Mechanistic understanding of the sensing process by analyzing response curves of TiO₂ based humidity sensors

Abstract

Sensors function by interacting with an appropriate stimulus, undergo a change in property, which is then diagnosed by making some measurements. For any sensor, the type of interaction between analyte and sensor surface determines its overall performance. This article explores the philosophy in which primary measurements like response curves can hold information on the “type” of interaction occurring between analyte and sensing material. As case study, titanium oxide (TiO2) pellet sensors fabricated by sol-gel growth of TiO2 nanoparticles (as-grown and annealed) are investigated for humidity sensing at room temperature. The sensors display a very fast response in the 0%–30% relative humidity (%RH) range and return to their initial state without applying any external heat treatment. The response curves are analysed in view of adsorption processes guided by Langmuir isotherms. Correlation between sensor microstructure, adsorption processes and response curve is used to build the mechanistic understanding of the sensing process. The results bring out a unique correlation between sensor microstructure, interaction of analyte with sensing material and profile of response curves. Further, the synthesised sensors exhibit a linear response in the 0%–30% RH range making them suitable for low humidity environments like food packaging industry.