Surface Restructuring and Ion Exchange in Molten Salts for Enhanced SrTiO₃ Water Splitting Photocatalysts

Semiconductor-based photocatalysis offers a promising method for generating solar fuels by directly converting water into hydrogen and oxygen under light exposure. However, its commercial application is limited by poor efficiency, primarily due to the recombination of photogenerated charge carriers and surface back reactions. Al-doped SrTiO₃ synthesized in a SrCl₂ melt, stands out as one of the most efficient photocatalysts for water splitting, exhibiting a quantum yield close to unity. Despite extensive discussions on the effect of Al³⁺ doping, the role of the molten SrCl₂ has yet to be fully understood. By analyzing the surface structure and chemistry of SrTiO₃ catalysts with AFM and XPS, we found that the treatment with SrCl₂ melt leads to a more negatively charged surface, attributed to an increased concentration of surface hydroxyl groups. The resulting potential drop varies with crystallographic orientation. Consequently, the potential difference between the (1 0 0) and (1 1 0) surfaces increases from 0.07 V to 0.21 V, generating a significantly stronger electric field within the particle and enhancing charge separation. Furthermore, we present a novel strategy for designing SrTiO₃ photocatalysts based on ion exchange reactions between perovskite oxides and molten salts. In this process, the A-site cations in oxides can be exchanged with alkali earth cations in molten salts. Notably, SrTiO₃ derived from BaTiO₃ via the reaction, BaTiO_3(s) + SrCl_2(l) = SrTiO_3(s) + BaCl_2(l), demonstrates exceptional photocatalytic performance (AQY = 11.4% under 380 nm) and produces hydrogen from pure water at a rate twice that of conventional SrTiO₃ without ion exchange. The enhanced photocatalytic efficiency is attributed to its nonequilibrium structure, featuring a Sr-excess phase near the surface, which compensates for donor defects that typically act as charge traps and recombination centers. Our results highlight the potential of using molten salts to design and synthesize highly efficient photocatalyst materials.