Understanding photocatalytic activity of S- and P-doped TiO2 under visible light from first-principles

Substitutional S- and P-doped anatase and rutile TiO2 crystals have been investigated by first-principles calculations on the basis of density functional theory (DFT). The results obtained show that S anion-doped anatase and rutile polymorphs of TiO2 exhibit different modifications of electronic band structure at ultrahigher and realtively lower S concentration. In S cation-doped anatase and rutile TiO2, the band gap has few changes but some mixing states of S 3s and O 2p are localized in the band gap, resulting in the reduction of photon absorption energy. In P anion-doped anatase TiO2, the band gap has a slight narrowing and some P 3p states locate in it, which is consistent with experimental absorption spectra measurements in P-doped anatase TiO2. In P anion-doped rutile TiO2, some of P 3p states locate in the band gap but it has little narrowing. Our electronic structure calculations for P cation-doped anatase and rutile TiO2 also indicate that the band gap has few modifications and no impurity energy levels, which favor recent experimental observations that higher photocatalytic activity in P cation-doped TiO2 results from the large surface area and the crystallinity of TiO2 rather than impurity energy levels in the band gap. In addition, our results indicate that S (P) anion-doped TiO2 is chemically more favored under Ti-rich condition while S (P) cation-doped TiO2 is preferred to be synthesized under O-rich condition.