Nonhydrolytic sol-gel processes

Condensation occurs at temperatures between 20 and 100 °C sometimes, a catalyst is needed (FeQs is often used). The kinetics of nonhydrolytic sol-gel processes depends on the nature of the metal, the nature of the oxygen donor, electronic effects of the group R, and the composition of the initial metal alkox-ide/metal chloride (carboxylate) mixture, but is generally slower than that for aqueous processes. 

Noteworthy features of this method are that nonhydrated oxides without residual hydroxo groups are obtained, due to the aprotic conditions, and that in bimetallic systems the metals M and M have an alternate order (no phase separation), due to the reaction mechanism (Eq. (1.7)). A limitation of nonhydrolytic processes is that the M/M ratio is not freely selectable if fully condensed products are targeted. Eor this reason, sol-gel processes of mixed metal systems are sometimes initiated by nonhydrolytic reactions (to obtain a high homogeneity) and then completed by hydrolytic reactions (to obtain complete hydrolysis and condensation). 

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In nonhydrolytic sol-gel processing, by low-temperature thermolysis of the precursor (A1 compound in Scheme 3) in solution [110]... 

Ceria NWs (1.2 nm in width) and tadpole-shaped nanocrystals (3.7 nm in width) are obtained by a nonhydrolytic sol-gel process in presence of diphenyl ether in a mixed high boiling solvent of oleic acid and oleyla-mine at 320 °C (Yu et al., 2005c Figure 7). The NWs reported by Yu et al. are among the thinnest ones known with a reasonable yield, showing a growth direction of [100] of the fluorite structure. Spherical ceria nanocrystals of 3.5 nm could also be obtained if oleic acid is absent from the system. 

Both stages are controlled by condensation chemistry that can include, as a first step, hydrolysis of hydrated metal ions or metal alkoxide molecules [6, 7] (hydrolytic sol-gel processing). The condensation chemistry in this case is based on olation/oxolation reactions between hydroxylated species. The hydroxylated species for further condensation can be formed also by a non-hydrolytic route, that is, by reactions between metal chlorides and alcohols with electron-donor substituents [8]. The nonhydrolytic sol-gel processing may... 

Another class of reactions leading to polycondensation of oxygenmetal precursors is the so-caUed aprotic condensation that proceeds during nonhydrolytic sol-gel processing where water is not required for precursor activation and not produced by condensation [9, 10]. It includes reactions between alkoxides of dissimilar metals differing in the polarity of M—O bonds, or of alkoxides with metal esters and metal chlorides. The condensation rate is strongly dependent on the OR/M ratio in mixed precursors, as well as on temperature, the presence of catalyst and structure of the R derivative. 

After several decades of intense research on the development of synthesis routes, it is nowadays possible to prepare nanoparticles with amazing structural, compositional, and morphological sophistication. But in comparison to the highly advanced synthesis know-how, knowledge about mechanistic aspects of nanoparticle formation has not yet reached the same leveL The reasons for this are not lack of interest or research efforts, but may be foimd in both the complexity of the processes involved in nanoparticle formation and the difficulty to monitor the reactions from the dissolution of the precursor to the formation of the final nanocrystalline product However, in the case of nonhydrolytic sol-gel processes, chemical reaction mechanisms are relatively well established. The reaction rates are typically slower than those in aqueous systems due to the moderate reactivity of the C—O bond (which plays the major role in nonhydrolytic reactions in contrast to the O—H bond in aqueous systems), and it is relatively straightforward to investigate the organic reactions that are correlated to nanoparticle formation and thus provide information about possible formation mechanisms [5]. 

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. 

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. 

In contrast to conventional sol-gel processes, which are based on the hydrolysis and condensation of metal precursors in an aqueous alcohol-mixed solvent, the nonhydrolytic sol-gel reaction proceeds by reactions of the metal precursors in organic media [16]. This nonhydrolytic sol-gel process has unique advantages... 

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