Stability of Multi-Wall Carbon Nanotube and Graphite in Acids

Abstract

Multi-walled carbon nanotubes (MWCNT) were treated in five commonly used acidic media with different pKa values. The stability of the MWCNT was evaluated by using various characterization techniques. The results indicated that oxidative damage of MWCNT was not closely related to acid strength, and that the MWCNT was more sensitive to the oxidizing nature of the acid. The oxidation of highly oriented pyrolytic graphite (HOPG) by chemical oxidants in air has been investigated. The oxidation of HOPG top surface was carried out by treating with H2SO4/HNO3 mixture. The samples for oxidation were prepared by molding in polyethylene. To evaluate the effects of oxidants on surface of graphite various techniques have been applied to study the damage on surface of HOPG. As a result, the MWCNT treated with the strongest acid, trifluoromethanesulfonic acid (pKa = -15.9), only reached oxygen content of 1.8%, while nitric acid (pKa = -1.5)/sulfuric acid (pKa = -3.0) treated MWCNT showed oxygen content as high as 19% after 24 h treatment. The remaining acidic media did not noticeably increase the oxygen content of the MWCNT up to 24 h treatment. The oxidized HOPG surface reached its oxygen content of 20.9% after 4 days of treatment. The results are supported by gradual increase of the intensity of D-band in Raman spectra with respect to time.