Alkoxides as precursors

Sol-gel processing almost exclusively involves metal alkoxides as precursor chemicals. An alkoxide is an alkane with an oxygen interposed between at least one of the carbon atoms and the metal—for example, (OC2H5)4Si, known as tetraethoxysilane (TEOS, a.k.a. tetratethyl orthosilicate). Other common alkoxide precursors are listed in Table 7.15. TEOS is used to produce the sol-gel-derived silica that generated the X-ray diffraction pattern in Figure 1.50. Let us use silica as an example to study the steps in the sol-gel process. 

Bismuth is an important element in many of the new high-temperature, oxide superconductors and in a variety of heterogeneous mixed oxide catalysts. Some of the methods employed in the preparation of these materials, namely sol-gel and chemical vapor deposition processes, require bismuth alkoxides as precursors and a number of papers on these compounds have recently been published.1 One synthetic route to bismuth alkoxides, which avoids the more commonly used trihalide starting materials and the often troublesome separation of alkali metal halides, involves the reaction between a bismuth amide and an alcohol according to the following equation ... 

Using alkoxides as precursors, alcohol as the supercritical drying agent, and a batch process the solvent exchange step was eliminated. 

Using alkoxides as precursors, carbon dioxide as tile drying agent, and a semicontinuous process the diying procedure became safer and faster. Introduction of organic aerogels. 

The thermal decomposition of vanadium alkxoides [24, 983, 1296] and their behavior in the gas phase (with the aid of mass-spectrometry) [11, 1613] were also studied in detail to optimize the CVD-techniques, applying the vanadium alkoxides as precursors. 

Using preformed sols instead of metal alkoxides as precursors is an attractive alternative in sol-gel preparation because recent advances in inorganic colloidal dispersions allow some control over the characteristics of the starting sols [11]. Often a colloidal suspension of sol particles is stabilized (i.e. prevented from flocculation) by pH adjustment. Thus, pH of the solution, which can be changed by the addition of either acid or base, is the single most important parameter in obtaining a gel from preformed sols. Other parameters that can impact on gel quality are the size and concentration of the starting sol particles. 

Using preformed sols instead of metal alkoxides as precursors is another effective way to change the pore structure, as recently demonstrated for the sol-gel preparation of zirconia [38], The two key parameters in this approach are the starting sol size and the intrinsic porosity of the sol particles. 

Bradley, D.C. (1989) Metal alkoxides as precursors for electronic and ceramic materials. Chemical Reviews, 89, 1317-1322. 

The replacement of alkoxo by chelating /3-diketonato ligands selectively modulates their hydrolyzability, which proved to be of considerable value in the sol-gel process for ceramic materials using alkoxides as precursors (3, 31, 33, 35). 

With the growing sophistication of X-ray crystallographic techniques, the definitive bonding modes of more and more alkoxide derivatives in the solid state are being determined. But with the rapidly advancing uses of metal alkoxides as precursors in the sol-gel process, a clearer understanding of their solution structures has become essential to understanding of the mechanistic aspects of hydrolysis and condensation as key steps in this process. 

A variety of papers have described the applications of Zr and Hf alkoxides as precursors for metal oxide thin films and ceramics. In addition, these species have been used as Lewis acid catalysts in a variety of organic transformations.375-381... 

Campion, J, F., Payne, D. A., Choe, H. K., Maurin, J. K., and Wilson, S. R., Synthesis of bimetallic barium titanium alkoxides as precursors for electrical ceramics. Molecular structure of the new barium titanium oxide alkoxide Ba4Tii30i2(OC2H40Me)24, Inorg. Chem., 30, 3245 (1991). 

The use of alkoxides as precursors to produce new materials [1] has increased in the last decade due to the advantages of the so-called sol-gel process [2] such as the preparation of compounds with high homogeneity, at room temperatures, in contrast with the typical high-temperature syntheses in solid-state chemistry. Such an experimental approach can lead not only to modified amorphous oxide surfaces, as discussed in Chapter 1, but also to nanostructured modified surfaces, as will be shown here. 

A commonly studied approach for synthesizing oxides has been to hydrolyze the appropriate metal alkoxides. There are several advantages to using metal alkoxides as precursors for ceramic powders. Most of the alkoxides of interest can be easily prepared or are commercially available and can be readily purified prior to use. Interaction of alkoxides with water yields precipitates of hydroxides, hydrates, and oxides. The precipitate particles usually range in size from 0.01 to 1 pm, depending on the hydrolysis conditions. So we can easily produce nanoparticles. 

Peter D, Ertel TS, Bertagnolli H. EXAFS study of zirconium alkoxides as precursor in the sol-gel process I. Structure investigation of the pure alkoxides. J Sol-Gel Sci Technol 1994 3(2) 91-9. 

T. Athar, O.H. Jeong, and S. Sang 11, Synthesis and characterization of fluoro-and chloro-bimetalhc alkoxides as precursors for luminescent metal oxide materials via sol-gel technique, Chin. J. Chem., vol. 25, no. 7, pp. 998-1004, 2007. 

In view of their importance in the sol-gel preparation of spinel type materials, detailed stepwise hydrolytic reaction studies of [Ca Al(OIV)4 2] and [Mg Al(OPr )4 2] ° have been carried out with similar results. With the synthesis of heteropolymetallic alkoxides as precursors, the importance of extending hydrolytic studies to such systems is obvious. " ... 

Sol-gel chemistry has allowed to bridge molecular chemistry and materials science. It opened up the way for the synthesis and processing of solid metal oxides under remarkably mild conditions by hydrolysis and condensation of molecular precursors [3]. The path using metal alkoxides as precursors has proved to be particularly adaptable [4]. 

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