Fabrication of Hierarchical TiO2 Nanotubes in a New HBF4-Based Electrolyte for Enhanced Morphology and Photocatalytic Activities

The present work introduces a new HBF4-based electrolyte for the preparation of anodic TiO2 nanotubes (NTs). It is shown that, by optimizing the preparation parameters, highly ordered and extremely smooth TiO2 NTs can be successfully fabricated in the HBF4-containing electrolyte via a two-step anodization approach (so-called the BF4-TiO2 NTs). The obtained BF4-TiO2 NTs have unique hierarchical upper-nanopore/lower-nanotube structure and show both enhanced photo-electrochemical and photocatalytic performances than the referenced TiO2 NTs formed in the F--containing electrolyte (i.e., the F-TiO2 NTs). We consider that the decomposition of BF4- into F- under high electric field is the key for the formation of anodic BF4-TiO2 NTs, and this allows the growth of BF4-TiO2 NTs going through a quite different way as compared to that of the F-TiO2 NTs. These findings may pave an alternative way for the preparation of TiO2 NTs with enhanced geometrical features and application properties.