I tried to study the response of ZnO chemiresistive sensor to CO2 gas at temperature > 250 degrees C and in presence of synthetic air. As per ionosorption theory, the resistance of n-type gas sensors increase with exposure to oxidizing gases. However, the resistance decreased with exposure to CO2 gas in this case. I have read fairly large number of papers related to CO2 gas sensing with metal oxides. Though I find contrasting results, I observed two things from the majority of the papers. (1) Resistance increases in presence of inert gas and (2) it decreases in presence of synthetic air.Could you please let me know the reason behind this?
Hi Srinivasulu, before answering your query I would like to know few more details about your material and experimental procedure.
1- Have you checked the conductivity type (n/p) of grown ZnO film?
2- Did you perform cross sensitivity measurement of your sensing device with other reducing gases such as H2, H2S or NH3?
Satyendra Kumar Mourya
1. The material is n-type.
2 . The resistance is decreasing with reducing gases H2S and NH3.
I observed the similar response to CO2 in the literature (Resistance decreasing with n-type and increasing with P-type materials)
1. Yadav, A.A., Lokhande, V.C., Bulakhe, R.N. and Lokhande, C.D., 2017. Amperometric CO2 gas sensor based on interconnected web-like nanoparticles of La2O3 synthesized by ultrasonic spray pyrolysis. Microchimica Acta, 184(10), pp.3713-3720.
2. Hunge, Y.M., Yadav, A.A., Kulkarni, S.B. and Mathe, V.L., 2018. A multifunctional ZnO thin film based devices for photoelectrocatalytic degradation of terephthalic acid and CO2 gas sensing applications. Sensors and Actuators B: Chemical, 274, pp.1-9.
Satyendra Kumar Mourya
I have just dropcasted ZnO on Polyimide and used as a chemiresistive gas sensor.
Synthetic air is used as the carrier gas. Temperature is from 250 C to 350 C. CO2 concentration is from 200 ppm to 1000 ppm.
Can you exclude any residual humidity being dosed with the CO2?
For many n-type gas sensing materials, H2O acts as a reducing compound. Even traces (a few ppm) or very minor changes of humidity can have a large impact on the sensor resistance. For most pristine metal oxides used for gas sensing, CO2 is regarded to cause no sensor signal, thus one should exclude other impurities as a possible reason for the signals towards CO2. By my experience, residual H2O or other reducing gases/vapours (e.g. alcohols) can lead to a sensor response when dosing a 'non-reactive' gas.
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