In Situ Monitoring of H-Induced Nonstoichiometry in CuO.

Using ambient-pressure X-ray photoelectron spectroscopy and Auger electron spectroscopy to monitor the reduction of CuO in H, we identify the formation of an intermediate, oxygen-deficient CuO phase and its progressive inward growth into the deeper region of the oxide. Complemented by atomistic modeling, we show that the oxygen-deficient CuO formation occurs via molecular H adsorption at the CuO surface, which results in the loss of lattice oxygen from the formation of HO molecules that desorb spontaneously from the oxide surface. The resulting oxygen-deficient CuO is a stable intermediate that persists before the CuO is fully reduced to metallic Cu. The oxygen vacancy-induced charge of the coordinating Cu atoms results in a satellite feature in Cu LMM, which can be used as a fingerprint to identify nonstoichiometry in oxides and local charge transfer induced by the nonstoichiometry.