Fabrication of an Efficient BiVO₄-TiO₂ Heterojunction Photoanode for Photoelectrochemical Water Oxidation

In this work, a simple planar BiVO₄/TiO₂ heterojunction photoanode was prepared on a fluorine-doped tin oxide (FTO) substrate for photoelectrochemical (PEC) water oxidation. The measurements of surface photovoltage, photocurrent transient behavior, and hole-scavenger-assisted PEC performance indicate that charge separation efficiency is improved compared to that of the BiVO₄/FTO photoanode. This improvement is caused by the formation of the staggered BiVO₄/TiO₂ heterojunction. However, the photocurrent densities of the BiVO₄/TiO₂/FTO photoanode are higher than those of the BiVO₄/FTO one only at potentials >1.2 V vs reversible hydrogen electrode, although the two BiVO₄ layers with comparable light harvesting efficiencies were prepared by the same method. The hole-scavenger-assisted PEC measurements reveal that the hole injection efficiency of the BiVO₄/TiO₂/FTO photoanode is inferior to that of the bare BiVO₄/FTO anode for oxygen evolution. It shows that the surface property of the BiVO₄ layers is altered as they are deposited on different substrates. On the basis of these characterizations, the cocatalyst cobalt phosphate (Co-Pi) was further deposited on the surface of BiVO₄/TiO₂/FTO photoanode to improve the hole injection efficiency. Subsequently, the photocurrent density and stability of the Co-Pi/BiVO₄/TiO₂/FTO photoanode were significantly improved compared to those of the bare BiVO₄/FTO photoanode.