Metal Clusters Of Sizes Research Articles

Cu and Pt clusters deposition on TiO2 powders by DC magnetron sputtering for photocatalytic hydrogen production

Titanium dioxide-based photocatalyst powders were obtained by deposition of copper and/or platinum clusters by means of pulsed direct current magnetron sputtering with different deposition times and plasma composition during Cu/Pt sputtering. A top-down configuration was employed with the sputtering source facing the powder holder mounted on a shaker mechanism, which continuously mixed the powder during the sputtering process. HRTEM analyses revealed the presence of well dispersed, subnanometric sized metal clusters, even for long deposition times, while XRPD analysis showed no modification of the TiO2 crystal structure upon metal deposition. The so obtained powders were tested as photocatalysts in methanol photo-steam reforming for hydrogen production. The presence of Pt clusters increased the photoactivity with respect to that of bare TiO2. The plasma composition during Cu sputtering was found to strongly affect the photoactivity of the obtained materials, Cu alone deposited as co-catalyst in an Ar-only atmosphere imparting better photoactivity than Cu sputtered in Ar/O2. When the deposition of Cu clusters was coupled with the deposition of Pt clusters, an additive effect of the two metals in increasing TiO2 photoactivity was observed if Cu clusters were sputtered in the absence of oxygen.

Luminescent centers in nanolayers of LiF crystals with embedded silver ions

The paper presents the results of studies of luminescent centers induced by irradiation of LiF crystals with flow of silver ions of a fluence varying in a range of 2×1013−5×1017 ions/cm2 and energy about of 150 keV. Two bands with maxima at 250 nm and 420 nm in the absorption spectra of the exposed crystals were observed. The former was due to the absorption of F colour centers and the latter consisted of F2 (F3+) colour centers band and a plasmon resonance band arose due to the embedded silver ions. The peak of the latter remained stable after annealing the sample at 400 °C unlike the colour centers bands. The luminescence spectra of the crystals under excitation with a laser irradiation of 375 nm wavelength showed a band with a peak at 450 nm along with the bands with peaks at 530 and 680 nm corresponding to F3+ and F2 colour centers, respectively. The peak of the former reached saturation, while the peaks of the colour centers bands increased with ion fluence increasing. These results show that sub-nanometre sized metal clusters Ag(n+) (n=3-6) are responsible for the observed luminescence, while the Ag particles of a nanometer scale are not luminescent.

ChemInform Abstract: Inorganic Molybdenum Octahedral Nanosized Cluster Units, Versatile Functional Building Block for Nanoarchitectonics

AbstractReview: 125 refs.

Inorganic Molybdenum Octahedral Nanosized Cluster Units, Versatile Functional Building Block for Nanoarchitectonics

Recently, Nanoarchitectonics has been introducedas a new concept that refers to a technology systemfor arranging nanoscale structural units in a requiredarchitecture. Multifunctional properties are achieved by acombination of several materials in a well-defined architecture.In the frame of this concept, composite nanoarchitecturesrepresent a new class of nanostructured entitiesthat integrate various dissimilar nanoscale building blocksincluding clusters, particles, wires and films. The heterogeneouscomposite nanostructured materials are composedby definition of multi-(nano)components, each tailored toaddress different requirements. As one of the nanocomponents,nanometer sized metal clusters (<2 nm), whichconsist of less than a few dozens of metal atoms, could bedefined as a link between atom and nanoparticle. In thispaper, we will focus on our results on new nano-compositesinvolving Mo6 atom nanosized cluster units for optical,nanobiotechnology, energy and environmental applications.

Sub-nanometre sized metal clusters: from synthetic challenges to the unique property discoveries

Sub-nanometre sized metal clusters, with dimensions between metal atoms and nanoparticles, have attracted more and more attention due to their unique electronic structures and the subsequent unusual physical and chemical properties. However, the tiny size of the metal clusters brings the difficulty of their synthesis compared to the easier preparation of large nanoparticles. Up to now various synthetic techniques and routes have been successfully applied to the preparation of sub-nanometre clusters. Among the metals, gold clusters, especially the alkanethiolate monolayer protected clusters (MPCs), have been extensively investigated during the past decades. In recent years, silver and copper nanoclusters have also attracted enormous interest mainly due to their excellent photoluminescent properties. Meanwhile, more structural characteristics, particular optical, catalytic, electronic and magnetic properties and the related technical applications of the metal nanoclusters have been discovered in recent years. In this critical review, recent advances in sub-nanometre sized metal clusters (Au, Ag, Cu, etc.) including the synthetic techniques, structural characterizations, novel physical, chemical and optical properties and their potential applications are discussed in detail. We finally give a brief outlook on the future development of metal nanoclusters from the viewpoint of controlled synthesis and their potential applications.

Dynamic screening and electron–electron scattering in low-dimensional metallic systems

The modification of dynamic screening in the electron–electron interaction in systems with reduced dimensionality and tunable one-particle electronic structure is studied. Two examples of such systems are considered, namely, the adsorbate-induced quantum well states at the Na adlayer covered Cu(1 1 1) surface, and metal clusters of sizes up to few nanometers. The dependence of the electron–electron decay rates on the Na coverage in the former case and on the cluster size in the latter is investigated. The role played by the dynamical screened interaction in such processes is addressed as well.

Nucleation and growth of 1B metal clusters on rutile TiO2(1 1 0): Atomic level understanding from first principles studies

Nanometer sized metal clusters dispersed on oxide supports often exhibit much higher activity than single-component metal catalysts. Their catalytic performance markedly depends on cluster size, shape and size distributions, along with support materials and support preparation methods. Supported metal nanoclusters can also easily rearrange and sinter during the course of thermally activated catalytic reactions even at moderate temperatures. An accurate assessment of the effects of cluster–support interactions on the growth, structure and reactivity of supported metal clusters, as well as the adsorbate-induced structural changes is therefore necessary to understand their catalytic performance under realistic operating conditions. The detailed understanding will also contribute to development of a new and reliable way to control their structural catalytic properties on the atomic scale. As a part of the effort to gain this atomic level understanding, we present our recent findings from density functional theory calculations, including: electronic structure of a reduced TiO2(110) surface and interactions between oxygen vacancies, with a brief introduction to the dynamics of oxygen molecules on the reduced surface, role of oxygen vacancies and oxygen adspecies in the nucleation of Au, Ag and Cu clusters.

Blue shift of plasmon resonance in Cu and Ag ion-exchanged and annealed soda-lime glass: an optical absorption study

Metal nanocluster composite glasses are formed by the ion-exchange technique of dipping the host matrix in the respective metal salt bath of interest. These ion-exchanged glasses are then annealed in air for 1 h at different temperatures. The optical absorption spectra of the ion-exchanged and annealed samples confirmed the presence of nano sized metal clusters embedded inside the glass matrix. With increase of the annealing temperature, the absorption peak of the Cu and Ag nanoparticles showed a blue shift which can be attributed to the change in particle size and volume fraction. Annealing of the sequential Cu and Ag ion-exchanged soda-lime glass resulted in the formation of complex metal nanocluster composite glass, with the optical spectra exhibiting two peaks corresponding to the surface plasmon resonance of both metals.

Mechanism of metal droplet nucleation from supersaturated vapor

An analysis of the full nonlinear mechanism of condensation nucleation from a supersaturated vapor suggests that the classical Markovian growth mechanism may not apply to systems, such as metals, that exhibit pronounced magic-numbered clusters. Packing of atoms in concentric layers and/or quantum confinement of valence electrons lead to particular sized metal clusters that are anomalously stable. They are called magic-numbered clusters. Magic-numbered clusters are not explicitly addressed in the Markovian mechanism that assumes that clusters change size by accommodation or evaporation of one monomer at a time. This is a justifiable approximation only if the frequency of all cluster-cluster collisions is negligible compared with the frequency of the succession of monomer-cluster collisions required to produce an equivalent sized cluster to the cluster-cluster collision. Since the stability of magic-numbered clusters increases the likelihood of collisions between clusters, magic-numbered clusters can affect the nucleation mechanism and may have a significant influence on the nucleation rate from supersaturated metal vapors.

Observation of weakly ionized dusty plasma in exhaust gas in a compact-size SF/sub 6/ gas circuit breaker

Measurement of exhausted gas was carried out in an exhaust chamber of a practical compact-size gas circuit breaker. The result show that the gas is weakly ionized, ablated metallic materials exist as metal clusters of sizes from 10 to 100 /spl mu/m, and their major constituent is iron.

X-Ray Mapping of Bimetallic Catalysts in Mesoporous Silica

The small electron probe available in a scanning transmission electron microscope (STEM), when combined with X-ray energy dispersive spectroscopy (XEDS), enables the acquisition of 2D compositional maps with high spatial resolution and sensitivity. In addition to the spectroscopic information available, images sensitive to atomic number (so called Z-contrast images) can be acquired simultaneously using high-angle annular dark field imaging (HAADF). These capabilities make the STEM an ideal tool to study elemental distributions in catalysts containing 1-2 nm sized metal clusters within a support of low atomic number. The material examined in this work comprises of mesoporous silica in a honeycomb structure with channels of ˜3 nm diameter (designated MCM-41) in which nano-clusters of the bimetallic Pd6Ru6 have been introduced into the channels using metal-cluster carbonyls as precursors. Details of the specimen preparation are similar to that outlined by Shepard et al. for a Cu-Ru bimetallic catalyst.Fig. 1 shows (a) a bright-field (BF) image, (b) a HAADF image and (c-f) X-ray elemental maps acquired on a VG 603 STEM, operated at 300 keV with probe size of ˜1.4 nm (FWTM) and probe current of ˜0.6 nA.

Magnetic properties of Ni and Fe clusters: a tight binding molecular dynamics study

We introduce an efficient approach which incorporates structural and magnetic ordering in transition metal clusters of sizes well beyond the range of ab initio techniques. The method combines the Hubbard model approximation with the tight binding molecular dynamics technique. Applications of the present scheme to Ni n ( n ⩽ 7), and Fe n ( n ⩽ 5) clusters show that while magnetic ordering does not affect the geometric ordering of Ni clusters, inclusion of magnetic effects strongly influences the structure of Fe clusters, forcing them to attain geometric configurations of higher symmetry than paramagnetic states.