Fluoride concentration controlled TiO2 nanotubes: the interplay of microstructure and photocatalytic performance

Abstract

Electrochemical anodization has been considered an efficient technique to prepare highly ordered TiO₂ nanotubes (NTs) for photocatalysis. Most researchers focus on the influence of preparation parameters on the micromorphology of TiO₂ NTs, while the correlation between micro-morphology and the photocatalytic performance of TiO₂ NTs has been ignored to some extent. Fluoride ions in the electrolyte are essential for the formation of TiO₂ NTs, as the F⁻ concentration influences both the microstructure and the corresponding photocatalytic performance of TiO₂ NTs. Here, we prepared a series of TiO₂ NTs by varying the [F⁻] of the electrolyte. The micro-morphology, crystallography information, and chemical compositions of the TiO₂ NTs were characterized by SEM, XRD and XPS, respectively. We found that an increase in [F⁻] in the electrolyte led to an increase in the TiO₂ NTs length and tube diameter, and excess [F⁻] resulted in a decrease in the integrity of the TiO₂ NTs surface and a shorter length due to over-etching of the TiO₂ NTs surface. The corresponding photoelectrocatalytic and photocatalytic performance were characterized by photocurrent response and photo-decolorization of rhodamine B (RhB). The results showed that longer TiO₂ NTs arrays prepared at higher [F⁻] enhanced both the photoelectrocatalytic and photocatalytic performance due to an improved separation and transportation of photo-generated charge carriers. However, excess [F⁻] in the electrolyte reduced both the length and integrity of TiO₂ NTs, which depressed the photoelectrocatalytic performance as a consequence of a faster recombination rate of the electron–hole pairs. A larger diameter was beneficial for the photo-decolorization of RhB because of the faster mass transfer of RhB molecules.