Time- and Depth-Resolved Chemical State Analysis of the Surface-to-Subsurface Oxidation of Cu by X-ray Absorption Spectroscopy at Near Ambient Pressure.

Metal surface oxidation is a well-known phenomenon, but the oxidation states of metal surfaces have not been observed and in real time because most techniques obtaining surface chemical states require high-vacuum conditions. Here, we achieved the real-time, observation of the initial stages of surface Cu oxidation using fluorescence-yield wavelength-dispersive X-ray absorption spectroscopy (XAS) in the soft X-ray region at sub-nanometer depth resolution. Further, the XAS data suggest a unique oxidation mechanism: CuO forms on the top surface, and subsequently, CuO forms in the subsurface layers (>1 nm from the surface), accompanied by the interdiffusion of Cu from the inner layer and that of CuO to the inner layer. The reported technique has applications for the analysis of surface phenomena at ambient pressure, especially oxidation processes, whose understanding is crucial in many fields, from chemistry to structural engineering.