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.     

Transmission of low-energy (< 10 eV) O ions through Au films on TiO2(110)

In an attempt to identify the fundamental processes that influence ion transport through metallic surface layers, we have studied the transmission of O⁺ ions through discontinuous Au films adsorbed on TiO₂(110). A low energy (< 10 eV) O⁺ ion beam is generated via electron stimulated desorption when an Au-dosed TiO₂(110) substrate is bombarded with a focused 250 eV electron beam. Low energy ion scattering data indicate that Au evaporated under ultrahigh vacuum conditions at 300 K forms three-dimensional clusters on TiO₂(110). As the Au coverage increases, the formation of Au clusters on TiO₂(110) blocks a fraction of the TiO₂ surface and the O⁺ yield is attenuated. However, for high coverages (≥30% Au covered substrate) the O⁺ signal decreases at a faster rate than the TiO₂ open area fraction. We attribute the attenuation of the O⁺ yield for high Au coverages mainly to blocking of O⁺ by Au clusters, to deflection of trajectories by the image force between ions and Au clusters, and to charge transfer between desorbing O⁺ and neighboring Au clusters.     

Field emission spectroscopy of gold on the (110) and (211) planes of tungsten

Adsorption of Au up to monolayer amounts on the (110) plane of a tungsten field emitter leads to a decrease in work function, and also to a decrease in effective emitting area. Energy distributions of field emitted electrons show no structure attributable to Au. These results can be explained by postulating that there is no tunneling from Au atoms or from beneath Au atoms. The most probable reason for this is that in (110) tunneling matrix elements from outward normal tungsten d-orbitals are stronger than the tunneling matrix element from the non-directional 6s Au orbital. Experiments on the (211) plane show behavior similar to that previously found on (111), i.e. a work function increase but strong emission from Au atoms.     

Synchrotron-Radiation Photoemission Study of Growth and Stability of Au Clusters on Rutile TiO2(110)-1×1

利用同步辐射高分辨光电子能谱研究了金团簇在部分还原TiO₂-(1×1)表面的生长和稳定性．价带谱实验结果观察到非常少量金团簇的沉积导致了Ti3+的3d峰完全消失,表明金团簇成核在TiO₂-(1×1)表面的氧缺陷位．Au4f芯电子光电子能谱实验结果证明了TiO₂-(1×1)表面氧缺陷位向金团簇转移电荷．还对比研究了化学剂量比和部分还原的TiO₂-(1×1)表面上金团簇的热稳定性．当金团簇尺寸相近时部分还原的TiO₂-(1×1)表面上金团簇要比化学剂量比的TiO₂-(1×1)面上金团簇稳定;在相同的表面上尺寸大的金团簇要比尺寸小的金团簇稳定．     

Photoemission spectroscopy of the strong-coupling superconducting transitions in lead and niobium

Scanning tunneling microscopy on roughened Au(110) reveals that the equilibrium shape of islands and pits on this surface is almondlike: each island contains two smoothly curved steps joined at two sharp corners. This shape has recently been predicted and finds its origin in the missing-row reconstruction of its fcc (110) surfaces (Au, Pt, etc.). We use the corner angles and the island shapes to determine the step energies. In addition we find that during the decay of an island on the Au(110) surface the shape changes and that the disappearance of the island involves the splitting of the layer below the island into two disconnected regions. The shape change has a dramatic influence on the decay rate of the islands.     

Bonding of gold nanoclusters to oxygen vacancies on rutile TiO2(110)

Through an interplay between scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we show that bridging oxygen vacancies are the active nucleation sites for Au clusters on the rutile TiO2(110) surface. We find that a direct correlation exists between a decrease in density of vacancies and the amount of Au deposited. From the DFT calculations we find that the oxygen vacancy is indeed the strongest Au binding site. We show both experimentally and theoretically that a single oxygen vacancy can bind 3 Au atoms on average. In view of the presented results, a new growth model for the TiO2(110) system involving vacancy-cluster complex diffusion is presented.     

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}     

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     

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.     

CEMS analysis of ferromagnetic interfaces

In continuation of previous work on the Fe(110)/Ag interface, an experimental analysis of hyperfine interactions, with true monolayer resolution, is performed for interfaces of Fe(110) with Cu, Au and W(110), using in situ CEMS analysis of ultrathin epitaxial films of Fe(110) on W(110), containing appropriate probe layers of Fe-57.     

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.     

Step patterns on vicinal reconstructed surfaces

Step patterns on vicinal (2 × 1) reconstructed surfaces of noble metals Au(110) and Pt(110), miscut towards the (100) orientation, are investigated. The free energy of the reconstructed surface with a network of crossing opposite steps is calculated in the strong chirality regime when the steps cannot make overhangs. It is explained why the steps are not perpendicular to the direction of the miscut but form in equilibrium a network of crossing steps which make the surface to look like a fish skin. The network formation is the consequence of competition between the — predominantly elastic — energy loss and entropy gain. It is in agreement with recent scanning tunnelling microscopy observations on vicinal Au(110) and Pt(110) surfaces.     

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.     

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.     

Nanoscale probing of adsorbed species by tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) is based on the optical excitation of localized surface plasmons in the tip-substrate cavity, which provides a large but local field enhancement near the tip apex. We report on TERS with smooth single crystalline surfaces as substrates. The adsorbates were CN- ions at Au(111) and malachite green isothiocyanate (MGITC) molecules at Au(111) and Pt(110) using either Au or Ir tips. The data analysis yields Raman enhancements of about 4 x 10(5) for CN- and up to 10(6) for MGITC at Au(111) with a Au tip, probing an area of less than 100 nm radius.     

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.     

Surface diffusion of Pd and Au on W single crystal planes: I. Spreading behaviour of Pd and Au layers

In two subsequent publications we report on the diffusion behaviour of Pd and Au on W(110) and stepped W(110) vicinals. The first part deals with the concentration profiles obtained by Pd(Au) spreading from linear sources and recorded by a scanning Auger microscope (SAM). The diffusion experiments were carried out in the temperature range 600–900°C and revealed the formation of Pd(Au) layers of distinct coverages ($\text{1}\text{2}$,1,$\text{3}\text{2}$, 2 monolayers) during the spreading process. Whereas on the W(110) plane essentially a Pd monolayer phase proceeds with a sharp boundary and a square root of time dependence, the concentration profiles observed on the stepped W(10 9 0), W(650) and W(750) surfaces exhibit somewhat rounded shapes. The spreading behaviour can be qualitatively rationalized by considering the gain in binding energy upon the formation of Pd-W bonds and strong lateral Pd(Au)-Pd(Au) attractive interactions. The activation energy for Pd diffusion deduced from the temperature dependence of the spreading rate amounts to 40–45 kcal/mole. In the case of the stepped surfaces this energy applies to both spreading directions parallel and perpendicular to the steps. The preexponential factor however differs by one order of magnitude favouring diffusion along the steps. Combined diffusion experiments involving Pd and Au diffusion simultaneously reveal that Au is the faster moving species. The following paper reports the eminent role of step edges as diffusion paths and dwells on the anisotropic diffusion behaviour caused by regular step structures.     

Anomalous proximity effect in gold coated (110) YBa2Cu3O7-delta films: penetration of the Andreev bound states

Scanning tunneling spectroscopy of (110)YBa(2)Cu(3)O(7-delta)/Au bilayers reveal a proximity effect markedly different from the conventional one. While proximity-induced mini-gaps rarely appear in the Au layer, the Andreev bound states clearly penetrate into the metal. Zero bias conductance peaks are measured on Au layers thinner than 7 nm with a magnitude similar to those detected on the bare superconductor films. The peaks then decay abruptly with Au thickness and disappear above 10 nm. This length is shorter than the normal coherence length and corresponds to the (ballistic) mean free path.     

Quantum-size effects in the electronic structure of low-dimensional metallic systems

By angle-resolved photoemission the electronic structure of quantum films of Mg, Ag, and Au has been compared on W(110) and Mo(110) substrates which are structurally and electronically very similar but differ in atomic number. In all cases, substrate-induced states with characteristic dispersions are observed in the region of a bulk band gap of the substrate. Based on the comparison between Mo and W, we can exclude that previously observed Mg states are spin-orbit split but observe a spin-orbit splitting in Ag and Au monolayers. This splitting is mainly caused by the substrate because it does not differ much between Ag and Au overlayers despite the large difference in atomic number.     

Atomic exchange mean field study of intermixing of Au on Ag(110)

A mean field method to study heteroepitaxial thin film growth is applied to growth, intermixing and surface reconstructions of Au on Ag(110). The results are in accordance with experimentally observed “burrowing” at submonolayer coverages and growth of elongated, (1 × 3) reconstructed, Au clusters at higher coverages. At coverages of a few monolayers the surface between the clusters has a high concentration of Ag, and ordered rows of Au are formed just beneath the surface.