Purificação e caracterização de nanotubos de carbono produzidos pelo método de descarga em arco elétrico

Hélio Ribeiro

The objective of this work was to study the purification of a sample of commercial carbon nanotubes (CNTs) produced by the Carbolex arc-discharge method using the catalyst mixture Ni/Y (4/1 at. %). Another objective of this work was to investigate an alternative functionalization route for the introduction of hydroxyl groups (-OH) onto CNT sidewalls and to evaluate the dispersability of the as-grown, purified, and functionalized samples in different solvents. A soft purification method based on the combination of gas-phase oxidation treatments, followed by the liquid-phase oxidation steps in an aqueous HCl solution with a minimal introduction of defects and functional groups was investigated. In view of the great challenges faced in purification studies, most of the work was dedicated to this stage. The CNT morphology and structural properties were characterized after each purification procedure to monitor the degree of efficiency of the purification procedures, as well as the effect of them on tube integrity. The characterization of the asgrow sample by several techniques, revealed a high degree of metallic impurities (42 at% of Ni, or 59% w/w of Ni2O3), amorphous carbon and graphitic nanospheres with different degrees of graphitization. TEM showed that most of the metallic nanoparticles were encapsulated by graphitic layers and covered by amorphous carbon. Part of the metal could be analyzed by XPS, which indicated the presence of metallic Ni (69%) and Ni2O3 (31%). TG demonstrated that approximately 9% of the material presented a high degree of graphitization, burning at temperatures around 700 C. Two purification protocols were investigated. In protocol I, the sample was submitted to two thermal oxidation treatments followed by reflux in HCl solution. In protocol II, an additional stage of reflux in acid was carried out soon after the first burning in argon. Both protocols were preceded by a common stage of extraction of fullerenes in toluene. Purification processes I and II resulted in a significant reduction of the metallic level, reaching 15 and 10 at% of Ni, respectively. XPS could not detect Ni and Y in the samples purified using protocols I and II, indicating that all the remaining metallic material was encapsulated. As for the presence of undesirable carbons, TEM and TG/DTG showed that even though both protocols were efficient in removing all amorphous carbon, part of the carbon nanospheres remained. However, it was noticed by the TEM that there were several empty graphitic rings , proving that the multi-stage procedure allowed for breaching the carbon layer for the acid attack of the metals. After protocol II and taking into account the TG/DTG levels, nanospheres with a high degree of graphitization were not noticed anymore, which proved that the method was efficient in modifying the nanoparticles surface. Raman spectroscopy revealed the presence of extractor groups, such as carboxylics and hydroxilics, which were not detected after the purification protocols. This suggests that these functionalities were not added extensively. The increase in the ratio between the intensities of bands D and G after protocol II may be associated with the introduction of defects and/or functionalization of the carbon impurities still present. The differences observed in the dispersability in the solvent of as-grow and purified samples were also attributed to the presence of functionalized carbon impurities. Although the purity of the sample obtained using protocol II was not ideal yet, a chemical procedure based on the reaction with NaOH catalyzed by tetrabutyl ammonium hydroxide (TBAH) was employed to introduce OH groups. The functionalized sample presented a significantly increased dispersability in regard to all the solvents tested, with the sole exception of hexane, in which no dispersion happened. The best results were observed using dichlorinemethane and THF; good dispersability was also observed in water and in solvents containing amine and hydroxyl groups. The strong chemical change of the sample has been demonstrated, even though it was not possible to characterize the extension of the SWNT chemical changes, due to presence high levels of carbon impurities in the sample. This result encourages further research using this route of functionalization.

Purificação e caracterização de nanotubos de carbono produzidos pelo método de descarga em arco elétrico

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