The effect of double impurity cluster of Ni and Co in TiO₂ bulk; a DFT study

The geometrical structure, electronic properties and stability energy of different arrangements of double cluster of Ni-Ni, Co-Co and Ni-Co in rutile and anatase bulk were investigated. Our calculated results showed that Co-Co preferred to state in the nearest distance (nn arrangement) in both anatase and rutile unit cells. While, the Ni-Ni and Ni-Co clusters showed the most stability with the nn form in rutile structure, they preferred the n arrangement (next to the nearest neighbor) in anatas one. These results can be explained by the crystalline structure of rutile that provides the stronger metal-metal interaction by lower distance and also the activity of Co rather than Ni. The band structure results showed a high, moderate and slight decline in the band gap of anatase and rutile in the presence of Co-Co, Ni-Co and Ni-Ni clusters, respectively. An interesting result was shown by altering the direct band gap of pure rutile to the indirect band gap in effect of Co-Co cluster that introduces the Co/rutile structure with an obvious photo-activity. Ni-Ni and Ni-Co impurities show an inverse behavior in anatase phase by changing the indirect band gap of pure anatase to the direct band gap. This may be the main reason to explain the lower photo-efficiency of Ni/TiO₂ than pure TiO₂ under UV illumination. In the last section we show that the Co impurity energetically prefer to state in TiO₂ bulk versus the Ni one by energy of 5 eV.