Surface State-Assisted Delayed Photocurrent Response of Au Nanocluster/TiO 2 Photoelectrodes.

Gold nanoclusters (NCs) can be used as sensitizers to extend the absorption capabilities of TiO 2 as photoelectrodes. However, the adsorption of NCs also creates additional surface states on the TiO 2 surface, which gives rise to intricacies in the understanding of various interfacial phenomena occurring in NC-sensitized TiO 2 . 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-TiO 2 system, which is unprecedented in other photoelectrode systems. In the presence of an efficient hole scavenger (Na 2 SO 3 ), the hole transfer in Au 18 -TiO 2 occurs directly through the highest occupied molecular orbital (HOMO) of Au 18 NCs. However, in the presence of a less efficient hole scavenger (ethylenediaminetetraacetic acid), hole transfer in Au 18 -TiO 2 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 Au 18 -TiO 2 . Evidence for this exotic hole-transfer mechanism shift is provided by photoelectrochemical electrochemical impedance spectroscopy, and its implications are discussed.