Facile Formation of Self-Organized TiO2 Nanotubes in Electrolyte Containing Ionic Liquid-Ethylammonium Nitrate and Their Remarkable Photocatalytic Properties

摘要

The oriented TiO2 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] [NO3], 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] [NO3] 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 mu m, 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] [NO3], revealing the improved ability to light photoabsorption and suppression of recombination rate. The increase of the contact angle from 9.3 degrees to 13.1 degrees 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 degrees.