Preparation of hollow silica microspheres with controlled shell thickness in supercritical fluids

This article presents a novel route to prepare hollow silica microspheres with well-defined wall thickness by using cross-linked polystyrene (PS) microspheres as templates with the assistance of supercritical carbon dioxide (SC-CO₂). In this approach, the cross-linked PS templates can be firstly prepared via emulsifier-free polymerization method by using ethylene glycol dimethacrylate or divinylbenzene as cross-linkers. Then, the silica shell from the sol–gel process of tetraethyl orthosilicate (TEOS) which was penetrated into the PS template with the assistance of SC-CO₂ was obtained. Finally, the hollow silica spheres were generated after calcinations at 600 °C for 4 h. The shell thickness of the hollow silica spheres could be finely tuned not only by adjusting the TEOS/PS ratio, which is the most frequently used method, but also by changing the pressure and aging time of the SC-CO₂ treatment. Fourier transform infrared spectroscopy, transmission electron microscopy, and scanning electron microscope were used to characterize these hollow silica spheres.     

O/W/O double emulsion-assisted synthesis and catalytic properties of CeO2 hollow microspheres

CeO₂ hollow microspheres have been fabricated through a simple thermal decomposition of precursor approach. The precursor with an average size of 10 μm was prepared in a reverse microemulsions containing Ce(NO₃)₃·6H₂O and CO(NH₂)₂ at 160 °C. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAED) and scanning electron microscopy (SEM). The possible formation mechanism of hollow spheres was discussed. In addition, the CeO₂ hollow microspheres modified glassy carbon electrode exhibit excellent sensing performance towards methyl orange, which provide a new application of CeO₂ hollow spheres. The catalytic activity of CeO₂ hollow spheres on the thermal decomposition of ammonium perchlorate (AP) also was investigated by TGA. The catalytic performance of CeO₂ hollow spheres is superior to that of commercial CeO₂ powder.     

Porous carbon and carbon composite hollow spheres

In this communication, we propose a template approach toward synthesis of carbon hollow spheres by direct carbonization of highly crosslinked sulfonated polystyrene gel hollow spheres (sPS). The sulfonic acid group can facilitate carbonization. Moreover, the acid group can also induce a favorable growth of other materials within the sPS gel-forming carbon-based composite shell such as bi-continuous C/SiO₂ and their derivative ceramic SiC. Release of small molecules during polymers decomposition results porous shell.     

Preparation and properties of nitrogen-containing hollow carbon nanospheres by pyrolysis of polyaniline-lignosulfonate composites

Polyaniline-lignosulfonate composite hollow spheres were synthesized by using one-step unstirred polymerization of aniline in the presence of lignosulfonate. Novel nitrogen-containing hollow carbon nanospheres were prepared by direct pyrolysis of the polyaniline-lignosulfonate composite spheres at different temperatures under a nitrogen atmosphere. Thermal behavior of the polyaniline-lignosulfonate composite spheres was studied by TG-DTG, FTIR and element analyze instruments. The resultant carbon spheres were characterized by SEM, XRD and nitrogen adsorption-desorption measurement. It was found that the pyrolysis products of the polyaniline-lignosulfonate composite spheres were made up of uniform hollow carbon nanospheres with an average diameter of 135 nm. Furthermore, the hollow carbon nanospheres exhibit high BET surface area range from 381.6 m² g⁻¹ to 700.2 m² g⁻¹. The hollow carbon nanospheres could be used as adsorbents of papain. The papain adsorption capacity for the carbon spheres prepared at 1200 °C was up to 1161 mg g⁻¹ at an initial papain concentration of 10 mg mL⁻¹ at 25 °C.     

Preparation of hollow silica nanospheres in O/W microemulsion system by hydrothermal temperature changes

Hollow silica nanospheres with wrinkled or smooth surfaces were successfully fabricated through a hydrothermal method. In this method, oil-in-water microemulsion (composed of cyclohexane, water, ethanol, and cetyltrimethylammonium bromide), and polyvinylpyrrolidone were utilized as template and capping agent, respectively. In such a facile synthesis, we can well realize the morphological transformation of spheres with radially oriented mesochannels to hollow structures of silica nanoparticle only by regulating the hydrothermal temperature from 100 °C to 200 °C. Synthesized samples with different mesostructures were then used as supports to immobilize Candida rugosa lipase (CRL). The immobilized CRL was employed as a new biocatalyst for biodiesel production through the esterification of heptanoic acid with ethanol. The conversion ratio of heptanoic acid with ethanol catalyzed by the immobilized CRL was also evaluated. Results of this study suggest that the prepared samples have potential applications in biocatalysis.     

One-step pyrolysis process to synthesize dispersed Pt/carbon hollow nanospheres catalysts for electrocatalysis

An effective method for loading Pt nanoparticles on monodispersed hollow carbon nanospheres by one-step pyrolysis of polystyrene spheres (PS) adsorbed with platinum (IV) ions was developed. The polystyrene spheres were firstly enwrapped with a layer of sucrose and cetyltrimethyl ammonium bromide (CTAB) micelles. Adsorption of platinum (IV) ions onto the polystyrene spheres was carried out via electrostatic interaction between the negatively charged platinum salt and the positively charged amino group in the CTAB. Pyrolysis of the PS-Pt (IV) precursors at 600 °C under nitrogen atmosphere resulted in the simultaneous decomposition of the sucrose to carbon and the adsorbed platinum complex to metallic Pt. During this process the polystyrene spheres was removed and hollow sphere of PtC formed. Nanocomposites of hollow carbon nanospheres with different platinum loading were synthesized and their electrocatalytic activity was evaluated using methanol as a model molecule. Results showed that the as-prepared hollow carbon nanospheres supported platinum catalysts have high electrocatalytic activity and long-term stability towards the oxidation of methanol. The present method is promising for the fabrication of carbon supported platinum catalysts for the direct methanol fuel cell.     

A novel approach to produce monodisperse hollow pure silica spheres

In this work, we report an efficient method to produce pure hollow silica spheres (HSS) using phenyltrimethoxysilane (PTMS) compound. The production of HSS was carried out via hydrolysis of PTMS in the aqueous media and followed by a condensation reaction to form silica spheres with phenyl groups. The product was then calcined to remove phenyl groups and obtain pure silica spheres with >95% fine structure. The chemical nature of pure silica was confirmed by Fourier transforms infrared spectroscopy. The calcined HSS were stable beyond the temperature of 900 °C as confirmed by thermal gravimetric analysis (TGA). The calcined spheres preserved their spherical appearance and hollow core as shown by SEM and TEM micrographs. Interestingly, the average size of the spheres was reduced significantly after calcination from 760 to 510 nm, confirming further the removal of phenyl groups. The calcined HSS offered much higher surface area (A_s) when analysed by BET; A_s for calcined product was ～406 and mere ～4.8 m²/g for uncalcined HSS. Finally, drug release study of cisplatin/HSS showed over 45% of steady cumulative release for 72 h. The prepared HSS can be dispersed in water opening the possibility of many novel bio/non-bio applications.     

A novel and facile preparation method of hollow silica spheres containing small SiO2 cores

This article presents a novel and facile preparation method of hollow silica spheres with loading small silica inside. In this approach, positively charged SiO₂/polystyrene core‐shell composite particles were synthesized first, when the silica shells from the sol‐gel process of tetraethoxysilane were then coated on the surfaces of composite particles via electrostatic interaction, the polystyrene was dissolved subsequently even synchronously in the same medium to form hollow silica spheres with small silica cores. TEM, SEM, and FTIR measurements were used to characterize these composite spheres. Based on this study, some inorganic or organic compounds could be loaded into these hollow silica spheres. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3431–3439, 2007     

Influence of thermal process on microstructural and physical properties of ambient pressure dried hydrophobic silica aerogel monoliths

The experimental results of thermal process on the microstructural and physical properties of ambient pressure dried hydrophobic silica aerogel monoliths are reported and discussed. With sodium silicate as precursor, ethanol/hexamethyldisiloxane/hydrochloric acid as surface modification agent, the crack-free and high hydrophobic silica aerogel monoliths was obtained possessing the properties as low density (0.096 g/cm³), high surface area (651 m²/g), high hydrophobicity (~147°) and low thermal conductivity (0.0217 Wm/K). Silica aerogels maintained hydrophobic behavior up to 430 °C. After a thermal process changing from room temperature to 300 °C, the hydrophobicity remained unchanged (~128°), of which the porosity was 95.69% and specific density about 0.094 g/cm³. After high temperature treatment (300–500 °C), the density of final product decreased from 0.094 to 0.089 g/cm³ and porosity increased to 96.33%. With surface area of 466 m²/g, porosity of 91.21% and density about 0.113 g/cm³, silica aerogels were at a good state at 800 °C. Thermal conductivities at desired temperatures were analyzed by the transient plane heat source method. Thermal conductivity coefficients of silica aerogel monoliths changed from 0.0217 to 0.0981 Wm/K as temperature increased to 800 °C, revealed an excellent heat insulation effect during thermal process.     

Formation of Triple‐Shelled Molybdenum–Polydopamine Hollow Spheres and Their Conversion into MoO2/Carbon Composite Hollow Spheres for Lithium‐Ion Batteries

Unique triple‐shelled Mo‐polydopamine (Mo‐PDA) hollow spheres are synthesized through a facile solvothermal process. A sequential self‐templating mechanism for the multi‐shell formation is proposed, and the number of shells can be adjusted by tuning the size of the Mo‐glycerate templates. These triple‐shelled Mo‐PDA hollow spheres can be converted to triple‐shelled MoO₂/carbon composite hollow spheres by thermal treatment. Owing to the unique multi‐shells and hollow interior, the as‐prepared MoO₂/carbon composite hollow spheres exhibit appealing performance as an anode material for lithium‐ion batteries, delivering a high capacity of ca. 580 mAh g^?1 at 0.5 A g^?1 with good rate capability and long cycle life.     

Low‐Temperature Facile Template Synthesis of Crystalline Inorganic Composite Hollow Spheres

This report presents a facile approach for the low‐temperature synthesis of crystalline inorganic‐oxide composite hollow spheres by employing the bulk controlled synthesis of inorganic‐oxide nanocrystals with polymer spheres as templates. The sulfonated polystyrene gel layer can adsorb the target precursor and induce inorganic nanocrystals to grow on the template in situ. The crystalline phase and morphology of the composite shell is tunable. By simply adjusting the acidity of the titania sol, crystalline titania composite hollow spheres with tunable crystalline phases of anatase, rutile, or a mixture of both were achieved. The approach is general and has been extended to synthesize the representative perovskite oxide (barium and strontium titanate) composite hollow spheres. The traditional thermal treatment for crystallite transformation is not required, thus intact shells can be guaranteed. The combination of oxide properties such as high refractive index, high dielectric constant, and catalytic ability with the cavity of the hollow spheres is promising for applications such as opacifiers, photonic crystals, high‐κ‐gate dielectrics, and photocatalysis.     

A general protocol to coat titania shell on carbon-based composite cores using carbon as coupling agent

We present herein a general protocol to coat titania shell on carbon-based composite cores via the hydrolysis of titanium tetrabutoxide in a mixed solvent with the catalysis of ammonia. As coupling agent, their carbonaceous component can adsorb counter-charged NH₄⁺, and then the negatively charged ≡TiO⁻ species are adsorbed via electrostatic interaction to form TiO₂ shell. Various carbon-based cores with different functional components, such as Ag/C core/shell spheres, Ag/C nanocables, as well as magnetic (C-Fe₃O₄) hollow spheres can be coated with titania shell to form corresponding core/shell composites. Their optical or magnetic property is also investigated. Finally, anatase-phase titania shell with rough surface can be generated via hydrothermal treatment of aforementioned titania coated composites, which can further introduce the properties of anatase-phase titania to the composites.     

Modified silica-based isoprene rubber composite by a multi-functional silane: Preparation and its mechanical and dynamic mechanical properties

Modified silica-based isoprene rubber (IR) composite has been designed and prepared by using a multi-functional silane, 2-aminoethyl-2-(3-triethoxysilylpropyl)aminoethyl disulfide (ATD), as coupling agent. Such modification significantly improved the dispersity of silica in the corresponding composites, as verified by SEM observation. And the hardness, tensile strength, stress at definite elongation, tear strength and temperature rise as well as the value of dynamic loss coefficient ranging from 0 °C to 80 °C of silica/IR vulcanized composites, are significantly improved, especially with low ATD dosage (2–4 phr). This modification of silica-based IR composite by employing ATD as coupling agent provides a facile and effective method to prepare silica-based rubber composites with improved mechanical properties and low hysteresis.     

Nanosized aluminum nitride hollow spheres formed through a self-templating solid-gas interface reaction

Nanosized aluminum nitride hollow spheres were synthesized by simply heating aluminum nanoparticles in ammonia at 1000 °C. The as-synthesized sphere shells are polycrystalline with cavity diameters ranging from 15 to 100 nm and shell thickness from 5 to 15 nm. The formation mechanism can be explained by the nanoscale Kirkendall effect, which results from the difference in diffusion rates between aluminum and nitrogen. The Al nanoparticles served as both reactant and templates for the hollow sphere formation. The effects of precursor particle size and temperature were also investigated in terms of product morphology. Room temperature cathode luminescence spectrum of the nanosized hollow spheres showed a broad emission band centered at 415 nm, which is originated from oxygen related luminescence centers. The hollow structure survived a 4-h heat treatment at 1200 °C, exhibiting excellent thermal stability.     

Template synthesis of tin-doped indium oxide (ITO)/polymer and the corresponding carbon composite hollow colloids

SnO₂, In₂O₃, and Sn-doped In₂O₃ (ITO)/polymer and the corresponding carbon composite hollow colloids are template synthesized. It is essential that the sulfonated gel shell of the cross-linked polystyrene hollow colloid can favorably induce adsorption of target precursors. After being calcined in air to remove the template, SnO₂, In₂O₃, and ITO hollow colloids are obtained. Because the cross-linked polymer gel can be transformed into carbon in nitrogen at higher temperature such as 800 °C, metal oxide/carbon hollow colloids are consequently derived, whose shells are mesoporous. The SnO₂-, In₂O₃-, and ITO-containing polymer or carbon composite hollow colloids will be promising in sensors, catalysts, and fuel cells as electrode materials. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Template-Free Hydrothermal Synthesis of Hollow α-FeOOH Urchin-Like Spheres and Their Conversion to α-Fe2O3 Under Low-Temperature Thermal Treatment in Air

Hollow α-FeOOH urchin-like spheres were synthesized by a simple hydrothermal method at 160 °C for 12 h and their thermal conversion to hollow α-Fe₂O₃ urchin-like spheres was performed at 300 °C for 2 h in air. The results from X-ray diffraction and electron microscopy analyses reveal that hollow α-FeOOH urchin-like spheres were completely transformed to hollow α-Fe₂O₃ urchin-like spheres without a significant morphological change. Also, the effect of hydrothermal treatment temperature (170–200 °C for 12 h) on the phase structure and morphology of the final product was investigated. Pure α-FeOOH, the mixture of α-FeOOH and α-Fe₂O₃, and pure α-Fe₂O₃ with different morphologies were obtained at <180, 180–190 and 200 °C, respectively. The obtained materials can be used in the photodegradation of organic pollutants under visible light irradiation.     

Aerosol-assisted synthesis of mesoporous titania nanoparticles with high surface area and controllable phase composition

Mesoporous titania nanoparticles (denoted as MTN) with high surface area (e.g., 252 m² g⁻¹) were prepared using tetrapropyl orthotitanate (TPOT) as a titania precursor and 10–20 nm or 20–30 nm silica colloids as templates. Co-assembly of TPOT and silica colloids in an aerosol-assisted process and immediate calcination at 450 °C resulted in anatase/silica composite nanoparticles. Subsequent removal of the silica colloids from the composite by NaOH solution created mesopores in the TiO₂ nanoparticles with pore size corresponding to that of silica colloids. Effects of silica colloids’ contents on MTN porosity and crystallites’ growth at a higher calcination temperature (e.g., 1000 °C) were investigated. Silica colloids suppressed the growth of TiO₂ crystallites during calcination at a higher calcination temperature and controllable contents of the silica colloids in precursor solution resulted in various atomic ratios of anatase to rutile in the calcinated materials. The mesostructure and crystalline structure of these titania materials were characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD), differential thermal analysis (DTA)-thermo-gravimetric analysis (TGA), and N₂ sorption.     

Preparation of hollow silica beads via soft template calcinating route

Heat treated hollow silica beads have been prepared via a two-step method: the fast synthesis of mesoporous hollow silica beads via soft template method and the closing of the porosity of as prepared hollow silica beads in a vertical furnace at more than 1,000 °C. The experiment results showed that the size of the as prepared hollow silica beads was greatly affected by the size of the octylamine vesicles, which increased as the mixing rate decreased, and the optimal synthesis time was 5 min. Fourier transform-infrared spectroscopy results indicated that the soft template was incorporated in as prepared hollow silica beads. The optical photographs and the X-ray diffraction results indicated that the shell of as prepared hollow silica beads was fully densified in the high temperature furnace. In addition, the morphology observation of polypropylene/polyolyaltha olfin/heat treated hollow silica beads ternary composites indicated that this type of silica beads possessed high intensity and strength to blend with the polypropylene/polyolyaltha olfin composites.     

Development and characterization of fully bio‐based polybenzoxazine‐silica hybrid composites for low‐k and flame‐retardant applications

In the present work, new classes of bio‐based polybenzoxazines were synthesized using eugenol as phenol source and furfurylamine and stearylamine as amine sources separately through solventless green synthetic process routes and were further reinforced with varying percentages (1, 3, 5, and 10 wt%) of silica (from rice husk) to attain hybrid composites. The molecular structure, cure behaviour, thermal stability, dielectric properties, and flame‐retardant behaviour of both benzoxazine monomers and benzoxazine composites were characterized using appropriate modern analytical techniques. The eugenol‐based benzoxazines synthesized using furfurylamine (FBz) and stearylamine (SBz) were cured at 223°C and 233°C, respectively. The differential scanning calorimetry (DSC) data reveal the glass transition temperatures (T_g) of FBz and SBz were 157°C and 132°C, respectively, and the maximum decomposition temperature (T_max) as obtained from thermogravimetric analysis (TGA), were found to be 464°C and 398°C for FBz and SBz, respectively. The dielectric constants for FBz and SBz obtained at 1 MHz were 3.28 and 3.62, respectively. Furthermore, varying weight percentages (1, 3, 5, and 10 wt%) of 3‐mercaptopropyltrimethoxysilane (3‐MPTMS) functionalized bio‐silica reinforced the composite materials as evidenced by their improved thermal stability and lower dielectric constant. Data obtained from thermal and dielectric studies suggested that these polybenzoxazines could be used in the form of adhesives, sealants, and composites for high performance inter‐layer low‐k dielectric applications in microelectronics.     

Formation of carbon nanotubes from a silicon carbide/carbon composite

The reaction of a SiC/C composite powder in an arcing plasma forms carbon nanotubes in good yield. Besides carbon nanotubes, a Si/C composite composed of β SiC covered with a shell of graphite is formed. The graphitic carbon surface layers of the carbon shell of this composite reacts further to form carbon nanotubes when heated to 600 °C. This process seems highly effective since only a small overall low weight loss, indicative for a complete carbon shell oxidation is observed by thermal analysis. The formation of the carbon nanotubes from SiC is unlikely since no SiO₂ has been found when heating the SiC/C core shell composite to its reaction temperature of 600 °C under O₂. The CNTs formed are of good quality with 3 to 6 concentric walls and high aspect ratio. Occasionally even single walled carbon naotubes have been observed.