Comparative Study of Zeolite Overlayers on ZnO Gas Sensors: Effects of Framework Structure on Sensing Performance

Zeolite incorporation into metal oxide semiconductor (MOS) gas sensors offers a promising way of improving sensor sensitivity, vapour discrimination, and operating performance. However, zeolite overlayers may also affect vapour diffusion, response kinetics, and responsiveness to target vapours. In this study, screen-printed ZnO thick-film sensors were modified with four structurally different zeolites (H-Y, H-Beta, Na-A, and H-ZSM-5) to investigate the effect of framework properties on sensing behaviour. The resulting sensor array was evaluated towards nitrogen dioxide (NO₂), ethanol, acetone, propane, and carbon monoxide. Na-A modification (sensor ZnO-LTA) produced the greatest enhancement towards NO₂, increasing the response to 900 ppb NO₂ by 2.8-fold (R_max = 28) at 350 °C and reducing response and recovery times by 20-30 s. ZnO-LTA exhibited the highest response towards 60 ppm ethanol, reaching R₀/R = 62.3 at 350 °C, while ZnO-MFI showed the highest acetone response, achieving R₀/R = 177.4 at 350 °C. Structural characterisation by XRD, Raman spectroscopy, SEM, and EDX confirmed preservation of both the ZnO phase and zeolite framework structures following sensor fabrication. This manuscript provides the first systematic comparison of four zeolite frameworks applied as screen-printed overlayers on ZnO and shows how zeolite framework characteristics influence sensor behaviour, operating temperature, and response dynamics. The findings highlight the potential of zeolite-modified ZnO sensors for vapour discrimination in MOS-based sensor arrays.