Simple Synthesis of ZnCo2O4 Nanoparticles as Gas-sensing Materials

Sensors & Transducers
Volume 134 Issue 11
November 2011
www.sensorsportal.com ISSN 1726-5479
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Sensors & Transducers Journal (ISSN 1726-5479) is a peer review international journal published monthly online by International Frequency Sensor Association (IFSA).
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Sensors & Transducers Journal
Contents
Volume 134
Issue 11
November 2011
www.sensorsportal.com ISSN 1726-5479
Research Articles
Nanomaterials and Chemical Sensors
Sukumar Basu and Palash Kumar Basu ............................................................................................ 1
Fabrication of Phenyl-Hydrazine Chemical Sensor Based on Al- doped ZnO Nanoparticles
Mohammed M. Rahman, Sher Bahadar Khan, A. Jamal, M. Faisal, Abdullah M. Asiri ..................... 32
Nanostructured Ferrite Based Electronic nose Sensitive to Ammonia at room temperature
U. B. Gawas, V. M. S. Verenkar, D. R. Patil....................................................................................... 45
A Humidity Sensor Based on Nb-doped Nanoporous TiO 2 Thin Film
Mansoor Anbia, S. E. Moosavi Fard................................................................................................... 56
A Resistive Humidity Sensor Based on Nanostructured WO 3 -ZnO Composites
Karunesh Tiwari, Anupam Tripathi, N. K. Pandey.............................................................................. 65
Highly Sensitive Cadmium Concentration Sensor Using Long Period Grating
A. S. Lalasangi, J. F. Akki, K. G. Manohar, T. Srinivas, Prasad Raikar, Sanjay Kher
and U. S. Raikar ................................................................................................................................. 76
MEMS Based Ethanol Sensor Using ZnO Nanoblocks, Nanocombs and Nanoflakes as
Sensing Layer
H. J. Pandya, Sudhir Chandra and A. L. Vyas ................................................................................... 85
Simple Synthesis of ZnCo 2 O 4 Nanoparticles as Gas-sensing Materials
S. V. Bangale,S. M. Khetre D. R. Patil and S. R. Bamane................................................................. 95
Nanostructured Spinel ZnFe 2 O 4 for the Detection of Chlorine Gas
S. V. Bangale, D. R. Patil and S. R. Bamane..................................................................................... 107
Fabrication of Polyaniline-ZnO Nanocomposite Gas Sensor
S. L. Patil, M. A. Chougule, S. G. Pawar, Shashwati Sen, A. V. Moholkar, J. H. Kim and V. B. Patil 120
Synthesis and Characterization of Nano-Crystalline Cu and Pb 0.5 -Cu 0.5 - ferrites by
Mechanochemical Method and Their Electrical and Gas Sensing Properties
V. B. Gaikwad ,S. S. Gaikwad, A. V. Borhade and R. D. Nikam........................................................ 132
Surface Modification of MWCNTs: Preparation, Characterization and Electrical Percolation
Studies of MWCNTs/PVP Composite Films for Realization of Ammonia Gas Sensor
Operable at Room Temperature
Sakshi Sharma, K. Sengupta, S. S. Islam.......................................................................................... 143
An Investigation of Structural and Electrical Properties of Nano Crystalline SnO2: Cu Thin
Films Deposited by Spray Pyrolysis
J. Podder and S. S. Roy ..................................................................................................................... 155

Nanocrystalline Cobalt-doped SnO2 Thin Film: A Sensitive Cigarette Smoke Sensor
Patil Shriram B., More Mahendra A., Patil Arun V.............................................................................. 163
Effect of Annealing on the Structural and Optical Properties of Nano Fiber ZnO Films
Deposited by Spray Pyrolysis
M. R. Islam, J. Podder, S. F. U. Farhad and D. K. Saha.................................................................... 170
Amperometric Acetylcholinesterase Biosensor Based on Multilayer Multiwall Carbon
Nanotubes-chitosan Composite
Xia Sun, Chen Zhai, Xiangyou Wang................................................................................................. 177
Fabrication of Biosensors Based on Nanostructured Conducting Polyaniline (NSPANI)
Deepshikha Saini, Ruchika Chauhan and Tinku Basu....................................................................... 187
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Please visit journal’s webpage with preparation instructions: http://www.sensorsportal.com/HTML/DIGEST/Submition.htm
International Frequency Sensor Association (IFSA).

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 95-106
Sensors & Transducers
ISSN 1726-5479
© 2011 by IFSA
http://www.sensorsportal.com
Simple Synthesis of ZnCo 2 O 4 Nanoparticles
as Gas-sensing Materials
*
S. V. Bangale, * S. M. Khetre ** D. R. Patil and * S. R. Bamane
*
Metal Oxide Research, Laboratory, Department of Chemistry, Dr. Patangrao Kadam
Mahavidyalaya, Sangli 416416 (M.S.) India
**
Bulk and Nano Materials Research Lab, Dept. of Physics, Rani Laxmibai College
Parola, Dist – Jalgaon, 425111
Tel.: 0233-2535993, fax: 0233-2535993
E-mail: bangale_sv@rediffmail.com
Received: 15 July 2011 /Accepted: 21 November 2011 /Published: 29 November 2011
Abstract: Semiconductive nanometer-size material ZnCo 2 O 4 was synthesized by a solution combustion
reaction of inorganic reagents of Zn(NO 3 ) 3 . 6H 2 O, Co(NO 3 ) 3 .6H 2 O and glycine as a fuel. The process was
a convenient, environment friendly, inexpensive and efficient preparation method for the ZnCo 2 O 4
nanomaterial. The synthesized materials were characterized by TG/DTA, XRD, EDX, SEM, and TEM.
Conductance responses of the nanocrystalline ZnCo 2 O 4 thick film were measured by exposing the film to
reducing gases like Acetone, Ethanol, Ammonia (NH 3 ), Hydrogen (H 2 ), Hydrogen sulphide (H 2 S),
Chlorine (Cl 2 ) and Liquefied petroleum gas (LPG). It was found that the sensors exhibited various
sensing responses to these gases at different operating temperature. Furthermore, the sensor exhibited a
fast response and a good recovery. The results demonstrated that ZnCo 2 O 4 can be used as a new type of
gas-sensing material which has a high sensitivity and good selectivity to Liquefied petroleum gas (LPG)
at 100 ppm. Copyright © 2011 IFSA.
Keyword: Nanostructure ZnCo 2 O 4 , XRD, SEM, TEM, Gas sensor.
1. Introduction
Among the various materials, ZnO is the most promising semiconductor [1] to detect the toxic and
hazardous gases. In recent years, one dimensional (1D) nanostructures of semiconducting metal oxides
with high surface to volume ratio and small grain size have received focused attention due to their
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Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 95-106
potential applications in fabricating gas sensors [2, 5], humidity sensors [6, 7] and nanoelectronic circuits
[9]. Cr2O3-activated ZnO was reported to be a humidity sensors [10], Cr2O3-modified ZnO thick film
resistors was reported to be LPG sensors [11]. Liquefied petroleum gas (LPG) is utilized in almost every
kitchen all over the world. It is therefore, referred as a town gas or cooking gas. It is utilized in large
extent for industrial purposes and in laboratories as fuel. Cooking gas consists chiefly of butane [12],
which is a colourless and odourless gas. It is usually mixed with compounds of sulphur (viz.methyl
mercaptan and ethyl mercaptan) having foul smell, so that its leakage can be noticed easily. This gas is
potentially hazardous because explosion accidents [13] might be caused, when it leaks out by mistake. It
has been reported that, at the concentration up to noticeable leakage, it is very much more than the lower
explosive limit (LEL) of the gas in air. Explosion accidents destroyed many industries, laboratories,
kitchens and houses, building, societies and what not? Many researchers are working on LPG sensor, but
could not meet the challenges up to the depth of demand by society. So there is a great demand from the
society of detecting LPG for the purpose of safety application in domestic and industrial fields.
Among p-type semiconducting metal oxide, Co 3 O 4 is a promising material due to its application potential
in many technological areas such as heterogeneous catalysis [14], anode material in lithium rechargeable
batteries [15], sensors [16], Zinc cobaltite ZnCo 2 O 4 is one of these compounds, which has long been used
as a ceramic color pigment or dying material [17,18]. In the same way, solid soluctions of nominal
composition Zn x Co 3-x O 4 exhibit improved adsorbent capabilities so they find application as selective
catalysts for oxidation and hydrogenation reaction [19], as well as for the monitoring and removal of
harmful species such as Cl 2 , NO 2 , CO 2 and H 2 S among others [20]. ZnCo 2 O 4 is a cobalt based spinel
oxide, where divalent Zn ions occupy the tetrahedral site in the cubic spinel structure and the trivalent Co
ions occupy the octahedral site [21]. Different methods such as typical co-precipitation [22], sol-gel route
[23], hydrothermal synthesis [24], high-temperature calcinations of hydroxide or carbonate precursor
mixtures [25] and combustion method have been proposed recently and developed which make it possible
to obtain spinel oxides in the form of ultrafine or nanoparticles. Chen and Coworkers [26] prepared by
MCo 2 O 4 ( M = Ni, Cu, Zn) nanotubes by using template assisted method and investigated their gas
sensing properties to Cl 2 , NO 2 , C 2 H 2 OH, SO 2 and CO. Du et al [27] synthesized ZnM 2 O 4 ( M = Fe, Co,
Cr) by microemulsion method and studied the gas sensing properties to Cl 2 , NO 2 , C 2 H 5 OH, H 2 S and
acetone. The aim of the present work is to develop LPG sensor by ZnCo 2 O 4 thick films. This could be
able to detect the trace amount of LPG. It was studied that the gas sensing performance of the material
can be improved by incorporating few additives into the base material and/ or surface activation of thick
films. Some well-known materials used for the fabrication of LPG sensor are SnO 2 [28], Ru-SnO 2 [29],
modified-SnO 2 [30], etc.
In the paper, we report a LPG sensor with gas response and good selectivity using cubic nanostructure
ZnCo 2 O 4 as the sensing material for the first time. The nanostructure ZnCo 2 O 4 was synthesized by
combustion route first time using mixed precursor (consisting of zinc nitrate, magnesium nitrate and
glycine) at 800 0 C in air for 4 h. The mixed precursor was prepared through a low cost and simple
combustion method.
2. Experimental
2.1. Powder Preparation
In this study, the as-synthesized ZnCo 2 O 4 powder were prepared using combustion synthesis different
molar composition of fuel, subsequently, the molar composition was maintained constant, and process
was carried out under varying synthesis temperatures.
Zinc cobalt oxide (ZnCo 2 O 4 ) was synthesized using the modified solution combustion technique, starting
from solution of Zn(NO 3 ) 2 6H 2 O (7.43g), (Co (NO 3 ) 2 6H 2 O (7.27g), and glycine (6.05g). glycine possesses
96

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 95-106
a high heat of combustion. It is an organic fuel and providing a platform for redox reactions during the
course of combustion. Initially the Zinc nitrates, Cobalt nitrates and Glycine are taken in the 1:1:4
stoichiometric amount and dissolved in a 250 ml beaker and made in homogenous peste. Peste formed
was evaporated on hot plate in temperature range of 70 0 C to 80 0 C resulting into a thick gel. The gel was
kept on a hot plate for auto combustion and heated in the temperature range of 170 0 C to 180 0 C. The
nanocrystalline ZnCo 2 O 4 powder was formed within a five minute. And it was sintered at about 300, 500,
800 and 1000 0 C for about 4 hours when we got a black color shining powder of nanocrystalline
ZnCo 2 O 4.
2.2. Thick Film Preparation
Zinc cobalt oxide powder was ground in an agate pastel-moter to ensure sufficiently fine particle size.
The fine powder was calcined at 800 0 C for 24 h in air and re-ground. The thixotropic paste [31, 32] was
formulated by mixing the resulting ZnCo 2 O 4 fine powder with a soluction of ethyl cellulose (a temporary
binder) in a mixture of organic solvents such as butyl carbitol acetate and turpineol. The ratio of inorganic
and organic path was kept as 75:25 in formulating the paste. The paste was then used to prepare thick
films. The thixotropic paste was screen printed on a glass substrate in desired patterns. The films prepared
were fired at 500 0 C for 24 h.
2.3. Characterization
The as –prepared samples were characterized by TG/DTA thermal analyzer (SDT Q600 V 20.9 Build 20),
XRD Philips Analytic X-ray B.V. (PW-3710 Based Model diffraction analysis using Cu-K α radiation),
scanning electron microscope (SEM, JEOL JED 2300) coupled with an energy dispersive spectrometer
(EDS JEOL 6360 LA), A JEOL JEM–200 CX transmission electron microscope operating at 200 kV
analysis.
3. Result and Discussion
3.1 TG-DTA
2C 2 NH 3 O 2 + (9/2) O 2 → N 2 + CO 2 + 5H 2 O (1)
Zn (NO 3 ) 3 + Co (NO 3 ) 3 + 4C 2 H 5 NO 2 → ZnCo 2 O 4 + 8H 2 O + 10H 2 O + 5N 2 (2)
TG-DTA analysis was performed at a heating rate of 10 K min -1 to investigate the thermal properties of
ZnCo 2 O 4. The TG spectrum and its 1 st derivative in Fig. 1 show the thermal decomposition of ZnCo 2 O 4 is
the curve indicates that the slight weight loss of about 13.2 % at temperature up to 140 0 C in ZnCo 2 O 4
powder due to little loss of moisture, carbon dioxide and nitrogen gas. The DTA curve of ZnCo 2 O 4
recorded in static air and in shown in Fig. 1. The curve shown that ZnCo 2 O 4 did not decompose, but
weight loss was due to dehydrogenation, decarboxylation and denitraction. Further weight loss of about
28 % between the temperature range 300 0 C and continuous loss in weight about 48 % up to 600 0 C is
attributed to loss of organic materials and yield final product at 650 0 C, this weight loss and weight gained
was very negligible. This weight change was in the range of 650 0 C these indicating that the synthesized
powder was almost stable from the begging. The formation temperature in the present work is found to be
comparatively similar than that reported for corresponding solid state reaction route [33].
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Fig. 1. TG-DTA curve of mixed precursorZnCo 2 O 4 .
3.2. XRD Results
The X-ray diffraction pattern of sample at different temperatures of synthesis. Is shown in Fig. 2. The
observed d values compared with standard d values and are in good agreement with standard d value
JCPDS data. (Card number 82-1042). The structure possesses the cubic may be attributed to the different
preparation method which may yield different structural defects. The crystalline size was determined
from full width of half maximum (FWHM) of the most intense peak obtained by shown scanning of
X-ray diffraction pattern. The crystallite size was calculated by using Scherrer equation [25-26]
d = 0.9λ/ βcosθ,
where, d is the crystalline size, λ is the X-ray wavelength of the Cu K α source (λ=1.54056 A 0 ), β is the
FWHM of the most predominant peak at 100 % intensity, θ is the Braggs angle at which peak is recorded.
In order to obtain pure nanocrystalline ZnCo 2 O 4 particles and understand the thermal characterizations,
the as prepared ZnCo 2 O 4 powder is further calcined at 180, 300, 500, 800 and 1000 0 C respectively (the
calcined temperature assigned as T C ). Fig. 2 presents XRD patterns for ZnCo 2 O 4 oxide nanoparticles. The
effects of the calcinations temperature on the crystallite size of ZnCo 2 O 4 particles can be demonstrated.
Traces of ZnCo 2 O 4 crystallites phases (111), (220), (311), (222), (400),(311), (422) and (511) are detected
in the XRD pattern for all calcined temperatures and then their intensities increase abruptly when the T C
above 1000 0 C .In general, the sharpness of the XRD peak (i.e. high crystallinity) is increased as the T C
increases. According to the (222) diffraction pattern of ZnCo 2 O 4 crystalline, the particle size of ZnCo 2 O 4
can be calculated from the full width at haif-maximum using the Scherrer equation. Obviously, the
particle size of ZnCo 2 O 4 changes as the Tc controlled fewer than 180, 300, 500, 800 and 1000 0 C, the
order is 7, 8, 9, 12 and 37 nm, respectively.
3.3. Particle Size Analyzer
Particle size distribution studies Fig. 3 have been carried out by using dynamic light scattering techniques
(DLS) via Laser input energy of 632 nm). It was observed that zinc cobalt oxide nanoparticles particles
have narrow size distribute within the range of about 25-30 nm. Which well matches are with calculated
from Debye-Scherrer equation?
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Intensity (arbitary unit)
111
220
222
311
1000 O C
800 O C
500 O C
300 O C
400
331
422
511
180 O C
20 40 60 80 100
Fig. 2. XRD pattern of calcinied mixed precursorZnCo 2 O 4 at 300 0 C, 500 0 C. 800 0 C, 1000 0 C in air for 4 h.
2
Fig. 3. Particle size Distribution of mixed precursor ZnCo 2 O 4 at 800 0 C in air for 4 h.
3.4. Energy dispersive X-ray microanalysis studies (EDX)
Fig. 4 shows the energy dispersive X-ray spectrum of ZnCo 2 O 4 . This was carried out to understand the
composition of zinc, cobalt and oxygen in the material. There was no unidentified peak observed in EDX.
This confirms the purity and the composition of the ZnCo 2 O 4 nanomaterial.
3.5. Scanning Electron Micrograph Study
The microstructure of the 800 0 C sintered samples can be visualized from scanning electron microscope
(SEM) tool. Fig. 5. epicts SEM images of ZnCo 2 O 4 powder. Shown the partical morphology of high
resolution, the partical are most irregular in shape with a nanosize range 100-200 nm some particles are
found as agglomerations containing very fine particles. It can be observed that ZnCo 2 O 4 have uniformed
size of about 5μm.It seems that surface are smooth, spongy pores are seen in the micrograph.
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Fig. 4. EDX pattern of mixed precursor ZnCo 2 O 4 at 800 0 C in air for 4 h.
(a)
(b)
Fig. 5. SEM images of mixed precursor ZnCo 2 O 4 at 800 0 C in air for 4 h (a) low resolution, and (b) high resolution.
3.6. Transmission Electron Microscopy (TEM)
The TEM image of the mixed precursor calcined at 800 0 C for 4 h are shown in Fig. 6(a, b).It indicates
the presence of ZnCo 2 O 4 nanoparticles with size 30-40 nm which form beed type of oriental aggregation
throughout the region. The selected area electron diffraction (SAED) pattern Fig. 6 (c) shows the spot
type pattern which is indicative of the presence of single crystalline particles. No evidence was found for
more than one pattern, suggesting the single phage nature of the material.
3.7. Thickness Measurement
The thicknesses of the films were observed to be in the range from 25-35 µm. The reproducibility of the
film thickness was achieved by maintaining the proper rheology and thixotropy of the paste.
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(a)
(b)
(c)
Fig. 6. TEM (a, b) images of nanostructured ZnCo 2 O 4 (c) SAED pattern.
4. Electrical Properties of the Sensors
4.1 I-V Characteristics
Fig. 7. depicts I-V characteristics of ZnCo 2 O 4 films. It is clear from the symmetrical I-V characteristics
that the silver contacts on the films were ohmic in nature.
4.2. Electrical Conductivity
Fig. 8 shows the variation of log (conductivity) with temperature. The conductivity values of sample
increase with operating temperature. The increase in conductivity with increasing temperature could be
attributed to negative temperature coefficient of resistance and semiconducting nature of ZnCo 2 O 4 . It is
observed from Fig. 8 that the electrical conductivities of the ZnCo 2 O 4 films are nearly linear in the
temperature range from 50- 400 0 C in air ambient.
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Fig. 7. I-V characteristics of the sensor.
Fig. 8. Variation log (conductivity) with reciprocal operating temperature (K -1 ).
5. Sensing Performance of Sensor
5.1 Gas Response, Selectivity, Response and Recovery Time
The relative response (S) to a target gas is defined as the ratio of the change in conductance of a sample
upon exposure of the gas to the original conductance in air. The relation for S is given as
S = G g -G a /G a ,
where G a is the conductance in air and G g the conductance in a sample gas.
Specificity or selectivity is defined as the ability of a sensor to respond to a certain gas in the presence of
different gases. Response time (RST) is defined as the time neede for a sensor to attain 90 % of the
maximum increases in conductance on exposure to the target gas.
Recovery time (RCT) is the time taken to get back 90 % of the original conductance in air.
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5.2. Gas Response and Operating Temperature
Fig. 9 depicts the variation of 100 ppm gas responses with operating temperature. The ZnCo 2 O 4 sensor
was observed to be a temperature dependents gas sensor. From Fig. 9, it is observed that the sensor better
responds to LPG gas at 350 0 C, H 2 S gas at 300 0 C, Ethanol gas at 300 0 C Acetone gas at 300 0 C, NH 3 gas
at 300 0 C, CI 2 gas at 300 0 C, H 2 gas at 350 0 C, CO 2 gas at 250 0 C. The sensitivity towards such gas
depends on temperature. The same sensors could be used to identify LPG, H 2 S, Ethanol, Acetone, NH 3 ,
Cl 2 , H 2 , and CO 2 just by varying the operating temperature. This is the most desirable characteristic of
this sensor.
Fig. 9. Variation of different gas responses with operating temperatures.
5.3. Gas Response and Gas Concentration
It is observed from Fig. 10 that the response of ZnCo 2 O 4 film was observed to increases continuously
with increasing the gas concentration up to 100 ppm, and it attains the maximum and saturates above
100 ppm. Therefore, the active region of the sensor would be up to 100 ppm, as the rate of rise of
response is larger during this region. At lower gas concentration, the unimolecular layer of gas molecules
would be expected to form on the surface, which would interact with the surface more actively giving
larger responses. There would be multilayers of gas molecules on the sensors surface at the higher gas
concentration (> 100 ppm) resulting in saturation in gas response.
5.4. Selectivity
Fig. 11 depicts the variation of gas responses of ZnCo 2 O 4 samples to LPG among the mixture of gases. It
is clear from the figure that the ZnCo 2 O 4 sample shows the selective response to LPG at 350 0 C among all
other gases.
5.5. Response and Recovery of the Sensor
The time taken for the sensor to attain 90 % of the maximum increases in conductance on exposure of the
target gas is known as response time. The time taken by the sensor to get back 90 % of the maximum
conductance when the flow of gas is switched off is known as recovery time. The response and recovery
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of the ZnCo 2 O 4 thick film sensors was quick ( ˜ 20 s) while the recovery was fast ( ˜ 44 s). The negligible
quantity of the surface reaction products and their high volatility explain the quick response to LPG and
fast recovery to its chemical status.
Fig. 10. Variation of gas response with gas concentration.
Fig. 11. Selectivity factor of sensor for various gases.
6. Discussion
Gas sensing mechanism is generally explained in terms of conductance either by adsorption of
atmospheric oxygen on the surface and/or by direct reaction of lattice oxygen or interstitial oxygen with
the test gases. In case of former, the atmospheric oxygen adsorbs on the surface by extracting an electron
from conduction band, in the form of super-oxide or peroxides, which are mainly responsible for the
detection of the test gases. At higher temperature, the adsorbed oxygen captures the electrons from
conduction band as:
O 2(air) + 2e - (cond.band) → 2O - (film surface).
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It would result in decreasing conductivity of the film. When LPG reacts with the adsorbed oxygen on the
surface of the film, it gets oxidized to CO 2 and H 2 O by following series of intermediate stage. This
liberates free electrons in the conduction band. The final reaction takes place as:
C 4 H 10(gas) + 13O - (filmsurface) → 4CO 2(gas) + 5H 2 O (gas) + 13e - (cond.band)
This shows n-type conduction mechanism. Thus generated electrons contribute to a sudden increase in
conductance of the thick film. The ZnCo 2 O 4 misfit regions dispersed on the surface would enhance the
ability of base materials to absorb more oxygen species giving high resistance in air ambient. Therefore
response was obtained to 100 ppm LPG.
7. Conclusions
(I) Nanocrystalline ZnCo 2 O 4 has been synthesized by self combustion route. This synthesis route may be
used for the synthesis of other metal oxide.
(II) Among all other additives tested, ZnCo 2 O 4 is outstanding in promoting the LPG gas sensing
mechanism.
(III) ZnCo 2 O 4 to be optimum and showed highest response to LPG gas at 350 0 C.
(IV)The sensor showed very rapid response and recovery to LPG gas.
(VI) Sensing mechanism of ZnCo 2 O 4 was the substitution of lattice oxygen by LPG gas. Material gains
electrons in this substitution.
(VII) The sensor has good selectivity to LPG against H 2 S, Acetone, NH 3 , CI 2 , H 2 and CO 2 .
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