As summarized above, different solid-state methods for CNT synthesis have been developed over the past decade, offering a wide range of selections for future specific applications. Nonetheless, the current status of solid-state CNT synthesis is just started. Although the solid-state methods promise possibilities for large-scale CNT production, the current research field is still far from its maturity. Many technical obstacles exist, compared to the existing vapor-phase methods. One of the unsolved difficulties is irregularity and polydispersivity of the CNTs prepared with these solid-state methods. Generally speaking, as exampled in the previous section, the overall quality of the CNTs is poorer than their vapor-phase synthesized counterparts. The second technical difficulty encountered in the solid-state synthesis is the alignment of the CNTs, because the synthetic processes are normally carried out in a less controllable manner, compared to much more sophisticated vapor-phase CNT fabrications. Furthermore, the solid precursors and starting chemicals used at this time may not be sufficiently economical and environmentally benign, as special design of the precursor molecules and more stringent reaction conditions are usually required.

In order to turn the solid-state syntheses of CNTs into practical approaches, several improvements should be made in the future. First, for example, we may have to search for new solid-precursor materials; perhaps more complex carbon-containing nanocomposites such as organic-inorganic hybrid materials will fulfill the requirements. Second, solid precursors with distinct structural anisotropy or well-defined crystal orientation are ideal candidates for the preparation of aligned CNTs (as demonstrated in Section 3.3). This class of materials may also include layered organic-inorganic self-assemblies in view of their excellent structural anisotropy and richness in carbon content. Third, the mesophase pyrolysis that has shown promising potential for large-scale CNT production is worthy of further investigations. This method may be further improved with introductions of supramolecular templates as CNT cores/micelles or tube-directing reagents in the synthesis to produce CNTs with a better monodispersivity. Perhaps pre-structured inorganic (including metal catalysts) or organic solid templates can also be introduced for carbonization and graphitiza-tion in this type of syntheses to achieve oriented growths of CNTs. Fourth, the hydrothermal synthesis of CNTs may further benefit from future investigations on template-directing growths in which organized metal nanoparticles in well-prepared thin films may act as both a catalyst and a nucle-ation template for aligned CNTs. Furthermore, in addition to the common thermal energy source used, high-energy electrons (as demonstrated in Section 3.4), photons, ions, and other extraordinary energy sources may be introduced to the solid-state synthesis of CNTs in order to generate new types of chemical reactions and thus product organizations.

It is anticipated that the vapor-phase syntheses will continue to take the lead of CNTs preparation in the foreseeable future while the solid-state synthesis merely plays a supplementary role. Nonetheless, it has been well conceived that future commercial applications of CNT materials depend on our development of cost-effective synthetic methods for large-scale CNT production. To this end, the solid-state synthesis of CNTs will gradually gain its strategic importance, and it may eventually take the lead when more sophisticated methods are in place in the future. In a broader sense, some of solid-state methods may be further coupled with gas-phase chemical reactions to devise simultaneous multiphase processes for CNT synthesis, in taking all the advantages of existing growth techniques, although this newer approach will remain challenging for many years ahead.

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