Fe-modified TiO₂ nanotube layer as photochemically versatile material for the degradation of organic pollutants in water

TiO₂ nanotube layers (TNT) are prepared by electrochemical anodization of Ti foil in an electrolyte composed of ethylene glycol, ammonium fluoride and water. The surface of TNT is modified by iron using spin-coating of Fe(NO₃)₃/IPA (isopropyl alcohol) solution of different concentrations. The as-prepared materials are annealed at 450 °C for 2 h to form crystalline Fe-TNT. The phase identification and surface morphology of the materials are investigated by XRD and SEM, respectively, while the energy bandgap (E_g) is analysed by DRS. The novelty of this work is based on the investigation of different photochemical processes that could occur simultaneously, and it includes photocatalysis but also Fenton-based processes since iron is a Fenton-active element. To this end, the degradation of organic pollutants (e.g., caffeine) is performed under solar-like radiation at pH = 3 using different systems that are Fe-TNT material, radical precursors (H₂O₂), and Fe-TNT combined with H₂O₂. It is worth noting the degradation mechanism of the organic pollutants is an advanced oxidation process where hydroxyl radicals have been identified by UV-visible fluorescence spectroscopy (using coumarin as probe molecule) as the main reactive oxygen species. One of the main goals of this work is to determine the contribution of the different involved photochemical processes (photocatalysis, photo-Fenton and photolysis) along with the potential synergy between all these processes. To resume, this work provides new insights into the concept of photochemical versatility, which is scarcely described in the literature.