Interface atom mobility and charge transfer effects on CuO and Cu2O formation on Cu3Pd(111) and Cu3Pt(111).

We bombarded [Formula: see text] and [Formula: see text] with a 2.3 eV hyperthermal oxygen molecular beam (HOMB) source, and characterized the corresponding (oxide) surfaces with synchrotron-radiation X-ray photoemission spectroscopy (SR-XPS). At [Formula: see text], CuO forms on both [Formula: see text] and [Formula: see text]. When we increase the surface temperature to [Formula: see text], [Formula: see text] also forms on [Formula: see text], but not on [Formula: see text]. For comparison, [Formula: see text] forms even at [Formula: see text] on Cu(111). On [Formula: see text], [Formula: see text] forms only after [Formula: see text], and no oxides can be found at [Formula: see text]. We ascribe this difference in Cu oxide formation to the mobility of the interfacial species (Cu/Pd/Pt) and charge transfer between the surface Cu oxides and subsurface species (Cu/Pd/Pt).

Quantitative Multilayer Cu(410) Structure and Relaxation Determined by QLEED.

Industrially relevant catalytically active surfaces exhibit defects. These defects serve as active sites; expose incoming adsorbates to both high and low coordinated surface atoms; determine morphology, reactivity, energetics, and surface relaxation. These, in turn, affect crystal growth, oxidation, catalysis, and corrosion. Systematic experimental analyses of such surface defects pose challenges, esp., when they do not exhibit order. High Miller index surfaces can provide access to these features and information, albeit indirectly. Here, we show that with quantitative low-energy electron diffraction (QLEED) intensity analyses and density functional theory (DFT) calculations, we can visualize the local atomic configuration, the corresponding electron distribution, and local reactivity. The QLEED-determined Cu(410) structure (Pendry reliability factor RP ≃ 0.0797) exhibits alternating sequences of expansion (+) and contraction (-) (of the first 16 atomic interlayers) relative to the bulk-truncated interlayer spacing of ca. 0.437 Å. The corresponding electron distribution shows smoothening relative to the bulk-determined structure. These results should aid us to further gain an atomic-scale understanding of the nature of defects in materials.

DFT and TPD study of the role of steps in the adsorption of CO on copper: Cu(4 1 0) versus Cu(1 0 0).

Adsorption of carbon monoxide (CO) was studied on stepped Cu(4 1 0) by temperature programmed desorption (TPD) and density-functional-theory (DFT) calculations. For comparison, the adsorption of CO was characterized also on Cu(1 0 0) by DFT calculations. On Cu(4 1 0) TPD reveals two desorption peaks: a high temperature peak (∼210 K) is attributed to the desorption of CO from step-edge sites and low temperature peak (∼170 K) to desorption from terrace sites. According to DFT calculations, CO prefers to adsorb at step-edges of Cu(4 1 0), although the step-edge versus terrace site preference is rather small at low coverage of 1/16 ML, about 0.05 eV; the respective DFT predicted CO binding energies are -0.89 and -0.84 eV at the step-edge and terrace top sites, whereas the value calculated at top sites of Cu(1 0 0) is -0.86 eV. Although this small step-edge over terrace site preference of 0.05 eV cannot explain the temperature difference of 40 K between the two TPD peaks, when the lateral intermolecular interactions are neglected, it is sufficient that the CO adsorbs almost exclusively at step-edges at low coverage (at 200 K the 0.05 eV corresponds to 3 kT). The emergence of the two TPD peaks on Cu(4 1 0) is therefore attributed to a combination of step-edge preference and lateral repulsion between CO molecules, which increases with increasing coverages and diminishes the net desorption energy of CO. DFT calculations further reveal that the reason for the significant increase of saturation coverage on Cu(4 1 0) compared to Cu(1 0 0) is related to the geometry of the step-edge that allows the CO molecules adsorbed thereon to tilt away from the nearest neighboring CO molecules adsorbed at the terrace and therefore to effectively reduce the lateral repulsion.

The effects of alloying and segregation for the reactivity and diffusion of oxygen on Cu3Au(111).

We report results of the segregation induced by the adsorption of O2 and the barrier of the formation of Cu2O in Cu3Au(111) with an experimental and theoretical approach. Oxidation by a hyperthermal O2 molecular beam (HOMB) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident-energy dependence of the measured O-uptake curve, dissociative adsorption of O2 is less effective on Cu3Au(111) than on Cu(111). The dissociative adsorption is accompanied by the Cu segregation on Cu3Au(111) as well as on Cu3Au(100) and Cu3Au(110). The obvious growth of Cu2O for a 2.3 eV HOMB cannot be observed and it suggests that the Au-rich protective layers prevent the diffusion of O atoms into the bulk. The theoretical approach based on density functional theory indicates that O adsorption shows the same behavior on Cu3Au(111) and on Cu(111). For the diffusion case, the barrier to diffuse into the subsurface in segregated Cu3Au(111) is higher than in Cu(111). This indicates that the segregated Au-rich layer in Cu3Au(111) works as a protective layer.

Initial stages of Cu3Au(111) oxidation: oxygen induced Cu segregation and the protective Au layer profile.

We report results of our experimental and theoretical studies on the Au concentration profile of Cu3Au(111) during oxidation by a hyperthermal O2 molecular beam at room temperature, using X-ray photoemission spectroscopy (XPS), in conjunction with synchrotron radiation (SR), and density functional theory (DFT). Before O2 exposure, we observe strong Au segregation to the top layer, i.e., Au surface enrichment of the clean surface. We also observe a gradual Cu surface enrichment, and Au enrichment of the second and third (subsurface) layers, with increasing O coverage. Complete Cu segregation to the surface occurs at 0.5 ML O surface coverage. The Au-rich second and third layers of the oxidized surface demonstrate the protective layer formation against oxidation deeper into the bulk.