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  • Author: L. Hammiche x
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Abstract

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.

Abstract

This investigation concerns the synthesis as well as structural and morphological characterizations of pure and Ce-doped ZnO nanorods. The samples were synthesized by simple low-temperature hydrothermal process using respectively NaOH and KOH as caustic bases. The as-synthesized nanorods were characterized in terms of their morphological, structural, compositional and vibrational properties. The sizes of the rods were found to be 1.5 μm to 2 μm in length and 250 nm to 300 nm in diameter. The presence of Ce ions in ZnO (NaOH) favored the agglomeration of the rods to form flower-like nanostructures. EDAX measurements showed Zn rich materials with high oxygen vacancy concentration. XRD results indicated that the synthesized ZnO nanorods possess a pure wurtzite structure with good crystallinity. It has also been found that Ce doping deteriorates the crystalline quality of ZnO (NaOH) and improves that of ZnO (KOH). The insignificant intensities observed in FT-IR signals confirm that the synthesized nanorods are of high purity. The Raman spectroscopy studies showed that Ce ions shift the vibrational modes towards lower frequencies. The peaks related to E2 (high) mode in ZnO (KOH) are relatively intense compared to those of ZnO (NaOH). The peaks are found to be shifted and asymmetrically broadened due to anharmonic effects originating from quantum-phonon-effect confinement.