XRD, XPS, optical, and Raman investigations of structural changes of nanoCo-doped ZnO

The mixed oxides Zn_1-xCo_xO (ZCO) (0.0 ≤ x ≤ 3.0) were prepared as nanopolycrystalline powders and thin films by a simple sol-gel process and dip coating method. Structural and microstructural analysis was carried out applying X-ray diffraction (XRD) and Rietveld method. Analysis showed that for x < 0.12, Co²⁺ replaces Zn²⁺ substitutionally yielding ZCO single phase, while for x ≥ 0.15 two phases are identified; ZCO and Co₃O₄. Replacing Zn⁺² by Co⁺² affects the lattice parameters in opposite ways, the parameter c decreases while a increases with an overall decrease in the ratio c/a of the wurtzite ZCO, which deviates the lattice gradually from the hexagonal structure as Co⁺² increases. The IR analysis shows red shift entirely dependent on Co concentration. With increased concentration of cobalt the E2high Raman mode exhibits a redshift attributed to the phonon softening caused by the ab-plane lattice expansion, while the E2low mode shows a blueshift due to the reduced mass effect caused by cation replacement.