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recovery and good selectivity
Sensors and Actuators B 238 (2017) 264–273
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Formaldehyde detection: SnO2 microspheres for formaldehyde gas sensor with high sensitivity, fast response/recovery and good selectivity Yuxiu Li a , Nan Chen b , Dongyang Deng a , Xinxin Xing a , Xuechun Xiao b,c , Yude Wang b,c,∗ a
School of Materials Science and Engineering, Yunnan University, 650091 Kunming, People’s Republic of China Department of Physics, Yunnan University, 650091 Kunming, People’s Republic of China c Yunnan Province Key Lab of Micro-Nano Materials and Technology, Yunnan University, 650091 Kunming, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 28 April 2016 Received in revised form 18 June 2016 Accepted 13 July 2016 Available online 15 July 2016 Keywords: SnO2 microspheres Hydrothermal synthesis Gas sensor Formaldehyde Stability
a b s t r a c t Tin oxide microspheres were successfully obtained through a facile hydrothermal method without any polymer templates or surfactant. The as-synthesized SnO2 microspheres are composed of large amount of small spheres with average diameters of about 250 nm, and every small sphere consists of numerous primary nanocrystallites with average sizes of about 8 nm. The resultant product was used as sensing material for gas sensor to detect the formaldehyde (HCHO) gas. The gas response, response and recovery time, selectivity and stability were carefully studied. It was found that the response value of the sensor to 100 ppm HCHO was 38.3 at the operating temperature of 200 ◦ C. The gas sensor based on SnO2 microspheres has excellent gas response, good response-recovery properties, linear dependence, repeatability and selectivity, making it to be a promising candidate for practical detectors for HCHO. © 2016 Elsevier B.V. All rights reserved.
1. Introduction HCHO is a gas of colorless and pungent-smelling, which is an important raw material in the world, is widely used in manufacturing processes with approximately ten megatons per year [1], and is found in more than 2000 products [2], because of its chemical reactivity, high purity and low cost [3]. Formaldehyde, therefore, can be emitted unremittingly by various sources such as building materials and numerous living goods (furniture, dry cleaning solutions, cosmetics, pharmaceutical etc) [4]. It is considered as one of the most important air pollutants in homes, offices and urban environments [5]. Formaldehyde also is one of the source materials which can cause sick building syndrome (SBS) [6], and exposuring to formaldehyde can cause many potential health risks. It is revealed by the U.S. Environmental Protection Agency that formaldehyde can cause eyes inflammation and upper airways infections, which can lead to central nervous system damage and immune system disorders [7,8]. Formaldehyde was also classified as a human carcinogen by the International Agency for Research on Cancer (IARC). Several safety standards toward formaldehyde have been set up.
∗ Corresponding author at: Department of Physics, Yunnan University, 650091 Kunming, People’s Republic of China. E-mail addresses: [email protected], [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.snb.2016.07.051 0925-4005/© 2016 Elsevier B.V. All rights reserved.
The World Health Organization (WHO) and National Institute for Occupational Safety and Health (NIOSH) have set safe exposure limit of 0.08 ppm and 1 ppm to formaldehyde [9], respectively. Therefore, a practical and effective way to detect formaldehyde is of great significance. Various ways have been extensively investigated for detecting formaldehyde. The most common methods are high-performance liquid chromatography (HPLC) [10], spectroscopy, enzyme electrodes [11] and the use of sensors [12]. Compared with sensors, many of these methods rely on trained personnel and expensive instrumentation, as well as require the formaldehyde to be put into on a membrane or a liquid solution before analysis. Thus those are unsuitable for real-time monitoring [13], and that is very slow, tiring and inconvenient. So the manufacture of a formaldehyde gas sensor is deemed to be a satisfactory method for detecting it in appropriate environment. In the last few years, for the detection of formaldehyde gas, a series of metal oxides have been reported, such as TiO2 or Ag-TiO2 [14–16], ZnO or ZnO-MnO2 [17], In2 O3 [18], WO3 [19], Co3 O4 [20], SnO2 [21], and so on. As a typical n-type semiconducting metal oxide with a wide band gap of 3.62 eV, SnO2 is the most potential metal oxide used a gas-sensing material on account of its good chemical stability and remarkable electrical properties. But most of all, it has a high gas response, rapid response, low power consump-
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tion and cost, as well as good reproducibility. The applications of SnO2 as a gas-sensing material have aroused great interest in the sensor field. The sensing mechanism of SnO2 is mainly gas reaction with the adsorbed oxygen on the surface and gas diffusion [22], and belongs to the surface-control type. This is determined by a lot of factors such as grain size, surface states and oxygen adsorption quantities. Recently, compared with bulk material of SnO2 , nanostructured SnO2 is drawing ever more attentions because its properties could more obviously improve from nanometer size effects [23]. On this account, the micro/nanostructures SnO2 materials were widely investigated, including nanowires [24], nanorods [25], microspheres [26], nanofibers [27], micro-rods [21], etc. Among them, spherical structure can cause the extensive concern on account of its high surface area and tailored structure, which could make for gas adsorption and desorption. Lin et al. reported that SnO2 microspheres have been successfully synthesized, it has also been reported that the detection toward formaldehyde [26]. The SnO2 microspheres also exhibited good response-recovery properties, but the gas response was relatively low. In this paper, we report the preparation of SnO2 microspheres via a facile hydrothermal method without any polymer templates or surfactant. We chose water and N,N-dimethylformamide (DMF) as solvents. The process is particularly simple and reproducible. The obtained SnO2 microspheres were used to fabricate a gas sensor which shows good sensing performance toward formaldehyde, especially for the selectivity and stability. Furthermore, the gas response of the obtained SnO2 microspheres is also high. The structure and morphology were also characterized to present a further understanding of the related mechanisms of spherical structure and gas sensing. 2. Experimental 2.1. Preparation of SnO2 microspheres All chemicals including tin (IV) chloride pentahydrate (SnCl4 ·5H2 O), sodium hydroxide (NaOH), N,N-dimethylformamide (DMF) and formaldehyde solution (HCHO) were analytical grade reagents and used without any further purification. A typical synthesis process of SnO2 microspheres was as following: 4.00 mmol SnCl4 ·5H2 O was first dissolved in 10 mL of distilled water under magnetic stirring until a homogenous solution was obtained. Afterwards, 12 mL DMF was completely dissolved into the above solution. After stirred for about 20 min, NaOH solution (20 mL, 2.00 M) was added into the solution under vigorous magnetic stirring for 30 min, and more distilled water was added into the above solution to make the whole volume of the solution reach 60 mL. The volume ratio of DMF to water is 1:4. Subsequently, the whole solution was transferred into a Teflon-lined stainless steel autoclave with a capacity of 90 mL and heated for 15 h at 160 ◦ C in an electric oven. The autoclave was cooled down to room temperature naturally, and precipitates were separated by centrifugation and washed with ethanol and distilled water for several times. The products were dried at 60 ◦ C for 24 h, followed by thermal treatment at 400 ◦ C for 1 h in air atmosphere. Finally, SnO2 microspheres were obtained for gas sensor fabrication. 2.2. Characterization of SnO2 microspheres Powder X-ray diffraction (XRD) pattern was recorded on a Rigaku TTRIII powder diffractometer equipped with Cu K˛ radiation ( = 1.54056 Å). The diffracted X-ray intensities were recorded by 2. The sample was scanned in the 2 range 10◦ –90◦ in steps of 0.02◦ . The operating voltage and current were 40 kV and 200 mA, respectively. The surface topography of the sample was
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inspected using scanning electron microscopy (SEM) taken on FEI QUANTA200 with microscope operating at 30 kV. The sample was coated with a thin gold layer before SEM imaging. The transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL JEM-2100 with an acceleration voltage of 200 kV. For TEM images, the microspheres were dispersed ultrasonically in ethanol, and then transferred onto copper grids before TEM observation. The surface chemistries of the sample were characterized using X-ray photoelectron spectroscopy (XPS). X-ray photoelectron spectroscopy (XPS) was carried out at room temperature in an ESCALAB 250 system with Al K␣ X-ray radiation at 15 kV. All XPS spectra were accurately calibrated by the C 1s peak at 284.6 eV to compensate for the charge effect. 2.3. Preparation of gas sensor To demonstrate the potential applications of the final SnO2 products, gas sensor of indirect heating was fabricated according to the literature [28]. At the beginning, the as-synthesized SnO2 microspheres were mixed with an appropriate deionized water to form paste. The paste was coated onto an alumina ceramic tube (4 mm in length, 1.2 mm in external diameter, and 0.8 mm in internal diameter) with thickness of about 0.6 mm. A pair of Au electrodes were previously printed on the alumina ceramic tube. Platinum lead wires attached to these Au electrodes were used as electrical contacts. Subsequently, the sensor sample was dried at 120 ◦ C for 1 h and calcined at 400 ◦ C for 1 h in air to reach the stability and repeatability of the material. Finally, a small Ni-Cr alloy coil (23 ) was inserted into the alumina ceramic tube as a heater, which provided the operating temperature of the gas sensor. The operating temperature can be adjusted by tuning the heat voltage (Vh ). To improve the long-term stability of the gas sensor, the asfabricated sensor was aged at the operating temperature of 350 ◦ C for 5 days. Gas sensing properties were measured by a WS-30A system (Weisheng Instruments Co., Zhengzhou, China) under laboratory condition. The relative humidity of the test is about 25–30%. The circuit voltage (Vc ) was set at 5 V, and the output voltage (Vout ) was set as the terminal voltage of the load resistor (RL ) [29]. The test gas was injected into a test chamber using a microsyringe after the base line of the sensor was stable. The desired concentrations of the testing gas are obtained by the volume of the analyte solution. The analyte solution was evaporated by an evaporator. At the same time, tow fans were installed to make the gas homogeneous. The gas response value () of the sensor in this paper is defined as follows:  = Ra /Rg
(1)
The Ra and Rg are the resistances of the gas sensor in air and target gas, respectively. The response time ( res ) is defined as the time to reach 90% of the final equilibrium value and the recovery time ( recov ) is defined as the time to decrease to 10% of the final equilibrium value. 3. Results and discussion The phase purity and crystal structure of the as-synthesized SnO2 microspheres were investigated by XRD. Fig. 1 illustrates the typical diffraction patterns of the SnO2 microspheres. All the peaks can be well indexed to the cassiterite SnO2 (JCPDS card no. 990024, a = b = 4.739 Å, c = 3.187 Å), space group P42 /mnm (136). No characteristic peaks were observed for other impurities, demonstrating the high purity of the as-synthesized SnO2 microspheres. The peak intensities indicate the excellent crystalline degree of
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Fig. 1. The XRD patterns (a) and the typical Rietveld refinement data (b) for as-synthesized SnO2 microspheres.
Fig. 2. SEM images of as-synthesized SnO2 microspheres at (a) low magnification and (b) high magnification, respectively.
Fig. 3. (a) TEM image and (b) magnified TEM image of as-synthesized SnO2 microspheres, (c) HRTEM image of SnO2 microspheres and (d) the corresponding HRTEM image with obvious SnO2 lattice fringes.
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Fig. 4. (a) XPS survey spectrum of the SnO2 microspheres, (b) high-resolution XPS spectrum of Sn 3d for SnO2 microspheres, (c) high-resolution XPS spectrum of O 1s for SnO2 microspheres. Table 1 Structural parameters for SnO2 microspheres are calculated by Rietveld refinement of the experimental XRD pattern. Space group
P42 /mnm (136)
Lattice parameters
Element coordinates
Sn
O
Average grain size (nm) Average maximum microstrain Rwp (%)
a0 (Å) b0 (Å) c0 (Å) x y z x y z
4.7420 4.7420 3.1844 0 0 0 0.2895 0.2895 0 8.0300 0.0045 13.43%
the as-synthesized SnO2 microspheres. The diffraction peaks of as-synthesized sample are broad, indicating the small grain size. Significantly, the structural information was further obtained by the refinement of the diffraction patterns with the Rietveld method [30]. The red dots, black curve and blue curve represent the experiment data, calculated pattern and the difference, respectively. The position of the Bragg reflections is shown in green bars. The average grain size of the SnO2 microspheres is calculated to be about 8 nm. All structural parameters obtained from Rietveld profile refinement, and summarized in Table 1. As can be obviously observed from Fig. 1, the calculated pattern and experimental data are in sat-
isfactory agreement as seen from the difference curve. The lattice parameters are refined to a = b = 4.742 Å and c = 3.184 Å, respectively, which is in good agreement with cassiterite SnO2 (JCPDS card no. 99-0024). These results confirm the successful synthesis of SnO2 via a facile hydrothermal method. In addition, in order to have a better understanding for the formation of as-synthesized SnO2 microspheres. The XRD pattern of precursor before the thermal treatment at 400 ◦ C was also obtained, and as shown in Fig. S1. The identified peaks for the precursor can be also indexed to cassiterite SnO2 (JCPDS card no. 99-0024). However, all diffraction peaks from the precursor obviously exhibit a broader and lower intensity relative to that of final SnO2 product, and the average grain is calculated to be about 3.88 nm. The morphology and the microstructure of the SnO2 microspheres were characterized by SEM, and displayed in Fig. 2. As can be obviously observed from the image in Fig. 2(a) (at low magnification), the SnO2 nano-architectures were obtained with mixture of water and DMF. It is clearly seen that the SnO2 nanoarchitectures are sphere-like. The more detailed structural feature of as-synthesized SnO2 microspheres was examined from highmagnification SEM image (Fig. 2(b)), suggesting that the product consists of large amount of small spheres with average diameters of about 250 nm. These microspheres are accumulational and not monodisperse. In addition, the surface of every small sphere is rough and composed of densely packed nanoparticles with several nanometers in size. For comparison, the SEM images of precursor before the thermal treatment at 400 ◦ C were also provided, and as
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shown in Fig. S2. It can be seen that the structure of the precursor is also globular. The more detailed structures of as-synthesized SnO2 microspheres were further examined with TEM and HRTEM. The image (Fig. 3(a)) of TEM confirms that SnO2 microspheres have been obtained. These microspheres are superimposed together. Furthermore, the SnO2 microspheres are irregular in size (but shape is highly regular), up to hundreds of nanometers in diameter, which is in good agreement with the above estimated size from SEM results. To gain further understanding of the micostructure, a magnified TEM image is displayed in Fig. 3(b). From the magnified TEM image, the as-synthesized SnO2 microspheres are composed of numerous primary nanocrystallites with average sizes of about 8 nm, which matches well with the XRD results. HRTEM image of the SnO2 microspheres is given in Fig. 3(c), and the clear well-developed lattice fringes imply a high crystallinity and random orientation of the SnO2 microspheres. In order to get a clearer image of lattice fringes, a HRTEM image of the edge part marked by the white dashed line in Fig. 3(c) is shown in Fig. 3(d). The interplanar spacing is estimated to be 0.334 nm and can be indexed to the (110) crystal planes of SnO2 . It is well known that the sensing mechanism is surface controlled process in which grain size, surface states and oxygen adsorption plays a fundamental role [31]. So the morphology of as-synthesized SnO2 microspheres plays an important role in gas-sensing properties. To further analysis and illuminate the composition and chemical state of every element, the XPS spectra were measured for the as-synthesized SnO2 . Fig. 4(a) represents XPS survey scan for the SnO2 microspheres sample. Apart from the peak of C 1s at 285 eV, as expected the spectrum was dominated by the lines of Sn and O. From the spectrum, the main peaks are Sn MNN (1062 eV), O KLL (977 eV), Sn 3P1/2 (759 eV), Sn 3P3/2 (717 eV), O 1 s (531 eV), Sn 3d3/2 (496 eV), Sn 3d5/2 (487 eV) and Sn 4d (27 eV), respectively, which confirms a good purity of the as-synthesized sample. The trace amounts of C 1s may be from the absorption of organic molecules in the air [22]. The high resolution Sn 3d and O 1s spectrums are shown in Fig. 4(b) and (c), which are matched by Multipak software, respectively. The Sn 3d spectrum (Fig. 4(b)) reveals two peaks of Sn 3d3/2 and Sn 3d5/2 with good symmetry. The peaks corresponding to binding energies for Sn 3d3/2 and Sn 3d5/2 appeared at 495.40 eV and 486.95 eV, respectively, giving a spin orbit coupling of 8.45 eV. The values correspond to the bind-
Fig. 5. The N2 adsorption-desorption isotherm of the SnO2 microspheres. The inset is the BJH pore-size distribution curve.
ing energies of the Sn4+ ion [32]. The high-resolution XPS spectrum of O 1s is shown in Fig. 4(c). One can find that the O 1s is an asymmetric peak at 530.90 eV, and the peak of O 1s can be separated into two peaks at Olat (530.90 eV) and Oads (Ox− , 532.15 eV) [33]. Olat is attributed to the lattice oxygen on the surface of SnO2 microspheres, which is thought to be has no contribution to the and O− ) is gas response but pretty stable [34]. However, Ox− (O− ads 2ads relative to the adsorbed oxygen ions of SnO2 microspheres, which has an important role in the gas sensing property [34]. The absorbed oxygen ions Ox− is reactable with the gas and then enhance the holes concentration [35]. Therefore, increasing the absorbed oxygen ions Ox− contributes to the gas response. The porous structure and the specific area of the SnO2 microspheres are performed by N2 adsorption-desorption isotherm and the corresponding BJH pore-size distribution curves, and as shown in Fig. 5. The SnO2 microspheres shows obvious hysteresis loop, indicating the existence of pores. As observed in inset of Fig. 5, the main peak is positioned at 6.196 nm have investigated for SnO2 microspheres, which suggested that the SnO2 microspheres pos-
Fig. 6. Schematic illustration of the formation process of SnO2 microspheres.
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sessed mesoporous structure. Moreover, the specific surface area of the SnO2 microspheres is calculated as 64.361 m2 /g, and it can supply an adequate surface for gas sensing reactions. In view of the above results, we supposed formation mechanism of SnO2 microspheres, as illustrated in Fig. 6. At first, a number of tiny primary nanocrystals were formed by hydrolysis. Afterwards, these nanoparticles tend to form the aggregated SnO2 microspheres. After calcined at 400 ◦ C for 1 h, the crystal phase of as-prepared sample became more pure due to the elimination of residual organics (DMF), and the average grain size of SnO2 microspheres become larger, and SnO2 microspheres were well crystallized with maintaining the original configuration. It is interesting to note that the chemical activities of semiconductor oxide sensors can be fully stimulated out at the optimum operating temperature. Therefore, the operating temperature strongly influences response property of a semiconductor oxide sensor, such as response, response-recovery time and selectivity. The temperature of 200 ◦ C was chosen as the operating temperature due to high gas response and fast response-recovery time. Fig. 7(a) shows the response of the SnO2 microspheres gas sensor under the seven different HCHO gas concentrations ranging from 1 to 500 ppm. It is clearly evident that the curve exhibits a stepwise change upon exposing the sensor to different concentrations of HCHO. The response amplitude of the sensor increases gradationally with increasing the gas concentration from 1 ppm to 500 ppm. More specifically, the gas responses toward 1, 3, 5, 50, 100, 200 and 500 ppm of HCHO are 5.72, 7.70, 10.85, 21.12, 38.26, 66.25 and 144.90, respectively. This result indicates that the SnO2 microspheres sensor is suitable for quantitative detection for HCHO at low concentrations. The responses of the SnO2 microspheres sensor as a function of the HCHO concentration are plotted in Fig. 7(b). The sensor responses increase nearly linearly with rising HCHO gas concentration. The relationship between the response ( = Ra /Rg ) and the gas concentration was fitted as follows:  = 0.28C + 8.19
(2)
where C is the concentration of HCHO. It is found that the experimental data and the theoretical curve show good agreements, and the correlative coefficient R2 is 0.99781. Such a good linear dependence of response on the gas concentration indicates that the SnO2 microspheres sensor can be used as promising materials for detecting HCHO. The response and recovery characteristics are important parameters for evaluating the performances of gas sensors [36]. The response and recovery times of gas sensor utilizing SnO2 microspheres as sensing material were investigated toward HCHO gas with the concentration of 100 ppm, as shown in Fig. 8. According to data of Fig. 8, the response time ( res ) and recovery time ( recov ) of the SnO2 microspheres are approximately 17 s and 25 s, respectively. The res value and recov value of the sensor are far more rapid than most reported SnO2 formaldehyde sensors [26,27]. The quick response and recovery of the sensor may be attributed to the special morphologies. These spherical structures can offer a sufficient active surface and good permeability for the fast adsorption and gas diffusion [26]. Therefore, we can conclude that SnO2 microspheres exhibit the excellent gas sensing performance. The gas sensing repeatability is another important parameter to evaluate the sensing ability of semiconductor materials [37]. So, the repeatability of the SnO2 microspheres gas sensor was studied by testing 100 ppm HCHO six times under the same conditions. In Fig. 9, one can see the response and recovery curve show neglectable change and the gas response in every test reaches about 38. On the other hand, the response time and recovery time are almost reproducible as well as the response values. The results indicate that the sensor has excellent reversibility and repeatability for the detection of HCHO.
Fig. 7. The gas sensing properties of the sensor based on SnO2 microspheres at an operating temperature of 200 ◦ C. (a) Dynamic response-recovery curve of the SnO2 microspheres gas sensor exposed to various concentrations of HCHO in a range of 1–500 ppm. (b) Linear dependence relation between resistance response sensitivity and gas concentration.
Furthermore, in order to clarify the potential gas-sensing applications, the selectivity of the sensor based on SnO2 microspheres was tested in 100 ppm (200 ◦ C) potential interference gases, such as acetone (CH3 COCH3 ), ammonia (NH3 ), xylene (C8 H10 ), methanol (CH4 O) and toluene (C7 H8 ). The results are compared in Fig. 10. It is obvious that the sensor shows higher response, and the response of the sensor to formaldehyde (HCHO) is 38.28, while to acetone, ammonia, xylene, methanol and toluene, the responses are 6.42, 1.74, 1.99, 19.38 and 1.55, respectively. The response toward HCHO is probably 6.0, 22.0, 19.2, 2.0, and 24.7 times higher than that toward acetone, ammonia, xylene, methanol and toluene, respectively. In conclusion, the sensor based on SnO2 microspheres exhibits a good selectivity toward HCHO. The selectivity of the sensor is influenced by various factors. For the study, the structures and bond dissociation energies of gas molecules are significant influences. The HCHO band is just 364 kJ/mol, which is smaller than other gases at a lower temperature. So the bond dissocia-
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Table 2 Comparison of the sensing performances of various SnO2 based gas sensors toward formaldehyde (HCHO). Materials
Optimum temperature (◦ C)
Response
Response/Recovery
Ref.
Porous SnO2 nanospheres Porous flower-like SnO2 Tin oxide nanofibers Hierarchical porous SnO2 micro-rods SnO2 nanowires Hollow SnO2 nanofibers SnO2 microspheres SnO2 microspheres
260 240 200 330 270 180 160 200
7.6 (10 ppm) 24.8 (100 ppm) 19.6 (50 ppm) 3.86 (1 ppm) 2.45 (10 ppm) 5.4 (100 ppm) 3.6 (100 ppm) 38.3 (100 ppm)
13 s/14 s (1 ppm) 9 s/13 s (50 ppm) 100 s/90 s (3 ppm) 4.3 s/19.3 s (1 ppm) 3.5 min/6.5 min (10 ppm) 12 s/22 s (100 ppm) 20 s/45 s (100 ppm) 17 s/25 s (100 ppm)
[40] [23] [27] [21] [24] [41] [26] This work
Fig. 8. Response and recovery characteristic of the gas sensor under HCHO concentration of 100 ppm at the operating temperature of 200 ◦ C. Fig. 10. Selectivity of the SnO2 microspheres gas sensor to 100 ppm of different gases at 200 ◦ C.
Fig. 9. Repeatability of the SnO2 microspheres gas sensor to 100 ppm HCHO at 200 ◦ C. Resistance response sensitivity changes with time in continuous six test cycles indicates the repeatability.
tion energies of formaldehyde can be easily broken to participate in the reaction with the sensing material during chemical adsorption. In addition, other gases because of their high bonding energy
would be reluctant to react at lower temperatures, but display lower response [22]. So the operating temperature is also an important factor. The operating temperature was decided by the orbital energy of gas molecule, adsorption mode and amount of gas and so on [38]. Therefore the selectivity may be distinct different operating temperatures. In a word, the high selectivity makes the SnO2 microspheres to be a remarkable sensing material in fabrication of formaldehyde sensor. From the view of the practical applications, in order to ensure the accuracy of the detention, gas sensors should maintain good long-term stability [39]. To verify the long-term stability of the as-fabricated sensor, the gas response evolutions were tested for 30 days under 100 ppm HCHO gas at the operating temperature of 200 ◦ C, as shown in Fig. 11. Even though the response changed every day, the response values are just floating around 38.79, and the standard deviation is statistically calculated to be 1.24–100 ppm formaldehyde. Clearly, the sensor based on the SnO2 microspheres had a good stability which can be put into various practical applications. In addition, the stability mechanism is complicated, and further analysis is required to get a definite understanding. Table 2 shows the brief summary of the gas-sensing performance of various SnO2 based gas sensors toward HCHO. As has been shown, the operating temperature of our SnO2 microspheres (200 ◦ C) is comparable with the literature results ranging from 160 to 330 ◦ C, though still higher than room temperature. It is worth noting that the sensor fabricated in this work exhibits better sensing performance as compared to those reported in the literatures
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[22,26,41,34,43]. The detection mechanism of HCHO can be interpreted in the following way (Fig. 12). When the sensor was exposed to the testing gas atmosphere, oxygen species absorbed on the surface and captured free electrons from the sensing materials. The or SnO2 microspheres can ionize to adsorbed oxygen ions Ox− (O− ads O− ) at grain boundaries. This process decreases the concentra2ads tion of electrons, leads to the formation of a thick space-charge layer and a high resistance of the sensor [26]. This process (as shown in Fig. 12(a)) can be expressed as follows [3,34]: O2 (gas) → O2 (ads)
(3)
−
−
O2 (ads) +e → O2 (ads)
(4)
O2 (ads)− +e− → 2O (ads)−
(5)
−
−
2−
O(ads) +e → O(ads)
(6)
When the sensor is exposed to HCHO, the HCHO gas will react with the adsorbed oxygen ions and release the trapped electrons back to SnO2 microspheres, leading to a thinner space-charge layer and lower potential barrier, and thus a decrease in resistance (as shown in Fig. 12(b)), which can be described as follows [3,44,45]: Fig. 11. Long-term stability of the SnO2 microspheres gas sensor to 100 ppm HCHO at 200 ◦ C.
(as listed in Table 2), such as porous SnO2 nanospheres [40], porous flower-like SnO2 [23], SnO2 nanofibers [27], SnO2 nanowires [24], SnO2 microspheres [26] and so on. For one thing, the gas response of the SnO2 microspheres is found higher than the literatures’ results. For another, the fast response time and recovery time (13 s/14 s [40], 9 s/13 s [23] and 4.3 s/19.3 s [21]) were reported but the operating temperatures were quite high at 260, 240 and 330 ◦ C, respectively. Simultaneously, it is clear that response time and recovery time (12 s/22 s) [41] of literature reports are almost the same with as-synthesized sensor (17 s/25 s), but the gas response is poor (5.4–100 ppm HCHO). In brief, the gas sensor based on SnO2 microspheres exhibits excellent response and good linear dependence to a wide rang of concentrations of formaldehyde (5 ppm to 500 ppm), fast response time and recovery time, good selectivity, remarkable repeatability and long-term stability. The results indicate that the as-synthesized SnO2 microspheres are a promising gas-sensing material for HCHO detection. SnO2 is one of the most representative n-type oxide semiconductor gas sensing materials and its sensing mechanism is explained as the change in electrical conductivity caused by the chemical interaction of gas molecules with the surface of the semiconductor metal oxides [42]. The gas-sensing mechanism of the SnO2 based sensor was discussed in previous work
HCHO(gas) → HCHO(ads)
(7)
HCHO(ads) + 2O(ads)2- → CO2 + H2 O(g) + 4e-
(8)
The excellent gas-sensing performance to HCHO may be attributed to two main aspects. The first can be ascribed to the special architecture of the sample. The as-synthesized SnO2 not only overlap with each other to exhibit a globular structure with numerous nanoparticles, but also show a large specific surface area of 64.361 m2 /g. The special architecture can allow easy adsorption of oxygen molecules and provide a more convenient pathway for the diffusion of HCHO to make the diffusion more effective [46]. Thus, the high response and fast response-recovery characters can be anticipated. Furthermore, it is well known that the gas-sensing performance is related to the thickness of surface electron depletion layer (Ld ) [47]. The gas-sensing performance can be extremely enhanced if the grain size could be reduced to a scale comparable to 2Ld . For SnO2 materials in air, Ld is about 3 nm [48]. The average grain size of as-synthesized SnO2 is about 8 nm, and is close 2Ld , which means the almost full depletion of electrons. It is helpful to improve the action rate between chemisorbed oxygen ions with HCHO molecules, resulting in the short response-recovery character. 5. Conclusions In summary, the SnO2 microspheres were successfully synthesized by a facile hydrothermal method using the chemical
Fig. 12. A schematic diagram of the proposed reaction mechanism of the SnO2 microspheres based sensor to HCHO: (a) in air, (b) in HCHO.
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reaction solvents of water and DMF at a reaction temperature of 160 ◦ C for 15 h. The obtained SnO2 are comprised of a large number of small spheres with hundreds nanometer in size, and every small sphere consists of numerous primary nanocrystallites with several nanometers in diameter. A gas sensor based on SnO2 microspheres was fabricated and exhibited high response, good response-recovery properties, linear dependence, repeatability, selectivity and long-term stability towards HCHO at the operating temperature of 200 ◦ C. It can be believed that these results will not only cause a novel way to synthesize SnO2 microspheres by a facile hydrothermal method, but also offer a material for gas sensor to HCHO gas with excellent sensing properties. Furthermore, the gas sensing mechanism of SnO2 microspheres to HCHO was carried out.
Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 51262029) and the Program for Excellent Young Talents, Yunnan University.
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.2016.07.051.
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Biographies Yuxiu Li received her B.S. degree in Department of Materials Science and Engineering in 2013. She is currently a graduate student in Yunnan University and devotes to inorganic functional materials for gas sensors. Nan Chen received her B.S. degree in Department of Materials Science and Engineering from Yunnan University in 2014. She is currently a graduate student in Yunnan University and devotes to nanostructured functional materials and their applications in supercapacitors and gas sensors.
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Dongyang Deng received her B.S. degree in Department of Materials Science and Engineering in 2013. She is currently a graduate student in Yunnan University and devotes to gas sensors and CO oxidation. Xinxin Xing received her B.S. degree in Department of Materials Science and Engineering from Yunnan University in 2012. She is currently a graduate student in Yunnan University and devotes to porous materials and their applications in gas sensors. Xuechun Xiao received her B.E. degree in Department of High Polymer Material from China Textile University in 1997 and M.E. degree in Department of Materials Science and Engineering from DongHua University in 2004. Currently, she is a teacher at the School of Materials Science and Engineering, Yunnan University. Her work is devoted to nanostructured functional materials and their applications in gas sensors. Yude Wang obtained his M.S. degree in Physics Condensed State from Yunnan University in 1997 and Ph.D. in Materials Physics and Chemistry from Tsinghua University in 2003. From 2005–2007, he was a guest scientist in Max-Planck-Institute of Metal and an Alexander von Humboldt fellow in Max-Planck-Institute of Colloids and Interfaces, Germany. Currently, he is a professor at the School of Physics and Astronomy, Yunnan University. His work is devoted to chemical and biochemical sensors, nanostructured functional materials and their applications.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Formaldehyde detection: SnO2 microspheres for formaldehyde gas sensor with high sensitivity, fast response/recovery and good selectivity Yuxiu Li a , Nan Chen b , Dongyang Deng a , Xinxin Xing a , Xuechun Xiao b,c , Yude Wang b,c,∗ a
School of Materials Science and Engineering, Yunnan University, 650091 Kunming, People’s Republic of China Department of Physics, Yunnan University, 650091 Kunming, People’s Republic of China c Yunnan Province Key Lab of Micro-Nano Materials and Technology, Yunnan University, 650091 Kunming, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 28 April 2016 Received in revised form 18 June 2016 Accepted 13 July 2016 Available online 15 July 2016 Keywords: SnO2 microspheres Hydrothermal synthesis Gas sensor Formaldehyde Stability
a b s t r a c t Tin oxide microspheres were successfully obtained through a facile hydrothermal method without any polymer templates or surfactant. The as-synthesized SnO2 microspheres are composed of large amount of small spheres with average diameters of about 250 nm, and every small sphere consists of numerous primary nanocrystallites with average sizes of about 8 nm. The resultant product was used as sensing material for gas sensor to detect the formaldehyde (HCHO) gas. The gas response, response and recovery time, selectivity and stability were carefully studied. It was found that the response value of the sensor to 100 ppm HCHO was 38.3 at the operating temperature of 200 ◦ C. The gas sensor based on SnO2 microspheres has excellent gas response, good response-recovery properties, linear dependence, repeatability and selectivity, making it to be a promising candidate for practical detectors for HCHO. © 2016 Elsevier B.V. All rights reserved.
1. Introduction HCHO is a gas of colorless and pungent-smelling, which is an important raw material in the world, is widely used in manufacturing processes with approximately ten megatons per year [1], and is found in more than 2000 products [2], because of its chemical reactivity, high purity and low cost [3]. Formaldehyde, therefore, can be emitted unremittingly by various sources such as building materials and numerous living goods (furniture, dry cleaning solutions, cosmetics, pharmaceutical etc) [4]. It is considered as one of the most important air pollutants in homes, offices and urban environments [5]. Formaldehyde also is one of the source materials which can cause sick building syndrome (SBS) [6], and exposuring to formaldehyde can cause many potential health risks. It is revealed by the U.S. Environmental Protection Agency that formaldehyde can cause eyes inflammation and upper airways infections, which can lead to central nervous system damage and immune system disorders [7,8]. Formaldehyde was also classified as a human carcinogen by the International Agency for Research on Cancer (IARC). Several safety standards toward formaldehyde have been set up.
∗ Corresponding author at: Department of Physics, Yunnan University, 650091 Kunming, People’s Republic of China. E-mail addresses: [email protected], [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.snb.2016.07.051 0925-4005/© 2016 Elsevier B.V. All rights reserved.
The World Health Organization (WHO) and National Institute for Occupational Safety and Health (NIOSH) have set safe exposure limit of 0.08 ppm and 1 ppm to formaldehyde [9], respectively. Therefore, a practical and effective way to detect formaldehyde is of great significance. Various ways have been extensively investigated for detecting formaldehyde. The most common methods are high-performance liquid chromatography (HPLC) [10], spectroscopy, enzyme electrodes [11] and the use of sensors [12]. Compared with sensors, many of these methods rely on trained personnel and expensive instrumentation, as well as require the formaldehyde to be put into on a membrane or a liquid solution before analysis. Thus those are unsuitable for real-time monitoring [13], and that is very slow, tiring and inconvenient. So the manufacture of a formaldehyde gas sensor is deemed to be a satisfactory method for detecting it in appropriate environment. In the last few years, for the detection of formaldehyde gas, a series of metal oxides have been reported, such as TiO2 or Ag-TiO2 [14–16], ZnO or ZnO-MnO2 [17], In2 O3 [18], WO3 [19], Co3 O4 [20], SnO2 [21], and so on. As a typical n-type semiconducting metal oxide with a wide band gap of 3.62 eV, SnO2 is the most potential metal oxide used a gas-sensing material on account of its good chemical stability and remarkable electrical properties. But most of all, it has a high gas response, rapid response, low power consump-
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tion and cost, as well as good reproducibility. The applications of SnO2 as a gas-sensing material have aroused great interest in the sensor field. The sensing mechanism of SnO2 is mainly gas reaction with the adsorbed oxygen on the surface and gas diffusion [22], and belongs to the surface-control type. This is determined by a lot of factors such as grain size, surface states and oxygen adsorption quantities. Recently, compared with bulk material of SnO2 , nanostructured SnO2 is drawing ever more attentions because its properties could more obviously improve from nanometer size effects [23]. On this account, the micro/nanostructures SnO2 materials were widely investigated, including nanowires [24], nanorods [25], microspheres [26], nanofibers [27], micro-rods [21], etc. Among them, spherical structure can cause the extensive concern on account of its high surface area and tailored structure, which could make for gas adsorption and desorption. Lin et al. reported that SnO2 microspheres have been successfully synthesized, it has also been reported that the detection toward formaldehyde [26]. The SnO2 microspheres also exhibited good response-recovery properties, but the gas response was relatively low. In this paper, we report the preparation of SnO2 microspheres via a facile hydrothermal method without any polymer templates or surfactant. We chose water and N,N-dimethylformamide (DMF) as solvents. The process is particularly simple and reproducible. The obtained SnO2 microspheres were used to fabricate a gas sensor which shows good sensing performance toward formaldehyde, especially for the selectivity and stability. Furthermore, the gas response of the obtained SnO2 microspheres is also high. The structure and morphology were also characterized to present a further understanding of the related mechanisms of spherical structure and gas sensing. 2. Experimental 2.1. Preparation of SnO2 microspheres All chemicals including tin (IV) chloride pentahydrate (SnCl4 ·5H2 O), sodium hydroxide (NaOH), N,N-dimethylformamide (DMF) and formaldehyde solution (HCHO) were analytical grade reagents and used without any further purification. A typical synthesis process of SnO2 microspheres was as following: 4.00 mmol SnCl4 ·5H2 O was first dissolved in 10 mL of distilled water under magnetic stirring until a homogenous solution was obtained. Afterwards, 12 mL DMF was completely dissolved into the above solution. After stirred for about 20 min, NaOH solution (20 mL, 2.00 M) was added into the solution under vigorous magnetic stirring for 30 min, and more distilled water was added into the above solution to make the whole volume of the solution reach 60 mL. The volume ratio of DMF to water is 1:4. Subsequently, the whole solution was transferred into a Teflon-lined stainless steel autoclave with a capacity of 90 mL and heated for 15 h at 160 ◦ C in an electric oven. The autoclave was cooled down to room temperature naturally, and precipitates were separated by centrifugation and washed with ethanol and distilled water for several times. The products were dried at 60 ◦ C for 24 h, followed by thermal treatment at 400 ◦ C for 1 h in air atmosphere. Finally, SnO2 microspheres were obtained for gas sensor fabrication. 2.2. Characterization of SnO2 microspheres Powder X-ray diffraction (XRD) pattern was recorded on a Rigaku TTRIII powder diffractometer equipped with Cu K˛ radiation ( = 1.54056 Å). The diffracted X-ray intensities were recorded by 2. The sample was scanned in the 2 range 10◦ –90◦ in steps of 0.02◦ . The operating voltage and current were 40 kV and 200 mA, respectively. The surface topography of the sample was
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inspected using scanning electron microscopy (SEM) taken on FEI QUANTA200 with microscope operating at 30 kV. The sample was coated with a thin gold layer before SEM imaging. The transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL JEM-2100 with an acceleration voltage of 200 kV. For TEM images, the microspheres were dispersed ultrasonically in ethanol, and then transferred onto copper grids before TEM observation. The surface chemistries of the sample were characterized using X-ray photoelectron spectroscopy (XPS). X-ray photoelectron spectroscopy (XPS) was carried out at room temperature in an ESCALAB 250 system with Al K␣ X-ray radiation at 15 kV. All XPS spectra were accurately calibrated by the C 1s peak at 284.6 eV to compensate for the charge effect. 2.3. Preparation of gas sensor To demonstrate the potential applications of the final SnO2 products, gas sensor of indirect heating was fabricated according to the literature [28]. At the beginning, the as-synthesized SnO2 microspheres were mixed with an appropriate deionized water to form paste. The paste was coated onto an alumina ceramic tube (4 mm in length, 1.2 mm in external diameter, and 0.8 mm in internal diameter) with thickness of about 0.6 mm. A pair of Au electrodes were previously printed on the alumina ceramic tube. Platinum lead wires attached to these Au electrodes were used as electrical contacts. Subsequently, the sensor sample was dried at 120 ◦ C for 1 h and calcined at 400 ◦ C for 1 h in air to reach the stability and repeatability of the material. Finally, a small Ni-Cr alloy coil (23 ) was inserted into the alumina ceramic tube as a heater, which provided the operating temperature of the gas sensor. The operating temperature can be adjusted by tuning the heat voltage (Vh ). To improve the long-term stability of the gas sensor, the asfabricated sensor was aged at the operating temperature of 350 ◦ C for 5 days. Gas sensing properties were measured by a WS-30A system (Weisheng Instruments Co., Zhengzhou, China) under laboratory condition. The relative humidity of the test is about 25–30%. The circuit voltage (Vc ) was set at 5 V, and the output voltage (Vout ) was set as the terminal voltage of the load resistor (RL ) [29]. The test gas was injected into a test chamber using a microsyringe after the base line of the sensor was stable. The desired concentrations of the testing gas are obtained by the volume of the analyte solution. The analyte solution was evaporated by an evaporator. At the same time, tow fans were installed to make the gas homogeneous. The gas response value () of the sensor in this paper is defined as follows:  = Ra /Rg
(1)
The Ra and Rg are the resistances of the gas sensor in air and target gas, respectively. The response time ( res ) is defined as the time to reach 90% of the final equilibrium value and the recovery time ( recov ) is defined as the time to decrease to 10% of the final equilibrium value. 3. Results and discussion The phase purity and crystal structure of the as-synthesized SnO2 microspheres were investigated by XRD. Fig. 1 illustrates the typical diffraction patterns of the SnO2 microspheres. All the peaks can be well indexed to the cassiterite SnO2 (JCPDS card no. 990024, a = b = 4.739 Å, c = 3.187 Å), space group P42 /mnm (136). No characteristic peaks were observed for other impurities, demonstrating the high purity of the as-synthesized SnO2 microspheres. The peak intensities indicate the excellent crystalline degree of
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Fig. 1. The XRD patterns (a) and the typical Rietveld refinement data (b) for as-synthesized SnO2 microspheres.
Fig. 2. SEM images of as-synthesized SnO2 microspheres at (a) low magnification and (b) high magnification, respectively.
Fig. 3. (a) TEM image and (b) magnified TEM image of as-synthesized SnO2 microspheres, (c) HRTEM image of SnO2 microspheres and (d) the corresponding HRTEM image with obvious SnO2 lattice fringes.
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Fig. 4. (a) XPS survey spectrum of the SnO2 microspheres, (b) high-resolution XPS spectrum of Sn 3d for SnO2 microspheres, (c) high-resolution XPS spectrum of O 1s for SnO2 microspheres. Table 1 Structural parameters for SnO2 microspheres are calculated by Rietveld refinement of the experimental XRD pattern. Space group
P42 /mnm (136)
Lattice parameters
Element coordinates
Sn
O
Average grain size (nm) Average maximum microstrain Rwp (%)
a0 (Å) b0 (Å) c0 (Å) x y z x y z
4.7420 4.7420 3.1844 0 0 0 0.2895 0.2895 0 8.0300 0.0045 13.43%
the as-synthesized SnO2 microspheres. The diffraction peaks of as-synthesized sample are broad, indicating the small grain size. Significantly, the structural information was further obtained by the refinement of the diffraction patterns with the Rietveld method [30]. The red dots, black curve and blue curve represent the experiment data, calculated pattern and the difference, respectively. The position of the Bragg reflections is shown in green bars. The average grain size of the SnO2 microspheres is calculated to be about 8 nm. All structural parameters obtained from Rietveld profile refinement, and summarized in Table 1. As can be obviously observed from Fig. 1, the calculated pattern and experimental data are in sat-
isfactory agreement as seen from the difference curve. The lattice parameters are refined to a = b = 4.742 Å and c = 3.184 Å, respectively, which is in good agreement with cassiterite SnO2 (JCPDS card no. 99-0024). These results confirm the successful synthesis of SnO2 via a facile hydrothermal method. In addition, in order to have a better understanding for the formation of as-synthesized SnO2 microspheres. The XRD pattern of precursor before the thermal treatment at 400 ◦ C was also obtained, and as shown in Fig. S1. The identified peaks for the precursor can be also indexed to cassiterite SnO2 (JCPDS card no. 99-0024). However, all diffraction peaks from the precursor obviously exhibit a broader and lower intensity relative to that of final SnO2 product, and the average grain is calculated to be about 3.88 nm. The morphology and the microstructure of the SnO2 microspheres were characterized by SEM, and displayed in Fig. 2. As can be obviously observed from the image in Fig. 2(a) (at low magnification), the SnO2 nano-architectures were obtained with mixture of water and DMF. It is clearly seen that the SnO2 nanoarchitectures are sphere-like. The more detailed structural feature of as-synthesized SnO2 microspheres was examined from highmagnification SEM image (Fig. 2(b)), suggesting that the product consists of large amount of small spheres with average diameters of about 250 nm. These microspheres are accumulational and not monodisperse. In addition, the surface of every small sphere is rough and composed of densely packed nanoparticles with several nanometers in size. For comparison, the SEM images of precursor before the thermal treatment at 400 ◦ C were also provided, and as
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shown in Fig. S2. It can be seen that the structure of the precursor is also globular. The more detailed structures of as-synthesized SnO2 microspheres were further examined with TEM and HRTEM. The image (Fig. 3(a)) of TEM confirms that SnO2 microspheres have been obtained. These microspheres are superimposed together. Furthermore, the SnO2 microspheres are irregular in size (but shape is highly regular), up to hundreds of nanometers in diameter, which is in good agreement with the above estimated size from SEM results. To gain further understanding of the micostructure, a magnified TEM image is displayed in Fig. 3(b). From the magnified TEM image, the as-synthesized SnO2 microspheres are composed of numerous primary nanocrystallites with average sizes of about 8 nm, which matches well with the XRD results. HRTEM image of the SnO2 microspheres is given in Fig. 3(c), and the clear well-developed lattice fringes imply a high crystallinity and random orientation of the SnO2 microspheres. In order to get a clearer image of lattice fringes, a HRTEM image of the edge part marked by the white dashed line in Fig. 3(c) is shown in Fig. 3(d). The interplanar spacing is estimated to be 0.334 nm and can be indexed to the (110) crystal planes of SnO2 . It is well known that the sensing mechanism is surface controlled process in which grain size, surface states and oxygen adsorption plays a fundamental role [31]. So the morphology of as-synthesized SnO2 microspheres plays an important role in gas-sensing properties. To further analysis and illuminate the composition and chemical state of every element, the XPS spectra were measured for the as-synthesized SnO2 . Fig. 4(a) represents XPS survey scan for the SnO2 microspheres sample. Apart from the peak of C 1s at 285 eV, as expected the spectrum was dominated by the lines of Sn and O. From the spectrum, the main peaks are Sn MNN (1062 eV), O KLL (977 eV), Sn 3P1/2 (759 eV), Sn 3P3/2 (717 eV), O 1 s (531 eV), Sn 3d3/2 (496 eV), Sn 3d5/2 (487 eV) and Sn 4d (27 eV), respectively, which confirms a good purity of the as-synthesized sample. The trace amounts of C 1s may be from the absorption of organic molecules in the air [22]. The high resolution Sn 3d and O 1s spectrums are shown in Fig. 4(b) and (c), which are matched by Multipak software, respectively. The Sn 3d spectrum (Fig. 4(b)) reveals two peaks of Sn 3d3/2 and Sn 3d5/2 with good symmetry. The peaks corresponding to binding energies for Sn 3d3/2 and Sn 3d5/2 appeared at 495.40 eV and 486.95 eV, respectively, giving a spin orbit coupling of 8.45 eV. The values correspond to the bind-
Fig. 5. The N2 adsorption-desorption isotherm of the SnO2 microspheres. The inset is the BJH pore-size distribution curve.
ing energies of the Sn4+ ion [32]. The high-resolution XPS spectrum of O 1s is shown in Fig. 4(c). One can find that the O 1s is an asymmetric peak at 530.90 eV, and the peak of O 1s can be separated into two peaks at Olat (530.90 eV) and Oads (Ox− , 532.15 eV) [33]. Olat is attributed to the lattice oxygen on the surface of SnO2 microspheres, which is thought to be has no contribution to the and O− ) is gas response but pretty stable [34]. However, Ox− (O− ads 2ads relative to the adsorbed oxygen ions of SnO2 microspheres, which has an important role in the gas sensing property [34]. The absorbed oxygen ions Ox− is reactable with the gas and then enhance the holes concentration [35]. Therefore, increasing the absorbed oxygen ions Ox− contributes to the gas response. The porous structure and the specific area of the SnO2 microspheres are performed by N2 adsorption-desorption isotherm and the corresponding BJH pore-size distribution curves, and as shown in Fig. 5. The SnO2 microspheres shows obvious hysteresis loop, indicating the existence of pores. As observed in inset of Fig. 5, the main peak is positioned at 6.196 nm have investigated for SnO2 microspheres, which suggested that the SnO2 microspheres pos-
Fig. 6. Schematic illustration of the formation process of SnO2 microspheres.
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sessed mesoporous structure. Moreover, the specific surface area of the SnO2 microspheres is calculated as 64.361 m2 /g, and it can supply an adequate surface for gas sensing reactions. In view of the above results, we supposed formation mechanism of SnO2 microspheres, as illustrated in Fig. 6. At first, a number of tiny primary nanocrystals were formed by hydrolysis. Afterwards, these nanoparticles tend to form the aggregated SnO2 microspheres. After calcined at 400 ◦ C for 1 h, the crystal phase of as-prepared sample became more pure due to the elimination of residual organics (DMF), and the average grain size of SnO2 microspheres become larger, and SnO2 microspheres were well crystallized with maintaining the original configuration. It is interesting to note that the chemical activities of semiconductor oxide sensors can be fully stimulated out at the optimum operating temperature. Therefore, the operating temperature strongly influences response property of a semiconductor oxide sensor, such as response, response-recovery time and selectivity. The temperature of 200 ◦ C was chosen as the operating temperature due to high gas response and fast response-recovery time. Fig. 7(a) shows the response of the SnO2 microspheres gas sensor under the seven different HCHO gas concentrations ranging from 1 to 500 ppm. It is clearly evident that the curve exhibits a stepwise change upon exposing the sensor to different concentrations of HCHO. The response amplitude of the sensor increases gradationally with increasing the gas concentration from 1 ppm to 500 ppm. More specifically, the gas responses toward 1, 3, 5, 50, 100, 200 and 500 ppm of HCHO are 5.72, 7.70, 10.85, 21.12, 38.26, 66.25 and 144.90, respectively. This result indicates that the SnO2 microspheres sensor is suitable for quantitative detection for HCHO at low concentrations. The responses of the SnO2 microspheres sensor as a function of the HCHO concentration are plotted in Fig. 7(b). The sensor responses increase nearly linearly with rising HCHO gas concentration. The relationship between the response ( = Ra /Rg ) and the gas concentration was fitted as follows:  = 0.28C + 8.19
(2)
where C is the concentration of HCHO. It is found that the experimental data and the theoretical curve show good agreements, and the correlative coefficient R2 is 0.99781. Such a good linear dependence of response on the gas concentration indicates that the SnO2 microspheres sensor can be used as promising materials for detecting HCHO. The response and recovery characteristics are important parameters for evaluating the performances of gas sensors [36]. The response and recovery times of gas sensor utilizing SnO2 microspheres as sensing material were investigated toward HCHO gas with the concentration of 100 ppm, as shown in Fig. 8. According to data of Fig. 8, the response time ( res ) and recovery time ( recov ) of the SnO2 microspheres are approximately 17 s and 25 s, respectively. The res value and recov value of the sensor are far more rapid than most reported SnO2 formaldehyde sensors [26,27]. The quick response and recovery of the sensor may be attributed to the special morphologies. These spherical structures can offer a sufficient active surface and good permeability for the fast adsorption and gas diffusion [26]. Therefore, we can conclude that SnO2 microspheres exhibit the excellent gas sensing performance. The gas sensing repeatability is another important parameter to evaluate the sensing ability of semiconductor materials [37]. So, the repeatability of the SnO2 microspheres gas sensor was studied by testing 100 ppm HCHO six times under the same conditions. In Fig. 9, one can see the response and recovery curve show neglectable change and the gas response in every test reaches about 38. On the other hand, the response time and recovery time are almost reproducible as well as the response values. The results indicate that the sensor has excellent reversibility and repeatability for the detection of HCHO.
Fig. 7. The gas sensing properties of the sensor based on SnO2 microspheres at an operating temperature of 200 ◦ C. (a) Dynamic response-recovery curve of the SnO2 microspheres gas sensor exposed to various concentrations of HCHO in a range of 1–500 ppm. (b) Linear dependence relation between resistance response sensitivity and gas concentration.
Furthermore, in order to clarify the potential gas-sensing applications, the selectivity of the sensor based on SnO2 microspheres was tested in 100 ppm (200 ◦ C) potential interference gases, such as acetone (CH3 COCH3 ), ammonia (NH3 ), xylene (C8 H10 ), methanol (CH4 O) and toluene (C7 H8 ). The results are compared in Fig. 10. It is obvious that the sensor shows higher response, and the response of the sensor to formaldehyde (HCHO) is 38.28, while to acetone, ammonia, xylene, methanol and toluene, the responses are 6.42, 1.74, 1.99, 19.38 and 1.55, respectively. The response toward HCHO is probably 6.0, 22.0, 19.2, 2.0, and 24.7 times higher than that toward acetone, ammonia, xylene, methanol and toluene, respectively. In conclusion, the sensor based on SnO2 microspheres exhibits a good selectivity toward HCHO. The selectivity of the sensor is influenced by various factors. For the study, the structures and bond dissociation energies of gas molecules are significant influences. The HCHO band is just 364 kJ/mol, which is smaller than other gases at a lower temperature. So the bond dissocia-
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Table 2 Comparison of the sensing performances of various SnO2 based gas sensors toward formaldehyde (HCHO). Materials
Optimum temperature (◦ C)
Response
Response/Recovery
Ref.
Porous SnO2 nanospheres Porous flower-like SnO2 Tin oxide nanofibers Hierarchical porous SnO2 micro-rods SnO2 nanowires Hollow SnO2 nanofibers SnO2 microspheres SnO2 microspheres
260 240 200 330 270 180 160 200
7.6 (10 ppm) 24.8 (100 ppm) 19.6 (50 ppm) 3.86 (1 ppm) 2.45 (10 ppm) 5.4 (100 ppm) 3.6 (100 ppm) 38.3 (100 ppm)
13 s/14 s (1 ppm) 9 s/13 s (50 ppm) 100 s/90 s (3 ppm) 4.3 s/19.3 s (1 ppm) 3.5 min/6.5 min (10 ppm) 12 s/22 s (100 ppm) 20 s/45 s (100 ppm) 17 s/25 s (100 ppm)
[40] [23] [27] [21] [24] [41] [26] This work
Fig. 8. Response and recovery characteristic of the gas sensor under HCHO concentration of 100 ppm at the operating temperature of 200 ◦ C. Fig. 10. Selectivity of the SnO2 microspheres gas sensor to 100 ppm of different gases at 200 ◦ C.
Fig. 9. Repeatability of the SnO2 microspheres gas sensor to 100 ppm HCHO at 200 ◦ C. Resistance response sensitivity changes with time in continuous six test cycles indicates the repeatability.
tion energies of formaldehyde can be easily broken to participate in the reaction with the sensing material during chemical adsorption. In addition, other gases because of their high bonding energy
would be reluctant to react at lower temperatures, but display lower response [22]. So the operating temperature is also an important factor. The operating temperature was decided by the orbital energy of gas molecule, adsorption mode and amount of gas and so on [38]. Therefore the selectivity may be distinct different operating temperatures. In a word, the high selectivity makes the SnO2 microspheres to be a remarkable sensing material in fabrication of formaldehyde sensor. From the view of the practical applications, in order to ensure the accuracy of the detention, gas sensors should maintain good long-term stability [39]. To verify the long-term stability of the as-fabricated sensor, the gas response evolutions were tested for 30 days under 100 ppm HCHO gas at the operating temperature of 200 ◦ C, as shown in Fig. 11. Even though the response changed every day, the response values are just floating around 38.79, and the standard deviation is statistically calculated to be 1.24–100 ppm formaldehyde. Clearly, the sensor based on the SnO2 microspheres had a good stability which can be put into various practical applications. In addition, the stability mechanism is complicated, and further analysis is required to get a definite understanding. Table 2 shows the brief summary of the gas-sensing performance of various SnO2 based gas sensors toward HCHO. As has been shown, the operating temperature of our SnO2 microspheres (200 ◦ C) is comparable with the literature results ranging from 160 to 330 ◦ C, though still higher than room temperature. It is worth noting that the sensor fabricated in this work exhibits better sensing performance as compared to those reported in the literatures
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[22,26,41,34,43]. The detection mechanism of HCHO can be interpreted in the following way (Fig. 12). When the sensor was exposed to the testing gas atmosphere, oxygen species absorbed on the surface and captured free electrons from the sensing materials. The or SnO2 microspheres can ionize to adsorbed oxygen ions Ox− (O− ads O− ) at grain boundaries. This process decreases the concentra2ads tion of electrons, leads to the formation of a thick space-charge layer and a high resistance of the sensor [26]. This process (as shown in Fig. 12(a)) can be expressed as follows [3,34]: O2 (gas) → O2 (ads)
(3)
−
−
O2 (ads) +e → O2 (ads)
(4)
O2 (ads)− +e− → 2O (ads)−
(5)
−
−
2−
O(ads) +e → O(ads)
(6)
When the sensor is exposed to HCHO, the HCHO gas will react with the adsorbed oxygen ions and release the trapped electrons back to SnO2 microspheres, leading to a thinner space-charge layer and lower potential barrier, and thus a decrease in resistance (as shown in Fig. 12(b)), which can be described as follows [3,44,45]: Fig. 11. Long-term stability of the SnO2 microspheres gas sensor to 100 ppm HCHO at 200 ◦ C.
(as listed in Table 2), such as porous SnO2 nanospheres [40], porous flower-like SnO2 [23], SnO2 nanofibers [27], SnO2 nanowires [24], SnO2 microspheres [26] and so on. For one thing, the gas response of the SnO2 microspheres is found higher than the literatures’ results. For another, the fast response time and recovery time (13 s/14 s [40], 9 s/13 s [23] and 4.3 s/19.3 s [21]) were reported but the operating temperatures were quite high at 260, 240 and 330 ◦ C, respectively. Simultaneously, it is clear that response time and recovery time (12 s/22 s) [41] of literature reports are almost the same with as-synthesized sensor (17 s/25 s), but the gas response is poor (5.4–100 ppm HCHO). In brief, the gas sensor based on SnO2 microspheres exhibits excellent response and good linear dependence to a wide rang of concentrations of formaldehyde (5 ppm to 500 ppm), fast response time and recovery time, good selectivity, remarkable repeatability and long-term stability. The results indicate that the as-synthesized SnO2 microspheres are a promising gas-sensing material for HCHO detection. SnO2 is one of the most representative n-type oxide semiconductor gas sensing materials and its sensing mechanism is explained as the change in electrical conductivity caused by the chemical interaction of gas molecules with the surface of the semiconductor metal oxides [42]. The gas-sensing mechanism of the SnO2 based sensor was discussed in previous work
HCHO(gas) → HCHO(ads)
(7)
HCHO(ads) + 2O(ads)2- → CO2 + H2 O(g) + 4e-
(8)
The excellent gas-sensing performance to HCHO may be attributed to two main aspects. The first can be ascribed to the special architecture of the sample. The as-synthesized SnO2 not only overlap with each other to exhibit a globular structure with numerous nanoparticles, but also show a large specific surface area of 64.361 m2 /g. The special architecture can allow easy adsorption of oxygen molecules and provide a more convenient pathway for the diffusion of HCHO to make the diffusion more effective [46]. Thus, the high response and fast response-recovery characters can be anticipated. Furthermore, it is well known that the gas-sensing performance is related to the thickness of surface electron depletion layer (Ld ) [47]. The gas-sensing performance can be extremely enhanced if the grain size could be reduced to a scale comparable to 2Ld . For SnO2 materials in air, Ld is about 3 nm [48]. The average grain size of as-synthesized SnO2 is about 8 nm, and is close 2Ld , which means the almost full depletion of electrons. It is helpful to improve the action rate between chemisorbed oxygen ions with HCHO molecules, resulting in the short response-recovery character. 5. Conclusions In summary, the SnO2 microspheres were successfully synthesized by a facile hydrothermal method using the chemical
Fig. 12. A schematic diagram of the proposed reaction mechanism of the SnO2 microspheres based sensor to HCHO: (a) in air, (b) in HCHO.
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reaction solvents of water and DMF at a reaction temperature of 160 ◦ C for 15 h. The obtained SnO2 are comprised of a large number of small spheres with hundreds nanometer in size, and every small sphere consists of numerous primary nanocrystallites with several nanometers in diameter. A gas sensor based on SnO2 microspheres was fabricated and exhibited high response, good response-recovery properties, linear dependence, repeatability, selectivity and long-term stability towards HCHO at the operating temperature of 200 ◦ C. It can be believed that these results will not only cause a novel way to synthesize SnO2 microspheres by a facile hydrothermal method, but also offer a material for gas sensor to HCHO gas with excellent sensing properties. Furthermore, the gas sensing mechanism of SnO2 microspheres to HCHO was carried out.
Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 51262029) and the Program for Excellent Young Talents, Yunnan University.
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.2016.07.051.
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Biographies Yuxiu Li received her B.S. degree in Department of Materials Science and Engineering in 2013. She is currently a graduate student in Yunnan University and devotes to inorganic functional materials for gas sensors. Nan Chen received her B.S. degree in Department of Materials Science and Engineering from Yunnan University in 2014. She is currently a graduate student in Yunnan University and devotes to nanostructured functional materials and their applications in supercapacitors and gas sensors.
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Dongyang Deng received her B.S. degree in Department of Materials Science and Engineering in 2013. She is currently a graduate student in Yunnan University and devotes to gas sensors and CO oxidation. Xinxin Xing received her B.S. degree in Department of Materials Science and Engineering from Yunnan University in 2012. She is currently a graduate student in Yunnan University and devotes to porous materials and their applications in gas sensors. Xuechun Xiao received her B.E. degree in Department of High Polymer Material from China Textile University in 1997 and M.E. degree in Department of Materials Science and Engineering from DongHua University in 2004. Currently, she is a teacher at the School of Materials Science and Engineering, Yunnan University. Her work is devoted to nanostructured functional materials and their applications in gas sensors. Yude Wang obtained his M.S. degree in Physics Condensed State from Yunnan University in 1997 and Ph.D. in Materials Physics and Chemistry from Tsinghua University in 2003. From 2005–2007, he was a guest scientist in Max-Planck-Institute of Metal and an Alexander von Humboldt fellow in Max-Planck-Institute of Colloids and Interfaces, Germany. Currently, he is a professor at the School of Physics and Astronomy, Yunnan University. His work is devoted to chemical and biochemical sensors, nanostructured functional materials and their applications.