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 degrees C). The pristine samples have different I-D/I-G ratios (ratio of the area of D peak to the area of G peak in the Raman spectra) between 0.19 and 2.6. I-D/I-G ratio represents defect density in graphitic structures. We find that as the annealing temperature is increased, I-D/I-G ratio first increases and then decreases. The annealing temperature at which the I-D/I-G ratio is maximum, decreases with increasing the pristine I-D/I-G value. We explain this by invoking a mathematical expression for I-D/I-G 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 I-D/I-G ratio. The contributions of various carbon phases to I-D/I-G, ratio on annealing have been discussed. (C) 2008 Elsevier B.V. All rights reserved.