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High-performance temperature-selective SnO2:Cu-based sensor
Materials Letters 57 (2003) 2177 – 2184 www.elsevier.com/locate/matlet
High-performance temperature-selective SnO2:Cu-based sensor P.S. More a,*, Y.B. Khollam a, S.B. Deshpande b, S.R. Sainkar b, S.K. Date b, R.N. Karekar c, R.C. Aiyer a a
Department of Physics, Centre for Advanced Studies in Material Science and Solid State Physics, University of Pune, Pune 411007, India b Physical and Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India c Department of Physics, R. Chandrashekhar Memorial Foundation Transducer Laboratory, University of Pune, Pune 411007, India Received 15 July 2002; accepted 5 September 2002
Abstract Systematic evaluation of gas-sensing characteristics of Cu-modified SnO2 pellets for CO, H2 and CH4 gases from ppm to percentage level is reported. The concentration level of Cu additive was varied systematically from 1 to 11 wt %. The highest values of sensitivity factor (SF) of 240 and 590 corresponding to CO and H2 gases were obtained at considerably lower temperatures of 160 and 230 jC, respectively, for 9 wt.% of Cu. The selectivity values are found to be maximum at 7 and 9 wt.% of Cu for H2 (c78) and CO (c1.9) gasses, respectively. For CH4 gas, on the contrary, the sensor showed quite inferior response. The sensitivity curve for our material follows the usual trend of three-region behaviour of standard Windischmann and Mark’s curve. The sensor tends to saturate at 1.5% and 3.6% for CO and H2, respectively. XRD and scanning electron micrograph (SEM) characterisation techniques are used to pinpoint the possible reasons for high performance of the sensor at 9 wt.% Cu. Our sensor material showed promising properties for gas-sensing (CO and H2) applications. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Gas sensor; Oxides; SnO2/Cu system; Sensitivity; Selectivity; Microstructure
1. Introduction The atmospheric pollution has lead to the research and development of a variety of sensors using different materials and technologies particularly for low cost and a lower operating temperatures [1 – 5]. Resistive pellet technology is probably the simplest and cheapest one. Tin dioxide (SnO2) is one of the most important and extensively used materials for the detection of gases [1 – 3,5– 17]. The changes in the * Corresponding author. E-mail addresses: [email protected] (P.S. More), [email protected] (R.C. Aiyer).
properties of SnO2 due to gas adsorption are related to the nonstoichiometry, average co-ordination number per grain and the neck size effect in the functional material. As such there is no specific report relating to its structure and sensing properties. The Schottky barrier at the grain boundaries is also reported to govern the thick and thin film sensor properties [1 –4,6]. Porosity of the sensor material is also one of the most important factors. It is also widely accepted that atmospheric oxygen is chemisorbed, reducing the conductivity of the sensor and on exposure to reducing gases like CO and H2, it increases at appropriate operating temperatures [1 – 24]. This phenomenon gives qualitative explanation
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01170-9
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for the change in the conductivity of gas sensor and its temperature selectivity. The change in conductance is normally proportional to the square root of partial pressure of the reducing gas in the air [2,3,6,7]. By the addition [2,3] of specific modifier/ promoter/catalyst, the sensing action and selectivity is often further enhanced for a particular gas at a particular temperature. Many references are available with CuO as an additive material in SnO2 [2,4,12,16,17] and most of them are for sensing H2S [4,11,12,14,20,24]. However, limited information about the SnO2/CuO system for gas-sensing characteristics for CO, H2 and CH4 is available in the literature [22]. Furthermore, very high concentration levels of CuO additive (mole %) are required. In view of this, an attempt is made in present study to see the effect of addition of Cu metal (instead of CuO) on the gas-sensing properties for CO, H2 and CH4. Further, the variation of sensitivity and selectivity of SnO2 pellets with the systematic variation of Cu (metal powder) as an additive (by wt.%) for CO, H2 and CH4 gases over a wider range and concentration from ppm to percentage level is visualised. The pellets are characterised by using XRD and SEM techniques to get an empirical idea about the effect of addition of Cu.
2. Experimental procedure Commercially available SnO2 powder (Thomas Baker) with x-wt.% of Cu metal powder (x = 1, 3, 5, 7, 9 and 11) is thoroughly mixed in an acetone as medium and dried under ambient condition. Each mixture is sintered at 600 jC/4 h and further pulverised to get still a fine powder. The powder is then cold-pressed (5 ton) to form pellets (diameter = 1.3 cm and thickness = 0.12 cm.) by using 3 wt.% polyvinyl alcohol (PVA) as binder material. The pellets are then fired at 300 jC in ambient air to burn PVA. Gas-sensing properties of these pellets are studied in a home-built system [15,18,19]. The Fig. 1. (a) SF variation with temperature at different concentration of Cu (wt.%) for 1000 ppm of CO. (b) SF variation with temperature at different concentration of Cu (wt.%) for 1000 ppm of H2. (c) Variation of sensitivity factor (SF) with temperature for 1000 ppm of CO, H2 and CH4 gases with 9 wt.% of Cu.
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Fig. 3. Variation of selectivity with wt.% of Cu for CO and H2.
tivity factor (SF) [15,18] and sensitivity (S) are defined as SF ¼ f½Gg Ga =Ga g 100 ¼ ðDG=Ga Þ 100; ð1Þ S ¼ D SF=D ðgas concentrationÞ;
Fig. 2. (a) Sample to sample repeatability and reproducibility for 1000 ppm of CO with 9 wt.% of Cu at optimum temperature of 160 jC. (b) Sample to sample repeatability and reproducibility for 1000 ppm of H2 with 9 wt.% of Cu at optimum temperature of 230 jC.
required gas concentration inside the system is achieved by injecting a known volume of the test gas in an airtight chamber (volume 15 l) at ambient conditions. The pellets are tested for a wide range of concentration of CO, H2 and CH4 gases separately as well as for their add-mixtures, from 50 ppm to f 3% level. To avoid the relative humidity effect, preheated and then cooled samples (in situ) are used for experimentation. The DC conductance of the pellet (with press contact electrodes) is obtained (at 20 V) by measuring the voltage drop across reference resistor (290 KV) connected in series. The percentage sensi-
ð2Þ
where Ga and Gg are the conductances of the sensor in air and (gas + air), respectively. Normally, three cycles are taken for each sample and two samples of each type are tested. For phase analysis and particle size estimation, the bulk XRD patterns [Philips, Model 1729] are studied. The microstructure of the pellets is studied using scanning electron microscope (Leica Stereoscan, 440 Model).
Fig. 4. Variation of sensitivity factor (SF) as a function of gas concentration for H2 at 230 jC and CO at 160 jC.
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3. Results 3.1. Sensor study Fig. 1(a) shows variations of SF with temperature at different concentration levels of Cu (wt.%) for CO gas (at 1000 ppm). It is clear that the SF is maximum of 240 for 9 wt.% of Cu at an optimum temperature of 160 jC. Fig. 1(b) shows results for an identical
experiment for H2 gas wherein SF is at maximum of 590 for 9 wt.% of Cu at the optimum temperature of 230 jC. However, in case of CH4 gas, very poor response for SF variation with temperature was observed (SF c 0.0, figure not shown). Fig. 1(c) shows variation of SF with temperature for three different gases H2, CO and CH4 for 9 wt.% of Cu. It shows maximum sensitivity for CO (SF = 240) at an operating temperature of 160 jC in comparison with
Fig. 5. XRD patterns of sintered pellets with (a) 1, (b) 3, (c) 5, (d) 7, (e) 9 and (f ) 11 wt.% of Cu for SnO2/Cu system.
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that obtained for H2 (SF = 100) and CH4 (SF = 80). At 230 jC, it gives a very high sensitivity for H2 (SF = 590) as compared to that obtained for CO (SF c 130) and CH4 (SF = 10). This gives scope for temperature selectivity for different gases. The response and recovery times for CO and H2 are f 4 and f 2, s respectively at 1000-ppm level under optimised conditions. Fig. 2(a) shows repeatability and reproducibility of samples with 9 wt.% of Cu for CO at an
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optimum temperature of 160 jC. Fig. 2(b) shows repeatability and reproducibility of samples with 9 wt.% of Cu for H2 at an optimum temperature of 230 jC. Fig. 3 shows variation of selectivity with Cu concentration (wt.%) for CO and H2. Here, the selectivity is defined as the ratio of SF for test gas and interfering gases (at same concentration of 1000 ppm) at the optimised operating temperature for the test gas. The optimum concentrations of Cu (wt.%) as
Fig. 6. SEM photographs of sintered pellets with (a) 1, (b) 3, (c) 5, (d) 7, (e) 9 and (f ) 11 wt.% of Cu for SnO2/Cu system.
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regards the selectivity for CO and H2 are 9 and 7 wt.%, respectively, at the corresponding optimum temperature. The sensors are also tested under optimum conditions (wt.% and temperature) for a test gas of 1000 ppm followed by injecting interfering gases (up to 1000 ppm). This produced no major change in response due to interfering gases used additionally. Fig. 4 shows the variation of SF with gas concentration from 50 ppm to f 4.5% for CO (at 160 jC) and H2 (at 230 jC) gases. This is used as sensitivity or calibration curve. It shows three-region behaviour as: (i) sharp initial rise in SF up to about 500 ppm, (ii) nearly linear intermediate region and (iii) a third region in which the sensors are saturated at 1.5% of CO and 3.6% of H2. In the major part (intermediate region), the sensitivities (i.e. the slopes) are 1 and 1.1, respectively, for CO and H2 gases which are higher than the corresponding reported values of 0.8 and 1 [9,17,18,22] indicating thereby enhancement of the sensitivity values. 3.2. Material characterisation In order to understand the phase symmetry in the final sintered powder samples, the X-ray study was undertaken. Fig. 5(a –f) shows the XRD patterns of sintered pellets with different concentration levels of Cu (wt % = 1, 3, 5, 7, 9 and 11) for SnO2/Cu system sintered at 600 jC for 4 h. The XRD pattern of commercially supplied SnO2 powder used in the present study exhibited pure tetragonal phase. Further, the XRD pattern of sintered commercial SnO2 powder shows the single-phase material with tetragonal symmetry. A single tetragonal phase was obtained for SnO2 for the entire range of concentration of Cu (wt.%). Furthermore, XRD patterns show additional reflections due to CuO. No reflections corresponding to Cu metal are observed. The reflection (2h = 35.54j) corresponding to CuO was observed only in case of 9 and 11 wt.% of Cu with the monoclinic symmetry. The absence or low intensity of the reflection (2h = 35.54j) corresponding CuO in the 1, 3, 5 and 7 wt.% of Cu may be due to the lower concentration of Cu. To have a better insight, our results on the SF variation for the present system are compared with the reported data [22]. The scanning electron micrographs (SEM) of fractured surfaces of sintered pellets for SnO2/Cu system (with 1, 3, 5, 7, 9 and 11 wt.% of
Cu ) are shown in Fig. 6(a – f). From the SEM photographs, it is clear that the grain sizes (and also voids) are large (f 5 Am) for 1, 3, 5, 7 and 11 wt % of Cu with varied size-distribution, whereas smaller grains (f 1 Am) are observed for the sample with 9 wt.% having nearly uniform distribution. This was confirmed by making additional samples and repeating the same procedure. This apparent fall in grain size (higher surface area) indicates the possible reason for the maximum sensitivity for the samples with 9 wt.% of Cu. The grain size reported for the SnO2/CuO system [22] at 1 mol% of CuO is f 1 Am, whereas for our system of SnO2/Cu, same results were obtained at extremely lower concentration of CuO (5 10 5 to 5.5 10 4 mol%).
4. Discussion It can be realized that the trends seen in these results (Figs. 1 –3) are as expected for the resistive gas sensor operation viz. (i) a critical optimum operating temperature, (ii) a critical optimum wt.% of an additive and lastly, (iii) nearly linear sensitivity (calibration) curve (the main region). Similar observations have been reported by Haeng Yu and Man Choi [22] in case of sensitivity curve. All these results pertaining to additive effect of Cu are discussed in details: (A) Optimum operating temperatures and temperature selectivity of SnO2/Cu pellet sensor for H2 and CO: It is well known that the optimum operating temperature is governed by the activation energy (Ea). As kT approaches Ea, there is an increase in surface reaction of the sensing material with the H2 or CO gases enhancing the conductance G with temperature [16 –24]. As mentioned in Results, we have obtained the lowest operating temperature particularly for CO sensing viz. 160 jC (Ea = 0.25 eV). The subsequent fall in G at higher temperatures may be governed by the desorption either of pre-adsorbed O2 or H2 or CO [1– 4,7– 10,17 – 22]. The optimized temperatures observed (Fig. 1) indicate that (Ea.)co < (Ea.)H2 for surface resistance-related adsorption reactions. The temperature window (defined at 0.7 of peak SF) of this ‘‘temperature-selective sensor’’ is 30 jC (140 – 170 jC) for CO and 70 jC (190 –260 jC) for H2.
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It is evident from the Figs. 1 and 2 that H2 sensor is more selective than CO sensor. It has been reported [1– 4] that the chemical reactions take place between (O2 – CO) or (O2 –H2) rather than with the sample material (e.g. SnO2) resulting thereby acceptance or release of an electron (or hole), which produces a change in the conductivity of the sample. However, the effect of these reactions on the actual adsorption-related activation energy or optimum temperature is not specified (neither the exact effects of the additives are clearly given nor its concentration (wt.%) is predicted). (B) Optimum weight percentage of additive: It is reported [24] that the additive produces new sites for gas adsorption at proper Ea level and that at some critical concentration (wt %) of an additive, the sensor produces maximum SF. This is expected because at lower concentration of an additive, the newly generated sites for the adsorption are less in number. Hence, not all the gas molecules are adsorbed, whereas for higher concentration (but specific), the additive may overshadow the functional material (by giving cover of an additive on the basic functional material). In our case, for all other concentrations (wt.%) of Cu, the SF value is less than half of the value for 9 (wt.%), for both H2 and CO (Fig. 1(a) and (b)). However, the variation of SF with the concentration (wt.%) of an additive is not very systematic. The sample with 9 wt.% of Cu has shown some differences in SEM photographs as compared to the samples prepared with 1, 3, 5, 7 and 11 wt.% of Cu. Furthermore, the grain size for the sample with 9 wt.% of Cu is smaller than the rest of the samples. To what extent, the above effects dominate the general theory for optimization of concentration (wt.%) of additives needs further experimentation. It is worth to mention that only few SnO2-based CO sensors are reported [one with CuO dopant [13,14,22] with nearly same optimum temperature (160 jC). However, in our work, we have added Cu metal in SnO2 which on sintering gives SnO2/CuO system with tetragonal phase of SnO2 at optimized concentration of 9 wt.%. of Cu (i. e. V 0.00045 mol% CuO). Further, the best selectivity values for H2 and
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CO gases are observed at 7 and 9 wt.% of Cu, respectively (Fig. 3). The reported selectivity values [22] for CO and H2 at their optimum conditions are f 2.5 and 1.9, respectively, for SnO2/CuO system (compared to ours f 1.9 and 78, respectively). The higher value of the selectivity for H2 sensor in our case may be due to the lower concentration level of CuO. (C) Sensitivity curve and sensor calibration: Fig. 4 shows the calibration curves which exhibits three regions viz. (i) high sensitivity (up to 500 ppm), (ii) intermediate (nearly linear) sensitivity and (iii) normal saturation region (1.5% and 3.6% for CO and H2, respectively). This range is wider as compared to the reported values (which is up to 0.9% for CO and 2.3% for H2) [1– 4,7– 10,16 – 22]. The first two regions in Fig. 4 show sharp change in the slopes suggesting thereby occurrence of two different independent adsorption processes. It is also reported [1– 3] that the dependence of the surface conductance on reducing gas concentration in SnO2 sensor follows the equations: Gg ¼ Ga þ Y ðPg Þ1=2 ,
ð3Þ
i.e. DG = ( Gg Ga) = Y ( Pg.)1/2, SF ¼ ðDG=Ga ÞaðPg Þ1=2 ,
ð4Þ
where Pg is the partial pressure of the test gas in air and Y is a constant of proportionality governed by the adsorption process. Our sensitivity curve data are re-plotted (figure not shown) to confirm the validity of the above Eq. (4) and it is found to be valid up to saturation level for both the gases (CO and H2).
5. Conclusions Our studies on SnO2/Cu gas sensor system revealed that 1. The optimum gas-sensing temperatures for CO and H2 are 160 and 230 jC, respectively. 2. This provides high sensitivity for CO (SF c 240) and H2 (SF c 590) and its wider range of 1.5% for CO and 3.6% for H2.
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3. A very high selectivity of 78 and 1.9 was obtained for H2 and CO, respectively. 4. These high sensitivity and wider range characteristics are achieved by using considerably lower concentration level of Cu additive as compared to the reported data.
Acknowledgements One of the authors (P.S.M) would like to acknowledge DST, Government of India, for the award of JRF during his M.Phil work.
References [1] J. Marc, S. Madaou, R. Morrison, Chemical Sensing with Solid State Devices, Academic Press, New York, 1989. [2] P. Moseley, B. Tofield, Solid State Gas Sensors (1987). [3] K.D. Schierbaum, U. Weimar, W. Gopel, Sens. Actuators, B 7 (1992) 709. [4] S. Tade, G.M. Jan, J. Choi, Am. Ceram. Soc. 81 (3) (1998) 695. [5] G. Sberveglieri, G. Faglie, S. Groppalli, P. Nelli, Sens. Actuators, B 8 (1992) 79. [6] G. Martineli, M.C. Carotta, Sens. Actuators, B 7 (1992) 717. [7] U. Weimar, K.D. Schierbaum, W. Gopel, Sens. Actuators, B 1 (1990) 93.
[8] K.H. Song, S.J. Park, J. Mater. Sci., Mater. Electron. 4 (1993) 249. [9] D. Hyok, J. Haeng Yu, G.M. Choi, Sens. Actuators, B 46 (1998) 15. [10] R.K. Srivastava, P. Lal, R. Dwivedi, S.K. Srivastava, Sens. Actuators, B 21 (1994) 213. [11] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators, B 3 (1991) 147. [12] J.T. Maekawa, N. Miura, N. Yamazoe, Sens. Actuators, B 9 (1992) 197. [13] G.S. Devi, S. Manorama, V.J. Rao, J. Electrochem. Soc. 142 (1995) 2754. [14] H. Yamaura, K. Moriya, N. Miura, N. Yamazoe, Sens. Actuators, B 65 (2000) 39. [15] G. Magamma, V. Jayaraman, T. Gnanasekaran, G. Periaswami, Sens. Actuators, B 53 (1998) 133. [16] T. Pagnier, M. Boulova, A. Galerie, A. Gaskov, G. Lucazeau, Sens. Actuators, B 71 (2000) 134. [17] J. Haeng Yu, G. Man Choi, Sens. Actuators, B 52 (1998) 251. [18] N.J. Dayan, S.R. Sainkar, R.N. Karekar, R.C. Aiyer, Thin Solid Films 325 (1998) 254. [19] S.G. Ansari, S.W. Gosdavi, S.A. Gangal, R.N. Karekar, R.C. Aiyer, J. Mater. Sci., Mater. Electron. 7 (1996) 267. [20] P.S. More, R.N. Karekar, R.C. Aiyer, National Symposium on Physics and Technology of Sensor, 1998, p. C18-1. [21] S.G. Ansari, P. Boroojerdian, S.R. Sainkar, R.N. Karekar, R.C. Aiyer, S.K. Kulkarni, Thin Solid Films 295 (1997) 271. [22] J. Haeng Yu, G. Man Choi, Sens. Actuators, B 3705 (2001) 1. [23] P.S. More, M. Phil Dissertation, University of Pune, July 1998. [24] N.J. Dayan, R.N. Karekar, R.C. Aiyer, J. Mater. Sci., Mater. Electron. 8 (1997) 277.
High-performance temperature-selective SnO2:Cu-based sensor P.S. More a,*, Y.B. Khollam a, S.B. Deshpande b, S.R. Sainkar b, S.K. Date b, R.N. Karekar c, R.C. Aiyer a a
Department of Physics, Centre for Advanced Studies in Material Science and Solid State Physics, University of Pune, Pune 411007, India b Physical and Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India c Department of Physics, R. Chandrashekhar Memorial Foundation Transducer Laboratory, University of Pune, Pune 411007, India Received 15 July 2002; accepted 5 September 2002
Abstract Systematic evaluation of gas-sensing characteristics of Cu-modified SnO2 pellets for CO, H2 and CH4 gases from ppm to percentage level is reported. The concentration level of Cu additive was varied systematically from 1 to 11 wt %. The highest values of sensitivity factor (SF) of 240 and 590 corresponding to CO and H2 gases were obtained at considerably lower temperatures of 160 and 230 jC, respectively, for 9 wt.% of Cu. The selectivity values are found to be maximum at 7 and 9 wt.% of Cu for H2 (c78) and CO (c1.9) gasses, respectively. For CH4 gas, on the contrary, the sensor showed quite inferior response. The sensitivity curve for our material follows the usual trend of three-region behaviour of standard Windischmann and Mark’s curve. The sensor tends to saturate at 1.5% and 3.6% for CO and H2, respectively. XRD and scanning electron micrograph (SEM) characterisation techniques are used to pinpoint the possible reasons for high performance of the sensor at 9 wt.% Cu. Our sensor material showed promising properties for gas-sensing (CO and H2) applications. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Gas sensor; Oxides; SnO2/Cu system; Sensitivity; Selectivity; Microstructure
1. Introduction The atmospheric pollution has lead to the research and development of a variety of sensors using different materials and technologies particularly for low cost and a lower operating temperatures [1 – 5]. Resistive pellet technology is probably the simplest and cheapest one. Tin dioxide (SnO2) is one of the most important and extensively used materials for the detection of gases [1 – 3,5– 17]. The changes in the * Corresponding author. E-mail addresses: [email protected] (P.S. More), [email protected] (R.C. Aiyer).
properties of SnO2 due to gas adsorption are related to the nonstoichiometry, average co-ordination number per grain and the neck size effect in the functional material. As such there is no specific report relating to its structure and sensing properties. The Schottky barrier at the grain boundaries is also reported to govern the thick and thin film sensor properties [1 –4,6]. Porosity of the sensor material is also one of the most important factors. It is also widely accepted that atmospheric oxygen is chemisorbed, reducing the conductivity of the sensor and on exposure to reducing gases like CO and H2, it increases at appropriate operating temperatures [1 – 24]. This phenomenon gives qualitative explanation
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01170-9
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for the change in the conductivity of gas sensor and its temperature selectivity. The change in conductance is normally proportional to the square root of partial pressure of the reducing gas in the air [2,3,6,7]. By the addition [2,3] of specific modifier/ promoter/catalyst, the sensing action and selectivity is often further enhanced for a particular gas at a particular temperature. Many references are available with CuO as an additive material in SnO2 [2,4,12,16,17] and most of them are for sensing H2S [4,11,12,14,20,24]. However, limited information about the SnO2/CuO system for gas-sensing characteristics for CO, H2 and CH4 is available in the literature [22]. Furthermore, very high concentration levels of CuO additive (mole %) are required. In view of this, an attempt is made in present study to see the effect of addition of Cu metal (instead of CuO) on the gas-sensing properties for CO, H2 and CH4. Further, the variation of sensitivity and selectivity of SnO2 pellets with the systematic variation of Cu (metal powder) as an additive (by wt.%) for CO, H2 and CH4 gases over a wider range and concentration from ppm to percentage level is visualised. The pellets are characterised by using XRD and SEM techniques to get an empirical idea about the effect of addition of Cu.
2. Experimental procedure Commercially available SnO2 powder (Thomas Baker) with x-wt.% of Cu metal powder (x = 1, 3, 5, 7, 9 and 11) is thoroughly mixed in an acetone as medium and dried under ambient condition. Each mixture is sintered at 600 jC/4 h and further pulverised to get still a fine powder. The powder is then cold-pressed (5 ton) to form pellets (diameter = 1.3 cm and thickness = 0.12 cm.) by using 3 wt.% polyvinyl alcohol (PVA) as binder material. The pellets are then fired at 300 jC in ambient air to burn PVA. Gas-sensing properties of these pellets are studied in a home-built system [15,18,19]. The Fig. 1. (a) SF variation with temperature at different concentration of Cu (wt.%) for 1000 ppm of CO. (b) SF variation with temperature at different concentration of Cu (wt.%) for 1000 ppm of H2. (c) Variation of sensitivity factor (SF) with temperature for 1000 ppm of CO, H2 and CH4 gases with 9 wt.% of Cu.
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Fig. 3. Variation of selectivity with wt.% of Cu for CO and H2.
tivity factor (SF) [15,18] and sensitivity (S) are defined as SF ¼ f½Gg Ga =Ga g 100 ¼ ðDG=Ga Þ 100; ð1Þ S ¼ D SF=D ðgas concentrationÞ;
Fig. 2. (a) Sample to sample repeatability and reproducibility for 1000 ppm of CO with 9 wt.% of Cu at optimum temperature of 160 jC. (b) Sample to sample repeatability and reproducibility for 1000 ppm of H2 with 9 wt.% of Cu at optimum temperature of 230 jC.
required gas concentration inside the system is achieved by injecting a known volume of the test gas in an airtight chamber (volume 15 l) at ambient conditions. The pellets are tested for a wide range of concentration of CO, H2 and CH4 gases separately as well as for their add-mixtures, from 50 ppm to f 3% level. To avoid the relative humidity effect, preheated and then cooled samples (in situ) are used for experimentation. The DC conductance of the pellet (with press contact electrodes) is obtained (at 20 V) by measuring the voltage drop across reference resistor (290 KV) connected in series. The percentage sensi-
ð2Þ
where Ga and Gg are the conductances of the sensor in air and (gas + air), respectively. Normally, three cycles are taken for each sample and two samples of each type are tested. For phase analysis and particle size estimation, the bulk XRD patterns [Philips, Model 1729] are studied. The microstructure of the pellets is studied using scanning electron microscope (Leica Stereoscan, 440 Model).
Fig. 4. Variation of sensitivity factor (SF) as a function of gas concentration for H2 at 230 jC and CO at 160 jC.
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3. Results 3.1. Sensor study Fig. 1(a) shows variations of SF with temperature at different concentration levels of Cu (wt.%) for CO gas (at 1000 ppm). It is clear that the SF is maximum of 240 for 9 wt.% of Cu at an optimum temperature of 160 jC. Fig. 1(b) shows results for an identical
experiment for H2 gas wherein SF is at maximum of 590 for 9 wt.% of Cu at the optimum temperature of 230 jC. However, in case of CH4 gas, very poor response for SF variation with temperature was observed (SF c 0.0, figure not shown). Fig. 1(c) shows variation of SF with temperature for three different gases H2, CO and CH4 for 9 wt.% of Cu. It shows maximum sensitivity for CO (SF = 240) at an operating temperature of 160 jC in comparison with
Fig. 5. XRD patterns of sintered pellets with (a) 1, (b) 3, (c) 5, (d) 7, (e) 9 and (f ) 11 wt.% of Cu for SnO2/Cu system.
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that obtained for H2 (SF = 100) and CH4 (SF = 80). At 230 jC, it gives a very high sensitivity for H2 (SF = 590) as compared to that obtained for CO (SF c 130) and CH4 (SF = 10). This gives scope for temperature selectivity for different gases. The response and recovery times for CO and H2 are f 4 and f 2, s respectively at 1000-ppm level under optimised conditions. Fig. 2(a) shows repeatability and reproducibility of samples with 9 wt.% of Cu for CO at an
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optimum temperature of 160 jC. Fig. 2(b) shows repeatability and reproducibility of samples with 9 wt.% of Cu for H2 at an optimum temperature of 230 jC. Fig. 3 shows variation of selectivity with Cu concentration (wt.%) for CO and H2. Here, the selectivity is defined as the ratio of SF for test gas and interfering gases (at same concentration of 1000 ppm) at the optimised operating temperature for the test gas. The optimum concentrations of Cu (wt.%) as
Fig. 6. SEM photographs of sintered pellets with (a) 1, (b) 3, (c) 5, (d) 7, (e) 9 and (f ) 11 wt.% of Cu for SnO2/Cu system.
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regards the selectivity for CO and H2 are 9 and 7 wt.%, respectively, at the corresponding optimum temperature. The sensors are also tested under optimum conditions (wt.% and temperature) for a test gas of 1000 ppm followed by injecting interfering gases (up to 1000 ppm). This produced no major change in response due to interfering gases used additionally. Fig. 4 shows the variation of SF with gas concentration from 50 ppm to f 4.5% for CO (at 160 jC) and H2 (at 230 jC) gases. This is used as sensitivity or calibration curve. It shows three-region behaviour as: (i) sharp initial rise in SF up to about 500 ppm, (ii) nearly linear intermediate region and (iii) a third region in which the sensors are saturated at 1.5% of CO and 3.6% of H2. In the major part (intermediate region), the sensitivities (i.e. the slopes) are 1 and 1.1, respectively, for CO and H2 gases which are higher than the corresponding reported values of 0.8 and 1 [9,17,18,22] indicating thereby enhancement of the sensitivity values. 3.2. Material characterisation In order to understand the phase symmetry in the final sintered powder samples, the X-ray study was undertaken. Fig. 5(a –f) shows the XRD patterns of sintered pellets with different concentration levels of Cu (wt % = 1, 3, 5, 7, 9 and 11) for SnO2/Cu system sintered at 600 jC for 4 h. The XRD pattern of commercially supplied SnO2 powder used in the present study exhibited pure tetragonal phase. Further, the XRD pattern of sintered commercial SnO2 powder shows the single-phase material with tetragonal symmetry. A single tetragonal phase was obtained for SnO2 for the entire range of concentration of Cu (wt.%). Furthermore, XRD patterns show additional reflections due to CuO. No reflections corresponding to Cu metal are observed. The reflection (2h = 35.54j) corresponding to CuO was observed only in case of 9 and 11 wt.% of Cu with the monoclinic symmetry. The absence or low intensity of the reflection (2h = 35.54j) corresponding CuO in the 1, 3, 5 and 7 wt.% of Cu may be due to the lower concentration of Cu. To have a better insight, our results on the SF variation for the present system are compared with the reported data [22]. The scanning electron micrographs (SEM) of fractured surfaces of sintered pellets for SnO2/Cu system (with 1, 3, 5, 7, 9 and 11 wt.% of
Cu ) are shown in Fig. 6(a – f). From the SEM photographs, it is clear that the grain sizes (and also voids) are large (f 5 Am) for 1, 3, 5, 7 and 11 wt % of Cu with varied size-distribution, whereas smaller grains (f 1 Am) are observed for the sample with 9 wt.% having nearly uniform distribution. This was confirmed by making additional samples and repeating the same procedure. This apparent fall in grain size (higher surface area) indicates the possible reason for the maximum sensitivity for the samples with 9 wt.% of Cu. The grain size reported for the SnO2/CuO system [22] at 1 mol% of CuO is f 1 Am, whereas for our system of SnO2/Cu, same results were obtained at extremely lower concentration of CuO (5 10 5 to 5.5 10 4 mol%).
4. Discussion It can be realized that the trends seen in these results (Figs. 1 –3) are as expected for the resistive gas sensor operation viz. (i) a critical optimum operating temperature, (ii) a critical optimum wt.% of an additive and lastly, (iii) nearly linear sensitivity (calibration) curve (the main region). Similar observations have been reported by Haeng Yu and Man Choi [22] in case of sensitivity curve. All these results pertaining to additive effect of Cu are discussed in details: (A) Optimum operating temperatures and temperature selectivity of SnO2/Cu pellet sensor for H2 and CO: It is well known that the optimum operating temperature is governed by the activation energy (Ea). As kT approaches Ea, there is an increase in surface reaction of the sensing material with the H2 or CO gases enhancing the conductance G with temperature [16 –24]. As mentioned in Results, we have obtained the lowest operating temperature particularly for CO sensing viz. 160 jC (Ea = 0.25 eV). The subsequent fall in G at higher temperatures may be governed by the desorption either of pre-adsorbed O2 or H2 or CO [1– 4,7– 10,17 – 22]. The optimized temperatures observed (Fig. 1) indicate that (Ea.)co < (Ea.)H2 for surface resistance-related adsorption reactions. The temperature window (defined at 0.7 of peak SF) of this ‘‘temperature-selective sensor’’ is 30 jC (140 – 170 jC) for CO and 70 jC (190 –260 jC) for H2.
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It is evident from the Figs. 1 and 2 that H2 sensor is more selective than CO sensor. It has been reported [1– 4] that the chemical reactions take place between (O2 – CO) or (O2 –H2) rather than with the sample material (e.g. SnO2) resulting thereby acceptance or release of an electron (or hole), which produces a change in the conductivity of the sample. However, the effect of these reactions on the actual adsorption-related activation energy or optimum temperature is not specified (neither the exact effects of the additives are clearly given nor its concentration (wt.%) is predicted). (B) Optimum weight percentage of additive: It is reported [24] that the additive produces new sites for gas adsorption at proper Ea level and that at some critical concentration (wt %) of an additive, the sensor produces maximum SF. This is expected because at lower concentration of an additive, the newly generated sites for the adsorption are less in number. Hence, not all the gas molecules are adsorbed, whereas for higher concentration (but specific), the additive may overshadow the functional material (by giving cover of an additive on the basic functional material). In our case, for all other concentrations (wt.%) of Cu, the SF value is less than half of the value for 9 (wt.%), for both H2 and CO (Fig. 1(a) and (b)). However, the variation of SF with the concentration (wt.%) of an additive is not very systematic. The sample with 9 wt.% of Cu has shown some differences in SEM photographs as compared to the samples prepared with 1, 3, 5, 7 and 11 wt.% of Cu. Furthermore, the grain size for the sample with 9 wt.% of Cu is smaller than the rest of the samples. To what extent, the above effects dominate the general theory for optimization of concentration (wt.%) of additives needs further experimentation. It is worth to mention that only few SnO2-based CO sensors are reported [one with CuO dopant [13,14,22] with nearly same optimum temperature (160 jC). However, in our work, we have added Cu metal in SnO2 which on sintering gives SnO2/CuO system with tetragonal phase of SnO2 at optimized concentration of 9 wt.%. of Cu (i. e. V 0.00045 mol% CuO). Further, the best selectivity values for H2 and
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CO gases are observed at 7 and 9 wt.% of Cu, respectively (Fig. 3). The reported selectivity values [22] for CO and H2 at their optimum conditions are f 2.5 and 1.9, respectively, for SnO2/CuO system (compared to ours f 1.9 and 78, respectively). The higher value of the selectivity for H2 sensor in our case may be due to the lower concentration level of CuO. (C) Sensitivity curve and sensor calibration: Fig. 4 shows the calibration curves which exhibits three regions viz. (i) high sensitivity (up to 500 ppm), (ii) intermediate (nearly linear) sensitivity and (iii) normal saturation region (1.5% and 3.6% for CO and H2, respectively). This range is wider as compared to the reported values (which is up to 0.9% for CO and 2.3% for H2) [1– 4,7– 10,16 – 22]. The first two regions in Fig. 4 show sharp change in the slopes suggesting thereby occurrence of two different independent adsorption processes. It is also reported [1– 3] that the dependence of the surface conductance on reducing gas concentration in SnO2 sensor follows the equations: Gg ¼ Ga þ Y ðPg Þ1=2 ,
ð3Þ
i.e. DG = ( Gg Ga) = Y ( Pg.)1/2, SF ¼ ðDG=Ga ÞaðPg Þ1=2 ,
ð4Þ
where Pg is the partial pressure of the test gas in air and Y is a constant of proportionality governed by the adsorption process. Our sensitivity curve data are re-plotted (figure not shown) to confirm the validity of the above Eq. (4) and it is found to be valid up to saturation level for both the gases (CO and H2).
5. Conclusions Our studies on SnO2/Cu gas sensor system revealed that 1. The optimum gas-sensing temperatures for CO and H2 are 160 and 230 jC, respectively. 2. This provides high sensitivity for CO (SF c 240) and H2 (SF c 590) and its wider range of 1.5% for CO and 3.6% for H2.
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3. A very high selectivity of 78 and 1.9 was obtained for H2 and CO, respectively. 4. These high sensitivity and wider range characteristics are achieved by using considerably lower concentration level of Cu additive as compared to the reported data.
Acknowledgements One of the authors (P.S.M) would like to acknowledge DST, Government of India, for the award of JRF during his M.Phil work.
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