A First-Principles Study of Energetics and Core Level Shifts of Anion-doped TiO2 Photo-catalysts

TiO2 is a very-known photocatalysis for splitting water, but the large band gap makes visible light unabsorbed. N-doped TiO2 which can lower the band gap shows a visible-light-active character, and anion-doped TiO2 has been study by many groups subsequently. However, the location of the anion is still not clear; there are two different doping types in TiO2: substitutional doping and interstitial doping. We first employed energetics calculations for experiment preparation. We found that substitutional anion-doping need an Oxygen vacancy first, which is 7.0 eV and 6.34 eV for anatase and rutile, respectively. We also calculated the formation energy of both substitutional and interstitial doping. The results tell that for anion-doped TiO2, formation of substitutional doping requires severe conditions, whereas interstitial doping only needs mild conditions (wet chemistry). In N-doped TiO2 preparation, they found that in XPS spectra there are two peaks around N 1s site: 396~397 eV and 399.6 eV. The difference between the two peaks is 2.6~3.6 eV, but not enough evidence can separate interstitial doping from substitutional doping. Di Valentin, C. et al had proformed DFT calculations with final state effect nelected, their results showed that interstitial doping is 1.6 eV higher than substitutional doping[1]. We have preformed DFT calculations with final state approximation, the results shows interstitial banding energy 2.6 eV larger than substitutional band energy, which agrees well with the experimental data. Core level shifts calculations have also been performed on B/F/P/S-doped TiO2, the conclusion that interstitial doping has a higher binding energy than substitutional one only reverses for F. Density of states have been studied to find out the change of the band gap after doping by anion.