Unraveling the Temperature Mediated Dual Band Edge in TiO_2 Nanotubes with Improved Photocatalytic Property

摘要

The resurgence of photocatalytic activity of TiO_2 is driven in past few decades owing to its potential to meet several contemporary global challenges, such as pollutant degradation, nonfossil fuel production from CO_2, or H_2 generation from aqueous solutions. Among three TiO_2 polymorphs anatase TiO_2 (A-TiO_2), rutile TiO_2 (R-TiO_2) and brookite TiO_2, first two are the mostly investigated phases for photocatalytic application. However, the use of TiO_2 photoelectrodes is limited owing to the recombination of photo-generated (e-h) pairs in the bulk, and also at the electrode/electrolyte interface. The overall efficiency associated with A-TiO_2 is still considered to be higher with respect to R-TiO_2 due to higher charge carrier mobility and slower charge recombination kinetics. Nevertheless, proper ratio of anatase-rutile phases (Degussa P25) is superior in photocatalytic activity than that of pure rutile or anatase phase. On the other hand, anodic TiO_2 nanotubes TNTs have attracted wide interest due to their high surface-to-volume ratio with preferential electron transport towards the Ti back-contact, and a scalable synthesis by electrochemical anodization. However, thermal annealing is needed to obtain anatase and rutile phases, besides achieving suitable ratio of mixed-phases for relatively high photocatalytic performance through efficient inter-granular charge transfer and minimizing the recombination processes. Temperature induced changes in surface morphology, grain size, crystal structure and composition also have significant influence on the photo-electrochemical properties of TNTs. Here, we report the synthesis of TNTs, and the correlation of optical band gap with structural transformation from anatase to rutile through intermediate mixed phases as a function of temperature and their corresponding effect on photocatalysis application. This is confirmed by means of UV-Vis spectroscopy, X-ray diffraction (XRD) and Raman Spectroscopy. X-ray Photoelectron Spectroscopy (XPS) analyses has also been carried out to understand the change in chemical behavior with annealing temperature, gives a clear evidence of sub-stoichiometric to stoichiometric TiO_2 (Ti~(4+)) phase transformation with increasing temperature via removal of the fluorine (F) species from the NT surface which eventually changes the crystallinity of the samples as observed in XRD patterns. Further, to comprehend the change in the local coordination environment of the constituent atoms in TiO_2 nanotubes with annealing temperature, the samples have been characterized by X-ray Absorption Spectroscopy (XAS). Density functional theory (DFT) has also been employed to understand the underlying mechanism.