Time dependent thermal treatment of oxidized MWCNTs studied by the electron and mass spectroscopy methods

Purified and functionalized in boiling concentrated (68%) HNO3 acid the oxidized multiwall carbon nanotubes (ox-MWCNTs) under thermal treatment from RT to 630 degrees C and at 350 degrees C time dependent (1-4 h) were investigated using the surface sensitive electron and mass spectroscopy methods. Mass spectroscopy indicates significant desorption of H-2 and H2O to about 300 degrees C. Higher H-2 desorption rate from RT up to about 100 degrees C is most likely caused by decomposition of organic acid impurities included within a bundle and in channels of the ox-MWCNTs after their functionalization by HNO3. In the range of 100-300 degrees C part of the detected H-2, accompanied by desorption of CO2, may origin from desorbed water. Above 300 degrees C, the small amount of desorbing H2O may result from transformation of carboxylic groups into carboxylic acid anhydride. Significant desorption of CO2 starting from 150 degrees C may result from decomposition of carboxylic groups, whereas desorption of CO starting at about 300 degrees C from decomposition of acid anhydride groups created from carboxylic groups during thermal dehydration. Desorption of CO and CO2 at about 470 degrees C may be due to decomposition of hydroxyl O-H and carbonyl C=O groups. Above 600 degrees C mainly decomposition of C=O groups takes place and results in small desorption of CO. Time dependent (1-4 h) thermal treatment of ox-MWCNTs at 350 degrees C shows in XPS spectra decreasing amount of C=O in carboxyl groups and increasing amount of C=O in carbonyl and acid anhydride groups arising from carboxyl groups decomposition. Between 350 degrees C and 470 degrees C the higher desorption rate of CO2 than CO indicates significant decomposition of carboxyl and carboxyl anhydride groups. At 350 degrees C the dynamic changes are indicated by the energy, intensity and full width at half maximum (FWHM) of the pi -> pi* interband transition and pi loss peak, and quasi-elastic peak FWHM. During 4 h at 350 degrees C no C sp(2) reconstruction is observed. For the applied procedure of MWCNTs oxidation, large amount of water and some organic acid impurities, resulting from the MWCNTs oxidation, remain in the CNTs channels, interstitial channels between tubes and at nanotubes surface. (C) 2012 Elsevier B.V. All rights reserved.