Direct Experimental Evidence of Transient Auδ+ Oxide in Au Electrooxidation

Abstract

Anodic electrooxidation of noble metals in acidic media represents a central topic in electrocatalysis and energy conversion due to its relevance for the oxygen evolution reaction (OER). In particular, gold electrodes are traditionally assumed to oxidize directly from the metallic state (Au0) to a trivalent state (Au3+) at high potentials relevant to OER. However, this long-standing paradigm has been increasingly challenged by recent in situ characterization studies. Here we report the direct observation of a metastable gold oxidation state (Auδ+, where 0 < δ < 3) preceding the formation of Au3+. This species is electrochemically induced on Au(111) in 0.1 M H2SO4 and observed at the electrode surface under ultrahigh vacuum (UHV) using quasi-in situ X-ray photoelectron spectroscopy (XPS). Auδ+ manifests a distinct chemical shift in XPS without conforming to a fully evolved Au3+ oxidation state. Atomic-resolution imaging via low-temperature scanning probe microscopy (LT-STM/nc-AFM) revealed that the Auδ+ phase forms an amorphous oxide layer. This study provides evidence of a two-step mechanism for Au electrooxidation accompanying the OER onset. Formation of the Auδ+ surface oxide precedes the formation of the bulk Au3+ oxide responsible for the OER. This mechanism strongly challenges the thermodynamically predicted one-step Au0 → Au3+ oxidation model. The two-step Au-oxidation mechanism parallels the proposed Pt electrooxidation pathway, suggesting a general oxidation mechanism for noble metals.