Because of their hydrophobic nature, carbon nanotubes tend to agglomerate hindering their dispersion is solvents or viscous polymer melts. Unsurprisingly, the resulting aggregates reduce the mechanical performance of the final composite. Effort has been directed at modifying the surface of the carbon nanotube to reduce the hydrophobicity and improve interfacial adhesion to a bulk polymer through chemical attachment. The carbon nanotube chemistry involves three main approaches, where reactions target: surface groups generated through acid-induced oxidation of the carbon nanotube surface; direct addition to the carbon nanotube sidewalls; groups attached to polycyclic aromatic hydrocarbons that are immobilised to the carbon nanotube surface through Van der Waals forces.
Contents |
The purification and oxidation of carbon nanotubes (CNTs) has been well represented in literature [1][2][3][4]. These processes were essential for low yield production of carbon nanotubes where carbon particles, amorphous carbon particles and coatings comprised a significant percentage of the overall material and are still important for the introduction of surface functional groups[5]. During acid oxidation, the carbon-carbon bonded network of the graphitic layers is broken allowing the introduction of oxygen units in the form of carboxyl, phenolic and lactone groups[6], which have been extensively exploited for further chemical functionalisation.[7]
In the mass production of single-walled carbon nanotubes in 1998, treatment with an aqueous base liberated yellow solutions[8], which were recognised as small polycyclic aromatic structures that contained oxygen groups. However, this purification step has not appeared in a large number of papers for the preparation and subsequent reaction of carboxylated CNTs. Studies in 2007 indicated that the liberated yellow alkaline solutions contained carbonaceous fragments from the oxidation of multi-walled carbon nanotubes (MWCNTs)[9]. These smaller fragments of the MWCNT lattice were determined to be humic acids and therefore are strongly immobilised on the MWCNT surface in acidic solution, becoming detached in basic solution, and account for up to 45% of the total number of oxygen containing groups[10]. The problem, therefore, is that the majority of research publications over the last 15 years have not used an alkaline purification procedure to remove these lattice fragments and it is highly likely that subsequent chemical modification of their acid-oxidised carbon nanotubes occurs through carboxylic groups on these detachable fragments rather than the carboxylic groups covalently attached to the carbon nanotube lattice. Moreover, the carboxylic group readily form carboxylates with amines and are quite stable to various treatments. So the problem with existing publications is compounded by the fact that such reports did not use an acid wash after coupling amines to the carbon nanotube carboxylic groups to ensure complete removal of the carboxylates. This renders ca. 98% of the existing research publications, which involve chemistry of acid-oxidised carbon nanotubes, potentially spurious.
The Nanoscience & Nanotechnology Group at the University of Brighton used acid-base (or Boehm) titrations with careful purification procedures for carbon nanotubes at each stage of the relevant process, it was demonstrated that after removal of acid-generated humic acids from the surface of multi-walled carbon nanotubes, the remaining carboxylic groups undergo covalent amidation reactions, through carbodiimide coupling, with around 50-75% conversion depending on the length of acid-oxidation initially used[11]. Moreover, the use of glutamic acid in the amidation step reveals that the intermediate complex of carbon nanotubes modified with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDAC) and N-Hydroxysuccinimide (NHS) do react to form the final amide. This work highlights the importance of using alkaline solutions to remove humic acids generated and immobilised in situ to the carbon nanotube surface when subjected to nitric acid reflux and the use of acids at key stages to remove carboxylates.
See Selective chemistry of single-walled nanotubes
Studies in 2007 revealed that the acid-oxidation of SWCNTs generated carbonaceous fragments, resulting from the acid-oxidation of the SWCNT structure, which are immobilised to the outer surface of the SWCNT. After purification and removal of these fragments indicated that the final SWCNT structure bore no acidic groups and that these carbonaceous fragments may be the sole carrier of the carboxylic groups[12].