Preparation and investigation of Al-doped ZnO thin films as a formaldehyde sensor with extremely low detection limit and considering the effect of RH

It is well known that humidity strongly affects the sensing performance of the resistance-based gas sensors. In the current research, pure and Al-doped ZnO thin films (with different Al contents) were prepared by the spray pyrolysis technique and studied as a formaldehyde sensor. The properties of the sensing materials were researched by X-ray diffraction (XRD) spectra, atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The sensing performance of the samples was studied as a function of Al content, temperature, formaldehyde concentration, and relative humidity. The sensing mechanism and the effect of RH on the sensing performance and reliability were also discussed. The investigations revealed that the Al-doping notably improved the sensitivity of the ZnO thin films while an increase in Al content over an optimum content of 2 at% destroyed the sensing performance of the samples due to the increase of the carrier concentration. The results also demonstrated that the sensing performance and reliability of the samples are strongly affected by the RH >= 60%.