Nondestructive Functionalization Of Carbon Nanotubes And Its Application

Having been a research topic for more than 20 years, the field of carbon nanotubes (CNTs) is currently focused on their technological applications, with developing functional CNT materials for demanding applications as the main research direction. To harness their full potential, both structural integrity and chemical functionality are needed. In this context, effective functionalization of CNTs without destorying their structure has been pursued consistently.High degree of functionalization can be achieved by using covalent chemistry, but it inevitably breaks the C=C of CNTs, leading to the distruption of the π-conjugated system. Another approach is the noncovalent functionalization, which is known as a strategy to preserve the structural integrity, but it is suffering from the drawback of instability and low-degree functionalization because the interactions between CNTs and modifiers are always quite weak.To endow CNTs with high degree of functionalization without destroying the π-system for practial applications, we perfomed the following studies.1. Wrapping CNTs with poly (sodium 4-styrenesulfonate)(PSS) and the mechanism for enhanced adsorption of methylene blue(MB)(1) The concept of "polymer wrapping" was used to nocovalently functionalize CNTs with PSS, and the nature of the wrapping interactions was investigated.Containing high density of negatively charged groups and aromatic moieties, PSS layer can simultaneously introduce extra π-electrons and surface charges to CNTs. In our experiment, CNT/PSS hybrid was prepared by simply stirring and sonicating CNTs in PSS aqueous solution. The colloid dispersibility, UV-Vis spectra, IR spectra, FESEM and TGA results demonstrated that a considerable amount of PSS was wrapped around CNTs. XPS was used to study the interactions between CNT and PSS. The results indicated that PSS was associated with the conjugated system of CNTs through π-π interactions and some π-electrons from the electron-rich CNTs might transfer to the electron-withdrawing-SO3- groups. Thus, for the first time, we proposed the π-π electron-donor-acceptor (EDA) interactions between CNTs and PSS. The π-π EDA interactions gained new insights into the noncovalent mechanism between polymer and CNTs.(2) CNT/PSS was used for enhanced adsorption of MB and the adsorption mechanism was revealed.Adsorption isotherms and kinetics were used to compare the adsorption performance of CNTs and CNT/PSS for MB, a typical aromatic pollutant in water. We found that a small amount of PSS around CNTs (10 wt%) significantly enhanced the adsorption of MB:CNT/PSS hybrid showed more than two times higher adsorption capacity and much faster adsorption kinetics than pristine CNTs. To study the adsorption mechanism, the surface chemitry of CNT/PSS-MB was carefully studied by XPS. It turned out that MB were adsorbed on CNT/PSS by two forces:π-π interactions (between MB planes and the CNT conjugated skeleton) and strong electrostatic attraction (between-SO3- groups of PSS and-NMe2 groups of MB). We further found more contributions of electrostatic attraction than π-π interactions to the overall adsorption. The advantage of electrostatic attraction over π-π interactions in adsorption lies in the spacesaving tilted orientation of MB plane on CNT surface and weakened electrostatic repulsion between adjacent MB molecules. This explination was further supported by theoretical calculations.2. Oxidation of CNTs with K2FeO4 and the realization of nondestructive covalent FunctionalizationA green oxidant K2FeVIO4 was employed to oxidize CNTs for the first time. The oxidation was found very efficient (less than 3 h) under mild condition (60℃) in H2SO4 medium, producing hydrophilic CNTs with abundant surface-COOH groups. This new oxidation method is facile, fast and green.More importantly, Fe (VI) oxidation of CNTs followed a defect-specific oxidation process, that is, only the sp3-C on CNT surface were oxidized while the C=C bonds remained unaffected. By systematic experiments and careful characterization, we provided concrete evidences to confirm this nondestructive/selective oxidation. One evidence is that, when all the accessible sp3-C have been oxidized, the oxidation reaction spontaneously stops (in only 3 h) without any excessive oxidation regardless of prolonged treatment. As a result of the nondestructive oxidation, oxidized CNT products were obtained in gratifying yields of above 100 wt%, which is very economical in industrial production.The prevailing HNO3/H2SO4 treatment, known as an effective but destructive oxidation method, was taken as a comparison, which helped to better illustrate the nondestructive oxidation mechanism.