Nitrogen oxide gas-sensing characteristics of hierarchical Bi2WO6 microspheres prepared by a hydrothermal method

Materials Science in Semiconductor Processing 40 (2015) 463–467

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Nitrogen oxide gas-sensing characteristics of hierarchical Bi2WO6 microspheres prepared by a hydrothermal method Jingkun Xiao, Wei Dong, Chengwen Song n, Yingtao Yu, Li Zhang, Chen Li, Yanyan Yin College of Environment Science and Engineering, Dalian Maritime University, 1 Linghai Road, Dalian 116026, China

a r t i c l e i n f o

abstract

Article history: Received 20 February 2015 Received in revised form 18 May 2015 Accepted 16 June 2015

Hierarchical Bi2WO6 microspheres are synthesized by a hydrothermal method. Morphology and structure of Bi2WO6 are analyzed by SEM, XRD, XPS, and Raman spectra. Gassensing properties of the as-prepared Bi2WO6 sensor are also systematically investigated. The results show the hierarchical Bi2WO6 microspheres are assembled by nanosheets and demonstrate good crystallinity. The optimal operating temperature of the Bi2WO6 sensors is 300 1C. At this operating temperature, the Bi2WO6 sensor exhibits a fast response– recovery to nitrogen oxide, suggesting its excellent potential application as a gas sensor for nitrogen oxide gas-sensing applications. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Preparation Bi2WO6 Nitrogen oxide Gas-sensing

1. Introduction Precise detection and early warning for flammable, toxic and explosive gases have become a major challenge in industry process due to the increasing concerns about the industrial safety [1]. In recent years, electronic nose has been explored and used as an effective tool for real-time detection of hazardous chemicals in the environment [2]. As we know, electronic nose is a complex system, which attempts to emulate the mammalian nose by using an array of chemical sensors that can simulate mammalian olfactory responses to odors [3–4]. Presently, many semiconductors, such as SnO2, In2O3, ZnO, TiO2, , WO3, etc., have been most widely used in electronic nose systems for the detection of hazardous chemicals because they possess a broad range of electronic, chemical, and physical properties that are often highly sensitive to changes in their chemical environment [5–6]. Bismuth tungstate (Bi2WO6), as the most important member in the Aurivillius family constructed by alternating

n

Corresponding author. Tel./fax: þ86 41 184 724 342. E-mail address: [email protected] (C. Song).

http://dx.doi.org/10.1016/j.mssp.2015.06.042 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

þ ðBi2 O2 Þ2n layers and perovskite-like ðWO4 Þn2n þ layers, has n received great interest over the past few years because of its potential applications in photoluminescence, microwave, optical fibers, catalysts, magnetic devices, and chemical sensors [7–12]. As we know, the morphologies play important roles in determining the properties of semiconducting materials [13]. Hence, Many different synthetic processes including mechanochemical synthesis, electrospinning, solid-state reaction, microwave-assisted method and hydrothermal method have been developed for the controllable synthesis of Bi2WO6 nanomaterials, and Bi2WO6 with various morphologies, such as microdiscs [14], microbelts [15], nanoparticles[16], nanoplates [17], nanosheets [18], spherelike [19], flower-like [20], curling-like [21], octahedron-like [22], etc., have been prepared successfully. These works revealed that different morphologies of Bi2WO6 nanomaterials could realize different physicochemical properties, which are beneficial for their applications as photocatalysts. However, to our knowledge, there are still very few studies concerning Bi2WO6 nanomaterials applied as gas sensors. Until recently, only several works on Bi2WO6 based sensors were reported. Wang et al. fabricated mesoporous Bi2WO6 by a hydrothermal method, which exhibited high sensitivity

J. Xiao et al. / Materials Science in Semiconductor Processing 40 (2015) 463–467

2.3. Fabrication and measurement of gas sensors Bi2WO6 was mixed with several drops of ethanol to form a slurry, and then the slurry was brush-coated onto the surfaces of an alumina tube with two Au electrodes and four Pt wires. A Ni–Cr heating wire was inserted into the alumina tube and used as a heater. The alumina tube was then welded onto a pedestal with six probes to obtain the final sensor unit. Gas sensing tests were performed on a WS-30A static gas-sensing system (HanWei Electronics Co., Ltd., Henan, China) using ambient air as the dilute and reference gas. The test gas with a calculated volume was introduced into the test chamber by a microsyringe [29].

3. Results and discussion 3.1. Effect of synthetic parameters on morphology of Bi2WO6 Fig. 1 shows the typical XRD pattern of the as-prepared sample. All the diffraction peaks are indexed to orthorhombic phase of Bi2WO6 [JCPDS No. 73-1126], and no other peaks are observed, suggesting pure orthorhombic Bi2WO6 is obtained [30]. The sharp and strong intensity of XRD peaks suggest that the samples have good crystallinity. Two factors, reaction temperature and time, which affect the morphology of as-prepared Bi2WO6 sample, were investigated. As shown in Fig. 2a–e, when reaction temperature is controlled at 110 1C, amorphous nanoparticles are observed. With the increase of reaction temperature to 140 1C, the Bi2WO6 sample shows the morphology of nonuniform hierarchical microsphere-shape nanostructures. Further increasing the reaction temperature increases to 170 1C, the size and shape of hierarchical microspheres tend to more regular. High-magnification SEM images reveal that each of the Bi2WO6 hierarchical structures is assembled by nanosheets. When the reaction temperature reaches 200 1C, loose hierarchical structures are observed. Fig. 2f and g presents the SEM images of the products obtained at different reaction time. By prolonging the reaction time from 12 h to 36 h, hierarchical structures of Bi2WO6 sample become more loose. From the HRTEM image of the Bi2WO6 sample (Fig. 2h), it can be seen that the fringe spacing of pure Bi2WO6 is about 0.313 nm, which corresponds to the interplanar spacing of (113) planes of orthorhombic Bi2WO6 [7]. The XPS wide scan spectra in Fig. 3a demonstrates the presence of W, Bi, and O. Fig. 3b–d depicts the highresolution spectra of Bi 4f, W 4f and O 1s. The binding energies of Bi 4f7/2 and Bi 4f5/2 peaks are at 159.28 and

20

30

40 50 60 2 Theta (degree )

Fig.1. XRD patterns of Bi2WO6 sample.

70

(333) (240)

X-ray diffraction (XRD) patterns of Bi2WO6 were recorded using a D/Max-2400 diffractometer (Cu Kα radiation, λ¼1.54055 Å) in a range of diffraction angle 2θ from 101 to 801 to analyze the diffraction peaks of Bi2WO6. The morphology of Bi2WO6 was observed by a Philips XL30 FEG scanning electron microscope (SEM) operated at 15 kV. X-ray photoelectron spectroscopy (XPS) of Bi2WO6 was carried out on a Thermo Scientific ESCALAB 250 spectrometer with a monochromatic Al Kα source. Raman spectra of Bi2WO6 were obtained by a LabRAM XploRa confocal Ramanscope spectrometer.

where Ra and Rg are the electrical resistance of the sensor in air and in test gas, respectively.

(040)

2.2. Characterizations

ð1Þ

(133) (313) (226) (218)

1.46 g of Bi(NO3)3  5H2O and 0.49 g of Na2WO4  2H2O were dissolved in 30 mL of H2O and 30 mL HNO3 (0.6 M) to obtain solution A and B, respectively. Then the solution B was added dropwise into the solution A with stirring for 30 min. After that, the above mixture was sealed in a Teflon-lined stainless steel autoclave of 100 mL capacity and heat at 170 1C for 24 h, and then cooled to room temperature. The resulting precipitates were collected by centrifugation and washed three times by deionized water and ethanol to remove possible impurities, and subsequently dried at 60 1C for 4 h.

Ra Rg

(220) (206)

2.1. Preparation of Bi2WO6

response ¼

(200) (020)

2. Experimental

The sensor response is defined as follows:

(113)

and a fast response–recovery to ethanol gas [23]. Lou et al. reported their works on curling-like Bi2WO6 synthesized by a solvothermal route, and its good sensing properties for ethanol detection [24]. Li et al. synthesized erythrocyte-like Bi2WO6 at a hydrothermal process, and found its gassensing property towards ethanol was obviously improved relative to the typical Bi2WO6 microspheres [25]. All those works inspired us to investigate the gas-sensing properties of Bi2WO6 based nanomaterials with different morphologies towards various gases. In this work, we synthesized hierarchical Bi2WO6 microspheres using a simple hydrothermal method [26–28], investigated their morphologies and structure characteristics, and further evaluate their gas sensing properties and potential in nitrogen oxide detection.

Intensity (a.u.)

464

80

J. Xiao et al. / Materials Science in Semiconductor Processing 40 (2015) 463–467

465

Fig. 2. SEM images of Bi2WO6 synthesized at (a) 110 1C, 24 h, (b) 140 1C, 24 h, (c) 170 1C, 24 h, (d) 170 1C, 24 h (high magnification), (e) 170 1C, 12 h, (f) 170 1C, 36 h, and HRTEM image (h) 170 1C, 24 h.

164.68 eV, which revealed the trivalent oxidation state of bismuth. The W 4f7/2 and W 4f5/2 peaks located at 35.68 eV and 37.68 eV indicate the W6 þ oxidation state. [31,32].

Raman spectra of Bi2WO6 microspheres is shown in Fig. 4. The peaks at 790 cm  1 and 820 cm  1 are assigned to antisymmetric and symmetric Ag modes of terminal O–

466

J. Xiao et al. / Materials Science in Semiconductor Processing 40 (2015) 463–467

Bi4f

Intensity (a.u.)

Intensity (a.u)

O1s

W4f

200

0

200

400

600

800

1000

1200

-1

400 600 800 1000 1200 Binding Energy (eV)

Raman shift (cm ) Fig. 4. Raman spectra of the Bi2WO6 sample.

Bi4f7/2

2.4

Intensity (a.u.)

Response (Rg/Ra)

Bi4f5/2

2.2 2.0 1.8 1.6 1.4

152

156 160 164 Binding Energy (eV)

240

168

280

320

360

400

o

Temperature ( C) Fig. 5. Response temperatures.

of

the

Bi2WO6

sensor

at

different

operating

W4f7/2 W4f5/2

Response (Rg/Ra)

Intensity (a.u.)

4.5 4.0 3.5 3.0 2.5 2.0

32

34

36 38 40 Binding Energy (eV)

42

Fig. 3. XPS spectra of the Bi2WO6 sample: (a) wide scan spectrum, (b) Bi 4f spectra, and (c) W 4f spectra.

1.5 10

20 30 40 Concentration (ppm)

50

Fig. 6. Response of Bi2WO6 sensor to different nitrogen oxide concentrations.

W–O. The peak at 310 cm  1 is associated with simultaþ6 neous motions of Bi3 þ andWO6 . The peak at 700 cm  1 is interpreted as an antisymmetric bridging mode associated with the tungstate chain [26]. All those give the insight that the sample is Bi2WO6. 3.2. Gas sensing properties of Bi2WO6 microspheres The operating temperature is a key factor in determining the gas sensing properties. Here, the gas sensing properties of Bi2WO6 sensor towards 20 ppm nitrogen oxide were studied as a function of operating temperature.

As shown in Fig. 5, the responses increase with operating temperature and reach a maximum of 2.42 at 300 1C, and then decrease with further increase in operating temperature. The reason may be attributed to the fact that nitrogen oxide gas does not have enough thermal energy to react with adsorbed oxygen species i.e., O  or O2  at lower operating temperature, while higher operating temperature makes the adsorbed oxygen species reduce and hence limits gas response towards nitrogen oxide gas [33]. Therefore, the optimal operating temperature of 300 1C is chosen for all other gas sensing tests.

J. Xiao et al. / Materials Science in Semiconductor Processing 40 (2015) 463–467

467

Acknowledgment

Resistance (ΚΩ)

8000

This work was supported by the Scientific Research Project of Education Department of Liaoning Province (L2013203), the Natural Science Foundation of Liaoning Province (2014025014), the Fundamental Research Funds for the Central Universities (3132015219) and the Scientific Public Research Foundation of Liaoning Province (No. 2013003007).

6000 4000 2000

References

0

100

200

300 400 Time (s)

500

600

Fig. 7. Response–recovery curves of the Bi2WO6 sensors to different nitrogen oxide concentrations.

Fig. 6 shows the variation of response of Bi2WO6 sensor with nitrogen oxide concentration in the range of 10– 50 ppm. It is obvious that the responses of Bi2WO6 sensor almost linearly increase as the nitrogen oxide concentrations increase, which favors the design of readout signal circuits [34]. Fig. 7 shows the response–recovery curves of the Bi2WO6 sensor to nitrogen oxide at different concentrations. The Bi2WO6 sensor exhibits fast response to 10, 20, 30, 40, and 50 ppm nitrogen oxide, and the response times are all less than 1 s. As we know, when Bi2WO6 are exposed in air, oxygen molecules adsorbed on the surfaces of Bi2WO6 sensor will trap electrons in the conduction band and form oxygen species (O  , O2 ), which will increase the resistance of Bi2WO6 sensor. Since nitrogen oxide with an unpaired electron is unstable state, and usually acts as a reducing gas, nitrogen oxide easily reacts with oxygen species to become stable NO2 and decrease the resistance of Bi2WO6 sensor when nitrogen oxide is introduced to the sensor. The fast response speed suggests that the diffusion of nitrogen oxide and its oxidation by oxygen species (O  , O2 ) are very rapid in hierarchical structure [35,36]. The recovery time upon exposure to 10, 20, 30, 40 and 50 ppm nitrogen oxide are 10 s, 12 s, 13 s, 14 s and 15 s, respectively, which are still short enough for practical application. 4. Conclusions In summary, we have synthesized hierarchical Bi2WO6 microspheres using a simple hydrothermal method. The responses of the as-prepared Bi2WO6 sensor to 20 ppm nitrogen oxide increase with operating temperature and then reach a maximum, and the optimal operating temperature is 300 1C. The Bi2WO6 sensor reveals fast response and recovery time for the detection of nitrogen oxide concentration in the range of 10–50 ppm, which may be attributed to their unique hierarchical structure, implying the Bi2WO6 is promising sensor materials for nitrogen oxide detection.

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