Synthesis of hollow α-Fe2O3–silica composites templated by poly(acrylic acid) colloidal aggregates

The synthesis of inorganic hollow nanoparticles has attracted more and more attention in recent research. In this article, hollow α-Fe₂O₃–silica composites were synthesized by a modified Stöber method. Tetraethyl orthosilicate (TEOS) formed oil drops, and ammonia solution facilitated the hydrolysis polymerization of TEOS. In the hydrolysis process of TEOS, we used cetyltrimethylammonium bromide and poly(acrylic acid) colloidal aggregates as the surfactant and template, respectively. The diameters of the composites we obtained are about 20 and 180 nm, respectively, which are smaller than that of pure silica spheres (200 nm). Moreover, Brunauer–Emmett–Teller surface areas of hollow α-Fe₂O₃–silica composites were determined to be in the range of 377–498 m²/g, and their pore sizes are around 2 nm as were determined by BJH method. The mesoporous silica was successfully coated around the α-Fe₂O₃. The synthesis process of the composites is a simple, one-step route, which exhibits the potential need for a great amount of synthesis for future research.     

Preparation of renewable porous TiO<Subscript>2</Subscript>/PVA composite sphere as photocatalyst for methyl orange degradation

Firstly, preparation of porous polyvinyl alcohol (PVA) spheres were investigated in detail by phase inversion method using N,N-dimethylacetamide (DMAc) and polyvinylpyrrolidone (PVP) as pore-forming additives. The morphology and pore structure of PVA spheres were characterized by SEM and BET measurements. It was found that the addition of DMAc and PVP increased the pore volume and the surface pore size of PVA sphere respectively. The maximum surface area of the porous PVA sphere reached 220 m²/g. Secondly; the synthesis of photoactive TiO₂ NPs (anatase type) at the low temperature was developed by controlling the aging process of the TiO₂ precursors. The crystallinity and photoactivity of TiO₂ NPs increased with the aging time. Finally, TiO₂ NPs/PVA composite spheres were prepared by immersing PVA sphere into TiO₂ precursor solution. Their structures were characterized by XRD pattern, TEM and TGA measurement. It was found that TiO₂ NPs were successfully immobilized into PVA spheres. The photodegradation of methyl orange (MO) under UV light by TiO₂/PVA spheres showed a good photocatalytic efficiency. Moreover, TiO₂/PVA spheres can be easily regenerated by the repeated immersion process. Overall, the porous TiO₂/PVA sphere displays a good photoactive property and an advantage of easier recovery, which facilitates its application in large-scale wastewater treatment.     

Preparation of hollow mesoporous silica spheres by a sol–gel/emulsion approach

Hollow mesoporous silica spheres were synthesized by a sol–gel/emulsion (oil-in-water/ethanol) approach, in which cetyltrimethylammonium bromide (CTAB) surfactant was employed to stabilize and direct the hydrolysis of oil droplets of tetraethoxysilane (TEOS). The diameters of the hollow spheres can be tuned in the range from 210 to 720 nm by varying the ratio of ethanol-to-water and their shell thickness can be mediated by changing the concentration of CTAB used in the system. BET surface areas of the hollow silica spheres are determined to be in the range of 924–1766 m² g^?1 and their pore sizes are around 3.10 nm as determined by BJH method.     

Preparation of hollow submicrocapsules with a mesoporous carbon shell

The preparation of carbon submicrocapsules with size up to 800 nm and a mesoporous shell by hard silica templating is reported. Washing and template synthesis conditions were varied to promote porosity and avoid deformation of the microcapsules. The silica template synthesis conditions analyzed were: silica nucleus formation time (0.25–6 h), octadecyltrimethoxysilane/tetraethylorthosilicate volume ratio for silica shell formation (0.2–0.6) and silica shell formation time (1–24 h). The samples were characterized by 77 K nitrogen adsorption/desorption, mercury porosimetry and electron microscopy. Under all the washing conditions tested the carbon submicrocapsules were deformed due to the large size of the hollow core and the thickness of the shell. Changes in the silica template synthesis conditions did not result in substantial improvement of the strength of the microcapsules. The synthesis of a silica template with a double shell allowed us to obtain thick shell carbon submicrocapsules without significant deflation and with higher porosity. The characterization of these microcapsules showed that they have a BET surface area of 1541 m²/g and a pore size distribution with peaks centered at 0.75, 0.86, 1.0 nm in the micropore range and 3.5 nm in the mesopore range. The pore volume in the 2–80 nm range was 1.7 cm³/g.     

Preparation of phenolic resin derived 3-D ordered macroporous carbon

Three-dimensionally ordered long-range macroporous carbon structures were prepared using commercially available phenolic resin by utilizing sacrificial colloidal silica crystalline arrays as templates that were subsequently removed by HF etching after pyrolysis in an argon atmosphere. SEM, TEM, and BET were employed to characterize the morphology and the surface area of the porous carbon structures. The pore size (150–1000 nm) and BET surface area, which reflect pore volume (298.6 m²/g (1.32 cm³/g) ∼ 93.7 m²/g (0.12 cm³/g)), of the macroporous carbon structures produced were approximately proportional to the size (150–1000 nm) of the sacrificial silica sphere templates used (annealing temp. 550°C). The achieved 550 nm porous carbon structures were examined to function as potential catalyst carriers and were successfully impregnated with Ag or Pt-Ru on their inner walls after borohydride reduction at room temperature. In addition, porous carbon patterns were fabricated using the ‘micromolding in capillary’ technique, which has potential applications in the microreaction technology.     

Nostoc calcicola Immobilized in Silica-coated Calcium Alginate and Silica Gel for Applications in Heavy Metal Biosorption

The present study reports the preparation and characterization of silica-based immobilization matrices for the purpose of metal accumulation using immobilized cyanobacterium Nostoc calcicola. Silica gel was prepared using aqueous sodium silicate and colloidal silica. Calcium alginate (CAG) beads were coated with silica using sodium silicate solutions. Microscopy observations and TTC tests confirmed that the immobilized cells were intact and viable. Ultrastructural studies with electron microscopy revealed a membrane thickness of approximately 10 μm around the CAG and the silica gel to be of mesoporous nature. BET surface area of silica gel-immobilized N. calcicola was 160 m² g^?1. The porous volume and average pore diameter were 0.40 cm³ g^?1 and ca. 100 Å, respectively, as calculated using the BJH model. Studies on silica-coated calcium alginate immobilized cells showed that these were superior to the uncoated CAG beads in terms of mechanical strength and metal accumulation. The silica matrices were found to be stable for repeated cycles of metal removal and with commonly used eluants for desorption processes. These matrices have potential applications in immobilization of industrially important biocatalysts.     

A novel route for synthesis of cross-linked polystyrene copolymer beads with tunable porosity using guar and xanthan gums from bioresources as alternative synthetic suspension stabilizers

Cross-linked polymer beads with different cross-linking agent loading were prepared by carrying out cross-linking suspension copolymerization of styrene-divinylbenzene (St- DVB) monomers using guar gum (GG) and xanthan gum (XG) from bioresources as eco-friendly suspension biopolymer stabilizers in the presence of non reactive diluents. The effects of GG and XG as suspension biostabilizers on the characteristics of the styrene copolymer beads were investigated regarding thermal properties, porosity characteristics, solvent swelling ratio, and surface morphologies using TGA, DSC, XRD, SEM, BET analyses. Spherical and regular beads with smooth surface were produced and the average particle size was in the range 170–290 μm (50–80 mesh size). The porosity characteristics of the produced beads including surface area and pore volume were in range 0.45 m²/g and 32–45 ml/g, respectively. Overall, the present article provided a novel route to prepare cross-linked polystyrene copolymer beads with tunable porosity suitable for catalyst support.     

Hierarchical porous carbon templated with silica spheres of a diameter of 14 nm from pure chitosan or a chitosan/ZnCl<Subscript>2</Subscript> solution

Nitrogen-containing mesoporous carbons with the use of colloidal silica spheres of (14 nm) and chitosan as a carbon precursor were obtained. A removal of such small template particles from carbonized silica–chitosan composite is difficult and HF with a minimum concentration of 15 wt% should be used. By varying the silica-to-chitosan ratio, the porous characteristic of products is controlled. The modification by ZnCl₂ with a molar Zn-to-C (in chitosan mass) ratio of ‘6’ results in the development of microporosity; however it is accompanied by a significant reduction of mesopore volume (V_mes). The addition of ZnCl₂ in a ratio of ‘5.25’ and pH adjustment to 5.8 increase the volumes of micropores, small mesopores, BET surface area to 1975 m²/g, and preserve V_mes of 4.15 cm³/g. The novelty of the presented strategy is the creation of microporosity in the hard-templated materials by incorporating ZnCl₂ into the mixture of Ludox HS-40 template and chitosan precursor, as well as the investigation on how the pH of synthesis influences the final porosity. The pH of a silica–chitosan–zinc solution, equal to 3.9, provides some coordination of Zn²⁺ by –OH and –NH₂ groups, whereas pH adjustment to 5.8 results in the precipitation of a new template—Zn(OH)₂.     

Monodispersed nanoporous starburst carbon spheres and their three-dimensionally ordered arrays

Controlled syntheses of highly monodispersed nanoporous carbon spheres via a nanocasting route are described. Previously reported monodispersed super-microporous or mesoporous silica spheres with hexagonally ordered pore channels were used as sacrificial templates, and the effect of pore sizes of the templates on the porous properties of the nanocast carbon spheres was comprehensively studied. The resultant carbon spheres exhibited a unique starburst structure derived from radially-aligned pore channels in the silica template, and had a BET surface area of over 1000 m²g^?1. It was found out that the radial alignment and sufficiently large pore size of hexagonally ordered pore channels in the silica spheres were effective to enhance the degree of order of the starburst structure in the nanocast carbon spheres and that ordered nanoporous carbon spheres could be obtained even from the MCM-41-type mesoporous silica. The diameters of the nanoporous carbon spheres were controlled in the sub-micrometer range by changing the sizes of silica templates. Furthermore, three-dimensionally ordered arrays consisting of nanoporous carbon spheres were successfully fabricated via the self-assembly of mesoporous silica/carbon composite spheres and the subsequent dissolution of the silica templates.     

Synthesis of benzxazine-based nitrogen-doped mesoporous carbon spheres for methyl orange dye adsorption

In this work, nitrogen-doped mesoporous carbon spheres (NMCS) were synthesized through a hard template method by using benzoxazine resin as precursor and ordered mesoporous silica spheres as template. The obtained N-doped mesoporous carbons were amorphous spherical nanoparticles with worm-like mesoporous channels and possessed high surface area of 789 m²/g, large pore volume of 0.49 cm³/g and high nitrogen content of 3.50 wt.%. The adsorption capacity of methyl orange (MO) by NMCS could attain 352.1 mg/g at an optimal condition, while the adsorption capacity of MO by non-doped mesoporous carbon spheres (MCS) was 251.9 mg/g at the same condition. The adsorption process fitted the pseudo-second-order kinetic model and the Langmuir isotherm well. Thermodynamic analysis indicated that the removal of MO by NMCS was spontaneous, endothermic and feasible process. In addition, the adsorption capacity of regenerated adsorbent was 89.04% of the initial level after four regeneration cycles.     

Synthesis and characterization of monolithic carbon/silicon carbide composite aerogels

Resorcinol–formaldehyde/silica composite (RF/SiO₂) aerogels were synthesized using sol–gel process followed by supercritical CO₂ drying. Monolithic carbon/silicon carbide composite (C/SiC) aerogels were formed from RF/SiO₂ aerogels after carbothermal reduction. X-ray diffraction and transmission electron microscopy demonstrate that β-SiC was obtained after carbothermal reduction. Scanning electron microscopy and nitrogen adsorption/desorption reveal that the as-prepared C/SiC aerogels are typical mesoporous materials. The pore structural properties were measured by nitrogen adsorption/desorption analysis. The resulting C/SiC aerogels possess a BET surface area of 564 m²/g, a porosity of 95.1 % and a pore volume of 2.59 cm³/g. The mass fraction of SiC in C/SiC aerogels is 31 %.     

Facile Synthesis of Monodispersed In2O3 Hollow Spheres and Application in Photocatalysis and Gas Sensing

Monodispersed, agglomerate‐free In₂O₃ hollow spheres have been prepared via a simple synthetic route involving permeation and anchoring of In³⁺ ions with carbonyl groups of swollen commercial polymer beads in tetrachloroethylene solvent followed by thermal removal of the template cores in ambient air. The as‐synthesized hollow spheres exhibit a narrow size distribution with tunable particle size (0.5–1.2 μm) and shell thickness (62–230 nm) over the process variables examined, i.e., InCl₃ precursor concentration (4.5 × 10^?3–6.7 × 10 ^{? 2} M), reaction temperature (55°C–95°C), and reaction time (1–6 h). Kinetics calculation reveals that the formation of permeating In³⁺‐rich shell in the swollen template beads becomes energetically less favorable to proceed as the reaction time increases. This limits the maximum shell thickness attainable at the given process variables. The shell is nanoporous with a Horvath‐Kawazoe (HK) pore size of ~3 nm, which remains essentially unchanged as the process variables alter. The In₂O₃ hollow spheres with an increased Brunauer‐Emmett‐Teller (BET) surface area (up to 329 m²/g) show an improved capability in photodegradation of aqueous methylene blue (MB) dye under UV exposure as well as an increased sensitivity for CO‐gas detection. This metal‐implantation scheme is general and can be extended to the synthesis of other hollow materials in various solvent liquids.     

Photocatalytic reduction of Hg using core–shell Fe/CeO2 hollow sphere nanocomposites

A sol–gel method was used to prepare Fe/CeO₂ hollow sphere nanocomposites. For comparison, a direct calcination of cerium nitrate was used to prepare CeO₂ nanoparticles and Fe/CeO₂ nanoparticles. The photocatalytic reduction of Hg was used to study the photocatalytic performance of the prepared nanocomposite photocatalysts using visible-light irradiation. The BET surface areas of the CeO₂ nanoparticles and CeO₂ hollow spheres were 76 and 160 m²/g, respectively. The BET surface area of the hollow sphere CeO₂ and CeO₂ nanoparticles decreased to 145 and 57 m²/g, respectively, by adding iron nanometal. The TEM results revealed that the shapes of the CeO₂ nanoparticle and hollow sphere materials are spherical nanoparticles and uniform nanospheres, respectively. The Fe/CeO₂ nanoparticles and Fe/CeO₂ hollow spheres are spherical nanoparticles and core–shell, respectively. The photocatalytic performance by the Fe/CeO₂ hollow spheres was 50, 3.9, and 1.4 times more efficient than that observed from the CeO₂ nanoparticles, Fe/CeO₂ nanoparticles, and CeO₂ hollow spheres, respectively.     

Facile syntheses of nanoporous organosilica spherical particles

Well-defined spherical particles of silica, containing phenyl group, with the diameter of ca. 2 microns were prepared by the co-condensation of tetraethoxysilane with phenyltriethoxysilane in basic aqueous methanol solutions of hexadecyltrimethylammonium chloride. The products were hydrophobic. The content of phenyl group was controlled by the ratio of tetraethoxysilane and phenyltriethoxysilane in the starting solution. Hexadecyltrimethylammonium was extracted with methanolic HCl to obtain nanoporous silica spherical particle containing phenyl group. The spherical particle possessed pore size of 1.8 nm and the BET surface area of 750 m² g^?1.     

Template-free fabrication of porous zinc oxide hollow spheres and their enhanced photocatalytic performance

Porous zinc oxide (ZnO) hollow spheres have been fabricated by calcination of a precursor complex in a furnace. The precursor was precipitated in a chemical solution at 80 °C. The field-emission scanning electron microscope and transmission electron microscope reveal the porous and hollow structure of the samples. The spherical hollow precursor is self-assembled in the solution under the coordination effect of citrate ions and the Kirkendall effect working together. The precursors can be converted to pure ZnO crystals by heating in a furnace above 300 °C. The Brunauer–Emmett–Teller (BET) specific surface area of this sample is 95.4 m²/g. The photocatalytic degradation of methyl blue solution test shows the ZnO hollow spheres have superior photocatalytic activity.     

Dispersion,crystallization behavior,and mechanical properties of poly(3-hydroxybutyrate) nanocomposites with various silica nanoparticles: effect of surface modifiers

Silica nanoparticles (NPs) with various surface properties were introduced in poly(3-hydroxybutyrate) (PHB) matrix and the their effect on the dispersion, crystallization behavior, and reinforcement in the nanocomposites was discussed in this article. Two kinds of commercial fumed silica NPs and two kinds of self-prepared sol-gel silica (bare and PEGylated) NPs were used. The cross-sectional SEM (scanning electron microscopy) images, provided the micrometer scale view (observation area: 12.6×8.2 μm²), showed that commercial fumed silica and PEG–silica NPs were aggregated and well-dispersed in PHB matrix, respectively. Similarly, Morisita’s analysis of TEM (transmission electron microscopy) images (observation area: 2.4×1.6 μm²) indicated that PEG-silica NPs were Poisson dispersed and commercial fumed silica NPs were serious aggregated in PHB matrix. However, SEM-EDX (energy dispersive X-ray analysis) Si-mapping micrographs, provided the millimeter scale view (observation area: 0.79×0.61 mm²), showed that four kinds of silica NPs were well-dispersed in PHB matrix. PLM (polarized light microscopy) images indicated that spherulite growth rate and morphology of PHB did not change obviously upon the addition of various silica NPs, except the PHB/PEG–silica system. PHB/PEG–silica showed a decreased spherulite growth rate, which was consistent with the DSC (differential scanning calorimetry) results, because the good miscibility between PHB and the grafted PEG chains on PEG–silica could decrease the polymer chain mobility during crystallization. The Young’s modulus and tensile strength of the PHB were enhanced by up to 34% and 63% by adding a small amount of PEG–silica. Fully well-dispersed PEG–silica NPs functioned as physical cross-linking centers for enhancing the mechanical properties of PHB but as retarding agents for reducing the crystallization rate.     

Synthesis of 3D porous ceramic scaffolds obtained by the sol-gel method with surface morphology modified by hollow spheres for bone tissue engineering applications

In the present work, we modified the surface morphology of 3D porous ceramic scaffolds by incorporating strontium phosphate (SrP) hollow nano-/microspheres with potential application as delivery system for the local release of therapeutic substances. SrP hollow spheres were synthesized by a template-free hydrothermal method. The influence of the reaction temperature, time and concentration of reactants on precipitates' morphology and size were investigated. To obtain a larger number of open hollow spheres, a new methodology was developed consisting of applying a second hydrothermal treatment to spheres by heating them at 120 °C for 24 h. The X-ray diffraction (XRD) analysis indicated that spheres consisted of a main magnesium-substituted strontium phosphate phase ((Sr_0.86Mg_0.14)₃(PO₄)₂). The scanning electron microscopy (SEM) micrographs confirmed that spheres had hollow interiors (～350 nm size) and an average diameter of 850 nm. Spheres had a specific surface area of 30.5 m²/g, a mesoporous shell with an average pore size of 3.8 nm, and a pore volume of 0.14 cm³/g. These characteristics make them promising candidates for drug, cell and protein delivery. For the attachment of spheres to scaffolds’ surface, ceramic structures were immersed in an ethanol solution containing 0.1 g of hollow spheres and kept at 37 °C for 4 h. The scaffolds with incorporated spheres were bioactive after being immersed in simulated body fluid (SBF) for 7 days and spheres were still adhered to their surface after 14 days.     

Catalysis Over Pore-Expanded MCM-41 Mesoporous Materials

Pore-expanded MCM-41 (PE-MCM-41) silica was obtained via a two-step strategy consisting of synthesis of MCM-41 in the presence of cetyltrimethylammonium cations (CTMA⁺) followed by hydrothermal treatment in the presence of an aqueous suspension of dimethyldecylamine (DMDA). It is believed that the two surfactants self-organize into concentric cylinders comprised of an inverted DMDA micelle within a regular CTMA⁺ micelle. The amine head groups point towards the pore center, thus creating a hydrophilic channel within the pores. Selective extraction of DMDA afforded PE-MCM-41E, a highly porous material with hydrophobic surface. Calcination of as-synthesized PE-MCM-41 or PE-MCM-41E gave rise to PE-MCM-41C which exhibits a unique combination of large pores (up to 20 nm), large pore volume (up to 3.5 cm³/g) and large surface area, often exceeding 1000 m²/g. All three mesophases were used as starting materials for the development of innovative catalysts and adsorbents. Moreover, post-synthesis alumination of PE-MCM-41 and PE-MCM-41C led to novel large pore aluminosilicates with different Si/Al ratios, which provided new opportunities in acid and bifunctional catalysis. This contribution is an overview of the catalytic applications of pore-expanded mesoporous silica and aluminosilica.     

Sacrificial PSS-doped CaCO3 templates to prepare chitosan capsules and their deformation under bulk pressure

To prepare microcapsules composed of chitosan (CS), a templating method is developed using poly(styrene sulfonate) (PSS)-doped porous calcium carbonate (CaCO₃) templates as sacrificial templates. First, CS is absorbed onto PSS-doped porous CaCO₃ templates, and then the absorbed CS is covalently cross-linked with each other by using glutaraldehyde. Porous CaCO₃ templates are dissolved with disodium ethylenediaminetetraacetate dihydrate and the resultant CS capsules ranged from 2 to 5 μm in diameter. Nitrogen adsorption–desorption analysis are applied to characterize the porous CaCO₃ templates, the BET surface area and total pore volume are 220 m²/g and 0.36 cm³/g, respectively. Field-emission scanning electron microscopy and transmission electron microscopy were used to characterize the CS capsules morphology. Confocal laser scanning microscopy images reveal that the capsules have been labeled with green fluorescein isothiocyanate. The gradual deformation of capsule in response to bulk osmotic pressure created by CS solutions has also been discussed.     

Microporous activated carbon spheres prepared from resole‐type crosslinked phenolic beads by physical activation

Microporous activated carbon spheres (ACSs) with a high specific Brunauer–Emmet–Teller (BET) surface area were prepared from resole‐type spherical crosslinked phenolic beads (PBs) by physical activation. The PBs used as precursors were synthesized in our laboratory through the mixing of phenol and formaldehyde in the presence of an alkaline medium by suspension polymerization. The effects of the gasification time, temperature, and flow rate of the gasifying agent on the surface properties of ACSs were investigated. ACSs with a controllable pore structure derived from carbonized PBs were prepared by CO₂ gasification. Surface properties of ACSs, such as the BET surface area, pore volume, pore size distribution, and pore diameters, were characterized with BET and Dubinin–Reduchkevich equations based on N₂ adsorption isotherms at 77 K. The results showed that ACSs with a 32–88% extent of burn‐off with CO₂ gasification exhibited a BET surface area ranging from 574 to 3101 m²/g, with the pore volume significantly increased from 0.29 to 2.08 cm³/g. The pore size and its distribution could be tailored by the selection of suitable conditions, including the gasification time, temperature, and flow rate of the gasifying agents. The experimental results of this analysis revealed that ACSs obtained under different conditions were mainly microporous. The development of the surface morphology of ACSs was also studied with scanning electron microscopy. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008