Effects of a SnO2 hole blocking layer in a BiVO4-based photoanode on photoelectrocatalytic water oxidation

Abstract

Bismuth vanadate (BiVO₄) has emerged as a promising photoanode for photoelectrochemical (PEC) water splitting due to its appropriate band edge positions for an oxygen evolution reaction (OER). However, poor charge transport and slow kinetics for an OER when used alone without a buffer layer have prevented BiVO₄ based photoanodes from achieving a high PEC performance that was theoretically expected. Here, we investigate the effects of a SnO₂ buffer layer in BiVO₄ based photoanodes; in particular, how varying the thickness of the SnO₂ layer affects the properties of the materials in the SnO₂, the BiVO₄ grown on top of the SnO₂ and the PEC performance for water oxidation. To our surprise, a simple change of the SnO₂ thickness resulted in substantial changes in the intrinsic properties of the BiVO₄, including the Bi/V stoichiometry, crystalline phases, grain size, film roughness, and even the band gap, all of which are relevant to the operation of PEC cells. With an optimal SnO₂ thickness, much enhanced PEC performances of SnO₂/BiVO₄ photoanodes were obtained; the photocurrent density of 0.95 and 3.76 mA cm⁻² (measured at 1.23 V vs. the reversible hydrogen electrode (RHE) under 1 sun illumination) was demonstrated for the water and sulfite oxidation reactions, respectively. Our findings emphasize the importance of tuning the properties of the buffer layer, which subsequently influences the material properties of BiVO₄, in achieving high PEC performance of BiVO₄-based heterojunctions.