Anatase versus Triphasic TiO2: Near-identical synthesis and comparative structure-sensitive photocatalytic degradation of methylene blue and 4-chlorophenol

Abstract

Studies on photocatalytic activity of monophasic and biphasic TiO2 have been well explored. However, detailed studies on the photocatalytic activity of triphasic titania, as opposed to monophasic or biphasic TiO2 are scarce. Here we report a comparative structure-sensitive photocatalytic study of triphasic versus anatase TiO2, both have been synthesized under near-identical conditions through a customized sol–gel approach. The composition of the phases is tuned just by varying the thermal pre-treatment conditions of TiO2 gel that has been subsequently subjected to calcination at 300 °C. Interestingly, when the pre-treatment temperature of the gel is systematically increased from 50 to 250 °C, a transition from anatase to triphasic (anatase, rutile, and brookite) and then again to anatase has been observed. The synthesized TiO2 phase compositions have been thoroughly characterized for their structural, optical, electrical, surface and morphological properties. Among the different phase compositions, triphasic titania having a significant proportion of rutile has been found to exhibit the highest photocatalytic activity, as probed using model organic pollutants, Methylene Blue (MB) and 4-Chlorophenol (4-CP). In addition to the earlier known factors such as effective heterojunction, and favorable position of the valence band (VB), an important contribution to the high photocatalytic activity of triphasic TiO2 has been experimentally found to stem from the additional electron density in VB that is attributed to the lattice contraction of anatase phase owing to the coexistence of other two phases. The study provides fundamental insights into the energetics that impact the photocatalytic activity of triphasic versus anatase TiO2.