The oriented TiO 2 nanotube arrays (NTs) are identified as a stable, active, and recyclable photocatalytic surface. However, their photoactivity is strictly dependent on morphology (especially length), which could be controlled by anodic oxidation parameters, including electrolyte properties. To control the morphology a series of NTs were successfully synthesized by a novel approach where ionic liquid (IL), ethylammonium nitrate [EAN][NO 3 ], was used as an addition to an organic electrolyte. Using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, diffuse reflectance UV−vis spectroscopy, and photoluminescence spectroscopy, we are able to show how electrolyte composition influences nanotubes' surface properties and photocatalytic activity. It was found that the change in the amount of [EAN][NO 3 ] in the electrolyte used for anodization in the range from 0.05 to 1.0 wt % affected dynamic viscosity, conductivity, and surface tension of the electrolyte and finally altered the morphology of the formed nanotubes resulting in a proportional increase of the outer diameter and tube length from 105 to 140 nm and from 6.0 to 8.1 μm, respectively. The highest photoactivity (achieving high reaction rate constant, equal to k = 0.0941 min −1 ) and wettability were found for the sample prepared in the electrolyte containing 0.05 wt % of [EAN][NO 3 ], revealing the improved ability to light photoabsorption and suppression of recombination rate. The increase of the contact angle from 9.3°to 13.1°with elongation of the tube diameter from 107 to 140 nm was also noted. It turned out that the IL_NTs' surface became more hydrophobic when stored in air ambience over 7 weeks after fabrication with approximately 20−52°.

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Modifying the geometric and surface parameters of oriented TiO 2 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 TiO 2 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 TiO 2 NT surface was demonstrated by X-ray photoelectron spectroscopy (XPS) results.

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