Interfacial trap states in gold nanocluster‐sensitized photoelectrochemical systems: Distinctive behaviors and charge‐transfer mechanisms

Gold nanoclusters (NCs) have emerged as promising sensitizers for photoelectrochemical (PEC) systems, yet their behavior at the NC–TiO 2 interface is fundamentally different from that of conventional molecular dyes. This review focuses on the formation and evolution of interfacial trap states in relation to the ligand environment, interfacial ionic effects, and the NCs' core structure. First, we show that structural and chemical modifications—including core restructuring, ligand engineering, and alkali‐metal‐ion incorporation—produce distinctive and often counterintuitive PEC behaviors that cannot be explained by conventional sensitizer models. Second, we synthesize evidence indicating that many of these anomalous behaviors are associated with interfacial trap states formed or modified at the NC–TiO 2 junction, which govern charge‐ and hole‐transfer pathways through non‐classical, trap‐mediated mechanisms. By integrating recent experimental and mechanistic findings, this review redefines the optimization of Au NC PEC systems as a problem of controlling the interfacial trap‐state landscape, rather than simply tuning intrinsic NC properties, and provides a mechanistic framework for the rational design of next‐generation solar energy conversion devices.