Anomalous Transition of Hole Transfer Pathways in Gold Nanocluster-Sensitized TiO₂ Photoelectrodes

Metal nanoclusters (NCs) are a recent addition to the family of traditional photosensitizers (e.g., dyes and quantum dots) for TiO2-based photoelectrodes. Despite the resemblance between NC-based and traditional sensitized photoelectrodes in their design architecture, recent studies have revealed that the photoelectrochemical behavior of NC–TiO2 photoelectrodes differs from that of their traditional counterparts. Here, we use impedance spectroscopy to shed light on another different behavior of NC–TiO2 photoelectrodes. Unlike dye- or quantum-dot-sensitized TiO2 photoanodes, in which surface states do not participate in the hole transfer process, hole transfer can occur through surface trap states and depends on the NC size. This unprecedented transition in hole transfer pathways is anomalous because it occurs even with the use of an efficient hole scavenger that typically activates direct hole transfer from a photoelectrode to an electrolyte. Along with evidence, the implications of this behavior on photoelectrochemical performance are discussed.