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

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 angstrom) can be attributed to the selective diffusion of smaller gas molecules through the ZIF-8 pores (3.4 angstrom) compared to molecules with a larger kinetic diameter such as ethanol (4.53 angstrom) and acetone (4.6 angstrom).