Bias-free visible light-driven photoelectrochemical water splitting of type II ZnO/CuS core/shell heterojunction nanotube arrays

Solar-driven water splitting of semiconductor photoelectrodes via photoelectrochemical (PEC) cell has been regarded as the most promising approach to mitigate the energy crisis and environmental issues in the future. In this work, CuS nanoparticles (NPs) are deposited on ZnO nanotube arrays (ZnO/CuS NTAs) via successive ion layer absorption and reaction method for PEC water splitting under visible light irradiation without applying bias. The excellent light harvesting capacity of CuS NPs from visible to near infrared region not only expands the light harvesting of ZnO NTAs into near infrared region, but also substantially boosts light absorption ranging from 300 to 800 nm. Moreover, CuS NPs coupled on ZnO NTAs can establish a type-II band alignment between ZnO and CuS. Consequently, the ZnO/CuS NTAs photoanode exhibits the significantly boosted PEC water splitting performance under visible light illumination (λ > 420 nm) without applying bias. The photocurrent density of the ZnO/CuS NTAs photoanode is 21.2 μA/cm2, which is increased by 9 times compared to that of the pure ZnO NTAs photoanode. The enhancement in PEC water splitting performance for ZnO/CuS NTAs is attributed to (i) the cooperative actions of ZnO and CuS; (ii) significant enhancement in light absorption from the visible to near infrared region achieved by CuS NPs and (iii) efficient charge carrier separation achieved by type-II band alignment.