In Situ Monitoring of H₂-Induced Nonstoichiometry in Cu₂O

Using ambient-pressure X-ray photoelectron spectroscopy and Auger electron spectroscopy to monitor the reduction of Cu₂O in H₂, we identify the formation of an intermediate, oxygen-deficient Cu₂O phase and its progressive inward growth into the deeper region of the oxide. Complemented by atomistic modeling, we show that the oxygen-deficient Cu₂O formation occurs via molecular H₂ adsorption at the Cu₂O surface, which results in the loss of lattice oxygen from the formation of H₂O molecules that desorb spontaneously from the oxide surface. The resulting oxygen-deficient Cu₂O is a stable intermediate that persists before the Cu₂O 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.