Unraveling the Photocatalytic Activity of Multiwalled Hydrogen Trititanate and Mixed-Phase Anatase/Trititanate Nanotubes: A Combined Catalytic and EPR Study

A detailed study of the photocatalytic activity of hydrogen trititanate nanotubes (H-TNT), formed by a hydrothermal treatment, Was carried out. H-TNT show a limited activity toward pyridinium chloride degradation under UV-light and even no activity under visible light. In contrast, H-TNT show activity toward the degradation of rhodamine 6G (R6G), both under UV and visible light. EPR spectroscopy is used to gain insight into this difference. UV-light excitation of H-TNT leads to the predominant formation of Ti3+ centers by trapping of electrons at Ti sites, whereas almost no reactive oxygen-based species are formed. Upon visible light excitation of these nanotubes, no light-induced EPR signals are observed. The activity toward R6G degradation thus stems from the excitation of R6G (under both UV and visible light) and the subsequent transfer of electrons into the conduction band of TiO2. After a short calcination process at 623 K, the H-TNT undergo a partial phase transformation into anatase, without affecting the shape and morphology of the nanotubes, and the photocatalytic activity increases to a great extent. The EPR analysis now reveals the formation of different types of species characterized by g values larger than g(e), both upon UV and visible light excitation. These reactive species, such as O-2(-) and O-, are known to play an important role in the photocatalytic process.