Electrolysis

Although the vapor-phase preparation of CNTs will still remain as a major processing method in the foreseeable future, solid-state synthetic methods become more and more important because of their promise for large-scale production. In 1995, multiwalled CNTs were produced for the first time by the electrolytic conversion of a graphite cathode in molten lithium chloride. By passing an electric current between carbon electrodes, it had been found that formation of CNTs in condensed phase is also possible [19-21]. In these experiments, a high-purity carbon crucible (as the anode, diameter 2.5 cm, depth 3 cm) was filled with around 1 g of LiCl and heated to its melting point at 600 °C in air. A high-purity carbon rod (diameter 3 mm) was immersed in the LiCl melt serving as the cathode for electrolysis cell. During the reaction, 30 A current was maintained for 1 min. The immersed part of the cathode was eroded (small pits formed on the surface), and particulate materials (10-30 mg) were found dispersed throughout the melt. After the separation and purification, onion-like carbon polyhedral particles and nanotubes (2-10 nm in diameter, and the length longer than 500 nm) were observed with TEM technique. Interestingly, some of the nanotubes are found encapsulated with lithium chloride, oxide, or possibly lithium in their cavities. All nanocarbons prepared in this way are multiwalled, consisting of 5-20 concentric layers. Unlike those prepared with the vapor-phase techniques, the thus-prepared nanotubes are not straight, easily forming loops. Nonetheless, this electrolysis investigation shows important implications for continuous methods of nanotube production, as well as facilitating encapsulation of material within the inner cavities of carbon nanotubes.

The actual mechanism of the formation is not clear at this time. It is noted that the carbon electrode probably dissolves in the molten droplets of alkali metal [19-21]. However, one can also propose that the formation of CNTs may occur not only from pure carbon clusters/aggregates, but also from common hydrocarbon radicals like CH3- or C2H-with subsequent dehydrogenation similar to the CVD processes [32]. In addition to the electrolysis methods in fused alkali salts [19-21], a new electrochemical method for mul-tiwalled CNTs from acetylene solution in liquid ammonia has been developed at 233 K (below normal room temperature!) [32]. Single-crystal silicon («-type Si(100) wafers) was used as electrodes and no metal catalysts were further used. It is further proposed that chain radical reactions may be involved in the growth process, and some of these reactions are listed below [32]:

where C* indicates a carbon product from the synthesis (carbon nanotubes, or graphitic and turbostratic carbon, or amorphous carbon). However, unlike the carbon electrode cases [19-21], as the precursor used in this case was a hydrocarbon compound rather than pure carbon element, this electrochemical CNT synthesis does not exactly belong to "solid-state" type, but to a general condensed-phase method.

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