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SnO2 heterojunctions for highly H2-selective sensors
Sensors and Actuators B 244 (2017) 694–700
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Modulation of potential barrier heights in Co3 O4 /SnO2 heterojunctions for highly H2 -selective sensors Lianping Huo a,b , Xi Yang a,∗ , Zengwei Liu a , Xin Tian a , Tianjiao Qi a , Xinfeng Wang a , Kun Yu a , Jie Sun a , Meikun Fan b,∗ a b
Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, China Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu 610031, China
a r t i c l e
i n f o
Article history: Received 10 October 2016 Received in revised form 4 January 2017 Accepted 9 January 2017 Available online 10 January 2017 Keywords: Gas sensor Potential barrier Co3 O4 /SnO2 Heterojunction H2 Selectivity
a b s t r a c t Chemiresistive gas sensors employing p-n heterostructures offer a compelling combination of high sensitivity and specific selectivity due to the synergic effects at interface. In this study, the p-Co3 O4 /n-SnO2 composites with different molar ratio of Co/Sn have been prepared using a simple soak-calcination method and their sensing properties are systematically investigated. The sensors demonstrate exclusive H2 sensing properties with p-type sensing response, and n-type sensing response to the typical reducing gases such as CO, H2 S and NH3 . We propose that the abnormal sensing behaviors might be associated with the modulation of potential barrier heights formed in p-Co3 O4 /n-SnO2 heterojunctions, namely the modulation from the asymmetric gas sensing reactivity of SnO2 and Co3 O4 to the reducing gases. This work may open up a general approach for tailoring the sensing selectivity of gas sensors via the modulation of potential barrier heights in p-n heterojunctions. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen (H2 ) is regarded as one of the most promising energy sources considering the increasing shortage of fossil fuels with environmental issues such as air pollution and global warming [1–3]. On the other hand, as one of the most dangerous gases, H2 can explode and flame when the concentration becomes higher than 4% in air [4]. Since H2 is a small molecular with high diffusion coefficient, the detection of H2 leakage has also become a crucial issue to ensure the safety during the storage, delivery, and usage of H2 [5]. Therefore, there has been a huge demand for sensitive and selective H2 gas sensors [6–8]. Chemiresistive gas sensors based on metal oxide semiconductors (MOSs) have been intensely investigated for the detection of toxic and explosive gases, owing to their ease of operation, low cost and superb sensitivity [9–11]. So far, a large number of MOSs have been investigated, such as SnO2 , ZnO, WO3 and In2 O3 [12–14]. Among them, SnO2 has gained particular attention owing to its outstanding sensitivity, high conductivity and excellent chemical stability [15–18]. Although SnO2 -based gas sensors have already been very popular in the market, there is vast room for modifica-
∗ Corresponding authors. E-mail addresses: [email protected] (X. Yang), [email protected] (M. Fan). http://dx.doi.org/10.1016/j.snb.2017.01.061 0925-4005/© 2017 Elsevier B.V. All rights reserved.
tions of the sensing properties, such as sensitivity, selectivity, speed (response/recovery rate), and stability, namely the “4s”, to meet the ever-expanding demands. Poor selectivity is one of the worst disadvantages of SnO2 -based gas sensors for the practical applications [19,20]. Surface modification with Pd nanoparticles as the sensitizer [21–23], is the most commonly used approach to increase the H2 -selectivity owing to the high activity of Pd nanoparticles for the H2 adsorption [24]. However, the enhancement is still not satisfactory and the crosssensitivity still occurs among some reducing gases. Decorating the H2 -selective sieves (such as MOFs [25–28], polymers [29]) on the surfaces of gas sensing materials also has been explored to improve the H2 selective sensitivity. The dense deposition of H2 -selective sieves will influence the sensitivity and response/recovery rate. Meanwhile, the p-n heterojunction strategy has been employed to improve the sensing properties of SnO2 -based gas sensors due to the synergic effects at interface. Unfortunately, most of the latest works have been focused on improving their sensitivity and response/recovery rate by decreasing grain size, enlarging specific surface area, and improving gas accessibility [13,30–33]. The enhancement is contributed by the modulation of potential barriers encountered by an electron crossing the interface and/or narrowing of the charge conduction channel via the p-n depletion region [34]. Little effort has been made to improve the selectivity.
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Fig. 1. a) The photographs of interdigitated Au-alloy electrode and gas sensor. The photographs of b) gas sensing analysis system, c) gas testing chamber, composed of heater and measurement probes, and d) two gas sensors under test.
Herein, we demonstrated that the selectivity can be adjusted via the modulation of potential barrier heights in hererojunctions utilizing the asymmetric gas sensing reactivity of heteromaterials. The Co3 O4 /SnO2 p-n heterojunction composites were prepared through a simple soak-calcination process, and their sensing properties were systematically investigated. The sensors exhibited exclusive H2 sensing properties, which show an n-type sensing response to the reducing gases of CO, H2 S and NH3 , whereas a p-type sensing response towards H2 . The mechanism of the abnormal H2 -sensing behavior was proposed to be the asymmetric gas sensing reactivity of SnO2 and Co3 O4 to the typical reducing gases of CO, H2 S, NH3 and H2 . 2. Experimental 2.1. Preparation of p-Co3 O4 /n-SnO2 composites The Co3 O4 /SnO2 composites were synthesized using a simple soak-calcination method. In a typical synthesis, cobaltous nitrate (Co(NO3 )2 ·6H2 O) was dissolved in ethanol to obtain a clear solution. Then, SnO2 with particle size of 50–70 nm (Purchased from Mackin Biochemical Co., Ltd) was added to the Co(NO3 )2 ethanol solution and thoroughly mixed under sonication for 30 min. Finally, the as-prepared mixture was dried at 100 ◦ C, and calcined at 600 ◦ C for 2 h under air atmosphere with a heating rate of 10 ◦ C min−1 to obtain the final sample. The molar ratio (mol%) of Co/Sn was varied from 0 to 100% and the Co3 O4 /SnO2 composites were named as Co/Sn-X, where Co and Sn stand for Co3 O4 and SnO2 , respectively, and X indicates the Co3 O4 content (mol%).
X-ray photoelectron spectroscopy (XPS) was carried out using AXIS HIS 165 spectrometer (Kratos Analytical) with a monochromatized Al K␣ X-ray source (1486.71 eV photons).
2.3. Sensor fabrication and measurement For the sensor fabrication, the as-synthesized materials were mixed with ethanol to form pastes, which were then coated on Al2 O3 ceramic substrates pre-patterned with interdigitated Aualloy electrodes (Fig. 1a). The gas sensors were aged at a proper temperature to stabilize the sensitivity. The measurements of gas sensing properties were operated on CGS-4TPs intelligent gas sensing analysis system (Beijing Elite Tech Co., Ltd., China), as shown in Fig. 1b–d. The gas concentration was controlled by mass flow controllers in order to maintain a balanced mixing ratio of highly purified test gases (500 ppm in air) and dry air. The total flow rate was set at 1000 sccm and the humidity was controlled under 10 RH% before the measurements. The sensor response was defined as: S = (Rg − Ra )/Ra × 100%, where Ra is the sensor resistance in the air, Rg is the sensor resistance in the target gas.
2.2. Characterization Powder X-ray diffraction (XRD) measurements were performed on a Bruker D8 diffractometer with Cu-K␣ radiation ( = 1.5406 Å). The morphology and microstructure were analyzed by a field emission scanning electron micro-scope (SEM) and transmission electron microscopy (TEM). The SEM was performed on a HITACHI SU8200 with a typical acceleration voltage of 10 kV. The TEM and HR-TEM were carried on Philips T20ST and the STEM and STEMEDX was carried on an FEI TECNAI F20 attached with high angle annular dark field (HAADF) detector and energy disperse X-ray spectroscopy (EDX), with a typical acceleration voltage of 200 kV.
Fig. 2. XRD patterns of Co3 O4 /SnO2 composites with different mol% of Co/Sn.
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3. Results and discussion 3.1. Microstructure characterization of Co3 O4 /SnO2 composites The crystalline structures of the Co3 O4 /SnO2 composites with different mol% of Co/Sn were analyzed by XRD. As shown in Fig. 2, the patterns are composed of two types of diffraction peaks and all the recognizable diffraction peaks can be assigned to tetragonal SnO2 (JCPDS 41-1445) and cubic Co3 O4 (JCPDS 65-3103), respectively. Accordingly, we confirmed that the Co(NO3 )2 was decomposed to form Co3 O4 nanocrystals after the calcination treatment. Furthermore, at low mol% of Co/Sn (≤10%), there is no recognizable Co3 O4 diffraction peaks, probably due to the low content of Co3 O4 nanocrystals. However, the presence of Co3 O4 can still be identified by the XPS spectra (Fig. S1). In addition, at higher mol% of Co/Sn (≥10%), the intensity of Co3 O4 diffraction peaks was enhanced as the mol% of Co/Sn increases.
The morphology and microstructure of Co3 O4 /SnO2 composite (Co/Sn-10%) were investigated by SEM and TEM characterizations. The low- and high-magnification SEM images of Co/Sn-10% composite are shown in Fig. 3a and b, respectively. The Co/Sn-10% composite displays the similar morphology with that of SnO2 nanoparticles (as shown in Fig. S2), indicating that the Co3 O4 nanoparticles were uniformly dispersed in the composites. The TEM image shown in Fig. 3c indicates irregular nanoparticles with diameters ranging from 50 to 70 nm. This is consistent with the particle size of SnO2 nanoparticles. Fig. 3d shows a lattice-resolved HR-TEM image of Co/Sn-10%. Co3 O4 nanoparticles with diameter of ∼10 nm were decorated on the surfaces of SnO2 nanoparticles; the distances between the lattice fringes were estimated to be 0.241 nm and 0.233 nm, matching the d311 value of a cubic Co3 O4 phase and the d200 value of a tetragonal SnO2 phase, respectively. The bright field STEM image and the corresponding STEM-EDX Co mapping image are shown in Fig. 3e, f. The STEM-EDX element mapping
Fig. 3. The morphology and microstructure of Co/Sn-10% composite. a) Low- and b) high-magnification SEM images. c) TEM and d) HR-TEM images. e) Bright field STEM image and f) STEM-EDX Co mapping image.
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Fig. 4. a) Dynamic resistance curves towards 50 ppm of H2 and b) Electrical resistances in air of Co3 O4 /SnO2 composites with different mol% of Co/Sn at the operating temperature of 300 ◦ C. c) Response versus the operating temperature of Co/Sn-10% to 50 ppm of H2 .
characterization also suggested a uniform dispersion of Co3 O4 in the composite. 3.2. Gas sensing properties of Co3 O4 /SnO2 composites The sensing properties of Co3 O4 /SnO2 composites were shown in Fig. 4. The dynamic resistance curves of Co3 O4 /SnO2 composites with different mol% of Co/Sn towards 50 ppm of H2 at the operating temperature of 300 ◦ C were shown in Fig. 4a. The sensor response towards H2 exhibited an interesting n-to-p transition depending on the Co/Sn mol%. To be more specific, the SnO2 and Co/Sn-0.1% had an n-type sensing response to 50 ppm of H2 where the electrical resistance decreased upon exposure to the target gas, which then switched to a p-type sensing response when the mol% of Co/Sn was between 1% and 75% (i.e., the electrical resistance increased upon exposure to the target gas). However, the Co/Sn-100% showed negligible response towards 50 ppm of H2 . The corresponding electrical resistances of Co3 O4 /SnO2 composites measured in air (Ra ) were recorded in Fig. 4b. With the increase of Co/Sn mol% from 0 to 10%, Ra increased sharply from ∼8 k to ∼130 k, and it decreased to ∼50–60 k with the mol% of Co/Sn between 25% and 75%. At 100 mol% of Co/Sn, the Ra is similar with that of Co3 O4 (Fig. S3). The electrical resistance increased sharply from 0 to 10 mol% of Co/Sn should be worthy mentioned. Such a sharp increase in Ra with increasing the amount of additives has been reported elsewhere for the p-n heterojunctions [35,36]. The Fermi levels across the interface of p-n heterojunctions can equilibrate to the same energy, usually resulting in the formation of a charge depletion layer and potential barriers in p-n heterojunctions to restrict the flow of electrons. Here the Co3 O4 /SnO2 system seemed to have the same mechanism. The p-type Co3 O4 can trap the electrons from the n-type SnO2 , leading to an additional depletion layer and forming the heterojunction. As a result, the resistance of Co3 O4 /SnO2 composites increased greatly. In addition, the sensor response varied
depending on the mol% of Co/Sn, and Co/Sn-10% had the optimal response, as shown in Fig. S4. The effect of operating temperature on the sensing response of Co/Sn-10% towards 50 ppm of H2 was also studied. As shown in Fig. 4c, the Co/Sn-10% showed a ptype sensing response in a wide operating temperature range from 250 ◦ C to 400 ◦ C, and the sensing response reached its maximum at an optimal operating temperature of 300 ◦ C. The sensing properties of Co/Sn-10% towards H2 were further studied, as shown in Fig. 5. Fig. 5a shows two successive cycles of repeated p-type dynamic response curves of Co/Sn-10% to different concentrations of H2 from 5 to 200 ppm, which exhibited similar dynamic response curves, indicating the excellent response stability. Upon increasing H2 concentration from 5 to 200 ppm, the response increased gradually. Moreover, as H2 was injected into the testing chamber, the response rapidly increased, and the response can almost recover to its initial value upon air purging, indicating a good reversibility. It is worth noting that the sensor was sensitive to H2 down to 5 ppm. The responses of Co/Sn-10% to different H2 concentrations had a typical dependence of the concentrations of H2 , as shown in Fig. S5. The response tended to saturation with increasing H2 concentrations, possibly due to the partial decrease of the available adsorption sites for the gas molecules under high H2 concentration. To demonstrate the excellent reproducibility of Co/Sn-10%, the sensor was exposed to 50 ppm of H2 for 24 successive cycles (Fig. 5b). Furthermore, the response of a Co/Sn-10% based sensor to 50 ppm of H2 was evaluated after 32 days of storage. The dynamic resistance curves have little changed, as shown in Fig. S6, which indicates the excellent long-term stability of Co/Sn-10%. H2 , CO, H2 S, and NH3 are the typical reducing gases, so they were chosen for the H2 selectivity study of Co/Sn-10% in this work. The dynamic resistance curves of Co/Sn-10% to 50 ppm of H2 , CO, H2 S and NH3 are shown in Fig. 6a. The sensor resistance was expect to decrease upon exposure to reducing gases including H2 , CO, H2 S, and NH3 . However, while having an n-type sensing response to
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Fig. 5. a) Two cycles of repeated dynamic resistance curves of Co/Sn-10% towards H2 of different concentrations (5–200 ppm). b) Dynamic resistance curves of repeatability test to 50 ppm of H2 . Note that the sensing properties of Co/Sn-10% towards H2 in Fig. a) and b) were characterized by two different sensor devices.
Fig. 6. a) Dynamic resistance curves of Co/Sn-10% to the typical reducing gases of H2 , CO, NH3 and H2 S. b) The sensing mechanism diagrams of Co3 O4 /SnO2 composites.
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50 ppm of CO, H2 S and NH3 , respectively, the sensor exhibited a ptype sensing response to 50 ppm of H2 . In this respect, Co/Sn-10% was highly selective to H2 against CO, H2 S, and NH3 . Interestingly, we observed a p-to-n transition for the sensor response toward H2 , which was dependent on the Co/Sn mol% (Fig. 4a). Taking together with the above experimental observations, we proposed the mechanism of the abnormal sensing behaviors as shown in Fig. 6b–f. In an air environment at the operating temperature of 300 ◦ C, the absorbed oxygen molecules will extract electrons from the conduction bands of SnO2 and Co3 O4 , and converted to oxygen species, such as O2− , O− and O2 − , covering the surfaces of nanograins [37,38]. Simultaneously, the band bending occurs and junctions form at the grain boundaries. At low mol% of Co/Sn, most of junctions are homojunctions formed at SnO2 -SnO2 nanograin boundaries, where the potential barriers in SnO2 -SnO2 homojunctions restrict the flow of electrons, as shown in Fig. 6b. In this case, when the sensor was exposed to H2 , the subsequent reaction (H2 + Ox− → H2 O + xe− ) would release electrons back into the conduction band of SnO2 , reducing the width of depletion layer and decreasing the height of potential barriers, and thereby the sensor resistance decreased (Fig. 6c) [39]. On the other hand, at the high mol% of Co/Sn, lots of SnO2 -Co3 O4 heterojunctions simultaneously generated at SnO2 -Co3 O4 nanograins boundaries in addition to the SnO2 -SnO2 homojunctions. When the n-type SnO2 and ptype Co3 O4 nanograins come into contact, electrons will flow from n-type SnO2 to p-type Co3 O4 due to the difference in their work function until reaching an equilibrium with the same Fermi level on each side. As a result, the resistances of Co3 O4 /SnO2 composites increased greatly [35,36], as actually observed in Fig. 4b. The potential barriers in SnO2 -Co3 O4 heterojunctions were shown in Fig. 6d, which was qualitatively consistent with the previously report [36,40]. Upon exposure to H2 , as SnO2 has highly reactivity to H2 , the conduction electrons are given back to the conduction band of SnO2 , resulting in the decrease of band bending. However, Co3 O4 is almost inactive to H2 , as shown in Fig. S7, the conduction band of Co3 O4 remains almost unchanged. So that the height of potential barriers in SnO2 -Co3 O4 heterojunctions is increased (Fig. 6e), which results in the increase of the sensor resistance towards H2 (Fig. 6a) [40,41]. Considering the fact that the sensors showed an ntype sensing response to 50 ppm of CO, H2 S and NH3 , it is possible that the reaction between CO, H2 S, NH3 with the oxygen species may occur on both Co3 O4 and SnO2 surfaces (Fig. S7), so that the electrons are given back to both the conduction bands of Co3 O4 and SnO2 , decreasing the height of potential barriers (Fig. 6f). In this way, the resistances of gas sensors towards CO, H2 S and NH3 decreased and shown as an n-type response.
4. Conclusion A series of p-Co3 O4 /n-SnO2 composites with different Co/Sn mol% were prepared using a simple soak-calcination method and their sensing properties were systematically investigated. The sensors exhibited interesting Co/Sn mol%-dependent transition of the response type towards H2 . At low mol% of Co/Sn, the sensor shows an n-type sensing response to 50 ppm of H2 , but switches to a ptype sensing response at high mol% of Co/Sn. When Co/Sn mol% is 10%, the sensor shows an n-type sensing response towards the typical reducing gases such as CO, H2 S and NH3 , whereas a p-type sensing response towards H2 was observed. The potential barriers formed in p-Co3 O4 /n-SnO2 heterojunctions may play a significant role for the abnormal sensing behaviors. The height of potential barriers could be modulated by the asymmetric gas sensing reactivity of SnO2 and Co3 O4 to the reducing gases, so that n- or ptypes of sensing response was observed. This study might open up
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an effective strategy for tailoring the selectivity of gas sensors by modulation of potential barriers heights in p-n heterojunctions. Acknowledgments The authors gratefully acknowledge financial support from NSFC (Grants 11472252), Innovative Research Fund (KJCX-201413) and Science and Technology Projects (KJZX-201502) of ICM. 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.2017.01.061. References [1] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Storage of hydrogen in single-walled carbon nanotubes, Nature 386 (1997) 377–379. [2] S. Dunn, Hydrogen futures: toward a sustainable energy system, Int. J. Hydrogen Energy 27 (2002) 235–264. [3] C.J. Winter, N. Joachim, Hydrogen as an Energy Carrier: Technologies, Systems, Economy, Springer Science & Business Media, 2012. [4] J.G. Firth, A. Jones, T.A. Jones, The principles of the detection of flammable atmospheres by catalytic devices, Combust. Flame 20 (1973) 303–311. [5] X. Bévenot, A. Trouillet, C. Veillas, H. Gagnaire, M. Clément, Hydrogen leak detection using an optical fiber sensor for aerospace applications, Sens. Actuators B: Chem. 67 (2000) 57–67. [6] F.d.r. Favier, E.C. Walter, M.P. Zach, T. Benter, R.M. Penner, Hydrogen sensors and switches from electrodeposited palladium mesowire arrays, Science 293 (2001) 2227. [7] G. Korotcenkov, S.D. Han, J.R. Stetter, Review of electrochemical hydrogen sensors, Chem. Rev. 109 (2009) 1402–1433. [8] J. Kong, M.G. Chapline, H. Dai, Functionalized carbon nanotubes for molecular hydrogen sensors, Adv. Mater. 13 (2001) 1384–1386. [9] G. Korotcenkov, Metal oxides for solid-state gas sensors: what determines our choice, Mater. Sci. Eng. B 139 (2007) 1–23. [10] N. Barsan, D. Koziej, U. Weimar, Metal oxide-based gas sensor research: how to, Sens. Actuators B: Chem. 121 (2007) 18–35. [11] M.E. Franke, T.J. Koplin, U. Simon, Metal and metal oxide nanoparticles in chemiresistors: does the nanoscale matter? Small 2 (2006) 36–50. [12] X.J. Huang, Y.K. Choi, Chemical sensors based on nanostructured materials, Sens. Actuators B: Chem. 122 (2007) 659–671. [13] M. Tiemann, Porous metal oxides as gas sensors, Chem. Eur. J. 13 (2007) 8376–8388. [14] G. Shen, P.C. Chen, K. Ryu, C. Zhou, Devices and chemical sensing applications of metal oxide nanowires, J. Mater. Chem. 19 (2009) 828–839. [15] N. Barsan, M. Schweizer-Berberich, W. Göpel, Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: a status report, Fresenius J. Anal. Chem. 365 (1999) 287–304. [16] E.R. Leite, I.T. Weber, E. Longo, J.A. Varela, A new method to control particle size and particle size distribution of SnO2 nanoparticles for gas sensor applications, Adv. Mater. 12 (2000) 965–968. [17] T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, S. Matsushima, Development of SnO2 -based ethanol gas sensor, Sens. Actuators B: Chem. 9 (1992) 63–69. [18] B. Wang, L.F. Zhu, Y.H. Yang, N.S. Xu, G.W. Yang, Fabrication of a SnO2 nanowire gas sensor and sensor performance for hydrogen, J. Phys. Chem. C 112 (2008) 6643–6647. [19] N. Shirahata, W. Shin, N. Murayama, A. Hozumi, Y. Yokogawa, T. Kameyama, Y. Masuda, K. Koumoto, Reliable monolayer-template patterning of SnO2 thin films from aqueous solution and their hydrogen-sensing properties, Adv. Funct. Mater. 14 (2004) 580–588. [20] H. Huang, H. Gong, C.L. Chow, J. Guo, T.J. White, M.S. Tse, O.K. Tan, Low-Temperature growth of SnO2 nanorod arrays and tunable n-p-n sensing response of a ZnO/SnO2 heterojunction for exclusive hydrogen sensors, Adv. Funct. Mater. 21 (2011) 2680–2686. [21] N. Van Toan, N. Viet Chien, N. Van Duy, H. Si Hong, H. Nguyen, N. Duc Hoa, N. Van Hieu, Fabrication of highly sensitive and selective H2 gas sensor based on SnO2 thin film sensitized with microsized Pd islands, J. Hazard. Mater. 301 (2016) 433–442. [22] C. Liewhiran, N. Tamaekong, A. Wisitsoraat, A. Tuantranont, S. Phanichphant, Ultra-sensitive H2 sensors based on flame-spray-made Pd-loaded SnO2 sensing films, Sens. Actuators B: Chem. 176 (2013) 893–905. [23] Lim C. b, S. Oh, Microstructure evolution and gas sensitivities of Pd-doped SnO2 -based sensor prepared by three different catalyst-addition processes, Sens. Actuators B: Chem. 30 (1996) 223–231. [24] F.D. Manchester, A. San-Martin, J.M. Pitre, The H-Pd (hydrogen-palladium) system, J. Phase Equilib. 15 (1994) 62–83. [25] G. Lu, J.T. Hupp, Metal-organic frameworks as sensors: a ZIF-8 based Fabry-Pérot device as a selective sensor for chemical vapors and gases, J. Am. Chem. Soc. 132 (2010) 7832–7833.
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Biographies Lianping Huo received her B.S. degree in Sichuan Normal University (China) in 2014. She is currently working toward the M.S. degree in the College of Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University (China). Xi Yang received his B.S. and Ph.D. degrees from Nankai University (China) in 2009 and 2014, respectively. Now, he is a Research Associate in Institute of Chemical Materials, China Academy of Engineering Physics. His research is focused on the preparation of functional materials and development of chemical sensors. Zengwei Liu received his B.S. degree in Qingdao University of Science and Technology (China) in 2014. He is currently working toward the M.S. degree in the College of Chemical Engineering, Qingdao University of Science and Technology (China). Xin Tian received her B.S. degree from Sichuan University (China) in 2005 and Ph.D. degree from Louzhou University (China) in 2014. Now, she is a Research Associate in Institute of Chemical Materials, China Academy of Engineering Physics. Her current research is preparation of functional materials for gas sensor applications. Tianjiao Qi received her B.S. degree from Chengdu University of Technology (China) in 2007 and M.S. degree from China Academy of Engineering Physics in 2010. Now, she is a Research Associate in Institute of Chemical Materials, China Academy of Engineering Physics. She is engaged in the synthesis and characterization of semiconducting functional materials and gas sensors. Xinfeng Wang received his B.S. degree from Xi’an Jiaotong University (China) in 1994 and M.S. degree from China Academy of Engineering Physics in 2003. His interests are sensor signal processing, modeling and the trace level gas detection. Kun Yu received her B.S. degree from Sichuan University (China) in 1992. Afterwards, she joined in Institute of Chemical Materials, China Academy of Engineering Physics, to work on the trace level gas detection. Jie Sun received his B.S. degree from Northeast Normal University (China) in 1993, M.S. degree in 1998 from China Academy of Engineering Physics, and Ph.D. degree in 2004 from Beijing Institute of Technology (China). Now, he is a professor of Institute of Chemical Materials, China Academy of Engineering Physics. His current research is the development of gas detection technology and instrument. Meikun Fan obtained his M.S. degree in Southwest China Normal University (China) in 2002, then worked as a university lecturer in Dalian University of Technology (China). In 2010, he received his Ph.D. degree from University of Victoria (Canada). Now, he is a professor of Southwest Jiaotong University (China). His research mainly focused on the fabrication, modification and self-assembly of metallic nanoparticles for sensing applications.
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Modulation of potential barrier heights in Co3 O4 /SnO2 heterojunctions for highly H2 -selective sensors Lianping Huo a,b , Xi Yang a,∗ , Zengwei Liu a , Xin Tian a , Tianjiao Qi a , Xinfeng Wang a , Kun Yu a , Jie Sun a , Meikun Fan b,∗ a b
Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, China Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu 610031, China
a r t i c l e
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Article history: Received 10 October 2016 Received in revised form 4 January 2017 Accepted 9 January 2017 Available online 10 January 2017 Keywords: Gas sensor Potential barrier Co3 O4 /SnO2 Heterojunction H2 Selectivity
a b s t r a c t Chemiresistive gas sensors employing p-n heterostructures offer a compelling combination of high sensitivity and specific selectivity due to the synergic effects at interface. In this study, the p-Co3 O4 /n-SnO2 composites with different molar ratio of Co/Sn have been prepared using a simple soak-calcination method and their sensing properties are systematically investigated. The sensors demonstrate exclusive H2 sensing properties with p-type sensing response, and n-type sensing response to the typical reducing gases such as CO, H2 S and NH3 . We propose that the abnormal sensing behaviors might be associated with the modulation of potential barrier heights formed in p-Co3 O4 /n-SnO2 heterojunctions, namely the modulation from the asymmetric gas sensing reactivity of SnO2 and Co3 O4 to the reducing gases. This work may open up a general approach for tailoring the sensing selectivity of gas sensors via the modulation of potential barrier heights in p-n heterojunctions. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen (H2 ) is regarded as one of the most promising energy sources considering the increasing shortage of fossil fuels with environmental issues such as air pollution and global warming [1–3]. On the other hand, as one of the most dangerous gases, H2 can explode and flame when the concentration becomes higher than 4% in air [4]. Since H2 is a small molecular with high diffusion coefficient, the detection of H2 leakage has also become a crucial issue to ensure the safety during the storage, delivery, and usage of H2 [5]. Therefore, there has been a huge demand for sensitive and selective H2 gas sensors [6–8]. Chemiresistive gas sensors based on metal oxide semiconductors (MOSs) have been intensely investigated for the detection of toxic and explosive gases, owing to their ease of operation, low cost and superb sensitivity [9–11]. So far, a large number of MOSs have been investigated, such as SnO2 , ZnO, WO3 and In2 O3 [12–14]. Among them, SnO2 has gained particular attention owing to its outstanding sensitivity, high conductivity and excellent chemical stability [15–18]. Although SnO2 -based gas sensors have already been very popular in the market, there is vast room for modifica-
∗ Corresponding authors. E-mail addresses: [email protected] (X. Yang), [email protected] (M. Fan). http://dx.doi.org/10.1016/j.snb.2017.01.061 0925-4005/© 2017 Elsevier B.V. All rights reserved.
tions of the sensing properties, such as sensitivity, selectivity, speed (response/recovery rate), and stability, namely the “4s”, to meet the ever-expanding demands. Poor selectivity is one of the worst disadvantages of SnO2 -based gas sensors for the practical applications [19,20]. Surface modification with Pd nanoparticles as the sensitizer [21–23], is the most commonly used approach to increase the H2 -selectivity owing to the high activity of Pd nanoparticles for the H2 adsorption [24]. However, the enhancement is still not satisfactory and the crosssensitivity still occurs among some reducing gases. Decorating the H2 -selective sieves (such as MOFs [25–28], polymers [29]) on the surfaces of gas sensing materials also has been explored to improve the H2 selective sensitivity. The dense deposition of H2 -selective sieves will influence the sensitivity and response/recovery rate. Meanwhile, the p-n heterojunction strategy has been employed to improve the sensing properties of SnO2 -based gas sensors due to the synergic effects at interface. Unfortunately, most of the latest works have been focused on improving their sensitivity and response/recovery rate by decreasing grain size, enlarging specific surface area, and improving gas accessibility [13,30–33]. The enhancement is contributed by the modulation of potential barriers encountered by an electron crossing the interface and/or narrowing of the charge conduction channel via the p-n depletion region [34]. Little effort has been made to improve the selectivity.
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Fig. 1. a) The photographs of interdigitated Au-alloy electrode and gas sensor. The photographs of b) gas sensing analysis system, c) gas testing chamber, composed of heater and measurement probes, and d) two gas sensors under test.
Herein, we demonstrated that the selectivity can be adjusted via the modulation of potential barrier heights in hererojunctions utilizing the asymmetric gas sensing reactivity of heteromaterials. The Co3 O4 /SnO2 p-n heterojunction composites were prepared through a simple soak-calcination process, and their sensing properties were systematically investigated. The sensors exhibited exclusive H2 sensing properties, which show an n-type sensing response to the reducing gases of CO, H2 S and NH3 , whereas a p-type sensing response towards H2 . The mechanism of the abnormal H2 -sensing behavior was proposed to be the asymmetric gas sensing reactivity of SnO2 and Co3 O4 to the typical reducing gases of CO, H2 S, NH3 and H2 . 2. Experimental 2.1. Preparation of p-Co3 O4 /n-SnO2 composites The Co3 O4 /SnO2 composites were synthesized using a simple soak-calcination method. In a typical synthesis, cobaltous nitrate (Co(NO3 )2 ·6H2 O) was dissolved in ethanol to obtain a clear solution. Then, SnO2 with particle size of 50–70 nm (Purchased from Mackin Biochemical Co., Ltd) was added to the Co(NO3 )2 ethanol solution and thoroughly mixed under sonication for 30 min. Finally, the as-prepared mixture was dried at 100 ◦ C, and calcined at 600 ◦ C for 2 h under air atmosphere with a heating rate of 10 ◦ C min−1 to obtain the final sample. The molar ratio (mol%) of Co/Sn was varied from 0 to 100% and the Co3 O4 /SnO2 composites were named as Co/Sn-X, where Co and Sn stand for Co3 O4 and SnO2 , respectively, and X indicates the Co3 O4 content (mol%).
X-ray photoelectron spectroscopy (XPS) was carried out using AXIS HIS 165 spectrometer (Kratos Analytical) with a monochromatized Al K␣ X-ray source (1486.71 eV photons).
2.3. Sensor fabrication and measurement For the sensor fabrication, the as-synthesized materials were mixed with ethanol to form pastes, which were then coated on Al2 O3 ceramic substrates pre-patterned with interdigitated Aualloy electrodes (Fig. 1a). The gas sensors were aged at a proper temperature to stabilize the sensitivity. The measurements of gas sensing properties were operated on CGS-4TPs intelligent gas sensing analysis system (Beijing Elite Tech Co., Ltd., China), as shown in Fig. 1b–d. The gas concentration was controlled by mass flow controllers in order to maintain a balanced mixing ratio of highly purified test gases (500 ppm in air) and dry air. The total flow rate was set at 1000 sccm and the humidity was controlled under 10 RH% before the measurements. The sensor response was defined as: S = (Rg − Ra )/Ra × 100%, where Ra is the sensor resistance in the air, Rg is the sensor resistance in the target gas.
2.2. Characterization Powder X-ray diffraction (XRD) measurements were performed on a Bruker D8 diffractometer with Cu-K␣ radiation ( = 1.5406 Å). The morphology and microstructure were analyzed by a field emission scanning electron micro-scope (SEM) and transmission electron microscopy (TEM). The SEM was performed on a HITACHI SU8200 with a typical acceleration voltage of 10 kV. The TEM and HR-TEM were carried on Philips T20ST and the STEM and STEMEDX was carried on an FEI TECNAI F20 attached with high angle annular dark field (HAADF) detector and energy disperse X-ray spectroscopy (EDX), with a typical acceleration voltage of 200 kV.
Fig. 2. XRD patterns of Co3 O4 /SnO2 composites with different mol% of Co/Sn.
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3. Results and discussion 3.1. Microstructure characterization of Co3 O4 /SnO2 composites The crystalline structures of the Co3 O4 /SnO2 composites with different mol% of Co/Sn were analyzed by XRD. As shown in Fig. 2, the patterns are composed of two types of diffraction peaks and all the recognizable diffraction peaks can be assigned to tetragonal SnO2 (JCPDS 41-1445) and cubic Co3 O4 (JCPDS 65-3103), respectively. Accordingly, we confirmed that the Co(NO3 )2 was decomposed to form Co3 O4 nanocrystals after the calcination treatment. Furthermore, at low mol% of Co/Sn (≤10%), there is no recognizable Co3 O4 diffraction peaks, probably due to the low content of Co3 O4 nanocrystals. However, the presence of Co3 O4 can still be identified by the XPS spectra (Fig. S1). In addition, at higher mol% of Co/Sn (≥10%), the intensity of Co3 O4 diffraction peaks was enhanced as the mol% of Co/Sn increases.
The morphology and microstructure of Co3 O4 /SnO2 composite (Co/Sn-10%) were investigated by SEM and TEM characterizations. The low- and high-magnification SEM images of Co/Sn-10% composite are shown in Fig. 3a and b, respectively. The Co/Sn-10% composite displays the similar morphology with that of SnO2 nanoparticles (as shown in Fig. S2), indicating that the Co3 O4 nanoparticles were uniformly dispersed in the composites. The TEM image shown in Fig. 3c indicates irregular nanoparticles with diameters ranging from 50 to 70 nm. This is consistent with the particle size of SnO2 nanoparticles. Fig. 3d shows a lattice-resolved HR-TEM image of Co/Sn-10%. Co3 O4 nanoparticles with diameter of ∼10 nm were decorated on the surfaces of SnO2 nanoparticles; the distances between the lattice fringes were estimated to be 0.241 nm and 0.233 nm, matching the d311 value of a cubic Co3 O4 phase and the d200 value of a tetragonal SnO2 phase, respectively. The bright field STEM image and the corresponding STEM-EDX Co mapping image are shown in Fig. 3e, f. The STEM-EDX element mapping
Fig. 3. The morphology and microstructure of Co/Sn-10% composite. a) Low- and b) high-magnification SEM images. c) TEM and d) HR-TEM images. e) Bright field STEM image and f) STEM-EDX Co mapping image.
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Fig. 4. a) Dynamic resistance curves towards 50 ppm of H2 and b) Electrical resistances in air of Co3 O4 /SnO2 composites with different mol% of Co/Sn at the operating temperature of 300 ◦ C. c) Response versus the operating temperature of Co/Sn-10% to 50 ppm of H2 .
characterization also suggested a uniform dispersion of Co3 O4 in the composite. 3.2. Gas sensing properties of Co3 O4 /SnO2 composites The sensing properties of Co3 O4 /SnO2 composites were shown in Fig. 4. The dynamic resistance curves of Co3 O4 /SnO2 composites with different mol% of Co/Sn towards 50 ppm of H2 at the operating temperature of 300 ◦ C were shown in Fig. 4a. The sensor response towards H2 exhibited an interesting n-to-p transition depending on the Co/Sn mol%. To be more specific, the SnO2 and Co/Sn-0.1% had an n-type sensing response to 50 ppm of H2 where the electrical resistance decreased upon exposure to the target gas, which then switched to a p-type sensing response when the mol% of Co/Sn was between 1% and 75% (i.e., the electrical resistance increased upon exposure to the target gas). However, the Co/Sn-100% showed negligible response towards 50 ppm of H2 . The corresponding electrical resistances of Co3 O4 /SnO2 composites measured in air (Ra ) were recorded in Fig. 4b. With the increase of Co/Sn mol% from 0 to 10%, Ra increased sharply from ∼8 k to ∼130 k, and it decreased to ∼50–60 k with the mol% of Co/Sn between 25% and 75%. At 100 mol% of Co/Sn, the Ra is similar with that of Co3 O4 (Fig. S3). The electrical resistance increased sharply from 0 to 10 mol% of Co/Sn should be worthy mentioned. Such a sharp increase in Ra with increasing the amount of additives has been reported elsewhere for the p-n heterojunctions [35,36]. The Fermi levels across the interface of p-n heterojunctions can equilibrate to the same energy, usually resulting in the formation of a charge depletion layer and potential barriers in p-n heterojunctions to restrict the flow of electrons. Here the Co3 O4 /SnO2 system seemed to have the same mechanism. The p-type Co3 O4 can trap the electrons from the n-type SnO2 , leading to an additional depletion layer and forming the heterojunction. As a result, the resistance of Co3 O4 /SnO2 composites increased greatly. In addition, the sensor response varied
depending on the mol% of Co/Sn, and Co/Sn-10% had the optimal response, as shown in Fig. S4. The effect of operating temperature on the sensing response of Co/Sn-10% towards 50 ppm of H2 was also studied. As shown in Fig. 4c, the Co/Sn-10% showed a ptype sensing response in a wide operating temperature range from 250 ◦ C to 400 ◦ C, and the sensing response reached its maximum at an optimal operating temperature of 300 ◦ C. The sensing properties of Co/Sn-10% towards H2 were further studied, as shown in Fig. 5. Fig. 5a shows two successive cycles of repeated p-type dynamic response curves of Co/Sn-10% to different concentrations of H2 from 5 to 200 ppm, which exhibited similar dynamic response curves, indicating the excellent response stability. Upon increasing H2 concentration from 5 to 200 ppm, the response increased gradually. Moreover, as H2 was injected into the testing chamber, the response rapidly increased, and the response can almost recover to its initial value upon air purging, indicating a good reversibility. It is worth noting that the sensor was sensitive to H2 down to 5 ppm. The responses of Co/Sn-10% to different H2 concentrations had a typical dependence of the concentrations of H2 , as shown in Fig. S5. The response tended to saturation with increasing H2 concentrations, possibly due to the partial decrease of the available adsorption sites for the gas molecules under high H2 concentration. To demonstrate the excellent reproducibility of Co/Sn-10%, the sensor was exposed to 50 ppm of H2 for 24 successive cycles (Fig. 5b). Furthermore, the response of a Co/Sn-10% based sensor to 50 ppm of H2 was evaluated after 32 days of storage. The dynamic resistance curves have little changed, as shown in Fig. S6, which indicates the excellent long-term stability of Co/Sn-10%. H2 , CO, H2 S, and NH3 are the typical reducing gases, so they were chosen for the H2 selectivity study of Co/Sn-10% in this work. The dynamic resistance curves of Co/Sn-10% to 50 ppm of H2 , CO, H2 S and NH3 are shown in Fig. 6a. The sensor resistance was expect to decrease upon exposure to reducing gases including H2 , CO, H2 S, and NH3 . However, while having an n-type sensing response to
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Fig. 5. a) Two cycles of repeated dynamic resistance curves of Co/Sn-10% towards H2 of different concentrations (5–200 ppm). b) Dynamic resistance curves of repeatability test to 50 ppm of H2 . Note that the sensing properties of Co/Sn-10% towards H2 in Fig. a) and b) were characterized by two different sensor devices.
Fig. 6. a) Dynamic resistance curves of Co/Sn-10% to the typical reducing gases of H2 , CO, NH3 and H2 S. b) The sensing mechanism diagrams of Co3 O4 /SnO2 composites.
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50 ppm of CO, H2 S and NH3 , respectively, the sensor exhibited a ptype sensing response to 50 ppm of H2 . In this respect, Co/Sn-10% was highly selective to H2 against CO, H2 S, and NH3 . Interestingly, we observed a p-to-n transition for the sensor response toward H2 , which was dependent on the Co/Sn mol% (Fig. 4a). Taking together with the above experimental observations, we proposed the mechanism of the abnormal sensing behaviors as shown in Fig. 6b–f. In an air environment at the operating temperature of 300 ◦ C, the absorbed oxygen molecules will extract electrons from the conduction bands of SnO2 and Co3 O4 , and converted to oxygen species, such as O2− , O− and O2 − , covering the surfaces of nanograins [37,38]. Simultaneously, the band bending occurs and junctions form at the grain boundaries. At low mol% of Co/Sn, most of junctions are homojunctions formed at SnO2 -SnO2 nanograin boundaries, where the potential barriers in SnO2 -SnO2 homojunctions restrict the flow of electrons, as shown in Fig. 6b. In this case, when the sensor was exposed to H2 , the subsequent reaction (H2 + Ox− → H2 O + xe− ) would release electrons back into the conduction band of SnO2 , reducing the width of depletion layer and decreasing the height of potential barriers, and thereby the sensor resistance decreased (Fig. 6c) [39]. On the other hand, at the high mol% of Co/Sn, lots of SnO2 -Co3 O4 heterojunctions simultaneously generated at SnO2 -Co3 O4 nanograins boundaries in addition to the SnO2 -SnO2 homojunctions. When the n-type SnO2 and ptype Co3 O4 nanograins come into contact, electrons will flow from n-type SnO2 to p-type Co3 O4 due to the difference in their work function until reaching an equilibrium with the same Fermi level on each side. As a result, the resistances of Co3 O4 /SnO2 composites increased greatly [35,36], as actually observed in Fig. 4b. The potential barriers in SnO2 -Co3 O4 heterojunctions were shown in Fig. 6d, which was qualitatively consistent with the previously report [36,40]. Upon exposure to H2 , as SnO2 has highly reactivity to H2 , the conduction electrons are given back to the conduction band of SnO2 , resulting in the decrease of band bending. However, Co3 O4 is almost inactive to H2 , as shown in Fig. S7, the conduction band of Co3 O4 remains almost unchanged. So that the height of potential barriers in SnO2 -Co3 O4 heterojunctions is increased (Fig. 6e), which results in the increase of the sensor resistance towards H2 (Fig. 6a) [40,41]. Considering the fact that the sensors showed an ntype sensing response to 50 ppm of CO, H2 S and NH3 , it is possible that the reaction between CO, H2 S, NH3 with the oxygen species may occur on both Co3 O4 and SnO2 surfaces (Fig. S7), so that the electrons are given back to both the conduction bands of Co3 O4 and SnO2 , decreasing the height of potential barriers (Fig. 6f). In this way, the resistances of gas sensors towards CO, H2 S and NH3 decreased and shown as an n-type response.
4. Conclusion A series of p-Co3 O4 /n-SnO2 composites with different Co/Sn mol% were prepared using a simple soak-calcination method and their sensing properties were systematically investigated. The sensors exhibited interesting Co/Sn mol%-dependent transition of the response type towards H2 . At low mol% of Co/Sn, the sensor shows an n-type sensing response to 50 ppm of H2 , but switches to a ptype sensing response at high mol% of Co/Sn. When Co/Sn mol% is 10%, the sensor shows an n-type sensing response towards the typical reducing gases such as CO, H2 S and NH3 , whereas a p-type sensing response towards H2 was observed. The potential barriers formed in p-Co3 O4 /n-SnO2 heterojunctions may play a significant role for the abnormal sensing behaviors. The height of potential barriers could be modulated by the asymmetric gas sensing reactivity of SnO2 and Co3 O4 to the reducing gases, so that n- or ptypes of sensing response was observed. This study might open up
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Biographies Lianping Huo received her B.S. degree in Sichuan Normal University (China) in 2014. She is currently working toward the M.S. degree in the College of Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University (China). Xi Yang received his B.S. and Ph.D. degrees from Nankai University (China) in 2009 and 2014, respectively. Now, he is a Research Associate in Institute of Chemical Materials, China Academy of Engineering Physics. His research is focused on the preparation of functional materials and development of chemical sensors. Zengwei Liu received his B.S. degree in Qingdao University of Science and Technology (China) in 2014. He is currently working toward the M.S. degree in the College of Chemical Engineering, Qingdao University of Science and Technology (China). Xin Tian received her B.S. degree from Sichuan University (China) in 2005 and Ph.D. degree from Louzhou University (China) in 2014. Now, she is a Research Associate in Institute of Chemical Materials, China Academy of Engineering Physics. Her current research is preparation of functional materials for gas sensor applications. Tianjiao Qi received her B.S. degree from Chengdu University of Technology (China) in 2007 and M.S. degree from China Academy of Engineering Physics in 2010. Now, she is a Research Associate in Institute of Chemical Materials, China Academy of Engineering Physics. She is engaged in the synthesis and characterization of semiconducting functional materials and gas sensors. Xinfeng Wang received his B.S. degree from Xi’an Jiaotong University (China) in 1994 and M.S. degree from China Academy of Engineering Physics in 2003. His interests are sensor signal processing, modeling and the trace level gas detection. Kun Yu received her B.S. degree from Sichuan University (China) in 1992. Afterwards, she joined in Institute of Chemical Materials, China Academy of Engineering Physics, to work on the trace level gas detection. Jie Sun received his B.S. degree from Northeast Normal University (China) in 1993, M.S. degree in 1998 from China Academy of Engineering Physics, and Ph.D. degree in 2004 from Beijing Institute of Technology (China). Now, he is a professor of Institute of Chemical Materials, China Academy of Engineering Physics. His current research is the development of gas detection technology and instrument. Meikun Fan obtained his M.S. degree in Southwest China Normal University (China) in 2002, then worked as a university lecturer in Dalian University of Technology (China). In 2010, he received his Ph.D. degree from University of Victoria (Canada). Now, he is a professor of Southwest Jiaotong University (China). His research mainly focused on the fabrication, modification and self-assembly of metallic nanoparticles for sensing applications.