Global optimization of bimetallic cluster structures. II. Size-matched Ag-Pd, Ag-Au, and Pd-Pt systems

Genetic algorithm global optimization of Ag-Pd, Ag-Au, and Pd-Pt clusters is performed. The 34- and 38-atom clusters are optimized for all compositions. The atom-atom interactions are modeled by a semiempirical potential. All three systems are characterized by a small size mismatch and a weak tendency of the larger atoms to segregate at the surface of the smaller ones. As a result, the global minimum structures exhibit a larger mixing than in Ag-Cu and Ag-Ni clusters. Polyicosahedral structures present generally favorable energetic configurations, even though they are less favorable than in the case of the size-mismatched systems. A comparison between all the systems studied here and in the previous paper (on size-mismatched systems) is presented.     

单原子分散的Au/Cu(111)表面合金的表面结构与吸附性质

Au-Cu双金属合金纳米颗粒对包括CO氧化和CO2还原等在内的多个反应有较好的催化活性,然而关于其表面性质的研究却相当匮乏。在此工作中,我们通过对低覆盖度的Au/Cu(111)和Cu/Au(111)双金属薄膜退火,制备出了单原子级分散的Au/Cu(111)和Cu/Au(111)合金化表面,并利用高分辨扫描隧道显微镜(STM)和扫描隧道谱(STS)进一步研究了掺杂原子的电子性质及其对CO吸附行为的影响。研究发现,分散在Cu(111)表面的表层和次表层Au单原子在STM上表现出不同衬度。在-0.5 e V附近,前者表现出相较于Cu(111)明显增强的电子态密度,而后者则明显减弱。吸附实验表明表层Au单原子对CO的吸附能力并没有得到增强,甚至会减弱其周围Cu原子的吸附能力。与Au在Cu(111)表面较好的分散相反,Cu原子倾向于钻入Au(111)的次表层,并且形成多原子聚集体。且Cu原子受Au(111)衬底吸电子作用的影响,其对CO的吸附能力明显减弱。这个研究结果揭示了合金表面的微观结构与性质的关联,为进一步阐明Au-Cu双金属催化剂的表面反应机理提供参考。     

Comparison of small metal clusters: Ni, Pd, Pt, Cu, Ag, Au

Structures of small clusters of Ni, Pd, Pt, Cu, Ag, and Au with n=2–34 and n=55 atoms are calculated as functions of number of atoms and temperature by the simulated annealing Monte Carlo method using an embedded atom potential.     

Structural optimization of Cu-Ag-Au trimetallic clusters by adaptive immune optimization algorithm

The putative global minimum structures of Cu-Ag-Au trimetallic clusters with 19 and 55 atoms are obtained by adaptive immune optimization algorithm (AIOA) with the Gupta potential. For the 19-atom trimetallic clusters, the results indicate that all of them have double-icosahedral motifs. For the optimized structures of Cu(13)Ag(n)Au(42-n) (n = 1-41), the clusters can be categorized into 19 Mackay icosahedral structures, 1 6-fold pancake structure, and 21 ring-like structures linked by three face-sharing double-icosahedra. Furthermore, the segregation phenomena of the Cu, Ag, and Au atoms in the Cu-Ag-Au trimetallic clusters are studied to provide useful information for geometric character. Results show that Cu and Ag atoms prefer to locate in the inner-shell and on the surface, respectively, whereas Au atoms mainly locate in the middle-shell and tend to solve into Cu and Ag atoms.     

Structural and energetic properties of Ni-Cu bimetallic clusters

The lowest-energy structures for all compositions of Ni n Cu m bimetallic clusters with N = n + m up to 20 atoms, N = 23, and N = 38 atoms have been determined using a genetic algorithm for unbiased structure optimization in combination with an embedded-atom method for the calculation of the total energy for a given structure. Comparing bimetallic clusters with homoatomic clusters of the same size, it is shown that the most stable structures for each cluster size are composed entirely of Ni atoms. Among the bimetallic clusters in the size range N = 2-20, the Ni N-1 Cu 1 clusters possess the highest stability. Further, it has been established that most of the bimetallic cluster structures have geometries similar to those of pure Ni clusters. The size N = 38 presents a special case, as the bimetallic clusters undergo a dramatic structural change with increasing atom fraction of Cu. Moreover, we have identified an icosahedron, a double, and a triple icosahedron with one, two, and three Ni atoms at the centers, respectively, as particularly stable structures. We show that in all global-minimum structures Ni atoms tend to occupy mainly high-coordination inner sites, and we confirm the segregation of Cu on the surface of Ni-Cu bimetallic clusters predicted in previous studies. Finally, it is observed that, in contrast to the bulk, the ground-state structures of the 15-, 16-, and 17-atom bimetallic clusters do not experience a smooth transition between the structures of the pure copper and the pure nickel clusters as a function of the relative number of the two types of atoms. For these sizes, the concentration effect on energy is more important than the geometric one.     

Density functional study of structural and electronic properties of small bimetallic silver-nickel clusters

Theoretical study on the structure and electronic properties of small AgmNip (m + p < or = 6) clusters has been carried out in the framework of density functional theory. Structural features, cohesive energies, vertical ionization potentials, and charge transfers are evaluated for each Ag/Ni ratio. In all the AgmNip clusters, the nickel atoms are brought together, yielding a maximum of Ni-Ni bonds, and the silver atoms are located around a Ni core with a maximum of Ag-Ni bonds. The ionization potential and the highest occupied molecular orbital shape are directly related to the two- or three-dimensional character of the cluster's geometry. A very low electronic charge transfer from Ni to Ag is found, and the magnetic moment is located on Ni atoms but with a low spin polarization on silver in the Ni-rich clusters.     

Sol-Gel Synthesis of Metal Nanoclusters-Silica Composite Films

In a program on the development of metal nanoclusters in sol-gel derived thin films, attempts were made to synthesize pure and mixed metal clusters, control the cluster size and increase the volume fract f the clusters. Thus, Ag, Cu and Ag-Cu nanoclusters were prepared in silica films using dip- and spin-coating techniques. The annealing of Ag/SiO₂ films in different atmospheres (air, argon and 5% H₂-95% N₂ gas) caused modifications of Ag nanoclusters resulting in changes in their surface plasmon resonance (SPR) peak positions. The Cu and Ag-Cu codoped films were annealed in reducing atmosphere (5% H₂-95% N₂ gas). In order to prepare Cu nanoclusters of different sizes, the concentrations of Cu in Cu/SiO₂ composite films were varied from 8 to 30 mol% and annealed at 800°C for different times for growth. The size of the Cu nanoclusters was measured from the half band width of Cu SPR peak (appearing within 570–557 nm range) and X-ray diffraction. In this way Cu-nanoclusters of size ranges from about 3.5 to 10 nm (average diameters) were prepared . The Ag-Cu nanocluster-containing silica films show the existence of both Ag and Cu SPR peaks with some blue shifting in comparison with to their pure analogues depending on the Ag:Cu ratio.     

Fabrication of Cu-Ag core-shell bimetallic superfine powders by eco-friendly reagents and structures characterization

Superfine bimetallic Cu-Ag core-shell powders were synthesized by reduction of copper sulfate pentahydrate and silver nitrate with eco-friendly ascorbic acid as a reducing agent and cyclodextrins as a protective agent in an aqueous system. The influence of Ag/Cu ratio on coatings was investigated. Ag was homogeneously distributed on the surface of Cu particles at a mole ratio of Ag/Cu=1. FE-SEM showed an uniformity of Ag coatings on Cu particles. Antioxidation of Cu particles was improved by increasing Ag/Cu ratio. TEM-EDX and UV-vis spectra also revealed that Cu cores were covered by Ag nanoshells on the whole. The surface composition analysis by XPS indicated that only small parts of Cu atoms in the surface were oxidized. It was noted that the hindrance of cyclodextrins chemisorbed on particles plays an important role in forming high quality and good dispersity Cu-Ag (Cu@Ag) core-shell powders.     

Au/Ag核一壳结构复合纳米粒子形成机制的研究

在已制备好的Au纳米粒子表面,通过化学还原的方法沉积生长Ag包覆层,通过 控制Au, Ag的比列,制备了粒度均匀且粒径可控的Au/Ag核-壳结构纳米粒子。利用 UV-vis吸收光谱和透射电子显微镜（TEM）对SAu, Ag摩尔比为1：10的复合纳米粒 子的光学性质和形态进行随时监测,直接观察了核-壳结构纳米粒子的生长过程： 一部分Ag+在Au核表面还原生长,溶液中其余Ag+还原形成银的纳米团簇向粒子表面 的继续沉积生长,壳层增厚。     

CO在M55（M=Cu,Ag,Au）团簇上吸附的密度泛函研究

在全电子相对论BVP86/DNP水平下对CO在Au55,Ag55和Cu55团簇上的吸附进行了比较研究,并考察了电荷对吸附的影响.计算结果表明,CO在Au55团簇上吸附能最大,其次为Cu55团簇,最弱的为Ag55团簇.团簇电荷对C—O键活化和CO与团簇表面原子成键影响较小.金团簇的电荷对吸附能影响较大,而银和铜团簇的电荷对吸附能影响较小.CO吸附到团簇上导致团簇上电子向CO转移.C—O键活化强度与吸附位置密切相关,其中孔位吸附导致C—O键活化程度最大,最弱的为顶位吸附.CO在金团簇上吸附具有较好选择性,而在银和铜团簇上吸附无选择性.     

O2 induced Cu surface segregation in Au-Cu alloys studied by angle resolved XPS and DFT modelling

Surface segregation effects on polycrystalline Au-Cu alloys (Au(0.80)Cu(0.20), Au(0.85)Cu(0.15) and Au(0.90)Cu(0.10)) were studied at room temperature by angle resolved XPS (ARXPS) and density functional theory (DFT) before and after exposure to O(2). Au surface enrichment was found as predicted from calculations showing that this process is energetically favourable, with a segregation energy for Au in a Cu matrix of -0.37 eV atom(-1). Surface enrichment with Cu was observed after exposure to O(2) due to its dissociative adsorption, in agreement with DFT calculations that predicted an energy gain of -1.80 eV atom(-1) for the transfer of Cu atoms to a surface containing adsorbed oxygen atoms, thus leading to an inversion in surface population.     

DFT studies of Pt/Au bimetallic clusters and their interactions with the CO molecule

Density functional theory (DFT) calculations were performed to study Pt/Au clusters of different size, structure, and composition as well as their interactions with a CO molecule. Among the Pt/Au isomers studied here, the planar structure is the most stable structure in many Pt compositions, although three-dimensional structures become more stable with increasing Pt composition. Furthermore, structures with the Pt atoms surrounded by Au atoms are more stable among homotops. However, these conclusions will be altered if ligands are attached to the Pt/Au bimetallic clusters, as evidenced from the results of CO adsorption. When both Au and Pt sites are exposed, CO adsorption at the Pt site is stronger. If only a Au site is available for CO adsorption, the strongest adsorption occurs at approximately 25% Pt composition, which may correlate with the experimentally observed reactivity of the core-shell structured Pt/Au nanoparticles.     

Observations on PVP-protected noble metallic nanoparticle deposits upon heating via in situ synchrotron radiation X-ray diffraction

Through monitoring the evolution of the X-ray diffraction peaks, the phase transformation of PVP-protected Ag and Au nanoparticle deposits (NPDs) on electronic substrates of Cu and Ni upon heating in air was investigated via in situ synchrotron radiation X-ray diffraction. With an increasing temperature, the broad diffraction peak of nano-sized Ag and Au particles with the original average diameters of 4.2 nm and 9.6 nm, respectively, became sharp because of particle coarsening and coalescence. Complex phase transitions among Au, Cu, AuCu(3) and CuO(x) were observed, mainly due to the negative enthalpy of mixing between Au and Cu. The interactions between NPDs and the substrates affected the shift of diffraction peaks to lower angles, caused by thermal expansion and also the temperature for the oxide formation. Compared to Au, Ag NPDs did not form intermetallic compounds with Cu and the formation of copper oxides can also be retarded mainly due to the phase separation feature of the Ag-Cu system.     

Electronic and structural shell closure in AgCu and AuCu nanoclusters

The structures of AgCu clusters containing 40 atoms are investigated. The most promising structural families (fcc clusters, capped decahedra, and two types of capped polyicosahedra) are singled out by means of global optimization techniques within an atom-atom potential model. Then, representative clusters of each family are relaxed by means of density-functional methods. It is shown that, for a large majority of compositions, a complex interplay of geometric and electronic shell-closure effects stabilizes a specific polyicosahedral family, whose clusters are much lower in energy and present large HOMO-LUMO gaps. Within this family, geometric and quantum effects concur to favor magic structures associated with core-shell chemical ordering and high symmetry, so that these clusters are very promising from the point of view of their optical properties. Our results also suggest a natural growth pathway of AgCu clusters through high-stability polyicosahedral structures. Results for AuCu clusters of the same size are reported for comparison, showing that the interplay of the different effects is highly material specific.     

Reactivity of the non stoichiometric Ni3O4 phase supported at the Pd(100) surface: interaction with Au and other transition metal atoms

In this work we have studied, employing ab initio periodic calculations, the interaction between the non-stoichiometric Ni(3)O(4) monolayer (a rhombic distribution of vacancies hereafter referred to as RH-Ni(3)O(4)) supported on the Pd(100) surface and several transition metal atoms (Ni, Cu, Pd, Pt, Ag, Au). The interaction produces a regular array of metal centers uniformly distributed in size and shape. According to the size of the atom and the ionization potential, the nature of the interaction ranges from an essentially electrostatic one to a polar-covalent one. The chemical reactivity versus the CO molecule of the overlayer resulting from the saturation with Au atoms of the Ni vacancies has been investigated.     

Use of helium nanodroplets for assembly, transport, and surface deposition of large molecular and atomic clusters

The utility of continuous beam of helium droplets for assembly, transport, and surface deposition of metal and molecular clusters is studied. Clusters of propyne having from about 10 to 10(4) molecules were obtained via sequential pickup of molecules by He droplets with average sizes in the range of 10(4)-10(7) atoms. The maximum attainable flux of the propyne molecules carried by He droplets was found to be in the range of (5-15)x10(15) molecules sr(-1) s(-1), being larger in larger droplets. The size of the clusters and the flux of the transported species are ultimately limited by the evaporative extinction of the entire helium droplet upon capture of particles. It is shown that the attenuation of the He droplet beam in the process of the cluster growth can be used in order to obtain the average size and the binding energy of the clusters. Furthermore, we used He droplets for assembling and surface deposition of gold and silver clusters having about 500 atoms. Typical deposition rate of metal atoms of about 3 x 10(15) atoms sr(-1) s(-1) is comparable to or larger than obtained with other beam deposition techniques. We propose that doping of He droplets by Au and Ag atoms in two separate pickup chambers leads to formation of the bimetal clusters having core-shell structure.     

Synthesis and spectroscopic characterization of Ag-Cu alloy nanoparticles prepared in various ratios

Ag-Cu alloy nanoparticles of various compositions were synthesized by the polyol process and characterized by UV-Visible, fluorescence and XRD techniques. The particle size calculated by Debye Scherrer's equation was found to decrease from 28 to18 nm with the increase in mole fraction of Cu in Ag-Cu alloy nanoparticles due to crystal lattice contraction. The appearance of the spectral peaks of alloy nanoparticles between the peaks of pure Ag and Cu revealed the formation of alloy nanoparticles. The optical properties were found to vary with composition and the Ag-Cu alloy of 1:1 composition showed a maximum value of extinction coefficient. The results of fluorescence spectroscopy revealed Cu as a quencher. Physical parameters, such as the total number of atoms in alloy nanoparticle, number of binding sites, binding constant and free energy of binding were calculated from fluorescence data.     

Bimetallic (Ag)Au nanoparticles prepared by the seed growth method: two-dimensional assembling, characterization by energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and surface enhanced raman spectroscopy, and proposed mechanism of growth

Layered core-shell bimetallic silver-gold nanoparticles were prepared by overdeposition of Au over Ag seeds by the seed-growth method using tetrachloroauric acid, with hydroxylamine hydrochloride as the reductant. The effects of pH, reduction rate, and seeding conditions on the morphology and surface plasmon extinction of the bimetallic nanoparticles were investigated. Nanoparticles prepared by a rapid reduction in the neutral ambient and assembled into two-dimensional nanoparticulate films by adsorption of 2,2'-bipyridine were characterized by energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, surface-enhanced Raman scattering spectroscopy, and transmission electron microscopy. The results are consistent with Ag core and Ag/Au-alloyed shell composition of the nanoparticles. Evidence of the presence of Ag on the surface of the nanoparticles, of enrichment of the Ag/Au alloy shell by Ag toward or at the nanoparticle surface, and of modification of the nanoparticle surface by adsorbed chlorides is also provided. Reduction of the size of the Ag seeds, alloying of Ag and Au in the shell of the nanoparticles, and modification of their surfaces by adsorbed chlorides are tentatively attributed to positive charging of the nanoparticles during the electrocatalytic overdeposition of Au over Ag seeds.     

Structure evolution of gold cluster anions between the planar and cage structures by isoelectronic substitution: Au(n)- (n = 13-15) and MAu(n)- (n = 12-14; M = Ag, Cu)

The structural and electronic effects of isoelectronic substitution by Ag and Cu atoms on gold cluster anions in the size range between 13 and 15 atoms are studied using a combination of photoelectron spectroscopy and first-principles density functional calculations. The most stable structures of the doped clusters are compared with those of the undoped Au clusters in the same size range. The joint experimental and theoretical study reveals a new C(3v) symmetric isomer for Au(13)(-), which is present in the experiment, but has hitherto not been recognized. The global minima of Au(14)(-) and Au(15)(-) are resolved on the basis of comparison between experiment and newly computed photoelectron spectra that include spin-orbit effects. The coexistence of two isomers for Au(15)(-) is firmly established with convincing experimental evidence and theoretical calculations. The overall effect of the isoelectronic substitution is minor on the structures relative to those of the undoped clusters, except that the dopant atoms tend to lower the symmetries of the doped clusters.     

Evolution of the structural stability of large Cu,Ni, Pd,and Ag clusters with size: An analysis within the embedded atom method

A study of the structural stability of clusters made up of a single component has been carried out within the Embedded Atom Method. Perfect icosahedral and cuboctahedral Cu, Ni, Pd, and Ag clusters with up to 5083 atoms have been compared. The icosahedron is found to be the stable structure for small clusters, and a change of structure from icosahedral to cuboctahedral is found as the cluster size increases. A contraction of the interatomic distances results when the cluster size decreases.