Gas Sensors Based on Perovskite Structured Material

Abstract

Gas sensing using metal oxides can be a highly cost effective and reliable technology in a variety of medical and industrial applications. However, selectively sensing a specific gas in a complex gas mixture continues to be a significant challenge in many applications, in particular for closely related analyte gases such as acetylene and ethylene. For example, the specific individual concentrations of dissolved gases (C2H2, C2H4, CH4, C2H6, CO, CO2 and H2) that emerge in electrical power transformer oils provide critical diagnostic information about the transformer’s stable operation and safety. It is therefore useful to have sensors that can selectively differentiate between these individual gases. To achieve this, I used the LaFeO3 perovskite and investigated its gas sensing mechanism in detail. The powders of LaFeO3 perovskite were obtained by two different synthesis methods, namely solid state reactions and sol gel processes. I then used this material as an active sensing layer during exposure to dissolved gases. All of my sensors showed a significant response to unsaturated hydrocarbons, namely acetylene and ethylene, but not to the other gases that I tested. I further improved this high selectivity of my sensors, to only detect acetylene and not ethylene, by controlling the operating temperature. The effects of different background conditions, such as humidity and CO2 levels, on the LaFeO3 sensors were also characterized. To understand the origin of the sensing mechanism of my sensors, I combined catalytic conversion measurements with simultaneously performed operando DRIFT (Diffuse Reflectance Infrared Fourier Transform) spectroscopy and DC resistance measurements. I applied the operando investigation technique to the relevant analytes, CO2, C2H4 and C2H2, in order to identify the type and role of different adsorbates in mediating selective gas sensing. DRIFT spectra revealed that at 150°C, the reaction with both acetylene and ethylene resulted in surface formate species; at higher temperatures, this was the case only for acetylene. Accordingly, the LaFeO3 responds to both gases at 150°C but only to acetylene at 250°C. Finally, I could identify the mechanism by which the formate species are responsible for the sensor response, and the essential additional role played by Pt electrodes used in my sensors in the detection of ethylene and the temperature dependent selectivity.