Several methodologies have been developed for the synthesis of metal nanoparticles. Their first natural classification is obviously into ‘‘top—down” and ‘‘bottom—up’which can also be considered as physical and chemical methods. Respectively. Within the bottom—up category. a wide variety of synthetic methods have been developed for preparing nanoparticles and reported in the literature such as sol—gel process, hydrothermal synthesis. Thermal decomposition of organo metallic compounds. Electrochemical reduction. Radiolytic reduction. Photochemical reduction, chemical reduction of metal salts, etc. ‘‘in a typical synthesis of metal nanoparticles, a precursor compound is either decomposed or reduced to generate zero valent atoms,—the building blocks of a metal nanoparticle. The Colloidal chemical synthesis of the metal nanoparticles in aqueous and/or organic solution (often termed “wet chemistry”, in which chemical reaction takes place in the liquid phase) is the most widely practiced method. Chemical reduction of the metal salt(s) precursor in solution in the presence of a stabilizing agent. And stabilizing agent prevents aggregation and improves the chemical stability of the formed nanoparticles is the most commonly used technique. In many cases the surface—modifying agents. Used throughout the chemical synthesis, act both as stabilizers and shape directing agents.
In general there are two routes for the preparation of Nanopartciles -Top Down and Bottom-Up. These two routes are those that reduce the macroscopic particles to the nano scale. Top Down route is not very suitable to prepare uniform particles of very small sizes. In contrast, with the bottom-up it is possible to obtain uniform particles (usually of different shapes and structures). These two routes start from atoms that can be added (either in solution or gas phase) to form larger particles.
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