WO₃/BiVO₄ Photoanodes: Facets Matching at the Heterojunction and BiVO₄ Layer Thickness Effects

Photoelectrochemical solar energy conversion offers a way to directly store light into energy-rich chemicals. Photoanodes based on the WO₃/BiVO₄ heterojunction are most effective mainly thanks to the efficient separation of photogenerated charges. The WO₃/BiVO₄ interfacial space region in the heterojunction is investigated here with the increasing thickness of the BiVO₄ layer over a WO₃ scaffold. On the basis of X-ray diffraction analysis results, density functional theory simulations show a BiVO₄ growth over the WO₃ layer along the BiVO₄ {010} face, driven by the formation of a stable interface with new covalent bonds, with a favorable band alignment and band bending between the two oxides. This crystal facet phase matching allows a smooth transition between the electronic states of the two oxides and may be a key factor ensuring the high efficiency attained with this heterojunction. The photoelectrochemical activity of the WO₃/BiVO₄ photoanodes depends on both the irradiation wavelength and the thickness of the visible-light-absorbing BiVO₄ layer, a 75 nm thick BiVO₄ layer on WO₃ being best performing.