Direct Experimental Evidence of Transient Au^δ+ Oxide in Au Electrooxidation

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 (Au^δ+, 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). Au^δ+ 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 Au^δ+ 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 Au^δ+ 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.