Similarities and differences in O2 chemisorption on graphene nanoribbon vs. carbon nanotube

A computational chemistry study was conducted to reveal similarities and differences in the adsorption of molecular oxygen on the edge sites of a carbon nanotube (CNT) and a graphene nanoribbon. Two prototypical CNT molecules with a carbene and a carbyne active site were selected, and this in turn defined two corresponding graphene molecules obtained by CNT unzipping. Their electronic and thermochemical properties before and after O-2 adsorption were compared using density functional theory at the B3LYP/3-21G' level, as implemented in the Gaussian03 software. The sensitivity of the results to the basis set used and the selected CNT diameter was also assessed. Despite significant curvature in a subnanometer-diameter CNT, more similarities than differences were revealed with respect to graphene, both in their charge density distributions and thermochemical properties. Contrary to intuitive expectations, the intrinsic activity of an edge site (at least in the prototypical O-2 chemisorption process) is therefore not significantly modified when graphene is rolled up into a nanotube possessing a relatively large degree of pyramidalization. Greater differences exist between armchair and zigzag edges in both CNT and graphene. Both undergo a two-step mechanism of O-2 adsorption, but O-2 dissociates only on the armchair edge. Non-dissociative adsorption on an isolated zigzag site has both a lower affinity and a higher activation energy than the dissociative adsorption on the armchair site. (C) 2011 Elsevier Ltd. All rights reserved.