Nonhydrolytic sol-gel methods

The fundamental goal of nanoparticle research is to assemble atoms in a controllable way and design nanostructured materials with the desired physical and chemical properties. A major part of the research in the field of nanoscience is dedicated to the development of synthesis routes to nanoparticles and nanostructures. Conventionally, solid-state reactions between powders have successfully been employed for the low-cost production of bulk metal oxides. However, to obtain metal oxide nanoparticles with well-defined shape, size, and composition, these solid-state routes are unsuitable. In contrast to these high-temperature processes, liquid-phase synthesis routes, and in particular sol-gel routes, offer better possibility to control the variation of structural, compositional, and morphological features of the final nanomaterials [1,2]. 

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

The Sol—Gel Handbook Synthesis, Characterization, and Applications, First Edition. 

Metal Halides Metal Acetates Metal Alkoxides Metal Acetylacetonates 

Alcohols (e g.. Benzyl Alcohol) Ketones (e.g., Acetophenone) Amines (e.g., Benzylamine) 

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This method is closely related to the nonhydrolytic sol-gel method. For example, titania is prepared by etherolysis/condensation of TiCl4 by diisopropyl ether (Equation 2.4) or by direct condensation between TiCl4 and Ti(0- Pr)4 (Equation 2.5). Detailed chemistry of the reaction was examined by means of nuclear magnetic resonance (NMR), and it has been reported that the tme precursors are titanium chloroisopropoxides in equilibrium through fast ligand exchange reactions. A variety of metal oxides, " nomnetal oxides," multicomponent oxides" " were studied, and the nonhydrolytic sol-gel method was surveyed by Vioux and Leclercq. ... 

A relatively new nonhydrolytic sol-gel method, based on the reaction between alkoxides and halides of Al, Si, Ti, etc., or between metal halides and oxygen containing organic compounds (ethers, aldehydes, ketones, etc.), was developed by Vioux and coworkers [133-138]. Structural investigations of the obtained material using Al NMR have revealed the presence of Al Al, and metastable Al sites. 

Cestari A, Bandeira LC, Calefi PS, Nassar EJ, Ciuffi KJ. Preparation of calcium fluoro-aluminosilicate glasses containing sodium and phosphorus by the nonhydrolytic sol-gel method. J Alloys Compd 2009 472(l) 299-306. 

In this section, we will provide a few selected examples for both the one- and the two-step assembly strategy, however, always restricted to nanoparticles produced by nonhydrolytic sol-gel methods. 

The description is limited to oxides obtained by the conventional sol-gel method, that is, the synthesis of oxide networks by the hydrolysis and condensation of suitable precursors, or by nonhydrolytic sol-gel methods, even if it is also possible to synthesize sulfides, phosphates, and so on. The description is provided in two parts, the first devoted to binary oxide structures and the second devoted to more complex structures. 

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