Size Of Metal Clusters Research Articles

Structure Sensitivity in l-Arabinose Oxidation over Au/Al2O3 Catalysts

Several Au/Al2O3 catalysts were prepared and evaluated in the oxidation of l-arabinose. Metal cluster size was varied by application of different preparation methods (DPU and DIE), washing agents (ammonia or water), calcination temperatures, and concentrations of the initial precursor solutions. The structure sensitivity of the reaction over these catalysts was proven. A volcano relationship of activity with the mean particle size was observed, the catalyst prepared by DPU being the most active one. The catalyst electric potential was correlated with the particle size by an inverse volcano shape. At 30% of conversion, the DPU sample exhibited the lowest potential.

Metal-Filled Polytetrafluoroethylene Nanostructered Thin Film

Thin metal-filled polyterafluoroethylene films with various metal concentration were produced by co-deposition in vacuum. Metal nanocluster size increased with metal concentration. Films were heated up to 300 degrees C, their optical spectra were recorded during heating. The changes in plasmon band shape and wavelength of the nanocluster ensemble during heating are not linearly related with metal concentration and heating temperature. This is caused by different thermal behavior of the complex processes, which are taking place in each of the two materials present in the film. The metal cluster size and optical properties of the whole ensemble can be purposefully formed by varying metal nature, its concentration and annealing temperature of the film. Nano- and micro-domains with properties different from original film were generated by focused excimer laser or electron beam. Gold-filled PTFE nano-structured films were used as substrate for surface enhanced Raman scattering measurements of ultrathin film of Rhodamine 6G dye.

Evolution of the electronic properties of transition metal nanoclusters on graphite surface

The electronic properties of nanoclusters of transition (Ni, Co, Cr) and noble (Au, Cu) metals deposited on the surface of highly oriented pyrolytic graphite (HOPG) are studied using the method of X-ray photoelectron spectroscopy. The laws of variation of a change ΔE b in the binding energies of core-level electrons in the initial (ΔE i) and final (ΔE f) states of atoms in nanoclusters, the intrinsic widths γ of photoelectron lines, and their singularity indices α as functions of the metal cluster size d are determined. A qualitative difference in behavior of the ΔE i(d) and α(d) values in metals of the two groups (Ni, Cr versus Co, Cu) is found. The values of the final-state energy (ΔE f 0) in the clusters of all metals studied vary in a similar manner. It is shown that a significant contribution to E i is due to a transfer of the valence-shell electrons at the cluster-substrate interface, which is caused by the contact potential difference. The value of an uncompensated charge per nanocluster is determined as a function of the cluster size and the number of atoms in the cluster. The behavior of ΔE f(d) is controlled by the Coulomb energy of a charged cluster and by a decrease in the efficiency of electron screening, which is different in the metals studied. The broadening of photoelectron lines is determined by a spread of the cluster sizes and by lower electron screening in the final Fermi system. An asymmetry of the core-level electron spectra of nanoclusters can be explained using notions about the electron-hole pair excitation near the Fermi level. The effect of the structure of the density of electron states in the d band of transition metals on the asymmetry of photoelectron lines is considered and it is concluded that this structure near the Fermi level qualitatively changes with a decrease in the nanocluster size. The obtained results indicate that the behavior of the electron subsystem of clusters of the d-metals in a size range of 2–10 nm under consideration is close to the behavior of a normal Fermi system.

Study of the thermal transformations of Co- and Fe-exchanged zeolites A and X by “in situ” XRD under reducing atmosphere

“In situ” high temperature X-ray diffraction under reducing atmosphere is used for the first time to study the thermal stability and transformations of Co- and Fe-exchanged A and X zeolites. TG-DTA and “ex situ” XRD characterization were also carried out. The temperature of incipient crystallization of metallic phase was found to be 700 °C in Fe-zeolites and 800 °C in Co-zeolites. Moreover, ex situ X-ray experiments, after thermal treatment both under inert and reducing atmosphere, revealed the formation of ceramic phases upon the thermal collapse of the zeolitic framework. Metal nanoparticles were obtained by reduction and the size of metal clusters was found to range between 24 and 40 nm.

Computational Study of Copper(II) Complexation and Hydrolysis in Aqueous Solutions Using Mixed Cluster/Continuum Models

We use density functional theory (B3LYP) and the COSMO continuum solvent model to characterize the structure and stability of the hydrated Cu(II) complexes [Cu(MeNH(2))(H(2)O)(n-1)](2+) and [Cu(OH)(x)(H(2)O)(n-x)](2-x) (x = 1-3) as a function of metal coordination number (4-6) and cluster size (n = 4-8, 18). The small clusters with n = 4-8 are found to be the most stable in the nearly square-planar four-coordinate configuration, except for [Cu(OH)(3)(H(2)O)](-), which is three-coordinate. In the presence of the two full hydration shells (n = 18), however, the five-coordinate square-pyramidal geometry is the most favorable for Cu(MeNH(2))(2+) (5, 6) and Cu(OH)(+) (5, 4, 6), and the four-coordinate geometry is the most stable for Cu(OH)(2) (4, 5) and Cu(OH)(3)(-) (4). (Other possible coordination numbers for these complexes in the aqueous phase are shown in parentheses.) A small energetic difference between these structures (0.23-2.65 kcal/mol) suggests that complexes with different coordination numbers may coexist in solution. Using two full hydration shells around the Cu(2+) ion (18 ligands) gives Gibbs free energies of aqueous reactions that are in excellent agreement with experiment. The mean unsigned error is 0.7 kcal/mol for the three consecutive hydrolysis steps of Cu(2+) and the complexation of Cu(2+) with methylamine. Conversely, calculations for the complexes with only one coordination shell (four equatorial ligands) lead to a mean unsigned error that is >6.0 kcal/mol. Thus, the explicit treatment of the first and the second shells is critical for the accurate prediction of structural and thermodynamic properties of Cu(II) species in aqueous solution.

Photocatalytic performance of TiO2 thin films connected with Cu micro-grid

Aiming at reducing the recombination of photo-induced carriers in semiconductor photocatalytic process, we prepared TiO2 thin film with its surface modified by a connected Cu micro-grid via a microsphere lithography strategy, which showed higher photocatalytic activity than the pure TiO2 film. The improvement of photocatalytic activity of Cu micro-grid to the TiO2 film is due to the charge carrier separation and electron transfer by the conducting metal grid. The photocatalytic activity was improved as metal loading increased, which obtained the best performance at a certain loading amount, and then decreased at higher loading amount. This phenomenon was attributed to the metal’s bulk effect which could be explained by the relationship between the energetic positions and the metal cluster size.

The nature of interactions between clusters of Mg and Zn with HCN from symmetry-adapted perturbation theory based of DFT

The donor-acceptor complexes HCN-Mg(n) and HCN-Zn(n) (n=1,...,4), which were recently detected in helium nanodroplet infrared spectroscopy experiments by Miller and co-workers [Science 292, 481 (2001); J. Phys. Chem. A 110, 5620 (2006)] are investigated by the symmetry-adapted perturbation theory based on the density functional monomer description [SAPT(DFT)]. The interaction energy components, such as the electrostatic, exchange, induction, and dispersion, are calculated as a function of the metal cluster size. We find that the donor-acceptor interactions manifest themselves by the large induction and dispersion interactions, which counteract the unusually large exchange repulsion. The dependence of the components on the clusters size n follows different patterns in the complexes of magnesium and zinc. In HCN-Mg(n) the induction effect increases in magnitude much faster than the dispersion effect. In HCN-Zn(n) there is a slight decrease in both dispersion and induction terms between n=2 and n=3. Then dispersion rises faster than induction between n=3 and n=4. The exchange effects are also much different in both types of complexes. The first-order exchange energy rises much faster with n in the magnesium complexes than in the zinc complexes. Furthermore, in the latter there is a significant drop in the exchange energy between n=2 and n=3. The second-order exchange effects tend to quench a larger percentage of the induction and dispersion contributions in the Mg(n) complexes than in Zn(n). These different patterns of the interaction energy variations with n are related to the different nature of nonadditive effects in the neat metal clusters.

Palladium clusters in nanoporous carbon samples: Structural properties

The structure of nanoporous carbon samples (prepared from silicon and boron carbides) with incorporated palladium clusters was studied. X-ray and electron diffraction studies show that most Pd clusters have an fcc lattice. Using small-angle X-ray scattering measurements, the metal cluster sizes are determined under certain assumptions. The sizes are not very close to the cluster sizes (4–14 nm) that are found from photomicrographs obtained using a transmission electron microscope (TEM). The difference is likely due to the local character of the TEM measurements. Apart from relatively coarse clusters of the sizes indicated above, the samples contain very small clusters, which are smaller in size than a micropore. Such small clusters are particularly large in number in C(SiC)B: Pd, where they are 0.5–0.7 nm in size. The content of small clusters in C(B4C)B: Pd is substantially lower, and they are somewhat larger in size (1.2–1.6 nm). The possible reasons of the ferromagnetism observed in these samples are discussed. It is assumed that the magnetism may be due to the small clusters, which do not have cubic symmetry.

Reactivity of Au nanoparticles supported over SiO 2 and TiO 2 studied by ambient pressure photoelectron spectroscopy

The influence of the metal cluster size and the support on the reactivity of gold-based catalysts has been studied in the CO oxidation reaction. To overcome the structural complexity of the supported catalysts, gold nanoparticles synthesized from colloidal chemistry with precisely controlled size have been used. Those particles were supported over SiO 2 and TiO 2 and their catalytic activity measured in a flow reactor. The reaction rate was dependent on the particle size and on the support, suggesting two reaction pathways in the CO oxidation reaction. In parallel, ambient pressure photoelectron spectroscopy (APPS) has been performed under reaction conditions using bidimensional model catalysts prepared by supporting Au nanoparticles over planar polycrystalline SiO 2 and TiO 2 thin films. The nanoparticles were transferred from a water surface where they have been dispersed by means of the Langmuir–Blodgett (LB) technique. In this way, the catalytically active surface was characterized under real reaction conditions, revealing that during CO oxidation gold remains in the metallic state.

Plasmonic properties of vapour-deposited polymer composites containing Ag nanoparticles and their changes upon annealing

The effect of temperature on microstructure and optical properties of nanocomposite films containing Ag nanoparticles embedded in a polymer matrix of Teflon AF was investigated in detail. Temperature effects were studied in two modes: the effect of temperature during preparation of the nanocomposites and post-deposition heat treatment. Substrate heating during deposition leads to a decrease in the condensation coefficient up to the glass transition temperature of the polymer and increases beyond this temperature. During heat treatment after deposition metal diffuses into the polymer and leads to a change in the microstructure of the nanocomposites resulting in larger metal cluster size, an increase in the interparticle distance and more spheroidal shaped clusters. An increase in the substrate temperature during deposition can lead to both blue shifts and red shifts of the plasmon peak position. Changes in the microstructure upon heat treatment above the glass transition temperature are caused by diffusion of metal clusters into the polymers. In this regime, the peak wavelength of the plasmon band shifts towards shorter wavelength with increasing temperature.

Strategy to improve the reproducibility of colloidal SERS

AbstractThere is a pressing need to improve the reproducibility of surface enhanced Raman spectroscopy (SERS) measurements, if the technique is to be used routinely for trace analysis. This is particularly true for colloidal SERS, in which data reproducibility is dominated by the final shape and size of metal clusters produced during colloid aggregation. This study presents general guidelines for designing appropriate measurement strategies that can be used to identify and optimise crucial steps in a protocol that leads to better reproducibility of the results. We show that the data reproducibility can be improved by optimising vortexing time during colloid aggregation, which we attribute to the formation of more reproducible metal clusters under conditions of ‘forced convection’. The study also investigated the effects of different storage conditions on the data reproducibility of SERS during a 6‐month study period. Storage conditions did not significantly influence the SERS reproducibility. However, at the end of 6 months, colloids that were stored (in plastic containers) at room temperature showed a difference in their quality, as mirrored by their different opto‐physical properties. This was made apparent through the analysis of UV‐vis spectroscopy measurements by principal component analysis. Copyright © 2007 John Wiley & Sons, Ltd.

Preparation of Pd–B/TiO2 amorphous alloy catalysts and their performance on liquid-phase hydrogenation of p-nitrophenol

The Pd–B/TiO2 amorphous alloy catalyst were prepared by the liquid-phase chemical reduction with KBH4 aqueous solution. The catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC) and high reaction transmission electron microscopy (HRTEM). Analysis results show that the active metal clusters Pd–B are homogeneously dispersed on the surface of the TiO2 particles and the size of most metal clusters is below 10 nm. The as-prepared Pd–B/TiO2 catalysts exhibit higher catalytic properties (activity, selectivity, and stability) than the corresponding Pd-based catalysts and Ni-based catalysts during the liquid-phase p-nitrophenol (PNP) hydrogenation to p-aminophenol (PAP), which indicates the promoting effect of the alloying B and TiO2 support. Enriched electron state of Pd active sites and high dispersion of the Pd–B active sites on TiO2 support are mainly attributed to the improvement of catalytic activity of Pd–B/TiO2 amorphous alloy catalyst. The effects of different reaction parameters such as support, pH value, Pd loading and the thermal treatment on the catalytic activity of the Pd–B/TiO2 amorphous alloy catalyst were studied in detail. The maximum preparation condition is achieved by using TiO2 (P25), pH 12 and 0.5% amount of Pd loading. © 2007 Elsevier B.V. All rights reserved.

Nucleation of metal clusters on plasma treated multi wall carbon nanotubes

The influence of a preliminary RF-plasma treatment of the multi wall carbon nanotube (MWCNTs) surface on the resulting overlayer morphology (dispersion, shape and size of metal clusters) formed by thermally evaporating Ag, Ni or Au atoms is studied. RF plasma treatments successfully bind polystyrene, oxygen, amine and fluorine chemical groups depending on the type of gas used for the treatment (styrene, O 2, Ar, NH 3 or CF 4) of the MWCNT surface. Transmission electron microscopy (TEM) analysis showed that the dispersion and size of the metal particles on MWCNT surface depend on the gas type used for plasma treatment, on the nature of the evaporated metal, on the MWCNT curvature and on the evaporation time.

A Simple Method for Measuring the Size of Metal Nanoclusters in Solution

The connection between quantum size effects and the surface plasmon resonance of metal nanoclusters is introduced and the pros and cons of in situ and ex situ cluster analysis methods are outlined. A new method for estimating the size of nanoclusters is presented. This method combines core/shell cluster synthesis, UV-visible spectroscopy, and Mie theory. The core/shell approach enables the estimation of metal cluster sizes directly from the UV-visible spectra, even for transition metal nanoclusters such as Pd that have no distinct surface-plasmon peak in UV-visible region. Pd/Au and Au/Pd core/shell clusters as well as Au-Pd alloy clusters are synthesized and used as test cases for simulations and spectroscopic measurements. The results of the simulations and UV-visible spectroscopy experiments are validated with transmission electron microscopy.

06/01901 Genetic fuzzy control applied to the inverter of solid oxide fuel cell for power quality improvement: Jurado, F. and Valverde, M. Electric Power Systems Research, 2005, 76, (1–3), 93–105

The influence of the metal cluster size and the support on the reactivity of gold-based catalysts has been studied in the CO oxidation reaction. To overcome the structural complexity of the supported catalysts, gold nanoparticles synthesized from colloidal chemistry with precisely controlled size have been used. Those particles were supported over SiO2 and TiO2 and their catalytic activity measured in a flow reactor. The reaction rate was dependent on the particle size and on the support, suggesting two reaction pathways in the CO oxidation reaction. In parallel, ambient pressure photoelectron spectroscopy (APPS) has been performed under reaction conditions using bidimensional model catalysts prepared by supporting Au nanoparticles over planar polycrystalline SiO2 and TiO2 thin films. The nanoparticles were transferred from a water surface where they have been dispersed by means of the Langmuir–Blodgett (LB) technique. In this way, the catalytically active surface was characterized under real reaction conditions, revealing that during CO oxidation gold remains in the metallic state.

Size dependence of the contact angle of liquid clusters of Bi and Sn supported on SiO2, Al2O3, graphite, diamond and AlN

Crystalline clusters of Bi and Sn with the diameters ranging from 100 to 5 nm were prepared by in situ evaporation on to different kinds of substrates in a transmission electron microscope. These metal clusters were then heated to the molten state and behaviour of the metal liquid clusters observed in situ in the transmission electron microscope. The contact angle of liquid clusters of Bi and Sn on different kinds of substrates changes as a function of the size of liquid metal clusters. It was concluded that the interfacial energy between the substrates and liquid metals depends on the size of liquid metal clusters and changes significantly at around 20 nm.

On the Face of It

CHEMISTRY Varying the size of a nanometer-scale metal cluster can alter its catalytic activity. This phenomenon is usually attributed either to geometrical effects (such as the distribution of defect atoms or step sites) or to electronic effects (such as the scaling of metallic character with

Current–voltage curves for molecular junctions: Metal basis set vs. cluster size

A 21 Au atom description of the contacts supports previous work that used only six Au atoms. Using the LANL1 (3s3p3d)/[1s1p1d] Au basis set in conjunction with the six atom cluster leads to solutions with excess charge on the Au atoms. While using the (3s3p3d)/[1s1p1d] Au basis set for all atoms in a larger 21 Au atom cluster also yields a non-physical solution, it is possible to use the full minimal basis set on the Au atoms that are not at the edge of the cluster.

Self-organized metal networks at ion-etched Cu∕Si and Ag∕Si interfaces

We report self-organized metal nanopatterns on Si substrates produced by ion beam etching. We have deposited thin layers of metal such as Cu or Ag on Si substrates and then etched the deposited layers by a 1–5keV Ar+ ion beam at room temperature. At the stage when the metal-Si interface is reached, we have observed networks of metal clusters on the Si substrate with the characteristic size of 30–60nm for Cu and 100–200nm for Ag. The Cu patterns are sensitive to the ion energy. At 1keV energy, we observe a well-defined Cu network, whereas at 3–5keV energy, the Cu pattern becomes fuzzy without clear boundaries. To systematize and explain our results, we have suggested a kinetic model that combines ion etching and coarsening of the metal clusters on Si substrates. From our kinetic Monte Carlo simulations, we have found that the cooperative effect of coarsening and etching has a regime when the size of metal clusters can be approximated by the expression a(4D∕aR)1∕3, where D is the surface diffusivity of metal adatoms on the Si substrate, R is the etch rate, and a is the interatomic distance. Our synergistic model of coarsening and sputtering explains the observed difference in Cu and Ag cluster sizes and predicts the fuzzy Cu patterns at elevated ion energies.

Organization of metal nanoclusters on fatty amine films using ion-dipole interaction

A new technique for the organization of metallic nanoparticles on fatty amine films is demonstrated. A selected polymer (PVP) is used which can both control the size of metal clusters in colloidal form and can help the clusters to be attached with the fatty amine films for a wide range of pH limit of the colloidal solution. The amount of metal clusters incorporated on the fatty amine surface can be controlled by controlling the time of exposure and pH of the colloidal solution. The ion-dipole interaction between the kyton group of PVP and the amino group of the amine is utilized for the incorporation of PVP capped metal clusters on the amine substrate. The same technique can be utilized for the organization of any metal particle on any substrate. Mechanism and optimum incorporation conditions are studied by microscopy and spectroscopic techniques.