Toward Eco-Friendly and Highly Efficient Solar Water Splitting Using In₂S₃/Anatase/Rutile TiO₂ Dual-Staggered-Heterojunction Nanodendrite Array Photoanode

The TiO₂-based heterojunction nanodendrite (ND) array composed of anatase nanoparticles (ANPs) on the surface of the rutile ND (RND) array is selected as the model photoanode to demonstrate the strategies toward eco-friendly and efficient solar water splitting using neutral electrolyte and seawater. Compared with the performances in alkaline electrolyte, a non-negligible potential drop across the electrolyte as well as impeded charge injection and charge separation is monitored in the ANP/RND array photoanode with neutral electrolyte, which are, respectively, ascribed to the series resistance of neutral electrolyte, the fundamentally pH-dependent water oxidation mechanism on TiO₂ surface, as well as the less band bending at the interface of TiO₂ and neutral electrolyte. Accordingly, a TiO₂-based dual-staggered heterojunction ND array photoanode is further designed in this work to overcome the issue of less band bending with the neutral electrolyte. The improvement of charge separation efficiency is realized by the deposition of a transparent In₂S₃ layer on the ANP/RND array photoanode for constructing additional staggered heterojunction. Under illumination of AM 1.5G (100 mW cm^-2), the improved photocurrent densities acquired both in neutral electrolyte and seawater at 1.23 V vs reversible hydrogen electrode (RHE), which approach the theoretical value for rutile TiO₂, are demonstrated in the dual-staggered-heterojunction ND array photoanode. Faradaic efficiencies of ∼95 and ∼32% for solar water oxidation in neutral electrolyte and solar seawater oxidation for 2 h are acquired at 1.23 V vs RHE, respectively.