Purifying double-walled carbon nanotubes by vacuum high-temperature treatment

Double-walled carbon nanotubes (DWNTs), produced by the chemical vapour deposition method with Fe/MgO catalyst, were purified by acid dissolution and high-temperature evaporation. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetry analysis (TGA), Raman spectra and Brunauer-Emmett-Teller analysis (BET) were used to characterize the specimens in these processes. The results show that the MgO and Si impurities in the raw specimen are effectively dissolved by HCl and HF solutions, respectively, and the residual Fe particles, covered by a carbon shell, can be further removed by evaporation at vacuum pressures 10(-3) -10 Pa and high temperature 1700-2200 degrees C. The initial burning temperature of the purified DWNTs increases up to 738 degrees C with temperature treatment, mainly because of the removal of the catalyst and defects in DWNTs. Although high-temperature treatment is good for the Fe evaporation process, it may destroy the structure of DWNTs if the temperature is too high. It is indicated that 1700 degrees C is an appropriate temperature for purifying DWNTs. After 1700 degrees C treatment, the carbon content of the specimen rises above 99% and the specific surface area is 598 m(2) g(-1).