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Highly sensitive and selective butanone sensors based on cerium-doped SnO2 thin films
Sensors and Actuators B 145 (2010) 667–673
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Highly sensitive and selective butanone sensors based on cerium-doped SnO2 thin films Zhongwei Jiang a,b , Zheng Guo a , Bai Sun a , Yong Jia a,c , Minqiang Li a,∗ , Jinhuai Liu a a Key Laboratory of Biomimetic Sensing and Advanced Robot Technology, Institute of Intelligent Machines, Chinese Academy of Sciences, Science Island, Shushan District, Hefei 230031, Anhui, PR China b Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China c School of Pharmacy, Anhui University of Traditional Chinese Medicine, Hefei 230031, PR China
a r t i c l e
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Article history: Received 6 July 2009 Received in revised form 8 January 2010 Accepted 8 January 2010 Available online 20 January 2010 Keywords: Gas sensor Butanone SnO2 thin films Cerium
a b s t r a c t Tin oxide (SnO2 ) thin films, doped with different concentrations of cerium (Ce), were prepared via a simple sol–gel and dip-coating technique. The surface morphologies and microstructures of the thin films were characterized by field emission scanning electron microscope, atomic force microscopy, X-ray diffraction and Raman spectra. It was revealed that the Ce-doped SnO2 thin films with rougher surface were composed of smaller crystallites compared with undoped ones. The disordered structures and boundaries of the Ce-doped SnO2 thin films were also enhanced. Furthermore, the influence factors of gas-sensing properties for butanone, such as cerium concentration, calcination temperature, the layers of thin films and humidity, were investigated. The results indicated that four-layer 1 at% Ce-doped SnO2 thin films calcined at 500 ◦ C presented the best response. At the optimal working temperature of 210 ◦ C, the response to 100 ppm of butanone vapor was about 181 in dry air. In addition, the gas sensor had a good selectivity for butanone. Finally, the possible mechanism of the Ce addition on the gas-sensing properties was also discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, easily made drug chemicals have been seriously monitored and detected, because they could often be illegally used to produce drugs. Moreover, some of them, especially butanone and acetone, could cause some accidents in the public owing to their volatile and inflammability. Accordingly, the detection of butanone and acetone is not only important but also necessary for human health and safety. So far, acetone gas sensors have been widely reported. For example, many sensing materials such as SmFe1−x Mgx O3 perovskite oxides [1], cobalt-doped SnO2 thin films [2] and In2 O3 nanowires [3] have been fabricated as gas sensors to detect acetone. However, there are few reports of gas sensors for the detection of butanone till now. SnO2 semiconductor gas sensors, especially thin film sensors, have been widely applied to the detection of various poisonous and combustible gases [4–6]. However, how to improve the sensitivity and selectivity of gas sensor is still a challenge for their practical application so far [7,8]. Doping as an efficient method to improve the sensing-performance of gas sensor was widely used. Many metals were doped into SnO2 , including Cu [9], Sb [10], La [11,12] and Pd [13]. Cerium (Ce) as a dopant has received great
attention due to its peculiar properties arising from the availability of the 4f shell. For example, Ce-doped SnO2 nanomaterials have been used to improve ethanol response selectivity in presence of CO, LPG and CH4 [14]. Based on Ce-doped SnO2 thin films by using SnCl2 ·2H2 O and Ce(NH4 )2 (NO3 )6 as precursors, Fang et al. [15] prepared a high sensitivity H2 S sensor at room temperature. Besides, Ce-doped ZnO thin-film gas sensors were also fabricated by dip-coating method, starting from Zn(CH3 COO)2 ·2H2 O and Ce(NO3 )3 ·6H2 O, which exhibited good responses to volatile organic compounds [16]. Moreover, CeO2 nanowires have been synthesized by a hydrothermal method and applied in fast detecting CO [17] and humidity [18]. Herein, highly sensitive and selective butanone sensor based on Ce-doped SnO2 thin films was proposed. A series of SnO2 thin films doped with different concentrations of cerium were prepared via a sol–gel method and dip-coating technique. The surface morphologies and microstructures of thin films were investigated in detail. The influence factors of gas-sensing properties, such as cerium concentration, calcination temperature, the layers of thin films and humidity, have been also investigated. 2. Experimental details 2.1. Preparation of thin films
∗ Corresponding author. Tel.: +86 551 5592385; fax: +86 551 5592420. E-mail address: [email protected] (M. Li). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.01.014
All the reagents and solvents, which were purchased from Shanghai Chemical Reagent Ltd. Co., were analytical grade and
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used as received without further purification. Undoped and Cedoped SnO2 thin films were prepared by a sol–gel and dip-coating process using SnCl2 ·2H2 O and CeCl3 ·7H2 O as precursors. The starting sols of undoped and Ce-doped SnO2 with five different molar concentrations (0.5, 1, 3, 5 and 10 at% cerium) were prepared by a simple procedure. A certain amount of CeCl3 ·7H2 O and SnCl2 ·2H2 O were dissolved in 20 ml ethanol. After refluxing at 100 ◦ C for 8 h under magnetic stirring, homogeneous mixtures of precursors were formed. Then aging for two days at room temperature, the transparent sols were obtained. Subsequently, the sols were dip-coated on the ceramic substrate, then dried at 80 ◦ C for 10 min, 150 ◦ C for 20 min and 80 ◦ C for 10 min step by step for the formation of initial thin films. After repeated several times for different layers (2, 3, 4 and 5 times corresponding to 2, 3, 4, and 5-layer, respectively), the samples were calcined in air at 400, 450, 500, 550 or 600 ◦ C for 1 h via program heating, respectively. Finally, undoped and Ce-doped SnO2 thin films were formed.
2.2. Physical characterization The morphologies of as-prepared thin films were investigated with a Quanta 200 FEG environmental scanning electronic microscopy (ESEM) and a Veeco Autoprobe CP atomic force microscopy (AFM). The crystalline structural analysis was performed with a Philips X’Pert Pro X-ray diffractometer (XRD) with Cu K␣ radiation (1.5418 Å). The Raman spectra of SnO2 thin films doped with different concentrations of cerium were taken at room temperature in backscattering geometry. Thermo Fisher DXR Raman spectrometer with semiconductor laser beam was carried out at the excitation wavelength of 532 nm.
2.3. Gas-sensing measurement The gas-sensing properties of thin films were performed with a gas-sensing measurement system similar to our previous reports [19–21]. All results shown in this paper were performed in a closed plastic box (1000 ml) equipped with appropriate inlets and outlets for gas flows. There is a gas container with air and target gases inputs which can be alternated for the requirement (air flow = 1800 ml/min). A Keithley 6487 picoammeter/voltage sourcemeter was employed to serve as both voltage source and current reader. The relative response was defined as: S=
Ra Rg
where Ra is the resistance in atmospheric air and Rg is the resistance of the sensor device exposed in the detected gases. The results of the gas-sensing properties were obtained from the repeated measurements of three or more sensors of each type, which were shown in the form of Error bars standing for standard deviation. In addition, the response and recovery time were obtained by the time for 90% of final resistance change. 3. Results and discussion 3.1. Surface morphology and microstructure of thin films After calcination at 500 ◦ C in air, the surface morphologies of four-layer undoped and Ce-doped SnO2 thin films were investigated by SEM, and the results were shown in Fig. 1(a)–(f). It was obvious that the Ce-doped SnO2 thin films turned rougher than undoped ones. The particle sizes were slightly increased with the concentration of cerium increasing from 0 at% to 10 at%. Fur-
Fig. 1. Low-magnification and high magnification (shown in the insets) SEM images of four-layer SnO2 thin films calcined at 500 ◦ C with different concentrations of cerium doped: (a) undoped; (b) 0.5 at%; (c) 1 at%; (d) 3 at%; (e) 5 at%; (f) 10 at%.
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Fig. 4. XRD patterns of undoped SnO2 and different concentrations of cerium-doped SnO2 thin films calcined at 500 ◦ C.
Fig. 2. SEM image of the cross-section for the four-layer sensing film.
thermore, high magnification SEM images shown in the insets of Fig. 1(a)–(f) suggested that all the thin films were composed of uniform grains. Fig. 2 showed a cross-section of four-layer thin films. It could be observed that the thickness of four-layer thin film was about 500 ± 50 nm. The thickness of the thin film was relatively increased with the increase of layers. Fig. 3(a)–(f) showed the AFM images of the surface morphologies of the thin films. All the thin films appeared to be granular structure with narrow and uniform distribution of the particle size. It was also observed that the surface agglomerates of Ce-doped SnO2 thin films were elongated compared with undoped ones. These results were consis-
tent with the above SEM images and similar with previous reports of SnO2 thin films doped with Fe and Mo [22,23]. Fig. 4 showed the XRD patterns of undoped and Ce-doped SnO2 thin films after calcination at 500 ◦ C in air. All the diffraction peaks could be indexed to the tetragonal rutile structure of SnO2 (JCPDS No. 41-1445) and the cubic structure CeO2 (JCPDS No. 81-0792) [14,24,25]. With the increasing of the concentration of cerium, the diffraction peaks of SnO2 turned broader and broader. According to the Sherrer formula, the SnO2 crystallite size decreased with the increasing concentration of dopant, which was in agreement with the previous reports [14,23]. Owing to the smaller crystallite sizes leading to high surface energy, the Ce-doped SnO2 thin films were easily agglomerated on the surface, which was consistent with the images of SEM and AFM. The calculated lattice parameter
Fig. 3. AFM micrographs of four-layer SnO2 thin films calcined at 500 ◦ C with different concentrations of cerium doped: (a) undoped; (b) 0.5 at%; (c) 1 at%; (d) 3 at%; (e) 5 at%; (f) 10 at%.
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Fig. 5. XRD patterns of 1 at% Ce-doped SnO2 thin films calcined at 400, 450, 500, 550, 600 ◦ C, respectively.
of the undoped SnO2 by least-squares fit were a = 0.4746 nm and c = 0.3188 nm, which were in agreement with the reported value in JCPDS No. 41-1445 (a = 0.4738 nm and c = 0.3187 nm). However, the lattice parameters of 1 at% Ce-doped SnO2 were a = 0.4762 nm and c = 0.3205 nm, which implied that partial cerium cations could be incorporated into the SnO2 lattice. But it was necessary to mention that the separated phase CeO2 also exist on the surface of SnO2 particles besides the partial incorporation of Ce4+ ions into the SnO2 lattice for all the investigated Ce-doped SnO2 thin films. When the content of cerium increased to 10 at%, the diffraction peaks of CeO2 could be obviously observed in Fig. 4. Fig. 5 showed the XRD patterns of 1 at% Ce-doped SnO2 thin films calcined at different temperature. The diffraction peaks turned sharper with the increasing of calcination temperature, which could be attributed to the improvement of crystallization and the growth of the SnO2 crystallites [24]. Based on Raman spectra presented in Fig. 6, the undoped SnO2 displayed the distinct Raman peaks at 473, 631, and 770 cm−1 , which corresponded to the first order Raman-active modes Eg (translational), A1g (symmetric Sn–O stretching) and B2g (asymmetric Sn–O stretching) vibration modes of SnO2 [26,27],
Fig. 6. Raman spectra of undoped SnO2 and different concentrations of ceriumdoped SnO2 thin films.
Fig. 7. Relationship between relative response and working temperature of different concentrations of cerium-doped four-layer SnO2 thin films to 100 ppm of butanone vapor. Error bars stand for ±S.D. (standard deviation).
respectively. Moreover, the peak at 300 cm−1 could be ascribed to vibrational modes of vacancy-related defect [28]. However, the A1g modes shifted to lower wavenumber with the increasing of the cerium concentration, which could be ascribed to the increase of the disordered structures and boundaries in the doped thin films [26]. The results were similar to the Eu-doped SnO2 thin films [26,27]. The peak shifted to 625 cm−1 when the content of cerium increased to 10 at%. The peak at 458 cm−1 , corresponding to the triply degenerate Raman-active mode with the F2g symmetry of CeO2 [29], could be observed when the content of cerium increased to 3 at%. Then, the intensity increased sharply when the concentration increased to 5 at% and 10 at%. The results of physical characterization indicated that the Cedoped SnO2 thin films with rougher surface, more disordered structures and boundaries were composed of smaller crystallites compared with undoped ones. It was believed that the gas-sensing properties could be greatly improved by optimizing the surface morphology and microstructure of SnO2 thin films. 3.2. Gas-sensing properties It was well accepted that the sensing response of gas sensors was greatly influenced by the working temperature and the amount of additives [2,22,23]. In order to obtain the best response to butanone, the responses of different concentrations of four-layer Ce-doped SnO2 thin films to 100 ppm of butanone vapor were investigated in detail with working temperature from 150 to 300 ◦ C in dry air. The results were shown in Fig. 7. It could be found that the optimal working temperature of Ce-doped SnO2 was about 210 ◦ C, which was lower than that of the undoped SnO2 thin films. For the undoped SnO2 thin films, the optimal working temperature was 270 ◦ C. The reason could be explained on the basis of the change in the energy barrier height due to the doping of cerium in the thin films [30]. Furthermore, for the undoped SnO2 thin films, the maximal response to 100 ppm of butanone vapor was about 36.3. However, the response of 0.5 at% Ce-doped SnO2 thin films to 100 ppm of butanone vapor was about 90. When the cerium content was increased to 1 at%, the sensor response was increased to about 181. However, with the concentration of cerium increasing to 3 at%, 5 at% and 10 at%, the sensor responses were decreased to about 104, 58.7 and 4.6, respectively. It was concluded that the gas sensors based on 1 at% Ce-doped SnO2 thin films presented the best response to butanone.
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Fig. 8. Relationship between relative response and working temperature of 1 at% Ce-doped four-layer SnO2 thin films calcined at different temperatures to 100 ppm of butanone vapor. Error bars stand for ±S.D.
For the gas sensor of metal oxide thin films, the gas-sensing responses were also affected by calcination temperature [6] and the thickness of thin films [31]. Fig. 8 showed the relationship between calcination temperature and the response of the four-layer 1 at% Ce-doped SnO2 thin films at different working temperature. It could be found that the thin films showed the best sensing-performance at calcination temperature of 500 ◦ C (relative response of 181–100 ppm of butanone vapor). At the lower calcination temperature, the responses were also lower, which were 119 and 126 at 400 and 450 ◦ C, respectively. The reason was that it was difficult to obtain a better form of polycrystalline thin films below 500 ◦ C. However, the crystal lattice of SnO2 became more ordered at higher calcination temperature, resulting in the increase of crystallite size. It was also unfavorable to improve the sensing response. The sensor responses to 100 ppm of butanone vapor were decreased to about 152 and 95 at the calcination temperature of 550 and 600 ◦ C, respectively. Thus, the optimal calcination temperature was about 500 ◦ C for the detection of butanone. The effect of the layers on the sensor response was also investigated. As shown in Fig. 9, four-layer 1 at% Ce-doped SnO2 thin films exhibited the best gas-sensing properties to butanone. When the layers of 1 at%
Fig. 9. Relationship between relative response and working temperature of 1 at% Ce-doped four-layer SnO2 thin films with different layers to 100 ppm of butanone vapor. Error bars stand for ±S.D.
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Fig. 10. Gas-sensing response curves of 1 at% Ce-doped four-layer SnO2 thin films to 100 ppm of butanone vapor at different relative humidity.
Ce-doped SnO2 thin films increased from two layers to four layers, the maximal response were increased from about 123 to 181 to 100 ppm of butanone vapor at 210 ◦ C. However, the maximal response decreased to 105 when the thin films increased to five layers. It was similar to the ethanol sensor based on Al-doped ZnO thin films [31]. It was noteworthy that the optimum working temperature was decreased to 180 ◦ C for five-layer Ce-doped SnO2 thin films, which could be explained by the diffusion-reaction model [32,33]. Due to diffusion warming effect, more layered thin films exhibited lower kinetic barriers for triggering detection reaction [33]. Accordingly, it could obtain the best of response under lower working temperature. In addition, the gas-sensing response curves of 1 at% Ce-doped four-layer SnO2 thin films to 100 ppm of butanone vapor at different relative humidity were shown in Fig. 10. The baseline of the sensor varied with the relative humidity. The conductance of the thin film rose with the increase of relative humidity, owing to the formation of multilayer physically absorbed water molecules and hydroxyl ions produced by the dissociation reaction of chemisorbs water vapor on the surface of sensing materials [18,34]. At the optimal working temperature of 210 ◦ C, the sensors exhibited the response of 181 to 100 ppm of butanone vapor in dry air. However, the gas sensors provided relative weak responses to 100 ppm of butanone vapor at 40% and 60% RH, which were about 66 and 43, respectively. The real time response curves of four-layer 1 at% Ce-doped SnO2 thin films to different concentrations of butanone were shown in Fig. 11. At the working temperature of 210 ◦ C, the sensors still held evident response value about 8–10 ppm of butanone vapor. Furthermore, the response sharply increased with gas concentration increased from 10 to 400 ppm. At the same time, the gas sensors exhibited a short response and recovery time. The selectivity of Ce-doped SnO2 thin films was also explored, and the results were shown in Fig. 12. For the four-layer 1 at% Cedoped SnO2 thin films, it had relatively weak responses to diethyl ether, ethyl acetate, chloroform, toluene, ammonia and so on. However, the gas sensor provided with good responses to ethanol and acetone; the responses were about 60 and 79 for 100 ppm of ethanol vapor and 100 ppm of acetone vapor, respectively. But, compared with that of the response to 100 ppm of butanone vapor, they were still lower. Obviously, the gas sensor based on 1 at% Ce-doped SnO2 thin films showed the good selectivity for butanone. Based on the above gas-sensing results, a possible mechanism of the Ce addition on the gas-sensing properties of SnO2 -based sensors was offered. Compared with the undoped SnO2 thin films, the Ce-doped SnO2 thin films were composed of smaller crystal-
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4. Conclusions In conclusion, different concentrations of Ce-doped SnO2 thin films were fabricated via the sol–gel method and dip-coating technique. The as-prepared Ce-doped SnO2 thin films kept regular and rough surface, which were composed of uniform grains. The gas-sensing results showed that the approach of doping cerium had greatly improved the gas-sensing response and decreased the working temperature. When the doping concentration of cerium was 1 at%, the gas sensor showed the best response, which was about 181–100 ppm of butanone vapor in dry air. Most importantly, the gas sensor presented selective response to butanone among all investigated gases. Accordingly, the gas sensors based on 1 at% Ce-doped SnO2 thin films have a promising application for the detection of butanone in practical security inspection environment. Acknowledgments
Fig. 11. Gas-sensing response curve of 1 at% Ce-doped four-layer SnO2 thin films to different concentrations of butanone vapor at 210 ◦ C.
lites and possessed rougher surface, more disordered structures and boundaries, which were positive effects to improve the gassensing properties. Additionally, with the existence of CeO2 as one of high-activity catalysts [35], the detected gases could be easily decomposed. Accordingly, for low concentration of Ce-doped SnO2 thin films (0.5 at% and 1.0 at%), the responses were both higher than that of undoped thin films. With the concentration of cerium increased to 3 at%, the response of 104 to butanone was still much better than 36.3 of undoped thin films. However, it was lower than 181 of 1 at% Ce-doped SnO2 thin films. It may be ascribed to the fact that the redundant CeO2 phase would partially enclose around SnO2 grain boundaries, which embarrassed the mobility of the carriers across the SnO2 grain boundaries. Then it was negative effect to improve the gas-sensing properties. Obviously, this negative effect was more notable in spite of abundant CeO2 phase with the high catalytic activities as the concentration of cerium continuously increased to 5 and 10 at%. As shown in Fig. 7, the response decreased to about 4.6 for 10 at% Ce-doped thin films, which was lower than 36.3 of undoped ones. Therefore, just appropriate concentration of cerium-doped SnO2 was propitious to the improvement of gassensing properties. It was similar with the gas-sensing model of Ce-doped ZnO thin films [16].
Fig. 12. Responses of 1 at% Ce-doped four-layer SnO2 thin films to 100 ppm of several kinds of gases or vapor at 210 ◦ C.
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Biographies Zhongwei Jiang received the B.S. degree in chemistry from Anhui University, China, in 2007. Since then, he has been a graduate student in Department of Chemistry at University of Science and Technology of China. He is doing his Master’s Thesis at Institute of Intelligent Machines, Chinese Academy of Sciences, China. His current research interests include semiconductor oxide thin films and their applications in gas nanosensors. Zheng Guo received the Ph.D. in 2008 from University of Science and Technology of China (Hefei, China). Currently, he works as an assistant researcher at Institute of Intelligent Machines, Chinese Academy of Sciences, China. His current research interests include synthesis of gas-sensing nanomaterials and biomimetric material and their applications in nanodevice. Bai Sun received the B.S. degree in 2002, Ph.D. in 2007, both from University of Science and Technology of China (Hefei, China). Currently, he works as an assistant researcher at Institute of Intelligent Machines, Chinese Academy of Sciences, China. His current research interests include synthesis of sensing nanomaterials and fabrication of gas sensors and cataluminescence sensors. Yong Jia received the B.S. degree in 1997, Ph.D. in 2009, both from Anhui University (Hefei, China). His research interests include synthesis of gas-sensing nanomaterials and fabrication of gas-sensing nanodevices. Minqiang Li received the B.S. degree in semiconductor material and device from University of Electronic Science and Technology of China in 1986. Since then, he has been at Institute of Intelligent Machines, Chinese Academy of Sciences, China, and currently as an associate researcher. His current research interests include semiconductor oxide gas nanosensors and their applications in detecting hazardous gases. Jinhuai Liu received the B.S. degree in inorganic chemistry from Yunnan Agricultural University, China, in 1982 and the Ph.D. degree in inorganic chemistry from Graduate University of Chinese Academy of Sciences, China, in 2003. Currently, he works as a Researcher at Institute of Intelligent Machines, Chinese Academy of Sciences. His current research interests include biomimetric material, gas-sensing nanomaterials and nanodevice, sensing technology and their applications in detecting hazardous gases and drug/explosive.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Highly sensitive and selective butanone sensors based on cerium-doped SnO2 thin films Zhongwei Jiang a,b , Zheng Guo a , Bai Sun a , Yong Jia a,c , Minqiang Li a,∗ , Jinhuai Liu a a Key Laboratory of Biomimetic Sensing and Advanced Robot Technology, Institute of Intelligent Machines, Chinese Academy of Sciences, Science Island, Shushan District, Hefei 230031, Anhui, PR China b Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China c School of Pharmacy, Anhui University of Traditional Chinese Medicine, Hefei 230031, PR China
a r t i c l e
i n f o
Article history: Received 6 July 2009 Received in revised form 8 January 2010 Accepted 8 January 2010 Available online 20 January 2010 Keywords: Gas sensor Butanone SnO2 thin films Cerium
a b s t r a c t Tin oxide (SnO2 ) thin films, doped with different concentrations of cerium (Ce), were prepared via a simple sol–gel and dip-coating technique. The surface morphologies and microstructures of the thin films were characterized by field emission scanning electron microscope, atomic force microscopy, X-ray diffraction and Raman spectra. It was revealed that the Ce-doped SnO2 thin films with rougher surface were composed of smaller crystallites compared with undoped ones. The disordered structures and boundaries of the Ce-doped SnO2 thin films were also enhanced. Furthermore, the influence factors of gas-sensing properties for butanone, such as cerium concentration, calcination temperature, the layers of thin films and humidity, were investigated. The results indicated that four-layer 1 at% Ce-doped SnO2 thin films calcined at 500 ◦ C presented the best response. At the optimal working temperature of 210 ◦ C, the response to 100 ppm of butanone vapor was about 181 in dry air. In addition, the gas sensor had a good selectivity for butanone. Finally, the possible mechanism of the Ce addition on the gas-sensing properties was also discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, easily made drug chemicals have been seriously monitored and detected, because they could often be illegally used to produce drugs. Moreover, some of them, especially butanone and acetone, could cause some accidents in the public owing to their volatile and inflammability. Accordingly, the detection of butanone and acetone is not only important but also necessary for human health and safety. So far, acetone gas sensors have been widely reported. For example, many sensing materials such as SmFe1−x Mgx O3 perovskite oxides [1], cobalt-doped SnO2 thin films [2] and In2 O3 nanowires [3] have been fabricated as gas sensors to detect acetone. However, there are few reports of gas sensors for the detection of butanone till now. SnO2 semiconductor gas sensors, especially thin film sensors, have been widely applied to the detection of various poisonous and combustible gases [4–6]. However, how to improve the sensitivity and selectivity of gas sensor is still a challenge for their practical application so far [7,8]. Doping as an efficient method to improve the sensing-performance of gas sensor was widely used. Many metals were doped into SnO2 , including Cu [9], Sb [10], La [11,12] and Pd [13]. Cerium (Ce) as a dopant has received great
attention due to its peculiar properties arising from the availability of the 4f shell. For example, Ce-doped SnO2 nanomaterials have been used to improve ethanol response selectivity in presence of CO, LPG and CH4 [14]. Based on Ce-doped SnO2 thin films by using SnCl2 ·2H2 O and Ce(NH4 )2 (NO3 )6 as precursors, Fang et al. [15] prepared a high sensitivity H2 S sensor at room temperature. Besides, Ce-doped ZnO thin-film gas sensors were also fabricated by dip-coating method, starting from Zn(CH3 COO)2 ·2H2 O and Ce(NO3 )3 ·6H2 O, which exhibited good responses to volatile organic compounds [16]. Moreover, CeO2 nanowires have been synthesized by a hydrothermal method and applied in fast detecting CO [17] and humidity [18]. Herein, highly sensitive and selective butanone sensor based on Ce-doped SnO2 thin films was proposed. A series of SnO2 thin films doped with different concentrations of cerium were prepared via a sol–gel method and dip-coating technique. The surface morphologies and microstructures of thin films were investigated in detail. The influence factors of gas-sensing properties, such as cerium concentration, calcination temperature, the layers of thin films and humidity, have been also investigated. 2. Experimental details 2.1. Preparation of thin films
∗ Corresponding author. Tel.: +86 551 5592385; fax: +86 551 5592420. E-mail address: [email protected] (M. Li). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.01.014
All the reagents and solvents, which were purchased from Shanghai Chemical Reagent Ltd. Co., were analytical grade and
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used as received without further purification. Undoped and Cedoped SnO2 thin films were prepared by a sol–gel and dip-coating process using SnCl2 ·2H2 O and CeCl3 ·7H2 O as precursors. The starting sols of undoped and Ce-doped SnO2 with five different molar concentrations (0.5, 1, 3, 5 and 10 at% cerium) were prepared by a simple procedure. A certain amount of CeCl3 ·7H2 O and SnCl2 ·2H2 O were dissolved in 20 ml ethanol. After refluxing at 100 ◦ C for 8 h under magnetic stirring, homogeneous mixtures of precursors were formed. Then aging for two days at room temperature, the transparent sols were obtained. Subsequently, the sols were dip-coated on the ceramic substrate, then dried at 80 ◦ C for 10 min, 150 ◦ C for 20 min and 80 ◦ C for 10 min step by step for the formation of initial thin films. After repeated several times for different layers (2, 3, 4 and 5 times corresponding to 2, 3, 4, and 5-layer, respectively), the samples were calcined in air at 400, 450, 500, 550 or 600 ◦ C for 1 h via program heating, respectively. Finally, undoped and Ce-doped SnO2 thin films were formed.
2.2. Physical characterization The morphologies of as-prepared thin films were investigated with a Quanta 200 FEG environmental scanning electronic microscopy (ESEM) and a Veeco Autoprobe CP atomic force microscopy (AFM). The crystalline structural analysis was performed with a Philips X’Pert Pro X-ray diffractometer (XRD) with Cu K␣ radiation (1.5418 Å). The Raman spectra of SnO2 thin films doped with different concentrations of cerium were taken at room temperature in backscattering geometry. Thermo Fisher DXR Raman spectrometer with semiconductor laser beam was carried out at the excitation wavelength of 532 nm.
2.3. Gas-sensing measurement The gas-sensing properties of thin films were performed with a gas-sensing measurement system similar to our previous reports [19–21]. All results shown in this paper were performed in a closed plastic box (1000 ml) equipped with appropriate inlets and outlets for gas flows. There is a gas container with air and target gases inputs which can be alternated for the requirement (air flow = 1800 ml/min). A Keithley 6487 picoammeter/voltage sourcemeter was employed to serve as both voltage source and current reader. The relative response was defined as: S=
Ra Rg
where Ra is the resistance in atmospheric air and Rg is the resistance of the sensor device exposed in the detected gases. The results of the gas-sensing properties were obtained from the repeated measurements of three or more sensors of each type, which were shown in the form of Error bars standing for standard deviation. In addition, the response and recovery time were obtained by the time for 90% of final resistance change. 3. Results and discussion 3.1. Surface morphology and microstructure of thin films After calcination at 500 ◦ C in air, the surface morphologies of four-layer undoped and Ce-doped SnO2 thin films were investigated by SEM, and the results were shown in Fig. 1(a)–(f). It was obvious that the Ce-doped SnO2 thin films turned rougher than undoped ones. The particle sizes were slightly increased with the concentration of cerium increasing from 0 at% to 10 at%. Fur-
Fig. 1. Low-magnification and high magnification (shown in the insets) SEM images of four-layer SnO2 thin films calcined at 500 ◦ C with different concentrations of cerium doped: (a) undoped; (b) 0.5 at%; (c) 1 at%; (d) 3 at%; (e) 5 at%; (f) 10 at%.
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Fig. 4. XRD patterns of undoped SnO2 and different concentrations of cerium-doped SnO2 thin films calcined at 500 ◦ C.
Fig. 2. SEM image of the cross-section for the four-layer sensing film.
thermore, high magnification SEM images shown in the insets of Fig. 1(a)–(f) suggested that all the thin films were composed of uniform grains. Fig. 2 showed a cross-section of four-layer thin films. It could be observed that the thickness of four-layer thin film was about 500 ± 50 nm. The thickness of the thin film was relatively increased with the increase of layers. Fig. 3(a)–(f) showed the AFM images of the surface morphologies of the thin films. All the thin films appeared to be granular structure with narrow and uniform distribution of the particle size. It was also observed that the surface agglomerates of Ce-doped SnO2 thin films were elongated compared with undoped ones. These results were consis-
tent with the above SEM images and similar with previous reports of SnO2 thin films doped with Fe and Mo [22,23]. Fig. 4 showed the XRD patterns of undoped and Ce-doped SnO2 thin films after calcination at 500 ◦ C in air. All the diffraction peaks could be indexed to the tetragonal rutile structure of SnO2 (JCPDS No. 41-1445) and the cubic structure CeO2 (JCPDS No. 81-0792) [14,24,25]. With the increasing of the concentration of cerium, the diffraction peaks of SnO2 turned broader and broader. According to the Sherrer formula, the SnO2 crystallite size decreased with the increasing concentration of dopant, which was in agreement with the previous reports [14,23]. Owing to the smaller crystallite sizes leading to high surface energy, the Ce-doped SnO2 thin films were easily agglomerated on the surface, which was consistent with the images of SEM and AFM. The calculated lattice parameter
Fig. 3. AFM micrographs of four-layer SnO2 thin films calcined at 500 ◦ C with different concentrations of cerium doped: (a) undoped; (b) 0.5 at%; (c) 1 at%; (d) 3 at%; (e) 5 at%; (f) 10 at%.
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Fig. 5. XRD patterns of 1 at% Ce-doped SnO2 thin films calcined at 400, 450, 500, 550, 600 ◦ C, respectively.
of the undoped SnO2 by least-squares fit were a = 0.4746 nm and c = 0.3188 nm, which were in agreement with the reported value in JCPDS No. 41-1445 (a = 0.4738 nm and c = 0.3187 nm). However, the lattice parameters of 1 at% Ce-doped SnO2 were a = 0.4762 nm and c = 0.3205 nm, which implied that partial cerium cations could be incorporated into the SnO2 lattice. But it was necessary to mention that the separated phase CeO2 also exist on the surface of SnO2 particles besides the partial incorporation of Ce4+ ions into the SnO2 lattice for all the investigated Ce-doped SnO2 thin films. When the content of cerium increased to 10 at%, the diffraction peaks of CeO2 could be obviously observed in Fig. 4. Fig. 5 showed the XRD patterns of 1 at% Ce-doped SnO2 thin films calcined at different temperature. The diffraction peaks turned sharper with the increasing of calcination temperature, which could be attributed to the improvement of crystallization and the growth of the SnO2 crystallites [24]. Based on Raman spectra presented in Fig. 6, the undoped SnO2 displayed the distinct Raman peaks at 473, 631, and 770 cm−1 , which corresponded to the first order Raman-active modes Eg (translational), A1g (symmetric Sn–O stretching) and B2g (asymmetric Sn–O stretching) vibration modes of SnO2 [26,27],
Fig. 6. Raman spectra of undoped SnO2 and different concentrations of ceriumdoped SnO2 thin films.
Fig. 7. Relationship between relative response and working temperature of different concentrations of cerium-doped four-layer SnO2 thin films to 100 ppm of butanone vapor. Error bars stand for ±S.D. (standard deviation).
respectively. Moreover, the peak at 300 cm−1 could be ascribed to vibrational modes of vacancy-related defect [28]. However, the A1g modes shifted to lower wavenumber with the increasing of the cerium concentration, which could be ascribed to the increase of the disordered structures and boundaries in the doped thin films [26]. The results were similar to the Eu-doped SnO2 thin films [26,27]. The peak shifted to 625 cm−1 when the content of cerium increased to 10 at%. The peak at 458 cm−1 , corresponding to the triply degenerate Raman-active mode with the F2g symmetry of CeO2 [29], could be observed when the content of cerium increased to 3 at%. Then, the intensity increased sharply when the concentration increased to 5 at% and 10 at%. The results of physical characterization indicated that the Cedoped SnO2 thin films with rougher surface, more disordered structures and boundaries were composed of smaller crystallites compared with undoped ones. It was believed that the gas-sensing properties could be greatly improved by optimizing the surface morphology and microstructure of SnO2 thin films. 3.2. Gas-sensing properties It was well accepted that the sensing response of gas sensors was greatly influenced by the working temperature and the amount of additives [2,22,23]. In order to obtain the best response to butanone, the responses of different concentrations of four-layer Ce-doped SnO2 thin films to 100 ppm of butanone vapor were investigated in detail with working temperature from 150 to 300 ◦ C in dry air. The results were shown in Fig. 7. It could be found that the optimal working temperature of Ce-doped SnO2 was about 210 ◦ C, which was lower than that of the undoped SnO2 thin films. For the undoped SnO2 thin films, the optimal working temperature was 270 ◦ C. The reason could be explained on the basis of the change in the energy barrier height due to the doping of cerium in the thin films [30]. Furthermore, for the undoped SnO2 thin films, the maximal response to 100 ppm of butanone vapor was about 36.3. However, the response of 0.5 at% Ce-doped SnO2 thin films to 100 ppm of butanone vapor was about 90. When the cerium content was increased to 1 at%, the sensor response was increased to about 181. However, with the concentration of cerium increasing to 3 at%, 5 at% and 10 at%, the sensor responses were decreased to about 104, 58.7 and 4.6, respectively. It was concluded that the gas sensors based on 1 at% Ce-doped SnO2 thin films presented the best response to butanone.
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Fig. 8. Relationship between relative response and working temperature of 1 at% Ce-doped four-layer SnO2 thin films calcined at different temperatures to 100 ppm of butanone vapor. Error bars stand for ±S.D.
For the gas sensor of metal oxide thin films, the gas-sensing responses were also affected by calcination temperature [6] and the thickness of thin films [31]. Fig. 8 showed the relationship between calcination temperature and the response of the four-layer 1 at% Ce-doped SnO2 thin films at different working temperature. It could be found that the thin films showed the best sensing-performance at calcination temperature of 500 ◦ C (relative response of 181–100 ppm of butanone vapor). At the lower calcination temperature, the responses were also lower, which were 119 and 126 at 400 and 450 ◦ C, respectively. The reason was that it was difficult to obtain a better form of polycrystalline thin films below 500 ◦ C. However, the crystal lattice of SnO2 became more ordered at higher calcination temperature, resulting in the increase of crystallite size. It was also unfavorable to improve the sensing response. The sensor responses to 100 ppm of butanone vapor were decreased to about 152 and 95 at the calcination temperature of 550 and 600 ◦ C, respectively. Thus, the optimal calcination temperature was about 500 ◦ C for the detection of butanone. The effect of the layers on the sensor response was also investigated. As shown in Fig. 9, four-layer 1 at% Ce-doped SnO2 thin films exhibited the best gas-sensing properties to butanone. When the layers of 1 at%
Fig. 9. Relationship between relative response and working temperature of 1 at% Ce-doped four-layer SnO2 thin films with different layers to 100 ppm of butanone vapor. Error bars stand for ±S.D.
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Fig. 10. Gas-sensing response curves of 1 at% Ce-doped four-layer SnO2 thin films to 100 ppm of butanone vapor at different relative humidity.
Ce-doped SnO2 thin films increased from two layers to four layers, the maximal response were increased from about 123 to 181 to 100 ppm of butanone vapor at 210 ◦ C. However, the maximal response decreased to 105 when the thin films increased to five layers. It was similar to the ethanol sensor based on Al-doped ZnO thin films [31]. It was noteworthy that the optimum working temperature was decreased to 180 ◦ C for five-layer Ce-doped SnO2 thin films, which could be explained by the diffusion-reaction model [32,33]. Due to diffusion warming effect, more layered thin films exhibited lower kinetic barriers for triggering detection reaction [33]. Accordingly, it could obtain the best of response under lower working temperature. In addition, the gas-sensing response curves of 1 at% Ce-doped four-layer SnO2 thin films to 100 ppm of butanone vapor at different relative humidity were shown in Fig. 10. The baseline of the sensor varied with the relative humidity. The conductance of the thin film rose with the increase of relative humidity, owing to the formation of multilayer physically absorbed water molecules and hydroxyl ions produced by the dissociation reaction of chemisorbs water vapor on the surface of sensing materials [18,34]. At the optimal working temperature of 210 ◦ C, the sensors exhibited the response of 181 to 100 ppm of butanone vapor in dry air. However, the gas sensors provided relative weak responses to 100 ppm of butanone vapor at 40% and 60% RH, which were about 66 and 43, respectively. The real time response curves of four-layer 1 at% Ce-doped SnO2 thin films to different concentrations of butanone were shown in Fig. 11. At the working temperature of 210 ◦ C, the sensors still held evident response value about 8–10 ppm of butanone vapor. Furthermore, the response sharply increased with gas concentration increased from 10 to 400 ppm. At the same time, the gas sensors exhibited a short response and recovery time. The selectivity of Ce-doped SnO2 thin films was also explored, and the results were shown in Fig. 12. For the four-layer 1 at% Cedoped SnO2 thin films, it had relatively weak responses to diethyl ether, ethyl acetate, chloroform, toluene, ammonia and so on. However, the gas sensor provided with good responses to ethanol and acetone; the responses were about 60 and 79 for 100 ppm of ethanol vapor and 100 ppm of acetone vapor, respectively. But, compared with that of the response to 100 ppm of butanone vapor, they were still lower. Obviously, the gas sensor based on 1 at% Ce-doped SnO2 thin films showed the good selectivity for butanone. Based on the above gas-sensing results, a possible mechanism of the Ce addition on the gas-sensing properties of SnO2 -based sensors was offered. Compared with the undoped SnO2 thin films, the Ce-doped SnO2 thin films were composed of smaller crystal-
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4. Conclusions In conclusion, different concentrations of Ce-doped SnO2 thin films were fabricated via the sol–gel method and dip-coating technique. The as-prepared Ce-doped SnO2 thin films kept regular and rough surface, which were composed of uniform grains. The gas-sensing results showed that the approach of doping cerium had greatly improved the gas-sensing response and decreased the working temperature. When the doping concentration of cerium was 1 at%, the gas sensor showed the best response, which was about 181–100 ppm of butanone vapor in dry air. Most importantly, the gas sensor presented selective response to butanone among all investigated gases. Accordingly, the gas sensors based on 1 at% Ce-doped SnO2 thin films have a promising application for the detection of butanone in practical security inspection environment. Acknowledgments
Fig. 11. Gas-sensing response curve of 1 at% Ce-doped four-layer SnO2 thin films to different concentrations of butanone vapor at 210 ◦ C.
lites and possessed rougher surface, more disordered structures and boundaries, which were positive effects to improve the gassensing properties. Additionally, with the existence of CeO2 as one of high-activity catalysts [35], the detected gases could be easily decomposed. Accordingly, for low concentration of Ce-doped SnO2 thin films (0.5 at% and 1.0 at%), the responses were both higher than that of undoped thin films. With the concentration of cerium increased to 3 at%, the response of 104 to butanone was still much better than 36.3 of undoped thin films. However, it was lower than 181 of 1 at% Ce-doped SnO2 thin films. It may be ascribed to the fact that the redundant CeO2 phase would partially enclose around SnO2 grain boundaries, which embarrassed the mobility of the carriers across the SnO2 grain boundaries. Then it was negative effect to improve the gas-sensing properties. Obviously, this negative effect was more notable in spite of abundant CeO2 phase with the high catalytic activities as the concentration of cerium continuously increased to 5 and 10 at%. As shown in Fig. 7, the response decreased to about 4.6 for 10 at% Ce-doped thin films, which was lower than 36.3 of undoped ones. Therefore, just appropriate concentration of cerium-doped SnO2 was propitious to the improvement of gassensing properties. It was similar with the gas-sensing model of Ce-doped ZnO thin films [16].
Fig. 12. Responses of 1 at% Ce-doped four-layer SnO2 thin films to 100 ppm of several kinds of gases or vapor at 210 ◦ C.
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Biographies Zhongwei Jiang received the B.S. degree in chemistry from Anhui University, China, in 2007. Since then, he has been a graduate student in Department of Chemistry at University of Science and Technology of China. He is doing his Master’s Thesis at Institute of Intelligent Machines, Chinese Academy of Sciences, China. His current research interests include semiconductor oxide thin films and their applications in gas nanosensors. Zheng Guo received the Ph.D. in 2008 from University of Science and Technology of China (Hefei, China). Currently, he works as an assistant researcher at Institute of Intelligent Machines, Chinese Academy of Sciences, China. His current research interests include synthesis of gas-sensing nanomaterials and biomimetric material and their applications in nanodevice. Bai Sun received the B.S. degree in 2002, Ph.D. in 2007, both from University of Science and Technology of China (Hefei, China). Currently, he works as an assistant researcher at Institute of Intelligent Machines, Chinese Academy of Sciences, China. His current research interests include synthesis of sensing nanomaterials and fabrication of gas sensors and cataluminescence sensors. Yong Jia received the B.S. degree in 1997, Ph.D. in 2009, both from Anhui University (Hefei, China). His research interests include synthesis of gas-sensing nanomaterials and fabrication of gas-sensing nanodevices. Minqiang Li received the B.S. degree in semiconductor material and device from University of Electronic Science and Technology of China in 1986. Since then, he has been at Institute of Intelligent Machines, Chinese Academy of Sciences, China, and currently as an associate researcher. His current research interests include semiconductor oxide gas nanosensors and their applications in detecting hazardous gases. Jinhuai Liu received the B.S. degree in inorganic chemistry from Yunnan Agricultural University, China, in 1982 and the Ph.D. degree in inorganic chemistry from Graduate University of Chinese Academy of Sciences, China, in 2003. Currently, he works as a Researcher at Institute of Intelligent Machines, Chinese Academy of Sciences. His current research interests include biomimetric material, gas-sensing nanomaterials and nanodevice, sensing technology and their applications in detecting hazardous gases and drug/explosive.