Nitroxides adsorption on carbon nanotubes

CNTs are one of the most attractive alternatives for detection of environmentally harmful gaseous species. The assessment of the interaction betweenthe nanotubes' external walls and the molecules, however, remains a challenging issue. As pilot experiment, we investigated the physicochemical effects in the interaction of NO_x and the outer wall of SWCNTs sorted according to their metallic character.
Observations on the lineshape and shifts in the C 1s and N 1s core levels and the Franck Condon satellites resolved in the XAS spectra (related to NO dimers) strongly suggest that the reaction pathways of the NO_x molecules are very different to what had been reported so far. The adsorption reaction is charge transfer mediated physisorption, and is more effective for metallic CNTs. Moreover adsorbed NO₂ transforms into NO dimers following the endothermic chemical reaction of 2NO₂ → 2NO+ 2O. The oxygen from the chemical reaction with the SWCNTs may be physisorbed as O₂ or
 

chemisorbed on pristine SWCNTs forming epoxide groups or at defects as carbonyl or ketene groups. The amount of reaction products, however, is smaller than observed for mixed CNTs, the semiconducting structures being the more active ultrapure material.
Complementary experiments on the desorption of the system show a very fast recovery and hence provide evidence of the reversibility of the process.

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