Role of Au x Pt1–x Clusters in the Enhancement of the Electrochemical Activity of ZnO Nanorod Electrodes

This study quantitatively elucidates the role of metal clusters in the electrochemical activation of metal-oxide nanostructured electrodes. Through the deposition of nearly monodisperse Au x Pt1–x (x = 0, 0.5, 1) clusters, smaller than 3 nm, on the ZnO nanorod (NR) electrode surface, a controlled enhancement of charge transfer and activation of electrocatalytic processes was achieved. The interfacial electrical states of the hybrid electrodes were probed by electrochemical impedance spectroscopy (EIS). Analysis of the charge-transfer resistance and interface capacitance, estimated by modeling EIS curves in different bias regimes, indicated the presence of a large amount of active donor states (∼1020 cm–3) at the surface of the ZnO NRs. Decoration of the ZnO NRs with Au x Pt1–x clusters strongly increased the charge-transfer process at the cluster–ZnO/electrolyte interface. This induced a more effective depletion of the electron charge available in the donor states of the ZnO NRs, leading to the formation of a positively charged layer at the interface between ZnO and the clusters. These two effects, intrinsically linked with the alignment between the electronic states of the Au x Pt1–x clusters and ZnO, strongly enhance the interface reactivity of the ZnO NR electrodes toward the redox reaction of potassium ferricyanide. This is particularly relevant for understanding and improving the performance of electrochemical biosensors.