Effect of Annealing Temperature on the Capacitive and Oxidant-generating Properties of an Electrochemically Reduced TiO2 Nanotube Array

Abstract

Recently, the electrochemically reduced TiO2 nanotube array (r-TiO2 NTA) has been an attractive promising material for supercapacitors and anodes for water treatment because of its significantly enhanced capacitive and oxidant-generating electrocatalytic properties. However, most of the previous studies have focused on r-TiO2 NTA prepared at a specific annealing temperature (approximately 450°C). Accordingly, the effect of annealing temperature on the electrochemical properties of r-TiO2 NTA remained unclear. This study aimed to investigate the capacitive and electrocatalytic properties of r-TiO2 NTAs prepared at various annealing temperatures ranging from 350 to 750°C. The anatase-dominant r-TiO2 NTAs prepared at low annealing temperatures (350 and 450°C) exhibited significantly better capacitive and electrocatalytic properties than the rutile-dominant r-TiO2 NTAs prepared at high annealing temperatures (650 and 750°C). The areal capacitance (14.9mF/cm2) of r-TiO2 NTA (annealed at 350°C) was approximately 150 times higher than the areal capacitance of TiO2 NTAs (annealed at 750°C) (0.1mF/cm2). The significant production of hydroxyl radicals was found in r-TiO2 NTAs annealed at 350 and 450°C, whereas no production of hydroxyl radicals was found in r-TiO2 NTA annealed at a higher temperature. This result is attributed to the anatase-dominant phase of r-TiO2 NTAs developed at a low annealing temperature, which contributes to a high doping level, as well as its structural advantage for proton intercalation, yielding a large production of Ti3+.