Sulfur-doping of rutile-titanium dioxide by ion implantation: Photocurrent spectroscopy and first-principles band calculation studies

Abstract

Sulfur (S)-doped titanium dioxide $({\mathrm{TiO}}_{\mathrm{2}})$ was synthesized by ion implantation and subsequent thermal annealing. The S ions were implanted into the single crystals of rutile ${\mathrm{TiO}}_{\mathrm{2}}$ at a fluence of ${\text{8×10}}^{15}$ ${\mathrm{ions/cm}}^{\mathrm{2}}.$ According to the results of Rutherford backscattering spectroscopy and ion channeling analysis, the irradiation damage recovered by annealing at 600 °C in air. In the annealed crystal, the S atoms occupied oxygen sites for form Ti-S bonds, as confirmed by x-ray photoelectron spectroscopy. Compared to the pure ${\mathrm{TiO}}_{\mathrm{2}},$ a photocurrent was observed in the lower-energy regions for the S-doped ${\mathrm{TiO}}_{\mathrm{2}}.$ Based on the theoretical analyses by the first-principles band calculations using the full potential linearized augmented plane-wave methods within the generalized gradient approximation, the mixing of the S $\text{3p}$ states with the valence band (VB) was found to contribute to the increasing width of the VB. This leads to the band gap narrowing in the S-doped ${\mathrm{TiO}}_{\mathrm{2}}.$ Therefore, the photon-to-carrier conversion was induced during irradiation by visible light above 420 nm (<2.9 eV).