Design of a metal-oxide solid solution for selective detection of ethanol with marginal influence by humidity

Emerging nanotechnologies improved solid-state sensors design and capabilities and enabled the use of semiconducting metal oxides-based gas sensors for new and increasingly demanding applications. Among them, ethanol monitoring is of great interest because it is an important chemical raw material widely used in chemical industry, medicine, paint and cosmetics. Water vapour is the most common interferent during operating conditions, affecting the baseline stability of sensors, and reducing their sensitivity and accuracy. In this work, a strategy based on the formation of stoichiometric solid solutions of Sn, Ti and Nb oxides have been sought to address humidity dependence of sensor signals and to enhance the sensing response vs. ethanol. Powders were synthesised through sol-gel technique, by keeping the Sn:Ti ratio constant at the optimal value for ethanol detection with different Nb concentrations (1.5, 3.5 and 5 at%), and thermally treated at 650 °C and 850 °C. Such solid solutions exploited the good sensitivity of SnO2 towards most analytes and the low cross-sensitivity of TiO2 to humidity. Nb was added to lower the film resistance and to prevent grains coalescence at temperatures required for the formation of the solid solutions and the anatase-to-rutile phase transition. The results show that the niobium concentration and the heating treatment of powders are fundamental parameters to optimise the sensing features of the film towards ethanol monitoring applications and to lower the influence of humidity on its performance.