Influence of annealing on the structural, optical and photoluminescence properties of ZnO thin films for enhanced H2 sensing application

Nanostructured zinc oxide (ZnO) thin film sensors were prepared by spray pyrolysis, and their structural, optical, photoluminescence and morphological properties were investigated by X-ray diffractometer, UV–vis spectrometer, photoluminescence spectrometer, and scanning electron microscope (SEM), respectively. The post-annealing of ZnO film in air at 400 °C was found to be effective for the distribution of grains and their sizes, which favors the c-axis orientation of the film. This enhancement is accompanied by an increase in the optical band gap from 3.4 eV to 3.53 eV, which confirms the uniformity of ZnO film prepared by using a specially designed spray nozzle. SEM micrograph after heat treatment revealed uniform distribution of particles with well grown grains of ZnO. Hydrogen sensing measurement indicated the annealed ZnO film to show much higher response than the as deposited film. To understand the enhancement of the sensing performance of the annealed ZnO film, the gas sensing mechanism of the film was proposed and discussed. The magnitudes of the sensor response as well as its dependence on annealing differ significantly depending on the crystallite size of the film.