Formation and electrochemical characterization of anodic ZrO2–WO3 mixed oxide nanotubular arrays

Abstract

ZrO2–WO3 mixed oxide nanotubes were synthesized by a simple electrochemical anodization route. The oxide nanotubes contained a mixture of metastable hexagonal WO3 and monoclinic (and orthorhombic) ZrO2 phases, as well as a mixed-oxide ZrW2O8 phase that showed a metastable tetragonal symmetry. Evaluation of photo-activity of the materials showed generation of photo-potentials of −85mV and −230mV in the as-anodized and annealed conditions. Because of the mismatch in the band edge positions of the WO3 and ZrO2 phases and the resultant relaxation of photo-generated charge carriers, no significant photo-current density could be observed. The arrays of oxide nanotubes are considered for electrochemical capacitor application because of their morphology-assisted fast charge/discharge kinetics and large surface area. Presence of a large concentration of charge defects (on the order of 1021cm−3) and the reported high proton conductivity of the ZrO2–WO3 mixed oxide rendered high capacitance, which decreased with an increase in the scan rate of cyclic voltammetry. The highest measured capacitance was 40.03mF/cm2 at a scan rate of 10mV/s and the lowest was 1.93 mF/cm2 at 1V/s in 1M sulfuric acid solution.