Labeling the defects of single-walled carbon nanotubes using titanium dioxide nanoparticles

We describe here a new method for labeling the defects of single-walled carbon nanotubes (SWNTs) using TiO2 nanoparticles as markers. SWNTs were prepared by chemical vapor deposition, and dilute nitric acid (2.6M) oxidation was used to introduce carboxylic acid groups at the defect sites. Characterization of the SWNTs using ultrastructural and spectroscopic analyses was carried out following introduction of TiO2 nanoparticles. The results indicated that TiO2 nanoparticles were chemically absorbed at SWNT defect sites via an ester-type linkage between carboxylic acid groups at the defect sites and hydroxyl groups at the surface of the TiO2 nanoparticles. In addition, the adsorption behavior of TiO2 nanoparticles on SWNTs was determined following oxidization of the SWNTs using different processes. The results indicated that gas-phase oxidation introduces very few defect sites as evidenced by the low adsorption density of TiO2 nanoparticles. Refluxing in dilute nitric acid can be considered as a mild oxidation for SWNTs, affecting only those defects already present and causing no further damage. In contrast, sonication of SWNTs in H2SO4/H2O2 increased the incidence of carboxylic acid groups, not only at original defect sites but also at newly created defect sites along the walls of SWNTs, resulting in a higher density of TiO2 nanoparticles. In conclusion, labeling of SWNT defect sites using TiO2 nanoparticles permits direct determination of the density, distribution, and location of the defects and offers new possibilities for the creation of heterojunctions between nanotubes and nanoparticles in the future.