Unveiling Temperature-Mediated Dual-Band Edge In Tio2 Nanotubes With Enhanced Photocatalytic Effect

An annealing temperature-driven appearance of a sharp dual-band edge in the ultraviolet-visible absorption spectrum of electrochemically anodized TiO2 nanotubes (TNTs) and the corresponding impact on the photodegradation of organic dyes is reported based on the evolution of an anatase (A)/rutile (R) phase junction. Systematic X-ray diffraction followed by a micro-Raman analysis illustrates the impact of annealing between 500 and 900 degrees C in promoting the anatase-to-rutile phase transformation with a higher degree of crystallinity through an evolution of an intermediate mixed phase. X-ray photoelectron spectroscopy (XPS) further reveals a clear transition from the sub-stoichiometric to stoichiometric TNTs with increasing annealing temperature. Moreover, the valence band (VB) XPS along with the ultraviolet photoelectron spectroscopy (UPS) analysis suggests the possible band alignment at the A/R interface, whereas the X-ray absorption spectroscopy (XAS) at the Ti L- and O K-edges confirms a systematic change from an anatase to a rutile phase owing to the modification in local symmetry. To validate our experimental results, especially the formation of a dual-band edge at the A/R phase junction in TNTs, theoretical calculations using density functional theory (DFT) were also performed. Here, a one-to-one analysis of our theoretical and experimental results not only shows a good agreement to each other but also illustrates the microscopic origin behind the formation of a dual-band edge in mixed phase TNTs as well as the observed variation in Raman and XPS spectra with annealing temperature. Finally, an improved photocatalytic degradation of methylene blue aqueous solution is demonstrated in the presence of a mixed phase and explained in the framework of an efficient transfer of photogenerated electrons from the conduction band (CB) of R to the CB of A and holes from the VB of A to the VB of R across the phase junction in tube walls.