragments a) Au-, b) AuS-, c) C3H4Au-, d) AuN3-. For every image, the first spectrum corresponds to the bare surface, the second spectrum to the hydroxylated surface, the third spectrum to the silanised surface, the fourth spectrum to the adding of glutaraldehyde, the fifth spectrum to the gold nanoparticle labelled hairpin probe (probe A) grafting and the sixth spectrum to the non-labelled hairpin probe (probe C) grafting.

On the other hand, phosphates fragments can be detected using ToF-SIMS. As can be observed in Figure III.18, appearance of the H4PO4+ fragment located at a mass of 98.987 Da Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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is observed only after probe grafting on Sb doped SnO2 films. This fragment is obtained whatever the type of probe is employed. Similar results are obtained on ITO films. 2 x 10

N 1s

N 1s

34

Intensity (CPS)

32

B

B

A 30

28

A C

26

C

24 412

410 408

406

404 402 400 398 396 394 Binding Energy (eV)

Figure III.17. XPS high resoltuion spectrum of N 1s after probes (A or C) grafting on functionalised ITO thin film.

Bare

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Silanization Probe A Probe C

Figure III.18. ToF-SIMS spectra of H4PO4+ fragment (mass 98.986 Da), obtained on a doped SnO2 film. First spectrum corresponds to the bare surface, second spectrum to the silanized surface, third spectrum to grafted gold nanoparticle labelled hairpin probes (probe A) and fourth spectrum to non-labelled hairpin probes (probe C).

Other relevant fragments obtained by ToF-SIMS can be detected. Among these fragments, the base fragments are characteristic of DNA molecule. They are generated from cytosine, adenine, thymine, and guanine. Each base generates its own numerous fragments. We only studied those which are the closest from the base regarding the composition and which are the most studied in the literature, e.g. base-H (negative fragments) and base+H (positive fragments). In Figure III.19.a, b, c and d, we present fragments of the negative spectrum corresponding to cytosine (Fig a), to adenine (Figure b), to thymine (Figure c) and to guanine (Figure d). These fragments are obtained from SnO2 films. Similar results are

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obtained on ITO films. It is clear that, for both probes, the respective fragment of each base appears only after probe grafting.

III-2-c. Hybridization Step The hybridization step of the Au nanoparticle labelled probes with complementary targets induces the unfolding of the hairpin, providing a helicoidal double-strand. The formation of this duplex structure with relative rigidity forces the Au nanoparticle away from the oxide surface. As a result, the Au nanoparticle is located at the top of the duplex. As for the grafting step, the hybridization phenomenon can be studied using several techniques.

III-2-c-1. Microscopy techniques Optical microscopy and electronic microscopy Drops of DNA molecular beacons hybridized on the surface cannot be visualized using optical microscopy because of nanometre dimensions of each compound. On the other hand, the HQS enhancement treatment of Au nanoparticles makes it possible to visualize the drops optically. In Figure III.20, a light image of a drop edge on ITO film can be observed. The DNA drop appears with a bright and homogeneous colour due to the silver coating surrounding Au nanoparticles. a) b) Bare Silanization

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Probe A Probe C

c)

d)

Figure III.19. ToF-SIMS spectra obtained on doped SnO2 films of fragments a) C4H4N3O-, b) C5H4N5-, c) C5H5N2O2-, d) C5H4N5O-. For both images, first spectrum corresponds to the bare surface second spectrum to silanized surface, third spectrum to grafted gold nanoparticle labelled hairpin probes (probe A) and fourth spectrum to non-labelled hairpin probes (probe C).

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Figure III-20. Light microscopy picture (magnification × 50) of hybridized DNA molecular beacon drop on ITO film after 8 minutes of HQS enhancement. The image shows the clear visual distinction between silver enhancement inside and outside the DNA drop.

A deeper insight of this DNA area may be obtained by TEM analyses after extraction of a piece of drop area with a diamond tip. The bright field image (Figure III.21.a) shows the presence of round spheres like a bunch of grapes. On the high resolution image of one nanoparticle (Figure III.21.b) one can see crystalline domains with different orientations. From the indexing of the electron diffraction pattern (Figure III-21.c), we deduce the presence of Ag and/or Au (by comparison with the simulated powder X-Ray diffraction spectrum of Ag or Au). The crystallographic structures of Ag and Au are similar, consequently we can not distinguish them only by electron diffraction. The energy dispersive spectrometry (EDS) evidences the presence of Ag but not the presence of Au due to the very weak contribution from Au. We note that a silver sphere may contain one or more Au nanoparticles. The surface density of silver nanoparticles (Figure III.21.a) ranges between 8 and 12.1010 nanoparticles.cm-2, which is in agreement with the densities of 2.1.1011 nanoparticle.cm-2 found by AFM (Figure III.3c, d).

Figure III.21. a: TEM image in bright field of DNA molecular beacon hybridization on ITO film after HQS enhancement treatment.(III.21.b) High Resolution TEM image performed on one sphere of Figure III.21.a. The scale bar corresponds to 5 nm.

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Fluorescence intensity of Cy3 (arbitrary units), exposition time 1 s

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Figure III.21. c: The indexing of the electron diffraction pattern is made from a comparison with the simulated powder X-ray diffraction spectrum of the Ag Fm-3m (fiche JCPDS 03-065-8428)

700

Probes A B Probes C

600 500 400 300 200 100 0

SiO /Si 2

SnO /Glass 2

ITO/Glass

-100

Figure III.22. Average fluorescence intensity of the Cy3 dye after complementary hybridization on SiO2/Si, SnO2/glass and ITO/glass. Probes are either gold nanoparticle labelled (Probes A) or non labelled (Probes B). Each intensity and standard deviation were measured on three or four spots from three samples. The error bars were determined from the average standard deviations.

Fluorescence microscopy Fluorescence intensity of the Cy3 fluorophore after hybridization has been studied in the case of both probes (i) Au nanoparticle labelled probes (probe A) and (ii) non-labelled probes (probes C). Results are reported in Figure III.22 for both oxide films: SiO2, SnO2 and ITO. When comparing Cy3 fluorescence intensity according to the type of probe, it can be observed that it is always less intense in case of probes A whatever the oxide surface. The Au

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nanoparticle located very close to the Cy3 behaves like a quencher for the Cy3 fluorescence. The quenching rates are 86.5 ± 3.8 % on SiO2 films, 52.6 ± 15.2 % on SnO2 films and nearly 100 % on ITO films. The Cy3 quenching phenomenon due to the proximity of Au nanoparticles confirms the hybridization. This issue is considered further(discussion part).

III-2-a-1. Spectroscopy techniques XPS analyses XPS analyses were performed on both ITO and SnO2 films after hybridization of DNA molecular beacons. On ITO, spectra are characterized by a slight increase of the intensities of the O 1s, N 1s and C 1s peaks. In particular the N 1s increase is more pronounced as can be seen on Figure III.7.a and b. This increase is related to DNA duplex formation leading to an increase (theoretically by a factor of two) of organic material. These results confirm the reaction of hybridization. On the other side, as expected, the Au atomic fraction stays at a constant value. Similar trends are obtained on SnO2 films (Figure III.8.a and b). Finally, we remark on the appearance of the P 2p peak located respectively at 133.1 and 134.0 eV for ITO and SnO2 films (Figure III.23.a). Its intensity is very low: 0.3 % for ITO and 0.1% for SnO2. On ITO, its atomic fraction decreases with probe concentration (Figure III.15). .In the case of SnO2 films, its proximity with the Sn 4s peak located at 138.9 eV does not facilitate the analysis of its intensity. Indeed, the base of Sn 4s peak is quite large and consequently, the P 2p peak is disturbed. Three reasons at least can explain the weak intensity of P peak: (i) weak emission coefficient of P element, (ii) in comparison with N element, P is less abundant, it represents less than 10 % of DNA atomic elements, and (iii) the surface density of DNA represents less than 1% of the surface coverage. Elsewhere some doubts can be formulated regarding its appearance observed only after hybridization. Indeed, if the reaction hybridization is performed using a SSC hybridization solution containing no phosphate salts, its intensity decreases to 0.01 % (Figure III.23.b). This leads to this conclusion: that the P peak seems to originate mainly from phosphate salt residues derived from the standard hybridization solution. ToF-sims ToF-SIMS studies were performed on both films after hybridization and showed no significant change in the fragments that we have studied after the grafting step (see section III-2-b-2). Fragments involving Au, phosphate salts or bases are still present. Regarding phosphate or base fragments, the increase of organic material can not be evaluated using ToFSIMS as this technique is, in general, not deployed as a quantitative technique due to complications arising from matrix effects. However, other new fragments corresponding to hydrogen bonded bases may be studied to further in the ToF-SIMS characterization of hybridization.

III-2-d. Denaturation Step Denaturation was only performed on ITO films biomodified with either Au nanoparticle labelled probes (probe A) or non labelled probes (probe B). This reaction requires basic

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conditions (NaOH). The aim was twofold: first, this was to check that the system returns to the initial state (closed configuration) after denaturation, and second to determine if the Au nanoparticles were still present.

III-2-c-1. Microscopy techniques The fluorescence images show no fluorescence indicating that no fluorescent duplex material remains on the surface.

III-2-c-2. Spectroscopy techniques The atomic fractions are reported in Figure III.7.a and b. The indium atomic fraction increases slightly whereas C, O and N atomic fractions decrease. This indicates that a loss of organic material took place in relation with deshybridization of DNA duplexes. Moreover, the Au is still present indicating that the Au-S bonding is not altered by the basic conditions required for denaturation.

IV. DISCUSSION

1 x 10

a)

P 2p

370

P 2p

x 101

375 P 2p

355 A

365

350 360

Intensity (CPS)

Intensity (CPS)

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The grafting and hybridization of molecular beacons on both oxides could be confirmed by complementary techniques including morphology and chemistry. By coupling AFM with SEM FEG, we confirm that the islands observed by AFM in phase mode on rough SnO2 surface correspond to Au nanoparticles, the surface density of which is 2.1 ± 0.3 x 1011 islands.cm-2. These values can be compared to the data on ITO by TEM after HQS enhancement.

355 350

345

340

345 340

335 335 140

138

136 134 132 Binding Energy (eV)

130

128

142

140

138

136

134

132

130

128

Binding Energy (eV)

Figure III.23. a: XPS P 2p high resolution spectrum on ITO films after hybridization performed with standard hybridization solution (III.23.b ): XPS P 2p high resolution spectrum ITO films after hybridization performed with the SCC hybridization solution.

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V. Stambouli, V. Lavalley, A. Bionaz, et al. Table IV.1. Calculated Au/Si ratio and oligonucleotide surface density from XPS measurements on ITO and SnO2 films.

Au/Si ratio 2,52 x 10-4

ITO Films oligonucleotides.cm-2 3,7 x 1011

Au/Si ratio 6,41 x 10-4

SnO2 Films oligonucleotides.cm-2 9,6 x 1011

ToF-SIMS analyses emphasize fragments characteristic of DNA molecule such as (i) bases fragments, (ii) some sugar fragments and (iii) phosphate fragments as well as gold fragments. By XPS, we evidence: (i) the increase of the N 1s peak which is a characteristic of DNA molecules and (ii) the presence of Au 4d peak. Their intensity decrease with probe concentration. The monolabelling of DNA hairpin probe (Au/Hairpin = 1) is helpful to provide another way to estimate the probe surface density on both ITO and SnO2 surfaces. Let us propose that only one Au monolabelled probe can be linked to one APTES molecule which involves 1 Si atom. From the ratio “number of Au nanoparticle per Si atom” given by XPS, and by knowing the APTES surface density, it is possible to estimate the probe density. The ratio “Au Nanoparticle per Si atom” is given by the following equation:

NPAu Au peak area Si sensitivity factor 1 = * * Si Au sensitivity factor Si peak area Nb atoms in 1 Au nanoparticle

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where Au 4f sensitivity factor = 1,9 Si 2p sensitivity factor = 0,17 Number of atoms in one Au nanoparticle = 55. In our case, the Au 4d sensitivity factor can be calculated from the ratio Au 4f/Au 4d obtained from a bulk Au XPS spectrum. The ratio Au4f/Au4d is equal to 3. Naturally, this transformation can increase the error magnitude of the surface density estimation. The surface density of Si atoms originating from APTES coverage is not known. A very optimistic approximation can be made by considering that it corresponds to a SiO2-like surface which gives 1.5.1015 Si.cm-2. The resulting oligonucleotide densities are reported for both ITO and SnO2 surfaces in table IV-1. Taking into account approximations, the values are close to the previous data obtained with AFM and TEM. The hybridization of Au monolabelled hairpin probes with Cy3 target induces the unfolding of hairpins with the gold nanoparticle at the top of the duplexes, near the Cy3. The fluorescence measurements evidence the quenching of the Cy3 by the closely positioned Au nanoparticles. The phenomenon of fluorescent dyes quenched by gold nanoparticles has already been investigated in liquid medium [22, 50]. Wu et al. [50] have adsorbed linear DNA probes on gold nanoparticles and hybridized these probes with targets functionalized by TAMRA. After hybridization, the dye was quenched at 82% by the gold nanoparticle. Dubertret et al. [22], using hairpins coupled at their 3’ end by a fluorescein dye and at their 5’end by a gold

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nanoparticle, found a fluorescence quenching of 99.42 ± 0.02% when hairpins were closed. As mentioned before, Du et al. [15] have immobilized hairpins functionalized by a fluorescent dye, rhodamine, on a gold surface and used this surface as a quencher for the dye. They found a fluorescence quenching of 95 ± 4%. As a consequence, gold is an effective quencher for several dyes; even for dyes emitting near the infrared region [22]. In the field of optical molecular rulers, Yun et al.. [51] studied the influence of the distance between the dye and a small gold nanoparticle (1.4 nm diameter) using linear DNA probes monofunctionalized by a gold nanoparticle and complementary DNA target functionalized by a fluorescein with increasing number of bases. They found the fluorescence quenching was more efficient when the dye and the nanoparticle were closed together. They explained this phenomenon on the basis of the Förster resonance energy transfer (FRET). FRET is a mechanism by which energy is transferred from an electronically excited donor molecule to a ground-state acceptor via through-space resonant dipolar coupling. The efficiency of the process is strongly distance dependent. If the acceptor and donor are in close proximity to each other, the acceptor can absorb the excitation energy of the donor via FRET, as long as the donor–acceptor separation is within the Förster distance. The Förster distance is a characteristic of a donor–acceptor pair, and depends on factors including refractive index of the surrounding medium, the donor quantum yield, the relative orientation between donor emission and acceptor absorption dipoles, and the degree of spectral resonance between the two species [52]. In the case of experiments by Yun et al. [51], the gold nanoparticle was the energy acceptor and the fluorescein was the energy donor. It was a successful application of a dipole-surface type energy transfer of a molecular dipole to a gold nanoparticle. In comparison with pure FRET mechanism [52], as the measurable distances for energy transfer mechanism were extended in the case of the gold nanoparticle, the phenomenon was named as “nanosurface energy transfer” (NSET) by Yun et al. [51]. In our case, we can consider that the nanogold® is the energy acceptor and the Cy3 is the energy donor. Chemically, the hybridization leads to the increase of the XPS N1s peak whereas the Au 4d peak stays at a constant value. The appearance of the P 2p peak seems to be related to the phosphate residues from the hybridization solution. The different fragments already observed by ToFSIMS after the grafting step are still present.

V. CONCLUSIONS The grafting and hybridization of molecular beacons on two different types of conductive oxide films, ITO and doped SnO2, were confirmed from modifications of surface morphology and surface chemistry as well as fluorescence. Thanks to Au nanoparticle monolabelled hairpin probes and by combining the results of different techniques (XPS, TEM, AFM), an estimation of density grafting leads to 2.1 ± 0.3 x 1011 oligonucleotide.cm-2 on tin oxide films. Similar results were obtained on indium oxide films. This study provides a better knowledge of the surface of biomodified conductive oxide electrode, that is crucial in the aim of the electrochemical-based detection using such kind of electrode.

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

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[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

Mandelkern, M; Elias, J; Eden, D; Crothers, D. J Mol Biol., 1981, 152 (1), 153-161. Lakowicz, J. Topics in Fluorescence Spectrometry : Fluorescence Sensibility, Vol 7 DNA Technology, Springer US, 2003. Manesse, M; Stambouli, V; Boukherroub, R; Szunerits, S. in press in The Analyst, 2008. Peterson, AW; Heaton, RJ; Georgiadis, R. Nucleic Acids Res., 2001, 29, 5163. Smith, EAW. MJ., Cheng, Y; Barreira, SVP; Frutos, AG; Corn, RM. Langmuir, 2001, 17, 2502. 65. Zhang, N; Schweiss, R; Zong, Y; Knoll, W. Electrochim. Acta, 2007, 52, 2869. Rogers, KR. Molecular biotechnology, 2000, 14, 109-130. McKendry, R; Zhang, J; Arntz, Y; Strunz, T. et al, PNAS, 2002, 99, 15, 9783-9788. Cai, W; Peck, JR; van der Weide, DW; Hamers, RJ. Biosens. Bioelectron, 2004, 19, 1013. Zebda, A; Stambouli, V; Labeau, M; Guidicci, C; Diard, JP; Le Gorrec, B. Biosens. Bioelectron, 2006. 22, 178. Berggren, C; Stalhandske, P; Brundell, J; Johansson, G. Electroanalysis, 1999, 11, 156. 75. Li, A; Yang, F; Ma, Y; Yang, X. Biosens. Bioelectron, 2007, 22, 1716. Ito, T; Hosokawa, K; Maeda, M. Biosens. Bioelectron, 2007, 22, 1816. Pan, S; Rothberg, L. Langmuir, 2005, 21, 1022. Souteyrand, E; Clorec, JP; Martin, JR; Wilson, C; Lawrence, I; Mikkelsen, S; Lawrence, MF. J. Phys. Chem., B, 1997, 101, 2980. Berney, H; West, J; Haefele, E; Alderman, J; Lane, W; Collins, JK. Sensors Actuators B, 2000, 68, 100. Pouthas, F; Gentil, C; Cote, D; Bockelmann, U. Appl. Phys. Lett, 2004, 84(9), 1594. SPR commercial Biacor Reference. Bonnet, G; Tyagi, S; Libchaber, A; Russell Kramer, F. Proceeding of the National Academy of Sciences, 1999, 96, 6. Bonnet, G; Krichevsky, O; Libchater, A. Proceeding of the National Academy of Sciences, 1998, 95, 8602. Tyagi, S; Kramer, FR. Nature Biotechnology, 1996, 14, 303. Dubertret, B; Calame, M; Libchaber, AJ. Nature biotechnology, 2001, 19, 365. Kim, JH; Morikis, D; Ozkan, M. Sensors and actuators B, 2004, 102, 315. Piestert, O; Barsch, H; Buschmann, V; Heinlein, T; Knemeyer, JP; Weston, KD; Sauer, M. Nanoletters, 2003, 3, 979. Liu, X; Tan, W. Analytical biochemistry, 1999, 71, 5054. Wang, H; Li, J; Liu, H; Liu, Q; Mei, Q; Wang, Y; Zhu, J; He, N; Lu, Z. Nucleic Acids research, 2002, 30, e61. Du, H; Disney, MD; Miller, BL; Krauss, TD. Journal of the American Chemical Society, 2003, 125, 4012. Du, H; Strohsahl, CM; Miller, BL; Krauss, TD. Journal of the American Chemical Society, 2005, 127, 7932. Fan, C; Plaxco, KW; Heeger, AJ. Proc. Natl. Acad. Sci. USA, 2003, 100, 9134-9137.

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[30] Wei, F; Sun, B; Liao, W; Ouyang, J; Zhao, XS. Biosensors and bioelectronics, 2003, 18, 1149. [31] Wei, Y; Cao, C; Jin, R; Mirkin, CA. Science, 2002, 297, 1536. [32] Park, SJ; Taton, TA; Mirkin, CA. Science, 2002, 295, 1503. [33] Jin, Y; Yao, X; Liu, Q; Li, J. Biosensors and bioelectronics, 2006, 22, 1126. [34] Mao, Y; Luo, C; Ouyang, Q. Nucleic Acids Res., 2003, 31, e108. [35] Steichen, M; Buess-Herman, C. Electrochemistry Communications, 2005, 7, 416. [36] Simon, A; Cohen-Bouhacina, T; Port´e, MC; Aime, JP; Baquey, C. J. Colloid Interface Sci., 2002, 251, 278-283. [37] Cloarec, JP; Deligianis, N; Martin, JR; Lawrence, I; Souteyrand, E; Polychronakos, C; Lawrence, MF. Biosensors and Bioelectronics. 2002, 17, 405-412. [38] Marquette, CA; Lawrence, I; Polychronakos, C; Lawrence, MF. Talanta, 2002, 56, 763768. [39] Xu, J; Zhu, JJ; Huang, Q; Chen, HY. Electrochem. Commun, 2001, 3, 665-669. [40] Yang, IV; Thorp, HH. Anal. Chem., 2001, 73, 5316-5322. [41] Stambouli, V; Labeau, M; Matko, I; Chenevier, B; Renault, O; Guiducci, C; Chaudouët, P; Roussel, H; Nibkin, D; Dupuis, E. Sensors and Actuators B, 2006, 113, 1025-1033. [42] Stambouli, V; Zebda, A; Appert, E; Guiducci, C; Labeau, M; Diard, JP; Le Gorrec, B; Brack, N; Pigram, PJ. Electrochemica Acta, 2006, 51, 5206-5214. [43] Popovich, ND; Eckhardt, AE; Mikulecky, JC; Napier, ME; Thomas, RS. Talanta, 2002, 56, 821-828. [44] Bras, M; Dugas, V; Bessueille, F; Cloarec, JP; Martin, JR; Cabrera, M; Chauvet, JP; Souteyrand, E; Garrigues, M. Biosens. Bioelectron, 2004, 20, 797-806. [45] Lambacher, A; Fromherz, P. Appl. Phys. A, 1996, 63, 207-216. [46] Labeau, M; Redoux, V; Dhahri, D; Joubert, JC. Thin Solid Films, 1986, 136, 257-262. [47] http://www.nanoprobes.com/ [48] Rouillat, MH ; Dugas, V ; Martin, JR. Phaner-Goutorbe, M. Applied Surface Science, 2005, 252, 1765-1771. [49] Mateo-Marty, E; Briones, C; Pradier, CM; Martin-Gago, JA. Biosensors and Bioelectronics, 2007, 22, 1926-1932. [50] Wu, ZS ; Jiang, JH ; Fu, L; Shen, GL; Yu, RQ. Anal. Biochem, 2006, 353, 11-29. [51] Yun, CS; Javier, A; Jennings, T; Fisher, M; Hira, S; Peterson, S; Hopkins, B; Reich, NO; Strouse, GF. J. Am. Chem. Soc., 2005, 127, 3115-3119. [52] Massey, M; Russ Algar, W; Krull, UJ. Anal. Chim. Acta, 2006, 568, 181-189. [53] Dulkeith, E; Morteani, AC; Niedereiechholz, T; Klar, TA; Feldmann, J; Levi, SA; van Veggel, FCJM; Reinhoudt, DN; Moller, M; Gittins, DI. Phys. Rev. Lett, 2002, 89, 203002-1-203002-4.

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In: Nanoparticles: Properties, Classification,… Editor: Aiden E. Kestell et al., pp. 323-338

ISBN: 978-1-61668-344-3 © 2010 Nova Science Publishers, Inc.

Chapter 7

SYNTHESIS AND OPTICAL PROPERTIES OF POLYMER FUNCTIONALIZED INORGANIC NANOPARTICLES Zhiguo Wang1,2, Xiaotao Zu1 and Jingbo Li2 1

Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu, 610054, P.R. China. 2 State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P.R. China.

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ABSTRACT Nanoparticles possess many special characteristics that can be used in many fields such as optics, electricity, magnetism and catalysis. However, it is difficult to obtain a uniform dispersion system of nanoparticles owing to their strong tendency of agglomeration for their high surface energy, so surface modification is urgently needed. Using polymer to modify the surface performance of nanoparticles is an interesting method. One major topical areas on preparing polymer nanoparticles are summarized in this chapter. Poly (methyl methacrylate) functionalized nanoparticles (anatase, γ-Al2O3, SiO2, and ZnO) are prepared using γ and electron radiation. A blue luminescence peak (at ~430 nm) can be observed for the nanocomposite. Annealing in air atmosphere destroys the carboxylate bonds and induces the decrease of luminescence density.

1. INTRODUCTION Nanomaterials are defined as materials with grain sizes below 100 nm or more stringent, as materials with special properties depending on their small grain size. Many crystallized or amorphous nanomaterials exhibit interesting physical properties [1]. These properties are related to the special electronic structure of the small particles. Whereas the special properties will be lost if the particles interact with each other. To avoid interaction between the particles they have to be kept in a certain distance depending on the type of interaction. However, it is difficult to obtain a uniform dispersion system of nanoparticles owing to their strong tendency

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of agglomeration for their high surface energy. Many efforts have been made to stabilize these nanoparticles, including employing organic ligands [2], coating nanoparticles with inorganic shells [3], and initiating polymerization on nanoparticles surfaces [4]. These considerations lead to nanocomposites consisting of an active phase, carrying the special physical property and a passive one, acting as distance holder. In recent years, the nanoparticles/polymer composites have attracted much attention [5-7]. The polymer can effectively improve the dispersion of the nanoparticles, and the surface can be tailed to special physical, optical, electronic, chemical, and biomedical properties by coating a thin film of materials on the surface of nanoparticles. Many approaches have been tried to change the surface characters of the nanoparticles using different techniques such as the vapor deposition [8], precursor technique [9], nanoreactor technique [10], supermolecular selfassembly process [11], ultrasonic irradiation [12] and plasma polymerization [13,14]. Luminescence is one of the most important properties of nanoparticles. The luminescence property from nanoparticles is important for application in medical, biological and pharmaceutical areas [15,16]. With respect to nanoparticles, quantum dots based on sulphides, selenides, or tellurides of zinc and cadmium show the best luminescence efficiency [17,18]. These materials are as well toxic as carcinogenic and show limited thermodynamic stability against oxidation. Applications are found primarily in medical, biological, or pharmaceutcal areas. These severe disadvantages make application difficult. Luminescence of oxide nanoparticles is subject of rapid aging caused by formation of hydroxides at the surface of oxide nanoparticles. Zou et al. [19] found that TiO2 ultrafine particles coated with a layer of stearic acid can exhibit 540 nm fluorescence. The ZnO can generate stable luminescence peaks down to the 465 nm (blue region) after coated with poly (ethylene glycol) [20], whereas the poly styrene coated ZnO nanoparticles show an yellow-green luminescence under a UV lamp [21]. In this chapter a simple method to initiate polymerization by irradiation, without any added initiators, was reviewed. Such irradiation initiated radical polymerization can be precisely controlled to form a thin polymer shell around nanoparticles. The coated nanoparticles show stable blue luminescence at room temperature.

EXPERIMENTAL DETAILS 2.1 Materials The investigation had been carried out on commercial anatase (20±10 nm), γ-Al2O3 (60±10 nm), ZnO (20±5 nm) and two types of SiO2 (SP1: 10±5 nm and specific surface area 640 m²/g; SS1: 15±5 nm and specific surface area 160 m²/g) nanoparticles. The morphology of the nanoparticles has been characterized by transmission electron microscope (JEM 2010FEG/STEM field emission TEM), showing that the ZnO, anatase-TiO2 and γ-Al2O3 are crystalline and both SS1-SiO2 and SPI-SiO2 are amorphous particles. Before irradiation all the nanoparticles were heated at 120 oC for 8 hrs in order to eliminate the possible adsorbed water on the surface of the nanoparticles. Methyl methacrylate (MMA), which was purchased from Chinese Shanghai First Chemical Work of Reagent, was distilled at reduce pressures and preserved at 4 oC before

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electron irradiation. All other reagents were analytical pure and were used without further purification.

2.2 Preparation of Polymer/Inorganic Nanoparticles The dried nanoparticle (w0) were accurately weighed out and fully mixed with prepared MMA solution (20% in volume) in cultural dishes. The solvent of MMA solution was the mixture of n-heptane /chloroform (2:3 in volume). The solution was radiated to doses of 30, 60, 90 and 120 kGy by γ radiation from a 60Co source at a dose rate of 10 kGy/h or a dose of 10, 20 and 30 kGy by electron radiation in an Electron Electrostatic Accelerator with 1.6 MeV electron beam at a dose rate of 60 kGy/h to in air at room temperature. After the irradiation, the synthetic polymer/nanopowder composites were wrapped with filter paper, and extracted using a Soxhlet extractor with boiling xylene (the homopolymer is thought to be completely removed by this way). The extracted composites were dried in air at 70 oC until a constant weight (w) was reached. The extracted composite after irradiation is termed as nanocomposite.

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2.3 Characterizations X-ray diffraction (XRD) analyses were performed in a X-ray diffractometer Type PHILIPS X’Pert Pro MPD with Cu-Kα (λ = 0.15406 nm). Fourier transform infrared (FTIR) spectra of the sample in KBr pellets were recorded using a Nicolet 560 FTIR spectrometer. The Spectra were collected from 4000 to 400 cm-1, with a 4 cm-1 resolution over 20 scans. The samples were also analyzed by XPS using a XSAM 800 Flexo electron spectrometer with monochromatic Al-Kα X-ray source (hν = 1486 eV). The instrument was standardized against the C1s spectral line at 285 eV, and the spectra were interpreted and deconvoluted using the KRATOS computer software package. Steady state photoluminescence measurements were carried out on the dispersion of nanopowders in purified water. For the optical measurements, nanopowders were first suspended in purified water. The solutions were then dispersed with 50W KQ-50B ultrasonic irradiation. Photoluminescence was recorded using a Shimadzu RF-5301PC fluorometer employing a 150 W Xe lamp as the light source. Excitation and emission monochromators were on mutually perpendicular directions.

3. RESULTS 3.1 Photoluminescence from PMMA Coated Nanopartilces Synthesized Through Electron Irradiation Figure 1 shows the steady state luminescence curves for pure anatase TiO2 and γ-Al2O3 nanoparticles and the corresponding nanocomposite synthesized through electron irradiation at room temperature. Excitation wavelength was kept constant at 305nm. No luminescence

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peaks appear in the pure nanoparticles. However, luminescence peaks at 415 nm and 420 nm can be observed for the nanocomposites of anatase TiO2 and γ-Al2O3, respectively. Figure 2 shows the X-ray diffraction patterns of pure anatase and the corresponding nanocomposites. The characteristic Bragg diffraction peaks of anatase can be observed in the nanocomposites. No obvious changes can be found in the XRD patterns after electron irradiation-induced polymerization, indicating that the irradiation-induced polymerization has no influence on the crystal structure.

Figure 1. Photoluminescence of the pure anatase and γ-Al2O3 nanopowders and the corresponding nanocomposites with the excitation wavelength = 305nm at room temperature.

Figure 2. X-ray diffraction patterns for the pure anatase and rutile nanopowders and the corresponding nanocomposites. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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Figure 3. FTIR spectra of the pure anatase and γ-Al2O3 nanopowders and the corresponding nanocomposites.

The comparison of FTIR spectra between the nanocomposite obtained after 24 h extraction and pure nanoparticles is shown in Figure 3. As compared with pure samples, a new peak attributed to carbonyl stretching vibration appears at about 1725 cm-1 after electron irradiation, which is close to the characteristic peak of PMMA at 1730 cm-1. The new peak is an indication of the existence of unextractable PMMA. The carbonyl vibration peaks of two nanoparticle/PMMA nanocomposites both shift to the lower wavenumbers. In contrast, the characteristic absorption peaks of anatase (1630 cm-1), r and γ-Al2O3 (1642 cm-1) shift to higher wavenumbers (1645 cm-1) after irradiation-induced polymerization. These results show that there are strong interactions between the PMMA and nanoparticles. High-resolution XPS collections of the C 1s binding energy regions are shown in Figure 4. Carbon is present in the pure nanoparticles because they can easily absorb pollutant in air due to their extremely high specific surface energy and numerous surface defects. The carbon is so tightly absorbed that it cannot be eliminated by vacuum during XPS measurement. For the pure nanoparticles, the photoelectron spectra of C 1s curve can be fitted by two peaks at 282.5eV and 285.0 eV for anatase, and 282.7eV and 285.2 eV for γ-Al2O3, respectively. The peak 285 eV is attributed to the C-C bond resulting from diffusion pump oil. The 282 eV may be due to carbide contaminant [22,23]. The C 1s curves for the nanocomposites can be fitted by three peaks at 282.7 eV, 285.2 eV, 288.0 eV for anatase and 282.0 eV, 285.2 eV, 288.0 eV for γ-Al2O3, respectively. The new peak at 288 eV is corresponding to carbonyl groups C=O [24]. The C-O at 286.67 eV for the methoxy group of the ester chemical function overlaps with the peak at 285 eV; thus, it was not deconvoluted.

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Anatase

Al2O3

Figure 4. C1s narrow scan X-ray photoelectron spectra of pure anatase and γ-Al2O3 and the corresponding nanocomposites.

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Anastase Anatase

Al 2O3

Pure

Pure

Nanocomposite

Nanocomposite

540

536

532

528

524

540

536.

532

528

524

Binding Energy (eV) Figure 5. O1s narrow scan X-ray photoelectron spectra of pure anatase and γ-Al2O3 and the corresponding nanocomposites.

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High-resolution XPS collections of the O 1s binding energy regions are shown in Figure 5. For the pure nanoparticles, the photoelectron spectra of O 1s curve can be fitted by two peaks at 530.0 eV, 532.7 eV for anatase, and 531.3 eV, 533.3 eV for γ-Al2O3, respectively. The high binging energy component is usually attributed to the presence of loosely-bound oxygen on the surface of nanoparticles. The other component is the binding energy of interior of nanoparticles. The O 1s curves for the nanocomposites can be fitted by three peaks at 529.6 eV, 531.2 eV, 532.8 eV for anatase and 529.1 eV, 531.6 eV, 532.1 eV for γ-Al2O3, respectively. The new peak at about 529 eV is an indication of the surface modification of the nanoparticles by PMMA. The solvent extraction, FTIR and XPS results clearly show that there exist some unextractable PMMA in the nanocomposites. The unextractable polymer indicates the presence of chemical bonds between the polymer and the nanoparticles. The polymerization reaction mechanism is based on the free radical mechanism on the nanocrystal surface. Oxygen atom defects are generated by the electron irradiation. It was believed that radiation caused the lose of the oxygen atom bonding with aluminum or titanium and produced radiation default in aluminum oxide (Al2O3) or titanium oxide (TiO2). One electron of the double bond was opened in vinyl monomers, and coordinated to aluminum or titanium of the nanocrystals; the other initiated free radical graft polymerization of MMA on the surface. The same results have been reported on micro-meter Al2O3 [25,26]. Grafting polystyrene onto sorbate-modified titanium dioxide surface possesses similar graft style between titanium atom and sorbet [27]. After the irradiation-induced polymerization on the anatase TiO2 and γ-Al2O3 nanoparticles, a new photoluminescence peak at about 415 and 420nm can be found at room temperature. Zou et al [19] found that TiO2 ultrafine particles coated with a layer of stearic acid can have 540 nm fluorescence. The mechanism of the photoluminescence is unclear, but it should be induced by the surface modification of the nanoparticle by the irradiation-induced polymerization. The photoluminescence maybe caused by the carbonyl adjacent to the surface of the nanoparticle. For biacetyl, CH3-(C=O)-(C=O)-CH3, it is well known that the carbonyl group is responsible for luminescence in aliphatic compounds [28]. Vollath et al. [29, 30] also found PMMA coated oxide core can emit blue emission at about 420 nm, originated from the carbonyl group of the coating polymer. The PMMA coated oxide core has a similar structure to our work. The polymerized nanoparticles might be readily dispersed into some polymer matrix due to the surface modification.

3.2 Photoluminescence from PMMA Coated Nanopartilces Synthesized through Γ Irradiation Figure 6 shows the photoluminescence of the modified TiO2 nanocrystals at room temperature after radiated to different dose. It is obvious that a broad photoluminescence spetra occur at around 420 nm after the radiation. The mechanism of the photoluminescence is still unclear, but it should be induced by the surface modification of the radiation induced polymerization.

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Figure 6. Visible photoluminescence of TiO2 and PMMA modified TiO2 prepared through γ radiation

Figure 7. FT-IR spectra of anatase-TiO2 nanocrystals before and after γ radiation at doses of 30, 50 and 90 kGy. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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The comparison between the nanocomposite obtained after 72hrs extraction and pure nanoparticles is shown in Figure 7. Compared with the curves of pure samples, it can be seen that the new peaks attributed to carbonyl stretching vibration appear at about 1730 cm-1 after the γ irradiation, which is the characteristic peak at 1730 cm-1 of PMMA. The peak at ~1399 cm-1 is attributed to the bend vibration of –CH3 group of polymer. The new peak is an indication the existence of unextractable PMMA. The characteristic absorption peak of anatase (1630 cm-1) shift to higher wavenumber (about 1635 cm-1) after radiation induced polymerization. These results show that the composites are not the simple mixtures of polymer and TiO2 nanocrystals. There should be strong interactions between the PMMA and nanoparticles, maybe new chemical bonds are formed on the interface between PMMA and TiO2.

Figure 8. Structure of the connection of PMMA to the surface of nanoparticles through (a) graft polymerization (b) ester like linkage.

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Figure 9. Room temperature photoluminescence spectra of the PMMA modified anatase nanoparticles prepared by (a) electron or (b) γ radiation and annealing.

The unextractable polymer indicates the presence of chemical bonds between the polymer and the nanoparticles. The oxygen on TiO2 surface can initiate the graft polymerization during the radiation. The radiation produced anion free radical on the TiO2 nanoparticle surface, and anion free radicals have higher activation. Brailsford et al. [31] suggested that the anion free radical species come from decomposed or are produced by trapped electron reacting with oxygen atom. Wong et al [32] has discussed the presence of anion free radical species on MgO surface by irradiation after studying wave function of selfspin state by EPR spectroscopy, the electron which dose not pair is in 2Pz orbit in the main, namely, 2Px22Py22Pz1. The anion free radicals induce the polymerization of MMA. The connection of the PMMA to the particles is given in figure 8 (a). The PMMA maybe also bound to to the nanoparticle with an ester like linkage [33], as shown in figure 8 (b). The photoluminescence maybe caused by the carbonyl adjacent to the surface of the nanoparticle. For biacetyl, CH3-(C=O)-(C=O)-CH3, it is well know tat the carbonyl group is responsible for luminescence in aliphatic compounds [28].

3.2 Annealing Induced the Decrease of Luminescence Density Figure 9(a) and (b) show the room temperature photoluminescence spectra of the PMMA modified anatase nanoparticles prepared by electron and γ radiation combined with annealing, respectively. It is obvious that a broad photoluminescence peak appears at around 430 nm

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after the treatment. The excitation wavelength used is 310 nm. The intensity of PL decreases with the increasing annealing time, and the PL almost disappears for the anatase+MMA radiated up to a dose of 90 kGy with γ radiation and 30kGy with electron radiation after annealing for 5min.

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Figure 10. Room temperature photoluminescence spectra of the PMMA modified SiO2 nanoparticles prepared by γ radiation and annealing: (a) SP1, (b) SS1.

Figure 11. Luminescence intensity of the surface modified nanoparticles as a function of annealing time.

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Figure 10(a) and (b) show the room temperature photoluminescence spectra of the PMMA modified SP1 and SS1 nanoparticles prepared by γ radiation and annealing, respectively. A visible emission centered at 440 nm can be clearly observed. The excitation wavelength used is 310 nm. But the intensity of PL from SP1 is stronger than that from the SS1. The result shows that the intensity of PL is related to the specific surface area. Visible emission centered at 440 nm and 410 nm can be clearly observed for the modified γ-Al2O3 and ZnO nanoparticles, respectively. The intensity of PL decreases with the increasing of annealing time. However, the micron γ-Al2O3 powders do not show any photoluminescence after the same treatment of nanoparticles. This result indicates that the PL is related to the nanoscale of the particles. Figure 11 shows the luminescence intensity of the surface modified nanoparticles as a function of annealing time. The luminescence intensity decreases with the increase of annealing time. The results indicate that an extended annealing process obviously destroys carboxylate bonds and luminescence. Figure 12 shows the X-ray diffraction patterns of the as-received anatase, PMMA coated anatase, and annealed anatase. The characteristic Bragg diffraction peaks of anatase can be observed in these samples. There is no obvious change in the XRD patterns after radiation induced polymerization and annealing, indicating the radiation induced polymerization and annealing have no influence on the crystal structure.

Figure 12. X-ray diffraction patterns of as received anatase, PMMA coated anatase, and annealed anatase after PMMA coating.

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Synthesis and Optical Properties of Polymer Functionalized Inorganic…

Figure 13. FTIR spectra for the as received and PMMA modified (a) anatase and (b) γ-Al2O3 nanoparticles. Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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Figure 14. High-resolution XPS collections of the O1s (a) and C1s (c) binding energy for the as received ZnO (1#) samples and ZnO+MMA radiated to a dose of 90 kGy (2#) and ZnO+MMA radiated to a dose of 90 kGy (3#) and annealed for 1min and 5min (4#).

Figure 13 (a) and (b) show the FTIR spectra for the as-received and modified anatase and γ-Al2O3 nanoparticles. Compared with the curves of the as-received samples, it can be seen that the new peaks attributed to carbonyl stretching vibration appear at about 1728 cm-1 after the radiation, which is due to the characteristic peak at 1730 cm-1 of PMMA. The peak at ~1400 cm-1 is attributed to the bend vibration of –CH3 group of polymer. The new peak is an indication of the existence of PMMA. These characteristic peaks of PMMA (~1728 cm-1 and ~1400 cm-1) disappear after the samples were annealed, which is an indication that the PMMA has burn out. High-resolution XPS collections of the O1s and C1s binding energy regions for the ZnO nanoparticles have been shown in Figure 14(a) and (b), respectively. For the as-received ZnO nanoparticles, the photoelectron spectra of O1s curve can be fitted by two peaks centered at 530.461 eV and 532.169 eV. The high-binding energy component is usually attributed to the presence of loosely bound oxygen on the surface of ZnO nanocystals [34]. The low-binding energy component is attributed to O2- ions on the wurtzite structure of the hexagonal Zn2+ ion array, surrounded by Zn atoms [35,36]. For the ZnO+MMA radiated to a dose of 90 kGy, the photoelectron spectra of O1s curve can be fitted by three peaks centered at 530.080 eV, 531.4460 eV and 533.249 eV. The high binding energy component is attributed to the carbonyl groups C=O, which indicates that the MMA polymerized after the radiation. The O 1s curve can be fitted by two peaks at 530.433 eV and 532.020 eV for the sample annealed for 1min, and 530.481 eV and 532.110 eV for the sample annealed for 5min. The binding

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Synthesis and Optical Properties of Polymer Functionalized Inorganic…

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energy of 533.249 eV disappears and the O1s recovers to its original value. For the asreceived ZnO nanoparticles, the photoelectron spectra of C1s curve can be fitted by one peak at 285.015 eV which is attributed to the diffusion pump oil representing C-C. For the ZnO+MMA radiated to a dose of 90 kGy, the photoelectron spectra of C1s curve can be fitted by two peaks centered at 285.035 eV and 289.04 eV. The new peak at 289.04 eV is corresponding to carbonyl groups C=O [37]. The C1s curve can be fitted by two peaks at 284.921 eV and 289.5 eV for the sample annealed for 1min, and one peak at 285.200 eV for the sample annealed for 5min. The results of C1s are consistent with those of the O1s. MMA is polymerized induced by the radiation. Annealing destroys the carbonyl, and the PL decreases even disappears.

4. CONCLUSION PMMA coated nanoparticles are successfully synthesized by γ or electron radiation, The modified anatase, SiO2, γ-Al2O3 and ZnO show blue emission centered at about 430 nm, 440 nm, 410nm and 420nm, respectively. The photoluminescence are related to the organic molecules. Annealing in air atmosphere destroys carboxylate bonds and induces the decrease of luminescence density.

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ACKNOWLEDGMENTS Z. Wang was financially supported by the National Natural Science Foundation of China (10704014) and the Young Scientists Foundation of Sichuan (09ZQ026-029) and UESTC (JX0731). J. Li gratefully acknowledges financial support from the "One-Hundred Talents Plan" of the Chinese Academy of Sciences.

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

Gleiter, H. Prog. Mat. Sci., 1989, 33, 223. Aldana, J; Lavelle, N; Wang, Y; Peng, X. J. Am. Chem. Soc., 2005, 127, 2496. Kim, S; Bawendi, MG. J. Am. Chem. Soc., 2003, 125, 14652. Xiong, HM; Wang, ZD; Xia, YY. Adv. Mater, 2006, 18, 748. Levine KL; Iroh, JO; Kosel, PB. Appl. Surf. Sci., 2004, 230, 24. Guo, L; Yang, SH; Yang, CL; Yu, P; Wang, JN; Ge, WK; Wong, GKL. Appl. Phys. Lett, 2000, 76, 2901. [7] Guo, L; Yang, SH; Yang, CL; Yu, P; Wang, JN; WK. Ge, WK; Wong, GKL. Chem. Mater, 2000, 12, 2268. [8] Akamatsu, K; Deki, S. Nanostructured Mater, 1997, 8, 1121. [9] Watkins, JJ; Mccarthy, T. J. Polym. Mater. Sci. Eng., 1995, 73, 158. [10] Mayer, A; Antonetti, M. Colloid Polym. Sci., 1998, 276, 769. [11] Weller, H. Adv. Mater, 1993, 5, 193.

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[12] Xia, HS; Wang, Q; Qiu, GH. Chem. Mater, 2003, 15, 3879. [13] Shi, DL; Wang, SX; Van Ooij Wim, J; Wang, LM; Zhao, JG; Yu, Z. Appl. Phy. Lett, 2001, 78, 1243. [14] SHi, DL; Lian, J; He, P; Wang, LM; Van Ooij, Wim J; Schulz, M; Liu, Y; Mast, DB. Appl. Phy. Lett, 2002, 81, 5216. [15] Bruchez, Jr. M; Moronne, M; Gin, P; Weiss, S; Alivastos, AP. Science, 1998, 281, 2013. [16] Chan, WCW; Nie, S. Science, 1998, 281, 2016. [17] Tian, Y; Newton, T; Kotov, NA; Guldi, DM; Fendler, JH. J. Phys. Chem., 1996, 100, 8927. [18] Porteanu, HE; Lifshitz, E; Pflughoefft, M; Eychmuller A; Weller, H. Phys. Sol. B, 2001, 226, 219. [19] Zou, BS; Xiao, LZ; Li, T; Zhao, JL; Lai, ZY; Gu, SW; Appl. Phys. Lett, 1991, 59, 1826. [20] Abdullah, M; Morimoto, T; Okuyama, K. Adv. Func. Mater, 2003, 13, 800. [21] Hong, RY; Chen, LL; Li, JH; Li, HZ; Zheng, Y; Ding, J. Polym. Adv. Technol, 2007, 18, 901-909. [22] Zhang, LH; Koka, RV. Mater. Chem. Phy., 1998, 57, 23. [23] Santerre, F; El Khakani, MA ; Chaker, M; Dodelet, JP. Appl. Surf. Sci., 1999, 148, 24. [24] Benamor, S; Baud, G; Jacquet, M. Appl. Surf. Sci., 2000, 153, 172. [25] Huang, GL; Wang, J. Polymer. Mater. Sci. Eng., 1993, 9, 40. [26] Wang, J; Huang, GL. Nucl. Sci. Tech., 1993, 4, 245. [27] Nakatsuka, T; Kawasaki, H; Itadani, K; Yamashita, S. J. Appl. Polym. Sci., 1979, 23, 3139. [28] Parker, CA. Photoluminescence of Solutions, Elsvier, 1968, Amsterdam, 21. [29] Vollath, D; Szabo, DV; Schlabach, S. J. Nanoparticles Res., 2004, 6, 181. [30] Vollath, D; Szabo, DV. Adv. Eng. Mater, 2004, 6, 3. [31] Brailsford, JR; Morton, JR. J. Chem. Phys., 1969, 51, 4797. [32] Wong, NB., Lunsford, JH. J. Chem. Phys., 1971, 55, 3007. [33] Weng, YX; Li, Y; Liu, Y; Wang L; Yang, GZ. J. Phys. Chem., B. 2003, 107, 4356. [34] Cebulla, R; Weridt R; Ellmer, K. J. Appl. Phys., 1998, 83, 1087. [35] Rao, LK; Vinni, V. Appl. Phys. Lett, 1993, 63, 608. [36] Fan, JCC. J. Appl. Phys., 1977, 48, 3524. [37] Ben Amor, S; Baud G; Jacquet, M. Appl. Surf. Sci., 2000, 153, 172.

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INDEX

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A  abatement, 229  absorption spectra, 86, 93, 97, 107  absorption spectroscopy, 30, 33, 110  accessibility, 290  accommodation, 9, 184, 206  accuracy, 50, 197  acetic acid, 111, 117, 216, 228, 240  acetone, 112, 117, 120, 240, 295  acetonitrile, 295  achievement, 230, 276  acid, 15, 44, 56, 100, 112, 117, 118, 120, 122, 217,  220, 228, 231, 233, 242, 246, 296, 297, 324, 329  acidity, 15, 16, 223  activation, 14, 40, 214, 234, 276, 332  activation energy, 14, 40  active oxygen, 243  active radicals, 240  active site, 47, 233  actuators, 261, 320  adamantane, 224  additives, x, 213, 238, 240, 245  adenine, 46, 289, 312  adhesion, 18, 42, 238  adhesion properties, 18  adjustment, 110  adsorption, 14, 15, 42, 44, 49, 56, 57, 100, 120, 216,  224, 233, 243, 279  aerogels, 21  AFM, vii, xi, 1, 30, 37, 214, 229, 230, 233, 288, 292,  297, 298, 299, 300, 301, 302, 303, 305, 306, 307,  308, 314, 317, 318, 319  ageing, 227, 231, 247  agent, 25, 28, 55, 217, 220, 233, 238  aggregates, 44, 114, 121, 233, 236 

      aggregation, 44, 119, 236  aging, 16, 21, 231, 232, 324  air pollutants, x, 213, 215, 239  alcohol, 21, 47, 104, 119, 122, 219, 220, 227  alcohols, 96, 118, 229  aldehydes, 239, 240  algorithm, 179, 189, 212  aliphatic compounds, 329, 332  alkenes, 118  alloys, ix, 8, 22, 25, 30, 149, 151, 154, 155  alternative, 46, 86, 154, 168, 227  alters, 107  aluminum, 16, 20, 169, 171, 329  aluminum oxide, 16, 329  ammonia, 104, 117, 122, 168, 217  ammonium, 22, 24, 25  amplitude, 40, 48, 91, 93, 94, 96, 234, 236, 276  aniline, 263  animals, 55, 57  anisotropy, 13  annealing, 8, 9, 217, 243, 273, 332, 333, 334  annihilation, 90  antibody, 42, 45, 49, 51, 57  anticancer drug, 56  antigen, 42, 45, 46, 49, 51  anti‐inflammatory drugs, 56  antimony, 46  apoptosis, 55  aqueous solutions, 111, 112, 118, 125, 217  aqueous suspension, 217  argon, 18, 94, 96, 153, 158, 159, 162, 163, 166, 169,  170, 171, 172, 177, 183, 206, 207, 208, 211  arthritis, 56  assumptions, 15, 153, 176, 180, 184 

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Index

atmospheric pressure, 150, 153, 176, 207, 208, 212,  227, 293  atomic distances, 32  atomic force, 37  atomic nucleus, 36  atoms, x, 2, 11, 14, 18, 21, 28, 31, 32, 33, 34, 177,  183, 214, 225, 255, 256, 257, 260, 261, 275, 277,  293, 318, 336  attachment, 46, 49, 121  attacks, 56  attractiveness, 25, 120  attribution, 305  Au nanoparticles, xi, 56, 288, 292, 310, 313, 314,  316, 317, 318  Australia, 287  availability, 55 

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B  backscattering, 36  bacteria, 49, 50, 112, 120, 122  band gap, viii, 6, 7, 11, 79, 82, 83, 85, 86, 90, 91, 93,  101, 103, 105, 107, 108, 109, 123, 214, 224, 227,  243  barriers, 7, 11, 119  base catalysis, 220  base pair, 289, 295  basicity, 16  behavior, viii, ix, 2, 9, 13, 16, 38, 42, 43, 48, 54, 79,  80, 81, 109, 168, 176, 180, 184, 189, 207, 256,  263, 269  Beijing, 323  bending, 98, 99, 123, 257, 261, 262, 263, 267, 268,  275  beneficial effect, 234  benign, 23  benzene, 22, 112, 118, 223  bias, 40, 264  binding, xi, 15, 33, 42, 44, 49, 50, 55, 56, 57, 87, 266,  288, 301, 302, 308, 327, 329, 336  binding energies, 15, 301, 302, 308  binding energy, 33, 301, 302, 327, 329, 336  bioassay, 52  bioavailability, 56  biochemistry, 320  biocompatibility, 42, 43, 47, 53, 57  biological systems, 37  biomarkers, 43, 53  biomass, 124  biomaterials, 41, 54 

biomedical applications, 2, 57  biosensors, viii, xi, 2, 41, 42, 44, 45, 46, 47, 49, 50,  57, 287, 289  biotechnology, 42, 52, 320  bleaching, 52, 53, 93, 94, 95, 96, 107  blood, 53, 55  Boltzmann constant, 98, 152, 206  bonding, 15, 33, 44, 225, 256, 257, 276, 317, 329  bonds, x, xii, 16, 118, 220, 255, 259, 261, 265, 270,  272, 273, 275, 276, 277, 289, 323, 334, 337  bone marrow, 55  boundary surface, 40  branching, 266  breast cancer, 42  buffer, 16, 56, 295, 297, 306  Bulgaria, 255  bulk materials, vii, 1, 6, 30, 103, 150  burn, 56, 336  burning, 154  by‐products, 220 

C  cadmium, 104, 105, 106, 108, 109, 111, 113, 114,  116, 324  calcination temperature, 220, 223  calcium, 244  calcium carbonate, 244  calibration, 301, 302  cancer, 42, 51, 53, 56, 124, 125  cancer cells, 42, 51, 53  candidates, 155  carbides, 151, 155  carbon atoms, x, 255, 256, 257, 259, 260, 273, 279  carbon dioxide, 103  carbon film, 297  carbon materials, x, 113, 255, 257, 260  carbon monoxide, 243  carbon nanotubes, x, 19, 23, 25, 26, 46, 113, 124,  255, 258, 265, 266, 267, 272, 273, 275, 276, 277,  279  carbonization, 118  carbonyl groups, 327, 336  carboxylic acids, 118  carrier, 56, 91, 99, 102, 172, 176, 180  catalyst, 14, 16, 19, 21, 23, 24, 41, 168, 216, 228,  230, 232, 240, 245, 260  catalytic activity, 14, 16, 168, 224  catalytic properties, 14, 16, 32, 46, 49, 80, 150  catalytic system, 46 

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Index category a, 24  cation, 44, 91, 156  cell, 2, 10, 32, 47, 49, 52, 54, 55, 56, 82, 122, 124,  168, 211, 260, 270  cell membranes, 55, 122  cell surface, 55  ceramic, 10, 21, 56, 124, 152, 168, 169, 209, 210  cerium, 11, 16, 24, 46  CGC, 294, 295  channels, 2, 26, 27  charge density, 93  charge migration, viii, 79, 98  charge trapping, viii, 79, 90, 99, 100, 123  chemical bonds, 15, 38, 329, 331, 332  chemical properties, 4, 52, 214, 219, 229, 248, 291  chemical reactions, 99, 123, 151, 152  chemical reactivity, ix, 43, 149, 150, 151, 152, 154  chemical stability, 42, 93  chemical vapor deposition, 17, 19  chemical vapour deposition, 292  chemisorption, 16  chemotherapy, 56  China, 323, 337  chirality, 262, 263, 269, 271  chloride anion, 228  chloroform, 325  cholesterol, 46  classification, iv, 3, 57, 81, 91, 119  cleaning, 215, 234, 238, 293  cleavage, 118, 295  closure, 187  clusters, 15, 85, 191, 225, 237, 272  CMC, 26  coagulation, ix, 150, 176, 185, 187, 188, 191, 192,  193, 194, 197, 199, 200, 201, 202, 212  coagulation process, 201, 202  coatings, 21, 124, 156, 210, 238, 240, 241, 242, 243,  244, 246, 248  cobalt, 194  collisions, 151  combined effect, 109, 225  combustion, x, 150, 213, 215, 216  combustion processes, 150  communication, 46, 47  community, 23  compatibility, 238  competition, 50, 223  complementary DNA, 51, 297, 319  complexity, 49  compliance, 56 

341

complications, 316  components, xi, 25, 33, 37, 40, 94, 111, 112, 113,  122, 156, 158, 169, 170, 171, 177, 217, 241, 245,  248, 273, 287, 288  composites, 24, 29, 208, 242, 273, 325, 331  compounds, ix, 11, 34, 40, 50, 80, 101, 102, 105,  124, 205, 239, 240, 241, 243, 294, 308  computation, ix, 150, 176, 180, 189, 201  computed tomography, 51  computer software, 325  computing, 261, 274  conception, 81  concrete, 124  condensation, ix, 20, 21, 149, 150, 152, 155, 156,  163, 166, 168, 172, 176, 184, 185, 187, 188, 189,  191, 193, 194, 196, 200, 201, 206, 208, 212, 219,  220, 235  conduction, viii, 5, 11, 79, 82, 85, 88, 89, 90, 91, 92,  93, 97, 101, 102, 103, 104, 105, 107, 108, 109,  110, 114, 115, 116, 121, 178, 179, 214  conductivity, 10, 11, 40, 45, 46, 154, 169, 179, 186,  206, 229, 264, 268, 300  conductor, 11, 40, 170  configuration, 44, 214, 266, 271, 273, 305, 307, 317  confinement, viii, 6, 11, 12, 43, 46, 79, 83, 84, 85,  86, 87, 101, 102, 103, 105, 113, 114  conjugation, 53, 55, 56, 294, 296  conservation, 11, 177, 188, 198, 199  consolidation, 17  constituent materials, 157  construction, x, 47, 49, 50, 55, 255  consumers, 124  consumption, 188, 191, 194, 195, 201, 202, 203,  241, 293  contaminant, 327  contamination, 8, 16, 28, 38, 150, 293  continuity, 177  control, vii, 1, 17, 19, 20, 23, 24, 25, 41, 55, 56, 80,  150, 151, 156, 162, 163, 166, 171, 174, 184, 211,  214, 217, 219, 227, 234  convergence, 179  conversion, viii, 2, 54, 112, 113, 122, 124, 140, 151,  174, 201, 202, 203, 206, 214, 223, 226, 237, 246,  275  cooling, 150, 155, 159, 172, 176, 179, 180, 191, 194,  211, 212, 234  cooling process, 159  copolymers, 238, 241  copper, 118, 153, 297  correlation, x, 85, 87, 162, 163, 233, 256, 310 

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342

Index

corrosion, 93, 109, 168, 169, 214  Coulomb interaction, 83, 85  coupling, xi, 39, 53, 103, 233, 234, 288, 296, 317,  319  covalent bond, 272, 293, 297  covalent bonding, 272, 293, 297  creep, 7, 169  critical value, 12, 206  crystal growth, 20, 23, 220, 228  crystal structure, 2, 29, 30, 31, 82, 83, 216, 326, 334  crystalline, 2, 19, 20, 24, 31, 33, 37, 170, 214, 215,  220, 227, 231, 232, 314, 324  crystallinity, 26, 30, 217, 220, 223, 227, 231, 232,  233  crystallites, 32, 37, 220, 225, 229, 234, 236, 237  crystallization, 26, 28, 217, 221, 227, 230, 232, 233,  234, 235  crystals, 5, 17, 19, 23, 80, 81, 82, 83, 90, 100, 103,  104, 106, 109, 114, 119, 123, 124, 170, 214  CTA, 294, 295  curing, vii, 1, 238  CVD, 17, 19, 20, 21, 210, 292  cyanide, 121  cyst, 47  cytometry, 55  cytoplasm, 42  cytosine, 289, 312 

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D  data processing, 41  database, 31  decay, 54, 91, 92, 94, 95, 97, 125, 241, 245  decomposition, 100, 105, 112, 113, 114, 117, 118,  125, 166, 168, 216, 217, 235, 239, 242, 266, 267  deconvolution, 305  defect formation, 256  defects, viii, x, 10, 21, 30, 33, 36, 39, 79, 83, 89, 91,  99, 100, 223, 237, 255, 256, 257, 258, 260, 263,  264, 265, 266, 269, 275, 276, 277, 278, 279, 327,  329  definition, 32  deformation, x, 9, 29, 255, 260, 263, 275, 276  degradation, x, 55, 118, 213, 215, 216, 218, 223,  224, 225, 227, 232, 233, 234, 235, 237, 238, 239,  240, 244, 245, 246, 247  degradation rate, 235  degree of crystallinity, 217, 223  Degussa, 99, 100, 114, 120, 216, 217, 224, 232, 233,  236, 242 

denaturation, 292, 297, 303, 317  density, xi, xii, 10, 15, 23, 28, 45, 54, 82, 89, 90, 93,  100, 107, 152, 168, 181, 206, 214, 256, 260, 269,  288, 290, 307, 310, 311, 314, 316, 317, 318, 319,  323, 337  deposition, ix, 17, 18, 19, 20, 25, 26, 28, 29, 46, 114,  116, 117, 121, 149, 152, 173, 207, 210, 246, 292,  293, 296, 302, 324  deposition rate, 18, 19, 293, 302  depression, 99  derivatives, 103, 112, 113, 117, 118, 261  desorption, 14, 123  destruction, 99, 113, 117, 118, 120, 121, 122  detection, xi, 38, 42, 43, 44, 45, 46, 47, 48, 49, 50,  51, 52, 53, 55, 229, 288, 289, 290, 291, 302, 308,  310, 319  deviation, 31, 32, 152, 153  dielectric constant, 6, 84, 98, 169  dielectrics, 51  differential scanning, 45  differentiation, 54  diffraction, 30, 31, 32, 35, 36, 39, 172, 214, 300,  314, 326, 334  diffraction spectrum, 300, 314  diffusion, ix, 9, 14, 28, 40, 108, 123, 150, 152, 174,  176, 185, 188, 189, 197, 199, 206, 212, 327, 337  diffusion process, 9  diffusion rates, 14  diffusivity, 16, 23  dimensionality, 3, 194  dimerization, 106  direct measure, 151  direct observation, 35  discharges, 152  discontinuity, 83  discretization, 89, 197, 198  dislocation, 7, 8, 10, 271  dispersion, xii, 21, 91, 100, 235, 238, 257, 323, 325  dispersity, 100  displacement, 176  dissociation, 152, 153, 169, 208  dissolved oxygen, 94  distilled water, 229  distribution, 4, 24, 25, 32, 34, 36, 41, 56, 82, 83, 91,  163, 168, 171, 172, 180, 184, 186, 187, 191, 194,  196, 197, 202, 203, 204, 205, 206, 218, 225, 234,  235, 307  distribution function, 184, 194, 196, 204, 205, 206  diversity, 18, 21, 23, 41  donors, 96, 111, 118, 122 

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Index dopants, 10, 219, 225  doping, 10, 11, 56, 111, 224, 291  double bonds, 272  double helix, 289  drug delivery, viii, 2, 41, 42, 56, 57  drug release, 55  drugs, 55, 56  drying, 21, 220, 221, 222, 223, 238  DSL, 281  DSM, 241, 242  ductility, 7, 9, 150  durability, 238, 244  duration, 220  dyes, x, 39, 44, 51, 52, 53, 54, 100, 103, 105, 111,  112, 113, 117, 118, 120, 121, 122, 125, 213, 216,  318 

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

E  elasticity, 238  electric charge, 10, 184, 206  electric current, 179  electric field, 29, 92, 98, 151, 152, 178, 206, 257  electrical conductivity, 11, 57, 156, 179, 206, 291  electrical properties, 46, 49, 163  electricity, xii, 323  electrochemical deposition, 26  electrochemistry, 17, 43, 44, 46  electrodes, viii, xi, 2, 28, 40, 46, 47, 48, 49, 112, 121,  122, 150, 288, 291  electrolyte, 28, 40, 80, 97, 107, 145  electromagnetic, ix, 4, 31, 36, 56, 150, 151, 156,  163, 174, 176, 177, 178, 180, 211, 230  electromagnetic fields, 180  electron beam lithography, vii, 2  electron diffraction, 20, 30, 36, 314, 315  electron microscopy, 35, 36  electron paramagnetic resonance, 30  electron state, 82, 89, 90  electronic structure, 6, 81, 82, 273, 279, 323  electrophoresis, 29  emission, viii, 2, 39, 42, 43, 50, 51, 52, 89, 91, 153,  171, 178, 240, 245, 292, 298, 316, 319, 324, 325,  329, 334, 337  emitters, 261, 264  empirical potential, 273  emulsions, 25, 245  encapsulation, 53, 56  energy efficiency, 230  energy transfer, 44, 50, 53, 56, 89, 319 

343

enlargement, 48  entropy, 82  environment, 25, 47, 56, 124, 125, 150, 215, 239  environmental control, 46  environmental effects, 238  environmental impact, 238  enzyme immobilization, 46, 47  enzymes, 38, 42, 45, 47, 49, 52, 53  epitaxial films, 19  epoxy resins, 238  equilibrium, 8, 50, 70, 107, 151, 152, 153, 155, 207,  208, 230  ESR, 33  ester, 295, 327, 331, 332  ethanol, 47, 96, 106, 108, 112, 120, 229, 234, 293  ethyl acetate, 238  ethyl alcohol, 115  ethylene, 112, 267, 324  ethylene glycol, 324  europium, 224  evacuation, 18  evaporation, ix, 17, 18, 19, 20, 149, 152, 155, 168,  170, 171, 174, 180, 181, 182, 183, 184, 189, 198,  210  evolution, xi, 21, 46, 103, 105, 111, 113, 118, 119,  120, 121, 122, 124, 201, 210, 219, 288, 292, 303,  305, 310  EXAFS, 33  excitation, 12, 33, 39, 50, 53, 92, 96, 214, 224, 292,  298, 319, 326, 333, 334  exciton, viii, 5, 6, 79, 81, 83, 84, 85, 86, 87, 89, 91,  92, 101, 102, 103, 105, 113, 114  exponential functions, 91  exposure, 109, 238, 241, 242, 243, 266, 294, 299  extinction, 89, 97, 171  extraction, 25, 53, 231, 314, 327, 329, 331 

F  fabrication, iv, 3, 17, 19, 21, 41, 42, 44, 57, 174  FAD, 46  family, 32, 33, 37, 261, 269, 273  FCC, 29  Fermi level, 15, 93, 107, 116, 119, 256, 263  ferrite, 155, 208, 209  fibers, 21, 42  fillers, 125, 169  film thickness, 240, 293  filters, 125  filtration, 24 

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344

Index

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

financial support, 337  flame, 164, 166, 171, 172, 173, 174  flatness, 269  flexibility, 219  flight, 151, 170, 181, 210  floating, 181, 183, 206  flocculation, 238  flood, 298  flow field, 154, 179, 180, 186, 194  fluctuations, 32, 276  fluid, ix, 150, 154, 176, 206  fluorescence, xi, 39, 43, 44, 50, 51, 52, 53, 288, 290,  291, 292, 293, 298, 303, 315, 317, 318, 319, 324,  329  fluorine, 224  fluorophores, 39, 52, 53, 290  focusing, 18  food, 125, 289  food industry, 289  Ford, 56, 78  formaldehyde, 118, 122, 240, 243, 245  France, 287, 294, 297  free energy, 25, 102, 157, 158, 161, 162, 163, 172  free radicals, 33, 332  FTIR, 38, 246, 247, 325, 327, 329, 335, 336  fuel, 3, 10, 16, 46, 124, 168, 215, 260  fullerene, 260, 266, 267, 272, 273  functionalization, 41, 44, 53, 291, 293, 294, 295,  296, 300, 301, 303, 306, 310 

G  gallium, 18  gas sensors, 170  gases, 18, 19, 33, 122, 152, 172, 208, 245  gel, 17, 21, 28, 53, 219, 220, 221, 222, 223, 232, 233,  234, 235, 236, 240, 243  gelation, 21, 219  gene, 41, 55, 57  gene expression, 55  gene therapy, 55  generation, 10, 44, 100, 112, 171, 184, 191, 223, 230  genes, 55  genetic defect, 55  genetic information, 289  Germany, 298  glasses, 21, 215  glucose, 45, 46, 47, 122  glucose oxidase, 46, 47  glutamate, 53 

glutathione, 26, 56  glycerol, 88  glycine, 215, 216  glycol, viii, 2, 22, 57, 223, 238  glycoproteins, 50  gold, xi, 22, 38, 41, 44, 46, 48, 49, 50, 51, 52, 56, 57,  114, 119, 169, 288, 290, 291, 292, 293, 294, 295,  296, 298, 307, 308, 309, 311, 312, 313, 315, 318,  319  gold compound, 293  gold nanoparticles, xi, 22, 44, 46, 50, 51, 53, 56, 288,  293, 294, 295, 296, 298, 307, 308, 318  government, iv  grain boundaries, 7, 8, 11, 29, 40  grains, 24, 29, 218, 225, 307  graph, 304  graphite, 47, 113, 168, 256, 259, 260, 266, 269  gravitational force, 180, 181, 206  gravity, 21  grids, 297  groups, xi, 16, 26, 38, 50, 52, 219, 220, 233, 239,  240, 256, 288, 289, 293, 307, 308, 337  growth factor, 57  growth mechanism, 194, 197  growth rate, ix, 149, 152, 185, 193, 200, 206  growth theory, 209  guanine, 289, 312  guidelines, 296  Guinea, 281 

H  hairpins, xi, 287, 290, 305, 307, 318  halos, 37  Hamiltonian, 87  hands, 263  hardness, 7, 9, 10, 150, 168, 169  health, 57  health effects, 57  heat, ix, 11, 14, 15, 18, 56, 82, 88, 92, 149, 151, 152,  169, 171, 174, 179, 180, 182, 183, 184, 206, 211,  223, 230, 273  heat capacity, 82, 179  heat transfer, ix, 149, 180, 182, 183, 206  heating, 18, 56, 168, 176, 178, 179, 206, 211, 215,  216, 217, 220, 230, 234, 266  heating rate, 230  heavy particle, 151, 152  height, 298, 305, 306, 307  helicity, 263 

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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Index hemoglobin, 45  hepatitis, 45  heptane, 118, 217, 325  herbicide, 100  heterogeneous catalysis, 100  homogeneity, 219  Honda, 207, 211  hot spots, 234  HRTEM, vii, 1, 30  humidity, 223, 239, 245  hybrid, viii, xi, 2, 46, 56, 209, 287, 288, 292, 295  hybridization, xi, 42, 48, 51, 288, 289, 290, 291, 292,  297, 299, 300, 301, 303, 308, 313, 314, 315, 316,  317, 318, 319  hydrazine, 25, 96  hydrides, 29  hydrocarbons, 38, 124, 238  hydrofluoric acid, 228  hydrogen, 16, 29, 44, 46, 47, 54, 56, 84, 102, 103,  105, 111, 112, 113, 118, 119, 120, 121, 122, 124,  153, 154, 170, 207, 208, 289, 316  hydrogen atoms, 153, 154  hydrogen bonds, 56, 289  hydrogen peroxide, 47  hydrolysis, 21, 22, 25, 217, 219, 220, 234, 235  hydrophobicity, 43  hydroquinone, 46  hydrothermal process, 228, 231, 232, 234  hydrothermal synthesis, 23, 228, 234, 235  hydrothermal synthesis method, 236  hydroxide, 24, 29, 217  hydroxyl, 20, 21, 217, 220, 233, 243 

I  identification, 30, 31, 38, 50  illumination, ix, 37, 80, 96, 106, 107, 108, 109, 112,  113, 116, 216, 224  image analysis, 298  images, xi, 30, 34, 36, 37, 218, 222, 225, 226, 228,  229, 230, 233, 235, 236, 288, 297, 298, 299, 300,  301, 308, 313, 317  imaging modalities, 54  immersion, 247  immobilization, 42, 43, 49, 290, 305  impurities, 30, 39, 91, 98  in transition, 85  in vitro, 51, 52, 55, 56, 57  in vivo, 51, 52, 53, 54, 55, 56, 57  incidence, 31, 49 

345

inclusion, 166  indexing, 314, 315  indication, 240, 269, 327, 329, 331, 336  indices, 270  indium, 267, 317, 319  induction, ix, 18, 149, 150, 155, 158, 176, 179, 207,  208, 209, 210, 211, 212, 271  industry, vii, 2, 19, 23, 46, 57, 168  inelastic, 36  inertia, 184  infinite, 13, 259  initial state, 317  injections, 211  insertion, 267, 270  insight, xi, 287, 314  instability, 164, 194  instruments, 36, 44, 49  insulation, 169  insulators, 12  integration, 95  interaction, 20, 33, 44, 49, 51, 53, 80, 83, 89, 102,  151, 154, 174, 230, 323  interactions, xi, 12, 13, 44, 46, 50, 51, 89, 152, 272,  278, 287  interface, 13, 16, 26, 46, 49, 97, 99, 106, 116, 290,  331  interference, 33, 47, 297  intermetallic compounds, ix, 7, 29, 149, 154, 155,  174, 193  intermetallics, 30  internalizing, 54  interval, 233  intravenously, 55, 56  ionization, 15, 33, 83, 90, 153, 206, 208  ions, xi, 10, 11, 15, 18, 21, 22, 28, 29, 44, 99, 114,  117, 151, 177, 181, 183, 215, 224, 227, 288, 297,  336  IR spectra, 330  iron, 8, 13, 15, 42, 46, 54, 56, 57, 184, 191, 193, 210,  211, 238, 267  irradiation, 22, 124, 216, 217, 218, 224, 230, 233,  234, 239, 241, 242, 245, 246, 248, 256, 266, 273,  324, 325, 326, 327, 329, 331, 332  isomerization, 16, 271  isomers, 261  iteration, 179 

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Index

J  Japan, 149, 158, 163, 172, 207, 208, 209, 210, 211,  252, 293 

K  KBr, 325  ketones, 106, 118, 238, 239, 240  kinetic constants, 218  kinetic curves, 91  kinetics, 49, 91, 151, 152, 207, 230, 264 

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

L  labeling, viii, 2, 42, 52, 54  lactase, 46  lactate dehydrogenase, 53  laminar, 176, 212  lanthanide, 52, 54, 224  laser ablation, 19, 260  lattice parameters, 37  lattices, 32  lens, 36, 297  Lewis acids, 21  ligand, 21, 53, 293  light conditions, 53  light scattering, 54, 186, 233  light transmission, 35  light transmittance, 291  light‐emitting diodes, 168  limitation, 18, 40  linkage, 331, 332  liposomes, 54, 56  liquid crystal phase, 221  liquid phase, 21, 25, 26, 206, 217, 219, 293  localization, viii, 79  location, 37, 260  low temperatures, 18, 19, 227, 234, 256  luciferase, 53  luminescence, xii, 11, 12, 52, 53, 91, 323, 324, 325,  329, 332, 334, 337  luminescence efficiency, 324  lymph node, 53  lysozyme, 26 

M  macromolecules, 38 

magnetic field, 18, 33, 54, 112, 151, 178, 269  magnetic moment, 13, 33, 269  magnetic particles, 13, 54  magnetic properties, 7, 13, 82, 155, 208  magnetic resonance, viii, 2, 51, 55  magnetic resonance imaging, viii, 2, 51  magnetic resonance spectroscopy, 55  magnetism, xii, 323  magnetization, 12, 13  manganese, 46  manipulation, 3, 280  manufacturer, 296  manufacturing, 57  market, 238, 243  marrow, 55  mass loss, 242  material surface, 37  materials science, 31  matrix, x, 46, 47, 49, 213, 215, 224, 238, 244, 246,  296, 316  measurement, 34, 37, 40, 47, 300, 327  mechanical degradation, 239  mechanical properties, 7, 37, 41, 241, 248, 269  media, 25, 228, 232, 248  melt, 170, 206  melting, 4, 8, 14, 82, 150, 151, 156, 157, 158, 162,  163, 166, 168, 180, 182, 194, 206, 289  melting temperature, 8, 82, 157, 158, 162, 166  membranes, 21, 42, 125  memory, 274  mercury, 39, 298  mesoporous materials, 35, 119, 120  mesoporous semiconductors, 81, 110  metal carbides, 166  metal hydroxides, 24  metal nanoparticles, viii, 2, 22, 25, 41, 47, 49, 150,  193  metal oxides, 7, 29, 38, 42  metal salts, 21, 25  metals, 3, 8, 9, 10, 15, 25, 29, 30, 32, 33, 54, 93, 116,  121, 154, 155, 157, 158, 161, 162, 166, 168, 191,  216, 224, 265  metastatic cancer, 56  methanol, 100, 112, 117, 118, 120, 121, 122, 216,  227, 229, 293  methyl methacrylate, xii, 323  mice, 54, 56  microelectronics, 163  microemulsion, 27, 53  micrometer, 3, 4, 37, 100, 150 

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Index microscope, 35, 36, 37, 39, 231, 297, 298, 307, 324  microscopy, xi, 34, 37, 55, 114, 288, 289, 303, 305,  313, 314, 315  microspheres, 22, 54, 120, 122  microstructure, 29, 36, 218, 237  microwave heating, 230  microwave radiation, 34  microwaves, x, 34, 213, 215, 230, 237  migration, 16, 90, 101, 120, 123  miniaturization, vii, 2  mixing, 20, 24, 38, 116, 156, 223, 227, 239  MMA, 324, 325, 329, 332, 333, 336  mobility, 16, 181, 191, 206  MOCVD, 17, 19, 292  modeling, 87, 100, 152, 153, 154, 171, 207, 208  models, ix, 55, 85, 86, 92, 150, 211, 256, 276  molecular beam, 19, 20  molecular beam epitaxy, 19, 20  molecular dynamics, 266, 272, 273, 275  molecules, xi, 14, 15, 22, 25, 26, 27, 28, 38, 39, 44,  49, 51, 52, 91, 121, 214, 269, 272, 273, 287, 288,  289, 296, 318, 337  molybdenum, 163, 184, 191, 201, 202  momentum, 83, 177, 180  monolayer, 47, 156, 260  monomers, ix, 29, 80, 185, 201  morphology, xi, 16, 17, 57, 114, 163, 170, 214, 218,  220, 228, 231, 256, 264, 276, 288, 292, 302, 303,  307, 317, 319, 324  motion, 7, 10, 152, 181, 211, 243  movement, 6, 11, 83, 85, 91, 121, 271  MRI, viii, 2, 51, 54, 55 

N  NaCl, 24, 233, 297  NAD, 47  NADH, 47  nanobelts, 3, 57  nanocomposites, 29, 43, 81, 110, 112, 113, 117,  125, 324, 326, 327, 328, 329  nanocrystalline metals, 8  nanocrystals, 3, 4, 6, 11, 12, 21, 53, 81, 82, 83, 85,  86, 89, 90, 92, 94, 97, 98, 99, 105, 108, 115, 121,  124, 125, 208, 217, 225, 229, 233, 329, 330, 331  nanodevices, vii, 1  nanodots, 307  nanoelectronics, viii, 2, 279  nanofibers, vii, 2  nanohorns, 256 

347

nanomaterials, vii, 2, 3, 4, 5, 22, 23, 25, 26, 27, 30,  37, 41, 46, 57, 80, 124, 214, 219, 253, 323  nanometer, vii, ix, 1, 3, 4, 11, 26, 27, 29, 36, 100,  103, 114, 119, 120, 123, 149, 152, 168, 305  nanometer scale, vii, 1, 4, 26, 27, 168  nanometers, vii, 25, 29, 36, 102, 123, 218, 236  nanophases, 24  nanophotonics, viii, 2  nanorods, 21, 23, 26, 28, 30, 57, 115, 116, 121, 233  nanoscale structures, 41  nanostructured materials, vii, 1, 2, 3, 4, 17, 19, 21,  25, 30, 36, 38, 41, 42, 43, 47, 49, 50, 52, 53, 57,  234  nanostructures, vii, x, 1, 3, 4, 5, 17, 23, 26, 28, 30,  37, 42, 57, 110, 111, 112, 113, 114, 115, 116, 117,  118, 119, 122, 123, 125, 168, 255, 256, 260, 264,  267, 268, 269, 273, 276, 279, 280  nanotechnology, vii, 1, 30, 56, 57, 58, 249, 280  nanotube, 26, 27, 57, 122, 260, 261, 263, 264, 265,  266, 269, 270, 271, 273, 274, 275, 276, 277, 278  nanowires, vii, 2, 3, 19, 23, 26, 28, 30, 57, 121  NATO, 144  necrosis, 57  Netherlands, 67, 68, 70  network, x, 21, 219, 221, 232, 255, 256, 257, 258,  260, 261, 262, 263, 265, 267, 268, 269, 271, 273,  276, 277, 279  neurodegenerative diseases, 55  nickel, 120, 208  niobium, 46, 121  NIR, 53, 56  nitrates, 219  nitric oxide, 56  nitrides, ix, 149, 151, 155, 156, 162, 169  nitrobenzene, 118  nitrogen, x, 112, 113, 117, 122, 153, 158, 159, 162,  169, 207, 213, 224, 225, 229  nitrogen oxides, x, 112, 113, 117, 213  NMR, 54, 281  noble metals, 22  nodes, 197, 199  nuclear magnetic resonance, 54  nuclei, ix, 24, 150, 176, 184, 185, 191, 193, 194, 196,  198, 201, 202  nucleic acid, 42, 49, 51  nucleophilicity, 228  nucleus, 42, 185, 191  numerical analysis, 171, 176 

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

348

Index

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

O  observations, 34, 208, 209, 297, 306, 308  oil, 24, 27, 327, 337  oligomers, 219  one dimension, vii, 2, 3, 6, 17  opacity, 53  operator, 198, 199  optical density, 95, 109  optical microscopy, 240, 313  optical properties, viii, 4, 30, 57, 79, 86, 93, 101, 224  optoelectronic properties, 17  orbit, 332  organ, 55, 292  organic compounds, x, 118, 125, 213, 224, 238, 246  organic matter, 239  organic solvents, 125, 227, 238  orientation, 2, 32, 261, 263, 271, 272, 279, 300, 301,  319  oxalate, 23  oxidation, 13, 33, 40, 47, 52, 99, 100, 102, 104, 106,  111, 112, 113, 114, 117, 118, 120, 121, 122, 123,  124, 150, 151, 169, 170, 171, 210, 214, 217, 224,  229, 244, 245, 246, 324  oxidative destruction, 120  oxide nanoparticles, 22, 24, 42, 43, 46, 54, 171, 174,  210, 324  oxides, ix, xi, 7, 10, 11, 13, 14, 15, 16, 19, 21, 24, 26,  90, 149, 151, 155, 214, 223, 288, 291, 293, 297,  317  oxygen, 10, 11, 15, 16, 20, 40, 42, 57, 94, 95, 96, 97,  100, 105, 119, 122, 125, 153, 168, 170, 171, 172,  207, 208, 219, 224, 225, 229, 230, 246, 305, 329,  332, 336  oxygen plasma, 171, 229, 230  oxygen sensors, 10  ozone, 117 

P  PAA, 26  paints, x, 124, 213, 215, 238, 239, 240, 241, 243,  245, 246, 247, 248  palladium, 25  parameter, 3, 32, 152  partial differential equations, 48  particle morphology, 14, 16, 210, 227  passive, 290, 324  pathogenesis, 55 

pathogens, 50  PCR, 44, 51  pepsin, 45  peptides, 41, 53  perchlorate, 216  periodicity, 258, 259, 264  permeability, 155, 206  permit, 238  peroxide, 124  PET, 51  pH, 21, 28, 45, 56, 57, 106, 217, 218, 220, 224, 225,  227, 228, 229, 233, 294, 295, 297  phase diagram, 168, 205  phase transformation, 220  phase transitions, 82  phenol, 99, 100, 103, 112, 113, 117, 118, 119, 120,  121, 227  phenol oxidation, 119, 121  phonons, 83, 89  phosphates, xi, 288, 308, 311  phospholipids, 55  phosphorous, 311  phosphorus, 224, 297  photoabsorption, 33  photobleaching, 54  photocatalysis, viii, 79, 81, 100, 120, 221, 222, 245,  246  photocatalysts, 22, 81, 103, 104, 105, 111, 112, 113,  117, 120, 121, 123, 124, 125, 223, 224, 227, 231,  232, 234  photocorrosion, 96, 108, 109, 111  photodegradation, x, 103, 213, 215, 221, 223, 224,  227, 231, 236, 240, 242, 245, 246, 248  photographs, 172  photoluminescence, viii, 6, 39, 79, 91, 325, 329, 330,  332, 333, 334, 337  photolysis, 94, 97  photonics, 82  photons, 32, 36, 106  photooxidation, 106, 227, 232  photopolymerization, 104, 109, 125  photovoltaic cells, 170  physical properties, vii, 1, 4, 56, 168, 206, 323  physics, 80, 126, 151, 176  piezoelectricity, 42  pitch, 268  PL spectrum, 39  Planck constant, 34, 84  plasma, ix, 18, 19, 55, 149, 150, 151, 152, 153, 154,  155, 158, 159, 161, 162, 163, 164, 166, 171, 172, 

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Index 174, 176, 177, 178, 179, 180, 181, 182, 183, 184,  186, 194, 206, 207, 208, 209, 210, 211, 212, 324  plasma membrane, 55  plastic deformation, 9, 276  platinum, 25, 46, 212  PMMA, 325, 327, 329, 330, 331, 332, 333, 334, 335,  336, 337  point mutation, 46  polarity, 28, 40  polarization, viii, 44, 80, 101, 107, 108, 109, 116,  123  pollutants, 215, 223, 224, 243, 245  pollution, 151, 245  polycarbonate, 25  polycondensation, 25, 219, 220  polycondensation process, 219  polymer, xii, 5, 11, 22, 25, 26, 45, 49, 50, 51, 53, 56,  110, 113, 120, 125, 239, 241, 289, 323, 324, 325,  329, 331, 332, 336  polymer composites, 324  polymer matrix, 11, 45, 329  polymerase, 44  polymerase chain reaction, 44  polymeric materials, viii, 2, 22  polymerization, ix, 21, 80, 103, 104, 125, 260, 272,  324, 326, 327, 329, 331, 332, 334  polymers, 42, 45, 113, 118, 124, 125, 219, 241, 290  polystyrene, 329  polyurethane, 238  poor, 223, 264  population, 93  porosity, 8, 9, 225, 244  porous materials, 25, 26, 27, 120  ports, 171  Portugal, 254  positron, 51  positron emission tomography, 51  potassium, 38, 119  power, ix, 10, 18, 23, 93, 96, 149, 155, 158, 167,  176, 208, 214, 234, 238, 239, 245  praseodymium, 224  precipitation, x, 17, 21, 23, 24, 213, 215, 217, 221,  222, 223, 227, 228, 229, 233, 235  prediction, 256  preservative, 239, 240  pressure, ix, 18, 19, 23, 31, 33, 40, 149, 152, 155,  156, 158, 163, 166, 168, 170, 171, 184, 185, 191,  193, 194, 195, 201, 206, 207, 220, 227, 228, 298  prevention, 242  probability, 14, 82, 87, 88, 89, 90, 100, 101, 106 

349

probe, xi, 31, 37, 44, 53, 84, 168, 214, 219, 236, 264,  288, 290, 292, 293, 295, 297, 299, 306, 307, 308,  310, 311, 312, 313, 315, 316, 318  production, 21, 25, 32, 118, 120, 124, 150, 151, 154,  158, 163, 169, 180, 184, 187, 189, 191, 197, 199,  200, 206, 209, 234, 238, 245, 267, 277  proliferation, 54  promoter, 55, 225  protective coating, 214, 215  proteins, 38, 41, 42, 43, 44, 50, 51, 52, 53, 54, 56  proteolysis, 53  proteolytic enzyme, 53  protocol, 44  protons, 15, 54  prototype, 18  pulse, ix, 18, 54, 80, 92, 94, 96, 97, 107, 110, 116,  208  pumps, 33  pure water, 22  purification, 124, 150, 215, 296, 325  PVAc, 240  PVC, 238, 243  pyrimidine, 289  pyrolysis, 292 

Q  quanta, 89, 96, 120  quantitative technique, 316  quantization, 90  quantum confinement, 4, 5, 168, 214  quantum dot, vii, viii, 2, 3, 4, 5, 17, 20, 38, 41, 42,  43, 52, 54, 56, 79, 81, 125, 290, 324  quantum dots, vii, viii, 2, 3, 4, 17, 20, 38, 41, 42, 43,  52, 54, 56, 79, 81, 125, 324  quantum mechanics, 81  quantum phenomena, 124  quantum well, 20  quantum yields, 110, 111, 119  quantum‐chemical calculations, 92  quartz, 45, 158, 238, 289 

R  radiation, xii, 22, 31, 33, 38, 54, 178, 179, 182, 206,  211, 323, 325, 329, 330, 331, 332, 333, 334, 336,  337  radical formation, 223  radical mechanism, 329 

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350

Index

radical polymerization, 324  radio, ix, 18, 52, 149, 207, 208, 209, 210  radius, 5, 6, 15, 24, 81, 84, 85, 86, 98, 206, 258, 270,  308  rain, 242  Raman spectra, 51  Raman spectroscopy, 50  raw materials, ix, 149, 150, 151, 155, 168, 176, 215,  216, 238, 239  reactant, 14, 24, 42, 47, 86, 169  reactants, 100, 101, 108, 230  reaction mechanism, 329  reaction rate, 40, 100  reaction temperature, 227, 234  reaction time, 28, 150  reactivity, 15, 214, 224, 293  reagents, 52, 325  real time, 31  reality, 258  reception, 44  receptors, 42, 49  recognition, 42, 44, 46, 50, 57  recombination, 80, 89, 90, 91, 97, 98, 99, 100, 104,  110, 111, 116, 119, 122, 123, 152, 153, 154, 214,  223, 225, 233, 243  recombination processes, 98, 110  recovery, 54, 94, 96  red shift, 11  redox groups, 215  reflection, 120  reflectivity, 49, 169  refractive index, 49, 292, 293, 319  regenerate, 248  rejection, 55  relationship, 37, 47, 85, 86, 87, 90, 93, 95, 104, 106  relationships, 85, 87, 89, 105, 118, 162  relative size, 295  relaxation, 8, 39, 40, 54, 89, 90, 256, 271  relaxation process, 40  relaxation processes, 40  relevance, 291  reliability, 290  remediation, 214, 241  residues, xi, 288, 316, 319  resins, x, 213, 238, 241, 245, 248  resistance, 40, 49, 54, 168, 169, 238, 245, 246  resolution, 30, 31, 36, 37, 38, 54, 55, 156, 274, 290,  298, 303, 305, 307, 308, 309, 310, 314, 317, 325,  327, 329, 336  resorcinol, 117 

restructuring, 15  retardation, 119  retention, 120  returns, 50, 317  reusability, 43  reverse reactions, 110, 122  rings, x, 255, 256, 257, 260, 264, 268, 269, 271, 273,  275, 279  RNA, 44  rods, 17, 41, 233  rolling, 258, 263, 269  Romania, 22  room temperature, 18, 232, 245, 247, 266, 293, 297,  324, 325, 326, 329, 332, 334  root‐mean‐square, 32  rotations, 272, 274, 275  roughness, 15, 242, 293, 298, 299, 300, 302, 303,  306, 307  Royal Society, 228, 235  rutile, 121, 170, 172, 210, 215, 216, 217, 219, 220,  228, 229, 230, 233, 234, 235, 241, 242, 243, 245,  326 

S  safety, 53, 56  salt, 24, 46, 225, 293, 316  salts, 21, 24, 25, 114, 121, 219, 307, 316  sample, 8, 9, 18, 31, 32, 33, 34, 35, 36, 39, 46, 52,  155, 216, 218, 221, 225, 228, 233, 236, 237, 242,  265, 278, 296, 297, 325, 336  saturation, ix, 13, 149, 156, 157, 158, 174, 184, 185,  191, 193, 194, 201, 206, 216  Saudi Arabia, 1  scanning electron microscopy, 30  scarcity, 168  scatter, 97  scattering, 31, 32, 36, 43, 50, 257  schema, 307  screw dislocations, 259  search, 110, 124, 272  segregation, 11  selected area electron diffraction, 30  selectivity, 14, 16, 42, 43, 45, 46, 100, 121, 290  self‐assembly, 25, 28, 231, 324  self‐destruction, 55, 125  sensing, 41, 42, 44, 46, 49, 50  sensitivity, xi, 15, 38, 42, 43, 45, 46, 47, 49, 50, 113,  120, 288, 289, 290, 318  sensors, 17, 44, 46, 47, 49, 50, 170, 261, 264 

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

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Index separation, 25, 37, 43, 49, 51, 53, 97, 98, 110, 111,  113, 114, 115, 121, 122, 123, 224, 264, 319  series, 2, 18, 21, 37, 41, 272  shape, viii, x, 9, 13, 17, 23, 25, 34, 40, 41, 49, 79, 82,  85, 110, 115, 116, 120, 121, 216, 217, 219, 225,  230, 234, 237, 255, 258, 264, 265, 267, 277, 310  shock, 168, 169, 236  shock waves, 236  Si3N4, 169  side effects, 56  signal transduction, 42  signals, 36, 40, 42, 48, 49, 50, 55, 97, 117  signal‐to‐noise ratio, 54  silane, 293, 303  silica, xi, 25, 43, 46, 49, 53, 54, 240, 242, 288  silicon, 12, 16, 19, 37, 122, 125, 168, 169, 174, 184,  191, 193, 194, 196, 197, 200, 201, 202, 205, 206,  209, 210, 211, 212, 292  silver, xi, 38, 41, 46, 48, 52, 227, 288, 296, 313, 314  similarity, 31  simulation, 256, 273  single crystals, 23, 24  sintering, 24, 121, 217, 220, 227, 230, 232, 243  SiO2, viii, xi, xii, 2, 13, 26, 43, 46, 51, 112, 172, 173,  174, 245, 288, 291, 292, 293, 299, 300, 302, 303,  305, 306, 307, 308, 315, 316, 318, 323, 324, 333,  337  SiO2 films, xi, 288, 291, 292, 299, 303, 307, 316  SiO2 surface, 46, 291, 307, 308  skeleton, 219  sodium, 22, 24, 106, 122, 216, 297, 306  sodium hydroxide, 24  software, 212, 298  solar cells, 5, 100, 124  sol‐gel, x, 21, 22, 213, 215, 219, 220, 221, 223, 224,  227, 231, 233, 234, 236, 240, 244  solid oxide fuel cells, 10  solid solutions, 118  solid state, 31  solid surfaces, 290  solid tumors, 57  solidification, 161  solubility, 23, 53  solvents, x, 25, 53, 213, 238  solvothermal synthesis, 23  Soxhlet extractor, 325  species, 16, 20, 21, 24, 33, 38, 40, 42, 47, 51, 89,  179, 183, 215, 217, 219, 223, 224, 225, 243, 246,  319, 332  specific heat, 206 

351

specific surface, 2, 103, 105, 120, 123, 217, 220,  223, 224, 229, 232, 235, 243, 324, 327, 334  specificity, 42, 44, 50  spectroscopy, 30, 33, 38, 48, 50, 125, 171, 303, 332  spectrum, 5, 10, 11, 31, 33, 34, 40, 49, 50, 52, 54,  83, 92, 159, 164, 165, 166, 167, 169, 210, 224,  279, 300, 301, 302, 309, 310, 311, 312, 313, 315,  317, 318  speed, 23, 206  spin, 12, 33, 229, 296, 332  stability, 8, 38, 43, 46, 52, 53, 57, 82, 214, 238, 241,  260, 269, 291, 324  stabilization, 82  stabilizers, 97, 119  standard deviation, 172, 184, 187, 206, 315  Stark effect, 92, 93  STM, vii, 1, 37  stoichiometry, 11, 19, 31  storage, 29, 41, 42, 56, 57  strain, 9, 11, 36, 269, 271, 276  strategies, 41, 53, 55, 214  strength, viii, 7, 9, 10, 11, 15, 33, 79, 87, 89, 90, 93,  108, 152, 163, 169, 206, 238, 294  stress, 9, 271  stressors, 270  stretching, 38, 258, 327, 331, 336  strong interaction, 327, 331  structural changes, 38  styrene, 238, 240, 241, 245, 324  substitution, 216, 224  substrates, 19, 20, 26, 27, 45, 90, 97, 98, 103, 104,  106, 120, 121, 169, 240, 292, 300  sucrose, 111, 112, 118  sugar, xi, 288, 289, 308, 318  sulfur, ix, 80, 114, 116, 117, 118, 124, 227  sulphur, 224, 225  summaries, 3  superconductivity, 168  superplasticity, 9  supply, ix, 18, 149, 158  suppression, 96, 110  surface area, viii, 2, 4, 16, 41, 42, 46, 49, 57, 81, 91,  100, 114, 120, 168, 206, 216, 217, 220, 221, 222,  223, 225, 227, 228, 229, 231, 232, 233, 234, 235,  236, 237, 243, 244, 324  surface chemistry, xi, 20, 56, 288, 319  surface energy, xii, 119, 273, 323, 324  surface layer, 81, 259  surface modification, xii, 53, 323, 329  surface properties, 225 

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

352

Index

surface reactions, 168  surface region, 32  surface structure, 14, 91, 105, 208  surface tension, 119, 157, 172, 174, 185, 193, 206  surfactant, 22, 24, 25, 26, 27, 28, 120, 221, 231, 235  surplus, 89, 95, 96, 107  SWNTs, 26, 266  symmetry, 3, 259, 260, 261, 263, 265, 271, 276  synergistic effect, 226 

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

T  talc, 238  tantalum, 161, 168  targets, xi, 51, 288, 292, 294, 299, 313, 318  technology, vii, 1, 29, 45, 50, 80, 124, 151, 152, 154,  230, 253, 291  tension, 9, 119, 172, 271, 276, 277  TEOS, 25  therapeutics, 41  therapy, 57, 124, 125  thermal activation, 271  thermal energy, 13  thermal evaporation, 17, 18, 19  thermal expansion, 169  thermal stability, 24, 234, 235  thermal treatment, 152, 227, 230, 232  thermalization, 89, 90  thermodynamic equilibrium, 151, 208  thermodynamic parameters, 156, 162, 163  thermodynamics, 11  thin films, vii, 2, 3, 19, 20, 21, 30, 31, 152, 227, 229,  230, 233, 291, 292  threshold, 81, 82, 86, 91  thymine, 289, 312  time increment, 200  tin, 46, 267, 292, 319  tin oxide, 46, 267, 292, 319  tissue, viii, 2, 52, 53, 54  titania, 98, 99, 111, 113, 117, 121, 122, 124, 125,  215, 216, 217, 219, 221, 224, 227, 228, 229, 230,  233, 234, 235, 236, 238, 241, 244, 247  titanium, 22, 29, 46, 99, 113, 114, 118, 163, 169,  170, 201, 202, 207, 209, 216, 217, 221, 223, 224,  231, 238, 241, 243, 248, 329  titanium isopropoxide, 22, 223  toluene, 112, 117, 118, 218, 223, 225, 240, 242  total energy, 273  toxicity, 42, 52, 56, 57, 214  trajectory, ix, 150, 180 

transducer, 42, 48  transduction, 42, 44  transferrin, 55, 57  transformation, x, 83, 90, 104, 119, 121, 256, 263,  272, 273, 276, 318  transformations, 95, 272, 275  transgene, 55  transition, 6, 11, 15, 16, 21, 34, 38, 39, 80, 82, 87,  88, 89, 90, 92, 93, 123, 214, 228, 258  transition metal, 15, 16, 214  transitions, viii, 6, 31, 39, 79, 82, 83, 86, 91, 93, 125,  151, 276, 308  translation, 271  transmission, 30, 35, 45, 95, 156, 274, 324  transmission electron microscopy, 30, 156  transport, vii, 2, 4, 7, 11, 23, 43, 98, 151, 153, 179,  180, 182, 183, 186, 264  transport processes, 151  tuberculosis, 56  tumor, 56  tumor growth, 57  tumors, 56  tungsten, 18, 153, 211  tunneling, 266 

U  UK, 284, 298  Ukraine, 79  ultrasound, x, 22, 51, 115, 213, 215, 234, 235, 236,  237  uncertainty, 309  uniform, xii, 26, 37, 180, 191, 217, 225, 230, 233,  234, 235, 289, 307, 323  universal gas constant, 95  urea, 45, 46, 217  UV, 30, 38, 125, 156, 215, 216, 217, 223, 224, 225,  227, 229, 231, 233, 234, 235, 236, 238, 239, 242,  245, 246, 247, 248, 295, 296, 324  UV irradiation, 216, 225, 229, 231, 233, 235, 238,  239, 246, 247  UV light, 156, 216, 224, 234  UV spectrum, 295 

V  vacancies, 10, 11, 31, 91, 100, 256, 273  vacuum, 14, 15, 18, 19, 20, 33, 206, 232, 327 

Nanoparticles: Properties, Classification, Characterization, and Fabrication : Properties, Classification, Characterization, and Fabrication, Nova

Index valence, viii, 5, 11, 50, 79, 82, 85, 88, 89, 91, 93, 94,  96, 97, 98, 99, 101, 102, 104, 105, 115, 116, 121,  214, 224  vapor, ix, 17, 18, 19, 26, 117, 120, 122, 149, 150,  151, 155, 156, 159, 163, 166, 168, 169, 171, 172,  174, 176, 179, 180, 184, 185, 188, 189, 191, 193,  194, 195, 201, 202, 203, 206, 324  variables, 151, 174, 179  variation, 2, 7, 17, 45, 50, 54, 86, 180, 182, 183, 238  vector, 11, 32, 178, 206, 270  velocity, ix, 149, 150, 152, 176, 177, 184, 186, 206  vibration, 327, 331, 336  vinyl monomers, 125, 329  viruses, 50  viscosity, 23, 86, 179, 206, 238 

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

W  Wales, 271, 272, 278, 279, 284  wall temperature, 179  waste treatment, 151  wave vector, 11  wavelengths, 11, 83, 86, 113, 216, 224  weight loss, 241, 247  weight ratio, 16, 241  wetting, 238  windows, 156, 163  wires, 17, 41  withdrawal, 96  workers, 22, 28, 50, 168 

353

X  XPS, xi, 30, 32, 216, 225, 233, 288, 292, 297, 298,  300, 301, 302, 303, 304, 305, 308, 309, 310, 312,  316, 317, 318, 319, 325, 327, 329, 336  X‐ray diffraction, 30, 31, 36, 172, 173, 315, 325, 326,  334  X‐ray diffraction (XRD), 30, 31, 325  X‐ray photoelectron spectroscopy (XPS), 33  XRD, vii, 1, 85, 86, 87, 159, 160, 161, 162, 164, 165,  166, 167, 221, 224, 225, 233, 326, 334 

Y  yield, 7, 21, 34, 41, 99, 100, 104, 105, 110, 118, 120,  121, 122, 171, 172, 173, 174, 233, 319  ytterbium, 224  yttrium, 24 

Z  zeolites, 105, 125  zinc, 26, 46, 103, 105, 106, 108, 111, 114, 118, 209,  324  zinc oxide, 46, 108, 114, 118  zirconium, 46  ZnO nanorods, 19, 23, 26, 114, 115, 116  ZnO nanostructures, 18, 112 

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