| dc.description.abstract | Water in most rivers in India does not meet the standards for drinking and even for bathing purposes due to the high concentration of coliforms, organic and inorganic contaminants. Overuse, uncontrolled release and careless disposal of drug, pharmaceutical components from hospital and domestic waste imparts a crucial role in developing a resistance towards antibiotics in microbes over a period of time. The class of bacteria affected by antibiotics are hereby called susceptible bacteria and the class that shows resistance towards antibiotics are known as antibiotic resistant bacteria. Therefore, presence of both class of microbes in water pose a serious question on the public health. The conventional ways to remove microbes possess the limitations of generating harmful by-products, fouling, high energy requirement and so on. In this regard, heterogeneous photocatalysis eliminates all the above-mentioned limitations by providing ease of separation, energy and cost benefits. However, there are some limitations associated with it such as recovery of the catalyst, reusability of the catalyst, photo-corrosion in the semiconductor material and so on. In this work, efforts have been made to rectify the issues associated with the use of photocatalysts at a larger scale.
Semiconductor based photocatalysis possesses capability to facilitate surface redox reaction for generation of highly oxidizing and reducing radicals for mineralization of contaminants into non-harmful by-products as a result of absorption of photons. Commercially available catalysts such as TiO2, ZnO are wide band gap semiconductors and are highly efficient under UV irradiation. However, their wide band gap limits their viable usage in visible/ longer wavelength light. Due to limited access of UV wavelength from solar light (3-4 %), there is extensive need to design visible light responsive semiconductors or composites. Lowering the band gap, alteration in charge dynamics of the semiconductors and usage of novel lower band gap semiconductors are the ways to improve the photo response of the semiconductors. This study contains all the possible aspects of lowering of band gap by doping and charge dynamics alteration by band engineering with Type-1, Type-2 and Z-scheme. Metal substitution of Cu and interstitial doping of N in ZnO lattice and drastic improvement in optical properties were studied for inactivation of susceptible E. coli. Morphological aspect of the metal oxides is also a very crucial parameter in charge dynamics of the excitons, in this regard, ZnO/CdS/Ag nanorods and nanoparticles were synthesised to understand the photocatalytic mechanism involving surface plasmon effect due to the presence of Ag impregnation. Combined effect of doping and impregnation was also analysed by doping of Fe due effective coupling of orbitals due to its half-filled 3d orbitals. Ag is a widely used expensive antimicrobial agent. Therefore, CuO was introduced to increase the cost effectiveness with excellent photocatalytic properties. Leaching of metal ions from the semiconductor reduces the repeatability and stability of the catalysts. Therefore, metal free semiconductor C3N4 was coupled with CuWO4 in order to understand the Z-scheme mechanism of photocatalysis which is analogous to the photosynthesis. This Z-scheme composite was exploited for simultaneous inactivation of gram positive and gram-negative bacteria and extensive analysis of kinetics was studied for both the scenarios. These ways increase the charge separation and diffusion length and increases the carrier lifetime. However, recovery of these catalysts after usage is another concern which raise a question for this method to be commercialized. Also, the cost involved in the separation of catalysts in slurry form is another limitation which has been addressed in the present study.
In order to address the problem of recovery and the separation of the catalyst particles in slurry form immobilization of the catalyst on substrates such as FTO, ITO, glass slide, cellulose acetate and so on can be performed. Vertically aligned ZnO nanorods coupled with CuI were grown on a conducting substrate (FTO) to augment the photo response in visible light. These substrates with grown catalysts were used as working electrodes for photo-electro-catalysis. A model was derived considering all the possible aspects of interaction of catalysts with the pollutants for various cases such as electrolysis, photolysis, electrocatalysis, photocatalysis and photoelectrocatalysis. Kinetics and mechanistic aspects of all the processes were touched and explored for simultaneous inactivation of drug (chloramphenicol) and susceptible/ antibiotic resistant bacteria (E.coli). The insights obtained from the above studies will be useful in better understanding of designing, synthesis and the charge dynamics of the semiconductors | en_US |
