Oxidação de nanotubos de carbono em meio ácido: um estudo teórico

Abstract

Due to the chemical inertia of pristine (as-synthesised) carbon nanotubes (CNTs), activation steps are necessary prior to chemical modification. In this sense, oxidative processes should be highlighted as a procedure for purification (removing amorphous carbon and catalyst particles) and surface activation. The interaction of oxidizing reagents with CNTs, similar to the alkenes, may result in the formation of C-OH, C=O, C-O-C, and O=C-OH functional groups. Among the main oxidizing species, the most important for liquid phase processes are HNO3 and H2SO4/HNO3 mixtures. For both reagents, the NO2+ (nitronium ion) species acts as the oxidizing agent, but the elementary steps and the topology of the chemically modified positions are not completely understood at the molecular level. Given the importance of the oxidation processes of CNTs, in the present thesis, two chemical ways were used to study the process. At the first model, distinct structures were proposed for an ionic CNT-NO2+ adduct, with the CNT represented by an armchair (5,5) tube with and without a Stone-Wales defect. Among several reactions analyzed one lead to an oxygen transfer to the tube surface, yielding ether like group. This specific process was notably favorable and, therefore, can be taken as the beginning for the oxidation of CNT in that acid medium. At the second model, the system chosen was a single-walled armchair (6,6) with a monovacancy defect. The oxidizing agent used again was the NO2+ ion resulting from an acid-base HNO3 dissociation, considering a neutral system due to its NO3- counter ion. An overall favorable oxidation pathway was proposed and includes an initial NO2+ exothermic electrophilic attack followed by an endothermic oxaziridine formation and a carbonyl group generation. For both chemical models, the computational methodology employed was the density functional theory using periodic boundary conditions. Structural, electronic, vibrational, and thermodynamic properties were calculated and discussed.