Raman study of the temperature-dependence of plasma-induced defect formation rates in carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs) were treated with a low-pressure water plasma, and the formation of defects on the MWCNT surface was monitored via the changes in the Raman D band to G band intensity ratio that occurred with different plasma treatment times and different temperatures. A kinetic model with two competing processes (defect formation and defect scavenging) was adopted to interpret the observed nonlinear time-dependent intensity ratio trends. The fitted activation energy (Ea) for the defect formation process was found to be higher than that of the defect scavenging process. This was ascribed to the fact that the OH radicals were more effective in reacting with carbon defects and impurities than with pure CNTs. The Raman-determined Ea's were also found to be excitation energy-dependent, with maximum values for red light. Such a unique dependence is characteristic of highly π-conjugated carbon systems.