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The control of adhesion at metal/oxide interfaces is of key importance in modern applications, whenever three-dimensional metal clusters or two-dimensional metal overlayers are to be synthesized on an oxide support. By focusing on the zinc/alumina system, we address here one of the long-standing issues in this context, which is the poor wetting of wide bandgap oxides by noble and post-transition metals. It has recently been recognized to have detrimental industrial consequences for the adhesion of anti-corrosive zinc coatings to new high strength steels grades. We have combined photoemission, thermal desorption and plasmonics with atomistic simulation to describe the energetics of zinc deposits on dry and hydroxylated α-Al 2 O 3 (0001) surfaces. Both experimental and computational results show that an activated reaction of the metal with the OHcovered surface, followed by hydrogen desorption, produces dispersed interfacial moieties involving both oxidized Zn species and undercoordinated oxygen ions, that lead to a significant improvement of adsorption/adhesion characteristics on the hydroxylated surface. In particular, the key role of interfacial undercoordinated anions, remnants of the hydroxylation layer, is highlighted for the first time, pointing to a general mechanism by which surface hydroxylation appears as a promising route towards a systematic improvement of wide band gap oxide wetting by metals. 1

The configurations associated with reversible and irreversible adsorption of hydrogen on MgO escape consensus. Here, we report the dissociation of H 2 on MgO nanocubes, which was examined by combining Fourier transform infrared spectroscopy and density functional theory (DFT)-based simulations. We found that the use of ultrahigh vacuum is essential for identifying the very first adsorption stages. Hydrogen pressure was varied from 10 −8 mbar to near ambient, resulting in IR spectra of richer complexity than in current state of the art. Models with oxygen at regular corners (O 3C) and Mg at inverse corners (Mg IC) were identified to be the most reactive and to split H 2 irreversibly already in the lowest pressure regime (P H 2 < 10 −3 mbar). The continuous increase in intensity of the corresponding IR bands (3712/1140 cm −1) in the intermediate range of pressures (10 −3 −1 mbar), along with the appearance of bands at 3605/1225 cm −1 , was demonstrated to stem from cooperative adsorption mechanisms, which could be therefore considered as the main origin of irreversible hydrogen adsorption. At P H 2 > 1 mbar, fully reversible adsorption was shown to occur at O 4C (either on mono-or diatomic steps) and Mg 3C sites. Another OH/MgH couple (3697/1030 cm −1) that became reinforced at high P H 2 but remained stable upon pumping was correlated to O 3C and Mg IC in multiatomic steps. The difference in adsorption and desorption sequences confirmed the proposed cooperative adsorption of H 2 molecules. Our study provides new insights into the mechanisms that can be beneficial for understanding the chemistry of H 2 and other hydrogen-containing molecules, such as CH 4 , on oxide surfaces, but also for the advancement of hydrogen-storage technologies.

The formation of an ultra-thin aluminum oxide film at Fe0.85Al0.15(110) surface (A2 random alloy) has been studied by a variety of surface sensitive techniques (X-ray photoemission, low-energy electron diffraction, surface X-ray diffraction and scanning tunneling microscopy) supplemented by ab initio atomistic simulations. Since iron is not oxidized in the used conditions, the study focused on the coupling between aluminum oxidation and segregation processes. Compared to the bare surface, whose average composition (Fe0.6Al0.4) is closer to the B2-CsCl structure over a ∼ 3 nm depth, the oxidation hardly affects the subsurface segregation of aluminum. All the structural and chemical fingerprints point to an oxide film similar to that found on NiAl(110). It is a bilayer (∼ 7.5Å thick) with a composition close to Al10O13 and a large (18.8 × 10.7)Å 2 nearly rectangular unit cell; an almost perfect match between substrate periodicity and the (1 × 2) oxide supercell is found. Nevertheless, microscopy reveals the presence of anti-phase domain boundaries. Measured Al 2p and O 1s core level shifts match calculated ones; their origin and the relative contributions of initial/final state effects are discussed. The ubiquity of the present oxide on different supports asks for the origin of its stability.

Thanks to a dedicated modelling of intensities, the depth sensitivity of X-ray photoemission is used to probe the segregation profile of aluminium at the (110), (100) and (111) low index surfaces of the body-centred Fe 0.85 Al 0.15 random alloy. Sputtered surface composition is close to the nominal bulk one, thus excluding preferential sputtering. Surface enrichment in aluminium upon annealing starts at around 700 K before reaching a stationary state above 1000 K. The average surface composition is close to Fe 0.5 Al 0.5 , corresponding to the B 2 CsCl structure on the phase diagram. The impacted depth, that is in the range of 2.5-3.5 nm, is quite significant. Although not evidenced previously in surface science conditions at FeAl single crystal surfaces, it is qualitatively in agreement with the segregation at grain boundaries and shear planes of Al-alloyed steels. This segregation tendency is rationalized through ab initio calculations.

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The experimental study of the effect of temperature on the complex optical response of matrix-embedded noble metal nanoparticles has been carried out. A thin silica film containing gold nanoparticles with 7% volume fraction has been elaborated. Its thermo-optical refraction and absorption coefficients have then been extracted from temperature-dependent spectroscopic ellipsometry measurements in the visible range. The results, in agreement with our theoretical approach, exhibit strong dispersion, with amplitude and sign changes due to the local electromagnetic field enhancement associated with the surface plasmon resonance in gold nanoparticles.

Band alignment at the interface between evaporated silver films and Zn-or O-terminated polar orientations of ZnO is explored by combining soft and hard X-ray photoemissions on native and hydrogenated surfaces. Ultraviolet Photoemission Spectroscopy (UPS) is used to track variations of work function, band bending, ionization energy and Schottky barrier during silver deposition.The absolute values of band bending and the bulk position of the Fermi level are determined on continuous silver films by HArd X-ray PhotoEmission Spectroscopy (HAXPES) through a dedicated modeling of core levels. Hydrogenation leads to the formation of ∼ 0.3 monolayer of donor-like hydroxyl groups on both ZnO-O and ZnO-Zn surfaces and to the release of metallic zinc on ZnO-Zn. However no transition to an accumulation layer is observed. On bare surfaces, silver adsorption is cationic on ZnO(0001)-O (anionic on ZnO(0001)-Zn) at the earliest stages of growth as expected from polarity healing before adsorbing as a neutral species. UPS and HAXPES data appear quite consistent. The two surfaces undergo rather similar band bendings for all types of preparation. The downward band bending of V bb,ZnO−O = −0.4 eV and V bb,ZnO−Zn = −0.6 eV found for the bare surfaces are reinforced for upon hydrogenation (V bb,ZnO−O+H = −1.1 eV, V bb,ZnO−Zn+H = −1.2 eV). At the interface with Ag, a unique value of band bending of -0.75 eV is observed. While exposure to atomic hydrogen modulates strongly the energetic positions of the surface levels, a similar Schottky barrier of 0.5-0.7 eV is found for thick silver films on the two surfaces.