Cocatalyst is matter: Cocatalyst engineering of photocatalysts on hydrogen evolution and carbon dioxide reduction

Photocatalysis provides a promising and sustainable pathway for converting solar energy into chemical fuels, particularly via the hydrogen evolution reaction (HER) and carbon dioxide reduction reaction (CO 2 RR). The integration of metal cocatalysts with semiconductor photocatalysts offers an effective strategy to enhance three fundamental steps of photocatalysis: light absorption, charge separation, and surface reactions. In this review, we present a comprehensive overview of cocatalyst deposition techniques and engineering strategies, with emphasis on five key parameters—chemical composition, size, morphology, loading density, and spatial distribution. We discuss their individual effects and synergistic roles in governing photocatalytic performance for HER and CO 2 RR. In addition, we highlight recent progress in the use of in situ characterization techniques, which provide critical insights into dynamic structural and chemical changes of cocatalyst–semiconductor systems under operating conditions. Finally, we outline the remaining challenges—such as long‐term stability, scalable synthesis, and mechanistic understanding—and suggest that future advances will be accelerated by integrating in situ techniques with computational approaches and machine learning. This review aims to guide the rational design of next‐generation cocatalyst–semiconductor photocatalysts for efficient solar‐to‐chemical energy conversion.