Promdet, Premrudee; (2022) Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances of ZnO thin films. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis focuses on the development of photocatalytic ZnO coatings using aerosol assisted chemical vapor deposition (AACVD). The work emphasises the influence of deposition parameters on the properties of the resultant film, and the fabrication of heterostructures based on ZnO to enhance photocatalytic property. The materials have been designed with a wide range of potential applications, from degradation of organic pollutants to energy production. Where possible, tests have been conducted to assess the efficiency of the material for these applications. The first chapter highlights a number of technologically important applications of photocatalytic material to demonstrate the motivation for research in this area. Fundamental principles of semiconductor photocatalysis and theoretical concepts are presented which are basic to understanding the behaviour of semiconductor. The principles and application of chemical vapour deposition, as well as the fundamental behind the photocatalytic tests used in this work are discussed. The second chapter focusses on the modification of morphology, crystal orientation and oxygen vacancy of ZnO thin films by acetic acid treatment under AACVD. It was found that acetic acid promotes preferential growth and exposure of polar facets, which enhances photocatalytic activity. The initial enhanced efficiency of ZnO thin film was also correlated to structural defects, likely oxygen vacancies, which the defect could be suppressed by annealing under air. This work provides the understanding of the fundamental properties of ZnO thin films, which is a crucial step in the development of ZnO photocatalysts. Although ZnO has received tremendous attention to address energy and environmental issues, its application is still limited because of low photocatalytic efficiency resulting from fast recombination of photogenerated carriers and poor solar energy utilization. In this thesis, to enhance charge separation and extend light absorption of ZnO thin films, the formation of ZnO composite materials or heterojunctions have been applied. For ZnO heterostructures, we successfully developed high-performance Cu nanoparticles: ZnO composite photoanode for water splitting photoelectrochemical cell (PEC). The material was deposited on FTO by single-step AACVD using readily available chemicals as precursors. It was found that the incorporation of Cu nanoparticles in ZnO photoanode significantly improved the photocatalytic activity of the photoanode compared with ZnO thin film. Studies using photoluminescence spectroscopy indicate that the enhanced photocatalytic performance could be attributed to hot electron generated from surface plasmon resonance (SPR). The Cu metal in the composite material functions as a sensitizer to supply electrons to the semiconductor resulting in enhanced electron density for redox reactions. Another heterostructure of ZnO that was fabricated and studied in this thesis is p-Cu2O/n-ZnO heterostructure thin films which were applied as photocathode in water splitting PEC. In this work, we have demonstrated a facile and low-cost synthesis of the p-Cu2O/n-ZnO thin films by single-step AACVD using compressed air. The as-synthesized p-Cu2O/n-ZnO thin film provided higher photocatalytic activity than that of pure Cu2O. This enhanced photocatalytic activity can be ascribed to the efficient charge separation in the heterostructure thin films. In this thesis, the preparation and photocatalytic performance of ZnGa2O4 thin films was studied. ZnGa2O4 is a binary compound oxide of ZnO and Ga2O3. By using various ratios of the Zn and Ga precursors, we successfully prepared ZnGa2O4-β-Ga2O3 heterostructure thin films via aerosol-assisted chemical vapor deposition (AACVD). The ZnGa2O4-β-Ga2O3 thin films showed enhanced photocatalytic activity compared with ZnGa2O4. The photocatalytic enhancement of the ZnGa2O4-β-Ga2O3 is explained by the formation of type-II band alignment at the interfaces between ZnGa2O4 and Ga2O3, resulting in enhanced photoinduced charge separation in the system. In conclusion, in this thesis, simple AACVD approaches to fabricate high-performance photocatalytic thin films by using readily available chemicals were studied. This thesis demonstrates the key findings from the experimental work and possible avenues for further research.

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