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Synthesis and Study of Photocatalytic and Conducting Nanoparticles And ABSTRACT This work entitled “Synthesis and Study of Photocatalytic and Conducting nanoparticles and nanocomposites” presents the synthesis, characterization and study of photo-electrochemical and photocatalytic properties of nanocomposites. This thesis is divided into six (06) chapters and the organization of each chapter is as follows: Chapter 1: Introduction and Review of Literature This chapter gives an introductory description of nanoparticles and semiconductor nanocomposites. It includes a brief account of the principle of photocatalysis by semiconductor nanomaterials and its advancements in the past few decades. The term “nanomaterials” is employed to describe the designing and exploitation of materials with structural features in between those of atoms and giant materials, having at least one of its dimensions in between 0.1nm and 500nm range (1nm = 10- 9m). The various physical properties (viz; dynamic, thermodynamic, mechanical, optical, electronic, magnetic) and chemical properties of nanomaterials can be significantly altered relative to their bulk counterparts. Semiconductor nanoparticles (SC NPs) exhibit different size-dependent properties like electronic band gap energies, solid-solid phase transition temperatures, melting temperatures and pressure responses. To understand photoconductivity, electrical conductivity and related phenomena viz; photocatalysis, it is necessary to understand the energy bands and doping of semiconductors. The reason was expressed by Reithmaier as follows, “the properties of a solid can change dramatically if its dimensions or the dimensions of the constituent phases, become smaller than some critical length associated with these properties” In semiconductors, when an electron gains the extra energy required to get excited into the nearby higher conduction band (CB), it can move freely carrying an electric current. This leaves a „gap‟ or „hole‟ in the lower valence band (VB) which can also move in direction opposite to an electron. Further, by supplying an extra energy from outside, or by clever designing of the SC, the way the semiconductor conducts electric current can be controlled. The principle of the semiconductor photocatalytic reaction is a light induced photochemical reaction. In a photocatalytic process, the illumination of a semiconductor photocatalyst with ultraviolet (UV) or visible radiation activates the photocatalyst, generating a redox environment in the aqueous solution. 1 Semiconductors act as sensitizers for light induced redox processes due to their electronic structure, having a completely filled valence band and an empty conduction band. The semiconductor photocatalyst absorbs impinging photons with quantum energy (i.e. wavelength) that hits an electron in the occupied valence band and excites that electron to the unoccupied conduction band leading to excited state conduction band electrons and positive valence band holes. However, the antagonism between charge-carrier recombination and charge-carrier trapping followed by the race between recombination and interfacial charge transfer actually determines the overall quantum efficiency for interfacial charge transfer. Further, the band positions or flat band potentials of the semiconductor material has an important role. These determine the thermodynamic limitations for these photoreactions. There have been a huge number of semiconductor nanomaterials exploited as the photocatalysts and in other photochemical devices. The elements which form these nanomaterial photocatalysts are classified into four groups (i) to form energy structure and crystal structure (ii) to form crystal structure but not energy structure, (iii) to construct impurity levels as dopants and (iv) to be exploited as co-catalysts. The history of heterogeneous photocatalysis indicates that it appeared as a new emerging “Advanced Oxidation Process” (AOP) at the end of 20th century with more than 2000 publications registered on the subject. Currently, more than 1000 articles are being published yearly on the topic. Although, it is important to mention that the discovery of the Honda–Fujishima effect is one of the most important discoveries in chemistry which opened up and extensively promoted the research field of photocatalysis, though, it is not an origin of photocatalysis. Actually, reports on photocatalytic oxidation of organic compounds by titania powders had been published before the discovery of this effect. Titania was considered as the most suitable photocatalyst because of its good photochemical properties and non toxic nature for three decades. However, it is now an established fact that the use of the bare TiO2 phases poses some limitations like; (i) small visible light response, (ii) high recombination rate for the photoinduced charge carriers (iii) doping with foreign species that often act as recombination centres, (iv) difficult to support powdered TiO2 on some materials. As a consequence, the research in heterogeneous photocatalysis has promisingly modified some morphological and electronic properties of TiO2 so as to improve its photocatalytic efficiency. 2 In order to improve the photocatalytic activity of the colloidal and bulk TiO2 particles, interfacial charge-transfer reactions need to be enhanced. Significant charge separation and inhibition of charge carrier recombination is imperative for improving the overall quantum efficiency by interfacial charge transfer. This can be achieved by modifying the properties of the particles by different methods. The different approaches include surface modification of the semiconductor particles with redox couples or noble metals. Another efficient approach involves the coupling of two semiconductor particles with different electronic energy levels to form the heterostructures. The various other approaches include cationic doping, anionic doping or organic-inorganic hybrids etc. The Current State of Research in the field of Photocatalysis is of the belief that TiO2 is still the predominant photocatalyst because no satisfactory alternative has been clearly identified and developed. Despite the prominent progress achieved by photocatalysis in the last decade, there are still various challenges ahead for its full development. The different areas of interest during current times are artificial photocatalysis and photocatalytic water splitting. Nevertheless, the obvious interest in the implementation of more durable processes, surely a brilliant trajectory of photocatalysis in the way to its development has continued. The three main trends forward can be outlined with a reasonable degree of confidence in the near future: (i) the fine control of increasingly complex nanoarchitectures, (ii) the use of novel non-oxide materials and (iii) the coupling with photovoltaic components in a single device. During the long four-decade course of this field, several presentations and concepts are erroneous or misleading and have accumulated wrongly in the literature on this topic of photocatalysis. A few examples viz; the concept of quantum efficiency, activity, reaction rate, normalized photocatalytic experiments, Langmuir- Hinshelwood mechanism and the concept of doping have been discussed. All the above aspects related to the field of photochemical and photocatalytic properties of semiconductor nanomaterials have been discussed in this chapter. Chapter 2: Methodology This chapter presents the various methods of synthesis and characterization of the semiconductor nanoparticles and nanocomposites. It also includes the methods of evaluation of photocatalytic experiments. The various methods of synthesis include 3 the sol gel method, the sol method, the chemical precipitation method, the hydrothermal method and the solvothermal method. The techniques of characterization include the X-Ray diffraction, Electron microscopy (Scanning Electron Microscopy and Transmission Electron Microscopy), selected area electron diffraction (SAED), Fourier Transform Infra Red spectroscopy (FTIR), Electron Dispersive spectroscopy (EDS), EDS Mapping, BET surface analysis, Photoluminescence, Electrochemical Impedance spectroscopy and Cyclic Voltammetry. To evaluate the photocatalytic property of the semiconductor photocatalyst, this chapter presents the methodology for the experimental set up and evaluation of other phenomena like; Optical absorption spectroscopy, Scavenging experiments and chemical oxygen demand (COD) analysis. Chapter 3: Synthesis, Characterization and Optimization of Photocatalytic Activity of TiO2/ZrO2 Nanocomposite Heterostructures This chapter presents the work on the synthesis of TiO2/ZrO2 nanocomposite heterostructures and the evaluation of their photocatalytic activity in presence of UV irradiation. ZrO2 coupled TiO2 photocatalytic nanoparticles were synthesized via a hybrid sol–gel method followed by a suitable calcination treatment. Tetragonal structure of TiO2/ZrO2 nanocomposite particles with stabilized anatase phase was confirmed by XRD studies. The synthesized TiO2/ZrO2 nanocomposite exhibits unique optical properties as the band gap increases on Zr addition but, incorporation of intermediate energy levels expands its absorption edge into the visible light region. Results showed a considerable decrease in recombination rate on ZrO2 addition and Impedance spectroscopy showed a significant decrease in dielectric characteristics on ZrO2 addition. The TiO2/ZrO2 composites show an efficient photocatalytic activity for degradation of the organic pollutants such as aqueous PBS. The optimum loading
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