Black titania with increased defective sites for phenol photodegradation under visible light

Herein, the effect of an acid pre-treatment over TiO 2 reduction was studied to obtain black titania with increased defectives sites. A commercial TiO 2 (anatase > 99%; Merck) was pre-treated with aqueous solutions of boric, nitric, phosphoric and sulfuric acids at 353 K. Afterwards, the solids were annealed for 2 h at 773 K under 50 mL min −1 H 2 flow. The above materials were analyzed by XRD, FTIR, XPS and UV–Vis to relate the chemical changes induced by the acid pre-treatment with crystalline and optical characteristics and; hence, to the photoactivity. The O/Ti surface ratio nor the energy band gap value of the TiO 2 was altered by the pre-treatment using sulfuric acid; therefore, the photocatalytic activity was not modified using this acid. Despite the fact that pre-treatment with boric and phosphoric acid decreased the energy band gap to 2.1 and 2.9; respectively (which is favorable for the absorption of visible light), the incorporation of borate and phosphate species in the surface was detrimental for the photocatalytic process. The O/Ti ratio dropped from 2.0 to 1.5 after the reduction of the sample pretreated with nitric acid. Since more oxygen vacancies are introduced by using nitric acid, the visible-light absorption was increased and the phenol photodegradation was boosted. The modulation of the defective sites allows to engineering the TiO 2 band-gap for solar-driven applications.