Uniform CrO_x Shells on Pt Cocatalyst Derived from Low-Toxicity Cr(III) Precursor for Improved Photocatalytic Water...

The efficiency of one-step and Z-scheme photocatalytic water-splitting systems is often limited by undesired reverse reactions, such as water formation from H₂ and O₂, oxygen reduction, and backward electron transfer to redox mediators. Although coating noble metal cocatalysts (e.g., Pt) with a CrO_x shell can effectively suppress these reactions, conventional CrO_x shell formation methods use toxic Cr(VI) precursors. To overcome this issue, CrCl₃, a previously overlooked low-toxicity Cr(III) precursor, is demonstrated to construct a superior noble metal core ─CrO_x shell via a distinct mechanism. Structural analyses reveal that CrCl₃ forms a highly uniform CrO_x shell that is thinner (∼1.2 nm) than conventional K₂CrO₄-derived shells (∼1.7 nm). Electrochemical studies suggest a novel deposition pathway involving a Cr(III) → Cr(VI) → Cr(III) redox cycle, wherein transiently generated Cr(VI) species allow for thickness control. Consequently, the CrCl₃-derived Pt-CrO_x core-shell cocatalyst effectively suppresses backward electron transfer toward redox mediators while preserving high intrinsic H₂ evolution activity. In a visible-light-driven Z-scheme water-splitting system, the photocatalyst loaded with the CrCl₃-derived core-shell cocatalyst exhibits higher H₂ and O₂ evolution rates than that loaded with the K₂CrO₄-derived counterpart, with sustained stability. This work establishes CrCl₃ as an effective, eco-friendly alternative for high-performance core-shell cocatalysts.