Detailed surface study of adsorbed nickel on Al12N12 nano-cage

The geometries and electronic structures of Ni-adsorbed Al12N12 (AlN) nano-cage are studied by density functional theory. A number of different structures varying in the position and orientation of nickel on the surface of the nano-cage are studied. Binding energies, band structures, total density of states, natural bond orbital charges, and electron density differences of Ni-adsorbed Al12N12 nano-cage were calculated, and analyzed in comparison with pure AlN nano-cage. The adsorption energies are in the range of (-110)-(-113) kcal mol(-1) for three adsorption sites (P1-P3), and about -86 kcal mol(-1) for P4. High binding energies for these geometries suggest strong chemisorption of nickel on AlN nano-cage. Moreover, dissociation of Al-N bond is observed concurrent with the formation of Ni-Al and Ni-N bonds when the Ni atom is adsorbed on the nano-cage. Results from the calculations also reveal that there is significant charge transfer from the nano-cage to the metal atom which changes the location of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) resulting in reductions in the E-gap (E-HOMO-E-LUMO). The HOMO-LUMO gap for P4 geometry is 1.79 eV which is much lower than the band gap for pure AlN nano-cage (3.93 eV). The band gap for P1, P2 and P3 geometries is 2.90, 2.79 and 3.18 eV, respectively. The decrease in band gap is attributed to the push of outer d electrons of the transition metals to become more diffuse like. The HOMOs in all geometries have densities on Ni atom whereas the densities in LUMO are distributed on the entire skeleton. The results revealed that Ni adsorption provides N-type conduction which makes it an ideal adsorbent for practical applications in magnetism, catalysis and other areas. Reactivity descriptors indicate an increase in electrophilic character of Ni-adsorbed nano-cages compared to pure AlN nano-cage. The electron affinity and softness are highest for P4 geometry whereas the lowest ionization potential and hardness are calculated for P4. (C) 2016 Elsevier B.V. All rights reserved.