Bandgap Engineering in TiO2 By Sulfurization

Previous studies have proposed bandgap narrowing by anion (e.g. N, C, or S) or cation doping into the lattice of TiO₂. Sulfur is of particular interest, as TiS₂ has a close-to-zero bandgap. By mixing TiO₂ and TiS₂ into oxysulfide, a new semiconductor with a suitable bandgap for photovoltaics or solar water splitting may emerge. In this work, we investigate sulfurization of TiO₂ thin films through H₂S gas-phase diffusion. The TiO₂ films were prepared by the sol-gel method. Transmittance and X-ray diffraction analyses indicate that the TiO₂ films have an anatase phase with a direct bandgap of 3.2 eV. The films are sulfurized in H₂S under different pressure, temperature and time. It is found that sulfurization results in a second, narrower bandgap phase in TiO₂. At 600˚C, the anatase TiO₂ almost completely disappears. The results suggest that sulfurization by H₂S is effective in narrowing the bandgap of TiO₂.