Development of α-Fe2O3/TiO2 3D hierarchical nanostructured photocatalysts through electrochemical anodization and sol–gel methods

Abstract

Titanium dioxide (TiO2) nanotubes have attracted much attention due to their formation, properties, and functionality in recent years. Many methods have been used to obtain the nanotubes, such as template synthesis, hydrothermal, sol–gel, and electrochemical anodization. Compared to others, the electrochemical anodization method makes it possible to grow dense and well-aligned nanotube layers on pure and/or alloyed titanium surfaces. However, for enhanced photocatalytic activities, the parameters of the anodization process such as applied potential, time, and electrolyte composition should be tightly controlled to obtain regular, well-aligned, and continuous nanotube arrays. Alternatively, by the formation of heterojunctions such as α-Fe2O3/TiO2, the performances of the photocatalysts can be boosted due to the shifts of absorption range into the visible region and narrower band gaps of the hierarchical structures. In this study, TiO2 nanotubes were obtained by electrochemical anodization on Ti foil in ethylene glycol, ammonium fluoride and distilled water-based electrolyte under constant voltage and varying anodization durations. Herein, it was aimed to observe the effects of the anodization time on the length and diameter of the nanotubes, which have significant roles on the photocatalytic activity. Besides the investigation of the anodization parameters, the heterogeneous structure, α-Fe2O3/TiO2, was formed on anodized surfaces to scrutinize the improvement of the photocatalytic properties. The TiO2 nanotubes were characterized through XRD and SEM to determine the phase structure and morphology, respectively. The variation of optical bandgap values of α-Fe2O3/TiO2 samples depending on the processing parameters was determined by using a UV–Vis spectrophotometer. The photocatalytic performances of the α-Fe2O3/TiO2 photocatalysts were revealed using an aqueous solution of methylene blue. Photocatalytic degradation rates and kinetic study of α-Fe2O3/TiO2 photocatalysts were evaluated by a comparative approach. The highest degradation efficiency was achieved as 85% using the α-Fe2O3 coated photocatalyst with the anodization time of 30 min and anodization voltage of 30 V.