Effect of annealed temperature on no2 gas-Sensing performances of sno2 nanowire sensors - Do Dang Trung

Journal of Science and Technology 54 (1A) (2016) 214-220 EFFECT OF ANNEALED TEMPERATURE ON NO2 GAS-SENSING PERFORMANCES OF SnO2 NANOWIRE SENSORS Do Dang Trung 1, * , Nguyen Que Phuong 2 , Nguyen Van Duy 2 , Nguyen Van Hieu 2,* 1 Department of Basic Sciences, University of Fire Fighting and Prevention, No 243 Khuat Duy Tien, Thanh Xuan, Hanoi, Vietnam 2 International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No 1 Dai Co Viet, Hai Ba Trung, Hanoi, Vietnam * Email: trungdo81@gmail.com and hieu@itims.edu.vn Received: 30 August 2015; Accepted for publication: 26 October 2015 ABSTRACT NO2 gas is a highly toxic gas and emitted from vehicles such as motocycles and internal engine transportations. It is one of the major reasons which cause air pollution in recent years, especially in big cities. Therefore, the development of NO2 gas sensor for environmental monitoring has been gained research attention in the global. In this work, we report a simple and effective method to prepare SnO2 nanowire gas sensors. The SnO2 nanowires were directly grown on the unplosihed Al2O3 substrate equipped with a pair of Pt-electrodes. The morphology and microstructure of as-grown nanowires have been investigated via X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transimission electron microscopy (TEM). The results indicated that the diameters and lengths of nanowires are about 60 – 100 nm and tens of µm, respectively. The gas-sensing performance of the SnO2 nanowires sensors annealed at different temperatures have also investigated. The results revealed that the annealed temperatures of 400 C and 500 o C do not affected on gas-sensing performance of SnO2 nanowires sensors, while the annealed temperature of 600 o C results in strong decrease in NO2 gas response as compared with as-grown SnO2 nanowires sensor. Keywords: SnO2, NO2, gas sensors, nanowires. 1. INTRODUCTION Recent years, the rapid growth of the number of vehicles in operation, the air pollutants emitted from these vehicles have contributed to air pollution, especially in large cities. The levels of pollution by CO, NO2, SO2, CO2 and NH3 have increased many times the level allowed standards. Therefore, the development of gas sensors for the detection of such gases has attracted considerable interest in recent years [1]. Many metal oxide semiconductors including ZnO, SnO2, WO3 and In2O3 have been applied for gas sensors [2]. Among them, SnO2 has been emerged as one of the most promising candidate due to its high sensitivity [3], good selectivity [4] and low cost [5]. Many techniques for growing SnO2 nanowires have been successfully Effect of annealed temperature on NO2 gas sensing performances of SnO2 nanowires sensors 215 developed such as hydrothermal growth [6], pulse laser deposition [7], sol-gel [8], and chemical vapor deposition [9]. The most commontly used method for fabricating SnO2 nanowires is tube furnace type thermal evaporation due to the simplicity, high-quality nanowires and low cost of this approach. Nitrogen dioxide (NO2) is highly harmful as an environmental pollutant which causes photochemical smog and acid rain. For environmental and safety reasons, gas sensors are required for monitoring concentration of NO2 gas in air. In this work, we report the synthesis and physical characterizations of SnO2 nanowires produced using thermal evaporation technique for gas sensor application. Besides, the gas-sensing performance of the SnO2 nanowire sensors annealed at different temperatures have also investigated. The results show that the sensors exhibit good gas sensing performance to NO2 gas. Especially, the sensors can detect NO2 gas at low concentration range (ppb). 2. EXPERIMENTAL The SnO2 nanowires were grown by a chemical vapor depositon (CVD) system as shown in Figure 1. Pure Sn powder (Merck, 99.8 %) was placed in an alumina boat as the evaporation materials sources. The substrates with a previously deposited 10 nm Au catalyst layer were placed approximately 2–3 cm from the sources on both sides (up-stream and down-stream). The growth process was divided into two steps. Figure 1. Schematic diagram of CVD system to synthesis SnO2 nanowires. Initially, the quartz tube was evacuated to 10-2 Torr and purged several times with Ar gas (99.999 %). Subsequently, the quartz tube was evacuated to 10 Torr again, and the furnace temperature was increased from room-temperature to 750 C in 20 minutes. After the furnace temperature reached the synthesized temperatures, oxygen gas was added to the quartz tube at a flow rate of 0.5 sccm, and the growth process was maintained for another 30 minutes. The as- synthesized SnO2 nanowires were analyzed by X-Ray Diffraction (XRD, D8 Advance, Brucker, Germany), Field Emission Scanning Electron Microscopy (FE-SEM, S4800, Hitachi) and Transmission Electron Microscopy (TEM, JEM-100CX). The gas-sensing properties were then measured with a standard flow rate of 400 sccm for both dry air balance and analytic gases. During sensing measurement, resistance of the sensors was continuously measured using a Keithley 2700 instrument interfaced with a computer while the dried air and analytic gases were switched on/off in each cycle. Sensors response was defined as S = Rg/Ra, where Rg and Ra are resistances of sensor in target gas and dry air, respectively. Do Dang Trung, Nguyen Que Phuong, Nguyen Van Duy, Nguyen Van Hieu 216 3. RESULTS AND DISCUSSION 3.1. Material characterizations Figure 2(a) shows the XRD pattern of SnO2 nanowires. It can be seen from the diffraction patterns that there are typical peaks for tetragonal tin dioxide, which are (110), (101), and (211) planes. All the peaks are well indexed to the tetragonal structure with lattice constants a = 4.73Å nm, c = 3.18 Å, which are in good agreement with those in the standard data card (JCPDS Card No. 77–0452). The morphologies of the SnO2 fabricated by thermal evaporation were characterized by FE- SEM and TEM images and the results were indicated in Figure 3(b-d). It can be seen that the SnO2 nanowires were successfully synthesized at the temperature of 750 C on Al2O3 substrate (Figure 3b). The high magnification of FESEM and TEM images of the SnO2 nanowires (Figures 3c and 3d) are revealed a smoothness and uniformity along the surface of the wire axis. Uniform SnO2 nanowires were produced on a very large area of the substrate. The average diameters and lengths of the SnO2 nanowires ranged from 60 to 100 nm, and from 50 to 150 µm, respectively. 20 30 40 50 60 70 (3 0 1 ) (1 1 2 ) (3 1 0 ) (0 0 2 ) (2 2 0 ) (2 1 1 ) (2 0 0 ) (1 0 1 ) In te n s it y ( a .u .) 2 (degree) (1 1 0 ) (b) (c) (d) (a) Figure 2. Materials charatcerizations: (a) XRD pattern; (b,c) FE-SEM and (d) TEM image of the SnO2 nanowires fabricated by thermal evaporation. 3.2. Gas sensing properties In order to investigate gas-sensing performance of the SnO2 nanowire sensors, a dynamic gas-sensing test was performed. The NO2 gas-sensing properties of as-grown SnO2 nanowire sensor are summarized in Figure 3. A good response-recovery characteristic was obtained under sensing temperature range of 150 – 250 C and 0.1-1 ppm NO2 gas. As can be seen, the sensor (C) Effect of annealed temperature on NO2 gas sensing performances of SnO2 nanowires sensors 217 could detect NO2 gas at low concentrations (su-ppm). However, the highest response was obtained at working temperature of 150 C, in which the sensor resistance increased rapidly upon exposure to target gas and reached saturation within few minutes. The response (Rg/Ra) increased from about 10 to 160 when target gas concentration increased from 100 to 1000 ppb. The resistance of sensors exhibited a upward trend when exposure to NO2 gas, because SnO2 material is known as an n-type semiconductor with free electrons as carriers due to the vacancy of oxygen, while NO2 is a oxidation gas. Thus, when NO2 was adsorbed on the surface, the NO2 molecules accepted the electron from the tin oxide nanowires, decreased the carrier density, and resulted in the increase sensors’ resistance [10]. 1000 2000 3000 4000 30M 60M 90M 120M 200 400 600 800 1000 1200 1400 5M 10M 15M 20M 200 400 600 800 1000 1200 1M 2M 3M 4M 200 400 600 800 1000 40.0 80.0 120.0 160.0 1 ppm 0.5 ppm 0.25 ppm 0.1 ppm SnO 2 as-grown @ 150 o C & NO 2 gas Time(s) R ( ) R ( ) 1 ppm 0.5 ppm 0.25 ppm 0.1 ppm SnO 2 as-grown @ 200 o C & NO 2 gas Time(s) SnO 2 as grown @ 250 o C & NO 2 gas 1 ppm 0.25bppm 0.5 ppm 0.1 ppm Time(s) R ( ) S ( R a /R g ) @ 150 o C @ 200 o C @ 250 o C NO2 (ppb) Figure 3. The NO2 gas response of the as-growth SnO2 at different temperatures: (a) 250 C, (b) 200 C, (c) 150 C and (d) The response with different gas concentration. In this study, we investigated the effect of annealed temperature on gas sensor response. The SnO2 nanowires were annealed at 400 C, 500 C and 600 C in 5 hours and measured at operating temperature of 150, 200 and 250 o C and NO2 gas concentrations of 100, 250, 500 and 1000 ppb NO2 gas. The transient response of the three sensors was shown in Figure 4(a-c). Apparently, the annealed SnO2 nanowire sensors exhibited a good response-recovery characteristic. The gas response as a function of gas concentration was shown in Figure 4(d-e). It can be seen clearly that the annealed temperature strongly depends on the gas response. In more details, the responses of the sensor annealed at 400, 500 and 600 C upon exposure to 1000 ppb NO2 at 200 o C are 68, 60 and 25, respectively. As compared with the as-grown SnO2 nanowires, the sensors annealed at 400 and 500 o C have similar response. When annealed temperature increased up to 600 o C, the response is significantly decreased. This could explain further study for plausible explanation. Do Dang Trung, Nguyen Que Phuong, Nguyen Van Duy, Nguyen Van Hieu 218 600 1200 1800 2400 3000 50M 100M 150M 300 600 900 1200 8M 16M 24M 200 400 600 800 1000 5M 10M 15M @250 o C @200 o C @150 o C 1 ppm 0.5 ppm 0.1 ppm 0.25 ppm 0.1 ppm 0.25 ppm 0.5 ppm 1 ppm Time (s) 0.1 ppm 0.25 ppm 0.5 ppm 1 ppm 600 1200 1800 2400 3000 50M 100M 150M 300 600 900 1200 1500 8M 16M 24M 300 600 900 1200 1500 8M 16M 24M @250 o C @200 o C @150 o C 1 ppm 0.5 ppm 0.1 ppm 0.25 ppm 0.1 ppm 0.25 ppm 0.5 ppm 1 ppm Time (s) 0.1 ppm 0.25 ppm 0.5 ppm 1 ppm 600 1200 1800 2400 3000 50M 100M 150M 300 600 900 1200 20M 40M 60M 300 600 900 1200 4M 8M 12M @250 o C @200 o C @150 o C 1 ppm 0.5 ppm 0.1 ppm 0.25 ppm 0.1 ppm 0.25 ppm 0.5 ppm 1 ppm Time (s) 0.1 ppm 0.25 ppm 0.5 ppm 1 ppm 200 400 600 800 1000 20 40 60 80 100 120 140 160 180 200 200 400 600 800 1000 200 400 600 800 1000 NO 2 (ppb)NO 2 (ppb) As-grown HT@400 o C HT@500 o C HT@600 o C S (R g /R a ) NO 2 (ppb) @NO2 &150 o C @NO2 &200 o C As-grown HT@400 o C HT@500 o C HT@600 o C @NO2 &250 o C As-grown HT@400 o C HT@500 o C HT@600 o C R ( ) (a) (b) (c) (d) (e) (f) Figure 4. The transient response of SnO2 nanowire sensors annealed at 400 o C (a), 500 o C (b) and 600 o C (c). The response as function of gas concentration measured at 200 C (d), 150 C (e) and 250 o C (f). The effect of source materials amount on gas sensing properties was shown in Figure 5. The results indicated that using 100 mg g of Sn powder to fabricate the on-chip sensor based on SnO2 nanowires gave the highest response (around 25), while the sensor grown from 30 mg only gave the response of nearly 8 and the one grown from 150 mg had a response of about 13. Using 150 mg Sn powder resulted in a thicker layer of SnO2 nanowires, thus making the adsorption of NO2 onto the surface of the nanowires more difficult. In the case of using 30 mg source material, the as-grown layer was too thin, therefore the sensitivity was not as high as the sensor fabricated from 100 mg Sn powder. 200k 400k 600k 800k 3M 6M 9M 12M 50 100 150 200 250 30k 60k 90k 120k R ( ) @ 500 ppb & 200 o C @ 500 ppb & 200 o C 150 mg Sn powder 100 mg Sn powder 30 mg Sn powder @ 500 ppb & 200 o C Time (s) 5 10 15 20 25 30 S (R g /R a ) 150 mg Sn 100 mg Sn @ 500 ppb NO 2 & 200 o C 30 mg Sn Source (mg) (a) (b) Figure 6. The transient response of SnO2 nanowires grown from 30 mg (a), 100 mg (b) and 150 mg (c) Sn powder upon exposure to 500 ppb NO2 at 200 C and comparison response of the three sensors (d). Effect of annealed temperature on NO2 gas sensing performances of SnO2 nanowires sensors 219 4. CONCLUSION The SnO2 nanowires were synthesized directly on the Al2O3 substrate equipped with electrodes by thermal evaporation route. This is a potential method that can be applied for mass- production of SnO2 nanowire gas sensor. 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Do Dang Trung, Nguyen Que Phuong, Nguyen Van Duy, Nguyen Van Hieu 220 TÓM TẮT ẢNH HƯỞNG CỦA NHIỆT ĐỘ Ủ ĐẾN TÍNH CHẤT NHẠY KHÍ NO2 CỦA CẢM BIẾN DÂY NANO SnO2 Đỗ Đăng Trung1,*, Nguyễn Quế Phương2, Nguyễn Văn Duy2, Nguyễn Văn Hiếu2, * 1Bộ môn cơ sở ngành, Trường Đại học PCCC, 243 Khuất Duy Tiến, Thanh Xuân, Hà Nội 2Viện Đào tạo Quốc tế về Khoa học vật liệu (ITIMS), Trường Đại học Bách khoa Hà Nội, Số 1 Đại Cồ Việt, Hà Nội; * Email: trungdo81@gmail.com and hieu@itims.edu.vn Trong những năm gần đây tình trạng ô nhiễm môi trường do khí NO2 sinh ra từ các phương tiện giao thông đang trở thành một vấn đề cấp bách, đặc biệt ở các thành phố lớn. Do đó, việc phát triển cảm biến khí NO2 để kiểm soát chất lượng không khí đang thu hút được sự quan tâm nghiên cứu của các nhà khoa học trên thế giới. Trong nghiên cứu này, chúng tôi giới thiệu quy trình chế tạo dây nano SnO2 bằng phương pháp bốc bay nhiệt từ bột Sn ở 750 oC. Hình thái, cấu trúc và tính chất của vật liệu được khảo sát bằng nhiễu xạ điện tử tia X (XRD), hiển vi điện tử phát xạ trường (FE-SEM). Kết quả chế tạo vật liệu chỉ ra rằng, dây nano SnO2 có đường kính khoảng 60 - 100 nm và chiều dài tới vài chục micro-mét. Tính chất nhạy khí NO2 của cảm biến trên cơ sở dây nano SnO2 xử lí nhiệt ở các nhiệt độ khác nhau cũng được khảo sát. Kết quả khảo sát cho thấy, khi xử lí nhiệt ở 400 oC và 500 oC không ảnh hưởng đến đặc trưng nhạy khí của cảm biến dây nano SnO2. Tuy nhiên, khi xử lí ở nhiệt độ 600 o C làm suy giảm đáng kể độ đáp ứng của cảm biến với khí NO2. Từ khóa: SnO2, NO2, cảm biến khí, nhạy khí, dây nano.