Dumbbell to Core–Shell Structure Transformation of Ni–Au Nanoparticle Driven by External Stimuli

Conversion of CO₂ gas to CO fuels is one of the most promising solutions for the increasing threat of global warming and energy crisis. The efficient catalyst Ni–Au dumbbell converting CO₂ into CO at elevated temperatures has high CO product selectivity; however, the accompanied atomic diffusion and subsequent surface reconstruction affect the catalytic efficiency of chemical reaction. Atomic scale characterization of structural evolution of the catalyst, which is essential to correlate the functional mechanism to active catalyst surfaces, is yet to be studied. Here, in situ transmission electron microscopy experiments and atomistic simulations are performed to characterize the structural evolution of Ni–Au dumbbell nanoparticles under two different external stimuli. In the condition of high temperature and vacuum, the Ni–Au nanostructure reveals a clear shape reconstruction from the initial dumbbell to core–shell‐like, which is induced by capillary force to minimize free surface energy of the system. The shape transformation involves two stages of processes, initial fast Au diffusion followed by slow source‐controlled diffusion. At ambient temperature, the combination of CO₂ and electron flux surprisingly induces analogous structural transformation of Ni–Au nanostructure, where the associated chemical reaction and CO absorption stimulate the Au migration on Ni surface. Such surface reconstruction can be widely present in catalytic reactions in different environmental conditions, and the results herein demonstrate the detailed processes of Ni–Au structure evolution, which provide important insights for understanding the catalyst performance.     

Growth and characterization of Au,Ni and Au–Ni nanoclusters on 6H-SiC(0001) carbon nanomesh

The nucleation and thermal stability of Au, Ni, and Au–Ni nanoclusters on 6H-SiC(0001) carbon nanomesh as well as the interaction between Au–Ni bimetallic clusters and reactive gases have been studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Both Au and Ni atoms grow as three-dimensional (3D) clusters. Annealing the Au/carbon nanomesh surface up to 1150 °C leads to complete desorption of the Au clusters, while interfacial reaction occurs between Ni clusters and the substrate surface when the Ni clusters are subjected to the same annealing process. The nucleation of Au–Ni clusters depends critically on the deposition sequence. Au atoms preferentially nucleate on the existing Ni clusters, leading to the formation of bimetallic clusters with Au enriched on the surface. If the deposition sequence is reversed, a part of Ni atoms nucleate between the Au clusters. The thermal stability of the Au–Ni clusters resembles that of the Ni/carbon nanomesh surface, irrespective of the deposition sequence. XPS characterization reveals that Ni atoms in Au–Ni bimetallic clusters are oxidized upon exposure to 5.0 × 10^? 7 mbar O₂ for 5 min at room temperature while negligible structure change can be detected when the bimetallic clusters are exposed to CO gas under the similar conditions.     

The role of phase separation for self-organized surface pattern formation by ion beam erosion and metal atom co-deposition

We investigate the ripple pattern formation on Si surfaces at room temperature during normal incidence ion beam erosion under simultaneous deposition of different metallic co-deposited surfactant atoms. The co-deposition of small amounts of metallic atoms, in particular Fe and Mo, is known to have a tremendous impact on the evolution of nanoscale surface patterns on Si. In previous work on ion erosion of Si during co-deposition of Fe atoms, we proposed that chemical interactions between Fe and Si atoms of the steady-state mixed Fe _x Si surface layer formed during ion beam erosion is a dominant driving force for self-organized pattern formation. In particular, we provided experimental evidence for the formation of amorphous iron disilicide. To confirm and generalize such chemical effects on the pattern formation, in particular the tendency for phase separation, we have now irradiated Si surfaces with normal incidence 5 keV Xe ions under simultaneous gracing incidence co-deposition of Fe, Ni, Cu, Mo, W, Pt, and Au surfactant atoms. The selected metals in the two groups (Fe, Ni, Cu) and (W, Pt, Au) are very similar regarding their collision cascade behavior, but strongly differ regarding their tendency to silicide formation. We find pronounced ripple pattern formation only for those co deposited metals (Fe, Mo, Ni, W, and Pt), which are prone to the formation of mono and disilicides. In contrast, for Cu and Au co-deposition the surface remains very flat, even after irradiation at high ion fluence. Because of the very different behavior of Cu compared to Fe, Ni and Au compared to W, Pt, phase separation toward amorphous metal silicide phases is seen as the relevant process for the pattern formation on Si in the case of Fe, Mo, Ni, W, and Pt co-deposition.     

液态三元Fe-Sn-Si/Ge偏晶合金相分离过程的实验和模拟研究

本文采用自由落体实验技术和格子玻尔兹曼计算方法研究了低重力条件下液态Fe-Sn-Si/Ge合金的相分离过程. 实验发现, 二种合金液滴在自由下落过程中均发生显著的液相分离, 形成了壳核和弥散组织. 当Fe-Sn-Si合金中的Si元素被等量的Ge元素替换后, 壳核组织中富Fe区和富Sn区的分布次序会发生反转. 计算表明, 在液相分离过程中冷却速率、Marangoni对流和表面偏析对壳核构型的选择和弥散组织的形成起决定性作用.     

Backscattering correction for quantitative Auger analysis: II. Verifications of the backscattering factors through quantification by AES

The validity and utility of the backscattering correction factors obtained from Monte Carlo calculations for quantitative analysis by Auger electron spectroscopy (AES) were examined through practical quantification of surface concentrations of binary alloys. Quantifications were attempted, first, to access the surface composition of a sputter-deposited NiPt layer, which is probably the most appropriate test-sample with known surface composition for surface analysis. The quantification by AES has led to the result that the surface composition of the layer agrees well with the bulk composition of the sputtered NiPt alloy, as expected. The composition of a sputtered AuCu alloy surface was, then, examined according to the same correction procedure as for the NiPt layer, leading to the confirmation that no preferential sputtering is observed for AuCu alloys by AES as Färber et al. reported.     

Estimation of surface tension for high temperature melts of the binary Ag-X alloys

ABSTRACT

Surface tension is a key property to materials. In this work, the surface tension of the binary alloys Ag-X (Au, Cu, Ce, Bi, Sn, Sb, In, Ni, Y, Pd) is carried out by using Butler Model over enter composition ratio at a certain temperature. According to calculation results, the increasing surface tension of the Ag-X (Cu, Au, Ni, Y, Pd) alloy is accompanied by the composition increases. For Ag-Sn alloy, the surface tension calculated by Butler model is consistent with the experimental result at temperature 1273?K. However, other Ag-X alloys can’t be compared due to the lack of the related experimental data. Although the experimental data about surface tension of the Ag-X alloy are limited, we are possible to make a comparison between the calculated results for the surface tension in this study and the available experimental data. Taken together, the surface tension calculated by Butler model that especially the Ag-Sn alloy are consistent with the experimental results at temperature 1273?K.     

Admetal-induced substrate surface restructuring during metal-on-metal electrochemical deposition studied by in situ scanning tunneling microscopy

Based on time-dependent in situ scanning tunneling microscopy (STM) studies, we demonstrate that for Ni on Ag(111) and Ru on Au(111), electrochemical metal-on-metal deposition can result in pronounced substrate surface restructuring. For Ni/Ag(111), we observe that at low deposition flux and low coverage, Ni submonolayer islands at steps are partly embedded in the Ag terraces, whereas at higher deposition flux and higher coverage, substrate restructuring results in the formation of monolayer bays in the Ag terraces. We suggest that this restructuring process proceeds predominantly via step edge diffusion of Ag atoms. For Ru/Au(111), the formation of fjords and monolayer holes in the Au terraces is observed at low and high Ru coverage, respectively. The importance of the Au surface mobility for the restructuring process is demonstrated by comparing experiments in H₂SO₄ and HCl solutions, in which Au exhibits strongly different surface mobilities. For this system, restructuring involves Au diffusion along Au steps, Au atom detachment from the Au steps, and upward exchange diffusion. According to these observations and their comparison with similar findings for vacuum deposition, we conclude that this restructuring requires (i) a high substrate surface mobility and (ii) a stronger bonding of substrate atoms to deposit islands than to the substrate.     

Chemisorption of carbon monoxide on copper-nickel alloy surfaces

The adsorption of CO on Cu-Ni alloy surfaces has been studied at 300 and 120 K using LEED, AES, TDS, and work function measurement. The alloys have been prepared as thin (111) epitaxial films evaporated on mica, and as poly crystalline foils. At 300 K the alloy surfaces show an adsorption behavior similar to that of pure Ni: the work function increases to a saturation value which is higher for Ni-rich surfaces than for Cu-rich. The isosteric heat of adsorption (106 kJ/mole) is nearly as high as with pure Ni. At 120 K the alloys exhibit a more Cu-like adsorption behavior: the work function passes through a minimum which becomes deeper at higher Cu surface concentration. The isosteric heat of adsorption at low temperatures (50 kJ/mole) is nearly as low as for pure Cu. From TDS it can be shown, that the binding energy of the highest (Ni-like) adsorption states increases with increasing Ni surface concentration. At the (111) alloy surfaces no LEED superstructures due to CO adsorption could be observed.     

Segregation at the surface of an Au/Pd alloy exposed to CO

We use density functional theory (DFT) with the generalized gradient approximation (GGA) and the revised Perdew-Burke-Ernzerhoff (rPBE) functional, to study the surface composition of the (1 1 1) and (1 0 0) dilute Pd/Au alloy. We find that the energy of Pd atoms is lower when they substitute an Au atom in the bulk than when they substitute an Au atom in the surface layer, or when they are adsorbed on the surface. Whether they are in the surface layer or in the bulk, the Pd atoms interact very weakly with each other. CO adsorbs on the Pd atom in the surface layer and the energy of this complex is lower than that of CO in gas and Pd atom in the bulk. The interaction between the PdCO complexes formed when CO adsorbs on a Pd atom imbedded in the surface layer, is also negligible. We use these energies, equilibrium thermodynamics, and a simple lattice-gas model to examine the equilibrium composition of the surface layer, as a function of temperature, CO pressure and the Pd/Au ratio. We find that the surface Pd concentration for a nanoparticle of an Au/Pd alloy differs from that in a bulk sample. The difference is due mainly to the fact that in a nanoparticle the migration of Pd atoms to the surface depletes the bulk concentration while in a large sample; the bulk provides an infinite source of Pd atoms to populate the surface sites. This system is of interest because Pd/Au alloys are selective catalysts for vinyl acetate synthesis when the Pd concentration on the surface is very low.     

First-principles investigations of the Ni3Sn alloy at steam reforming conditions

The structure and surface composition of a Ni₃Sn alloy at conditions relevant for the steam reforming reaction was investigated using density functional theory calculations. Both the flat Ni₃Sn(0 0 0 1) surface and a surface with steps in the closed packed direction [1 0  0]were considered. The adsorption geometries and energies of the species CO, C, OH and H were calculated. Chemical potentials were used to map out which adsorbates are on the surface under varying conditions. It was found that adsorbates preferably bind to Ni as nearest neighbor with Sn as second-nearest neighbor. The binding energy is slightly stronger than on pure Ni. Adsorbate binding to Sn was found to be very unfavorable. Binding free energies indicate that at high temperature the alloy surface will be predominantly covered by CO and C, and at low temperatures one may find H and almost no OH. Even though the nominal composition of the investigated alloy is Ni₃Sn, the surface composition may differ significantly depending on temperature and pressure of the gas phase. This effect was investigated by calculating segregation energies both in the absence and in the presence of adsorbates. For the flat surface, it was found that only the bulk termination is present under relevant conditions. In contrast, it was found that for steps preferential adsorption of CO and C on Ni sites may lead to adsorption-induced segregation at temperatures below 400 °C. When taking segregation into account, the most stable Ni₃Sn surfaces will not bind CO or C at the same condition that Ni does. This is in excellent agreement with the previously proven ability of Ni-Sn alloys to inhibit graphite formation.     

强流脉冲电子束表面改性CuFe10合金的显微组织和性能研究

研究了不同脉冲次数强流脉冲电子束表面改性对CuFe10合金组织及性能的影响。强流脉冲电子束处理CuFe10合金的重熔表面出现了火山坑和直径为100 nm到1 m的富铁球,表明了强流脉冲电子束处理CuFe10合金表面发生了液相分离。强流脉冲电子束脉冲轰击30次后,CuFe10合金表面的显微硬度与耐蚀性能均得到显著改善,主要是由于强流脉冲电子束轰击处理CuFe10合金表层引发的快速熔凝过程中表面发生了液相分离及晶粒细化的缘故。     

Adsorption an Einkristall-Oberflächen von Cu/Ni-legierungen. I

The adsorption of O₂ and CO on the (110) face of a Cu/Ni alloy (55 at% Cu) has been studied by means of low energy electron diffraction (LEED), Auger electron spectroscopy, work function measurements, and flash desorption. A comparison with the behavior of Cu(110) and Ni(110) is made. It is shown that the height of an Auger peak is proportional to the surface concentration of the corresponding species and that the surface composition of the alloy is identical with the composition of the bulk. Adsorption of oxygen leads to the formation of an ordered 2 × 1 structure, as is the case for Cu(110) and Ni(110). Further exposure causes disordered adsorption in contrast to the pure components where c6 × 2 respectively 3 × 1 structures are formed. Oxygen increases the work function of Cu and Cu/Ni by about 0.25 eV whereas for Ni the increase is > 1 eV. CO is not irreversibly adsorbed on Cu at 25°C, but forms a stable 1 × 1 structure on Ni(110). With the alloy two ordered phases (2 × 1 and 2 × 2) are observed. The flash desorption spectrum shows three maxima which are similar to the binding states of CO on Ni(110) and Ni(100). The results are discussed in view of the electronic structure of Cu/Ni alloys and the parameters influencing the configuration of adsorbed particles.     

GaN基紫光LED的高反射率p型欧姆接触

研究用于GaN基大功率倒装焊(Flip-chip)紫光LED(UV-LED)的高反射率p型欧姆接触的电学和光学性能。用磁控溅射的方法在GaN基LED外延片表面沉积了不同厚度Ag,Al,Au和Pd四种金属,测量了样品的反射率和透射率。结合同步辐射高强度X射线衍射和AFM对金属薄膜的晶体结构进行分析,并对表面形貌进行了观测,对由金属薄膜构成的多层膜结构及其对光反射率的作用机理进行了研究。测量结果表明,在入射光波长为400nm时,Ni/Au/Ag和Ni/Au/Al电极的反射率比Ni/Au的反射率提高了三倍。同时与p-GaN有良好的欧姆接触特性。     

Competition between order and phase separation in Au-Ni

We have measured and theoretically analyzed the diffuse scattering in the binary alloy system Au-Ni, which has been proposed as a testing ground for theories of alloy phase stability. We found strong evidence that in the alloys Au3Ni and Au3Ni2, fluctuations of both ordering- and clustering-type are competing with each other. Our results resolve a long-standing controversy on the balance of relaxation and mixing energies in this alloy system and explain recent findings of ordering in thin Au-Ni films.     

Au80Sn20合金焊料制备工艺

针对高功率二极管激光器的封装要求,通过磁控溅射的方法制备了Au80Sn20合金焊料,使用扫描电子显微镜（SEM）观察其微结构和表面形貌;利用能谱仪（EDX）和X射线荧光测试仪分析其成分;采用差热分析法（DTA）测试其熔化温度,并用制备的Au80Sn20合金焊料进行了可焊性实验。结果表明:磁控溅射法可以制备Au80Sn20合金焊料,其制备的Au80Sn20合金焊料表面无明显缺陷,结构致密;成分与理论值接近;熔点与理论熔点接近;焊接浸润性好,空洞率小,强度大。     

“Long” and “short” range electronic effects in the bonding of CO and H2 on CuNi alloy

The thermal desorption data of CO from the CuNi alloys cannot be explained by a local binding site picture alone. The CO activation energy of desorption from a Ni binding site decreases linearly with increased surface Cu concentration indicating the presence of a ligand (long range electronic) effect. In contrast, the H₂ activation energy of desorption is constant with alloy composition.     

Temperature induced effects in the coadsorption system: CO/D2/Ni(100)

The temperature dependent changes in the CO/D/Ni(100) system have been examined using thermal desorption spectroscopy, work function change measurements, ultra-violet photoelectron spectroscopy and X-ray photoelectron spectroscopy. Three forms of CO(a) have been found, one of which exists only in the intermediate temperature range of 130–215 K. This form is believed to be a CO(a) tilted with respect to the surface normal, with significant ONi interaction. In addition, a new interpretation of several UPS parameters measured for CO(a) is presented. It is proposed that for CO(a), the 5σ?1π separation is a probe of bonding geometry, the 4σ?1π separation reflects COCO repulsion, and the $\text{4σ}\text{5σ}$ intensity ratio reflects the NiCO bond strength.     

Interfacial segregation in Ag-Au,Au-Pd,and Cu-Ni alloys: I. (100) surfaces

Atomistic simulations of segregation to (100) free surface in Ag–Au, Au–Pd, and Cu–Ni alloy systems have been performed for a wide range of temperatures and compositions within the solid solution region of these alloy phase diagrams. In addition to the surface segregation profiles, surface free energies, enthalpies, and entropies were determined. These simulations were performed within the framework of the free energy simulation method, in which an approximate free energy functional is minimized with respect to atomic coordinates and atomic site occupation. The effects of the relaxation with respect to either the atomic positions or the atomic concentrations are discussed. For all alloy bulk compositions (0.05 C 0.95) and temperatures (400 T(K) 1,100) examined, Ag, Au, and Cu segregates to the surface in the Ag–Au, Au–Pd, and Cu–Ni alloy systems, respectively. The present results are compared with several theories for segregation. The resultant segregation profiles in Au–Pd and Ag–Au alloys are shown to be in good agreement with an empirical segregation theory, while in Cu–Ni alloys the disagreement in Ni-rich alloys is substantial. The width of the segregation profile is limited to approximately three to four atomic planes. The surface thermodynamic properties depend sensitively on the magnitude of the surface segregation, and some of them are shown to vary linearly with the magnitude of the surface segregation.     

Synthesis of NiAu alloy and core–shell nanoparticles in water-in-oil microemulsions

NiAu alloy nanoparticles with various Ni/Au molar ratios were synthesized by the hydrazine reduction of nickel chloride and hydrogen tetrachloroaurate in the microemulsion system. They had a face-centered cubic structure and a mean diameter of 6–13 nm, decreasing with increasing Au content. As Au nanoparticles did, they showed a characteristic absorption peak at about 520 nm but the intensity decreased with increasing Ni content. Also, they were nearly superparamagnetic, although the magnetization decreased significantly with increasing Au content. Under an external magnetic field, they could be self-organized into the parallel lines. In addition, the core–shell nanoparticles, Ni₃Au₁@Au, were prepared by the Au coating on the surface of Ni₃Au₁ alloy nanoparticles. By increasing the hydrogen tetrachloroaurate concentration for Au coating, the thickness of Au shells could be raised and led to an enhanced and red-shifted surface plasmon absorption.