Sensors and Actuators B 55 (1999) 90 – 95
Semiconducting gas sensor for chlorine based on inverse spinel
nickel ferrite
C.V. Gopal Reddy, S.V. Manorama *, V.J. Rao
Materials Science Group, Indian Institute of Chemical Technology, Hyderabad 500 007, india
Received 19 May 1998; received in revised form 2 February 1999; accepted 4 February 1999
Abstract
Nickel ferrite, a p-type semiconducting oxide with an inverse spinel structure has been used as a gas sensor to selectively detect
chlorine in air. This compound was prepared by two different routes namely, the citrate and co-precipitation method and sensor
properties of the resulting compounds from both the methods were compared. X-ray diffraction was used to confirm the structure.
The gas sensing characteristics were obtained by measuring the sensitivity as a function of various controlling factors like dopant,
concentration of the dopant, operating temperature, concentration of the gas and finally the response time. The sensitivity to
chlorine has been compared with that of other interfering gases. A probable explanation has been proposed to explain the selective
sensitivity to oxidising gases like chlorine. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Nickel ferrite; p-Type semiconducting oxide; Oxidising gases; Chlorine sensor
1. Introduction
There is currently a lot of interest in the development
of new materials as possible gas sensors. This is partly
due to the non-selective nature of tin dioxide, which has
been conventionally used to detect almost all kinds of
gases and secondly, the discovery of novel materials
with peculiar and extraordinary gas sensing capabilities.
In a novel attempt, nickel ferrite (NiFe2O4), has been
successfully used for the first time as a sensor to detect
low concentrations of chlorine gas. Chlorine is a greenish yellow, oxidising gas, which is intensely irritating. A
lot of controversy surrounds the quantification of its
toxicity. Exposure to chlorine is known to affect the
lungs and also produce Oedema. There are several
methods employed to ascertain the presence of chlorine.
Some of them are g-spectrophotometric, potentiometric
[1,2], chemical analysis and chemical sensors [3], electrochemical method [4] etc. More recently optochemical
sensors have been developed wherein different indicator
dyes have been immobilised in a transparent membrane
[5 – 7] and are being used for chlorine detection. There
* Corresponding author. Fax: +91-40-7173757.
E-mail address: [email protected] (S.V. Manorama)
are a few reports in the literature of attempts to develop
sensors for chlorine based on semiconducting oxides.
Palladium doped SnO2 [8] is shown to detect Cl2 gas
but a sparker is necessary to improve the sensing characteristics. Thin films of indium tin oxide (ITO) [9] and
hetero-contacts of SiC/ZnO [10] have also shown sensitivity to Cl2. In the latter case the sensitivity has been
explained to be due to the interaction between the test
gas and the humidity. There is also a galvanic cell type
Cl2 sensor that makes use of an Ag + -ion conductivity
electrolyte coated on one side with a thin film of AgCl
containing material [11].
We have developed a sensor to detect chlorine, based
on a p-type semiconductor nickel ferrite (NiFe2O4).
This is a soft ferrimagnetic material with an inverse
spinel (Trevorite) structure [12]. This material is found
to detect low concentrations of Cl2 in air.
In technologies where ferrites are to be used for
magnetic or electrical applications, high-density materials are generally required and the ferrites are often
prepared by high temperature solid-state reactions between finely ground powders. Although most applications of ferrites as ceramic materials require high
densities to achieve the desired properties, there are
many applications for which lower densities and high
0925-4005/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S 0 9 2 5 - 4 0 0 5 ( 9 9 ) 0 0 1 1 2 - 4
C.V. Gopal Reddy et al. / Sensors and Actuators B 55 (1999) 90–95
Fig. 1. X-ray diffraction pattern of NiFe2O4 prepared by (a) co-precipitation; and (b) the citrate method.
Fig. 2. Sensitivity versus operating temperature of virgin and 1 wt. % Pd doped NiFe2O4 prepared by the co-precipitation method.
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Fig. 3. Sensitivity as a function of concentration of Cl2 in air.
surface area are preferred. Lower temperature methods
offer many potentially attractive features for materials
synthesis, particularly when the materials are to be used
as simulated corrosion products or heterogeneous catalysts or, as in the present application, as a gas sensing
material. There are several routes adopted to prepare
these spinels and a range of nickel ferrite powders have
been prepared from co-precipitated nickel-iron oxalate
precursors by firing in the range of 300 – 1100°C and the
variation in crystallite size and surface area studied [13].
The present paper reports the results of our work
pertaining to the preparation, characterisation of the
NiFe2O4 and its application as a suitable sensor for Cl2
detection.
2. Experimental details
The synthesis of nickel ferrite was accomplished from
the corresponding nitrates by two methods, namely the
co-precipitation and the citrate.
2.1. Co-precipitation method
Nickel and iron nitrate were taken in a 1:2 mole ratio
and dissolved in de-ionised water. This mixture was
co-precipitated with urea at 90°C under constant stirring. The precipitate was filtered and washed several
times with de-ionised water to get rid of the nitrates,
finally the mixture was dried in an oven overnight at
120°C. This material was calcined at different temperatures. The crystalline single phase is formed above
550°C, so the material was calcined at 600°C for 4 h. The
homogeneity of the compound was confirmed by XRD.
2.2. Citrate method
In this method the same nickel and iron nitrates were
taken in a 1:2 mole ratio along with 3 moles of citric acid
and dissolved in de-ionised water with continuous stirring. This mixture was slowly evaporated and then dried
in an oven at 120°C overnight. The dried material was
crushed and calcined at 600°C for 4 h.
C.V. Gopal Reddy et al. / Sensors and Actuators B 55 (1999) 90–95
93
3. Results and discussions
Fig. 4. Cross sensitivity of the 1 wt. % Pd:NiFe2O4 Cl2 sensor to O2,
LPG, CH4 and Br2 vapour.
The X-ray diffractograms were recorded on a
Siemens/D.5000 X-ray diffractometer (Cu –Ka radiation
l= 1.5406 Å). NiFe2O4 thus prepared, is used to fabricate sensor elements in the following way. A total of 2
wt. % poly-vinyl alcohol (PVA) was used as a binder to
form a paste and the material was coated onto aluminium tube substrates provided with platinum wire
electrodes for electrical contacts. Finally, the sensor
element is sintered at 550°C for 2 h, to make it rigid
and impart it ceramic properties. The electrical resistance of the element was measured in the presence and
absence of chlorine and other test gases. For gas sensing measurements the sensor element was provided with
a heater fixed inside the aluminium tube coated with
the sensor material. The schematic of the sensor assembly has been already described [14]. The sensitivity, S is
defined as the ratio of change in resistance of the sensor
in the presence of gas, DR, to the value of the resistance
in air, Ra.
S =DR/Ra = Ra − Rg/Ra
X-ray diffraction studies were used to confirm the
structure [15] and compare the crystallite size using the
Scherrer formula [16]. Fig. 1 shows the XRD pattern of
NiFe2O4 prepared by both the methods. The material
prepared by the co-precipitation method has a lower
crystalllite size 175 Å, as compared to that prepared
by the citrate method 275 Å. The difference in the
crystallite size manifests itself even in the gas sensing
characteristics. The value of sensitivity for the sensor
using NiFe2O4 prepared by the co-precipitation method
is higher. This observation is not an altogether unexpected result because a smaller crystallite size implies a
larger surface area exposed to the test gas, which
increases the probability of gas–solid interaction, which
increases the sensitivity. Hence we have adopted the
co-precipitation technique in all our further
experimentation.
Virgin NiFe2O4 senses Cl2 with a maximum sensitivity of 0.75 at 300°C but the sensor responds very slowly
and it has a very long fall time. We tried different metal
oxides like CoO and other noble metals like Pt but the
sensitivity of the sensor does not improve. Fig. 2 shows
the sensitivity versus operating temperature of virgin
and 1 wt. % Pd doped NiFe2O4. 1 wt. % of PdCl2 was
incorporated by impregnating corresponding palladium
chloride solution on the NiFe2O4. Palladium is known
to catalyse surface oxidation reactions by oxygen or
chlorine [5,17] and hence has been incorporated into
NiFe2O4 to improve the gas sensing characteristics. On
incorporating Pd into NiFe2O4 the operating temperature came down to about 250°C and the sensitivity
improved to about 0.9. Addition of palladium is also
shown to improve the selectivity by decreasing the
sensitivity to other oxidising gases like O2, Br2 vapour
and reducing gases like LPG and CH4. This material
also responds to reducing gases but in the reverse way,
i.e. the resistance of the element increases when it
comes into contact with reducing gases, as opposed to
a decrease in resistance when exposed to oxidising gases
[18].
Fig. 3 shows the calibration curve, i.e. the sensitivity
of the sensor to different concentrations of chlorine in
air. The x-axis has been plotted on a log scale to
accommodate the range from 100 ppm to 100% of Cl2
in air. The sensitivity of the sensor to other gases is
compared in Fig. 4. Chlorine has been diluted to 1000
ppm in dry air whereas the other gases have been taken
as pure gases or saturated vapour, as in the case of
bromine, to highlight the selectively sensitive nature of
the chlorine sensor. The sensor has a very high sensitivity to even 1000 ppm of Cl2 as compared to the other
gases. The bar diagram of Fig. 4 clearly brings out the
selective sensitivity of the sensor to chlorine. Efforts are
being made to improve the sensitivity further with the
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Fig. 5. Response characteristics of the 1 wt. % Pd:NiFe2O4 element to 1000 ppm Cl2 in air.
main aim of fabricating an industrially viable sensor to
detect even traces of chlorine in air.
The response characteristics give an idea of the rise
time, i.e. the time taken by the sensor to respond to the
presence of a gas and the fall time, i.e. the time taken by
the sensor to come back to its original value once the test
gas is removed. Fig. 5 gives the response curve of
NiFe2O4: Pd (1 wt. %) to 1000 ppm Cl2 in dry air. It shows
a rise time of 20–30 s and fall time of 60 – 90 s.
Thermo-emf studies were carried out to determine the
type of conductivity in NiFe2O4. It is observed that holes
are the majority carriers and so NiFe2O4 is a p-type
semiconducting oxide. The basic mechanism of sensing
in NiFe2O4 to any oxidising gas can be explained as
follows. At the operating temperature, when chlorine or
any oxidising gas comes in contact with this semiconductor surface, the gas gets reduced and draws electrons from
the semiconductor. This increases the hole concentration
in the semiconductor, which are the majority carriers, and
thereby the conductivity. This is exactly parallel to any
n-type semiconductor like SnO2, ZnO [19] etc., detecting
reducing gases like LPG, methane etc. Further confirmation of our observation is that NiFe2O4 is seen to respond
to reducing gases like LPG, CH4 but with an increase in
resistance. We have included the sensitivities of these
gases in the cross sensitivity graph to reiterate our stand
that NiFe2O4 senses primarily chlorine from among all
other interfering gases both oxidising and reducing.
4. Conclusions
In conclusion we have shown that a p-type semiconductor can be effectively used to sense oxidising gases.
Earlier reports on the role of palladium [5,17] show that
its role is mostly to catalyse surface reactions which
property is applicable even in the present case. The sensor
does detect lower concentrations (10 ppm and less) but
the sensitivity is still less. These results are preliminary
and efforts are on to improve the sensitivity further so
that the sensor becomes practically viable for the detection of traces (B 1 ppm) of chlorine in air. Experiments
are also underway to establish the exact role and chemical
nature of palladium in the sensor, which is responsible
in improving the sensitivity of NiFe2O4 particularly to
chlorine. In-situ, Mossbauer and X-ray photoelectron
spectroscopy studies would be beneficial in observing the
change in the state of Fe in the presence and absence of
test gas. This would assist in proposing an exact mechanism of sensing.
C.V. Gopal Reddy et al. / Sensors and Actuators B 55 (1999) 90–95
Acknowledgements
One of the authors C.V. Gopal Reddy acknowledges
CSIR for the grant of Senior Research Fellowship. All
the authors gratefully acknowledge the financial support of DST, New Delhi.
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Biographies
C.V.Gopal-Reddy, graduated from the Osmania University with an M.Sc. in Chemistry. He is currently working
for his Ph.D thesis as a Senior Research Fellow in the
Materials Science group at IICT, Hyderabad. Currently
he is involved in the development of materials for gas
sensors for LPG, H2, CO and Cl2 gas sensors based on
semiconducting oxides like SnO2, spinel, perovskites and
mixed oxides.
S.V. Manorama, received her Ph.D. degree in Physics
from Poona University, Pune in 1990. Her postgraduate
research involved the study of the electronic properties
of modified gallium arsenide surfaces and interfaces. She
worked on the study of plasma polymer passivated
GaAs surfaces at the University of Wales, College of
Cardiff, Cardiff, Wales, UK, on a Commonwealth fellowship. Presently she is a Scientist at the Indian Institute of Chemical Technology, Hyderabad. As a project
leader, her research interests are mainly directed towards
the development of nanoparticles of different materials
and semiconducting gas sensors based on SnO2 and
other oxides with spinel and perovskite structures.
V.J. Rao, obtained his Ph.D. Degree in chemistry in 1969
from the University of Poona and did post-doctoral
research on photovoltaics at the University of Reading,
CNET at Bagneaux, Paris and SUNY, Buffalo, USA.
Presently he is a senior Scientist at the Indian Institute
of Chemical Technology, Hyderabad. His current interests are mainly in the growth of hard coatings like c-BN,
diamond etc., by MOCVD, developing phosphor materials for colour television and also on the development
of gas sensors based on semiconductors such as SnO2
and ZnO, both thin and thick films. His earlier work was
related to the surfaces and interfaces of GaAs for
MISFET’s and optoelectronic devices.