The reactivity of the clean and contaminated (0001̄) polar and the (101̄0) prism ZnO surfaces to chlorine

The reactivities of the (0001̄) and (101̄O) surfaces of zinc oxide to chlorine gas have been studied by a range of techniques. In the case of the (0001̄) oxygen polar surface investigations were made with the surface both atomically clean and with a known level of carbon and calcium contamination. Comparison is made with our earlier results on the (0001) surface which showed a high level of reactivity due to the increased electrostatic stability on adsorption of the electronegative gas. Both the oxygen polar and the prism surface showed a much lower reactivity to chlorine than the zinc face: contamination by carbon and calcium on the former surface reduced the reactivities still further. This result conflicts with comparable data for oxygen adsorption where previous work has shown a greater take-up of oxygen on the oxygen face than the zinc face. Unlike the zinc face, no LEED superstructures were observed on any of, the three surfaces, but in common with the (0001) there were significant electron beam desorption effects. Two states could be identified: one was rapidly removed in ~10 μA min exposure to the beam, the other in much longer periods. Work function and ELS data were consistent with atomic adsorption of chlorine on all surfaces. An exception was the (101̄O) at high exposures where a work function decrease took place following the initial increase: this may indicate a second molecular state.     

The effect of hydrogen chemisorption on GaAs(100) and GaAs(1̄1̄1̄)

The interaction of hydrogen with the polar (100) and (1̄1̄1̄) surfaces of GaAs has been studied with LEED, angle-resolved photoemission and core level spectroscopy. It was found that the properties of the hydrogen-covered surface were independent of the composition of the initial surface. The core levels also showed an increase in the surface As concentration for initially Ga-rich surfaces. Angle-resolved photoemission results for GaAs(100) and GaAs(100):H are presented and the dispersion of a hydrogen-induced state is shown.     

Composition,structure, surface states,and O2 sticking coefficient for differently prepared GaAs(1̄1̄1̄)As surfaces

The polar GaAs(1̄1̄1̄)As surface can be prepared in three stable and ordered states: two by molecular beam epitaxy (MBE), namely the As-stabilized and the Ga-stabilized states and one simply by ion bombardment and annealing at 770 K. The respective LEED structures are $\text{(2 × 2), (}\text{19}\text{×}\text{19}\text{)}\text{R}\text{23.4°}$, and (1 × 1) with a diffuse faint (3 × 3) superstructure. Auger measurements and the comparison with the stoichiometric cleaved (110) surface show that there are different As concentrations in the first atomic layer associated with each of these three surfaces. Whereas about 10 to 15% of the first As layer appears to be missing on the (2 × 2) surface, about 50% is missing on the $\text{19}$ surface. On the (1 × 1) surface the first As layer is removed completely. The intensity of emission from the surface sensitive states between 1 and 4 eV below the valence band edge, as seen by angular resolved UPS, roughly corresponds to the amount of As at the surface thus confirming their interpretation as As-derived surface states. The inital sticking coefficent for oxygen depends strongly on the surface structure: ～10^?8 for the (2 × 2), ～10^?7 for the $\text{19}$, and ～10^?4 for the (1 × 1) surface. The sticking coefficient does not depend on the surface concentration of As but rather on the degree of saturation of dangling bonds on Ga atoms.     

Angular resolved ups of surface states on GaAs(1̄1̄1̄) prepared by molecular beam epitaxy

The As-rich (2 × 2), a newly found (√3 × √3) and the (√19 × √19) surfaces of GaAs(1̄1̄1̄) are studied by angular resolved UPS (ARUPS). The (2 × 2) surface is prepared by molecular beam epitaxy and the others by mild annealing. For the (2 × 2) surface emission from surface states is observed, which shows dispersion periodic within the (2 × 2) surface Brillouin zone. Using s-polarized light and the known symmetry selection rules the uppermost surface bands between 1 and 2 eV below the valence band maximum are assigned to the As dangling bond orbital. The bands near 4 and 7 eV assigned to the backbonds. From the strong decrease of emission intensity of the As-derived surface states between the (2 × 2) and the annealed surfaces it is concluded that the character of the As dangling bond orbital must have been changed from sp³-hybridic to s-like. This gives further evidence for our recently proposed model for the (√19 × √19) surface, which is particularly applicable for the (√3 × √3) surface.     

The orientation of formate and carbonate on ZnO(101̄0)

Polarisation dependent CK-edge NEXAFS spectra of carbonate and formate species on ZnO(101̄0) indicate that both species are aligned by the substrate. The carbonate species, formed by adsorption of CO₂, is oriented with its molecular plane close to the [0001] azimuth. This suggests a bond geometry in which one O atom of CO²⁻₃ is in the substrate, and a second interacts with a surface cation, consistent with the results of previous cluster calculations. In contrast, the formate species, formed by adsorption of formic acid, is not azimuthally ordered, although the molecular rotation axis is aligned close to the surface normal, consistent with bidentate bonding to a single cation.     

Iodine etching of the GaAs(1̄1̄1̄)As surface studied by LEED,AES, and mass spectroscopy

The iodine interaction with the GaAs(1̄1̄1̄)As surface prepared by molecular beam epitaxy has been studied by LEED, LEED intensity measurements, Auger electron spectroscopy (AES) and computer controlled mass spectroscopic study of the whole desorption spectrum. It is shown that an iodine beam hitting the GaAs(1̄1̄1̄)As face at 300 K under UHV conditions etches the surface continuously. After this etching there remains an adsorbate of GaI_x where x is a number between 0 and 3. By thermal desorption of this GaI_x adsorbate an As stabilized GaAs(1̄1̄1̄)As surface showing a (2 × 2) structure can be prepared, which up to the present could be done only by molecular beam epitaxy.     

A new type of reconstruction on the InSb(1̄1̄1̄) surface determined by grazing incidence X-ray diffraction

The (3×3) reconstruction of the InSb(1̄1̄1̄) surface has been investigated by grazing incidence X-ray diffraction and scanning tunneling microscopy. The structure is characterized by 6-atom rings on top of a slightly buckled InSb top double layer. Two types of rings have been found, an elliptic ring consisting of 4 In and 2 Sb atoms and a trigonal ring with 3 In and 3 Sb atoms. The bond angles and lengths are consistent with the concept of rehybridization and depolarization which explains the reconstructions of the (111) and (110) surfaces.     

Surface core level shift on Be( 112̄0)

A photoemission study of the Be(112̄0) surface carried out at a sample temperature of 100 K is reported. A surface shifted Be 1s component, having a shift of - 410 meV, is resolved on this surface. The extracted surface to bulk intensity ratio indicate that this component originates from atoms in the surface layer only. This is opposite to previous observations on both the close-packed Be(0001) surface and the Be(101̄0) surface where sub-surface shifted Be 1s levels were unambiguously identified. Among these three surfaces a surface layer atom is expected to have the lowest coordination on the (112̄0) surface but the surface layer shift is found to be smallest on this surface. Compared to findings on other metals this is unusual and reasons contributing to this behaviour are suggested and discussed.     

Atomic relaxation of the BeO (101̄0) surface

The atomic relaxation of the nonpolar (101̄0) surface of BeO has been calculated by minimizing the surface energy within the framework of the ab initio Hartree-Fock method. A six-layer two-dimensionally periodic slab model was used, permitting a full symmetry-conserving relaxation of the two outer layers. The BeO surface bonds show a small rotation angle of about 4 accompanied by a large (about 10%) reduction in surface bond length. Significant contraction of backbonds and a small rotation of second layer bonds are also found. The relaxed BeO (101̄0) surface is thus predicted to be similar to the ZnO (101̄0) surface but different from the corresponding surfaces of all other II–VI compounds. Various explanations for this difference are discussed, and evidence from a bond population analysis is presented which suggests that this behavior can be described in terms of partial double bond character in the surface bonds. Since multiple bonding is related to small atomic radii, it would follow that the small radius of the oxygen atom is the ultimate cause of the type of surface relaxation we predict.     

Observation of extrinsic surface states on (112̄0) CdS

Surface states have been detected by surface photovoltage spectroscopy on (112̄0) CdS surfaces subjected to various treatments in UHV and studied by Auger electron spectroscopy and LEED. All surface electronic features can be related to chemical contamination or lattice nonstoichiometry. Energy level spectra of air-exposed CdS exhibit a set of discrete states due to adsorption of C, O, and Cl. Ion bombardment generates a pair of states 2.35 eV and ~0.8 eV above the valence band edge due to S interstitials and vacancies, respectively. Oxygen adsorption produces a broad continuum of states. Changes in surface atomic order show no direct effect on these electronic features. No intrinsic surface states, filled or empty, are observed by surface photovoltage spectroscopy on clean, stoichiometric (112̄0) faces of CdS.     

The decomposition of formic acid on Ru(101̄0)

The decomposition of formic acid was studied on a clean Ru(101̄0) surface adsorption temperature between 100 and 460 K by means of flash thermal desorption. The decomposition products observed were H₂, CO₂, H₂O and CO. HCOOH itself was also desorbed, although at low exposures no formic acid was observed. The H₂ and CO₂ products were desorbed in identical first order peaks, with a peak temperature of 395 K. The H₂O product desorbed in a second order peak at 813 K, in contrast to H₂O desorption from low coverage H₂O adsorption which occurs in two peaks in the region of 220 and 265 K. The CO product desorbed in a first order peak at 488 K, identical to CO from CO adsorption. The dependence of the product peaks on adsorption temperature of the Ru surface was also studied. These results suggest a model involving the formation and decomposition of a surface intermediate species.     

Structure determination of the clean Co(112̄0) surface by LEED

The atomic structure of the (112&#x0304;0) surface of cobalt has been determined by LEED using six intensity spectra at normal incidence. The surface exhibits the truncated bulk structure with a contraction of the first interlayer spacing by about 8.5% with respect to the bulk value. Quantitative evaluation of the LEED spectra was done using Zanazzi and Jona's and Pendry's r-factors. The minimum averaged r-factors are $\text{r}{}_{\mathrm{<mtext>ZJ</mtext>}}\text{= 0.09}$ and $\text{r}{}_{\mathrm{<mtext>P</mtext>}}\text{= 0.22}$. No change of the interatomic distances within the plane could be detected and no rearrangement of the surface structure takes place up to temperatures shortly below the transition temperature.     

The initial stages of deposition of Cu on ZnO(101&#x0304;0) and MgO(001) surfaces studied by electron energy loss spectroscopy

The copper deposition on single crystal ZnO(101&#x0304;0) and MgO(001) surfaces has been studied by electron energy loss spectroscopy (EELS) in UHV at room temperature. The initial deposited Cu (well below one monolayer) induces a loss peak at about 2 eV on both oxide surfaces and at 4.3 eV on the MgO(001) surface. Based upon heat treatment and oxidation experiments the 2 eV structure is assigned to the electronic resonance of Cu(I) from the Cu deposit on the oxide matrix substrates. On the basis of the experiments the colour-center-related loss peaks, at 2.6 eV for MgO(001) and at 1.9 eV for ZnO(101&#x0304;0), are believed to be due to electronic resonance of a V⁻_s center, and the metal ion vacancies are suggested to be active centers which interact with the submonolayer copper deposits. Finally, the electronic energy loss spectra from the Cu-covered oxide surfaces are discussed in the framework of electronic band structures.     

The synthesis of methanol and the reverse water-gas shift reaction over Zn-deposited Cu(100) and Cu(110) surfaces: comparison with Zn/Cu(111)

The catalytic activity of Zn vapor-deposited Cu(100) and Cu(110) surfaces for methanol synthesis by the hydrogenation of CO₂ and the reverse water-gas shift reaction were studied using an XPS apparatus combined with a high-pressure flow reactor (18 atm). At a reaction temperature of 523 K, no promotional effect of Zn was observed for the methanol synthesis on both Zn/Cu(100) and Zn/Cu(110). The results were quite different from those for Zn/Cu(111), on which a significant promotion of methanol synthesis activity appeared to be due to the deposition of Zn, indicating that the promotional effect of Zn was sensitive to the surface structure of Cu. However, hysteresis was observed in the catalytic activity for methanol synthesis over the Zn/Cu(110) surface upon heating above 543 K in the reaction mixture. The activity became twice that measured before heating, which was close to the methanol synthesis activity of Zn/Cu(111) at the same Zn coverage. On the other hand, no such hysteresis was observed for the reverse water-gas shift reaction on Zn/Cu(110), indicating that the active site for methanol synthesis was not identical to that for the reverse water-gas shift reaction. In the post-reaction surface analysis, formate species was detected on both Zn/Cu(100) and Zn/Cu(110), whose coverage increased with increasing Zn coverage at 0<Θ_Zn<0.2. No correlation between the formate coverage and the methanol synthesis activity was obtained, which was in contrast to the results for Zn/Cu(111). Thus, the structure sensitivity observed in the catalytic activity of methanol synthesis over Zn-deposited Cu surfaces is ascribed to the significant difference in the reactivity of the formate intermediate.     

Temperature dependence of Cu2O orientations in oxidation of Cu (111)/ZnO (0001) by oxygen plasma

The role of temperature on the oxidation dynamics of Cu₂O on ZnO (0001) was investigated during the oxidation of Cu (111)/ZnO (0001) by using the oxygen plasma as oxidant. A transition from single crystalline Cu₂O (111) orientation to micro-zone phase separation with multiple orientations was revealed when the oxidation temperature increased from 300 ℃ to higher. The experimental results clearly showed the effect of oxidation temperature with the assistance of oxygen plasma on changing the morphology of Cu (111) film and enhancing the lateral nucleation and migration abilities of cuprous oxides. A vertical top-down oxidation mode and a lateral migration model were proposed to explain the different nucleation and growth dynamics of the temperature-dependent oxidation process in the oxidation of Cu (111)/ZnO (0001).     

Interfacial potential approach for Ag/ZnO （0001） interfaces

Systematic approaches are presented to extract the interfacial potentials from the ab initio adhesive energy of the interface system by using the Chen–M ¨obius inversion method. We focus on the interface structure of the metal（111）/Zn O（0001）in this work. The interfacial potentials of Ag–Zn and Ag–O are obtained. These potentials can be used to solve some problems about Ag/Zn O interfacial structure. Three metastable interfacial structures are investigated in order to check these potentials. Using the interfacial potentials we study the procedure of interface fracture in the Ag/Zn O（0001） interface and discuss the change of the energy, stress, and atomic structures in tensile process. The result indicates that the exact misfit dislocation reduces the total energy and softens the fracture process. Meanwhile, the formation and mobility of the vacancy near the interface are observed.     

Surface chemistry of the non-basal planes of cobalt: The structure,stability, and reactivity of Co(101&#x0304;2)-CO

The structural and chemisorptive properties of the stepped, non-unique, (101&#x0304;2) surface of cobalt have been investigated by standard LEED/Auger/Δφ/thermal desorption methods. The clean surface is well-ordered, unreconstructed, and reversibly undergoes the predicted structural changes on cycling through the phase transition. CO chemisorption is rapid and non-dissociative at 300 K, leading ultimately to a (3 × 1) structure with a COCO spacing of 3.8 Å. Heating of the adlayer can, depending on the conditions, lead to competitive desorption and dissociation reactions. The data suggest that the transition state to desorption is mobile whereas that for dissociation is localised. Dissociation is accompanied by diffusion of oxygen into the bulk and formation of a very well-ordered (2 × 3) carbon structure. This structure is interpreted in terms of epitaxial growth of the (001) plane of Co₃C. The carbide surface is still capable of chemisorbing a substantial amount of CO, but cannot dissociate it. Some other ordered phases of the CoCCO system are also observed, and an attempt is made to interpret them in a consistent way. The CO chemistry of the (101&#x0304;2) surface is very different from that of the basal plane.     

A molecular beam study of the adsorption dynamics of CO on Ru(0001), Cu(111) and a pseudomorphic Cu monolayer on Ru(0001)

We have investigated the sticking coefficient of CO on Ru(0001), a pseudomorphic Cu monolayer on Ru(0001), and a fully relaxed Cu(111) multilayer as function of kinetic energy, surface coverage, and surface temperature. At a low kinetic energy of 0.09 eV, the initial sticking coefficients, S₀, on these surfaces are determined to be 0.92, 0.96 and 0.87, respectively. In all cases, a decrease of S₀ with increasing beam energy was observed, yielding values of 0.58, 0.14 and 0.07, respectively, at a kinetic energy of 2.0 eV. For all three surfaces the coverage dependent sticking coefficients, S(Θ), display very characteristic behavior at low kinetic energies: S(Θ) remains more or less constant up to coverages close to saturation, indicative of precursor adsorption kinetics. However, characteristic minima at intermediate coverages are observed, which are correlated to the formation of well ordered adsorbate phases. For high kinetic energies we observe a transition towards a linear decrease of S(Θ) for Ru(0001). In contrast, for the pseudomorphic Cu monolayer and for Cu(111) we find an increase in the sticking coefficients at low coverages, followed by a decrease close to saturation. This behavior is attributed to adsorbate assisted sticking, that is, to a higher sticking coefficient on adsorbate covered regions than on the bare surface. The comparison between the pseudomorphic monolayer and Cu(111) reveals that the CO bond strength to the former is larger by 40%. The initial sticking coefficients for both surfaces are very similar at low kinetic energies; at high kinetic energies, S₀ for the pseudomorphic Cu monolayer is, however, larger by a factor of two.