Growth conditions of double-walled carbon nanotubes in arc discharge

Preparation conditions for large-scale synthesis of double-walled carbon nanotubes (DWCNTs) by using electric arc discharge were examined. Iron, cobalt and nickel were used as catalysts, and sulfur was added as a promoter. Hydrogen and rare gases were used as buffer gases of the arc discharge. The yield of DWCNTs was sensitively dependent on the catalytic metal and the atmosphere gas, and it was deduced that the addition of sulfur into an electrode and the presence of hydrogen in atmosphere are indispensable for the selective formation of DWCNTs. In the optimal condition, DWCNTs occupied more than 90% of the synthesized nanotubes, though trace amounts of single-walled carbon nanotubes are also grown. The diameters of DWCNTs are in a range from 2 to 5 nm, and the spacings between the inner and the outer walls are about 0.38 nm wider than those usually observed for multiwall carbon nanotubes.