Revealing the role of defect-induced trap levels in sol-gel-derived TiO2 samples and the synergistic effect of a mixed phase in photocatalytic degradation of organic pollutants

Sol-gel-derived TiO2 samples were synthesized by annealing a xerogel at three different temperatures, namely 350, 550, and 750 degrees C, which resulted in near-pure anatase, an anatase/rutile mixed phase, and a pure rutile phase, respectively. With the increase in annealing temperature, the average crystallite size increased from ca. 9 to 48 nm, while the specific surface area decreased from ca. 110.9 to 2.3 m2/g. The mixed-phase sample with a rutile-rich outer layer showed the highest photocatalytic activity, even though the specific surface area of this sample (7.4 m2/g) was much lower than that of the anatase sample (110.9 m2/g). In solar photocatalytic degradation of Methylene Blue dye, the rate constant over this sample (6.25 x 10-2 min-1) was more than three times that over the commercially available photocatalyst Degussa P25 (1.97 x 10-2 min-1). The synergistic increase in the photodegradation performance is attributed to interfacial charge transfer between the anatase and rutile phases. The photocatalytic performances of the anatase and rutile phases can be correlated to the presence of different types of defects. The majority of defects in the anatase sample are shallow levels associated with Vo+, while rutile contains shallow levels related to Ti3+ as well as deep levels corresponding to Vo++.