Effect of cerium on structural and optical properties of ZnO aerogel synthesized in supercritical methanol

Pure ZnO and Ce-doped (5 at.%) ZnO aerogels were prepared by dissociation of dihydrate zinc acetate and cerium nitrate in methanol, followed by drying in supercritical conditions of the solvent. The concentration of zinc acetate solution and the atomic ratio Ce/Zn were fixed at 0.2 mol/L and 0.05 mol/L, respectively. XRD results showed a hexagonal wurtzite structure of ZnO aerogel crystallites and a formation of a separate second phase of ceria cubic phase with fluorite structure and 7.4 nm in size. The introduction of cerium in the solution had a negative effect on the aerogel crystalline quality. The crystallites size was found to be 16 nm in pure ZnO and 27 nm in Ce-doped ZnO aerogels. The grains of Ce-doped ZnO aerogel had torus shaped morphology with hollow centers, but those of pure ZnO were flattened semispheres. The calculated values of different structural parameters showed that cerium ions introduced into the ZnO lattice occupied interstitial sites and Zn ions substituted Ce ones in ceria lattice during the formation process. FT-IR and UV-Vis absorption spectra have not revealed any particularities due to the presence of cerium atoms in ZnO, indicating that Ce (5 at.%) doping of ZnO crystallites synthesized in supercritical methanol did not strongly affect the optical gap of the semiconductor. Micro-Raman studies confirmed the formation of cubic fluorite structure ceria in ZnO aerogel and showed that Raman active modes of ZnO are amplified with the presence of ceria.

Synthesis and Structure of Zinc Oxide Aerogel

The zinc-based aerogel was synthesized via a sol-gel route that it was processed using Zinc chloride as precursor, polyacrylic acid as the template and propylene oxide as the gelation agent. The Zn-based aerogel is formed the length and cross-link of the zinc chloride hydroxide chain, owing to a dominant influence of PAA on the nucleation and growth position of sol nucleus. Nevertheless, ZnO aerogel was prepared with the method that the zinc-based aerogel was calcined at 400°C. Consequently, the RWP value was in reasonable 7.35% according to X-ray diffraction pattern (XRD) refinement. The crystal system of the ZnO aerogel was monoclinic.