Synthesis of crystal-phase and color tunable mixed anion co-doped titanium oxides and their controllable photocatalytic activity

B and N mixed anions co-doped titania with various crystal phases such as anatase, brookite, and rutile were successfully synthesized by a hydrothermal synthesis followed by heat treatment in an ammonia gas atmosphere at 550–650°C (denoted as BN-Ana_x, BN-Bro_x, and BN-Rut_x, x is the treatment temperature). The colors of as-prepared BN-Ana, BN-Bro, and BN-Rut are red, yellow-green, and cyan-green, respectively. The color changing mechanism of titania was related to their various band gap structure and the existence of B–N bonding. The nitridation temperature exhibits effective color changing compared to that of nitridation time. The different phases of the mixed anion co-doped titania possess different photocatalytic deNO_x activity. The BN-Ana and BN-Rut show poor photocatalytic deNO_x activity, while the BN-Bro shows excellent photocatalytic deNO_x activity, better than that of standard titania photocatalyst Degussa P25. The colorful titania with low-photocatalytic activity is heavy metal elements free, indicating their possible applications as nontoxic color pigments or novel cosmetic raw materials.