Multilayered and Chemiresistive Thin and Thick Film Gas Sensors for Air Quality Monitoring

Selective detection of gases such as nitrogen dioxide (NO 2 ), carbon monoxide (CO), carbon dioxide (CO 2 ), and various volatile organic components (VOCs) is necessary for air quality monitoring. Detection of hydrogen (H 2 ) is equally important as it is a flammable gas and poses serious threat of explosion when exposed to oxygen gas. We have studied the sensing characteristics of these gases using thin film deposited by chemical solution deposition as well as relatively thicker films deposited by atmospheric plasma spray (APS) process. The chapter starts with the sensing mechanism of chemiresistive sensors followed by the definition of gas sensing parameters. Subsequently, we have demonstrated selective NO 2 sensing characteristics of zinc oxide-graphene (ZnO-G) multilayered thin film followed by CO and H 2 sensing characteristics of ZnO thin film and SnO 2 thick film. Cross-sensitivity among CO and H 2 gases has been addressed through the analysis of conductance transients with the determination of activation energy, E a , and heat of adsorption, Q. The concepts of reversible and irreversible sensing have also been discussed in relation to CO and H 2 gases. CO 2 sensing characteristics of LaFe 0.8 Co 0.2 O 3 (LFCO)-ZnO thin film have been elucidated. Interference from CO has been addressed with principal component analyses and the ascertaining of E a and Q values. Additionally, the variation of response with temperature for each gas was simulated to determine distinct parameters for the individual gases. Further, VOC sensing characteristics of copper oxide (CuO) thin film and WO 3 -SnO 2 thick film were investigated. Principal component analysis was performed to discriminate the gases in CuO thin film. The interaction of WO 3 -SnO 2 thick film with various VOCs was found to obey the Freundlich adsorption isotherm based on which E a and Q values were determined.