Oxidação controlada e funcionalização de nanotubos de carbono de parede única: uma abordagem experimental e teórica

Abstract

The chemical treatment of carbon nanotubes (CNT) is necessary to improve their properties, applications and to remove impurities. Treatments with strong acids as H2SO4 and HNO3 is the mostly used alternative. The mixture of these strong acids produces the electrophilic species NO2+, the nitronium ion that is a potential oxidizing with concentration depending on the H2SO4:HNO3 proportion. In this work the interaction between the nitronium ion and a single-walled carbon nanotube (SWCNT) was explored experimentally e theoretically. Experimentally, the H2SO4:HNO3 solution was analyzed at different proportions (1:1, 2:1, 5:2, 3:1, 4:1, 5:1, 6:1, 7:1 and 8:1 v/v) and the nitronium ion concentration obtained using a calibration plot constructed from a standard solution of NO2BF4 in H2SO4. All the species in the acid mixture were characterized by Raman spectroscopy. The results showed that the concentration of nitronium ion in the acid mixtures varied from 0 to 4.53 mol/L. The mixtures 2:1, 5:2 and 3:1 were then used for the chemical oxidation of single-walled CNT for 4, 8 and 12 hours. The final samples were analyzed by Raman spectroscopy, thermal gravimetric analysis (TGA) and energy dispersive X-ray spectroscopy (EDS). It was observed by Raman spectroscopy a higher structural disorder in the system after the oxidation, with significant changes in RBM modes, such as disappearance of bands of small diameter tubes, and in the ID/IG ratio, which increases from 0.027 until 0.59 to CNT oxidized with 3:1 mixture. The TGA showed an increase in the temperature of the tube decomposition of at least 30ºC relative to the pristine form, suggesting a significant oxidation degree. The EDS data point to considerable increase of the oxygen amount from 7% to at least 20%, increasing with the reaction time and nitronium ion concentration. Theoretically the interaction between nitronium ion and SWCNT was studied by quantum mechanical calculations. In addition to the pristine (P) form of an armchair (5,5) SWCNT, two other species containing Stone-Wales (SW) and mono-vacancy (V1) defects were considered in order to model the distinct defective regions on the carbon nanotube surface. For the P and SW regions, the ether (COC) functional group was predicted as the main product, with an epoxide (CCO) found as a reactive intermediate. The Gibbs free energy barriers were predicted to be 31.7 (P) and 37.8 kcal mol-1 (SW) in aqueous solution at 298.15 K and 1 atm. The mechanism involving the V1 region leads to the carbonyl group (CO) as the main product, which is formed spontaneously upon NO2+ adsorption without energy barrier. A higher energy mechanism was also described for V1 region, passing through a transition

state characterized as an oxaziridine-like ring. Through this pathway an alkoxy (CO-) is firstly formed and reacts with the neighbor carbon yielding the ether (COC) functional group. The activation Gibbs free energies were 4.5 and 11.2 kcal mol-1 for the first (CO- formation) and second (COC formation) steps, respectively. The results reported here suggest that at the beginning of oxidation in acid medium, the vacancy regions (V) are firstly oxidized leading to the carbonyl (CO) functional groups, followed by reactions at the topological defective parts (P and SW) of the tube surface where the ether (COC) function is the main product.