Abstract: The superior ability of perovskite-type SrNbO₂N to absorb intensive visible light makes it a potential semiconductor to produce hydrogen and oxygen by photoelectrochemical (PEC) water splitting under sunlight. The surface morphologies, such as shape and structure, of the oxynitride strongly affect its photoactivity, although the mechanism has been hardly studied. Herein, we report a two-dimensional (2D) porous SrNbO₂N plate with Zr doping, nitrided from layered perovskite Sr₅Nb₄O₁₅ and also its largely enhanced PEC water splitting activity. Zr⁴⁺ was doped in Sr₅Nb₄O₁₅ during flux-assisted calcination using KCl, producing 2D-type truncated-octahedral Sr₅Nb₄O₁₅:Zr plates approximately 50 nm in thickness. The nitridation completely transformed Sr₅Nb₄O₁₅:Zr to 2D single-crystalline SrNbO₂N:Zr with a large surface area, which was subsequently used to fabricate a thin and uniform photoanode by the spin coating method. As a result, the Co(OH)_x/SrNbO₂N:Zr/FTO photoanode capable of absorbing visible light of up to 680 nm exhibited an activity of 2.0 mA cm^-2 at 1.23 V vs the reversible hydrogen electrode for water splitting under AM 1.5G simulated sunlight. This improvement in photoactivity mainly originated from the 2D surface morphology of SrNbO₂N:Zr, which is clearly distinguishable from 3D-type oxynitrides. According to electrochemical analyses, the 2D structure of SrNbO₂N:Zr boosted the separation and accelerated the transfer of charges photogenerated during the water splitting, thus driving the reaction further. Therefore, the result empirically demonstrates that controlling the surface morphology of SrNbO₂N is an effective strategy to suppress the recombination of charges and minimize their diffusion pathway, eventually enhancing the PEC activity.

Key words: Solar water splitting, Water oxidation, BaNbO₂N, Doping engineering, Flux-assisted calcination, Spin coating