A study on the sensing of NO₂ and O₂ utilizing ZnO films grown by aerosol spray pyrolysis

Abstract The present paper addresses the preparation and characterization of ZnO nanostructured thin films obtained using aerosol spray pyrolysis method at different deposition periods. Aiming at understanding the chemical composition, structural and morphological properties of the samples, characterization was performed using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscopy (AFM) and focused ion beam scanning electron microscopy (FIB-SEM), respectively. Defect structures were characterized by photoluminescence (PL), Raman, and electron paramagnetic resonance (EPR) techniques. The purity and crystallinity of the deposited films with increasing growth time were confirmed from XRD studies supported by Raman studies. AFM and SEM revealed morphology transition from uniform small particles to rod-like structures fused together growing perpendicular to the surface with prolonged deposition time. Optical absorption analyses revealed the decrease in the optical band-gap energy from 3.38 to 3.27 eV with prolonged growth time. The dominant emission at 2.80 eV (445 nm) in the PL spectra signifies the presence of Zn_i in the ZnO nanostructures which is shown to slightly decrease with an increase in growth time. EPR analyses revealed higher ferromagnetic (FM) signal for ZnO films grown for 20 min. Improved sensitivity to both NO₂ and O₂ was observed for ZnO nanostructured film grown for 20 min owing to higher Zn_i and V_O as compared to its counterpart. Film grown for longer periods showed a decrease on the FM and sensitivity due to reduced Zn_i and V_O defects induced by larger grain sizes.