Chromium Adsorption Reveals a Persistent Hydroxylation of Vacuum-Annealed α-Al₂O₃(0001)

We address herein the question of the termination of the Al-terminated α-Al₂O₃(0001) surface. Over decades, various analyses made by different groups repeatedly suggested the presence of a residual coverage of surface OH groups on Al₂O₃ crystals and powders after annealing in vacuum. However, other authors came to contrary conclusions, thus maintaining a persistent blur on the issue. The present work examines the Cr/alumina interface via a Cr K-absorption edge analysis (extended X-ray absorption fine structure and X-ray absorption near-edge structure) and photoemission with the support of density functional theory (DFT) calculations. Experiments support the presence of surface OH groups to account for the observed environment as well as the oxidation state of Cr adatoms (Cr³⁺). Following a comprehensive DFT-based analysis of Crᶻ⁺-OₙHₘ configurations (z = 0–6; n and m = 0–3), the Cr³⁺-O₂H alumina-supported surface moieties are found to successfully fit the X-ray absorption spectroscopy edge calculations. Most importantly, the combination of experiment and theory that is developed unambiguously demonstrates the presence of surface OH groups on α-Al₂O₃(0001) after annealing in vacuum.