Fig. 1. Flow charts of anode oxidation (a) and hydrothermal synthesis and annealing (b)

Fig. 2. Surface morphology of TiO₂ with different NH₄F mass fractions (a) 0.2%, (b) 0.3%, (c) 0.4%, (d) 0.5%

Fig. 3. Oxidation currents with different NH₄F mass fractions

Fig. 4. Surface micro-morphologies of TiO₂ under different oxidation durations (a) 2 h, (b) 4 h, (c) 6 h, (d) 8 h

Fig. 5. Surface morphologies of TiO₂ under different oxidation voltages of 40 V (a), 50 V (b), 60 V (c), and the voltage-tube diameter curve (d)

Fig. 6. Surface morphologies of polished/unpolished titanium sheet and TiO₂ nanotube (a) Unpolished Ti sheet, (b) Polished Ti sheet, (c) Unpolished Ti sheet surface nanotubes, (d) Polished Ti sheet surface nanotubes

Fig. 7. Surface morphologies of TiO₂ and BaTiO₃ under different hydrothermal reaction time(a) TiO₂ nanotubes without hydrothermal treatment, BaTiO₃ nanotubes under (b) 2 h, (c) 4 h, (d) 8 h, (e) 20 h, (f) 24 h hydrothermal reaction time

Fig. 8. Surface morphologies of BaTiO₃ nanotubes under different annealing temperatures (a, b) 700 ℃, (c, d) 800 ℃

Fig. 9. XRD patterns of samples under different hydrothermal reaction time

Fig. 10. Raman spectra of BaTiO₃ under different hydrothermal durations

Fig. 11. XRD pattern (a) and Raman spectra (b) of BaTiO₃ under different annealing temperatures

Fig. 12. Fluorescence spectrum of the solution after the sample reaction