Humidity-enhanced NO₂ gas sensing performance of UV-activated ZnO–polyaniline composite nanomaterials at room temperature

In this study, we present a UV-activated ZnO–polyaniline (PANI) composite gas sensor capable of highly sensitive and selective detection of nitrogen dioxide (NO₂) at room temperature, with a remarkable performance enhancement observed under humid conditions. The sensor was fabricated through a drop-casting process, by depositing different amounts of p-type PANI on n-type zinc oxide (ZnO) nanoparticles, forming a well-defined p-n heterojunction. Under 365 nm ultraviolet (UV) illumination, the photogenerated electron–hole pairs in ZnO significantly enhance the surface reactivity, enabling efficient adsorption and charge transfer processes with NO₂ molecules. At the same time, the presence of PANI not only facilitates charge transport through its conjugated polymer network but also contributes to the redox interaction with NO₂, especially via proton doping mechanisms that are highly effective in humid environments. This counterintuitive behavior provides a promising strategy for designing robust gas sensors with reliable operation under ambient and humid conditions.