- Email: [email protected]
ZnO core–shell nanorods for gas sensors
Sensors and Actuators B 119 (2006) 52–56
Fe2O3/ZnO core–shell nanorods for gas sensors Shufeng Si a,b , Chunhui Li a,b , Xun Wang a,b , Qing Peng a,b , Yadong Li a,b,∗ a
b
Department of Chemistry, Tsinghua University, Beijing 100084, PR China National Center for Nanoscience and Nanotechnology, Beijing 100084, PR China
Received 8 May 2005; received in revised form 23 November 2005; accepted 25 November 2005 Available online 10 January 2006
Abstract Fe2 O3 /ZnO core/shell nanorods were prepared by a solution phase controlled hydrolysis method, and were characterized by XRD, BET and TEM techniques. The surface area of these core/shell nanorods is higher than that of bulk ZnO sensor materials, and in our experiments, the resultant Fe2 O3 /ZnO gas sensor exhibited a high response, good stability and a short response/recovery time in the detection of low concentrations of various combustible gases. The response/recovery time was less than 20 s, and the response decreased slightly after 4 months. © 2005 Elsevier B.V. All rights reserved. Keywords: ZnO; Fe2 O3 ; Nanorods; Gas sensor; Combustible gas
1. Introduction The increasing concern on the safety in laboratory and industrial activities has generated great interests in fast reliable gas detection. In recent years, zero-dimensional nanoparticles and one-dimensional nanoscale materials, for example, ZnO, SnO2 , WO3 and MnO2 nanoparticles and nanorods, have been investigated to fabricate new semiconductor gas sensors [1–4], due to their fine particle size and large surface area. The gas-sensing mechanism involves the chemisorption of oxygen on the surface of these oxides, followed by charge transfer during the reaction of oxygen with target gas molecules [5], which will cause a resistance change on the surface of the sensor. Zinc oxide is an important oxide semiconductor for sensing applications to toxic and combustible gases [6–12]. Generally, ZnO sensors provide a wide variety of advantages, such as low cost, short response time, easy manufacturing, and small size, compared to the traditional analytical instruments. However, its working temperature is rather high, normally at 400–500 ◦ C, and the selective response ability is fairly poor. In recent years, the study on ZnO gas-sensing materials has become one of the major research topics, and the research is focused on improving their preparation method and decreasing their working temperature [13–15]. In this paper, we report a solution phase con∗
Corresponding author. Tel.: +86 1062772350; fax: +86 1062788765. E-mail address: [email protected] (Y. Li).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.11.050
trolled hydrolysis method to synthesize Fe2 O3 /ZnO core/shell nanorods. Compared to other sensor elements, these core/shell nanorods exhibit a high gas sensitivity, short response/recovery time and can work at relatively low temperatures. 2. Experimental Fe2 O3 /ZnO core/shell nanorods were prepared through a hydrolysis process of Zn2+ in the presence of Fe2 O3 nanorods. Uniform Fe2 O3 nanorods were also prepared through a hydrolysis process of ferric chloride at 80 ◦ C as literature described [16]. Hundred milligram of Fe2 O3 nanoshuttles were dispersed in 200 mL deionized water by ultrasonication for more than 30 min. Then 20 mg Zn(Ac)2 ·6H2 O was introduced to the solution, and the suspension was heated in an oil bath at 40 ◦ C under vigorous stirring. Twenty millilitres of 5% ammonia was added into the suspension in 0.5 h and the reaction was maintaining the temperature for 1 h. Finally, the colloids were centrifuged and sintered at 400 ◦ C for 2 h, yielding the desired Fe2 O3 /ZnO nanorods. These Fe2 O3 /ZnO nanorods were then coated on Al2 O3 tubes (4 mm in length, 1.2 mm in external diameter and 0.8 mm in internal diameter), which had a Pt electrode at each end. These devices were sintered at 400 ◦ C for 2 h after drying under air. A small Ni–Cr alloy coil with the resistance of about 28 was placed through the tube as a heater. The temperature was controlled through adjusting the heating power.
S. Si et al. / Sensors and Actuators B 119 (2006) 52–56
53
Fig. 1. TEM images of (a) Fe2 O3 /ZnO nanorods and (b) Fe2 O3 nanoparticles.
X-ray diffraction spectra (XRD) have been obtained by using a D8 advance X-ray diffractometer (Bruker Instruments Inc., Billerica, MA, USA) using the Cu K␣ radiation. The BET surface measurements have been performed by using an ASAP 2010 instrument. The particle size and morphology of all the particulate samples were examined by using a Hitachi H-800 transmission electron microscope (TEM) at accelerating voltages up to 200 kV. Measurements of electrical resistance in air at various temperatures and gas-sensing properties were carried out on a commercial HW-30 under laboratory conditions (25 ◦ C, 60% relative humidity) after the gas sensors were pretreated at 300 ◦ C for 10 days in air. The gas response, R, is given by R = Vg /Va , where Vg and Va express the output voltages of a loading resistor in series with the sensor in a detecting gas and in air, respectively. 3. Result and discussion The obtained core/shell Fe2 O3 /ZnO nanorods were 30–35 nm in width and 110–150 nm in length, as shown in Fig. 1a. For comparison, the morphology of the original Fe2 O3 nanoshuttles is shown in Fig. 1b, which indicates that they were 25–32 nm in width and about 110–150 nm in length. This result indicates that the thickness of the ZnO shell coated on Fe2 O3 is about 3 nm. The film thickness of the core/shell Fe2 O3 /ZnO nanorods coated on an Al2 O3 ceremic tube observed by a light section microscope is around 100 m. Surface area of as-synthesized materials was obtained by BET method using nitrogen adsorption. The area was 48.6 m2 g−1 , indicating that the devices from Fe2 O3 /ZnO nanorods presents a higher surface area than the film materials prepared by other common methods. The chemical composition of the samples was characterized by XRD patterns. Fig. 2 shows the XRD patterns of ZnO and Fe2 O3 , and all the reflection peaks can be readily indexed to a wurtzite phase of ZnO (JCPDS 79-0205) and a hematite phase of Fe2 O3 (JCPDS 82-1503). The apparent broadening of these peaks indicates fine size of the as-obtained nanorods, which is estimated to be about 3 nm from its XRD data. The fact that no distinct peaks existed except the patterns of ZnO and Fe2 O3
indicates that no third phase was formed at the sintering temperature of 400 ◦ C. Fig. 3a and b presents the SEM images of Fe2 O3 /ZnO nanorods after heat treatment. The nanorods have a randomly packed structure, which can provide a plenty of pores into which target gases can migrate. In the case of n-type semiconductor oxides [17,18], the intrinsic conductance increases with the increasing temperature, whereas the adsorbed oxygen molecules transform into oxygen ions (O− , O2− ) by capturing free electrons from the oxide, which causes a decrease in conductance of the oxide with the increasing temperature. The gas-sensing mechanism of ZnO-based sensors belongs to the surface-controlled type, and the change of resistance is dependent on the species and the amount of chemisorbed oxygen on the surface. When a combustible gas is introduced to the system, oxidation reactions will take place on the surface of the oxide. For Fe2 O3 /ZnO nanorods, there are two remarkable features when they are applied as a sensor: (1) due to the thin thickness of the ZnO layer (only about 3 nm) outside the Fe2 O3 nanorods, all ZnO microcrystallines will respond to target gases within a short time, and (2) better sensing performance may be realized because the randomly packed nanorods provide a larger number of pores and a higher surface area than that of
Fig. 2. An X-ray diffraction pattern of Fe2 O3 /ZnO nanoparticles (# peaks of ZnO).
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S. Si et al. / Sensors and Actuators B 119 (2006) 52–56
Fig. 3. SEM images showing the morphology of Fe2 O3 /ZnO nanorods on an Al2 O3 tube: (a) low and (b) high magnification.
Fig. 4. Responses to four kinds of gases of 50 ppm in air as a function of heating voltage (RT 25 ◦ C and RH 60%).
other types of sensors. Thus, the sensors made by the Fe2 O3 /ZnO nanorods are expected to be highly sensitive and have a short response/recovery time. The responses to gasoline, ethanol, cyclohexane and acetone of Fe2 O3 /ZnO nanorods are shown as a function of operating temperature in Fig. 4. From the temperature dependence, the temperature TM(g) was evaluated at which the sensor showed the highest response to a certain target gas. This is an important parameter that describes the sensing behavior of a sensor. The maximum response to ethanol and acetone was observed at 200 ◦ C, and the TM(g) was up to about 320 ◦ C for gasoline and cyclohexane.
The concentration dependences of the response were measured in the range from 5 (about 0.05 g m−3 ) to 500 ppm at 200 ◦ C for ethanol and acetone and at 320 ◦ C for gasoline and cyclohexane. The sensor showed a reversible and reproducible response (see Table 1). The response to the combustible vapors increased quasilinearly with increasing gas concentration, without showing a saturating tendency up to 500 ppm. For comparison, the devices of pure nanoshuttle Fe2 O3 were prepared by the same process similar to that of Fe2 O3 /ZnO. The responses to gasoline and ethanol, are also shown in Fig. 4. Under the same condition, the device of pure Fe2 O3 shows low sensitivity to ethanol gas. The material shows distinctive response until the gas concentration reach 500 ppm. At 320 ◦ C, the response of the material to gasoline rises. Comparing with Fe2 O3 /ZnO, however, the device also shows weak response and long response time. In a word, the material of Fe2 O3 /ZnO, as a sensitive device, exhibits better sensitivity and more improved response time than pure Fe2 O3 . The present sensor gives a better response and shorter response/recovery time than that reported in literatures [17–19]. The voltage showed a drastic rise with the injection of target gases and quickly restored to its initial value after the test gases were released. As shown in Fig. 5, the conductance of the specimen increased rapidly and reached the maximum within 20 s when they were exposed to 5 ppm ethanol and gasoline vapor, while other sensors prepared by conventional SnO2 , Cu2 O and ZnO materials needed a longer response time of one or several minutes. Even if the concentration of ethanol and gasoline vapors increased, the fast response characteristics were unchanged.
Table 1 Response results (Vgas /Vair ) to various gases (ppm) (operating temperatures was 200 ◦ C for ethanol and acetone and 320 ◦ C for gasoline and cyclohexane; RT 25 ◦ C and RH 60%) Concentration (ppm)
5
90#petroleum Cyclohexane Ethanol Acetone
2.73 1.5 4.01 3.53
10
25
50
100
250
500
5.21 2.57 6.48 5.34
6.57 2.89 7.34 6.28
9.28 3.15
5.02 4.12
3.60 2.14 5.92 4.86
13.39 3.57 9.67 8.84
S. Si et al. / Sensors and Actuators B 119 (2006) 52–56
55
of the nanorods film easily. Second, the resistance of ZnO is hundred times smaller than that of Fe2 O3 , so that the resistance change of the ZnO shell is the decisive factor to determine the performance of the sensor. The thickness of the ZnO shell is about 2–3 nm, which is shorter than its Debye length, so that the resistance of Fe2 O3 /ZnO film can respond quickly to the change of oxygen ions adsorbed on the surface. The long-term stability is another important factor for gas sensors. Repeated experiments were carried out every 2 weeks, and the sensors showed good response performance even after 4 months (shown in Fig. 6). This indicates that the present sensor based on the Fe2 O3 /ZnO nanorods may be applied to an air quality monitor to detect various kinds of vapors of low concentration safely and stably. 4. Conclusion The thickness of the ZnO shell coated on the surface of Fe2 O3 nanorods was estimated to be about 2–3 nm, which was smaller than that in conventional ZnO-based sensor devices. This Fe2 O3 /ZnO-based sensor showed higher responses to various combustible gases with a faster response/recovery time less than 20 s. In our experiments, the sensor also showed a good long-term stability. These favorable gas-sensing features make the present Fe2 O3 /ZnO core/shell nanorods to be particularly attractive as a promising practical sensor. Acknowledgements Fig. 5. Responses to gasoline at 320 ◦ C and to ethanol at 200 ◦ C (RT 25 ◦ C and RH 60%).
The better response/recovery characteristics of the Fe2 O3 /ZnO sensor are considered to arise from the following two structural features. First, the film of Fe2 O3 /ZnO nanorods deposited on an Al2 O3 tube has a loosely packed structure, which provides larger channels or pores, and a higher surface area than the film of spherical or granular particles. Therefore, target gas molecules can access the inner
Fig. 6. The long-term response values of Fe2 O3 /ZnO to 100 ppm ethanol at 200 ◦ C (RT 25 ◦ C and RH 60%).
This work was supported by NSFC (50372030, 20025102, 20151001), the foundation for the author of National Excellent Doctoral Dissertation of PR China and the state key project of fundamental research for nanomaterials and nanostructures (2003CB716901). References [1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Nanobelts of semiconducting oxides, Science 291 (2001) 1947–1949. [2] P.D. Yang, H.Q. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R.R. He, H.J. Choi, Controled growth of ZnO nanowires and their optical properties, Adv. Funct. Mater. 12 (2002) 323–331. [3] X. Wang, Y.D. Li, Selected-control hydrothermal synthesis of ␣- and MnO2 single crystal nanowire, J. Am. Chem. Soc. 124 (2002) 2880–2881. [4] X.L. Li, J.F. Liu, Y.D. Li, Large scale synthesis of tungsten oxide nanowire with high aspect ratio, Inorg. Chem. 42 (2003) 921–924. [5] P. Esser, W. Gopel, Physical adsorption on single crystal zinc oxide, Surf. Sci. 97 (1980) 309–318. [6] A. Jones, T.A. Jones, B. Mann, J.G. Griffith, The effect of the physical form of the oxide on the conductivity changes produced by CH4 , CO, and H2 O on ZnO, Sens. Actuators 5 (1984) 75–88. [7] S. Saito, M. Miyayama, K. Koumoto, H. Janagida, Gas-sensing characteristics of porous ZnO and Pt/ZnO ceramics, J. Am. Ceram. Soc. 68 (1985) 40–43. [8] S. Pizzini, N. Butta, D. Norducci, M. Palladino, Thick film ZnO resistive gas sensors analysis of their kinetic behavior, J. Electrochem. Soc. 136 (1989) 1945–1948. [9] H. Nanto, T. Minami, S. Takata, Zinc oxide thin film ammonia gas sensors with high sensitivity and excellent selectivity, J. Appl. Phys. 60 (1986) 482–484.
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[10] T. Yamazaki, S. Wada, T. Noma, T. Suzuki, Gas-sensing properties of ultrathin Zinc oxide films, Sens. Actuators B 13–14 (1993) 594–595. [11] S. Basu, A. Dutta, Modified heterojunction-based zinc oxide thin film for hydrogen gas sensor application, Sens. Actuators B 22 (1994) 83–87. [12] S. Strassler, A. Reis, Simple models for N-type metal oxide gas sensors, Sens. Actuators 4 (1983) 465–472. [13] X. Lou, Development of ZnO series ceramic semiconductor gas sensors, J. Sens. Trans. Technol. 3 (1991) 1–5. [14] S. Pan, S. Mei, Sol–gel preparation of ZnO gas-sensing film, J. Sens. Trans. Technol. 3 (1993) 18–20. [15] J.Q. Xu, Q.Y. Pan, Y.A. Shun, Z.Z. Tian, Grain size control and gas-sensing properties of ZnO gas sensor, Sens. Actuators B 66 (2000) 277–279. [16] A.K. Mukhopadhyay, P. Mitra, D. Chattopadhyay, H.S. Maiti, Influences of fabrication techniques and doping on hydrogen sensitivity of zinc oxide sensors, J. Mater. Sci. Lett. 15 (1996) 431. [17] B.L. Zhu, C.S. Xie, W.Y. Wang, K.J. Huang, J.H. Hu, Improvement in gas sensitivity of ZnO thick film to volatile organic compounds (VOCs) by adding TiO2 , Mater. Lett. 58 (2004) 624–629. [18] U. Hoefer, J. Frank, M. Fleischer, High temperature Ga2 O3 -gas sensors and SnO2 -gas sensors: a comparison, Sens. Actuators B 78 (2001) 6–11. [19] P. Mitra, A.P. Chatterjee, H.S. Maiti, ZnO thin film sensor, Mater. Lett. 35 (1998) 33–38.
Biographies S. Si received his Ph.D. degree in inorganic chemistry from Nankai University in 2002. His study focuses on the field of characterization of gas-sensing properties and catalysis of metal oxide materials. C. Li is studying in Y. Li’s group for his Ph.D. degree in department of chemistry, Tsinghua University in China since 2003. He has been working in the field of characterization and devices of functional materials. X. Wang received his Ph.D. in chemistry from Tsinghua University in China in 2004. He has been working in the field of characterization and devices of transition metal oxides. Q. Peng received his Ph.D. in chemistry from Tsinghua University in China in 2003. He has been working in the field of characterization and devices of transition metal oxides. Y. Li received his Ph.D. degree in chemistry from Science and Technical University of China in 1998. He is one of leaders of National Center for Nanoscience and Nanotechnology and his researches are on semiconductor and devices of Inorganic materials. His present research interests include material science and surface science, focusing on catalysis and sensitivity of metal oxide deposits.
Fe2O3/ZnO core–shell nanorods for gas sensors Shufeng Si a,b , Chunhui Li a,b , Xun Wang a,b , Qing Peng a,b , Yadong Li a,b,∗ a
b
Department of Chemistry, Tsinghua University, Beijing 100084, PR China National Center for Nanoscience and Nanotechnology, Beijing 100084, PR China
Received 8 May 2005; received in revised form 23 November 2005; accepted 25 November 2005 Available online 10 January 2006
Abstract Fe2 O3 /ZnO core/shell nanorods were prepared by a solution phase controlled hydrolysis method, and were characterized by XRD, BET and TEM techniques. The surface area of these core/shell nanorods is higher than that of bulk ZnO sensor materials, and in our experiments, the resultant Fe2 O3 /ZnO gas sensor exhibited a high response, good stability and a short response/recovery time in the detection of low concentrations of various combustible gases. The response/recovery time was less than 20 s, and the response decreased slightly after 4 months. © 2005 Elsevier B.V. All rights reserved. Keywords: ZnO; Fe2 O3 ; Nanorods; Gas sensor; Combustible gas
1. Introduction The increasing concern on the safety in laboratory and industrial activities has generated great interests in fast reliable gas detection. In recent years, zero-dimensional nanoparticles and one-dimensional nanoscale materials, for example, ZnO, SnO2 , WO3 and MnO2 nanoparticles and nanorods, have been investigated to fabricate new semiconductor gas sensors [1–4], due to their fine particle size and large surface area. The gas-sensing mechanism involves the chemisorption of oxygen on the surface of these oxides, followed by charge transfer during the reaction of oxygen with target gas molecules [5], which will cause a resistance change on the surface of the sensor. Zinc oxide is an important oxide semiconductor for sensing applications to toxic and combustible gases [6–12]. Generally, ZnO sensors provide a wide variety of advantages, such as low cost, short response time, easy manufacturing, and small size, compared to the traditional analytical instruments. However, its working temperature is rather high, normally at 400–500 ◦ C, and the selective response ability is fairly poor. In recent years, the study on ZnO gas-sensing materials has become one of the major research topics, and the research is focused on improving their preparation method and decreasing their working temperature [13–15]. In this paper, we report a solution phase con∗
Corresponding author. Tel.: +86 1062772350; fax: +86 1062788765. E-mail address: [email protected] (Y. Li).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.11.050
trolled hydrolysis method to synthesize Fe2 O3 /ZnO core/shell nanorods. Compared to other sensor elements, these core/shell nanorods exhibit a high gas sensitivity, short response/recovery time and can work at relatively low temperatures. 2. Experimental Fe2 O3 /ZnO core/shell nanorods were prepared through a hydrolysis process of Zn2+ in the presence of Fe2 O3 nanorods. Uniform Fe2 O3 nanorods were also prepared through a hydrolysis process of ferric chloride at 80 ◦ C as literature described [16]. Hundred milligram of Fe2 O3 nanoshuttles were dispersed in 200 mL deionized water by ultrasonication for more than 30 min. Then 20 mg Zn(Ac)2 ·6H2 O was introduced to the solution, and the suspension was heated in an oil bath at 40 ◦ C under vigorous stirring. Twenty millilitres of 5% ammonia was added into the suspension in 0.5 h and the reaction was maintaining the temperature for 1 h. Finally, the colloids were centrifuged and sintered at 400 ◦ C for 2 h, yielding the desired Fe2 O3 /ZnO nanorods. These Fe2 O3 /ZnO nanorods were then coated on Al2 O3 tubes (4 mm in length, 1.2 mm in external diameter and 0.8 mm in internal diameter), which had a Pt electrode at each end. These devices were sintered at 400 ◦ C for 2 h after drying under air. A small Ni–Cr alloy coil with the resistance of about 28 was placed through the tube as a heater. The temperature was controlled through adjusting the heating power.
S. Si et al. / Sensors and Actuators B 119 (2006) 52–56
53
Fig. 1. TEM images of (a) Fe2 O3 /ZnO nanorods and (b) Fe2 O3 nanoparticles.
X-ray diffraction spectra (XRD) have been obtained by using a D8 advance X-ray diffractometer (Bruker Instruments Inc., Billerica, MA, USA) using the Cu K␣ radiation. The BET surface measurements have been performed by using an ASAP 2010 instrument. The particle size and morphology of all the particulate samples were examined by using a Hitachi H-800 transmission electron microscope (TEM) at accelerating voltages up to 200 kV. Measurements of electrical resistance in air at various temperatures and gas-sensing properties were carried out on a commercial HW-30 under laboratory conditions (25 ◦ C, 60% relative humidity) after the gas sensors were pretreated at 300 ◦ C for 10 days in air. The gas response, R, is given by R = Vg /Va , where Vg and Va express the output voltages of a loading resistor in series with the sensor in a detecting gas and in air, respectively. 3. Result and discussion The obtained core/shell Fe2 O3 /ZnO nanorods were 30–35 nm in width and 110–150 nm in length, as shown in Fig. 1a. For comparison, the morphology of the original Fe2 O3 nanoshuttles is shown in Fig. 1b, which indicates that they were 25–32 nm in width and about 110–150 nm in length. This result indicates that the thickness of the ZnO shell coated on Fe2 O3 is about 3 nm. The film thickness of the core/shell Fe2 O3 /ZnO nanorods coated on an Al2 O3 ceremic tube observed by a light section microscope is around 100 m. Surface area of as-synthesized materials was obtained by BET method using nitrogen adsorption. The area was 48.6 m2 g−1 , indicating that the devices from Fe2 O3 /ZnO nanorods presents a higher surface area than the film materials prepared by other common methods. The chemical composition of the samples was characterized by XRD patterns. Fig. 2 shows the XRD patterns of ZnO and Fe2 O3 , and all the reflection peaks can be readily indexed to a wurtzite phase of ZnO (JCPDS 79-0205) and a hematite phase of Fe2 O3 (JCPDS 82-1503). The apparent broadening of these peaks indicates fine size of the as-obtained nanorods, which is estimated to be about 3 nm from its XRD data. The fact that no distinct peaks existed except the patterns of ZnO and Fe2 O3
indicates that no third phase was formed at the sintering temperature of 400 ◦ C. Fig. 3a and b presents the SEM images of Fe2 O3 /ZnO nanorods after heat treatment. The nanorods have a randomly packed structure, which can provide a plenty of pores into which target gases can migrate. In the case of n-type semiconductor oxides [17,18], the intrinsic conductance increases with the increasing temperature, whereas the adsorbed oxygen molecules transform into oxygen ions (O− , O2− ) by capturing free electrons from the oxide, which causes a decrease in conductance of the oxide with the increasing temperature. The gas-sensing mechanism of ZnO-based sensors belongs to the surface-controlled type, and the change of resistance is dependent on the species and the amount of chemisorbed oxygen on the surface. When a combustible gas is introduced to the system, oxidation reactions will take place on the surface of the oxide. For Fe2 O3 /ZnO nanorods, there are two remarkable features when they are applied as a sensor: (1) due to the thin thickness of the ZnO layer (only about 3 nm) outside the Fe2 O3 nanorods, all ZnO microcrystallines will respond to target gases within a short time, and (2) better sensing performance may be realized because the randomly packed nanorods provide a larger number of pores and a higher surface area than that of
Fig. 2. An X-ray diffraction pattern of Fe2 O3 /ZnO nanoparticles (# peaks of ZnO).
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S. Si et al. / Sensors and Actuators B 119 (2006) 52–56
Fig. 3. SEM images showing the morphology of Fe2 O3 /ZnO nanorods on an Al2 O3 tube: (a) low and (b) high magnification.
Fig. 4. Responses to four kinds of gases of 50 ppm in air as a function of heating voltage (RT 25 ◦ C and RH 60%).
other types of sensors. Thus, the sensors made by the Fe2 O3 /ZnO nanorods are expected to be highly sensitive and have a short response/recovery time. The responses to gasoline, ethanol, cyclohexane and acetone of Fe2 O3 /ZnO nanorods are shown as a function of operating temperature in Fig. 4. From the temperature dependence, the temperature TM(g) was evaluated at which the sensor showed the highest response to a certain target gas. This is an important parameter that describes the sensing behavior of a sensor. The maximum response to ethanol and acetone was observed at 200 ◦ C, and the TM(g) was up to about 320 ◦ C for gasoline and cyclohexane.
The concentration dependences of the response were measured in the range from 5 (about 0.05 g m−3 ) to 500 ppm at 200 ◦ C for ethanol and acetone and at 320 ◦ C for gasoline and cyclohexane. The sensor showed a reversible and reproducible response (see Table 1). The response to the combustible vapors increased quasilinearly with increasing gas concentration, without showing a saturating tendency up to 500 ppm. For comparison, the devices of pure nanoshuttle Fe2 O3 were prepared by the same process similar to that of Fe2 O3 /ZnO. The responses to gasoline and ethanol, are also shown in Fig. 4. Under the same condition, the device of pure Fe2 O3 shows low sensitivity to ethanol gas. The material shows distinctive response until the gas concentration reach 500 ppm. At 320 ◦ C, the response of the material to gasoline rises. Comparing with Fe2 O3 /ZnO, however, the device also shows weak response and long response time. In a word, the material of Fe2 O3 /ZnO, as a sensitive device, exhibits better sensitivity and more improved response time than pure Fe2 O3 . The present sensor gives a better response and shorter response/recovery time than that reported in literatures [17–19]. The voltage showed a drastic rise with the injection of target gases and quickly restored to its initial value after the test gases were released. As shown in Fig. 5, the conductance of the specimen increased rapidly and reached the maximum within 20 s when they were exposed to 5 ppm ethanol and gasoline vapor, while other sensors prepared by conventional SnO2 , Cu2 O and ZnO materials needed a longer response time of one or several minutes. Even if the concentration of ethanol and gasoline vapors increased, the fast response characteristics were unchanged.
Table 1 Response results (Vgas /Vair ) to various gases (ppm) (operating temperatures was 200 ◦ C for ethanol and acetone and 320 ◦ C for gasoline and cyclohexane; RT 25 ◦ C and RH 60%) Concentration (ppm)
5
90#petroleum Cyclohexane Ethanol Acetone
2.73 1.5 4.01 3.53
10
25
50
100
250
500
5.21 2.57 6.48 5.34
6.57 2.89 7.34 6.28
9.28 3.15
5.02 4.12
3.60 2.14 5.92 4.86
13.39 3.57 9.67 8.84
S. Si et al. / Sensors and Actuators B 119 (2006) 52–56
55
of the nanorods film easily. Second, the resistance of ZnO is hundred times smaller than that of Fe2 O3 , so that the resistance change of the ZnO shell is the decisive factor to determine the performance of the sensor. The thickness of the ZnO shell is about 2–3 nm, which is shorter than its Debye length, so that the resistance of Fe2 O3 /ZnO film can respond quickly to the change of oxygen ions adsorbed on the surface. The long-term stability is another important factor for gas sensors. Repeated experiments were carried out every 2 weeks, and the sensors showed good response performance even after 4 months (shown in Fig. 6). This indicates that the present sensor based on the Fe2 O3 /ZnO nanorods may be applied to an air quality monitor to detect various kinds of vapors of low concentration safely and stably. 4. Conclusion The thickness of the ZnO shell coated on the surface of Fe2 O3 nanorods was estimated to be about 2–3 nm, which was smaller than that in conventional ZnO-based sensor devices. This Fe2 O3 /ZnO-based sensor showed higher responses to various combustible gases with a faster response/recovery time less than 20 s. In our experiments, the sensor also showed a good long-term stability. These favorable gas-sensing features make the present Fe2 O3 /ZnO core/shell nanorods to be particularly attractive as a promising practical sensor. Acknowledgements Fig. 5. Responses to gasoline at 320 ◦ C and to ethanol at 200 ◦ C (RT 25 ◦ C and RH 60%).
The better response/recovery characteristics of the Fe2 O3 /ZnO sensor are considered to arise from the following two structural features. First, the film of Fe2 O3 /ZnO nanorods deposited on an Al2 O3 tube has a loosely packed structure, which provides larger channels or pores, and a higher surface area than the film of spherical or granular particles. Therefore, target gas molecules can access the inner
Fig. 6. The long-term response values of Fe2 O3 /ZnO to 100 ppm ethanol at 200 ◦ C (RT 25 ◦ C and RH 60%).
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Biographies S. Si received his Ph.D. degree in inorganic chemistry from Nankai University in 2002. His study focuses on the field of characterization of gas-sensing properties and catalysis of metal oxide materials. C. Li is studying in Y. Li’s group for his Ph.D. degree in department of chemistry, Tsinghua University in China since 2003. He has been working in the field of characterization and devices of functional materials. X. Wang received his Ph.D. in chemistry from Tsinghua University in China in 2004. He has been working in the field of characterization and devices of transition metal oxides. Q. Peng received his Ph.D. in chemistry from Tsinghua University in China in 2003. He has been working in the field of characterization and devices of transition metal oxides. Y. Li received his Ph.D. degree in chemistry from Science and Technical University of China in 1998. He is one of leaders of National Center for Nanoscience and Nanotechnology and his researches are on semiconductor and devices of Inorganic materials. His present research interests include material science and surface science, focusing on catalysis and sensitivity of metal oxide deposits.