Uniform Shell Thickness Research Articles

Visible Mie Scattering from Hollow Silica Particles with Particulate Shells

A series of colloidal nanocomposite dispersions are synthesized by alcoholic dispersion polymerization of styrene in the presence of an ultrafine silica sol. The original core/shell polystyrene/silica nanocomposite particles have mean diameters ranging from 321 to 471 nm, as determined by dynamic light scattering. Upon calcination of the polystyrene cores, some shrinkage occurs but intact hollow silica shells are observed by transmission electron microscopy. On visual inspection, these silica residues display remarkable colors that vary depending on the particle diameter. When examined in transmittance mode (i.e., with an illuminated background) the silica powders appear yellow to red in color, but when viewed in reflectance (i.e., with a dark background) relatively intense blue/green colors are observed. The latter phenomenon has been analyzed by visible reflectance spectroscopy and the reflectance maximum depends on the dimensions of the silica shell, which are in turn dictated by the initial nanocomposite particle diameter. Small-angle X-ray scattering is used to determine the packing density of the silica nanoparticles, both in the original polystyrene/silica nanocomposite particles and in the calcined silica shells. Combined with geometrical considerations, this allows the equivalent uniform silica shell thickness to be calculated for a particulate silica shell and this parameter is then related to the theoretical predictions made by Retsch et al. for hollow particles comprising uniform silica shells (see Retsch, M.; Schmelzeisen, M.; Butt, H. J.; Thomas, E. L. Nano Lett., 2011, 11, 1389).

Facile microwave fabrication of CdS nanobubbles with highly efficient photocatalytic performances

A new fabrication method of CdS nanobubbles with high photocatalytic activity has been explored under ambient conditions using SiO2 hard templates and microwave irradiation. The one-step Stober method has been used to produce SiO2 core nanospheres, onto which CdS shells have been readily and efficiently coated by 10 min microwave irradiation. The acid etching of the SiO2 cores has produced monodispersive CdS nanobubbles with outer diameters of 300, 370, and 400 nm with a uniform shell thickness of 11 nm. The CdS nanobubbles have been found to photocatalyze the degradation of rhodamine B efficiently under Xe-lamp irradiation via simultaneous reaction pathways of N-deethylation and conjugated-ring attack. It has been found that the degradation rate increases linearly with the concentration increase of the nanocatalyst but decreases nonlinearly with increasing nanobubble diameters. The pseudo-first-order degradation rate constant via the nanobubbles (0.081 min−1) was found to be about 20 times larger than that via the SiO2@CdS core@shell nanocomposites (0.0044 min−1) due to the nanoreactor confinement effect leading to a decrease in the activation energy of the reaction.

Facile fabrication of platinum nanobubbles having efficient catalytic degradation performances

Platinum nanobubbles having a uniform shell thickness of 20nm with average outer diameters of 150, 320, and 420nm have been synthesized readily by etching the silica cores of SiO2@Pt core–shell nanospheres and they have been found to catalyze the degradation of rhodamine B efficiently in the presence of KBH4 compared with SiO2@Pt core–shell nanospheres. The catalytic rate constant of the nanobubbles (0.030min−1) with an activation energy of 10.7kcalmol−1 is larger by 23 times than that of the core–shell nanospheres with an activation energy of 30.5kcalmol−1. The catalytic activation energy and the entropy of activation obtained from the Arrhenius and the Eyring plots, respectively, have been found to increase with the size increase of platinum nanobubbles, due to the reduction of the nanoreactor confinement effect of platinum nanobubbles. The existence of a good compensation effect between activation energies and frequency factors has been evidenced to support that the catalytic degradation reaction takes place within the nanocavities of platinum nanocatalysts.

Synthesis and Photoluminescence Properties of Eu3+‐Doped Silica@Coordination Polymer Core–Shell Structures and Their Calcinated Silica@Gd2O3:Eu and Hollow Gd2O3:Eu Microsphere Products

The conjugation of Eu(3+)-doped coordination polymers constructed from Gd(3+) and isophthalic acid (H(2)IPA) with silica particles is investigated for the production of luminescent microspheres. A series of doping ratio-controlled silica@coordination polymer core-shell spheres is easily synthesized by altering the amounts of metal nodes used in the reactions, where the ratios of Gd(3+) and Eu(3+) are 10:0 (1a), 9:1 (1b), 8:2 (1c), 7:3 (1d), 5:5 (1e), and 0:10 (1f). The formation of monodisperse uniform core-shell structures is achieved throughout the entirety of a series. Investigations of the photoluminescence property of the resulting series of silica@coordination polymer core-shell spheres reveal that 20% Eu(3+)-doped product (1c) has the strongest emission intensity. The subsequent calcination process on the silica@coordination polymer core-shell structures (1a-f) results in the formation of a series of doping ratio-controlled silica@Gd(2)O(3):Eu core-shell microspheres (2a-f) with uniform shell thickness. During the calcination step, the coordination polymers within silica@coordination polymer core-shells are transformed into metal oxides, resulting in silica@Gd(2)O(3):Eu core-shell structures. The final etching process on the silica@Gd(2)O(3):Eu core-shell microspheres (2a-f) produces a series of hollow Gd(2)O(3):Eu microspheres (3a-f) as a result of the elimination of silica cores. The luminescence intensities of silica@Gd(2)O(3):Eu core-shell (2a-f) and hollow Gd(2) O(3):Eu microspheres (3a-f) also vary depending upon the doping ratio of Eu(3+) ions.

Highly efficient and stable nonplatinum anode catalyst with Au@Pd core–shell nanostructures for methanol electrooxidation

This paper reports the easy synthesis of Au@Pd core–shell nanoparticles (NPs) with uniform shell thickness and demonstrates their viability as a nonplatinum catalyst for the electrooxidation of methanol in alkaline anion-exchange membrane fuel cells. The synthesis involves the first preparation of Au core NPs, followed by a three-phase-transfer procedure to coat Pd shells, through which homogeneous Pd shell growth on Au cores can be achieved. The as-prepared Au@Pd NPs have an activity more than 40 times higher than that of the Pd catalyst for the methanol oxidation. Moreover, these Au@Pd NPs possess excellent stability (over seven times more stable than Pd catalysts). The remarkable performance enhancement is mainly attributed to the finely tailored electronic structure of the Pd shell achieved by the underlying Au core. The easily synthesized Au@Pd core–shell NPs represent a promising class of nonplatinum anode catalysts with high activity and durability for alkaline fuel cell applications.

Monodisperse double-walled microspheres loaded with chitosan-p53 nanoparticles and doxorubicin for combined gene therapy and chemotherapy

We have designed and evaluated a dual anticancer delivery system to provide combined gene therapy and chemotherapy. Double-walled microspheres consisting of a poly(d,l-lactic-co-glycolic acid) (PLGA) core surrounded by a poly(lactic acid) (PLA) shell were fabricated via the precision particle fabrication (PPF) technique. We make use of the advantages of double-walled microspheres to deliver chitosan–DNA nanoparticles containing the gene encoding the p53 tumor suppressor protein (chi-p53) and/or doxorubicin (Dox), loaded in the shell and core phases, respectively. Different molecular weights of PLA were used to form the shell layer for each formulation. The microspheres were monodisperse with a mean diameter of 65 to 75μm and uniform shell thickness of 8 to 17μm. Blank and Dox-loaded microspheres typically exhibited a smooth surface with relatively few small pores, while chi-microspheres containing p53 nanoparticles, with and without Dox, presented rough and porous surfaces. The encapsulation efficiency of Dox was significantly higher when it was encapsulated alone compared to co-encapsulation with chi-p53 nanoparticles. The encapsulation efficiency of chi-p53 nanoparticles, on the other hand, was not affected by the presence of Dox. As desired, chi-p53 nanoparticles were released first, followed by simultaneous release of chi-p53 nanoparticles and Dox at a near zero-order rate. Thus, we have demonstrated that the PPF method is capable of producing double-walled microspheres and encapsulating dual agents for combined modality treatment, such as gene therapy and chemotherapy.

Optimizing Compliance and Thermal Conductivity of Plasma Sprayed Thermal Barrier Coatings via Controlled Powders and Processing Strategies

The properties and performance of plasma-sprayed thermal barrier coatings (TBCs) are strongly dependent on the microstructural defects, which are affected by starting powder morphology and processing conditions. Of particular interest is the use of hollow powders which not only allow for efficient melting of zirconia ceramics but also produce lower conductivity and more compliant coatings. Typical industrial hollow spray powders have an assortment of densities resulting in masking potential advantages of the hollow morphology. In this study, we have conducted process mapping strategies using a novel uniform shell thickness hollow powder to control the defect microstructure and properties. Correlations among coating properties, microstructure, and processing reveal feasibility to produce highly compliant and low conductivity TBC through a combination of optimized feedstock and processing conditions. The results are presented through the framework of process maps establishing correlations among process, microstructure, and properties and providing opportunities for optimization of TBCs.

Facilitating growth of InAs–InP core–shell nanowires through the introduction of Al

InAs nanowires were grown on GaAs substrates by the Au-assisted vapour–liquid–solid (VLS) method in a gas source molecular beam epitaxy (GSMBE) system. Passivation of the InAs nanowires using InP shells proved difficult due to the tendency for the formation of axial rather than core–shell structures. To circumvent this issue, AlxIn1−xAs or AlxIn1−xP shells with nominal Al composition fraction of x=0.20, 0.36, or 0.53 were grown by direct vapour–solid deposition on the sidewalls of the InAs nanowires. Characterisation by transmission electron microscopy revealed that the addition of Al in the shell resulted in a remarkable transition from the VLS to the vapour–solid growth mode with uniform shell thickness along the nanowire length. Possible mechanisms for this transition include reduced adatom diffusion, a phase change of the Au seed particle, and surfactant effects. The InAs–AlInP core-shell nanowires exhibited misfit dislocations, while the InAs–AlInAs nanowires with lower strain appeared to be free of dislocations.

Design of Aerosol Coating Reactors: Precursor Injection

Particles are coated with thin shells to facilitate their processing and incorporation into liquid or solid matrixes without altering core particle properties (coloristic, magnetic, etc.). Here, computational fluid and particle dynamics are combined to investigate the geometry of an aerosol reactor for continuous coating of freshly-made titanium dioxide core nanoparticles with nanothin silica shells by injection of hexamethyldisiloxane (HMDSO) vapor downstream of TiO2 particle formation. The focus is on the influence of HMDSO vapor jet number and direction in terms of azimuth and inclination jet angles on process temperature and coated particle characteristics (shell thickness and fraction of uncoated particles). Rapid and homogeneous mixing of core particle aerosol and coating precursor vapor facilitates synthesis of core-shell nanoparticles with uniform shell thickness and high coating efficiency (minimal uncoated core and free coating particles).

Reduced Stiffness Buckling Analysis of Aboveground Storage Tanks with Thickness Changes

The Reduced Stiffness Analysis (RSA) to compute lower bounds to buckling loads of shells has been employed by a number of researchers as a simple way to evaluate the buckling capacity of shells that display unstable behavior and imperfection-sensitivity. It allows the use of simple eigenvalue analysis, without having to perform incremental nonlinear analysis, and is based on the physical behavior of the shell which recognizes that a significant contribution to the stability of a shell under lateral pressure is provided by its membrane stiffness. Unstable post-critical behavior is associated with the loss of this stabilizing membrane contribution. Past use of the approach has been mainly restricted to cases of uniform shell thickness and uniform pressures in the circumferential direction, in which case analytical solutions are possible. Recent applications by the authors and other researchers have shown ways to compute the lower bounds using finite element analysis, for which a modified eigenvalue analysis is constructed by neglecting the membrane contributions to the matrix containing the initial stresses. This paper illustrates the application of the methodology to cases of pressure loaded shells with thickness changes in the meridional direction. A semi-analytical finite element code has been employed for the buckling analysis when uniform pressures act on aboveground steel tanks. The tanks are representative of those constructed for the oil industry, with diameter to thickness ratios of the order of 3000, and height to diameter ratios lower than one.

Synthesis and characterization of polyfunctional aziridine/polyester microcapsules by multiple emulsion-solvent evaporation method

Polyfunctional aziridine/polyester microcapsules as control-release waterborne cross-linker were synthesized by multiple emulsion-solvent evaporation method. The results show that, a lower surface free energy with shell polyester is more favourable for the formation of microcapsules. Full encapsulating microcapsules are synthesized with the polyester with a surface free energy of 34.5 mJ/m2. Shell-to-core feeding mass ratio has a significant influence on the morphology and core content of the resulting microcapsules. Well defined spherical microcapsules with uniform shell thickness and core content at around 22% are produced at a shell-to-core mass ratio of 1:1. When 2.5% of colloid stabilizer is used, hollow spherical microcapsules are obtained. A high solvent evaporation rate results in wrinkling and porosity of the microcapsules, and an evaporation rate equivalent to solvent elimination in about 2 h provides a uniform rate of surface hardening. The characterization of the microcapsules by SEM and FTIR demonstrates that polyfunctional aziridine is encapsulated at the centre of the microcapsule. The microcapsules synthesized can be broken at a high shear rate.

Laser-induced fabrication of platinum nanoshells having enhanced catalytic and Raman properties

Platinum nanoclusters topped on silica nanospheres, prepared by a seeded-growth method, have been transformed into platinum nanoshells having uniform shell thickness of 6 nm via irradiation of nanosecond laser pulses. The thickness of platinum nanoshells can be tuned readily by adjusting the concentration of H 2PtCl 6 for the growth of platinum nanoparticles on silica nanospheres. Compared with Pt nanoclusters-topped silica nanospheres, platinum nanoshells coated on silica nanospheres have been found to catalyze the degradation of rhodamine B very rapidly in the presence of KBH 4, and they show highly active surface-enhanced Raman scattering.

Hollow spheres with α-cyclodextrin nanotube assembled shells

Monodisperse hollow spheres with radially oriented α-cyclodextrin nanotube assembled shells were synthesized using PEG-grafted silica as templates. (3-Aminopropyl)triethoxysilane-modified silica core was synthesized by the Stöber method and subsequently grafted with mPEG-succinimidyl carbonate through amide linkage. Maleic anhydride modified α-cyclodextrins were threaded onto the grafted-PEG chains, forming tubular polypseudorotaxanes. Adjacent polypseudorotaxanes were immobilized by crosslinking. After the extraction of the inner silica template by etching and the included PEG template by heating, hollow spheres of uniform size with α-CD nanotube assembled shells were prepared. The products were composed of intact and dispersed hollow spheres with diameter of about 50 nm, possessed uniform shell thickness of 12 nm as confirmed by transmission electron microscopy (TEM) measurement. α-CD nanotubes have a function of molecular recognization with PEG, which may endow the hollow nanospheres as promising target drug delivery vehicles by forming inclusion complexes with end-functionalized PEG.

Core–shell nanostructures and nanocomposites of Ag@TiO2: effect of capping agent and shell thickness on the optical properties

Two different shell-forming reagents viz. titanium isopropoxide and titanium hydroxyacylate, have been employed to obtain core-shell nanostruc- tures of Ag@TiO2. However, nanocomposites were formed when the shell-forming agent, titanium iso- propoxide, was added before breaking the micelles. Titanium hydroxyacylate has been used for the first time as a shell-forming agent which resulted in uniform core-shell structures of Ag@TiO2 with core diameter ranging from 10 to 40 nm and a shell thickness of 10-50 nm. The low rate of hydrolysis of titanium hydroxyacylate than titanium isopropoxide (used in other methods) appears to be responsible for the uniform shell thickness. The presence of capping agent (2-mercaptoethanol) disrupts the formation of a uniform shell structure of Ag@TiO2. HRTEM, IR, and XPS studies of Ag@TiO2 synthesized using capping agent show the formation of Ag2S coated with an amorphous layer of TiO2. A red shift of 25 and 10 nm was observed in the surface plasmon band of silver for Ag@TiO2 core-shell structures (com- pared with that of silver nanoparticles) synthesized using titanium hydroxyacylate and titanium isoprop- oxide, respectively. The presence of capping agent (2-mercaptoethanol) masks the surface plasmon peak. Photoluminescence studies show an increase in the emission intensity for the core-shell structures when compared to that of TiO2 nanoparticles.

Facile one-pot synthesis and drug storage/release properties of hollow micro/mesoporous organosilica nanospheres

Novel hollow micro/mesoporous organosilica nanospheres (HMOSNs) of uniform diameter and shell thickness of about 90 nm and 15 nm, respectively, and with wormlike micro/mesoporous shell full of uramido groups, have been successfully fabricated by a facile one-pot route. The micro/mesoporosity of the synthesized HMOSNs has been characterized by small-angle and wide-angle X-ray diffraction (XRD), scan electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen adsorption–desorption measurements. The drug storage and release properties of the synthetic HMOSNs are measured by using ibuprofen (IBU) as a model drug, and a high drug storage capacity of 531 mg IBU per gram HMOSNs and a steady drug release behavior are exhibited.

Oxidation−Reduction Reaction Driven Approach for Hydrothermal Synthesis of Polyaniline Hollow Spheres with Controllable Size and Shell Thickness

Different polyaniline (PANI) micro/mesostructures have been synthesized by one-pot polymerization of aniline using hydrogen peroxide (H2O2) as oxidant and Fe3+ as catalyst under hydrothermal conditions. Well-defined PANI hollow spheres with relatively uniform sizes and controllable shell thickness can be prepared in case of low concentrations of monomer and oxidant. The oxidation−reduction reaction between the benzenoid unit and O2 is a driving force for the formation of hollow spheres. This approach provides a unique route for the preparation of well-defined PANI hollow spheres in the absence of any sacrificial templates and organic surfactants and can be potentially extended to synthesize other polymer hollow spheres.

Synthesis and characterization of carbon sphere-silica core–shell structure and hollow silica spheres

Carbon sphere-silica core–shell structured material was prepared in the presence of cetyltrimethylammonium bromide using tetraethyl orthosilicate as precursor of silica by a sol–gel method combined with Stöber method. Carbon sphere cores with an average diameter of 300 nm were prepared by the pyrolysis of acetylene. After carbon sphere cores were removed by calcinations, hollow silica spheres with smooth surface and uniform shell thickness around 110 nm were obtained. The core–shell structure of carbon sphere-silica composites and the hollow structure of silica spheres were characterized by FESEM, HRTEM, XRD, XPS and FTIR. The results indicate that the silica was coated on the surface of carbon spheres. The inner diameter of the hollow silica spheres with amorphous shell was about 290 nm. A possible formation mechanism of the core–shell structure of carbon sphere-silica and hollow silica spheres was proposed.

Morphology and dispersivity modulation of hollow microporous spheres synthesized by a hard template route

Uniform hollow microporous silica spheres (HMS) have been synthesized in an easy route using polystyrene spheres (PS) and cetyltrimethylammonium bromide (CTAB) as co-templates at room temperature in concentrated aqueous ammonia. The hexagonally ordered pore channels are formed on the shell of the HMS. Effects of various processing parameters such as the tetraethyl orthosilicate (TEOS) adding rate, the amount of aqueous ammonia and ultrasonic treatment, on the dispersivity and morphology, were investigated. The slow syringing addition of TEOS favors the formation of highly dispersed hollow spheres with a certain and uniform shell thickness. More aqueous ammonia and proper time of ultrasonic are helpful to the synthesis of uniform hollow silica spheres with good dispersivity. The morphology, dispersivity and micropore structure of as-synthesized HMS were characterized using X-ray diffraction, N 2 sorption isotherms, scanning electron microscope and transmission electron microscope.

A Novel Technique for Synthesizing Nanoshell Hollow Alumina Particles

A novel and versatile synthesis route has been developed to fabricate hollow Al2O3 microspheres with nanoporous shell structures. The method involves a preferential “implantation” of Al species into polymeric template particles to form a core–shell structure in solvent. The template cores can then be removed by thermal pyrolysis to form hollow interiors. This unique “implantation” process allows for synthesis of monodisperse hollow spheres with a nonaggregated character. The implantation of Al species is confirmed by electron spectroscopy for chemical analysis/Auger analysis in depth profiles. The shell structure changes from amorphous to γ‐Al2O3 at 900°C; because of which, the shell is nanoporous in structure. The porous shell then transforms to α‐Al2O3 as temperature is further raised above 1000°C. Transmission electron microscopy examination reveals a uniform shell thickness of ca. 20–40 nm, and grain growth appears to be constrained by the thin shell walls at 1100°C, leading to void formation at the triple grain junctions.

Fabrication of hollow Fe 3O 4-polyaniline spheres with sulfonated polystyrene templates

Hollow Fe 3O 4-polyaniline (PANI) spheres with uniform cavity size and shell thickness were prepared by an easy and effective approach. In this method, sulfonated polystyrene (PS) spheres were used as templates; aniline was oxidized and polymerized in the shell of sulfonated PS. Since PANI was in situ doped by sulfonic acid, the as-synthesized PANI/PS had good conductivity. Moreover, the resulting PANI/PS layer could adsorb the target precursor, and then formed Fe 3O 4-PANI/PS composite capsules. Finally, sulfonated PS cores were dissolved and removed from the composite capsules by solvent extract, which resulted in producing hollow Fe 3O 4-PANI spheres with uniform size. The hollow composite spheres exhibited a super-paramagnetic behavior at room temperature. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray powder scattering (XRD) were applied to characterize the nanocomposite spheres.