Dissociative adsorption of O2 on negatively charged nitrogen-doped single-walled carbon nanotubes: first-principles calculations

Abstract

Spin unrestricted density functional theory (DFT) calculations have been used to investigate the molecular and dissociative adsorption of O₂ on achiral substitutional nitrogen-doped single-walled carbon nanotubes (N-SWNTs) with and without additional charges. Adsorption (dissociation) of O₂ on the charged tubes is quite complex. The N-SWNTs with an additional electron with different orientations exhibit different behaviors. The armchair N-SWNTs carrying an extra electron have lower chemisorption (up to 0.6 eV) and reaction (up to 1.2 eV) energies for O₂ compared to the corresponding neutral N-SWNTs. For (10, 10) N-SWNTs, the dissociation barrier decreases from 1.36 eV (in neutral) to 0.76 eV (in negatively charged). The reactivity of the armchair N-SWNTs with low nitrogen content can be increased by adding extra electrons. For the metallic zigzag N-SWNTs, an additional electron affects the O₂ adsorption and dissociation marginally. However, negatively charged zigzag N-SWNTs, which are semiconducting when un-doped, have significantly lower O₂ adsorption, reaction and dissociation barrier energies than the neutral N-SWNTs. We also studied the molecular and dissociative adsorption of O₂ on (10, 10) single-walled nanotubes doped with two substitutional nitrogen atoms (N₂-SWNTs), as a function of extra charge. The chemisorption and reaction energies decrease linearly with the increasing number of extra electrons. The barrier for O₂ dissociation on N₂-SWNTs is found to increase with the increasing number of additional electrons. Our results point out that the dissociative adsorption of an oxygen molecule on N-SWNTs depends on many parameters: curvature, chirality, and charge. The effect of additional charge on the reactivity of the N-doped SWNTs is rather independent of the tube diameter, but depends on the tube orientation.