Promoting effect of interfacial hole accumulation on photoelectrochemical water oxidation in BiVO4 and Mo-doped BiVO4

Abstract

Hole transfer at the semiconductor-electrolyte interface is a key elementary process in (photo)electrochemical (PEC) water oxidation. However, up to now, a detailed understanding of the hole transfer and the influence of surface hole density on PEC water oxidation kinetics is lacking. In this work, we propose a model for the first time in which the surface accumulated hole density in BiVO₄ and Mo-doped BiVO₄ samples during water oxidation can be acquired via employing illumination-dependent Mott-Schottky measurements. Based on this model, some results are demonstrated as below: (1) Although the surface hole density increases when increasing light intensity and applied potential, the hole transfer rate remains linearly proportional to surface hole density on a log-log scale. (2) Both water oxidation on BiVO₄ and Mo-doped BiVO₄ follow first-order reaction kinetics at low surface hole densities, which is in good agreement with literature. (3) We find that water oxidation active sites in both BiVO₄ and Mo-doped BiVO₄ are very likely to be Bi⁵⁺, which are produced by photoexcited or/and electro-induced surface holes, rather than VO_x species or Mo⁶⁺ due to their insufficient redox potential for water oxidation. (4) Introduction of Mo doping brings about higher OER activity of BiVO₄, as it suppresses the recombination rate of surface holes and increases formation of Bi⁵⁺. This surface hole model offers a general approach for the quantification of surface hole density in the field of semiconductor photoelectrocatalysis.