Energy barriers of single-adatoms diffusion on unreconstructed and reconstructed (110) surfaces

The present paper is aimed mainly to investigate theoretically the diffusion of Ag, Cu, Au and Pt adatoms on the (1 × 1) unreconstructed geometry for Ag, Cu and Pt (110), and reconstructed geometries ((1 × 2), (1 × 3) and (1 × 4)) for Pt and Au (110) surfaces. We consider the single adatom diffusion when additional atoms are deposited in adjacent row. For this study, we have used the molecular statics simulations combined with the embedded atom method. For several systems, we have calculated the activation barriers for hopping mechanism. For the diffusion on the unreconstructed surfaces, the trends for the activation barriers are the same for all considered systems except for Cu/Ag (110) system, where the activation barrier do not change. Further, our results indicate that additional atoms lead to a small decreasing of activation barriers for diffusion on reconstructed surfaces for some systems, while for other systems; the activation barrier remains practically unchanged.     

An efficientmethod to determine chemical potential of mixtures in the isothermal and isobaric bulk phase with kineticMonte Carlo simulation

A kinetic Monte Carlo (kMC) scheme in the NPT ensemble (constant number of molecules, pressure and temperature) has been developed to determine accurate chemical potentials for all components in a homogeneous mixture. The simulation requires two moves: (1) a displacement move and (2) a volume change move. In the former, the mobility rate of a selected molecule is determined by its interaction with all the other molecules in the system and is moved to a random position within the simulation box, according to the Rosenbluth algorithm, without any rejections (entropic sampling). The volume change move is decided by a comparison between either the instant pressure or the partial average pressure (with long-range correction) and the specified pressure and is carried out much less frequently than the displacement move. We applied this NPT scheme to a number of mixtures in both the gaseous and liquid phases, and show that the derived chemical potentials are accurate and reproducible. The method is recommended for obtaining chemical potentials in mixtures that are required as input in a grand canonical ensemble simulation.     

Monitoring the transitions of the charge-induced reconstruction of Aau(110) by reflectance anisotropy spectroscopy

Missing-row reconstructions on Au(110) immersed in electrolytes have been studied by in situ reflectance anisotropy spectroscopy. Transitions between the 1 x 3, 1 x 2, and 1 x 1 surface structures were monitored as a function of the applied potential. A kinetic model allowed us to reproduce the data satisfactorily. These results confirm the theoretical predictions showing that the surface charge determines the surface reconstruction. The transition potentials and the activation barriers were determined.     

高指数晶面Au-Pd纳米合金粒子的稳定结构研究

基于蒙特卡罗方法, 本文采用了紧束缚势和量子修正Sutton-Chen型多体势两种势能函数对具有不同比例、不同尺寸二十四面体Au-Pd合金纳米粒子的稳定结构、表面原子分布、核壳分布和化学短程序值进行了研究分析. 结果表明: 两种势函数得到的表面原子分布趋势一致, 即Au-Pd合金纳米粒子中的Au原子趋向于分布在纳米粒子的外层, 而Pd原子趋向于分布在纳米粒子的内层, 这有利于降低纳米粒子的总能; 在Au原子比例较小时, 两种势函数下得到的稳定结构均呈现出核壳分离的结构, 随着Au比例的增大, 紧束缚势函数下得到的纳米粒子稳定结构将趋向于洋葱状的多壳层的结构; 相比于紧束缚势, 量子修正Sutton-Chen型多体势作用下得到的Au-Pd纳米粒子的稳定结构偏聚程度更高.     

Atomistic structure of stepped surfaces

Atomistic computer simulation with embedded atom method (EAM) interatomic forces was used to study the structure of surface steps on the {111} unreconstructed surface in fcc metallic materials. The energetics and local atomic relaxation behavior of ledges parallel to the 110 direction were studied using a potential describing lattice properties of Au. The vacancy formation energies in the stepped surfaces was also studied, and it was found that the energy of formation of a vacancy in a terrace is the same as that in the perfect unstepped surface. This value is 30% lower than that of the bulk. The vacancy formation energy in the ledge is reduced by a factor of two with respect to that of the terraces. The structure of the “up ledge” (A step) is different from the “down ledge” (B step). These differences do not significantly affect the energy of the ledges, although they do affect the vacancy formation energies in sites in the second surface layer near the ledge. The implications of the results for the formation of kinks and the general structure of high index surfaces are discussed.     

DFT calculations for Au adsorption onto a reduced TiO2 (110) surface with the coexistence of Cl

Residual chlorines, which originate from HAuCl₄, enhance the aggregation of gold (Au) nanoparticles and clusters, preventing the generation of highly active supported Au catalysts. However, the detailed mechanism of residual-chlorine-promoted aggregation of Au is unknown. Herein to investigate this mechanism, density functional theory (DFT) calculations of Au and Cl adsorption onto a reduced rutile TiO₂ (110) surface were performed using a generalised gradient approximation Perdew, Burke, and Ernzerhof formula (GGA–PBE) functional and plane-wave basis. Although both Au and Cl atoms prefer to mono-absorb onto oxygen defect sites, Cl atoms have a stronger absorption onto a reduced TiO₂ (110) surface, abbreviated as rTiO₂ (110) in the following, than Au atoms. Additionally, co-adsorption of a Cl atom and a Au atom or Au nanorod onto a rTiO₂ surface was investigated; Cl adsorption onto an oxygen defect site weakens the interaction between a Au atom or Au nanorod and rTiO₂ (110) surface. The calculation results suggest that the depletion of interaction between Au and rTiO₂ surface is due to strong interaction between Cl atoms at oxygen defect sites and neighbouring bridging oxygen (O^B) atoms.     

Role of Au(+) in supporting and activating Au(7) on TiO(2)(110)

The adhesion properties and catalytic activity of rutile TiO(2)(110)-supported Au(7) nanoclusters in different oxidation states are investigated by means of density functional theory. The calculations cover both surface science conditions of reduced TiO(2) and real catalyst conditions of oxidized (alkaline) TiO(2) supports. Large adhesion energies of Au(7) are found only when modeling real catalysts where the cluster becomes cationic with Au(+) ions in Au-O or Au-OH bonds. The full catalytic cycle for oxidation of CO by O(2) over Au(7) on alkaline TiO(2)(110) is calculated and found to involve only small activation barriers. In the presence of the CO reductant, the Au(+) sites are capable of cycling between bonding of atomic and molecular oxygen. We confirm our findings by comparison of calculated and experimental infrared stretch frequency data for adsorbed CO.     

Au在Zr掺杂的CeO2(110)面吸附的第一性原理研究

采用基于广义梯度近似的投影缀加平面波(projector augmented wave)雁势和具有三维周期性边界条件的超晶胞模型,用第一性原理方法,计算并分析了Au在CeO₂(110)和Zr掺杂的CeO₂(110) 面的吸附能,吸附结构和电子结构等特征.从而得出Zr掺杂对Au/CeO₂(110)吸附体系的影响.结果表明：Zr的掺杂增大了Au在CeO₂(110) 面的吸附能,并改变了最强吸附位置,且导致了吸附体系中衬底结 关键词： Au Zr掺杂 2')" href="#">CeO₂ 吸附     

Influence of the metal substrate on the adsorption properties of thin oxide layers: Au atoms on a thin alumina film on NiAl(110)

Thin oxide films grown on metal substrates are widely used in surface science to model bulk oxides, assuming their chemical and electronic properties to be similar. In some cases, however, this might not be justified as the present scanning tunneling microscopy studies demonstrate for Au atoms on a thin alumina film on NiAl(110). Au atoms were evaporated onto the oxide film at a sample temperature of approximately 10 K. At low coverage, this leads to the formation of one-dimensional clusters with unusually large Au-Au distances of 5.6-6.0 A. A direct interaction between the Au atoms can be excluded, and a substrate-mediated mechanism is supposed instead. This assumption is strengthened by the finding that the Au chains exhibit a preferential orientation: They are almost aligned with the [001] direction of the NiAl(110) substrate, clearly indicating that the metal substrate participates in the binding of the Au atoms.     

Electrocompression of the Au(111) surface layer during Au electrodeposition

In situ grazing-incidence x-ray diffraction studies of reconstructed Au(111) electrodes in aqueous electrolyte solutions are presented, which reveal a significantly increased compression of the Au surface layer during Au electrodeposition as compared to Au(111) surfaces under ultrahigh vacuum conditions or in the Au-free electrolyte. The compression increases towards more negative potentials, reaching 5.3% at the most negative potentials studied. It may be explained within a simple thermodynamic model by a release of potential-induced surface stress.     

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.     

Au(110)表面结构和氧原子吸附的第一性原理研究

用密度泛函理论(DFT)研究了金属Au(110)表面结构以及氧原子的吸附状态.计算得到Au(110)-(1×2)缺列再构表面原子的弛豫分别是-15.0%(Δd₁₂/d₀)和-1.1%(Δd₂₃/d₀),表面能为52.7 meV/²,功函数Φ=5.00 eV;Au(110)-(1×3)缺列再构表面的Δd_{1 关键词： 缺列再构Au(110)表面 STM图像 氧原子吸附}     

A Monte Carlo study of the thermal properties of Cu3Au low index surfaces

The thermal dependencies of composition and order of the (111), (100) and (110) Cu₃Au surfaces are studied at the atomic scale by means of Monte Carlo simulations in the “transmutational” ensemble at constant volume. The question addressed is the extent to which such simulations carried on with a model N-body potential designed on the basis of bulk energetic and mechanical properties allow predictions consistent with experimental observations of the surface. Although the currently available experimental data still leave unanswered questions, many of them allow for comparison with modelling. Qualitative agreement is found for temperature dependencies of both surface composition and order, and the simulation results are discussed in detail. Some clear discrepancies are found as well, in particular (but not only) in the case of the (110) surface and its first neighbouring layer. Although the origin of such differences is not yet clear, it is suggested that they may serve to assess and to improve the model in the light of quantitative surface studies.     

低温下Au959团簇负载于MgO(100)表面后结构变化的分子动力学研究

采用基于Chen-Mbius反演方法,从金属/金属氧化物界面第一性原理计算的粘结能结果中推导出的Au/MgO原子间相互作用势的正则系综(NVT)分子动力学,模拟了在10 K条件下,Au959团簇负载于MgO(100)表面后团簇结构的变化.根据原子对分析技术和对分布函数的分析表明,由于团簇界面处原子间距与载体原子间距相匹配,置于载体上的Au团簇经过一个变形过程后,较其孤立自由表面时的团簇体积变大.     

A calculation of surface diffusion coefficients of adsorbates on the (110) plane of tungsten

An attempt to calculate the prefactors of the diffusion coefficients of tungsten, xenon and oxygen atoms on the W(110) plane in the zero-coverage limit was made, using a Fokker-Planck formalism. Pairwise substrate-substrate and adsorbate-substrate atom interactions were assumed and expressed by appropriate Morse potentials. The vibrations of the substrate were approximated by an Einstein model of independent oscillators. When Morse potentials were so adjusted as to give good agreement between experimental and calculated activation energies, a prefactor in excellent agreement with experiment was calculated for W/W(110). For Xe/W(110) and O/W(110) the calculated values were ～10^?4 cm²/s, i.e. essentially “normal”, while the experimental values are much lower. Possible reasons for the discrepancies are discussed.     

Programming configurational changes in systems of functionalised polymers using reversible intramolecular linkages

ABSTRACT

We study polymers functionalised by complexes forming intramolecular linkages. Simulations of chains forming reversible linkages are difficult due to entropic barriers that hamper the sampling of different connectivity states specified by the list of pairs of reacted complexes. We address this problem by devising Monte Carlo (MC) moves that change the connectivity state of the system by regrowing parts of the chain while simultaneously reacting bond-forming complexes. Along with moves that link/unlink pairs of complexes, we develop two types of bond-exchange moves. We use these algorithms to study self-assembly of single chain polymeric nanoparticles. When considering monofunctional precursors, we find branched and linear nanoparticle morphologies dominated by long and short intramolecular loops, respectively, along with hierarchical structures in which complexes belonging to different loops are cross-linked. In the strong association limit, equilibrium structures are only reached when using bond-exchange MC moves. We also consider bifunctional precursors in which two different types of complexes decorate the two halves of the chain. We find different types of morphologies featuring different amounts of linkages between complexes of different types. Such findings corroborate our method as a valuable tool to design and predict self-assembly of functional polymers.     

Stability of gold nanostructures on rutile TiO2(110) surface

The adsorption of gold atoms and formation of nanostructures on the rutile TiO₂(110) surface with different degree of oxygen reduction was studied from first principles. The Au atoms adsorb strongest at oxygen vacancy sites. Starting from a very low coverage limit the potential energy profiles or diffusion paths of the adsorbed Au monomers and dimers were calculated. Stable structures of two to nine Au atoms arranged in finite and infinite rows and in the shape of finite-size clusters were determined. All these structures are found to bind to the reduced surface stronger than 2 eV/atom. The elongated Au row-like structures bind by about 0.1 eV stronger than 3D clusters, suggesting a preference for the 1D-like Au growth mode on the missing-row reconstructed TiO₂(110).     

Predicting XAFS scattering path cumulants and XAFS spectra for metals (Cu,Ni, Fe,Ti, Au) using molecular dynamics simulations

The ability of molecular dynamics (MD) simulations to support the analysis of X‐ray absorption fine‐structure (XAFS) data for metals is evaluated. The low‐order cumulants (ΔR, σ², C₃) for XAFS scattering paths are calculated for the metals Cu, Ni, Fe, Ti and Au at 300 K using 28 interatomic potentials of the embedded‐atom method type. The MD cumulant predictions were evaluated within a cumulant expansion XAFS fitting model, using global (path‐independent) scaling factors. Direct simulations of the corresponding XAFS spectra, χ(R), are also performed using MD configurational data in combination with the FEFFab initio code. The cumulant scaling parameters compensate for differences between the real and effective scattering path distributions, and for any errors that might exist in the MD predictions and in the experimental data. The fitted value of ΔR is susceptible to experimental errors and inadvertent lattice thermal expansion in the simulation crystallites. The unadjusted predictions of σ² vary in accuracy, but do not show a consistent bias for any metal except Au, for which all potentials overestimate σ². The unadjusted C₃ predictions produced by different potentials display only order‐of‐magnitude consistency. The accuracy of direct simulations of χ(R) for a given metal varies among the different potentials. For each of the metals Cu, Ni, Fe and Ti, one or more of the tested potentials was found to provide a reasonable simulation of χ(R). However, none of the potentials tested for Au was sufficiently accurate for this purpose.     

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.