Valence State Tuning of Gold Nanoparticles in the Dewetting Process: An X-ray Photoelectron Spectroscopy Study

Gold nanoparticles (AuNPs) are commonly synthesized using the citrate reduction method, reducing Au³⁺ into Au¹⁺ ions and facilitating the disproportionation of aurous species to Au atoms (Au⁰). This method results on citrate-capped AuNPs with valence single states Au⁰. Here, we report a methodology that allows obtaining AuNPs by the dewetting process with three different valence states (Au³⁺, Au¹⁺, and Au⁰), which can be fine-tuned with ion bombardment. The chemical surface changes and binding state of the NPs were investigated using core-level X-ray photoelectron spectroscopy (XPS). This is achieved by recording high-resolution Au 4f XPS spectra as a function of ion dose exposure. The results obtained show a time-dependent tuning effect on the Au valence states using low-energy 200 V acceleration voltage Ar⁺ ion bombardment, and the valence state conversion kinetics involves the reduction from Au³⁺ and Au¹⁺ to Au⁰. Proper control of the reduction in the valence states is critical in surface engineering for controlling catalytic reactions.