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

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 CrOx shell can effectively suppress these reactions, conventional CrOx 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 ─CrOx shell via a distinct mechanism. Structural analyses reveal that CrCl3 forms a highly uniform CrOx 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-CrOx 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.