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1.
The temperature dependent adsorption of sulfur on TiO 2(1 1 0) has been studied with X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). Sulfur adsorbs dissociatively at room temperature and binds to fivefold coordinated Ti atoms. Upon heating to 120°C, 80% of the sulfur desorbs and the S 2p peak position changes from 164.3±0.1 to 162.5±0.1 eV. This peak shift corresponds to a change of the adsorption site to the position of the bridging oxygen atoms of TiO 2(1 1 0). Further heating causes little change in S coverage and XPS binding energies, up to a temperature of 430°C where most of the S desorbs and the S 2p peak shifts back to higher binding energy. Sulfur adsorption at 150°C, 200°C, and 300°C leads to a rich variety of structures and adsorption sites as observed with LEED and STM. At low coverages, sulfur occupies the position of the bridging oxygen atoms. At 200°C these S atoms arrange in a (3×1) superstructure. For adsorption between 300°C and 400°C a (3×3) and (4×1) LEED pattern is observed for intermediate and saturation coverage, respectively. Adsorption at elevated temperature reduces the substrate as indicated by a strong Ti 3+ shoulder in the XPS Ti 2p 3/2 peak, with up to 15.6% of the total peak area for the (4×1) structure. STM of different coverages adsorbed at 400°C indicates structural features consisting of two single S atoms placed next to each other along the [0 0 1] direction at the position of the in-plane oxygen atoms. The (3×3) and the (4×1) structure are formed by different arrangements of these S pairs. 相似文献
2.
The co-adsorption of oxygen and deuterium at 100 K on a Pd(110) surface has been studied by measurements of the change in work function (Δφ) and by thermal desorption spectroscopy (TDS). When the surface with co-adsorbed species is heated, the adsorbates O and D react to form D 2O which desorbs from the surface at T > 200 K. The D 2O desorption peaks shift continuously to lower temperatures as the surface D coverage (θ D) increases. The maximum production of D 2O is estimated to be 0.26 ML (1 ML = 9.5 × 10 14 atoms cm −2), resulting from reaction in a layer containing 0.65 ML D and 0.3 ML O. The maximum work function increase caused by adsorption of D to saturation onto oxygen precovered Pd(110) decreases almost linearly with Δφ O of the oxygen precovered surface. On a surface with pre-adsorbed D however, the maximum Δφ increase contributed by oxygen adsorption decreases abruptly at Δφ D > 200 mV. This sharp change occurs at θ D > 1 ML and is believed to be associated with the development of the reconstructed (1 × 2) phase of D/Pd(110). 相似文献
3.
A complete inspection of the capabilities of reflectance anisotropy spectroscopy (RAS) in studying the adsorption of molecules or atoms on the Si(0 0 1)-(2 × 1) surface is presented. First, a direct comparison between RA spectra recorded on the clean Si(0 0 1)-(2 × 1) and the corresponding topography of the surface obtained using scanning tunneling microscopy (STM) allows us to quantify the mixing of the two domains that are present on the surface. Characteristic RA spectra recorded for oxygen, hydrogen, water, ethylene, benzene are compared to try to elucidate the origin of the optical structures. Quantitative and qualitative information can be obtained with RAS on the kinetics of adsorption, by monitoring the RA signal at a given energy versus the dose of adsorbate; two examples are presented: H 2/Si(0 0 1) and C 6H 6/Si(0 0 1). Very different behaviours in the adsorption processes are observed, making of this technique a versatile tool for further investigations of kinetics. 相似文献
4.
Atomic oxygen coverages of up to 1.2 ML may be cleanly adsorbed on the Au(111) surface by exposure to O 3 at 300 K. We have studied the adsorbed oxygen layer by AES, XPS, HREELS, LEED, work function measurements and TPD. A plot of the O(519 eV)/Au(239 eV) AES ratio versus coverage is nearly linear, but a small change in slope occurs at ΘO=0.9 ML. LEED observations show no ordered superlattice for the oxygen overlayer for any coverage studied. One-dimensional ordering of the adlayer occurs at low coverages, and disordering of the substrate occurs at higher coverages. Adsorption of 1.0 ML of oxygen on Au(111) increases the work function by +0.80 eV, indicating electron transfer from the Au substrate into an oxygen adlayer. The O(1s) peak in XPS has a binding energy of 530.1 eV, showing only a small (0.3 eV) shift to a higher binding energy with increasing oxygen coverage. No shift was detected for the Au 4f 7/2 peak due to adsorption. All oxygen is removed by thermal desorption of O 2 to leave a clean Au(111) surface after heating to 600 K. TPD spectra initially show an O 2 desorption peak at 520 K at low ΘO, and the peak shifts to higher temperatures for increasing oxygen coverages up to ΘO=0.22 ML. Above this coverage, the peak shifts very slightly to higher temperatures, resulting in a peak at 550 K at ΘO=1.2 ML. Analysis of the TPD data indicates that the desorption of O 2 from Au(111) can be described by first-order kinetics with an activation energy for O 2 desorption of 30 kcal mol −1 near saturation coverage. We estimate a value for the Au–O bond dissociation energy D(Au–O) to be 56 kcal mol −1. 相似文献
5.
The structure and the electronic valence state occupation of ultrathin K, Rb, and Cs films grown on a GaAs(1 0 0)-(4×2) surface have been studied by means of metastable He atom scattering (MHAS), He atom scattering (HAS), and low-energy electron diffraction (LEED) at temperatures ranging from 150 to 400 K. From the survival probability of the scattered He * atoms, detailed information on the coverage-dependent filling of the alkali metal valence states and their emptying upon subsequent exposure to oxygen were derived. These data reveal for K and Rb a nearly linear band filling with increasing coverage starting at about 0.5 ML whereas a more rapid filling is observed for Cs which is almost completed at about 0.7 ML. Subsequent oxygen adsorption causes a demetallization of the metallic alkali metal monolayers. In case of Cs, a distinct minimum of the He * signal appears at an oxygen exposure of about 0.8 L, presumably indicating the onset of subsurface oxidation. 相似文献
6.
We investigated the adsorption of sodium on the (1 0 0) surface of germanium with LEED, STM and electron spectroscopy (XPS). Upon adsorption at room temperature a metastable p(4 × 1) and a p(2 × 1) superstructure have been found. Annealing of these structures, accompanied by thermal desorption, results in the formation of a commensurate p(3 × 2) phase after an incommensurate state has been passed. The formation of structures observed after annealing requires the rearrangement of substrate atoms. In addition strong evidence was found that all ordered phases discussed in this paper contain one adsorbate atom per unit mesh. 相似文献
7.
The spatially inhomogeneous decomposition and desorption reaction of oxide layers with coverage 1-0.3 monolayers (ML) from a silicon (100) surface has been studied using scanning tunneling microscopy (STM). After desorption, microscopic changes to the (2 × 1) reconstruction produce two variations on the dimer row reconstruction with decreased surface atom density. A (2 × n) vacancy chain reconstruction and a c(4 × 4) incomplete row reconstruction were observed; a structure for the latter is proposed. Both reconstructions are metastable, reforming the (2 × 1) reconstruction upon heating. At greater length scales during desorption from an initial 1.0 ML coverage, the mesoscopic changes to the surface structure include pitting and roughening, with up to a measured 20 fold increase in the edge density as compared to the clean Si(100) surface. These structural changes suggest a reaction mechanism involving a substantial rearrangement of the substrate silicon. From an initial 1.0 ML oxygen coverage, using measured void size distributions at total desorption levels of 13% and below — before voids have begun to coalesce — the evolution of void sizes during initial desorption can be followed. A mechanism for desorption is proposed in which silicon atoms must diffuse from adjacent clean surface area to the oxide boundary, producing a reactive complex from which SiO is desorbed. Void growth rates derived from two rate limiting cases for this desorption reaction mechanism can be compared to measured results. We show that the measured void area evolution is consistent with a reaction mechanism where the rate limiting step for monolayer desorption is the promotion of a silicon atom in a lattice site to a mobile monomer within the void. 相似文献
8.
We studied reaction of oxygen atoms with D-terminated Si(1 1 1) surfaces from a desorption point of view. As the D (1 ML)/Si(1 1 1) surface was exposed to O atoms D 2 and D 2O molecules were found to desorb from the surface. The desorption kinetics of D 2 and D 2O molecules exhibited a feature characterized with a quick rate jump at the very beginning of O exposure, which was followed by a gradual increase with a delayed maximum and then by an exponential decrease. The O-induced D 2 desorption spectra as a function of Ts appeared to be very similar to the H-induced D 2 desorption spectrum from the D/Si(1 1 1) surfaces. Possible mechanisms for the O-induced desorption reactions were discussed. 相似文献
9.
The TiO 2(110)-(1 × 1) surface and its reconstruction as a (1 × 2) form have been studied with low energy electron diffraction (LEED), electron stimulated desorption ion angular distribution (ESDIAD) and scanning tunnelling microscopy (STM). Oxygen ion desorption occurs within a lobe perpendicular to the (1 × 1) surface, changing to two off-normal lobes for the (1 × 2) reconstruction. This transformation in the ESDIAD pattern is consistent with the added Ti 2O 3 row model of the (1 × 2) reconstruction proposed by Onishi and Iwasawa. STM studies of the stoichiometric and electron irradiated surfaces reinforce the association of the O + ESD contribution with majority sites at the surface. Adsorption of acetic acid on the (1 × 1) surface produces a (2 × 1) overlayed and induces a reconstruction of the underlying substrate. ESDIAD reveals H + ions emitted off-normally from dissociatively adsorbed acetate, and along the surface normal from surface hydroxyls. Adsorption of acetic acid on the (1 × 2) surface does not modify the LEED pattern, but ESDIAD reveals H + desorption with a weaker off-normal contribution consistent with the Ti 2O 3 model of the reconstruction. 相似文献
10.
The adsorption of CO and CO 2 on K-predosed Pd{1 1 0} at room temperature has been examined via reflection–absorption infrared spectroscopy (RAIRS). CO 2 adsorbs on 0.37 ML K-predosed Pd{1 1 0} with high sticking probability and a reactive chemisorbed intermediate, CO 2−, is detected in RAIRS at room temperature. Reaction of this species ultimately yields carbonate. The same high K precoverage induces dissociation of CO at low CO exposure. Carbonate is detected at higher CO exposure and is probably produced via stepwise oxidation of molecularly adsorbed CO. In contrast at low K precoverage (0.11 ML), CO remains intact but the C–O bond is considerably weakened with respect to CO chemisorbed on clean Pd{1 1 0}. These findings illustrate a dual promoter mechanism of K in the adsorption and reaction of CO or CO 2 at high K coverage. The alkali metal induces dissociation of these molecules and directly participates in the formation of a surface compound, K 2CO 3. 相似文献
11.
Diffusion length of Ga on the GaAs(0 0 1)-(2×4)β2 is investigated by a newly developed Monte Carlo-based computational method. The new computational method incorporates chemical potential of Ga in the vapor phase and Ga migration potential on the reconstructed surface obtained by ab initio calculations; therefore we can investigate the adsorption, diffusion and desorption kinetics of adsorbate atoms on the surface. The calculated results imply that Ga diffusion length before desorption decreases exponentially with temperature because Ga surface lifetime decreases exponentially. Furthermore, Ga diffusion length L along
and [1 1 0] on the GaAs(0 0 1)-(2×4)β2 are estimated to be
and L[110]200 nm, respectively, at the incorporation–desorption transition temperature ( T860 K). 相似文献
12.
The adsorption of CO and the reaction of CO with pre-adsorbed oxygen at room temperature has been studied on the (2 × 1)ORh(1 1 1) surface and on vanadium oxideRh(1 1 1) “inverse model catalyst” surfaces using scanning tunnelling microscopy (STM) and core-level photoemission with synchrotron radiation. Two types of structurally well-defined model catalyst V 3O 9Rh(1 1 1) surfaces have been prepared, which consist of large (mean size of 50 nm, type I model catalyst) and small (mean size <15 nm, type II model catalyst) two-dimensional oxide islands and bare Rh areas in between; the latter are covered by chemisorbed oxygen. Adsorption of CO on the oxygen pre-covered (2 × 1)ORh(1 1 1) surface leads to fast CO uptake in on-top sites and to the removal of half (0.25 ML) of the initial oxygen coverage by an oxidation clean-off reaction and as a result to the formation of a coadsorbed (2 × 2)O + CO phase. Further removal of the adsorbed O with CO is kinetically hindered at room temperature. A similar kinetic behaviour has been found also for the CO adsorption and oxidation reaction on the type I “inverse model catalyst” surface. In contrast, on the type II inverse catalyst surface, containing small V-oxide islands, the rate of removal of the chemisorbed oxygen is significantly enhanced. In addition, a reduction of the V-oxide islands at their perimeter by CO has been observed, which is suggested to be the reason for the promotion of the CO oxidation reaction near the metal-oxide phase boundary. 相似文献
13.
The Sb adsorption process on the Si(1 1 1)–In(4×1) surface phase was studied in the temperature range 200–400 °C. The formation of a Si(1 1 1)–InSb (2×2) structure was observed between 0.5 and 0.7 ML of Sb. This reconstruction decomposes when the Sb coverage approaches 1 ML and Sb atoms rearrange to
and (2×1) reconstructions; released In atoms agglomerate into islands of irregular shapes. During the phase transition process from InSb(2×2) to Sb
(θ Sb>0.7 ML), we observed the formation of a metastable (4×2) structure. Possible atomic arrangements of the InSb(2×2) and metastable (4×2) phases were discussed. 相似文献
14.
The surface structure and properties of the HfB 2(0 0 0 1) (Hafnium diboride, HfB 2) surface have been investigated with X-ray photoelectron spectroscopy, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Annealing temperatures above 1900°C produce a sharp (1×1) LEED pattern, which corresponds to STM images showing flat (0 0 0 1) terraces with a very low contamination level separated by steps 3.4 Å in height, corresponding to the separation of adjacent Hf planes in the HfB 2 bulk structure. For lower annealing temperatures, extra p(2×2) spots were observed with LEED, which correspond to intermediate terraces of a p(2×1) missing row structure as observed with STM. 相似文献
15.
The mechanism and energetics are presented of the dimerization of two adsorbed surface SiH 2 groups on the H-terminated Si(0 0 1)-(2 × 1) surface to form Si 2H 4 species during the initial stages of growth in plasma deposition of hydrogenated amorphous silicon (a-Si:H) films. The reactions are observed during classical molecular-dynamics (MD) simulations of a-Si:H film deposition from SiH 2 radical precursors impinging on an initially H-terminated Si(0 0 1)-(2 × 1) surface and substrate temperature, T, over the range 500 T700 K. The Si 2H 4 species resulting from the surface SiH 2 dimerization reactions undergo surface conformational changes resulting in either a non-rotated (NRD) or a rotated dimer (RD) configuration. The RD configuration is found to be the energetically favorable one. The MD simulation results for the structure of the NRD and RD surface Si 2H 4 configurations corroborate with ab initio calculations of optimized adsorption configurations of SiH 2 radicals on crystalline Si surfaces, as well as results of STM imaging of the thermal decomposition of disilane on Si(0 0 1). 相似文献
16.
The surface morphological change at an initial stage of thermal oxidation on Si(0 0 1) surface with O 2 was investigated as a function of oxide coverage by a real-time monitoring method of Auger electron spectroscopy (AES) combined with reflection high energy electron diffraction (RHEED). At 653 °C where oxide islands grow laterally, protrusions were observed to develop under the oxide islands as a consequence of concurrent etching of the surface. The rate of etching was measured from a periodic oscillation of RHEED half-order spot intensity I(1/2,0) and I(0,1/2). At 549 °C where Langmuir-type adsorption proceeds, it was observed that both I(1/2,0) and I(0,1/2) decrease more rapidly in comparison with an increase of oxide coverage and the intensity ratio between them decreases gradually with O 2 exposure time. These suggest that Langmuir-type adsorption occurs at sites where O 2 adsorbs randomly, leading to subdivision of the 2×1 and 1×2 domains by oxidized regions, and that Si atoms are ejected due to volume expansion in oxidation to change the ratio between 2×1 and 1×2 domains. 相似文献
17.
The reactions of H 2S with predosed surface oxygen on Ni(110) surfaces were studied for a variety of coverage conditions. The primary reaction product is H 2O, but the details of the water formation and desorption depends on the coverage of both O and H 2S. For high coverages of oxygen (p(2 × 1)−O; 0.5 ML), the reaction to form water is quantitative. The loss of oxygen from the surface (as measured by AES) is equal to the increase in sulfur coverage. XPS and HREELS measurements indicate the presence of chemisorbed H2O immediately following large exposures of H2S on the oxygen predosed surface at 110 K. Deuterium incorporation results suggest that the primary mechanism for these coverage conditions involves direct transfer of hydrogen from SH or H2S moieties to the oxygen. A second mechanism involving reaction of surface hydroxyl groups with surface hydrogen was also identified. This mechanism is particularly important for high coverages of oxygen (0.5 ML) and low coverages of H2S (0.15 ML), where water desorption was observed at 235 K, but was not observed spectroscopically at 110 K. The sequential addition of two surface hydrogen atoms to surface oxygen is not an important mechanism in this system. These reactions were modeled using a bond-order conservation method, and the model successfully reproduced the important mechanistic conclusions. 相似文献
18.
A crystallographic analysis is reported using low-energy electron diffraction (LEED) in the tensor LEED approach for the electrodeposited coadsorption (3×3) structure with 4/9 monolayer (ML) of silver and 4/9 ML of iodine on the Pt(1 1 1) surface. The structure approximates a two-layer slice of bulk AgI cut parallel to its (1 1 1) plane and superimposed on the substrate with the Ag atoms in contact with the topmost Pt(1 1 1) layer, and the I atoms forming an overlayer on the Ag atoms. There are two types of Ag atoms in the (3×3) unit mesh; one type bonds to a single Pt atom, while the other type bonds to three Pt atoms. The average Ag–Pt bond distances are close to 2.48 and 2.82 Å respectively for the one and three-coordinate Ag atoms, but both types of Ag atoms bond to three I atoms with an average Ag–I distance of 2.67 Å. No significant corrugation is observed for either the I layer or the Ag layer. 相似文献
19.
The interaction of SO 2 with oxygen-sputtered Au(1 1 1) ( θoxygen 0.35 ML) was studied by monitoring the oxygen and sulfur coverages as a function of SO 2 exposure. The morphology of the sputtered Au is relatively smooth on a long length scale, but rough on a finer scale with islands averaging 15 nm. The rough surface is not stable to scanning with the STM. Two reaction regimes were observed: oxygen depletion followed by sulfur deposition. An enhanced, transient sulfur deposition rate is observed at the oxygen depletion point. This effect is specifically pronounced if the Au surface is continuously exposed to SO 2. The enhanced reactivity towards S deposition seems to be linked to the presence of highly reactive, under-coordinated Au atoms. Adsorbed oxygen appears to stabilize, but also to block these sites. In absence of the stabilization effect of adsorbed oxygen, i.e. at the oxygen depletion point, the enhanced reactivity decays on a timescale of a few minutes. These observations shed a new light on the catalytic reactivity of highly dispersed gold nanoparticles. 相似文献
20.
The adsorption of CO 2 on the NaCl(100) surface was studied with a high-resolution LEED-system. Measurements without charging up at low electron energies and without damage by the e-beam could be performed by using ultrathin epitaxial films on a conducting Ge(100) substrate. The adsorption behavior was recorded as a function of time and pressure at constant substrate temperatures of 78 and 83 K and CO 2 partial pressures from 4 × 10 −8−2 × 10 −3 Pa. The adsorption system shows a first-order two-dimensional phase transition to a (2 × 1) superstructure including glide planes (herringbone-like structure) at p = 7.2 × 10 −8Pa ( T = 78 K). The condensation of the CO 2 solid is starting at p = 1.5 × 10 −4 Pa ( T = 78 K). The LEED-pattern shows in this c(2 × 2) superstructure, which corresponds to the pyrite-like structure of the CO 2 solid. Both observed superstructures are commensurable with the NaCl(100) surface. Observation of island growth shows that the domains of the (2 × 1) superstructures have already at coverage of 5% of a monolayer an average lateral size of at least 200 A. 相似文献
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