Functionalization of Single-Wall Zigzag Carbon Nanotubes by Carboxyl Groups: Clustering Effect

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 pair carboxylation increases with tube diameter. Furthermore, we find that pair adsorption is not kinetically hindered and shows similar activation energies to that of the isolated adsorption. The electronic mechanism for the clustering effect is discussed.