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Nanoclusters
Selected AbstractsChemical, Mechanical, and Antibacterial Properties of Silver Nanocluster,Silica Composite Coatings Obtained by Sputtering,ADVANCED ENGINEERING MATERIALS, Issue 7 2010Monica Ferraris Abstract Silver nanocluster,silica matrix composite coatings have been deposited by radio frequency (RF) co-sputtering on silica substrates. Field emission scanning electron microscopy and X-ray diffraction spectra of the as deposited and heated samples (150,600,°C) revealed the presence of metal silver nanoclusters, their size depending on the heating treatment. The antibacterial activity of the as deposited and heated samples has been measured in accordance to National Committee for Clinical Laboratory Standards, and it has been demonstrated on samples heated up to 450,°C in contact mode and for samples heated at 600,°C in a liquid environment. Their antibacterial activity was still present after gamma ray and ethylene oxide gas (EtO) sterilization of the samples. Silver leaching tests on the as deposited and heated samples has been measured by graphite furnace atomic absorption spectrometer, revealing an amount ranging from 0.1 to 0.9,µg mm,2, over 28 days. Tape resistance (ASTM D3359-97) and scratch resistance tests have been done on each sample revealing a good adhesion of the coatings on silica. [source] A Nanoscopic 3D Polyferrocenyl Assembly: The Triacontakaihexa(ferrocenylmethylthiolate) [Ag48(,4 -S)6(,2/3 -SCH2Fc)36],ANGEWANDTE CHEMIE, Issue 26 2010Siawash Ahmar Der molekulare Nanocluster [Ag48(,4 -S)6(,2/3 -SCH2Fc)36] (1) (siehe Bild; Ag blau, S gelb, Fe orange, C grau; Fc=Ferrocenyl) wurde in guten Ausbeuten hergestellt und durch Einkristall-Röntgenbeugung charakterisiert. Die 36 Ferrocenylmethylthiolat-Liganden hüllen den Ag-S-Kern vollständig ein, und cyclovoltammetrische Messungen von 1 ergeben eine einzelne reversible Oxidationswelle. [source] Ein Gadolinium-verbrückter Polywolframarsenat(III)-Nanocluster: [Gd8As12W124O432(H2O)22]60,,ANGEWANDTE CHEMIE, Issue 48 2009Firasat Hussain Dr. POMs de terre: Die Kombination wolframhaltiger Polyoxometallat(POM)-Fragmente mit Gadoliniumkationen führt zu besonders anisotropen Polyoxowolframaten. So ist das Titelpolyanion (siehe Struktur; W grün, As pink, Gd blau, O rot, H2O cyan) fast 5,nm lang , dies macht seine Charakterisierung in Lösung anspruchsvoll, eröffnet aber auch neue Perspektiven zur Implementierung von POMs in der Nanotechnologie. [source] ChemInform Abstract: Nucleation Process in the Cavity of a 48-Tungstophosphate Wheel Resulting in a 16-Metal-Center Iron Oxide Nanocluster.CHEMINFORM, Issue 19 2008Ulrich Kortz Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] A Highly Charged Ag64+ Core in a DNA-Encapsulated Silver NanoclusterCHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2010Konrad Koszinowski Dr. Highly charged: Electrospray-ionization mass spectrometry shows that single-stranded oligonucleotide dC12 -encapsulated silver nanoclusters contain Ag64+ cores at basic pH values (see section of spectrum). The high positive charge of the cluster cores is presumably essential for preventing the multiply deprotonated dC12 ligand from unfolding. [source] Bromide-Induced Formation of a Highly Symmetric Silver Thiolate Cluster Containing 36 Silver Atoms from an Infinite Polymeric Silver ThiolateEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 14 2010Xiaomin Liu Abstract Owing to its good affinity with Ag+, Br, is able to truncate a silver thiolate polymer and induce the formation of a high-nuclearity silver thiolate nanocluster, [Br@Ag36(SC6H4tBu-4)36],. The nanocluster has a disc-like structure with one Br, anion trapped at the center of the cluster in an octahedral coordination. [source] Chemical, Mechanical, and Antibacterial Properties of Silver Nanocluster,Silica Composite Coatings Obtained by Sputtering,ADVANCED ENGINEERING MATERIALS, Issue 7 2010Monica Ferraris Abstract Silver nanocluster,silica matrix composite coatings have been deposited by radio frequency (RF) co-sputtering on silica substrates. Field emission scanning electron microscopy and X-ray diffraction spectra of the as deposited and heated samples (150,600,°C) revealed the presence of metal silver nanoclusters, their size depending on the heating treatment. The antibacterial activity of the as deposited and heated samples has been measured in accordance to National Committee for Clinical Laboratory Standards, and it has been demonstrated on samples heated up to 450,°C in contact mode and for samples heated at 600,°C in a liquid environment. Their antibacterial activity was still present after gamma ray and ethylene oxide gas (EtO) sterilization of the samples. Silver leaching tests on the as deposited and heated samples has been measured by graphite furnace atomic absorption spectrometer, revealing an amount ranging from 0.1 to 0.9,µg mm,2, over 28 days. Tape resistance (ASTM D3359-97) and scratch resistance tests have been done on each sample revealing a good adhesion of the coatings on silica. [source] A Quantum-Chemical Study on Understanding the Dehydrogenation Mechanisms of Metal (Na, K, or Mg) Cation Substitution in Lithium Amide NanoclustersADVANCED FUNCTIONAL MATERIALS, Issue 12 2010Lanlan Li Abstract The hydrogen-releasing activity of (LiNH2)6,LiH nanoclusters and metal (Na, K, or Mg)-cation substituted nanoclusters (denoted as (NaNH2)(LiNH2)5, (KNH2)(LiNH2)5, and (MgNH)(LiNH2)5) are studied using ab initio molecular orbital theory. Kinetics results show that the rate-determining step for the dehydrogenation of the (LiNH2)6,LiH nanocluster is the ammonia liberation from the amide with a high activation energy of 167.0,kJ,mol,1 (at B3LYP/6-31,+,G(d,p) level). However, metal (Na, K, Mg)-cation substitution in amide,hydride nanosystems reduces the activation energies for the rate-determining step to 156.8, 149.6, and 144.1,kJ,mol,1 (at B3LYP/6-31,+,G(d,p) level) for (NaNH2)(LiNH2)5, (KNH2)(LiNH2)5, and (MgNH)(LiNH2)5, respectively. Furthermore, only the ,NH2 group bound to the Na/K cation is destabilized after Na/K cation substitution, indicating that the improving effect from Na/K-cation substitution is due to a short-range interaction. On the other hand, Mg-cation substitution affects all ,NH2 groups in the nanocluster, resulting in weakened N,H covalent bonding together with stronger ionic interactions between Li and the ,NH2 group. The present results shed light on the dehydrogenation mechanisms of metal-cation substitution in lithium amide,hydride nanoclusters and the application of (MgNH)(LiNH2)5 nanoclusters as promising hydrogen-storage media. [source] Predicting the low energy landscape of nanoscale silica using interatomic potentialsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2006S. T. BromleyArticle first published online: 18 APR 200 Abstract The energies of 52 of the lowest lying structural isomers of the (SiO2)12 nanocluster are accurately calculated via energy minimisations employing density functional theory (DFT) and also with two silica interatomic potentials (IPs). Of the tested IPs, one was specifically parameterised with respect to small silica nanoclusters, and the other was biased to accurately recover bulk silica properties, although having been applied numerous times to silica nanosystems. The predicted energetic ordering of the nanocluster isomers resulting from the IP optimisations are compared with respect to their deviance from benchmark nanocluster energies from DFT calculations. Although both IPs predict the DFT ground state isomer to be a very low energy cluster and thus are of use in global optimisation studies, large fluctuations in the IP energies of other low lying isomers (relative to the respective DFT energies) shed doubt on their wider applicability to nanoscale silica systems. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] CVD growth of carbon nanotubes using molecular nanoclusters as catalystPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11-12 2009K. Goss Abstract The growth of isolated carbon nanotubes (CNTs) using chemical vapour deposition process catalysed by the molecular nanocluster {Mo72Fe30} of the Keplerate structure type in different oxidation states is studied on different substrates. The molecule in its oxidized state (MoVI/FeIII) agglomerates in solution. Thus, nucleation from large catalyst particles cannot provide a narrow CNT diameter distribution. However, a partially reduced and charged form of the same molecule (MoV/MoVI) enables growth from small nucleation sites as evidenced by atomic force microscope imaging, albeit the yield in this case is dramatically reduced. [source] Chemical, Mechanical, and Antibacterial Properties of Silver Nanocluster,Silica Composite Coatings Obtained by Sputtering,ADVANCED ENGINEERING MATERIALS, Issue 7 2010Monica Ferraris Abstract Silver nanocluster,silica matrix composite coatings have been deposited by radio frequency (RF) co-sputtering on silica substrates. Field emission scanning electron microscopy and X-ray diffraction spectra of the as deposited and heated samples (150,600,°C) revealed the presence of metal silver nanoclusters, their size depending on the heating treatment. The antibacterial activity of the as deposited and heated samples has been measured in accordance to National Committee for Clinical Laboratory Standards, and it has been demonstrated on samples heated up to 450,°C in contact mode and for samples heated at 600,°C in a liquid environment. Their antibacterial activity was still present after gamma ray and ethylene oxide gas (EtO) sterilization of the samples. Silver leaching tests on the as deposited and heated samples has been measured by graphite furnace atomic absorption spectrometer, revealing an amount ranging from 0.1 to 0.9,µg mm,2, over 28 days. Tape resistance (ASTM D3359-97) and scratch resistance tests have been done on each sample revealing a good adhesion of the coatings on silica. [source] Silver Surface Iodination for Enhancing the Conductivity of Conductive CompositesADVANCED FUNCTIONAL MATERIALS, Issue 16 2010Cheng Yang Abstract The electrical conductivity of a silver microflake-filled conductive composites is dramatically improved after a filler surface treatment. By a simple iodine solution treatment, nonstoichiometric silver/silver iodide nanoislands form on the silver filler surface. Evidence of the decrease of surface silver oxide species is provided by TOF-SIMS and the redox property of the nanoclusters is studied using cyclic voltammetry and TOF-SIMS depth profile analyses. The redox property of the nanoclusters on silver flakes helps enhance the electrical conductivity of the conductive composites. The electrical resistivity of the improved conductive composites is measured by four-point probe method; the reliability of the printed thin film resistors is evaluated by both the 85 °C/85% relative humidity moisture exposure and the ,40 , 125 °C thermal cycling exposure. The conductive composite printed radio frequency identification (RFID) antennas with 27.5 wt% of the modified silver flake content show comparable performance in the RFID tag read range versus copper foil antennas, and better than those commercial conductive adhesives that require much higher silver content (i.e., 80 wt%). This work suggests that a surface chemistry method can significantly reduce the percolation threshold of the loading level of the silver flakes and improve the electrical conductivity of an important printed electronic passive component. [source] Silver Surface Iodination for Enhancing the Conductivity of Conductive CompositesADVANCED FUNCTIONAL MATERIALS, Issue 16 2010Cheng Yang Abstract The electrical conductivity of a silver microflake-filled conductive composites is dramatically improved after a filler surface treatment. By a simple iodine solution treatment, nonstoichiometric silver/silver iodide nanoislands form on the silver filler surface. Evidence of the decrease of surface silver oxide species is provided by TOF-SIMS and the redox property of the nanoclusters is studied using cyclic voltammetry and TOF-SIMS depth profile analyses. The redox property of the nanoclusters on silver flakes helps enhance the electrical conductivity of the conductive composites. The electrical resistivity of the improved conductive composites is measured by four-point probe method; the reliability of the printed thin film resistors is evaluated by both the 85 °C/85% relative humidity moisture exposure and the ,40 , 125 °C thermal cycling exposure. The conductive composite printed radio frequency identification (RFID) antennas with 27.5 wt% of the modified silver flake content show comparable performance in the RFID tag read range versus copper foil antennas, and better than those commercial conductive adhesives that require much higher silver content (i.e., 80 wt%). This work suggests that a surface chemistry method can significantly reduce the percolation threshold of the loading level of the silver flakes and improve the electrical conductivity of an important printed electronic passive component. [source] A Quantum-Chemical Study on Understanding the Dehydrogenation Mechanisms of Metal (Na, K, or Mg) Cation Substitution in Lithium Amide NanoclustersADVANCED FUNCTIONAL MATERIALS, Issue 12 2010Lanlan Li Abstract The hydrogen-releasing activity of (LiNH2)6,LiH nanoclusters and metal (Na, K, or Mg)-cation substituted nanoclusters (denoted as (NaNH2)(LiNH2)5, (KNH2)(LiNH2)5, and (MgNH)(LiNH2)5) are studied using ab initio molecular orbital theory. Kinetics results show that the rate-determining step for the dehydrogenation of the (LiNH2)6,LiH nanocluster is the ammonia liberation from the amide with a high activation energy of 167.0,kJ,mol,1 (at B3LYP/6-31,+,G(d,p) level). However, metal (Na, K, Mg)-cation substitution in amide,hydride nanosystems reduces the activation energies for the rate-determining step to 156.8, 149.6, and 144.1,kJ,mol,1 (at B3LYP/6-31,+,G(d,p) level) for (NaNH2)(LiNH2)5, (KNH2)(LiNH2)5, and (MgNH)(LiNH2)5, respectively. Furthermore, only the ,NH2 group bound to the Na/K cation is destabilized after Na/K cation substitution, indicating that the improving effect from Na/K-cation substitution is due to a short-range interaction. On the other hand, Mg-cation substitution affects all ,NH2 groups in the nanocluster, resulting in weakened N,H covalent bonding together with stronger ionic interactions between Li and the ,NH2 group. The present results shed light on the dehydrogenation mechanisms of metal-cation substitution in lithium amide,hydride nanoclusters and the application of (MgNH)(LiNH2)5 nanoclusters as promising hydrogen-storage media. [source] Ultrafast Hole-Transfer Dynamics in Polymer/PCBM Bulk HeterojunctionsADVANCED FUNCTIONAL MATERIALS, Issue 10 2010Artem A. Bakulin Abstract Ultrafast dynamics of the hole-transfer process from methanofullerene to a polymer in a polymer/PCBM bulk heterojunction are directly resolved. Injection of holes into MDMO-PPV is markedly delayed with respect to [60]PCBM excitation. The fastest component of the delayed response is attributed to the PCBM,polymer hole-transfer process (30,±,10,fs), while the slower component (,150,fs) is provisionally assigned to energy transfer and/or relaxation inside PCBM nanoclusters. The charge generation through the hole transfer is therefore as fast and efficient as through the electron-transfer process. Exciton harvesting efficiency after PCBM excitation crucially depends on the concentration of the methanofullerene in the blend, which is related to changes in the blend morphology. Ultrafast charge generation is most efficient when the characteristic scale of phase separation in the blend does not exceed ,20,nm. At larger-scale phase separation, the exciton harvesting dramatically declines. The obtained results on the time scales of the ultrafast charge generation after PCBM excitation and their dependence on blend composition and morphology are instrumental for the future design of fullerene-derivative-based photovoltaic devices. [source] Gold-Nanocluster-Based Fluorescent Sensors for Highly Sensitive and Selective Detection of Cyanide in WaterADVANCED FUNCTIONAL MATERIALS, Issue 6 2010Yanlan Liu Abstract A novel, gold-nanocluster-based fluorescent sensor for cyanide in aqueous solution, which is based on the cyanide etching-induced fluorescence quenching of gold nanoclusters, is reported. In addition to offering high selectivity due to the unique Elsner reaction between cyanide and the gold atoms of gold nanoclusters, this facile, environmentally friendly and cost-effective method provides high sensitivity. With this sensor, the lowest concentration to quantify cyanide ions could be down to 200,×,10,9,M, which is approximately 14 times lower than the maximum level (2.7,×,10,6,M) of cyanide in drinking water permitted by the World Health Organization (WHO). Furthermore, several real water samples spiked with cyanide, including local groundwater, tap water, pond water, and lake water, are analyzed using the sensing system, and experimental results show that this fluorescent sensor exhibits excellent recoveries (over 93%). This gold-nanocluster-based fluorescent sensor could find applications in highly sensitive and selective detection of cyanide in food, soil, water, and biological samples. [source] Thermo-Switchable Charge Transport and Electrocatalysis Using Metal-Ion-Modified pNIPAM-Functionalized ElectrodesADVANCED FUNCTIONAL MATERIALS, Issue 15 2009Michael Riskin Abstract Metal ions (Ag+, Cu2+, Hg2+) are incorporated into an electropolymerized, poly(N -isopropyl acrylamide), pNIPAM, thermosensitive polymer associated with an electrode using the "breathing-in" method. The ion-functionalized pNIPAM matrices reveal ion-dependent gel-to-solid phase-transition temperatures (28,±,1,°C, 25,±,1,°C, 40,±,1,°C for the Ag+, Cu2+, and Hg2+ -modified pNIPAM, respectively). Furthermore, the ion-functionalized polymers exhibit quasi-reversible redox properties, and the ions are reduced to the respective Ag0, Cu0, and Hg0 nanocluster-modified polymers. The metal-nanocluster-functionalized pNIPAM matrices enhance the electron transfer (they exhibit lower electron-transfer resistances) in the compacted states. The electron-transfer resistances of the metal-nanocluster-modified pNIPAM can be cycled between low and high values by temperature-induced switching of the polymer between its contracted solid and expanded gel states, respectively. The enhanced electron-transfer properties of the metal nanocluster-functionalized polymer are attributed to the contacting of the metal nanoclusters in the contracted state of the polymers. This temperature-switchable electron transfer across a Ag0 -modified pNIPAM was implemented to design a thermo-switchable electrocatalytic process (the temperature-switchable electrocatalyzed reduction of H2O2 by Ag0 -pNIPAM). [source] Metal Chalcogenide Clusters on the Border between Molecules and Materials,ADVANCED MATERIALS, Issue 18 2009John F. Corrigan Abstract The preparative and materials chemistry of high nuclearity transition metal chalcogenide nanoclusters has been in the focus of our research for many years. These polynuclear metal compounds possess rich photophysical properties and can be understood as intermediates between mononuclear complexes and binary bulk phases. Based on our previous results we discuss herein recent advances in three different areas of cluster research. In the field of copper selenide clusters we present the synthesis of monodisperse, nanostructured , -Cu2Se via the thermolysis of well-defined cluster compounds as well as our approaches in the synthesis of functionalized clusters. In case of silver chalcogenides we established a strategy to synthesis cluster compounds containing several hundreds of silver atoms with the nanoclusters arranging in a closely packed crystal lattice. Finally the presented chalcogenide clusters of the group 12 metals (Zn, Cd, Hg) can be taken as model compounds for corresponding nanoparticles as even the smallest of frameworks display a clear structural relationship to the bulk materials. [source] Synthesis and Characterization of Iron Oxide Derivatized Mutant Cowpea Mosaic Virus Hybrid Nanoparticles,ADVANCED MATERIALS, Issue 24 2008Alfredo A. Martinez-Morales A novel nanoparticle hybrid is attained by the covalent attachment of iron oxide (, -Fe2O3) nanoclusters onto the surface of a mutagenized cow pea mosaic virus (CPMV-T184C). Using a stepwise substrate-based integration, monodisperse CPMV-IO hybrids are anchored on a gold substrate. The physical and magnetic properties of individual CPMV-IO hybrids are qualitatively investigated by atomic/magnetic force microscopy (AFM/ MFM). During MFM characterization a ,boundary-effect' is observed at the CPMV/IO interface. [source] Quantized Double-Layer Charging of Rhodium2057(Tridecylamine)321 Clusters Using Differential Pulse and Cyclic Voltammetry,ADVANCED MATERIALS, Issue 2 2007A. Kakade Rhodium nanoclusters show a series of quantized double-layer-charging events in solution-phase voltammetry at room temperature (see figure). The unusual variation in the FWHM for both the cathodic and anodic regions in differential pulse voltammetry experiments can be explained by several complex factors including reorganization and disproportionation of charged clusters coupled with electron-transfer processes pertaining to the Rh,Rh bonds. [source] Controlling the Morphology of Carbon Nanotube Films by Varying the Areal Density of Catalyst Nanoclusters Using Block-Copolymer Micellar Thin Films,ADVANCED MATERIALS, Issue 17 2006D. Bennett The morphologies of carbon nanotube (CNT) thin films are controlled by varying the areal density of catalytic nanoclusters synthesized from a block-copolymer micellar thin film. The morphologies of the CNT films vary from a tangled and sparse arrangement of individual CNTs, through a transition region with locally bunched and self-aligned CNTs, to a rapid growth of thick vertical CNT films (see figure). [source] Assemblies of Metal Nanoparticles and Self-Assembled Peptide Fibrils,Formation of Double Helical and Single-Chain Arrays of Metal Nanoparticles,ADVANCED MATERIALS, Issue 11 2003X. Fu Double helical and single-chain arrays of Au and Pd metal nanoclusters have been achieved by assembling the corresponding metal nanoparticles on peptide nanofibril templates at different pH values. The assembled metal nanoparticles are useful for studying the superstructures of the peptide nanofibrils. Pd nanowires of different shapes have also been prepared by varying the amount of metal deposition. [source] Phase behavior of ionic clusters down to nanoscale.INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2010A review of recent work Abstract As finding an exact and manageable partition function for nanoclusters is a desirable but, so far, unattainable task, approximated treatments are proposed to explain and predict phase changes and phase coexistence at these size scales. In this article, a review of those approaches is presented, mainly focusing the authors work on the subject. The foundations and limitations of the proposed models are discussed and perspectives for extended treatments are given. The discussions are illustrated with new molecular dynamics simulations of unconstrained NaI and NaCl clusters. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] Synchronization patterns in spaghetti-like nanoclustersINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2008Acep Purqon Abstract Spaghetti-like nanoclusters concern disordered shapes and irregular shape fluctuations in few correlated biological lipids. We evaluate the shape fluctuations by introducing Symmetry- S as a physical parameter for measuring symmetry degrees and detecting shape transitions. From numerical simulation of few correlated lipids of POPC and POPE at 300 K and 340 K by using molecular dynamics, we investigate the symmetry dynamics for each individual cluster by analyzing both spatiotemporal and frequency. From spatiotemporal analysis, we find several jump motions in S -dynamics and non-Gaussian distributions in S -distribution. Interestingly, the jump motions likely contribute on the existence of transitions in the non-Gaussian distributions. Additionally, even number of lipids show more symmetric than the odd number of lipids and the symmetry distributions shift at higher temperature, while, from three dimension of actual position of symmetry dynamics, they are not easy to configure high symmetry as well as showing certain patterns. From power spectra density analysis, each individual cluster shows nearly random fluctuation. Besides individual clusters, we also investigate mutual clusters. Surprisingly, although individual clusters show fluctuations randomly, mutual clusters show certain direction correlations. Moreover, they show certain patterns in delayed time analysis such as mutual fluctuations periodically occur for same number of lipids. It indicates that an existence of synchronization patterns occur in shape fluctuations of spaghetti-like nanoclusters. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Surface segregation in bimetallic nanoclusters: Geometric and thermodynamic effectsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2001Daniela S. Mainardi Abstract Nanocluster morphology (size, crystallographic faces, surface defects), coupled to thermodynamic driving forces, influence surface segregation phenomena. These geometric and thermodynamic effects are studied at several temperatures and overall compositions for Cu,Ni, Ag,Cu, and Ag,Rh. The bond order metal simulation model [Zhu, L.; DePristo, A. E. J Catal 1997, 167, 400] is used in Monte Carlo simulations to describe interatomic interactions. Nanoclusters from 110 to 729 atoms are arranged on stacks of layers corresponding to face-centered cubic crystals. The exposed surfaces are (111), (110), and (331) faces, and the overall composition of the clusters defined in terms of the segregating atoms ranges from 5 to 95%. The effect of surface steps on surface segregation is also investigated. It is found that as overall concentration increases, the effect of surface defects on segregation phenomena becomes more pronounced. The increase in the number of low coordination sites due to the presence of surface steps modifies the layer-by-layer composition profile, causing surface and inner sites in the proximity of the step to become occupied by the segregating atoms. The population on other surfaces becomes depleted, resulting in a lowered total surface segregation. These effects are more noticeable not only at high overall composition but also at high temperatures, especially for the less segregating system, Ag,Cu. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] Hydroxypropyl-,-Cyclodextrin-Capped Palladium Nanoparticles: Active Scaffolds for Efficient Carbon-Carbon Bond Forming Cross-Couplings in WaterADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 14-15 2009Jaqueline Abstract A new approach for the preparation of palladium nanoparticles in water from a renewable source, 2-hydroxypropyl-,-cyclodextrin (,-HPCD), which acts both as a reductant and capping agent, is presented. The palladium nanoparticles were characterized by using dynamic light scattering (DLS), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), which revealed the formation of spherical particles in the size range of 2,7,nm. Further analysis by Fourier-transform infrared spectroscopy (FT-IR) and 1H,NMR did not show covalent bonds between cyclodextrins and palladium nanoparticles, suggesting that ,-HPCD is only physically adsorbed on the nanoparticle surface, presumably through hydrophobic interactions which limit the mutual coalescence of nanoclusters. The catalytic activity was tested in Suzuki, Heck and Sonogashira reactions in neat water, providing good yields and selectivities of coupling products under very low Pd loadings (0.5,0.01,mol%). Remarkably, the nanocatalyst showed significant stability hence the aqueous phase remained active for four subsequent runs. The combination of a binding site for substrates (the HPCD cavity) and a reactive centre (Pd core) provides a potential to explore functional catalysis in aqueous medium. [source] Aerobic Oxidation of Alcohols under Mild Conditions Catalyzed by Novel Polymer-Incarcerated, Carbon-Stabilized Gold NanoclustersADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2008Céline Lucchesi Abstract Polymer-incarcerated, carbon-stabilized gold nanoclusters were found to be highly active in heterogeneous catalysis for the oxidation of secondary alcohols using molecular oxygen in aqueous medium. After optimization of catalyst preparation methods, highly loaded and highly effective catalysts were obtained, and a broad range of secondary alcohols could be oxidized by using these catalysts under mild conditions. The catalysts could be recovered and reused several times without significant loss of activity. Moreover, kinetics of the oxidation reaction with (±)-1-phenylethanol was investigated. [source] Structure of magnetic poly(oxyethylene),siloxane nanohybrids doped with FeII and FeIIIJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2003N. J. O. Silva Hybrid organic,inorganic nanocomposites doped with FeII and FeIII ions and exhibiting interesting magnetic properties have been obtained by the sol,gel process. The hybrid matrix of these ormosils (organically modified silicates), classed as di-ureasils and termed U(2000), is composed of poly(oxyethylene) chains of variable length grafted to siloxane groups by means of urea crosslinkages. Iron perchlorate and iron nitrate were incorporated in the di-ureasil matrices, leading to compositions within the range 80 ,n, 10, n being the molar ratio of ether-type O atoms per cation. The structure of the doped di-ureasils was investigated by small-angle X-ray scattering (SAXS). For FeII -doped samples, SAXS results suggest the existence of a two-level hierarchical structure. The primary level is composed of spatially correlated siloxane clusters embedded in the polymeric matrix and the secondary, coarser level consists of domains where the siloxane clusters are segregated. The structure of FeIII -doped hybrids is different, revealing the existence of iron oxide based nanoclusters, identified as ferrihydrite by wide-angle X-ray diffraction, dispersed in the hybrid matrix. The magnetic susceptibility of these materials was determined by zero-field-cooling and field-cooling procedures as functions of both temperature and field. The different magnetic features between FeII - and FeIII -doped samples are consistent with the structural differences revealed by SAXS. While FeII -doped composites exhibit a paramagnetic Curie-type behaviour, hybrids containing FeIII ions show thermal and field irreversibilities. [source] Non-destructive Raman study of the glazing technique in lustre potteries and faience (9,14th centuries): silver ions, nanoclusters, microstructure and processingJOURNAL OF RAMAN SPECTROSCOPY, Issue 3 2004Philippe Colomban Abstract The oldest known nanotechnology dates back to the fabrication of the first lustre potteries. A lustre is a thin film formed just below the surface of medieval Islamic glazed potteries which contains silver and/or copper in the metallic and ionic form. Raman studies of the lustre films of different ceramics excavated from Fustât (near Cairo, Egypt, 11,12th century) or from the Silk Road (Termez, 13,14th centuries) showed that they associate many layers of different compositions (with or without cassiterite). Energy-dispersive spectroscopic analysis shows that all studied glazes are Ca- (and K)-rich, nearly free of Al silicates, with some addition of lead. Comparison is made with a copy of three-colour Tang ceramics made in Bassorah or Baghdad, in the 9th century, which is among the first known ,faiences', i.e. ceramics enamelled with an Sn-containing glaze. Surprisingly, Sn is not present in the form of a cassiterite (SnO2) precipitate but as a Ca,K-rich salt. Composition analysis and Raman spectra show that all glazes have been processed with similar technology. The distribution of elemental Ag and Cu is very heterogeneous in the lustre decor. The main Raman signature (50,100 cm,1 peaks) of the lustre film is assigned to Ag+ ions. The additional low-wavenumber features could be due to the Ag0 [or (Agn)m+] nanocluster modes. It is clear that the lustre colour arises from the combination of iridescence (diffraction) and absorption/diffusion. Raman criteria are proposed for a sample classification as a function of processing (cassiterite content, processing temperature). The glazing technique is discussed on the basis of experimental evidence and ancient potters' reports. Exothermic burning of acetate residus is proposed as the key step for the preparation of polychrome lustre. Copyright © 2004 John Wiley & Sons, Ltd. [source] Thermodynamically Stable SiwCxNyOz Polymer-Like, Amorphous Ceramics Made from Organic PrecursorsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2008Riham M. Morcos This communication reports new results on the enthalpy of formation of pseudo-amorphous ceramic compounds constituted from silicon, carbon, oxygen, and nitrogen (SiCNO), made from the polymer route. Again, like the SiCO materials, although with one exception, the enthalpy of formation from crystalline components (SiO2 cristobalite, ,-Si3N4, SiC, and excess C) is negative. Some of the alloyed oxygen,nitrogen compositions yield enthalpies that are much more negative (,100 kJ/g·atom) in comparison with compositions that contain mainly oxygen or nitrogen (,20 kJ/g·atom). The exception, having a N/O ratio near 2, has a positive value for the enthalpy. This may reflect the presence of nanoclusters of stoichiometric Si2N2O instead of the pseudo-amorphous nanodomain structure seen for the other samples. [source] | |||||||||||||||||||||||||||||||