Types Of Synthetic Methods

There have been numerous synthetic methods for nano-tubular and other low-dimensional materials [1-6]. For example, the preparation methods of CNTs range from the most common techniques such as high-voltage arc-discharge, double laser vaporization, pyrolysis, and chemical vapor deposition [1-6], to the rare types like synthesis of CNTs under a focused sunlight [5]! Concerning the mass transport during the synthesis, these methods can be broadly divided into two categories—vapor-phase and condensed-phase processes. The vapor-phase methods involve processes in which gaseous reactants are transported through vapor phase and converted into solid/liquid phase in the growing fronts of materials. These methods include arc-discharge [7], thermal decompositions (pyrolysis) [8, 9], chemical vapor deposition [10-12], laser catalytic growth (LCG) [13, 14], and pulsed laser ablation [15, 16], and various physical (possibly also with additional chemical) vapor depositions [17, 18], etc. The technical merits of vapor-phase processes have been laid largely on the production of highly oriented crystalline materials such as thin films with excellent crystal perfection, although their growth rates and thus the product yields are normally not high. Compared to the vapor-phase methods, conventional syntheses with condensed-phase reactions can offer much higher production yields. In this regard, solidstate reactions and electrochemical processes have also been widely used nowadays in nanostructured materials synthesis. In fact, soon after the discovery of the first CNTs [7],

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 10: Pages (57-66)

condensed-phase preparation of CNTs had been attempted with an electrolysis method by passing an electric current between carbon electrodes in molten lithium chloride that acted as an electrolyte [19, 20]. Other electrochemical methods, such as anodization processes, have also been utilized in preparations of alumina oxide membranes with controlled 1D channels and individual alumina nanotubes [22]. In recent years, low-energy and environmentally benign processes such as liquid-phase deposition (LPD) analogous to "biomimetic" mineralization, have generated significant interests [23-29] in the nanomaterials research. For instance, TiO2 crystalline films and nanostructures with desired crys-tallographic phase(s) have been prepared by hydrolyzing titanium inorganic salts (for example, (NH4)2TiF6, TiF4, and TiCl4, etc) in aqueous solution at a low reaction temperature on various substrates including metal oxides and organic polymeric materials [23-29]. It should be mentioned that in the above LPDs, source chemicals exist in the movable forms of molecules (or complexes), ions, or clusters in the liquid phase prior to transport/conversion to solid phases. In this sense, these condensed-phase reactions are analogous to the vapor-phase reactions, except for the solvation effects for the starting chemicals and other known physical processes in the solution phase.

In this chapter, we will limit ourselves only to material syntheses involving conventional solid-state reactions; that is, one or more source chemicals are in the form of solid precursors, and most likely at elevated temperatures or with other external energy inputs (e.g., electrolysis), for the formation of nanotubular materials. Certainly, we realize that a strict classification of the reviewing scope cannot be done so simply, as in many cases multiple reaction steps (and thus possibly multi-techniques) are applied to a particular synthesis. In such cases, the context of "solid-state synthesis" of this chapter is extended to any synthetic processes in which one or more of steps involve solid-state reactions, including dissolutions of solid phases. It should be mentioned that although multiwalled CNTs have been transformed directly from solid carbon precursors without using metal catalysts in high-temperature arc furnaces at 2200-3000 °C [30, 31], the synthesis of this type still belongs to the arc-discharge method in general.

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