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.

The morphology and adhesion energy of nanosized metal particles supported on dielectrics is a puzzling issue since, due to the increasing contribution of surfaces and interfaces in their energetics, their equilibrium shape escapes the rules established for large objects.

The eect of additives on metal/oxide interfaces is explored in situ and in real time on evaporated lms by a combination of surface science techniques, among which a very exible optical method shows a unique ability to scrutinize the growth and wetting properties of supported clusters that involve several elements. The study focuses on Cr at the Zn/α-Al 2 O 3 (0001) interface at 300 K. A particular interest of the present interface is that Zn does not stick at all on bare alumina. The sticking and morphology of both Cr and Zn lms are analyzed from submonolayer to multilayer thicknesses, during their growth. After an initial oxidation reaction with residual OH groups, shown to be detrimental to Zn adhesion, Cr growth proceeds through the formation of high aspect ratio particles that percolate around an average thickness of 10 Å. As regards to Zn growth on a Cr deposit, two very distinct stages can be distinguished. In the submonolayer thickness range, Cr forms a seed layer that drastically increases the Zn sticking coecient from zero to nearly one due to a diusion length of physisorbed Zn adatoms before desorption larger than Cr island separation; Zn clusters are anchored on the Cr seeds that they encapsulate, but their wetting behavior is dictated by the interaction with alumina. In a second stage, as soon as the Cr lm percolates, it forms an adhesion layer on which Zn grows in a nearly 2D mode. In all cases, Cr lms are stable upon annealing. On Cr-covered alumina, the Zn desorption energy is enhanced as compared to bare surfaces which, in line with atomistic simulations, is assigned to the formation of more favorable Cr-Al 2 O 3 and Cr-Zn than Zn-Al 2 O 3 bonds. Generally speaking, the ability demonstrated herein of small amounts of additives to dramatically increase the adhesion of lms is of great practical interest. It shows that non-continuous and partially oxidized lms of additives, closer to realistic cases of application, can strongly enhance the sticking of lms. Also, anchoring a functional lm by discrete pre-deposited seeds can keep its properties intact.

The Cr/$\alpha$-Al$_2$O$_3$(0001) interface has been explored by X-ray photoemission spectroscopy, X-ray absorption spectroscopy (XAS) and {\it ab initio} first-principle calculations of core level shifts including final state effects. After an...