CuO-doped BSST thick film resistors for ppb level H2S gas sensing at room temperature

Sensors and Actuators B 123 (2007) 246–253

CuO-doped BSST thick film resistors for ppb level H2S gas sensing at room temperature G.H. Jain, L.A. Patil ∗ Materials Research Lab, P.G. Department of Physics, Pratap College, Amalner 425401, India Received 23 May 2006; received in revised form 12 August 2006; accepted 18 August 2006 Available online 26 September 2006

Abstract An effort has been made to develop a heterojunction type CuO-(Ba0.8 Sr0.2 )(Sn0.8 Ti0.2 )O3 (BSST) gas sensor which could detect an extremely small amount of H2 S gas at room temperature. The BSST solid solution was prepared mechanochemically. Copper was doped in the BSST ceramics by adding CuCl2 , followed by sintering at 550 ◦ C for 2 h. Sintering converts CuCl2 into CuO. CuO is known to be p-type and BSST as an n-type semiconductor. Copper oxide grains in association with BSST form p-CuO/n-BSST heterojunctions. The thick films of CuO-BSST were prepared by screen-printing technique. The CuO-BSST heterojunction gas sensor is a new type gas sensor and has many advantages, such as low cost, simple processing, and convenient testing. This sensor was observed to be highly sensitive and selective to a ppb level of H2 S gas at room temperature. © 2006 Elsevier B.V. All rights reserved. Keywords: p-CuO; n-BSST; Heterojunctions; Gas response; Room temperature H2 S sensor; Response time; Recovery time

1. Introduction Hydrogen sulfide is a toxic gas, often produced in coal, coal oil or natural gas manufacturing. The threshold limit value (TLV) for H2 S is 10 ppm. Even at low concentration it produces severe effects on the nervous system. Therefore, reliable sensors with low cost, low energy consumption having high sensitivity, selectivity, and operable in a sub ppm range of H2 S are in high demand for environmental safety and industrial control purposes. The catalyst in semiconducting ceramic gas sensors has been studied extensively because it plays a very important part in determining the sensitive property of devices. In recent years, heterocontacts between p- and n-type semiconducting grains have been developed for detecting various gases [1–4]. These semiconducting gas sensors possess a gas sensing mechanism different from single oxide semiconductors and can be used to detect small amounts of toxic gases that exist in air. Research for new good gas-sensing materials and the new properties of conventional materials has become an active research field. Concerning the detection of dilute H2 S less than 1 ppm, thin film or thick film sensors using CuO–SnO2 [5,6],

WO3 [7–9], and CuO-doped SnO2 [10] have been reported to show excellent performance. Recently, nanophased WO3 -based H2 S gas sensors have been reported [11] for the detection of ppb-level H2 S. It is reported that CuO-surface modified BSST (using dipping technique) sensors respond to 100 ppm H2 S gas at 300 ◦ C. Instead of such modification of the film surface, BSST was doped with CuO in this article. The resultant CuO-BSST sensors could detect ppb-level H2 S at room temperature. The most general gas sensing mechanism [12–17] of semiconductor gas sensors proposed is a simple resistivity change due to adsorption–desorption of gases. The p–n heterocontact concept was used in the present investigation [18–27]. There is an intrinsic difference in the behavior between the heterocontact type sensors and the sensors based on catalytic oxidation/reduction of gas molecules. The heterocontact type sensors work on the principle of barrier mechanism, which needs no adsorption and desorption of oxygen for the detection of H2 S gas. 2. Experimental 2.1. Powder and thick film preparation

∗

Corresponding author. Tel.: +91 2587 224226. E-mail address: [email protected] (L.A. Patil).

0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.08.026

The (Ba0.8 Sr0.2 )(Sn0.8 Ti0.2 )O3 powder was prepared by mechanochemical processing (MCP). The AR grade powders

G.H. Jain, L.A. Patil / Sensors and Actuators B 123 (2007) 246–253

of Ba(OH)2 ·8H2 O, Sr(OH)2 , SnO2 and TiO2 (with molar concentrations of Ba:Sr = 4:1 and Sn:Ti = 4:1) were ball milled to mix thoroughly, followed by sintering at 1000 ◦ C for 6 h. The fine-grained powder was obtained by milling in a planetary ball mill for 2 h. The repeated deformation, fracturing, and cold welding of powder particles (in MCP) increase the area of contacts between the starting material particles due to a decrease in particle size and allows the surfaces formed to come continually into contact. This permits the reaction to continue without the necessity for diffusion through the product layer. As a result, chemical reactions that normally need high temperatures will take place at lower temperatures during milling without any external heating. The XRD spectrum of as-prepared powder confirmed the polycrystalline mixed tetragonal and hexagonal pervoskite phases. Microarea EDS analysis also confirmed the expected weight and atomic percentages of the elements in composition. CuO-BSST powders with different wt% of CuO were obtained by mixing CuCl2 into BSST, followed by firing at 550 ◦ C for 30 min. The thixotropic paste was formulated by mixing the fine powder of CuO-BSST with a solution of ethyl cellulose (a temporary binder) in a mixture of organic solvents such as butyl cellulose, butyl carbitol acetate, terpineol, etc. The ratio of the inorganic to organic part was kept at 75:25 in formulating the paste. This paste was screen-printed [16,17] on glass a substrate in the desired pattern. The films were fired at 550 ◦ C for 30 min. 2.2. Thickness measurements The thicknesses range of the films was observed to be from 65 to 75 ␮m. The reproducibility in thickness of the films was possible by maintaining the proper rheology and thixotropy of the paste. 2.3. Thermoelectric power measurements The p- or n-type semiconductivity of the thick films of CuO and BSST were confirmed by measuring the thermoelectromotive force of the thick film samples. The CuO was observed to be p-type and BSST the n-type material. 2.4. Characterization and sample testing The structural properties of the powder were studied using a Rigaku model DMAX-2500 X-ray diffractometer (XRD) with ˚ The microstructure and Cu K␣ radiation, having λ = 1.5406 A. chemical compositions of the films were analyzed using a scanning electron microscope (SEM, JEOL JED 6300) coupled with an energy dispersive spectrometer (EDS, JEOL JED 2300LA). The thicknesses of the thick films were measured using a Taylor–Hobson (Talystep, UK) system. Fourier transformation infrared (FTIR) spectra of the films were recorded using a Spectrum-2000 FTIR spectrophotometer in the range of 650–4000 cm−1 . The sensing performance of the sensors was examined using a ‘static gas sensing system’ explained elsewhere [17].

247

Fig. 1. X-ray diffractograms of: (a) BSST and (b) CuO-BSST (6.6 wt% CuO) thick films.

3. Characterization results 3.1. Structural properties Fig. 1 shows the X-ray diffractograms of screen-printed BSST and CuO-BSST (6.6 wt% CuO) thick films fired at 550 ◦ C in air atmosphere. It is revealed from the XRD that the material is polycrystalline in nature with mixed tetragonal and hexagonal pervoskite phases. The peaks in the XRD pattern are matching well with the ASTM data book [28]. The average grain size was determined using Scherrer formula and was estimated to be 37.6 nm. The XRD analysis showed no diffraction peaks except those of CuO and BSST, which revealed that the CuO-BSST composite was a mechanical mixture of CuO and BSST [29]. 3.2. Microstructural analysis Fig. 2(a) depicts the SEM image of the unmodified BSST thick film fired at 550 ◦ C. The film consists of voids and wide distribution of grains with grain sizes ranging from 0.25 to 1.0 ␮m distributed nonuniformaly. Fig. 2(b) is the SEM image of the CuO-BSST film with 6.6 wt% CuO. It is clear from the figure that the average grain size of the CuO-modified film was smaller than the grain size associated with the unmodified BSST. Fig. 2(c) is the SEM image of the CuO-BSST film with 28.9 wt% CuO. It shows a high degree of agglomeration of grains with a wide range of particle size distribution. Due to the high degree of agglomeration the porosity of the film was observed to be decreased, which in turn decreased the in-pore adsorption of gas, leading to a smaller surface to volume ratio. 3.3. Elemental analysis The quantitative elemental compositions of the BSST films with different amounts of CuO, analyzed using an energy dispersive spectrometer, are represented in Table 1. The stoichiometric composition of BSST is 87.68 and 12.32 wt% for cations (Ba + Sr + Sn + Ti) and anions (O), respectively. The concentrations of these constituent elements

248

G.H. Jain, L.A. Patil / Sensors and Actuators B 123 (2007) 246–253

Fig. 2. SEM images of: (a) unmodified BSST, (b) CuO-BSST (6.6 wt% CuO), and (c) CuO-BSST (28.9 wt% CuO) films.

in the present samples were not in the stoichiometric proportion and all the samples were observed to be oxygen deficient, leading to the semiconducting nature to the BSST. Fig. 3 represents the variation of O/BSST and O/CuO ratios with the amount of CuO. It is clear from the figure that

the wt% oxygen goes on decreasing with an increase of the CuO wt%. 3.4. FTIR spectra Fig. 4(a–d) represents the FTIR spectra of the BSST, CuOBSST and CuO, and the difference of BSST and CuO-BSST, respectively. The FTIR pattern of the CuO-BSST material looks like a combination of those of BSST (Fig. 4(a)) and CuO (Fig. 4(c)). The profile of the spectrum consists of peaks around 1613 and 1422 cm−1 corresponding to CuO. The peaks between 746 and 1370 cm−1 are a mixture of these two materials and the peaks between 1446 and 3020 cm−1 are typical for BSST. Fig. 4(c and d) are matching with each other. This reveals that CuO has maintained its status after the processing. 3.5. Electrical properties

Fig. 3. Variation of O/BSST and O/CuO ratios with the amount of CuO.

Fig. 5 shows the temperature dependence of conductivity of CuO-BSST films in air and gas ambient. Electrical conductivity of these films goes on increasing with an increase in temperature in air and gas (H2 S) ambient, indicating the semiconducting nature of the films.

G.H. Jain, L.A. Patil / Sensors and Actuators B 123 (2007) 246–253

249

Table 1 Quantitative elemental analysis (in wt%) (Ba + Sr + Sn + Ti) O BSST CuO

89.23 10.77 100 0

93.38 6.53 99.9041 0.09959

93.37 5.75 99.0794 0.9306

89.9 5.77 95.4302 4.5798

88.31 5.46 93.4298 6.5702

84.6 5.73 89.7760 10.2240

Fig. 4. FTIR spectra of: (a) unmodified BSST, (b) CuO-BSST, (c) CuO, and (d) difference of BSST and CuO-BSST.

Fig. 5. Variation of conductivity with temperature in air and H2 S gas (10 ppm) ambient.

68.35 3.75 71.0538 28.9462

250

G.H. Jain, L.A. Patil / Sensors and Actuators B 123 (2007) 246–253

4. Sensing performance 4.1. Definition of sensing characteristics Gas response (S) is defined as the ratio of the change in conductance of the sample on exposure to gas to the conductance in air. S=

G Gg − Ga = Ga Ga

where Gg and Ga are the conductances of the sample in the presence and absence of the test gas, respectively, and G is change in conductance. The ability of a sensor to respond to certain gas in the presence of other gases is known as selectivity. The time taken for the sensor to attain 90% of the maximum change in resistance on exposure to the gas is the response time. The time taken for the sensor to get back 90% of the original resistance is the recovery time [30]. 4.2. Response and H2 S gas concentration 4.2.1. Active region of the gas sensor The variation of log (gas response) of CuO-BSST samples with H2 S gas concentration is represented in Fig. 6(a) at room temperature. Each film was exposed to varying concentrations of H2 S gas (1–100 ppm).

For all samples, the response values were observed to increase continuously with the gas concentration up to 100 ppm at room temperature. The rate of the increase in response was relatively larger up to 20 ppm, smaller during 20–50 ppm and then approximately saturated after 50 ppm. Thus, the active region of the sensor would be between 0.1 and 20 ppm. 4.2.2. Ppb-level gas sensing Fig. 6(b) is the histogram showing the gas response of a BSST sensor doped with 6.6 wt% CuO to H2 S up to 10 ppm gas concentration at room temperature. At lower gas concentrations, a monolayer of the gas molecules would be expected to be formed on the surface, which could interact with the surface more actively, giving larger responses. There would be multilayers of gas molecules on the sensor surface at the higher gas concentrations, resulting in saturation in response. 4.3. Gas response and operating temperature The variation of H2 S gas response (10 ppm) of the CuO-BSST films with operating temperature is shown in Fig. 7. The CuOBSST films showed very high electrical resistance of the order of 109  in air and very low resistance of the order of 105  upon exposure to 10 ppm H2 S at room temperature. The sample with 6.6 wt% CuO was observed to be the most sensitive of all. It showed the response of 1711 and 3140 to 10 ppm H2 S at room temperature and at 50 ◦ C, respectively. The room temperature response could be attributed to the heterojunction type sensing mechanism. 4.4. CuO dopant and gas response Fig. 8 is the histogram indicating the gas response to 1 ppm H2 S as a function of the CuO content at room temperature and at 50 ◦ C. The sensor with 6.6 wt% CuO was observed to be most sensitive. This could be attributed to the optimum number of misfits dispersed adequately on the surface of the film, forming an optimum number of p–n heterocontacts on the surface. The resistance of such CuO-doped BSST would be very high in air ambient. On exposure to H2 S gas, CuO molecules on the surface

Fig. 6. (a) Variation in H2 S gas response with concentration at room temperature and (b) ppb-level H2 S gas response of a BSST sensor with 6.6 wt% CuO at room temperature.

Fig. 7. Variation of H2 S gas response (10 ppm) with operating temperature.

G.H. Jain, L.A. Patil / Sensors and Actuators B 123 (2007) 246–253

251

would be converted into conducting CuS, the p-CuO/n-BSST junctions would be disrupted and resistance of the sensor would suddenly be decreased, giving very high response even to 1 ppm H2 S gas. 4.5. Selectivity of H2 S against various gases Fig. 9 depicts the selectivity profile of the CuO-BSST (6.6 wt% CuO) film for H2 S (1 ppm) gas at room temperature, 50 and 350 ◦ C. The sensor showed high selectivity for H2 S and could distinguish an extremely small amount (1 ppm) of H2 S among 1000 ppm of the following gases: NH3 , ethanol, LPG, CO, CO2 , and H2 at 350 ◦ C. Fig. 8. Gas response (1 ppm) values of different CuO content samples.

4.6. Response and recovery profile The response and recovery profile of the most sensitive film (6.6 wt% CuO) film is represented in Fig. 10. The response was quick (10 s) even to a trace amount (1 ppm) of H2 S at room temperature, and the sensor could recover within 60 s after heating at 200 ◦ C. 5. Discussion 5.1. Model to represent CuO-BSST thick films

Fig. 9. Selectivity for H2 S gas from gas mixtures.

Fig. 10. Gas response and recovery of the p-CuO/n-BSST film (1 ppm).

Fig. 11(a and b) represents an equivalent resistance model for the CuO-BSST thick film in air and H2 S gas ambient. In air ambient, the CuO-doped BSST thick film can be looked upon as a resistor (Rair ) resulted from an intergrannular resistance R connected in series with parallel combination of surface and bulk resistances due to heterojunctions (p-CuO/n-BSST), as represented in Fig. 11(a). In air ambient, Rair was observed to be of the order of 109 . On exposure to H2 S gas, the heterojunctions (p-CuO/nBSST) on the surface would be disrupted due to conversion of CuO into well conducting CuS [31], and the surface resistance would decrease suddenly, while the bulk resistance would remain as it was. Therefore, the film on exposure of gas could be looked upon as a resistor (Rgas ) resulted from an intergrannular resistance (R) connected in series with very low

Fig. 11. (a and b)Resistance model of the CuO-BSST thick film.

252

G.H. Jain, L.A. Patil / Sensors and Actuators B 123 (2007) 246–253

Fig. 12. (a and b)Effect of dispersion of CuO misfits.

effective resistance [resultant of parallel combination of very low surface resistance (CuS.n-BSST) and high bulk resistance (p-CuO/n-BSST)], as shown in Fig. 11(b). A ppb-level H2 S gas response was due to a very large change in resistance [109  (air) to105  (gas)] of the film on exposure to gas. On heating the film in air, CuS on the film surface would be oxidized and converted back to CuO and the p–n junctions would be recovered back as follows:

semiconductor particle could have a control on the small portion of the semiconductor surface, while adequate catalyst dispersion on the semiconductor surface could produce a uniform depletion region on the surface of the semiconductor and could control the conductivity more effectively. If the dispersion of CuO misfits is poor, then the depletion regions would be discontinuous and there would be the paths to pass electrons from one to another grain and the conductance would be relatively larger.

heat

CuS + 23 O2 −→CuO + SO2 ↑

6. Summary

5.2. Effect of CuO content

From the results, following statements can be made for the sensing performance of the CuO-BSST sensors.

For the optimum CuO (6.6 wt%) content in BSST, the H2 S gas sensitivity is maximum. This could be attributed to the adequate dispersion of CuO misfits and the optimum number of p-CuO/n-BSST heterojunctions formed on the surface. The proportion of p-CuO/n-BSST heterojunctions in samples is crucial for the resistance of the samples, and the resistance of p-CuO/nBSST heterojunctions becomes a control factor to the surface resistance of the samples [32]. If the amount of CuO in BSST is smaller than the optimum (6.6 wt%) and if the CuO misfits are poorly dispersed, the number of heterojunctions formed on the surface would be insufficient to change the overall resistance drastically and the sensor would show poor gas response. When the amount of CuO dispersed on the surface is larger than the optimum (6.6 wt%), only some part of CuO would be utilized for the formation of p–n junctions, and unused CuO would remain idle, resisting the H2 S gas to reach to the p-CuO/nBSST junctions. Due to this, the resistance of the sensor could not change drastically and the sensor response would be comparatively smaller. 5.3. Dispersion of CuO misfits Fig. 12(a and b) illustrates the effect of the dispersion of CuO misfits on the conductivity. The misfits dominate the depletion of electrons from the semiconductor. A few CuO misfits on the

1. The H2 S gas sensors fabricated in this work were of heterojunction type. 2. The room temperature sensing of H2 S gas was possible due to the heterocontact type sensing mechanism. 3. The barrier height of the n-CuO/p-BSST heterojunctions decreased markedly due to the chemical transformation of highly resistive p-CuO into well conducting CuS, leading to a drastic decrease in resistance. 4. The sensors fabricated in the present article could detect ppblevel H2 S gas (response = 4–10 ppb), which is less than its occupational exposure limit (10 ppm). 5. The sensor showed very quick response (10 s) and fast recovery time (60 s) to H2 S gas. References [1] E. Traversa, M. Miyayama, H. Yanagida, Gas sensitivity of ZnO/La2 CuO4 heterocontacts, Sens. Actuators B 17 (1994) 257–261. [2] J. Tamaki, T. Maekawa, N. Miura, N. Yamazoe, CuO–SnO2 element for highly sensitive and selective detection of H2 S, Sens. Actuators B 9 (1992) 197–203. [3] X. Zhou, Q. Cao, Y. Hu, J. Gao, Y. Xu, Sensing behavior and mechanism of La2 CuO4 –SnO2 gas sensors, Sens. Actuators B 77 (2001) 443–446. [4] X. Zhou, Q. CaO, H. Huang, P. Yang, Y. Hu, Study on sensing mechanism of CuO–SnO2 gas sensors, Mater. Sci. Eng. B 99 (2003) 44–47. [5] J. Tamaki, K. Shimanoe, Y. Yamada, Y. Yamamoto, N. Miura, N. Yamazoe, Dilute hydrogen sulfide sensing properties of CuO–SnO2 thin film pre-

G.H. Jain, L.A. Patil / Sensors and Actuators B 123 (2007) 246–253

[6]

[7]

[8]

[9]

[10]

[11]

[12] [13] [14] [15] [16]

[17]

[18]

[19]

[20] [21]

pared by low-pressure evaporation method, Sens. Actuators B 49 (1998) 121. D.J. Yoo, J. Tamaki, S. Park, N. Miura, N. Yamazoe, Copper oxide loaded tin dioxide thin film for detection of dilute hydrogen sulfide, Jpn. J. Appl. Phys. 34 (1995) L455. J.L. Solis, S. Saukko, L.B. Kish, C.G. Granqvist, V. Lantto, Nanocrystalline tungsten oxide thick films with high sensitivity to H2 S at room temperature, Sens. Actuators B 77 (2001) 316–321. J.L. Solis, S. Saukko, L.B. Kish, C.G. Granqvist, V. Lantto, Semiconductor gas sensors based on nanostructured tungsten oxide, Thin Solid Films 391 (2001) 255–260. J.L. Solis, A. Hoel, L.B. Kish, C.G. Granqvist, S. Saukko, V. Lantto, Gassensing properties of nanocrystalline WO3 films made by advanced reactive gas deposition, J. Am. Ceram. Soc. 84 (2001) 1504–1508. R. Kumar, A. Khanna, P. Tripathi, R.V. Nandedkar, S.R. Potdar, S.M. Chaudhari, S.S. Bhatti, CuO–SnO2 element as hydrogen sulphide gas sensor prepared by sequential electron beam evaporation technique, J. Appl. Phys. 36 (2003) 2377–2381. R. Ionescu, A. Hoel, C.G. Granqvist, E. Liobet, P. Heszler, Low-level detection of ethanol and H2 S with temperature-modulated WO3 nanoparticle gas sensors, Sens. Actuators B 104 (2005) 132–139. T. Seiyama, F. Era, Gas detecting materials, Zairyo-Kagaku Jpn. 8 (1971) 232–239. S.R. Morrison, Selectivity in semiconducting gas sensors, Sens. Actuators B 12 (1987) 425–440. S. Pizzini, N. Butta, D. Narducci, M. Palladino, Thick film ZnO resistive gas sensors, J. Electrochem. Soc. 136 (1989) 1945–1948. P.T. Moseley, Materials selection for semiconductor gas sensors, Sens. Actuators B 6 (1992) 149–156. M.S. Wagh, G.H. Jain, D.R. Patil, S.A. Patil, L.A. Patil, Modified zinc oxide thick films resistors as NH3 gas sensors, Sens. Actuators B 115 (2006) 125–133. G.H. Jain, L.A. Patil, M.S. Wagh, D.R. Patil, S.A. Patil, D.P. Amalnerkar, Surface modified BaTiO3 thick film resistors as H2 S gas sensor, Sens. Actuators B 117 (2006) 159–165. K. Kawakami, H. Yanagida, Effects of water vapor on the electrical conductivity of the interface of semiconductor ceramic–ceramic contact, Yogyo Kyokaishi Jpn. 67 (1979) 112–115. Y. Nakamura, T. Tsurutani, M. Miyayama, K. Koamoto, H. Yanagida, The detection of carbon monoxide by the oxide-semiconductor heterocontacts, J. Chem. Soc. Jpn. (1987) 477–483. H. Ito, S. Fujisu, M. Miyayama, K. Koamoto, H. Yanagida, Alcohol sensor using p–n semiconductor contact, J. Ceram. Soc. Jpn. 100 (1992) 888–893. Y. Ushio, M. Miyayama, H. Yanagida, Fabrication of thin film CuO/ZnO hetero-junction and its humidity sensing properties, Sens. Actuators B 12 (1993) 135–139.

253

[22] K. Hikita, M. Miyayama, H. Yanagida, New gas sensing method for detecting carbon monoxide by use of the complex impedance of CuO/ZnO heteroconductor under a dc bias voltage, J. Am. Ceram. Soc. 77 (1994) 1761–1764. [23] H. Hirota, M. Mayayama, Y. Nakamura, H. Yanagida, Quantitative analysis of ethanol in water solution by a heterocontact type sensor of CuO/ZnO, Chem. Lett. Jpn. (1994) 1793–1796. [24] D.J. Yoo, J. Tamaki, S.J. Park, N. Miura, N. Yamazoe, Copper oxide loaded tin dioxide thin film for detection of dilute hydrogen sulphide, Jpn. J. Appl. Phys. 34 (1995) 455–457. [25] S.J. Jung, H. Yanagida, The characterization for a CuO/ZnO heterocontact type gas sensor having selectivity for CO gas, Sens. Actuators B 37 (1996) 55–60. [26] R.B. Vasilier, M.N. Rumyantseva, M.N. Yakovlev, A.M. Gaskov, CuO/SnO2 thin film heterostructures as chemical sensors to H2 S, Sens. Actuators B 50 (1998) 186–193. [27] M.S. Wagh, L.A. Patil, D.P. Tanay Seth, D.P. Amalnerkar, Surface cupricated SnO2 –ZnO thick films as a H2 S gas sensor, Mater. Chem. Phys. 84 (2004) 228–233. [28] ASTM Data manuals, pp. 5–626; 34–129. [29] L. Bo, L. Hongzhun, Y. Jianhong, Testing system for calibrating characteristics of carbon dioxide gas sensor, Sens. Technol. 16 (1997) 43– 45. [30] T. Ishihara, K. Kometani, M. Hashida, Y. Takita, Application of mixed oxide capacitor to the selective carbon dioxide sensor, J. Electrochem. Soc. 138 (1991) 173–177. [31] Y. Qui, et al., Handbook of Chemistry, Sandong Science and Technology Press, People’s Republic of China, 1985. [32] Y. Hu, X. Zhou, Q. Han, Q. CaO, Y. Huang, Sensing properties of CuO–ZnO heterojunction gas sensors, Mater. Sci. Eng. B 99 (2003) 41–43.

Biographies G.H. Jain is a lecturer in physics, at NDMVP’s, A.C.S. College, Nandgaon, India. He received his MSc degree in physics from University of Pune in 1989. He is working on gas sensors for his PhD degree, Material Research Laboratory, Pratap College, Amalner, India. L.A. Patil is a reader in physics at Pratap College, Amalner, India. He received his MPhil in applied electronics and PhD in material science. His topics of interest are ceramic gas sensors, photoconducting and luminescent materials, art of growing crystals, dielectric properties of materials, nanomaterials, thin and thick film physics. He is the member of “Management Council”, North Maharashtra University, Jalgaon, India.