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A novel tin oxide-based recoverable thick film SO2 gas sensor promoted with magnesium and vanadium oxides
Sensors and Actuators B 160 (2011) 1328–1334
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A novel tin oxide-based recoverable thick film SO2 gas sensor promoted with magnesium and vanadium oxides Soo Chool Lee a , Byung Wook Hwang a , Soo Jae Lee b , Ho Yun Choi a , Seong Yeol Kim a , Suk Yong Jung a , Dhanusuraman Ragupathy a , Duk Dong Lee c , Jae Chang Kim a,∗ a
Department of Chemical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea Fuel Cell R&D Center, GS Caltex Co., Seoul 134-848, Republic of Korea c School of Electrical Engineering and Computer Science, Kyungpook National University, Daegu 702-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 5 March 2011 Received in revised form 21 September 2011 Accepted 22 September 2011 Available online 29 September 2011 Keywords: Sensor Tin oxide Sulfur dioxide MgO V2 O5
a b s t r a c t An SnO2 -based thick film gas sensor was developed to detect sulfur dioxide (SO2 ) gas at the ppm level. The SnO2 -based sensors were prepared by mixing tin oxide with various promoters such as V2 O5 , MoO3 , Sb2 O3 , Al2 O3 , CeO2 , and MgO. Their sensor responses and recovery behaviors were investigated in a flow system. It was found that the SnO2 -based gas sensor promoted by simultaneous mixing of both 5 wt% MgO and 2 wt% V2 O5 , showed not only a 44% sensor response for the detection of SO2 at the 1 ppm level, but also good repeatability by means of thermal treatment in air. In particular, this sensor showed a response of approximately 20% even at the level of 0.1 ppm (100 ppb). The sensor recovery properties, as well as the sensor response, could be enhanced by the simultaneous addition of both MgO and V2 O5 promoters as compared with the SnO2 sensor. These results are due to the promoting effects of MgO and V2 O5 on response and recovery properties of the SnO2 -based sensor, respectively. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Substantial interest has arisen in terms of protecting the environment from various air pollutants generated by combustion exhausts. Sulfur dioxide (SO2 ) is emitted from the many industrial processes that use sulfur-containing compounds, such as fossil fuels [1]. SO2 is one of the most hazardous atmospheric pollutants because it directly contributes to acid rain. Health effects caused by exposure to high levels of SO2 include breathing problems, respiratory illnesses, changes in the lung’s defences, and worsening respiratory and cardiovascular disease [2]. Several human-based studies have shown that repeated exposure to low levels of SO2 (<5 ppm) can cause permanent pulmonary impairment [3]. The long-term and short-term exposure limits for SO2 gas are 2 ppm and 5 ppm, respectively, though the acceptable limit for SO2 exposure in ambient air is much lower [4]. In recent years, there has been an increasing demand for sensing devices that monitor low concentrations of toxic gases, including SO2 [4–6]. SnO2 -based gas sensors have been used to detect toxic gases and chemical agent stimulants [7–19]. SnO2 is an ntype semiconductor oxide with a wide-energy-gap. The advantages
∗ Corresponding author. Tel.: +82 53 950 5622; fax: +82 53 950 6615. E-mail address: [email protected] (J.C. Kim). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.09.070
of a sensor fabricated with SnO2 include its high level of sensor response, simple design, low weight, and low cost. Das et al. reported that vanadium doped tin dioxide could detect SO2 at concentrations down to 5 ppm [4]. However, until now, there have been very few reports concerning the use of SnO2 -based gas sensors for very low concentrations of SO2 . In particular, there have not yet been any reports of SnO2 -based thick film gas sensors demonstrating excellent sensor response and complete recovery properties for the detection of SO2 at very low concentration levels below 1 ppm. The aim of this study was to develop a new recoverable SnO2 based thick film gas sensor to detect very low concentrations of SO2 at the level of <1 ppm. The effects of promoters on the response and recovery properties of SnO2 -based gas sensors were also investigated. 2. Experimental 2.1. Preparation of materials and sensors Tin oxide (SnO2 ) prepared by a precipitation method [SnO2 (P)] was used as a source for the SnO2 sensors in this study. SnO2 (P) was prepared by a precipitation method using tin chloride dihydrate (SnCl2 ·2H2 O, Aldrich, 99%) and an ammonium solution [16]. SnO2 -based materials were prepared by the physical mixing of two or three components with SnO2 (P) and promoters such as V2 O5 ,
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3. Results and discussion 3.1. Effects of the promoter on the sensor response and recovery properties
Fig. 1. Schematic diagram of the experimental system.
MoO3 , Sb2 O3 , Al2 O3 , CeO2 , and MgO. We denoted the materials as SnO2 (P)Mo5 , SnO2 (P)V5 , and SnO2 (P)Mg5 V2 ; where SnO2 (P) represents tin oxide prepared by a precipitation method, Mo represents MoO3 , Mg represents MgO, V represents V2 O5 , and 5 or 2 represents the amount of metal oxide added (wt%). All products were calcined in a muffle furnace at 600 ◦ C for 4 h. The thick-film sensors were fabricated on an alumina substrate with Pt electrodes and a Pt heater [16–18] by means of a screenprinting technique using various pure materials and an organic binder (␣-Terpineol, Aldrich, 90%). The printed thick-film sensors were dried and calcined at 600 ◦ C for 1 h. A detailed description of the sensor device and preparation method can be found in our previous papers [16–18].
2.2. Apparatus and procedure Fig. 1 shows a schematic diagram of the experimental system. Sensing behaviors were examined in a flow system with a 0.1 L chamber. The SO2 gas was diluted by dry air to a concentration of <1 ppm and then introduced into the chamber. The total gas flow rate diluted by air was 400 ml/min and the SO2 gas was injected for 200 s. In this study, the sensor response was defined by the following equation: sensor response (%) = [(Ra − Rg )/Ra ] × 100, where Ra and Rg represent the electric resistance in air and in the SO2 gas, respectively [16–18].
2.3. Characterization of materials To identify crystalline phases in the materials, X-ray diffraction (XRD; Philips, X’PERT) and Fourier transform infrared (FT-IR; Mattson Instruments Inc., Galaxy 7020A) were performed. A Philips X’PERT instrument using Cu K␣ radiation was used for these procedures. The morphologies of materials were observed by field emission scanning electron microscope (FE-SEM; JEOL, JSM-6701F).
To develop a new recoverable SnO2 -based thick film gas sensor to detect very low concentrations of SO2 at the level of <1 ppm, the response curves of an SnO2 sensor and SnO2 -based gas sensors promoted with various metal oxides such as V2 O5 , MoO3 , Sb2 O3 , Al2 O3 , CeO2 , and MgO were investigated at a low concentration of SO2 (1 ppm) in a flow system at 400 ◦ C. Table 1 shows responses of the SnO2 (P)-based gas sensors promoted with various metal oxides at a low concentration of SO2 (1 ppm) and 400 ◦ C and their 40% and 70% recovery times in air at 400 ◦ C. Sensor response levels and recovery times of the SnO2 -based sensors were calculated from their response curves. 40% and 70% recovery times were defined as the time taken by the sensor to get back 40% and 70% of the original resistance, respectively. The sensor response decreased in the following order: SnO2 (P)Mg5 > SnO2 (P)Ce5 > SnO2 (P)Sb5 > SnO2 (P) > SnO2 (P)Al5 > SnO2 (P)V5 > SnO2 (P)Mo5 . As shown in Table 1, the SnO2 (P)Mg5 and SnO2 (P)Ce5 sensors showed higher sensor response than that of the SnO2 (P) sensor. In particular, the SnO2 (P)Mg5 sensor, which was prepared by a physical mixing method with 95 wt% of SnO2 (P) and 5 wt% of MgO, showed the highest sensor response level of approximately 52% even at a concentration of 1 ppm. This result indicates that the SnO2 (P)Mg5 sensor promoted with MgO is more effective than SnO2 -based sensors promoted with other metal oxides for the detection of SO2 gas. However, 40% recovery time of the SnO2 (P)Mg5 sensor rapidly increased as compared with that of the SnO2 (P) sensor and it was not recovered up to 70% level of the original response. This means that the recovery of the SnO2 (P)Mg5 sensor is impossible under air at 400 ◦ C after the detection of SO2 gas. On the other hand, 70% recovery time was found in the case of the sensor promoted with 5 wt% V2 O5 [SnO2 (P)V5 ], even though the sensor response decreased. This result indicates that the recovery properties of the SnO2 -based sensor can be improved due to the promoting effect of V2 O5 .
3.2. Effects of V2 O5 promoter on the recovery properties In addition to the sensor response for the detection of SO2 , the recovery properties of the SnO2 -based sensor is one of the most important factors to be considered. To improve both the response and the recovery properties of the SnO2 -based sensor, new SnO2 based sensors were prepared by simultaneous addition of MgO and V2 O5 which were selected during screen tests. Fig. 2 shows the response curves of SnO2 -based sensors promoted with 1 wt% V2 O5 and various amounts of MgO from 1 wt% to 10 wt% at 1 ppm of SO2 at 400 ◦ C. The response of the SnO2 -based sensor increased from 19% to 50% as the level of the MgO promoter was increased from 1 wt% to 10 wt%. This result indicates that the MgO promoter directly contributed to the sensor response. However, in the case
Table 1 Responses of the SnO2 (P)-based gas sensors promoted with various metal oxides at a low concentration of SO2 (1 ppm) and 400 ◦ C and their recovery times in air at 400 ◦ C. Sensors
Composition (wt%) SnO2
SnO2 (P) SnO2 (P)V5 SnO2 (P)Mo5 SnO2 (P)Sb5 SnO2 (P)Al5 SnO2 (P)Ce5 SnO2 (P)Mg5
100 95 95 95 95 95 95
Sensor response (%)
40% Recovery time (s)
70% Recovery time (s)
34.0 9.7 7.6 34.1 30.9 39.7 52.1
90 290 246 – 590 – 1255
– 1195 – – – – –
Promoters – 5 (V2 O5 ) 5 (MoO3 ) 5 (Sb2 O3 ) 5 (Al2 O3 ) 5 (CeO2 ) 5 (MgO)
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Table 2 Responses and recovery times of the SnO2 (P) sensor and the SnO2 (P)-based gas sensors calculated from Figs. 2 and 3. Sensors
Composition (wt%)
SnO2 (P)Mg5 SnO2 (P)Mg1 V1 SnO2 (P)Mg5 V1 SnO2 (P)Mg10 V1 SnO2 (P)Mg5 V2 SnO2 (P)Mg5 V5
Sensor response (%)
MgO
V2 O5
5 1 5 10 5 5
– 1 1 1 2 5
52.1 18.2 38.4 51.3 44.2 24.4
80
Sensor response (%)
(a) SnO2(P)Mg1V1 (b) SnO2(P)Mg5V1
60
(c) SnO2(P)Mg10V1
(c)
40 (b)
20 (a)
0 0
500
1000
1500
2000
2500
Time (sec) Fig. 2. Response curves of the SnO2 -based sensors promoted with 1 wt% V2 O5 and various amounts of MgO: (a) SnO2 (P)Mg1 V1 , (b) SnO2 (P)Mg5 V1 , and (c) SnO2 (P)Mg10 V1 .
of 10 wt% MgO, the recovery properties of the sensor deteriorated as compared with those of other sensors. Fig. 3 shows the response curves of SnO2 -based sensors promoted with 5 wt% MgO and various amounts of V2 O5 . The sensing behaviors of SnO2 -based sensors promoted with both 5 wt% MgO and various amounts of V2 O5 ranging from 1 wt% to 5 wt% were investigated under the same conditions. It was found that a SnO2 (P)Mg5 V2 sensor promoted with both 5 wt% MgO and 2 wt% V2 O5 showed the excellent sensor response and recovery properties as shown in Fig. 3. To explain in detail the responses and recovery properties of the SnO2 (P) sensor and the SnO2 (P)-based sensors promoted with metal oxides, sensor responses, the sensor responses and 40% and
80
Sensor response (%)
(a) SnO2(P)Mg5V1 (b) SnO2(P)Mg5V2
60
(c) SnO2(P)Mg5V5 (b)
40 (a)
20 (c)
0 0
500
1000
1500
2000
2500
Time (sec) Fig. 3. Response curves of SnO2 -based sensors promoted with 5 wt% MgO and various amounts of V2 O5 : (a) SnO2 (P)Mg5 V1 , (b) SnO2 (P)Mg5 V2 , and (c) SnO2 (P)Mg5 V5 .
40% Recovery time (s)
1255 130 250 1940 230 180
70% Recovery time (s)
– 325 770 – 625 415
70% recovery times were calculated from Figs. 2 and 3. These results are shown in Table 2. The response of the SnO2 -based sensor was enhanced by adding MgO to SnO2 (P) as previously mentioned, while its recovery time rapidly increased. However, when the amount of V2 O5 was increased from 1 wt% to 5 wt%, the recovery times of the SnO2 (P)-based sensors reduced from 770 s to 415 s. In particular, the SnO2 (P)Mg5 V2 sensor that was promoted with both 5 wt% MgO and 2 wt% V2 O5 showed 40% and 70% recovery times of about 230 and 625 s, respectively, as well as an excellent sensor response of 44% at 1 ppm. It is clear that both the sensor response and recovery time can be improved by promoting the sensor with both MgO and V2 O5 . From these results, it was concluded that the MgO promoter plays an important role in the sensor response and that the V2 O5 promoter plays an important role in the recovery properties. Even though further study is required to verify the sensing mechanism and the reason for the enhancement in sensor response, it is thought that the enhanced response of the SnO2 -based sensor promoted with MgO is due to the SO2 sorption capacity of MgO, as well as SnO2 . Lee et al. reported that Mg-based sorbents promoted with cerium and iron oxides could absorb SO2 and that the MgSO4 was formed during SO2 absorption [20,21]. The ceria promoter played an important role in the catalytic oxidation of SO2 to SO3 which could be easily absorbed by the MgO [20]. In this study, it is thought that SnO2 worked as an oxidation catalyst and adsorbent as other researchers have mentioned [4,22]. In addition, further study is necessary to verify the catalytic role of the V2 O5 promoter in the enhancement of the recovery properties. 3.3. Structure identification of the materials and SnO2 -based sensors SEM morphologies of the thick layer of the SnO2 (C) sensor and surfaces of the SnO2 (C), SnO2 (P), SnO2 (P)Mg5 , SnO2 (P)V5 , and SnO2 (P)Mg5 V2 sensors were observed at ×2 K and ×100 K, respectively, and these results are shown in Fig. 4. Commercial tin oxide [SnO2 (C)] has a particle size range between about 30 nm and 200 nm. On the other hand, the tin oxides [SnO2 (P)] prepared by the precipitation method were composed of nano-sized particles of 20–40 nm with narrow size distribution. The film thickness of the SnO2 (C) sensor was observed to be approximately 20 m as shown in Fig. 4a. In a separate experiment, it was observed that the thicknesses of other sensors were almost similar to that of the SnO2 (C) sensor. Fig. 5 shows the XRD patterns of the SnO2 (C), SnO2 (P), SnO2 (P)Mg5 , SnO2 (P)V5 , and SnO2 (P)Mg5 V2 materials. SnO2 (C) showed a perfect tetragonal structure like SnO2 (JCPDS No. 880287), while SnO2 (P) prepared by the precipitation method showed an orthorhombic structure (JCPDS No. 29-1484), as well as a tetragonal structure (JCPDS No. 29-1484) as reported in our previous papers [16]. As shown in Fig. 5c, the XRD peaks of SnO2 (P)Mg5 showed only the two types of SnO2 , the tetragonal structure and the orthorhombic structure, due to the SnO2 being prepared by the precipitation method, without an MgO phase. The XRD patterns of SnO2 (P)V5 showed a V2 O5 phase in addition to two types of
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Fig. 4. SEM images of (a) a thick layer of the SnO2 (C) sensor and the surfaces of the (b) SnO2 (C), (c) SnO2 (P), (d) SnO2 (P)Mg5 , (e) SnO2 (P)V5 , and (f) SnO2 (P)Mg5 V2 sensors.
SnO2 . The new diffraction lines appeared with a 2 value of 20.26, 26.12, and 31.00, which were assigned to the V2 O5 phase (JCPDS No. 41-1426). The XRD patterns of SnO2 (P)Mg5 V2 showed only the SnO2 phases, as shown in Fig. 5e. Unfortunately, the MgO and V2 O5 phases were not observed in these XRD results. To precisely identify the structure of these materials, the FTIR experiments for the SnO2 (C), SnO2 (P)Mg5 , SnO2 (P)V5 , and SnO2 (P)Mg5 V2 materials were carried out in a range from 4000 cm−1 to 400 cm−1 , and the results are shown in Fig. 6. As expected, the absorption peaks of SnO2 appeared at 497–790 and 1625 cm−1 , those of MgO at 1420 and 1493 cm−1 , and those of V2 O5 at 820 and 1009 cm−1 . These results are in agreement with
those obtained by Nyquist and Kagel [23]. It is clear that the MgO and V2 O5 phases play an important role in the response of the SnO2 (P)Mg5 V2 sensor, as well as in terms of its recovery properties. In other words, the sensor response and the recovery properties were directly related to promoters that contained the MgO and V2 O5 phases. 3.4. Sensing behaviors at various temperatures and concentrations Fig. 7 shows the responses of the SnO2 (P) and SnO2 (P)Mg5 V2 sensors at 1 ppm of SO2 and a temperature range between 350 ◦ C
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SnO2(P)
Sensor response (%)
(a)
Intensity (a.u.)
(b)
SnO2(P)Mg5V2
60
40
20
0 (c)
350
400
450 o
Temperature ( C) Fig. 7. Responses of the SnO2 (P) and SnO2 (P)Mg5 V2 sensors at 1 ppm of SO2 and a temperature range between 350 ◦ C and 450 ◦ C.
(d)
and 450 ◦ C. The SnO2 (P) sensor showed a very low sensor response of about 30% at a temperature range between 350 ◦ C and 450 ◦ C. The response of the SnO2 (P)Mg5 V2 sensor could be enhanced at all temperatures resulting from the effect of promoters such as MgO and V2 O5 . Fig. 8 shows the responses of the SnO2 (P)Mg5 V2
(e)
a
20
30
40
50
60
70
100
80
350oC 400oC 450oC
80
Fig. 5. XRD patterns of the (a) SnO2 (C), (b) SnO2 (P), (c) SnO2 (P)Mg5 , (d) SnO2 (P)V5 , and (e) SnO2 (P)Mg5 V2 materials: () SnO2 (tetragonal); (䊉) SnO2 (orthorhombic); () V2 O5 .
(a)
Sensor response (%)
2θ (deg.)
60
40
20
0 0.0
0.4
0.6
0.8
1.0
1.2
The concentration of SO2 (ppm) b
100 o
350 C o 400 C o 450 C
80
Sensor response (%)
Transmittance (A.U.)
(b)
0.2
(c)
(d)
60
40
20
0 0
4000
3500
3000
2500
2000
1500
1000
500
Wave number (cm-1) Fig. 6. FT-IR spectra of the (a) SnO2 (P), (b) SnO2 (P)Mg5 , (c) SnO2 (P)V5 , and (d) SnO2 (P)Mg5 V2 materials.
500
1000
1500
2000
2500
Time (s) Fig. 8. (a) Responses of the SnO2 (P)Mg5 V2 sensor at a concentration range between 0.1 ppm and 1 ppm and various detection temperatures and (b) their response curves at 1 ppm SO2 .
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100
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level was maintained during multiple cycles without deactivation, unlike the results of the SnO2 sensor.
SnO2(P)Mg5V2 SnO2(P)
Senssor response (%)
80
Acknowledgments 60
This work was supported by the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (2010201020007A). This work was supported by Energy Efficiency and Resources R&D program (2008CCD11P020000) under the Ministry of Knowledge Economy, Republic of Korea.
(a)
40
20
(b) 0 0
2000
4000
6000
8000
10000
12000
Time (sec) Fig. 9. Response repeatabilities of the (a) SnO2 (P) and (b) SnO2 (P)Mg5 V2 sensors at 1 ppm of SO2 and 400 ◦ C.
sensor at a concentration range between 0.1 ppm and 1 ppm and various detection temperatures and their response curves at 1 ppm SO2 . All responses of those sensors increased almost linearly in the concentration range between 0.1 ppm and 1 ppm at all temperatures including 350 ◦ C, 400 ◦ C, and 450 ◦ C. In particular, the SnO2 (P)Mg5 V2 sensor showed a relatively good sensor response of about 21% even at a very low concentration of 0.1 ppm SO2 at 400 ◦ C. As shown in Fig. 8b, the SnO2 (P)Mg5 V2 sensor showed a higher recovery properties at 400 ◦ C and 450 ◦ C than that at 350 ◦ C. Considering that sensing behaviors such as sensor response and the recovery properties at very low concentration levels are very important factors for detecting SO2 gas, the recoverable SnO2 (P)Mg5 V2 sensor developed in this study has great potential for the detection of SO2 at the ppm level. 3.5. Repeatability characteristics of SnO2 -based sensors Fig. 9 shows the response repeatabilities of the SnO2 (P) and SnO2 (P)Mg5 V2 sensors at 1 ppm of SO2 and 400 ◦ C. As shown in Fig. 9, the response of the SnO2 (P) sensor decreased during the multiple cycles of detection and recovery, while the SnO2 (P)Mg5 V2 sensor showed a relatively high sensor response level of about 44% and its sensor response was maintained after 3 cycles. In other words, the important point to note is that the SnO2 -based thick film gas sensor promoted with MgO and V2 O5 demonstrated excellent repeatability, as well as an enhanced sensor response, when compared with the SnO2 (P) sensor. It is concluded that the recoverable SnO2 -based sensor promoted with MgO and V2 O5 developed in this study is an excellent thick film gas sensor for the detection of SO2 gas of a very low ppm level in that it satisfies the requirements for excellent sensor response and recovery properties. 4. Conclusion An SnO2 -based thick film gas sensor was developed in this study for the detection of SO2 gas. It shows a high sensor response and excellent recovery properties. The best SnO2 -based thick film gas sensor from among the several sensors tested was prepared by the simultaneous addition of both the MgO and V2 O5 promoters. It was found that the MgO promoter played an important role in sensor response. In addition, it was found that the recovery properties and sensor response could be enhanced if both MgO and V2 O5 were simultaneously added to SnO2 (P). In particular, the SnO2 -based sensor [SnO2 (P)Mg5 V2 ] promoted with both 5 wt% MgO and 2 wt% V2 O5 showed a sensor response of about 44%, and its response
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Biographies Soo Chool Lee received a BSc degree from Keimyung University, South Korea in 1998, and MSc and PhD degrees from Kyungpook National University in 2000 and 2006, respectively. He is currently a research professor at the Research Institute of Advanced Energy Technology in Kyungpook National University, South Korea. His current research interests include CO2 removal sorbents and metal oxide gas sensors. Byung Wook Hwang received a BSc degree from the Department of Environmental Engineering, Kyungpook National University, South Korea, in 2010. He is currently studying for an MSc degree in the Department of Chemical Engineering, Kyungpook National University, Daegu, South Korea. His current research interests include SnO2 gas sensors. Soo Jae Lee received a BSc degree from Keimyung University, South Korea in 2002 and MSc and PhD degrees from Kyungpook National University in 2004 and 2009, respectively. He is currently working for Gs Fuel Cell Co., Ltd. His current research interests include SO2 absorbents and fuel cells. Ho Yun Choi received BSc and MSc degrees from Kyungpook National University in 2005 and 2007, respectively. He is currently studying for a PhD in the Department of Chemical Engineering, Kyungpook National University, Daegu, South Korea. His current research interests include SnO2 gas sensors. Seong Yeol Kim received BSc and MSc degrees from the Department of Chemical Engineering, Kyungpook National University, South Korea, in 1999 and 2005, respectively. He is currently studying for a PhD in the Department of Chemical Engineering, Kyungpook National University, Daegu, South Korea. His current research interests include SnO2 gas sensors.
Suk Yong Jung received a BS degree from the Department of Chemical Engineering at Kyungil University in 2001, MSc and PhD degrees from the Department of Chemical Engineering, Kyungpook National University in 2003 and 2008, respectively. He is currently a Post Doctoral Fellow in the Department of Chemical Engineering at Kyungpook National University, Daegu, South Korea. His current research interests include H2 S removal, DME synthesis, fuel cells, and reforming. Dhanusuraman Ragupathy received an MSc degree in chemistry from Bharathiar University, Coimbatore, India (2005). He completed his PhD degree in the Department of Chemistry, Kyungpook National University, South Korea (2010). Currently, he is a Post Doctoral Fellow in the Department of Chemical Engineering, Kyungpook National University, South Korea. He has published 10 research articles in international journals. His research field includes the development of new nanomaterials for biosensor and electrocatalytic applications. Duk Dong Lee received a BSc degree in physics and an MS degree in electronics from Kyungpook National University, Daegu, South Korea, in 1966 and 1974, respectively, and a PhD from Yonsei University, Seoul, South Korea, in 1984. He is currently a professor at the School of Electrical Engineering and Computer Science in Kyungpook National University, Daegu, South Korea. He has performed research on semiconductor gas sensors since 1978, and has also carried out research in the field of electronic nose. Jae Chang Kim received his BS from the Department of Chemical Engineering in 1982 from Yonsei University, and his MSc and PhD from the Department of Chemical Engineering from the Korea Advanced Institute of Science and Technology (KAIST) in 1984 and 1988, respectively. He is currently a professor in the Department of Chemical Engineering at Kyungpook National University, Daegu, South Korea. His current research interests include CO2 capture, SO2 removal, H2 S removal, DME synthesis, and metal oxide gas sensors.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A novel tin oxide-based recoverable thick film SO2 gas sensor promoted with magnesium and vanadium oxides Soo Chool Lee a , Byung Wook Hwang a , Soo Jae Lee b , Ho Yun Choi a , Seong Yeol Kim a , Suk Yong Jung a , Dhanusuraman Ragupathy a , Duk Dong Lee c , Jae Chang Kim a,∗ a
Department of Chemical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea Fuel Cell R&D Center, GS Caltex Co., Seoul 134-848, Republic of Korea c School of Electrical Engineering and Computer Science, Kyungpook National University, Daegu 702-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 5 March 2011 Received in revised form 21 September 2011 Accepted 22 September 2011 Available online 29 September 2011 Keywords: Sensor Tin oxide Sulfur dioxide MgO V2 O5
a b s t r a c t An SnO2 -based thick film gas sensor was developed to detect sulfur dioxide (SO2 ) gas at the ppm level. The SnO2 -based sensors were prepared by mixing tin oxide with various promoters such as V2 O5 , MoO3 , Sb2 O3 , Al2 O3 , CeO2 , and MgO. Their sensor responses and recovery behaviors were investigated in a flow system. It was found that the SnO2 -based gas sensor promoted by simultaneous mixing of both 5 wt% MgO and 2 wt% V2 O5 , showed not only a 44% sensor response for the detection of SO2 at the 1 ppm level, but also good repeatability by means of thermal treatment in air. In particular, this sensor showed a response of approximately 20% even at the level of 0.1 ppm (100 ppb). The sensor recovery properties, as well as the sensor response, could be enhanced by the simultaneous addition of both MgO and V2 O5 promoters as compared with the SnO2 sensor. These results are due to the promoting effects of MgO and V2 O5 on response and recovery properties of the SnO2 -based sensor, respectively. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Substantial interest has arisen in terms of protecting the environment from various air pollutants generated by combustion exhausts. Sulfur dioxide (SO2 ) is emitted from the many industrial processes that use sulfur-containing compounds, such as fossil fuels [1]. SO2 is one of the most hazardous atmospheric pollutants because it directly contributes to acid rain. Health effects caused by exposure to high levels of SO2 include breathing problems, respiratory illnesses, changes in the lung’s defences, and worsening respiratory and cardiovascular disease [2]. Several human-based studies have shown that repeated exposure to low levels of SO2 (<5 ppm) can cause permanent pulmonary impairment [3]. The long-term and short-term exposure limits for SO2 gas are 2 ppm and 5 ppm, respectively, though the acceptable limit for SO2 exposure in ambient air is much lower [4]. In recent years, there has been an increasing demand for sensing devices that monitor low concentrations of toxic gases, including SO2 [4–6]. SnO2 -based gas sensors have been used to detect toxic gases and chemical agent stimulants [7–19]. SnO2 is an ntype semiconductor oxide with a wide-energy-gap. The advantages
∗ Corresponding author. Tel.: +82 53 950 5622; fax: +82 53 950 6615. E-mail address: [email protected] (J.C. Kim). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.09.070
of a sensor fabricated with SnO2 include its high level of sensor response, simple design, low weight, and low cost. Das et al. reported that vanadium doped tin dioxide could detect SO2 at concentrations down to 5 ppm [4]. However, until now, there have been very few reports concerning the use of SnO2 -based gas sensors for very low concentrations of SO2 . In particular, there have not yet been any reports of SnO2 -based thick film gas sensors demonstrating excellent sensor response and complete recovery properties for the detection of SO2 at very low concentration levels below 1 ppm. The aim of this study was to develop a new recoverable SnO2 based thick film gas sensor to detect very low concentrations of SO2 at the level of <1 ppm. The effects of promoters on the response and recovery properties of SnO2 -based gas sensors were also investigated. 2. Experimental 2.1. Preparation of materials and sensors Tin oxide (SnO2 ) prepared by a precipitation method [SnO2 (P)] was used as a source for the SnO2 sensors in this study. SnO2 (P) was prepared by a precipitation method using tin chloride dihydrate (SnCl2 ·2H2 O, Aldrich, 99%) and an ammonium solution [16]. SnO2 -based materials were prepared by the physical mixing of two or three components with SnO2 (P) and promoters such as V2 O5 ,
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3. Results and discussion 3.1. Effects of the promoter on the sensor response and recovery properties
Fig. 1. Schematic diagram of the experimental system.
MoO3 , Sb2 O3 , Al2 O3 , CeO2 , and MgO. We denoted the materials as SnO2 (P)Mo5 , SnO2 (P)V5 , and SnO2 (P)Mg5 V2 ; where SnO2 (P) represents tin oxide prepared by a precipitation method, Mo represents MoO3 , Mg represents MgO, V represents V2 O5 , and 5 or 2 represents the amount of metal oxide added (wt%). All products were calcined in a muffle furnace at 600 ◦ C for 4 h. The thick-film sensors were fabricated on an alumina substrate with Pt electrodes and a Pt heater [16–18] by means of a screenprinting technique using various pure materials and an organic binder (␣-Terpineol, Aldrich, 90%). The printed thick-film sensors were dried and calcined at 600 ◦ C for 1 h. A detailed description of the sensor device and preparation method can be found in our previous papers [16–18].
2.2. Apparatus and procedure Fig. 1 shows a schematic diagram of the experimental system. Sensing behaviors were examined in a flow system with a 0.1 L chamber. The SO2 gas was diluted by dry air to a concentration of <1 ppm and then introduced into the chamber. The total gas flow rate diluted by air was 400 ml/min and the SO2 gas was injected for 200 s. In this study, the sensor response was defined by the following equation: sensor response (%) = [(Ra − Rg )/Ra ] × 100, where Ra and Rg represent the electric resistance in air and in the SO2 gas, respectively [16–18].
2.3. Characterization of materials To identify crystalline phases in the materials, X-ray diffraction (XRD; Philips, X’PERT) and Fourier transform infrared (FT-IR; Mattson Instruments Inc., Galaxy 7020A) were performed. A Philips X’PERT instrument using Cu K␣ radiation was used for these procedures. The morphologies of materials were observed by field emission scanning electron microscope (FE-SEM; JEOL, JSM-6701F).
To develop a new recoverable SnO2 -based thick film gas sensor to detect very low concentrations of SO2 at the level of <1 ppm, the response curves of an SnO2 sensor and SnO2 -based gas sensors promoted with various metal oxides such as V2 O5 , MoO3 , Sb2 O3 , Al2 O3 , CeO2 , and MgO were investigated at a low concentration of SO2 (1 ppm) in a flow system at 400 ◦ C. Table 1 shows responses of the SnO2 (P)-based gas sensors promoted with various metal oxides at a low concentration of SO2 (1 ppm) and 400 ◦ C and their 40% and 70% recovery times in air at 400 ◦ C. Sensor response levels and recovery times of the SnO2 -based sensors were calculated from their response curves. 40% and 70% recovery times were defined as the time taken by the sensor to get back 40% and 70% of the original resistance, respectively. The sensor response decreased in the following order: SnO2 (P)Mg5 > SnO2 (P)Ce5 > SnO2 (P)Sb5 > SnO2 (P) > SnO2 (P)Al5 > SnO2 (P)V5 > SnO2 (P)Mo5 . As shown in Table 1, the SnO2 (P)Mg5 and SnO2 (P)Ce5 sensors showed higher sensor response than that of the SnO2 (P) sensor. In particular, the SnO2 (P)Mg5 sensor, which was prepared by a physical mixing method with 95 wt% of SnO2 (P) and 5 wt% of MgO, showed the highest sensor response level of approximately 52% even at a concentration of 1 ppm. This result indicates that the SnO2 (P)Mg5 sensor promoted with MgO is more effective than SnO2 -based sensors promoted with other metal oxides for the detection of SO2 gas. However, 40% recovery time of the SnO2 (P)Mg5 sensor rapidly increased as compared with that of the SnO2 (P) sensor and it was not recovered up to 70% level of the original response. This means that the recovery of the SnO2 (P)Mg5 sensor is impossible under air at 400 ◦ C after the detection of SO2 gas. On the other hand, 70% recovery time was found in the case of the sensor promoted with 5 wt% V2 O5 [SnO2 (P)V5 ], even though the sensor response decreased. This result indicates that the recovery properties of the SnO2 -based sensor can be improved due to the promoting effect of V2 O5 .
3.2. Effects of V2 O5 promoter on the recovery properties In addition to the sensor response for the detection of SO2 , the recovery properties of the SnO2 -based sensor is one of the most important factors to be considered. To improve both the response and the recovery properties of the SnO2 -based sensor, new SnO2 based sensors were prepared by simultaneous addition of MgO and V2 O5 which were selected during screen tests. Fig. 2 shows the response curves of SnO2 -based sensors promoted with 1 wt% V2 O5 and various amounts of MgO from 1 wt% to 10 wt% at 1 ppm of SO2 at 400 ◦ C. The response of the SnO2 -based sensor increased from 19% to 50% as the level of the MgO promoter was increased from 1 wt% to 10 wt%. This result indicates that the MgO promoter directly contributed to the sensor response. However, in the case
Table 1 Responses of the SnO2 (P)-based gas sensors promoted with various metal oxides at a low concentration of SO2 (1 ppm) and 400 ◦ C and their recovery times in air at 400 ◦ C. Sensors
Composition (wt%) SnO2
SnO2 (P) SnO2 (P)V5 SnO2 (P)Mo5 SnO2 (P)Sb5 SnO2 (P)Al5 SnO2 (P)Ce5 SnO2 (P)Mg5
100 95 95 95 95 95 95
Sensor response (%)
40% Recovery time (s)
70% Recovery time (s)
34.0 9.7 7.6 34.1 30.9 39.7 52.1
90 290 246 – 590 – 1255
– 1195 – – – – –
Promoters – 5 (V2 O5 ) 5 (MoO3 ) 5 (Sb2 O3 ) 5 (Al2 O3 ) 5 (CeO2 ) 5 (MgO)
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Table 2 Responses and recovery times of the SnO2 (P) sensor and the SnO2 (P)-based gas sensors calculated from Figs. 2 and 3. Sensors
Composition (wt%)
SnO2 (P)Mg5 SnO2 (P)Mg1 V1 SnO2 (P)Mg5 V1 SnO2 (P)Mg10 V1 SnO2 (P)Mg5 V2 SnO2 (P)Mg5 V5
Sensor response (%)
MgO
V2 O5
5 1 5 10 5 5
– 1 1 1 2 5
52.1 18.2 38.4 51.3 44.2 24.4
80
Sensor response (%)
(a) SnO2(P)Mg1V1 (b) SnO2(P)Mg5V1
60
(c) SnO2(P)Mg10V1
(c)
40 (b)
20 (a)
0 0
500
1000
1500
2000
2500
Time (sec) Fig. 2. Response curves of the SnO2 -based sensors promoted with 1 wt% V2 O5 and various amounts of MgO: (a) SnO2 (P)Mg1 V1 , (b) SnO2 (P)Mg5 V1 , and (c) SnO2 (P)Mg10 V1 .
of 10 wt% MgO, the recovery properties of the sensor deteriorated as compared with those of other sensors. Fig. 3 shows the response curves of SnO2 -based sensors promoted with 5 wt% MgO and various amounts of V2 O5 . The sensing behaviors of SnO2 -based sensors promoted with both 5 wt% MgO and various amounts of V2 O5 ranging from 1 wt% to 5 wt% were investigated under the same conditions. It was found that a SnO2 (P)Mg5 V2 sensor promoted with both 5 wt% MgO and 2 wt% V2 O5 showed the excellent sensor response and recovery properties as shown in Fig. 3. To explain in detail the responses and recovery properties of the SnO2 (P) sensor and the SnO2 (P)-based sensors promoted with metal oxides, sensor responses, the sensor responses and 40% and
80
Sensor response (%)
(a) SnO2(P)Mg5V1 (b) SnO2(P)Mg5V2
60
(c) SnO2(P)Mg5V5 (b)
40 (a)
20 (c)
0 0
500
1000
1500
2000
2500
Time (sec) Fig. 3. Response curves of SnO2 -based sensors promoted with 5 wt% MgO and various amounts of V2 O5 : (a) SnO2 (P)Mg5 V1 , (b) SnO2 (P)Mg5 V2 , and (c) SnO2 (P)Mg5 V5 .
40% Recovery time (s)
1255 130 250 1940 230 180
70% Recovery time (s)
– 325 770 – 625 415
70% recovery times were calculated from Figs. 2 and 3. These results are shown in Table 2. The response of the SnO2 -based sensor was enhanced by adding MgO to SnO2 (P) as previously mentioned, while its recovery time rapidly increased. However, when the amount of V2 O5 was increased from 1 wt% to 5 wt%, the recovery times of the SnO2 (P)-based sensors reduced from 770 s to 415 s. In particular, the SnO2 (P)Mg5 V2 sensor that was promoted with both 5 wt% MgO and 2 wt% V2 O5 showed 40% and 70% recovery times of about 230 and 625 s, respectively, as well as an excellent sensor response of 44% at 1 ppm. It is clear that both the sensor response and recovery time can be improved by promoting the sensor with both MgO and V2 O5 . From these results, it was concluded that the MgO promoter plays an important role in the sensor response and that the V2 O5 promoter plays an important role in the recovery properties. Even though further study is required to verify the sensing mechanism and the reason for the enhancement in sensor response, it is thought that the enhanced response of the SnO2 -based sensor promoted with MgO is due to the SO2 sorption capacity of MgO, as well as SnO2 . Lee et al. reported that Mg-based sorbents promoted with cerium and iron oxides could absorb SO2 and that the MgSO4 was formed during SO2 absorption [20,21]. The ceria promoter played an important role in the catalytic oxidation of SO2 to SO3 which could be easily absorbed by the MgO [20]. In this study, it is thought that SnO2 worked as an oxidation catalyst and adsorbent as other researchers have mentioned [4,22]. In addition, further study is necessary to verify the catalytic role of the V2 O5 promoter in the enhancement of the recovery properties. 3.3. Structure identification of the materials and SnO2 -based sensors SEM morphologies of the thick layer of the SnO2 (C) sensor and surfaces of the SnO2 (C), SnO2 (P), SnO2 (P)Mg5 , SnO2 (P)V5 , and SnO2 (P)Mg5 V2 sensors were observed at ×2 K and ×100 K, respectively, and these results are shown in Fig. 4. Commercial tin oxide [SnO2 (C)] has a particle size range between about 30 nm and 200 nm. On the other hand, the tin oxides [SnO2 (P)] prepared by the precipitation method were composed of nano-sized particles of 20–40 nm with narrow size distribution. The film thickness of the SnO2 (C) sensor was observed to be approximately 20 m as shown in Fig. 4a. In a separate experiment, it was observed that the thicknesses of other sensors were almost similar to that of the SnO2 (C) sensor. Fig. 5 shows the XRD patterns of the SnO2 (C), SnO2 (P), SnO2 (P)Mg5 , SnO2 (P)V5 , and SnO2 (P)Mg5 V2 materials. SnO2 (C) showed a perfect tetragonal structure like SnO2 (JCPDS No. 880287), while SnO2 (P) prepared by the precipitation method showed an orthorhombic structure (JCPDS No. 29-1484), as well as a tetragonal structure (JCPDS No. 29-1484) as reported in our previous papers [16]. As shown in Fig. 5c, the XRD peaks of SnO2 (P)Mg5 showed only the two types of SnO2 , the tetragonal structure and the orthorhombic structure, due to the SnO2 being prepared by the precipitation method, without an MgO phase. The XRD patterns of SnO2 (P)V5 showed a V2 O5 phase in addition to two types of
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Fig. 4. SEM images of (a) a thick layer of the SnO2 (C) sensor and the surfaces of the (b) SnO2 (C), (c) SnO2 (P), (d) SnO2 (P)Mg5 , (e) SnO2 (P)V5 , and (f) SnO2 (P)Mg5 V2 sensors.
SnO2 . The new diffraction lines appeared with a 2 value of 20.26, 26.12, and 31.00, which were assigned to the V2 O5 phase (JCPDS No. 41-1426). The XRD patterns of SnO2 (P)Mg5 V2 showed only the SnO2 phases, as shown in Fig. 5e. Unfortunately, the MgO and V2 O5 phases were not observed in these XRD results. To precisely identify the structure of these materials, the FTIR experiments for the SnO2 (C), SnO2 (P)Mg5 , SnO2 (P)V5 , and SnO2 (P)Mg5 V2 materials were carried out in a range from 4000 cm−1 to 400 cm−1 , and the results are shown in Fig. 6. As expected, the absorption peaks of SnO2 appeared at 497–790 and 1625 cm−1 , those of MgO at 1420 and 1493 cm−1 , and those of V2 O5 at 820 and 1009 cm−1 . These results are in agreement with
those obtained by Nyquist and Kagel [23]. It is clear that the MgO and V2 O5 phases play an important role in the response of the SnO2 (P)Mg5 V2 sensor, as well as in terms of its recovery properties. In other words, the sensor response and the recovery properties were directly related to promoters that contained the MgO and V2 O5 phases. 3.4. Sensing behaviors at various temperatures and concentrations Fig. 7 shows the responses of the SnO2 (P) and SnO2 (P)Mg5 V2 sensors at 1 ppm of SO2 and a temperature range between 350 ◦ C
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SnO2(P)
Sensor response (%)
(a)
Intensity (a.u.)
(b)
SnO2(P)Mg5V2
60
40
20
0 (c)
350
400
450 o
Temperature ( C) Fig. 7. Responses of the SnO2 (P) and SnO2 (P)Mg5 V2 sensors at 1 ppm of SO2 and a temperature range between 350 ◦ C and 450 ◦ C.
(d)
and 450 ◦ C. The SnO2 (P) sensor showed a very low sensor response of about 30% at a temperature range between 350 ◦ C and 450 ◦ C. The response of the SnO2 (P)Mg5 V2 sensor could be enhanced at all temperatures resulting from the effect of promoters such as MgO and V2 O5 . Fig. 8 shows the responses of the SnO2 (P)Mg5 V2
(e)
a
20
30
40
50
60
70
100
80
350oC 400oC 450oC
80
Fig. 5. XRD patterns of the (a) SnO2 (C), (b) SnO2 (P), (c) SnO2 (P)Mg5 , (d) SnO2 (P)V5 , and (e) SnO2 (P)Mg5 V2 materials: () SnO2 (tetragonal); (䊉) SnO2 (orthorhombic); () V2 O5 .
(a)
Sensor response (%)
2θ (deg.)
60
40
20
0 0.0
0.4
0.6
0.8
1.0
1.2
The concentration of SO2 (ppm) b
100 o
350 C o 400 C o 450 C
80
Sensor response (%)
Transmittance (A.U.)
(b)
0.2
(c)
(d)
60
40
20
0 0
4000
3500
3000
2500
2000
1500
1000
500
Wave number (cm-1) Fig. 6. FT-IR spectra of the (a) SnO2 (P), (b) SnO2 (P)Mg5 , (c) SnO2 (P)V5 , and (d) SnO2 (P)Mg5 V2 materials.
500
1000
1500
2000
2500
Time (s) Fig. 8. (a) Responses of the SnO2 (P)Mg5 V2 sensor at a concentration range between 0.1 ppm and 1 ppm and various detection temperatures and (b) their response curves at 1 ppm SO2 .
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level was maintained during multiple cycles without deactivation, unlike the results of the SnO2 sensor.
SnO2(P)Mg5V2 SnO2(P)
Senssor response (%)
80
Acknowledgments 60
This work was supported by the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (2010201020007A). This work was supported by Energy Efficiency and Resources R&D program (2008CCD11P020000) under the Ministry of Knowledge Economy, Republic of Korea.
(a)
40
20
(b) 0 0
2000
4000
6000
8000
10000
12000
Time (sec) Fig. 9. Response repeatabilities of the (a) SnO2 (P) and (b) SnO2 (P)Mg5 V2 sensors at 1 ppm of SO2 and 400 ◦ C.
sensor at a concentration range between 0.1 ppm and 1 ppm and various detection temperatures and their response curves at 1 ppm SO2 . All responses of those sensors increased almost linearly in the concentration range between 0.1 ppm and 1 ppm at all temperatures including 350 ◦ C, 400 ◦ C, and 450 ◦ C. In particular, the SnO2 (P)Mg5 V2 sensor showed a relatively good sensor response of about 21% even at a very low concentration of 0.1 ppm SO2 at 400 ◦ C. As shown in Fig. 8b, the SnO2 (P)Mg5 V2 sensor showed a higher recovery properties at 400 ◦ C and 450 ◦ C than that at 350 ◦ C. Considering that sensing behaviors such as sensor response and the recovery properties at very low concentration levels are very important factors for detecting SO2 gas, the recoverable SnO2 (P)Mg5 V2 sensor developed in this study has great potential for the detection of SO2 at the ppm level. 3.5. Repeatability characteristics of SnO2 -based sensors Fig. 9 shows the response repeatabilities of the SnO2 (P) and SnO2 (P)Mg5 V2 sensors at 1 ppm of SO2 and 400 ◦ C. As shown in Fig. 9, the response of the SnO2 (P) sensor decreased during the multiple cycles of detection and recovery, while the SnO2 (P)Mg5 V2 sensor showed a relatively high sensor response level of about 44% and its sensor response was maintained after 3 cycles. In other words, the important point to note is that the SnO2 -based thick film gas sensor promoted with MgO and V2 O5 demonstrated excellent repeatability, as well as an enhanced sensor response, when compared with the SnO2 (P) sensor. It is concluded that the recoverable SnO2 -based sensor promoted with MgO and V2 O5 developed in this study is an excellent thick film gas sensor for the detection of SO2 gas of a very low ppm level in that it satisfies the requirements for excellent sensor response and recovery properties. 4. Conclusion An SnO2 -based thick film gas sensor was developed in this study for the detection of SO2 gas. It shows a high sensor response and excellent recovery properties. The best SnO2 -based thick film gas sensor from among the several sensors tested was prepared by the simultaneous addition of both the MgO and V2 O5 promoters. It was found that the MgO promoter played an important role in sensor response. In addition, it was found that the recovery properties and sensor response could be enhanced if both MgO and V2 O5 were simultaneously added to SnO2 (P). In particular, the SnO2 -based sensor [SnO2 (P)Mg5 V2 ] promoted with both 5 wt% MgO and 2 wt% V2 O5 showed a sensor response of about 44%, and its response
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Biographies Soo Chool Lee received a BSc degree from Keimyung University, South Korea in 1998, and MSc and PhD degrees from Kyungpook National University in 2000 and 2006, respectively. He is currently a research professor at the Research Institute of Advanced Energy Technology in Kyungpook National University, South Korea. His current research interests include CO2 removal sorbents and metal oxide gas sensors. Byung Wook Hwang received a BSc degree from the Department of Environmental Engineering, Kyungpook National University, South Korea, in 2010. He is currently studying for an MSc degree in the Department of Chemical Engineering, Kyungpook National University, Daegu, South Korea. His current research interests include SnO2 gas sensors. Soo Jae Lee received a BSc degree from Keimyung University, South Korea in 2002 and MSc and PhD degrees from Kyungpook National University in 2004 and 2009, respectively. He is currently working for Gs Fuel Cell Co., Ltd. His current research interests include SO2 absorbents and fuel cells. Ho Yun Choi received BSc and MSc degrees from Kyungpook National University in 2005 and 2007, respectively. He is currently studying for a PhD in the Department of Chemical Engineering, Kyungpook National University, Daegu, South Korea. His current research interests include SnO2 gas sensors. Seong Yeol Kim received BSc and MSc degrees from the Department of Chemical Engineering, Kyungpook National University, South Korea, in 1999 and 2005, respectively. He is currently studying for a PhD in the Department of Chemical Engineering, Kyungpook National University, Daegu, South Korea. His current research interests include SnO2 gas sensors.
Suk Yong Jung received a BS degree from the Department of Chemical Engineering at Kyungil University in 2001, MSc and PhD degrees from the Department of Chemical Engineering, Kyungpook National University in 2003 and 2008, respectively. He is currently a Post Doctoral Fellow in the Department of Chemical Engineering at Kyungpook National University, Daegu, South Korea. His current research interests include H2 S removal, DME synthesis, fuel cells, and reforming. Dhanusuraman Ragupathy received an MSc degree in chemistry from Bharathiar University, Coimbatore, India (2005). He completed his PhD degree in the Department of Chemistry, Kyungpook National University, South Korea (2010). Currently, he is a Post Doctoral Fellow in the Department of Chemical Engineering, Kyungpook National University, South Korea. He has published 10 research articles in international journals. His research field includes the development of new nanomaterials for biosensor and electrocatalytic applications. Duk Dong Lee received a BSc degree in physics and an MS degree in electronics from Kyungpook National University, Daegu, South Korea, in 1966 and 1974, respectively, and a PhD from Yonsei University, Seoul, South Korea, in 1984. He is currently a professor at the School of Electrical Engineering and Computer Science in Kyungpook National University, Daegu, South Korea. He has performed research on semiconductor gas sensors since 1978, and has also carried out research in the field of electronic nose. Jae Chang Kim received his BS from the Department of Chemical Engineering in 1982 from Yonsei University, and his MSc and PhD from the Department of Chemical Engineering from the Korea Advanced Institute of Science and Technology (KAIST) in 1984 and 1988, respectively. He is currently a professor in the Department of Chemical Engineering at Kyungpook National University, Daegu, South Korea. His current research interests include CO2 capture, SO2 removal, H2 S removal, DME synthesis, and metal oxide gas sensors.