Doping Metal Elements of WO3 for Enhancement of NO2-Sensing

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Doping metal elements for enhancement of WO3 to NO2 sensing performance at room temperature Shouli Bai, Yaqiang Ma, Xin Shu, Jianhua Sun, Yongjun Feng, Ruixian Luo, Dianqing Li, and Aifan Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03055 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017

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Industrial & Engineering Chemistry Research

Doping metal elements for enhancement of WO3 to NO2 sensing performance at room temperature

Shouli Bai,a Yaqiang Ma,a Xin Shua, Jianhua Sun,a,b Yongjun Feng*,a, Ruixian Luo,a Dianqing Li*,a, Aifan Chena a

State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Environmentally Harmful Chemicals Analysis, Beijing University of Chemical Technology, Beijing 100029, China b

Guangxi Key Laboratory of Petrochemical Resource Processing and Process

Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China

1

ACS Paragon Plus Environment


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ABSTRACT: WO3 nanoparticles doped with Sb, Cd and Ce were synthesized by a chemical method to enhance the sensing performance of WO3 to NO2 at room temperature. The doping of Sb element can significantly enhance the NO2-sensing properties of WO3. Upon exposure to 10 ppm of NO2, particularly, the 2 wt% Sb-doped WO3 sample exhibits a 6.8-times higher response and an improved selectivity at room temperature compared with undoped WO3. The enhanced NO2-sensing mechanism of WO3 by doping was discussed in detail, which is mainly ascribed to the increase of oxygen vacancies in the doped WO3 samples as confirmed by Raman, PL and XPS spectra. Besides, the narrower band gap may also be responsible for the enhancement of response as observed from the corresponding UV-Vis spectra. Keywords: WO3; Sb-doping; oxygen vacancies; NO2; room temperature sensor

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1. INTRODUCTION In recent years, there has been increasing interest in the detection of toxic gases due to the deterioration of our living environment.1-3 NO2 is one of toxic gases emitted from vehicles and industrial processes, which is the main source of acid rain and photochemical

smog.4-5

Although

some

traditional

methods

like

gas/liquid

chromatography, gas detection tube, and electrochemistry analysis are powerful for gas detection, the expensive equipment and complex detection process seriously limit their wide applications.6-10 Gas sensors have been viewed as one of the most effective detecting devices due to low cost, fast and on-line detection.11-13 Especially the sensors operating at room temperature are much desired owing to low power consumption, high safety, and long life time, which mainly depends on the corresponding gas-sensing materials. Therefore, it is highly desirable and important to develop excellent gas-sensing materials to monitor NO2 at room temperature.14-16 Recently, tungsten oxide (WO3), as one of important n-type semiconductors with a wide band-gap of 2.6-2.8 eV, has attracted extensive interest due to high sensing response to NO2 because of the existence of the intrinsic non stoichiometry (WO3−x) with native oxygen vacancy point defects.

17-19

However, WO3 generally exhibits very low response to NO2 at room temperature. Up to date, some techniques have been developed to enhance NO2-sensing performance of WO3 at room temperature such as forming composite with carbon materials,20-22 and metal oxides (e.g. NiO)23. For example, Jie et al.21 reported the graphene-wrapped WO3 composite with an improved response value from 1.4 to 56 ppm NO2 at room temperature. 3



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Yet, it remains a big challenge and an increasing interest to explore NO2-sensing materials with high sensitivity and high selectivity at room temperature. In this work, we prepared WO3 nanoparticles and three metal (Sb, Cd and Ce) doped WO3 samples as NO2-sensing materials by a chemical method, and carefully investigated the corresponding sensors’ performance as well as the improved NO2-sensing mechanism. The obtained results demonstrate that the 2 wt% Sb-doped WO3 sample is one of the promising NO2-sensing materials for room-temperature sensor, and moreover doping of non-noble metal elements is a facial and effective way to enhance the gas-sensing properties of semiconducting metal oxides. 2. EXPERIMENTAL SECTION 2.1 Preparation of doped-WO3 nanoparticles Sb, Cd, Ce-doped WO3 nanoparticles were synthesized by a chemical method. For Sb-doped WO3, typically, 1 g of sodium tungstate dehydrate (Na2WO4·2H2O, analytical grade) was dissolved in 60 mL of deionized water, and certain amounts (Sb/WO3=1.0, 2.0, 3.0 and 5.0 wt%) of antimony trichloride (SbCl3) was added in the W-containing solution. Subsequently a certain aqueous solution of HCl was added to the above W, Sb-containing solution under stirring at 25 oC for half an hour. Later the resulting solution was aged at 25 oC for another 24 h, and then 15 mL of cetyltrimethyl ammonium bromide (CTAB, 0.15 mol/L) solution was added to the above solution to produce flocculent precipitate. Finally the Sb-doped WO3 samples were collected after ultrasonication treatment for 40 min, washing and centrifugation with deionized water and ethanol for five cycles, drying 4


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at 50 oC overnight, and calcination at 600 oC for 2 h. Cd-, and Ce-doped WO3 samples were prepared following the same procedure with that for Sb-doped WO3 using Cadmium nitrate (Cd(NO3)2·4H2O) and Cerium nitrate (Ce(NO3)3·6H2O) as the corresponding resource of Cd, and Ce as well as undoped WO3 sample was obtained without any addition of metal salts in solution. 2.2 Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-2500 X-ray diffractometer with Ni-filtered Cu Kα radiation (λ= 0.15406 nm) at 30 kV and 100 mA with the scanning speed of 10o min−1 for 2θ in a range of 10–70o. Morphologies and elemental composition analysis were examined on Field emission scanning electron microscopy (FESEM, Hitachi S-4700, and 20.0 kV) with an attached EDX spectrometer. Raman spectra were measured using a Raman spectrometer (HR-800, λexc = 532 nm). UV–Vis absorption spectra in methanol solvent were recorded in the range of 200–800 nm on a Shimadzu UV-2550 UV–Vis spectrophotometer. Photoluminescence (PL) spectra were recorded from 350 to 600 nm with a 325 nm excitation (RF-5301PC spectrometer). X-ray photoelectron spectra (XPS) were examined on a VG ESCALAB-MK electron spectrometer with Al Kα as the excitation source. 2.3 Sensor fabrication and response measurement The undoped WO3 and Sb, Cd, Ce-doped WO3 nanoparticles were individually mixed with ethanol to form the corresponding paste, and then the paste was drop-coated onto the surface of a ceramic tube with Pt electrodes and a Ni-Cr 5



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heating coil that was inserted through the ceramic tube to construct the corresponding sensor. The sensors were aged at the certain temperature for several days. The gas sensing properties of these prepared sensors were measured using a JF02E gas sensor test system (Guiyan Jinfeng Technology Co., Kunming, China). NO2 was used as the test gas to evaluate the gas-sensing performance of samples at 20 oC. Test gases with different concentrations were introduced into an 18 L air chamber via syringes with various volumes, such as 5 mL, 10 mL, 20 mL, 50 mL, 100 mL etc. Meantime, the air in syringes was evacuated before sucking up test gases. After every test, the chamber was opened to diffuse test gas away. The resistance of the sensor in air (Rair) and in the air–test gas mixture (Rgas) were recorded to define the response for oxidizing gas of NO2 as the ratio of Rgas/Rair (the corresponding gas sensing test electric circuit is shown in Figure S1 in the Supporting Information). In order to confirm the repeatability, notably, three sensors were prepared for each gas-sensing material sample to evaluate the gas-sensing performance and the average value was presented in this work. The relative humidity was controlled by configuring various saturated salt solutions in the hermetic air chamber over 24h at the operating temperature before the gas-sensing measurements. In this work, for example, CH3COOK, MgCl2, K2CO3, NaBr, and KBr saturated solutions were used for the relative humidity value 25%, 30%, 40%, 60%, and 80% at 20 oC, respectively. 3. RESULTS AND DISCUSSION 6


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3.1 Structure, morphology and band gap Figure 1 shows the XRD pattern of WO3, 2%Sb-WO3, 2%Cd-WO3, and 2%Ce-WO3 with a typical orthorhombic WO3 phase (JCPDS no. 20-1324).24 No peak of Sb, Cd and Ce is observed due to the low concentration of doped elements. Compared with the WO3, the diffraction peaks of 2%Sb-WO3 obviously shifts to high angle, suggesting that Sb was doped into the lattice of tungsten oxide.25 Moreover, the diffraction intensities of 2%Cd-WO3 and 2%Ce-WO3 are stronger than that of WO3, probably resulting from the enhanced crystallinity for doped WO3 because the incorporation of Cd and Ce into the WO3 lattice network reduces the density of nucleation centers, and then favors the growth of crystal grains.26 To further analyze the variations of doped and undoped WO3 samples, lattice constants of each sample were systematically determined from the (001), (020), (200) peaks (see Table 1). The lattice strains ε of the samples have been calculated by using the following Eqn. (1):16 ε = β/4 tan θ

(1)

Among the investigated samples, 2%Sb-doped WO3 has the largest lattice strain while WO3 has the least one. The larger the lattice strain is, the more the structure defects are.

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Figure 1. XRD patterns of (a) WO3, (b) 2%Sb-WO3, (c) 2%Cd-WO3, and (d) 2%Ce-WO3.

Table 1. The lattice parameters and lattice strain of undoped and metal elements doped WO3 samples Lattice parameters (Å)

Lattice strain

Samples a

b

c

ε (%)

WO3

7.3558

7.5772

3.8580

1.94

2%Ce-WO3

7.3482

7.5654

3.8638

2.82

2%Cd-WO3

7.3080

7.5458

3.8475

3.00

2%Sb-WO3

7.3790

7.4925

3.8537

4.53

Figure 2 (a) and (b) exhibits the typical morphologies of WO3 samples as observed by scanning electron microscopy (SEM). Both of them are irregular nanoparticles with the

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diameter between ca. 100 and 200 nm. Ca. 0.48 wt% Sb was doped on WO3 as determined by EDX in Figure 2 (c).

Figure 2 FESEM micrographs of (a) WO3, (b) 2%Sb-WO3 and (c) EDX spectrum of 2%Sb-WO3. Figure 3 shows the UV-Vis absorption spectra and the plot of (αhν)2 versus (hν) of four investigated samples. All the samples exhibit an absorption band in the range of 300-500 nm. Furthermore, the band gap energy (Eg) is estimated from a plot of (αhν)1/2 versus photon energy (hν) based on αhν= A (hν − Eg) n/2, herein, α, ν, Eg, A and n are the absorption coefficient, light frequency, band gap energy, a constant, and 4 for indirect band gap semiconductor, respectively.

27, 28

, while its value can be obtained by the

intercept of the tangent to the X axis. The estimated indirect band gap of pure WO3 is 2.79 eV and that of 2%Sb-WO3 is the 2.65 eV. From Figure S2 we can draw the conclusion that band gap gradually decreases with the increase of Sb wt% in WO3 nanoparticles. The narrower band gap is caused by formation of impurity level.29, 30

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Figure 3 (a) UV-Vis absorption spectra; (b) the plot of (αhν)1/2 versus (hν) of undoped WO3, 2% Sb-, 2% Cd-, and 2% Ce-WO3 samples. 3.2 Structural defect analysis Figure S3 also confirms the formation of oxygen vacancies in WO3 samples. In the Raman spectra, four peaks result from lattice modes (133 and 183 cm-1), the W4+–O mode (272 cm-1), and the W5+–O mode (327 cm-1), respectively.31 Another two sharp peaks are attributed to two O–W6+–O stretching modes for WO3 with an orthorhombic phase: one located at 718 cm-1 is for a short W6+–O bond and the other at 807 cm-1 is for a long W6+–O bond.32 Among the four samples, 2%Sb-WO3 shows the weakest intensity of the peak located at 718 cm-1 for the W6+–O short mode, indicating the predominance of asymmetric vibration modes, which probably results from numerous defects or oxygen vacancies. Figure S4 further demonstrates the PL spectra of WO3, 2%Sb-, 2%Cd- and 2%Ce-WO3 recorded with an excitation wavelength of 325 nm. All of the samples present luminescence in both the ultraviolet and the visible regions. The near-UV

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photoluminescence band is described as the near band edge emission (NBE), which is for the recombination of electrons in the conduction band and holes in the valence band.33 The visible PL band is mainly attributed to defects such as oxygen vacancies, which are related to deep level or trap state emissions.34 High intensity blue emission peaks occurred at 405 nm and 432 nm are ascribed to defects such as oxygen deficiencies or vacancies. Among the four samples, undoped WO3 shows the lowest visible emission intensity, while 2%Sb-WO3 exhibits the highest visible intensity, indicating the most defects among the investigated samples. EDX was used to detect the existence of W, O and Sb elements in WO3 sample, which indicates that the Sb element has been doped and no other impurity was imported in prepared sample. In addition, XPS surface analysis was carried out at the same time to further determine the chemical compositions and the defect states of the sample. The content of Sb is measured to be 1.93wt% in WO3, which is close to the feeding value in the synthesis process. Figure 4 displays high-resolution XPS spectra of W4f and O1s for the undoped WO3 and 2%Sb-WO3. Both of W4f core-level spectra in Figure 4 (a) and (b) are decomposed into two components (W6+ and W5+). The peak positions corresponding to W6+ state occur at 35.6 and 37.6 eV, while for the W5+ state these occur at 35.1 and 36.2 eV in the undoped WO3.35 Because the reduction of W6+ to W5+ relates to the changes of oxygen, the concentration of W5+ can indirectly reveal the content of oxygen vacancies. Actually, the molecular formula of WO3 could be written as WO3−x, where x represents the number of oxygen vacancies. By calculating the area ratio of W6+ to W5+ 11



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from XPS spectra, the x is confirmed to be 0.14 for the undoped WO3 and 0.24 for the 2%Sb-WO3, indicating that the molecular formulas of undoped WO3 and 2%Sb-WO3 should be written as WO2.86 and WO2.76, respectively.36 The improved non-stoichiometric feature of 2%Sb-WO3 implies its potential gas-sensing applications.37 The O 1s spectra in Figure 4 (c) and (d) are carefully deconvoluted into three peaks (Oa, Ob, and Oc) by using Gaussian fitting. The Oa peak at 530.3 eV is attributed to the lattice oxygen or inherent O associated with W. The Ob peak at 531.2 eV is associated with the oxygen-deficient region caused by oxygen vacancies and the Oc peak at 532.5 eV is assigned to the chemically absorbed oxygen site.38, 39 It is obvious that 2%Sb-WO3 possesses more Ob and Oc species than undoped WO3, for example, it has a high donor content (Vo) compared with undoped WO3. Many researchers have reported that the presence of oxygen vacancies on the surface results in increasing adsorption of gas molecules. That is to say, 2%Sb-WO3 can adsorb more oxygen on the surface. In addition, the chemical shifts of W4f and O1s to lower energy in Figure 4(e) and (f) suggest that Sb has been doped into WO3. In the case of doped samples, Sb3+ ions more easily substitute for W6+ ions in WO3 lattice because the ion radius of Sb3+ is closer that of W6+ than that of Cd2+ and Ce3+. When W6+ ions in the WO3 lattice are substituted by Sb3+, the defect equilibrium of the WO3 crystals is changed. Therefore, negative 3 of valence charges of the substituted W site must be compensated in the form of oxygen vacancy to maintain electrical neutral, which can be described in Eqn. (2):

12


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ௐைయ

ᇱᇱᇱ ܾܵଶ ܱଷ ሱۛሮ 2ܾܵ௪ + 3ܱ௢௫ + 6 ܸ௢•

(2)

ᇱᇱᇱ is a Sb ion sitting on a wolfram lattice site Where in the Kroger–Vink notation, ܾܵ௪

with three negative charges. The ܱ୭௫ denotes a neutral oxygen atom on oxygen site and ܸ௢• represents a vacancy on an oxygen site with positive charge of +1.40,

41

For

2%Sb-WO3, the substitution of Sb3+ for W6+ ions in the WO3 lattice changes the defect equilibrium of the WO3 crystals, that is, increase the oxygen vacancy, which may favor to enhances the NO2-sensing response. These results indicate that Sb doping effectively modify the surface state, such as by increasing the defects.

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Figure 4 W4f XPS spectra of (a) WO3 and (b) 2%Sb-WO3; O1s XPS spectra of (c) WO3 and (d) 2%Sb-WO3; (e) W4f XPS spectra of the pure and 2%-doped WO3; (f) O1s XPS spectra of undoped WO3 and 2%-doped WO3. 3.3 Gas sensing performance Figure 5 depicts the responses of the sensors based on the undoped WO3 and 2%Sb-WO3 samples to 1-10 ppm NO2 at 20 oC and relative humidity of 25%. The response of 2%Sb-WO3 based sensor to 10 ppm NO2 reaches 51, which is 6.8 times

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higher than that of undoped WO3 of 7.5, even though the response of undoped WO3 in the work is higher than that reported in literatures. A comparison of the sensing performances of the sensors in this work and reported in literatures is summarized in Table 2.23, 41-44 From the table, one observes that the sensors based on undoped WO3 and 2%Sb-WO3 to NO2 have higher response and lower working temperature than those reported in literatures. Therefore, they are the promising sensing materials for the detection of trace NO2.

Figure 5 Responses of sensors based on undoped WO3 and 2%Sb-WO3 to 1-10 ppm NO2 at 20 oC.

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Table 2 Responses of WO3 and its composites to NO2 at room temperature in the present work and those reported in the literatures. NO2

Operating

Sensor material concentration

Response

Reference

1.34

41

temperature Room temperature

WO3

100 ppm o

(RT, 38 C) o

NiO/WO3

10 ppm

RT ( 25 C)

3

23

WO3

15 ppm

RT (No description)

6

42

56 ppm

RT (25 C)

o

1.4

43

2 ppm

RT (25 C)

o

3.37

44

7.5

Present

51

work

Graphene-WO3 nanocomposites WO3 nanoparticles/porous silicon WO3 o

RT (20 C)

10 ppm Sb-WO3

Figure 6 (a) shows the responses of sensors based on undoped WO3, 2%Sb-, 2%Cdand 2%Ce-WO3 to 1-10 ppm NO2 at 20 oC and the relative humidity of 25%, which are derived from Figure 5 and Figure S5. Among them, 2%Sb-WO3 has the highest response to 10 ppm NO2, and the responses of Sb, Cd and Ce doped samples are all higher than that of undoped WO3. Thus we can draw a conclusion that doping is an effective way to improve the WO3 room-temperature NO2-sensing properties. Figure 6 (b) describes the 16


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responses for doped-WO3 with different mass loading of Sb to 10 ppm NO2 at 20 oC. The response of Sb-doped WO3 increases with the increase in the weight percentage of Sb and the optimized value is 2 wt% in the range of 0-4 wt%. It reveals that the Sb dopant plays an important role in the improved gas sensing performance for WO3, and the excess dopants hinder the carrier through the grain boundaries of the WO3 to reduce the gas sensing properties. Besides the high response, the selectivity is another key parameter for the gas-sensing sensors with practical applications. Figure 6 (c) represents the responses of undoped WO3 and 2%Sb-WO3 based sensors to 10 ppm of NO2 and 100 ppm of acetone, formaldehyde, NH3, ethanol and methanol at 20 oC. One observes that the sensor based on 2%Sb-WO3 has the highest response to NO2 compared with the other gases, which suggests that the sensor based on 2%Sb-WO3 has excellent selectivity to NO2 at room temperature, and 2%Sb-WO3 is one of promising gas-sensing materials for detection of trace NO2 at room temperature. Additionally, the humidity has an obvious influence on sensing response at temperature lower than 100 ◦C especially at room temperature. To determine the influence of humidity, we measured the sensor’s responses to 10 ppm NO2 under different relative humidity as shown in Figure 6 (d). The response obviously decreases with increasing of the relative humidity, but in the range from 20% to 40%, the response just shows a slight change. Therefore, the sensor was operated at relative humidity at 25%.

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Figure 6 (a) Responses of sensors based on undoped WO3, 2%Sb-, 2%Cd- and 2%Ce-WO3 to 1-10 ppm NO2 at 20 oC; (b) Responses of Sb-doped WO3 with different dopant of Sb to 10 ppm NO2 at 20 oC; (c) Responses of sensors based on undoped WO3 and 2%Sb-WO3 to different test gases at 20 oC; (d) Response of 2%Sb-WO3 to 10 ppm NO2 at 20 oC under different relative humidity.

3.4 Mechanism of gas sensing For n-type semiconducting metal oxides, the gas sensing mechanism is primarily governed by the surface adsorbed oxygen species which play important roles in sensing

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properties of sensor. The Sb doping causes larger lattice strain and narrower band gap, which is helpful to the enhancement of response because the dopant forms impurity energy between the conduction band and the valence band as confirmed by XRD and UV-Vis analysis, respectively. When the sample is exposed in air, oxygen molecules in air adsorb on the surface of WO3 and capture electrons from conduction band of WO3 to create oxygen species such as O2− (below 100 oC), O− (100–300 oC), and O2− (above 300 oC) as described in Figure 7.16,44 Meanwhile, the electron depletion layer and the voltage barrier are formed at surface of WO3. Once the sample is exposed in oxidizing gas of NO2, the NO2 molecules not only capture electrons from the conduction band of WO3 but also react with the oxygen species chemisorbed on the surface of WO3. NO2 has higher electronegativity than oxygen, which causes an increase of the electron depletion layer and the voltage barrier, and then leads to the increase in resistance of sensor in NO2, that is, the response increases according to the response definition of Rg/Rair for oxidizing gases. The reducing reactions in Eqn. (3-6) result in the increase of the corresponding resistance, which enhances the gas-sensing response of the doped sample. ି ܱଶ(௚） + ݁ ି ⟶ ܱଶ(ୟୢୱ)

(3)

ܱܰଶ(௚) ⟶ ܱܰଶ(௔ௗ௦)

(4)

ି ܱܰଶ(௔ௗ௦) + ݁ ି ⟶ ܱܰଶ(௔ௗ௦)

(5)

ܱܰଶ(௔ௗ௦) + O2− ⟶

ି ܱܰଶ(௔ௗ௦) + O2

(6) 19



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Figure 7. (a) Schematic diagram of sensing for Sb-doped WO3 based sensor exposed in air and NO2 gas, respectively; (b) the electron depletion layer and voltage of the sensor in air; (c) the electron depletion layer and voltage barrier of the sensor in NO2.

4. CONCLUSIONS In summary, WO3 nanoparticles with orthorhombic phase were successfully prepared by a chemical method, which exhibit a good gas-sensing performance to 10 ppm of NO2 at 20 oC. Among the investigated WO3, the sensor based on 2%Sb-WO3 exhibits the highest response to 10 ppm NO2, and the best selectivity to NO2 in 100 ppm of VOCs including acetone, formaldehyde, NH3, ethanol and methanol ethanol. Furthermore, the 2

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% Sb-WO3 sensor is more suitable for the NO2 detection with concentrations above 10 ppm at room temperature. The enhanced NO2-sensing response at room temperature is attributed to the increase of oxygen vacancies and the narrower band gap due to the doping of Sb in a certain range. Therefore, it is expected that the WO3 nanoparticles can be potentially used as one of promising sensing materials for detection of trace NO2 at room temperature by doping of inexpensive Sb metal element.

ASSOCIATED CONTENT Supporting information. The supporting information including five additional figures (gas sensing test electric circuit, band gap energy for different dopant Sb-WO3, Raman spectra, Room temperature photoluminescence (PL) spectra, and Responses of sensors) is available free of charge on ACS Publication website at DOI: 10.1021/acs.iecr.XXXX. AUTHOR INFORMATION Corresponding Author Fax and Tel: + 86-10-64436992. E-mail: [email protected], [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS

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Table of Contents

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Doping Metal Elements of WO3 for Enhancement of NO2-Sensing

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