Facet-Engineered Octadecahedral Rh/Cr Co-doped SrTiO3 for Z-Scheme Overall Water Splitting

Facet engineering is an effective strategy for enhancing the photocatalytic performance of semiconductor single crystals. However, conventional high-temperature solid-state synthesis often suffers from coupled crystallization and grain coarsening, making it difficult to simultaneously achieve high crystallinity, controlled facet exposure, small particle size, and good dispersibility. Herein, we report an integrated strategy combining solid-state recrystallization and facet engineering for the synthesis of small-sized SrTiO3:Rh single crystals with co-exposed (100)/(110) facets. Specifically, this approach employs partially crystallized SrTiO3 nanosphere precursors, which are subsequently subjected to SrCl2 molten-salt-assisted high-temperature recrystallization to enable crystal reconstruction, while suppressing excessive grain growth. The incorporation of Rh, together with the SrCl2 molten-salt environment, influences crystal growth behavior and facilitates selective facet exposure, which is associated with the formation of octadecahedral SrTiO3:Rh single crystals. The resulting particles simultaneously exhibit high crystallinity, anisotropic facet exposure, reduced particle size, and enhanced suspension stability in aqueous media. These structural features are closely correlated with improved charge carrier separation behavior and higher mass-normalized activity under low catalyst loading conditions. Compared with conventional Rh2O3-derived SrTiO3:Rh, the optimized samples exhibit significantly enhanced charge separation behavior and photocatalytic hydrogen evolution performance. Furthermore, upon Cr co-doping, the SrTiO3:Rh,Cr/BiVO4 system with the highest activity among the investigated Cr concentrations, coupled with the Fe3+/Fe2+ redox mediator, achieves an apparent quantum yield (AQY) of 4% under 420 nm monochromatic irradiation and demonstrates stable overall water splitting activity in a liquid-phase Z-scheme configuration.