Computational insights into the adsorption mechanisms of anionic dyes on the rutile TiO2 (1 1 0) surface: Combining SCC-DFT tight binding with quantum chemical and molecular dynamics simulations

Abstract

Metal oxides are gaining momentum rapidly for application in water pollutant remediation. In this study, the adsorption proprieties of two anionic dyes, i.e., acid yellow 36 (AY36) and acid orange 6 (AO6) on the (110) surface of rutile titanium dioxide (TiO2) in an aqueous medium were investigated using computational methods. The density functional theory (DFT) was used to determine the reactivity of organic molecules by calculating the frontier molecular orbital energies, energy gap (ΔEgap), chemical hardness (η), chemical softness (σ), electronegativity (χ), chemical potential (μ), electrophilicity (ω), the fraction of electrons transferred (ΔN), back-donation energy (ΔEback-donation), Mulliken charge, and Fukui indices. The obtained results showed that the AY36 molecule is more reactive than the AO6 molecule and may have a good adsorption capability compared to the AO6 dye. The most favorable adsorption configurations of AY36 and AO6 molecules were investigated using molecular dynamics (MD) simulation. The calculated interaction energies by MD simulation showed that the TiO2 (110) surface has a high sensitivity to interact with the two anionic dyes, with more affinity toward the AY36 molecule. Furthermore, to get deep insights into the chemistry of interactions between the anionic dyes and the TiO2 (110) surface, the self-consistent charge density functional tight-binding (SCC-DFTB) method was carried out. Results showed that anionic dyes adsorbed on the TiO2 (110) surface by forming covalent bonds between oxygen atoms of the sulfonic group and Ti atoms. Theoretical insights from this work would serve as a guide for researchers to explore the application of oxides in water pollutant remediation.