Multi-walled carbon nanotubes were treated with a low-pressure water plasma, and the formation of defects on the multi-walled carbon nanotube 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, defect scavenging) was adopted to interpret the observed non-linear time-dependent intensity ratio trends. The fitted activation energy for the defect formation process was found to be higher than that of the defect scavenging process. This was attributed to the fact that the OH radicals were more effective in reacting with carbon defects and impurities than with pure carbon nanotubes. The Raman-determined activation energies were also found to be excitation energy-dependent, with maximum values for red light. Such a unique dependence was characteristic of highly π-conjugated carbon systems.

Raman Study of the Temperature-Dependence of Plasma-Induced Defect Formation Rates in Carbon Nanotubes. S.Lee, Y.C.Liu, C.H.Chen: Carbon, 2012, 50[14], 5210-6