Computational Investigation of Adsorption of 2,4,6-Trinitrotoluene on (0001) Surface of (4 x 4) α-Alumina

Adsorption of 2,4,6-trinitrotoluene (TNT) on a (0001) surface of (4 x 4) Al-terminated alpha-Al2O3 was investigated at the plane-wave density functional theory (DFT) level utilizing the PBE variant of the generalized-gradient approximation. Interactions of valence electrons with atomic cores were treated by ultrasoft pseudopotentials. Both the perpendicular and parallel orientations of TNT with respect to the substrate surface were considered. It was revealed that TNT is adsorbed strongly on the alumina surface. Such adsorption is characterized by the coordination of surface aluminum ions with nitro groups of TNT. In the perpendicular orientation of TNT, only a single nitro group is involved in the interaction with the surface while all nitro groups are involved in interaction with alumina surface in the parallel orientation resulting in much stronger binding than the perpendicular orientation. Further, a methyl hydrogen atom was also found to be involved in weak interactions with nearby oxygen atoms of the second layer. Thus, a parallel orientation of TNT will be more favorable for adsorption on the Al-terminated alpha-Al2O3 (0001) surface. Consequent to adsorption, the positions of coordinated Al ions are significantly distorted (pulled up toward TNT) as compared to the plane containing the rest of the Al atoms. Moreover, the buildup of electronic charge in the nitro oxygen-surface Al bonding region indicated the covalent nature of the TNT adsorption on the Al-terminated (0001) surface of alpha-alumina.