Insights into Effects of Metal Cations on the Adsorption of Benzotriazole on Halloysite Nanotubes: An Experimental and DFT Study

Natural halloysite nanotubes (HNTs) have attracted great interest as smart nanocontainers of corrosion inhibitors. The adsorption behavior of corrosion inhibitors on HNTs will affect their loading capacity, a crucial property of smart nanocontainers. This work modified the surface of HNTs by preadsorbing metal cations (M2+: Fe2+, Zn2+, or Cu2+) to improve the adsorption of benzotriazole (BTA) on HNTs. The results based on the thermal gravimetric analysis showed that the loading capacities of BTA in Fe2+-, Zn2+-, and Cu2+-adsorbed HNTs were 8.25, 8.39, and 9.74 wt %, respectively, which are significantly higher than those in pristine HNTs of 4.57 wt %. Furthermore, density functional theory calculations exhibited that the adsorption energies of BTA on the M2+-adsorbed surfaces increased compared to that on the clean surface. Notably, the increasing trend of adsorption energies is in good agreement with that of measured loading capacities. Moreover, the presence of metal cations significantly enhances the charge transfer between BTA and the surface. The density of states analysis indicated that the adsorbed metal cations shifted the valence bands of the surface close to the Fermi level, thereby enhancing the coupling between the occupied bands of HNTs and the unoccupied orbitals of BTA.