Optimization of Chemical Vapor Deposition Process for Reducing the Fiber Diameter and Number of Graphene Layers in Multi Walled Carbon Nanocoils

Multi walled carbon nanocoils (MWCNCs) were synthesized by chemical vapor deposition (CVD) and the experimental parameters were optimized to reduce their fiber diameter. The conditions for the synthesis of the thinnest MWCNC in this experiment were as follows: reaction temperature, 700 °C; C2H2/N2 pressure, 0.67 kPa; and C2H2/N2 ratio, 0.01. A low C2H2 gas flow rate and a low partial gas pressure were important in reducing the fiber diameter. The reaction temperature affected both the MWCNC fiber diameter and purity, which depends on the content of MWCNCs and multi walled carbon nanotubes (MWCNTs). At high temperatures (≥750 °C), MWCNTs were predominant and their crystallinity increased, which was confirmed by the detection of the radial breathing mode and high intensity ratios of the G peak to the D peak in the Raman spectra. By contrast, MWCNCs were produced preferentially at low temperatures (approximately 700 °C). Transmission electron microscopy showed that the fiber diameter of the thinnest MWCNC was less than 5 nm at both the helix and tip and that the thinnest MWCNC had a triple walled structure. Under optimized conditions, the vacuum deposition of a thin film of Sn on a Si substrate and the mounting of Fe catalyst supported zeolite on a Sn/Si substrate effectively increased MWCNC purity. MWCNC purity was improved by up to 30%, which is the highest purity we have observed thus far.