Controlled Band Offsets in Ultrathin Hematite for Enhancing the Photoelectrochemical Water Splitting Performance of Heterostructured Photoanodes

Formation of type II heterojunctions is a promising strategy to enhance the photoelectrochemical performance of water-splitting photoanodes, which has been tremendously studied. However, there have been few studies focusing on the formation of type II heterojunctions depending on the thickness of the overlayer. Here, enhanced photoelectrochemical activities of a Fe₂O₃ film deposited-BiVO₄/WO₃ heterostructure with different thicknesses of the Fe₂O₃ layer have been investigated. The Fe₂O₃ (10 nm)/BiVO₄/WO₃ heterojunction photoanode shows a much higher photocurrent density compared to the Fe₂O₃ (100 nm)/BiVO₄/WO₃ photoanode. The Fe₂O₃ (10 nm)/BiVO₄/WO₃ trilayer heterojunction anodes have sequential type II junctions, while a thick Fe₂O₃ overlayer forms an inverse type II junction between Fe₂O₃ and BiVO₄. Furthermore, the incident-photon-to-current efficiency measured under back-illumination is higher than those measured under front-illumination, demonstrating the importance of the illumination sequence for light absorption and charge transfer and transport. This study shows that the thickness of the oxide overlayer influences the energy band alignment and can be a strategy to improve solar water splitting performance. Based on our findings, we propose a photoanode design strategy for efficient photoelectrochemical water splitting.