Tuning the band gap of Bismuth Vanadate via Z-Scheme heterojunction

Abstract

Metal oxide semiconductors such as TiO2 and ZnO have been exploited many times in photocatalysis and photoelectrochemical applications under UV light irradiation. Alternatively, new sets of metal oxide semiconductors such as Cu2O, Fe2O3, and BiVO4 have attracted widespread attention because of their ability to absorb light in the visible region. However, most of this category of photocatalyst is impacted by electron-hole recombination, slow charge transportation, and photo-corrosion. Furthermore, the bandgap of most metal oxides is not positioned to allow the reduction of water to hydrogen. Creating a Z-scheme heterojunction is a practical approach to total photoelectrochemical water splitting by combining a photoanode and a photocathode in one photocatalyst. This architecture allows spatial separation between the H2 and the O2 generation processes and improves the electron/hole separation. In this thesis, we describe a new approach to enhance the photocatalytic activity of BiVO4 with other bismuth-based metal oxides, namely Bi2Ru2O7, and BixGayOz to produce the Z-scheme photocatalyst Bi2Ru2O7/BiVO4, and BixGayOz/BiVO4, respectively. These heterogeneous Z-scheme architectures are constructed by temperature-controlled hydrothermal synthetic methods. A relatively low-temperature hydrothermal synthesis at 80 °C and high-temperature hydrothermal synthesis at temperatures ranging from 140 to 180 °C have been followed to tune the composition of the BiVO4-based Z-scheme photocatalysts. These materials are further characterized by transmission electron microscopy, x-ray crystallographic diffraction, x-ray photoelectron spectroscopy, and UV-vis diffuse reflectance spectroscopy. Among all the nano-heterojunction materials, the Bi2Ru2O7/BiVO4 Z-scheme heterojunction displayed an improved photocatalytic activity in the degradation of the organic dye, rhodamine B. The Bi2Ru2O7/BiVO4 heterojunction degrades rhodamine B completely in 80 min compared to 300 min and 120 min for the single oxides Bi2Ru2O7 and BiVO4, respectively. The photoelectrochemical characterization indicated that the Z-scheme heterojunction improved the charge transfer and enhanced the electron/hole separation in Bi2Ru2O7/BiVO4 photocatalyst compared to the single catalyst. This novel bismuth vanadate/bismuth heterometal composite is a promising photocatalyst for photoelectrochemical water splitting and photodegradation of persistent organic contaminants.