co2 sensing characteristics of sm1-xba coo3 (x = 0, 0.1, 0...
TRANSCRIPT
CO2 sensing characteristics of Sm1-xBaxCoO3 (x = 0, 0.1, 0.15,
0.2) nanostructured thick film
G.N. Chaudhari, P.R. Padole, S.V. Jagatap, M.J. Pawar*
Nanotechnology Research Laboratory, Dept. of Chemistry,
Shri Shivaji Science College, Amravati, M.S. India.
Abstract:
SmCoO3 gas sensor has been developed with high sensitivity for CO2 gas by doping Ba.
Nanostructured SmCoO3 and Sm1-xBaxCoO3 (x = 0, 0.1, 0.15, 0.2) were obtained by EDTA-
Glycol method. The operation temperature falls and sensitivity increases from 425 to 370OC
when Ba concentration in SmCoO3 changes from x = 0 to x = 0.1. Ag impregnation over
Sm0.9Ba0.1CoO3 sensor, on exposure to CO2 at about 360OC showed an increased sensitivity as
well as the response time also decreases. The possible CO2 sensing mechanism is proposed on
the basis of available literatures.
Keywords: SmCoO3; Nanostructures; Gas Sensitivity; Selectivity; EDTA-Glycol Method;
1. INTRODUCTION
Growing environmental awareness and hazards of global warming has increased the need
for CO2 measurements in many countries. CO2 is one of the most common gases evolved
in the combustion; it is too stable chemically to be detected in a sensitive manner by
conventional gas sensors. The advanced technologies, such as air conditioning,
agriculture, biological technology and medical services have made the control and
measurements of CO2 concentration critical. Furthermore, the CO2 concentrations in the
atmosphere have increased on the global scale. Therefore, non-expensive and robust
detection systems are required for increasing carbon dioxide concentrations.
Unlike, optical and electrochemical sensors, solid state gas sensors based on
semiconductor metal oxides (SMOs) may be a promising alternative, since they offer
good sensor properties and can be easily mass-produced. Several SMOs viz. La-doped
SnO2 [1, 2], BaTiO3 [3, 4], etc are reported to the most reliable CO2 sensors. Among the
chemical sensors, LaCoO3, BaTiO3, LaFeO3, LaMnO3 etc are perovskite-type materials
of general formula ABO3 are extensively studied owing to their notable gas sensitivity
for different poisonous gases in addition to their magnetic, catalytic and other physical
properties. The perovskite-type metal oxide including the d-block and rare earth elements
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has attracted the attention of many researchers due to their homogeneity, interesting
structural, catalytic and gas sensing properties. Rare earth-cobaltites (LnCoO3; Ln =
Metals like La, Sm, Gd and Nd etc.) with perovskite-type structure exhibit outstanding
transport properties and high chemical activity, which make these materials suitable for
applications in areas of gas sensors, heterogeneous catalysis, gas separation membranes
and cathodes for solid oxide fuel cells. The materials of LnCoO3 type have been explored
for many technical applications technical applications, e.g. as electrodes in fuel cells [5]
and as auto-exhaust conversion catalysts [6, 7].
SmCoO3 material was studied as a catalyst for oxidation of CO in early eighties
[8].Sensing properties of Ba doped SmCoO3 has been studied at 410OC [9]. Many
approaches have been made to modify the sensing properties of these perovskite
materials in order to achieve high sensitivity, selectivity and to reduce the operation
temperature. In this direction different approaches are adopted including new methods of
material synthesis, doping suitable metals, physical or chemical filters etc. The purpose
of this work is to investigate and develop a new synthesis route for pure SmCoO3 and Ba
doped SmCoO3 as well as to study the effect of noble metals impregnation on gas
sensitivity of Sm1-xBaxCoO3.
Chemical sensors made of nanomaterials have great potential for an enhanced generation
of sensing devices that are smaller, consume less power, are higher-performing, and less
expensive than conventional sensors. Therefore, nanostuctured and single-phase
perovskites, Sm1−xBaxCoO3 (x = 0, 0.1, 0.15, 0.2), were prepared by an EDTA-glycol
method using stoichiometric amounts of the corresponding nitrates, ethylene glycol (EG)
and disodium salt of ethylene diamine tetra acetic acid (EDTA). So formed
nanostructered powders were characterized by X-ray diffraction and scanning electron
microscopy (SEM). XRD studies showed that the introduction of barium reduced the
temperature of formation of the perovskite and yielded nanostructured Sm0.9Ba0.1CoO3.
The evaluation of nanostructured Sm0.9Ba0.1CoO3 as a CO2 sensor was made through the
electrical characterization of sintered thick films.
2. EXPERIMENTAL
2.1 MATERIAL SYNTHESIS
The nanostructured compounds of Sm1-xBaxCoO3 were prepared for x = 0, 0.1, 0.15 and
0.2 by EDTA-glycol method. A calculated quantity of EDTA was firstly dissolved in a
small quantity of deionized water followed by the addition of stoichiometric ratio of
G.N. CHAUDHARI, P.R. PADOLE, S.V. JAGATAP, M.J. PAWAR, CO2 SENSING CHARACTERISTICS OF SM1-XBAXCOO3 (X = 0, 0.1, 0.15, 0.2) NANOSTRUCTURED THICK FILM
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nitrates Sm, Ba and Co. The mixture was magnetically stirred at 60OC for 3 hr in order to
obtain stable metal-EDTA complexes. After stirring, appropriate amount of ethylene
glycol (EG) was added to this solution. The solution so obtained was continuously stirred
on a hot plate with a magnetic stirrer at 90OC for 7 hr to remove the excess of water and
facilitate the polyesterification between EDTA and EG leading into formation of a resin
like mass. Heating at 350OC the polymeric resin is decomposed. The decomposed resin
was treated in a mantle heater at 400-450OC over 3 hr in order evaporate highly
combustible species and induce charring. The resulting ash were slightly ground in to
powder and subjected to calcination at 750OC for 6 hr as summarized in Table 1.
In order to enhance the sensitivity for CO2 gas, sample GIV was further loaded with 0.5
wt. % of Ag and Pd. Appropriate weights of silver nitrate and palladium nitrate were
dissolved separately in distilled water and then a calculated quantity of sample GIV was
added to this solution. These solutions were then stirred on magnetic stirrer to maintain
uniform ness. After several hours, the samples were slowly heated to dryness with
stirring and then heated at 4000C for 6 hr in air. The samples at the final were finely
grounded into powders and represented as GIV: Ag and GIV: Pd.
Sm1-xBaxCoO3 Calcination
Temperature (OC)
Calcination Time
(Hr) Sample Code
SmCoO3
Sm0.9Ba0.1CoO3
Sm0.85Ba0.15CoO3
Sm0.8Ba0.2CoO3
750
750
750
750
6
6
6
6
GI
GII
GIII
GIV
Table 1. Sensor samples with experimental conditions.
The samples were characterized by X-ray diffraction (XRD) analysis and scanning
electron microscopy (SEM). In the present article, results are discussed only for SEM
analysis.
2.2 GAS SENSING MEASUREMENTS
The sensitivity for CO2 gas was measured for all the samples by preparing their thick-
films. The sensor materials were deposited through screen-printing technique on the
alumina substrate. Two Platinum electrodes separated by distance of 2 mm were printed
on the surface of alumina tube for the measurement of gas sensing properties. The paste
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was prepared by mixing sensor samples with 30 wt. % organic binder, 30 wt. % solvent
and 0.15 ml. dispersant. Thick-films, after screen-printing on alumina substrate were
kept at 2000C for 2 hr for removal of the organic binder. The thick-film devices must be
heated to sufficiently high temperatures to ensure interconnectivity between individual
grains and film robustness. To measure gas sensitivity, the sensors were placed in test
chamber through which dry air and gas to be sensed were allowed to flow. The operating
temperature was measured by use of a thermocouple kept touched to the sensor system.
Before taking the measurement the sensor was heated to 3500C, cooled in dry air since
these sensors are influenced by humidity in atmosphere. Finally, the sensitivity of thick-
films was measured for CO2 gas for the temperature starting from 150 to 450 0C. The
sensitivity (S) to CO2 was defined as the ratio of resistance of an element in CO2 free air
(RAIR) to the resistance in a CO2 gas (RCO2), RAIR / RCO2. The change in resistance on
contact between target gas and sensor was recorded from multimeter.
3. RESULT AND DISCUSSION:
3.1 CHARACTERIZATION
From the XRD analysis, carried out on calcinated doped powders of SmCoO3, only the
signals corresponding to the oxides of Sm, Ba and Co were observed. No secondary
phase was observed in these samples though they are calcined at lower temperature;
750OC. These observations indicate a better mixing of the cations in the compounds
formed. In order to obtain the insight information about the surface morphology and
particle size of the sample, SEM analysis was performed. Fig.1 shows the SEM
micrograph of the sample GII. The formation of Sm0.9Ba0.1CoO3 aggregates comprising
very tiny three-dimensional disordered nanoparticles was visibly observed. The particle
size of the sample GII is quite uniform of about 60 nm.
Figure1. SEM photomicrograph of the Sm0.9Ba0.1CoO3
G.N. CHAUDHARI, P.R. PADOLE, S.V. JAGATAP, M.J. PAWAR, CO2 SENSING CHARACTERISTICS OF SM1-XBAXCOO3 (X = 0, 0.1, 0.15, 0.2) NANOSTRUCTURED THICK FILM
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3.2 SENSITIVITY MEASUREMENT OF NANOSTRUCTERED Sm1-XBaXCoO3
Fig. 2 shows the gas sensitivity for 1000 ppm CO2 gas for samples GI, GII, GIII and GIV
at different temperatures ranging from 150 to 450OC. The maximum sensitivity was
observed at about 425OC for undoped sample GI, which reduced to 370OC for sample
GII. The gas sensitivity increased as compared to GI on Ba addition. It could be due to
uniform distribution of dopant at the molecular level and the reduction of particle size on
Ba addition (for x=0.1). From the Fig.2 it can be seen that, for samples GIII and GIV no
significant rise in sensitivity though the concentration of Ba is increased from x = 0.15 to
0.2. Maximum gas sensitivity increased from about 0.35 to 0.87 from samples GI to GII
at 370OC. Even without the noble metal catalyst additions the gas sensitivity
enhancement in this case was about four times.
Fig. 3 shows the sensitivity at 370OC to CO2 as a function of the amount of Ba
dopants. The sensitivity of the element increases substantially with increasing amount of
Ba up to x = 0.1. However, the sensitivity is maintained in the range from x = 0.1 to x =
0.15 and further addition (x = 0.2) decrease the sensitivity for CO2. It seems that x = 0.1
is an enough amount of Ba as a dopant for increasing the sensitivity of SmCoO3.
1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 00 .0
0 .2
0 .4
0 .6
0 .8
1 .0
O p e r a t io n T e m p e r a tu r e ( O C )
Sens
itivi
ty (S
)
S a m p le G I S a m p le G II S a m p le G II I
0 .0
0 .2
0 .4
0 .6
0 .8
1 .0 S a m p le G IV
Fig. 2 CO2 sensing characteristics of the samples GI, GII, GIII and GIV as a function of operation
temperature (OC).
0 .0 0 .1 0 .20 .2
0 .4
0 .6
0 .8
1 .0
X in S m1 -x
B axC o O
3
Sens
itivi
ty (S
)
0 .2
0 .4
0 .6
0 .8
1 .0
Figure 3 CO2 sensing characteristics of SmCoO3 as a function of doped Ba at 370OC
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3.3 EFFECT OF Ag AND Pd ADDITIVES ON CO2 DETECTION
The noble metals, well known as active catalysts, have been confirmed to possess the
promoting effects on many semiconductor gas sensors [10–12]. Doping with the noble
metals such as Pt, Pd, Ag and Au can raise the adsorption activity of the semiconducting
oxides. These metals form cluster on the oxide surface, promoting adsorption and
dissociation of reducing gases [13-16], and thus are well known for enhancing the rate of
response with decreased operation temperature and raising selectivity to single gas. In
the present study, we have tested the effect of 0.5 wt. % Ag and Pd impregnation over
the sample GII as. The sample GII was only selected for the further study as it has shown
the higher sensor response for CO2 among all the samples.
150 200 250 300 350 4000.0
0.2
0.4
0.6
0.8
1.0
Operation Temperature (OC)
Sens
itivi
ty (S
)
Sample GII:Ag
0.0
0.2
0.4
0.6
0.8
1.0
Sample GII:Pd
Figure 4 CO2 sensing characteristics of Ag and Pd impregnated sample GII as a function of operation
temperature (OC)
Fig. 4 depicts the gas sensitivity of sample GII: Ag and GII: Pd as a function of operating
temperature. One can see from the fig. 4 that, the sensitivity to CO2 gas of sample GII
with 0.5 wt. % Ag is about twice as large as that for GII: Pd at about 360OC. The
addition of Ag gives a high sensitivity to CO2, but the operating temperature is not much
reduced. The high operating temperature is not desirable for practical applications and
long term stability. The mechanism for gas detection in these materials is based, in large
part, on reactions that occur at the sensor surface, resulting in a change in the
concentration of adsorbed oxygen. The enhancement of sensing property seems to be
brought about by changes in the surface property caused due to impregnation of Ag.
Since the amount of adsorbed CO2 increased greatly by the addition of Ag, as the added
Ag catalyses the surface carbonation of the sensor element.
G.N. CHAUDHARI, P.R. PADOLE, S.V. JAGATAP, M.J. PAWAR, CO2 SENSING CHARACTERISTICS OF SM1-XBAXCOO3 (X = 0, 0.1, 0.15, 0.2) NANOSTRUCTURED THICK FILM
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0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 340.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Sample GII
Sample GII:Ag
Time (min.)
Sens
itivi
ty (S
)
0.0
0.2
0.4
0.6
0.8
1.0
Figure 5 Response characteristics of sample GII (at 370OC) and sample GII:Ag (at 360OC)
100 200 300 400 500 600 700 800 900 10000.2
0.4
0.6
0.8
1.0
CO2 Gas Concentration (ppm)
Sens
itivi
ty (S
)
0.2
0.4
0.6
0.8
1.0
Figure 6 Sensing characteristics of sample GII: Ag as a function of CO2 concentration (ppm) at 360OC
In case of GII: Pd, a monotonic rise in sensitivity can be seen up to 200OC and beyond
this temperature, sensitivity falls. Though the PdO is a well known good catalyst for
chemical sensors but is not very active beyond 200OC may be because of its
decomposition at 200OC.
The change in sensitivity of GII: Ag element with the concentration of CO2 at 360OC is
shown in Fig. 5. The sensitivity monotonically increased with the increase in
concentrations of CO2 from 100 to 1200 ppm. Fig. 6 shows the response characteristics
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of the samples GII and GII: Ag. The Ag impregnated GII showed a high sensitivity as
compared to the undoped element.
For the CO2 sensors, selectivity is an important subject, and in particular, removing the
interference of from humidity is critical. To check the selectivity, the effect of 1.5 vol. %
water vapors on the gas sensitivity of sample GII was investigated at 360OC. The
presence of water vapors decreased the sensitivity to CO2 by the factor of about 5.4. The
similar types of results were also reported by Tamaki et al. [17]. The addition of Ag to
GII did not remove the interference of humidity in CO2 sensing.
3.5 SENSOR STABILITY
In practice, the stability of the sensors is one of the most important parameters in the
sensor technology. In order to check the long-term stability of the noble metal loaded
sensors, they were tested for 1000 ppm of CO2 after the period of four months at the
optimum temperature of both the samples. The results obtained in this measurement are
shown in Figure 7. It was observed that, the Ag loaded sample losses its sensitivity by
±20%. On the other hand, undoped sample stands with high stability and maximum loss
in sensitivity was not more than 15%.
50 100 150 200 2500.0
0.2
0.4
0.6
0.8
1.0
Sample GII:AgSample GIISamples
Sens
itivi
ty (S
)
0.0
0.2
0.4
0.6
0.8
1.0At 360OC
At 370OC
Figure 7 Sensitivity of sample GII (at 370OC) and GII:Ag (at 360OC) towards 1000 ppm of CO2 after the
time period of four months
3.4 SENSING MECHANISM
The sensing mechanism of SmCoO3 thick-film gas sensors is usually based on surface
properties of material. The molecular oxygen in the air gets adsorbed on the surface of
sensing material and dissociates to produce different species like O, O2- and O-. There is
a formation of a depletion layer among the adsorbed oxygen ions and the oxide particles
G.N. CHAUDHARI, P.R. PADOLE, S.V. JAGATAP, M.J. PAWAR, CO2 SENSING CHARACTERISTICS OF SM1-XBAXCOO3 (X = 0, 0.1, 0.15, 0.2) NANOSTRUCTURED THICK FILM
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caused due to transfer of conduction electron. This develops an electric field between
adsorbed oxygen species and the oxide surface. At the specific temperature CO2 can be
oxidized in to CO3- by interacting with adsorbed oxygen species. These carbonates
disappear when they are exposed to oxidizing conditions [18]. When the sensor response
measurement is carried out at higher temperature, the desorption of gas leads into the
sensitivity of material.
O2 + e-→ O2- (1)
O2 + 2e- → 2O2- (2)
CO2 + O- → CO3- (3)
4. CONCLUSIONS
In conclusion, the CO2 sensing property of SmCoO3 was improved by doping Ba. The
operation temperature was decreased from 425OC (for sample GI) to 370OC (for sample
GII). The sample GII (Sm0.9Ba0.1CoO3) impregnated with Ag showed rather high
sensitivity and stability with quick response at the reduced temperature of 360OC. A big
fall in the sensitivity for CO2 gas in presence of humidity is a major disadvantage of the
sample GII: Ag, which needs further improvement.
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