UV-Vis band gap from 3.10 eV to 3.24 eV

UV-Vis absorption spectra have been used to find out the changes in the
energy band structure of ZnO nanorods due to cobalt doping. Primarily it is
used to find out the band gap of the material. As shown in Fig. 4(a) the
optical absorption of all the three samples show a strong absorption around
370–390 nm and the absorption edge is
shifted to higher wavelength upon cobalt doping in ZnO. For undoped ZnO nanorods, there is very rapidly rising
absorption edge at around 376 nm. For the samples with 20% cobalt doping, this
absorption edge shifted from 375 nm to 389 nm. This redshift could be due to
the increase in compressive stress as doping level is increased. Also, the
absorption intensity slightly increases
with cobalt doping. This enhancement of absorbance in the visible region is
mainly due to the increase in defect concentration that creates deep levels in
the band gap of ZnO nanorods.74 The lattice defects increase with the cobalt concentration in ZnO, and the replacing of Co++ ions
in the ZnO lattices causes the increase in the absorption of light.75

Our previous study14 showed the decrease in the optical band gap
from 3.31 eV to 3.15 eV. A redshift of
the optical band gap in ZnO nanorods with cobalt
doping has been previously reported76 and which can be explained by the sp-d
exchange interactions between the band electrons and the localized d-electrons
of the Co++ that substitutes the Zn++
ions.77
Such a compositional-dependent shift in
the band gap energy has been previously reported in cobalt-doped ZnO nanorods by Wu et al.78 However, some other literatures79–82 showed a blue-shift in the cobalt
doped ZnO as the cobalt concentration is increased, that was attributed to the
Burstein–Moss (BM) effect.36,83 Chanda et al.58 observed the increase in band gap from 3.10
eV to 3.24 eV as cobalt concentration increased from 0% to 10%. It is believed
in some of the metal oxide systems that
particle size reduction results in a blue shift of bandgap due to quantum confinement
effect.84However, it is also known that quantum confinement
effect is not only the factor, doping can also modify the local lattice
symmetry and introduce defect centers into the lattice which can make an
alteration in the band structure and cause the large change in ZnO nanorods.85,86 A similar type of phenomena has been
observed by Ivill et al.,71 Mera et al.,87 Caglar,88 and Husain et al.89 in cobalt-doped
ZnO nanostructures.

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