Spectroscopic characterization of contaminants and interaction with gases in single-walled carbon nanotubes

Several spectroscopic techniques have been used to investigate the presence of contaminants in a commercial purified single-walled carbon nanotube (SWCNT) bucky paper, to determine their cleaning procedure in ultra-high-vacuum conditions and to study how impurities influence the interaction between SWCNTs and gas phase molecules. Nickel catalyst particles and sodium-containing species, likely a residual of the surfactant bath, were fully removed only after prolonged (>2 h) annealing at 1270 ± 30 K. Other impurity elements (S and Si) remain in the material as localised clusters that do not interact with the SWCNTs and do not interfere with their properties.A dramatic difference was observed when the Na-contaminated or the Na-free nanotubes interacted with molecular oxygen. O₂ adsorption was strongly altered by the Na traces, which simulated an intense sample oxidation causing a modification of the tube electronic properties. On the contrary, for the Na-free sample the lack of adsorbed oxygen and the stability of the C1s core level after large O₂ doses demonstrated the absence of any chemical bond between SWCNTs and O₂. Similarly, exposures to N₂, H₂O and CO do not have influence on the electronic properties of SWCNTs. Instead, a sizeable effect on the electronic spectra was observed for SO₂, NO and NO₂ adsorption. The sensitivity of the SWCNT electronic spectra to ppb quantities of nitrogen oxides and sulfur oxide undoubtedly foresees applications in the field of toxic gas sensing.