Visible-light driven photo-activity study : m-BiVO4 based nano-particles and thin film

• Main Author: Luo,Qiong
• Other Authors: Tan Ooi Kiang
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/139669
• Institution: Nanyang Technological University

Over the past few decades, semiconductor photocatalysts have received increasing amount of research interests for their environmental applications in various fields, such as water-splitting, water decontamination and air treatment. Titanium dioxide (TiO2) is reported to be the most efficient photocatalyst driven by UV-light irradiation. However, its applications are restricted to the UV-light range, which makes up of less than 5% of the solar spectrum, as a result of its wide bandgap of 3.2 eV. Alternative metal oxides with smaller bandgap are studied to develop a visible-light driven photocatalyst. Among them, bismuth vanadate (BiVO4) has become a promising candidate, as a result of its intrinsic narrow bandgap of 2.4 eV, which is suitable for visible light absorbance and proper band-edge position for water splitting. It can be synthesized by hydrothermal process, sonochemical method, co-precipitation and high temperature solid state reaction. In this work, monoclinic scheelite BiVO4 nano-particles were synthesized and modified for the purpose of improving photocatalytic efficiency. Furthermore BiVO4 thin films with photocatalytic property were developed and studied for their photo-response activity. Besides using photoelectrochemical (PEC) stations to investigate the photo-activity of BiVO4 nano-particles and thin film, a customized Kelvin probe coupled with multiple LED sources was developed to study the surface photovoltage (SPV) as a non-contact tool to understand the charge generation and separation that occurred at the semiconductor surfaces. BiVO4 nano-particles were synthesized by high energy ball mill process, which yielded a good throughput compared with wet-chemical approaches and produced fine nano-particles with relatively large surface areas. To improve the crystallinity, post annealing at 300°C to 600°C were followed. Nano-particles annealed at 400°C were found to exhibit the highest efficiency in photocatalytic performance. This can be explained by the trade-off between crystallinity, which increased with annealing temperature and favored the photocatalytic reactions by reducing the defects and recombination sites, and surface area distribution reduction, which generated less number of active sites for photocatalytic reactions to occur. In an attempt to further enhance the photocatalytic efficiency, p-type CuO was deposited onto BiVO4 nano-particle surface to form p-n heterojunctions. This helped to bring down the recombination rate and was reflected in terms of the suppression of photoluminescence (PL) signal, the enhancement of photo-currents upon light illumination, as well as large and fast SPV response. In terms of application on surface cleaning, thin film is more easily managed. Hence m- BiVO4 thin films were fabricated using spin coating method. The deposited films were further annealed at 350° to 500°C to form monoclinic scheelite BiVO4 following pyrolysis. The annealed thin film exhibited reasonable photo-degradation capability. The photocatalytic efficiency corresponded well with the SPV response. The best photo-efficiency came from BiVO4 thin film annealed at 450°C. This was the result of the increase in good crystallinity, and the most efficient charge transportation arising from the enhancement in hole movement due to lone-pair distortion in local crystal structure, benefiting the photocatalytic reactions by reducing recombination sites and increasing charge separation, respectively.