VOLUME 9, NUMBER 11
NOVEMBER 1997
© Copyright 1997 by the American Chemical Society
Editorial Preface to the Special Issue: Sol-Gel Derived Materials There is intense interest in the area of sol-gel processing, directed at both fundamental issues and promising applications. This interest derives in part from the recognition that sol-gel chemistry is finding an increasingly broader range of applications, as a route to new materials and as a means of processing materials in improved or unique ways. This Special Issue of Chemistry of Materials is dedicated to Sol-Gel Derived Materials. It originates from the enthusiastic participation of the Symposium on “Sol-Gel Synthesis of Catalysts and Advanced Materials” organized for the American Chemical Society National Meeting in San Francisco, April 1997. Featuring 21 invited talks and 33 contributed papers, this Symposium covered topics such as precursor chemistry, sol-gel derived ceramics and catalysts, synthesis of mesoporous structures, design of porous catalysts, and nanocrystalline and nanocomposite materials. The peer-reviewed papers in this special issue cover a broader spectrum of sol-gel research than that presented at the Symposium and are grouped under eight topics. Reviews and articles on structural evolution of sol-gel derived materials feature in situ studies of multicomponent network gel formation and crystallization and probes of local environment of porous matrices. The field also continues to witness new innovations in synthetic approaches that significantly expand the capability of sol-gel processing for tailoring different structural characteristics. Aero-sol-gel synthesis of nanostructured powders, use of commercial preformed sols, derivation of environmentally benign precursors, and development of nonhydrolytic syntheses represent several examples of novel sol-gel routes. Also noteworthy are synthesis of hybrid polymers as precursors for oxycarbide ceramics, microporous polymer replication with zeolitic templates, and fabrication of hierarchical macrostructures using bacterial superstructures as templates. Supercritical drying has been used to produce aerogels with ultrahigh porosity for decades. Recent advances include direct formation of aerogels with supercritical CO2 as the polymerization solvent, and derivation of aerogels with widely varied compositions. An example of the latter is alkaline-earth oxides, which are generated as nanoparticles via supercritical drying. These materials present unusually high surface reactivities for destructive adsorption of toxic chemicals. Besides using special drying media, highly porous structures can also be achieved with molecular imprinting and supramolecular templating during the S0897-4756(97)00999-X CCC: $14.00
© 1997 American Chemical Society
2248 Chem. Mater., Vol. 9, No. 11, 1997
Editorial
wet-chemical synthesis itself. The special issue includes several articles on porous structures and catalytic materials. Of particular interest is the derivation of well-defined mesoporous materials via assembly of surfactant templates. The development of transition-metal oxide molecular sieves is also remarkable, and these new materials are finding interesting catalytic applications. Porous systems also have a distinctive role in matrix-mediated synthesis and complexation. The porous host provides for the generation and stabilization of nanocrystallites and quantum dots; sol-gel processing is particularly attractive for its ability to process such nanocomposite structures in the form of thin films for optoelectronic applications. Porous materials further allow for the fixation of organometallic complexes for heterogeneous catalysis, providing for easy recovery of catalysts and shape- and size-selectivity associated with the pore structures. Sol-gel derived thin films and coatings are widely examined for electroceramic and membrane applications. Papers in this special issue address the challenges in solution deposition of defect-free coatings and phase-pure multicomponent oxide thin films. Sol-gel derived materials also find increasing interests in optical and electrochemical applications. Hybrid organic-inorganic sol-gel systems are useful as photochromic and photosensitized materials. The ease of dopant introduction and chemical homogeneity inherent in sol-gel processing also lend themselves to fine-tuned optical waveguides and microoptical devices. Promising prospects in sol-gel preparation of electrodes and solid electrolyte materials are also emerging in the literature. Sol-gel is attractive for processing such materials for its microstructure control, versatility in organic hybridization, and ease of surface functionalization. These features are also exploited in biochemical applications to create bioactive systems and optical biosensors. Sol-gel matrices allow for flexible biological modification via entrapment or binding of antibodies and hold great potential for selective immunochromatography, immunosensing, and catalytic conversion. The variety of topics covered reflects the level of activity in the materials chemistry community on sol-gel processing, and the increased interest in engineering sol-gel derived materials toward specific applications. There continues to be a need for better understanding of the sol precursor chemistry, gelation process, and post-treatment to tailor unique and, sometimes, metastable structures with designed morphology, pore structure, compositional variation, and surface functionality. The flexibility of this low-temperature wetchemical process is precisely what gives sol-gel research its distinctive richness and complexity. It is my hope that this special issue will contribute to the growing interest in this interdisciplinary field. Jackie Y. Ying, Guest Editor Massachusetts Institute of Technology Department of Chemical Engineering Cambridge, Massachusetts CM970999Y
