Despite substantial interest on the influence of heat treatment on the activity of anodic TiO2 films, there is still a lack of systematic studies addressing the differences in photocatalytic and photoelectrochemical performance of thermally annealed anodic TiO2 layers. Here we addressed this problem on hierarchical anodic titanium oxide (ATO) layers consisting of an outer layer with large pores (rings) and inner layer with sub-pores were obtained by a three-step anodization of titanium. The ATO layers were formed at 20 °C in an ethylene glycol solution containing 0.38 wt% NH4F and 1.79 wt% H2O at the constant anodizing voltage of 70 V. The resulting amorphous ATO was annealed at 400, 500, and 600 °C in order to obtain photoactive anatase and anatase-rutile mixed phases. The characterization of ATO was performed by electron scanning microscopy (SEM/EDS), X-ray diffraction (XRD), low-temperature nitrogen sorption measurements (BET surface area), and electrochemical impedance spectroscopy (EIS). A gradual decrease in the BET surface area of ATO was observed with increasing annealing temperature due to the clogging of the sub-pores resulting from the sintering of anatase grains. EIS study and the Mott-Schottky analysis suggested that conductivity of ATO changed due to the reduced fluorine content in the anodic oxide layer, with donor densities decreasing from 1.78⋅1018cm−3 to 4.08⋅1017cm−3. Raising the annealing temperature also caused a shift of the flat band potential towards more negative values, which is correlated with an increase in rutile content and consequently a decrease in band gap from 3.26 eV to 3.09 eV. The photoelectrochemical performance of the annealed TiO2 was studied under pulsed UV illumination. The highest photoconversion efficiencies were observed for the wavelength of 350 nm for the ATO sample with the dominant anatase content. The photocatalytic degradation of methyl red and toluidine blue dyes was highest on the ATO layers annealed at 500 °C. After 9 h, the total removal of each dye was about 80% and a decrease in total carbon (TC) by 55.3% was observed.
- Titanium oxide
ASJC Scopus subject areas
- Renewable Energy, Sustainability and the Environment
- Fuel Technology
- Condensed Matter Physics
- Energy Engineering and Power Technology