The mechanisms of gel formation in silicate systems derived from metal alkoxides were reviewed. There is compelling experimental evidence proving, that under many conditions employed in silica gel preparation, the resulting polysilicate species formed prior to gelation is not a dense colloidal particle of anhydrous silica but instead a solvated polymeric chain or cluster. The skeletal gel phase which results during desiccation is, therefore, expected to be less highly crosslinked than the corresponding melted glass, and perhaps to contain additional excess free volume. It is proposed that, during gel densification, the desiccated gel will change to become more highly crosslinked while reducing its surface area and free volume. Thus, it is necessary to consider both the macroscopic physical structure and the local chemical structure of gels in order to explain the gel to glass conversion.

Sol → gel → glass: I. Gelation and gel structure

In the sol-gel synthesis of silica, a silicon-containing raw material, a solvent, water, and a catalyst are utilized. By varying the catalyst, dramatic effects on gelation time, porosity, bulk and apparent density, and volume shrinkage on drying were observed. For example, porosities ranging from two to sixty-eight per cent can be obtained for dried and fired gels. Mechanisms for catalysis have been proposed for the catalysts considered.

Sol-gel processing of silica. II: The role of the catalyst

A novel method for synthesis of silica nanoparticles.

As on today, the conversion of carbon dioxide (CO 2 ) into value added chemicals including methanol using exclusively solar energy (i.e., artificial photosynthesis) has been considered as one of the top most research priorities all over the world as this process can indeed deal with (i) the mitigation of CO 2 associated global warming problem, (ii) creation of highly sustainable and renewable energy resources, and (iii) development of one of the best processes to store the energy in a most convenient form of liquid fuels with sufficiently high energy density in comparison to all the existing methods of energy storage. As the CO 2 sequestration process is expensive, there has been a quest for finding other alternative options. The CO 2 can be converted into several value added chemicals including methanol following stoichiometric reactions, thermochemical, electrochemical, photoelectrochemical, photocatalytic, etc., routes. Methanol synthesized from CO 2 can be employed directly in place of fossil fuels without disturbing the present existing energy distribution infrastructure. Further, if H 2 required for CO 2 reduction is produced from water using exclusively solar energy, and methanol is produced with minimum overpotentials with the help of suitable catalytic systems, this process can be employed to address the above-mentioned three major problems. The process for converting CO 2 into methanol can be developed by careful analysis and understanding of the information available on this process reported so far in the literature. Since, none of the existing review articles deals exclusively with this important process of converting CO 2 into methanol, in this review article, all the information including fundamental challenges and opportunities that are associated with the conversion of CO 2 into methanol following various routes has been summarized, discussed and meticulously presented by citing all the up to date relevant references.

Conversion of carbon dioxide into methanol – a potential liquid fuel: Fundamental challenges and opportunities (a review)

The sol-gel process is utilized to solve the notorious problem of incorporating an organic dye in an inorganic oxide thin film. Fluorescent thin films are prepared by this low temperature process with the aid of a surface active agent, with good homogeneity and reproducability. A variety of organic fluorescent molecules are embeded either in silica or in silica-titania films. The dye molecules are not leached out by water. Absorption and emission spectra, enhanced photostability, longer lifetimes and energy transfers between the trapped dye molecules are described and discussed in terms of the effects of molecule matrix-isolation on these properties. Potential uses of the special thin films are numerous, e.g., as laser or solar light guides.

Organic fluorescent dyes trapped in silica and silica-titania thin films by the sol-gel method. Photophysical, film and cage properties

Fundamental problems in preparing thin sheets and coating films of oxides have been discussed on the basis of experiments made in the author's laboratory. It has been found that thin sheet formation is only possible with the starting solutions exhibiting spinnability in the course of hydrolysis and polymerization of the metal alkoxide. This requirement is satisfied by the formation of linear polymers in the solution. Coating films have been prepared by the dip-withdrawal technique. It has been found that coating films of 0.05–0.5 μm in thickness can be provided by one dip-coating run and repetition of the application leads to thicker coating films. The preparation of SiO2, BaTiO3 and transition metal oxide-SiO2 coating films has been discussed.

K. Makita

Papers

Formation of sheets and coating films from alkoxide solutions

Optical absorption of transition metal ions in silica coating films prepared by the sol-gel technique