The growth process of TiO2 nanotube films formed on Ti by anodization in NH4 H2 PO4 - NH4 F electrolytes has been studied by electrochemical measurements, field emission-scanning electron microscopy (FE-SEM) observations, and ellipsometric analyses. The results of electrochemical measurements and FE-SEM observations showed that the change in current density during anodization partly reflects the formation process of nanotube films but does not give detailed information on the film growth process. Ellipsometry could be successfully applied to monitor the growth of nanotube films in situ. Ellipsometric data (Δ and ψ) obtained during anodization in 0.5 wt % NH4 F revealed that the growth process of TiO2 nanotubes can be divided into four stages, that is, the barrier layer growth (1st), the outer nanoporous layer formation and inner nanotube layer growth (2nd), the pure nanotube growth (3rd), and the tube mouth dissolution (4th) stages. The outer nanoporous layer in the 2nd stage is formed by F- -induced breakdown of the initially grown barrier layer. The final 4th stage is characterized by an inward spiral of the Δ-ψ plot and a significant reduction of the amplitude of a cyclic change in Δ. On the basis of these characteristics of ellipsometric data, one can control the end point of anodization for preparing ordered TiO2 nanotubes.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Renewable Energy, Sustainability and the Environment
- Surfaces, Coatings and Films
- Materials Chemistry