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Platinum dioxide activated porous SnO2 microspheres for the detection of trace formaldehyde at low operating temperature
Sensors and Actuators B 244 (2017) 475–481
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Platinum dioxide activated porous SnO2 microspheres for the detection of trace formaldehyde at low operating temperature Yi He a , Huihui Li a , Xiaoxin Zou a , Ni Bai b , Yanying Cao a , Yang Cao a , Meihong Fan a , Guo-Dong Li a,∗ a State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, International Joint Research Laboratory of Nano-Micro Architecture Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China b School of Mechanical and Metallurgical Engineering, Jiangsu University of Science and Technology, Zhangjiagang 215600, China
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Article history: Received 15 July 2016 Received in revised form 26 December 2016 Accepted 3 January 2017 Available online 4 January 2017 Keywords: Gas sensor PtO2 SnO2 Formaldehyde
a b s t r a c t Great efforts have been devoted on the detection of the volatile organic compounds indoor for human health. The detection of formaldehyde is one of the most important and popular issues for the wide usage of formaldehyde and its toxic. However, it is a challenge to monitor trace gaseous formaldehyde at lower working temperature. Herein, we present a sensor with a significantly improved sensing response and dramatically decreased operating temperature based on porous PtO2 /SnO2 microspheres. And the PtO2 /SnO2 -5 mol% composite shows the most outstanding sensing performance among all products. What’s more, the above sensor can detect gaseous formaldehyde in the ppb level (100 ppb) at low operating temperature (100 ◦ C). The excellent sensing performance should be attributed to the high catalytic activity of PtO2 nanoparticles decorated on the surface of SnO2 microspheres and the porosity of the composite. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, great concern is given on the detection of trace toxic volatile organic compounds (VOCs). As we all know, formaldehyde (HCHO) is the most popular one among VOCs emerged in room for their wide usage in many products, especially for furniture, adhesive agents, textiles, paints, and so on [1–3]. In addition, it has been classified as one of the people’s carcinogens by the international agency for research on cancer (IARC) and the world health organization (WHO) in 2004 [4,5]. However, it is difficult to selectively detect trace formaldehyde rapidly at low working temperature. Although several kinds of analytical technologies for formaldehyde detection were developed, including chromatography (gas chromatography and high-performance liquid chromatography) [6–8], spectrophotometry [9], colorimetric methods [3,10], mass spectrometric methods [11] and biosensors [12,13], they are either composed of costly instruments, complex operation procedures and/or lack of sensitivity. And it is impractical to popularize the above equipment for on-site detection [14]. Thus, portable gas sensors based on nano-sized metal oxide materials have been widely
∗ Corresponding author. E-mail address: [email protected] (G.-D. Li). http://dx.doi.org/10.1016/j.snb.2017.01.014 0925-4005/© 2017 Elsevier B.V. All rights reserved.
used in industries, medical, public safety and environmental diagnostics for its high sensitivity, high stability and low cost [15–18]. It is obvious that the more the surface area is, the more oxygen would be absorbed on the surface of the sensing material, resulting the higher response. Thus, many efforts have been devoted to obtain porous metal oxides, including SnO2 , In2 O3 , ZnO, Fe2 O3 , NiO, Co3 O4 , Cu2 O and WO3 , and so on [15,19–31]. Among the above metal oxide, tin oxide (SnO2 ) is the most important sensing material for its good stability, extensive sensitivity and low cost. Up to now, tin oxide has been widely used to detect reducing gases, for instance, ethanol, hydrogen, CO and formaldehyde et al. [15,19,32,33]. To enhance the sensing performance of SnO2 , many strategies have been developed to control its size, structure, and morphology, especially the surface area and porosity. However, the operating temperature of SnO2 sensors is still rather high, and the selectivity is poor towards formaldehyde. To increase the activity and selectivity, it is an effective way to integrate some catalytic nanoparticles into the surface of SnO2 nanomaterial. The additional catalytic particles can not only increase the reaction rate between the absorbed oxygen species and the target gases but also accelerate the electron exchange between sensing material and gases [34,35]. As we all know, noble metal Pt, one of the most active noble metal catalysts, can dramatically decrease the operating temperature and simultaneously improve the sensitivity as well
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as selectivity [36–38]. For example, Q. N. Abdullah reported that the sensor based on Pt-decorated GaN nanowires with improved H2 gas-sensing performance at room temperature [36], Y. Wang et al. found that the sensor based on Pt-functionalized NiO could enhance formaldehyde sensing performance distinctly and reduce the working temperature obviously [38]. Although there are a few PtO2 -decorated SnO2 materials that can used for formaldehyde detection, in those works formaldehyde is usually not studied as a target gas because the materials in those works are usually more sensitive for NH3 or methanol [39–41]. In the present work, porous PtO2 /SnO2 composites with good sensing performance towards formaldehyde were obtained. Moreover, it exhibits a rapid response to 100 ppb HCHO at a relatively low working temperature of 100 ◦ C. What’s more, it presents an excellent selectivity towards formaldehyde. The enhanced sensing properties should be attributed to the promotion of surface reactions assisted by PtO2 nanoparticles supported on the surface of porous SnO2 microspheres.
Fig. 1. The XRD patterns of porous SnO2 , PtO2 /SnO2 -2 mol%, PtO2 /SnO2 -5 mol%, PtO2 /SnO2 -8 mol% and PtO2 /SnO2 -10 mol%, respectively. The peaks corresponding to SnO2 are marked with “*”.
2. Experimental 2.1. Chemicals and reagents Stannic chloride pentahydrate (SnCl4 ·5H2 O) was purchased from Tianjin Fuchen Chemical Reagent Factory. Isopropanol and ethanol were purchased from Beijing Chemical Works. Chloroplatinic acid was purchased from Shanghai Bojing chemical company. Glycerol was purchased from Sinopharm Chemical Reagent Co. Ltd. All the above chemicals were of analytical grade and used without further purification. 2.2. Synthesis and characterizations of the porous PtO2 /SnO2 microspheres Porous SnO2 microspheres were prepared by a templatefree solvothermal method reported previously [42]. The porous PtO2 /SnO2 composites were prepared by an impregnation method [43,44]. Typically, 1 mmol SnO2 precursor was added in 2 mL ethanol containing different amount of H2 PtCl6 (0.02, 0.05, 0.08 and 0.10 mmol) and stirred at room temperature for 2 h, then dried at 80 ◦ C in air, finally calcined in air at 500 ◦ C for 2 h, the resulting products were denoted as PtO2 /SnO2 -2 mol%, PtO2 /SnO2 -5 mol%, PtO2 /SnO2 -8 mol% and PtO2 /SnO2 -10 mol%, where the number meant the molar ratio of Pt species to SnO2 . To obtain Pt/SnO2 composites sensor, PtO2 /SnO2 -5 mol% was reduced by hydrogen at 250 ◦ C for 2 h, as a result, a black product was obtained indicating the formation of Pt nano particles on the surface of PtO2 /SnO2 . The photo graph of fabricated sensor was shown in Fig. S5, Pt/SnO2 is black, while, the PtO2 /SnO2 -5 mol% was light yellow and the SnO2 was nearly white. The crystallization information of the final products were collected on a Rigaku D/Max 2550 X-ray diffractometer using Cu K␣ radiation operated at 200 mA and 50 kV, a HITACHI SU8020 scanning electron microscope (SEM) and a field-emission transmission electron microscope (FE-TEM, Philips-FEI Tecnai G2S-Twin). The composition and chemical state of the porous PtO2 /SnO2 composite was analyzed by X-ray photoelectron Spectroscopy (XPS, ESCALAB 250 X-ray photoelectron spectrometer) and selected-area electron diffraction (SAED, Philips-FEI Tecnai G2S-Twin). BET surface area and the corresponding Barrett-Joyner-Halenda (BJH) pore diameter were obtained by using a Micromeritics ASAP 2020 M system. 2.3. Sensor fabrication and measurement The fabrication process of gas sensor can be described as follows: an appropriate amount of as-synthesized porous PtO2 /SnO2 micro-
spheres were fully mixed with ethanol to prepare viscous slurry, which was then pasted onto a ceramic tube (a diameter around 1 mm and a length of 4 mm) positioned with a pair of Au electrodes and four Pt wires on both ends of the tube. After drying in air at room temperature, a nickel-chromium heating wire was inserted into the center of the tube as a heater, allowing us to control the working temperature (Scheme 1A). Then, the as-fabricated sensors were aged at 200 ◦ C for 12 h to ensure the good contact between the sensing material and the Au electrodes. All of the sensors with different composition were fabricated by the same method. Gas sensing tests were performed on a commercial CGS-8 Gas Sensing Measurement System (Beijing Elite Tech Company Limited). The gas sensing performances were evaluated by a static process, which was described in detail in our previous work [22] (Scheme 1B). The response was defined as Ra /Rg for reducing gases, where the resistance of the sensor in clean air and target gas was recorded as Ra and Rg , respectively. The response time and recovery time are defined according to previous reports [45]. All the experiments were conducted at room temperature (∼ 27 ◦ C) and the clean air with a relative humidity around 25% RH (27 ◦ C) was used to dilute the target gas to the desired concentration. 3. Results and discussion 3.1. Characterization of the composites The phase composition of the as-synthesized samples was confirmed by X-ray diffraction (XRD) pattern, and the typical diffraction patterns of porous SnO2 microsphere and PtO2 /SnO2 composites calcined at 500 ◦ C were shown in Fig. 1. All of the diffraction peaks in the typical XRD pattern could be attributed to tetragonal rutile SnO2 (JCPDS card no. 41–1445). No peaks related to Pt species were presented in the XRD patterns of PtO2 /SnO2 composites due to the trace Pt in the composites [46]. Moreover, FE-TEM and XPS were used to identify the presence of Pt nanoparticles. The morphology and size of SnO2 microspheres and assynthesized PtO2 /SnO2 composite was characterized by SEM and FE-TEM. As shown in Fig. 2, 2 S2 and Fig. 3A, the porous microspheres with the diameter of about 1–2 m are composed of nanoparticles with the size around 10 nanometers. Moreover, the surface of the as-synthesized PtO2 /SnO2 composite is smoother than that of the pure SnO2 spheres (Fig. S2) indicating the formation of tiny PtO2 species on the surface of SnO2 . In addition, the PtO2 /SnO2 composite is a kind of solid microsphere as shown in TEM image (Fig. 2D).
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Scheme 1. (A) A diagram of the sensor composed of ceramic tube, two Au electrodes, and a Ni-Cr wire. (B) A schematic diagram of the measurement circuit.
Fig. 2. SEM images of (A) porous SnO2 and (B-C) PtO2 /SnO2 -5 mol% composite. TEM image of (D) PtO2 /SnO2 -5 mol% composite.
Fig. 3. (A) TEM and (B) HRTEM image, (C) SAED pattern of PtO2 /SnO2 -5 mol% composite.
To understand the microstructure of the PtO2 /SnO2 composites, the HRTEM image and SAED were obtained, as shown in Fig. 3B and C. The fringe spacing of 0.34 nm and 0.27 nm can be clearly observed in Fig. 3B corresponds to the (110) plane of SnO2 and the (100) plane of PtO2 , respectively. The polycrystalline feature of the microspheres was also indicated by the SAED, which is in accordance with the HRTEM image. To further confirm the presence of PtO2 species on the porous SnO2 microspheres as well as the valence state, X-ray photoelectron spectroscopy (XPS) measurements were carried out to study the binding energy of Pt 4f. The survey spectrum (Fig. S1) revealed
the presence of Sn, Pt and O in the PtO2 /SnO2 composite, indicating a good purity of the as-synthesized product. Fig. 4 showed the Gaussian fitted core-level Pt 4f XPS of the as-synthesized PtO2 /SnO2 composite. The characteristic peaks of the valence state were located at BE 78.5 eV and 74.7 eV ascribed to Pt4+ 4f5/2 and 4f7/2 , respectively [47–50]. Thus, the XPS spectrum further confirmed the decoration of PtO2 nanoparticles on the surface of porous SnO2 microspheres during the calcination at 500 ◦ C in air. Moreover, the high-resolution XPS spectra of the O 1 s for each as-synthesized PtO2 /SnO2 composite were shown in Fig. S4. According to the area ratio of O 1 species from the fitting Sn-O peak and Pt-O peak, the
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ysis (Fig. 5B) indicated pore size-distribution of the as-synthesized products was in the range of 2–50 nm, further confirming the existence of mesopores in the material.
3.2. Sensing performance towards gaseous formaldehyde
Fig. 4. High-resolution Pt4+ 4f XPS spectra of PtO2 /SnO2 -5 mol% composite.
amount of Pt-O was increased with the increase of Pt species in the composite, confirming the formation of PtO2 on the surface of porous SnO2 . The porous structure of the SnO2 microspheres and the as-synthesized PtO2 /SnO2 composite were investigated by N2 adsorption and desorption measurements. Fig. 5 showed the nitrogen adsorption-desorption isotherms and the BJH pore diameter distribution obtained at 77 K. It is clear that the nitrogen adsorption-desorption isotherms of the synthesized products is a typical type-IV isotherm with an H3 hysteresis loop (Fig. 5A), indicating the presence of mesoporous in the products. And the BET specific surface area was around 40 m2 g−1 (Table S1). The BJH anal-
For the practical application of a gas sensor, the operating temperature is important for a gas sensor due to its great influence on the contact reaction during the gas-sensing process. Combining the peculiarity of porous SnO2 microspheres with the catalytic role of PtO2 NPs, we expect that the as-synthesized PtO2 /SnO2 composites would be promising candidates for high performance gas sensor to detect trace gaseous formaldehyde at lower operating temperature. The responses of PtO2 /SnO2 composites sensors as well as the pure SnO2 microspheres sensor to 100 ppm formaldehyde was recorded at elevated operating temperature (Fig. 6A). Obviously, the response of each sensor is strongly dependent on the operating temperature, and the optimum working temperature decreased greatly after the decoration of PtO2 on SnO2 . It is clear that the gas sensor based on the PtO2 /SnO2 −5 mol% composite material shows highest response (Ra /Rg = 70) at the optimal working temperature of 100 ◦ C. The time taken by the sensor resistance to change from Ra to Ra – 90% × (Ra – Rg) was defined as response time when the target gas was introduced to the sensor, and the time taken from Rg to Rg + 90% × (Ra – Rg) was defined as recovery time when the ambience was replaced by air [51]. In addition, the PtO2 /SnO2 −5 mol% composite sensor exhibits a short response time (3–130 s) in temperature range of 27–200 ◦ C (Fig. 6B). The resistance-time curves of the PtO2 /SnO2 −5 mol% and Pt/SnO2 composite to 100 ppm formaldehyde at different working temperature
Fig. 5. (A) N2 adsorption-desorption isotherm and (B) the corresponding BJH pore size distribution of PtO2 /SnO2 -5 mol% composite.
Fig. 6. (A) Response to 100 ppm formaldehyde at different operating temperature for SnO2 and PtO2 /SnO2 composites sensors, (B) Response and recovery time of PtO2 /SnO2 -5 mol% composite sensor towards 100 ppm formaldehyde at different working temperatures.
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Scheme 2. Schematic diagram of the proposed reaction mechanism of the PtO2 /SnO2 composite senor in (A) air and (B) formaldehyde.
Fig. 7. A response-recovery curve of the PtO2 /SnO2 -5 mol% sensor with increasing formaldehyde concentrations (0.1–100 ppm) at an operating temperature of 100 ◦ C.
Fig. 9. The response of PtO2 /SnO2 -5 mol% sensor towards different gases (100 ppm) at 100 ◦ C. The error bars represent the standard deviation of six measurements.
Fig. 8. The cycling performance of PtO2 /SnO2 -5 mol% sensor at an operating temperature of 100 ◦ C.
were presented in Fig. S3. It is worth mentioning that this sensor even give a response of 14 at room temperature (∼ 27 ◦ C), but the recovery time is too long (Fig. 6B). Thus, the sensing performance of the as-synthesized PtO2 /SnO2 -5 mol% composite was studied in details at the optimal working temperature (100 ◦ C). The dynamic response-recovery curve of the above sensor to different concentrations of gaseous formaldehyde (0.1–100 ppm) at 100 ◦ C is presented Fig. 7. The gas sensor showed a typical n-type gas sensing behavior, and the resistance decreases upon interaction with increased formaldehyde concentration [52]. Moreover, this material can be considered as a promising candidate for the detection of trace formaldehyde (as low as 100 ppb). To investigate the reproducibility of this sensor, 15 cycles were measured to 10 ppm formaldehyde at 100 ◦ C (Fig. 8). As a result, this sensor gives nearly identical response towards trace formaldehyde, indicating the excellent reproducibility of this sensor.
The selectivity towards the target gas is another important aspect for a sensor except the high response and excellent reproducibility for practical applications. Thus, we also measured the responses of this sensor towards 100 ppm hydrogen, CO, gaseous formaldehyde, methanol, acetone, benzene and methane at 100 ◦ C. It is clear that this sensor gives a quite high response to formaldehyde, but a very low response to hydrogen, CO, acetone, benzene and methane at the operating temperature of 100 ◦ C (Fig. 9). In addition, this sensor exhibits a relatively week response toward methanol at the operating temperature of 100 ◦ C. Furthermore, it is found that this sensor, in fact, is also very sensitive for the detection of acetone and methanol at higher operating temperature (Fig. S6). These results overall indicate that this sensor has a relatively good selectivity to formaldehyde at low operating temperature. 3.3. Sensing mechanism Typically, SnO2 , as an n-type semiconductor material, exhibits a resistance change according to an adsorption-oxidationdesorption process in which chemisorbed oxygen plays an important role. Usually, oxygen molecules are considered to be absorbed on the surface of SnO2 , and withdraw the free electrons from the conduction band of SnO2 to form O− , O2 − or O2− species, leading to the generation of depletion layers on the surface and the increased resistance of SnO2 . When the sensor fabricated by these SnO2 spheres is exposed to the HCHO gas, the HCHO molecules can be oxidized by these oxygen ions, and release the captured electrons back to the conduction band, resulting in a decreased resistance of the sensor based on SnO2 at relatively high working temperature [31,53].
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After decorated PtO2 NPs on the surface of the porous SnO2 material, the additional PtO2 NPs could significantly promote the adsorption of HCHO on the surface of porous PtO2 /SnO2 composites. As shown in Fig. S7, the electrons in the conduction band of SnO2 would be preferred to jump to the valance band of PtO2 for the rather narrow band gap (0.49 eV) at a low working temperature. Thus, the electron would be easily captured by oxygen on PtO2 to form active oxygens, which can react with formaldehyde. That is, PtO2 NPs may catalytically activate the dissociation of oxygen molecules on the surface of the composites and facilitate the migration of oxygen ions to react with formaldehyde. The sensing process is presented in Scheme 2. Also, the synergetic effect between PtO2 NPs and SnO2 material may also play an important role in catalytic oxidation of formaldehyde [54–56]. Eventually, the porous PtO2 /SnO2 microspheres sensor shows a significantly improvement on the sensing response and dramatically decrease the operating temperature compared with the sensor based on pure SnO2 microspheres. However, heavily decorated PtO2 NPs would cause the electrons to conduct along the PtO2 nanoparticles, leading to the depressed sensing response [46]. 4. Conclusions In summary, by decorating PtO2 NPs on the surface of porous SnO2 microspheres, the response to trace formaldehyde was enhanced greatly and the operating temperature was decreased dramatically. The obtained PtO2 /SnO2 -5 mol% composite shows most outstanding sensing performance among all composites due to its proper PtO2 amount. It may be a good candidate for monitoring gaseous formaldehyde at the ppb level. The enhanced formaldehyde sensing ability can be attributed to the high catalytic activity of PtO2 nanoparticles built on the surface of SnO2 microsphere. Acknowledgment This work was supported by the NSFC (21371070, 21401066, 21401016), the Natural Science Foundation of Jilin Province (20160101291JC) and Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education. 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.2017.01.014. References [1] T. Salthammer, Formaldehyde in the ambient atmosphere: from an indoor pollutant to an outdoor pollutant, Angew. Chem. Int. Ed. 52 (2013) 3320–3327. [2] M. Hauptmann, J.H. Lubin, P.A. Stewart, R.B. Hayes, A. Blair, Mortality from solid cancers among workers in formaldehyde industries, Am. J. Epidemiol. 159 (2004) 1117–1130. [3] L. Feng, C.J. Musto, K.S. Suslick, A simple and highly sensitive colorimetric detection method for gaseous formaldehyde, J. Am. Chem. Soc. 132 (2010) 4046–4047. [4] D. Coggon, E.C. Harris, J. Poole, K.T. Palmer, Extended follow-up of a cohort of british chemical workers exposed to formaldehyde, J. Natl. Cancer Inst. 95 (2003) 1608–1615. [5] N. Aoki, K. Kato, R. Aoyagi, M. Wakayama, Evaluation of the permeability of formaldehyde and water through a permeation tube for the preparation of an accurate formaldehyde reference gas mixture, Analyst 138 (2013) 6930–6937. [6] Z. Gu, G. Wang, X. Yan, MOF-5 metal-organic framework as sorbent for in-field sampling and preconcentration in combination with thermal desorption GC/MS for determination of atmospheric formaldehyde, Anal. Chem. 82 (2010) 1365–1370. [7] J.J. Michels, Improved measurement of formaldehyde in water-soluble polymers by high-performance liquid chromatography coupled with post-column reaction detection, J. Chromatogr. A 914 (2001) 123–129.
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Biographies Yi He is a full professor at College of Chemistry, Jilin University in China. She received PhD (2006) from Jilin University. Her research interests include the preparation and characterization of inorganic-organic hybrid materials and their functions. Hui-Hui Li was received her B.Sc. in Anyang Normal University in 2013. She is a master student in State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University in China. Her research interest is the synthesis of sensing nanomaterials. Xiaoxin Zou was awarded a Ph.D. in Inorganic Chemistry from Jilin University (China) in 06/2011; and then moved to the University of California, Riverside and Rutgers, The State University of New Jersey as a Postdoctoral Scholar from 07/2011 to 10/2013. He is currently an associate professor in Jilin University. His research interests focus on the design and synthesis of noble metal-free, nanostructured and/or nanoporous materials for water splitting and renewable energy applications. Ni Bai was awarded a Ph.D. in Inorganic Chemistry from Jilin University (China) in 12/2001, and then moved to Peking University as a Postdoctoral Scholar from 04/2002 to 01/2004. She is currently a Lecturer in Jiangsu University of Science and Technology. Her research interests focus on sensing materials, recycling solid waste materials, corrosion and protection technique of alloy, photocatalysis and others. Yanying Cao was received her B.Sc. in Daqing Normal University in 2014. She is a master student in State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University in China. Her research interest is the synthesis of nanomaterials. Yang Cao was received his B.Sc. in Huaiyin Normal University in 2011. He is a Ph.D candidate in State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University in China. His research interest is the synthesis of sensing nanomaterials. Mei-Hong Fan was received her B.Sc. in Anyang Normal University in 2013. She is a master student in State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University in China. Her research interest is the design and synthesis of nanostructured and/or nanoporous materials. Guo-Dong Li is a full professor at State Key Lab of Inorganic Synthesis & Preparative Chemistry, College of Chemistry, Jilin University in China. He received his B.Sc. (1995), M.Sc (1998) and PhD (2001) from Jilin University. His research interests include chemical sensors, lithium batteries and catalysts for water splitting.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Platinum dioxide activated porous SnO2 microspheres for the detection of trace formaldehyde at low operating temperature Yi He a , Huihui Li a , Xiaoxin Zou a , Ni Bai b , Yanying Cao a , Yang Cao a , Meihong Fan a , Guo-Dong Li a,∗ a State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, International Joint Research Laboratory of Nano-Micro Architecture Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China b School of Mechanical and Metallurgical Engineering, Jiangsu University of Science and Technology, Zhangjiagang 215600, China
a r t i c l e
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Article history: Received 15 July 2016 Received in revised form 26 December 2016 Accepted 3 January 2017 Available online 4 January 2017 Keywords: Gas sensor PtO2 SnO2 Formaldehyde
a b s t r a c t Great efforts have been devoted on the detection of the volatile organic compounds indoor for human health. The detection of formaldehyde is one of the most important and popular issues for the wide usage of formaldehyde and its toxic. However, it is a challenge to monitor trace gaseous formaldehyde at lower working temperature. Herein, we present a sensor with a significantly improved sensing response and dramatically decreased operating temperature based on porous PtO2 /SnO2 microspheres. And the PtO2 /SnO2 -5 mol% composite shows the most outstanding sensing performance among all products. What’s more, the above sensor can detect gaseous formaldehyde in the ppb level (100 ppb) at low operating temperature (100 ◦ C). The excellent sensing performance should be attributed to the high catalytic activity of PtO2 nanoparticles decorated on the surface of SnO2 microspheres and the porosity of the composite. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, great concern is given on the detection of trace toxic volatile organic compounds (VOCs). As we all know, formaldehyde (HCHO) is the most popular one among VOCs emerged in room for their wide usage in many products, especially for furniture, adhesive agents, textiles, paints, and so on [1–3]. In addition, it has been classified as one of the people’s carcinogens by the international agency for research on cancer (IARC) and the world health organization (WHO) in 2004 [4,5]. However, it is difficult to selectively detect trace formaldehyde rapidly at low working temperature. Although several kinds of analytical technologies for formaldehyde detection were developed, including chromatography (gas chromatography and high-performance liquid chromatography) [6–8], spectrophotometry [9], colorimetric methods [3,10], mass spectrometric methods [11] and biosensors [12,13], they are either composed of costly instruments, complex operation procedures and/or lack of sensitivity. And it is impractical to popularize the above equipment for on-site detection [14]. Thus, portable gas sensors based on nano-sized metal oxide materials have been widely
∗ Corresponding author. E-mail address: [email protected] (G.-D. Li). http://dx.doi.org/10.1016/j.snb.2017.01.014 0925-4005/© 2017 Elsevier B.V. All rights reserved.
used in industries, medical, public safety and environmental diagnostics for its high sensitivity, high stability and low cost [15–18]. It is obvious that the more the surface area is, the more oxygen would be absorbed on the surface of the sensing material, resulting the higher response. Thus, many efforts have been devoted to obtain porous metal oxides, including SnO2 , In2 O3 , ZnO, Fe2 O3 , NiO, Co3 O4 , Cu2 O and WO3 , and so on [15,19–31]. Among the above metal oxide, tin oxide (SnO2 ) is the most important sensing material for its good stability, extensive sensitivity and low cost. Up to now, tin oxide has been widely used to detect reducing gases, for instance, ethanol, hydrogen, CO and formaldehyde et al. [15,19,32,33]. To enhance the sensing performance of SnO2 , many strategies have been developed to control its size, structure, and morphology, especially the surface area and porosity. However, the operating temperature of SnO2 sensors is still rather high, and the selectivity is poor towards formaldehyde. To increase the activity and selectivity, it is an effective way to integrate some catalytic nanoparticles into the surface of SnO2 nanomaterial. The additional catalytic particles can not only increase the reaction rate between the absorbed oxygen species and the target gases but also accelerate the electron exchange between sensing material and gases [34,35]. As we all know, noble metal Pt, one of the most active noble metal catalysts, can dramatically decrease the operating temperature and simultaneously improve the sensitivity as well
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as selectivity [36–38]. For example, Q. N. Abdullah reported that the sensor based on Pt-decorated GaN nanowires with improved H2 gas-sensing performance at room temperature [36], Y. Wang et al. found that the sensor based on Pt-functionalized NiO could enhance formaldehyde sensing performance distinctly and reduce the working temperature obviously [38]. Although there are a few PtO2 -decorated SnO2 materials that can used for formaldehyde detection, in those works formaldehyde is usually not studied as a target gas because the materials in those works are usually more sensitive for NH3 or methanol [39–41]. In the present work, porous PtO2 /SnO2 composites with good sensing performance towards formaldehyde were obtained. Moreover, it exhibits a rapid response to 100 ppb HCHO at a relatively low working temperature of 100 ◦ C. What’s more, it presents an excellent selectivity towards formaldehyde. The enhanced sensing properties should be attributed to the promotion of surface reactions assisted by PtO2 nanoparticles supported on the surface of porous SnO2 microspheres.
Fig. 1. The XRD patterns of porous SnO2 , PtO2 /SnO2 -2 mol%, PtO2 /SnO2 -5 mol%, PtO2 /SnO2 -8 mol% and PtO2 /SnO2 -10 mol%, respectively. The peaks corresponding to SnO2 are marked with “*”.
2. Experimental 2.1. Chemicals and reagents Stannic chloride pentahydrate (SnCl4 ·5H2 O) was purchased from Tianjin Fuchen Chemical Reagent Factory. Isopropanol and ethanol were purchased from Beijing Chemical Works. Chloroplatinic acid was purchased from Shanghai Bojing chemical company. Glycerol was purchased from Sinopharm Chemical Reagent Co. Ltd. All the above chemicals were of analytical grade and used without further purification. 2.2. Synthesis and characterizations of the porous PtO2 /SnO2 microspheres Porous SnO2 microspheres were prepared by a templatefree solvothermal method reported previously [42]. The porous PtO2 /SnO2 composites were prepared by an impregnation method [43,44]. Typically, 1 mmol SnO2 precursor was added in 2 mL ethanol containing different amount of H2 PtCl6 (0.02, 0.05, 0.08 and 0.10 mmol) and stirred at room temperature for 2 h, then dried at 80 ◦ C in air, finally calcined in air at 500 ◦ C for 2 h, the resulting products were denoted as PtO2 /SnO2 -2 mol%, PtO2 /SnO2 -5 mol%, PtO2 /SnO2 -8 mol% and PtO2 /SnO2 -10 mol%, where the number meant the molar ratio of Pt species to SnO2 . To obtain Pt/SnO2 composites sensor, PtO2 /SnO2 -5 mol% was reduced by hydrogen at 250 ◦ C for 2 h, as a result, a black product was obtained indicating the formation of Pt nano particles on the surface of PtO2 /SnO2 . The photo graph of fabricated sensor was shown in Fig. S5, Pt/SnO2 is black, while, the PtO2 /SnO2 -5 mol% was light yellow and the SnO2 was nearly white. The crystallization information of the final products were collected on a Rigaku D/Max 2550 X-ray diffractometer using Cu K␣ radiation operated at 200 mA and 50 kV, a HITACHI SU8020 scanning electron microscope (SEM) and a field-emission transmission electron microscope (FE-TEM, Philips-FEI Tecnai G2S-Twin). The composition and chemical state of the porous PtO2 /SnO2 composite was analyzed by X-ray photoelectron Spectroscopy (XPS, ESCALAB 250 X-ray photoelectron spectrometer) and selected-area electron diffraction (SAED, Philips-FEI Tecnai G2S-Twin). BET surface area and the corresponding Barrett-Joyner-Halenda (BJH) pore diameter were obtained by using a Micromeritics ASAP 2020 M system. 2.3. Sensor fabrication and measurement The fabrication process of gas sensor can be described as follows: an appropriate amount of as-synthesized porous PtO2 /SnO2 micro-
spheres were fully mixed with ethanol to prepare viscous slurry, which was then pasted onto a ceramic tube (a diameter around 1 mm and a length of 4 mm) positioned with a pair of Au electrodes and four Pt wires on both ends of the tube. After drying in air at room temperature, a nickel-chromium heating wire was inserted into the center of the tube as a heater, allowing us to control the working temperature (Scheme 1A). Then, the as-fabricated sensors were aged at 200 ◦ C for 12 h to ensure the good contact between the sensing material and the Au electrodes. All of the sensors with different composition were fabricated by the same method. Gas sensing tests were performed on a commercial CGS-8 Gas Sensing Measurement System (Beijing Elite Tech Company Limited). The gas sensing performances were evaluated by a static process, which was described in detail in our previous work [22] (Scheme 1B). The response was defined as Ra /Rg for reducing gases, where the resistance of the sensor in clean air and target gas was recorded as Ra and Rg , respectively. The response time and recovery time are defined according to previous reports [45]. All the experiments were conducted at room temperature (∼ 27 ◦ C) and the clean air with a relative humidity around 25% RH (27 ◦ C) was used to dilute the target gas to the desired concentration. 3. Results and discussion 3.1. Characterization of the composites The phase composition of the as-synthesized samples was confirmed by X-ray diffraction (XRD) pattern, and the typical diffraction patterns of porous SnO2 microsphere and PtO2 /SnO2 composites calcined at 500 ◦ C were shown in Fig. 1. All of the diffraction peaks in the typical XRD pattern could be attributed to tetragonal rutile SnO2 (JCPDS card no. 41–1445). No peaks related to Pt species were presented in the XRD patterns of PtO2 /SnO2 composites due to the trace Pt in the composites [46]. Moreover, FE-TEM and XPS were used to identify the presence of Pt nanoparticles. The morphology and size of SnO2 microspheres and assynthesized PtO2 /SnO2 composite was characterized by SEM and FE-TEM. As shown in Fig. 2, 2 S2 and Fig. 3A, the porous microspheres with the diameter of about 1–2 m are composed of nanoparticles with the size around 10 nanometers. Moreover, the surface of the as-synthesized PtO2 /SnO2 composite is smoother than that of the pure SnO2 spheres (Fig. S2) indicating the formation of tiny PtO2 species on the surface of SnO2 . In addition, the PtO2 /SnO2 composite is a kind of solid microsphere as shown in TEM image (Fig. 2D).
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Scheme 1. (A) A diagram of the sensor composed of ceramic tube, two Au electrodes, and a Ni-Cr wire. (B) A schematic diagram of the measurement circuit.
Fig. 2. SEM images of (A) porous SnO2 and (B-C) PtO2 /SnO2 -5 mol% composite. TEM image of (D) PtO2 /SnO2 -5 mol% composite.
Fig. 3. (A) TEM and (B) HRTEM image, (C) SAED pattern of PtO2 /SnO2 -5 mol% composite.
To understand the microstructure of the PtO2 /SnO2 composites, the HRTEM image and SAED were obtained, as shown in Fig. 3B and C. The fringe spacing of 0.34 nm and 0.27 nm can be clearly observed in Fig. 3B corresponds to the (110) plane of SnO2 and the (100) plane of PtO2 , respectively. The polycrystalline feature of the microspheres was also indicated by the SAED, which is in accordance with the HRTEM image. To further confirm the presence of PtO2 species on the porous SnO2 microspheres as well as the valence state, X-ray photoelectron spectroscopy (XPS) measurements were carried out to study the binding energy of Pt 4f. The survey spectrum (Fig. S1) revealed
the presence of Sn, Pt and O in the PtO2 /SnO2 composite, indicating a good purity of the as-synthesized product. Fig. 4 showed the Gaussian fitted core-level Pt 4f XPS of the as-synthesized PtO2 /SnO2 composite. The characteristic peaks of the valence state were located at BE 78.5 eV and 74.7 eV ascribed to Pt4+ 4f5/2 and 4f7/2 , respectively [47–50]. Thus, the XPS spectrum further confirmed the decoration of PtO2 nanoparticles on the surface of porous SnO2 microspheres during the calcination at 500 ◦ C in air. Moreover, the high-resolution XPS spectra of the O 1 s for each as-synthesized PtO2 /SnO2 composite were shown in Fig. S4. According to the area ratio of O 1 species from the fitting Sn-O peak and Pt-O peak, the
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ysis (Fig. 5B) indicated pore size-distribution of the as-synthesized products was in the range of 2–50 nm, further confirming the existence of mesopores in the material.
3.2. Sensing performance towards gaseous formaldehyde
Fig. 4. High-resolution Pt4+ 4f XPS spectra of PtO2 /SnO2 -5 mol% composite.
amount of Pt-O was increased with the increase of Pt species in the composite, confirming the formation of PtO2 on the surface of porous SnO2 . The porous structure of the SnO2 microspheres and the as-synthesized PtO2 /SnO2 composite were investigated by N2 adsorption and desorption measurements. Fig. 5 showed the nitrogen adsorption-desorption isotherms and the BJH pore diameter distribution obtained at 77 K. It is clear that the nitrogen adsorption-desorption isotherms of the synthesized products is a typical type-IV isotherm with an H3 hysteresis loop (Fig. 5A), indicating the presence of mesoporous in the products. And the BET specific surface area was around 40 m2 g−1 (Table S1). The BJH anal-
For the practical application of a gas sensor, the operating temperature is important for a gas sensor due to its great influence on the contact reaction during the gas-sensing process. Combining the peculiarity of porous SnO2 microspheres with the catalytic role of PtO2 NPs, we expect that the as-synthesized PtO2 /SnO2 composites would be promising candidates for high performance gas sensor to detect trace gaseous formaldehyde at lower operating temperature. The responses of PtO2 /SnO2 composites sensors as well as the pure SnO2 microspheres sensor to 100 ppm formaldehyde was recorded at elevated operating temperature (Fig. 6A). Obviously, the response of each sensor is strongly dependent on the operating temperature, and the optimum working temperature decreased greatly after the decoration of PtO2 on SnO2 . It is clear that the gas sensor based on the PtO2 /SnO2 −5 mol% composite material shows highest response (Ra /Rg = 70) at the optimal working temperature of 100 ◦ C. The time taken by the sensor resistance to change from Ra to Ra – 90% × (Ra – Rg) was defined as response time when the target gas was introduced to the sensor, and the time taken from Rg to Rg + 90% × (Ra – Rg) was defined as recovery time when the ambience was replaced by air [51]. In addition, the PtO2 /SnO2 −5 mol% composite sensor exhibits a short response time (3–130 s) in temperature range of 27–200 ◦ C (Fig. 6B). The resistance-time curves of the PtO2 /SnO2 −5 mol% and Pt/SnO2 composite to 100 ppm formaldehyde at different working temperature
Fig. 5. (A) N2 adsorption-desorption isotherm and (B) the corresponding BJH pore size distribution of PtO2 /SnO2 -5 mol% composite.
Fig. 6. (A) Response to 100 ppm formaldehyde at different operating temperature for SnO2 and PtO2 /SnO2 composites sensors, (B) Response and recovery time of PtO2 /SnO2 -5 mol% composite sensor towards 100 ppm formaldehyde at different working temperatures.
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Scheme 2. Schematic diagram of the proposed reaction mechanism of the PtO2 /SnO2 composite senor in (A) air and (B) formaldehyde.
Fig. 7. A response-recovery curve of the PtO2 /SnO2 -5 mol% sensor with increasing formaldehyde concentrations (0.1–100 ppm) at an operating temperature of 100 ◦ C.
Fig. 9. The response of PtO2 /SnO2 -5 mol% sensor towards different gases (100 ppm) at 100 ◦ C. The error bars represent the standard deviation of six measurements.
Fig. 8. The cycling performance of PtO2 /SnO2 -5 mol% sensor at an operating temperature of 100 ◦ C.
were presented in Fig. S3. It is worth mentioning that this sensor even give a response of 14 at room temperature (∼ 27 ◦ C), but the recovery time is too long (Fig. 6B). Thus, the sensing performance of the as-synthesized PtO2 /SnO2 -5 mol% composite was studied in details at the optimal working temperature (100 ◦ C). The dynamic response-recovery curve of the above sensor to different concentrations of gaseous formaldehyde (0.1–100 ppm) at 100 ◦ C is presented Fig. 7. The gas sensor showed a typical n-type gas sensing behavior, and the resistance decreases upon interaction with increased formaldehyde concentration [52]. Moreover, this material can be considered as a promising candidate for the detection of trace formaldehyde (as low as 100 ppb). To investigate the reproducibility of this sensor, 15 cycles were measured to 10 ppm formaldehyde at 100 ◦ C (Fig. 8). As a result, this sensor gives nearly identical response towards trace formaldehyde, indicating the excellent reproducibility of this sensor.
The selectivity towards the target gas is another important aspect for a sensor except the high response and excellent reproducibility for practical applications. Thus, we also measured the responses of this sensor towards 100 ppm hydrogen, CO, gaseous formaldehyde, methanol, acetone, benzene and methane at 100 ◦ C. It is clear that this sensor gives a quite high response to formaldehyde, but a very low response to hydrogen, CO, acetone, benzene and methane at the operating temperature of 100 ◦ C (Fig. 9). In addition, this sensor exhibits a relatively week response toward methanol at the operating temperature of 100 ◦ C. Furthermore, it is found that this sensor, in fact, is also very sensitive for the detection of acetone and methanol at higher operating temperature (Fig. S6). These results overall indicate that this sensor has a relatively good selectivity to formaldehyde at low operating temperature. 3.3. Sensing mechanism Typically, SnO2 , as an n-type semiconductor material, exhibits a resistance change according to an adsorption-oxidationdesorption process in which chemisorbed oxygen plays an important role. Usually, oxygen molecules are considered to be absorbed on the surface of SnO2 , and withdraw the free electrons from the conduction band of SnO2 to form O− , O2 − or O2− species, leading to the generation of depletion layers on the surface and the increased resistance of SnO2 . When the sensor fabricated by these SnO2 spheres is exposed to the HCHO gas, the HCHO molecules can be oxidized by these oxygen ions, and release the captured electrons back to the conduction band, resulting in a decreased resistance of the sensor based on SnO2 at relatively high working temperature [31,53].
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After decorated PtO2 NPs on the surface of the porous SnO2 material, the additional PtO2 NPs could significantly promote the adsorption of HCHO on the surface of porous PtO2 /SnO2 composites. As shown in Fig. S7, the electrons in the conduction band of SnO2 would be preferred to jump to the valance band of PtO2 for the rather narrow band gap (0.49 eV) at a low working temperature. Thus, the electron would be easily captured by oxygen on PtO2 to form active oxygens, which can react with formaldehyde. That is, PtO2 NPs may catalytically activate the dissociation of oxygen molecules on the surface of the composites and facilitate the migration of oxygen ions to react with formaldehyde. The sensing process is presented in Scheme 2. Also, the synergetic effect between PtO2 NPs and SnO2 material may also play an important role in catalytic oxidation of formaldehyde [54–56]. Eventually, the porous PtO2 /SnO2 microspheres sensor shows a significantly improvement on the sensing response and dramatically decrease the operating temperature compared with the sensor based on pure SnO2 microspheres. However, heavily decorated PtO2 NPs would cause the electrons to conduct along the PtO2 nanoparticles, leading to the depressed sensing response [46]. 4. Conclusions In summary, by decorating PtO2 NPs on the surface of porous SnO2 microspheres, the response to trace formaldehyde was enhanced greatly and the operating temperature was decreased dramatically. The obtained PtO2 /SnO2 -5 mol% composite shows most outstanding sensing performance among all composites due to its proper PtO2 amount. It may be a good candidate for monitoring gaseous formaldehyde at the ppb level. The enhanced formaldehyde sensing ability can be attributed to the high catalytic activity of PtO2 nanoparticles built on the surface of SnO2 microsphere. Acknowledgment This work was supported by the NSFC (21371070, 21401066, 21401016), the Natural Science Foundation of Jilin Province (20160101291JC) and Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education. 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.2017.01.014. References [1] T. Salthammer, Formaldehyde in the ambient atmosphere: from an indoor pollutant to an outdoor pollutant, Angew. Chem. Int. Ed. 52 (2013) 3320–3327. [2] M. Hauptmann, J.H. Lubin, P.A. Stewart, R.B. Hayes, A. Blair, Mortality from solid cancers among workers in formaldehyde industries, Am. J. Epidemiol. 159 (2004) 1117–1130. [3] L. Feng, C.J. Musto, K.S. Suslick, A simple and highly sensitive colorimetric detection method for gaseous formaldehyde, J. Am. Chem. Soc. 132 (2010) 4046–4047. [4] D. Coggon, E.C. Harris, J. Poole, K.T. Palmer, Extended follow-up of a cohort of british chemical workers exposed to formaldehyde, J. Natl. Cancer Inst. 95 (2003) 1608–1615. [5] N. Aoki, K. Kato, R. Aoyagi, M. Wakayama, Evaluation of the permeability of formaldehyde and water through a permeation tube for the preparation of an accurate formaldehyde reference gas mixture, Analyst 138 (2013) 6930–6937. [6] Z. Gu, G. Wang, X. Yan, MOF-5 metal-organic framework as sorbent for in-field sampling and preconcentration in combination with thermal desorption GC/MS for determination of atmospheric formaldehyde, Anal. Chem. 82 (2010) 1365–1370. [7] J.J. Michels, Improved measurement of formaldehyde in water-soluble polymers by high-performance liquid chromatography coupled with post-column reaction detection, J. Chromatogr. A 914 (2001) 123–129.
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Biographies Yi He is a full professor at College of Chemistry, Jilin University in China. She received PhD (2006) from Jilin University. Her research interests include the preparation and characterization of inorganic-organic hybrid materials and their functions. Hui-Hui Li was received her B.Sc. in Anyang Normal University in 2013. She is a master student in State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University in China. Her research interest is the synthesis of sensing nanomaterials. Xiaoxin Zou was awarded a Ph.D. in Inorganic Chemistry from Jilin University (China) in 06/2011; and then moved to the University of California, Riverside and Rutgers, The State University of New Jersey as a Postdoctoral Scholar from 07/2011 to 10/2013. He is currently an associate professor in Jilin University. His research interests focus on the design and synthesis of noble metal-free, nanostructured and/or nanoporous materials for water splitting and renewable energy applications. Ni Bai was awarded a Ph.D. in Inorganic Chemistry from Jilin University (China) in 12/2001, and then moved to Peking University as a Postdoctoral Scholar from 04/2002 to 01/2004. She is currently a Lecturer in Jiangsu University of Science and Technology. Her research interests focus on sensing materials, recycling solid waste materials, corrosion and protection technique of alloy, photocatalysis and others. Yanying Cao was received her B.Sc. in Daqing Normal University in 2014. She is a master student in State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University in China. Her research interest is the synthesis of nanomaterials. Yang Cao was received his B.Sc. in Huaiyin Normal University in 2011. He is a Ph.D candidate in State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University in China. His research interest is the synthesis of sensing nanomaterials. Mei-Hong Fan was received her B.Sc. in Anyang Normal University in 2013. She is a master student in State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University in China. Her research interest is the design and synthesis of nanostructured and/or nanoporous materials. Guo-Dong Li is a full professor at State Key Lab of Inorganic Synthesis & Preparative Chemistry, College of Chemistry, Jilin University in China. He received his B.Sc. (1995), M.Sc (1998) and PhD (2001) from Jilin University. His research interests include chemical sensors, lithium batteries and catalysts for water splitting.