The structural parameters and
phase purity of ZnO nanorods have been studied using powder X-ray diffraction
(XRD) technique. Figure 2 (a) shows the typical XRD patterns of undoped and
cobalt doped ZnO nanorods at different concentrations of cobalt. All the peaks
on the diffraction patterns were well matched to the ZnO wurtzite-phase
structure (JCPDS No.36-1451) without any secondary phase. All the undoped and
cobalt doped ZnO are highly c-axis oriented,
and (002) peak position gradually shifted toward lower diffraction angle with
higher cobalt doping shown in the inset
of Fig. 2(a). The intensity of (002) peak decreased whereas the intensity of (100) and (110) peak increased with increase in cobalt
concentration. This suggests that an
increase in cobalt concentration depreciates the ZnO crystallinity, which may
be due to stress caused by the difference
in ionic radii between Zn++ and Co++, and the segregation
of dopant in grain boundaries at very high cobalt concentration35,36. Li et al.37 studied the lattice parameters and particles size for
0-7% of cobalt doped ZnO nanorods using XRD. Their results are presented in Table
1. As seen from the table, cobalt doping slightly decreases the lattice parameters
of ZnO nanostructures. Such a decrease in lattice parameter with cobalt doping
was observed in earlier reports38–40 as well, which strongly
suggest that Co++ are successfully substituted into the ZnO lattice
at the Zn++ site. The ionic radius of Co++ (0.58 Å) is
smaller than that of Zn++ (0.60 Å).20 Therefore, incorporation of
Co++ at Zn++ site generates strain and hence leads to a
decrease in the lattice constant. These findings essentially indicate that
cobalt was doped into the ZnO lattice without changing the wurtzite structure
of ZnO. Liu et al.41 studied 0-40% cobalt doped
ZnO nanorods and found that no secondary or impurities phase was detected by
XRD. However, they found that the
increase of the lattice parameter and the peak shift towards lower Bragg diffraction
angle. In our previous study14 on 0-9% cobalt doped ZnO nanorods, we observed that the
intensity of (002) diffraction peaks decreased, and the peaks’ position has shifted towards the lower diffraction angle
for a higher concentration of cobalt, which implies that the Co++ dopants
substituted the lattice of Zn++ ions. We believe that this variation observed
is 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 cobalt
doping. There are other several reports which indicate that cobalt has a
limited solubility in ZnO nanorod.45 Yeng et al.46 synthesized cobalt doped ZnO nanorods
by a hydrothermal process using zinc
nitrate and cobalt nitrate and observed
the appearance of the secondary impurity phase at 9.9% cobalt in ZnO.