Distinct Diameter Dependence of Redox Property for Armchair, Zigzag Single-walled, and Double-walled Carbon Nanotubes

Abstract

Multiscale density functional theories (DFTB, PBE, and M06-2X) were used to investigate redox properties, such as ionization potentials (IP), electron affinities (EA), electronegativities (χ), and Fermi levels (E f) for infinite armchair single-walled carbon nanotubes (SWNT) (n,n) (n = 3 − 16), zigzag SWNT (n,0) (n = 5 − 16), as well as double-walled carbon nanotubes (DWNT) (n,n)@(n + 5, n + 5) (n = 3, 5 and 6). These properties show strong and different diameter dependence. With increasing diameter, IPs of armchair SWNTs (n,n) decrease monotonically, while EAs increase monotonically. Although IPs of zigzag SWNTs (n,0) also generally decrease, there is an increase occurring just after (3k, 0) (k = 2, 3, 4, and 5) and shows a group behavior, in which every three neighbourhood (3k, 0), (3k − 1, 0) and (3k − 2, 0) form a group. However, opposite to the armchair SWNTs, the EAs of zigzag SWNTs decrease rapidly with increasing diameter till (11,0) and then gently increase. EAs of the zigzag SWNTs also exhibit a group behavior, yet are not synchronous with IPs. With increasing diameter, the IPs and EAs of both the armchair and zigzag SWNTs approach to approximately 4.7 and 3.9 eV. For the armchair SWNTs electronegativity (χ) and Fermi level (− E f) change very slightly with diameter, while for the zigzag they decrease rapidly till (9,0) and then gently oscillate to the similar levels to those of the armchair. The IPs and EAs for (n,n) @ (n + 5, n + 5) DWNTs have the same trend as armchair SWNTs. It was also found that these DWNTs characterize better redox properties than their constituents. These interesting findings are important for redox chemistry based on CNTs and may offer a new strategy for separation of CNTs. The biomedical implication of SWNT, C60 and Li@C60 was also discussed. Similar to Li@C60, the binding and HOMO analysis shows that SWNT may also be able to well protect DNA bases from radiation.