Chemical Reaction and Crystallization Mechanisms

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]. 

But the chemical reactions provide only half of the information on the way to establishing a complete nanoparticle formation mechanism. The other half includes all types of crystallization processes such as prenucleation, nucleation and growth, and assembly and agglomeration. Size, shape, and size distribution of the nanoparticles are strongly dominated by crystallization processes. The tremendous variety of nanoparticle morphologies and architectures made it necessary to expand classical crystallization theory to new concepts such as oriented attachment, particle-based crystallization, or mesocrystal formation [154]. 

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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