Synthesis and Optical Properties of Transition Metal Doped ZnO ...

of ZnO can be enhanced by creating more
defects on its surface. R. Wang et al. [9] has
demonstrated that silver ion doped ZnO has
improved photocatalytic activities owing to
the increased surface defects caused by the
enhanced oxygen vacancies. Doping of ZnO
with metal and transitional metals might
shift the optical absorption of ZnO to the
visible region i.e. to the longer wavelength.
This shifting of optical absorption of doped
ZnO will make this material capable to
operate at lower excitation energy and can
generate electron hole pair upon visible light
irradiation from solar spectrum. In a
technical study by K. Vanheusden et al. [13]
it has been observed that lead (Pb) doping in
ZnO narrows the effective band gap of ZnO
nanopowders and decreases both the
photoluminescence and the free-carrier
concentration. Doping of ZnO films with
Cobalt (Co) [12] has been reported to
significantly decrease the bang gap of ZnO
up to 2.75 ev. This decrease in the band gap
of cobalt-doped ZnO films resultantly
causes hyperchromic shift in its optical
absorption. Incorporating cupper and
manganese ions in ZnO thin films
separately, bring opposite effect on the grain
size of ZnO, cupper doping increases while
the manganese doping decreases the grain
size [14]. It has been reported that both the
dopant Cu and Mn resulted a slight decrease
in the optical band gap of ZnO films.
Similarly [5] band gap tailing effect has
been observed for aluminum doped ZnO,
which caused reduction in the band gap of
ZnO, likely due to the doping of donors. The
doping of ZnO nanocrystals with various
ions was accomplished by Y. S. Wang et al.
[15] using a family of dopants such as Cd,
Mg, Mn, and Fe ions. Shift in the band gap
was attributed to the effect of dopant, which
causes decrease in the band gap with Cd
doping while increase in the band gap for
other dopants. An enhancement in the
optical absorption has been found for
various dopants level in case of Fe and Mn
doped ZnO nanocrystals. It is well known
from the various studies that doping of ZnO
with metal and transition metals could
decrease the effective band gap and can
subsequently increase optical absorption of
this band gap tunable material. We
therefore, doped ZnO with manganese using
the co-precipitation techniques. This
material can be used as a better
photocatalyst and might have applications in
photoelectrochemical hydrogen production.
This study focused on the optical
characteristic of manganese doped ZnO
nanoparticles, synthesized by coprecipitation
techniques.
3. EXPERIMENT
The synthesis method described
earlier in our previous work [16] was
pursued with a little modification for
preparation of doped ZnO nanoparticles.
Two types of dopants, manganese and
cupper were investigated at the earlier
stages. In a typical process, 4 mili moles of
Zinc acetate dehydrate were dissolved in 40
ml of ethanol and heated at 50C along with
vigorous stirring for half an hour, thus
making precursor solution A. Since then 4
mili moles of Sodium hydroxide were
dissolved in 40 ml of ethanol and heated at
50 C along with vigorous stirring for one
hour, making precursor solution B. The
dopant solutions were also prepared by
dissolving 0.02 mili moles of manganese
acetate and cupper acetate each in 20 ml of
ethanol separately. The two solutions were
heated at 50 o C along with vigorous stirring
for half an hour.
In order to make ZnO:Mn 2+ /
ZnO:Cu 2+ colloids, a complex of 20 ml
precursor solution A and 20 ml dopant
(manganese acetate and cupper acetate)
solutions each were complexed and heated
at 80C for half an hour along with vigorous
stirring. After cooling to room temperature,
20 ml of precursor solution B (NaOH
solution) was mixed with the two complex
solutions (for hydrolysis), in order to
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