The surface analytical characterization of carbon fibers functionalized by H2SO4/HNO3 treatment

A systematic, time-dependent, surface-sensitive study has, for the first time, given new insights into the mechanism of the often-used, but unexplained, sulfuric/nitric acid oxidation of graphene-containing materials, such as carbon fibers and carbon nanotubes. We used X-ray photoelectron, photoacoustic FTIR and Raman spectroscopies to follow the evolution of the functionalization of carbon fibers sonicated in a 3:1 (v/v) mixture of concentrated acids at 60 °C. The study has revealed that oxidation occurs subsequent to acid attack, the attack serving to prepare sites for the ensuing oxidation, as witnessed by the presence of four O1s, two N1s and two S2p XPS peaks during the site preparation process. Their intensities varied discontinuously with treatment time, particularly in the early stages, rather than constantly increasing with time. Two of the O1s, and all the S2p and N1s peaks diminished in intensity with treatment time, eventually disappearing. The only nontransitory oxidized carbon functionality was COOH, confirmed by the two remaining O1s XPS peaks, which continued to increase long after the disappearance of the transitory species. This slow carboxylic acid production indicates that COOH formation is the rate-controlling step of the process, occurring subsequent to the site preparation initiated by the transitory species.