Atomically Precise Cluster Cocatalysts: Missing Link toward Heterogenized Photocatalytic Systems

This review critically examines the emerging role of atomically precise clusters - oxide, sulfide, and metallic in nature - as transformative cocatalysts for photocatalytic solar fuel production. Unlike structurally imprecise nanoparticles with ill-defined active sites or molecular organometallic complexes with limited stability, these clusters offer atomically resolved structures that bridge homogeneous precision with heterogeneous robustness. Their well-defined multinuclear architectures enable precise control over their active sites, facilitating systematic studies of structure-activity relationships and mechanistic insights critical to rational catalyst design. Compared to contemporary cocatalytic systems, these clusters offer tunable compositions and structures, enhanced stability on the surface, and the capacity to engage in multielectron redox processes. We review recent experimental developments, discuss strategies of their surface-anchoring, and highlight mechanistic insights provided by their use. Finally, we critically evaluate current challenges and propose future research directions to unlock the full potential of cluster-based cocatalysts as a tool for purposeful engineering of active and selective photocatalysts for light-driven solar fuel generation.