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Room temperature chlorine gas sensing using surface modified ZnO thick film resistors
Sensors and Actuators B 123 (2007) 546–553
Room temperature chlorine gas sensing using surface modified ZnO thick film resistors D.R. Patil, L.A. Patil ∗ Materials Research Laboratory, Pratap College, Amalner 425401, India Received 3 June 2006; received in revised form 25 September 2006; accepted 26 September 2006 Available online 13 November 2006
Abstract Thick films of pure and CuO-modified ZnO were prepared by screen-printing technique. Pure ZnO was observed to be almost insensitive to chlorine gas. Pure ZnO white powder was calcined at 1100 ◦ C for 24 h, to minimize the oxygen deficiency. CuO-modified ZnO thick films were obtained by dipping technique in which Cu2+ could be incorporated as an additive into the ZnO base material. The films were fired at 500 ◦ C for 24 h. The CuO-modified sensor was observed to be sensitive to 300 ppm Cl2 gas at room temperature. The effect of operating temperature, calcination temperature, gas concentration, amount of CuO in ZnO on the gas response was studied. The selectivity of the target gas against other gases, gas response and recovery times of the sensor were also presented. The quick response (∼18 s) and fast recovery (∼50 s) are the main features of the modified sensor. © 2006 Elsevier B.V. All rights reserved. Keywords: Room temperature; Chlorine gas sensor; CuO-modified ZnO; Thick films; Selectivity; Response and recovery time
1. Introduction Chlorine is a yellowish-green [1] gas having pungent smell, and is explosively utilized in industrial applications such as to bleach paper pulp, to disinfect sewage and drinking water, etc. As it has a wide range of applications, its toxicity [2–5] can affect the health of humans in contact. Chlorine has excellent bleaching ability, but once it is discharged in aquatic systems, it interacts with other industrial effluents to produce a host of chlorinated organics such as dioxin. Dioxin persists in the environment for prolonged periods and has a tendency to bioaccumulate in the food chains, which elicits toxic effects to humans, such as skin infection, psychological disorders and even liver damage. Therefore, it is needed to monitor chlorine gas at ordinary temperature. Gas chromatography, chemical detecting tubes and electrochemical sensing techniques are available for the detection of chlorine gas in the environment. A few Cl2 gas sensors have already been developed, but they do not work at room temperature. Transparent conducting oxide (TCO) thin film sensors operate at higher temperatures (∼300 ◦ C). However, it is inconvenient to operate
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Corresponding author. Tel.: +91 2587 224226. E-mail address: plalchand phy [email protected] (L.A. Patil).
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sensors at higher temperatures for monitoring. It is, therefore, essential to develop a sensor, which could detect Cl2 gas at room temperature. Zirconia tubes, MgO–In2 O3 , ZnO–In2 O3 , Zn2 In2 O5 –MgIn2 O4 (TCO), ZnO–CaO, etc., are the materials utilized in Cl2 sensing applications [6–21]. It is well known that the semiconducting oxides such as ZnO, SnO2, Fe2 O3 , Ga2 O3 , Sb2 O3 [22–28], etc., are sensitive to toxic and inflammable gases. It has also reported that surface additives [29–33] play an important role in enhancing the gas response and specificity to a particular gas. The additives like Al, In, Cu, Fe, Sn and Ru [34,35] are often added to improve the response and selectivity. In the present article, thick film surfaces are modified by dipping them into a Cu-precursor for different time intervals, followed by firing. Firing would convert the Cu-precursor into CuO. In the surface modification process, the grains of CuO would disperse on the grains of ZnO. The dipping technique adds a dimension to the usefulness of surfaces by allowing one to customize their properties. This type of sensor works on the principle of substitution of lattice oxygen by chlorine. Substitution of O2 by Cl2 [2] gas at room temperature on the surface of CuO-modified ZnO thick film causes a drastic change in resistance. Room temperature gas detection offers the advantages of low power drain and reduced tendency to provide a source of ignition [25].
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2. Experimental
Table 1 Elemental analysis of unmodified (pure) and CuO-modified films
2.1. Powder and paste preparation
Elemental (mass%)
Dipping time (min) 0
2
5
15
30
Zn O ZnO Cu CuO
91.95 8.05 100 0.00 0.00
91.16 8.77 99.91 0.07 0.09
88.38 11.30 99.59 0.33 0.41
91.48 8.11 99.49 0.41 0.51
88.49 11.18 99.58 0.33 0.42
AR grade zinc oxide powder (99.9% pure) was ground in an agate pastle–mortor to ensure sufficiently fine particle size. The fine powder was calcined at 1100 ◦ C for 24 h in air and re-ground. The thixotropic paste was formulated by mixing the resulting ZnO fine powder with a temporary binder as a mixture of organic solvents. The ratio of inorganic to organic part was kept as 75:25 in formulating the paste. The paste was then used to prepare thick films. 2.2. Thick film preparation The thixotropic paste was screen printed on a glass substrate in desired patterns [30–33]. The films prepared were fired at 500 ◦ C for 24 h. These films were surface modified by dipping them into a 0.01 M aqueous solution of cupric chloride for different intervals of time and were dried at 80 ◦ C under an IR lamp, followed by calcination at 500 ◦ C for 24 h in air ambient. The CuCl2 dispersed on the film surface was oxidized in calcination and sensor elements with different mass% of CuO were obtained. Silver contacts were made by vacuum evaporation for electrical measurements.
for the largest time interval (30 min), respectively. The unmodified ZnO film in Fig. 1(a) consists of randomly distributed grains with larger size and shape distribution. Fig. 1(b) depicts that the microstructure of CuO-modified film (15 min) consists of more spherical particles of Cu-species distributed uniformly with smaller size and shape on the ZnO grains. These smaller spherical particles would be attributed to Cu-misfits. Fig. 1(c) depicts larger grains than those associated with Fig. 1(b). Larger grains in Fig. 1(c) may be due to a smaller amount of CuO on the surface as some amount of copper percolates into the bulk portion of the film. 3.2. Elemental analysis
3. Materials characterization
The quantitative elemental composition of the films was analyzed using an energy dispersive spectrometer and mass% of Zn, O, ZnO, Cu and CuO are represented in Table 1. Pure stoichiometric ZnO is expected to be insulating. Stoichiometrically expected mass% of Zn and O in ZnO are 80.34 and 19.66, respectively. The pure ZnO powder turns white to yellow on calcination at higher temperature (∼1100 ◦ C for 24 h). It may be due to deficiency of oxygen [36]. The mass% of Zn and O in each sample were not as per the stoichiometric proportion and all samples were observed to be oxygen deficient (Table 1). Whenever in the crystal, there is an excess or deficiency of one type of atom, which results in a distortion in the band structure, with a corresponding increase in conductivity [36]. Zinc oxide loses oxygen on heating so that the zinc is then in excess. The oxygen is, of course, evolved as an electrically neutral substance, so that being associated with each excess zinc ion in the crystal, there will be two electrons which remain trapped in the solidmaterial, thus leading to non-stoichiometricity in the solid. This leads to the formation of n-type semiconductivity. It is clear from Table 1 that unmodified ZnO is more oxygen deficient and oxygen deficiency could be reduced by adding CuO into ZnO. The mass% of CuO goes on increasing with the dipping time, reaches to a maximum and then decreases with a further increase in dipping time. This could be attributed to the dispersion of Cu grains on the surface of the film. The mass% of CuO (0.51) is highest for a 15 min dipping time. Due to an increase in mass% of Cu there was a decrease in the mass% of Zn.
3.1. Microstructure-SEM
3.3. Thickness measurement
Fig. 1 depicts the SEM images of unmodified ZnO film, most sensitive CuO-modified film (15 min) and CuO-modified film
Film thicknesses were observed to be in the range from 25 to 30 m. The reproducibility of film thickness was achieved
2.3. Characterization The micro structural analysis was carried out by using SEM (JEOL-6300 LA, Germany) and chemical compositions of the films were analyzed using EDS (JOEL, JED-2300, Germany) coupled with an energy dispersive spectrometer. Thickness measurements were carried out using Taylor-Hobson’s (Talystep, UK). 2.4. Details of gas sensing system The sensing performance of the sensors was examined using a ‘static gas sensing system’. There were electrical feeds on the base plate. A heater was fixed on the base plate. The sample under test was mounted on the heater. A Cr–Al thermocouple was mounted on the heater to measure the operating temperature of the sensor. The output of thermocouple was feed to a temperature indicator. A gas inlet valve was fitted at one of the ports of base plate. The required gas concentration inside the static system was achieved by injecting a known volume of test gas by a gas injecting syringe. A constant voltage was applied to the sensor and current was measured by a current-meter. The air was allowed to pass into the glass dome after every Cl2 gas exposure cycle.
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Fig. 1. Micrographs of: (a) unmodified (pure), (b) modified (for 15 min), and (c) modified (for 30 min) samples.
by maintaining the proper rheology and thixotropy of the paste. 3.4. Thermoelectric power TEP measurement The p- or n-type semiconductivity of CuO and ZnO was confirmed by measuring thermo-electromotive force of the thick film samples. The ZnO was observed to be of n-type and CuO the p-type material.
4.2. Electrical resistivity Fig. 3 depicts the variation of log (resistivity) with operating temperature of pure and modified ZnO thick films. The semiconducting nature of ZnO is observed from the measurement of resistivity with temperature. The semiconductivity of ZnO
4. Electrical properties of the sensor 4.1. I–V characteristics I–V characteristics of pure and modified ZnO are observed to be symmetrical in nature, indicating the ohmic nature of silver contacts. Fig. 2 depicts the conductivities of pure and modified ZnO at room temperature. The conductivity of CuO-modified ZnO film was observed to be lower than that of pure ZnO at room temperature. This could be attributed to the ZnO–CuO intergrain boundaries and hence intergranular potential barrier. CuO grains may reside in the intergranular regions of ZnO, resulting in formation of intergrain boundaries and the intergranular potential barrier.
Fig. 2. I–V characteristics of the sensor.
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Fig. 4. Variation of response with operating temperature.
Fig. 3. Variation of log (resistivity) with operating temperature.
must be due to large oxygen deficiency in it which could adsorb oxygen species at higher temperatures (O2 − → 2O− → O2− ). The adsorption chemistry of the CuO-modified ZnO surface would be different from the pure ZnO thick film surface. The CuO misfits on the surface are the places where the oxygen species adsorb. The CuO misfits distributed evenly on the surface would have made it possible to adsorb the oxygen ions even at low temperatures. From Fig. 3, it is clear that the resistivity of CuO-modified film decreases with an increase in operating temperature, indicating a negative temperature coefficient of resistance. This behavior confirmed the semiconducting nature of the modified ZnO. 5. Sensing performance of the sensor 5.1. Measurement of gas response, selectivity, response and recovery time
5.2. Sensing performance of pure ZnO thick films Fig. 4 depicts the variation of gas response to 300 ppm Cl2 with operating temperature of pure ZnO thick film. The response to Cl2 gas goes on increasing with operating temperature, reaches to a maximum at 400 ◦ C (21) and decreases with a further increase of operating temperature. Response to a gas is related generally to substitution of oxygen ions adsorbed on the surface of the film with a target gas. In the present case the following reaction is conceivable. Cl2 + O2(ad) 2− → 2Cl(ad) − + O2 + 2e− If the film surface chemistry is favorable for adsorption, the response and selectivity would be enhanced. In the case of pure ZnO, oxygen substitution by chlorine seems to be poor and not easier, which could be the reason for poor response. In addition to this, substitution of oxygen from ZnO requires heat energy and the sensor is therefore necessarily operated at higher temperature to sense the gas. To improve the sensing performance of ZnO, it is essential to modify its surface. 5.3. Sensing performance of CuO-modified ZnO thick films
Gas response (S) is defined as the ratio of the change in conductance of the sensor on exposure to the target gas to the original conductance in air. The relation for S is as: S=
Gg − Ga Ga
5.3.1. Effect of operating temperature Fig. 5 depicts the variation of response to Cl2 gas (300 ppm) with operating temperature of pure and CuO-modified ZnO thick
where Ga and Gg are the conductance of sensor in air and in a target gas medium, respectively. Selectivity or specificity is defined as the ability of a sensor to respond to certain gas in the presence of other gases. Percent selectivity [37] of one gas over others is defined as the ratio of the maximum response of other gas to the maximum response of the target gas at optimum temperature. Sgas % selectivity = × 100% Starget gas The time taken for the sensor to attain 90% of the maximum increase in conductance on exposure to the target gas is the response time. The time taken by the sensor to get back 90% of the original conductance is the recovery time.
Fig. 5. Response to Cl2 gas (300 ppm) of pure and modified ZnO thick films.
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Fig. 6. Variation of gas response with calcination temperature and period.
films. The largest response of CuO-modified ZnO was observed to be 270 at 50 ◦ C and 195 at room temperature. The chlorine response at room temperature is expected to be monitored by substitution of ZnO lattice oxygen by chlorine. After each substitution of lattice oxygen by a Cl2 molecule, there would be the gain of two electrons to the base material. The cumulative effect would decrease the film resistance drastically, giving a response to chlorine at room temperature. The chlorine response of the CuO–ZnO sensor would follow the reaction: Cl2 + 2Oo x → 2Clo − + O2 + 2e− where Oo represents species occupying lattice oxygen sites. At room temperature, there would be no oxygen adsorption. Therefore the oxygen adsorption–desorption mechanism is not employed to sense the Cl2 gas. Thus, substitution of lattice oxygen by chlorine is more plausible. 5.3.2. Effect of calcination temperature and period Fig. 6 represents the variation of Cl2 gas response with the calcination temperature and period of the CuO-modified ZnO film. Figures show that the gas response increases with calcination temperature and period to reach to a maximum (195 at room temperature) for the film calcined at 1100 ◦ C for 24 h and falls with a further increase in calcination temperature. It is well known that the grain size of the film increases with calcination temperature. At 1100 ◦ C, the grain size would be optimum to get the largest effective surface area and critical porosity which would have favored and enhanced gas response to achieve its maximum. At higher temperatures (>1100 ◦ C) and larger intervals of calcination, average grain size would increase
Fig. 7. Variation of response with chlorine gas concentration at room temperature.
and therefore the effective surface area would decrease giving smaller gas response. 5.3.3. Effect of gas concentration at room temperature It is clear from Fig. 7 that, the gas response increases linearly with the gas concentration, attains the maximum and saturates at above 300 ppm gas. The excess gas would form multimolecular layers on the sensor surface and a part of gas amount would be idle and unable to interact with sensor surfaces. Hence response would not increase further. Thus, the active region for the sensor is up to 300 ppm. 5.3.4. Effect of dipping time and the amount of Cu-surfactant Fig. 8(a and b) depict the variation of gas response with dipping time and the amount of Cu-surfactant, respectively. The
Fig. 8. Variation of response with dipping time and the amount of CuO-surfactant.
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Fig. 9. Gas responses to various gases.
film dipped for 15 min showed the highest gas response (195). At a 15 min dipping time, the sensor would find CuO mass% to be optimum. The optimum mass% of CuO would cover the film surface uniformly. The lower gas responses at higher operating temperatures would be due to consumption of chlorine gas by adsorbed oxygen at higher temperatures. Fig. 7(b) represents the variation of Cl2 gas response with the mass% of Cu in the CuO-modified ZnO films. It is observed that the gas response is largest at 0.51 mass% CuO. At higher mass%, the Cu-surfactant would mask the base material (ZnO) and resist the gas to reach to the surface sites, so that the gas response would decrease.
Fig. 10. Response and recovery of the sensor.
5.3.5. Selectivity for Cl2 against various gases Fig. 9 depicts the selectivity of the CuO-modified ZnO (calcined at 1100 ◦ C for 24 h with 0.51 mass% of CuO) to 300 ppm of Cl2 gas (room temperature) against various gases. It is clear
Fig. 11. Gas sensing mechanism of CuO-modified samples at (a) 350 ◦ C and (b) room temperature.
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from Fig. 9 that in contrast to pure zinc oxide the sample shows not only enhanced response towards Cl2 but also high selectivity against LPG, NH3 , CO2 , C2 H5 OH, H2 and H2 S gases. 5.3.6. Response and recovery of the sensor The response and recovery of the CuO-modified sensor are represented in Fig. 10. The response of the sensor is quick (∼18 s) and the recovery is fast (∼50 s). 6. Discussion There are four adsorption behaviors of chlorine on the oxide surface [2,15]: Cl2 + O2(ad) 2− → 2Cl(ad) − + O2 + 2e−
(i)
Cl2 + 2Oo x → 2Clo − + O2 + 2e−
(ii)
Cl2 + 2e → 2Cl(ad) −
(iii)
Cl2 + 2Vo + 2e− → 2Clo −
(iv)
where subscripts ad and o mean the species adsorbed on the surface and the species occupying the lattice oxygen site, respectively. Vo is the oxygen vacancy. Pure ZnO showed response to chlorine at high temperature. This would be attributed to the replacement of adsorbed oxygen by chlorine (Fig. 11(a)), donating electrons to the base material (reaction (i)). When chlorine gas was exposed to CuO-modified ZnO film, it substitutes lattice oxygen on the film surface. The reaction (ii) is responsible for sensing of chlorine in which chlorine substituted for lattice oxygen to form Cl(ad) − , inducing electron donation into the oxide. It is clear from Table 1 that oxygen deficiency goes on decreasing with an increase of mass% of CuO, reaches to a minimum and goes on increasing with a further increase in mass% of CuO. Pure ZnO is known to be oxygen deficient. In the case of CuO-modified ZnO, CuO could satisfy the oxygen deficiency of ZnO material. ZnO–CuO forms p–n heterojunctions, giving very high resistance even at room temperature. When CuO–ZnO comes in contact with chlorine gas, lattice oxygen would be replaced by chlorine rupturing the heterojunctions (Fig. 11(b)) and the resistance drops down suddenly. In addition to this, the material gains electrons. 7. Summary From the results, following statements can be made for the sensing performance of the present CuO-modified sensors. (1) Pure zinc oxide was almost insensitive to chlorine gas at room temperature. (2) Pure zinc oxide showed gas response, though small, to chlorine at high operating temperature. (3) Among various additives tested, CuO is outstanding in promoting the room temperature Cl2 gas sensing by ZnO. (4) Sensing mechanism of pure ZnO was the replacement of surface adsorbed oxygen by chlorine gas. Material gains electrons in this replacement.
(5) Sensing mechanism of CuO-modified ZnO was the substitution of lattice oxygen by chlorine gas. Material gains electrons in this substitution. (6) Surface modification by dipping process is one of the most suitable methods of modifying the thick film surface. (7) A modified film with 0.51 mass% of CuO in ZnO was observed to be the most sensitive element to Cl2 gas. (8) CuO-modified ZnO has the potential of fabricating a room temperature Cl2 sensor. (9) The sensor showed very rapid response and recovery to Cl2 gas. (10) The sensor has good selectivity for Cl2 against LPG, NH3 , CO2 , H2 and C2 H5 OH gases at room temperature. Acknowledgements Authors are grateful to the Principal, Pratap College, Amalner for providing laboratory facilities, and also thankful to the Head of the P.G. Department of Physics, Pratap College, Amalner for his keen interest in this research project. Authors are also thankful to U.G.C. for providing financial assistance for this project. One of us (DRP) also acknowledges the co-operation rendered by Mr. B.V. Patil, Principal, R.L. College Parola, India. References [1] F.A. Cotton, G. Wilkinson, C. Murillo, M. Bochman, Advanced Inorganic Chemistry, sixth ed., John Wiley and Sons (Asia) Pte Ltd., 2003, pp. 550–570, 604. [2] D.H. Dawson, D.E. Williams, Gas sensitive resistors: surface interaction of chlorine with semiconducting oxides, J. Mater. Chem. 6 (1996) 409– 414. [3] G.S. Sodhi, Fundamental Concepts of Environmental Chemistry, first ed., Narosa Publishing House, New Delhi, 2002, pp. 344–350, 437. [4] H.W. Gehm, in: C.F. Gurnham (Ed.), Industrial Waste Water Control, Academic Press, New York, 1965, pp. 357–373. [5] J.W. Moore, E.A. Moore, Environmental Chemistry, Academic press, New York, 1976, pp. 406–410. [6] T. Miyata, T. Hikosaka, T. Minami, High sensitivity chlorine gas sensors using multicomponent transparent conducting oxide thin films, Sens. Actuators B 69 (2000) 16–19. [7] M. Miyayama, Chlorine gas sensing properties of ZnO–CaO ceramics, J. Electroceram. 2 (1998) 45–48. [8] Y. Yan, N. Miura, N. Yamazoe, Solid state electrochemical chlorine sensor using stabilized Zirconia tube and chloride auxiliary phase, in: Tech. digest of 5th IMCS (1994) 366. [9] S. Jain, A.B. Samui, M. Patri, V.R. Hande, S.V. Bhoraskar, FEP/Polyaniline based multilayered Chlorine Sensor, Sens. Actuators B 106 (2005) 609–613. [10] A. Galdikas, Z. Martunas, A. Setkus, SnInO-based chlorine gas sensor, Sens. Actuators B 7 (1992) 633–639. [11] T. Miyata, T. Minami, Chlorine gas sensors with high sensitivity using Mg-phthalocyanine thin films, Appl. Surf. Sci. 244 (2005) 563– 567. [12] N. Yamaguchi, M. Yang, Development and evaluation of a micro chemical gas sensor with an inner-circulation diffuser pump, Sens. Actuators B 103 (2004) 369–374. [13] R. Sathiyamoorthi, R. Chandrasekaran, T. Mathanmohan, B. Muralidharan, T. Vasudevan, Study of electrochemical based gas sensors for fluorine and chlorine, Sens. Actuators B 99 (2004) 336–339. [14] X.-F. Chu, Z.-M. Cheng, High sensitivity chlorine gas sensors using CdSnO3 thick film prepared by co-precipitation method, Sens. Actuators B 98 (2004) 215–217.
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[30] L.A. Patil, D.R. Patil, Heterocontact type CuO modified SnO2 sensor for the detection of a ppm level H2 S gas at room temperature, Sens. Actuators B 120 (2006) 316–323. [31] M.S. Wagh, L.A. Patil, T. Seth, D.P. Amalnerkar, Surface cupricated SnO2 ZnO thick films as a H2 S gas sensor, Mater. Chem. Phys. 84 (2004) 228–233. [32] M.S. Wagh, G.H. Jain, D.R. Patil, S.A. Patil, L.A. Patil, Modified zinc oxide thick film resistors as NH3 gas sensor, Sens. Actuators B 115 (2006) 128–133. [33] 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. [34] F. Paraguay, M. Yoshida, J. Morales, J. Solis, W. Estrada, Influence of Al, In, Cu, Fe and Sn dopants on the response of thin film ZnO gas sensor to ethanol vapor, Thin Solid films 373 (2000) 137–140. [35] M. Aslam, V.A. Chaudhary, I.S. Mulla, S.R. Sainkar, A.B. Mandale, A.A. Belhekar, K. Vijayamohanan, A highly selective ammonia gas sensor using surface ruthenated zinc oxide, Sens. Actuators B 75 (1999) 162– 167. [36] K.J. Laidler, Chemical Kinetics, second ed., TMG Publication Comp. Ltd., 1965, p. 310. [37] G.H. Jain, L.A. Patil, V.B. Gaikwad, Studies on gas sensing performance of (Ba0.8 Sr0.2 )(Sn0.8 Ti0.2 )O3 thick film resistors, Sens. Actuators B 122 (2007) 605–612.
Biographies D.R. Patil is a lecturer in physics at R.L. College, Parola. He received his MSc degree in physics from University of Pune in 1989. He is working on gas sensing by thick film technology for his PhD degree, in Materials Research Laboratory, Pratap College, Amalner, Maharashtra, India. L.A. Patil is a reader in physics, Pratap College, Amalner Maharashtra, India. He received his MPhil in applied electronics and PhD in material science. His topics of interest are ceramic gas sensors, photoconducting and photoluminescent materials, art of growing crystals, dielectric properties of materials, nanomaterials, thin and thick film physics. He is also the member of Management Council, North Maharashtra University, Jalgaon.
Room temperature chlorine gas sensing using surface modified ZnO thick film resistors D.R. Patil, L.A. Patil ∗ Materials Research Laboratory, Pratap College, Amalner 425401, India Received 3 June 2006; received in revised form 25 September 2006; accepted 26 September 2006 Available online 13 November 2006
Abstract Thick films of pure and CuO-modified ZnO were prepared by screen-printing technique. Pure ZnO was observed to be almost insensitive to chlorine gas. Pure ZnO white powder was calcined at 1100 ◦ C for 24 h, to minimize the oxygen deficiency. CuO-modified ZnO thick films were obtained by dipping technique in which Cu2+ could be incorporated as an additive into the ZnO base material. The films were fired at 500 ◦ C for 24 h. The CuO-modified sensor was observed to be sensitive to 300 ppm Cl2 gas at room temperature. The effect of operating temperature, calcination temperature, gas concentration, amount of CuO in ZnO on the gas response was studied. The selectivity of the target gas against other gases, gas response and recovery times of the sensor were also presented. The quick response (∼18 s) and fast recovery (∼50 s) are the main features of the modified sensor. © 2006 Elsevier B.V. All rights reserved. Keywords: Room temperature; Chlorine gas sensor; CuO-modified ZnO; Thick films; Selectivity; Response and recovery time
1. Introduction Chlorine is a yellowish-green [1] gas having pungent smell, and is explosively utilized in industrial applications such as to bleach paper pulp, to disinfect sewage and drinking water, etc. As it has a wide range of applications, its toxicity [2–5] can affect the health of humans in contact. Chlorine has excellent bleaching ability, but once it is discharged in aquatic systems, it interacts with other industrial effluents to produce a host of chlorinated organics such as dioxin. Dioxin persists in the environment for prolonged periods and has a tendency to bioaccumulate in the food chains, which elicits toxic effects to humans, such as skin infection, psychological disorders and even liver damage. Therefore, it is needed to monitor chlorine gas at ordinary temperature. Gas chromatography, chemical detecting tubes and electrochemical sensing techniques are available for the detection of chlorine gas in the environment. A few Cl2 gas sensors have already been developed, but they do not work at room temperature. Transparent conducting oxide (TCO) thin film sensors operate at higher temperatures (∼300 ◦ C). However, it is inconvenient to operate
∗
Corresponding author. Tel.: +91 2587 224226. E-mail address: plalchand phy [email protected] (L.A. Patil).
0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.09.060
sensors at higher temperatures for monitoring. It is, therefore, essential to develop a sensor, which could detect Cl2 gas at room temperature. Zirconia tubes, MgO–In2 O3 , ZnO–In2 O3 , Zn2 In2 O5 –MgIn2 O4 (TCO), ZnO–CaO, etc., are the materials utilized in Cl2 sensing applications [6–21]. It is well known that the semiconducting oxides such as ZnO, SnO2, Fe2 O3 , Ga2 O3 , Sb2 O3 [22–28], etc., are sensitive to toxic and inflammable gases. It has also reported that surface additives [29–33] play an important role in enhancing the gas response and specificity to a particular gas. The additives like Al, In, Cu, Fe, Sn and Ru [34,35] are often added to improve the response and selectivity. In the present article, thick film surfaces are modified by dipping them into a Cu-precursor for different time intervals, followed by firing. Firing would convert the Cu-precursor into CuO. In the surface modification process, the grains of CuO would disperse on the grains of ZnO. The dipping technique adds a dimension to the usefulness of surfaces by allowing one to customize their properties. This type of sensor works on the principle of substitution of lattice oxygen by chlorine. Substitution of O2 by Cl2 [2] gas at room temperature on the surface of CuO-modified ZnO thick film causes a drastic change in resistance. Room temperature gas detection offers the advantages of low power drain and reduced tendency to provide a source of ignition [25].
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2. Experimental
Table 1 Elemental analysis of unmodified (pure) and CuO-modified films
2.1. Powder and paste preparation
Elemental (mass%)
Dipping time (min) 0
2
5
15
30
Zn O ZnO Cu CuO
91.95 8.05 100 0.00 0.00
91.16 8.77 99.91 0.07 0.09
88.38 11.30 99.59 0.33 0.41
91.48 8.11 99.49 0.41 0.51
88.49 11.18 99.58 0.33 0.42
AR grade zinc oxide powder (99.9% pure) was ground in an agate pastle–mortor to ensure sufficiently fine particle size. The fine powder was calcined at 1100 ◦ C for 24 h in air and re-ground. The thixotropic paste was formulated by mixing the resulting ZnO fine powder with a temporary binder as a mixture of organic solvents. The ratio of inorganic to organic part was kept as 75:25 in formulating the paste. The paste was then used to prepare thick films. 2.2. Thick film preparation The thixotropic paste was screen printed on a glass substrate in desired patterns [30–33]. The films prepared were fired at 500 ◦ C for 24 h. These films were surface modified by dipping them into a 0.01 M aqueous solution of cupric chloride for different intervals of time and were dried at 80 ◦ C under an IR lamp, followed by calcination at 500 ◦ C for 24 h in air ambient. The CuCl2 dispersed on the film surface was oxidized in calcination and sensor elements with different mass% of CuO were obtained. Silver contacts were made by vacuum evaporation for electrical measurements.
for the largest time interval (30 min), respectively. The unmodified ZnO film in Fig. 1(a) consists of randomly distributed grains with larger size and shape distribution. Fig. 1(b) depicts that the microstructure of CuO-modified film (15 min) consists of more spherical particles of Cu-species distributed uniformly with smaller size and shape on the ZnO grains. These smaller spherical particles would be attributed to Cu-misfits. Fig. 1(c) depicts larger grains than those associated with Fig. 1(b). Larger grains in Fig. 1(c) may be due to a smaller amount of CuO on the surface as some amount of copper percolates into the bulk portion of the film. 3.2. Elemental analysis
3. Materials characterization
The quantitative elemental composition of the films was analyzed using an energy dispersive spectrometer and mass% of Zn, O, ZnO, Cu and CuO are represented in Table 1. Pure stoichiometric ZnO is expected to be insulating. Stoichiometrically expected mass% of Zn and O in ZnO are 80.34 and 19.66, respectively. The pure ZnO powder turns white to yellow on calcination at higher temperature (∼1100 ◦ C for 24 h). It may be due to deficiency of oxygen [36]. The mass% of Zn and O in each sample were not as per the stoichiometric proportion and all samples were observed to be oxygen deficient (Table 1). Whenever in the crystal, there is an excess or deficiency of one type of atom, which results in a distortion in the band structure, with a corresponding increase in conductivity [36]. Zinc oxide loses oxygen on heating so that the zinc is then in excess. The oxygen is, of course, evolved as an electrically neutral substance, so that being associated with each excess zinc ion in the crystal, there will be two electrons which remain trapped in the solidmaterial, thus leading to non-stoichiometricity in the solid. This leads to the formation of n-type semiconductivity. It is clear from Table 1 that unmodified ZnO is more oxygen deficient and oxygen deficiency could be reduced by adding CuO into ZnO. The mass% of CuO goes on increasing with the dipping time, reaches to a maximum and then decreases with a further increase in dipping time. This could be attributed to the dispersion of Cu grains on the surface of the film. The mass% of CuO (0.51) is highest for a 15 min dipping time. Due to an increase in mass% of Cu there was a decrease in the mass% of Zn.
3.1. Microstructure-SEM
3.3. Thickness measurement
Fig. 1 depicts the SEM images of unmodified ZnO film, most sensitive CuO-modified film (15 min) and CuO-modified film
Film thicknesses were observed to be in the range from 25 to 30 m. The reproducibility of film thickness was achieved
2.3. Characterization The micro structural analysis was carried out by using SEM (JEOL-6300 LA, Germany) and chemical compositions of the films were analyzed using EDS (JOEL, JED-2300, Germany) coupled with an energy dispersive spectrometer. Thickness measurements were carried out using Taylor-Hobson’s (Talystep, UK). 2.4. Details of gas sensing system The sensing performance of the sensors was examined using a ‘static gas sensing system’. There were electrical feeds on the base plate. A heater was fixed on the base plate. The sample under test was mounted on the heater. A Cr–Al thermocouple was mounted on the heater to measure the operating temperature of the sensor. The output of thermocouple was feed to a temperature indicator. A gas inlet valve was fitted at one of the ports of base plate. The required gas concentration inside the static system was achieved by injecting a known volume of test gas by a gas injecting syringe. A constant voltage was applied to the sensor and current was measured by a current-meter. The air was allowed to pass into the glass dome after every Cl2 gas exposure cycle.
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Fig. 1. Micrographs of: (a) unmodified (pure), (b) modified (for 15 min), and (c) modified (for 30 min) samples.
by maintaining the proper rheology and thixotropy of the paste. 3.4. Thermoelectric power TEP measurement The p- or n-type semiconductivity of CuO and ZnO was confirmed by measuring thermo-electromotive force of the thick film samples. The ZnO was observed to be of n-type and CuO the p-type material.
4.2. Electrical resistivity Fig. 3 depicts the variation of log (resistivity) with operating temperature of pure and modified ZnO thick films. The semiconducting nature of ZnO is observed from the measurement of resistivity with temperature. The semiconductivity of ZnO
4. Electrical properties of the sensor 4.1. I–V characteristics I–V characteristics of pure and modified ZnO are observed to be symmetrical in nature, indicating the ohmic nature of silver contacts. Fig. 2 depicts the conductivities of pure and modified ZnO at room temperature. The conductivity of CuO-modified ZnO film was observed to be lower than that of pure ZnO at room temperature. This could be attributed to the ZnO–CuO intergrain boundaries and hence intergranular potential barrier. CuO grains may reside in the intergranular regions of ZnO, resulting in formation of intergrain boundaries and the intergranular potential barrier.
Fig. 2. I–V characteristics of the sensor.
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Fig. 4. Variation of response with operating temperature.
Fig. 3. Variation of log (resistivity) with operating temperature.
must be due to large oxygen deficiency in it which could adsorb oxygen species at higher temperatures (O2 − → 2O− → O2− ). The adsorption chemistry of the CuO-modified ZnO surface would be different from the pure ZnO thick film surface. The CuO misfits on the surface are the places where the oxygen species adsorb. The CuO misfits distributed evenly on the surface would have made it possible to adsorb the oxygen ions even at low temperatures. From Fig. 3, it is clear that the resistivity of CuO-modified film decreases with an increase in operating temperature, indicating a negative temperature coefficient of resistance. This behavior confirmed the semiconducting nature of the modified ZnO. 5. Sensing performance of the sensor 5.1. Measurement of gas response, selectivity, response and recovery time
5.2. Sensing performance of pure ZnO thick films Fig. 4 depicts the variation of gas response to 300 ppm Cl2 with operating temperature of pure ZnO thick film. The response to Cl2 gas goes on increasing with operating temperature, reaches to a maximum at 400 ◦ C (21) and decreases with a further increase of operating temperature. Response to a gas is related generally to substitution of oxygen ions adsorbed on the surface of the film with a target gas. In the present case the following reaction is conceivable. Cl2 + O2(ad) 2− → 2Cl(ad) − + O2 + 2e− If the film surface chemistry is favorable for adsorption, the response and selectivity would be enhanced. In the case of pure ZnO, oxygen substitution by chlorine seems to be poor and not easier, which could be the reason for poor response. In addition to this, substitution of oxygen from ZnO requires heat energy and the sensor is therefore necessarily operated at higher temperature to sense the gas. To improve the sensing performance of ZnO, it is essential to modify its surface. 5.3. Sensing performance of CuO-modified ZnO thick films
Gas response (S) is defined as the ratio of the change in conductance of the sensor on exposure to the target gas to the original conductance in air. The relation for S is as: S=
Gg − Ga Ga
5.3.1. Effect of operating temperature Fig. 5 depicts the variation of response to Cl2 gas (300 ppm) with operating temperature of pure and CuO-modified ZnO thick
where Ga and Gg are the conductance of sensor in air and in a target gas medium, respectively. Selectivity or specificity is defined as the ability of a sensor to respond to certain gas in the presence of other gases. Percent selectivity [37] of one gas over others is defined as the ratio of the maximum response of other gas to the maximum response of the target gas at optimum temperature. Sgas % selectivity = × 100% Starget gas The time taken for the sensor to attain 90% of the maximum increase in conductance on exposure to the target gas is the response time. The time taken by the sensor to get back 90% of the original conductance is the recovery time.
Fig. 5. Response to Cl2 gas (300 ppm) of pure and modified ZnO thick films.
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Fig. 6. Variation of gas response with calcination temperature and period.
films. The largest response of CuO-modified ZnO was observed to be 270 at 50 ◦ C and 195 at room temperature. The chlorine response at room temperature is expected to be monitored by substitution of ZnO lattice oxygen by chlorine. After each substitution of lattice oxygen by a Cl2 molecule, there would be the gain of two electrons to the base material. The cumulative effect would decrease the film resistance drastically, giving a response to chlorine at room temperature. The chlorine response of the CuO–ZnO sensor would follow the reaction: Cl2 + 2Oo x → 2Clo − + O2 + 2e− where Oo represents species occupying lattice oxygen sites. At room temperature, there would be no oxygen adsorption. Therefore the oxygen adsorption–desorption mechanism is not employed to sense the Cl2 gas. Thus, substitution of lattice oxygen by chlorine is more plausible. 5.3.2. Effect of calcination temperature and period Fig. 6 represents the variation of Cl2 gas response with the calcination temperature and period of the CuO-modified ZnO film. Figures show that the gas response increases with calcination temperature and period to reach to a maximum (195 at room temperature) for the film calcined at 1100 ◦ C for 24 h and falls with a further increase in calcination temperature. It is well known that the grain size of the film increases with calcination temperature. At 1100 ◦ C, the grain size would be optimum to get the largest effective surface area and critical porosity which would have favored and enhanced gas response to achieve its maximum. At higher temperatures (>1100 ◦ C) and larger intervals of calcination, average grain size would increase
Fig. 7. Variation of response with chlorine gas concentration at room temperature.
and therefore the effective surface area would decrease giving smaller gas response. 5.3.3. Effect of gas concentration at room temperature It is clear from Fig. 7 that, the gas response increases linearly with the gas concentration, attains the maximum and saturates at above 300 ppm gas. The excess gas would form multimolecular layers on the sensor surface and a part of gas amount would be idle and unable to interact with sensor surfaces. Hence response would not increase further. Thus, the active region for the sensor is up to 300 ppm. 5.3.4. Effect of dipping time and the amount of Cu-surfactant Fig. 8(a and b) depict the variation of gas response with dipping time and the amount of Cu-surfactant, respectively. The
Fig. 8. Variation of response with dipping time and the amount of CuO-surfactant.
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Fig. 9. Gas responses to various gases.
film dipped for 15 min showed the highest gas response (195). At a 15 min dipping time, the sensor would find CuO mass% to be optimum. The optimum mass% of CuO would cover the film surface uniformly. The lower gas responses at higher operating temperatures would be due to consumption of chlorine gas by adsorbed oxygen at higher temperatures. Fig. 7(b) represents the variation of Cl2 gas response with the mass% of Cu in the CuO-modified ZnO films. It is observed that the gas response is largest at 0.51 mass% CuO. At higher mass%, the Cu-surfactant would mask the base material (ZnO) and resist the gas to reach to the surface sites, so that the gas response would decrease.
Fig. 10. Response and recovery of the sensor.
5.3.5. Selectivity for Cl2 against various gases Fig. 9 depicts the selectivity of the CuO-modified ZnO (calcined at 1100 ◦ C for 24 h with 0.51 mass% of CuO) to 300 ppm of Cl2 gas (room temperature) against various gases. It is clear
Fig. 11. Gas sensing mechanism of CuO-modified samples at (a) 350 ◦ C and (b) room temperature.
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from Fig. 9 that in contrast to pure zinc oxide the sample shows not only enhanced response towards Cl2 but also high selectivity against LPG, NH3 , CO2 , C2 H5 OH, H2 and H2 S gases. 5.3.6. Response and recovery of the sensor The response and recovery of the CuO-modified sensor are represented in Fig. 10. The response of the sensor is quick (∼18 s) and the recovery is fast (∼50 s). 6. Discussion There are four adsorption behaviors of chlorine on the oxide surface [2,15]: Cl2 + O2(ad) 2− → 2Cl(ad) − + O2 + 2e−
(i)
Cl2 + 2Oo x → 2Clo − + O2 + 2e−
(ii)
Cl2 + 2e → 2Cl(ad) −
(iii)
Cl2 + 2Vo + 2e− → 2Clo −
(iv)
where subscripts ad and o mean the species adsorbed on the surface and the species occupying the lattice oxygen site, respectively. Vo is the oxygen vacancy. Pure ZnO showed response to chlorine at high temperature. This would be attributed to the replacement of adsorbed oxygen by chlorine (Fig. 11(a)), donating electrons to the base material (reaction (i)). When chlorine gas was exposed to CuO-modified ZnO film, it substitutes lattice oxygen on the film surface. The reaction (ii) is responsible for sensing of chlorine in which chlorine substituted for lattice oxygen to form Cl(ad) − , inducing electron donation into the oxide. It is clear from Table 1 that oxygen deficiency goes on decreasing with an increase of mass% of CuO, reaches to a minimum and goes on increasing with a further increase in mass% of CuO. Pure ZnO is known to be oxygen deficient. In the case of CuO-modified ZnO, CuO could satisfy the oxygen deficiency of ZnO material. ZnO–CuO forms p–n heterojunctions, giving very high resistance even at room temperature. When CuO–ZnO comes in contact with chlorine gas, lattice oxygen would be replaced by chlorine rupturing the heterojunctions (Fig. 11(b)) and the resistance drops down suddenly. In addition to this, the material gains electrons. 7. Summary From the results, following statements can be made for the sensing performance of the present CuO-modified sensors. (1) Pure zinc oxide was almost insensitive to chlorine gas at room temperature. (2) Pure zinc oxide showed gas response, though small, to chlorine at high operating temperature. (3) Among various additives tested, CuO is outstanding in promoting the room temperature Cl2 gas sensing by ZnO. (4) Sensing mechanism of pure ZnO was the replacement of surface adsorbed oxygen by chlorine gas. Material gains electrons in this replacement.
(5) Sensing mechanism of CuO-modified ZnO was the substitution of lattice oxygen by chlorine gas. Material gains electrons in this substitution. (6) Surface modification by dipping process is one of the most suitable methods of modifying the thick film surface. (7) A modified film with 0.51 mass% of CuO in ZnO was observed to be the most sensitive element to Cl2 gas. (8) CuO-modified ZnO has the potential of fabricating a room temperature Cl2 sensor. (9) The sensor showed very rapid response and recovery to Cl2 gas. (10) The sensor has good selectivity for Cl2 against LPG, NH3 , CO2 , H2 and C2 H5 OH gases at room temperature. Acknowledgements Authors are grateful to the Principal, Pratap College, Amalner for providing laboratory facilities, and also thankful to the Head of the P.G. Department of Physics, Pratap College, Amalner for his keen interest in this research project. Authors are also thankful to U.G.C. for providing financial assistance for this project. One of us (DRP) also acknowledges the co-operation rendered by Mr. B.V. Patil, Principal, R.L. College Parola, India. References [1] F.A. Cotton, G. Wilkinson, C. Murillo, M. Bochman, Advanced Inorganic Chemistry, sixth ed., John Wiley and Sons (Asia) Pte Ltd., 2003, pp. 550–570, 604. [2] D.H. Dawson, D.E. Williams, Gas sensitive resistors: surface interaction of chlorine with semiconducting oxides, J. Mater. Chem. 6 (1996) 409– 414. [3] G.S. Sodhi, Fundamental Concepts of Environmental Chemistry, first ed., Narosa Publishing House, New Delhi, 2002, pp. 344–350, 437. [4] H.W. Gehm, in: C.F. Gurnham (Ed.), Industrial Waste Water Control, Academic Press, New York, 1965, pp. 357–373. [5] J.W. Moore, E.A. Moore, Environmental Chemistry, Academic press, New York, 1976, pp. 406–410. [6] T. Miyata, T. Hikosaka, T. Minami, High sensitivity chlorine gas sensors using multicomponent transparent conducting oxide thin films, Sens. Actuators B 69 (2000) 16–19. [7] M. Miyayama, Chlorine gas sensing properties of ZnO–CaO ceramics, J. Electroceram. 2 (1998) 45–48. [8] Y. Yan, N. Miura, N. Yamazoe, Solid state electrochemical chlorine sensor using stabilized Zirconia tube and chloride auxiliary phase, in: Tech. digest of 5th IMCS (1994) 366. [9] S. Jain, A.B. Samui, M. Patri, V.R. Hande, S.V. Bhoraskar, FEP/Polyaniline based multilayered Chlorine Sensor, Sens. Actuators B 106 (2005) 609–613. [10] A. Galdikas, Z. Martunas, A. Setkus, SnInO-based chlorine gas sensor, Sens. Actuators B 7 (1992) 633–639. [11] T. Miyata, T. Minami, Chlorine gas sensors with high sensitivity using Mg-phthalocyanine thin films, Appl. Surf. Sci. 244 (2005) 563– 567. [12] N. Yamaguchi, M. Yang, Development and evaluation of a micro chemical gas sensor with an inner-circulation diffuser pump, Sens. Actuators B 103 (2004) 369–374. [13] R. Sathiyamoorthi, R. Chandrasekaran, T. Mathanmohan, B. Muralidharan, T. Vasudevan, Study of electrochemical based gas sensors for fluorine and chlorine, Sens. Actuators B 99 (2004) 336–339. [14] X.-F. Chu, Z.-M. Cheng, High sensitivity chlorine gas sensors using CdSnO3 thick film prepared by co-precipitation method, Sens. Actuators B 98 (2004) 215–217.
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Biographies D.R. Patil is a lecturer in physics at R.L. College, Parola. He received his MSc degree in physics from University of Pune in 1989. He is working on gas sensing by thick film technology for his PhD degree, in Materials Research Laboratory, Pratap College, Amalner, Maharashtra, India. L.A. Patil is a reader in physics, Pratap College, Amalner Maharashtra, India. He received his MPhil in applied electronics and PhD in material science. His topics of interest are ceramic gas sensors, photoconducting and photoluminescent materials, art of growing crystals, dielectric properties of materials, nanomaterials, thin and thick film physics. He is also the member of Management Council, North Maharashtra University, Jalgaon.