Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.248
Publication date: 18th July 2019
Due to their ease of synthesis, low production cost, and potential long-term stability, multinary, semiconducting metal oxide materials have received much attention for use as photoanode materials in photoelectrochemical (PEC) water splitting devices. Among the multinary oxides investigated, bismuth vanadate (BiVO4) has garnered particularly high interest and remains the highest performing multinary oxide photoanode material to date. Reactions at the BiVO4/electrolyte interface may give rise to, or passivate, surface states. These surface states can act as relay sites for charge injection into the electrolyte, or as electron and hole traps that can enhance recombination rates. A detailed understanding of the chemical composition at the BiVO4/electrolyte interface and its dependence on specific conditions (applied potential and illumination) would provide valuable input for strategies to suppress surface recombination and to further optimize BiVO4-based photoanode materials.
We have used ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) to understand the light induced changes at the BiVO4/aqueous electrolyte interface. AP-HAXPES can be used to directly interrogate a solid surface under a bulk-like electrolyte film that is tens of nanometers thick. Using AP-HAXPES we have studied the open-circuit behaviour of the thin-film BiVO4/aqueous electrolyte interface under dark conditions and when illuminated with a solar simulator. In a phosphate buffer electrolyte and under illumination, we observe spectral changes consistent with the formation of a thin bismuth phosphate layer and the repulsion of anions away from the interface. In a borate buffer solution no chemical changes at the interface are observed. Furthermore, by studying a series of electrolytes, consisting of a sodium citrate buffer, a sodium phosphate buffer and a combined sodium citrate-phosphate buffer, each with a pH of ~6.2, we show that the addition of citrate suppresses the formation of the thin bismuth phosphate layer. These results provide fundamental information about the complex chemical behaviour of semiconductor/electrolyte interfaces used in water splitting devices and indicate that judicious choice of electrolyte may provide a means of controlling the interfacial properties.
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