Morphological Modification of MWCNT Functionalized with HNO<SUB>3</SUB>/H<SUB>2</SUB>SO<SUB>4</SUB> Mixtures

Abstract

The acidic oxidation with HNO3/H2SO4 mixtures is widely reported as an effective method to functionalize multi-walled carbon nanotubes (MWCNT). Although effective, a bad control of the oxidation conditions frequently cause serious modifications of carbon nanotube network, limiting further potential applications. Investigations about the effect of functionalization operating conditions on the morphological, chemical and chemical-physical properties of MWCNT can be useful for a proper setting of oxidation reactions of MWCNT according to their specific applications. In this work the effect of HNO3/H2SO4 ratio on the morphological and chemical-physical properties and on the degree of functionalization of MWCNT was investigated. Electron microscopy, thermogravimetric, X-ray diffraction, titration and water dispersion analyses clearly revealed that the increase of the amount of concentrated sulphuric acid in the HNO3/H2SO4 mixture lead to an increase of the amount of functional groups on the MWCNT surface but also to an increase of structural damage in terms of tube cutting and generation of additional defects in the graphitic network of pristine

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“…The oxidative treatments used in this study (H 2 SO 4 /HNO 3 mixture) remove amorphous carbon materials, while simultaneously introducing sidewall defects. Thus, in agreement with our previous results [37] and with literature data [42], the I D /I G band intensity ratio decreases significantly compared to the pristine MWCNT, and this effect is presumably due to the effective removal of amorphous carbon from the MWCNT surface. Raman analyses of pristine and oxidized MWCNT, recorded both exciting the sample with k 532 and 633 nm show a structural modification after the oxidative treatment while not structural modification was observed in the final conjugated system, after the last functionalization steps, in accordance with what expected.…”

“…MWCNT were produced by catalytic chemical vapor deposition (CCVD) from isobutane on a Fe/Al 2 O 3 catalyst; synthesized MWCNT were then subjected to purification as previously reported, giving pristine MWCNT (p-MWCNT) with purity >95% [36]. The oxidized MWCNT (ox-MWCNT) sample was prepared by oxidation of purified pristine MWCNT using a mixture of nitric acid and sulfuric acid (1:3) according to our previously reported procedure [37]. Lamivudine was purchased from Aldrich.…”

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“…HNO 3 is the most common reagent for the oxidation of nanotubes, which can selectively remove amorphous carbon and metal catalysts and, at the same time, generate oxygenated groups at the exposed CNT surface [23]. However, acid attack, most frequently performed under boiling conditions, is accompanied by severe degradation effects, including material’s loss and selective removal of metallic nanotubes, tube shortening, as well as the formation of structural defects and carbonaceous debris [24,25,26]. The latter (e.g., SN) were obtained upon the application of a more aggressive acid treatment with a mixture of HNO 3 and H 2 SO 4 that not only promoted the functionalization with oxygenated groups, but also shortened their original length to ca.…”

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“…After activation, nano tube fragments with various oxygen-containing groups (mainly carboxyl groups) attached at the ends and sidewalls were obtained [12] Figure2a and b presents the schematic of the activation of MWCNT using HNO 3 and a mixture of 1:1 ratio of H 2 SO 4 +HNO 3 During the activation process of MWCNT the long carbon nano tubes get ruptured and the carbon atoms at the rupture point become reactive centers for further oxidation. Several functional group containing oxygen (CO, COOH, OH, lactones, phenol, hydroquinones and ether groups) will be inserted on the surface of carbon nano tubes [8]. Ultrasonic agitation also aids the oxidation and consequently the dispersion of MWCNT in water due to increased functionalisation.…”

“…Additionally, the CNT tips have to be activated to improve their physical and chemical affinity to the solvents and other additives in the bath. One way would be to oxidize them by using appropriate acids which leads to attachment of different functional group to the CNT structure [8]. Infact, activation with acids also improves their dispersion.…”