Effective Surface Engineering for Defect Passivation and Reduction of Water Oxidation Overpotential in Benchmark 2D 𝛼‐SnWO ₄ Nanoplate Photoanodes

Abstract Stannous tungstate ( α ‐SnWO 4 ) is a highly anticipated next‐generation metal oxide photoanode for photoelectrochemical (PEC) water oxidation because of its narrow bandgap (1.9 eV) and favorable band edge positions. Despite its high theoretical photocurrent density, its practical applicability is constrained because of poor charge transfer ability and severe surface charge recombination due to the surface states leading to slow water oxidation kinetics. Here, the effective nanoarchitectural design and surface Cl‐modification of solvothermally fabricated (121) facet 2D α ‐SnWO 4 nanoplates arrays for PEC water splitting are demonstrated. The Cl: α ‐SnWO 4 photoanode delivers the benchmarking photocurrent density of 1.9 mA. cm −2 at 1.23 V RHE under AM1.5G radiation (100 mW cm −2 ). Surface Cl‐modification improves the visible light harvesting performance and reduces nonradiative photocarrier recombination through surface defect passivation. The DFT studies confirm the favorable tuning of electronic structure and increased delocalization of the surface Sn orbital due to Cl‐doping in SnWO 4 boosting the photogenerated hole mobility and injection at the interface. DFT simulations reveal that the surface Cl‐doping also reduces the water oxidation overpotential, increasing the OER kinetics of the Cl‐SnWO 4 photoanode. This study establishes practical and straightforward strategies to empower the water‐splitting performance of the α ‐SnWO 4 photoanode through nanoscale architecture, facet, and surface engineering.