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Electrical and CO-sensing properties of SmFe0.7Co0.3O3 perovskite oxide
Materials Science and Engineering B 171 (2010) 139–143
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Electrical and CO-sensing properties of SmFe0.7 Co0.3 O3 perovskite oxide Ru Zhang a , Jifan Hu a,∗ , Ma Zhao a , Zhouxiang Han a , Jianying Wei a , Zhanlei Wu a , Hongwei Qin b , Kaiying Wang c a
Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, China School of Physics, Shandong University, Jinan 250100, China c Vestfold University College, Institute for Microsystem Technology, Raveien 197, Horten, 3184, Norway b
a r t i c l e
i n f o
Article history: Received 21 December 2009 Received in revised form 18 March 2010 Accepted 31 March 2010 Keywords: Perovskite Semiconductor gas sensor CO-sensing Sol–gel method
a b s t r a c t In this work, nanocrystalline powders of perovskite-type compounds SmFe1−x Cox O3 (x = 0, 0.3) were prepared by sol–gel method. The morphology, electrical and carbon monoxide (CO) sensing properties of the nanocrystalline powders were studied by X-ray diffraction technique, Scanning Electron Microscopy (SEM), and electrical resistance measurements from 40 to 400 ◦ C. The optimal value of CO-sensing response (S = Rg /Ra , here Ra = resistances in atmospheric air with humidity = 45%, Rg = resistances in CO within the background of atmospheric air with humidity = 45%) for nanocrystalline SmFe0.7 Co0.3 O3 -based sensor is about 8.43 at 150 ◦ C as the CO concentration is equal to 500 ppm. In addition, the SmFe0.7 Co0.3 O3 sample shows a good selectivity to the gas CO. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The detection of carbon monoxide (CO) has attracted great research interests in scientific and engineering fields such as materials, energy, environment, and human health [1–5]. It is well known that the gas CO is one of the most poisonous and harmful gases in atmosphere for human being. When molecules of the gas CO enter into human body, they combine with hemoglobin much faster than that with oxygen molecules and will remain in blood for longer time. The chemical combination of CO with hemoglobin deteriorates the transportation of oxygen in blood and will induce fatal problems to our life. The maximum tolerance level of CO concentration for human body is just 50 ppm for a time weighted average limit (TWA) in air [1]. Therefore, timely and accurate detection of CO gas in the surrounding environment is very important. From our knowledge, perovskite-type oxide materials are very promising candidates to detect gas CO, such as La0.68 Pb0.32 FeO3 [1], La0.8 Pb0.2 Fe0.8 M0.2 O3 (M = Co, Ni) [2,3], LaFeO3 [4] and La0.8 Sr0.2 Co0.5 Ni0.5 O3 [5]. Besides, the perovskite-type compound SmFeO3 prepared by thermal decomposition and subsequently screen-printing on alumina substrates shows excellent response to the gases, such as O3 , NO2 [6–8], CO [9,10]. Later, Itagaki et al. [11] investigated the sensing properties of the compounds SmFe1−x Cox O3 by substituting atoms Fe with atoms Co in the per-
∗ Corresponding author. Tel.: +86 531 8856 6143; fax: +86 531 8856 5167. E-mail address: [email protected] (J. Hu). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.03.087
ovskite structure. They discovered that the Co-added oxides show higher conductivity and better response-recovery to O3 and NO2 than that of compound SmFeO3 . In one of our previous work, we reported that the compounds SmFe1−x Cox O3 perovskite oxides presented proper ethanol sensing properties and the maximum response of the compound SmFe0.7 Co0.3 O3 can reach S = 80.78 with ethanol concentration = 300 ppm at 215 ◦ C [12,13]. In this paper, the morphology and electrical properties of the compounds SmFe1−x Cox O3 (x = 0, 0.3) have been studied in details. A CO-sensing sensor has been made based on the nanocrystalline materials SmFe0.7 Co0.3 O3 and its response properties are reported. 2. Experimental 2.1. Preparation of powders The powders of compounds SmFeO3 and SmFe0.7 Co0.3 O3 were prepared by sol–gel citrate method. Firstly, stoichiometric powders of oxide Sm2 O3 (99.95%) were dissolved in nitric acid to prepare Sm(NO3 )3 ·6H2 O, followed by the addition of Fe(NO3 )3 ·9H2 O (Analytical Reagent, A.R.), Co(NO3 )2 ·6H2 O (A.R.) and citric acid. Then the PEG (polyethylene glycol, with the help of the micellar solubilization of PEG, the solubility of the inorganic salts in the sols was greatly increased; as a result, the homogeneous sols could be prepared conveniently [14]) was added into the mixed solution under stirring at 70 ◦ C to obtain the sol and the sol was dried to form gel, then the gel pieces were ground to form fine powders. Finally, the fine powders were annealed at 800 ◦ C for 2 h in an oven. When
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R. Zhang et al. / Materials Science and Engineering B 171 (2010) 139–143 Table 1 The lattice constants and unit cell volumes of the samples. Samples
SmFeO3 (standard) SmFeO3 SmFe0.7 Co0.3 O3
Fig. 1. The XRD patterns of SmFcO3 and SmFc0.7 Co0.3 O3 powders.
the powders temperature dropped to room temperature, ground the powders again. The structures of the powders were characterized by X-ray diffraction (XRD, Bruker D8 Advance) with Cu K␣ radiation. The morphology of the powders was observed by Scanning Electron Microscopy (SEM, Hitachi S2500). The specific surface area of the powders was measured by BET surface area tester (3H2000BET). 2.2. Fabrication of sensors The final obtained powders of compounds SmFeO3 and SmFe0.7 Co0.3 O3 were mixed respectively with terpineol (C10 H18 O, molecular weight 154.25) and ground into pastes. These pastes were packed into Al2 O3 tubes with electrodes mounted at both ends, then the tubes were calcined at 400 ◦ C for 2 h to enhance the stability of sensors. The inner and outer diameters of the ceramic tube are 4 mm and 6 mm, respectively with 10 mm in length.
Volume (Å3 )
Lattice constant (Å) a
b
c
5.400 5.3962 5.3951
5.597 5.5953 5.5887
7.711 7.7352 7.6941
233.0557 233.5517 231.9894
LaCoO3 and LaFe1−x Cox O3 compounds, the valence of Fe and Co ions is 3+. When oxygen vacancies occur in nanocrystalline SmFe0.7 Co0.3 O3 (the exact structure formula of the samples should be SmFe0.7 Co0.3 O3−ı ), Co2+ may occur in order to maintain the charge neutrality. The average particle sizes D were calculated from X-ray patterns based on the Scherrer’s equation. The D values were about 34.24 and 25.14 nm for SmFeO3 and SmFe0.7 Co0.3 O3 powders, respectively. The result of the D decrease indicates that the doping with element cobalt in SmFeO3 may prevent the grain growth during high temperature treatment. The BET specific surface areas of the nanocrystalline SmFeO3 and SmFe0.7 Co0.3 O3 are 2.865 m2 /g and 5.407 m2 /g. At the same time, surface morphology of the compounds was examined by Scanning Electron Microscopy and given in Fig. 2. The grain sizes of the nanocrystalline SmFeO3 and SmFe0.7 Co0.3 O3 are 47 nm and 36 nm. Since the compound SmFe0.7 Co0.3 O3 has a smaller grain size as compared with the compound SmFeO3 , it has a bigger total surface area while they have same weights. The relatively large specific surface area is an advantage for gas sensing application.
2.3. Measurement of sensors A conventional circuit in which the sensor was connected with an external resistor in series was used to measure the sensor resistance. The conductances of the sensors were measured in atmospheric air with humidity = 45%. A suitable amount of tested gas (i.e. CO) was injected into a test chamber having atmospheric air with humidity = 45%. The gas sensing properties of the sensors were measured in a temperature range of 40–400 ◦ C. The tested CO concentrations were 100, 300, 500, 700 and 900 ppm, respectively. The responses of sensors were defined as S = Rg /Ra [15], where Ra and Rg were the resistances measured under atmospheric air with humidity = 45% and a tested gas (within the background of atmospheric air with humidity = 45%), respectively. 3. Results and discussion 3.1. Phase composition The X-ray diffraction patterns of the powders SmFeO3 and SmFe0.7 Co0.3 O3 are shown in Fig. 1. It can be seen that all the diffraction peaks can be indexed to the compound SmFeO3 (JCPDS: 74-1474) with an orthorhombic structure. The lattice parameters of the samples calculated from XRD patterns are listed in Table 1, which also includes the experimental and standard lattice parameters. The experimental data shows that the lattice parameters and unit cell volume of the compound SmFe0.7 Co0.3 O3 are smaller than those of compound SmFeO3 , since the radius of Co3+ (63 pm) is smaller than that of that of Fe3+ (64 pm). Xray photoelectron spectroscopy analysis indicated that in LaFeO3 ,
Fig. 2. SEM images of SmFeO3 (a) and SmFe0.7 Co0.3 O3 (b) nanoparticles calcined at 800 ◦ C.
R. Zhang et al. / Materials Science and Engineering B 171 (2010) 139–143
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Fig. 3. Temperature dependence of the conductance of SmFeO3 and SmFe0.7 Co0.3 O3 in atmospheric air with humidity = 45%. Fig. 4. Ln G of the sensors in atmospheric air with humidity = 45% and 500 ppm CO (in atmospheric air with humidity = 45%) as a function of the inverse of temperature.
3.2. Electrical properties Fig. 3 presents the temperature dependence of conductance of the compounds SmFeO3 and SmFe0.7 Co0.3 O3 based sensors from 40 to 400 ◦ C in atmospheric air with humidity = 45%. It can be seen that the conductance values of the two samples increase with increasing temperature and the SmFe0.7 Co0.3 O3 -based sensor shows a larger conductance in the whole tested temperature range as compared with the SmFeO3 -based one. As we know, the compound SmFeO3 is a p-type semi-conductive material, its charge carriers are holes x ]: (h• ) which are produced by Sm3+ vacancy defects [VSm
x VSm → VSm + 3h•
(1)
In addition to Co3+ , when Co2+ also occurs in nanocrystalline SmFe0.7 Co0.3 O3−ı , amount of holes will increase due to the ionization of [CoxFe ] for the partial substitution of Fe3+ by Co2+ : CoxFe → CoFe + h•
(2)
which increases the conductivity of p-type SmFe0.7 Co0.3 O3−ı . Meanwhile, similar to the double exchange interaction between Mn3+ and Mn4+ ions in alkali–metal ion doped LaMnO3 [16], the possible double exchange interaction between Co2+ and Co3+ ions in nanocrystalline SmFe0.7 Co0.3 O3−ı may also enhance the conductivity. Of course, the double exchange interaction should not occur in ideal SmFe0.7 Co0.3 O3 without oxygen vacancies, where Co2+ is absent. On the other hand, the difference of the conductance may have a relationship with the grain boundary resistance, which is a major factor for the conduction of a semiconductor [17]. Another advantage of the SmFe0.7 Co0.3 O3 -based sensor is that it needs less activation energy. The activation energy can be calculated by the Arrhenius plot of the conductance [18]: G = G0 exp(−Ea /kT )
compound SmFe0.7 Co0.3 O3 upon exposure to CO could be understood through CO chemical reduction to the p-type semiconductor material SmFe0.7 Co0.3 O3 . 3.3. The CO sensing properties Fig. 5 shows the temperature dependence of the response of SmFeO3 and SmFe0.7 Co0.3 O3 -based sensors in CO concentration 500 ppm. For the SmFeO3 and SmFe0.7 Co0.3 O3 , the optimal responses are 3.12 at 220 ◦ C and 8.43 at 150 ◦ C. In addition, there are different tendencies between the response curves of the SmFeO3 and SmFe0.7 Co0.3 O3 -based sensors. With increasing operating temperature from 40 to 400 ◦ C, the response value of SmFe0.7 Co0.3 O3 increased gradually from 1.5 to 8.2 and then decreased to 1. However, the response of the SmFeO3 does not decrease all the time after reaching the optimal response, but after a slight decrease, it starts to increase again. Therefore, the optimal operating temperature for the SmFeO3 may be more than 400 ◦ C. The SmFe0.7 Co0.3 O3 -based sensor shows a higher response than that of the SmFeO3 and it may be related to its smaller particle size and larger specific surface area. A larger surface area allows more of the On− (ads) adsorption, so
(3)
where G is the conductance, k is the Boltzmann’s constant, T is the absolute temperature and Ea is the activation energy. Plotting ln G against 1/T: ln G = ln G0 −
Ea kT
(4)
Then Ea of the two compounds SmFeO3 and SmFe0.7 Co0.3 O3 were obtained from Fig. 4. Their activation energies are 0.550 eV and 0.475 eV for the compounds SmFeO3 and SmFe0.7 Co0.3 O3 in atmospheric air with humidity = 45%. The activation energy of the compound SmFe0.7 Co0.3 O3 is increased to 0.548 eV while CO concentration increases to 500 ppm. This active energy increase of the
Fig. 5. Temperature dependence of the response of SmFcO3 and SmFe0.7 CO0.3 O3 based sensors to 500 ppm CO.
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R. Zhang et al. / Materials Science and Engineering B 171 (2010) 139–143
Fig. 6. Temperature dependence of the responses of SmFe0.7 CO0.3 O3 -based sensor to CO gas under different concentrations. (The insert figure shows the relationship between CO gas concentration and the response of SmFe0.7 CO0.3 O3 -based sensor at 150 ◦ C.)
Fig. 7. Log resistance vs. log CO concentration (in atmospheric air with humidity = 45%) of SmFe0.7 CO0.3 O3 -based sensor at 150 ◦ C. The symbol of black square represents the experimental point.
that it can accelerate the reaction of CO and On− [19]: CO + On−
(ads) → CO2 + ne−
(5)
Another explanation is that the appearance of oxygen vacancies. Due to the loss of metal atoms at the crank pints of cells in the calcined process at high temperature, oxygen vacancies should form in order to maintain the charge balance. Therefore, the oxygen absorbed increases [20]. So, the SmFe0.7 Co0.3 O3 -based sensor which parts of Fe3+ are replaced by Co2+ at the B site creates a relatively larger amount of oxygen adsorption sites and has the higher response. Fig. 6 shows the curves of response of the SmFe0.7 Co0.3 O3 -based sensor to CO at different operating temperatures. It can be seen obviously that the sensor shows high response to CO with a wide temperature range of 90–210 ◦ C and the optimal operating temperature is about 150◦ C. The dependence of the response on the concentration of CO gas at 150 ◦ C is shown in the insert of Fig. 6. The responses of 1.54, 5.14, 8.43, 10.92 and 12.54 were obtained in the different concentrations of CO (100, 300, 500, 700 and 900 ppm, respectively). It can be observed clearly that the responses and CO gas concentration presents a good linear relationship. Fig. 7 shows the logarithmic dependence of the resistance of the SmFe0.7 Co0.3 O3 -based sensor on the CO concentration at 150 ◦ C. To illustrate the sensor with relatively large dependence on the concentration of CO gas, we calculated the value of dependence according to an empirical formula [21–23]: ˛ R = KCCO
(6)
where R is the resistance of the sensor, CCO is the concentration of CO, K and ˛ are the constants. As known from the literature, the ˛ value of the SnO2 sensor is probably between 1/6 and 1/2 [24]. While, the ˛ value of SmFe0.7 Co0.3 O3 -based sensor obtained from Fig. 6 is about 0.97. It is not only larger than that of SnO2 but also larger than those of other perovskite oxides, such as La0.8 Pb0.2 Fe0.8 Co0.2 O3 and La0.8 Pb0.2 Fe0.8 Ni0.2 O3 [2,3]. It means that the resistance of the SmFe0.7 Co0.3 O3 -based sensor is strongly dependent on the CO gas concentration. Generally speaking, there will have second gas or more in the surrounding circumstances when sensor detects a target gas. The second gas may have a great impact on the accurate detection of the target gas. Therefore, the selectivity of gas sensors is very important. In this paper, we studied the selectivity of the
Fig. 8. The selectivity of SmFe0.7 CO0.3 O3 -based sensor to CO (in atmospheric air with humidity = 45%).
SmFe0.7 Co0.3 O3 -based sensors to CO at 150 ◦ C. As temperature is at 150 ◦ C and all the gas concentration are 500 ppm, it can be seen in Fig. 8, the S values of CO, diethyl ether, ammonia, cyclohexane and toluene are 8.43, 1.08, 1.42, 1.28 and 1.39. Obviously, the response of the SmFe0.7 Co0.3 O3 -based sensors to CO is much higher than those to other gases. Therefore, the SmFe0.7 Co0.3 O3 based sensor has a good selectivity to CO. Furthermore, response and recovery times at 150 ◦ C for SmFe0.7 Co0.3 O3 -based sensor to 500 ppm CO were measured as 17 s and 33 s, respectively. However, the gas ethanol has significant side-effect to the CO sensing application since the response of the compound SmFe0.7 Co0.3 O3 is relatively high at 215 ◦ C, the detailed results are listed in Table 2. As a gas sensor, its stability is another important property. Stability determines a sensor whether can be reused. In order to study the stability of SmFe0.7 Co0.3 O3 -based sensor, we measured its resistance variation for 144 h at 150 ◦ C. As is shown in Fig. 9, the minimum value of resistance is 5.07 k; maximum value is 10.77 k, so the fluctuation in resistance is only 5.7 k in the whole measuring range. The results show that the sensor has good stability and repeatability at 150 ◦ C.
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Table 2 The comparison of gas sensing properties of sensors based on SmFe0.7 Co0.3 O3 nanocrystalline powders prepared at different temperatures. Sensors
Annealing temperature (◦ C)
Grain size (nm)
Tested gas
Highest sensitivity
Optimal operating temperature (◦ C)
Response, recovery time (s)
Literature
SmFe0.7 Co0.3 O3 SmFe0.7 Co0.3 O3
850 800
34.39 25.14
Ethanol (300 ppm) CO (500 ppm)
80.78 8.43
215 150
11, 17 17, 33
[12,13] This text
Acknowledgement This work was supported by National Natural Science Foundation of China (Nos: 50872074 and 50872069). References
Fig. 9. The stability of SmFe0.7 CO0.3 O3 -based sensor at 150 ◦ C.
4. Conclusion The perovskite oxides SmFeO3 and SmFe0.7 Co0.3 O3 powders were prepared by a sol–gel method. Their structures, electrical and CO-sensing properties were investigated in detail. The two samples are single phase with orthorhombic structure. The SmFe0.7 Co0.3 O3 sample has a smaller unit cell volume and grain size as compared with SmFeO3 . On the other hand, its electrical conductance is higher and activation energy is smaller than SmFeO3 in atmospheric air with humidity = 45%. The SmFe0.7 Co0.3 O3 -based sensor has a wide operating temperature range from 90 to 210 ◦ C and its optimal response could reach to 8.43 upon exposure to the gas CO with concentration = 500 ppm. The sensor has a stronger dependence on the concentration of CO than that of SnO2 based sensor or other oxides. Moreover, the sensor shows good selectivity and stability. All the conclusions indicated that the SmFe0.7 Co0.3 O3 nanocrystalline is a promising CO gas sensor material.
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Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Electrical and CO-sensing properties of SmFe0.7 Co0.3 O3 perovskite oxide Ru Zhang a , Jifan Hu a,∗ , Ma Zhao a , Zhouxiang Han a , Jianying Wei a , Zhanlei Wu a , Hongwei Qin b , Kaiying Wang c a
Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, China School of Physics, Shandong University, Jinan 250100, China c Vestfold University College, Institute for Microsystem Technology, Raveien 197, Horten, 3184, Norway b
a r t i c l e
i n f o
Article history: Received 21 December 2009 Received in revised form 18 March 2010 Accepted 31 March 2010 Keywords: Perovskite Semiconductor gas sensor CO-sensing Sol–gel method
a b s t r a c t In this work, nanocrystalline powders of perovskite-type compounds SmFe1−x Cox O3 (x = 0, 0.3) were prepared by sol–gel method. The morphology, electrical and carbon monoxide (CO) sensing properties of the nanocrystalline powders were studied by X-ray diffraction technique, Scanning Electron Microscopy (SEM), and electrical resistance measurements from 40 to 400 ◦ C. The optimal value of CO-sensing response (S = Rg /Ra , here Ra = resistances in atmospheric air with humidity = 45%, Rg = resistances in CO within the background of atmospheric air with humidity = 45%) for nanocrystalline SmFe0.7 Co0.3 O3 -based sensor is about 8.43 at 150 ◦ C as the CO concentration is equal to 500 ppm. In addition, the SmFe0.7 Co0.3 O3 sample shows a good selectivity to the gas CO. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The detection of carbon monoxide (CO) has attracted great research interests in scientific and engineering fields such as materials, energy, environment, and human health [1–5]. It is well known that the gas CO is one of the most poisonous and harmful gases in atmosphere for human being. When molecules of the gas CO enter into human body, they combine with hemoglobin much faster than that with oxygen molecules and will remain in blood for longer time. The chemical combination of CO with hemoglobin deteriorates the transportation of oxygen in blood and will induce fatal problems to our life. The maximum tolerance level of CO concentration for human body is just 50 ppm for a time weighted average limit (TWA) in air [1]. Therefore, timely and accurate detection of CO gas in the surrounding environment is very important. From our knowledge, perovskite-type oxide materials are very promising candidates to detect gas CO, such as La0.68 Pb0.32 FeO3 [1], La0.8 Pb0.2 Fe0.8 M0.2 O3 (M = Co, Ni) [2,3], LaFeO3 [4] and La0.8 Sr0.2 Co0.5 Ni0.5 O3 [5]. Besides, the perovskite-type compound SmFeO3 prepared by thermal decomposition and subsequently screen-printing on alumina substrates shows excellent response to the gases, such as O3 , NO2 [6–8], CO [9,10]. Later, Itagaki et al. [11] investigated the sensing properties of the compounds SmFe1−x Cox O3 by substituting atoms Fe with atoms Co in the per-
∗ Corresponding author. Tel.: +86 531 8856 6143; fax: +86 531 8856 5167. E-mail address: [email protected] (J. Hu). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.03.087
ovskite structure. They discovered that the Co-added oxides show higher conductivity and better response-recovery to O3 and NO2 than that of compound SmFeO3 . In one of our previous work, we reported that the compounds SmFe1−x Cox O3 perovskite oxides presented proper ethanol sensing properties and the maximum response of the compound SmFe0.7 Co0.3 O3 can reach S = 80.78 with ethanol concentration = 300 ppm at 215 ◦ C [12,13]. In this paper, the morphology and electrical properties of the compounds SmFe1−x Cox O3 (x = 0, 0.3) have been studied in details. A CO-sensing sensor has been made based on the nanocrystalline materials SmFe0.7 Co0.3 O3 and its response properties are reported. 2. Experimental 2.1. Preparation of powders The powders of compounds SmFeO3 and SmFe0.7 Co0.3 O3 were prepared by sol–gel citrate method. Firstly, stoichiometric powders of oxide Sm2 O3 (99.95%) were dissolved in nitric acid to prepare Sm(NO3 )3 ·6H2 O, followed by the addition of Fe(NO3 )3 ·9H2 O (Analytical Reagent, A.R.), Co(NO3 )2 ·6H2 O (A.R.) and citric acid. Then the PEG (polyethylene glycol, with the help of the micellar solubilization of PEG, the solubility of the inorganic salts in the sols was greatly increased; as a result, the homogeneous sols could be prepared conveniently [14]) was added into the mixed solution under stirring at 70 ◦ C to obtain the sol and the sol was dried to form gel, then the gel pieces were ground to form fine powders. Finally, the fine powders were annealed at 800 ◦ C for 2 h in an oven. When
140
R. Zhang et al. / Materials Science and Engineering B 171 (2010) 139–143 Table 1 The lattice constants and unit cell volumes of the samples. Samples
SmFeO3 (standard) SmFeO3 SmFe0.7 Co0.3 O3
Fig. 1. The XRD patterns of SmFcO3 and SmFc0.7 Co0.3 O3 powders.
the powders temperature dropped to room temperature, ground the powders again. The structures of the powders were characterized by X-ray diffraction (XRD, Bruker D8 Advance) with Cu K␣ radiation. The morphology of the powders was observed by Scanning Electron Microscopy (SEM, Hitachi S2500). The specific surface area of the powders was measured by BET surface area tester (3H2000BET). 2.2. Fabrication of sensors The final obtained powders of compounds SmFeO3 and SmFe0.7 Co0.3 O3 were mixed respectively with terpineol (C10 H18 O, molecular weight 154.25) and ground into pastes. These pastes were packed into Al2 O3 tubes with electrodes mounted at both ends, then the tubes were calcined at 400 ◦ C for 2 h to enhance the stability of sensors. The inner and outer diameters of the ceramic tube are 4 mm and 6 mm, respectively with 10 mm in length.
Volume (Å3 )
Lattice constant (Å) a
b
c
5.400 5.3962 5.3951
5.597 5.5953 5.5887
7.711 7.7352 7.6941
233.0557 233.5517 231.9894
LaCoO3 and LaFe1−x Cox O3 compounds, the valence of Fe and Co ions is 3+. When oxygen vacancies occur in nanocrystalline SmFe0.7 Co0.3 O3 (the exact structure formula of the samples should be SmFe0.7 Co0.3 O3−ı ), Co2+ may occur in order to maintain the charge neutrality. The average particle sizes D were calculated from X-ray patterns based on the Scherrer’s equation. The D values were about 34.24 and 25.14 nm for SmFeO3 and SmFe0.7 Co0.3 O3 powders, respectively. The result of the D decrease indicates that the doping with element cobalt in SmFeO3 may prevent the grain growth during high temperature treatment. The BET specific surface areas of the nanocrystalline SmFeO3 and SmFe0.7 Co0.3 O3 are 2.865 m2 /g and 5.407 m2 /g. At the same time, surface morphology of the compounds was examined by Scanning Electron Microscopy and given in Fig. 2. The grain sizes of the nanocrystalline SmFeO3 and SmFe0.7 Co0.3 O3 are 47 nm and 36 nm. Since the compound SmFe0.7 Co0.3 O3 has a smaller grain size as compared with the compound SmFeO3 , it has a bigger total surface area while they have same weights. The relatively large specific surface area is an advantage for gas sensing application.
2.3. Measurement of sensors A conventional circuit in which the sensor was connected with an external resistor in series was used to measure the sensor resistance. The conductances of the sensors were measured in atmospheric air with humidity = 45%. A suitable amount of tested gas (i.e. CO) was injected into a test chamber having atmospheric air with humidity = 45%. The gas sensing properties of the sensors were measured in a temperature range of 40–400 ◦ C. The tested CO concentrations were 100, 300, 500, 700 and 900 ppm, respectively. The responses of sensors were defined as S = Rg /Ra [15], where Ra and Rg were the resistances measured under atmospheric air with humidity = 45% and a tested gas (within the background of atmospheric air with humidity = 45%), respectively. 3. Results and discussion 3.1. Phase composition The X-ray diffraction patterns of the powders SmFeO3 and SmFe0.7 Co0.3 O3 are shown in Fig. 1. It can be seen that all the diffraction peaks can be indexed to the compound SmFeO3 (JCPDS: 74-1474) with an orthorhombic structure. The lattice parameters of the samples calculated from XRD patterns are listed in Table 1, which also includes the experimental and standard lattice parameters. The experimental data shows that the lattice parameters and unit cell volume of the compound SmFe0.7 Co0.3 O3 are smaller than those of compound SmFeO3 , since the radius of Co3+ (63 pm) is smaller than that of that of Fe3+ (64 pm). Xray photoelectron spectroscopy analysis indicated that in LaFeO3 ,
Fig. 2. SEM images of SmFeO3 (a) and SmFe0.7 Co0.3 O3 (b) nanoparticles calcined at 800 ◦ C.
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Fig. 3. Temperature dependence of the conductance of SmFeO3 and SmFe0.7 Co0.3 O3 in atmospheric air with humidity = 45%. Fig. 4. Ln G of the sensors in atmospheric air with humidity = 45% and 500 ppm CO (in atmospheric air with humidity = 45%) as a function of the inverse of temperature.
3.2. Electrical properties Fig. 3 presents the temperature dependence of conductance of the compounds SmFeO3 and SmFe0.7 Co0.3 O3 based sensors from 40 to 400 ◦ C in atmospheric air with humidity = 45%. It can be seen that the conductance values of the two samples increase with increasing temperature and the SmFe0.7 Co0.3 O3 -based sensor shows a larger conductance in the whole tested temperature range as compared with the SmFeO3 -based one. As we know, the compound SmFeO3 is a p-type semi-conductive material, its charge carriers are holes x ]: (h• ) which are produced by Sm3+ vacancy defects [VSm
x VSm → VSm + 3h•
(1)
In addition to Co3+ , when Co2+ also occurs in nanocrystalline SmFe0.7 Co0.3 O3−ı , amount of holes will increase due to the ionization of [CoxFe ] for the partial substitution of Fe3+ by Co2+ : CoxFe → CoFe + h•
(2)
which increases the conductivity of p-type SmFe0.7 Co0.3 O3−ı . Meanwhile, similar to the double exchange interaction between Mn3+ and Mn4+ ions in alkali–metal ion doped LaMnO3 [16], the possible double exchange interaction between Co2+ and Co3+ ions in nanocrystalline SmFe0.7 Co0.3 O3−ı may also enhance the conductivity. Of course, the double exchange interaction should not occur in ideal SmFe0.7 Co0.3 O3 without oxygen vacancies, where Co2+ is absent. On the other hand, the difference of the conductance may have a relationship with the grain boundary resistance, which is a major factor for the conduction of a semiconductor [17]. Another advantage of the SmFe0.7 Co0.3 O3 -based sensor is that it needs less activation energy. The activation energy can be calculated by the Arrhenius plot of the conductance [18]: G = G0 exp(−Ea /kT )
compound SmFe0.7 Co0.3 O3 upon exposure to CO could be understood through CO chemical reduction to the p-type semiconductor material SmFe0.7 Co0.3 O3 . 3.3. The CO sensing properties Fig. 5 shows the temperature dependence of the response of SmFeO3 and SmFe0.7 Co0.3 O3 -based sensors in CO concentration 500 ppm. For the SmFeO3 and SmFe0.7 Co0.3 O3 , the optimal responses are 3.12 at 220 ◦ C and 8.43 at 150 ◦ C. In addition, there are different tendencies between the response curves of the SmFeO3 and SmFe0.7 Co0.3 O3 -based sensors. With increasing operating temperature from 40 to 400 ◦ C, the response value of SmFe0.7 Co0.3 O3 increased gradually from 1.5 to 8.2 and then decreased to 1. However, the response of the SmFeO3 does not decrease all the time after reaching the optimal response, but after a slight decrease, it starts to increase again. Therefore, the optimal operating temperature for the SmFeO3 may be more than 400 ◦ C. The SmFe0.7 Co0.3 O3 -based sensor shows a higher response than that of the SmFeO3 and it may be related to its smaller particle size and larger specific surface area. A larger surface area allows more of the On− (ads) adsorption, so
(3)
where G is the conductance, k is the Boltzmann’s constant, T is the absolute temperature and Ea is the activation energy. Plotting ln G against 1/T: ln G = ln G0 −
Ea kT
(4)
Then Ea of the two compounds SmFeO3 and SmFe0.7 Co0.3 O3 were obtained from Fig. 4. Their activation energies are 0.550 eV and 0.475 eV for the compounds SmFeO3 and SmFe0.7 Co0.3 O3 in atmospheric air with humidity = 45%. The activation energy of the compound SmFe0.7 Co0.3 O3 is increased to 0.548 eV while CO concentration increases to 500 ppm. This active energy increase of the
Fig. 5. Temperature dependence of the response of SmFcO3 and SmFe0.7 CO0.3 O3 based sensors to 500 ppm CO.
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Fig. 6. Temperature dependence of the responses of SmFe0.7 CO0.3 O3 -based sensor to CO gas under different concentrations. (The insert figure shows the relationship between CO gas concentration and the response of SmFe0.7 CO0.3 O3 -based sensor at 150 ◦ C.)
Fig. 7. Log resistance vs. log CO concentration (in atmospheric air with humidity = 45%) of SmFe0.7 CO0.3 O3 -based sensor at 150 ◦ C. The symbol of black square represents the experimental point.
that it can accelerate the reaction of CO and On− [19]: CO + On−
(ads) → CO2 + ne−
(5)
Another explanation is that the appearance of oxygen vacancies. Due to the loss of metal atoms at the crank pints of cells in the calcined process at high temperature, oxygen vacancies should form in order to maintain the charge balance. Therefore, the oxygen absorbed increases [20]. So, the SmFe0.7 Co0.3 O3 -based sensor which parts of Fe3+ are replaced by Co2+ at the B site creates a relatively larger amount of oxygen adsorption sites and has the higher response. Fig. 6 shows the curves of response of the SmFe0.7 Co0.3 O3 -based sensor to CO at different operating temperatures. It can be seen obviously that the sensor shows high response to CO with a wide temperature range of 90–210 ◦ C and the optimal operating temperature is about 150◦ C. The dependence of the response on the concentration of CO gas at 150 ◦ C is shown in the insert of Fig. 6. The responses of 1.54, 5.14, 8.43, 10.92 and 12.54 were obtained in the different concentrations of CO (100, 300, 500, 700 and 900 ppm, respectively). It can be observed clearly that the responses and CO gas concentration presents a good linear relationship. Fig. 7 shows the logarithmic dependence of the resistance of the SmFe0.7 Co0.3 O3 -based sensor on the CO concentration at 150 ◦ C. To illustrate the sensor with relatively large dependence on the concentration of CO gas, we calculated the value of dependence according to an empirical formula [21–23]: ˛ R = KCCO
(6)
where R is the resistance of the sensor, CCO is the concentration of CO, K and ˛ are the constants. As known from the literature, the ˛ value of the SnO2 sensor is probably between 1/6 and 1/2 [24]. While, the ˛ value of SmFe0.7 Co0.3 O3 -based sensor obtained from Fig. 6 is about 0.97. It is not only larger than that of SnO2 but also larger than those of other perovskite oxides, such as La0.8 Pb0.2 Fe0.8 Co0.2 O3 and La0.8 Pb0.2 Fe0.8 Ni0.2 O3 [2,3]. It means that the resistance of the SmFe0.7 Co0.3 O3 -based sensor is strongly dependent on the CO gas concentration. Generally speaking, there will have second gas or more in the surrounding circumstances when sensor detects a target gas. The second gas may have a great impact on the accurate detection of the target gas. Therefore, the selectivity of gas sensors is very important. In this paper, we studied the selectivity of the
Fig. 8. The selectivity of SmFe0.7 CO0.3 O3 -based sensor to CO (in atmospheric air with humidity = 45%).
SmFe0.7 Co0.3 O3 -based sensors to CO at 150 ◦ C. As temperature is at 150 ◦ C and all the gas concentration are 500 ppm, it can be seen in Fig. 8, the S values of CO, diethyl ether, ammonia, cyclohexane and toluene are 8.43, 1.08, 1.42, 1.28 and 1.39. Obviously, the response of the SmFe0.7 Co0.3 O3 -based sensors to CO is much higher than those to other gases. Therefore, the SmFe0.7 Co0.3 O3 based sensor has a good selectivity to CO. Furthermore, response and recovery times at 150 ◦ C for SmFe0.7 Co0.3 O3 -based sensor to 500 ppm CO were measured as 17 s and 33 s, respectively. However, the gas ethanol has significant side-effect to the CO sensing application since the response of the compound SmFe0.7 Co0.3 O3 is relatively high at 215 ◦ C, the detailed results are listed in Table 2. As a gas sensor, its stability is another important property. Stability determines a sensor whether can be reused. In order to study the stability of SmFe0.7 Co0.3 O3 -based sensor, we measured its resistance variation for 144 h at 150 ◦ C. As is shown in Fig. 9, the minimum value of resistance is 5.07 k; maximum value is 10.77 k, so the fluctuation in resistance is only 5.7 k in the whole measuring range. The results show that the sensor has good stability and repeatability at 150 ◦ C.
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Table 2 The comparison of gas sensing properties of sensors based on SmFe0.7 Co0.3 O3 nanocrystalline powders prepared at different temperatures. Sensors
Annealing temperature (◦ C)
Grain size (nm)
Tested gas
Highest sensitivity
Optimal operating temperature (◦ C)
Response, recovery time (s)
Literature
SmFe0.7 Co0.3 O3 SmFe0.7 Co0.3 O3
850 800
34.39 25.14
Ethanol (300 ppm) CO (500 ppm)
80.78 8.43
215 150
11, 17 17, 33
[12,13] This text
Acknowledgement This work was supported by National Natural Science Foundation of China (Nos: 50872074 and 50872069). References
Fig. 9. The stability of SmFe0.7 CO0.3 O3 -based sensor at 150 ◦ C.
4. Conclusion The perovskite oxides SmFeO3 and SmFe0.7 Co0.3 O3 powders were prepared by a sol–gel method. Their structures, electrical and CO-sensing properties were investigated in detail. The two samples are single phase with orthorhombic structure. The SmFe0.7 Co0.3 O3 sample has a smaller unit cell volume and grain size as compared with SmFeO3 . On the other hand, its electrical conductance is higher and activation energy is smaller than SmFeO3 in atmospheric air with humidity = 45%. The SmFe0.7 Co0.3 O3 -based sensor has a wide operating temperature range from 90 to 210 ◦ C and its optimal response could reach to 8.43 upon exposure to the gas CO with concentration = 500 ppm. The sensor has a stronger dependence on the concentration of CO than that of SnO2 based sensor or other oxides. Moreover, the sensor shows good selectivity and stability. All the conclusions indicated that the SmFe0.7 Co0.3 O3 nanocrystalline is a promising CO gas sensor material.
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