The interaction of deep eutectic solvents with pristine carbon nanotubes and their associated defects: A density functional theory study

In this study, the interaction of four deep eutectic solvents (DESs): [Choline chloride][Urea] ([ChCl][U]), [Choline chloride][Ethylene glycol] ([ChCl][EG]), [Choline chloride][Glycerol] ([ChCl][Gly]) and [Choline chloride][Benzoic acid] ([ChCl][BA]), with pristine carbon nanotube (CNT) and its defects: double-vacancy and Stone–Wales structures (CNT-DV and CNT-SW) is investigated using density functional theory (DFT) calculations. The geometry optimization, electronic property calculations, noncovalent interaction analysis and optical properties of the DES@nanotube complexes were carried out at the M06-2X/cc-pVDZ level of theory. The adsorption energy (E_ads) calculations show that the presence of the DV and SW defects on the CNT increases the adsorption strength of the DESs, DES@CNT-SW > DES@CNT-DV > DES@CNT. On the other hand, the adsorption energy values increase with an increase in the volume of DESs due to the increase of noncovalent interactions, following the order [ChCl][BA] > [ChCl][Gly] > [ChCl][U] > [ChCl][EG]. The calculation of the HOMO-LUMO energy gap (E_g) and chemical hardness (η) of the DES@nanotube complexes indicates that the DES@CNT-SW complexes have the largest E_g and η values and thus the lowest chemical reactivity. The analysis of the interactions between the nanotubes and the DESs using noncovalent interaction (NCI) plots and energy decomposition analysis (EDA) suggests that the DESs adsorb onto the nanotubes through van der Waals interactions and that dispersive interactions dominate (dispersion interaction energy (ΔE_disp) > electrostatic interaction energy (ΔE_elec) > orbital interaction energy (ΔE_orb)). Predicted ultraviolet–visible absorption spectra of the complexes show that the adsorption of DESs on the nanotubes has only a very marginal effect on the optical response of the nanotubes. Transition density matrix heat maps reveal that the electrons and holes localize to the CNT, CNT-DV and CNT-SW surfaces in the DES@nanotube complexes, indicating that the charge transfer occurs mostly on the surfaces.