Study of chemisorption on copper low-index and stepped surfaces

Adsorption of CHCl₃, O₂, and hydrocarbons has been studied on Cu(111) and stepped surfaces using LEED, AES, and UPS at room temperature. We find that ordered Cl overlayers form upon Cu(111), Cu[3(111) × (100)], and Cu[5(111) × (100)] surfaces upon exposure to CHCl₃. Exposure to O₂ results in rearrangement of the Cu[5(111) × (100)] surface to hill-and-valley regions with large (111) areas, whereas Cu[2(111) × (100)] is stable for the same exposure. The photoemission spectra show new energy levels due to C1 above and below the Cu d band region and a small splitting of the halogen p orbitals. Effects consistent with interaction with the Cu d band are observed. Similar effects are observed with oxygen adsorption. The initial rate of Cl or O₂ chemisorption as measured by photoemission is proportional to the density of steps on these surfaces. Apparently, structural effects play an important role in chemisorption on metals (such as copper) with low density of states at the Fermi energy.     

UPS spectra of H2O,CH3OH and C5H9OH adsorbed onto Cu(111)/Na and Na(cp)

The present communication presents ultraviolet photoemission spectra (UPS) of three different “alcohols”; water (H₂O), methanol (CH₃OH), and cyclopentanol (C₅H₉OH), chemisorbed onto a Cu(111) surface partially covered by sodium atoms as well as onto closely packed sodium films, a free electron adsorbent. Whereas all three alcohols ROH bind reversibly and associatively to Cu(111) they react with adsorbed sodium atoms to metal bound alcoxides RO. The chemisorption bond, characterized by the interaction between O 2pπ orbitals and metal atoms as an electron donor, the alcoxide being the acceptor, is similar for all groups R. The O 2pπ orbitals shift to higher UPS binding energies with increasing electron density, i.e. decreasing r_s/a_o of the sodium overlayer. Only for HONa, the sterically smallest group R, does the alcoxide growth continue in three dimensions. Although, possibly failing to reproduce the electron density profile of a free electron surface, Hartree-Fock-Slater cluster calculations of small models ROH and RONa₃ enable correlations to be made between UPS intensity peaks and one electron orbitals.     

Study of chemisorption on silver surfaces using ultraviolet photoelectron spectroscopy (UPS)

Ultraviolet photoelectron spectroscopy (UPS) was used to study the chemisorption of halogens on stepped [3(111) × (100)] and low-index (111) silver surfaces. The initial rate of halogen adsorption using CHCl₃ exposure on the silver stepped surface is approximately twice that on the low-index surface. This indicates that steps play an important role in chemisorption even on metals with a low density of states at the Fermi level. The adsorbate-induced levels on silver were correlated with halogen p valence orbitals using model extended Hückel calculations. Changes in the silver d band are interpreted as due to p?d orbital interactions.     

CH3NO2在Cu(111)面吸附态的研究

用CNDO/2半经验量化计算方法对CH₃NO₂分子在Cu(111)面四个吸附位上25种吸附态进行了优化计算,得到以CH₃NO₂分子中的-NO₂取向吸附在Cu(111)面的桥位上,且CH₃NO₂分子中的ONO面与Cu-Cu键成60°时为最稳吸附态。计算得到的这一稳定吸附态的吸附取向和吸附体系的态密度结果与我们的实验结果是一致的;从吸附态的轨道成分分析表明, 关键词：     

Nitrogen doping concentration influence on NaNbO3 from first-principle calculations

The nitrogen concentration effects on electronic band structures and photocatalytic performance of N-doped sodium niobate (NaNbO₃) have been investigated by first-principles calculations based on density functional theory (DFT). At lower nitrogen doping levels, some localized N 2p states are formed above the valence band (O 2p) in N-doped NaNbO₃, leading to the reduction of the photon transition energy in comparison to that of undoped compound. Under higher doping levels, the N 2p states mix with O 2p states and then move the top of valence band upward. Two possible mechanisms for increasing visible light absorbance in N-doped NaNbO₃ are tentatively put forward according to the doping levels, which would be of importance in understanding and developing the visible-light-sensitive nitrogen-doped multimetal oxide.     

Cyanogen on Ni(110): an experimental and theoretical study

Adsorption of cyanogen, C₂N₂, on a Ni(110) surface has been investigated by combining low energy electron diffraction, thermal desorption and angular resolved ultra-violet photoemission experiments as well as model cluster calculations using the linear combination of Gaussian-type orbitals local density functional method as well as a force field approach. Results of mirror plane photoemission experiments on the ordered c(2 × 2) monolayer could be rationalized by invoking adsorbates bonded to the surface via the π electrons, with their axis oriented along the [001] direction, across the grooves of the (110) surface, leading to an assignment of all six adsorbate-derived valence orbitals. However, unlike in a previous study on the analogous chemisorption system Pd(110)/C₂N₂, only one mirror plane was detected. A possible tilt of the adsorbates in the (1 0) plane was related to crowding on the closer spaced nickel surface by estimating the lateral interaction within the adsorption layer using force field models. Electronic structure calculations on various chemisorption model clusters confirmed the experimental orbital assignment and the orientation of the adsorbate axis essentially along the [001] direction. Best agreement with UPS data was found for the orbital splitting pattern of an adsorption geometry where the C-N groups bind sideways on-top of nickel atoms in the first crystal plane. On the other hand, bonding along in the troughs of the (110) surface leads to a distinctly different ordering of the valence orbitals, at variance with the experimental assignment.     

Interaction of Cu cluster anions (Cu<Superscript>-</Superscript><Subscript>n</Subscript>, n = 8–11) with oxygen

The reaction of oxygen with Cu cluster anions consisting of 6–11 atoms was studied by means of Time-of-Flight mass (TOF) spectroscopy and Ultraviolet Photoelectron Spectroscopy (UPS). Using molecular oxygen, we found that a Cu^-_n cluster (n=6-11) can react only with one single oxygen molecule, which adsorbs molecularly, implying that the clusters studied here are less reactive towards oxygen chemisorption compared to the smaller clusters (n<5). This result indicates that chemical properties can alter significantly with increasing cluster size. Depending on the cluster source conditions, different cluster structures and reactivity patterns were found. These results are used to qualitatively describe the chemisorption energetics of oxygen on Cu cluster anions.     

Cluster model calculation and photoemission studies of molecularly adsorbed NO on Ni

Self-consistent scattered wave cluster model calculations for the NONi system are presented with NO bonded N-down in a fourfold coordination site. The results suggest that the charge is transferred from metal into the single occupied 2π¹ molecular orbital of NO which is thus significantly perturbed. The interaction involves the metal s, p, and d-derived states. We calculate total and local “density of states” defined within the framework of the cluster model and compare the results with Ultraviolet Photoemission experiments recorded at 80 K. At 300 K partial dissociation occurs as judged from UPS and XPS. Increased photoemission observed at about 2 eV below the Fermi level in the molecularly adsorbed state is attributed to the alteration of density of states due to the interaction of the 2π¹ molecular orbital of NO with the substrate. A detailed analysis of the nature of the relevant molecular orbitals is also given.     

O2、 CO2和H2O在UN(001)表面化学吸附的密度泛函理论研究

采用广义梯度近似GGA,修正Perdew-Burke-Ernzerhof交换-关联泛函,以及周期性切片模型对O₂、CO₂和H₂O在UN(001)表面的化学吸附行为进行非自旋极化水平的密度泛函理论计算. 在四个对称性化学位置条件下,对化学吸附能与分子和UN(001)表面之间距离的关系曲线进行优化. 结果表明O₂、CO₂和H₂O分子的最稳定吸附位置分别为桥式平行、空心平行和桥式H     

Molecular orbital calculations of the dissociative adsorption of a hydrogen molecule on a 14 atom nickel cluster

MINDO/SR molecular orbital calculations have been performed for the Ni₁₄H₂ system in order to study the various steps of the dissociative chemisorption process of H₂ over nickel. Particular attention was placed in the determination of physisorbed “precursor” states. It was found that such states may occur at border sites of our model cluster and that dissociative chemisorption may take place preferentially over a two-fold nickel site. The present results indicate that starting from this site, dissociation occurs following an essentially barrierless path. Changes in both electronic and binding properties along the path were studied in order to determine the relative importance of the several factors affecting chemisorption. In this respect, the role of d orbitals has been carefully analyzed with reference to the changes in density of states. The present results also indicate that after dissociation, the hydrogen atoms may easily move away from the border, into the surface, through low-barrier potential energy channels.     

LCAO studies of hydrogen chemisorption on nickel: I. Tight-binding calculations for adsorption on periodic surfaces

The model we have used to study hydrogen chemisorption on nickel surfaces is a tightbinding Extended Hückel method applied to finite (periodic) crystals up to about 250 atoms, the non-orthogonal basis set comprising five 3d orbitals, one 4s orbital and three 4p orbitals per atom. After calculating the band structure of fcc nickel, we have examined, by this model, the effect of the (100), (110) and (111) surfaces on the local density of states and the charge distribution. The results agree closely with moment calculations of the density of states in semi-infinite crystals and with experimental (XPS, UPS and INS) spectra. Extensive studies have been made of the influence of adsorption on the (partial) densities of states in order to illuminate the nature of the chemisorption bond. Particularly, we have concluded that both the 3d electrons and the conduction electrons take part in this bond. Equilibrium positions for adsorption on various sites have been determined and the adsorption energy has been computed and compared with experimental data. We find that the stability of adsorption decreases in the order (110) > (100) > (111) and Atop > Bridge > Centred.     

低温下Cu13团簇负载于Cu(001)表面上结构变化的分子动力学研究

应用分子动力学方法研究温度为10和50 K时具有二十面体结构的Cu₁₃团簇以不同接触条件与Cu（001）表面结合后的结构变化,原子间的相互作用势采用Johnson的嵌入原子方法模型.通过基于原子密度分布函数的分析表明,负载团簇与表面的结合能主要受团簇与载体相接触的最低层原子数及这些原子所具有的不同几何构型影响,同时更高层的原子呈现出不同的几何结构.温度为10 K时,负载团簇的初始位置对团簇几何结构和结合能影响较大. 关键词： 分子动力学 团簇 表面 计算机模拟     

无序二元合金（NixCu1－x）不同解理面上O吸附对Cu偏析的影响

应用计算机编程构造出了存在和不存在表面偏析的无序二元合金Ni_xCu_1-x (x=0.4)(１００)表面及(１１０)表面的原子集团模型，然后按覆盖度θ=0.5，构造 出了O吸附后的原子集团模型，应用Recursion方法计算了O在Ni_xCu_{1-x(存在偏析和不存在偏析时)无序二元合金(１００)和(１１０)表面吸附的电子结构.由此 得出：1)O吸附使合金表面态密度峰降低，带宽加宽，并且表面Ni原子的d电子与吸附质O原 子的s，p电子的共价作用比Cu更强烈；2)O吸附在NixCu1-x(x=0.4) (１１０)表面比(１００)表面更稳定；3)O的吸附抑制了Cu在表面富集，且这种作用主要表 现在表面一层. 关键词： 化学吸附 表面偏析 Recursion方法 态密度}     

A density functional theory study on the adsorption of CO and O2 on Cu-terminated Cu2O （111） surface

The adsorptions of CO and 02 molecules individually on the stoichiometric Cu-terminatcd Cu20 （111） surface are investigated by first-principles calculations on the basis of the density functional theory. The calculated results indicate that the CO molecule preferably coordinates to the Cu2 site through its C atom with an adsorption energy of-1.69 eV, whereas the 02 molecule is most stably adsorbed in a tilt type with one O atom coordinating to the Cu2 site and the other O atom coordinating to the Cul site, and has an adsorption energy of -1.97 eV. From the analysis of density of states, it is observed that Cu 3d transfers electrons to 2π orbital of the CO molecule and the highest occupied 5σ orbital of the CO molecule transfers electrons to the substrate. The sharp band of Cu 4s is delocalized when compared to that before the CO molecule adsorption, and overlaps substantially with bands of the adsorbed CO molecule. There is a broadening of the 2π orbital of the 02 molecule because of its overlapping with the Cu 3d orbital, indicating that strong 3d-2π interactions are involved in the chemisorption of the 02 molecule on the surface.     

Stabilities,electronic and magnetic properties of Cu-doped nickel clusters: a DFT investigation

In this paper, we investigate the electronic and magnetic properties of Cu-doped nickel clusters by means of density functional theory. The stabilities of these clusters have also been studied in terms of the binding energies, second-order difference of energies, fragmentation energies and HOMO–LUMO energy gaps. The obtained results reveal that the N₄Cu, N₅Cu and Ni₇Cu clusters are found to be more stable that than all other clusters. Higher HOMO–LUMO gap was observed for Ni₅Cu cluster (2.265 eV), indicating its higher chemical stability. A half-metallic behaviour has also been observed for the Ni_nCu clusters, which suggests that these clusters can be employed as nanocatalysts for several catalytic processes, particularly for hydrogenation and dehydrogenation reactions. The magnetism calculations show that the magnetic moment is mostly located on the Ni atoms, and the contribution of the Cu atom to the total magnetic moment in the Ni_nCu clusters is very small. Furthermore, partial density of states analysis indicates that the 3d orbitals in Ni atoms are mostly responsible for the magnetic behaviour of these clusters, and the s orbitals have a very little contribution to the total magnetic moment.     

用Xα-DV方法研究CO,NO在Ⅷ族过渡金属表面的化学吸附(Ⅰ)——CO吸附在Rh(111)面的电子结构

本文用自洽Hartree-Fock-Slater分子丛方法计算了CO在Rh(111)面上(θ≤1/3)的电子结构。计算了分子丛的总能量、基态能级随吸附高度的变化。从总能量曲线确定的最佳键长为1.85?与实验值1.95±0.1?符合得较好。相应的吸附能为0.98eV比实验值1.3eV略小。在以上最佳键长处计算了总态密度,考虑终态和弛豫效应后与UPS实验结果符合更好。通过CO分子接近表面时各分子轨道能量本征值的变化,讨论了各轨道的成键、反键特征。通过Mulliken总数分析和用CO分子波函数展开总波函数的系数分析着重讨论了CO分子被过渡金属Rh吸附前后的电荷转移。这种电荷转移导致被吸附CO分子的活化。 关键词：     

Ultraviolet photoelectron spectroscopy of Pd-Si glasses

In order to determine systematic changes in the density of states with alloy composition, photoelectron spectra at hv=21.2 eV were measured for several amorphous alloys based on the well-known Pd-Si glass system. Three binary alloys with 15, 20, and 25 at. % Si, two ternaries, Pd₈₀ Si₁₇ Cu₃ and Pd₈₀ Si₁₄ Cu₆, and polycrystalline Pd were analyzed. Compared to Pd, both the density of states at the Fermi energy and the d-band width are reduced in the glasses. The d-bands display an overall shift of 0.4 eV over the range of alloy compositions studied. Partial agreement with recent density of states calculations was obtained.     

Electronic structure and optical properties of RbPb2Br5

We report on density functional theory (DFT) calculations of the total and partial densities of states of rubidium dilead pentabromide, RbPb₂Br₅, employing the augmented plane wave+local orbitals (APW+lo) method as incorporated in the WIEN2k package. The calculations indicate that the Pb 6s and Br 4p states are the dominant contributors to the valence band: their main contributions are found to occur at the bottom and at the top of the band, respectively. Our calculations reveal that the bottom of the conduction band is formed predominantly from contributions of the unoccupied Pb 6p states. Data of total DOS derived in the present DFT calculations are found to be in agreement with the experimental X-ray photoelectron valence-band spectrum of this compound. The predominant contributions of the Br 4p states at the top of the valence band of rubidium dilead pentabromide are confirmed by comparison on a common energy scale of the X-ray emission band representing the energy distribution of the valence Br p states and the X-ray photoelectron valence-band spectrum of the RbPb₂Br₅ single crystal. Main optical characteristics of RbPb₂Br₅, such as dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity are explored for RbPb₂Br₅ by the DFT calculations.     

Structural and phase transformations during initial stages of copper condensation on Si(001)

Auger-electron spectroscopy, electron-energy loss spectroscopy, low-energy electron diffraction, and atomic-force microscopy are employed to investigate the growth mechanism, composition, structural and phase states, and morphology of Cu films (0.1–1 nm thick) deposited on a Si(001)-2 × 1 surface at a lower temperature of Cu evaporation (900°C) and room temperature of a substrate. The Cu film phase is shown to start growing on the Si(001)⁻² × 1 surface after three Cu monolayers (MLs) are condensed. It has been revealed that atoms of Cu and Si(001) are mixed, a Cu₂Si film phase is formed, and, thereafter, Cu₃Si islands arise at a larger coating thickness. Annealing of the first Cu ML leads to reconstruction of the Si(001)-1 × 1-Cu surface layer, thereby modifying the film growth mechanism. As a consequence, the Cu₂Si film phase arises when the thickness reaches two to four MLs, and bulk Cu₃Si silicide islands begin growing at five to ten MLs. When islands continue to grow, their height and density reach, respectively, 1.5 nm and 2 × 10¹¹ cm⁻² and the island area is 70% of the substrate surface at a thickness of ten MLs.     

Cu/N表面沉积共掺杂TiO_2光催化剂作用机理的理论研究

本文采用基于周期性密度泛函理论研究了Cu/N表面沉积共掺杂对锐钛矿相TiO2(001)面的修饰作用.计算了Cu在不同位置掺杂TiO2(101)面和(001)面的形成能,并在此基础上计算N不同位置掺杂TiO2(001)面及Cu/TiO2(001)面的形成能,通过形成能的比较获得了表面共掺杂的最优化结构.在此基础上计算了最稳定结构的能带结构及态密度,并与S单掺杂TiO2(001)面最稳定结构进行了对比.通过对结果的分析发现:Cu/N在(001)表面的沉积共掺杂有效降低了TiO2的禁带宽度,并在表面形成CuO2相,更利于提高其光催化活性.