Visible-light-responsive titania nanotubes (TNTs) are attracting attention as functional photoactive nanostructures for visible-light harvesting. Conventional dye-sensitization methods suffer from dye degradation over time, motivating the development of dye-free approaches. In this study, we fabricated oxygen-deficient TNTs by introducing oxygen deficiencies through electrochemical reduction of anatase-type TNTs in molten NaCl–CaCl2 at 843 K. Electrochemical measurements revealed that short-pulse reduction at −2.2 V and −2.3 V (vs Ag(I)/Ag) induced oxygen removal from TNTs. Structural analyses were consistent with reduction-induced oxygen deficiency in anatase TNTs: XRD showed asymmetric broadening of the anatase (101) peak by ∼0.1°, and Raman spectroscopy revealed an Eg mode shift from 396.6 cm–1 (before electrolysis) to 395.5 cm–1 at −2.2 V and 395.8 cm–1 at −2.3 V. Optical spectra showed a pronounced extension of absorption into the visible region up to ∼730 nm (apparent absorption edge), and the absorption-edge behavior was evaluated using Tauc-like analysis assuming absorbance is proportional to the absorption coefficient. Furthermore, EDS analysis indicated significant Ti/O ratio variations depending on electrolysis cycles (e.g., from 95:5 before electrolysis to 38:62 after 25 cycles), indicating cycle-dependent changes in the apparent Ti/O atomic ratio (at%). Compared with conventional plasma-treated TiO2 (showing visible response up to ∼600 nm), the electrochemically reduced TNTs showed a pronounced extension of visible-range absorption with an apparent absorption edge reaching ∼730 nm, indicating enhanced visible light absorption and suggesting potential utility for visible-light harvesting. These results demonstrate that electrochemical reduction in molten salts provides a controllable and scalable route to engineer oxygen-deficient TNTs with superior optical properties.
Shiomi et al. (Wed,) studied this question.