Meta-Hybrid
Density Functional Theory Study of Adsorption
of Imidazolium- and Ammonium-Based Ionic Liquids on Graphene Sheet
- Publication date
- 2015
- Publisher
Abstract
In this study, two types of ionic
liquids (ILs) based on 1-butyl-3-methylimidazolium
[Bmim]<sup>+</sup> and butyltrimethylammonium [Btma]<sup>+</sup> cations,
paired to tetrafluoroborate [BF<sub>4</sub>]<sup>−</sup>, hexafluorophosphate
[PF<sub>6</sub>]<sup>−</sup>, dicyanamide [DCA]<sup>−</sup>, and bis(trifluoromethylsilfonyl)imide [Tf<sub>2</sub>N]<sup>−</sup> anions, were chosen as adsorbates to investigate the influence of
cation and anion type on the adsorption of ILs on the graphene surface.
The adsorption process on the graphene surface (circumcoronene) was
studied using M06-2X/cc-pVDZ level of theory. Empirical dispersion
correction (D3) was also added to the M06-2X functional to investigate
the effects of dispersion on the binding energy values. The graphene···IL
configurations, binding energies, and thermochemistry of IL adsorption
on the graphene surface were investigated. Orbital energies, charge
transfer behavior, the influence of adsorption on the hydrogen bond
strength between cation and anion of ILs, and the significance of
noncovalent interactions on the adsorption of ILs on the graphene
surface were also considered. ChelpG analysis indicated that upon
adsorption of ILs on the graphene surface the overall charge on the
cation, anion, and graphene surface changes, enabled by the charge
transfer that occurs between ILs and graphene surface. Orbital energy
and density of states calculations also show that the HOMO–LUMO
energy gap of ILs decreases upon adsorption on the graphene surface.
Quantum theory of atoms in molecules analysis indicates that the hydrogen-bond
strength between cation and anion in ILs decreases upon adsorption
on the graphene surface. Plotting the noncovalent interactions between
ILs and graphene surface shows the role and significance of cooperative
π···π, C–H···π,
and X···π (X = N, O, F atoms from anions) interactions
in the adsorption of ILs on the graphene surface. The thermochemical
analysis also indicates that the free energy of adsorption (Δ<i>G</i><sub>ads</sub>) of ILs on the graphene surface is negative,
and thus the adsorption occurs spontaneously