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

Abstract

We address herein the question of the termination of the Al-terminated α-Al 2 O 3 (0001) surface. Over decades, various analyses made by different groups repeatedly suggested the presence of a residual coverage of surface OH groups on Al 2 O 3 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 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 (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 (Cr 3+). Following a comprehensive DFT-based analysis of Cr z+-O n H m configurations (z=0 to 6, n and m=0 to 3), the Cr 3+-O 2 H alumina-supported surface moieties are found to successfully fit the XAS edge calculations. Most importantly, the combination of experiment and theory that is developed unambiguously demonstrates the presence of surface OH groups on α-Al 2 O 3 (0001) after annealing in vacuum.

Core level shifts as indicators of Cr chemistry on hydroxylated α-Al2O3(0001): a combined photoemission and first-principles study

“…Contaminants, in particular carbon, were below the detection limit of XPS, except for calcium which segregates from bulk up to a maximum apparent coverage of 1 Å. Since such segregated Ca does not impact the long-range surface crystalline structure of alumina [46][47][48] , is absent from the local environment of Cr probed by XAS 24 and does not evolve upon Cr deposition (i.e. the Ca 2p level is unaffected), its presence was assumed not to affect the chemical state of Cr.…”

“…Layer stacks stable under operating conditions are commonly obtained through the pre-deposition of intermediate layers of early transition metals, the so-called buffers, such as Ni 1,17 , Ti 2,4,5,8,18 , Nb 1,19 and Cr 1,2,4,5,7 . In particular, we have focused in the recent years on the capacity of pre-deposited Cr to enhance the sticking, spreading and adhesion of Zn on α-Al 2 O 3 (0001) [20][21][22][23][24][25] . The subject was inspired by advanced high strength steels which are alloyed with electropositive elements (including aluminium) and used to lighten car weight to comply with energy saving rules.…”

“…Cr clusters are not entirely metallic. Xray absorption spectroscopy (XAS) combined with first principles Density Functional Theory (DFT) calculations showed that, at the onset of its deposition, Cr reacts with residual OH groups 24 to mainly form Cr 3+ -O 2 H surface moieties while Cr does not reduce the alumina substrate 24 . However, the observation of reduced Al at the Cr/α−Al 2 O 2 (1012) interface 33,34 or of oxidizied Cr on OH-free alumina thin film on NiAl(110) 35,36 question this finding.…”

“…However, the observation of reduced Al at the Cr/α−Al 2 O 2 (1012) interface 33,34 or of oxidizied Cr on OH-free alumina thin film on NiAl(110) 35,36 question this finding. As the Cr film thickens, XAS 24 and UV-visible plasmonics 25 reveal a metallic behaviour but, in the absence of quantitative data, the frontier between the two media remains unclear. In particular, the formation of intermediate oxidation states at the Cr/Cr-O 2 H interface cannot be excluded.…”

“…Film crystallinity and ordering are analysed via XAS oscillations at the Cr K-edge and electron diffraction. In an original way, photoemission data are supplemented by ab initio calculations of core-level shifts prior to checking the consistency of the findings with the previous analysis of the same system 24,25 .…”