Hemispherical Particles Research Articles

Simple preparation of hemispherical polystyrene particles

We show the simple preparation of the hemispherical polymer particles. Submicron sized hemispherical polystyrene particles were prepared by adding a poor solvent (water) into a tetrahydrofran (THF) solution of polystyrene and following evaporation of THF. When the concentration of the polymer solution was higher than 0.4 g/L, spherical particles were formed. On the other hand, the hemispherical nano-particles were formed in the case of the dilute solution whose concentration was lower than 0.2 g/L. Turbidity change of the polymer solution due to THF evaporation starts from the surface of the solution. The light scattering experiment suggests that the hemispherical nano-particles were formed by precipitation at the air/solution interface.

Production of hydrogen via partial oxidation of methanol over Au/TiO 2 catalysts

Selective production of hydrogen by partial oxidation of methanol (CH 3OH + (1/2)O 2 → 2H 2 + CO 2) over Au/TiO 2 catalysts, prepared by a deposition–precipitation method, was studied. The catalysts were characterized by XRD, TEM, and XPS analyses. TEM observations show that the Au/TiO 2 catalysts exhibit hemispherical gold particles, which are strongly attached to the metal oxide support at their flat planes. The size of the gold particles decreases from 3.5 to 1.9 nm during preparation of the catalysts with the rise in pH from 6 to 9 and increases from 2.9 to 4.3 nm with the rise in calcination temperature up to 673 K. XPS analyses demonstrate that in uncalcined catalysts gold existed in three different states: i.e., metallic gold (Au 0), non-metallic gold (Au δ+ ) and Au 2O 3, and in catalysts calcined at 573 K only in metallic state. The catalytic activity is strongly dependent on the gold particle size. The catalyst precipitated at pH 8 and uncalcined catalysts show the highest activity for hydrogen generation. The partial pressure of oxygen plays an important role in determining the product distribution. There is no carbon monoxide detected when the O 2/CH 3OH molar ratio in the feed is 0.3. Both hydrogen selectivity and methanol conversion increase with increasing the reaction temperature. The reaction pathway is suggested to consist of consecutive methanol combustion, partial oxidation and steam reforming.

Electrocrystallization of Rhodium Clusters on Thiolate-Covered Polycrystalline Gold

This study reports on the electrochemical deposition of rhodium metal clusters on a polycrystalline gold electrode, modified with a monolayer of dodecanethiol through self-assembly from solution. The deposition process was investigated using cyclic voltammetry, chronoamperometry, and electrochemical quartz crystal microbalance. It is shown that the presence of the thiol monolayer drastically alters the nucleation and growth mechanism compared with the mechanism on the bare gold electrode. The small uncovered gold domains, located at the imperfections in the thiolate monolayer which are induced by the gold nanoroughness, act as nucleation sites for small rhodium clusters. At longer times, these clusters can outgrow the organic monolayer. The resulting surface morphology was characterized by scanning electron microscopy. Rhodium electrocrystallization on the bare gold substrate resulted in an ensemble of a very large amount of very small clusters that are difficult to distinguish from the gold roughness. In contrast, in the presence of a self-assembled monolayer (SAM) of dodecanethiol covalently attached to the gold electrode, the resulting deposit consisted of an ensemble of hemispherical particles. The size distribution of the rhodium particles obtained by using double step chronoamperometry was compared to the ones obtained with cyclic voltammetry and "classical" chronoamperometry. It is shown by X-ray photoelectron spectroscopy that the SAM is still present after rhodium deposition on the thiolate-covered gold substrate. Because the rhodium clusters are directly attached to the gold substrate and can thus easily be electrified, the resulting interface could be used as a composite electrode consisting of a random array of gold supported rhodium nano/microparticles separated from each other by an organic phase. On the other hand, it is shown that the SAM is easily removed by electrochemical oxidation without dissolving the rhodium clusters and, thus, leaving a different array of rhodium clusters on the gold surface compared with the topography obtained in the absence of the SAM. From this point of view, substrate modification with such "removable" organic monolayers was found to be an interesting tool to tune the nano- or microtopography of electrochemically deposited rhodium.

Formation and properties of stannate conversion coatings on AZ61 magnesium alloys

The effects of solution composition and temperature on the microstructure and corrosion resistance of stannate conversion coatings on AZ61 magnesium alloys were investigated. The conversion coating consisted of a porous layer as under layer intimately contacted with the magnesium plate and a hemispherical particle layer as major overlay formed right on top of the porous layer. During the coalescence of the hemispherical particles to form a complete coating on the magnesium alloy, some sites of discontinuity inevitably left and determined the corrosion resistance of the coating evaluated using a salt spray test. Increasing bath stannate ion concentration and lowering bath pH increased the population density of the hemispherical particles whose size was accordingly reduced. The corrosion resistance of the conversion coating was improved with finer particles, which were preferably formed at less alkaline solution with higher stannate ion contents. Furthermore, the conditions favoring the formation of finer particles also reduced the immersion time necessary for producing the conversion coating with optimal corrosion resistance.

Preparation and characterization of biotinylated and enzyme‐immobilized heterobifunctional latex particles as nanobio devices

AbstractA novel biotinylated and enzyme‐immobilized nanobio device was prepared with heterobifunctional composite latex particles. Hemispherical poly(glycidyl methacrylate‐co‐divinylbenzene)/polystyrene [P(GMA‐DVB)/PSt] particles with epoxy and hydroxyl groups were prepared by soap‐free seeded emulsion polymerization with P(GMA‐DVB) seed particles. Hydroxyl groups were introduced to PSt chain terminals in the seeded stage by adding 2‐mercaptoethanol as a chain‐transfer agent. To obtain the desired hemispherical structure particles, we studied the effects of the preswelling process, the type and amount of solvents added in the seeded polymerization step, the weight ratio of the secondary monomer (styrene) to the seed particle (M/P), and the type of initiators. Under suitable conditions, heterobifunctional P(GMA‐DVB)/PSt was obtained, which was confirmed by observing the binding of streptavidin–colloidal gold with transmission electron microscopy (TEM). To obtain biotinylated and enzyme‐immobilized particles, 5‐(N‐succinimidyloxycarbonyl)pentyl D‐biotinamide was first reacted with the hydroxyl group on the PSt domain of the particle. Pyruvate kinase (PK) was then directly immobilized to the biotinylated particles through a reaction with the epoxy group in the PGMA domain. The monolayer of PK on the latex particle surface was considered to be formed by covalent binding. The activity of immobilized PK was almost conserved, even after being stored at 4 °C for 48 days. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 562–574, 2005

New micrometer-sized monodispersed self-assembled amphiphilic polystyrene/poly( n-butyl methacrylate) composite particles of hemispherical morphology: synthesis and characterization

Uniform micron-sized polystyrene template particles surface grafted with polyvinylpyrrolidone were prepared by dispersion polymerization of styrene in alcoholic solvents in the presence of polyvinylpyrrolidone as stabilizer. Poly(acrylic acid, sodium salt) derivatized polystyrene/ poly( n-butyl methacrylate) micron-sized composite particles of hemispherical morphology and narrow size distribution were formed by a single-step swelling of the uniform polystyrene template microspheres dispersed in water with emulsion droplets of the monomer n-butyl methacrylate containing the initiator benzoyl peroxide. n-Butyl methacrylate was then polymerized at 73 °C within the template microspheres in the presence of the water-soluble monomer acrylic acid, sodium salt. The sodium ions of the poly(acrylic acid) derivatized composite particles were then replaced with dodecylammonium ions by interacting these composite particles with excess dodecylamine hydrochloride. As a result of this process the surface of the hemispherical composite particles composed of hydrophilic polyvinylpyrrolidone chains on the polystyrene phase and hydrophobic dodecyl groups on the poly(acrylic acid) derivatized poly( n-butyl methacrylate) phase. These newly formed micron-sized composite particles show amphiphilic characteristics and aggregate to form micelle-like spherical clusters. The ability to control the template polystyrene microspheres’ dimensions and various polymerization parameters may open a wide variety of self-assembled spherical and other structures composed of micron-sized particles for various fundamental and practical applications such as self-assembly processes and the formation of photonic bandgap crystals.

Particle sizing of tetrahedral and hemispherical particles from small-angle scattering experiments

Particle sizing of tetrahedral and hemispherical particles from small-angle scattering experiments

Simulation of the Influence of Reactant Depletion on Nucleation Rate in Electrodeposition

Numerical simulations are used to study the electrochemical nucleation of hemispherical particles followed by diffusion-controlled growth. The impact of reactant depletion on nucleation rate is considered by introduction of a concentration-dependent rate law. The effect is only important when the nucleation rate is comparable to or slower than the growth rate. For instantaneous nucleation, particles are born before surface depletion is significant. Simulated current transients and particle statistics are discussed and compared to previous experimental and theoretical attempts to address the problem. A concentration-dependent nucleation rate may have a relatively small impact on the predicted current transient but may have a significant effect on particle density and size distribution. © 2004 The Electrochemical Society. All rights reserved.

Mathematical Model for the Emulsion Polymerization Reaction Kinetics of Two Phase Latex Particles

Many of today's industrial applications of emulsion polymers require the creation of polymer particles with specifically designed structures. A fair amount of work has been done on predicting and controlling particle morphology, and it is recognized that the reaction kinetics often play an important role in determining that morphology. The present work describes a procedure to calculate the reaction rate for two‐phase latex particles by extending the classic Smith–Ewart concepts to the more complicated situation. The model involves a complete account of radical events in both polymeric phases. Eight rate parameters are necessary to completely describe the system. A particle population distribution is generated to reflect the probability of specific particles to have two distinct numbers of radicals in each phase. Simple statistics on this population allow the calculation of the average number of radicals for each polymeric phase. The model collapses to the correct answers when applied to single‐phase particles and results in the prediction of the number of radicals in each of the phases when applied to structured particles. Several examples are described for core–shell (or inverted core–shell) and hemispherical particles in which the polymer Tg, monomer concentration and radical entry rates are varied. In addition, the effects of radical transfer rates between the polymer phases are highlighted. Such effects are found to be important for some polymer systems and not so important for others. The model is readily extendible to occluded structures. Since the reaction kinetics are concluded to be significantly dependent upon the particle morphology, it is clear that morphology development and reaction kinetics are coupled processes and need to be treated simultaneously in order to produce an effective overall model for two‐phase emulsion polymers.

Anodic Electrosynthesis of a Thin Film of Cu2S on a Gold Electrode. A Voltammetric, Nanoelectrogravimetric, and I/t Transient Study

The electroformation of a Cu2S phase, obtained by sulfidization of a thin film of copper placed by the spin-coating technique onto the gold surface of a gold/quartz crystal that is part of an electrochemical quartz crystal microbalance (EQCM), was studied by cyclic voltammetry (CV) and electrochemical quartz crystal microgravimetry. The electrolytic medium employed was a bisulfide-containing aqueous buffered solution (pH 9.2). CV parameters showed that the formation of the Cu2S phase occurred by means of an irreversible diffusion-controlled mechanism, where a first uncomplicated electron transfer was the rate-determining step. The EQCM data showed that the initially formed phase was Cu2S, which was later oxidized to nonstoichiometric compounds (Cu(2-x)S) and CuS. Furthermore ΔQ/Δm data analysis established that the electron reaction valence (n) was close to 2 for all electrochemical processes where Cu/S compounds were involved. The nucleation and growth mechanism (NGM) of the Cu2S phase corresponded to a bidimensional instantaneous mechanism containing two contributions, one controlled by diffusion and the other controlled by charge transfer. Atomic force microscopy (AFM) revealed that the Cu2S phase electroformed on the gold/crystal quartz electrode was deposited as hemispherical particles (sized between 40 and 130 nm) on the terraces and in the cavities present in the gold film.

Interference technique in grazing-emission electron probe microanalysis of submicrometer particles

The distribution of X-ray radiation from a small particle, situated on a flat support and excited by an electron beam, is analyzed theoretically. The interference pattern, appearing within the region of grazing take-off angles, is shown to contain valuable information about the particle structure and composition. It also provides the resources for extracting this information that are valid even in conditions of extremely low signals (e.g. for very small samples), when the effect of background suppression for angles of the total reflection region is insufficient. Characteristic elements of the interference pattern are found that are relatively independent on the noise caused by the support radiation. Algorithms for this noise correction and elimination are proposed. An approximate formalism for the particle structure retrieval, based on the Fourier-transformation properties, is described. Potential applications of the proposed technique are illustrated with numerical models of spherical and hemispherical particles, both homogeneous and of ‘core-and-shell’ structures, with total sizes between 30 and 200 nm. The additional possibilities of studying the shape, structure and elemental composition of small particles have been demonstrated. The general conditions of the experimental realization of proposed methods are discussed.

The Effect of Li on Structure of Supported Rh Particles in Zeolite

We prepared Rh ion exchanged Y-type zeolite (RhY) and Li-doped RhY (Li/RhY) catalysts and studied the structure transformation of Rh intercalated species during CO adsorption and CO2 hydrogenation by EXAFS and TEM. We found that Rh particles in RhY initially supported inside the zeolite cage as 1.3 nm spherical or hemispherical particles. CO exposure of the fresh RhY caused formation of atomically dispersed Rh species. After CO2 hydrogenation and following reduction, the Rh particles aggregated to form 3.3 nm particles on the outer surface. Meanwhile, Rh particles in Li/RhY were embedded in the zeolite cages, which were narrowed by Li deposition on the wall. The size of the Rh particles was 0.8 nm in diameter. The Rh particles in Li/RhY were stable under reduction, CO adsorption, and CO2 hydrogenation conditions. We concluded Li additive changed the stability of Rh particles in the supercage by modifying surface hydroxyl group concentration.

Catalyst genesis studied by atomic force microscopy

Atomic force microscopy has been used to study the genesis of realistic Ag–α-alumina model catalysts from an aqueous precursor (AgNO 3), using conditions identical to those used in the synthesis of practical dispersed catalysts. The various stages in the evolution of the final catalyst from the initially deposited precursor were successfully imaged. It is found that the structure of the alumina substrate exerts a dramatic influence on the structure and stability of the silver deposit. On Al 2O 3(0001), sintering in 1 bar oxygen at 780 K causes a quasi-2D network of metallic Ag to collapse to large well-formed hemispherical particles on the bare oxide surface. On Al 2O 3(112̄0), precursor decomposition leads initially to the formation of quasi-1D Ag nanowires that decorate the faceted step edges and extend for many hundreds of nanometres. This is entirely different from the behaviour of vacuum-deposited Ag. These nanowires are much more stable against sintering in 1 bar oxygen at 780 K. Their formation and stability suggest a particularly favourable wetting interaction between Ag and the (11̄02) plane of α-alumina.

Shadow-hiding effect in inhomogeneous layered particulate media

Shadow hiding is studied for particulate media consisting of opaque, spherical particles with Lambertian scattering phase functions. The present computer modeling allows geometric optics calculations of the photometric characteristics of layered particulate media with accuracies better than 1%. For statistically homogeneous particulate media, packing density is the single parameter that characterizes the opposition effect due to shadow hiding. The opposition effects become sharper with decreasing packing density both for monodisperse and polydisperse particulate media. There are no differences between the phase curves of monodisperse and polydisperse media with equal packing densities. A monolayer of hemispherical particles placed randomly on a plane-parallel, Lambertian background medium always produces phase curves that are convex upward. A monolayer of spherical particles can give almost linear phase curves in the phase angles of 0–40°. In a double-layer model, an optically thin layer of small particles can sharpen the opposition effect, but that depends strongly on the method of conjugation of the two layers. A dependence of the opposition effect on the size of the upper-layer particles really exists (Hillier's effect), but it is rather small at about 5% according to our calculations. Sparsely distributed large particles on a plane-parallel dusty surface can also produce sharpening of the phase function near opposition.

A Surface Science Study of Model Catalysts. 1. Quantitative Surface Analysis of Wet-Chemically Prepared Cu/SiO2 Model Catalysts

Cu/SiO2 model catalysts containing nanometer-sized Cu particles on a flat silica model support were wet-chemically prepared and characterized in detail by a variety of surface science techniques. The particle size and shape, particle number density, metal surface coverage, total metal loading, and oxidation state of the particles were determined by ultrahigh vacuum atomic force microscopy, electron microscopy, low-energy ion scattering, Rutherford backscattering spectrometry, and X-ray photoelectron spectroscopy. Deposition of a Cu precursor on a flat Si wafer with a SiO2 top layer by spin-coating was followed by calcination in air at 450 °C. This preparation method produces both homogeneously distributed hemispherical CuO particles with an average height of 8 nm and highly dispersed oxidic Cu species. Subsequent reduction in hydrogen at 250 °C results in metallic and more rounded Cu particles with an average height of 8 nm.

Selective Vapor-Phase Epoxidation of Propylene over Au/TiO2Catalysts in the Presence of Oxygen and Hydrogen

Gold has long been thought to be chemically inert, however, it has recently been proven that its catalytic performance is dramatically tunable by control of the particle size and by careful selection of the support metal oxide. A typical example is the selective oxidation of propylene in a gas containing oxygen and hydrogen. When gold is deposited on TiO2by a deposition–precipitation technique as hemispherical particles with diameters smaller than 4.0 nm it produces propylene oxide with selectivities higher than 90% and conversions of 1–2% at temperatures of 303–393 K. The oxidation of hydrogen to form water is depressed by propylene, whereas propylene oxidation is not only enhanced but also restricted to partial oxidation by hydrogen. The depression of hydrogen combustion by the presence of propylene and a new peak due to gold deposition in TPD spectra have indicated that propylene is adsorbed on the surfaces of both gold particles and the TiO2support. The reaction rate is almost independent on the concentration of propylene and increases linearly with increasing concentrations of O2and H2. The above results suggest that propylene adsorbed on a gold surface may react with oxygen species formed at the perimeter interface between the gold particles and the TiO2support through the reaction of oxygen with hydrogen. The effect of gold loading is surprising in that the reaction product switches from propylene oxide to propane when gold loading is decreased to 0.1 wt%. Careful TEM observation indicates that gold particles larger than 2.0 nm in diameter produce propylene oxide, whereas smaller gold particles produce propane.

Characterization of metal clusters (Pd and Au) supported on various metal oxide surfaces (MgO and TiO2)

Au clusters supported on the TiO2(001)/Mo(100) surface and Pd clusters supported on the MgO(100)/Mo(100) surface have been studied using temperature programmed desorption, ion scattering spectroscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy. Three dimensional growth has been observed at 300 K for both cases. The adsorption energy of Au on the TiO2(001) surface has been measured to be 50 kcal/mol using temperature programmed desorption, compared to the adhesive energy of 90 kcal/mol for the bulk Au metal. At an equivalent Au coverage of 1.8 ML, very homogenous, hemispherical Au particles with an average diameter of 25 Å and a height of 10 Å are present on a TiO2(001)/Mo(100) surface. Increasing the Au coverage to 3 equivalent monolayers produces ∼60 Å diameter clusters. The scanning tunneling spectroscopy measurements show a gradual development of the metallic character of the Au clusters with increasing cluster size. Very similar results have been obtained for the Pd clusters supported on the MgO(100)/Mo(100) surface.

Size- and support-dependency in the catalysis of gold

The adsorption properties and reactivities of gold are summarized in terms of their size dependency from bulk to fine particles, clusters and atoms. The catalytic performances of gold markedly depend on dispersion, supports, and preparation methods. When gold is deposited on select metal oxides as hemispherical ultra-fine particles with diameters smaller than 5 run, it exhibits surprisingly high activities and/or selectivities in the combustion of CO and saturated hydrocarbons, the oxidation-decomposition of amines and organic halogenated compounds, the partial oxidation of hydrocarbons, the hydrogenation of carbon oxides, unsaturated carbonyl compounds, alkynes and alkadienes, and the reduction of nitrogen oxides. The unique catalytic nature of supported gold can be explained by assuming that the gold-metal oxide perimeter interface acts as a site for activating at least one of the reactants, for example, oxygen. Some examples and future prospects in applications are also briefly described.

Local electronic structure of metal particles on metal oxide surfaces: Ni on alumina

Ni supported on thin alumina films epitaxially grown on the Re(0001) surface has been studied using scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS). Ni, deposited at 300 K onto the Al2O3(111)/Re(0001) surface, forms hemispherical particles with a very narrow size distribution. The transition from layer-by-layer growth to three-dimensional cluster formation has been found to occur at an Ni coverage as low as 0.03 monolayers (ML), indicating a Volmer–Weber growth mode for Ni on Al2O3. Upon heating to 900 K, the Ni layer in contact with the Al2O3 surface reacts with Al2O3 and becomes indistinguishable from the substrate via STM, while Ni in the second and subsequent layers agglomerates. STS was used to study the local electronic structure and to identify the features observed in the STM topographic images.

プラズマジェット法による金属‐窒化物複合超微粒子の作製

The formation of metal and metal-nitride composite ultrafine powder (UFP) using a plasma jet method was examined. Pure elemental powder of Al, Fe, Cr, Cu, Ni, Si, Ti and pre-mixed elemental powders of Al-Cu, Al-Ni, Ni-Ti, Al-Ti, Si-Ti were injected into the plasma jet of Ar-N2 working gas. The main results obtained were as follows; (1) When UFP was produced from pure metal powders of Cu, Fe, Ni and Si, nitride UFP did not form at all. On the other hand, UFP of A1N, Cr2N and TiN was formed from pure metal powders of Al, Cr and Ti, respectively. (2) Formation of nitride UFP of Si, Cr, Al, Ti was not enhanced by using working gas containing a small amount of H2 On the contrary, AIN was eliminated by using working gas containing a small amount of H2. (3) The structure of composite UFP of Cu-AlN and Ni-A1N consisted of two different particle types. In one type, a number of hemispherical particles was attached on the surface of a hexagonal prismatic particle. In the second type, a spherical particle was attached on one side of a cylindrical particle. (4) The structure of ultrafine composite particles of Ni-TiN consisted of three different particle types. One type was dumbbell-like and second type was dice-like morphologies. In the last type, one or two large hemispherical particles were attached on faces of a cubic particle. (5) When UFP was produced from pre-mixed powder of the Si-Ti system, string-shaped UFP was observed.