Increased conductivities of Cr doped Al2-xCrxO 3 powders due to band gap narrowing

Article abstract of DOI:10.1016/j.physb.2013.11.059

A high Cr content in the synthesized Al_2-xCr_xO ₃ materials was achieved via a new synthesis route, the self propagating combustion method, for investigation of the effect of Cr substitution on the electrical, optical band gap and structural characteristics of the modified Al₂O₃ materials. X-ray diffraction (XRD) results showed that all the samples were pure and that Cr was successfully substituted in the crystal lattice. The cell parameters and volume are linearly dependent on the Cr content. AC impedance spectroscopy results show that conductivity of the Cr doped samples increases exponentially with Cr content. This is attributed to band gap narrowing of the Al_2-xCr _xO₃ powders as obtained from UV-visible spectrophotometric studies.

Full text of DOI:10.1016/j.physb.2013.11.059

Physica B 437 (2014) 32–35
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Physica B
journal homepage: www.elsevier.com/locate/physb
Increased conductivities of Cr doped Al2ꢀxCrxO3 powders due to band
gap narrowing
a,b
Nurhanna Badar a,b, Norlida Kamarulzaman a,b, , Roshidah Rusdi
,
n
Nor Diyana Abdul Aziz a,b, Hoong Kun Fun c,d
a Centre for Nanomaterials Research, Institute of Science, Level 3 Block C, Universiti Teknologi MARA, Shah Alam, Selangor 40450, Malaysia
b School of Physics and Materials Studies, Faculty of Applied Sciences, Universiti Teknologi MARA, Shah Alam, Selangor 40450, Malaysia
c School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia
d Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, P.O. Box 2457, Riyadh 11451, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
A high Cr content in the synthesized Al2ꢀxCrxO3 materials was achieved via a new synthesis route, the
self propagating combustion method, for investigation of the effect of Cr substitution on the electrical,
optical band gap and structural characteristics of the modified Al2O3 materials. X-ray diffraction (XRD)
results showed that all the samples were pure and that Cr was successfully substituted in the crystal
lattice. The cell parameters and volume are linearly dependent on the Cr content. AC impedance
spectroscopy results show that conductivity of the Cr doped samples increases exponentially with Cr
content. This is attributed to band gap narrowing of the Al2ꢀxCrxO3 powders as obtained from UV–visible
spectrophotometric studies.
Received 18 August 2013
Received in revised form
27 November 2013
Accepted 29 November 2013
Available online 18 December 2013
Keywords:
Ceramics
Powder technology
X-ray techniques
Structural
& 2013 Elsevier B.V. All rights reserved.
Electrical properties
1. Introduction
It is also found that as the Cr content increases, band gap values
decrease.
Corundum or
α
-Al2O3 is the most stable form of aluminium
oxide. It is known that Al2O3 is highly insulating with a large band
gap of 8.8 eV [1–3]. The electrical properties of materials can be
changed by doping [4]. Not much work has been done on the
electrical effect of substitutional doping of metal oxide insulators
such as Al2O3. It is therefore interesting to study the effects of
doping on the electrical characteristics of Al2O3.
Corundum is a very stable compound with a high melting point
of 2054 1C [5]. Therefore, it will not be easy to synthesize pure and
single phase Cr substituted samples, Al2ꢀxCrxO3, unless a suitable
synthesis method is used and the thermal annealing is done at the
correct temperature. In this work, substitutional doping of Cr in
Al2O3 is done via a self propagating combustion synthesis method.
This synthesis method is found to be useful in obtaining pure and
single phase metal oxides with complex stoichiometries [6–8]. The
conductivities of the Al2ꢀxCrxO3 have been found to increase
by the order of one in the highest Cr content sample, Al1.7Cr0.3O3.
2. Experimental
Al2O3 and Al2ꢀxCrxO3 (x¼0.1, 0.2, 0.3) materials were prepared
using a self-propagating combustion method. Starting materials of
aluminium nitrate nonahydrate and chromium nitrate nonahy-
drate (Merck) both of 99.9% purity, were dissolved in ultra pure
deionised water (from TKA Labtower (EDI:15–10 M
Ω
cm)). They
were slowly heated at 200 1C until combustion occurs and then
calcined at 1300 1C for 48 h for Al2O3 sample and for 30 h for
Al2ꢀxCrxO3 samples. The samples were designated as A, R1, R2 and
R3 for Al2O3, Al1.9Cr0.1O3, Al1.8Cr0.2O3 and Al1.7Cr0.3O3, respectively.
Chromium oxide (Cr2O3) (Merck 99.9% purity) was used for
comparison and designated as sample C.
The samples were then studied by X-ray powder diffraction
(XRPD) (Pan Analytical X0Pert Pro XRD System) for structural
studies. The Rietveld refinement software used was the PANaly-
tical X0pert Highscore Plus. The refinement was done using the
hexagonal crystal structure setting of the ICSD 75479 and ICSD
64988 for the Al2O3 and Al2ꢀxCrxO3 respectively. Conductivity
studies were done via AC Impedance Spectroscopic technique
using a WONATECH WEIS510 Multichannel Electrochemical Impe-
dance Spectroscopy (EIS) System with a frequency range between
n
Corresponding author. Tel.: þ60 193 115 359/60 355 444 481;
fax: þ60 355 443 870.
E-mail addresses: norlyk@salam.uitm.edu.my,
norly003@yahoo.co.uk (N. Kamarulzaman).
0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.physb.2013.11.059

N. Badar et al. / Physica B 437 (2014) 32–35
33
1 Hz and 1 MHz. The band gap studies were measured using a UV–
vis NIR spectrophotometer (Perkin-Elmer Lambda 950) using a
reflection optical geometry.
method for producing doped Al2ꢀxCrxO3 materials. Fig. 1 of
the indexed XRD patterns of Al2ꢀxCrxO3 shows that the materials
are pure and single phase. Crystallographic parameters listed
in Table 1 are obtained from Rietveld refinements of the XRD
datasets (shown in Fig. 2(a)–(e)). It is observed that with increas-
ing Cr content, there is a linear increase of the cell parameters,
a and c, as well as volume, V, as shown in Fig. 3. The high R2 values
in the linear regression graphs of Fig. 3 show a remarkable
accuracy of the site occupancy factor determination from the
Rietveld refinements. The increase of the cell parameters is due
to the larger ionic radius of Cr (ionic radius of Al3þ ¼0.54 Å and
Cr3þ ¼0.62 Å for coordination number 6 [10]) compared to Al. The
XRD results prove that Cr has been substituted for Al in the crystal
lattice.
3. Results and discussion
Cr substitution in Al2O3 is difficult and can only be done at high
temperatures. So, far, only 4% substitutional doping of Cr has been
achieved by Cava et al. [9] who employed a polymeric precursor
method. Our combustion synthesis method has successfully
yielded a higher Cr content of 6 to 26% in the Al2ꢀxCrxO3 powders.
This illustrates that the combustion method is a good synthesis
Fig. 4(a)–(e) shows Nyquist plots of the Al2O3, Al2ꢀxCrxO3 and
Cr2O3 materials. Only one semi-circle is observed implying a single
time constant or Debye response for all the samples. The plots are
analyzed using the ZMAN software with a circular fit. A Randles
equivalent circuit is used in the fitting and the bulk impedance, Rb,
are obtained in this way. The conductivity values of the samples
are calculated using the well known Eq. (1) below.
t
s
¼
ð1Þ
ARb
where
s
is the conductivity, t is the pellet thickness, A is the
contact area of pellet with the electrode and Rb is the bulk
resistance of the material [11].
Conductivity values are given in Table 1 and conductivity
versus Cr content is shown in the graph of Fig. 4(f). The con-
ductivity values seem to increase according to an exponential
relation with Cr content. This is a new and novel finding.
To further understand the increase of conductivity with the
dopant amount, band gap values are studied via the absorption
edges of the UV–vis spectrum of the samples. The equation used is
the well-known Tauc relation [12] given below,
x
ðα
h
υÞ ¼ Aðh
υ
ꢀEgÞ
ð2Þ
where
value of the wavelength
α
is the absorption coefficient of the material at a certain
, h is Planck0s constant, A is a propor-
λ
tionality constant, is the frequency of light, Eg is the band energy
ν
and x¼½ (for direct transition mode materials) [13]. The electronic
band transitions of all the materials are obtained from the Tauc
plots shown in Fig. 5 and listed in Table 1. From the listed values,
it is obvious that there is band gap narrowing of the samples as the
Cr content increases. The decrease of the band gap explains why
the conductivity of the Cr doped samples increases. Electrons from
the highest occupied molecular orbital (HOMO) can jump into the
lowest unoccupied molecular orbital (LUMO) in an easier manner
when the band gap is narrower. Thus, conductivity rises. Con-
sidering the orbitals of the Al3þ ions, it can be seen that on
bonding with oxygen, all the 2p electrons are paired and there is
Fig. 1. XRD results of the indexed XRD patterns of sample A, R1, R2 and R3.
Table 1
Crystallographic parameters of the samples from the Rietveld refinements of the XRD datasets (s.o.f.¼site occupancy factor), conductivities and band gap energy of the
samples.
Sample ID
Chemical formula
Cr (synthesized values)
(%)
a¼b (Å)
c (Å)
V (Å3)
Rw
χ2
s.o.f of
Cr
Conductivity
(S cmꢀ1
s
Band gap energy70.001
(eV)
)
A
Al2O3
0.00
0.05
0.10
0.15
1.00
4.7592
4.7711
4.7826
4.7936
4.9600
12.9924
13.0215
13.0514
13.0805
13.5969
254.8497
256.6982
258.5362
260.3075
289.6869
10.83
9.61
9.06
9.00
8.83
2.97
2.27
2.30
2.23
1.80
–
2.57 ꢁ 10ꢀ10
4.36 ꢁ 10ꢀ10
5.98 ꢁ 10ꢀ10
6.83 ꢁ 10ꢀ9
3.17 ꢁ 10ꢀ4
8.8a
R1
R2
R3
C
Al1.9Cr0.1O3
Al1.8Cr0.2O3
Al1.7Cr0.3O3
Cr2O3
0.031
0.070
0.130
0.950
4.900
4.800
4.490
3.230
a
Values taken from R.H. French [1].

34
N. Badar et al. / Physica B 437 (2014) 32–35
Fig. 2. Rietveld refinements of XRD patterns of (a) sample A, (b) sample R1, (c) sample R2 (d) sample R3 and (e) Sample C.
Fig. 3. Graphs showing the relationship of (a) a with s.o.f. of Cr (b) c with s.o.f. of Cr and (c) V with s.o.f. of Cr.
no crystal splitting occurring in the Al2O3. There are also no
unpaired electrons in the electronic configuration of O2ꢀ. The
gap between the HOMO and LUMO in Al2O3 is large making it
difficult for the electrons to jump into the conduction band. This
explains why Al2O3 is an insulator. This also explains why Al2O3
has a very stable electronic configuration. The situation is different
for the doped materials. The presence of the 3d transition metal
ion, Cr3þ, results in three unpaired electrons and this causes
crystal field splitting. The electrons in the dxy, dxz and dyz orbitals
doping introduces more energy levels in the band structure of
Al2ꢀxCrxO3 materials making them more conducting.
4. Conclusions
Higher Cr content of the Al2ꢀxCrxO3 materials have been success-
fully obtained using the combustion method. On doping with Cr, the
energy levels of the substituted compounds are drastically modified
by the presence of the transition metal ion, Cr3þ, increasing the
2
2 2
can undergo transitions to the dz and dxꢀy orbitals. Thus, Cr

N. Badar et al. / Physica B 437 (2014) 32–35
35
Fig. 4. Analyzed Nyquist plots using Zman software and fitted to the equivalent circuit shown for sample (a) Al2O3, (b) Al1.9Cr0.1O3, (c) Al1.8Cr0.2O3, (d) Al1.7Cr0.3O3, (e) Cr2O3
and (f) graph showing the relationship between conductivity of the materials with Cr content.
Universiti Teknologi MARA for internal funds used in this research
work. The authors also extend their appreciation to the Deanship of
Scientific Research at King Saud University for funding this work
through research group no. RGP-VPP-207.
References
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Cr2O3 (C).
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The authors would like to thank the Ministry of Higher Education,
Malaysia for the FRGS grant on Al2O3 and the Institute of Science,