Titania nanotubes from weak organic acid electrolyte: Fabrication, characterization and oxide film properties

• TiO2 nanotubes were synthesized in a citric acid and sodium fluoride environment.
• Wettability measurements show that both as-formed and annealed nanotubes exhibited hydrophilic behavior.
• TiO2 nanotube layer behaves as an n-type semiconductor.
• Annealed TiO2 nanotubes had a higher impedance magnitude compared to as-formed nanotubes.

In this study, TiO2 nanotubes were fabricated using anodic oxidation in fluoride containing weak organic acid for different durations (0.5 h, 1 h, 2 h and 3 h). Scanning electron microscope (SEM) micrographs reveal that the morphology of titanium oxide varies with anodization time. Raman spectroscopy and X-ray diffraction (XRD) results indicate that the as-formed oxide nanotubes were amorphous in nature, yet transform into crystalline phases (anatase and rutile) upon annealing at 600 °C. Wettability measurements show that both as-formed and annealed nanotubes exhibited hydrophilic behavior. The electrochemical behavior was ascertained by DC polarization and AC electrochemical impedance spectroscopy (EIS) measurements in 0.9% NaCl solution. The results suggest that the annealed nanotubes showed higher impedance (105–106 Ω cm2) and lower passive current density (10− 7 A cm− 2) than the as-formed nanotubes. In addition, we investigated the influence of post heat treatment on the semiconducting properties of the oxides by capacitance measurements. In vitro bioactivity test in simulated body fluid (SBF) showed that precipitation of Ca/P is easier in crystallized nanotubes than the amorphous structure. Our study uses a simple strategy to prepare nano-structured titania films and hints the feasibility of tailoring the oxide properties by thermal treatment, producing surfaces with better bioactivity.