Conformally coated BiVO4 nanodots on porosity-controlled WO₃ nanorods as highly efficient type II heterojunction photoanodes for water oxidation

Highlights: • This is the first systematic study between PEC properties and band structures of BiVO₄-based heterojunction. • Control of active areas of WO₃ is the key to achieve high PEC efficiency of BiVO₄/WO_3. • Conformally coated 3D BiVO₄ nanodots on the surface of WO₃-NRs was firstly reported. • Our BiVO₄/WO₃ show the highest photocurrent without catalyst or doping reported to date. Bismuth vanadate (BiVO₄) has become a hot topic for solar hydrogen generation by water oxidation due to its appropriate band edges, and narrow band gap. However, the actual conversion efficiency achieved with BiVO₄-based photoanodes is considerably less than the theoretical values because of drawbacks such as poor electron transportation and slow kinetics of oxygen evolution. In these respects, formation of nanostructured heterojunctions is considered to have a marked effect in terms of improving the photoactivities of BiVO₄ photoanodes since it is possible to enhance the charge transfer at the interface and increase light absorption significantly. Herein, we report the synthesis and photoelectrochemical (PEC) properties of various BiVO₄-based heterojunction anodes based on stoichiometric BiVO₄ by pulsed electrodeposition. We show that WO₃ is the optimum bottom layer for a type II band system with BiVO₄ among various metal oxides. It is revealed that the photocurrent density of bare WO₃ nanorods can be changed drastically by controlling the porosity and aspect ratio and thus BiVO₄/WO₃ heterojunction anodes show as similar tendency to bare WO₃ nanorods. Under the optimum conditions, a BiVO₄/WO₃ heterojunction anode can lead to a high photocurrent density of 4.55 mA/cm² and an incident photon to current conversion efficiency of 80% at 1.23 V versus a reversible hydrogen electrode without additional catalyst. Cross-sectional transmission electron microscopy shows that dot-like BiVO₄ is well decorated on the entire surface of WO₃ nanorods. This systematic study provides a viewpoint on the crucial role of the active area of the bottom layer for high-performance BiVO₄-based type II heterojunction photoanodes, and the proposed concept is applicable to various photoelectrode systems.