Co chemisorption on PtCu surfaces III. CO on PtCu(111)

Selected thermal desorption and valence band photoemission data on the chemisorption of CO on PtCu(111) surfaces are presented. The main objective is to make a comparison with CO chemisorption on an annealed (1 × 3) reconstructed Pt_0.98Cu_0.02(110) surface. The (111) alloy surfaces are unreconstructed (1 × 1) surfaces, with average near-surface Cu concentrations ranging from ? 7.5% to ? 20% as indicated by the Cu 920 eV Auger signal. It is observed that the effect of alloying Pt(111) with Cu is to progressively lower the desorption peak temperature and hence the free energy of CO desorption from Pt sites. A second observation is that the energy distribution of the Cu 3d-derived states is little affected by CO adsorption on Cu sites at 155 K. Both these results offer a contrast to the results for CO/Pt_0.98Cu_0.02(110) reported earlier.     

Cu在Pt表面扩散激活能和扩散系数的ASED-MO计算

本文应用ASED-MO方法,计算研究了Cu在Pt(111),(100),(110)表面的扩散问题。Cu原子在上述三个表面上的扩散激活能的计算结果分别为0.167eV,0.162eV和0.668eV;300K时的扩散系数分别为3.04×10¹⁰m²/s,3.69×10^-10m²/s和2.42×10^-18m²/s。计算结果表明,Cu原子在Pt(111)和(100)面上,扩散激活能很小,极易迁移,而在(110)面上,激活能较大,扩散系数甚小。     

Effect of Oxygen on the Quantum,Magnetic, and Thermodynamic Properties of Co Nanowires on the Reconstructed Anisotropic (1 × 2)/Au(110) and (1 × 2)/Pt(110) Surfaces: Ab Initio Approach

Ab initio theoretical study of the quantum magnetic properties of Co nanowires on the pure and oxygen-reconstructed (1 × 2)/Au(110) and (1 × 2)/Pt(110) surfaces is performed. Their structures and electronic configurations are calculated using the electron density functional theory. High values of magnetic moment and magnetic anisotropy energies of Co atoms are found on both pure and oxygen-reconstructed (1 × 2)/Au(110) and (1 × 2)/Pt(110) surfaces. The adsorption of oxygen atoms on the (1 × 2)/Au(110) substrate is shown to affect the structural arrangement of Co nanowire atoms on this substrate and to increase the magnetic anisotropy energy (by 1.91 meV per nanowire atom). The adsorption of oxygen on the Pt(110) substrate substantially decreases the magnetic anisotropy energy of the Co nanowire on it (by 5.98 meV per atom). The origin of these changes is revealed by analyzing the local densities of states of the d electrons of nanowire atoms. The temperature ranges of the states with the lowest free surface energy are determined using the atomistic thermodynamics methods. These data and the available experimental data are used to predict the possibility of observing the structures under study in experiments.     

Comment on the “cross-channel diffusion mechanism on the Pt(110) surface” BY S.H. Garofalini and T. Halicioglu

It is known that the Pt(110) surface exhibits in its stable state a 1 × 2 reconstruction. It seems therefore not appropriate to compare theoretical surface diffusion coefficients calculated for the regular, non-reconstructed Pt(110) surface with the experimental data obtained by field ion microscopy. In particular, the process of cross-channel diffusion by atom exchange which is observed on the regular (110) surface by molecular dynamics simulations may have no relationship to the experimental findings of cross-channel adatom diffusion on reconstructed Pt(110) and Ir(110) surfaces. An alternate mechanism for easy cross-channel adatom diffusion on reconstructed (110)?1 × 2 surfaces of Pt and Ir is proposed.     

Calculation of the structural energy of the unreconstructed and (1 × 2) reconstructed Pt(110) surface

Describing the binding energy of both d and s valence electrons within the LCAO formalism, and by including repulsive Born-Mayer type interactions, we study the structural stability of the reconstructed and unreconstructed Pt(110) surfaces. Our main result is that amongst the various models for the (1 × 2) reconstruction the “Bonzel-Ferrer” model is unfavoured, while the “missing-row” model seems to be energetically degenerate with the unreconstructed surface. Our calculation predicts also a small surface concentration, which, however, has only a minor effect on the total energy of the system.     

Predicted diffusion rates on fcc (001) metal surfaces for adsorbate/substrate combinations of Ni, Cu, Rh, Pd, Ag, Pt, Au

We investigate the diffusion of a single metal atom on the surface of a fcc (001) metal. Two points concerning the application of kinetic models to diffusion were considered. First, we test the assumption of kinetic models that diffusion occurs via a sequence of uncorrelated jumps. Second, when kinetic models are applicable we predict reasonable values of the kinetic rate constants.

Direct molecular dynamics (MD) simulations were performed for Ag on Ag(001) and Rh on Rh(001) systems. Diffusion was found to obey an Arrhenius-type dependence on temperature in both systems. The barriers and prefactors extracted from the MD results agree with estimates made from transition state theory (TST) and the experimental values for the Rh system. We conclude that kinetic models are applicable to diffusion on fcc (001) surfaces.

Transition state theory was then used to estimate diffusion parameters for all other adsorbate/ substrate combinations of the metals Ni, Cu, Rh, Pd, Ag, Pt, and Au. These results indicate that the characteristics of diffusion are primarily a property of the adsorbate. We also predict Ag atoms to have an anomalously low diffusion barrier on all of the substrates in this study. We use the accurate many-body density functional based MD/MC-CEM potential energy surface which allows us to consistently treat these multi-component systems.     

Electric-dipole layer on Au(111) surfaces

We measured the barrier height (BH) a UHV scanning tunneling microscope on Au(111) surfaces with Au, Pt, and carbon nanotube tips. The 222?{^[¯] }3^[¯]\sqrt\Box3\Box]] reconstruction was observed with all the tips, and the current-voltage relation reflected the density of states of the tips. The BH measured on the reconstructed Au(111) surface using a modulation method showed a bias-polarity dependence as 30%, at low currents (<100 pA) irrespective of the tip material, while on unreconstructed Au surfaces, BH values were independent of the bias polarity, suggesting a dipole layer originating from the reconstructed Au(111) surface.     

NO在Ag/Pt(110)-(1×2)双金属表面的吸附：亚硝酸盐/硝酸盐表面物种的生成

以俄歇电子能谱、X射线光电子能谱和热脱附谱研究了室温下NO在Ag/Pt(110)-(1×2)双金属表面的吸附. 在该双金属表面上观察到了可能的亚硝酸盐/硝酸盐表面物种,其在更高温度下分解生成N₂. 然而,室温下NO在清洁Pt(110)表面和Ag-Pt合金表面上并不会生成这种亚硝酸盐/硝酸盐表面物种. 亚硝酸盐/硝酸盐表面物种的形成归因于高度配位不饱和Ag粒子的高活性及其与Pt基底之间的协同作用.     

吸附原子在Ag,Pt,Au(110)表面上的自扩散现象

采用表面嵌入势，用静态计算和分子动力学方法研究了Ag,Pt,Au单个吸附原子在(110)表面上的自扩散现象.分别给出了跳跃机制和交换机制所对应的能量变化曲线及相关原子的运动轨迹，分析了这三种不同金属(110)表面上的自扩散特点，结果与分子动力学模拟及有关的实验结果相符合.     

Mechanism of an adsorbate-induced surface phase transformation: CO on Pt(100)

The mechanism of the (5 × 20)→(1 × 1) transition of Pt(100) during adsorption of CO has been investigated using a fast Video-LEED technique. Analysis of the coverage-dependence of the intensities of several LEED spots on different surfaces leads to a straightforward model. The phase transition occurs by a nucleation-trapping mechanism of the adsorbed CO. The driving force is the difference in stabilities of CO adsorbed on the reconstructed and unreconstructed Pt surfaces.     

A surface analysis of CuAu alloy by leed and Auger electron spectroscopy

Surface structures and compositions of the CuAu alloys have been investigated, which were prepared by depositing gold on (110) and (111) surfaces of copper and by subsequent heating. By this method the structure of alloy surfaces corresponding to different compositions can be observed by LEED. A series of the LEED patterns, streak, (1 × 2), (1 × 1)I, complex, c(3 × 1), (1 × 1)II, (2 × 2) and (1 × 1) have been observed on the (110) surface with decreasing gold composition. On the (111) surface (1 × 1) pattern, weak (2/√3 × 2/√3)R30° and (2 × 2) patterns are observed. The mean surface composition is determined by analysing the data of Auger electron spectroscopy. Most surface periodicities observed are different from those expected if one passes a mathematical plane through the crystal (unreconstructed surface).     

Hydrogen chemisorption on Ni(110) by high-resolution electron energy loss spectroscopy

High-resolution electron energy loss spectra of hydrogen-covered Ni(110) surfaces both at 100 and 300 K are presented. The adsorbed sites of hydrogen atoms are discussed.High-resolution electron energy loss spectra of hydrogen covered Ni(110) surfaces have been studied. Tentative models for the adsorbed sites of hydrogen atoms are as follows: (1) For the (2 × 1)-H surface, hydrogen is adsorbed in the three-coordinated sites of the rudimentary (111) face of the unreconstructed Ni(110) substrate. (2) For the low-temperature (1 × 2)-H surface, hydrogen is adsorbed in the three-coordinated sites and, probably, in the two-fold hollow sites of the distorted Ni(110) substrate. (3) For the room-temperature (1 × 2)-H surface, hydrogen is disorderedly adsorbed in the three-coordinated, two-fold hollow and short-bridge sites and, possibly, in the octahedral sites of the distorted Ni(110) substrate. Some of the unresolved problems in the above assignments are summarized: (1) Strictly, the three-coordinated sites above are somewhat different from those discussed in the molecular-beam diffraction study [5]. (2) For the low-temperature (1 × 2)-H surface, the loss associated with hydrogen in the two-fold hollow sites is apparently not observed. (3) Intensity changes of the three losses for the room-temperature (1 × 2)-H surface with increasing hydrogen pressure (Fig. 2) are not well understood.     

Diffusion processes of trimers on missing row surfaces: \mathrm{Cu }_{3}/\mathrm{Ag }(110) and \mathrm{Ag }_{3}/\mathrm{Cu }(110)

A semi-empirical potential according to the embedded atom, has been applied to investigate the diffusion of trimers by computing the energy barriers for different mechanisms. Our attention was more focused on the leapfrog process which is likely to occur on missing row surfaces. The activation barriers of this mechanism are calculated using drag method at 0K. These barriers are found to be 0.64 and 0.68 eV for hopping out the channel for $\mathrm{Cu }_{3}/\mathrm{Ag }(110) \mathrm{and Ag }_{3}/\mathrm{Cu }$ (110) respectively. While for hopping down at the other side they are about 0.42 and 0.32 eV. Moreover, a deep metastable position is observed during leapfrog diffusion leading to some spectacular trimer motion. At high temperature and essentially for $\mathrm{Cu }_{3}/\mathrm{Ag }$ (110), we also observed a competition between leapfrog process and concerted jump mechanism with a deformation of trimer geometry. Implications of these findings are briefly discussed.     

Reconstruction behaviour of fcc(110) transition metal surfaces and their vicinals

The fcc(110) surfaces are well known for their strong tendency to missing-row (MR) type reconstructions either in the clean state (Au, Pt) or driven by adsorbates (Ni, Cu, Pd, Ag). The present knowledge on the different reconstruction behaviour of flat (110) surfaces is reviewed. The survey focuses on recent scanning tunneling microscopy (STM) studies, which for the first time also elucidate the dynamics of the reconstruction process for the various systems. An overview of our recent STM and low energy electron diffraction studies on vicinal Au(110) and Ni(110) surfaces is given, aiming for a deeper understanding of the influence of steps on reconstruction behaviour of fcc(110) surfaces on the one hand, and on the stability of reconstructing vicinal surfaces on the other. Finally, we report on the reconstruction behaviour of Ir(110), which stabilizes in the clean state by formation of mesoscopic (331) facets and dereconstructs to the (1×1) phase upon oxygen adsorption at 700–900 K.     

Co chemisorption on PtCu surfaces II. (1 × 1)-CO/Pt0.98Cu0.02(110)

The chemisorption of CO on the Pt atoms of an initially (1 × 3) reconstructed Pt_0.98Cu_0.02(110) surface at ~ 373 K can lead to the formation of a (1 × 1) surface. Comparisons are made with (1 × 3)-CO surfaces formed by CO exposures at 293 or 155 K. Thermal desorption shows that the (1 × 1)-CO surface has an enhanced population of high temperature CO peak ( ~ 543 K) from Pt sites. The CO-induced structural conversion also leads to a decrease in the subsequent CO uptake on the low temperature Pt sites and on the Pt-Cu “mixed” sites, with a concomitant increase in adsorption on the Cu-like sites. Such a reduction in the number of the Pt-Cu “ mixed” sites is also reflected in the CO-induced changes of the Cu 3d-derived states and the $\text{Cu 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ core levels. A dynamic interplay between chemisorption and surface structure is thus demonstrated.     

异质扩散过程中ES势垒的计算

利用分子动力学中的静态结构计算方法对Pd，Ag及Cu原子在面心立方铜的台阶表面扩散过程中的Ehrlich-Schwoebel(ES)势垒进行了模拟计算，研究了各种台阶表面情况下增原子扩散过程中的ES势垒；讨论了与衬底互溶的金属和与衬底不互溶的金属增原子扩散的ES势垒的异同，并将模拟结果与同质情况的研究结果进行了对比. 结果表明： 1)在同质和异质扩散过程中ES势垒随着台阶高度的变化关系是相似的，即随着台阶高度的增加，ES势垒逐渐增加；当台阶高度达到某一高度时ES势垒将趋于定值. 2)在跳跃机理下，与Cu互溶的金属(Pd)在Cu表面台阶上扩散的ES势垒最大，其次是Cu，最小的是与Cu不互溶的金属 (Ag)；而在交换机理下，与Cu不互溶的金属(Ag)在Cu表面台阶上扩散的ES势垒最大，其次是Cu，最小的是与Cu互溶的金属(Pd). 3)对大多数台阶的情况，交换机理支配着原子在台阶边缘的扩散行为；且表面台阶高度对交换扩散过程影响较大.     

The growth and chemisorptive propertles of Ag and Au monolayers on platinum single crystal surfaces: An AES,TDS and LEED study

The growth and chemisorptive properties of monolayer films of Ag and Au deposited on both the Pt(111) and the stepped Pt(553) surfaces were studied using Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). AES studies indicate that the growth of Au on Pt(111) and Pt(553) and Ag on Pt(111) proceeds via a Stranski-Krastanov mechanism, whereas the growth of Ag on the Pt(553) surface follows a Volmer-Weber mechanism. Au dissolves into the Pt crystal bulk at temperatures > 800 K, whereas Ag desorbs at temperatures > 900 K. TDS studies of Ag-covered Pt surfaces indicate that the AgPt bond (283 kJ mol^?1) is ～25 kJ mol^?1 stronger than the AgAg bond (254 kJ mol^?1). On the Pt(553) surface the Au atoms are uniformly distributed between terrace and step sites, but Ag preferentially segregates to the terraces. The decrease in CO adsorption on the Pt crystal surfaces is in direct proportion to the Ag or Au coverage. No CO adsorption could be detected for Ag or Au coverages above one monolayer at 300 K and 10^?8 Torr. The heat of adsorption of CO on Pt is unaltered by the presence of Ag or Au.     

Molecular dynamics simulation of the formation of metal nanocontacts

The process of the formation of nanocontacts has been studied by the molecular dynamics methods for a group of metals (Cu, Rh, Pd, Ag, Pt, Au). It has been shown that the disruption forces of nanocontacts substantially depend on the orientation ((100), (110), or (111)) of the contact-surface interface. The possibility of forming linear atomic chains as a result of the disruption of nanocontacts has been analyzed for different orientations of the electrode surfaces. The possibility of forming quasi-one-dimensional nanostructures from the Co/Au alloy, which represent a periodic alternation of gold atoms and cobalt trimers, has been predicted.     

Quantum-well states and resonances in thin single-crystal layers of noble metals on W(110) substrates

This paper reports on the first experimental observation of quantum-well states and sp-type resonances in thin single-crystal gold, silver, and copper layers formed on single-crystal W(110) surfaces, which result from spatial localization of Bloch-type electronic wave functions in a quantum well with potential barriers at the vacuum/metal and metal/W(110) interfaces. The quantization of the valence-band electronic structure in Au/W(110), Ag/W(110), and Cu/W(110) systems was studied experimentally using angle-resolved photoelectron spectroscopy.     

A systematic DFT study of hydrogen diffusion on transition metal surfaces

Density functional theory calculations of the diffusion of hydrogen atoms on 23 transition metal surfaces in their closed-packed structure have been carried out. The d-metals chosen are all the metals in the 4th, 5th and 6th periods, from Sc to Au, except Mn, Tc, and Hf. Potential energy surfaces of H atom on these metals are constructed and the diffusion barrier from one minima to another is compared with nudged elastic band calculations. Most of the minimum energy paths have a single activation barrier, except on two surfaces where a dip in the bridge position (W and Pt) is observed. Trends in the adsorption and activation energies are observed where the former is explained with the d-band model. All the activation energies for diffusion are relatively low, or from 0.04 eV for Pt to 0.28 eV on Y and Zr. Finally, we estimate the temperature where tunneling effects should start to take place.