小尺寸金属团簇熔化过程的分子动力学模拟

运用分子动力学方法模拟了小尺寸金属团簇的熔化过程, 原子之间的作用采用嵌入原子法(EAM)模型, 计算了均方根键长涨落δ随温度的变化, 以及升温过程中团簇热容的变化. 包含55、56个原子的面心立方(FCC)结构Au团簇的熔化过程是基本相同的. 而同样结构和数目Cu团簇的熔化过程却呈现出不同的趋势. Cu55、Cu56在模拟过程中都出现了FCC结构到二十面体结构的转变. 但由于表面多出了一个原子, Cu56的热容曲线比Cu55多了一个峰, 体系出现了预熔化现象. 这表明小尺寸团簇的固液转变的过程与团簇的原子类型、几何结构和原子数目密切相关.     

Pt-Au合金团簇热力学性质分子动力学模拟研究

采用分子动力学方法和原子嵌入法模型势模拟了Pt原子和Au原子合金纳米团簇的熔化过程,研究了这些金属原子纳米团簇熔点与团簇组分的关系,发现不同组分纳米团簇的熔点不是单调变化的,同时均出现了负热容现象.通过对各种团簇溶化前后结构的比较研究,分析了导致这种现象的原因.     

中介尺度Au纳米团簇熔化的分子动力学模拟

采用分子动力学模拟技术，研究了原子个数为16～8628的 Au纳米团簇的熔化过程.采用 Johnson的EAM (embedded atom method) 模型,模拟结果表明，金属纳米团簇存在一中介尺度区域.对Au纳米团簇而言，当原子个数N >456时，团簇的热力学性质与团簇尺寸呈线性关系，熔化首先从表面开始，逐步向中心区域推进，且满足Tmb－Tmc(N)＝aN(－1/3)的关系.另外，计算了中介区域的团簇的尺寸、熔化温度、表面能、熵、焓等热力学量以及均方根位移（RMSD）等动力学量，为研究纳米团簇提供定量数据.     

Aun(n=2-9)团簇与乙醇分子相互作用的第一性原理研究

利用密度泛函理论研究了Au_n(n=2-9)团簇吸附一个乙醇分子的结构和电子性质. 研究结果表明: Au_n(n=2-9)团簇的最稳定构型为二维平面结构, Au6团簇最稳定; 吸附过程是通过金团簇上一个特定的金原子与乙醇分子中氧原子相互作用完成, 形成了20种稳定构型; 金原子的配位数对吸附作用影响明显; 作为吸附主体的金团簇和被吸附的乙醇分子在吸附前后构型无明显变化, 它们之间为弱相互作用.     

Au12M (M=Na,Mg, Al,Si, P,S, Cl)团簇的结构、稳定性和电子性质

采用基于密度泛函理论的第一性原理方法系统地研究了Au12M(M=Na,Mg,Al,Si,P,S,Cl)团簇的结构、稳定性和电子性质.对团簇的平均结合能、镶嵌能、垂直离化势、最高占据分子轨道(HOMO)和最低未占据分子轨道(LUMO)的能级差、电荷布居分析、自然键轨道(NBO)进行了计算和讨论.对于Au12M(M=Na,Mg,Al)团簇,它们形成了内含M原子的最稳定的笼状结构.然而对于Au12M(M=Si,P,S,Cl)团簇,它们却形成了以M元素为顶点的稳定锥形结构.在这些团簇中发现Au12S团簇相对是最稳定的,这是由于Au12S团簇形成了稳定的满壳层的电子结构.自然电荷布居分析表明:对于所有的Au12M(M=Na,Mg,Al,Si,P,S,Cl)团簇电荷总是从Au原子转向M原子.自然键轨道和HOMO分析表明Au12M团簇中发生了Au原子的s-d轨道和M原子的p轨道间的杂化现象.     

Au_(5~25)Pd团簇结构与性质的理论研究

用basin-hopping算法结合密度泛函PBE方法系统搜索了Au5~25Pd团簇的全局稳定结构,并对团簇几何结构,稳定性,Fermi能级,化学硬度和前线轨道进行了分析。计算结果表明,Au5Pd,Au7Pd和Au11Pd团簇为平面结构,其他团簇均为立体结构,与相同原子数纯金团簇结构类似。所有团簇中,Pd原子均位于配位数较高位置。团簇平均结合能随金原子数增大而逐渐增大,并有收敛到某点的趋势。偶数金原子团簇较相邻的奇数金原子团簇稳定。团簇的Fermi能级随团簇增大呈奇偶振荡,偶数金原子团簇的Fermi能级较相邻奇数金原子团簇的低,与金团簇Fermi能级变化类似。Au7Pd,Au12Pd,Au16Pd和Au18Pd团簇化学活性较高。Au5~19Pd团簇中Pd原子优先与CO,烯烃,炔烃等分子形成配位键。CO等小分子仍然吸附到Au20~25Pd团簇的顶点或面中心的金原子。     

Au@Gen (n=2-9)团簇的几何结构和电子性质的理论研究

本文选用密度泛函B3LYP方法在Lan L2DZ基组上对Au Gen+(n=2~9)团簇的几何结构和电子性质进行了理论研究,其中包括结构优化、平均键能、HOMO-LUMO能隙和电荷转移等。结果表明,随着锗原子数的不断增加,这些掺杂团簇逐渐形成了三维立体结构,并发现Au Ge7+和Au Ge9+两个掺杂团簇是相对稳定的,而且这些掺杂团簇的电荷转移主要是由金原子到锗原子骨架上。此外,还模拟了这些掺杂团簇的红外光谱,为以后实验研究提供有价值的理论参考。     

单原子催化剂的制备、表征及催化性能

单原子催化剂(SACs)是指金属以单原子形式均匀分散在载体上形成的具有优异催化性能的催化剂.与传统载体型催化剂相比,SACs具有活性高、选择性好及贵金属利用率高等优点,在氧化反应、加氢反应、水煤气变换、光催化制氢以及电化学催化等领域都具有广泛应用,是目前催化领域的研究热点之一.常见的SACs制备方法有共沉淀法、浸渍法、置换反应法、原子层沉积法以及反奥斯瓦尔德熟化法等.实验及理论研究表明,单原子催化剂高的活性和选择性可归因于活性金属原子和载体之间的相互作用及由此引起的电子结构改变.载体是影响单原子催化剂性能的重要因素之一.目前常用的SACs载体有金属氧化物、二维材料和金属纳米团簇等,本文着重综述了这三种负载型SACs的制备、表征、催化性能及催化机理,并概述了SACs未来可能的发展方向和应用.研究表明,共沉淀法、湿浸渍法和反奥斯瓦尔德熟化法等方法可用来制备氧化物负载的SACs.高角环形暗场像-扫描透射电子显微镜(HAADF-STEM)表明金属是以单原子形式均匀分散在载体上,近边X射线吸收精细结构(XANES)结果表明金属原子与载体之间存在着强相互作用.实验和理论研究均表明该类催化剂在CO氧化反应、水煤气转化及乙炔加氢生成乙烯等反应中具有高的催化活性和稳定性.采用化学气相沉积法和原子层沉积法等方法可以将金属原子稳定地负载在具有缺陷活性位点的石墨烯、MXene及六方氮化硼等二维材料上并相应制备出SACs.X射线吸收精细结构谱(EXAFS)和XANES分析表明样品中金属以单原子形式存在,而且金属原子与载体之间也存在着强相互作用,理论计算表明金属原子与二维载体之间的电荷转移是SACs活性高的主要原因.置换反应法和连续还原法是制备溶胶型SACs的有效方法,其中置换反应法可将活性金属原子原位组装在金属模板团簇的顶点位置,连续还原法可将活性原子负载于金属模板团簇的表面.DFT计算表明活性原子和金属模板团簇之间存在电荷转移效应,这是溶胶型SACs具有非常高的催化活性的主要原因.SACs下一步的研究方向可能是:(1)研究开发新型SACs,尽可能提高催化剂中活性金属原子的含量;(2)深入研究SACs的结构、活性以及催化机理之间的关系;(3)尝试将SACs大规模应用于工业催化.     

单原子催化剂的制备、表征及催化性能（英文）

单原子催化剂(SACs)是指金属以单原子形式均匀分散在载体上形成的具有优异催化性能的催化剂.与传统载体型催化剂相比,SACs具有活性高、选择性好及贵金属利用率高等优点,在氧化反应、加氢反应、水煤气变换、光催化制氢以及电化学催化等领域都具有广泛应用,是目前催化领域的研究热点之一.常见的SACs制备方法有共沉淀法、浸渍法、置换反应法、原子层沉积法以及反奥斯瓦尔德熟化法等.实验及理论研究表明,单原子催化剂高的活性和选择性可归因于活性金属原子和载体之间的相互作用及由此引起的电子结构改变.载体是影响单原子催化剂性能的重要因素之一.目前常用的SACs载体有金属氧化物、二维材料和金属纳米团簇等,本文着重综述了这三种负载型SACs的制备、表征、催化性能及催化机理,并概述了SACs未来可能的发展方向和应用.研究表明,共沉淀法、湿浸渍法和反奥斯瓦尔德熟化法等方法可用来制备氧化物负载的SACs.高角环形暗场像-扫描透射电子显微镜(HAADF-STEM)表明金属是以单原子形式均匀分散在载体上,近边X射线吸收精细结构(XANES)结果表明金属原子与载体之间存在着强相互作用.实验和理论研究均表明该类催化剂在CO氧化反应、水煤气转化及乙炔加氢生成乙烯等反应中具有高的催化活性和稳定性.采用化学气相沉积法和原子层沉积法等方法可以将金属原子稳定地负载在具有缺陷活性位点的石墨烯、MXene及六方氮化硼等二维材料上并相应制备出SACs.X射线吸收精细结构谱(EXAFS)和XANES分析表明样品中金属以单原子形式存在,而且金属原子与载体之间也存在着强相互作用,理论计算表明金属原子与二维载体之间的电荷转移是SACs活性高的主要原因.置换反应法和连续还原法是制备溶胶型SACs的有效方法,其中置换反应法可将活性金属原子原位组装在金属模板团簇的顶点位置,连续还原法可将活性原子负载于金属模板团簇的表面.DFT计算表明活性原子和金属模板团簇之间存在电荷转移效应,这是溶胶型SACs具有非常高的催化活性的主要原因.SACs下一步的研究方向可能是:(1)研究开发新型SACs,尽可能提高催化剂中活性金属原子的含量;(2)深入研究SACs的结构、活性以及催化机理之间的关系;(3)尝试将SACs大规模应用于工业催化.     

金镍二元团簇结构和电子性质的理论研究

在UBP86/LANL2DZ和UBP86/def2-TZVP水平下详细研究了AumNin (m＋n≤6)团簇的几何结构和电子性质. 详细地分析了团簇的结构特征, 平均结合能, 垂直电离势, 垂直电子亲和能, 电荷转移以及成键特征. 所有混合团簇中, 镍原子趋于聚集到一起, 形成最多Ni—Ni键, 金原子分布在镍原子聚集体周围以形成最多Au—Ni键. Ni原子较少团簇的电子性质与纯金团簇类似, 呈现一定奇偶振荡. 混合团簇中存在镍到金原子间的电荷转移. Ni原子较少团簇中, 自旋电子主要定域在Ni原子上, Ni原子较多团簇中, Au原子明显受到自旋极化. 混合团簇的分波态密度表明, AuNi混合团簇对小分子的反应活性要高于纯金团簇.     

Melting and freezing characteristics and structural properties of supported and unsupported gold nanoclusters

Molecular dynamics simulations in conjunction with MEAM potential models have been used to study the melting and freezing behavior and structural properties of both supported and unsupported Au nanoclusters within a size range of 2 to 5 nm. In contrast to results from previous simulations regarding the melting of free Au nanoclusters, we observed a structural transformation from the initial FCC configuration to an icosahedral structure at elevated temperatures followed by a transition to a quasimolten state in the vicinity of the melting point. During the freezing of Au liquid clusters, the quasimolten state reappeared in the vicinity of the freezing point, playing the role of a transitional region between the liquid and solid phases. In essence, the melting and freezing processes involved the same structural changes which may suggest that the formation of icosahedral structures at high temperatures is intrinsic to the thermodynamics of the clusters, rather than reflecting a kinetic phenomenon. When Au nanoclusters were deposited on a silica surface, they transformed into icosahedral structures at high temperatures, slightly deformed due to stress arising from the Au-silica interface. Unlike free Au nanoclusters, an icosahedral solid-liquid coexistence state was found in the vicinity of the melting point, where the cluster consisted of coexisting solid and liquid fractions but retained an icosahedral shape at all times. These results demonstrated that the structural stability in the structures of small Au nanoclusters can be enhanced through interaction with the substrate. Supported Au nanoclusters demonstrated a structural transformation from decahedral to icosahedral motifs during Au island growth, in contrast to the predictions of the minimum-energy growth sequence: icosahedral structures appear first at very small cluster sizes, followed by decahedral structures, and finally FCC structures recovered at very large cluster sizes. The simulations also showed that island shapes are strongly influenced by the substrate, more specifically, the structural characteristic of a Au island is not only a function of size, but also depends on the contact area with the surface, which is controlled by the wetting of the cluster to the substrate.     

Substrate effect on the melting temperature of gold nanoparticles

Previous experimental, molecular dynamics, and thermodynamic researches on the melting temperature of Au nanoparticles on tungsten substrate provide entirely different results. To account for the substrate effect upon the melting point of nanoparticles, three different substrates were tested by using a thermodynamic model: tungsten, amorphous carbon, and graphite. The results reveal that the melting point suppression of a substrate-supported Au nanoparticle is principally ruled by the free surface-to-volume ratio of the particle or the contact angle between the particle and the substrate. When the contact angle θ is less than 90°, a stronger size-dependent melting point depression compared with those for free nanoparticles is predicted; when the contact angle θ is greater than 90°, the melting temperature of the supported Au nanoparticles are somewhat higher than those for free nanoparticles.     

Ab initio molecular dynamical investigation of the finite temperature behavior of the tetrahedral Au19 and Au20 clusters

Density functional molecular dynamics simulations have been carried out to understand the finite temperature behavior of Au19 and Au20 clusters. Au20 has been reported to be a unique molecule having tetrahedral geometry, a large HOMO-LUMO energy gap, and an atomic packing similar to that of the bulk gold (Li, J.; et al. Science 2003, 299, 864). Our results show that the geometry of Au19 is exactly identical with that of Au20 with one missing corner atom (called a vacancy). Surprisingly, our calculated heat capacities for this nearly identical pair of gold clusters exhibit dramatic differences. Au20 undergoes a clear and distinct solid-like to liquid-like transition with a sharp peak in the heat capacity curve around 770 K. On the other hand, Au19 has a broad and flat heat capacity curve with continuous melting transition. This continuous melting transition turns out to be a consequence of a process involving a series of atomic rearrangements along the surface to fill in the missing corner atom. This results in a restricted diffusive motion of atoms along the surface of Au19 between 650 to 900 K during which the shape of the ground state geometry is retained. In contrast, the tetrahedral structure of Au20 is destroyed around 800 K, and the cluster is clearly in a liquid-like state above 1000 K. Thus, this work clearly demonstrates that (i) the gold clusters exhibit size sensitive variations in the heat capacity curves and (ii) the broad and continuous melting transition in a cluster, a feature that has so far been attributed to the disorder or absence of symmetry in the system, can also be a consequence of a defect (absence of a cap atom) in the structure.     

Density functional study of the interaction between small Au clusters, Au(n) (n=1-7) and the rutile TiO(2) surface. I. Adsorption on the stoichiometric surface

This is the first paper in a series of four dealing with the adsorption site, electronic structure, and chemistry of small Au clusters, Au(n) (n=1-7), supported on stoichiometric, partially reduced, or partially hydroxylated rutile TiO(2)(110) surfaces. Analysis of the electronic structure reveals that the main contribution to the binding energy is the overlap between the highest occupied molecular orbitals of Au clusters and the Kohn-Sham orbitals localized on the bridging and the in-plane oxygen of the rutile TiO(2)(110) surface. The structure of adsorbed Au(n) differs from that in the gas phase mostly because the cluster wants to maximize this orbital overlap and to increase the number of Au-O bonds. For example, the equilibrium structures of Au(5) and Au(7) are planar in the gas phase, while the adsorbed Au(5) has a distorted two-dimensional structure and the adsorbed Au(7) is three-dimensional. The dissociation of an adsorbed cluster into two adsorbed fragments is endothermic, for all clusters, by at least 0.8 eV. This does not mean that the gas-phase clusters hitting the surface with kinetic energy greater than 0.8 eV will fragment. To place enough energy in the reaction coordinate for fragmentation, the impact kinetic energy needs to be substantially higher than 0.8 eV. We have also calculated the interaction energy between all pairs of Au clusters. These interactions are small except when a Au monomer is coadsorbed with a Au(n) with odd n. In this case the interaction energy is of the order of 0.7 eV and the two clusters interact through the support even when they are fairly far apart. This happens because the adsorption of a Au(n) cluster places electrons in the states of the bottom of the conduction band and these electrons help the Au monomer to bind to the five-coordinated Ti atoms on the surface.     

Size-dependent dynamics in excited states of gold clusters: from oscillatory motion to photoinduced melting

Femtosecond time resolved photoelectron spectroscopy in combination with direct ab initio molecular dynamics "on the fly" based on density functional theory has been used to study the relaxation dynamics of optically excited states in small mass selected anionic gold clusters (Au(n) (-); n = 5-8). The nature of the dynamics strongly depends on the cluster size and structure. Oscillatory wavepacket motion (Au(5)(-)), a long lived excited state (Au(6)(-)), as well as photoinduced melting (Au(7)(-),Au(8)(-)) is observed in real time. This illustrates nonscalable properties of excited states in clusters in the size regime, in which each atom counts.     

Gold cluster carbonyls: saturated adsorption of CO on gold cluster cations, vibrational spectroscopy, and implications for their structures

We report on the interaction of carbon monoxide with cationic gold clusters in the gas phase. Successive adsorption of CO molecules on the Au(n)(+) clusters proceeds until a cluster size specific saturation coverage is reached. Structural information for the bare gold clusters is obtained by comparing the saturation stoichiometry with the number of available equivalent sites presented by candidate structures of Au(n)(+). Our findings are in agreement with the planar structures of the Au(n)(+) cluster cations with n < or = 7 that are suggested by ion mobility experiments [Gilb, S.; Weis, P.; Furche, F.; Ahlrichs, R.; Kappes, M. M. J. Chem. Phys. 2001, 116, 4094]. By inference we also establish the structure of the saturated Au(n)(CO)(m)(+) complexes. In certain cases we find evidence suggesting that successive adsorption of CO can distort the metal cluster framework. In addition, the vibrational spectra of the Au(n)(CO)(m)(+) complexes in both the CO stretching region and in the region of the Au-C stretch and the Au-C-O bend are measured using infrared photodepletion spectroscopy. The spectra further aid in the structure determination of Au(n)(+), provide information on the structure of the Au(n)(+)-CO complexes, and can be compared with spectra of CO adsorbates on deposited clusters or surfaces.     

A new magic titanium-doped gold cluster and orientation dependent cluster-cluster interaction

The stability and structures of titanium-doped gold clusters Au(n)Ti (n=2-16) are studied by the relativistic all-electron density-functional calculations. The most stable structures for Au(n)Ti clusters with n=2-7 are found to be planar. A structural transition of Au(n)Ti clusters from two-dimensional to three-dimensional geometry occurs at n=8, while the Au(n)Ti (n=12-16) prefer a gold cage structure with Ti atom locating at the center. Binding energy and second-order energy differences indicate that the Au(14)Ti has a significantly higher stability than its neighbors. A high ionization potential, low electron affinity, and large energy gap being the typical characters of a magic cluster are found for the Au(14)Ti. For cluster-cluster interaction between magic transition-metal-doped gold clusters, calculations were performed for cluster dimers, in which the clusters have an icosahedral or nonicosahedral structure. It is concluded that both electronic shell effect and relative orientation of clusters are responsible for the cluster-cluster interaction.     

Scanning tunneling microscopy studies of metal clusters supported on TiO2 (110): Morphology and electronic structure

A brief review of our laboratory's recent scanning tunneling microscopy (STM) studies on nanoclusters supported on TiO₂(110) is presented. Particular emphasis is placed on the system Au/TiO₂(110). The nucleation and growth of the clusters, which were vapor-deposited on TiO₂(110) under ultra high vacuum (UHV) conditions, were investigated using STM. It was found that Au, Pd, and Ag clusters all grow in a three-dimensional (3D) (Volmer-Weber) fashion on TiO₂(110), but that at low coverages, quasi-two dimensional (quasi-2D) Au and Pd clusters were observed. These quasi-2D clusters are characterized by heights of 1–2 atomic layers. Annealing studies show that Au and Pd clusters form large microcrystals with well-defined hexagonal shapes. Al clusters, which have a strong interaction with the substrate, are oxidized upon deposition, “wetting” the surface and forming small clusters. In addition to the topographic studies, the local electronic properties of these clusters have been studied using scanning tunneling spectroscopy (STS) to measure the cluster band gaps. The electronic structure was found to be cluster size-dependent, as seen by the appearance of a band gap as the cluster size decreased. More specifically, the onset of cluster metallic properties correlates with the transition from quasi-2D to 3D cluster growth.