Effect of Temperature on Methanol and Ethanol Measurement Using Noble Metal Doped Tin Oxide Sensors

Abstract

In this work, the results of the sensing assays using the sensors based on tin oxide doped with Pd and Pt (0.1; 0.2; 0.3 and 0.5%) with and without zeolite coating are presented in contact with 3% methanol and 12% ethanol. In these assays, the stability and reproducibility of the sensing signal at different working temperatures (200, 220, 240, 260 and 280⁰C) were analyzed.The intensity of the response signal of the sensors in contact mainly with solutions of 3% methanol and 12% ethanol was determined. And it was found that the working temperature with which the highest intensity in the response signal was obtained was 260 ⁰C. At this temperature the sensors with the best results were: 0.1% Pd / SnO2; 0.1% Pd / SnO2-Z; 0.2% Pd / SnO2-Z; 0.1% Pt / SnO2; 0.2% Pt / SnO2; 0.1% Pt / SnO2-Z and 0.2% Pt / SnO2-Z.In most of the cases studied, it was observed that the sensing signal was repetitive and stable, maintaining its amplitude and shape during the analysis. However, at the temperature of 280 ° C the signal loses intensity and becomes distorted.The response signals of the zeolite-coated sensors presented greater intensity and stability compared to those obtained with the same sensors without zeolite coating and with the tin oxide sensors without doping.For example, in detecting 12% ethanol, as the sensing temperature increases from 200 ° C to 260 ° C, the sensitivity of doped sensors increases with respect to SnO2 and this effect is greater in sensors with zeolite coating. This is possibly due to the semiconductor nature of tin oxide that becomes more sensitive as the temperature increases, its surface becomes richer in electrons due to the increase of the more reactive species (O-adsorbed), which reacts with the alcohols (reducing substances), producing a flow of electrons towards the surface of the oxide that increases the conductivity of the sensor. This effect is increased by incorporating small amounts of palladium or platinum and adding a layer of zeolite. The sensing signal is not very stable and the distortion is greater at a temperature of 280 ° C where, in addition, a decrease in the intensity of the sensor response is observed.