Overview of the Main Chemical Reactions

Most of the principles of nonhydrolytic sol-gel chemistry have been developed a long time ago. At that time, gels rather than nanoparticles were in the focus of interest. For example, Bearing and Reid reported already in 1928 the preparation of a nonaqueous sihca gel by reacting ethyl orthosilicate with acetic acid [155]. 

In spite of a few other subsequent publications, nonhydrolytic routes became popular to a wider commimity only in the 1990s, when especially Corriu, Vioux, and coworkers published their studies on monolithic gels [156,157]. However, due to space limitation, we will restrict our overview to metal oxide nanoparticles. More information on earlier work, including gek, can be found in a review article published in 1997 [15]. Several other reviews followed, providing a more up-to-date overview of nonhydrolytic sol-gel chemistry [4,8,10,158-161]. 

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. 

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We will start with a brief overview of the main chemical reactions involved in the formation of metal oxide nanoparticles in nonaqueous systems, followed by a short introduction to the main principles of classical and nonclassical crystallization. We restricted these first two subtopics to a minimum, because on the one hand there is already a large amount of literature available (selected references are provided), and, on the other hand, we have more space for the last part, which is dedicated to the presentation of selected and recently reported studies on mechanistic aspects of nanoparticle formation. The chosen examples come... 

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