Enhancing the Efficiency of Hematite and Bismuth Vanadate Photoanodes for Solar Hydrogen Production Via Nano-engineering and Surface Modification

Solar energy is the most abundant renewable energy source which can satisfy the entire global energy demand. Its diurnal and intermittent nature however limits its use in direct applications. Harvesting and storing the solar energy in the form of chemical fuel, like H2, is thus a tremendously important step for enabling its use as a fuel for nighttime and transportations. The development of photoelectrochemical cells for solar H2 production using a cheap and abundant material is always a promising solution. Many semiconductors have been investigated as a photoanode or a photocathode for photoelectrochemical water splitting; but, they are either unstable or exhibit a low efficiency. In fact, no materials yet known have both high stability and high efficiency. In contrast to many investigated photoelectrodes, hematite and BiVO4 demonstrated high chemical stability in aqueous solutions. They have suitable valence band position for water oxidation reaction, whereas, the position of their conduction bands are too low to generate H2 efficiently, which can be overcome by applying an additional bias potential. They have an ideal optical band gap around 2.2 and 2.4 eV, respectively, allowing them to convert solar energy into H2 with efficiencies of more than 16 % and 9 %, respectively. Due to the ideal optical band gap and the relatively high theoretical efficiency, hematite and BiVO4 are attractive candidates as photoanodes for photoelectrochemical solar H2 production; however, they have some drawbacks and significant challenges limiting its practical performance for the water oxidation process. These include short hole-diffusion length, poor carrier conductivity, and slow kinetics of water oxidation. In this project, attempts to overcome these problems by nano-engineering the structure of hematite and BiVO4 electrodes will be investigated. The fabrication of single-crystalline nanostructured photoanodes with lateral dimension that matches the short hole-diffusion length is highly expected to greatly reduce the bulk recombination. Whereas the modification of hematite and BiVO4 surfaces with passivation overlayer and/or oxygen evolution co-catalyst is highly expected to effectively suppress the surface recombination. The kinetic of charge carriers transfer and recombination will be investigated to evaluate the effectiveness of the proposed approaches in enhancing the photoactivity of the fabricated hematite and BiVO4 electrodes