Enhanced photoelectrochemical water oxidation of WO₃/R-CoO and WO₃/B-CoO photoanodes with a type II heterojunction

Tungsten trioxide (WO₃) has been conceived as a promising photoanode material for photoelectrochemical (PEC) water oxidation. Therefore, many efforts have been made to improve its PEC performances. Herein, a novel heterojunction is fabricated through combining rocksalt CoO (R-CoO) or blende CoO (B-CoO) nanosheets with WO₃ nanoplates using a spin-coating method. The typical type II heterojunctions, e.g., WO₃/R-CoO and WO₃/B-CoO, both have exhibited higher photocurrent densities than pristine WO₃ photoanode. The photocurrent densities of WO₃/R-CoO, WO₃/B-CoO and WO₃ are 0.53 mA cm⁻², 0.45 mA cm⁻² and 0.31 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode, respectively. For the WO₃/R-CoO photoanode, the surface charge separation efficiency is 50.95% and the photoconversion efficiency is 0.062%, which are both higher than the WO₃ and WO₃/B-CoO photoanodes. The enhanced PEC performances are due to the type II heterojunction between WO₃ and R-CoO (or B-CoO), which facilitates the absorption of visible light and charge transport. The better performance of WO₃/R-CoO than that of WO₃/B-CoO may be due to the deeper valence band position of R-CoO. Our work demonstrates that R-CoO (or B-CoO) can couple with WO₃ to form a type II heterojunction to improve the PEC water oxidation performance.