Au/TiO2 Nanotube/Ti Chemicapacitive Devices: An Approach for Optimal Gas Sensing

Abstract

VOC sensing response of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TiO₂</i> nanotube array in capacitive mode with the effect of temperature modulation and frequency variation is presented in this paper. The optimal response of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TiO₂</i> nanotube towards acetone, 2-butanone, methanol, ethanol, and 2-propanol was obtained by identifying the resonant frequency over the frequency range of 0.05-225 kHz. The shifts in the resonant frequency upon exposure to VOCs with reference to that in air might be a simple strategy for selective detection of the VOCs. Such resonance frequency shifts have been associated with the drastic change in the dielectric ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\varepsilon _r}$</tex-math></inline-formula> ) medium under different vapor exposure at the inner and outer pore of the nanotubes. The experimental finding reveals that <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TiO₂</i> nanotube showed resonant frequency at 2.2 kHz in air which was shifted to 2.1 kHz, 1.5 kHz, 0.84 kHz, 0.52 kHz, and 0.4 kHz for the exposure of 2-butanone, acetone, 2-propanol, ethanol, and methanol, respectively. Combination of optimum temperature (125 °C) in association with resonant frequency offers a maximum sensor response of ∼180%, 225%, 406%, 556%, and 669%, towards 2-butanone, acetone, 2-propanol, ethanol, and methanol, respectively. The maximum response magnitude to methanol is possibly attributed to the high oriented polarization of the methanol molecules at relatively lower resonant frequency (0.4 kHz). An equivalent circuit model in association with complex impedance data has been analyzed to correlate the experimental findings.