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Synthesis and characterization of Pd-doped α-Fe2O3 H2S sensor with low power consumption
Materials Science and Engineering B 140 (2007) 98–102
Synthesis and characterization of Pd-doped ␣-Fe2O3 H2S sensor with low power consumption Yan Wang, Fanhong Kong, Baolin Zhu, Shurong Wang, Shihua Wu ∗ , Weiping Huang Department of Chemistry, Nankai University, Tianjin 300071, China Received 30 January 2007; received in revised form 28 March 2007; accepted 15 April 2007
Abstract Pd-doped ␣-Fe2 O3 nanoparticles were synthesized by chemical coprecipitation method and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The gas sensing properties of undoped and Pd-doped ␣-Fe2 O3 sensors were investigated. Compared with the undoped one, the doped sensors exhibited higher response, better selectivity, and faster response/recovery to H2 S. The operating temperature of ␣-Fe2 O3 to H2 S is decreased after the addition of Pd, which result in the relative low power consumption in H2 S detection. Among all the doped sensors, the sensor of 1.5 wt% Pd/␣-Fe2 O3 showed the largest response (128.3) to 100 ppm H2 S at 160 ◦ C. © 2007 Elsevier B.V. All rights reserved. Keywords: Gas sensor; Pd-doped ␣-Fe2 O3 ; H2 S; Low power consumption
1. Introduction The increasing concern on environmental protection and human health has generated great interests in efficient gas detection [1–3]. ␣-Fe2 O3 is an n-type metal oxide semiconductor, and has been used as gas sensing material since the 1980s of the last century [4,5]. There have been many reports about good response and selectivity of ␣-Fe2 O3 sensors to combustible gases and organic vapors in recent years, such as ethanol, acetone, gasoline and LPG, etc. [6,7], while their gas sensing properties to H2 S have been seldom reported until now. Recently, Zhang et al. found that ␣-Fe2 O3 exhibited sensitivity to H2 S based on the catalytic chemiluminescence at 360 ◦ C [8]. Wang et al. reported the ␣-Fe2 O3 sensors synthesized by microwave hydrolysis had a high sensitivity at 300 ◦ C [9]. However, their application is limited by the high operating temperature. As a consequence, it is important to design new type of low power consumption H2 S sensor. Noble metal doping is an effective approach to improve the gas sensing properties of sensors. For instance, Kobayashi et al. developed CO sensor based on Au-doped ␣-Fe2 O3 [10]. Shen et al. found that the response of ␣-Fe2 O3 sensor to CO was greatly improved after it was doped with PdO [11]. In this paper, Pd∗
Corresponding author. Tel.: +86 22 2350 5896; fax: +86 22 2350 2458. E-mail address: [email protected] (S. Wu).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.04.004
doped ␣-Fe2 O3 nanoparticles were prepared by coprecipitation method. The gas sensing properties of the sensors were also investigated. 2. Experimental All the reagents are of analytical grade and used as purchased. Pd/␣-Fe2 O3 powders were prepared by a coprecipitation method [12]. A small quantity of polyglycol was added to an aqueous solution of PdCl2 (0.25, 0.5, 1.0, 1.5, 2.0 and 3.0 wt%) and Fe(NO3 )3 ·9H2 O. The aqueous mixture was then added dropwise to an aqueous solution of Na2 CO3 under vigorous stirring at 80 ◦ C. The pH of the solution was adjusted by diluted Na2 CO3 aqueous solution in the reaction process. After stirring for 1 h, a solid precipitate was formed and kept digesting overnight at room temperature. Then the precipitate was washed with deionized water, dried at 80 ◦ C and calcined at 400 ◦ C for an hour, a series of 0.25, 0.5, 1.0, 1.5, 2.0 and 3.0 wt% Pd-doped ␣-Fe2 O3 powders were obtained. X-ray diffraction (XRD) analyses were performed on D/MAX-RAX diffractometer operating at 40 kV and 100 mA, ˚ scanning range 2u: using Cu K␣ radiation (λ = 1.5418 A, 20–758). Diffraction peaks of crystalline phases were compared with those of standard compounds reported in the JCPDS Data File. Transmission electron microscopy (TEM) was carried out on a Philips–T20ST electron microscope, operating at 200 kV.
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Fig. 1. Schematic diagram of the Pd/␣-Fe2 O3 thick film sensor: (1) Pt wire; (2) Ni–Cr heated wire; (3) Al2 O3 tube; (4) Pd/␣-Fe2 O3 thick film; (5) Au electrode.
The gas sensing behavior was investigated by using the commercial gas sensing measurement system of HW-30A from Henan Hanwei Electronical Technology Co. Ltd. An alumina substrate tube with 4 mm length was used for the heater and sensing base. The schematic diagram of a typical gas sensor is shown in Fig. 1. A small Ni–Cr alloy coil was placed through the tube to supply the operating temperatures from 100 to 500 ◦ C. Electrical contacts were made with two platinum wires attached to each gold electrode. The Pd/␣-Fe2 O3 powder was mixed with terpineol to form a paste. Then the paste was coated onto the outside surface of the alumina tube. In order to improve their stability and repeatability, the gas sensors were sintered at 300 ◦ C for 10 days in air. Gas sensing properties of the sensors were tested in a glass chamber with a volume of 15 L. The test gases were injected into the closed chamber by a microinjector. Gas sensitivity of the side-heated gas sensors was measured under a steady-state condition. The schematic representation and the measuring principle of the gas sensor are shown in Fig. 2. The operating voltage (Vh) was supplied to either of the coils for heating the sensors and the circuit voltage (Vc = 10 V) was supplied across the sensors and the load resistor (RL = 1 M) connected in series. The signal voltage across the load, which changed with sort and concentration of gas, was measured. In the gas sensitivity measurement, a given amount of sample gases were injected into a closed chamber by a microinjector and mixed by a fan for 20 s (liquids were
Fig. 2. Graphic of testing principle.
Fig. 3. XRD pattern of 1.5 wt% Pd/␣-Fe2 O3 .
firstly evaporated and then mixed by a fan for 20 s). The gas response S is defined as the ratio Ra /Rg , where Ra and Rg are the resistances measured in air and in a test gas, respectively. 3. Result and discussion 3.1. Material characterization Fig. 3 shows the XRD pattern of ␣-Fe2 O3 doped with 1.5 wt% Pd additions. The diffraction pattern of ␣-Fe2 O3 (1.5 wt% Pd) matched perfectly with the standard ␣-Fe2 O3 reflections (JCPDS No. 33-664). However, no obvious Pd peaks was observed, which may be due to high dispersion of Pd particles. The sharp peaks suggest that the crystal of ␣-Fe2 O3 is perfect. The mean size of the crystals is around 40 nm, calculated by the Deby–Scherrer equation. TEM image of 1.5 wt% Pd/␣-Fe2 O3 is shown in Fig. 4. It can be seen that the morphology of the particles is spherical.
Fig. 4. TEM image of 1.5 wt% Pd/␣-Fe2 O3 .
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Fig. 5. Responses of undoped ␣-Fe2 O3 and Pd/␣-Fe2 O3 to 100 ppm H2 S.
Fig. 6. Responses of 1.5 wt% Pd-doped ␣-Fe2 O3 to various gases at different operating temperatures.
The size of the particles is 30–50 nm in diameter, which agrees with the results from XRD pattern.
As has been generally reported, ␣-Fe2 O3 is a typical n-type semiconductor, and its gas-sensing mechanism belongs to the surface-controlled type. The change of resistance is dependent on the species and chemisorbed oxygen on the surface. The oxygen absorbed on the surface of an ␣-Fe2 O3 sensor cause the electron depletion, consequently the resistance of the sensor increases. The process can be expressed in the following reactions:
3.2. Gas sensing properties The gas sensing properties of undoped ␣-Fe2 O3 and Pddoped ␣-Fe2 O3 were studied. It is well known that the gas sensitivity is greatly influenced by the operating temperature and the amounts of additives. In order to determine the optimum operating temperature and additive amount, responses of all the sensors to 100 ppm H2 S gas at different operating temperatures were examined. The results are shown in Fig. 5. It can be seen that the response of the sensors to H2 S varies with not only the amount of Pd but also the operating temperature. Each curve reveals a maximum at an optimum operating temperature. The undoped sensor has the maximum gas response at 200 ◦ C, while all the doped sensors have the maximum gas response about 160 ◦ C. The operating temperature of Pd doped sensors is lower, compared with previous reports [13–15] about H2 S sensor. The lower operating temperature would result in the lower energy consumption, which may be attributed to the presence of Pd. Furthermore, all the doped sensors exhibit much higher response than the undoped one. Especially, the sensor doped with 1.5 wt% Pd exhibits the largest response to H2 S at 160 ◦ C. Selectivity is an important parameter in gas sensing, and the sensor has to have rather high selectivity for its application. As far as we know from the recent researches, ␣-Fe2 O3 sensor also responds to other gases such as ethanol and acetone. Therefore, we examined the responses of the 1.5 wt% Pd-doped ␣-Fe2 O3 sensor to other seven gases of 1000 ppm at different operating temperatures. From Fig. 6, it can be observed that the sensor exhibits the largest response to H2 S, moderate responses to ethanol and acetone, and negligible responses to n-hexane, NH3 , H2 and CO. On the other hand, the optimum operating temperature to ethanol and acetone is 200 ◦ C, which is higher than that to H2 S. The selectivity to H2 S is higher when the operating temperature is about 160 ◦ C. As a result, it is easier to detect H2 S at lower operating temperatures.
O2 (gas) → O2 (ads)
(1)
O2 (ads) + e− → O2 − (ads)
(2)
O2 − (ads) + e− → 2O− (ads)
(3)
O− (ads) + e− → O2− (ads)
(4)
With introduction of H2 S gas, it would be oxidized by these chemisorbed oxygen species (O2 − , O− , O2− ) on the surface of the sensor. During this reaction, the electrons back into the semiconductor, resulting in a decrease in resistance of the sensor. When the sensor is exposed in air again, the gases are desorbed as H2 O and SO2 . This reaction process may be as Eq. (5). H2 S + 3O2− → H2 O + SO2 + 6e−
(5)
The effect of noble metal addition on the gas sensing properties of metal oxide has been widely studied [16–19]. This influence has formally been classified as chemical or electronic according to two basic sensitization mechanisms. The two mechanisms are so-called spill-over and Fermi-level effect, respectively. Pd is typically considered to be related with the electronic one [20–22]. In the electronic mechanism, the reaction with gas molecules takes place on the dopants surface. These dopants change the electrostatic potential due to their different electron affinities, which gives rise to a variation of the surface barrier height. This results in the corresponding resistance change of the base semiconductor. On the other hand, Pd dopants, as a catalyst, enhance the adsorption of gas molecules and accelerate the electron exchange between the sensor and the
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Fig. 7. Comparison of response of 1.5 wt% Pd-doped ␣-Fe2 O3 to various gases.
test gas. All these factors together contribute to the improvement of gas sensing properties of the Pd-doped ␣-Fe2 O3 sensors. The gas response of the undoped and 1.5 wt% Pd-doped ␣-Fe2 O3 sensors to various gases at their optimum operating temperatures are compared in Fig. 7. The responses of 1.5 wt% Pd-doped ␣-Fe2 O3 sensor to all of these gases are much higher than that of the undoped one, especially to H2 S. Fig. 8 shows the relationship between the response of the 1.5 wt% Pd-doped ␣-Fe2 O3 sensor and the H2 S concentration (10–200 ppm) at 160 ◦ C. It can be seen that the response increases with the H2 S concentration, and the concentration dependence presents good linearity, which suggests that the Pd/␣-Fe2 O3 could meet the applications in H2 S sensing. Furthermore, the sensor still shows excellent response to low concentration H2 S, and its response to 10 ppm H2 S is 46.6. Response and recovery times are the basic parameters of gas sensors, which are defined as the time required to reach 90% of the final resistance. Fig. 9 shows the response–recovery characteristics of 1.5 wt% Pd-doped ␣-Fe2 O3 to H2 S at different concentrations. As can be seen from the curve, the sensor responds very rapidly after introduction of H2 S and recovers
Fig. 9. Response–recovery characteristics of 1.5 wt% Pd-doped ␣-Fe2 O3 to H2 S of different concentrations.
Fig. 10. Long-time stability of 1.5 wt% Pd-doped ␣-Fe2 O3 sensor to 100 ppm H2 S.
immediately when it is exposed to air. The result indicates that the sensor can meet the practical application in H2 S detection. The stability of the 1.5 wt% Pd-doped ␣-Fe2 O3 sensor was measured at a level of 100 ppm H2 S after ageing for 60 days. The results are shown in Fig. 10, from which it is seen that the sensor presented nearly constant response to H2 S even after 2 months. We believe that Pd/␣-Fe2 O3 sensors have promising applications in H2 S sensing in the future. 4. Conclusions
Fig. 8. Gas response vs. H2 S concentration of 1.5 wt% Pd-doped ␣-Fe2 O3 .
In summary, the Pd-doped ␣-Fe2 O3 sensors have been prepared by chemical coprecipitation method. Compared with the undoped one, the doped sensors present much higher sensitivity, better selectivity and rather lower optimum operating temperature to H2 S than the undoped one. The optimum doping amount is 1.5 wt% and the optimum operating temperature is 160 ◦ C. The doped sensors also present long-term stability and short response/recovery time. Obtained results indicate that the Pd-
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doped ␣-Fe2 O3 sensor should be a type of promising H2 S sensor at relatively low working temperature. Acknowledgement We gratefully appreciate the financial support of the 973 program of China (No. 2005Cb623607). References [1] [2] [3] [4] [5] [6]
N. Yamazoe, N. Miura, Sens. Actuators B 20 (1994) 95. W. Gopel, Sens. Actuators B 18 (1994) 1. O.K. Tan, W. Cao, Y. Hu, W. Zhu, Ceram. Int. 30 (2004) 1127. Y. Nakatani, M. Matsuoka, Jpn. J. Appl. Phys. 21 (1982) 1758. W.Y. Chung, Thin Solid Film 200 (1991) 329. L.H. Huo, Q. Li, H. Zhao, L.J. Yu, S. Gao, J.G. Zhao, Sens. Actuators B 107 (2005) 915. [7] J. Chen, L.N. Xu, W.Y. Li, Adv. Mater. 17 (2005) 582. [8] Z.Y. Zhang, H.J. Jiang, Z. Xing, X.R. Zhang, Sens. Actuators B 102 (2004) 155.
[9] H.X. Wang, D.M. Zhu, J.Q. Xu, Chin. J. Sens. Actuators B 19 (2006) 947. [10] N. Funazaki, A. Hemmi, S. Ito, Y. Asano, S. Yamashita, T. Kobayashi, M. Haruta, Sens. Actuators B 14 (1993) 536. [11] F.S. Shen, J.D. Lin, et al., Appl. Chem. 18 (2001) 295. [12] D. Andreeva, V. Idakiev, T. Tabakova, A. Andreev, R. Giovanoli, Appl. Catal. A 134 (1996) 275. [13] G.N. Caudhari, D.R. Bambole, A.B. Bodade, P.R. Padole, J. Mater. Sci. 41 (2006) 4860. [14] G.H. Jian, L.A. Patil, M.S. Wagh, D.R. Patil, S.A. Patil, D.P. Amalnerkar, Sens. Actuators B 117 (2006) 159. [15] J.Q. Xu, X.H. Wang, J.N. Shen, Sens. Actuators B 115 (2006) 642. [16] J. Mizsei, P. Sipila, V. Lantto, Sens. Actuators B 47 (1998) 139. [17] Y.L. Liu, H. Wang, Y. Yang, Z.M. Liu, H.F. Yang, G.L. Shen, R.Q. Yu, Sens. Actuators B 102 (2004) 148. [18] T. Maosong, G.R. Dai, D.S. Gao, Appl. Surf. Sci. 171 (2001) 226. [19] T. Ishihara, H. Fujita, Y. Takita, Sens. Actuators B 52 (1998) 100. [20] N. Yamazoe, Sens. Actuators B 5 (1991) 7. [21] A. Cabot, J. Arbiol, J.R. Morante, U. Weimar, N. Barsan, W. Gopel, Sens. Actuators B 70 (2000) 87. [22] M. Schweizer-Berberich, J.G. Zheng, U. Weimar, W. Gopel, N. Barsan, E. Pentia, A. Tamescu, Sens. Actuators B 31 (1996) 71.
Synthesis and characterization of Pd-doped ␣-Fe2O3 H2S sensor with low power consumption Yan Wang, Fanhong Kong, Baolin Zhu, Shurong Wang, Shihua Wu ∗ , Weiping Huang Department of Chemistry, Nankai University, Tianjin 300071, China Received 30 January 2007; received in revised form 28 March 2007; accepted 15 April 2007
Abstract Pd-doped ␣-Fe2 O3 nanoparticles were synthesized by chemical coprecipitation method and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The gas sensing properties of undoped and Pd-doped ␣-Fe2 O3 sensors were investigated. Compared with the undoped one, the doped sensors exhibited higher response, better selectivity, and faster response/recovery to H2 S. The operating temperature of ␣-Fe2 O3 to H2 S is decreased after the addition of Pd, which result in the relative low power consumption in H2 S detection. Among all the doped sensors, the sensor of 1.5 wt% Pd/␣-Fe2 O3 showed the largest response (128.3) to 100 ppm H2 S at 160 ◦ C. © 2007 Elsevier B.V. All rights reserved. Keywords: Gas sensor; Pd-doped ␣-Fe2 O3 ; H2 S; Low power consumption
1. Introduction The increasing concern on environmental protection and human health has generated great interests in efficient gas detection [1–3]. ␣-Fe2 O3 is an n-type metal oxide semiconductor, and has been used as gas sensing material since the 1980s of the last century [4,5]. There have been many reports about good response and selectivity of ␣-Fe2 O3 sensors to combustible gases and organic vapors in recent years, such as ethanol, acetone, gasoline and LPG, etc. [6,7], while their gas sensing properties to H2 S have been seldom reported until now. Recently, Zhang et al. found that ␣-Fe2 O3 exhibited sensitivity to H2 S based on the catalytic chemiluminescence at 360 ◦ C [8]. Wang et al. reported the ␣-Fe2 O3 sensors synthesized by microwave hydrolysis had a high sensitivity at 300 ◦ C [9]. However, their application is limited by the high operating temperature. As a consequence, it is important to design new type of low power consumption H2 S sensor. Noble metal doping is an effective approach to improve the gas sensing properties of sensors. For instance, Kobayashi et al. developed CO sensor based on Au-doped ␣-Fe2 O3 [10]. Shen et al. found that the response of ␣-Fe2 O3 sensor to CO was greatly improved after it was doped with PdO [11]. In this paper, Pd∗
Corresponding author. Tel.: +86 22 2350 5896; fax: +86 22 2350 2458. E-mail address: [email protected] (S. Wu).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.04.004
doped ␣-Fe2 O3 nanoparticles were prepared by coprecipitation method. The gas sensing properties of the sensors were also investigated. 2. Experimental All the reagents are of analytical grade and used as purchased. Pd/␣-Fe2 O3 powders were prepared by a coprecipitation method [12]. A small quantity of polyglycol was added to an aqueous solution of PdCl2 (0.25, 0.5, 1.0, 1.5, 2.0 and 3.0 wt%) and Fe(NO3 )3 ·9H2 O. The aqueous mixture was then added dropwise to an aqueous solution of Na2 CO3 under vigorous stirring at 80 ◦ C. The pH of the solution was adjusted by diluted Na2 CO3 aqueous solution in the reaction process. After stirring for 1 h, a solid precipitate was formed and kept digesting overnight at room temperature. Then the precipitate was washed with deionized water, dried at 80 ◦ C and calcined at 400 ◦ C for an hour, a series of 0.25, 0.5, 1.0, 1.5, 2.0 and 3.0 wt% Pd-doped ␣-Fe2 O3 powders were obtained. X-ray diffraction (XRD) analyses were performed on D/MAX-RAX diffractometer operating at 40 kV and 100 mA, ˚ scanning range 2u: using Cu K␣ radiation (λ = 1.5418 A, 20–758). Diffraction peaks of crystalline phases were compared with those of standard compounds reported in the JCPDS Data File. Transmission electron microscopy (TEM) was carried out on a Philips–T20ST electron microscope, operating at 200 kV.
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Fig. 1. Schematic diagram of the Pd/␣-Fe2 O3 thick film sensor: (1) Pt wire; (2) Ni–Cr heated wire; (3) Al2 O3 tube; (4) Pd/␣-Fe2 O3 thick film; (5) Au electrode.
The gas sensing behavior was investigated by using the commercial gas sensing measurement system of HW-30A from Henan Hanwei Electronical Technology Co. Ltd. An alumina substrate tube with 4 mm length was used for the heater and sensing base. The schematic diagram of a typical gas sensor is shown in Fig. 1. A small Ni–Cr alloy coil was placed through the tube to supply the operating temperatures from 100 to 500 ◦ C. Electrical contacts were made with two platinum wires attached to each gold electrode. The Pd/␣-Fe2 O3 powder was mixed with terpineol to form a paste. Then the paste was coated onto the outside surface of the alumina tube. In order to improve their stability and repeatability, the gas sensors were sintered at 300 ◦ C for 10 days in air. Gas sensing properties of the sensors were tested in a glass chamber with a volume of 15 L. The test gases were injected into the closed chamber by a microinjector. Gas sensitivity of the side-heated gas sensors was measured under a steady-state condition. The schematic representation and the measuring principle of the gas sensor are shown in Fig. 2. The operating voltage (Vh) was supplied to either of the coils for heating the sensors and the circuit voltage (Vc = 10 V) was supplied across the sensors and the load resistor (RL = 1 M) connected in series. The signal voltage across the load, which changed with sort and concentration of gas, was measured. In the gas sensitivity measurement, a given amount of sample gases were injected into a closed chamber by a microinjector and mixed by a fan for 20 s (liquids were
Fig. 2. Graphic of testing principle.
Fig. 3. XRD pattern of 1.5 wt% Pd/␣-Fe2 O3 .
firstly evaporated and then mixed by a fan for 20 s). The gas response S is defined as the ratio Ra /Rg , where Ra and Rg are the resistances measured in air and in a test gas, respectively. 3. Result and discussion 3.1. Material characterization Fig. 3 shows the XRD pattern of ␣-Fe2 O3 doped with 1.5 wt% Pd additions. The diffraction pattern of ␣-Fe2 O3 (1.5 wt% Pd) matched perfectly with the standard ␣-Fe2 O3 reflections (JCPDS No. 33-664). However, no obvious Pd peaks was observed, which may be due to high dispersion of Pd particles. The sharp peaks suggest that the crystal of ␣-Fe2 O3 is perfect. The mean size of the crystals is around 40 nm, calculated by the Deby–Scherrer equation. TEM image of 1.5 wt% Pd/␣-Fe2 O3 is shown in Fig. 4. It can be seen that the morphology of the particles is spherical.
Fig. 4. TEM image of 1.5 wt% Pd/␣-Fe2 O3 .
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Fig. 5. Responses of undoped ␣-Fe2 O3 and Pd/␣-Fe2 O3 to 100 ppm H2 S.
Fig. 6. Responses of 1.5 wt% Pd-doped ␣-Fe2 O3 to various gases at different operating temperatures.
The size of the particles is 30–50 nm in diameter, which agrees with the results from XRD pattern.
As has been generally reported, ␣-Fe2 O3 is a typical n-type semiconductor, and its gas-sensing mechanism belongs to the surface-controlled type. The change of resistance is dependent on the species and chemisorbed oxygen on the surface. The oxygen absorbed on the surface of an ␣-Fe2 O3 sensor cause the electron depletion, consequently the resistance of the sensor increases. The process can be expressed in the following reactions:
3.2. Gas sensing properties The gas sensing properties of undoped ␣-Fe2 O3 and Pddoped ␣-Fe2 O3 were studied. It is well known that the gas sensitivity is greatly influenced by the operating temperature and the amounts of additives. In order to determine the optimum operating temperature and additive amount, responses of all the sensors to 100 ppm H2 S gas at different operating temperatures were examined. The results are shown in Fig. 5. It can be seen that the response of the sensors to H2 S varies with not only the amount of Pd but also the operating temperature. Each curve reveals a maximum at an optimum operating temperature. The undoped sensor has the maximum gas response at 200 ◦ C, while all the doped sensors have the maximum gas response about 160 ◦ C. The operating temperature of Pd doped sensors is lower, compared with previous reports [13–15] about H2 S sensor. The lower operating temperature would result in the lower energy consumption, which may be attributed to the presence of Pd. Furthermore, all the doped sensors exhibit much higher response than the undoped one. Especially, the sensor doped with 1.5 wt% Pd exhibits the largest response to H2 S at 160 ◦ C. Selectivity is an important parameter in gas sensing, and the sensor has to have rather high selectivity for its application. As far as we know from the recent researches, ␣-Fe2 O3 sensor also responds to other gases such as ethanol and acetone. Therefore, we examined the responses of the 1.5 wt% Pd-doped ␣-Fe2 O3 sensor to other seven gases of 1000 ppm at different operating temperatures. From Fig. 6, it can be observed that the sensor exhibits the largest response to H2 S, moderate responses to ethanol and acetone, and negligible responses to n-hexane, NH3 , H2 and CO. On the other hand, the optimum operating temperature to ethanol and acetone is 200 ◦ C, which is higher than that to H2 S. The selectivity to H2 S is higher when the operating temperature is about 160 ◦ C. As a result, it is easier to detect H2 S at lower operating temperatures.
O2 (gas) → O2 (ads)
(1)
O2 (ads) + e− → O2 − (ads)
(2)
O2 − (ads) + e− → 2O− (ads)
(3)
O− (ads) + e− → O2− (ads)
(4)
With introduction of H2 S gas, it would be oxidized by these chemisorbed oxygen species (O2 − , O− , O2− ) on the surface of the sensor. During this reaction, the electrons back into the semiconductor, resulting in a decrease in resistance of the sensor. When the sensor is exposed in air again, the gases are desorbed as H2 O and SO2 . This reaction process may be as Eq. (5). H2 S + 3O2− → H2 O + SO2 + 6e−
(5)
The effect of noble metal addition on the gas sensing properties of metal oxide has been widely studied [16–19]. This influence has formally been classified as chemical or electronic according to two basic sensitization mechanisms. The two mechanisms are so-called spill-over and Fermi-level effect, respectively. Pd is typically considered to be related with the electronic one [20–22]. In the electronic mechanism, the reaction with gas molecules takes place on the dopants surface. These dopants change the electrostatic potential due to their different electron affinities, which gives rise to a variation of the surface barrier height. This results in the corresponding resistance change of the base semiconductor. On the other hand, Pd dopants, as a catalyst, enhance the adsorption of gas molecules and accelerate the electron exchange between the sensor and the
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Fig. 7. Comparison of response of 1.5 wt% Pd-doped ␣-Fe2 O3 to various gases.
test gas. All these factors together contribute to the improvement of gas sensing properties of the Pd-doped ␣-Fe2 O3 sensors. The gas response of the undoped and 1.5 wt% Pd-doped ␣-Fe2 O3 sensors to various gases at their optimum operating temperatures are compared in Fig. 7. The responses of 1.5 wt% Pd-doped ␣-Fe2 O3 sensor to all of these gases are much higher than that of the undoped one, especially to H2 S. Fig. 8 shows the relationship between the response of the 1.5 wt% Pd-doped ␣-Fe2 O3 sensor and the H2 S concentration (10–200 ppm) at 160 ◦ C. It can be seen that the response increases with the H2 S concentration, and the concentration dependence presents good linearity, which suggests that the Pd/␣-Fe2 O3 could meet the applications in H2 S sensing. Furthermore, the sensor still shows excellent response to low concentration H2 S, and its response to 10 ppm H2 S is 46.6. Response and recovery times are the basic parameters of gas sensors, which are defined as the time required to reach 90% of the final resistance. Fig. 9 shows the response–recovery characteristics of 1.5 wt% Pd-doped ␣-Fe2 O3 to H2 S at different concentrations. As can be seen from the curve, the sensor responds very rapidly after introduction of H2 S and recovers
Fig. 9. Response–recovery characteristics of 1.5 wt% Pd-doped ␣-Fe2 O3 to H2 S of different concentrations.
Fig. 10. Long-time stability of 1.5 wt% Pd-doped ␣-Fe2 O3 sensor to 100 ppm H2 S.
immediately when it is exposed to air. The result indicates that the sensor can meet the practical application in H2 S detection. The stability of the 1.5 wt% Pd-doped ␣-Fe2 O3 sensor was measured at a level of 100 ppm H2 S after ageing for 60 days. The results are shown in Fig. 10, from which it is seen that the sensor presented nearly constant response to H2 S even after 2 months. We believe that Pd/␣-Fe2 O3 sensors have promising applications in H2 S sensing in the future. 4. Conclusions
Fig. 8. Gas response vs. H2 S concentration of 1.5 wt% Pd-doped ␣-Fe2 O3 .
In summary, the Pd-doped ␣-Fe2 O3 sensors have been prepared by chemical coprecipitation method. Compared with the undoped one, the doped sensors present much higher sensitivity, better selectivity and rather lower optimum operating temperature to H2 S than the undoped one. The optimum doping amount is 1.5 wt% and the optimum operating temperature is 160 ◦ C. The doped sensors also present long-term stability and short response/recovery time. Obtained results indicate that the Pd-
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