Nitrogen/hydrogen codoping of anatase: A DFT study

The stability of nitrogen incorporation at protonated titanium vacancy sites, VTi+nH, n = 0-4, and interstitial sites in anatase under oxygen-rich growth conditions has been investigated using density functional theory quantum mechanical modeling. The 0 K DFT energy results were corrected using thermodynamic free energy data to obtain defect formation energies, Ed, at a typical sol-gel calcination temperature of 700 K. Doping of sol-gel anatase was simulated using a 2 × 2 × 1 defected anatase supercell as host, in which one Ti atom was replaced by 4 H atoms, giving [H4]Ti15O32. A number of different nitrogen configurations were found to be stable at VTi sites in the defected anatase, having negative or small positive Ed values at 700 K. These included nitrogen bonded to one, two and three framework oxygen atoms and NH bonded to two framework oxygens, HNO2. Maximum stability due to codoping with H was obtained with one H atom per VTi, although models with up to 3 codoped H atoms gave negative Ed values. An interesting observation was that extra stability was obtained in models where the structure relaxed to give bonding between the N at the VTi site and one of the surrounding Ti atoms, with a Ti-N distance of 2 Å. Electronic structure calculations showed that the 2p orbitals of N at a VTi site mix with both O 2p states at the top of the valence band and with Ti 3d states at the bottom of the conduction band. A small amount (0.1 eV) of band gap narrowing occurs for N doping at VTi.