CHINESE JOURNAL OF PHYSICS
VOL. 53, NO. 7
December 2015
The Gas-Sensing Properties of Ni-Zn Ferrite (Ni0.6 Zn0.4 Fe2 O4 ) Nanoparticles
Prepared by the Microwave Method
Y. Zohrabi, M. E. Ghazi,∗ and M. Izadifard
Department of Physics, Shahrood University, Shahrood, 36155-316, Iran
(Received November 25, 2014; Revised June 7, 2015)
In this paper, we report on the gas sensing properties of Ni0.6 Zn0.4 Fe2 O4 nanoparticles.
The samples were synthesized by the microwave method. The synthesized nanoparticles
were characterized by X-Ray diffraction, which confirmed the formation of a cubic spinel
structure without any detectable impurity phases. The results showed that the average
crystallite size was ∼ 25 nm. These results were further confirmed by scanning electron
microscopy images. The gas sensing properties of the samples were investigated in the
presence of acetone and methanol gases. The Ni-Zn ferrite indicated a good response toward
the acetone gas compared to the methanol gas, and the best operating temperature for both
gases was 250 ◦ C.
DOI: 10.6122/CJP.20150713B
PACS numbers: 81.20.ka, 07.07.Df
I. INTRODUCTION
The study of gas sensor materials has become an active field in recent years. The aim
is to achieve the best sensitivity at low concentrations of the sensing gases and lower working
temperatures. Much effort has been made on semi-conducting oxide gas sensors such as
ZnO, SnO2 , WO3 , and TiO2 or on doping these oxides with other elements to increase the
sensitivity. Among these, the ferrites have attracted a considerable amount of attention.
Soft ferrites with the general formula MFe2 O4 (M being a metallic element such as Ni,
Mn, Zn, and Co) are magnetic materials having a spinal structure. These materials have
promising properties, such as high electrical resistivity and low hysteresis loss, that make
them good candidates for applications in medicine, electronic devices, and magnetic cores.
Most of the investigations have been on their magnetic properties. It has also been found
that they are a good candidate for gas sensing applications. Their conductivity changes
when they are exposed to gas flow due to the interaction of their surface atoms with the
gas. The results obtained have indicated that surface morphology and higher surface area
are essential factors affecting their sensitivity to gas detection.
Among these, nickel ferrite has shown to be a very good sensor for detecting reducing
gases like acetone [1], chlorine, and liquid petroleum gas (LPG) [2]. Among the reducing
gases, acetone is an organic compound with the chemical formula (CH3 )2 CO, which is a
colorless and flammable liquid [3]. This gas is an important solvent, and it is also used
∗
Electronic address: ebrahim ghazi@yahoo.com, mghazi@shahroodut.ac.ir
http://PSROC.phys.ntu.edu.tw/cjp
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c 2015 THE PHYSICAL SOCIETY
⃝
OF THE REPUBLIC OF CHINA
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THE GAS-SENSING PROPERTIES OF Ni-Zn FERRITE (Ni0.6 Zn0.4 Fe2 O4 ) . . . VOL. 53
typically for cleaning purposes in the laboratory. When the concentration of acetone in
the air is high, it is irritating, and may affect the central nervous system. Methanol with
the chemical formula CH3 OH is highly toxic, and is used as a solvent, anti-freeze, and fuel.
Therefore, detecting these gases in our environment is necessary for our safety and health.
Several chemical and physical methods such as sol-gel, hydrothermal, spray pyrolysis,
and high energy milling have been used to fabricate sensor materials as well as ferrites [1, 4–
8]. Recently, the use of microwave irradiation for the synthesis of some materials has grown
because the method is cleaner, faster than other methods, and cheaper [8]. In this method,
microwave radiation is absorbed by materials and its energy is converted into thermal
energy.
The purpose of this work is to synthesize Ni-Zn ferrite nanoparticles (Ni0.6 Zn0.4 Fe2 O4 )
by the microwave method and to study their structural and gas sensing properties towards
acetone and methanol.
II. EXPERIMENTAL DETAILS
Ni-Zn ferrite nanoparticles were synthesized using the microwave method. A
stoichiometric mixture of nitrates, ferric nitrate [Fe(NO3 )3 .9H2 O], and nickel nitrate
[Ni(NO3 )2 .6H2 O] was mixed with urea (to serve as fuel) and deionized water. The resultant mixture was stirred magnetically at 70 ◦ C for 120 minutes to get a homogeneous
mixture. The solution was put in a microwave oven at a power of 720 W for 330 seconds.
The solution initially boiled and then underwent dehydration followed by decomposition
with the mutation of a large amount of gas. When the solution reached the point of spontaneous combustion, it began to burn. At this stage, lots of heat was released, vaporizing
all of the solution and the nanoparticles formed.
In order to make the sensor, the sintered nanoparticles were mixed with PVA
(polyvinyl alcohol) as a binder. The resulting paste was pressed into a disk shape under 30
Mpa pressure. The ferrite disks were then calcined at 700 ◦ C for 4 h in order to improve
the crystalline structure of the particles. The disks were cut to about 2 mm thickness and
25 mm in diameter. For resistance measurements, two copper wires were connected to the
disk using silver paste.
The ferrite disk was situated on the alumina plate (as an insulator), which was itself
placed on a heater. For gas sensing measurements with temperature, the sensor element
was heated at the operating temperatures of 50–350 ◦ C with a heater. The temperature
was controlled by a micro controller.
The test acetone gas was injected into the closed container (volume of 600 ml) made
from Teflon through an inlet. The sensing performance of the sensors was examined using the ‘static gas sensing system’. The sensor response (S) can be calculated using the
relation [9]
S=
(Ra − Rg )
△R
=
,
Ra
Ra
(1)
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Y. ZOHRABI, M. E. GHAZI, AND M. IZADIFARD
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where Ra is the sensor resistance in air, and Rg is the sensor resistance in the presence of a
testing gas at a certain temperature. The sensor was used to detect acetone and methanol
gases at different working temperatures and also concentrations. The structures of the
synthesized samples were characterized by X-ray diffraction (XRD) using a B8 advance
(BRUKER) X-ray diffractometer using Cu Kα radiation. The data was collected over a 2θ
range of 25–70 degrees. The surface morphology of the prepared nanoparticles was studied
using a Hitachi S.4160 scanning electron microscope (SEM).
III. RESULT AND DISCUSSION
The X-ray diffraction pattern of the sample prepared by the microwave method and
annealed at 700 ◦ C for 4 h is presented in Figure 1. The pattern shows the formation
of a cubic spinel structure without any signature of impurity phases. The broadening of
the peaks indicates that the particles are at the nanometer scale. The crystallite size was
calculated with the help of the Debye-Scherrer formula [10]
D=
0.9λ
,
β cos θ
(2)
where λ is the wavelength of X-ray, β is the full-width at half maximum (FWHM) of the
peak, D is the average crystallite size, and θ the Bragg angle of the peak. The average
crystallite size of the sample was estimated using the FWHM of diffraction from the (311),
(400), and (511) planes; it was found to be ∼ 25 nm.
The lattice constant for the Ni0.6 Zn0.4 Fe2 O4 compound calculated from the XRD data
was 0.8401 nm. The reported lattice constant for undoped NiFe2 O4 is 0.838 nm [8]. This
small increase could be attributed to the difference in the ionic radii of the Ni cation (0.78
Å) and Zn cation (0.82 Å) [11].
Figure 2 shows the SEM image of the Ni0.6 Zn0.4 Fe2 O4 nanoparticles calcined at 700
◦ C for 4 h. As it can be seen in the figure, the morphology of the particles is almost
spherical. The average particle size is the same as that found from the XRD data around
25–30 nm.
Figure 3 shows the relationship between the operating temperature and resistance
(in logarithmic scale) of the Ni0.6 Zn0.4 Fe2 O4 sensor in air. The resistance decreases greatly
from several thousand kΩ to a few kΩ with an increase of temperature due to the thermally
activated resistance. The gas sensing measurements were performed in the temperature
range of 100 ◦ C to 350 ◦ C.
We studied the sensing of the sensor for acetone and methanol gases. The resistance
of the sensing element was measured in air atmosphere before introducing the gas in the
Teflon lacuna. Then a quantity of 1000 ppm of gas was inserted into the closed container.
Figure 4 shows the gas sensing measurements of the Ni0.6 Zn0.4 Fe2 O4 compound at
different operating temperatures, ranging between 150–350 ◦ C. The element was tested
for the two reducing gases acetone and methanol at various operating temperatures. The
concentrations of the gases were maintained at 1000 ppm. The sensing characteristics
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THE GAS-SENSING PROPERTIES OF Ni-Zn FERRITE (Ni0.6 Zn0.4 Fe2 O4 ) . . . VOL. 53
FIG. 1: X-Ray Diffraction spectra of the Ni0.6 Zn0.4 Fe2 O4 nanoparticles sintered at 700 ◦ C.
FIG. 2: SEM image of Ni0.6 Zn0.4 Fe2 O4 nanoparticles calcined at 700 ◦ C for 4 h.
indicate that the element shows a higher response (around two times) to the acetone gas
compared to the methanol gas. For both gases, the sensor response is much higher at 250
◦ C (around 80%) compared to all the other operating temperatures.
A similar study was performed on nanocrystalline Pd doped-Ni0.6 Zn0.4 Fe2 O4 [12]
synthesized by the ethylene glycol mediated citrate sol–gel method. Undoped samples have
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Y. ZOHRABI, M. E. GHAZI, AND M. IZADIFARD
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FIG. 3: Resistance versus temperature for Ni0.6 Zn0.4 Fe2 O4 sensor measured at air.
FIG. 4: Sensor response curves as a function of operating temperature for Ni0.6 Zn0.4 Fe2 O4 sensor.
shown high response rates (60%) to 100 ppm ethanol gas at 300 ◦ C. Pd modified samples
(0.5–1.5 wt. %) showed notable enhancement (up to 80%) in the response and a reduction
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THE GAS-SENSING PROPERTIES OF Ni-Zn FERRITE (Ni0.6 Zn0.4 Fe2 O4 ) . . . VOL. 53
in the optimal operating temperature (275 ◦ C).
Tudorache and coworkers [13] have reported that ZnFe2 O4 shows considerable sensitivity to alcohol and poor response to acetone and the sensitivity maximum appears at
350 ◦ C. While NiFe2 O4 ferrite exhibits a selective sensitivity to acetone and the sensitivity
maximum appears at 250 ◦ C. They also measured the sensing properties of Ni1−x Cox Fe2 O4
nanocrystalline thin films toward acetone gas (150 ppm gas concentration). They observed
that the response to acetone vapor increases from 49% to 61% when the cobalt content was
decreased from x = 0.75 to x = 0.25 at an operating temperature around 300 ◦ C.
As was mentioned, the sensing mechanism for these kinds of gas sensors is based upon
the surface reactions of the semi-conducting oxide sensor and gas. Oxygen vacancies on the
surface of the semi-conductor are activated electrically and chemically. During the thermal
treatment in the air, the oxygen species bind chemically onto the surface layer of the sensor
−
in the forms of O2− , O−
2 , and O depending on the operating temperature [10, 14]. This
oxygen entangles electrons from the surface of the semi-conductor, leading to a reduction
of the conductivity. When the reducing gas is brought into contact with the sensor, the
entangled electrons are brought back into the semi-conductor conduction band leading to
an enhancement of the conductivity [9, 15].
It is known that the Ni0.6 Zn0.4 Fe2 O4 nanoparticles show a typical n-type semiconducting behavior. With increasing temperature, the state of oxygen adsorbed on the
surface of the Ni0.6 Zn0.4 Fe2 O4 sensor undergoes the following reaction [16]:
O2 (gas)+e− ↔O−
2 (ads),
−
−
O−
2 (ads)+e ↔ 2O (ads).
(3)
The general case of a reducing gas interacting with the adsorbed oxygen at the element
sensor surface can be explained by the following reaction [9]:
R+O− →RO+e− ,
(4)
where R stands for the reducing gas. With acetone exposure, the trapped electrons are
transferred back to the semi-conductor according to the reaction [17]
CH3 COCH3 (gas)+O− →CH3 CO+ +CH3 O− + e− .
(5)
The response time, which is defined as the time for reaching 90% of the final signal,
is an important parameter of the gas sensors. The response curve of the sample for 1000
ppm acetone and methanol at an operating temperature of 250 ◦ C is shown in Fig. 5. The
figure shows that the time taken by the sensor element to reach its maximum response at
the operating temperature of 250 ◦ C is about 90 s for acetone and 55 s for methanol. When
the sensor reached a steady response, the gas was removed from the closed Teflon lacuna
and the response was recorded again. The change in response was found to be not good.
The poor recovery observed is due to the bulky nature of the sensing element. When the
Ni-Zn Ferrite (Ni0.6 Zn0.4 Fe2 O4 ) sensor is exposed to the gas, it goes deep into the pellet
and comes out very slowly. This causes a long recovery time [18]. Response and recovery
times of 2 and 4 minutes have been reported for ZnFe2 O4 ferrites [13]. In a 1.5% Ag
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modified NiFe2 O4 sensor which was prepared by a simple solid-state reaction route [19], the
sensitivity to acetone gas was 43%, but this sensor showed a good response and recovery
time of 1 s and 10 s, respectively. It has also been reported that ZnFe2 O4 ferrite shows
their maximum response to both acetone (57%) and LPG (54%) at operating temperatures
of 250 ◦ C and 375 ◦ C, respectively [20].
FIG. 5: Response characteristics of Ni0.6 Zn0.4 Fe2 O4 at an operating temperature of 250 ◦ C for 1000
ppm (a) acetone and (b) methanol.
The results obtained from the measurements on both gases are collected in Table I.
The comparatively lower response to the methanol than acetone gas may be due to the
weaker interaction between the test gas and the Ni0.6 Zn0.4 Fe2 O4 surface.
TABLE I: Sensor response and recovery time of the Ni0.6 Zn0.4 Fe2 O4 sensor to 1000 ppm gases at
250 ◦ C.
Name of the Vapor
Maximum response (%)
Response time (s)
Recovery time (min.)
Acetone
72
90
8–9
Methanol
38
55
10
Figure 6 shows the dependence of the sensor response of the sensor on the concentration of acetone and methanol at 250 ◦ C. The sensor exhibits gas concentration dependence
at low ppm, but it shows nearly a saturated behavior for more than 1000 ppm gas. An increase in the gas concentration raises the surface coverage, eventually leading to a saturation
level.
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FIG. 6: Sensor response as a function of acetone and methanol gases concentrations at an operating
temperature of 250 ◦ C.
IV. CONCLUSION
We prepared nanoparticles of Ni0.6 Zn0.4 Fe2 O4 using the microwave method and calcined at 700 ◦ C. The XRD pattern of the sample confirms a cubic spinel structure without
any signature of impurity phases with the average crystallite size of ∼ 25 nm. The results
observed from the SEM image were in agreement with the XRD results. The sensor response measurements versus temperature indicate that the element shows a higher sensor
response (around two times) to acetone gas compared to methanol gas. For both gases, the
sensor response is much higher at 250 ◦ C compared to all the other operating temperatures.
Hence, according to the results, the Ni-Zn Ferrite (Ni0.6 Zn0.4 Fe2 O4 ) is a better candidate
for acetone gas sensors.
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