Low-Temperature Transformation of Titania Thin Films from Amorphous Nanowires to Crystallized Nanoflowers for Heterogeneous Photocatalysis

Titania thin films with ordered nanostructures are of general interest in fields of photocatalysis, gas sensors, energy storages, energy conversions, etc. In this study, we report a low-temperature crystallization and the simultaneously occurred morphology change of titania thin films in a dilute H2SO4 solution. Amorphous titania nanowire arrays were fabricated by a Ti-H2O2 interaction, which transformed to crystallized nanoflower arrays through a dissolution-precipitation route during the subsequent acid treatments. The nanoflowers were doped with nitrogen and also incorporated with sulfate ions. An increasing H2SO4 concentration resulted in larger nanoflowers with higher anatase content; but the crystallinity reduced. The low-temperature-derived nanoflower arrays possessed high density of surface hydroxyl groups and defect V-O-Ti3+ sites, which contributed to a high absorption and enhanced photodegradation efficiency of rhodamine B in water under the illumination of UV-visible light during the first several runs of photocatalytic evaluations. The beneficial surface defects diminished gradually with increasing runs; however, the average reaction rate constants of the acid treated films are still superior to that of the calcinated one, which can be attributed to several structural parameters such as the nanoflower morphology, incorporation of sulfate ions, and the coexistence of anatase and rutile.