Improved adsorption and degradation performance by S-doping of (001)-TiO ₂

In this work, sulfur-doped (S-doped) TiO ₂ with the (001) face exposed was synthesized by thermal chemical vapor deposition at 180 or 250 °C using S/Ti molar ratios R _S/Ti of 0, 0.5, 1, 2, 3, 4 and 5. The S-doped samples synthesized at 250 °C exhibit a significantly improved photocatalytic performance. More precisely, S-doping has the following effects on the material: (1) S can adopt different chemical states in the samples. Specifically, it exists in the form of S ²⁻ replacing O ²⁻ at a ratio of R _S/Ti = 1 and also in the form of S ⁶⁺ replacing Ti ⁴⁺ at R _S/Ti ≥ 2. As a result, S-doping causes a lattice distortion, because the ionic radii of S ²⁻ and S ⁶⁺ differ from that of the O ²⁻ and Ti ⁴⁺ ions. (2) S-doping increases the adsorption coefficient A _e for methylene blue (MB) from 0.9% to 68.5% due to the synergistic effects of the oxygen vacancies, increased number of surface chemical adsorption centers as a result of SO ₄ ²⁻ adsorption on the TiO ₂ surface and the larger pore size. (3) S-doping increases the MB degradation rate from 6.9 × 10 ⁻² min ⁻¹ to 18.2 × 10 ⁻² min ⁻¹ due to an increase in the amount of •OH and •O ²⁻ radicals.