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

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.