Production and Characterization of Arc-Discharged Carbon Nanotubes Using High Purity Natural Vein Graphite as Electrodes

Carbon nanotubes (CNTs) have garnered worldwide attention in the past few years due to their exceptional electrical, thermal, elastic, and functionalization properties. CNTs are a tubular form of carbon made up of a lattice of carbon hexagons folded into nanometer-scale multi-walled(MWCNTs) or single-walled (SWCNTs) tubes. Manufacturing high-quality carbon nanotubes have been expensive and challenging for industrial applications. Although production methods such as chemical vapor deposition (CVD) are currently used for large-scale production of CNTs, their quality and purity do not meet the demands of specific applications. Therefore, the arc-discharge method has been suggested as a practical approach to alleviating this problem in producing CNTs. In the arc-discharge process, carbon electrodes generate the arc between the two electrodes inside the inert gas-filled chamber where CNT is built on the cathode while consuming the anode. The cost associated with purchasing high-purity carbon electrodes hinders the production of high-quality CNT using the arc discharge method. Alternatively, high purity natural graphite electrodes, readily available in some parts of the world, can be used as electrodes obviating the need to use commercially purchased carbon electrodes. Hence an arc-discharge system was built using high-purity vein graphite as electrodes (Carbon purity of > 99.5 % wt, obtained from the Bogala mines in Sri Lanka). The two electrodes were mounted inside a closed vessel (14.34 L), where the position of the anode was fixed, and the cathode was moved back and forth using a linear actuator consisting of a stepper motor. The arc-discharging was performed inside the closed vessel filled with Argon (Ar) under the pressure of 100 mmHg and with a direct current (DC) supply of 100 A and 70 V, maintaining a 1-2 mm gap between the electrodes. Thus, by varying the arc-discharging time, CNT samples were prepared and collected carefully by scraping the cathode tip. Without further purification, the as-produced samples were characterized using Raman spectroscopy and Scanning electron microscopy (SEM). G band, D band, and noticeable radial breathing mode (RBM) were observed in the Raman spectrograph of the sample produced at 40 s(Sample A) constant arch-discharging. A slight D` split was also observed in the G band peak, indicating that the CNTs should be MWCNTs; Moreover, the D/G ratio was 0.83 (83%), which revealed a high depletion ratio, but the samples were unpurified and contained copious of amorphous carbon. In contrast, only G and D band peaks were recognizable in the sample produced with 60 s (Sample B) constant arc discharging, where no distinguishable RBM peak was observed, which is a sign of the non-existence of CNTs. The above results were confirmed by the further characterization of samples using SEM, where the dispersed CNTs were observed in the SEM image of sample A, while no CNTs were observed in sample B. In conclusion, the newly-home-built arc-discharge system was capable of producing CNTs; however, further research is required to determine the correlation between production variables and characteristics of CNTs.