Catalytic oxidation of cyclohexane by size-selected palladium clusters pinned on graphite

We report the catalytic oxidation of cyclohexane to CO and CO₂ over size-selected palladium clusters (Pd _N clusters, N = 10–120) supported on graphite as a function of cluster size. The stability of the pinned clusters (nanoparticles) under reaction conditions is investigated by scanning tunnelling microscopy measurement both before and after reaction. Temperature-programmed reaction experiments at 800 Torr show that the turnover rates (per surface Pd atom) for both CO and CO₂ increase significantly as cluster size decreases and correlate with the number of Pd perimeter atoms at the graphite interface. Under oxygen-rich conditions, the activity of the clusters increases by a factor of 3 while the product ratio CO:CO₂ rises by an order of magnitude.     

Deposition of size-selected metal clusters generated by magnetron sputtering and gas condensation: a progress review

This paper presents a topical review of the production and deposition of size-selected metal clusters generated by magnetron sputtering and gas condensation. Clusters with up to 75,000 atoms can be obtained by controlling the gas pressure in the sputtering source, prior to size selection with a novel (time-of-flight) mass filter. The clusters are deposited on the model graphite substrate to study the cluster-surface interaction. Two regimes for cluster deposition have been identified at elevated impact energies: (i) 'pinning' (ca. 10 eV per atom) and (ii) implantation (ca. 100 eV per atom). Of particular importance is the pinning regime, since this allows the fabrication of monodispersed cluster arrays, which are stable against diffusion at room temperature (and above). The deposition of size-selected metal clusters represents a novel method of preparing surface nanostructures, with potential applications including model catalyst studies and the immobilization of biological molecules.     

Size-evolution towards metallic behavior in nano-sized gold and platinum clusters as revealed by 197Au Mössbauer spectroscopy

Four types of molecular Au₅₅ clusters, with different types of ligands, have been studied using ¹⁹⁷Au Mössbauer spectroscopy. Emission Mössbauer measurements were also performed on the larger Pt309 cluster, in which ¹⁹⁷Au was produced by neutron activation. There is a large influence of the ligands on the charge densities at the surface atoms of the metal atom cluster cores. For the Au₅₅ clusters, the inner core atoms have a different charge density at the nucleus than the bulk metal, and it depends also slightly on the type of ligands on it. For the Pt₃₀₉ compound, the inner core atoms have the same charge density at the nucleus as in the corresponding bulk metal. We, therefore, conclude that the inner core of the Pt₃₀₉ cluster is metallic with respect to the electronic charge densities, while this is not yet the case for the Au₅₅ cluster.     

Surface modification and chemical sputtering of graphite induced by low-energy atomic and molecular deuterium ions

The surface morphology, and chemical/structural modifications induced during chemical sputtering of ATJ graphite by low-energy (<200 eV/D) deuterium atomic and molecular ions are explored by Scanning Electron Microscopy (SEM), Raman and Auger Electron Spectroscopy (AES) diagnostics. At the lowest impact energies, the ion range may become less than the probe depth of Raman and AES spectroscopy diagnostics. We show that such diagnostics are still useful probes at these energies. As demonstration, we used these surface diagnostics to confirm the characteristic changes of surface texture, increased amorphization, enhanced surface reactivity to impurity species, and increased sp³ content that low-energy deuterium ion bombardment to steady-state chemical sputtering conditions produces. To put these studies into proper context, we also present new chemical sputtering yields for methane production of ATJ graphite at room temperature by impact of D₂⁺ in the energy range 10-250 eV/D, and by impact of D⁺ and D₃⁺ at 30 eV/D and 125 eV/D, obtained using a Quadrupole Mass Spectroscopy (QMS) approach. Below 100 eV/D, the methane production in ATJ graphite is larger than that in HOPG by a factor of ∼2. In the energy range 10-60 eV/D, the methane production yield is almost independent of energy and then decreases with increasing ion energies. The results are in good agreement with recent molecular dynamics simulations.     

Electrochemical and <Emphasis Type="Italic">in situ</Emphasis> spectroelectrochemical studies of gold nanoparticles immobilized Nafion matrix modified electrode

Electrochemical and in situ spectroelectrochemical behaviours of phenosafranine (PS⁺) were studied at the gold nanoparticles (Au_Nps) immobilized Nafion (Nf) film coated glassy carbon (GC) and indium tin oxide (ITO) electrodes. Cyclic voltammetric studies showed that the PS⁺ molecules strongly interact with the Au_Nps immobilized in the Nf matrix through the electrostatic interaction. The presence of Au_Nps in the Nf film improved the electrochemical characteristics of the incorporated dye molecules. The emission spectra of Nf-Au_Nps-PS⁺ films showed that the incorporated PS⁺ was quenched by Au_Nps and it could be explained based on the electronic interaction between the Au_Nps and PS⁺ molecules. The in situ spectroelectrochemical study showed an improved electrochemical characteristic of the incorporated PS⁺ molecules at the ITO/Nf-Au_Nps electrode when compared to the ITO/Nf electrode.     

Nanostructured surfaces from size-selected clusters

The deposition of ionized beams of size-selected atomic clusters onto well-defined substrates represents a new method of preparing nanostructured surfaces, with lateral feature sizes in the range 1-10 nm. 'Pinning' of the incident clusters prevents the diffusion of the clusters on the surface, and thus preserves the gas-phase cluster size, even at room temperature and above. At the same time, advances in diblock copolymer techniques allow the preparation of ordered two-dimensional arrays of clusters. Here we discuss the creation and applications of these nanostructured surfaces, ranging from the fabrication of semiconductor nanostructures to the immobilization of protein molecules.     

Sputtering of thin films of bariated molecules of arachidic acid by large noble gas clusters

Coarse-grained molecular dynamics computer simulations have been employed to investigate the sputtering process of a multilayer organic system composed of long, well-organized linear molecules induced by an impact of slow clusters composed of large number of noble gas atoms. The organic system is represented by Langmuir–Blodgett multilayers formed from bariated molecules of arachidic acid. The sputtering yield, surface modifications and the angular distributions of ejected species have been analyzed as a function of the kinetic energy of an Ar₈₇₂ cluster projectile and the thickness of the deposited multilayer. It has been shown that the physics of ejection by these large and slow clusters is distinct from the ejection events stimulated by the popular SIMS clusters: C₆₀, Au₃ and SF₅. The organic molecules are not ejected by interaction with the energized substrate but by direct interactions with the projectile atoms.     

NH3 and H2S adsorption on Au3Pt3 cluster studied by a first principles calculation

The equilibrium atomic structures of the Au₃Pt₃ cluster and NH₃, H₂S molecules are determined by Density Functional Theory calculations. The adsorption of NH₃ and H₂S molecules on Au₃Pt₃ cluster is then studied. The energy levels and the corresponding charge densities, adsorption energies, charge transfer and magnetization are calculated for different adsorption sites. We find that the adsorption generally modifies the structure of the Au₃Pt₃ cluster and the adsorbate. There exists strong interaction between Pt and N atoms. We observe that H₂S dissociate at a specific site for the local density approximation (LDA) and one of the H atoms binds to Pt. In general, there is charge transfer from the molecules NH₃ and H₂S to Au₃Pt₃ cluster, for almost all adsorption sites. This makes the Au₃Pt₃ cluster semiconducting.     

Au8 cluster adsorption on Si(100):2 × 1 asymmetric surface studied by a first principles calculation

In this work, we investigate the adsorption of Au₈ cluster onto H-terminated Si(100):2 × 1 asymmetric surface by density functional theory within local density and generalized gradient approximations. We study the site-dependent shape of Au₈ cluster, adsorption energies, band structures and the corresponding charge distribution. We show that the electronic properties of the cluster and substrate complex change with the location of the cluster on the surface.     

Polymorphism of Phosphine‐Protected Gold Nanoclusters: Synthesis and Characterization of a New 22‐Gold‐Atom Cluster

A new Au₂₂ nanocluster, protected by bis(2‐diphenyl‐phosphino)ethyl ether (dppee or C₂₈H₂₈OP₂) ligand, has been synthsized and purified with high yield. Electrospray mass spectrometry shows that the new cluster has a formula of Au₂₂(dppee)₇, containing 22 gold atoms and seven dppee ligands. The cluster is found to be stable as a solid, but metastable in solution. The new cluster has been characterized by UV‐Vis‐NIR absorption spectroscopy, collision‐induced dissociation, and ³¹P‐NMR. The properties of the new cluster have been compared with the previous Au₂₂(dppo)₆ nanocluster (dppo = 1,8‐bis(diphenyl‐phosphino)octane or C₃₂H₃₆P₂), which contains two fused Au₁₁ units. All the experimental data indicate that the new Au₂₂(dppee)₇ cluster is different from the previously known Au₂₂(dppo)₆ cluster and represents a new Au₂₂ core, which contains most likely one Au₁₁ motif with several Au₂(dppee) or Au(dppee) units. The Au₂₂(dppee)₇ cluster provides a new example of the ligand effects on the nuclearity and structural polymorphism of phosphine‐protected atom‐precise gold nanoclusters.     

Simulation of ion sputtering of copper clusters from single crystal graphite surface

The sputtering of clusters consisting of 13 and 75 copper atoms from a (0001) graphite surface bombarded by normally incident 200-eV Ar⁺ ions was studied by molecular dynamics method. The angular distribution of sputtered copper atoms, their energies, and the sputtering yields are discussed.     

Simulation of low-energy argon ion scattering from surface copper clusters

The scattering of normally incident 200-eV Ar⁺ ions from individual clusters consisting of 13 and 39 copper atoms on a (0001) graphite surface was simulated by the molecular dynamics method. The angular distribution of scattered argon ions and their energies and reflection coefficients are discussed.     

Effect of insertion of a thin bathocuproine layer at Au/Zn-phthalocyanine interface on energy level alignment and morphology

The effects of the insertion of a thin bathocuproine (BCP) layer at the Au/Zn-phthalocyanine (ZnPc) interface on the energy level alignment and morphology of each layer are investigated using photoelectron spectroscopy, atomic force microscopy and X-ray diffraction. The energy differences between the highest occupied molecular orbital (HOMO) of ZnPc and Fermi level of Au (E_F^Au) and between the HOMO of BCP and E_F^Au are about 0.7 eV and 2.4 eV, respectively. The HOMO level of ZnPc on Au is shifted to higher binding energies with increasing ZnPc thickness. The HOMO level of ZnPc on BCP, however, indicates an energy shift in the opposite direction to the lower binding energies as the thickness of ZnPc increases. Atomic force microscopy reveals that the roughness of the ZnPc surface is reduced by the insertion of the BCP layer. X-ray diffraction patterns show that the existence of the thin BCP layer promotes crystallization of ZnPc over the layer and the molecular planes of ZnPc are perpendicular to the BCP surface.     

苯甲酸钠改性石墨作为锂离子电池负极材料的性能

将不同浓度的苯甲酸钠改性的石墨电极作为锂离子电池的负极备用材料,并使用恒流充放电、循环伏安和交流阻抗等电化学方法表征电池的性能.结果表明,与初始的石墨电极相比,被改性后的石墨电极表现出更好的循环效率和稳定性,且在0.5C条件下,首次的充放电比容量分别为293.9mAh/g和326.4 mAh/g.主要原因是改性后的石墨电极的表面形成的SEI膜能有效抑制石墨材料的膨胀,并且更有利于锂离子的迁移.同时,采用量子化学方法计算了溶剂分子和苯甲酸钠的最低空轨道和最高占据轨道能量值.结合电化学表征和量子计算结果,苯甲酸钠改性石墨电极的最佳浓度为1.0％.此外,还研究了最佳浓度改性石墨电极的高温性能.     

Metal‐Cluster‐Decorated TiO2 Nanotube Arrays: A Composite Heterostructure toward Versatile Photocatalytic and Photoelectrochemical Applications

Recent years have witnessed increasing interest in the solution‐phase synthesis of atomically precise thiolate‐protected gold clusters (Au_x); nonetheless, research on the photocatalytic properties of Au_x–semiconductor nanocomposites is still in its infancy. In this work, recently developed glutathione‐capped gold clusters and highly ordered nanoporous layer‐covered TiO₂ nanotube arrays (NP‐TNTAs) are employed as nanobuilding blocks for the construction of a well‐defined Au_x/NP‐TNTA heterostructure via a facile electrostatic self‐assembly strategy. Versatile photocatalytic performances of the Au_x/NP‐TNTA heterostructure which acts as a model catalyst, including photocatalytic oxidation of organic pollutant, photocatalytic reduction of aromatic nitro compounds and photoelectrochemical (PEC) water splitting under simulated solar light irradiation, are systematically exploited. It is found that synergistic interaction stemming from monodisperse coverage of Au_x clusters on NP‐TNTAs in combination with hierarchical nanostructure of NP‐TNTAs reinforce light absorption of Au_x/NP‐TNTA heterostructure especially within visible region, hence contributing to the significantly enhanced photocatalytic and PEC water splitting performances. Moreover, photocatalytic and PEC mechanisms over Au_x/NP‐TNTA heterostructure are elucidated and corresponding reaction models were presented. It is anticipated that this work could boost new insight for photocatalytic properties of metal‐cluster‐sensitized semiconductor nanocomposites.     

Quantum-chemical modeling of interaction between gold nanoclusters and thiols

Ab initio calculations are used to model small Au _n nanoclusters and Au _m SH clusters. The results for the Au₆, Au₈, and Au₂₀ clusters demonstrate that the substitution of a SH group for a Au atom gives a stable cluster of the same geometry if the Au atom has an acute bond angle and a negative effective charge. The example of the Au₁₀ cluster suggests that SH substitution for Au has a stabilizing effect. The modeling results are discussed with application to self-organizing thiol monolayers on gold clusters.     

Oxidation of gold metal particles supported on TiO<Subscript>2</Subscript>: an FTIR study by means of low-temperature CO adsorption

Two Au/TiO₂ samples with different gold loadings (0.7 and 4.0 wt.% Au) were prepared by deposition-precipitation with urea and calcined at 673 K. TEM revealed gold particles of 3.2 and 3.9 nm for the 0.7 and 4.0 wt.% samples, respectively. The samples were subjected to different red-ox treatments and then the state of gold was determined by the FTIR spectra of CO adsorbed at low temperature. Several kinds of gold carbonyl species were detected during the experiments: (i) Au⁰–CO at around 2107 cm⁻¹; (ii) Au⁺–CO at ca. 2175 cm⁻¹; (iii) Au^δ+–CO in the region of 2140–2137 cm⁻¹ and (iv) Au^δ′+–CO (δ^′ > δ) at around 2155 cm⁻¹. The 4.0 wt.% sample contained mainly metallic gold after evacuation at 673 K. Subsequent interaction with oxygen at 373 K leads to oxidation of a fraction of the surface metallic gold sites to Au^δ+ sites. These sites were considered as cations located on the surface of the metal particles with a partially positive charge δ+ (0 < δ < 1) because of electron transfer from the gold bulk. Evacuation at 673 K leads to back reduction of the Au^δ+ sites to metallic gold. The oxidation of gold particles was more efficient when performed with a NO + O₂ mixture. It resulted in creation of Au^δ′+ sites with a higher positive charge than that of the Au^δ+ sites. In this case the oxidation involved a higher number of Au⁰ sites. A similar treatment of the 0.7 wt.% Au sample, however, resulted in formation of “isolated” Au⁺ species. The results indicate that small metal particles are more easily oxidized by a NO + O₂ mixture. A model of the formation of the different sites, explaining well the experimental results, is proposed.     

Mixage ionique sur des structures Au/InP

The reactions induced by Zn⁺ implantations near the interface of Au/InP contacts have been studied by using scanning electron microscopy, X-ray diffraction, He⁺ Rutherford backscattering, secondary ion mass spectrometry and current-voltage measurements. A 5 × 10¹⁴ Zn ions cm^-2 dose does not induce compound formation but accelerates the growth of Au₃In and Au₂P₃ patches during post-annealing treatment. After a 5 × 10¹⁵ Zn ions cm^-2 implantation, many compounds, different from those obtained by a thermal anneal, are detected. These compounds, which depend on the implantation temperature (25 or 200°C), have a layered structure. In this case no Au₂P₃ is observed. However, for the range of doses (from 10¹⁴ to 5 × 10¹⁵ Zn ions cm^-2), the temperatures of implantation (25 and 200°C) and the range of annealing temperatures (from 320 to 450°C) that were studied, no contact with a low resistivity is formed. The electrical properties are in fact limited by an InP layer damaged by the ion implantation in which the zinc atoms are trapped in an electrically inactive form.     

SnO<Subscript>2</Subscript>(Au<Superscript>0</Superscript>, Co<Superscript>II,III</Superscript>) nanocomposites: A synergistic effect of the modifiers in CO detection

Nanocrystalline SnO₂ has been synthesized and its surface has been modified with Au⁰ and Co(II, III). The distribution of the modifiers over the nanocomposites has been studied by X-ray diffraction, inductively coupled plasma mass spectrometry, and transmission electron microscopy. The effect of the modifiers on the hydrogen reduction of nanocrystalline SnO₂ has been assessed. The CO sensing properties of the synthesized materials (10 ppm CO in air) have been studied in situ by electrical conductance measurements. The addition of both Au⁰ and Co(II, III) allows the working temperature of the SnO₂-based semiconductor sensor to be lowered to 215°C.     

Structural features and energy of small water clusters

Collisions between water clusters (H₂O)_n, n=27, with relative velocities V=1, 3, or 10 km/s and various initial temperatures, were studied by the molecular dynamics method within the framework of a polarization model. It was found that the clusters may either stick together (at V=1 km/s), break into fragments (V=3 km/s), or form compressed (excited) molecules (V=10 km/s) dissociating into H⁺ and OH⁻. Inside the cluster, the charge is transferred by the H⁺ hopping mechanism. Upon fragmentation, the clusters separate into H⁺(H₂O)_i and OH⁻(H₂O)_j ions.