Ab-initio calculations of synergistic chromium–nitrogen codoping effects on the electronic and optical properties of anatase TiO2

Abstract

Electronic and optical properties of compensated and noncompensated (Cr, N) codoped TiO2 have been investigated using density functional theory with plane wave basis set and pseudopotential. To investigate the formation of defect pair in the codoped models, defect pair binding energy was calculated. Compensated codoped model has two Cr atoms doped at Ti sites, one N atom at O sites along with an oxygen vacancy that gave stable configuration, better electronic and optical properties. Defect pair binding energy of this model showed that, individual defects would bind each other leading to stable configuration compared to mono-doped models. Band structure results showed that compensated (Cr, N) codoping introduced substantially broaden intermediate states in the forbidden band along with narrowed band gap. Furthermore, the Fermi level was shifted from top of the valence band to middle of the forbidden band describing half metallic character. Cr doping changed the nature of N 2p states from unoccupied to occupied which will improve electron–hole pair separation. Optical properties comparison showed that all doped models effectively shifted the absorption edge of TiO2 towards visible light. Compensated (Cr, N) codoped TiO2 has better optical properties and covered wide absorption band in the visible light region, attributed to the stable configuration, narrowed band gap and widely distributed states in the band gap. Our results provide reasonable explanation of the experimental findings.