Formation of Nanostructured Carbons under Hydrothermal Conditions

The formation of carbon nanostructures (fullerenes, nanotubes, nanofilaments, etc.) in gaseous media and/or vacuum has been widely studied. On the other hand, their formation from liquids or high-pressure, high-temperature fluids has received limited attention. We have recently demonstrated synthesis of various carbon nanomaterials under severe hydrothermal conditions. Multiwall open-end and closed carbon nanotubes (MWNT) with the wall thickness from five to more than 100 carbon layers were produced from polyethylene and ethylene glycol with Ni under hydrothermal conditions at 700–800 °C under 60–100 MPa. An important feature of hydrothermal nanotubes is a small wall thickness, which is about 10% of the large inner diameter of 20–800 nm. They combine a large inside diameter and a highly graphitic wall structure. The exceptional quality of the tube walls is demonstrated by the straight and perfect lattice fringing in the tube walls and also by the electron diffraction spot patterns, which are free of diffuse amorphous rings. Raman spectral evidence shows a high similarity to the spectra obtained from high-purity natural graphite. Closed nanotubes were leak-tight by virtue of holding encapsulated water at high vacuum in a transmission electron microscope (TEM). Other carbon sources, such as fullerenes, single-wall carbon nanotubes (SWNT), and amorphous carbons can produce similar MWNTs under similar hydrothermal conditions. Tubes were also synthesized without water in C-H and C-H-O systems, but these tubes had multiple internal caps while those produced with water present had very few internal obstructions and a large inside diameter. Bamboo-like filaments can be produced by decomposition of paraformaldehyde with no water added. Narrow-channel polyhedral nanotubes and other carbon nanoparticles were obtained by varying the synthesis conditions. Starting from amorphous carbon materials, we obtained hollow nanotubes with 10–50 nm outer and 2–8 nm inner diameters and aspect ratios of 20–100. Unique chain-like nanocells with graphitic walls of 30–200 nm have been prepared from amorphous carbons above 600 °C.