Increasing Rutile Phase Amount in Chromium-Doped Titania by Simple Stirring Approach for Photodegradation of Methylene Blue under Visible Light

The amount of rutile phase in chromium-doped titania photocatalyst was controlled by varying stirring time (0.5–2.0 h) at room temperature during a sol–gel synthesis process. The percentage of rutile phase increased from 15.1 % to 28.6 % when stirring time was prolonged from 0.5 to 1.5 h. Further increases in the stirring time had negligible effect on the rutile phase amount. As evidenced by analyses using diffuse reflectance ultraviolet–visible spectroscopy and X-ray photoelectron spectroscopy, a sufficient stirring time was important for more substitution of Cr3+ for Ti4+ in the lattice, resulting in anatase-to-rutile phase transformation. The formation of more rutile phase in Cr-doped TiO2 not only reduced the band gap energy, but also induced surface defects that retarded electron–hole recombination. It has been demonstrated that the Cr-doped TiO2 prepared with a stirring time of 1.5 h possessed the lowest band gap energy of 1.89 eV, and hence it achieved the highest photodegradation of methylene blue under visible light irradiation.