Two-Step Oxidation of Pb（111） Surfaces

We report on a two-step method for oxidation of Pb（111） surfaces, which consists of low temperature （90K） adsorption of 02 and subsequent annealing to room temperature. In situ scanning tunnelling microscopy observation reveals that oxidation of Pb（111） can occur effectively by this method, while direct room temperature adsorption results in no oxidation. Temperature-dependent adsorption behaviour suggests the existence of a precursor state for 02 adsorption on Pb（111） surfaces and can explain the oxidation-resistance of clean Pb（111） surface at room temperature.     

Ar adsorptions on Al (111) and Ir (111) surfaces: a first-principles study

We investigate the adsorptions of Ar on Al (111) and Ir (111) surfaces at the four high symmetry sites, i.e., top, bridge, fcc- and hcp-hollow sites at the coverage of 0.25 monolayer (ML) using the density functional theory within the generalized gradient approximation of Perdew, Burke and Ernzerhof functions. The geometric structures, the binding energies, the electronic properties of argon atoms adsorbed on Al (111) and Ir (111) surfaces, the difference in electron density between on the Al (111) surface and on the Ir (111) surface and the total density of states are calculated. Our studies indicate that the most stable adsorption site of Ar on the Al (111) surface is found to be the fcc-hollow site for the (2 × 2) structure. The corresponding binding energy of an argon atom at this site is 0.538 eV/Ar atom at a coverage of 0.25 ML. For the Ar adsorption on Ir (111) surface at the same coverage, the most favourable site is the hcp-hollow site, with a corresponding binding energy of 0.493 eV. The total density of states (TDOS) is analysed for Ar adsorption on Al (111) surface and it is concluded that the adsorption behaviour is dominated by the interaction between 3s, 3p orbits of Ar atom and the 3p orbit of the base Al metal and the formation of sp hybrid orbital. For Ar adsorption on Ir (111) surface, the conclusion is that the main interaction in the process of Ar adsorption on Ir (111) surface comes from the 3s and 3p orbits of argon atom and 5d orbit of Ir atom.     

Directional UV photoemission from clean and sulphur saturated (100), (110) and (111) nickel surfaces

Angle resolved photoemission energy distribution curves (EDC's) were obtained on clean and sulphur saturated (100), (110) and (111) nickel surfaces for excitation energies equal to 10.2, 13.5, 16.8 and 21.2 eV. The EDC's of clean surfaces are weakly structured at θ = 0° and become more rich in features for oblique angles. In the explored energy range, adsorption of sulphur produces two extra-structures at initial energies depending on surface orientation. One of which situated at about ?4.5 eV below the Fermi level is in good agreement with Hagstrum ion neutralization spectroscopy results, the other at around ?1.8 cV has never been observed before. A remarkable similarity of adsorption effects on the three surfaces is found. These results are compared with experimental data obtained previously on equivalent sulphur saturated surfaces by INS and discussed in relation to recent theoretical calculations on chalcogen adsorption on nickel.     

Atomic and molecular oxygen adsorption on Ag(111)

The adsorption of O₂ on Ag(111) between 150 and 650 K has been studied with thermal desorption spectroscopy, Auger and photoelectron spectroscopies, and low-energy electron diffraction. A molecularly adsorbed O₂ species is populated with extremely low sticking probability (~ 5 X 10^?6) at 150 K. This species desorbs, with little dissociation, at 217 K. An atomically adsorbed species, with an O(1s) BE of 528.2 eV, is populated at 490 K with a sticking probability near 10^?6. This species exists in islands of local coverage θ_O ? 0.41, displaying a p(4 X 4)-O LEED pattern. It associatively desorbs at 579 K as O₂, and can be titrated at room temperature with CO to produce CO₂. There is also evidence for a subsurface oxygen species which reactivates below 600 K. Surface carbonate (CO_3,a) can be produced from O_a and CO₂ gas. These results are compared with similar species on Ag(110). A kinetic model is developed which describes the interaction of O₂ with these surfaces over a broad range of temperatures, and provides energetic values for the O₂/Ag interaction potential.     

Iodine adsorption on a platinum stepped surface: Pt(s)[6(111) × (111)]

I₂ adsorption on Pt(s)[6(111) × (111)] surfaces under vacuum and atmospheric pressure conditions was studied by LEED, AES and thermal desorption. In contrast to smooth Pt(111), the surface structures were composed of multiple phase domains having (3 × 3) or ($\text{3}\text{×}\text{3}$)R30° local geometry and structural coincidence of the adjacent terraces. No special stability or instability of iodine adsorption at steps was observed.     

Rectification behaviour of molecular layers on Si(111)

Reproducible and strong diode-like behaviour is observed for molecular films of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) on n-type Si(111)- 7×7 surfaces studied by scanning tunnelling microscopy (STM) and spectroscopy (STS) at 77 K. The mechanism behind the rectification is likely to be related to the electron distribution at the molecule-silicon interface. We suggest that the adsorption of the molecular layer profoundly modifies the electronic structure of the Si(111)- 7×7 surface.     

A DFT study of H adsorption on Pt(111) and Pt-Ru(111) surfaces

In this work a comparative analysis between different Pt-Ru(111) surface models and pure Pt(111) surface is presented. Some aspects of the electronic structure of the surfaces and hydrogen adsorption are analysed based on density functional theory calculations. The hydrogen adsorption energy is significantly reduced when Ru is present on the surface. The substitution of Pt atoms by Ru atoms reinforce the Pt-H bond while the metal-metal bond is strongly modified, making the system less stable.     

采用(meta)-GGA泛函对Ag(111)、Rh(111)和Ir(111)上乙烯吸附的理论研究

本文针对在多种催化反应的重要中间体乙烯,使用(meta)-GGA等级的包含PBE,BEEF-vdW,SCAN以及SCAN+rVV10在内的多种交换关联泛函,系统研究了在过渡金属表面(Ag,Rh和Ir)上乙烯吸附势能面对泛函的依赖关系. 研究发现,对于乙烯在贵金属Ag(111)上的吸附,除了PBE外,BEEF-vdW,SCAN以及SCAN+rVV10均能预测出物理吸附态的存在. 对于乙烯在Rh(111)面的吸附,SCAN和SCAN+rVV10预测在化学吸附位之前存在有物理吸附前驱态,而基于PBE和BEEF-vdW的势能面并没有发现前驱态的存在. 而对于乙烯在Ir(111)上的吸附,BEEF-vdW也能微弱地预测出化学吸附前驱态的存在. 研究结果表明,无论在哪一种金属表面上,四种泛函中SCAN+rVV10给出的吸附能最强,其次是SCAN,最后是PBE或者BEEF-vdW.     

2D Ising Model for Enantiomer Adsorption on Achiral Surfaces: L- and D-Aspartic Acid on Cu(111)

The 2D Ising model is well-formulated to address problems in adsorption thermodynamics. It is particularly well-suited to describing the adsorption isotherms predicting the surface enantiomeric excess, ees, observed during competitive co-adsorption of enantiomers onto achiral surfaces. Herein, we make the direct one-to-one correspondence between the 2D Ising model Hamiltonian and the Hamiltonian used to describe competitive enantiomer adsorption on achiral surfaces. We then demonstrate that adsorption from racemic mixtures of enantiomers and adsorption of prochiral molecules are directly analogous to the Ising model with no applied magnetic field, i.e., the enantiomeric excess on chiral surfaces can be predicted using Onsager’s solution to the 2D Ising model. The implication is that enantiomeric purity on the surface can be achieved during equilibrium exposure of prochiral compounds or racemic mixtures of enantiomers to achiral surfaces.     

A vibrational study of ammonia chemisorbed on Ni(110) and Ni(111): whither goest the metal-nitrogen stretching mode on FCC (111) surfaces?

The adsorption of ammonia on the Ni(110) and Ni(111) surfaces has been studied with high resolution (≤ 65 cm_?1) electron energy loss spectroscopy (EELS) combined with thermal desorption spectroscopy. The EELS spectra of the initial chemisorbed layer or α state on each surface are very different. Ammonia chemisorbed on the Ni(110) surface exhibits a strong Ni-N stretching mode at 570 cm^?1 which is absent on the Ni(111) surface. The Ammonia adsorption site appears to be different on the Ni(110) and Ni(111) surfaces. We suggest that the absence of the M-N stretching mode on the Ni(111) surface is a general characteristic of the ammonia adsorption site on the (111) surfaces of fcc Group VIII metals.     

Structural Stabilities of Ordered Nb4 Clusters on the Cu（111） and Cu（100） Surfaces

Based on first-principles calculations, we show that very high-density periodic arrays of Nb4 clusters with both the tetrahedron and quadrangle configurations can be stably absorbed on the Cu（111） and Cu（100） surfaces, with the quadrangle configurations more stable than the tetrahedron ones. The strong covalent bonding between atoms within the Nb4 clusters contributes to the stability of Nb4 adsorptions on the Cu surfaces. The energy barriers for the tetrahedron to the quadrangle-Nb4 on Cu（111） and （100） are around 1.21 eV and 0.94 eV/cluster, respectively. The stable adsorption of high-density Nb4 on these surfaces should have important applications.     

Interactions of CO molecules adsorbed on oxidised Cu(111) and Cu(110)

Reflection absorption infrared spectroscopy has been used in conjunction with LEED and surface potential measurements to study low temperature CO adsorption on the oxidised Cu surfaces Cu(111)O|$\text{3}\text{2}\text{?}\text{2}$|, Cu(110)O(2 × 1) and Cu(110)Oc(6 × 2). On all three surfaces adsorption at 80 K yields surface potential changes in excess of 0.6 V and does not lead to the formation of an ordered overlayer. At high coverages the adsorption enthalpy is lower than on the clean surfaces. Infrared spectra show the growth of a doublet band with components initially at 2100 and 2117 cm^?1 on the oxidised Cu(111) surface. Similar features seen on the oxidised Cu(110) surfaces are accompanied by a band at 2140 cm^?1: a very weak band at the same frequency on oxidised Cu(111) is attributed to defect sites. Studies of the temperature dependence of the spectrum from oxidised Cu(111) lead to the conclusion that two different binding sites are occupied. Spectra of ${}^{12}\text{CO}{}^{13}\text{CO}$ mixtures show that the molecules occupying these sites are in close proximity to each other, and that the spectrum is subject to large but opposing coverage-dependent frequency shifts.     

Surface interactions of Au(I) cyclo-trimer with Au(111) and Al(111) surfaces: A computational study

A plane-wave density functional theory (DFT) study on surface interactions of a cyclo-[Au(μ-Pz)]₃ monolayer (denoted as T), Pz = pyrazolate, with Au(111) and Al(111) surfaces (denoted as M′) has been performed. Structural and electronic properties at the M′–T interfaces are determined from individually optimized structures of M′, T and M′–T. Results show that the gold pyrazolate trimer (T) binds more strongly on the Au(111) surface than on Al(111). Charge redistribution has been observed at both M′–T interfaces, where charge is “pushed” back towards the Au(111) surface from the trimer monolayer in Au(111)–T system, while the opposite happens in the Al(111)–T system where the charge is being pushed toward the trimer monolayer from the Al(111) surface. Considerable changes to the work function of Au(111) and Al(111) surfaces upon the trimer adsorption which arise from monolayer vacuum level shifts and dipole formation at the interfaces are calculated. The interaction between cyclo-[Au(μ-Pz)]₃ with metal surfaces causes band broadening of the gold pyrazolate trimer in M′–T systems. The present study aids better understanding of the role of intermolecular interactions, bond dipoles, energy-level alignment and electronic coupling at the interface of metal electrodes and organometallic semiconductor to help design metal–organic field effect transistors (MOFETs) and other organometallic electronic devices.     

Evidence for two different adsorption sites of CO on Pt(111) from infrared reflection spectroscopy

The adsorption of CO on Pt(111) surfaces has been studied under clean conditions by a highly surface sensitive double-beam infrared reflection spectroscopy (IRS). In contrast to results of other authors two stretching vibrations of adsorbed CO rather than one are detected near 2100cm⁻¹ and 1870cm⁻¹. This is in agreement with recent findings in high-resolution electron energy loss spectroscopy (ELS). The results are discussed in terms of two adsorption sites: CO adsorbed in on-top positions and double coordinated on bridging sites, respectively. Furthermore, a precursor state and a preferential adsorption in islands at low coverage is taken into account.     

Comparison between the adsorption properties of Pd(111) and PdCu(111) surfaces for carbon monoxide and hydrogen

Thermal desorption spectroscopy and LEED have been used to investigate the interaction of CO and hydrogen with a Pd_0.75Cu_0.25(111) single crystal surface with surface composition of about Pd_0.7Cu_0.3. The main objective was to make a comparison with the previously studied Pd_0.67Ag_0.33(111) (surface composition Pd_0.1Ag_0.9) and Pd(111) surfaces. In addition, the effect of preadsorbed H on subsequent CO dosage and the effect of adsorbed CO on postdosed hydrogen are described. Marked differences were found in the adsorption behaviour of the three surfaces towards CO and hydrogen. The maximum amount of H and CO that can be adsorbed at 250 K and pressures below 10⁻⁹ mbar is much lower on the PdCu surface than expected on the basis of the surface composition. This effect appears to be caused by a low heat of adsorption of hydrogen and CO and Pd singlet sites. Arguments are presented that singlet Pd sites or isolated Pd atoms in a Cu or Ag matrix are able to trap and dissociate the hydrogen molecule at 250 K. The CO desorption spectra are not influenced by pre- or postexposed hydrogen. Adsorbed CO hampers the uptake of hydrogen upon subsequent exposure to hydrogen. Postdosed CO causes adsorbed H adatoms to move to the bulk (adsorbed H). CO exposure at 250 K results in a very broad desorption plateau between 310 and 425 K with hardly discernable maxima. The results can be explained in terms of the size and relative concentration of the various Pd sites present on the surface (triplet, doublet and singlet sites). It can be concluded that for Pd (111) the heat of adsorption of both CO and H differ appreciably for the triplet, doublet and singlet sites. The effect of site has a larger contribution to the decrease of the heat of adsorption with coverage than the effect of lateral interaction in the adlayer. For Pd(111), PdCu(111) and PdAg(111) the effect of the available Pd sites is the major effect that determines the heat of adsorption, followed by the effect of lateral interaction and for the alloy surfaces the electronic or ligand effect.     

New experimental studies on the adsorption of K on Si(100) and Si(111)

We report additional experimental characterization of the adsorption of K on Si(100) and Si(111). Our AES data reveal a layer-by-layer growth of K at room temperature on both surfaces. Measurements of the work function change on K adsorption present a minimum at a K coverage of half a monolayer. Furthermore, we find evidence for an incomplete charge transfer in contradiction to recent calculations. Our data could provide a solid basis to perform calculations in this interesting model system.     

A density-functional theory investigation on disorption of O2 on Sn(111) and its comparison with initial oxidation on the X(111) (X=Si,Ge, Sn,Pb) surfaces

The first-principles calculations are performed to investigate the adsorption of O₂ molecules on an Sn(111) 2×2 surface. The chemisorbed adsorption precursor states for O₂ are identified to be along the parallel and vertical channels, and the surface reconstructions of Sn(111) induced by oxygen adsorption are studied. Based on this, the adsorption behaviours of O₂ on X(111) (X=Si, Ge, Sn, Pb) surfaces are analysed, and the most stable adsorption channels of O₂ on X(111) (X=Si, Ge, Sn, Pb) are identified. The surface reconstructions and electron distributions along the most stable adsorption channels are discussed and compared. The results show that the O₂ adsorption ability declines gradually and the amount of charge transferred decreases with the enhancement of metallicity.     

Adsorption and diffusion of OH on Mo modified Pt(111) surface: First-principles theory

A first-principles study of adsorption and diffusion of OH on Pt and PtMo(111) surfaces is described. It confirms that the dissociation of water is much easier on PtMo than on pure Pt. Furthermore, we also found that OH binds most strongly at Mo atop site with adsorption energy of −3.32 eV, which is ∼1 eV stronger than binding to the pure Pt(111) surface. OH is much more localized on the PtMo alloy surface than on pure Pt. Both the stranger bond and the higher localization of OH contribute to the enhanced fuel cell performance with PtMo electrodes compared to pure Pt.     

Atomic scale characterization of oxygen adsorbates on Al(111) by scanning tunneling microscopy

We present atomic scale STM pictures of clean and oxygen containing Al(111) surfaces. Little influence of the surface oxygen on the topography of the surfaces is found. Three different oxygen species can be distinguished. One of them is associated with adsorbed oxygen and found to grow in small islands upon adsorption at 300 K. Characteristic hexagonal nuclei, created upon annealing of a dilute oxygen adlayer, represent the second one. By comparison with existing spectroscopic data these are assigned to nuclei of a surface oxide.     

Methanol decomposition on platinum (111)

The interaction of methanol with clean and oxygen-covered Pt(111) surfaces has been examined with high resolution electron loss spectroscopy (EELS) and thermal desorption spectroscopy (TDS). On the clean Pt(111) surface, methanol dehydrogenated above 140 K to form adsorbed carbon monoxide and hydrogen. On a Pt(111)-p(2 × 2)O surface, methanol formed a methoxy species (CH₃O) and adsorbed water. The methoxy species was unstable above 170 K and decomposed to form adsorbed CO and hydrogen. Above room temperature, hydrogen and carbon monoxide desorbed near 360 and 470 K, respectively. The instability of methanol and methoxy groups on the Pt surface is in agreement with the dehydrogenation reaction observed on W, Ru, Pd and Ni surfaces at low pressures. This is in contrast with the higher stability of methoxy groups on silver and copper surfaces, where decomposition to formaldehyde and hydrogen occurs. The hypothesis is proposed that metals with low heats of adsorption of CO and H₂ (Ag, Cu) may selectively form formaldehyde via the methoxy intermediate, whereas other metals with high CO and H₂ chemisorption heats rapidly dehydrogenate methoxy species below room temperature.