Ordered TiO2 Nanotubes with Improved Photoactivity through Self-organizing Anodization with the Addition of an Ionic Liquid: Effects of the Preparation Conditions

Modifying the geometric and surface parameters of oriented TiO2 nanotubes (NTs) is beneficial to the utilization of solar energy for chemical reactions, and this performance may be further improved. Thus, the effects of adding an ionic liquid (IL), 1-butylpiridinium chloride [BPy] [Cl], and the effects of the water content and preparation conditions on the surface morphological, physicochemical, photocatalytic, and photoelectrochemical properties of TiO2 NTs were thoroughly examined and presented. The anodic oxidation of Ti foil in an organic electrolyte containing a small amount of [BPy] [Cl] allowed for obtaining IL_NTs with a well-ordered structure, even at low voltages (10 V). Additionally, the application of [BPy] [Cl] enabled one of the geometric parameters of the IL_NTs, namely, the diameter, to be tuned, whereas the length remained the same (i.e., control over the diameter was first noted). Improvements in the photocatalytic and photoelectrochemical activities of the IL_NTs in comparison with those of the pristine sample were observed (approximately 2.5 times higher due to the better utilization of incident photons). It is evident that the most significant parameter to influence the photoactivity was the electrolyte composition, especially the water and IL contents (15 vol. % and 0.01 wt. %, respectively). Consequently, the interaction between [BPy] [Cl] and the TiO2 NT surface was demonstrated by X-ray photoelectron spectroscopy (XPS) results.