Impact of sonication pretreatment on carbon nanotubes: A transmission electron microscopy study

Sonication treatments are commonly used for debundling and dispersing carbon nanotubes (CNTs) in liquid media prior to chemical functionalization. However, this step may lead to the stripping of the outer graphitic layers and the scission of the CNTs, and can therefore have a deleterious effect on the achievable properties of the functionalized CNTs. Thus, knowledge on the structural integrity of the modified CNTs is required to understand its influence on the device performance of hybrid nanocarbon-based composites. Here we report on the impact of a sonication pretreatment on the structure of multiwalled CNTs, and on the role of the induced modifications on the subsequent attachment of ferrimagnetic Fe3O4 nanoparticles. Decoration of the CNTs with Fe3O4 nanoparticles is achieved by a microwave-assisted synthesis route involving the reaction of iron acetylacetonate with 2-pyrrolidinone. Employing a combination of atomic resolution transmission electron microscopy, electron energy-loss spectroscopy, energy-filtered transmission electron microscopy and electron tomography, we provide evidence that significant degradation of the CNT structure takes place during the dispersion process. Moreover we find that the sp2 system is more heavily disrupted at the interface between the CNTs and the surface-deposited nanoparticles suggesting that nucleation of Fe3O4 preferentially occurs at the nanotube defect sites.