SYNTHESIS OF NANOSTRUCTURED ZrO2 FOR GAS SENSING APPLICATION …s2is.org/Issues/v5/n3/papers/paper8.pdf · SYNTHESIS OF NANOSTRUCTURED ZrO 2 FOR GAS SENSING APPLICATION . Pratap G

SYNTHESIS OF NANOSTRUCTURED ZrO2 FOR GAS SENSING

APPLICATION

Pratap G. Patil1, D. D. Kajale 2, V. P. Patil 3 , G. E. Patil 2 and G. H. Jain 2 * 1 Department of Physics, Ramnarain Ruia College, Matunga, Mumbai, India

2 Materials Research Lab., K.T.H.M. College, Nashik 422 002, India 3 Department of Physics, BHAVAN’s College, Mumbai 400 058, India

Corresponding Author: [email protected]

Submitted: June 25, 2012 Accepted: Aug. 5, 2012 Published: Sep. 1, 2012

Abstract- Nanocrystalline ZrO2 (Zirconia) has been synthesized by a conventional precipitation method.

The structural, morphological, microstructural, optical and gas-sensing properties of ZrO2 were

investigated by using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM),

transmission electron microscopy (TEM), UV-vis spectroscopy and static gas sensing unit, respectively.

X-ray diffraction pattern and TEM of the synthesized product reveal their nano-crystalline nature with

grain size 18 nm and 20 nm, respectively. Gas sensing properties of their thick films, which were

fabricated by screen-printing to various gases (O2, NO2, C2H5OH, CO, CO2, NH3, LPG, H2S and H2)

were tested in ambient air. The ZrO2 thick films showed a high response and selectivity to H2S gas. The

effect of operating temperature, gas concentration on the sensing characteristics of these films towards

H2S was discussed..

Index terms: zirconia nanopowder, conventional precipitation method, characterization, H2S sensor.

INTERNATIONAL JOURNAL ON SMART SENSING AND INTELLIGENT SYSTEMS, VOL. 5, NO. 3, SEPTEMBER 2012

673

mailto:[email protected]�

I. INTRODUCTION

It is inevitable to have a continuous control of the hazardous gases in atmosphere. The gases

such as CO, H2, C2H5OH, CO2, NO2, O2, CH4, NH4, H2S, C2H2, C2H4, C3H6, LPG etc. have to

be controlled for the betterment of living beings. Fuels are widely consumed for transport

services all over the world. During combustion, various polluting and toxic gases are released

resulting in environmental pollution which can cause serious health hazards. Upon burning,

toxic hydrogen sulfide gas is oxidized to sulfur dioxide by atomic oxygen, molecular oxygen

or ozone [1]. Combustions of petroleum and coal are the predominant sources of the gases

containing sulfur [2]. The gases containing sulfur can result in undesirable disastrous

deformations such as infection to respiratory track and lung cancer [2, 3]. Thus H2S is

harmful to human body and the environment. According to the safety standards established

by American conference of Government Industrial Hygienists, the threshold limit value

(TLV) for H2S is 10 ppm. Even at low concentration its effect on the nervous system is severe

[4]. The emission of H2S and release of sulfur needs effective monitoring which can allow

one to control its free release to environment. Using suitable gas sensors often does Suxh

monitoring. Currently there are a number of materials being used for such sensoric

applications but these are often not so effective and suffer from various drawbacks, such as

low sensitivity, poor selectivity, slow response and recovery times. Therefore there is a need

for low cost and more effective H2S sensor operable in sub-ppm range. In addition,

concentration of H2S varied from the types of oil or natural gas used. The concentration of

H2S thus is different for different source of oil or natural gases mines. Therefore, the

detection and monitoring of H2S should be considered of high importance for both resource

exploitations and human health.

In the recent years, a number of semiconductor sensors have been found to be suitable for

H2S gas e.g. SnO2, WO3, In2O3, ZnO2 and a few perovskite-type materials like NdFeO3 and

NiFeO4 [5-12]. This is an approach of having ZrO2 nanocrystalline thick film as a H2S gas

sensor.

Generally these sensors are fabricated by several techniques like thick film technology, by

deposition of a film by screen printing method and the chemical bath deposition method etc.

In this study we used conventional precipitation method to synthesize nanocrystalline ZrO2.

Pratap G. Patil, D. D. Kajale, V. P. Patil, G. E. Patil and G. H. Jain, Synthesis of Nanostructured ZrO2 for Gas Sensing Application

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II. EXPERIMENTAL AND CHARACTERIZATION

a. Synthesis of Nanocrystalline ZrO2

All the chemicals used in present study are of analytical grade. ZrO2 supports were prepared by

conventional precipitation method (denoted as CP method) [13]. Firstly, ZrO(NO3) 2 aqueous

solution (0.1M) was added dropwise into diluted NH3 solution with vigorous stirring, which were

continued for another 30 min after titration. Then the white precipitate of ZrO(OH) 2 was aged at

room temperature for 10 h. Subsequently, the precipitate was filtered and washed thoroughly

with deionized water till pH 7. ZrO(OH) 2 precipitate was dried at 110 oC for 10 h in air and then

calcined at 550 oC for 5 h, yielding ZrO2 powder.

b. Preparation of Nanocrystalline ZrO2Thick Films

The synthesized powder of ZrO2 was calcined at 200oC for 1 h. The thixotropic paste was

formulated by mixing the fine powder of ZrO2 with a solution of ethyl cellulose (a temporary

binder) in a mixture of organic solvents such as butyl cellulose, butyl barbital acetate and

terpineol etc. The weight ratio of the inorganic to organic part was kept at 75:25 in formulating

the paste. This paste was screen printed on a glass substrate in a desired pattern [14]. The films

were fired at 550oC for 30 min. Silver contacts are made for electrical measurements.

c. Characterization of Synthesized ZrO2

The crystalline structure, as prepared nanopowder, was analyzed with X-ray diffractogram

(Rigaku DMAX 2500) by using radiation with a wavelength of 1.5418 Å. The surface

topography and elemental composition of the nanopowder were analyzed with scanning electron

microscopy (JOEL JED 2300), coupled with an energy-dispersive spectrometer (6360 LA). The

microstructure of nanopowder was studied with a transmission electron microscope [Tecnai 20

G2 (FEI make)]. A UV–visible spectrophotometer (Simadzu 2450 UV-VIS) was used to study

the optical properties of nanopowder. Electrical and gas-sensing properties were measured by

using a static gas-sensing system.


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d. Static Gas Sensing System

Figure 1. Schematic diagram of static gas system.

The basic electrical characterization is the resistance versus temperature measurement by using

home-built static gas characterization system as shown in Fig. 1. A heater (2000 watt) was used

to change the operating temperature of the sensor from room temperature to 500oC. The sensing

performances of the samples were tested for different gases. The highest gas-sensing performance

characteristics of the material prepared under the optimized processing parameters are further

checked and confirmed by using different measurements viz. repeatability, reproducibility, gas

concentration range, response and recovery time.

The sensing performance of the sensors was examined using a „static gas sensing system‟, shown in Fig. 1. There were electrical feeds through the base plate. The heater was fixed on the

base plate to heat the sample under test up to required operating temperatures. The current

passing through the heating element was monitored using a relay operated with an electronic

circuit with adjustable ON-OFF time intervals. A Cr-Al thermocouple was used to sense the

operating temperature of the sensor. The output of the thermocouple was connected to a digital

temperature indicator. A gas inlet valve was fitted at one of the ports of the base plate. The

required gas concentration inside the static system was achieved by injecting a known volume of

a test gas using a gas-injecting syringe. A constant voltage was applied to the sensor, and the

current was measured by a digital picoammeter. The air was allowed to pass into the glass

chamber after every gas exposure cycle.


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III. RESULT AND DISCUSSION

a. Structural Analysis of Nanocrystalline ZrO2 by XRD

Figure 2 shows the X-ray Diffractogram (XRD)of the as prepared ZrO2 nanopowder. The

observed peaks match well with the standard JCPDS data of ZrO2 [15]. The broad peaks are due

to the nanocrystalline nature of ZrO2.

Figure 2. XRD spectra of synthesized ZrO2 powder.

Scherrer‟s Formula for Grain Size Calculation:

t = 0.9λ / βcosθ (1)

where, λ= wavelength of X-ray; β = FWHM of peak; θ = corresponding angle of the peak.

The average grain size calculated from Scherrer‟s formula was about 18 nm.

b. Surface Topography Using SEM

Scanning electron micrograph is shown in figure 3 by using topography of the film surface.

Scanning electron microscope could not resolve nanoparticles associated with the film even at

very high magnification. It may be due to very small nanocrystalline particles associated with the

film.


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Figure 3. Scanning electron micrograph of nanocrystalline ZrO2.

c. Quantitative Elemental Analysis

The quantitative elemental composition of the ZrO2 was analyzed using an energy dispersive

spectrometer (EDS). Observed At% of Zr and O is: 30.33 and 69.67, respectively. There is little

deviation from stoichiometry of the prepared film.

d. Transmission Eelectron Microscopy (TEM)

(a) (b)

Figure 4. (a) Transmission electron micrograph and (b) SAED pattern of nanocrystalline ZrO2.


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The TEM technique was used to know the exact grain size, shape, and distribution of the

crystallites associated with the powder. It is clear from the TEM image [Fig. 4(a)] that there are

uniformly distributed grains with an average grain size of 20 nm. Additional structural

characterization of ZrO2 nanopowder was carried out by electron diffraction shown in Fig. 4(b).

Spherical rings in electron diffraction patterns suggest that the nanopowder has good

crystallinity. The images clearly show the spotty fringes corresponding to the lattice planes of

ZrO2.

e. Absorption Spectra

Fig. 5 shows the absorption spectra of the as-prepared nanocrystalline ZrO2 powder. The sharp of

the absorption edge suggests a single phase. The band-gap energy calculated from the absorption

spectra is 3.97 eV, smaller than the reported band gap energy (5.2 eV) of ZrO2 [16]. It is

interesting to note that the nanocrystalline nature does not have a significant effect on the band

gap of the nanocrystalline ZnO2 powder.

Figure 5. Absorbance spectra nanocrystalline of ZrO2 powder.

f. Gas Sensing Performance of the Sensor Fabricated from Nanocrystalline ZrO2 Powder

The gas response (S) is defined as the ratio of the change in the conductance of the sensor on

exposure to the target gas to the original conductance in air and is given by the following relation

[17]:


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S = Gg – Ga/ Ga (2)

where Ga is the conductance of sensor in air and Gg is the conductance of sensor in a target gas.

i. Gas Response with the Operating Temperature of the Sensor

Figure 6. Variation of gas response with operating temperature.

Figure 6 depicts the variation of gas response with operating temperature of the nanocrystalline

ZrO2 sensor for 1000 ppm CO, CO2, NH3, NO2, H2S, Cl2, H2 and LPG. It is clear from the figure

that the largest response to H2S at 250 oC is shown compared to the responses of CO, CO2, NH3,

NO2, Cl2, H2 and LPG. The sensors reported in this paper show reasonably better sensitivity at

lower operating temperatures. The enhancement of sensitivity at a relatively lower operating

temperature may be due to the nanocrystalline nature of ZrO2 powder employed to fabricate the

sensors.

ii. Selectivity of Sensor

Selectivity can be defined as the ability of a sensor to respond to a certain gas in the presence of

different gases [18]. It is observed from figure 9 that the ZrO2 thick film sensor gives maximum

response to H2S (1000 ppm) at 250oC. The sensor showed highest selectivity for H2S against all

other tested gases: NH3, LPG, Cl2, CO, CO2, O2, H2 and NO2. Hence it is highly selective to H2S

gas.


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Figure 7. Selectivity of ZrO2 sensor.

iii. Response Time and Recovery Time.

The response/ recovery time is an important parameter used for characterizing a sensor. It is

defined as the time required to reach 90 % of the final change in current, when the gas is turned

on and off respectively [18-20]. The response was quick (8 s) while the recovery was fast (< 18

s). The quick response may be due to faster oxidation of gas.

IV. CONCLUSION

A conventional precipitation method is used to prepare ZrO2 nanocrystalline powder. The

technique is simple and inexpensive and it may be useful for the production of metal–oxide

nanopowders. XRD analysis confirmed that the synthesized powder to be that of the ZrO2. The

TEM image showed roughly spherical particles with an average size of 20 nm. The average grain

size calculated from XRD was 18 nm and from TEM, it was 20 nm. The band-gap energy

calculated from an absorption spectrum was 3.97 eV. This value matches exactly with the

reported value. The response of the nanocrystalline ZrO2-based sensor was observed to be the

largest to H2S gas. The sensor showed good selectivity to H2S gas against NH3, LPG, Cl2, CO,

CO2, O2, H2 and NO2 gases.


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ACKNOWLEDGEMENTS

The authors are thankful to the Principal, Arts, Commerce and Science College, Nandgaon, and

Ramnarain Ruia College, Matunga for providing laboratory facilities for this work. Authors are

grateful to Dr. D. C. Kothari, Nanomaterials Research Lab., University of Mumbai for their

valuable cooperation rendered for characterizations of the material.

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SYNTHESIS OF NANOSTRUCTURED ZrO2 FOR GAS SENSING APPLICATION …s2is.org/Issues/v5/n3/papers/paper8.pdf · SYNTHESIS OF NANOSTRUCTURED ZrO 2 FOR GAS SENSING APPLICATION . Pratap G