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Environmental pollution and industrialization on a global scale has drawn attention to the vital need for developing new hygienic and environmentally friendly purification technologies. The most common indoor and outdoor purification process have their corresponding limitations of electricity and/or other energy sources. To address such enormous tasks, advance oxidation technology like heterogeneous photocatalytic systems via metal oxide semiconductors such as TiO2, ZnO, that are capable to operate effectively and efficiently under UV and visible light must be established. Due to the wide band gap of semiconductors, they are unable to absorb visible light. Hence, numerous research efforts have been done to increase the photocatalytic activity and optical absorption of semiconductors by various methods. This thesis presents the study of enhanced absorption capability of metal modified, especially silver modified, Ti02 and ZnO and silver-ceria co-doping on Ti02. All the experiments carried out and characterization techniques used were explained in detail. The present work deals with the synthesis of unmodified and with various mol% of metal modified Ti02, and ZnO. Photocatalytic activity is analysed using a model dye, rhodamine 6G (R6G). A noble metal such as silver is used as the metal modifier and optimised the concentration of silver giving the highest photocatalytic activity in both cases. The synthesis of Ti02 through a modified sol-gel route and effect of silver modification to enhance the photocatalytic activity of Ti02 is explored. In the case of Ti02, two methods are adopted to introduce silver, such as the light irradiation method and the direct calcination method. Of the two methods, the latter is found to produce a more effective photocatalytic material (6-50% improvement in catalytic efficiency), which is attributed to the fact that the silver is homogeneously dispersed throughout the material. 5 mol% is found as the optimum giving the highest rate of photocatalytic activity. Results demonstrate that silver modification retains the anatase phase stability up to 700°C. In addition, adsorption experiments on Ti02 which undergo further degradation. Photocatalytic activity tests of various % of urea modified Ti02 indicate urea modification can extend the anatase phase stability for higher temperatures and 1:1 TTIP:Urea modified sample at 900°C show high photocatalytic activity among other samples. Analysis on the effect of silver modification on ZnO and mechanism of photocatalytic enhancement is illustrated. In the case of Zn0, a wet chemistry approach is used for the silver modification. Materials are synthesized at various temperatures ranging from 300 to 1000°C. It is observed from the characterization that 3 mol % silver-modified ZnO at 400 °C shows approximately a four time higher rate of degradation than that of unmodified ZnO and a three times higher rate of degradation than that of unmodified ZnO and a three times higher rate of degradation than that of unmodified ZnO and a three times higher rate than that of commercial Ti02, photocatalyst standard, Degussa P-25. A possible mechanism of photocatalytic activity in presence of sensitising dye is also discussed in light of photoemission studies. It is reasoned that the presence of silver facilitates the interfacial charge transfer processes in such a way to utilise the CB electrons for enhancing the photocatalytic activity. Investigated the visible light absorption capability of Ti02, as a result of novel silverceria co-doping. The effect in structural and optical properties associated with co-doping is demonstrated. Analysis on results shows silver-ceria co-doping results in a large shift in absorption towards visible wavelength resulting in visible light active materials. Furthermore, the structural analysis clearly proves the high temperature stability of the anatase phase and it is believed that there is a silver-ceria interaction at low temperature on the Ti02 surface. In overview, the work produced and results derived form the current study have different potential applications in the field of semiconductor photocatalysis in the near future.
Georgekutty, Reenamole. Enhanced absorption metal oxides for photocatalytic applications. Dublin : Dublin Institute of Technology. 2009