Defect-dependent annealing behavior of multi-walled carbon nanotubes

We report Raman spectroscopy and electron microscopy studies on multi-walled carbon nanotubes annealed in hydrogen atmosphere at low temperatures (<1300 °C). The pristine samples have different ID/IG ratios (ratio of the area of D peak to the area of G peak in the Raman spectra) between 0.19 and 2.6. ID/IG ratio represents defect density in graphitic structures. We find that as the annealing temperature is increased, ID/IG ratio first increases and then decreases. The annealing temperature at which the ID/IG ratio is maximum, decreases with increasing the pristine ID/IG value. We explain this by invoking a mathematical expression for ID/IG ratio, consisting of an exponential and a stretched exponential term for production and annealing of defects, respectively. The obtained activation energies and the pre-exponential factors of the two contributions are found to vary systematically with pristine ID/IG ratio. The contributions of various carbon phases to ID/IG ratio on annealing have been discussed.