Influence of electrochemical reduction on the optical properties of TiO2 nanotubes under ambient conditions

Titania nanotubes (TNTs) are attractive for a variety of applications. In this study, amorphous TNTs have been synthesized by anodization. Annealing of anodized TNTs has been performed to get the anatase phase. Amorphous and annealed TNTs have been electrochemically reduced using 1 M KOH solution. For characterization of amorphous, annealed, electrochemically reduced amorphous and annealed TNTs, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) techniques were used. The presence of C, F and K is detected from the full scan of X-ray photoelectron spectroscopy (XPS) analysis. The effect of electrochemical reduction on optical properties of TNTs under ambient conditions is studied using photoluminescence (PL) spectroscopy. The electrochemical reduction does not cause any appreciable morphological changes (evident from SEM images). However, XRD results show that this treatment produces strain in the anatase phase as a result of the increase in ‘d’ spacing between (101) and (202) planes. Photoluminescence spectroscopy of TNTs indicates that the defect states lie in the visible region for all the samples. These defects states have been found at 2.93 eV, 2.67 eV, 2.53 eV and 2.35 eV energies for amorphous TNTs. For annealed TNTs, these states have been observed at 2.67 eV and 2.35 eV. PL signal for amorphous TNTs is higher than the annealed TNTs. The electrochemical reduction treatment of the amorphous TNTs efficiently removes defects like F, K and C in addition to creating oxygen vacancies as compared to annealed TNTs. As these electrochemically reduced amorphous TNTs are exposed to ambient air for 7 days, the oxygen vacancies are filled. Moreover, in addition to removal of oxygen vacancies, these exposed and electrochemically reduced amorphous TNTs are devoid of other defects like F, K and C. This results in the significant reduction in PL intensity for such amorphous TNTs samples 7 days after electrochemical reduction. Due to the oxygen scavenging ability of electrochemically reduced TNTs, they could be used for vacuum improvement in various devices. This type of electrochemical reduction/recovery cycle makes these TNTs useful for the solar cell application under special (reduced/absence of oxygen) conditions.