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

Synthesis and Optical Properties of Transition Metal
Doped ZnO Nanoparticles
Ruh Ullah1, Joydeep Dutta2
1Department of Electronic Engineering NUST Institute of Information Technology Chaklala Scheme III,
2Center o f Excellence in Nanotechnology, Industrial Systems Engineering School of Engineering &
2Technologies Asian Institute of Technolo
1email: ruhullah@niit.edu.pk, ruhullahg@gmail.com
2email: joy@ait.ac.th
1. Abstract
Enhancement in the optical absorption of
metal and transition metal doped ZnO nanoparticles
could make this material capable to work as an
efficient photocatalysts. Doped ZnO nanoparticles
were synthesized via co-precipitation techniques.
Doping of ZnO with manganese (Mn2 +) and cupper
was intended to enhance the surface defects of ZnO.
These can subsequently be used as efficient centers
for optical ab sorption. Nanoparticles prepared with
these techniques, which were characterized with
transmission electron microscopy (TEM), infrared
spectroscopy (FTIR), photo-co-relation spectroscopy
(PCS) and UV/VIS-spectroscopy showed significant
enhancement in the optical absorption when
compared with the undoped ZnO. Enhancement in
the optical absorption of Mn-doped ZnO indicates
that it can be used as an efficient photocatalyst under
visible light irradiation and might have applications
in the photoelectrochemical hydrogen production. It
was found that manganese-doped ZnO (ZnO:Mn2+ )
absorbs more visible light in comparison to cupper-
doped ZnO (ZnO:Cu2+ ) when exposed to tungsten
bulb. Key words: ZnO, manganese doping, photocatalysts,
optical absorption
2. INTRODUCTION
optoelectronic devices [2], ferromagnetic
devices and heterogeneous photocatalysts
[3-4]. ZnO is an interesting wide band gap
(3.3ev) metal oxide semiconductor martial
and is currently under investigation owing to
its tremendous applications in response to its
tunable properties [5-6]. Magnetic,
electrical, optical and catalytic properties of
ZnO are mostly size and surface dependent.
Attempts [6-9] have been made to improve
these properties and enhance the industrial
applications of ZnO nanoparticles\nano-
devices. Since surface properties such as
oxygen deficiencies and area play important
role in heterogeneous photocatalysis, these
can be enhanced by doping ZnO with metal
ion [9]. In addition, doping of ZnO with
metal and transition metal ions has been
reported to control the size of ZnO crystal
and improve the magnetic properties of
ZnO. Viswanatha et al, [10] has doped ZnO
with Mn to control the growth of particle
and get smaller sized single crystallites and
was deemed to have applications in
spintronics owing to some unusual magnetic
behavior. Blue shift in the absorption edge
compared to the bulk was attributed to the
quantum confinement effect. S. Sakthivel et
al. [11] has demonstrated that ZnO can
comparatively absorb more light than TiO2
in the region where the light absorption
occurs due to band gap excitation. The study
ZnO has been extensively studied as
a potential material for various applications
such as sensors, varistors, piezoelectric [1]
transducers, surface acoustic wave devices,
phosphors, transparent conducting oxides,
further suggests that, the optical absorption
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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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transform zinc hydroxide to ZnO. The
solutions were kept in water bath at
60~65C for 2 hours. It was observed that
solutions started precipitating after one hour
in water bath. Subsequently to the 2 hours
water bath, the solutions were cold down to
room temperature followed by 4 hours
aging. The colloidal solutions were
centrifuged for 20 minutes at 4 k rpm to
remove the large sized agglomerates. It was
observed that nanoparticles of almost
uniform size were suspended in the solution.
ZnO:Mn 2+ / ZnO:Cu 2+ nanoparticles thus
synthesized were then used for further
experimental analysis.
Optical characteristics of doped ZnO
(ZnO:Mn 2+ , ZnO:Cu 2+ ) were determined
with double beam UV/VIS
spectrophotometer (Model SL 164 from
ELICO). Manganese doped ZnO
nanoparticles were further studied based on
the enhanced optical absorption when
compared with ZnO:Cu 2+ . Therefore
structural characterizations of only
ZnO:Mn 2+ were carried out with
Transmission Electron Microscope
(JEOL/JEM-2100F version) operated at
200KV, Fourier Transform Infrared
Spectroscope (System 2000 FTIR, Perkin–
Elmer).We used PCS machine from
MALVERN Instrument Zetasizer Nano
Model ZS Zen3600 fitted with a red laser
(633nm) which can measure particle size
within a range of 0.6nm to 600nm. Folded
Capillary Cell (DTS1060) was used for zeta
potential measurements and Disposable low
volume polystyrene (DST0112) cuvette was
used for size measurement
4. Results and discussion
Synthesis of doped ZnO was
performed in alcoholic solution
consecutively to avoid formation of ZnOH
[17]. Therefore zinc acetate, manganese
acetate, cupper acetate and NaOH all were
dissolved in ethanol. The nucleation and
aggregation of nanoparticles are strongly
solvent dependent, and are increasing with
decreasing the dielectric constant of solvent
[ 1 8]. Water has a dielectric constant of about
80 while for ethanol it is 24.3. The
nucleation and growth of ZnO is faster in
ethanol than in water and hence ZnO doped
colloids were synthesized in ethanol in order
to avoid oxidation of dopant ions. UV/VISspectroscopy
of both the cupper doped ZnO
(ZnO: Cu 2+ ) and manganese doped ZnO
(ZnO:Mn 2+ ) as well as undoped [16] newly
prepared nanoparticles showed evidence of a
significant divergence in the absorption
intensity in the blue region, as shown in
figure 1. This enhancement in the absorption
intensity within the visible region is
attributed to the doping of ZnO with Cu, and
Mn. Figure 1 further illustrates that Mn ions
affect the absorption characteristic of the
nanoparticles more markedly than Cu ions.
This increase in the absorption intensity in
the blue region can be attributed to the more
pronounced doping of ZnO with manganese
ion [12, 19]. It demonstrates that manganese
doping in ZnO creates more defects sites as
compared to Cu doping [20].
Figure 1. UV Visible spectroscopy of cupper
doped, manganese doped and undoped ZnO
Fourier Transform Infrared
Spectroscopy of the hydrolysed particles
(figure 2) shows strong peaks at 1562 cm -1
indicating the formation of ZnO [ 21 ] and
peak at 1404 cm -1 that may be assigned to
the symmetric stretching of carboxylate
group (COO - ) probably from the un-reacted
acetates. We assume here that solubility of
Cu in ZnO is less than that of the
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manganese; therefore strong peaks of ZnO
are observed when manganese acetate
solution was mixed with zinc acetate
solution.
% Transmission
Wave number (cm-1 )
Figure. 2 FTIR spectroscopy Of Zinc acetate
complexed with Cu and Mn
Based on the highest absorption
characteristic of ZnO:Mn 2+ and solubility of
manganese ion in ZnO, we further carried
out study only on ZnO:Mn 2+ nanoparticles.
The main goal of this work was to increase
the optical absorption of ZnO nanoparticles
by doping it with metal and/or transition
metal. The optimum dopant (Mn)
concentration was found to be 1%, because
increase in the dopant concentration causes
reduction in the optical absorption as shown
in figure 3.
Figure 3. Effect of dopant concentration on
optical absorption of ZnO:Mn 2+
This decrease in the optical
absorption at higher dopant concentration
has been demonstrated to modify the
morphology and growth of ZnO i.e. changes
ZnO from crystalline form to amorphous
form [12]. However we assume that being a
highly reactive, Mn may react more readily
with oxygen to form MnOx instead of taking
interstitial or substitutional site in ZnO
crystal. Transmission electron micrograph of
the ZnO:Mn 2+ shows polycrystalline
structure. The image shown in figure 4
reveals that about 3-5 nm sized crystalline
have been agglomerated into 30-40 nm
particles. Photo-co-relation spectroscopy
(PCS) of the colloids also represents the
same average size of particle as
demonstrated by TEM.
Reverse FFT
(a)
FFT
5 nm
TEM micrograph (b)
Figure 4. TEM micrograph of ZnO:Mn 2+ .
Figure (a) TEM image showing particle size
of 37 nm, while Figure (b)on right hand side
(top) is the Fast Fourier Transform analysis
of TEM image, on the top lift of this image is
reverse FFT representing crystallographic
plans.
Crystallographic planes (shown in
figure 4) of ZnO:Mn 2+ nanoparticles were
determined by applying Fast Fourier
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Transform (FFT) and inverse FFT to the
transmission electron micrographs. The
measured lattice spacing of 2.81Å and 1.9 Å
correspond to the (100) and (102) planes of
the Wurtzite structure [22, 23].
Modification in the band gap
structure of ZnO through manganese doping
has been reported to cause ferromagnetic
ordering [24]. We therefore assume that
doping of ZnO with metal and transition
metals add tails within the conduction and
valence band. Thus visible light energy will
be enough to excite electrons from tails state
to the conduction states. Further, ZnO doped
with Mn 2+ is more effective than
ZnO:Cu 2+ in absorbing visible light This
highest absorption intensity of Mn 2+ doped
ZnO is attributed to the lowest dopant level
of Mn 2+ with in the band gap of ZnO as
compared to that of Cu 2+ ion.
5. Conclusion
In summary we have shown that manganese
doping of ZnO nanoparticles have decrease the native
band gap of ZnO and created more defects sites on
the ZnO surface. The nanoparticles synthesized by
co-precipitation techniques have poly crystalline
structure with average particles size of 37 nm. The
increased surface defects are capable to absorb more
visible light. This newly synthesized manganese
doped ZnO will have applications in
electrophotochemical hydrogen production and
heterogeneous photocatalysis. It will make the
photocatalyst capable to work with only visible light
irradiation and will eliminate the need of UV light.
The preliminary results suggest that manganese
doped ZnO nanoparticles can be used as immobilized
photocatalysts for water and environmental
detoxification from organic compounds.
6. Acknowledgment
The author gratefully acknowledge the
financial and intellectual support from Asian
Institute of Technology Bangkok, Thailand
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