Effect of annealing temperature on growth of Ce-ZnO nanocomposite thin films: X-ray photoelectron spectroscopy study

Ce-doped ZnO nanocomposite thin films with Ce/Zn ratio fixed at optimum value (10 at.%) have been prepared via sol-gel method at different annealing temperatures varied from 180 to 500 °C. The synthesized samples were characterized employing atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) techniques. According to AFM analysis, the average grain size increased from about 70 nm to 150 nm by increasing the annealing temperature from 300 to 500 °C. Moreover, based on the XPS data analysis, it was found that three major metal ions namely Ce3+, Ce4+, and Zn2+ coexist on the surface of the nanocomposite films. XPS data analysis also revealed that Ce3+ ion is oxidized to Ce4+ ion with increasing annealing temperature. Due to oxidation, the ratio of [Ce3±]/[Ce total] changed from 68.8 to 38.1% by increasing the annealing temperature from 180 to 500 °C. In addition, the Ce/Zn ratio increased from 0.21 to 0.42 when increasing the annealing temperature from 180 to 500 °C indicating migration of Ce ions toward the surface at higher temperatures. Finally, the XRD measurements determined that the ZnO thin films have a hexagonal wurtzite structure and CeO2 crystallites are formed at 500 °C in the Ce-doped ZnO nanocomposite thin films.