Hydrothermal Synthesis

Hydrothermal and solvothermal methods have long been used for inorganic materials synthesis. Since the finding of CNTs, these methods have also been utilized widely for separation and purification of as-grown CNTs, including both single- [77, 78] and multiwalled CNTs [79]. More importantly, hydrothermal processes for the formations of amorphous carbon as well as CNTs had been developed in recent years [80-87].

In these hydrothermal reactions, solid-state carbon-containing materials such as amorphous carbon [84], fullerene carbon (e.g., C60 [85]), and polymeric carbon (e.g., polyethylene [86]) have been adopted as starting materials of carbon source. Using amorphous carbon and distilled water, carbon nanocells can be prepared at 600 °C in an autoclave without using metal catalysts [84]. The straight segments of the polygonal cell walls show a well-defined lattice with an interlayer spacing of about 0.33 nm which corresponds to the 002 distance of a graphitic lattice. The graphitic multiwalls grew as a result of the hydrothermal treatment of amorphous carbon. The interpenetrating carbon nanocells form a very porous texture. At 800 °C and 100 MPa, the carbon products showed as aggregates of needle-like or polygonal nanoparticles. The CNTs formed have diameters in the range of tens and lengths in the range of hundreds of nanometers. It is recognized that the mechanism by which amorphous carbon rearranges into curled graphitic onions is more complex than in multisheet graphitic carbon produced in the gas-phase reactions and involves debonding of graphitic clusters from the bulk carbon material due to a catalytic effect of hydrothermal fluids [84].

The behaviors of fullerene C60 have also been investigated with the similar hydrothermal treatments in the temperature range of 200 to 800 °C and 100 MPa [85]. It is found that the fullerene molecules can survive in water up to 400 °C for 48 h under the studied hydrothermal conditions. However, they gradually transform to amorphous carbon when the heating temperature or time is increased. Note that C60 is thermally stable without water. Molecular dynamic studies have shown that C60 is stable up to over 4000 ° C in the carbon-only system [88]! At 700 °C, high-quality open-ended multiwalled CNTs are formed in the vicinity of nickel particles (which act as a catalyst). The typical outer diameter of the hydrothermally formed CNTs is about 30-40 nm, and the wall thickness is ca. 5 nm. The observed results clearly indicate that water strongly accelerates the transformation of fullerenes. This hydrothermal process is kinetically controlled, and longer reaction time will shift the stability range to lower reaction temperatures.

Using polyethylene (PE, high density) sheets or ethylene glycol (EG) as a carbon source, the hydrothermal formation of nanocarbons has been investigated in a greater depth with a two-step process [86]. In step one, a post-pyrolysis C—H—O equilibrium is established, and in step two, the growth of graphitic carbon occurs with an increase in reaction pressure. Depending on the size of metal particles (Ni as a catalyst), flake-like graphite or CNTs (70-1300 nm in diameter) were generated. Without using water, the CNTs synthesized had multiple internal caps. On the contrary, very few internal obstructions and a large inner diameter were observed when water was used in the synthesis. The quality of the CNTs is exceptional, while Ni particles in the carbon tube tips can be easily found. The presence of water appears to be responsible for the large internal channels and highly graphitic nature of the carbon deposited. More importantly, high integrity of hydrothermally synthesized

CNT walls

Liquid Phase Gas Bubble

Figure 7. CNTs synthesized with hydrothermal methods. Both gas bubbles and liquid entrapments can be observed in the inner cavity of the as-prepared CNTs.

CNTs is demonstrated by the existence of gas/liquid inclusions trapped inside the tube cavities, as shown in Figure 7.

Regarding the formation mechanism of CNTs under hydrothermal conditions, it has been recognized that the precursor chemistry is not critical in the process, with the most crucial factor being the C—H—O equilibrium established in step one. Because EG/water and PE/water mixtures yield very similar pyrolytic carbon products, it is believed that the C—H—O equilibrium compositions in the two systems are very similar, and the equilibrium species of importance are CO, CO2, H2O, CH4, and H2 in these cases. The injected gaseous species can be converted to desired nano-carbons via catalytic carbon condensation at higher temperatures or pressures. Finally, it should be mentioned that the liquid-encapsulated CNTs synthesized in this type of hydrothermal methods can be used as nanofluidic devices and as lightweight reinforcement for composites [86, 87].

The above hydrothermal synthesis can be further extended to organic solvent conditions (also called solvothermal synthesis). Recently, synthesis of CNTs had been carried out using a so-called "benzene-thermal-reduction-catalysis" route [89]. In this method, a refluxed mixture of FeCl3 • 6H2O and HAuCl4 • 3H2O (Au:Fe molar ratio = 1:1) in thionyl chloride (SOCl2) and AgCl were used respectively as catalysts. A typical synthesis can be summarized as follows: 8 mL of tetrachloroethylene (C2Cl4), 10 mL of benzene, 5 g of potassium, and 100 mg of catalyst precursor were added sequentially into a stainless steel autoclave, which was then heated at 200 ° C for 27 h, and cooled down to room temperature naturally. The TEM investigation for the synthesized products indicates that CNTs are produced in a straight form with lengths of 1.8 ^m, inner diameters of 60 nm, and outer diameters of 80 nm on average. The walls of CNTs are multiwalled, and the yield is about 15% from the original reagents through electron microscopic observation. In addition to the CNTs, carbon nanorods are also formed with this method, with a yield of about 60%. It is believed that the freshly reduced C2 (from C2Cl4) may also assemble into one-dimension conjugate carbon chain clusters. This could be the reason for the formation of carbon nanorods observed in the presence of Ag catalyst in the synthesis [89].

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