A family of highly ordered mesoporous (20−300 A) silica structures have been synthesized by the use of commercially available nonionic alkyl poly(ethylene oxide) (PEO) oligomeric surfactants and poly(alkylene oxide) block copolymers in acid media. Periodic arrangements of mescoscopically ordered pores with cubic Im3m, cubic Pm3m (or others), 3-d hexagonal (P63/mmc), 2-d hexagonal (p6mm), and lamellar (Lα) symmetries have been prepared. Under acidic conditions at room temperature, the nonionic oligomeric surfactants frequently form cubic or 3-d hexagonal mesoporous silica structures, while the nonionic triblock copolymers tend to form hexagonal (p6mm) mesoporous silica structures. A cubic mesoporous silica structure (SBA-11) with Pm3m diffraction symmetry has been synthesized in the presence of C16H33(OCH2CH2)10OH (C16EO10) surfactant species, while a 3-d hexagonal (P63/mmc) mesoporous silica structure (SBA-12) results when C18EO10 is used. Surfactants with short EO segments tend to form lamellar mesost...

Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures

Mesoporous organic-inorganic hybrid materials, a new class of materials characterized by large specific surface areas and pore sizes between 2 and 15 nm, have been obtained through the coupling of inorganic and organic components by template synthesis. The incorporation of functionalities can be achieved in three ways: by subsequent attachment of organic components onto a pure silica matrix (grafting), by simultaneous reaction of condensable inorganic silica species and silylated organic compounds (co-condensation, one-pot synthesis), and by the use of bissilylated organic precursors that lead to periodic mesoporous organosilicas (PMOs). This Review gives an overview of the preparation, properties, and potential applications of these materials in the areas of catalysis, sorption, chromatography, and the construction of systems for controlled release of active compounds, as well as molecular switches, with the main focus being on PMOs.

Silica-based mesoporous organic-inorganic hybrid materials.

Colloidal metal nanoparticles are emerging as key materials for catalysis, plasmonics, sensing, and spectroscopy. Within these applications, control of nanoparticle shape lends increasing functionality and selectivity. Shape-controlled nanocrystals possess well-defined surfaces and morphologies because their nucleation and growth are controlled at the atomic level. An overall picture of shaped metal particles is presented, with a particular focus on solution-based syntheses for the noble metals. General strategies for synthetic control are discussed, emphasizing key factors that result in anisotropic, nonspherical growth such as crystallographically selective adsorbates and seeding processes.

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Shape Control of Colloidal Metal Nanocrystals

On the Controllable Soft-Templating Approach to Mesoporous Silicates

Organic porous materials—a class of advanced materials— possess enormous potential for many high-tech applications, such as bioreactors, dielectrics, sensors, microelectrophoresis, thermal insulation, and catalysts. In general, they can be prepared by phase separation, and a hard templating approach, such as those employing colloidal particles. Phase separation can be derived from organic– organic phases, while the pore structures can be formed after etching, or by dissolving one block (A) from the assembled block copolymer (A–B). However, most of the resulting porous polymer structures are disordered with wide pore size distributions because of the contraction and swelling from changes in volume, as well as the structured defects formed during template removal. Large porosities have rarely been reported. Furthermore, the resistance of the pore structure to heat and solvents is rather low because the materials are formed by weak van der Waals forces and physical twists between polymer chains, which means that the framework is not connected by covalent bonds. Recently, a procedure for cross-linking lyotropic liquid crystals (LLC) in water was introduced to prepare periodic porous organic mesostructures. Unfortunately, polymerization only occurs between nearest neighboring head groups, and the mesostructured channels are fully occupied with solution. Therefore, it is not surprising that porosity has yet to be reported. Carbon materials, including nanotubes and fullerenes, have attracted considerable attention because of their remarkable properties. The traditional carbonization process for active carbon and related materials can only generate

Ordered Mesoporous Polymers and Homologous Carbon Frameworks: Amphiphilic Surfactant Templating and Direct Transformation

Thin films of surfactant-templated mesoporous materials1,2 could find applications in membrane-based separations, selective catalysis and sensors. Above the critical micelle concentration of a bulk silica–surfactant solution, films of mesophases with hexagonally packed one-dimensional channels can be formed at solid–liquid and liquid–vapour interfaces3,4,5. But this process is slow and the supported films3,5 are granular and with the pore channels oriented parallel to the substrate surface, so that transport across the films is not facilitated by the pores. Ogawa6,7 has reported a rapid spin-coating procedure for making transparent mesoporous films, but their formation mechanism, microstructure and pore accessibility have not been elucidated. Here we report a sol–gel-based dip-coating method for the rapid synthesis of continuous mesoporous thin films on a solid substrate. The influence of the substrate generates film mesostructures that have no bulk counterparts, such as composites with incipient liquid-crystalline order of the surfactant–silica phase. We are also able to form mesoporous films of the cubic phase, in which the pores are connected in a three-dimensional network that guarantees their accessibility from the film surface. We demonstrate and quantify this accessibility using a surface-acoustic-wave nitrogen-adsorption technique. We use fluorescence depolarization to monitor the evolution of the mesophase in situ, and see a progression through a sequence of lamellar to cubic to hexagonal structures that has not previously been reported.

Continuous formation of supported cubic and hexagonal mesoporous films by sol–gel dip-coating

Results appear to confirm the concept of surfactant-templating of thin film mesostructures. Final film pore structure depends on starting surfactant and water concentrations and process time scale (governed by evaporation rate). Surfactant ordering at substrate-film and film-vapor interfaces orients the porosity of adjoining films, leading to graded structures. SAW experiments show that depending on processing conditions, the porosity may be open or closed (restricted). Open porosity is monosized. Upon pyrolysis, lamellar structures collapse, while the hexagonal structures persist; when both hexagonal and lamellar structures are present, the hexagonal may serve to pillar the lamellar, avoiding its complete collapse. Thick lamellar films can be prepared because the surfactant mechanically decouples stress development in adjoining layers. Upon drying and heating, each individual layer can shrink due to continuing condensation reactions without accumulating stress. During surfactant pyrolysis, the layers coalesce to form a thick crack-free layer. Formation of closed porosity films is discussed.

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Fundamentals of sol-gel film deposition

Processing can be controlled to produce a family of mesoporous silica films with either disordered, lamellar, hexagonal, or cubic pore distributions[l]. These films, formed by surfactant-templated synthesis and exhibiting a unimodal pore size, promise potential use as inorganic membranes, catalysts, and optically-based sensors[l,2]. The mesoporous films can be formed from initially homogeneous silica sols by a continuous, surfactant-templated process, which relies upon solvent evaporation during the sol-gel dip-coating process. Films of 100-500 nm thick are formed within seconds in a continuous coating operation. The microstructure of the films is dependent upon the cationic surfactant concentration CTAB (CH3(CH2)15N+(CH3)3Br-) and the dip-coating rate. Several films, processed under differing conditions, were investigated by TEM to characterize pore size, structure, and orientation. Figures 1 a & b show the plan view and cross-sectional microstructure of a 2-d hexagonal mesoporous silica film deposited on silicon; the sample was calcined at 400 °C for 3 hours in air. The images were obtained on a Philips CM30 TEM, operated at 300 kV.

R. Ganguli

Papers

Continuous formation of supported cubic and hexagonal mesoporous films by sol–gel dip-coating

Fundamentals of sol-gel film deposition

Microstructural Analysis of Disordered and Ordered Mesoporous Silica Films