Fabrication, purification and characterization of multiwall carbon nanotubes

Abstract

Multiwall carbon nanotubes (MWCNTs) have numerous potential applications which can be attributed to their outstanding electrical conductivity, mechanical strength and thermal conductivity. However, as-grown multiwall carbon nanotubes (a-MWCNTs) usually contain large amount of impurities or by-products in the form of carbonaceous and metal particles, which hinder and limit the technological implementation of MWCNTs. This study is focused on fabricating MWCNTs by arc discharge and developing an efficient purification route based on gas and liquid phase oxidation. The MWCNTs are fabricated by developing arc discharge carbon plasma between graphite rods by applying 20 V dc voltage and 100 A current at atmospheric pressure. The purification of as-grown MWCNTs is performed in two stages. In first stage, the as-grown MWCNTs are subjected to heat treatment at 750 oC at atmospheric pressure. The second stage involves chemical oxidation using mixture of HNO3 and H2O2 in different ratios via refluxing, sonication and drying processes. The role and impact of oxidizing agents such as HNO3 and H2O2 are investigated in details and optimized. The synthesized and purified MWCNTs are characterized by thermogravimetric analysis (TGA), Raman spectroscopy, X-ray diffraction (XRD) and field emission electron microscopy (FESEM). The shift in the (002) peak towards lower angle in XRD spectra and low value of ID/IG ratio from Raman spectra suggest improvement and increase in the presence of tubular structure for the chemically oxidized MWCNTs with HNO3:H2O2 ratio of 3:0. A decrease in amorphous structures with no nanoparticles attached with nanotubes is observed for the MWCNTs samples treated by thermal and chemical oxidation for HNO3:H2O2 ratio of 3:0 and 2:1 in the scanning electron microscopic images. Significant increase in the as-grown MWCNT density was observed from MWCNT samples thermally oxidized at temperature 750 ºC and chemically oxidized with HNO3:H2O2 ratios of 3:0 and 2:1.