Hemispherical Particles Research Articles

A facile template synthesis of asymmetric gold silica heteronanoparticles

Silica hemispheres containing gold nanoparticle cores have been synthesized via immobilization of gold nanoparticles on a substrate and site-selective growth of silica followed by removal of the hemispherical particles. The structure of these asymmetric heteronanoparticles allows selective etching or overgrowth of the core gold seeds, which results in the respective formation of hemispherical capsules or gold homodimers.

Biocompatible Shaped Particles from Dried Multilayer Polymer Capsules

We demonstrated a simple and facile approach to fabricate biocompatible monodisperse hollow microparticles of controlled geometry. The hemispherical, spherical, and cubical microparticles are obtained by drying multilayer capsules of hydrogen-bonded poly(N-vinylpyrrolidone)/tannic acid (PVPON/TA)n. Drying spherical capsules results in hemispherical particles if 15 < n < 20. This shape transformation is controlled by capsule stiffness, which is regulated by the layer number, capsule diameter, and PVPON molecular weight. Cubical and spherical hollow particles maintaining their three-dimensional shapes in the dry state are obtained if n ≥ 25.5. A 17-fold stiffness increase is required to lead from totally collapsed (PVPON/TA)5.5 to dried self-supporting (PVPON/TA)25.5 particles of 2 μm in dimensions. All hollow particles could be further resuspended in aqueous solutions while retaining their shapes upon rehydration. The cell growth and viability studies using human cancer cells revealed noncytotoxic properties of the (PVPON/TA) multilayer particles. Both spherical and hemispherical capsules were internalized by macrophages with the uptake of the hemispherical particles per cell two times more efficient. The method presented here allows for a robust preparation of biocompatible shaped particles whose shape and dimensions can be easily tuned by controlling capsule size and wall thickness. The reported structures can be potentially useful for biomedical applications such as shape-controlled cellular uptake and flow dynamics.

Effective thermal conductivity of sintered porous media: Model and experimental validation

A model to estimate the effective thermal conductivity of sintered porous media for heat pipes is proposed in this paper. An elementary cell of a porous media is physically modeled as two metallic hemispheres in contact with a fluid film around them. The electrical circuit analogy is employed to determine the heat leaving the top and reaching the bottom of the cell. The thermal circuit consists of two parallel resistance paths, one for the solid spheres in contact and the other for the heat transfer in the fluid. A literature model is employed to calculate the thermal resistance within the hemispherical particles. Also, literature models are used for the determination of the geometry of the neck produced between particles during the sintering process. The neck dimensions are used to estimate the neck thermal resistance, which is in series with the hemisphere resistances. Effective thermal conductivity experimental data were obtained for porous materials produced with atomized copper powder, with particle diameters ranging from 20 to 50μm. The comparison between present model and data is good. Statistics of the particle size distribution is employed to determine average particle dimensions. The porosity and permeability of the material tested was characterized in the laboratory. The samples were tested in three conditions: vacuum and saturated with distilled water or methanol. Literature models for the effective thermal conductivity for bed packed (not sintered) porous media were also compared with the present model and data results.

Highly Efficient Dehydrogenation of Primary Aliphatic Alcohols Catalyzed by Cu Nanoparticles Dispersed on Rod-Shaped La2O2CO3

Copper nanoparticles dispersed rod-shaped La2O2CO3 efficiently catalyzed transfer dehydrogenation of primary aliphatic alcohols with an aldehyde yield of up to 97%. This high efficiency was achieved by creating a catalytically active nanoenvironment for effective reaction coupling between alcohol dehydrogenation and styrene hydrogenation via hydrogen transfer. The {110} planes on the La2O2CO3 nanorods not only provided substantial amounts of medium-strength basic sites for the activation of alcohol but also directed the selective dispersion of hemispherical Cu particles of about 4.5 nm on their surfaces, which abstracted and transferred hydrogen atoms for styrene hydrogenation. This finding provides a new strategy for developing highly active alcohol-dehydrogenation catalysts by tuning the shape of the oxide support and consequently the metal-oxide interfacial nanostructure.

Trivalent ion cross-linked pH sensitive alginate-methyl cellulose blend hydrogel beads from aqueous template

pH sensitive alginate-methyl cellulose blend hydrogel beads were prepared by single water-in-water (w/w) emulsion gelation method in a complete aqueous environment. The influence of different variables like total polymer concentration, gelation time and crosslinker content on in vitro physico-chemical characteristics and drug release rate in different medium were investigated. Drug loaded beads were evaluated through Fourier Transform Infra-red (FTIR), X-ray diffraction (XRD) and Differential Scanning Calorimetry (DSC) analysis. Scanning electron microscopy (SEM) picture of the dried beads suggested the formation of hemispherical particles. FTIR analysis indicated the stable nature of the drug in the blend hydrogel beads. DSC and XRD analysis revealed amorphous state of drug after encapsulation. The drug release profile in acidic medium was considerably less in compared in alkaline media. Formulations showed non-Fickian type transport mechanism. This trivalent ion crosslinked beads not only improves drug encapsulation efficiency but also enhances drug release in alkaline media.

Nanoparticle modified electrodes can show an apparent increase in electrode kinetics due solely to altered surface geometry: The effective electrochemical rate constant for non-flat and non-uniform electrode surfaces

The voltammetry of micro- and nano-particle modified electrodes and other electrodes of partially covered and non-planar geometry is investigated by simulation. Building on existing theory, it is demonstrated that for a simple one-electron process (assuming that the diffusion fields of neighbouring electroactive regions strongly overlap such that diffusion to the entire surface is linear), the apparent electrochemical rate constant of the reaction, kapp, is equal to the product of the true rate constant, k0, and the ratio, Ψ, of the total electroactive surface area to the geometric surface area of the substrate. It is demonstrated that for a given value of Ψ, the voltammetry is independent of the surface geometry; surfaces covered by, for example, long thin bands of electroactive material, or electroactive hemispherical or spherical particles, show the same voltammetry if they have the same surface area of electroactive material per area of substrate. Distributions of, most importantly, electroactive nanoparticles, with Ψ>1, will display an apparent catalytic effect compared to the bulk material which can be solely due to the geometry of the surface and not necessarily related to changes in kinetics at the nanoscale, for example by altered structural or electronic properties. Further, if an electrode surface is modified by a fixed mass of nanocatalyst per unit area, then the response will reflect the size and shape of the modified particles.

The surface morphology of thin Au films deposited on Si(001) substrates by sputter deposition

Thin (100nm) gold films were deposited on the native oxide of Si(001) substrates by direct current magnetron sputtering. The effects of the deposition rate, the target/substrate distance, the substrate orientation, substrate bias, and substrate temperature on the surface morphology of the gold film have been studied using atomic force microscopy. It was observed that the deposition of Au on Si(001) is characterized, in most cases, by hemi-spherical particles distributed uniformly on the surface. Decreasing the target/substrate distance caused both the average particle diameter and the height to increase. However, at a target/substrate distance of 5cm using a deposition rate of 0.1Å/s, samples with different morphologies were produced characterized by the coalescence of the surface Au particles. Increasing the relative angle from 0° to 90° between the substrate and the sputtering target caused the average particle diameter to decrease by approximately 8.7% and the average particle height to increase by approximately 52.3%. Applying a high negative voltage to the substrate during the deposition caused a small decrease of approximately 10.2% in the average particle size and a change of 26.2% to the average particle height. Lowering the substrate temperature from 12 to −11°C during the deposition process caused a decrease of 21.6% in the average grain size and an increase of 44.7% in the average grain height. By adjusting the above parameters it was possible to obtain samples with an average particle size ranging from 25.8 to 180nm with root-mean-square roughness values ranging from 0.8 to 21nm. The results of this work provide an understanding on how to produce a wide range of surface morphologies of thin Au films on Si(001+native oxide) for applications such as cantilever sensing and for producing bio-functionalized tips for atomic-force microscopy.

A Voltammetry Study of Ethanol Oxidation on Carbon Supported, Non Alloyed Platinum-Tungsten Catalysts

The synthesis and characterization of bimetallic catalysts with Pt and W as active phases, supported on carbon, were carried out. A synthesis method based on the thermolysis of metal precursors was developed with systems in which an aqueous phase, an organic one and solid carbon are present. The Pt loading in the synthesized catalysts was kept constant and the amounts of W and carbon changed. It was found that the Pt phase forms hemispherical particles with average diameter of 3 nm. The W is predominantly in the form of hexagonal WO3 crystals with average dimensions 35 by 15 nm. No evidence of alloy formation is found. The electrochemical characterization of ethanol oxidation includes cyclic voltammetry and current–sampled voltammetry techniques. Tungsten exerts a promoting role in the activity of the catalysts, revealed by considerably increased current densities with respect to carbon–supported Pt. Details of this enhanced activity are revealed by cyclic voltammetry with different operating conditions and electrolyte formulations. These experiments allowed to identify the oxidation of adsorbed organic intermediates influenced by the tungsten phase. WO3 appears as well to cause the decrease in the rate at which Pt oxides reduce, leading to higher surface coverage of OH• species.

Novel Mini-Reactor of Silicone Oil Droplets for Synthesis of Morphology-Controlled Polymer Particles

Inside spaces of emulsion droplets can be used as mini-reactors for material synthesis. The novel application of sol-gel derived silicone oil droplets as mini-reactors was examined for the case of polymerization of styrene (St) and comonomers with the oil-soluble initiator 2,2'-azobis(2,4-dimethylvaleronitrile). Polydimethylsiloxane (PDMS) droplets prepared from dimethylsiloxane were used as the mini-reactors, in which the polymerization of St without comonomers was first conducted. In the polymerization, the St/PDMS volume ratio was varied from 0.025 to 0.10. After the polymerization, each PDMS droplet contained a polystyrene (PSt) particle. The St/PDMS ratio of 0.05 enabled the synthesis of micrometer-sized, spherical PSt particles with low polydispsersity. Copolymerization of St with comonomers having hydrophilic groups deformed the spherical shape of particles to lens-like or disk-like morphologies that were obtained with acrylic acid or sodium 4-styrene sulfonate, respectively. In another copolymerization, with divinylbenzene used as a cross-linker, hemispherical polymer particles were formed. To diversify the particle morphologies further, the proposed mini-reactor synthesis was combined with the recently proposed silicone oil droplet templating method (Ohta et al., 2012). Around the PDMS droplets containing a polymer particle, polymeric shells with a depression were successfully formed with the proposed method. The remaining PDMS oil inside the polymeric shells was extracted with ethanol, which caused hemispherical polymeric bowl-shaped capsules having a protrusion on the inside.

Preparation of hemispherical polyimide particles by inverse emulsion technique from poly(amic acid) ammonium salts

Nonspherical polymer particles have attracted increasing attention recently. In this paper, micron-scale hemispherical polyimide (PI) particles were fabricated using water-soluble poly(amic acid) ammonium salts (PAAS) by a novel inverse emulsion technique. In the process, liquid paraffin was used as a continuous phase, the mixed solution of PAAS and water as a dispersed phase and sorbitan monooleate (Span80) as a surfactant. The research suggested that water as a stabilizing agent played an important role in forming stable emulsion. As the amount of water increased, stability of the emulsion increased gradually and morphology of PI particles transformed from sphere to ellipsoid, and finally to hemisphere. The concentration of PAAS solution and Span80 both affected the shape of particles, which changed from spherical to hemispherical by increasing the PAAS/Span80 concentration. The mechanism of forming hemispherical PI particles was discussed based on interfacial tension and interfacial free energy changes. Via adjusting the composition of the system to change the corresponding interfacial tension, we could get the particles with different morphologies. Furthermore, the change in structure characterized by FT-IR spectroscopy demonstrated that PAAS had been converted to PI after adding the dehydrating agent to the emulsion. And TGA results showed that the obtained PI particles had excellent thermal stability.

Preparation of Hemispherical Polymer Particles by Cleavage of a Janus Poly(methyl Methacrylate) /Polystyrene Composite Particle†Part CCCLVII of the series “Studies on Suspension and Emulsion”.

Micrometer-sized, hemispherical particles were successfully prepared as a result of the cleavage of Janus PMMA/PS composite particles by dispersion into acetone/water (9/1-10/0 v/v) media or a THF/water (8/2 v/v) medium. The spherical composite particles having a Janus structure were prepared by the slow evaporation of toluene from homogeneous PMMA/PS/toluene droplets dispersed in an aqueous medium in advance. It was clarified that the difference in affinity between PMMA and PS phases with respect to the media caused the cleavage of the composite particles. This method is expected to be a novel approach to the preparation of nonspherical polymer particles.

High-Speed in Situ X-ray Scattering of Carbon Nanotube Film Nucleation and Self-Organization

The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs.

Preparation of hemispherical polystyrene particles utilizing the solvent evaporation method in aqueous dispersed systems

Various nonspherical polystyrene (PS) particles were prepared by slow evaporation of toluene (used common good solvent) from homogeneous PS/hexadecane (HD)/toluene droplets dispersed in surfactant aqueous solutions at room temperature, followed by the rapid removal of HD from PS/HD particles with various phase-separated morphologies. The morphology of PS/HD particles was controlled by tuning the interfacial tension with various types of surfactants. Hemispherical PS particles with flat surfaces were obtained from phase-separated PS/HD/toluene droplets having a Janus structure, when polyoxyethylene nonylphenyl ether with an average ethylene oxide chain length of 30.8 was used as the surfactant.

The formation of asymmetric polystyrene/saponite composite nanoparticles via miniemulsion polymerization

AbstractThe synthesis of asymmetric spherical nanoparticles has attracted great interest because their anisotropic structure can be used as unique building blocks for constructing advanced materials. In this article, we report the formation of hemispherical or truncated polystyrene/nanosaponite composite particles via one‐pot miniemulsion polymerization. It was found that the morphology of final composite latex particles strongly depends on the size of the nanoclay and its surface properties. Hemisphere or truncated sphere is the dominant morphology if the size of the nanoclay is larger than 100 nm. With the increase of the nanoclay content (up to 30 wt %), the fraction of hemispherical or truncated polystyrene/nanosaponite composite latex particles increased accordingly. The formation of hemispherical particles is possibly attributed to either the asymmetric growth of polymer chains on one side of the hydrophobically modified clay or the mechanical peeling‐off of large spherical particles between polymer and saponite. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Preparation of hemispherical polymer particles via phase separation induced by microsuspension polymerization

Micrometer-sized, hemispherical polystyrene (PS) particles were successfully prepared by microsuspension polymerization of homogeneous styrene/hexadecane (HD) droplets dispersed in polyoxyethylene nonylphenyl ether (Emulgen 931) aqueous solution, followed by rapid removal of HD from formed PS/HD particles with a “Janus” structure. It was important for the formation of the morphology of Janus particles in thermodynamically stable state to carry out the polymerization slowly. The formation of by-product small PS particles by emulsion polymerization was suppressed by the additions of CuCl2 as a water-soluble inhibitor and NaCl to decrease the solubility of styrene in the aqueous phase.

TiO2 Nanostructures Obtained in Non-Aqueous Electrolytes at Low Potential

In this paper, the synthesis of TiO2 porous arrays on titanium substrates in non-aqueous solution of ethylene glycol (1 vol.% H2O) + 0.5 wt.% NH4F was investigated. Electrochemical anodization of titanium was carried out at 3V during 39 to 72 hours. The microstructural analysis of samples by scanning electron microscopy and atomic force microscopy suggests that the thickness of porous layers decrease with the anodization time, whereas the polydispersity in the pore size increases by the integration of small pores formed early during the anodization process. At 72 hours the presence of a double wall of the porous is indicative of the incipient formation of nanotubes. By the other hand, the pore depth suggests that the oxide film was formed by the coalescence of 20-50 nm diameter hemispherical titania particles.

Synthesis of titanium oxide nanocups by electrospraying

Porous titanium oxide nanocups were synthesized via a 550 °C thermal treatment of hemispherical cup-shaped composite particles prepared by electrospraying a nitromethane solution of titanium tetraisopropoxide and polymethylmethacrylate (PMMA) polymer. During the calcination process, as-electrosprayed cup-like PMMA–TiO 2 composites were converted into miniaturized TiO 2 nanocups without a significant change in shape, via selective removal of PMMA. The TiO 2 nanocups with an outer diameter of around 500 nm were confirmed to form by sintering polycrystalline anatase crystals with an average size of about 20 nm.

Continuous Production of Functionalized Polymer Particles Employing the Phase Separation in Polymer Blend Films

This study reports a continuous prepartion of spherical or hemispherical polymer particles simply utilizing the phase separation in polymer blend films during the coating process. We took an advantage of the strong phase separation between a water-soluble crystalline polymer as a matrix and hydrophobic polymers as minor components. We demonstrated the prepartion of water-soluble polystyrene (PS) particles, nitrilotriacetic acid (NTA)-functionalized PS particles for protein separation, and semiconducting poly(3-hexylthiophene) (P3HT) particles. The sizes of the particles could be controlled by adjusting the film thickness and weight fraction of the minor component polymers in the blend film. It provides a simple facile way to prepare polymer particles in a continous process.

Cyclic Voltammetry of the EC′ Mechanism at Hemispherical Particles and Their Arrays: The Split Wave

The EC′ (catalytic) mechanism, (1) A + e– ⇌ B; (2) B + X → K2 A + P, in which the reduction of ‘X’ to ‘P’ is mediated by the A/B redox couple, is studied at a regularly distributed array of hemispherical particles on a planar surface using simulated cyclic voltammetry. It is assumed that the supporting surface itself is not electroactive and therefore the electron transfer in process (1) occurs exclusively on the surface of the particles. Two-dimensional finite difference simulations are performed for a range of scan rates, particle surface coverages, and second-order rate constants, K2. Additionally, for the case of an isolated particle, the effect of the concentration of reactant species ‘X’ is also examined. Particular attention is paid to the ‘split-wave’ phenomenon, where two peaks are observed in the forward scan of a cyclic voltammogram, which tends to occur when the values of [A] and [X] are similar and the second-order rate constant, K2, is relatively high. The conditions under which two peaks are resolvable are elucidated and expressions are presented for the first peak current and potential for the isolated case.

Direct Synthesis of Vertically Interconnected 3-D Graphitic Nanosheets on Hemispherical Carbon Particles by Microwave Plasma CVD

High-quality, free-standing, and vertically interconnected three-dimensional (3-D) graphitic nanosheets (GNSs) were synthesized over the surface of hemispherical carbon particles/GaN at 700 °C by microwave plasma chemical vapor deposition (CVD) in presence of methane gas, whereas the hemispherical carbon particles have been directly deposited on GaN/sapphire template. The GNSs are ∼1–5 nm in thickness and have a graphitic flake structure on hemispherical carbon particles. The vertically interconnected 3-D GNSs on hemispherical carbon particles have been characterized by scanning electron microscopy, transmission electron microscopy, selective area electron diffraction pattern, X-ray diffraction, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and nitrogen gas adsorption-Brunauer-Emmet-Teller. The present CVD approach is capable of producing large quantities of GNSs with high purity. Moreover, a high-purity free-standing and vertically interconnected 3-D GNSs on hemispherical carbon particles have an enormous potential for applications in electronic devices, biological sensors, gas uptake and storage, fuel cells, lithium ion batteries, and more.