American Journal of Applied Sciences (Special Issue): 25-29, 2005
ISSN 1546-9239
© 2005 Science Publications
Photopyroelectric Spectroscopy of MnO Doped Ceramic ZnO
at Different Sintering Temperatures
B.Z. Azmi, Zahid Rizwan, M. Hashim, A.H. Shaari, W.M.M. Yunus and E. Saion
Photoacoustic Laboratory, Department of Physics, Faculty of Science, Universiti Putra Malaysia
43400 UPM Serdang, Selangor D.E., Malaysia.
Abstract: The band-gap energy, Eg, of ZnO doped with 0.1 to 2.0 mol% of MnO and sintered at
different isothermal sintering temperatures is studied by photopyroelectric spectroscopy in the
wavelength range of 300 to 800 nm. This energy is estimated from the plot (ρhυ)2 versus hυ and is
about 3.0 eV for the samples sintered at 850°C at all doping levels. Samples sintered at higher
temperatures (1050 and 1300°C), the value of Eg decreases with the MnO mol% and beyond 1 mol%,
Eg becomes constant at about 2.0 eV. The increase in steepness factor (σA, in A-region and σB, in Bregion) is related to sintering temperature at low doping level. The dielectric constant, ranging from
300 to 3100, increases with the increase in sintering temperature and decreases with the increase in
frequency indicating the formation of insulating layer near the grain boundaries. The X-ray
diffractrometry shows that the crystal structure of ZnO doped with different MnO mol% at all sintering
temperatures remains to be of hexagonal type but a small peak is found related to the new phase
ZnMn2O4 only at 1050°C sintering temperature and 2 mol% of MnO doping level. The grain size
ranges from 2 - 30 µm, increases with the MnO mol% and sintering temperature. The density is
decreased from 95.5 to 87% with the increase of sintering temperature and doping level.
Key words: Photopyroelectric, band-gap energy, ceramic ZnO
present the photopyroelectric spectroscopy of MnO
doped ceramic ZnO at various sintering temperatures.
INTRODUCTION
The ZnO based ceramic semi-conductors are
widely used as gas sensors[1], piezoelectric transducers,
electrode for solar cells, phosphors, transparent
conducting films[2] and varistors. Regarding ZnO
varistor, it posses high-energy absorption capability
against various surges and is extensively used as
protective device. It has super-fast response to overvoltage transients as they sense and clamp transients in
nano-second speed, repeatedly in thousands of times
without being destroyed[2]. The varistors are typically
fabricated by sintering of ZnO with other metal oxides
of small amounts such as Bi2O3, Co3O4, Cr2O3, MnO,
Sb2O3, Al2O3, BaO, Y2O3, Pr6O11 etc. These additives
are the main tools that are used to improve the nonlinear response and the stability of ZnO varistor[3].
Much have been done in I-V studies on ZnO based
varistor by previous workers[3,4] and there are few
investigations have been carried out on optical
absorption measurements ceramic ZnO doped with
metal oxides because of the difficulty of conventional
transmitting methods. It is necessary to get information
on optical absorption of ceramic ZnO doped with metal
oxides for investigation on electronic states of ceramic
ZnO and doped impurities at different levels during
sintering process that can help the understanding of the
electrical behaviour of doped ZnO. In this study we
MATERIALS AND METHODS
Sample preparation: ZnO (99.9% purity, Alfa Aesar)
was doped with MnO (99.5% purity) varied as 0.1, 0.4,
0.7, 1.0, 1.3, 1.6, 2.0 mol% (mole percentage). The 30
g product at each mol% was mixed with ethanol and
was stirred for 24 h. The slurry was filtered and dried
in air for 48 h. The mixture was ground to make a
powder of fine particles before pre-sintering. The
powder of each mol% was divided into three parts and
was pre-sintered at temperatures 800, 1000 and 1250°C
for 2 h at the rate of 10°C min¯1. Then each sample was
ground and polyvinyl alcohol (1 wt %) was mixed as a
binder to give strength and also to avoid cracks in the
final pressed product. The mixture was pressed in a
Specac press under a force of 2 tons using zinc stearate
as a lubricant to form a disk of 10 mm diameter with 1
mm thickness. Finally the pellets were sintered at 850,
1050 and 1300°C for 1 h in air at heating and cooling
rate of 4°C min¯1. The density of the ceramic ZnO
doped with MnO was measured by the Archimedes
method. The disk from each sample was ground for 2 h
and granulated by sieving through a 75-mesh screen for
the photopyroelectric (PPE) spectroscopy and XRD
analysis.
Corresponding Author: B.Z. Azmi, Photoacoustic Laboratory, Department of Physics, Faculty of Science, Universiti Putra
Malaysia, 43400 UPM Serdang, Selangor D.E., Malaysia.
25
Am. J. Applied Sci., (Sp. Issue): 25-29, 2005
0
ZnO + 2mol% MnO Sintered at 1050 C
20
30
40
50
103
200
112
201
004
202
110
Pure ZnO
102
100
002
101
220
Intensity (arb. Unit)
ZnMn 2O4
Photopyroelectric
measurements:
The
PPE
spectroscopy, a powerful technique from photothermal
science is a non-radiative tool[5] to study optical
properties of the materials. The method is based on
photothermal effect where the pyroelectric (PE) film
transducer is used to detect the temperature variation
from the light-induced periodic heating in the sample.
When there is absorption of incident light, the nonradiative de-excitation processes with in the solid will
cause the sample temperature to fluctuate, through heat
diffusion to the surrounding PE film. Due to this
temperature change, a PE voltage at modulation
frequency (ω) is observed in the PE film and is given
by V(ω) = pI d < ∆T > /ε ,[6] where p is the PE
coefficient, Id is the film thickness, ε is the film
dielectric constant and < ∆T > is the average
temperature rise in the film. PPE signal amplitude was
measured for a range of optical excitation wavelengths
300 to 800 nm using the PPE spectrometer system as
described elsewhere[7] to produce a PPE spectrum
which is actually an excitation spectrum. For sample
treatment prior the PPE measurement, fine powder was
again ground in deionised water, then few drops of the
each mixture were dropped on an aluminium foil of
area 1.5 cm2 and dried in air to form a thin layer of
sample on the foil. The foil was placed in contact to a
Polyvinylidene Difluoride PPE film transducer[8] using
silver conductive grease. A high power 1 kW Xenon
arc lamp (Oriel 6921) was used as the light source in
the spectrometer system and the light beam was
mechanically chopped at 9 Hz. The PPE spectrum data
of the sample ZnO doped with MnO at room
temperature were accumulated in a personal computer
and normalized with respect to carbon black PPE
spectrum data to obtain the true sample spectrum.
For the determination of band-gap energy, it was
assumed that the fundamental absorption edge of ZnOX metal oxide is due to the direct allowed transition.
The optical absorption coefficient β varies with the
excitation light energy hυ[9] and is given by the
expression, (βhυ)2 = C (hυ-Eg) near the band gap,
where C is the constant independent of photon energy
hυ, and Eg is the direct allowed band-gap energy. The
PPE signal intensity ρ is directly proportional to β,
hence (ρhυ)2 is related to hυ linearly. From the plot of
(ρhυ)2 versus hυ, the value of Eg is obtained by
extrapolating the linear fitted regions to (ρhυ)2 = 0 , as
shown in Fig. 2.
60
70
80
0
Position ( 2Theta )
Fig. 1: XRD pattern for pure ZnO and doped ZnO with
MnO.
o
850 C, Eg=2.96eV
25
o
1300 C, Eg=1.89eV
20
15
o
(ρνh)
2
1050 C, Eg=2.06eV
10
5
0
1.5
2.0
2.5
3.0
3.5
4.0
Photon Energy,eV
Fig. 2: Band-gap energy of ZnO doped with 1 mol% of
MnO at different sintering temperatures.
Microstructure examination: The either surface of
samples was lapped and ground with SiC paper and
polished with 1 µm diamond suspension to a mirror like
surface. The polished samples were thermally etched at
the temperature 150°C below the sintering temperature
for 10 min. The surface microstructure was examined
by a scanning electron microscope (SEM). The average
grain size (d) was determined by the linear intercept
method, given by d = 1.56L/MN , where L is the
random line length on the micrograph, M is the
magnification of the micrograph and N is the number of
grain boundaries intercepted by lines. The X-ray
diffraction (XRD) with Cu Kα radiation using
PANAalytical (Philips) X’Pert Pro PW1830 was used
to identify the crystalline phases. The XRD data were
analysed by using X’Pert High Score software for the
identification of the crystalline phases.
Dielectric dispersion measurements: The frequency
dependence of dielectric constant for the MnO doped
ZnO ceramic samples at sintering temperatures 850,
1050, 1300°C at all MnO doping level were scanned
from 100 Hz to 1 MHz at room temperature[10], using
impedance analyzer (Hewlett Packard Model 4192A).
26
Am. J. Applied Sci., (Sp. Issue): 25-29, 2005
transition metal oxides, such as Mn, is involved in the
formation of interfacial states and deep bulk traps at
RESULTS AND DISCUSSION
The XRD pattern in Fig. 1 shows that the crystal
structure of pure ZnO and ZnO doped with 2 mol% of
MnO at all sintering temperatures 850, 1050, 1300oC
remain to be of hexagonal wurtzite-type structure. No
extra peak was found at 850 and 1300°C sintering
temperatures at all doping levels, indicating the
substitution of Mn2+ ions inside the lattice of ZnO and
the absence of any free MnO. The presence of a small
peak at the diffraction angle of 44.66o for the plane
(220) found in the pattern related to the new phase
ZnMn2O4 only at 1050°C sintering temperature at 2
mol% of MnO doping level, indicates the significant
amount of the new phase is only found at this
temperature and it may present at 850, 1300°C sintering
temperatures but in a very low quantity which cannot
be detected. It is also expected that low quantity that
detected only at 1050°C have no effect on the optical
absorption spectra. Therefore, it is considered that
presence of second phase ZnMn2O4 and an amorphous
Mn-rich thin film, in a nanometer scale that is not
detected in XRD pattern, is formed at the grain
boundaries at 850 and 1300°C sintering temperatures.
The presence of a second phase or amorphous film can
benefit grain boundary diffusion, thus promoting the
grain growth of ZnO at all sintering temperatures. The
density was decreased from 95.5% to 92.3% of
theoretical density 5.78 g/cm3 [4] from low to high
sintering temperatures upto 1mol% and then decreased
from 92.3% to 87% with the increase of sintering
temperature and doping level, but the decrement is very
prominent with sintering temperature. Therefore, it is
expected that there is an abnormal grain growth with
the increase of doping level and sintering temperature
as the abnormal grains were seen in SEM with irregular
shapes. The grain size is ranged from 2-5, 3-20 and 830 µm by increasing MnO mol% at 850, 1050, 1300°C
sintering temperatures, respectively, which indicates the
grain promotion property of MnO.
ZnO behaves as a n-type semiconductor, as its
conduction band arises from the 4s orbital of Zn is wide
enough to favour the effective charge transfer.
Electrons are excited from its valence state due to the
absorption of light so this process generates the
electron-hole pairs, which starts the conduction
mechanism. Above the band-gap energy limit, the PE
signal is due to non-radiative process and that of below
is due to the Urbach tail[11]. The value of Eg about
constant 3.0 eV that is very close to that of pure ZnO,
Fig. 3, indicates that at all doping levels the probability
of the substitution of the Mn2+ ions into the grain
boundaries is very low at 850°C sintering temperature.
In practice, there are a variety of inter-grain conduction
paths that operate in parallel[12], such as through the
bulk inter-granular material or through the grain
boundary region and are sensitive to the presence of
chemical additives.
It has been postulated that
3.4
3.2
o
850 C
3.0
2.8
2.6
2.4
Eg (eV)
o
1050 C
2.2
2.0
o
1.8
1300 C
1.6
0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
MnO doping mol %
Fig. 3:Dependence of energy band gap at MnO mol% at
different sintering temperatures.
1.0
0.9
At 2 mol% MnO
Intensity (arb.units)
0.8
0.7
o
1050 C
0.6
o
1300 C
0.5
o
0.4
850 C
0.3
300
400
500
600
700
800
Wavelength (nm)
Fig.4: PPE spectra for 2 mol% of MnO different
sintering temperatures.
At 2 mol% MnO
σΑ
0.9
σΒ
0 oC
0.8
σΒ
130
0.7
σΑ
0.6
850 o
C
1.0
105 o
0C
Ln(PPE Signal Intensity), arb. units
1.1
0.5
0.4
0.3
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Photon Energy (eV)
Fig. 5: PPE signal intensity spectra for different sintring
temperatures.
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Am. J. Applied Sci., (Sp. Issue): 25-29, 2005
and also the decrease in the Eg is very low ranging from
0.14
σB (1300 C)
At 2 mol% MnO
0.13
o
0.12
Steepness Factor (σA, σΒ)
0.11
0.10
0.09
σB (1050 C)
0.08
o
0.07
0.06
0.05
σA (1050 C)
0.04
o
0.03
σA (1300 C)
0.02
o
0.01
0.0
0.5
1.0
1.5
2.0
MnO mol%
Fig.6: Dependence of the steepness factor σA and σB on
MnO mol%.
3500
3000
2500
Dielectric Constant
grain boundaries, providing large potential barriers to
give better nonlinear characteristics when the I-V
characteristics of the material was studied as a
varistor[3]. The PPE spectra of samples can be seen in
Fig. 4 with the PPE signal intensities normalized to that
of lower wavelength range. The PPE intensity is higher
at high sintering temperatures indicating that the
phonon-phonon interaction is greater at the higher
wavelength. PPE signal intensities plotted against
photon energy (hυ) vary linearly just below the
fundamental absorption edge in two regions (denoted as
A and B) in accordance with Urbach’s rule (exponential
tail)[13]. Therefore, an empirical relation for the
dependence of PPE signal intensity (P) on the photon
energy and sintering temperature (T) has been fitted to
the data according to:
P = Po exp[σ(hυ - hυ o )K ]
where K = (kT) −1 , k is the Boltzman’s constant, P0 , υ o
are the fitting parameters[14] and σ is the steepness
factor that characterized by the exponential optical
absorption. Hence from these spectra in Fig. 4, the
steepness factor is obtain by the above equation and is
presented in Fig. 5. Steepness factor cannot be
observed at 850 o C sintering temperature. In Fig. 6, the
steepness factors σA (region A) and σB (region B) are
plotted against MnO mol% of doping for sintering
temperatures of 1050 and 1300 o C . It is noted that at
low doping about 0.1 mol% for both 1050 and 1300°C
sintering temperatures, σB and σA roughly have equal
value about 0.08 but later σB increases but σA decreases
with doping. At low doping (< 0.4 mol%) with the
increase of sintering temperature both σB and σA
increase indicating an increase in structural ordering[15].
At higher doping (> 0.4 mol%) with the increase of
sintering temperature, σB still increases indicating an
increase in structural ordering but σA decreasing
indicating a decrease in structural ordering. This
suggests that probing at higher photon energy and at
higher MnO doping level in itself disturbed the
orientation of the polycrystalline structure of doped
ZnO. Also, at higher doping (> 1.0 mol%), both σB and
σA are about constant with the increase of doping level
that indicates MnO mol% plays no further contribution.
Dielectric constant of bulk ZnO[16] is less than 10
but for the present doped ceramic ZnO it ranges from
300 to 3100 and decreases with the increase of
frequency at all MnO doping levels, as can be seen in
Fig. 7. However, for sample sintered at 1300°C and
sintered the decrement is very rapid with the rise of
frequency. The increase in the dielectric constant at
higher temperature indicates the segregation of MnO
forming insulating layer in or near the grain boundaries
and the decrease in Eg ranging from 3.2-2.0 eV is also
observed at higher temperatures indicating the
introduction of interfacial states.
The dielectric
constant has low value at lower sintering temperature
At 0.1 mol% MnO
2000
o
1500
1300 C
1000
500
o
1050 C
0
o
850 C
2
3
4
5
6
Log F (Hz)
Fig. 7: The frequency dependence of dielectric constant
for 0.1 mol% of MnO at different sintering
temperatures.
3.2-3.0 eV indicating the possibility of substitution of
Mn2+ ions in the grain boundaries.
CONCLUSION
The XRD results and MnO doping level are
correlated with the PPE spectroscopy results indicating
that the optimum concentration of MnO in ceramic ZnO
is possible upto certain mol% at certain ceramic
processing conditions. The PPE spectroscopy of ZnO
doped with different mol% of MnO shows the decrease
in band-gap energy, increase in steepness factor at high
sintering temperatures indicating the formation of
interfacial states and structural ordering. The dielectric
constant decreases with increasing frequency and has
higher values at higher temperatures indicating the
formation of microstructure of semiconducting ZnO
surrounded by insulating layer.
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Am. J. Applied Sci., (Sp. Issue): 25-29, 2005
8.
ACKNOWLEDGEMENTS
The authors would like to thank the Ministry of
Science, Technology and Environment of Malaysia for
the financial support of this work under IRPA Grant
No. 02-02-04-0132-EA001.
9.
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