Aqueous sol-gel processes

Aqueous sol-gel process It is an economical and ecological process. Agglomeration is a major drawback. 

Non-aqueous sol-gel process It is expensive with limited ecological and agglomeration properties. 

The generation of silica in situ, in the polymer matrix, has been performed by using the sol-gel process. The typical aqueous sol-gel process consists of the hydrolysis of metal alkoxides M(OR)x followed by condensation reaction. Reactions involved to prepare silica through the sol-gel process are reported as follows ... 

The sol-gel method is generally employed for the synthesis of metal oxide NPs as well as oxide nanocomposites. The sol-gel process involves the hydrolysis and condensation of metal precnrsors [43,44]. Further condensation and polymerization will lead to three-dimensional metal oxide networks forming the gel. The sol-gel process can be either in aqueous or non-aqueous medium. In the aqueous sol-gel process, oxygen for the formation of the oxide is supplied by water molecules. In the... 

Larbot A., Fabre J.-P, Guizard C., Cot L., Gillot J. New inorganic ultrafiltration membranes titania and zirconia membranes. J. Am. Ceram. Soc. 1989 72 257-261 Lee B., Zhang J. Preparation, structure evolution and dielectric properties ofBaTiOs thin films and powders by an aqueous sol-gel process. Thin Solid Films 2001 388 107-113 Lee D.-S., Liu T.-K. Preparation ofTi02 sol using TiC as a precursor. J. Sol-Gel Sci. Technol. 2002 25 121-136... 

Bhatia R.B., Brinker C.J., Gupta A.K., Singh A.K. Aqueous sol-gel process for protein encapsulation. Chem. Mate. 2000 12 2434-2441... 

Bogdanovidene, 1., Beganskiene, A., Tonsuaadu, K., Glaser, J., Meyer, H. and Kareiva, A. (2006) Galdum hydroxyapatite, Caio(P04)6(OH)2 ceramics prepared by aqueous sol-gel processing. Materials Research Bulletin, 41,1754-62. 

The aqueous sol-gel process is one of the most important processes employed for the production of oxide nanoparticles, particularly if particle redispersion in a solvent or surface functionalization is requested [1]. Generally, molecular compounds are hydrolyzed in an aqueous environment, the intermediates consisting of E—OH (E = semiconductor element or metal) groups are not stable under the given reaction conditions and condensation reactions occur. Thus, E—O—E bonds are formed and a three-dimensional network is also formed. 

The chemistry of the aqueous sol-gel process is quite complex due to metal and the double role of water as ligand and solvent as well as due to the large number of reaction parameters that have to be stricdy controlled (hydrolysis and condensation rate of the metal oxide precursors, pH, temperature, method of mixing, rate of oxidation, the nature and concentration of anions, etc.) in order to provide good reproducibility of the synthesis protocol. Another fundamental problem of aqueous sol-gel chemistry is that the as-synthesized precipitates are generally amorphous. The required postsynthetic annealing step to induce the crystallization process prevents any subtle control over crystal size and shape. For the preparation of bulk metal oxides, like the ceramic pigment case, these limitations play only a minor role however, in the case of nanoparticle synthesis, they constitute a major concern. 

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