Tuning the selectivity of highly sensitive chemiresistive nanoparticle networks by encapsulation with metal–organic frameworks

Abstract

Developing highly selective chemiresistive gas sensors is of great importance for non-invasive health diagnosis and environmental monitoring. There is a need for new materials and robust techniques to selectively detect specific gases in different environments. Here, we present a new approach for fabricating metal–organic framework (MOF) encapsulated metal oxide nanoparticle fractal networks for selective gas sensing applications. SnO₂ chemiresistors were fabricated using a flame spray pyrolysis technique. ZnO was then conformally deposited over the ultra-porous nanoparticle network (UNN) of SnO₂ using atomic layer deposition (ALD), which was subsequently converted to ZIF-8 using a chemical vapour conversion technique. The SnO₂ UNN helps in providing a large surface area for enhancing the reaction of the film with the analyte, while the ZIF-8 hinders the interaction with large gas molecules, increasing the selectivity towards smaller analytes such as NO₂. The compact sensor layer showed a higher response of 0.3 (R_a/R_g − 1) at 1 ppm for NO₂ as compared to ethanol (0.08 at 1 ppm). The increased selectivity towards NO₂ (3.2 Å) can be attributed to the selective diffusion of smaller gas molecules through the ZIF-8 pores (3.4 Å) compared to molecules with a larger kinetic diameter such as ethanol (4.53 Å) and acetone (4.6 Å).