Unraveling the Charge States of Au Nanoclusters on an Oxygen-Rich Rutile TiO₂(110) Surface and Their Triboelectrification Overturn by nc-AFM and KPFM

Using noncontact atomic force microscopy and Kelvin probe force microscopy at 78 K, we study the charge states of Au nanoclusters dispersed on an oxygen-rich rutile TiO₂(110) surface. The Au nanoclusters are catalytically active species with a single- or double-layered thickness and having sub 3 nm diameters. The Au nanoclusters are found by Kelvin probe force microscopy to be positively charged, and the charge transfer from Au nanoclusters to a substrate is demonstrated to be strongly size-dependent. Specifically, the charged defects play a key role in the discharge process by serving as a polaronic charge storage for the back-donated charge from the Au nanoclusters. Using the triboelectrification effect, we find that the redox states of the Au nanoclusters can be dynamically overturned from positive to negative by continuous long-time scanning. Our findings have a strong impact on the understanding of catalytic reactions based on dispersed noble-metal nanoclusters on transition-metal oxide surfaces.