May 3, 2013
Journal of Physical Chemistry C
Carboxylation of carbon nanotubes (CNTs) is a byproduct of acid oxidation treatments that are applied routinely for several purposes including cleaning of CNTs and as a first step of functionalization procedures. In this study, we employ density functional calculations to study the atomic and electronic structures of side-wall —COOH-functionalized zigzag CNTs and elucidate their dependence on the tube diameter and the metallic or semiconducting character. Adsorption of a —COOH group shows a covalent bonding character associated with a small charge transfer from the CNT to the carboxyl groups. The amount of charge transfer, as well as the binding energy, of the carboxyl to the CNT decreases with the tube diameter. We find that it is thermodynamically more favorable for —COOH to adsorb in pairs on top of two neighboring carbon atoms that are bonded along the tube axis. This clustering effect becomes more favorable for larger diameter CNTs, because the difference in adsorption energy between isolated and pai...