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1.
《Surface science》1993,298(1):L196-L202
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.  相似文献   

2.
《Surface science》1997,381(1):L563-L567
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.  相似文献   

3.
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.  相似文献   

4.
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.  相似文献   

5.
《Surface science》1994,321(3):L219-L224
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.  相似文献   

6.
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.  相似文献   

7.
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.  相似文献   

8.
《Surface science》1994,321(3):L225-L232
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.  相似文献   

9.
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 GaIx where x is a number between 0 and 3. By thermal desorption of this GaIx 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.  相似文献   

10.
M. Welz  W. Moritz  D. Wolf 《Surface science》1983,125(2):473-480
The atomic structure of the (112̄0) 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 rZJ = 0.09 and rP = 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.  相似文献   

11.
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 (2 × 2), (19 × 19)R23.4°, 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 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 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.  相似文献   

12.
The copper deposition on single crystal ZnO(101̄0) 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(101̄0), are believed to be due to electronic resonance of a Vs 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.  相似文献   

13.
Electron energy loss spectroscopy has demonstrated the existence of both a monodentate and a symmetric bidentate bridging formate as stable intermediates in the decomposition of formic acid on the Ru(001) surface. The monodentate formate converts upon heating to the bidentate formate which decomposes via two pathways: CH bond cleavage to yield CO2 and adsorbed hydrogen; and CO bond cleavage to yield adsorbed hydrogen, oxygen and CO. Thermal desorption spectra demonstrate the evolution of H2,H2O, CO and CO2 as gaseous products of the decomposition reaction. The observation of this product distribution from Ru(100), Ni(100) and Ni(110) had prompted the proposal of a formic anhydride intermediate, the existence of which is rendered questionable by the spectroscopic results reported here.  相似文献   

14.
The structural and chemisorptive properties of the stepped, non-unique, (101̄2) 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 (101̄2) surface is very different from that of the basal plane.  相似文献   

15.
The decomposition of HCOOD was studied on Ni(100). Low temperature adsorption of HCOOD resulted in the desorption of D2O, CO2, CO, and H2. The D2O was evolved below room temperature. CO2 and H2 were evolved in coincident peaks at a temperature above that at which h2 desorbed following H2 adsorption and well above that for CO2 desorption from CO2 adsorption; CO desorbed primarily in a desorption limited step. The decomposition of formic acid on the clean surface was found to yield equal amounts of H2, CO, and CO2 within experimental error. The kinetics and mechanism of the decomposition of formic acid on Ni (110) and Ni(100) single crystal surfaces were compared. The reaction proceeded by the dehydration of formic acid to formic anhydride on both surfaces. The anhydride intermediate condensed into islands due to attractive dipole-dipole interactions. Within the islands the rate of the decomposition reaction to form CO2 was given by:
Rate = 6 × 1015 exp{?[25,500 + ω(ccsat)]/RT} × c
, where c is the local surface concentration, csat is the saturation coverage for the particular crystal plane, and ω is the interaction potential. The interaction potential was determined to be 2.7 kcal/mole on Ni(110) and 1.4 kcal/mole on Ni(100); the difference observed was due to structural differences of the surfaces relating to the alignment of the dipole moments within the islands. These attractive interactions resulted in an autocatalytic reaction on Ni(110), whereas the interaction was not strong enough on Ni(100) to sustain the autocatalytic behavior. Formic acid decomposition oxidized the Ni(100) surface resulting in the formation of a stable surface oxide. The buildup of the oxide resulted in a change in the selectivity reducing the amount of CO formed. This trend indicated that on the oxide surface the decomposition proceeded via a formate intermediate as on Ni(110) O.  相似文献   

16.
The X-ray diffuse scatterinf along the 4̄ axis on urea gives the dispersion curves for the two longitudinal branches of translational motions. The experimental curves can be interpreted with a model of lattice dynamics. On the boundary Brillouin zone, the frequencies are 3.96 · 1012 Hz for the acoustical branch and 4.62 · 1012 Hz for the optical branch. These experimental results give the value of 4.66 Debyes for the dipole moment of urea.  相似文献   

17.
K. Ozawa  K. Edamoto 《Surface science》2003,547(3):257-267
The electronic structure and the chemical composition of the K-covered ZnO(1 0 1̄ 0) surface at temperatures between 300 and 1200 K are investigated by X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. Adsorption of K on ZnO(1 0 1̄ 0) at room temperature results in the formation of a two-dimensional disordered K overlayer and induces 0.2 eV downward bending of the substrate’s bands going from the bulk to the surface. Upon annealing the K-covered surface, initial downward bending turns to upward bending with maximum bending of 0.5 eV at 700-800 K. The thermally induced migration of the bulk O atoms and the resultant increase in the number of the O atoms on the surface is responsible for upward bending on the annealed surface. The accumulated O atoms interact with the predeposited K atoms on the surface to form non-stoichiometric K-O complexes with the O/K atomic ratio being 1.6-1.8 in the temperature range between 600 and 1000 K.  相似文献   

18.
《Surface science》1987,182(3):458-476
In an attempt to understand the relative importance of the various constituents of copper-zinc oxide catalysts for methanol synthesis (2H2 + CO → H3COH), we have prepared and characterized a number of single-crystal surface structures of Cu-ZnO. The model catalysts have also been tested in terms of their activity for methanol synthesis. The growth of vapor-deposited Cu overlayers on a ZnO(0001&#x0304;) (O face) single crystal has been investigated using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), He+ ion scattering spectroscopy (ISS) and low-energy electron diffraction (LEED). The results are consistent with a growth model in which, at room temperature, the first monolayer spreads uniformly across the surface in a p(1 × 1) structure. As more Cu is added, thick Cu(111) islands grow and these are separated by large regions of the p(1 × 1)-Cu monolayer. The Cu(111) islands are rotationally aligned with the ZnO substrate, and at high enough coverages grow together to cover the ZnO. Increasing temperature favors more agglomeration. A clean Cu(111) crystal and one containing a ZnOx (x ⋍ 3) monolayer were also studied. None of these model catalysts gave rates of methanol production which were measurable in our present experimental limits (TOF < 2 × 10−3 molecules site−1 s−1) at 500–600 K and CO + H2 pressures up to 1500 Torr. Under these “reaction” conditions, the Cu in direct contact with ZnO may be slightly oxidized; all the other Cu is completely metallic. The Cu does not change its character between deposition and reaction conditions, even if heavily oxidized to CuO in between. The addition of CO2 at very high levels under reaction conditions does not change the character of a Cu(111) model catalyst surface, and no surface oxygen is ever observable after treatment under reaction conditions.  相似文献   

19.
《Surface science》1982,122(1):L574-L578
High resolution electron energy loss spectroscopy has demonstrated the existence of a symmetrical bidentate bridging formate as a stable intermediate in the decomposition of formic acid on the Ru(001) surface. This formate decomposes with two pathways — via CH bond cleavage to give CO2 and adsorbed hydrogen, and via CO bond cleavage to give adsorbed hydrogen, oxygen and CO. Thermal desorption demonstrates the evolution of H2, H2O, CO and CO2. The observation of this product distribution from Ru(100), Ni(100) and Ni(110) had prompted the proposal of a formic anhydride intermediate. The spectroscopic results reported here bring the presence of this postulated, normally unstable species into question.  相似文献   

20.
Pt/Pd anode catalysts for direct formic acid polymer electrolyte membrane fuel cells outperform both Pt and Pd in steady-state electrooxidation trials. Temperature-programmed desorption (TPD) experiments in ultra-high vacuum (UHV) were performed with 1 L formic acid on clean Pt(1 1 0), 0.6 monolayers Pd/Pt(1 1 0), and multilayer Pd/Pt(1 1 0) to gain a better understanding of the effect of Pd additions to a Pt catalyst. Both dehydration and dehydrogenation of formic acid occur on all three surfaces. As Pd coverage increases, the activation barrier for formate decomposition to CO2 decreases, but the effect does not explain the unusual activity of Pt/Pd in the electrochemical environment.  相似文献   

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