Hyperthermal O2 Molecular Beam Research Articles

Experimental and Theoretical Studies on Oxidation of Cu-Au Alloy Surfaces: Effect of Bulk Au Concentration.

We report results of our experimental and theoretical studies on the oxidation of Cu-Au alloy surfaces, viz., Cu3Au(111), CuAu(111), and Au3Cu(111), using hyperthermal O2 molecular beam (HOMB). We observed strong Au segregation to the top layer of the corresponding clean (111) surfaces. This forms a protective layer that hinders further oxidation into the bulk. The higher the concentration of Au in the protective layer formed, the higher the protective efficacy. As a result, of the three Cu-Au surfaces studied, Au3Cu(111) is the most stable against dissociative adsorption of O2, even with HOMB. We also found that this protective property breaks down for oxidations occurring at temperatures above 300 K.

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.

Temperature dependence of Cu2O formation on Cu3Au(1 1 0) surface with energetic O2 molecular beams

We report a study of the surface temperature (Ts) dependence of oxidation on Cu3Au(110) induced by a hyperthermal O2 molecular beam (HOMB), using X-ray photoemission spectroscopy in conjunction with synchrotron radiation. From the Ts dependence of the O uptake curve, the direct dissociative adsorption process is dominant process in the oxidation at less than O coverage (Θ) of 0.2monolayer (ML) for the 2.2eV HOMB incidence. On the other hand, the oxidation, in particular, the Cu2O formation, depends on Ts at high Θ. The high Ts causes the efficient Cu segregation on the surface at high Θ, suggesting the dominant role of Cu migration in the Cu2O formation.

The temperature dependence of Cu2O formation on a Cu(110) surface with an energetic O2 molecular beam

We report a study of the surface temperature (Ts) dependence of Cu2O formation on a Cu(110) surface induced by a hyperthermal O2 molecular beam (HOMB), using x-ray photoemission spectroscopy in conjunction with synchrotron radiation. From the Ts dependence of the O uptake curves, the direct dissociative adsorption process mainly contributes to the formation of the p(2 × 1)-O structure with an O coverage (Θ) of 0.5 ML for 2.2 eV HOMB incidence. On the other hand, the rate of oxidation at Θ > 0.5 ML, particularly in Cu2O formation, strongly depends on the Ts. Thicker Cu2O islands were found inhomogeneously at 400 and 500 K, suggesting the dominant role of the migration of Cu atoms in the Cu2O formations on the Cu(110) surface.

The effect of step geometry in copper oxidation by hyperthermal O2 molecular beam: Cu(511) vs Cu(410)

Steps are known to be often the active sites for the dissociation of O(2) molecules and the nucleation sites of oxide films since they provide paths for subsurface migration and oxygen incorporation. In order to unravel the effect of their morphology on the oxidation of Cu surfaces, we present here a detailed investigation of the O(2) interaction with Cu(511) and compare it with previous results for Cu(410), a surface exhibiting terraces of similar size and geometry but different step morphology. As for Cu(410) we find, by x-ray photoemission spectroscopy performed with synchrotron radiation, that Cu(2)O formation gradually starts above half a monolayer oxygen coverage and that the ignition of oxidation can be lowered to room temperature by dosing O(2) via a supersonic molecular beam at hyperthermal energy. The oxidation rate for Cu(511) comes out to be lower than for Cu(410) at normal incidence, about the same when the O(2) molecules impinge towards the ascending step rise, but higher when they hit the surface along trajectories even slightly inclined towards the descending step rise. These findings can be rationalized by a collision induced absorption mechanism.

Active oxidation of Cu 3Au(1 1 0) using hyperthermal O 2 molecular beam

Oxidation of Cu 3Au(1 1 0) using a hyperthermal O 2 molecular beam (HOMB) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident energy dependence of the O-uptake curve, the precursor-mediated dissociative adsorption occurs, where the trapped O 2 molecule can migrate and dissociate at the lower activation-barrier sites, dominantly at thermal O 2 exposures. Dissociative adsorption of O 2 on Cu 3Au(1 1 0) is as effective at the thermal O 2 exposure as on Cu(1 1 0). On the other hand, at the incident energies of HOMB where the direct dissociative adsorption is dominant, it was determined that the dissociative adsorption of O 2 implies a higher activation barrier and therefore less reactivity due to the Au alloying in comparison with the HOMB oxidation of Cu(1 1 0). The dissociative adsorption progresses with the Cu segregation on Cu 3Au(1 1 0) similarly as on Cu 3Au(1 0 0). The growth of Cu 2O for 2 eV HOMB suggests that the diffusion of Cu atoms also contribute to the oxidation process through the open face, which makes the difference from Cu 3Au(1 0 0).

The Study of Oxidation on TiAl Surface with Photoemission Spectroscopy in Conjunction with Synchrotron Radiation

The oxidation processes on a TiAl surface induced by a hyperthermal O2 molecular beam (HOMB) with a translational energy of 2.2 eV was studied by X-ray photoemission spectroscopy in conjunction with synchrotron radiation. At a surface temperature of 300 K, the simultaneous growth of Al and Ti oxides accompanied with the segregation of Al2O3 near the surface was observed. The efficiency of oxidation for the HOMB incidence was smaller than that for O2 backfilling (25 meV). Furthermore, the chemical compositions of oxide species (Al2O3, Ti2O3, TiO2) on the TiAl surface were independent of the translational energy of incident O2 molecule. The present results suggest that the oxidation on TiAl surface proceed via precursor molecular states.

Study on Oxidation of Cu and Cu3Au Surfaces with Hyperthermal Oxygen Molecular Beam

Corrosion wastes more than a few percent of the world's GDP every year. The initial stage of the corrosion is one of the central topics in material science. The oxidation is one of the major corrosion processes of metals. Thus, the study of the oxidation process on metal surfaces is generally interesting in various fields of science and technology. The growth of a protective thin surface layer, which prevents further oxidation into bulk of a metal, requires the formation of a homogeneous film. One simple way for the protection of underlying metals is surface alloying, combining different substances to form multi-component surfaces. The surface alloying leads to the formation of a protective oxide layer due to the preferential oxidation of one component, possibly with surface segregation. Copper and copper alloys have wide industrial applications, and therefore are of interest for studies of oxidation mechanism, especially in the Cu2O formation. Cu forms the stable Cu2O, while Au does not form a stable oxide and is not soluble into stable Cu2O. Thus, the Cu-Au alloy system is ideal for investigating the effect of alloying on the formation of protective layer against further oxidation into bulk. Here, we introduce our recent comparative studies of the oxidation of Cu(100) and Cu3Au(100) with hyperthermal O2 molecular beam and discuss why Cu3Au(100) is protective against the oxidation.

Protective layer formation during oxidation of Cu3Au(100) using hyperthermal O2 molecular beam

Oxidation of Cu3Au(100) using 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 O-uptake curve, it was determined that the dissociative adsorption of O2 implies a higher activation barrier and therefore less reactivity compared to Cu, owing to the Au alloying. The dissociative adsorption progresses with the Cu segregation on the surface. No prominent growth of Cu2O even for 2eV HOMB suggests that the Au alloying of Cu can serve as a protective layer against further oxidation into the bulk.

Translational energy induced reconstruction and absorption in the oxidation processes of Cu{111}

The oxidation processes of Cu{111} with thermal and hyperthermal O 2 were studied using high-resolution X-ray photoemission spectroscopy (XPS) in conjunction with a synchrotron radiation (SR) source. We measured O-uptake curves for thermal O 2 exposure and hyperthermal O 2 molecular beam (HOMB) irradiation on a Cu{111} surface at ∼300 K. Under an O coverage ( Θ) ≤0.4 ML, oxidation with HOMB irradiation caused reconstruction of the Cu{111} surface to a distorted Cu{100} ( 2 × 2 )R45 °- O overlayer. The oxidation accompanied by reconstruction is more effective with HOMB irradiation than thermal O 2 exposure. Low-energy electron diffraction (LEED) observations and XPS measurements suggest that HOMB irradiation is useful for fabricating O-induced reconstructed surfaces, which are precursors to Cu 2O. Further oxidation under Θ≥0.4 ML proceeds slowly when irradiating with 2.3-eV HOMB, while a prominent increase in Θ ̇ over 0.4 ML was not observed when irradiating with 0.6-eV HOMB. The incident O 2 energy can control the oxidation processes of Cu{111}.

Photoemission study of the translational energy induced oxidation processes on Cu(111)

The oxidation processes of Cu(111) with a hyperthermal O2 molecular beam (HOMB) were studied using high-resolution x-ray photoemission spectroscopy in conjunction with a synchrotron radiation source. The O-uptake curves were precisely measured at ∼300 K when irradiating with 2.3 and 0.6 eV HOMB on a Cu(111) surface. A Langmuir-type adsorption model at an oxygen coverage (Θ)⩽0.4 ML describes the kinetics of dissociative adsorption in the HOMB incidence. On the other hand, further inefficient oxidation occurs even for the 2.3 eV HOMB irradiation at Θ⩾0.4 ML. A collision-induced absorption mechanism can interpret the slow oxidation process of Cu(111).

Actively controlled oxidation of Cu{100} with hyperthermal O2 molecular beam

The oxidation of Cu{100} with a hyperthermal O2 molecular beam (HOMB) was investigated using x-ray photoemission spectroscopy in conjunction with a synchrotron light source. The efficiency of oxidation with HOMB is higher than that with ambient thermal O2. Further oxidation under oxygen coverage (Θ)⩾0.5 ML occurs rather inefficiently even for the 2.3-eV-HOMB irradiation. We found that such slow oxidation of Cu corresponding to the initial stage of the Cu2O formation can be interpreted in terms of a collision-induced-absorption mechanism. The kinetics of the dissociative adsorption under Θ⩽0.5 ML is well described using the first-order kinetics in a simple Langmuir-type adsorption model.