Metal oxides are particularly attractive as photoelectrode materials for solar water splitting, mainly due to their general aqueous stability and low cost. However, their semiconducting properties are usually poor due to poor carrier transport (small polarons) and high defect densities. This is especially true for complex metal oxides, where even small deviations from the ideal cation stoichiometry can generate high defect concentrations (e.g., 0.1% deviation can result in 1019 defects per cm3). Further complications may arise from the presence of oxygen vacancies, and from non-crystalline phases that are often formed when depositing complex oxides at moderate temperatures. One way to alleviate these limitations is to apply a high temperature treatment in an oxygen-rich environment [1,2]. However, systematic studies on the effect of such heat treatments on the defect properties and their influence on the photoelectrochemical (PEC) performance are scarce in the literature.

In this work, we investigate the defect behaviour of BiVO4 at high temperature. BiVO4 is currently the highest performing metal oxide photoanode [3,4], yet little work is done on the effect of high temperature treatments. Upon heating to 700 °C in air, we observed crystal lattice restructuring. X-ray diffractogram still showed the monoclinic scheelite phase, but the crystal orientation changed from (1 2 1) to (0 4 0). Raman spectroscopy also revealed a shift of the symmetric V-O stretching mode peak at ~825 cm-1. To investigate this further, we performed a mass spectrometry coupled thermogravimetric analysis and found that vanadium species (in the form of V, VO and VO2) leave the lattice at this temperature. XPS analysis shows a consistent decrease of the V/Bi ratio after the high temperature treatment, indicating the presence of vanadium vacancies. We propose that these vacancies are responsible for the intraband absorption that is observed above 500 °C with in-situ UV-vis measurements. We were able to suppress the loss of vanadium by performing the anneal in a V-rich environment. This strategy offers a general approach to prevent changes in the cation stoichiometry during high temperature treatment of complex metal oxides. Finally, the implications of the high temperature treatment on the conductivity and PEC activity of BiVO4 will be discussed.

[1] Thalluri et al., Ind. Eng. Chem. Res. 52 (2013) 17414

[2] Sivula et. al., J. Am. Chem. Soc. 132 (2010) 7436

[3] Pihosh et al., Sci. Rep. 5:11141 (2015)

[4] Abdi et al., Nat. Commun. 4:2195 (2013)