Aggregate structure and crystallite size of platinum nanoparticles synthesized by ethanol reduction

Monodispersed platinum (Pt) nanoparticles were synthesized from reducing hydrated hydrogen hexachloroplatinic acid (H₂PtCl₆·nH₂O) with ethanol in the presence of polyvinylpyrrolidone (PVP) as a steric stabilizer. Concentration of both PVP and ethanol influenced the aggregate structure and crystallite size of the nanoparticles. When the molar ratio of monomeric unit of PVP to Pt, i.e., [PVP]/[Pt], was one, the synthesized Pt particles coagulated pronouncedly into an inter-connected particulate network or self-organized into spherical superstructures with an apparent diameter ranging from 60 to 80 nm, depending on the ethanol concentration. The geometry and structure of these complex aggregates were characterized by fractal analysis. Fractal dimensions of 2.13–2.23 in three dimensions were determined from the Richardson’s plot, which suggests that a reaction-limited cluster–cluster aggregation model (RCLA) was operative. The Pt colloids became apparently more stable when the [PVP]/[Pt] ratio was increased greater than 20. Crystallite size of the Pt nanoparticles was found to increase linearly with the ethanol concentration as the [PVP]/[Pt] was held at one. This suggests that the reduction rate of PtCl₆ ²⁻ ions in solution is critically important to the synthesized crystallite size.     

Strong influence of defects on the electronic structure of Pt adatoms and clusters on graphite

We study the specific impact of defects such as step edges at the graphite surface on the electronic configuration of adsorbed Pt atoms and Pt₈ clusters. Surface diffusion is strongly reduced by depositing Pt and Pt₈ into a thin rare gas layer. In this configuration a very narrow adatom Pt 4f spectrum is found at an exceptionally small binding energy, similar to Pt surfaces. Both, adatom and cluster spectra are strongly shifted towards higher binding energy when allowed to diffuse towards defects like step edges. The strong shifts are indicative of a chemical reaction at the step edges and are conjectured to be part of the particle size dependent binding energy shifts typically observed for transition metal clusters grown on the surface of graphite.     

Chiral structures and tunable magnetic moments in 3d transition metal doped Pt₆ clusters

The structural, electronic, and magnetic properties of transition metal doped platinum clusters MPt 6 (M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) are systematically studied by using the relativistic all-electron density functional theory with the generalized gradient approximation. Most of the doped clusters show larger binding energies than the pure Pt 7 cluster, which indicates that the doping of the transition metal atom can stabilize the pure platinum cluster. The results of the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps suggest that the doped clusters can have higher chemical activities than the pure Pt 7 cluster. The magnetism calculations demonstrate that the variation range of the magnetic moments of the MPt 6 clusters is from 0 μ B to 7 μ B , revealing that the MPt 6 clusters have potential utility in designing new spintronic nanomaterials with tunable magnetic properties.     

A Rapid Method for Growth of Metal Nanoparticles on Nanowire Substrates

The production of nickel and platinum nanoparticles on silica nanowire substrates using plasma-enhanced chemical vapor deposition has been investigated. Determination of particle size and particle size distribution was done using transmission electron microscopy (TEM). Ni nanoparticle diameters were found to be between 2 and 6 nm, with particle size increasing as the substrate temperature increased from 573 to 873 K. The size of Ni nanoparticles was found to be dependent on the chamber pressure during growth. The results indicate a competition between pressure-related diffusion within the vapor and dissociation of the precursor. Pt nanoparticle diameters were consistently found to be 2.5–3.0 nm at all deposition conditions. Insufficient thermal energy within the studied range results in a minimal contribution from surface diffusion, the primary mechanism for nanoparticle growth.     

All-electron scalar relativistic calculation on the interaction between nitric monoxide and small gold cluster

An all-electron scalar relativistic calculation on Au _n NO (n = 1–10) clusters has been performed by using density functional theory with the generalized gradient approximation at the PW91 level. The small gold cluster would like to bond with nitric and the nitric monoxide molecule prefers to occupy the on-top and single fold coordination site. The Au _n structures in all Au _n NO clusters are only distorted slightly and still keep the planar structures. With the bend of Au-N-O bond, the structures of Au _n NO clusters evolve from the 2D structure to 3D structure. The most favorable adsorption between small gold cluster and nitric monoxide molecule takes place in the case that nitric monoxide molecule is adsorbed onto an odd-numbered pure Au _n cluster and becomes odd-numbered Au _n NO cluster with even number of valence electrons. The scalar relativistic effect strengthens the Au–Au, Au–N interaction and weakens the N–O interaction, appearing as the shorter Au–Au, Au–N bond-length and the longer N–O bond-length. The differences between our work and previous work are believed to be the reflection of the scalar relativistic effect.     

Enhanced activity of chaperonin GroEL in the presence of platinum nanoparticles

The chaperonin protein GroEL was mixed with varying concentrations of K₂PtCl₄ followed by a 20-fold concentration of sodium borohydride to afford GroEL–platinum nanoparticle complexes in a ratio of between 1:25 and 1:2,000. Typical colour change, from colourless or pale yellow to brown, occurred that was dependent on the amount of platinum present. These complexes were characterised by UV/Vis, inductively coupled plasma optical emission spectroscopy, Fourier transform infra red, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy. TEM analysis revealed that the size of nanoparticles increased as the molar ratio of platinum to GroEL increased with an average size diameter of 1.72–3.5 nm generated with GroEL–platinum molar ratios of 1:125–1:2,000. Fourier-transform infrared spectroscopy (FTIR) spectra showed no distinct changes in the structure of GroEL but confirmed that the nanoparticles were attached to the protein. The effect of platinum nanoparticles on the ATPase activity of GroEL showed an activity of 5.60 μmol min⁻¹ ml⁻¹ (87 % increase over a control) at the molar ratio of GroEL–platinum nanoparticles of 1:25.     

Surface self-diffusion of a Pt adatom on cuboctahedral and truncated decahedral clusters,size dependence

The self-diffusion of single Pt adatom on the surface of cuboctahedral and truncated decahedral clusters with 561–10 179 atoms are studied within the context of the many-body potentials obtained via the embedded atom method. The minimum energy diffusion path and the corresponding energy barrier for adatom diffusion on the cuboctahedral and truncated decahedral clusters surfaces are determined through a combination of the quenched molecular dynamics and the nudged elastic band method. The calculated energy barriers are consistent with the available experimental data. The dependence of energy barrier for adatom diffusion across the step edge on the cluster size is found. For the larger cuboctahedral and truncated decahedral clusters, the simulations show that the movement of the adatom is confined to a central region, and the adatom may escape from the center region only at elevated temperatures. In addition, we also find that the truncated decahedral structure is more favorable over the cuboctahedral structure for smaller clusters. The cluster growth experiments support our results.     

Electronic structure and magnetism in (CoPt)n(n⩽5) clusters

The geometrical and magnetic properties of bimetallic clusters (CoPt)_n(1?n?5) have been studied by using the generalized gradient correction spin density formalisms. In general, the ground state structures of (CoPt)_n clusters are the three-dimension structures. We found that both the binding energy and magnetism per (CoPt) unit are increasing consistently with the size of the Co–Pt cluster (n). However, as the n increases, the magnetism shows a trace of convergence while the binding energy shows a linearly increasing pattern. Generally, Co average magnetic moment is enhanced when alloyed with Pt atoms than that in pure Co clusters.     

The influence of the composition of eight-atom Pt–Ir clusters on the magnetic properties

The energetic stability, electronic structure and magnetic properties of Pt_8–nIr_n clusters have been investigated by employing the spin-polarised generalised gradient approximation. The cubic structure is expected to be the effective building block in Ir-rich clusters after optimisation extensively. The average binding energy of all the clusters presents the linear increment trend with iridium atoms, due to the stronger interaction between Ir atoms than Pt atoms. Bader charge analysis shows how tiny charge transfers from iridium to platinum. The atomic moments of Ir are larger than that of Pt, and the Ir-rich clusters show greater moments than the Pt-rich cluster, with the exception of Ir₈ and Ir₇Pt. A unique magnetic property is found in the Pt₄Ir₄ cluster, where two Pt atoms show antiferromagnetic alignment and the other atoms are found to be aligned ferromagnetically.     

Thermal stability of Pt nanoclusters interacting to carbon sublattice

The catalytic activity of Pt clusters is dependent not only on the nanoparticle size and its composition, but also on its internal structure. To determine the real structure of the nanoparticles used in catalysis, the boundaries of the thermal structure stability of Pt clusters to 8.0 nm in diameter interacting with carbon substrates of two types: a fixed α-graphite plane and a mobile substrate with the diamond structure. The effect of a substrate on the processes melting of Pt nanoclusters is estimated. The role of the cooling rate in the formation of the internal structure of Pt clusters during crystallization is studied. The regularities obtained in the case of “free” Pt clusters and Pt clusters on a substrate are compared. It is concluded that platinum nanoparticles with diameter D ≤ 4.0 nm disposed on a carbon substrate conserve the initial fcc structure during cooling.

     

Ab initio study of neutral (TiO2)n clusters and their interactions with water and transition metal atoms

We have systematically investigated the growth behavior and stability of small stoichiometric (TiO(2))(n) (n = 1-10) clusters as well as their structural, electronic and magnetic properties by using the first-principles plane wave pseudopotential method within density functional theory. In order to find out the ground state geometries, a large number of initial cluster structures for each n has been searched via total energy calculations. Generally, the ground state structures for the case of n = 1-9 clusters have at least one monovalent O atom, which only binds to a single Ti atom. However, the most stable structure of the n = 10 cluster does not have any monovalent O atom. On the other hand, Ti atoms are at least fourfold coordinated for the ground state structures for n ≥ 4 clusters. Our calculations have revealed that clusters prefer to form three-dimensional structures. Furthermore, all these stoichiometric clusters have nonmagnetic ground state. The formation energy and the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap for the most stable structure of (TiO(2))(n) clusters for each n have also been calculated. The formation energy and hence the stability increases as the cluster size grows. In addition, the interactions between the ground state structure of the (TiO(2))(n) cluster and a single water molecule have been studied. The binding energy (E(b)) of the H(2)O molecule exhibits an oscillatory behavior with the size of the clusters. A single water molecule preferably binds to the cluster Ti atom through its oxygen atom, resulting an average binding energy of 1.1 eV. We have also reported the interaction of the selected clusters (n = 3, 4, 10) with multiple water molecules. We have found that additional water molecules lead to a decrease in the binding energy of these molecules to the (TiO(2))(n) clusters. Finally, the adsorption of transition metal (TM) atoms (V, Co and Pt) on the n = 10 cluster has been investigated for possible functionalization. All these elements interact strongly with this cluster, and a permanent magnetic moment is induced upon adsorption of Co and V atoms. We have observed gap localized TM states leading to significant HOMO-LUMO gap narrowing, which is essential to achieve visible light response for the efficient use of TiO(2) based materials. In this way, electronic and optical as well as magnetic properties of TiO(2) materials can be modulated by using the appropriate adsorbate atoms.     

密度泛函理论研究CO与Nin（n=1—6）团簇的相互作用

采用密度泛函理论对CO吸附在镍团簇表面进行了系统研究.结果表明,Ni_nCO团簇的最低能量结构是在Ni_n团簇最低能量结构的基础上吸附CO生长而成,CO的吸附没有改变Ni_n团簇的结构;CO分子在Ni_n团簇表面发生的是非解离性吸附,与优化的CO键长（0.1138?nm）相比,吸附后C—O键长变长（0.1180—0.1214?nm）,表明吸附后C—O键被削弱,CO分子被活化.自然键轨道分析表明,CO分子只与最近邻的Ni原子发生相互作用;CO分子与Ni原子相互作用的本质是CO分子内的杂化轨道与Ni原子3d, 4s, 4p轨道相互作用的结果. 关键词： nCO团簇')" href="#">Ni_nCO团簇 n团簇')" href="#">Ni_n团簇 平衡结构 电子性质     

Adsorption and diffusion of OH on Mo modified Pt(111) surface: First-principles theory

A first-principles study of adsorption and diffusion of OH on Pt and PtMo(111) surfaces is described. It confirms that the dissociation of water is much easier on PtMo than on pure Pt. Furthermore, we also found that OH binds most strongly at Mo atop site with adsorption energy of −3.32 eV, which is ∼1 eV stronger than binding to the pure Pt(111) surface. OH is much more localized on the PtMo alloy surface than on pure Pt. Both the stranger bond and the higher localization of OH contribute to the enhanced fuel cell performance with PtMo electrodes compared to pure Pt.     

In situ scanning tunneling microscopy studies of bimetallic cluster growth: Pt-Rh on TiO2(1 1 0)

The growth of Pt on clusters on TiO₂(1 1 0) in the presence and absence of Rh was investigated by scanning tunneling microscopy (STM) for Pt deposited on top of 0.3 ML Rh clusters (Rh + Pt). In situ STM studies of Pt growth at room temperature show that bimetallic clusters are produced when Pt is directly incorporated into existing Rh clusters or when newly nucleated clusters of pure Pt coalesce with existing Rh clusters. Low energy ion scattering experiments demonstrate that Rh is still present at the surface of the clusters even after deposition of 2 ML of Pt, indicating that Rh atoms can diffuse to the cluster surface at room temperature. Rh clusters were found to seed the growth of Pt clusters at room temperature as well as 100 K and 450 K. Furthermore, clusters as large as 100 atoms were observed to be mobile on the surface at room temperature and 450 K, but not at 100 K. Pt deposition at 100 K exhibited more two-dimensional cluster growth and higher cluster densities compared to room temperature experiments due to the lower diffusion rate. Increased diffusion rates at 450 K resulted in more three-dimensional cluster growth and lower densities for pure Pt growth, but cluster densities for Pt + Rh growth were the same as at room temperature.     

Density functional investigation of structural and electronic properties of small bimetallic silver–gold clusters

Structural and electronic properties of bimetallic silver–gold clusters up to eight atoms are investigated by the density functional theory using Wu and Cohen generalized gradient approximation functional. By substitution of Ag and Au atoms, in the optimized lowest energy structures of pure gold and silver clusters, we determine the ground state conformations of the bimetallic silver–gold ones. We reveal that Ag atoms prefer internal positions whereas Au atoms prefer exposed ones favoring charge transfer from Ag to Au atoms. For each size and composition, binding energy, HOMO–LUMO gap, magnetic moment, vertical ionization potential, electron affinity and chemical hardness were calculated. On increasing the size of the cluster by varying number of Ag atoms with fixed number of Au ones, vertical ionization potential and electron affinity show obvious odd–even oscillations consistent with the pure Ag and Au clusters. Au atoms inclusion in the cluster increases the binding energy and vertical ionization potential, indicating higher stability as the number of Au atoms grows. The variation of chemical hardness with the composition in a cluster with the same size shows peaks when the number of Ag atoms is greater than or equal to Au ones, corresponding to transition from planar to tri-dimensional structures. For clusters with even number of atoms, the peaks indicate that the clusters with the same number of Ag and Au atoms are the most stable ones. Analyzing the density of states, we found that increasing the concentration of Ag atoms affects the energy separation between the HOMO and the low lying occupied states.     

Morphology and chemical state of PVP-protected Pt,Pt–Cu,and Pt–Ag nanoparticles prepared by alkaline polyol method

Well-dispersed, uniform monometallic Pt and bimetallic Pt–Cu, and Pt–Ag nanoparticles protected with PVP have been synthesized by a modified-protocol alkaline polyol method. The nanoparticles were characterized by various methods (TEM, XPS, and XRD) to elucidate the relationship between morphology and preparation variables. The average of monodispersed nanoparticles ranged between 4.1 and 4.9 nm. Core–shell structure was obtained in the case of bimetallic nanoparticles. The core of bimetallic nanoparticles was found to be rich in platinum, whereas the shell contained mostly copper or silver. The final structure of bimetallic nanoparticles was found to be determined by the morphology particles resulted in the first reduction step. Explanations are advanced on the light of experimental results.     

合金效应加强水分子在PtRun团簇上的吸附作用

利用第一性原理计算方法,研究合金效应对PtRu_n-1（n=2-14）和H₂O-PtRu_n-1（n=2-14）体系的几何构型、稳定性及吸附水特性的影响.结果表明：铂原子替代钌原子的能量较低,容易与钌团簇形成合金,铂原子喜欢占据配位数较低位置.相对于纯钌团簇,合金效应很大程度上提高了水分子在PtRu_n团簇上的吸附能.考虑范德瓦尔斯力后,水分子在PtRu₇上的吸附能增大,分解势垒降低,水分子可以在PtRu₇上分解.铂钌合金更适合做分解水制取氢气的催化剂.     

Core binding energies for clusters deposited on different insulating substrates: esca spectra and theoretical electronic structure studies

The 4f core-level X-ray photoemission of small Pt clusters deposited on Teflon and SiO₂ substrates has been investigated. The experimental results are discussed and related to other data reported in the literature. Parallel results are presented for ab initio selfconsistent field, SCF, calculations of the core ionization potentials, IPs, of small Li clusters. These model calculations provide qualitative information about the nature and magnitude of the initial and final state contributions to the IPs for small clusters. We find that the binding energy shifts and linewidth broadening depend primarily on the cluster size and on the cluster substrate interaction. They do not result from electrostatic effects of the unit positive charge remaining on the ionized cluster. Our calculations show that this Coulomb effect does not apply to very small clusters.     

Structural deformation, melting point and lattice parameter studies of size selected silver clusters

Silver clusters have been produced by magnetron sputtering in a gas aggregation nanocluster source. Clusters are size selected using a quadrupole mass filter (3–8 nm) or by varying the aggregation tube length (9–20 nm) of the nanocluster source. Mass selected clusters are deposited on a Si(100) substrate at different bias voltages and are characterized by atomic force microscopy. We observe a significant flattening of clusters on the surface due to the increase of impact energy as a result of increasing substrate bias voltage. The behavior of lattice parameters for size selected clusters are investigated by X-ray diffraction. All measured lattice constants exhibit a tensile strain; it is found that the lattice constant slightly increases with increasing cluster size up to a size of 12 nm and then decreases. The melting temperature of deposited clusters is found to be size-dependent and significantly lower than for bulk material, in agreement with theoretical considerations.     

Structures of Pt clusters on graphene doped with nitrogen, boron, and silicon: a theoretical study

The structures of Pt clusters on nitrogen-,boron-,silicon-doped graphenes are theoretically studied using densityfunctional theory.These dopants(nitrogen,boron and silicon) each do not induce a local curvature in the graphene and the doped graphenes all retain their planar form.The formation energy of the silicon-graphene system is lower than those of the nitrogen-,boron-doped graphenes,indicating that the silicon atom is easier to incorporate into the graphene.All the substitutional impurities enhance the interaction between the Pt atom and the graphene.The adsorption energy of a Pt adsorbed on the silicon-doped graphene is much higher than those on the nitrogen-and boron-doped graphenes.The doped silicon atom can provide more charges to enhance the Pt-graphene interaction and the formation of Pt clusters each with a large size.The stable structures of Pt clusters on the doped-graphenes are dimeric,triangle and tetrahedron with the increase of the Pt coverage.Of all the studied structures,the tetrahedron is the most stable cluster which has the least influence on the planar surface of doped-graphene.