Geometry, Orbital Interaction, and Oxygen Chemisorption Properties of Chromium-Doped Nickel Clusters

Density functional theory (DFT) for generalized gradient approximation calculations has been applied to study the stability and orbital structures of Ni and Cr-doped nickel clusters varying in the range 2 <= n <= 13. Moreover, chemisorption of the oxygen atom has also been studied to understand the oxygen reactivity on Ni and Cr-doped nickel clusters. The binding energy per atom gradually increases with the increase of number of atoms for pure nickel and Cr-doped nickel clusters. The icosahedral Ni12Cr cluster is the most stable structure with binding energy of 2.87 eV per atom. This calculation noticed that the chromium atom prefers to exhibit center at the cluster (n >= 10) rather than edge. Furthermore, cluster stability has been gradually decreased with the increase of chromium concentration. Icosahedron structures containing more than four chromium atoms are less stable than the Ni-13 cluster. Metal atom 3d orbitals have a strong influence on the overall structure stability of these clusters. The interactions among the 3p and 4s with 3d orbitals also have a small effect on their stability. Oxygen adsorption on the hollow site is the highest among all sites of the pure nickel clusters (n >= 5), but oxygen adsorption sites have been modified with Cr doping into the cluster. The oxygen chemisorption energy of the Cr-doped cluster is higher than the pure nickel cluster even though a single chromium atom existed in the cluster. Chromium makes a strong bond with the oxygen that may initiate passive film formation at a certain place. The calculated results imply that oxygen chemisorption energies are increasing with the increase of chromium concentration. It reveals that the more protective film can be formed on the nanoalloy clusters by making a preferential bond between oxygen and chromium.