Enhanced ammonia gas sensing performance of in situ-polymerised ZnO/PANI–HCl-doped emeraldine base: Experimental and theoretical investigations

Abstract

Ammonia (NH₃) detection has an essential role in environmental monitoring, industrial safety, and medical diagnostics, particularly in the early-stage detection of possible diseases from human breath. In this study, we comprehensively investigated various gas sensors, focusing on the characteristics of an NH₃ gas sensor prepared using pure ZnO and polyaniline (PANI) and their nanocomposites at room temperature. Bare ZnO nanostructures were synthesised via electrical current heating, while PANI and ZnO/PANI were produced through in situ polymerisation. The prepared nanostructures were characterised using X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive spectrometry, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, and Raman spectroscopy. The ZnO/PANI with 95.1 %/4.9 % by weight ratio sensor (SZP3) demonstrated a notable performance with responses of 5990 and 5.5 at NH₃ concentrations of 500 ppm and 100 ppb, respectively. Density functional theory calculations helped elucidate the sensing mechanisms of the PANI, ZnO, and ZnO/PANI nanocomposites. The PANI sensor exhibited a deprotonation activation energy of 0.53 eV, correlating with the pre-adsorbed $O_2^{–}$ and OH⁻ mechanisms on ZnO surfaces. ZnO/PANI nanocomposites showed increased resistance due to charge settlement effects. The sensors displayed high selectivity toward NH₃ over other gases, with sustained performance and stability over 40 days. This comparative study of the gas sensing mechanisms provides both experimental and theoretical insights into the response of ZnO/PANI nanocomposites to NH₃, underscoring the potential for developing a highly selective NH₃ sensor. Our findings can help produce NH₃ sensor prototypes for non-invasive medical diagnostics using human breath.