Interfacial adsorption of ionic liquids (ILs) on graphitic carbon nitride (g-C3N4) nanosheets: A DFT study

The adsorption of nine common ionic liquids (ILs) on two well-known structures of graphitic carbon nitride (g-C₃N₄) nanosheets, including Triazine-g-C₃N₄ and Heptazine-g-C₃N₄ has been investigated using density functional theory (DFT) method in gas phase and water solvent. The interactions between the ILs and the nanosheets are characterized by several techniques, such as quantum theory of atoms in molecules (QTAIM) analysis, noncovalent interaction (NCI) plots, and energy decomposition analysis (EDA) method. Interestingly, the obtained results showed that electrostatic interactions have the highest contributions to the interaction energies between the ILs and the surfaces, followed by dispersion interactions and orbital interactions (ΔE_elec > ΔE_disp > ΔE_orb) The most important Van der Waals (vdW) interactions between the ILs and the surfaces include π–π, CH…N, CH…π (CN), and CN…X (X = F, N, O) interactions. On the other hand, the adsorption of ILs is accompanied by charge transfer from the surfaces to the ILs. The calculated adsorption energy (E_ads) showed that the [Bmim][PF₆] IL has the highest adsorption affinity on the Triazine-g-C₃N₄ (−28.28 kcal/mol) and Heptazine-g-C₃N₄ (−26.68 kcal/mol) surfaces. Thermochemistry calculations showed that the E_ads, ΔH_ads, and ΔG_ads values of IL@surface complexes decrease on moving from gas phase to water media. However, our results showed that the adsorption of ILs on the surfaces is still exothermic and proceeds spontaneously. Upon adsorption of ILs, a decrease in the HOMO–LUMO energy gap (E_g) is observed and is accompanied by an increase in the reactivity and electrophilic character of the surfaces. TD-DFT calculations showed that the weak adsorption of ILs leads to small red or blue shifts in the absorption bands of the Triazine-g-C₃N₄ and Heptazine-g-C₃N₄ surfaces. However, no significant changes are observed in the shape of the absorption spectra of the surfaces. Furthermore, the transition density matrix (TDM) heat maps of the IL@surface complexes showed that the electrons and holes are generally concentrated on the Triazine-g-C₃N₄ and Heptazine-g-C₃N₄ surfaces in the IL@surface complexes, signifying that the electronic transitions occur mainly on the surfaces.