Polyaniline–Silica Composite Conductive Capsules and Hollow Spheres

In this paper, we report on the preparation of monodisperse polyaniline (PANi)–silica composite capsules and hollow spheres on monodisperse core–gel‐shell template particles. An extension of the previously reported inward growth method was used. The samples were self‐stabilized without external additives. The core–gel‐shell particles were prepared by the inward sulfonation of monodisperse polystyrene particles. The introduced sulfonic acid and sulfone groups are responsible for the gel properties. The gel‐shell thickness and core size were synchronously controlled over the whole particle radius range. After aniline (ANi) monomer was preferentially absorbed in the sulfonated polystyrene shell, PANi was formed by polymerization. PANi was doped in situ with a sulfonic acid group to give the capsules a high conductivity. PANi hollow spheres were derived after the polystyrene cores were dissolved: their cavity size and shell thickness were synchronously controlled by using different core–gel‐shell particles. The PANi–silica composite capsules and hollow spheres were therefore prepared by a sol–gel process using tetraethylorthosilicate in the conducting shell. The PANi shell became more robust while maintaining the same conductivity level. Morphological results indicate that the PANi and silica formed a bicontinuous network. Fourier‐transform infrared (FTIR) spectra revealed that the hydrogen bonding in the PANi–gel shell was enhanced after the silica phase was incorporated, which could explain the high conductivity level after the silica phase was added. In a converse procedure, silica capsules and hollow spheres were prepared by a sol–gel process that incorporated tetraethylorthosilicate into the core–gel‐shell templates, which was followed by the absorption and polymerization of aniline in the silica shell thus forming PANi–silica composite capsules and hollow spheres. The silica capsules and hollow spheres thereby became conductive.     

Yolk‐like Micro/Nanoparticles with Superparamagnetic Iron Oxide Cores and Hierarchical Nickel Silicate Shells

Yolk‐like nano/microparticles with superparamagnetic iron oxide (SPIO) cores and hierarchical nickel silicate (NS) shells, designated yolk SPIO@NS, are fabricated by combining the versatile sol–gel process and the hydrothermal reaction, involving the coating of SPIO particles with SiO₂ and transformation of the SiO₂ shells into NS hollow spheres with hierarchical nanostructures. Various yolk/shell nanostructures with tunable NS shell thicknesses and SPIO core sizes are successfully prepared by controlling the experimental para­meters. Au nanoparticles can be impregnated into the yolk‐like microspheres in situ to form SPIO@NS/Au composite particles and the as‐prepared magnetic nanocatalysts show good catalytic activity, using the catalytic reduction of RhB as a model reaction. This facile method can be extended to the synthesis of other encapsulated particles with yolk‐like nanostructure.     

“Ship‐in‐a‐Bottle” Growth of Noble Metal Nanostructures

Noble metal nanostructures are grown inside hollow mesoporous silica microspheres using “ship‐in‐a‐bottle” growth. Small Au seeds are first introduced into the interior of the hollow microspheres. Au nanorods with synthetically tunable longitudinal plasmon wavelengths and Au nanospheres are obtained through seed‐mediated growth within the microspheres. The encapsulated Au nanocrystals are further coated with Pd or Pt shells. The microsphere‐encapsulated bimetallic core/shell nanostructures can function as catalysts. They exhibit high catalytic performance and their stability is superior to that of the corresponding unencapsulated core/shell nanostructures in the catalytic oxidation of o‐phenylenediamine with hydrogen peroxide. Therefore, these hollow microsphere‐encapsulated metal nanostructures are promising as recoverable and efficient catalysts for various liquid‐phase catalytic reactions.     

One‐Pot Preparation of Conducting Polymer‐Coated Silica Particles: Model Highly Absorbing Aerosols

Near‐monodisperse 0.50 μm and 1.0 μm silica particles are surface‐modified using 3‐(trimethoxysilyl)propyl methacrylate (MPS) and subsequently coated by aqueous deposition of an ultrathin polypyrrole (PPy) overlayer to produce PPy‐coated silica particles. The targeted degree of MPS modification and PPy mass loading are systematically varied to optimize the colloidal stability and PPy coating uniformity. MPS surface modification is characterized by contact angle goniometry and the PPy overlayer uniformity is assessed by scanning electron microscopy. HF etching of the silica cores produces hollow PPy shells, thus confirming the contiguous nature of the PPy overlayer and the core–shell morphology of the original particles. Four‐point probe measurements and XPS studies indicate that the electrical conductivity of pressed pellets of PPy‐coated silica particles increases with PPy surface coverage. Colloidal stabilities of the bare, MPS‐modified, and PPy‐coated silica particles in aqueous solution are assessed using disk centrifuge photosedimentometry. MPS surface modification results in weak flocculation, with subsequent PPy deposition causing further aggregation. In contrast, white light aerosol spectrometry indicates a relatively high degree of dispersion for PPy‐coated silica particles in the gas phase. Such PPy‐coated silica particles are expected to be useful mimics for silica‐rich micrometeorites and may also serve as a model highly absorbing aerosol.     

Formation of Gold and Silver Nanoparticle Arrays and Thin Shells on Mesostructured Silica Nanofibers

Mesostructured silica nanofibers synthesized in high yields with cetyltrimethylammonium bromide as the structure‐directing agent in HBr solutions are used as templates for the assembly of Au and Ag nanoparticles and the formation of thin Au shells along the fiber axis. Presynthesized spherical Au and Ag nanoparticles are adsorbed in varying amounts onto the silica nanofibers through bifunctional linking molecules. Nonspherical Au nanoparticles with sharp tips are synthesized on the nanofibers through a seed‐mediated growth approach. The number density of nonspherical Au nanoparticles is controlled by varying the amount of seeded nanofibers relative to the amount of supplied Au precursor. This seed‐mediated growth is further used to form continuous Au shells around the silica nanofibers. Both the Au‐ and Ag‐nanoparticle/silica‐nanofiber hybrid nanostructures and silica/Au core/shell fibers exhibit extinction spectra that are distinct from the spectra of Au and Ag nanoparticles in solution, indicating the presence of new surface plasmon resonance modes in the silica/Au core/shell fibers and surface plasmon coupling between closely spaced metal nanoparticles assembled on silica nanofibers. Spherical Au‐ and Ag‐nanoparticle/silica‐nanofiber hybrid nanostructures are further used as substrates for surface‐enhanced Raman spectroscopy, and the enhancement factors of the Raman signals obtained on the Ag‐nanoparticle/silica‐nanofiber hybrid nanostructures are 2 × 10⁵ for 4‐mercaptobenzoic acid and 4‐mercaptophenol and 7 × 10⁷ for rhodamine B isothiocyanate. These hybrid nanostructures are therefore potentially useful for ultrasensitive chemical and biological sensing by using molecular vibrational signatures.     

Preparation of Hollow Zeolite Spheres and Three‐Dimensionally Ordered Macroporous Zeolite Monoliths with Functionalized Interiors

A novel and flexible strategy involving hydrothermal transformation of guest‐incorporated zeolite‐seeded mesoporous silica spheres was proposed to prepare guest‐encapsulated hollow zeolite spheres and three‐dimensionally (3D) ordered macroporous zeolite monoliths. The guest species that were pre‐incorporated into the mesopores of silica spheres could be spontaneously encapsulated inside the formed hollow zeolite shells by consuming silica nutrition of the original mesoporous silica cores during the hydrothermal process. A wide range of guest materials with a size ranging from nanometers to micrometers, e.g., Ag and PdO nanoparticles, and mesoporous spheres of carbon and polymer of micrometer size were successfully encapsulated into both discrete hollow zeolite spheres and 3D ordered macroporous zeolite monoliths. Such materials are expected to find a variety of applications such as catalysis, adsorption, and novel microreactors for their special structures with active species inside and zeolitic porous shell outside.     

Inorganic Micelles (Hydrophilic Core@Amphiprotic Shell) for Multiple Applications

A facile approach for synthesizing superhydrophobic hollow silica micelles (SHSMs) with hydrophilic cores and amphiprotic (superhydrophobic/hydrophilic) shell structures that act as “all‐in‐one” smart nanomaterials is presented. The particles possess hydrophilic cores consisting of silica and a polyelectrolyte (PE) network and an amphiprotic shell consisting of superhydrophobic long‐chained hydrocarbons and hydrophilic PEs. Due to the unique hydrophilic cores and amphiprotic shells, the particles exhibit extraordinary performance in terms of amphiprotic catalytic reactions in organic and aqueous solutions, oil/water separation and pollutant purification, and an ultrahigh loading capacity of enzymes with significant stability and efficient recyclability. The amphiprotic functionalities of the SHSMs have the potential to allow for a rich range of applications to be explored.     

Colloidal Synthesis of Hollow Cobalt Sulfide Nanocrystals

Formation of cobalt sulfide hollow nanocrystals through a mechanism similar to the Kirkendall Effect has been investigated in detail. It is found that performing the reaction at > 120 °C leads to fast formation of a single void inside each shell, whereas at room temperature multiple voids are formed within each shell, which can be attributed to strongly temperature‐dependent diffusivities for vacancies. The void formation process is dominated by outward diffusion of cobalt cations; still, the occurrence of significant inward transport of sulfur anions can be inferred as the final voids are smaller in diameter than the original cobalt nanocrystals. Comparison of volume distributions for initial and final nanostructures indicates excess apparent volume in shells, implying significant porosity and/or a defective structure. Indirect evidence for fracture of shells during growth at lower temperatures was observed in shell‐size statistics and transmission electron microscopy images of as‐grown shells. An idealized model of the diffusional process imposes two minimal requirements on material parameters for shell growth to be obtainable within a specific synthetic system.     

Cover Picture: Colloidal Synthesis of Hollow Cobalt Sulfide Nanocrystals (Adv. Funct. Mater. 11/2006)

Hollow nanocrystals have been synthesized through a mechanism analogous to the Kirkendall Effect. When a cobalt nanocrystal reacts with sulfur in solution, the outward diffusion of cobalt atoms is faster than the inward diffusion of sulfur atoms through the sulfide shell. The dominating outward diffusion of cobalt cations produces vacancies that can condense into a single void in the center of the nanocrystal at high temperatures. This process provides a general route to the synthesis of hollow nanostructures of a large number of compounds and is described in the Full Paper by A. P. Alivisatos and co‐workers on p. 1389. Formation of cobalt sulfide hollow nanocrystals through a mechanism similar to the Kirkendall Effect has been investigated in detail. It is found that performing the reaction at > 120 °C leads to fast formation of a single void inside each shell, whereas at room temperature multiple voids are formed within each shell, which can be attributed to strongly temperature‐dependent diffusivities for vacancies. The void formation process is dominated by outward diffusion of cobalt cations; still, the occurrence of significant inward transport of sulfur anions can be inferred as the final voids are smaller in diameter than the original cobalt nanocrystals. Comparison of volume distributions for initial and final nanostructures indicates excess apparent volume in shells, implying significant porosity and/or a defective structure. Indirect evidence for fracture of shells during growth at lower temperatures was observed in shell‐size statistics and transmission electron microscopy images of as‐grown shells. An idealized model of the diffusional process imposes two minimal requirements on material parameters for shell growth to be obtainable within a specific synthetic system.     

Hollow Two‐Layered Chiral Nanoparticles Consisting of Optically Active Helical Polymer/Silica: Preparation and Application for Enantioselective Crystallization

This article reports for the first time a novel category of hollow organic@inorganic hybrid two‐layered nanoparticles (NPs), in which the inner layer is formed by optically active helical polyacetylene, and the outer layer by silica. Such NPs show remarkable optical activity and are successfully used for enantioselective crystallization. To prepare such NPs, n‐butyl acrylate undergoes radical polymerization to first form poly(n‐butyl acrylate) (PBA) cores two shells by catalytic polymerization of substituted acetylene and sol–gel approach of TEOS (tetraethyl orthosilicate), respectively. Removal of the PBA cores provides the expected hollow core/shell NPs. The intense dircular dichroism (CD) effects demonstrate that the hollow chiral NPs possess considerable optical activity, arising from the helical substituted polyacetylenes forming the inner layer. The hollow NPs are further used as chiral templates to induce enantioselective crystallization of racemic alanines, demonstrating the significant potential applications of the hollow chiral NPs in chiral technologies. Also of particular significance is the detailed process of the induced crystallization observed by TEM. The strategy for preparing the hollow hybrid chiral NPs should be highlighted since it combines free radical polymerization and catalytic polymerization with sol–gel process in a single system, by which numerous advanced materials will be accessible.     

Yolk–Shell Hybrid Materials with a Periodic Mesoporous Organosilica Shell: Ideal Nanoreactors for Selective Alcohol Oxidation

This contribution describes the preparation of multifunctional yolk–shell nanoparticles (YSNs) consisting of a core of silica spheres and an outer shell based on periodic mesoporous organosilica (PMO) with perpendicularly aligned mesoporous channels. The new yolk–shell hybrid materials were synthesised through a dual mesophase and vesicle soft templating method. The mesostructure of the shell, the dimension of the hollow space (4～52 nm), and the shell thickness (16～34 nm) could be adjusted by precise tuning of the synthesis parameters, as evidenced by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen sorption investigations. Various metal nanoparticles (e.g., Au, Pt, and Pd) were encapsulated and confined in the void space between the core and the shell using impregnation and reduction of adequate metal precursors. The selective oxidation of various alcohol substrates was then carried out to illustrate the benefits of such an architecture in catalysis. High conversion (～100%) and excellent selectivity (～99%) were obtained over Pd nanoparticles encapsulated in the hybrid PMO yolk–shell structures.     

Conversion of Fly Ash Cenosphere to Hollow Microspheres with Zeolite/Mullite Composite Shells

A seed‐induced in‐situ hydrothermal conversion technique is proposed to prepare novel hollow microspheres with zeolite/mullite composite shells from fly ash cenosphere (FAC), a solid waste with a hollow structure from coal power stations. Two groups of hollow microspheres were prepared, one with zeolite FAU/mullite composite shells and the other with zeolite LTA/mullite composite shells. The FAC in this study plays dual roles as both the template cores and the aluminosilicate nutrition contributor. The final products inherit the hollow spherical morphology of FAC and possessed bilayered shells, the outer dense shell of zeolite crystals and the inner porous shell of mullite. Such hollow zeolitic materials are expected to have many advantages in applications such as catalysis, adsorption, separation, and as releasing capsules.     

A Hollow‐Core,Magnetic, and Mesoporous Double‐Shell Nanostructure: In Situ Decomposition/Reduction Synthesis,Bioimaging, and Drug‐Delivery Properties

A novel in situ decomposition/reduction approach is developed to manu­facture hollow core, magnetic, and mesoporous double‐shell nanostructures (HMMNSs) via in situ decomposition and reduction of a β‐FeOOH nanorod core and organosilicate‐incorporated silica‐shell precursor. The formed HMMNSs are then aminated by silanization for further covalent conjugation to rhodamine B isothiocyanate (RBITC) and poly(ethylene glycol) (PEG) chains. The resultant RBITC‐grafted and PEGylated nanocomposites (HMMNS–R/Ps) have excellent blood compatibility and very low cytotoxicity towards HeLa and MCF‐7 cells, and can be taken up by cancer cells effectively in a dose‐dependent manner, as confirmed by in vitro flow cytometry, confocal luminescence imaging, and magnetic resonance imaging (MRI) studies. In vivo MRI studies coupled with Prussian blue staining of slides from different organs show that the nanocomposites preferentially accumulate in liver and spleen after intravenous injection, which suggests a potential application of the nanocomposites as MRI contrast agents. Importantly, the HMMNS–R/P nanocomposites show high loading capacity for water‐insoluble anticancer drugs (docetaxel or camptothecin) owing to the presence of a large inner cavity and enhanced surface area and pore volume. Furthermore, the drug‐loaded nanocomposites exhibit greater cytotoxicity than the corresponding free drugs. These results confirm that the HMMNS–R/P nanocomposites are promising candidates for simultaneous bioimaging and drug delivery.     

Synthesis of Inorganic and Organic–Inorganic Hybrid Hollow Particles Using a Cationic Surfactant with a Partially Fluorinated Tail

A new partially fluorinated cationic surfactant, 1‐(10‐perfluorooctyldecyl)pyridinium bromide monohydrate, is synthesized and used as the template for mesoporous ceramic and inorganic–organic hybrid particles. Several hydrolyzed alkoxide precursors are shown to co‐assemble with this surfactant to form hollow vesicle‐like particles, and the effect of changing the alkoxide chemical structure on the formation of these particles is examined. Tetramethoxysilane produces cubic or columnar particles without hollow cavities, but all other tetra‐n‐alkoxysilanes tested up to the n‐butoxide produce hollow particles. As the alkoxide length increases, the shell structure changes from multilayered (with Si(OC₂H₅)₄) to a single thin layer (with Si(OC₃H₇)₄) to a single thick layer (with Si(OC₄H₉)₄). The stability of the fluorocarbon bilayers allows similar vesicular structures to be obtained in organic–inorganic hybrids prepared with bridged alkoxysilanes. Ethylene‐bridged silanes display similar structures to tetraalkoxysilanes. However, the hollow structures appear to partially collapse when the bridging chain is too long (octylene) and no hollow particles are formed with bis(trialkoxysilylpropyl)amines.     

Some New Developments in the Synthesis,Functionalization, and Utilization of Monodisperse Colloidal Spheres

This article provides an overview of some recent developments related to the synthesis and functionalization of monodisperse colloidal spheres, a class of colloidal materials that has found widespread use in applications such as the fabrication of photonic crystals, optical sensing, and drug delivery. Traditionally, the choice of materials has been limited to polystyrene and silica. We and other groups have recently expanded the scope of materials by developing a number of methods for producing monodisperse colloidal spheres from various semiconductors and metals. This article is confined to our own work; it covers three different synthetic strategies: the bottom–up approach, the top–down approach, and template‐directed synthesis. The colloidal spheres may have a solid, hollow, or core–shell structure, and the chemical compositions can include Se, Bi, Pb, In, Sn, Cd, Pt, Ag₂Se, CdSe, PbS, or TiO₂. As an example to illustrate the attractive features of these colloidal spheres, we demonstrate the fabrication of Ag₂Se‐based photonic crystals whose stop bands can be thermally switched between two spectral positions.     

Templated Synthesis of Porous Capsules with a Controllable Surface Morphology and their Application as Gas Sensors

Porous capsules composed of hematite, silica, and hematite–silica composites are prepared by a templated synthesis method. Polyelectrolyte multilayer‐coated particles (PEMPs) are used to synthesize goethite nanocrystals and the resulting goethite‐nanocrystal‐embedded PEMPs (PEMP–goethite) are then used as templates to form porous capsules. The surface morphology and surface area of the porous capsules can be controlled by the size of the PEMP–goethite template, which is determined by the extent of growth of the goethite nanocrystals. By controlling the surface morphology and area, it is also possible to tune the sensitivity of the hematite capsules for use as gas‐sensing materials. This surfactant‐free approach is also used to synthesize silica and silica‐based composite capsules with a controllable porous shell thickness. This straightforward approach can also be extended to the synthesis of other porous capsules or particles with a controllable surface morphology.     

Gas Pickering Emulsion Templated Hollow Carbon for High Rate Performance Lithium Sulfur Batteries

A CO₂ in water nanoparticle stabilized Pickering emulsion is used to template micrometer sized hollow porous nitrogen doped carbon particles for high rate performance lithium sulfur battery. For the first time, nanoparticles serve the dual role of an emulsion stabilizer and a pore template for the shell, directly utilizing in situ generated CO₂ bubbles as template for the core. The minimalistic nature of this method does not require expensive surfactants or additional core templates. Upon polymerization of melamine formaldehyde onto CO₂, a robust polymer/silica composite shell is formed and transformed into a porous shell upon washing. The micrometer‐sized hollow morphology in combination with its nitrogen rich porous shell demonstrates impressive rate capabilities of 670 and 500 mAh g^?1 even at a high rate of 7C and 9C, respectively. This material also possesses excellent cycle durability, exhibiting a low capacity decay of 0.088%/cycle over 300 cycles. Measurement of the shuttle current and impedance provides interesting insight into the polysulfide mass transfer mechanism of hollow structured sulfur hosts.     

Facile Methods to Coat Polystyrene and Silica Colloids with Metal

To avoid the complex core surface functionalization or pretreatment that is necessary in order to coat latex and silica colloids with a uniform, complete metal shell, the solvent‐assisted route has been explored to prepare a complete metal (Ag or Au) shell with controlled thickness on polystyrene (PS) colloids and the electroless plating approach, based on electrostatic attraction, has been explored to prepare a complete silver shell with controlled thickness on silica colloids. Without any additional surface treatment, the as‐prepared complex core–shell colloids can be crystallized directly into long‐range‐ordered structures with photonic bandgaps, as reported here for the first time. These ordered structures may find potential applications as substrates or physical systems for the enhancement of Raman scattering studies, besides applications as photonic crystals. The optical plasmon resonance of the composite core–shell colloids changes with metal shell thickness, the wavelength varying over hundreds of nanometers. Our coating routes are facile and versatile, and can be extended to coat PS and silica colloids with any other metal whose ion or complex can be reduced in solution.     

一种新型空心镍纳米球的合成及其性能

以二氧化硅颗粒作为模板、金纳米颗粒作为表面晶种的方法制备了壳厚度可控的镍空心球。得到的空心镍纳米球是由似针状的面心立方的镍纳米颗粒构成的,碱溶液处理过程不影响镍纳米壳的形貌。高温处理显示镍空心球具有良好的热稳定性。研究了镍空心球电催化氧化甲醇和乙醇的活性,结果显示在碱性溶液中镍空心球是一种非常高效的甲醇和乙醇电催化氧化剂。     

Copper‐Assisted Weak Polyelectrolyte Multilayer Formation on Microspheres and Subsequent Film Crosslinking

The formation of weak polyelectrolyte films on planar and spherical supports has recently evoked major interest, as such coatings allow novel material properties to be tunable by pH and salt adjustment of the polyelectrolyte deposition conditions. We report on the build up of multilayers of the weak polyelectrolytes poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) on submicrometer‐sized polystyrene (PS) and silica colloid spheres (～ 500 nm) with the aid of copper ion templating. The copper ions complex to the carboxylate groups of PAA, facilitating the formation of PAA/PAH multilayers on the particles. Regular growth of the layers on the colloid spheres with each polyelectrolyte deposition step was confirmed by microelectrophoresis, single‐particle light scattering (SPLS), and transmission electron microscopy (TEM), with an average bilayer thickness of ～ 3 nm. The polyelectrolyte multilayer‐coated particles formed stable colloidal dispersions, with ζ‐potentials ranging from 30 mV (PAH outer layer) and –50 mV (PAA outer layer). Complementary quartz‐crystal microbalance and UV‐vis spectrophotometry studies on PAA/PAH multilayers formed on planar supports were performed to examine the film formation and the role of copper ion binding to the layers. PAA/PAH multilayers formed on colloid particles were also chemically crosslinked by using the activator 1‐ethyl‐3‐(3‐dimethylaminopropyl) carbodiimide (EDC). The degree of film crosslinking could be readily controlled by varying the concentration of EDC employed. Following solvent decomposition of the template particles coated with crosslinked PAA/PAH multilayers, intact hollow polymer capsules were obtained. These capsules were found to be impenetrable to polystyrene.