Nonaqueous sol-gel process

There are only a few reports on nonaqueous sol-gel process applied for the formation of silica. The published literature describes the formation of monolithic silica structures [25], mixed silica metal oxide compounds [26], and the generation of a silica phase within a polymer matrix [27]. There are no reports that describe the synthesis of silica nanoparticles via the nonhydrolytic sol-gel process. One of the inherent problems might be that this type of reaction does not generate charged silica surfaces, which are usually required for the stabilization of the particles. Contrarily, a large variety of binary and ternary metal oxides can be formed. 

Many different metal oxide nanoparticles have been produced in the last couple of years applying nonhydrolytic sol-gel reactions. One of the most used methods is the so-called benzyl alcohol route, in which various metal oxide precursors are 

NaNbOj, NaT.tO, TiOj, VOi52(OH)o77 BaTiOj, SrTiOg, (Ba.Sr)TIO, WO3 H2O, SnOj, HfOj LiNbOg, BaZrOg 

Examples of metal oxides produced with the benzyl alcohol route are SnOa [30], InNb04 [31], TiOa [32], CeOa [32], and Al ZnO [33] to mention a selection. The formed metal oxide nanopartides often show very small diameters of a few nanometers. They form different shapes, including spheres, platelets, and rods [34]. In most cases the partides are crystalline. 

Besides the formation of metal oxides, there were also efforts for the production of nonoxidic nanopartides. Only a few examples were reported in the 

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Rare earth hydroxides are rarely obtained in nonaqueous systems. However, Djerdj et al. reported the synthesis of La(OH)s nanorods/nanofibers and manganese oxide nanoparticles through a nonaqueous sol-gel process involving the reaction of La(OiPr)3 (lanthanum f-propoxide) and KMn04 with organic solvents such as benzyl alcohol, 2-butanone, and their mixture (Djerdj et al., 2007). 

Hsiue G-H, Chen J-K, Liu Y-L (2000) Synthesis and characterization of nanocomposite of polyimide-silica hybrid from nonaqueous sol-gel process. J Appl Polym Sci 76(11) 1609-1618... 

Niederberger and coworkers developed an interesting variation of the sol-gel method, the nonaqueous sol-gel process, that produces nanocrystalline... 

In this chapter, the particles are divided into two subgroups sihca and metal oxide particles, because both systems have different challenges. In addition, the chapter is also divided in accordance with the preparation routes toward the nanopartides, namely, aqueous and nonaqueous sol-gel processes. For the implementation of the nanopartides into devices, it is often necessary to carry out a surface functionalization, which is also part of this chapter. In the concluding paragraphs, the effect of the thus formed particles on properties of nanocomposites has been discussed. 

Silica Gels. The acid-catalyzed alkoxide sol-gel process produces gels (17). Frib-erg and coworkers (40-50) pioneered the extension of this process to silica synthesis in microemulsions both aqueous and nonaqueous microemulsions were used. For aqueous microemulsions, experiments were conducted mostly with the SDS/ pentanol/water/acid system. A representative flow diagram is shown in Figure 2.2.9. The nonaqueous microemulsion systems utilized included CTAB/decanol/ decane/formamide and AOT/decane/glycerol (44-46,49,50). The experimental approach followed the sequence nonaqueous microemulsion preparation, water addition, and then TEOS addition. 

Extensive research on sol—gel processing of silicic acid esters, eg, tetraethoxysilane (TEOS), Si(OC2H5)4, in alcohol—water mixtures has ducidated silica polymerization in nonaqueous solvents (54,55). The relative rates of hydrolysis and condensation depend on the substrate, water, and catalyst (acid or base) concentrations these rates determine the polymer structure. For example, acid-catalyzed hydrolysis of TEOS at low water concentrations produces linear polymers. These solutions yield fibers upon spinning. Conversely, high H20 /TEOS ratios favor the formation of highly cross-linked polymers under... 

Sol-gel Process Hydrolysis of precursor salts in an aqueous or nonaqueous environment to form a colloidal suspension of precipitates often in the form of hydroxide, oxalate, and carbonate Particle size control By reaction temperature and duration, precursor concentration, pH, surfactant additive, precursor materials, and solvent General attributes Spherical shapes further heat treatment is necessary for the nonoxide precipitates and often leads to severe agglomeration... 

Valentin C., Munoz M.C., Alarcdn J. Synthesis and characterization of vanadium-containing ZrSi04 solid solutions from gels. J. Sol-Gel Sci. Technol. 1999 15 221-230 Van Helden A.K., Jansen J.W., Vrij A. Preparation and characterization of spherical monodisperse silica dispersions in nonaqueous solvents. J. Colloid Interf. Sci. 1981 81 354-368 Woodhead J.L. Sol-gel processes to ceramic particles using inorganic precursors. J. Mater. Educ. 1984 6 887-925... 

Nonhydrolytic sol-gel routes are not necessarily completely water-free. Although the initial reaction mixture might be anhydrous, specific organic reactions are able to produce water in situ (e.g., aldol condensation and esterification reactions), rendering the system in principle hydrolytic. Therefore, nonhydrolytic sol-gel processes are often also called nonaqueous. But even in the absence of water, it is possible to have hydroxylation reactions. 

The sol-gel process - either aqueous or nonaqueous - is one of the most important processes for the preparation of oxidic nanopartides. For silica particles, the sol-gel-based Stober process is definitely the most used wet chemical preparation route. Particularly, the mild reaction conditions combined with the excellent control over nanopartides properties malce it a universal method for the production of colloids for various applications. In recent years, the metal oxide routes have also become more and more sophisticated. Another reason for the attractiveness of the preparation route is the activity of the derived particles toward surface functionalization. Such modified particles can be easily incorporated into polymer matrices to obtain nanocomposites with extraordinary properties. 

Greater flexibility in terms of the processing conditions. Because sol-gel polymerization involves mild reaction conditions, biological molecules, water soluble molecules, and thermally sensitive molecules can be utilized as templates. Such molecules are often difficult to use as templates when traditional free radical polymerization in nonaqueous solvents is employed. 

Although the electrodeposition of sol-gel films can be well manipulated based on the experimental data, the kinetics of the deposition process is still not quantified. This is desired to be solved in the near future as it is important for better understanding and controlling the process. Other future aspects may include the electrodeposition from nonaqueous solutions using other electrochemically induced catalytic reactions, and improving the spatial resolution of patterning. 

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