A review of the chemical manipulation of nanomaterials using solvents: Gelation dependent structures

The hydrolysis of Mg, Zr, and Ti alkoxides by water in solvent-alcohol mixtures has been examined in the absence of added acid or base catalysts, and herein, these data are reviewed. The presence of low dielectric constant solvents (benzene, toluene, or anisole), in varying ratios with methanol (or other alcohols), caused much more rapid gelation, subsequently yielding mesoporous materials with higher surface areas and larger pore volumes as fibrous, open web-like structures. The presence of high dielectric constant (polar aprotic) solvents such as acetone, acetonitrile, DMF, DMSO, or dimethyaniline was not effective in this way. A partial charge model was employed to explain why the low dielectric solvents forced more rapid hydrolysis, and it is concluded that the hydrolysis step is rate determining, approaching diffusion control, in this gelation process. The use of these “spectator solvents” helps simplify and enhance sol-gel and xerogel/aerogel synthesis schemes, and yields higher quality materials.