Calibration of reaction parameters for the improvement of thermal stability and crystalline quality of multi-walled carbon nanotubes

The results of Raman analysis on multi-walled carbon nanotubes, prepared by catalysed chemical vapour deposition, are used as a guide for the calibration of the growth parameters, directed to improve crystalline quality and resulting thermal stability of nanotubes. Under selective growth conditions, the resistance to oxidation in air, as assessed by thermogravimetry measurements, is found to increase with the establishment of the long-range graphitic order in radial tube direction, as signalled by the Raman G′/G intensity ratio enhancement. In the range of parameters explored (synthesis temperature: 500–700 °C; growth atmosphere: 120 cc/min i-C₄H₁₀–H₂–He mixture with He at 0–25%; i-C₄H₁₀/H₂ flow ratio: 1–3; metal load and reduction temperature of Fe/Al₂O₃ catalysts: 17–40 wt%, and 500 and 700 °C, respectively), the best crystalline quality and the highest oxidative resistance are achieved by carrying out the synthesis reaction at 700 °C in 1:1:0 i-C₄H₁₀–H₂–He atmosphere over 29 wt% Fe catalysts reduced at 700 °C. An additional relevant finding is the strong correlation evidenced between results of thermogravimetry and Raman analyses, suggesting the use of Raman spectroscopy for non-destructively evaluating the thermal stability of any graphitically ordered carbon species.