Increased demand of power, limited fuel resources and environmental concerns have recently prompted a huge thrust on research areas of alternative energy and photovoltaics have been hailed as energy source for future. Particularly, third generation solar cell configurations like dye-sensitized solar cells and quantum dot Schottky barrier solar cells have drawn more attention because of their ease of processability, cheap cost with decent performance, low payback time and portability. Quantum dots are very attractive materials as sensitizers because of their size dependent bandgap tunability, increased oscillator strength and hence higher absorption coefficient, possibility of multiple exciton generation and photochemical robustness. Hence syntheses of quantum dot based hybrid nanostructures have received huge attention among researchers for using it quantum dot sensitized solar cell configuration. This dissertation can be divided in two parts. In the first part two different methods have been reported to prepare interconnected mesoporous nanostructures of wide band gap semiconductors like TiO2 and ZnO which is very important in providing high surface area for absorption or attachment of the sensitizers. In the second part, methods have been developed to establish direct contacts between quantum dots and wide bandgap substrates without molecular linkers which are expected to increase the electron injection rate from quantum dots to TiO2/ZnO. The entire thesis based on the results and findings obtained from the present investigation is organised as follows: Chapter-I provides a general introduction on the working principle of different type of solar cells and then gives a detailed description of the structure and electronic process of dye sensitised solar cells. Then, benefits of quantum dots as sensitizer over dye molecules has been discussed followed by the modification needed in case of quantum dot sensitized solar cells. Chapter-II deals with the materials and methods which essentially gives the information about the materials used for the synthesis and the techniques utilized to characterize the materials chosen for the investigation. Chapter-III describes a hybrid sol-gel combustion technique to synthesize large quantities of highly crystalline and phase-pure anatase powder in a single step. Titanium isopropoxide reacts with oleic acid to form a viscous liquid (oxocarboxoalkoxide) which undergoes non-hydrolytic polycondensation to form TiO2 during combustion. Oleylamine takes part in formation of reverse micelle which expands during combustion giving rise to porous interconnected membrane like microstructure of pore size ~5 nm, BET surface area of ~100 m2/g and porosity of ~ 48%. More importantly, this porous powder having a pre-existing network can be used to form thicker film by doctor blade technique from its paste and at the same time is expected to have better transport properties due to its less particulate nature. Chapter-IV presents a general method to prepare mesoporous structure from rod-like morphologies by partial sintering of a green pellet. Material having inherent anisotropy in their crystal structure tends to grow in a particular direction rather undergoing equiaxial growth. For instance, hexagonal ZnO and tetragonal rutile usually grow as rod-shaped particles. A loose compact of these nanorods give nanoporous morphology upon heating. Advantage of this method is the tunability of pore size by tuning the aspect ratio of the nanorods. Preparation of porous TiO2, ZnO and hydroxyapatite has been demonstrated from their corresponding nanorods. Chapter-V deals with a solvothermal based technique that has been developed for in-situ deposition of nanoparticles on any plane or curved surfaces conformally. This has been demonstrated for nanoparticles of FeCo, Au, Co, CdS on substrates like glass, mica, Si, NaCl, Al2O3 M-plane and also conformal coating of Au nanoparticles on polystyrene latex spheres. CdSe on rutile nanorods, ZnO nanorods and CNTs are promising hybrid nanostructures for third generation photovoltaics and their successful preparation has been detailed in the chapter. The mechanism proposed is based on dominant attractive sphere-plate interaction under high temperature and high autogeneous pressure condition which at reduced density and surface tension of the solvent reduces the dispersibility of the nanoparticle and allow effective spreading of the nanoparticles on the substrate. This method is also advantageous for coating of complicated geometry like inner walls of porous structures. Chapter-VI presents a method to coat chalcogenide nanoparticles on mesoporous TiO2 without any molecular linker which can enhance the electron injection rate from the chalcogenide quantum dots to TiO2. CdS, PbS can be easily synthesized through aqueous chemistry. For deposition of these sulfides, the ion layer gas absorption and reaction (IGLAR) method was modified to form uniform dense nanoparticles on anatase and ZnO surfaces. Nitrate salts of corresponding metal ions are dried directly on the semiconductor surface and instead of exposing it to H2S gas, it was treated with a concentrated sulfide solution. This introduces two competitive process i) dissolution of nitrate salt ii) formation of the metal sulfide. This dissolution step was absent when treated with H2S gas (IGLAR) and hence lead to a continuous coating. We have successfully produced CdS-TiO2 and PbS-TiO2 composites using this approach. Photoelectrochemical measurements on CdSTiO2 composites show an overall efficiency of 2.8% which is among the highest values obtained for this system demonstrating the applicability of the method to engineer interfaces to achieve high efficiency solar cells. Chapter-VI explores the combination of strategies of nanocrystal conversion chemistry with previously described sol-gel combustion technique to create dense and uniformly coated QD sensitized TiO2 electrode without compromising heat-treatment routines which is essential for better adhesion and to enhance performance with reduced leakage. Intimate biphasic oxide mixtures of PbO and CdO with TiO2 are first synthesized by nonhydrolytic solgel process with is followed by combustion to produce porous morphology. This powder can be coated as electrode and can sustain high temperature heat treatment routines and finally can be selectively converted to sulfides with Na2S treatment as TiO2 is immune to sulfidation under this condition. Materials at different stages are characterised by XRD, TEM, EDS, UV-Vis and XPS.

Made available in DSpace on 2014-12-17T14:06:59Z (GMT). No. of bitstreams: 1 LucianoLMS.pdf: 2498297 bytes, checksum: 4e0c9395aebdb6d0be7a37737a51ca92 (MD5) Previous issue date: 2008-04-29 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / Oxide type spinel AB2O4 presents structure adjusted for application in the automobile industry. The spinel of cobalt has many practical applications had its excellent physical and chemical properties such as catalyst in hydrocarbon oxidation reaction. The CeO2 has been used in many of these processes because it assigns to a material with excellent thermal resistance and mechanics, high capacity of oxygen stockage (OSC) among others properties. This work deals with the synthesis, characterization and catalytic application of spinel of cobalt and CeO2 with fluorita structure, obtained for method of Pechini and method of Gel-Combustion. The process of Pechini, the puff was obtained at 300 ?C for 2 h in air. In the process of Gel-Combustion the approximately at 350 ?C material was prepared and burnt for Pyrolysis, both had been calcined at 500 ?C, 700 ?C, 900 ?C and 1050 ?C for 2 h in air. The materials of the calcinations had been characterized by TG/DTA, electronic microscopy of sweepings (MEV), spectroscopy of absorption in the infra-red ray (FTIR) and diffraction of X-rays (DRX). The obtained material reaches the phase oxide at 450 oC for Pechini method and 500 ?C for combustion method. The samples were submitted catalytic reaction of n-hexane on superficies of materials. The reactor function in molar ration of 0, 85 mol.h-1.g-1 and temperature of system was 450 ?C. The sample obtained for Pechini and support in alumine of superficial area of 178,63 m2.g-1 calcined at 700 ?C, give results of catalytic conversions of 39 % and the sample obtained for method of gel-combustion and support in alumina of 150 mesh calcined at 500 ?C result 13 % of conversion. Both method were selective specie C1 / O espin?lio de cobalto tem muitas aplica??es pr?ticas devido as suas excelentes propriedades f?sicas e qu?micas tal como catalisador nas rea??es de oxida??o de hidrocarboneto. O CeO2 tem sido utilizado em muitos processos porque designa um material com excelente resist?ncia t?rmica e mec?nica, alta capacidade de estocagem de oxig?nio (OSC) entre outras propriedades. Este trabalho trata da s?ntese e caracteriza??o e aplica??o catal?tica dos ?xidos de cobalto com estrutura espin?lio e CeO2 com estrutura fluorita, obtidos pelo m?todo de Pechini e pelo m?todo de gel-combust?o. No processo Pechini o puff foi obtido a 300 ?C por 2 h em atmosfera ambiente. No processo de gel-combust?o o material foi preparado e queimado a aproximadamente 350 ?C por igni??o espont?nea e, posteriormente, ambos os p?s foram calcinados a 500 ?C, 700 ?C, 900 ?C e 1050 ?C por 2 h em atmosfera ambiente. Os p?s-resultantes das calcina??es foram caracterizados por TG/DTA, microscopia eletr?nica de varredura (MEV), espectroscopia de absor??o no infravermelho (FTIR) e difra??o de raios X (DRX). O material obtido deve atingir a fase ?xida a 450 ?C, para Pechini e 500 ?C para gel-combust?o. As amostras foram submetidas a rea??es catal?ticas de n-hexano sobre os catalisadores de Ce/Co. O reator operou a uma raz?o F/W fluxo molar de reagente por grama de catalisador de 0,85 mol.h-1.g-1 e temperatura no leito de 450 ?C. Para amostras obtidas por Pechini calcinadas a 700 ?C e suportada em alumina com ?rea superficial de 178,63 m2.g-1 obteve-se 39 % de convers?o catal?tica. Para as amostras obtidas pelo m?todo de gel-combust?o calcinadas a 500 ?C e suportada em alumina da Porogel com 150 mesh obteve cerca de 13 % de convers?o catal?tica. Ambos os m?todos foram seletivos a esp?cie C1.