On the supercapacitor performance of microwave heat treated self organized TiO2 nanotubes: influence of the cathodic pre-treatment, water aging, and thermal oxide

The electrical and morphological properties as well as the assessment of self organized TiO2 nanotubes (TiO2-NTs) as supercapacitors were carried out for the first time for samples grown in organic medium and annealed in a microwave oven (MO) through a hybrid thermal heating. These samples were compared to those obtained by heat treatment in a conventional muffle furnace (MF). No significant differences were observed in the nanotubular morphology, crystalline structure (anatase phase) and of the band gap values; however, samples annealed in a MO showed an improvement in the donor density and a higher flat band potential. After a cathodic pre-treatment (-1.4 V vs. NHE for 60 s) to generate oxygen vacancies/Ti3+ (black TiO2) the MO annealed samples exhibited a superior areal capacitance than the MF annealed samples, even after 5000 cycles of galvanostatic charge-discharge experiments. The improved performance of the MO annealed samples is related to a higher amount of produced oxygen vacancies/Ti3+ on the surface of TiO2-NT. As the generated surface oxygen vacancies are unstable, reactions with adsorbed H2O lead to a more hydroxylated surface layer. Electrochemical impedance spectroscopy (EIS) measurements of the as-prepared black TiO2 exhibited a significant diminishment of the charge transfer resistance for the MO annealed samples; however, assessment of the water aging stability of these pre-treated films resulted in increasing values for the charge transfer resistance. The thermally grown oxide interface layer formed on the Ti substrate showed for both annealing methods similar EIS profiles and seems it has no influence on the values obtained for the areal capacitance. The results suggest that microwave annealing is an interesting option to convert amorphous TiO2-NTs into the crystalline anatase phase, especially because this procedure is very rapid and facile, resulting in an improvement of their areal capacitance after a cathodic pre-treatment.