Surface State-Assisted Delayed Photocurrent Response of Au Nanocluster/TiO₂ Photoelectrodes

Gold nanoclusters (NCs) can be used as sensitizers to extend the absorption capabilities of TiO₂ as photoelectrodes. However, the adsorption of NCs also creates additional surface states on the TiO₂ surface, which gives rise to intricacies in the understanding of various interfacial phenomena occurring in NC-sensitized TiO₂. One of the complexities that have recently been discovered is the size-dependent hole-transfer mechanism. In this work, we reveal another anomalous behavior in the hole-transfer process that the hole scavenging ability of the electrolyte also plays a role in determining the hole-transfer mechanism in the NC-TiO₂ system, which is unprecedented in other photoelectrode systems. In the presence of an efficient hole scavenger (Na₂SO₃), the hole transfer in Au₁₈-TiO₂ occurs directly through the highest occupied molecular orbital (HOMO) of Au₁₈ NCs. However, in the presence of a less efficient hole scavenger (ethylenediaminetetraacetic acid), hole transfer in Au₁₈-TiO₂ does not occur through the HOMO and shifts to surface state-assisted hole transfer. Due to surface state charging, this surface state-assisted hole-transfer mechanism results in delayed photocurrent response in Au₁₈-TiO₂. Evidence for this exotic hole-transfer mechanism shift is provided by photoelectrochemical electrochemical impedance spectroscopy, and its implications are discussed.