Spherical Core-shell Particles Research Articles

Magnetic nanocrystals embedded in silicate glasses studied by polarized SANS

Fe–Mn–O magnetic nanocrystals precipitate in a glass system SiO2/Na2O/Fe2O3/MnO during thermal annealing for different periods of time. We report here small angle neutron scattering investigations using polarized neutrons (pSANS) on these glass samples. Analysis of scattering data reveals the formation of spherical core–shell particles. The core of the particle is found to be magnetic and it is surrounded by a non-magnetic shell-like region. The size parameters of the nanocrystals calculated by pSANS data analyses are comparable with the small angle X-ray scattering (SAXS) investigations, previously reported. Contrary to other glasses in the studied system the diffusion of Fe and Mn ions is not impaired by a rigid shell. This is explained by decreasing of the glass transition temperature of the shell region while its composition departs from that of the matrix.

Short-time dynamics and high-frequency rheology of suspensions of spherical core–shell particles with thin-shells

Short-time dynamics and high-frequency rheology for suspensions of non-overlapping core–shell particles with thin shells were analyzed. In the thin-shell limit, the single-particle scattering coefficients were derived and shown to define a unique effective radius. This result was used to justify theoretically (in the thin-shell limit) the accuracy of the annulus approximation with the inner radius equal to the effective hydrodynamic radius of the core–shell particle. The two-particle virial expansion of the translational and rotational self-diffusion, sedimentation and viscosity was performed. The virial coefficients were evaluated and shown to be accurately approximated by the effective annulus model, in contrast to the imprecise effective hard sphere model.

Characterization of spherical core–shell particles by static light scattering. Estimation of the core- and particle-size distributions

A numerical method is proposed for the characterization of core–shell spherical particles from static light scattering (SLS) measurements. The method is able to estimate the core size distribution (CSD) and the particle size distribution (PSD), through the following two-step procedure: (i) the estimation of the bivariate core–particle size distribution (C–PSD), by solving a linear ill-conditioned inverse problem through a generalized Tikhonov regularization strategy, and (ii) the calculation of the CSD and the PSD from the estimated C–PSD. First, the method was evaluated on the basis of several simulated examples, with polystyrene–poly(methyl methacrylate) core–shell particles of different CSDs and PSDs. Then, two samples of hematite–Yttrium basic carbonate core–shell particles were successfully characterized. In all analyzed examples, acceptable estimates of the PSD and the average diameter of the CSD were obtained. Based on the single-scattering Mie theory, the proposed method is an effective tool for characterizing core–shell colloidal particles larger than their Rayleigh limits without requiring any a-priori assumption on the shapes of the size distributions. Under such conditions, the PSDs can always be adequately estimated, while acceptable CSD estimates are obtained when the core/shell particles exhibit either a high optical contrast, or a moderate optical contrast but with a high ‘average core diameter’/‘average particle diameter’ ratio.

Structural analysis of magnetic nanocrystals embedded in silicate glasses by anomalous small-angle X-ray scattering

The present work reports the structural analysis of magnetic nanocrystals of Fe- and Mn-containing phases formed during annealing of the glass 13.6Na2O–62.9SiO2–8.5MnO–15.0Fe2O3−x(mol%) at 823 K for different periods of time. X-ray diffraction investigation shows the formation of a mixture of MnFe2O4and Fe3O4phases. Furthermore, small-angle X-ray scattering and anomalous small-angle X-ray scattering (ASAXS) experiments were used to extract quantitative information about the structure of the formed nano-sized crystals, as well as their phase compositions, volume fractions and size distributions. ASAXS analysis reveals the formation of spherical core–shell particles with the size of the particles between 10 and 50 nm. The concentrations of iron atoms in the core are higher than those in the shell and in the remaining matrix. With increasing annealing time, the ratio of Mn to Fe increases in the nanoparticles and the crystalline phase moves toward the pure jacobsite phase. Evaluated compositions reveal the presence of SiO2in the shell. This shell acts as a diffusion barrier for the nanocrystal-forming ions, and the growth of the particles is kinetically constrained.

An analytical scattering function for polydisperse spherical core–shell particles with Schulz–Flory size distributions of both core size and shell thickness

The scattering function for polydisperse core–shell particles with Schulz–Flory size distributions of both the core radius and the shell thickness is analytically calculated. The results of the analytical solution are compared with the results of a numerical integration over both size distributions, as well as with available analytical solutions for homogeneous spheres. The CPU time required for the calculation of the scattering function can be reduced by several orders of magnitude compared with numerical integration. This is of tremendous importance in nonlinear curve fitting requiring repeated calculation of the theoretical function in an iterative process.

Black Cr/α-Cr2O3 nanoparticles based solar absorbers

Monodisperse spherical core–shell particles of Cr/α-Cr2O3 with high adhesion were successfully coated on rough copper substrates by a simple self-assembly-like method for the use in solar thermal absorbers. The structure and morphology of the core-shell particles of Cr/α-Cr2O3 were effectively controlled by deposition temperature and the pH of the initial precursor solution. Their characterizations were carried out with X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry and attenuated total reflection, as well as UV–vis diffuse reflectance spectroscopy. The samples aged for more than 40h at 75°C exhibit the targeted high absorbing optical characteristic “Black chrome” while those aged for ≤40h show a significant high UV–vis diffuse reflectance “green color”.

Structure and properties of core-shell microparticles in paraffin-ultradispersed polytetrafluoroethylene systems

Microsized spherical core-shell particles consisting of hydrocarbon cores encapsulated into fluoropolymer shells are obtained in supercritical carbon dioxide. Paraffins serve as a core material, while the polymer shell is formed from ultradispersed polytetrafluoroethylene. The morphology and molecular structure in the bulk and on the surface of the particles and the influence of conditions of particle formation on the shell thickness and the thermal properties of the materials are studied. The materials are composites comprised of paraffin cores coated with shells of loosened globular fluoropolymer particles with sizes of 0.2–1.7 μm. The shells is built of low- and high-molecular-mass fractions consisting of CF3(CF2)nCF3 molecular chains with different lengths. The shell thickness is governed by preparation conditions, exposure time, and the percentage of the polymer in the initial dispersion.

Preparation and properties of core–shell nanosilica/poly(methyl methacrylate–butyl acrylate–2,2,2‐trifluoroethyl methacrylate) latex

AbstractA core–shell nanosilica (nano‐SiO2)/fluorinated acrylic copolymer latex, where nano‐SiO2 served as the core and a copolymer of butyl acrylate, methyl methacrylate, and 2,2,2‐trifluoroethyl methacrylate (TFEMA) served as the shell, was synthesized in this study by seed emulsion polymerization. The compatibility between the core and shell was enhanced by the introduction of vinyl trimethoxysilane on the surface of nano‐SiO2. The morphology and particle size of the nano‐SiO2/poly(methyl methacrylate–butyl acrylate–2,2,2‐trifluoroethyl methacrylate) [P(MMA–BA–TFEMA)] core–shell latex were characterized by transmission electron microscopy. The properties and surface energy of films formed by the nano‐SiO2/P(MMA–BA–TFEMA) latex were analyzed by Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, scanning electron microscopy/energy‐dispersive X‐ray spectroscopy, and static contact angle measurement. The analyzed results indicate that the nano‐SiO2/P(MMA–BA–TFEMA) latex presented uniform spherical core–shell particles about 45 nm in diameter. Favorable characteristics in the latex film and the lowest surface energy were obtained with 30 wt % TFEMA; this was due to the optimal migration of fluorine to the surface during film formation. The mechanical properties of the films were significantly improved by 1.0–1.5 wt % modified nano‐SiO2. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

The preparation of silica–titania core–shell particles and their impact as an alternative material to pure nano-titania photocatalysts

In this study spherical core–shell particles in the range 200–800 nm were prepared. Heterocoagulation method was applied to deposit nano-crystalline titania on monodisperse Stöber silica spheres. Silica and titania were synthesized separately via sol–gel processes. Titania was obtained from tetrapropyl orthotitanate and titanium tetrachloride as the precursors. Particle sizes were analysed by photon cross correlation spectroscopy. Deposition of titania on the surface was verified by transmission electron micrographs. The extent of coverage was analysed by zeta-potential measurements and varied between 40 and 90%. The degradation of an aqueous methylene blue solution confirmed photocatalytic activity of the particles.

Synthesis and characterization of spherical core–shell particles SiO 2@AgEu(MoO 4) 2

Submicron spherical SiO 2 particles have been coated with AgEu(MoO 4) 2 phosphor layers by a sol–gel process, followed by surface reaction at high temperature, to get core/shell structured SiO 2@AgEu(MoO 4) 2 particles. X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) were used to characterize the structure and morphology of the resulted core–shell phosphors. The luminescent properties of the core–shell structured phosphors have also been measured at room temperature, and their photoluminescence (PL) spectra are similar to the pure AgEu(MoO 4) 2 phosphor prepared by the same sol–gel method exhibiting red emission.

The effects of sonification and TiO 2 deposition on the micro-characteristics of the thermally treated SiO 2/TiO 2 spherical core–shell particles for photo-catalysis of methyl orange

SiO 2/TiO 2 core–shell particles were prepared to decompose methyl orange (MO) in a laboratory scale study prior to its use in the treatment of waste containing azo dyes that might potentially pose adverse environmental consequences and health hazard to human. The preparation of silica cores produced perfectly spherical and smooth surfaces, but when TiO 2 shell/film was deposited using titanium butoxide (TBOT) precursor at concentration >0.24 mol L −1, the resulting shell was rougher and loose, giving rise to Ostwald ripening that caused the core–shell particles to agglomerate. The use of sonification improved the dispersion at the expense of thin shell. When multi-step shell coating was applied to increase the shell thickness, loose titania shells were observed despite increasing surface coverage at higher number of coating step. The isoelectric point of the core–shell particles was equivalent to that of pure titania (4.5) due to the complete encapsulation of silica cores by titania. The silica core crystals exhibited highly amorphous structures, whereas titania covered silica cores demonstrated pure anatase phase that grew in size upon heat treatment at 900 °C. Rutile phase started to develop at 5 coatings, suggesting that anatase crystallite size was attainable for as long as the critical crystalline size smaller than ca. 30 nm was sustained or equivalently, 3 coatings were applied. Complete decomposition of MO was achieved in less than 370 min using core–shell particles prepared from triple coating.

Formation of Star-Like and Core-Shell AuAg Nanoparticles during Two- and Three-Step Preparation in Batch and in Microfluidic Systems

Regular dendrit-like metal nanoparticles and core-shell nanoparticles were formed by the reduction of mixtures of tetrachloroaurate and silver nitrate solutions with ascorbic acid at room temperature in two- and three-step procedures. The formation of these particles was found in batch experiments as well as in micro flow-through processes using static micromixers. The characteristic diameters of 4-branched star particles were in the range between 60 and 100 nm. The typical particles consist of four metal cores which are embedded in a common shell. Additionally, particles with five and more metallic cores were formed, to some extent, and aggregates of the 4-branched particles also were formed. Larger aggregates and network-like structures of connected star particles were formed after sedimentation. The properties of the formed particles are dependent on the educt concentrations as well as on the order of mixing steps and on the time interval between them. Obviously, the relation of nucleation and particle growth in relation to the concentrations of metal ions determines the composition and the properties of formed nanoparticles. So, star-like particles are observed in case of nucleation of Au in absence of silver ions but with silver deposition after short nucleation time. Spherical core shell particles are formed in case of silver salt addition after complete reduction of tetrachloroaurate in flow-through experiments with sufficient residence time between both mixing steps. Polymer layers are always found in the form of a second outer shell even if the polymer solutions are added in an early stage of particle formation.

A small-angle neutron scattering study of sodium dodecyl sulfate-poly(propylene oxide) methacrylate mixed micelles

Mixed micelle of protonated or deuterated sodium dodecyl sulfate (SDS and SDSd25, respectively) and poly(propylene oxide) methacrylate (PPOMA) are studied by small-angle neutron scattering (SANS). In all the cases the scattering curves exhibit a peak whose position changes with the composition of the system. The main parameters which characterize mixed micelles, i.e., aggregation numbers of SDS and PPOMA, geometrical dimensions of the micelles and degree of ionisation are evaluated from the analysis of the SANS curves. The position q max of the correlation peak can be related to the average aggregation numbers of SDS–PPOMA and SDSd25–PPOMA mixed micelles. It is found that the aggregation number of SDS decreases upon increasing the weight ratio PPOMA/SDS (or SDSd25). The isotopic combination, which uses the “contrast effect” between the two micellar systems, has allowed us to determine the mixed micelle composition. Finally, the SANS curves were adjusted using the RMSA for the structure factor S ( q ) of charged spherical particles and the form factor P ( q ) of spherical core–shell particle. This analysis confirms the particular core–shell structure of the SDS–PPOMA mixed micelle, i.e., a SDS “core” micelle surrounded by the shell formed by PPOMA macromonomers. The structural parameters of mixed micelles obtained from the analysis of the SANS data are in good agreement with those determined previously by conductimetry and fluorescence studies.

Small-angle scattering from spherical core–shell particles: an analytical scattering function for particles with Schulz–Flory size distribution

The scattering function for Schulz–Flory distributed spherical core–shell particles is derived analytically. A constant ratio of core to shell radii is assumed. The analytical expression, which does not require any numerical integration, provides a fast way to model experimental data by nonlinear least-squares fitting. The asymptotic behavior for large momentum transfers coincides with the different power laws expected for homogenous spheres and thin spherical shells. In the second part, the derived expression is applied to describe experimental small-angle X-ray scattering data from core–shell particles with different particle sizes, polydispersity and ratio of core to shell radii. For large particles, a resolution correction by numerical convolution with a Gaussian resolution function is applied.

Enhancement of photoluminescence in manganese-doped ZnS nanoparticles due to a silica shell

Zinc sulphide nanoparticles doped with manganese (ZnS:Mn) have been stabilized using thioglycerol [HSCH2CH(OH)CH2OH] molecules. The nanoparticles (∼1.7 nm) are highly stable and exhibit photoluminescence at ∼600 nm when excited with ultraviolet light. For increasing luminescence and stability the particles are further treated with tetraethylorthosilicate (TEOS)[Si(C2H5O)4] in an aqueous medium, yielding either a disordered silica matrix or spherical core-shell particles of up to ∼900 nm size with strongly enhanced luminescence under certain conditions. Photoluminescence, excitation spectroscopy, transmission electron microscopy, energy dispersive analysis of x-rays, x-ray diffraction, Raman spectroscopy, and x-ray photoelectron spectroscopy measurements have been performed for the characterization of the ZnS:Mn nanoparticles alone, in the silica matrix as well as in spherical silica shells. Among other things, the analysis indicates that the thioglycerol capping has been affected by the coating neither in the silica matrix nor in core-shell particles.

Formation of luminescent spherical core-shell particles by the consecutive adsorption of polyelectrolyte and CdTe(S) nanocrystals on latex colloids

Functional core-shell particles were prepared by assembling a composite multilayer shell of charged polyelectrolytes and luminescent CdTe(S) nanocrystals (cadmium telluride with a certain content of sulfide) via their consecutive electrostatic adsorption from solution onto micron-sized latex particles. The formation of the composite shell has been confirmed by optical spectroscopy, confocal microscopy and high-resolution transmission electron microscopy. Variation of the size and surface chemistry of the semiconductor nanoparticles, the size and shape of the colloid templates, and the nature of the polyelectrolyte opens new avenues for the production of a variety of novel core-shell materials.