Bonding trends and dimensionality crossover of gold nanoclusters on metal-supported MgO thin films

Bonding of gold clusters, , 16, and 20, on MgO(100) and on thin MgO films supported on Mo(100) is investigated using first-principles density-functional theory. Enhanced adhesive bonding is found for clusters deposited on metal-supported MgO films of thickness up to about 1 nm, or 4 to 5 MgO layers, originating from electrostatic interaction between the underlying metal and metal-induced excess electronic charge accumulated at the cluster interface with the oxide film. The increased wetting propensity is accompanied by a dimensionality crossover from three-dimensional optimal cluster geometries on MgO(100) to energetically favored two-dimensional structures on the metal-supported films.     

First-principles studies on initial growth of Ni on MgO(0 0 1) surface

To elucidate the initial growth of metal on oxide surface, we studied adsorption of small nickel clusters, Ni_n (n = 1-5), on MgO(0 0 1) surface using first-principles method based on density-functional theory. It was found that the preferential adsorption site for an isolated Ni atom is directly above the surface oxygen atom. A strong covalent bond with partial ionic character is formed between the Ni adatom and the surface oxygen atom. Various structures were considered for the Ni_n isomers and 3D structures were found to be energetically more stable than 2D structures for clusters of more than two atoms. For the 2D clusters, metal-metal bonds prevail over metal-substrate bonds with increasing Ni coverage. The calculated work function and ionization energy were found to vary with Ni coverage which is attributed to the change of the surface dipole moment upon metal adsorption, while the evolution of Schottky barrier height at the initial growth stage is dominated by the adatom-induced gap states.     

Charging of metal atoms on ultrathin MgO/Mo(100) films

The chemical activity of supported metal nanoclusters is enhanced by electronic charging induced by the interaction with surface defects. We use density functional theory plane wave calculations to show that charging of metal atoms with high electron affinity like Au is possible also in the absence of defects by atom deposition on ultrathin MgO films (1 to 3 layers) grown on Mo(100). The Au 6s level falls below the Fermi level of Mo, leading to electron transfer from Mo to Au by direct tunneling through the insulating MgO film. The effect is not observed for Pd, whose 5s empty level falls just above the Fermi level of Mo, or for thicker MgO films.     

Effects of O defects on adsorption of small Ag clusterson a MgO(001) surface

The interaction of Ag atoms with a defective MgO(001) surface is systematically studied based on density functional theory. The Ag clusters are deposited on neutral and charged oxygen vacancies of the MgO(001) surface. The structures of Ag clusters take the shape of simple models of two- or three-dimensional (2D and 3D) metal particles deposited on the MgO surface. When the nucleation of the metal clusters occurs in the F_s (missing neutral O) centre, the interaction with the substrate is considerably stronger than that in the F_s⁺ (missing O^- ) centre. The results show that the adsorption of Ag atoms on the MgO surface with oxygen vacancy is stronger than on a clear MgO surface, thereby attracting more Ag atoms to cluster together, and forming atomic islands.     

A molecular dynamics study of the structural change differences between Au225 and Au369 clusters on MgO surfaces at low temperature

The differences in structural change between Au₂₂₅ and Au₃₆₉ clusters with their (111) facets supported on MgO(100) surfaces at 5 K are studied by using molecular-dynamics simulations with the atomic interchange potentials of the Au/MgO interface. The parameters are obtained from the ab initio energies using the Chen-Möbius inversion method. Analyses of the pair distribution functions show that the two Au clusters use different deformation processes to adjust the distances between the interface atoms, owing to the misfit between the atom distances among the clusters and the substrates. The local structural changes are identified by atomic density profiles.     

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.     

水在金属氧化物表面的吸附与解离

水(H2O)由H原子和O原子组成.地球上有大量的水,若能找到一种经济、实用的方法将H2O解离生成H2和O2,则在新能源的开发和应用方面,意义深远.水在固体表面的吸附现象极为普遍,在某些金属或金属氧化物表面,H2O被吸附并解离成OH-和H+.文章以有序氧化镁(MgO(100))薄膜和Pd/MgO(100)体系为例,在超高真空条件下,用光电子能谱和高分辨电子能量损失谱方法,研究了水在它们表面的吸附与解离.研究结果表明,H2O在MgO(100)表面可以被部分解离,而H2O在Pd/MgO(100)表面的解离与Pd的含量有关.了解水与固体表面的相互作用机理还需要做更多的基础研究工作.     

First-principles studies: Thiolated Au2Cr and Au6Cr clusters

The structural, energetic and magnetic properties of thiol-passivating Au₂Cr and Au₆Cr clusters are investigated by performing first-principles calculation based on density functional theory. We find that the adsorption of thiolate is energetically more favorable than thiol and that the thiolates favor “top” site adsorption on Cr atom in one methanethiolate adsorbed Au₂Cr cluster while they favor “bridge” site adsorption on top of the middle point of Cr–Au bonds in three methanethiolates adsorbed Au₂Cr cluster. In thiol-passivating Au₆Cr cluster, the thiol favor “top” site adsorption on top of atom Au while the thiolate favor “bridge” site adsorption on top of the middle point of Au–Au bonds. The energetics of the reactions indicates that these thiol-passivating Au₂Cr or Au₆Cr clusters can be used as hydrogen storage materials. There are large and positive spin populations on atom Cr. The spin populations of atoms Au, S and H are attributed to both the spin delocalization and the spin polarization mechanisms.     

利用FePt/Au多层膜结构制备垂直磁记录L10-FePt薄膜

利用磁控溅射方法在100℃的MgO单晶基片上制备了［FePt/Au］₁₀多层膜,并研究了采用FePt/Au多层膜结构对FePt薄膜的有序化温度、矫顽力（H_C）、垂直磁各向异性、晶粒尺寸以及颗粒间磁交换耦合作用的影响.磁性测试结果表明：FePt/Au多层膜在退火后具有较高的H_C、良好的垂直磁各向异性、较小的晶粒尺寸且无磁交换耦合作用.截面高分辨电镜分析表明：Au可以缓解MgO和FePt之间较大的晶格错配,从而促进薄 关键词： 0-FePt薄膜')" href="#">L1₀-FePt薄膜 有序化温度 垂直磁各向异性 磁交换耦合作用     

CO interaction with Au atoms adsorbed on terrace, edge and corner sites of the MgO(1 0 0) surface. Electronic structure and vibrational analysis from DFT

The interaction of CO with Au atoms adsorbed on terrace and low-coordinates sites (edge and corner) of the MgO(1 0 0) surface was studied using the density functional theory (DFT) in combination with embedded cluster models. Surface anionic (O²⁻) and neutral oxygen vacancy (F_s) sites were considered. In all the cases, the CO stretching frequencies are shifted with respect to free CO with values between −232 and −358 cm⁻¹. In particular, the values for Au on F_s at edge and corner are shifted to higher stretching frequencies by 100 and 59 cm⁻¹, respectively, with respect to the value on a perfect MgO(1 0 0) surface. This result is in agreement with recent scanning tunneling microscopy and infrared spectroscopy experiments where a corresponding shift of 70 cm⁻¹ was observed by comparing the measurements on perfect and O-deficient MgO(1 0 0) surfaces. However, these results are different than expected because Au atoms on F_s centers are negatively charged and, therefore, according to the generally accepted scheme the CO frequency should be red-shifted with respect to the adsorption on anionic five-coordinated site where the Au atom is essentially neutral. The following picture emerges from the present results: the single occupied HOMO(α) of Au atom on F_s at low-coordinated sites consists in two lobes extended sideward the Au atom. For symmetry reasons, this MO overlaps efficiently with the 2π^∗ MO of CO. This bonding contribution to the Au-CO link is counteracted by a Pauli repulsion between the 5σ MO of CO and more internal orbitals (the HOMO-1(α) and the HOMO(β)) centered on Au. In consequence, CO is forced to vibrate against a region with a high electron density. This is the so-called “wall effect” which by itself contributes to higher CO frequency values.     

Charging of metal adatoms on ultrathin oxide films: Au and Pd on FeO/Pt(111)

We present a combined experimental (STM/scanning tunneling spectroscopy) and theoretical (density functional theory) study on the deposition of Au and Pd metal atoms on FeO/Pt(111) ultrathin films. We show that while the Pd atoms are only slightly oxidized, the Au atoms form positive ions upon deposition, at variance to a charge transfer into the Au atoms as observed for MgO/Ag(100). The modulation of the adsorption properties within the surface Moiré cell and the charging induce the formation a self-assembled array of gold adatoms on FeO/Pt(111), whereas Pd atoms are randomly distributed.     

Charging effects on the vibrational properties of Au and Au2 on MgO(100)

Vibrational spectra of charge-neutral and charged Au and Au₂ on MgO(100) were investigated using ab-initio density functional perturbation theory. The calculated vibrational spectra showed vibrational features associated with the charge states of Au and Au₂ on MgO(100). Further analyses of surface in-plane and normal phonon modes of Au and Au₂ on MgO(100) were performed to extract vibrational features involving the Au modes. These features provide important information for experimentally explaining the charge states of Au and Au₂ on MgO(100).     

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).     

CO adsorption on Rh, Pd and Ag atoms deposited on the MgO surface: a comparative ab initio study

The adsorption properties of CO molecules adsorbed on Rh, Pd, and Ag atoms supported on various sites of the MgO surface have been studied by means of a density functional cluster model approach. The metal atoms are stabilized with different binding energies on the regular and morphological defect sites of the surface. Among others we considered oxide anions, neutral and charged anion vacancies (F centers) located at terraces, steps, edges, and corners. CO is used as a probe molecule to characterize where the metal atoms are located. This is done by analyzing how the metal-CO binding energy and the C-O stretching frequency change as function of the substrate site where the metal atom is bound.     

The configuration and electronic state of SO3 adsorbed on Au surface

We evaluated the adsorption of SO₃ molecule on Au (1 1 1) surface using first principles calculation by a slab model with a periodic boundary condition. We find that there are six stable adsorption configurations on an Au surface, where the SO₃ molecule is adsorbed above the three-fold fcc and hcp hollow sites and on the atop site. In two of these configurations, S and two O atoms are bound to the Au atoms, the next two configurations have all the three O atoms bound to the Au surface atoms, and the last two configurations have the S atom bound to an Au surface atom on the atop site and O atoms situated above the hollow sites. In these configurations, the electronic structures of SO₃ on the Au surface show that molecular orbitals of SO₃ and those of the Au surface are hybridized in the active metal d-band region, that the localized molecular orbitals in SO₃ are stabilized, and that charge is transferred from Au to S 3p by SO₃ adsorption on the Au surface though there is little other interaction of the S and O (bound to Au) component with Au. Moreover, the bond between the S and O atoms bound to Au is weakened due to SO₃ adsorption on the Au surface due to the charge polarization of the O-Au bond. This interaction is likely to encourage the S-O bond to break.     

Formation of hydroxyl and water layers on MgO films studied with ambient pressure XPS

To understand the interaction of water with MgO(100), a detailed quantitative assessment of the interfacial chemistry is necessary. We have used ambient pressure X-ray photoelectron spectroscopy (XPS) to measure molecular (H₂O) and dissociative (OH) water adsorption on a 4 monolayer (ML) thick MgO(100)/Ag(100) film under ambient conditions. Since the entire 4 ML metal oxide (Ox) film is probed by XPS, the reaction of the MgO film with water can be quantitatively studied. Using a multilayer model (Model 1) that measures changes in Ox thickness from O 1s (film) and Ag 3d (substrate) spectra, it is shown that the oxide portion of the MgO film becomes thinner upon hydroxylation. A reaction mechanism is postulated in which the top-most layer of MgO converts to Mg(OH)₂ upon dissociation of water. Based on this mechanism a second model (Model 2) is developed to calculate Ox and OH thickness changes based on OH/Ox intensity ratios from O 1s spectra measured in situ, with the known initial Ox thickness prior to hydroxylation. Models 1 and 2 are applied to a 0.15 Torr isobar experiment, yielding similar results for H₂O, OH and Ox thickness changes as a function of relative humidity.     

A molecular dynamics study of the structural change differences between Au225 and Au369 clusters on MgO surfaces at low temperature

The differences in structural change between Au 225 and Au 369 clusters with their(111) facets supported on MgO(100) surfaces at 5 K are studied by using molecular-dynamics simulations with the atomic interchange potentials of the Au/MgO interface.The parameters are obtained from the ab initio energies using the Chen-Mo¨bius inversion method.Analyses of the pair distribution functions show that the two Au clusters use different deformation processes to adjust the distances between the interface atoms,owing to the misfit between the atom distances among the clusters and the substrates.The local structural changes are identified by atomic density profiles.     

Theoretical study of interaction between atoms of Au, Ag, Cu and clean Si(1 1 1) surface

To evaluate the interactions between the atoms of Au, Ag and Cu and clean Si(1 1 1) surface, two types of silicon clusters Si₄H₇ and Si₁₆H₂₀ together with their metal complexes were studied by using hybrid (U)B3LYP density functional theory method. Optimized geometries and energies on different adsorption sites indicate that: (1) the binding energies at different adsorption sites are large (ranging from 1.2 to 2.6 eV depend on the metal atoms and adsorption sites), suggesting a strong interaction between metal atom and silicon surface; (2) the most favorable adsorption site is the on top (T) site. Mulliken population analysis indicated that in the system of on top (T) site, a covalent bond is formed between metal atom and dangling bond of surface Si atom.     

Geometric,stability, and electronic properties of gold-doped Pd clusters (Pd<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>Au, <Emphasis Type="Italic">n</Emphasis> = 3~20)

The structure, stability, and electronic properties of Pd _n Au (n = 3~20) clusters are studied by density functional theory. The results show that the clusters studied here prefer three-dimensional structures even with very small atom number. It is found that the binding energies of Pd _n Au clusters are higher than the corresponding pure Pd _n clusters with the same atom number. Most Pd _n Au clusters studied here are magnetic with magnetic moments ranging from 1.0 to 7.0 μ _B. The dissociation energies of Pd atoms are lower than the doped gold atom, that is the doped Au atom will increase the mother clusters stability and activity.