Colloidal Metal Particles Research Articles

Physicochemical properties of small metal particles in solution: "microelectrode" reactions, chemisorption, composite metal particles, and the atom-to-metal transition

The study of nanometer and subnanometer colloidal metal particles in aqueous solution complements the investigation of small particles in molecular beams, frozen solutions, and inorganic clusters. The electronic properties of the metal particles are changed by surface modification, for example by chemisorption of a nucleophilic molecule or by deposition of a second metal. The resulting changes in the chemical, photochemical, and optical properties are discussed. Methods are described which enable one to store excess electrons or positive holes on the particles in a controlled manner and to investigate the accompanying changes in the optical properties. Metal particles carrying excess electrons initiate electrochemical reactions such as the reduction of water or the deposition of metals

Synthesis and characterisation of colloidal metal and semiconductor particles prepared in microemulsions

The preparation of colloidal particles of platinum ‘in situ’ in water-in-oil microemulsions formed by the aerosol-OT (AOT)–water–heptane system and the hexadecyltrimethylammonium bromide (CTAB)–water–heptane–chloroform system is described. In both systems, particle growth is controlled and particle sizes of the order of a nm or so are obtained. In the AOT system, there is only a weak correlation between the particle size and droplet size. To obtain further insight into the mechanism of particle growth, the kinetics of the process were investigated. Nucleation particles (Pt°) are produced in an initial step by the reduction of PtCl2–6(PtIV) using hydrazine as reducing agent. The kinetics of this stage were found to be erratic and this was attributed to a reaction of AOT with hydrazine. This also had the effect of flocculating the microemulsion system (forming networks) which drastically perturbed the size measurements made using the sedimentation velocity (ultracentrifuge) method. Fractal growth was also indicated by light-scattering measurements.

Transferring colloidal metal particles from an organic to an aqueous medium and vice versa by ligand coordination

Colloidal noble metal particles dispersed in an organic medium have been transferred to an aqueous medium, and then transferred back to an organic medium by using a coordination capture technique.

Colloid filtration: A novel substrate preparation method for surface-enhanced Raman spectroscopy

A new method for preparing a monodisperse layer of metal colloid particles is presented. Metal colloid particles are filtered onto the surface of an aluminum oxide capillary pore surface capture membrane using both vacuum and syringe filtration devices. Scanning electron microscope (SEM) images reveal that the metal particles can be deposited with very little aggregation. Electronic absorption spectra show that the concentration of particles on the surface can be controlled by varying the silver colloid solution concentration. The linear dynamic range of absorbance of particles on the surface is demonstrated to be at least one order of magnitude. Surface-enhanced Raman (SER) activity is observed for the compound 9-aminoacridine (AA) at 514.5 nm using a filtered silver colloid solution. The SER activity of AA is observed to be related to the degree of colloid aggregation on the surface. Surfaces with no aggregation have no SER activity at this wavelength. The syringe colloid filtration technique is a fast, inexpensive method for producing surfaces with a highly controlled metal particle geometry, extremely useful for comparisons between theoretical and experimental results. These surfaces are also ideal for theoretical substrate design.

Probing the Nature of Divided Metals

ABSTRACTA review is given on the properties of metals which are neither quite macroscopic, nor microscopic in Nature. Both the geometric and electronic structure of such Divided Metals is discussed, with a special emphasis on recent studies of colloidal metal particles of the transition elements, Pd, Pt, Cu, Ag and Au.

Formation of mercury clusters during pulsed radiolysis of aqueous Hg 2 2+ solutions

Pulsed radiolysis, EPR, and optical spectroscopy were used to investigate the radiation-induced reduction of Hg22+ ions in aqueous solutions. It was shown that the Hg2+ ions that form as a result of the reduction reaction react rapidly with Hg22+ with formation of Hg43+. Constants of formation and disappearance of these ions were determined. The process of disappearance of this species results in the formation of more complex clusters containing six or more mercury atoms. Further complication of the clusters affords colloidal metal particles.

Air-stable alkali-metal colloids and the blue colour in Wurtz syntheses

POLYSILANES are used as resists in microlithography and as precursors for silicon carbide ceramics1–3. We have been investigating the formation of polysilanes by the reductive dechlorination polymerization of dichloro-organosilanes with sodium4—a modification of the well-known Wurtz reaction, in which organic halides are reductively coupled using sodium to form carbon-carbon single bonds. In all such reactions, after a period of time a blue precipitate is formed, but no characterization of this solid has so far been reported. It has been suggested (without spectroscopic evidence) that the colour in the polysilane synthesis is due to defects in sodium chloride1, sodium colour centres in sodium chloride3,5, reactive polysilane chain-ends3 or a stabilized organic radical6. The first of these suggestions has been tacitly accepted despite the fact that the most common defects in sodium chloride are F centres (anion vacancies with trapped electrons from excess sodium atoms) which give the salt a yellow colour7,8. Here we present spectroscopic results suggesting that the blue colour is due to colloidal alkali-metal particles formed during the reaction. These particles are contained in a matrix composed of an intimate mixture of polymer and alkali-metal halide, and are remarkably stable in air. It is most unusual for colloidal metal particles to be formed under such mild conditions.

Probing the electronic structure of small copper particles: 63Cu NMR at 1.5 K

We have investigated the 63Cu NMR (at 1.5 and 4.2 K) of a series of samples comprising silica-supported colloidal copper particles with average diameter (d) ranging from 1000 to 34 A. Substantial reductions have been observed in the metal Knight shift of the NMR of sub-80 A particles in our measurement field of 12.7 T. The magnitude of this reduction in the Knight shift is both temperature- and size-dependent; the effect being most pronounced for the smallest copper particles at our base temperature of 1.5 K.In the lower-size regime of colloidal copper particles (d⩽ 80 A), there exists a fundamental and qualitative difference in the electronic structure of the particles as compared with that of bulk copper metal. Specifically, the magnetic properties of the colloidal metal particles are dependent upon whether each individual particle contains either an even or an odd number of copper atoms. We find that the 63Cu NMR Knight shift distinguishes the behaviour of even (and odd) numbered copper particles. In the case of even-number particles, the reduction in the Knight shift arises from the loss of the paramagnetic susceptibility, this being most pronounced at low temperatures.The observed reduction in the NMR Knight shift of copper from its bulk metallic value is strong evidence for the existence of quantum size effects in colloidal copper particles at low temperatures.

Monodispersed colloidal metal particles from nonaqueous solutions: Catalytic behaviour in hydrogenolysis and isomerization of hydrocarbons of supported platinum particles

Platinum catalysts have been prepared by depositing on alumina monodispersed particles of platinum prepared in reversed micellar solution. After deposition, the particles are well dispersed on the support and the size distribution has a sharp maximum around 2 nm. The isomerization and hydrogenolysis of hexanes were studied in order to get more information about the particle size effects on the selectivities of Pt catalysts in these reactions.

Small-particle research: physicochemical properties of extremely small colloidal metal and semiconductor particles

Small-particle research: physicochemical properties of extremely small colloidal metal and semiconductor particles

Colloidal metal particles dispersed in monomeric and polymeric styrene and methyl methacrylate

Colloidal metal particles dispersed in monomeric and polymeric styrene and methyl methacrylate

Structure and growth of colloidal metal particles

In this paper we discuss the results of electron microscopic studies of colloidal gold and silver metal particles in the context of a model for their structure and growth. The images obtained are compared with the results of computer simulations for various candidate structures and the actual morphologies present are thereby deduced. With reference to earlier work on inert gas atom clusters we have developed a scheme which models the growth of such particles and enables the structures present to be predicted and rationalized. In particular, we deduce that particles belonging to the icosahedral point group have Mackay-type structures, in agreement with the experimental results. Comparisons are also made between the morphologies adopted by gold, silver, and argon clusters, and the relationship between the bonding in these systems is discussed.

Covalently Immobilized Colloidal Metal Particles as Selective Catalysts for Olefin Hydrogenation

Abstract Homogeneous colloidal dispersions of ultrafine noble metal particles have been prepared by the reduction of the corresponding metal ions in the presence of protective polymers. These colloidal metal particles show effective and selective catalyses in hydrogenation of olefins’. However, separation of these homogeneous catalysts from reaction mixtures for the repeated use is difficult. Thus, immobilization of these colloidal metal particles to supports is required. This paper reports immobilization of colloidal rhodium particles onto polymer support by use of covalent bonding between the protective polymer and the support. Activities and selectivities of the resultant immobilized catalysts for hydrogenation of olefins are shown.

Ion-Implanted Superionic Clusters

AbstractAg+ was implanted in silica glass and beta alumina crystal and studied using particle backscattering and optical absorption. We found that Ag aggregates into colloidal metallic particles and when annealed diffuses toward the surface. The activation energy for diffusion is 0.14 eV in silica glass and 0.16 eV in beta alumina. Computer simulations indicate fast Ag+ diffusion is due to large concentrations of vacancies deposited along the track of the implanted ion. The high ionic conductivity of beta alumina promotes uniform spreading of the implanted layer.

ChemInform Abstract: Morphology and Nanostructure of Colloidal Gold and Silver.

AbstractIt is demonstrated that detailed informations on the atomic arrangement of colloidal metal particles can be obtained by high‐resolution electron microscopy (HREM) combined with appropriate techniques to prepare ultrafine powders.

Clusters, colloids and catalysis

Metal cluster molecules containing easily displaceable ligands have been successfully exploited in organometallic cluster synthesis and to a lesser extent in homogeneous catalysis. We have extended this concept to elemental metal clusters in a fluid medium, by stabilizing colloidal transition metal particles in non-polar hydrocarbon solutions using polymers as tenuous ligands to prevent aggregation of the metal. The colloids were prepared by condensation of the metal vapor into a cold solution of the stabilizing polymer. In the absence of polymer, the metal atoms agglomerate into $ ̃ 100 μm aggregates of microcrystalline metal, containing crystallites of the order of 20–50 Å in diameter. In the presence of polymer, agglomeration of the metal proceeds only to the 20–50 Å stage, at which point the polymer sterically stabilizes the metal cluster and prevents further aggregation. The colloidal solutions thus obtained exhibit considerable thermal stability. The homogeneous catalytic properties of these solutions have been investigated in the hydrogenation of acenaphthylene. Colloidal metals prepared in this way also serve as sources of heterogeneous metal catalysts; copper-nickel colloids have been used in this way to probe the particle size dependency of filamentous carbon formation from ethane.

Optical properties of Cs2O and Ag–Cs2O thin films

The optical properties of well‐activated Cs2O and Ag–Cs2O thin films have been studied. These films are important for photoemission, especially for the ultrashort laser pulse detection. The curves for the index of refraction n, extinction coefficient k, and complex dielectric constant of Cs2O with different Cs impurity content are derived as a function of wavelengths of visible and infrared lights. The variations of front reflectance RF, vacuum reflectance RV, and transmittance T of the Cs2O thin film with an amount of Ag added into it were measured. The effects of sizes and shapes of metal colloidal particles embedded in semiconductor thin film on light absorption and scattering are discussed theoretically. The values of the absorption coefficient of Ag–Cs2O are also calculated.

The use of colloidal metal particles in protein blotting

AbstractTwo procedures are described for staining the overall protein pattern on transfer membranes. Both methods involve the use of colloidal metal particles. AuroDye staining is based on the interaction of anionic colloidal gold particles with cationic proteins at low pH. The method is highly sensitive, but limited to nitrocellulose and polyvinylidene difluoride membranes. FerriDye comprises an incubation in a cationic iron (hydrous) oxide sol and subsequent intensification with potassium hexacyanoferrate in acidic medium. FerriDye is of intermediate sensitivity, but there are no limitations concerning membranes. It can be used on nitrocellulose‐, nylon‐and polyvinylidene difluoride‐based membranes. An immunodetection procedure is described which uses colloidal gold‐labelled secondary antibodies to visualize antigen‐antibody interactions. This signal can be significantly enhanced by silver. The procedure explained uses a new light‐insensitive and user‐friendly silver enhancement system.

Monodisperse colloidal metal particles from nonaqueous solutions: Catalytic behavior in hydrogenation of but-1-ene of platinum, palladium, and rhodium particles supported on pumice

Metal catalysts have been prepared by depositing monodisperse particles of platinum (2–3 nm), rhodium (2–3 nm), or palladium (5 nm) prepared in reversed micellar solutions on pumice. The particles are well dispersed on the support whereas particles deposited from aqueous or alcoholic solution give large aggregates. The catalytic properties of these different catalysts in the deuteration, isomerization, and hydrogen-deuterium exchange of but-1-ene have been compared. The activities calculated per metal surface atom are similar. However, platinum prepared from microemulsions show unusually high selectivity in the isomerization reaction, and for such particles dehydrogenated species are active in the exchange reaction. The specificity of rhodium and palladium catalysts is independent of the mode of preparation. The reaction mechanisms are discussed.

LIGHT ABSORPTION AND SCATTERING OF SMALL COLLOIDAL PARTICLES IN METALLIC COLLOIDAL PARTICLES-SEMICONDUCTOR THIN FILMS

Metallic colloidal particles- semiconductor thin films show special electrical, optical and photoelectric properties. The formulas for light absorption coefficient and scattering coefficient of small colloidal particles dispersed in semiconductor thin film are given in this paper. The colloidal particles may be spheroids, prolate spheroids or oblate spheroids. As an example, the effects of sizes and shapes of Ag colloidal particcles in Ag-Cs2O semiconductor thin films on light absorption and scattering are discussed. The calculated curves for relative absorption coefficient and scattering coefficient vs. wavelength λ are also presented.