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

The mixed oxides Zn1−xCoxO (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 E 2 high Raman mode exhibits a redshift attributed to the phonon softening caused by the ab-plane lattice expansion, while the E 2 low mode shows a blueshift due to the reduced mass effect caused by cation replacement.