Phase-engineered nickel selenide cocatalyst on CdS for enhanced visible-light photocatalytic H2 production

• Four distinct phases of nickel selenide were prepared in a controlled manner by varying the reaction temperatures. • NiSe x /CdS nanocomposites were synthesized using an in-situ precipitation method. • The NiSe x cocatalysts enhance charge separation in CdS, which promotes H 2 evolution under visible light. • Orthorhombic-cubic NiSe 2 on CdS exhibits the highest charge transfer efficiency. To improve the efficiency of photocatalytic H 2 generation, cocatalysts are commonly utilized to suppress the recombination of photogenerated charge carriers and provide reaction active sites. Herein, four different crystalline phases of nickel selenide were controllably prepared via a simple one-pot approach. The nickel selenide underwent multiple phase transformations at reaction temperatures of 180, 240, 280, and 300 ℃, resulting in orthorhombic, orthorhombic-cubic, cubic, and cubic-hexagonal phases, respectively. Nickel selenides were combined with CdS using an in-situ precipitation method for photocatalytic H 2 generation reactions. All four crystalline phases of nickel selenide were found to be effective cocatalysts that boosted the photocatalytic H 2 production performance of CdS under visible light. Among these phases, the orthorhombic-cubic NiSe 2 demonstrated a superior H 2 generation rate of 3039 µmol h −1 g −1 , 6.5 times higher than CdS. The effective charge transfer between CdS and nickel selenide was revealed through photoluminescence (PL), electrochemical, and photoelectrochemical analyses. This work introduces a practical approach to designing cost-effectiveness NiSe x cocatalysts on metal sulfide semiconductors, leveraging phase control to optimize photocatalytic solar-to-H 2 conversion.