Characterization of multiwalled carbon nanotubes by DC ARC discharge in methane under magnetic field influence

Abstract

Carbon nanotubes (CNTs) have gained many interest among researchers over the last two decades due to its remarkable mechanical, electrical, optical and thermal properties. High quality CNTs are in demand especially for application in nano electronics where CNTs are required to be in high crystallinity, straight and aligned orientation, having uniform diameter and less impurities to achieve the best performance. Literally, hydrogen gas is reported as the best buffer gas in producing high crystallinity and less impurities attached to CNTs by arc discharge method. However, it is not suitable for large scale CNTs synthesis due to unstable plasma formation. Recently, methane gas which contains hydrogen atoms is being studied in producing multiwalled carbon nanotubes (MWCNTs). This leads to the opportunity of investigating methane as buffer gas in producing high quality CNTs. On the other hand, the usage of magnetic field in arc discharge has been reported to have the ability to enhance the quality of CNTs in terms of narrow and uniform diameter as well as reducing impurities. Thus, this work presents a comparative study on the effect of three different arc discharge configurations to the yield of MWCNTs in methane environment. The first configuration known as Configuration A where no magnetic field assistance during CNTs arc discharge synthesis. Configuration B utilises four (4) magnets which are placed surrounding inter electrode gap while two (2) magnets are placed at anode for Configuration C. Arc discharge is generated at fixed 750 mbar of chamber pressure and fixed current about 60 A in the voltage range of 30 ~ 32 V. As a result, needle like shapes with straight orientation of individual MWCNTs for all configurations is observed under Scanning Electron Microscope. Narrow diameters are observed in configuration B with standard deviation of 2.71 mm followed by configuration C of 5.7 mm and configuration A of 8.05 mm. The results show the influence of magnetic field in producing MWCNTs with narrow and uniform diameter compared to no magnetic field assistance. The diameter distribution trend is confirmed by X-Ray powder diffraction results. High crystalline MWCNTs is confirmed by Transmission Electron Microscope images for configuration B with uniform MWCNTs inner diameter at average 2 nm. Raman spectrum shows low ratio of D band intensity over G band intensity at 0.53 for configuration B while configuration A at 0.79 which suggest fewer wall defects of MWCNTs produced in configuration B. Therefore, magnetic field assistance in methane arc discharge is proved to produce smaller and uniform diameter of MWCNTs with less wall defects. MWCNTs produced in this study can be further investigated in nanoelectronics applications such as nanowires and conductive nanofiller