| Nanocrystals (NCs) of defined morphologies are the building blocks of many applications at the nanoscale. When materials shrink below certain critical sizes, unusual properties arise, e.g., semiconductors will have quantized electronic energy levels, and ferro-/ferri-magnetic materials will show superparamagnetism. Different properties' critical sizes may not coincide, but all remain at the nanoscale, i.e., dimensions of 10-9 to 10-7 m. The synthesis of NCs of controlled size and shape has attracted the attention of a large group of scientists, and a wide array of synthesis methods have been developed for doing so. In general, the methods can be categorized into "bottom-up" and "top-down" methods. In bottom-up approaches, the nanomaterials are constructed from smaller building blocks, e.g., atoms, molecules, or even component nanocrystals; in top-down approaches, the nanostructures are carved out of larger, even bulk materials. Solution phase synthesis methods, as a major category of bottom-up methods, have been expanded into a few subcategories differing in reagents and procedures, among which the high temperature decomposition (HTD) methods are considered highly facile and reliable. The details of solution phase synthesis have been laid out in Chapter 1, with a focus on transition metal oxides. Subcategories such as sol-gel, hydrothermal, solvothermal, HTD, and micelle/reverse micelle methods are introduced and compared in detail there. Representative morphologies of NCs are presented and classified based upon their dimensions. Common phases of titanium oxide, vanadium oxide, manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, and zinc oxide are listed and illustrated for comparison. A systematical review of the formation mechanism of nanocrystals in solution phase synthesis is presented to weave the methods together. The solution phase synthesis methods may differ from aspects such as reagents, apparatus, and reaction conditions, but they are interlocked in an overall mechanism atlas, Figure 72.;HTD methods have an average reaction temperature of ~250 °C and time of 1~2 h. The morphological control of the resultant NCs at such high temperature within such short duration has been a pressing issue. Here we studied the mechanism of the HTD methods with two model cases: monodisperse Fe3O 4 nanocrystals and hierarchical Ni3C nanostructures.;Magnetite (Fe3O4) nanocrystals (MNCs) are among the most-studied magnetic nanomaterials, and many reports of solution-phase synthesis of monodisperse MNCs have been published. However, lack of reproducibility of MNC synthesis is a persistent problem, and the keys to producing monodisperse MNCs remain elusive. In chapter 2, we define and explore the synthesis parameters thoroughly to reveal their effects on the final product MNCs. We demonstrate, for the first time, the essential role of benzaldehyde and benzyl benzoate produced by oxidation of benzyl ether, the solvent typically used for MNC synthesis, in producing monodisperse MNCs. This insight allowed us to develop stable formulae for producing monodisperse MNCs, and propose a model to rationalize MNC size and shape evolution. Solvent polarity controls the MNC size, while short ligands shift the morphology from octahedral to cubic. We also demonstrate preparation of specific assemblies with these MNCs. This standardized and reproducible synthesis of MNCs of well-controlled size, shape, and magnetic properties is expected to be an example of stabilizing and expanding the existing protocols for nanocrystal syntheses, and will drive practical advances including enhanced MRI contrast, higher catalytic selectivity, and more accurate magnetic targeting.;The similar categories of synthesis methods apply to carbides as well. Rhombohedral nickel carbide, Ni3C, is a common catalyst in petroleum refining and electrochemistry. For a fixed amount of material, hierarchical structures have larger surface area than the corresponding dense structure, e.g. a sphere, cube, or platonic solid. Very limited results of solution phase synthesis of Ni3C have been reported. We studied the synthesis of a hierarchical nonporous Ni3C structure with high specific surface area by thermal decomposition of an organometallic nickel precursor in a mixture of ligands in a non-coordinating solvent.;In short, we delved into the methodology of solution phase synthesis, and studied two model cases to develop high-quality nanocrystals for potential applications. |
