Atomic oxygen chemisorption on the sidewall of zigzag single-walled carbon nanotubes

Abstract

A theoretical approach is developed to study the chemisorption of a single oxygen atom on the outer surface of zigzag single-walled carbon nanotubes (ZSWCNTs). The adatom Green's function, the charge transfer between the nanotubes and the chemisorbed O atom, and the adsorption energy $\ensuremath{\Delta}{E}_{\mathrm{ads}}$ are calculated within the Anderson-Newns model, which takes account of Coulomb interaction between adsorbate electrons. Two different adsorption positions are considered, in which an oxygen atom forms a bridge between two nearest-neighbor carbon atoms: one is on top of an axial C-C bond (position I), and the other---on top of a zigzag C-C one (position II). According to our calculations carried out for a series of the ZSWCNTs $(p,0)$ with $p$ ranging from 9 to 18, the adsorption of a single O atom in both the above-mentioned positions is possible and equally probable from the energetic point of view as the corresponding adsorption energies, being negative, are almost identical. The absolute values of these energies generally fall into the range $1.1\ensuremath{-}2.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, larger $\ensuremath{\mid}\ensuremath{\Delta}{E}_{\mathrm{ads}}\ensuremath{\mid}$ values being associated with semiconducting tubes. For the latter ones, $\ensuremath{\Delta}{E}_{\mathrm{ads}}$ is found to be practically independent of the nanotube radius $R$, whereas for metallic tubes $\ensuremath{\Delta}{E}_{\mathrm{ads}}$ slightly decreases with increasing $R$, tending towards the ``infinite'' radius graphene case. The localized acceptor states created by the O adatom in the band gap of the semiconducting ZSWCNTs are found to be responsible for such a different behavior of $\ensuremath{\Delta}{E}_{\mathrm{ads}}$ as a function of $R$ for the two types of nanotubes (metallic and semiconducting), as well as for the lowering in $\ensuremath{\Delta}{E}_{\mathrm{ads}}$ (by about $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) for the semiconducting tubes as compared with the metallic ones. It is also shown that a fairly large charge transfer (of about 0.55 electron) to the O adatom occurs when it is chemisorbed on the outer surface of the ZSWCNTs, which suggests that their electronic transport properties can be significantly changed upon chemisorption of atomic oxygen.