The structural parameters andphase purity of ZnO nanorods have been studied using powder X-ray diffraction(XRD) technique. Figure 2 (a) shows the typical XRD patterns of undoped andcobalt doped ZnO nanorods at different concentrations of cobalt.

All the peakson the diffraction patterns were well matched to the ZnO wurtzite-phasestructure (JCPDS No.36-1451) without any secondary phase. All the undoped andcobalt doped ZnO are highly c-axis oriented,and (002) peak position gradually shifted toward lower diffraction angle withhigher cobalt doping shown in the insetof Fig. 2(a).

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The intensity of (002) peak decreased whereas the intensity of (100) and (110) peak increased with increase in cobaltconcentration. This suggests that anincrease in cobalt concentration depreciates the ZnO crystallinity, which maybe due to stress caused by the differencein ionic radii between Zn++ and Co++, and the segregationof dopant in grain boundaries at very high cobalt concentration35,36. Li et al.37 studied the lattice parameters and particles size for0-7% of cobalt doped ZnO nanorods using XRD. Their results are presented in Table1. As seen from the table, cobalt doping slightly decreases the lattice parametersof ZnO nanostructures.

Such a decrease in lattice parameter with cobalt dopingwas observed in earlier reports38–40 as well, which stronglysuggest that Co++ are successfully substituted into the ZnO latticeat the Zn++ site. The ionic radius of Co++ (0.58 Å) issmaller than that of Zn++ (0.60 Å).20 Therefore, incorporation ofCo++ at Zn++ site generates strain and hence leads to adecrease in the lattice constant. These findings essentially indicate thatcobalt was doped into the ZnO lattice without changing the wurtzite structureof ZnO. Liu et al.

41 studied 0-40% cobalt dopedZnO nanorods and found that no secondary or impurities phase was detected byXRD. However, they found that theincrease of the lattice parameter and the peak shift towards lower Bragg diffractionangle. In our previous study14 on 0-9% cobalt doped ZnO nanorods, we observed that theintensity of (002) diffraction peaks decreased, and the peaks’ position has shifted towards the lower diffraction anglefor a higher concentration of cobalt, which implies that the Co++ dopantssubstituted the lattice of Zn++ ions. We believe that this variation observedis attributed to the ionic radii difference between Co++ ions (0.58 Å) and Zn++ions (0.

60 Å).42–44 Also, the lattice parameters(a and c) obtained from XRD show an increase in value with the higher cobaltdoping. There are other several reports which indicate that cobalt has alimited solubility in ZnO nanorod.45 Yeng et al.46 synthesized cobalt doped ZnO nanorodsby a hydrothermal process using zincnitrate and cobalt nitrate and observedthe appearance of the secondary impurity phase at 9.9% cobalt in ZnO.

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