Modification of Multi-Wall carbon nanotubes using controlled microwave Irradiation: Experimental determination of the enthalpy change

Abstract

Irradiation of multi-wall carbon nanotubes (MWCNT) was performed at different energy densities to characterize structural changes induced by a highly localized 2.45 GHz electric field. For this purpose, a transversal electric (TE102) resonant microwave cavity was designed and coupled to a standard 800 W microwave oven magnetron. The nanotubes were sealed inside a Pyrex reactor at 10-4 Torr to avoid oxidation or reactions with the environment. Irradiated MWCNT properties were characterized using Raman Spectroscopy, Fourier Transform Infrared Spectroscopy, Thermogravimetry, Solubility tests and Transmission Electron Microscopy. In this work, the energy densities required to modify the MWCNT structure were obtained through an optical method. From an energy density of 82 J/cm3, nanotubes presented nondestructive structural changes, loss of catalytic particles and a decrease of amorphous carbon structures on the surface. A more effective modification with high purity and clearly better structured walls was observed for 167 J/cm3. Furthermore, it was determined that at an energy density of 6.9 KJ/mol a higher purification of MWCNTs is obtained. This value is in agreement with the enthalpy of formation reported for the MWCNTs. This experimental approach could be applied for the functionalization of carbon nanostructures. Finally, the purification process is easily scalable, environmentally friendly and makes maximum use of the irradiated energy.