CCVD SYNTHESIS OF SINGLE- AND DOUBLE-WALLED CARBON NANOTUBES

摘要

We describe the synthesis of single and double-walled carbon nanotubes by an original catalytic chemical vapor deposition method based on the selective reduction of oxide solid solutions in H_2-CH_4 atmosphere. For example, the reduction of an α-Al_(2-2x)Fe_(2x)O_3 solid solution produces pristine Fe nanoparticles at a temperature high enough to catalyze the decomposition of CH_4 and the in situ formation of carbon nanotubes (CNTs). The so-obtained products are CNTs-metal-oxide composite powders with a very homogeneous dispersion of the CNTs. This method was extended to Mg_(1-y)M_yAl_2O_4 and Mg_(1-z)M_zO solid solutions (with M = Fe, Co, Ni or a binary alloy), to obtain CNT-M-MgAl_2O_4 and CNT-M-MgO composites powders, respectively. The use of MgO-based catalysts is of special interest because of their easy removal by dissolution in an HCl aqueous solution, allowing the preparation of isolated CNTs. Some control of the number of walls and diameter distribution can be achieved through the CCVD parameters, such as the composition of the H_2-CH_4 atmosphere, the reduction temperature and the time spent at that temperature. A global overview of the CCVD synthesis of single- and double-walled carbon nanotubes is proposed through a review of some of the most striking results in the field.