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Fabrication of porous SnO2 nanowires gas sensors with enhanced sensitivity
Sensors and Actuators B 252 (2017) 79–85
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fabrication of porous SnO2 nanowires gas sensors with enhanced sensitivity Rui Li a,b,c , Shuai Chen a,b,c , Zheng Lou c,∗ , La Li c , Tingting Huang a,b,c , Yuanjun Song a,b , Di Chen a,b,∗ , Guozhen Shen c,d,∗ a
School of Mathematics and Physics, University of Science Technology Beijing, Beijing 100083, China Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, PR China c State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China d College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e
i n f o
Article history: Received 28 February 2017 Received in revised form 30 April 2017 Accepted 26 May 2017 Available online 31 May 2017 Keywords: Nanowires Electrospinning Hydrothermal etching treatment Sensors Porous Ethanol
a b s t r a c t With large surface area, metal oxide porous nanostructures usually exhibited enhanced sensitivities when used as chemical sensors. In this work, porous SnO2 nanowires (PNWs) with the diameter of about 130 nm and length up to 10 m are synthesized by a controlled two-step method including electrospinning followed with hydrothermal etching treatment by Na2 S solution, which were used to fabricate highperformance gas sensors. Studies found that, compared with the electrospun pristine SnO2 NWs without hydrothermal treatment, the SnO2 PNWs exhibited remarkably enhanced gas sensing performances, including two times higher responsivity to ethanol. The method used here may be easily extended to synthesize other metal oxide nanostructures for high performance chemical sensors. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With the severe environmental problems, monitoring and controlling air pollutant are more and more important and are attracting the attention of many researchers all over the world, recently. Ethanol vapour, which is one of the most popular flammable gases in our daily life, is considered seriously harmful to human life. [1] Developing high performance gas sensors to monitor ethanol in air is of great importance because it can reduce ethanol emissions, protect people from over-exposure to ethanol gas, and improve environmental quality. [2–5] Tin oxide (SnO2 ), as one of the most important n-type metaloxide semiconductors, has been widely investigated for the detection of gas pollutants, due to its unique properties including low cost, low detection limit, fast response and recovery time. [6–10] However, most of the SnO2 based sensors till now still
∗ Corresponding authors at: State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China E-mail addresses: [email protected] (Z. Lou), [email protected] (D. Chen), [email protected] (G. Shen). http://dx.doi.org/10.1016/j.snb.2017.05.161 0925-4005/© 2017 Elsevier B.V. All rights reserved.
exhibit low response and poor selectivity, which limits their applications in many fields. It is still a challenge for researchers to develop peculiar techniques for improving the sensing performance of SnO2 based gas sensors. For example, by doping SnO2 nanorod layered arrays with Nb, Zhao’s group prepared gas sensors with improved performance toward alcohol. [11] Cao’s group used Au nanoparticles to functionalize 3D SnO2 microstructures from the simple hydrothermal combining with subsequent annealing process to improve the gas sensing performance. [12] By controlling the microstructures of SnO2 nanostructures, our group fabricated several high-performance SnO2 based gas sensors. [13–16] All these results suggested that the microstructures, particle size and morphologies played significantly important roles in the sensing improvements of SnO2 based materials. Especially, the formation of porous SnO2 nanostructures have been proved to be an efficient way to get high-performance gas sensors, as porous nanostructures usually exhibited low density and large surface area. Herein, we reported the fabrication of high performance gas sensors with highly porous SnO2 nanowires (SnO2 PNWs) as the sensing elements. SnO2 PNWs used here is synthesized by a simple electrospinning process, followed with hydrothermal etching treatment in Na2 S solution. When fabricated into sensing device,
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Scheme 1. Schematic illustration of the synthesis of SnO2 PNWs.
the as-synthesized SnO2 PNWs consisting of nanoparticles with diameters of about 130 nm showed superior gas sensing properties compared with the pristine SnO2 nanowires (NWs) without hydrothermal etching treatment.
ucts was examined through measuring N2 adsorption-desorption isotherm (QS-18,0.01 M).
2. Experiment
To fabricate the gas sensors, in the experiments, we mixed the grinding SnO2 products with a small amount of deionized water to form a paste and then coated on a ceramic substrate with patterned gold electrodes. The gas-sensing properties of the sensor were measured by a CGS-1 TP intelligent gas sensitivity analysis system. The gas-sensing sensitivity was assessed through the response value of the electric resistance, which was defined as S = Ra/Rg (where Rg and Ra were the sensor resistance in target gas and in dry air, respectively).
2.1. Materials All materials used in our experiments are of analytical purity and used without further treatment. Stannic chloride pentahydrate (SnCl4 ·5H2 O), sodium sulfide nonahydrate (Na2 S·9H2 O), ethanol and N,N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagents Co., Shanghai, China. Polyvinylpyrrolidone (PVP, M = 13,00,000g mol-1) was supplied by Qi Fuqin Materials Technology Co., LTD. Shanghai, China. 2.2. Synthesis of material Synthesis of SnO2 nanowires: SnO2 nanowires were synthesized from the simple electrospinning process. First, the precursor solution was prepared by dissolving 3 g of SnCl4 ·5H2 O into 27 g of ethanol/DMF mixture solution with the weight rate of 1:1 under magnetic stirring. Then 3 g of PVP was added into the solution and kept stirring for 2 h at 60 ◦ C. Using a Medical propulsion pump, the transparency and uniformity precursor solution was delivered to a needle at a constant flow rate of 0.3 mL/h. Upon applying a high voltage of 18 kV, the solution was electrospun from the stainless steel needle, formed a fibrous mat on an aluminum foil collector. The distance between the needle and the collector was 22 cm. The electrospun fibers were then calcinated at 600 ◦ C for 4 h with a heating rate of 0.5 ◦ C/min. [17] Hydrothermal etching in Na2 S solution: To prepare SnO2 PNWs, the freshly electrospinning fabricated nanowires were added into 0.1 M Na2 S·9H2 O aqueous solution. After vigorous stirring for 30 min, the mixture was transferred into a 20 mL Teflon-lined autoclave and sealed. Finally, the system was heated at 100 ◦ C for 1 h. When cooling to room temperature, the obtained products were washed with deionized water 3 times and then dried in air for 6 h at 100 ◦ C. 2.3. Characterizations The morphology and microstructure of the obtained products were characterized by field emission scanning electron microscopy (FEM, SUPRA 55) and transmission electron microscope (TEM, JEM 2200FS). The elemental analyses of the products were determined by X-ray diffraction (XRD, DMAX-RB) and Xray photoelectron spectroscopy (XPS, Thermo escalab 250XI). The Brunauer–Emmett–Teller (BET) specific surface area of the prod-
2.4. Fabrication and measurement of gas sensor
3. Results and discussion The schematic diagram for the formation of the SnO2 PNWs is shown in Scheme 1. SnO2 nanowires composed of small nanoparticles were firstly prepared from the electrospinning process, followed with high temperature calcination. The as-prepared SnO2 NWs were then dispersed in Na2 S solution and performed hydrothermal etching reaction at 100 ◦ C for 1 h. Pure SnO2 NWs with porous structure were prepared, suggesting the possible chemical reactions occurred as following in the Na2 S solution: Na2 S + H2 O → H2 S + NaOH
(1)
SnO2 + H2 S + S2− → SnS3 2− + 2H2 O
(2)
Typically, in the Na2 S solution, SnO2 NWs reacted with H2 S and S2− to form SnS3 2− according to the above equations, which resulted in the formation of porous samples due to the partially etching reaction with short time of 1 h. Compared with SnO2 NWs, the SnO2 PNWs after etching showed increased BET surface area, which might exhibit superior properties in sensing field. The morphology and microstructure of the as-synthesized SnO2 NWs and PNWs were observed by SEM, respectively. Fig. 1a shows the SEM image of the SnO2 NWs after electrospinning/calcination process. SnO2 NWs with uniform diameter of about 130 nm were prepared on a large scale. Inset is a higher magnification SEM image of the SnO2 NWs, demonstrating that the NWs are composed of numerous particles with rough surface. TEM image of a typical SnO2 NW is shown in Fig. 1b, clearly displaying that the NWs are composed of nanoparticles with uniform size of about 30 nm. The high-resolution TEM (HRTEM) of a single SnO2 NW in Fig. 1c further illustrates that the SnO2 nanoparticles are of high crystallinity. The marked lattice spacing was estimated to be 1.76 nm and 3.35 nm, which are in good agreement with the (211) and (110) d-spacing of rutile SnO2 phase, respectively. The corresponding selected area electron diffraction (SAED) pattern shown in Fig. 1c inset revealed the polycrystalline nature of the SnO2 NW.
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Fig. 1. SEM and TEM images of the as-prepared (a–c) SnO2 NWs and (d-f) SnO2 PNWs. (g)XRD patterns of different samples (h,i) High resolution XPS spectra of SnO2 PNWs.
Fig. 2. Response of as-synthesized porous SnO2 nanowires from hydrothermal-etching process (a) with different concentrations of Na2 S solution (0.05–0.15 M) for 1 h and (b) for different time (0.5–2 h) with the same concentration of Na2 S solution (0.1 M) to 100 ppm ethanol at various temperatures (340–400 ◦ C).
SEM image of the SnO2 PNWs after Na2 S etching was shown in Fig. 1d. The surface of the product after etching became much rougher and looser due to the S2− etching effect while still kept the one-dimensional morphology of the SnO2 NWs and each PNW is composed of relatively low-density nanoparticles with an average diameter of about 20 nm, as depicted in Fig. 1e. The microstructure of the SnO2 PNWs was also investigated by HRTEM and the corresponding image was depicted in Fig. 1f. The clearly resolved lattice
fringes are measured to be 1.76 nm and 1.68 nm, corresponding well to the d-spacing of the (211) and (220) plane of rutile SnO2 . To study the composition of both products before and after hydrothermal etching, XRD technique was performed and the results are shown in Fig. 1g. The diffraction peaks on both patterns can be indexed to SnO2 with pure rutile phase (JCPDS card No.41-1445), confirming the formation of pure SnO2 products. Hydrothermal etching with Na2 S did not induce any impurity phase like SnS or
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Fig. 3. (a) Gas response of both sensors toward 100 ppm of ethanol at different operating temperatures (250–400). (b) The dynamic response-recovery curve of both sensors toward 100 ppm of ethanol at 380 ◦ C. (c) Response curve of porous SnO2 nanowire sensor toward different concentrations of ethanol at 380 ◦ C. (d) Response curve of SnO2 nanowire sensor toward different concentrations of ethanol at 380 ◦ C. (e) Cyclic response curves of both sensors toward 100 ppm of ethanol at 380 ◦ C. (f) Gas response of both sensors to ethanol with different concentrations at 380 ◦ C. (inset: response of the sensor to 1–20 ppm ethanol).
SnS2 . XPS analysis in Fig. 1h–i gave further information about the composition and chemical valence of the species of the SnO2 PNWs. Only signals from carbon, oxygen and tin are observed, further confirming the formation of pure porous SnO2 nanowires. To understand the etching process, we also studied the influence of etching time and the Na2 S concentration on the final SnO2 products. The corresponding SEM images were shown in Fig. S1. As shown in Fig. S1a–c, keeping the etching time for 1 h, the samples obtained with various Na2 S concentrations ranging from 0.05, 0.1, 0.15–0.2 M displayed different morphologies, respectively. Clearly,
when the concentration of Na2 S is lower (0.05 M), SnO2 nanowires with low pore densities were obtained (Fig. S1a). When the Na2 S concentration was increased to 0.15 M, although porous SnO2 nanowires were stilled obtained, they became much shorter and nanoparticles were observed coexistence with the nanowires (Fig. S1b). When we increased the Na2 S concentration to 0.2 M, no wirelike products were obtained, as shown in Fig. S1c. Based on these results, it can be concluded that the optimal Na2 S concentration is about 0.1 M for the preparation of porous SnO2 nanowires.
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Fig. S1d–f showed the SEM images of the as-synthesized samples after etched for from 0.5, 1–2 h, respectively, and kept other parameters constant. It can be seen that with the increase of etching time, the pore densities also increased. However, longer etching time, longer than 2 h for instance, resulted in the destroy of nanowires. Thus 1 h was set as the optimal etching time in this work. The ethanol sensing performances of the samples discussed above were also studied and the corresponding results were demonstrated in Fig. 2. Fig. 2a–b shows that the response curves of the samples with the Na2 S concentration ranging from 0.05, 0.1–0.15 M and the etching time for 0.5, 1 and 2 h at series of working temperatures, respectively. From these data, we can see that all samples showed the highest response to 100 ppm ethanol at the working temperature of 380 ◦ C. Compared with other sample, the porous nanowires obtained from the hydrothermal etching process with the reaction time of 1 h and the Na2 S concentration of 0.1 M show the best sensing performance under the same testing environment. We also measured the BET surface areas of the nanowires without etching process and the target porous nanowires. It showed that the SnO2 PNWs have larger surface area (38 m2 /g) than the common SnO2 nanowires with etching (22 m2 /g). With porous structure and large surface area, the SnO2 PNWs are thought to be good candidate for high performance gas sensors. The sensing performances of the SnO2 PNWs were shown in Fig. 3, along with the SnO2 NWs for comparison. As working temperature has a great effect on gas sensing performance, since it influences the adsorption and desorption process of oxygen molecules on the surface of sensor, we first studied the sensing response of both samples to ethanol at different temperatures ranging from 280 to 440 ◦ C and the results were demonstrated in Fig. 3a. The gas response of both samples first increased, reached to the maximum value and then quickly decreased. The optimum sensing temperature was thus about 380 ◦ C, as can be seen in the figure. The possible reason and detailed mechanism will be discussed later. Moreover, compared to pristine SnO2 NWs, the SnO2 PNWs exhibits about two times higher response to ethanol, which might be caused by the higher porous microstructure and larger BET surface area of the PNWs. The response-recovery time is an important parameter to evaluate the performance of gas sensors. Herein, we define the response and recovery time as the time required to achieve 90% of the total resistance variation when the gas goes in and out. Fig. 3b showed the real-time dynamic response curves to 100 ppm ethanol of both samples at 380 ◦ C. The response and recovery time of SnO2 PNWs can be estimated to be 22s and 18s, respectively, similar to those of the SnO2 NWs (21s and 16s). Reproducible and reversible sensing are another key factors to the gas sensors for trace target gas analysis. Fig. 3c–d shows the representative dynamic gas response of SnO2 NWs and SnO2 PNWs to ethanol with various concentrations (10, 20, 50, 100 and 200 ppm) when the sensors worked at 380 ◦ C, respectively. The gas response amplitude of both sensors increases with the increase of ethanol concentration. And the corresponding response of SnO2 PNWs is much larger than that of SnO2 NWs, demonstrating the outstanding performance of the porous structures with larger BET surface area. The cyclic response curves of both sensors to 100 ppm ethanol at 380 ◦ C with 5 cycles are shown in Fig. 3e. Obviously, both sensors exhibit good repeatability with fast response and recovery time, while the PNWs exhibited much higher responsivity, also confirming the outstanding sensing performance of the SnO2 PNWs. Fig. 3f displays the response of the SnO2 samples to ethanol with different concentrations. The SnO2 PNWs exhibited a very broad gas sensing range toward ethanol from 1 ppm to 5000 ppm. The sensing saturation concentration is found to be ∼2000 ppm for the SnO2 PNWs. On the contrary, the SnO2 NWs only showed sensing response to
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Fig. 4. Response of both sensors to various gases (100 ppm) at optimal operating temperature.
ethanol with shorter concentration range and the saturation concentration is 500 ppm. Besides, the minimum detection limit for SnO2 PNWs to ethanol is about 1 ppm and the response has a linear relationship with the ethanol concentration in the low concentration region, as can be seen in Fig. 3f inset. The sensing selectivity of the SnO2 PNWs was investigated by exposing the corresponding sensors to different volatile organic gases with concentrations of 100 ppm, including benzene, ammonia, formaldehyde, ethanol, acetone and dimethylbenzene, respectively. As shown in Fig. 4, it is obvious that the response of the SnO2 PNWs to ethanol is much higher than that to other gases at optimal working temperature, indicating that the SnO2 PNWs display superior selectivity to ethanol against other interference gases. Compared to the SnO2 NWs, SnO2 PNWs exhibited enhanced response to all the test gases, further confirming the superior sensing performance of the SnO2 PNWs. According to above results, we can deduce that the SnO2 PNWs here exhibited greatly enhanced sensing performance, including higher sensitivity and longer sensing concentration range. As a typical n-type semiconductor, the possible sensing mechanism of the SnO2 PNWs for detecting gases can be explained by the spacecharge layer mode widely discussed in many previous reports. [18–20] As shown in Fig. 5, comparing to SnO2 NWs, SnO2 PNWs composed of numerous particles have larger specific surface area (38 m2 /g), providing more active sites for target gas adsorption. It will be beneficial for the enhancement of sensing performance. Moreover, during the adsorption and desorption process of oxygen molecules on the surface of the SnO2 PNWs, the resistance of the sensors is changed when exposed to different target gas based on the space-charge layer mode. [22–25] When the sensors are exposed to air, oxygen molecules in air are adsorbed onto the surface of nanowire and extract electrons from the conduction band to generate chemisorbed oxygen species (O2− ), which leads to the formation of a thick depletion layer at the oxides surface, as shown in Fig. 5, resulted in relatively high resistance. When the sensors are exposed to reducing gas such as ethanol at the working temperature, the chemical reactions between the reducing gases and the oxygen species adsorbed on the surface of the sensors resulted in the feed of free electrons back into the conduction band of SnO2 . The reactions can be express by the following equation: CH3 CH2 OH + 6O2− → 2CO2 + 3H2 O + 12e−
(3)
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Fig. 5. Schematic gas sensing mechanism of porous SnO2 nanowire and SnO2 nanowire sensors.
Simultaneously, the depletion layer at the SnO2 surface is narrowed and the conductivity is increased as shown in Fig. 5. Thus, the resistance of the SnO2 PNWs is decreased. When ethanol gas is out, the sensors are exposed to air again and the resistance of the sensors is also increased. 4. Conclusion In summary, highly porous SnO2 nanowires with a large surface area of 38 m2 /g were synthesized by hydrothermal etching of electrospun SnO2 NWs in Na2 S solution. Compared with the pristine SnO2 NWs, the porous nanowires exhibit a much higher gas sensing responsivity (∼two times higher) to ethanol. Besides, it can detect a much wider ethanol concentrate region (1–5000 ppm) and has a higher saturation concentration of 2000 ppm. The remarkable enhanced sensing performance of the porous nanowires can be attributed to the highly effective surface interactions arising from the increased exposed surface after hydrothermally-etching process which enhances the response by enabling increased interactions between gas molecules and the SnO2 PNWs. The excellent gas sensing of the porous nanowires demonstrated here reveals that the combination of electrospinning and hydrothermal etching techniques may provide a very simple and efficient way to get porous nanostructures with enhanced sensing performance for practical applications. Acknowledgement The work was supported by National Natural Science Foundation of China (51672308, 61625404, 61504136), the Beijing Natural Science Foundation (4162062), Beijing Municipal Science and Technology Project (No. Z17111000220000) and the Key Research Program of Frontiers Sciences, CAS (QYZDY-SSW-JSC004). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2017.05.161. References [1] C. Coronado, J.J. Carvalho, J. Andrade, E. Cortez, F. Carvalho, J. Santos, A. Mendiburu, Flammability limits: a review with emphasis on ethanol for aeronautical applications and description of the experimental procedure, J. Hazard. Mater. 241–242 (2012) 32–54. [2] F. Bender, N. Barie, G. Romoudis, A. Voigt, M. Rapp, Development of a preconcentration unit for a SAW sensor micro array and its use for indoor air quality monitoring, Sens. Actuators B-Chem. 93 (2003) 135–141.
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Biographies
Rui Li received her BS degree from School of Physics, Qufu Normal University.She is a postgraduate at the School of Mathematics and Physics, University of Science and Technology Beijing. Her research interest focuses on flexible gas sensors and lithium ion batteries.
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Tingting Huang received her Bachelor’s degree from School of Physics, Hebei Normal University. Currently, she is a postgraduate at the School of Mathematics and Physics, University of Science and Technology Beijing. Her research interest is flexible gas sensors and supercapacitors.
Shuai Chen received his MSc degree from Qingdao University in 2015. Now he is a PhD candidate at the School of Mathematics and Physics, University of Science and Technology Beijing. He is also a visiting student at Institute of Semiconductors, Chinese Academy of Sciences. His research interest focuses on flexible electronic devices, including strain sensors and multifunctional integrated systems.
Yuanjun Song received his PhD degree in Condensed Matter Physics from Institute of Physics, Chinese Academy of Sciences (IOPCAS). Now he works in the School of Mathematics and Physics, University of Science and Technology Beijing. His research interest focuses on microstructure and phase transition study of magneto-photoelectrical composite via transmission electron microscopy (TEM) and X-ray diffraction. He has investigated some functional materials such as catalysts, magnetic nanomaterials and lithium battery materials, with understanding of the microstructure and phase transition give some advice on optimizing the performance.
Zheng Lou received his Ph.D. degree from Jilin University in 2014. He joined the Institute of Semiconductors, Chinese Academy of Sciences as an Assistant Professor in 2014. His current research focuses on flexible electronics, including gas sensors, pressure sensors, electronic-skin, transistors and photo-detectors.
Di Chen is a professor at University of Science and Technology Beijing. She received her PhD degree from the University of Science and Technology of China in 2005. Her current research interest is the advanced technology for designing nanostructure for sustainable energy applications, including energy storage and photocatalysts. She has published about 100 papers in international referred journals, including Advanced Materials, Advanced Functional Materials, Nano Letters, ACS Nano, etc.
La Li received her BS degree from the Department of Chemical Engineering, Tomsk Polytechnical University, Tomsk City, Russia, Currently, she is a Ph. D candidate at College of Physics, Jilin University, China. Her research interests mainly focus on flexible micro-supercapacitors.
Guozhen Shen received his Ph. D degree in 2003 from University of Science and Technology of China. From 2004–2009, he conducted his research in Hanyang University (Korea), National Institute for Materials Science (Japan), University of Southern California (USA) and Huazhong University of Science and Technology (China). He joined Institute of Semiconductors, Chinese academy of Sciences as a professor in 2013. His current research focuses on flexible electronics and printable electronics. He has published more than 200 papers with an H-factor of 54. He received several important awards, including the Research Fund for Distinguished Young Scientists, Mao Yi-Sheng Science and Technology Awards, etc.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fabrication of porous SnO2 nanowires gas sensors with enhanced sensitivity Rui Li a,b,c , Shuai Chen a,b,c , Zheng Lou c,∗ , La Li c , Tingting Huang a,b,c , Yuanjun Song a,b , Di Chen a,b,∗ , Guozhen Shen c,d,∗ a
School of Mathematics and Physics, University of Science Technology Beijing, Beijing 100083, China Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, PR China c State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China d College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e
i n f o
Article history: Received 28 February 2017 Received in revised form 30 April 2017 Accepted 26 May 2017 Available online 31 May 2017 Keywords: Nanowires Electrospinning Hydrothermal etching treatment Sensors Porous Ethanol
a b s t r a c t With large surface area, metal oxide porous nanostructures usually exhibited enhanced sensitivities when used as chemical sensors. In this work, porous SnO2 nanowires (PNWs) with the diameter of about 130 nm and length up to 10 m are synthesized by a controlled two-step method including electrospinning followed with hydrothermal etching treatment by Na2 S solution, which were used to fabricate highperformance gas sensors. Studies found that, compared with the electrospun pristine SnO2 NWs without hydrothermal treatment, the SnO2 PNWs exhibited remarkably enhanced gas sensing performances, including two times higher responsivity to ethanol. The method used here may be easily extended to synthesize other metal oxide nanostructures for high performance chemical sensors. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With the severe environmental problems, monitoring and controlling air pollutant are more and more important and are attracting the attention of many researchers all over the world, recently. Ethanol vapour, which is one of the most popular flammable gases in our daily life, is considered seriously harmful to human life. [1] Developing high performance gas sensors to monitor ethanol in air is of great importance because it can reduce ethanol emissions, protect people from over-exposure to ethanol gas, and improve environmental quality. [2–5] Tin oxide (SnO2 ), as one of the most important n-type metaloxide semiconductors, has been widely investigated for the detection of gas pollutants, due to its unique properties including low cost, low detection limit, fast response and recovery time. [6–10] However, most of the SnO2 based sensors till now still
∗ Corresponding authors at: State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China E-mail addresses: [email protected] (Z. Lou), [email protected] (D. Chen), [email protected] (G. Shen). http://dx.doi.org/10.1016/j.snb.2017.05.161 0925-4005/© 2017 Elsevier B.V. All rights reserved.
exhibit low response and poor selectivity, which limits their applications in many fields. It is still a challenge for researchers to develop peculiar techniques for improving the sensing performance of SnO2 based gas sensors. For example, by doping SnO2 nanorod layered arrays with Nb, Zhao’s group prepared gas sensors with improved performance toward alcohol. [11] Cao’s group used Au nanoparticles to functionalize 3D SnO2 microstructures from the simple hydrothermal combining with subsequent annealing process to improve the gas sensing performance. [12] By controlling the microstructures of SnO2 nanostructures, our group fabricated several high-performance SnO2 based gas sensors. [13–16] All these results suggested that the microstructures, particle size and morphologies played significantly important roles in the sensing improvements of SnO2 based materials. Especially, the formation of porous SnO2 nanostructures have been proved to be an efficient way to get high-performance gas sensors, as porous nanostructures usually exhibited low density and large surface area. Herein, we reported the fabrication of high performance gas sensors with highly porous SnO2 nanowires (SnO2 PNWs) as the sensing elements. SnO2 PNWs used here is synthesized by a simple electrospinning process, followed with hydrothermal etching treatment in Na2 S solution. When fabricated into sensing device,
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Scheme 1. Schematic illustration of the synthesis of SnO2 PNWs.
the as-synthesized SnO2 PNWs consisting of nanoparticles with diameters of about 130 nm showed superior gas sensing properties compared with the pristine SnO2 nanowires (NWs) without hydrothermal etching treatment.
ucts was examined through measuring N2 adsorption-desorption isotherm (QS-18,0.01 M).
2. Experiment
To fabricate the gas sensors, in the experiments, we mixed the grinding SnO2 products with a small amount of deionized water to form a paste and then coated on a ceramic substrate with patterned gold electrodes. The gas-sensing properties of the sensor were measured by a CGS-1 TP intelligent gas sensitivity analysis system. The gas-sensing sensitivity was assessed through the response value of the electric resistance, which was defined as S = Ra/Rg (where Rg and Ra were the sensor resistance in target gas and in dry air, respectively).
2.1. Materials All materials used in our experiments are of analytical purity and used without further treatment. Stannic chloride pentahydrate (SnCl4 ·5H2 O), sodium sulfide nonahydrate (Na2 S·9H2 O), ethanol and N,N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagents Co., Shanghai, China. Polyvinylpyrrolidone (PVP, M = 13,00,000g mol-1) was supplied by Qi Fuqin Materials Technology Co., LTD. Shanghai, China. 2.2. Synthesis of material Synthesis of SnO2 nanowires: SnO2 nanowires were synthesized from the simple electrospinning process. First, the precursor solution was prepared by dissolving 3 g of SnCl4 ·5H2 O into 27 g of ethanol/DMF mixture solution with the weight rate of 1:1 under magnetic stirring. Then 3 g of PVP was added into the solution and kept stirring for 2 h at 60 ◦ C. Using a Medical propulsion pump, the transparency and uniformity precursor solution was delivered to a needle at a constant flow rate of 0.3 mL/h. Upon applying a high voltage of 18 kV, the solution was electrospun from the stainless steel needle, formed a fibrous mat on an aluminum foil collector. The distance between the needle and the collector was 22 cm. The electrospun fibers were then calcinated at 600 ◦ C for 4 h with a heating rate of 0.5 ◦ C/min. [17] Hydrothermal etching in Na2 S solution: To prepare SnO2 PNWs, the freshly electrospinning fabricated nanowires were added into 0.1 M Na2 S·9H2 O aqueous solution. After vigorous stirring for 30 min, the mixture was transferred into a 20 mL Teflon-lined autoclave and sealed. Finally, the system was heated at 100 ◦ C for 1 h. When cooling to room temperature, the obtained products were washed with deionized water 3 times and then dried in air for 6 h at 100 ◦ C. 2.3. Characterizations The morphology and microstructure of the obtained products were characterized by field emission scanning electron microscopy (FEM, SUPRA 55) and transmission electron microscope (TEM, JEM 2200FS). The elemental analyses of the products were determined by X-ray diffraction (XRD, DMAX-RB) and Xray photoelectron spectroscopy (XPS, Thermo escalab 250XI). The Brunauer–Emmett–Teller (BET) specific surface area of the prod-
2.4. Fabrication and measurement of gas sensor
3. Results and discussion The schematic diagram for the formation of the SnO2 PNWs is shown in Scheme 1. SnO2 nanowires composed of small nanoparticles were firstly prepared from the electrospinning process, followed with high temperature calcination. The as-prepared SnO2 NWs were then dispersed in Na2 S solution and performed hydrothermal etching reaction at 100 ◦ C for 1 h. Pure SnO2 NWs with porous structure were prepared, suggesting the possible chemical reactions occurred as following in the Na2 S solution: Na2 S + H2 O → H2 S + NaOH
(1)
SnO2 + H2 S + S2− → SnS3 2− + 2H2 O
(2)
Typically, in the Na2 S solution, SnO2 NWs reacted with H2 S and S2− to form SnS3 2− according to the above equations, which resulted in the formation of porous samples due to the partially etching reaction with short time of 1 h. Compared with SnO2 NWs, the SnO2 PNWs after etching showed increased BET surface area, which might exhibit superior properties in sensing field. The morphology and microstructure of the as-synthesized SnO2 NWs and PNWs were observed by SEM, respectively. Fig. 1a shows the SEM image of the SnO2 NWs after electrospinning/calcination process. SnO2 NWs with uniform diameter of about 130 nm were prepared on a large scale. Inset is a higher magnification SEM image of the SnO2 NWs, demonstrating that the NWs are composed of numerous particles with rough surface. TEM image of a typical SnO2 NW is shown in Fig. 1b, clearly displaying that the NWs are composed of nanoparticles with uniform size of about 30 nm. The high-resolution TEM (HRTEM) of a single SnO2 NW in Fig. 1c further illustrates that the SnO2 nanoparticles are of high crystallinity. The marked lattice spacing was estimated to be 1.76 nm and 3.35 nm, which are in good agreement with the (211) and (110) d-spacing of rutile SnO2 phase, respectively. The corresponding selected area electron diffraction (SAED) pattern shown in Fig. 1c inset revealed the polycrystalline nature of the SnO2 NW.
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Fig. 1. SEM and TEM images of the as-prepared (a–c) SnO2 NWs and (d-f) SnO2 PNWs. (g)XRD patterns of different samples (h,i) High resolution XPS spectra of SnO2 PNWs.
Fig. 2. Response of as-synthesized porous SnO2 nanowires from hydrothermal-etching process (a) with different concentrations of Na2 S solution (0.05–0.15 M) for 1 h and (b) for different time (0.5–2 h) with the same concentration of Na2 S solution (0.1 M) to 100 ppm ethanol at various temperatures (340–400 ◦ C).
SEM image of the SnO2 PNWs after Na2 S etching was shown in Fig. 1d. The surface of the product after etching became much rougher and looser due to the S2− etching effect while still kept the one-dimensional morphology of the SnO2 NWs and each PNW is composed of relatively low-density nanoparticles with an average diameter of about 20 nm, as depicted in Fig. 1e. The microstructure of the SnO2 PNWs was also investigated by HRTEM and the corresponding image was depicted in Fig. 1f. The clearly resolved lattice
fringes are measured to be 1.76 nm and 1.68 nm, corresponding well to the d-spacing of the (211) and (220) plane of rutile SnO2 . To study the composition of both products before and after hydrothermal etching, XRD technique was performed and the results are shown in Fig. 1g. The diffraction peaks on both patterns can be indexed to SnO2 with pure rutile phase (JCPDS card No.41-1445), confirming the formation of pure SnO2 products. Hydrothermal etching with Na2 S did not induce any impurity phase like SnS or
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Fig. 3. (a) Gas response of both sensors toward 100 ppm of ethanol at different operating temperatures (250–400). (b) The dynamic response-recovery curve of both sensors toward 100 ppm of ethanol at 380 ◦ C. (c) Response curve of porous SnO2 nanowire sensor toward different concentrations of ethanol at 380 ◦ C. (d) Response curve of SnO2 nanowire sensor toward different concentrations of ethanol at 380 ◦ C. (e) Cyclic response curves of both sensors toward 100 ppm of ethanol at 380 ◦ C. (f) Gas response of both sensors to ethanol with different concentrations at 380 ◦ C. (inset: response of the sensor to 1–20 ppm ethanol).
SnS2 . XPS analysis in Fig. 1h–i gave further information about the composition and chemical valence of the species of the SnO2 PNWs. Only signals from carbon, oxygen and tin are observed, further confirming the formation of pure porous SnO2 nanowires. To understand the etching process, we also studied the influence of etching time and the Na2 S concentration on the final SnO2 products. The corresponding SEM images were shown in Fig. S1. As shown in Fig. S1a–c, keeping the etching time for 1 h, the samples obtained with various Na2 S concentrations ranging from 0.05, 0.1, 0.15–0.2 M displayed different morphologies, respectively. Clearly,
when the concentration of Na2 S is lower (0.05 M), SnO2 nanowires with low pore densities were obtained (Fig. S1a). When the Na2 S concentration was increased to 0.15 M, although porous SnO2 nanowires were stilled obtained, they became much shorter and nanoparticles were observed coexistence with the nanowires (Fig. S1b). When we increased the Na2 S concentration to 0.2 M, no wirelike products were obtained, as shown in Fig. S1c. Based on these results, it can be concluded that the optimal Na2 S concentration is about 0.1 M for the preparation of porous SnO2 nanowires.
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Fig. S1d–f showed the SEM images of the as-synthesized samples after etched for from 0.5, 1–2 h, respectively, and kept other parameters constant. It can be seen that with the increase of etching time, the pore densities also increased. However, longer etching time, longer than 2 h for instance, resulted in the destroy of nanowires. Thus 1 h was set as the optimal etching time in this work. The ethanol sensing performances of the samples discussed above were also studied and the corresponding results were demonstrated in Fig. 2. Fig. 2a–b shows that the response curves of the samples with the Na2 S concentration ranging from 0.05, 0.1–0.15 M and the etching time for 0.5, 1 and 2 h at series of working temperatures, respectively. From these data, we can see that all samples showed the highest response to 100 ppm ethanol at the working temperature of 380 ◦ C. Compared with other sample, the porous nanowires obtained from the hydrothermal etching process with the reaction time of 1 h and the Na2 S concentration of 0.1 M show the best sensing performance under the same testing environment. We also measured the BET surface areas of the nanowires without etching process and the target porous nanowires. It showed that the SnO2 PNWs have larger surface area (38 m2 /g) than the common SnO2 nanowires with etching (22 m2 /g). With porous structure and large surface area, the SnO2 PNWs are thought to be good candidate for high performance gas sensors. The sensing performances of the SnO2 PNWs were shown in Fig. 3, along with the SnO2 NWs for comparison. As working temperature has a great effect on gas sensing performance, since it influences the adsorption and desorption process of oxygen molecules on the surface of sensor, we first studied the sensing response of both samples to ethanol at different temperatures ranging from 280 to 440 ◦ C and the results were demonstrated in Fig. 3a. The gas response of both samples first increased, reached to the maximum value and then quickly decreased. The optimum sensing temperature was thus about 380 ◦ C, as can be seen in the figure. The possible reason and detailed mechanism will be discussed later. Moreover, compared to pristine SnO2 NWs, the SnO2 PNWs exhibits about two times higher response to ethanol, which might be caused by the higher porous microstructure and larger BET surface area of the PNWs. The response-recovery time is an important parameter to evaluate the performance of gas sensors. Herein, we define the response and recovery time as the time required to achieve 90% of the total resistance variation when the gas goes in and out. Fig. 3b showed the real-time dynamic response curves to 100 ppm ethanol of both samples at 380 ◦ C. The response and recovery time of SnO2 PNWs can be estimated to be 22s and 18s, respectively, similar to those of the SnO2 NWs (21s and 16s). Reproducible and reversible sensing are another key factors to the gas sensors for trace target gas analysis. Fig. 3c–d shows the representative dynamic gas response of SnO2 NWs and SnO2 PNWs to ethanol with various concentrations (10, 20, 50, 100 and 200 ppm) when the sensors worked at 380 ◦ C, respectively. The gas response amplitude of both sensors increases with the increase of ethanol concentration. And the corresponding response of SnO2 PNWs is much larger than that of SnO2 NWs, demonstrating the outstanding performance of the porous structures with larger BET surface area. The cyclic response curves of both sensors to 100 ppm ethanol at 380 ◦ C with 5 cycles are shown in Fig. 3e. Obviously, both sensors exhibit good repeatability with fast response and recovery time, while the PNWs exhibited much higher responsivity, also confirming the outstanding sensing performance of the SnO2 PNWs. Fig. 3f displays the response of the SnO2 samples to ethanol with different concentrations. The SnO2 PNWs exhibited a very broad gas sensing range toward ethanol from 1 ppm to 5000 ppm. The sensing saturation concentration is found to be ∼2000 ppm for the SnO2 PNWs. On the contrary, the SnO2 NWs only showed sensing response to
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Fig. 4. Response of both sensors to various gases (100 ppm) at optimal operating temperature.
ethanol with shorter concentration range and the saturation concentration is 500 ppm. Besides, the minimum detection limit for SnO2 PNWs to ethanol is about 1 ppm and the response has a linear relationship with the ethanol concentration in the low concentration region, as can be seen in Fig. 3f inset. The sensing selectivity of the SnO2 PNWs was investigated by exposing the corresponding sensors to different volatile organic gases with concentrations of 100 ppm, including benzene, ammonia, formaldehyde, ethanol, acetone and dimethylbenzene, respectively. As shown in Fig. 4, it is obvious that the response of the SnO2 PNWs to ethanol is much higher than that to other gases at optimal working temperature, indicating that the SnO2 PNWs display superior selectivity to ethanol against other interference gases. Compared to the SnO2 NWs, SnO2 PNWs exhibited enhanced response to all the test gases, further confirming the superior sensing performance of the SnO2 PNWs. According to above results, we can deduce that the SnO2 PNWs here exhibited greatly enhanced sensing performance, including higher sensitivity and longer sensing concentration range. As a typical n-type semiconductor, the possible sensing mechanism of the SnO2 PNWs for detecting gases can be explained by the spacecharge layer mode widely discussed in many previous reports. [18–20] As shown in Fig. 5, comparing to SnO2 NWs, SnO2 PNWs composed of numerous particles have larger specific surface area (38 m2 /g), providing more active sites for target gas adsorption. It will be beneficial for the enhancement of sensing performance. Moreover, during the adsorption and desorption process of oxygen molecules on the surface of the SnO2 PNWs, the resistance of the sensors is changed when exposed to different target gas based on the space-charge layer mode. [22–25] When the sensors are exposed to air, oxygen molecules in air are adsorbed onto the surface of nanowire and extract electrons from the conduction band to generate chemisorbed oxygen species (O2− ), which leads to the formation of a thick depletion layer at the oxides surface, as shown in Fig. 5, resulted in relatively high resistance. When the sensors are exposed to reducing gas such as ethanol at the working temperature, the chemical reactions between the reducing gases and the oxygen species adsorbed on the surface of the sensors resulted in the feed of free electrons back into the conduction band of SnO2 . The reactions can be express by the following equation: CH3 CH2 OH + 6O2− → 2CO2 + 3H2 O + 12e−
(3)
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Fig. 5. Schematic gas sensing mechanism of porous SnO2 nanowire and SnO2 nanowire sensors.
Simultaneously, the depletion layer at the SnO2 surface is narrowed and the conductivity is increased as shown in Fig. 5. Thus, the resistance of the SnO2 PNWs is decreased. When ethanol gas is out, the sensors are exposed to air again and the resistance of the sensors is also increased. 4. Conclusion In summary, highly porous SnO2 nanowires with a large surface area of 38 m2 /g were synthesized by hydrothermal etching of electrospun SnO2 NWs in Na2 S solution. Compared with the pristine SnO2 NWs, the porous nanowires exhibit a much higher gas sensing responsivity (∼two times higher) to ethanol. Besides, it can detect a much wider ethanol concentrate region (1–5000 ppm) and has a higher saturation concentration of 2000 ppm. The remarkable enhanced sensing performance of the porous nanowires can be attributed to the highly effective surface interactions arising from the increased exposed surface after hydrothermally-etching process which enhances the response by enabling increased interactions between gas molecules and the SnO2 PNWs. The excellent gas sensing of the porous nanowires demonstrated here reveals that the combination of electrospinning and hydrothermal etching techniques may provide a very simple and efficient way to get porous nanostructures with enhanced sensing performance for practical applications. Acknowledgement The work was supported by National Natural Science Foundation of China (51672308, 61625404, 61504136), the Beijing Natural Science Foundation (4162062), Beijing Municipal Science and Technology Project (No. Z17111000220000) and the Key Research Program of Frontiers Sciences, CAS (QYZDY-SSW-JSC004). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2017.05.161. References [1] C. Coronado, J.J. Carvalho, J. Andrade, E. Cortez, F. Carvalho, J. Santos, A. Mendiburu, Flammability limits: a review with emphasis on ethanol for aeronautical applications and description of the experimental procedure, J. Hazard. Mater. 241–242 (2012) 32–54. [2] F. Bender, N. Barie, G. Romoudis, A. Voigt, M. Rapp, Development of a preconcentration unit for a SAW sensor micro array and its use for indoor air quality monitoring, Sens. Actuators B-Chem. 93 (2003) 135–141.
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Biographies
Rui Li received her BS degree from School of Physics, Qufu Normal University.She is a postgraduate at the School of Mathematics and Physics, University of Science and Technology Beijing. Her research interest focuses on flexible gas sensors and lithium ion batteries.
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Tingting Huang received her Bachelor’s degree from School of Physics, Hebei Normal University. Currently, she is a postgraduate at the School of Mathematics and Physics, University of Science and Technology Beijing. Her research interest is flexible gas sensors and supercapacitors.
Shuai Chen received his MSc degree from Qingdao University in 2015. Now he is a PhD candidate at the School of Mathematics and Physics, University of Science and Technology Beijing. He is also a visiting student at Institute of Semiconductors, Chinese Academy of Sciences. His research interest focuses on flexible electronic devices, including strain sensors and multifunctional integrated systems.
Yuanjun Song received his PhD degree in Condensed Matter Physics from Institute of Physics, Chinese Academy of Sciences (IOPCAS). Now he works in the School of Mathematics and Physics, University of Science and Technology Beijing. His research interest focuses on microstructure and phase transition study of magneto-photoelectrical composite via transmission electron microscopy (TEM) and X-ray diffraction. He has investigated some functional materials such as catalysts, magnetic nanomaterials and lithium battery materials, with understanding of the microstructure and phase transition give some advice on optimizing the performance.
Zheng Lou received his Ph.D. degree from Jilin University in 2014. He joined the Institute of Semiconductors, Chinese Academy of Sciences as an Assistant Professor in 2014. His current research focuses on flexible electronics, including gas sensors, pressure sensors, electronic-skin, transistors and photo-detectors.
Di Chen is a professor at University of Science and Technology Beijing. She received her PhD degree from the University of Science and Technology of China in 2005. Her current research interest is the advanced technology for designing nanostructure for sustainable energy applications, including energy storage and photocatalysts. She has published about 100 papers in international referred journals, including Advanced Materials, Advanced Functional Materials, Nano Letters, ACS Nano, etc.
La Li received her BS degree from the Department of Chemical Engineering, Tomsk Polytechnical University, Tomsk City, Russia, Currently, she is a Ph. D candidate at College of Physics, Jilin University, China. Her research interests mainly focus on flexible micro-supercapacitors.
Guozhen Shen received his Ph. D degree in 2003 from University of Science and Technology of China. From 2004–2009, he conducted his research in Hanyang University (Korea), National Institute for Materials Science (Japan), University of Southern California (USA) and Huazhong University of Science and Technology (China). He joined Institute of Semiconductors, Chinese academy of Sciences as a professor in 2013. His current research focuses on flexible electronics and printable electronics. He has published more than 200 papers with an H-factor of 54. He received several important awards, including the Research Fund for Distinguished Young Scientists, Mao Yi-Sheng Science and Technology Awards, etc.