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Inorganics 2016, 4(3), 24; doi:10.3390/inorganics4030024

Article
Mesoporous WN/WO3-Composite Nanosheets for the Chemiresistive Detection of NO2 at Room Temperature
1
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
2
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA
*
Author to whom correspondence should be addressed.
Academic Editor: Rainer Niewa
Received: 13 June 2016 / Accepted: 21 July 2016 / Published: 26 July 2016

Abstract

:
Composite materials, which can optimally use the advantages of different materials, have been studied extensively. Herein, hybrid tungsten nitride and oxide (WN/WO3) composites were prepared through a simple aqueous solution route followed by nitriding in NH3, for application as novel sensing materials. We found that the introduction of WN can improve the electrical properties of the composites, thus improving the gas sensing properties of the composites when compared with bare WO3. The highest sensing response was up to 21.3 for 100 ppb NO2 with a fast response time of ~50 s at room temperature, and the low detection limit was 1.28 ppb, which is far below the level that is immediately dangerous to life or health (IDLH) values (NO2: 20 ppm) defined by the U.S. National Institute for Occupational Safety and Health (NIOSH). In addition, the composites successfully lower the optimum temperature of WO3 from 300 °C to room temperature, and the composites-based sensor presents good long-term stability for NO2 of 100 ppb. Furthermore, a possible sensing mechanism is proposed.
Keywords:
WN/WO3 composite; nanosheets; gas sensor; NO2; room temperature

1. Introduction

Gas sensors have been in increasing demand in recent years because of their wide applications, not only in environmental monitoring, but also in human health [1,2,3,4]. Metal oxide semiconductors (MOSs) based gas sensors have been intensely investigated for their high sensitivity, excellent selectivity, good portability, and low cost [5,6,7]. However, for detecting lower concentrations of toxic and disease signal gases, composite materials have shown greater potential than bare materials which exhibit low or no sensitivity in low gas concentrations (sub-ppm), as well as poor selectivity [8,9]. These composite materials include heterostructural materials, doped materials, and noble metal decorated materials, among which the heterostructural materials have been widely investigated because of super injection of carriers [10,11]. When different materials hybridize to form a heterojunction, an electron depletion layer (EDL) or an electron accumulation layer (EAL) will form at the interfaces. It is well acknowledged that the heterojunctions can serve as a lever in electron transfer, which can facilitate or restrain the electron transfer, resulting in enhanced sensing performance of a gas sensor [12]. Thus, designing composite sensing materials with heterostructures is regarded as one of the best strategies to achieve excellent gas sensing characteristics.
In the past few years, various sensing materials have been prepared that have exhibited excellent gas sensing characteristics [13,14]. However, to date and to the best of our knowledge, there have been very scarce reports about heterostructural metal nitride and oxide composites for sensing applications [15]. Metal nitrides can have varied properties: from metal-like, to semiconducting, to that of insulators. Thus, introducing metal nitrides into metal oxides could tune electrical properties that have great influence on gas sensing characteristics, such as carrier mobility and electrical resistivity [16]. In addition, other reports have demonstrated that establishing heterojunction could largely decrease the working temperature of sensing materials [17,18,19]. Thus, designing composites consisting of metal nitrides and oxides, and innovatively applying the composites as gas sensor materials, is of vital significance.
Tungsten trioxide (WO3) is a multifunctional material that has been widely investigated as a photocatalyst [20,21] and a gas sensor [22,23]. When applied as a gas sensing material, WO3 exhibits sensitivity to several gases, and has especially high-sensitivity to NO2, making it an ideal material for detecting NO2 [23,24,25]. However, because of its wide band gap, (2.5–3.5 eV) WO3-based sensors have to work at high working temperature or with the assistance of ultraviolet light [19,26,27]. Therefore, efforts should be focused on lowering working temperature while achieving optimal sensitivity. Tungsten nitride (WN) shows metallic properties with a resistivity of 10.9 × 10−5 Ω·m at room temperature [28]. Therefore, synthesis of WN-composite WO3 materials for detecting NO2 gas at low working temperature may be feasible and is highly desirable.
Herein, for the first time, we present an applicable strategy for the efficient synthesis of new gas sensing materials made of hybrid WN/WO3 composites. The strategy includes the synthesis of a porous WO3 nanosheets precursor and subsequent transformation to WN/WO3 composites by thermal annealing of the as-prepared precursor in NH3. The WN/WO3 composites sensing materials exhibit sensitive, selective and reliable detection of NO2 at room temperature.

2. Results and Discussion

2.1. Structure, Composition and Morphology

Powder X-ray diffraction (XRD) was used to characterize the phase composition and purity of the as-synthesized products. The yellow powder (precursors) obtained from the oil bath reaction agreed well with reported orthorhombic tungsten oxide hydrate (WO3·H2O, JCPDS No. 84-0886), as shown in Figure 1a (Pattern A) [29]. The WO3·H2O precursors were converted into WN and WN/WO3 composites by nitridation in NH3 atmosphere and oxidation in air, under heat treatment at 700 °C and 200 °C for 3 h, respectively. Figure 1a (Patterns B and C) shows the XRD of WN and WN/WO3 composites. As shown in pattern b, all the diffraction peaks matched well with those of hexagonal WN (JCPDS No. 65-2898). For the composite (Pattern C), the crystal phase was a mixture of hexagonal WN (JCPDS No. 65-2898), a big convex appearing in the position of 2θ 20°–30°, and a small convex in the position of 2θ 50°–60°, which can be attributed to amorphous WO3. Thus, the final product is a composite material composed of hexagonal WN and amorphous WO3 (WN/WO3). The composites underwent a mass change during the transformation from WN to WO3 by calcination in air. By using this mass change, we can calculate the ratio of WN to WO3. Thermogravimetric analyses of the products at temperature range of 0–1000 °C are shown in Figure 1b. The DSC-TGA results of the WN/WO3 composite nanosheets are displayed in Figure 1b. It was observed from the TGA curve that the specimen shows a weight loss of about 1%, which relates to adsorbed water and other trace substances. When the temperature reached about 211.5 °C, the weight of the specimen began to increase. When the temperature reaches about 457 °C, the weight nearly reaches stability. Moreover, the DSC curve exhibited an obvious exothermal peak at 426 °C, which corresponded to the obvious weight gain of approximately 8.86% between 211 and 457 °C in the TGA curves. The significant weight gain was attributed to the reaction of the WN with oxygen to generate WO3. According to the weight gain of approximately 8.86%, the WN content of the composites is calculated to be 55.9%.
The morphology of WN/WO3 composites was characterized through SEM and TEM analysis, and the results are shown in Figure 2. According to Figure 2a–c, the WN/WO3 composites exhibited square shaped nanosheets with uniform lateral dimensions mainly ranging from 80 to 150 nm (Figure 2a,b) and thickness of 10 to 30 nm (Figure 2c). In addition, relatively homogeneous pores with average diameter of 10 nm were dispersed uniformly at the surfaces of WN/WO3 composites. The porous feature may have contributed to the volume shrinkage caused by transformation of orthorhombic WO3·H2O to hexagonal WN. Figure 2d shows the HRTEM image of the WN/WO3 composites, where the lattice fringes d-spacings of 0.239 and 0.204 nm are identified and correspond to the (111) and (200) planes of hexagonal WN (JCPDS No. 65-2898), respectively. The inset of Figure 2d presents the selected area electron diffraction (SEAD) of WN/WO3 composites. The distinctive diffraction rings and spots on the SEAD pattern match the crystal structure identification obtained from the HRTEM and XRD. The diffraction rings are indexed to cubic WN (JCPDS No. 65-2898) (111), (200), (220), (311), (400), and (331) planes, respectively.

2.2. Gas Sensing Characteristics

Dynamic sensing performance of WN/WO3 composites-based sensors to various concentrations of NO2 at room temperature is shown in Figure 3a. NO2 is an oxidizing gas, so when an NO2 molecule adsorbs on the surface of WN/WO3 composites, it captures electrons from the composites, resulting in the increase of resistance of the composites-based sensors upon exposure to NO2. Figure 3b plots the sensor response as a function of NO2 concentration, and shows that with an increase in NO2 concentration, the response of the sensor also exhibits an increasing trend. Interestingly, the sensitivities to NO2 concentrations from 5 to 100 ppb and 100 to 1000 ppb exhibit linear dependence, fitting well into two linear curves which have different slopes. It is well understood that electrical conductance shows strong dependence on carrier and mobility (σ = neμn, where n is the concentration of electrons, e is electronic charge, and μn is electron mobility), and that electrical resistivity is the reciprocal of electrical conductance. When the WN/WO3 composites sensor is exposed to low concentration of NO2, the amount of adsorbed NO2 is so sparse that the degradation of carrier could be almost negligible, and the response of the sensor exhibits a “high increase model”. However, when exposed to high concentration of NO2, the effect of NO2 adsorption will be enhanced, and the degradation of carrier mobility becomes relatively remarkable, resulting in a decrease in the rate of sensitivity increase under abundant NO2 adsorption [30]. The detection limit of the WN/WO3 composites sensor for NO2 is calculated to be approximately 1.28 ppb based on signal-to-noise ratio of 3 (see the Section “Calculation of Theoretical Limit of Detection Using Signal/Noise Ratio” in Supplementary Materials), which is far below the immediately dangerous to life or health (IDLH) values (NO2: 20 ppm) defined by the U.S. National Institute for Occupational Safety and Health (NIOSH). The recommended NO2 exposure limit defined by the NIOSH is 1 ppm [31].
Selective detection of target gas from various interference gases is still an unsolved drawback for chemiresistive gas sensors [32]. The responses of the sensor based on WN/WO3 composites to common vehicle exhaust, including SO2, CO2, moisture, CO, NH3, ethanol and NO2, were measured to quantify the selectivity, which is shown in Figure 4. As clearly illustrated in Figure 4, the sensor shows a high response to NO2, which is more than 2 times higher than that for other interference gases, indicating the excellent selectivity to NO2 as opposed to other selected interference gases. The selective detection to NO2 gas might be due to the high reactivity and large electron affinity (2.28 eV) of NO2, in comparison with pre-adsorbed oxygen (0.43 eV) and other test gases [33]. In addition, the responses of the sensor based on WN/WO3 composites to 200 ppb NO2 at RT under different humidities were tested and the result is shown in Figure S1. It can be observed that, with increasing relative humidity, the response of the sensor decreases because of the competition between water molecules and NO2 molecules for the reacting sites [34]. However, the sensor shows a high response of 17.8 even at high humidity of 90 RH%.
Figure 5a shows the response and recovery times of the WN/WO3 composites-based sensor. With increasing NO2 concentration, the response time and recovery time both experience a decrease. The reason why the response and recovery times are substantially longer at low concentration is unclear and needs further study. The following can be considered as a plausible explanation. At low concentration of ambient NO2, the NO2 molecules occupy a lower percentage of air relative to oxygen molecules, so the probability of NO2 molecules arriving and being captured by the surface of the sensing materials (WN/WO3) is relatively smaller, resulting in a long response. When the concentration of NO2 increases, the probability of NO2 molecules arriving and being captured by the surface of sensing materials becomes higher, decreasing the response time. As for the recovery time: at low concentration of NO2, the amount of adsorbed NO2 is too small, making it difficult to desorb completely, resulting in a relatively longer recovery time. When the concentration is higher, the concentration difference between the surface of WN/WO3 and ambient atmosphere is large enough to desorb NO2 molecules quickly, reducing the recovery time. Similar observations have also been reported by other investigators [35,36]. In addition, the sensor shows excellent reversibility properties with a response time of less than 80 s and a recovery time of less than 180 s, regardless of the NO2 concentration. Stability is another key quality indicator in the development of gas sensors for real markets. Thus, we measured the response of the WN/WO3 composites-based sensor for 7 weeks, once a day (every 24 h) at room temperature for a week and then once a week for 6 weeks, shown in Figure 5b. We can observe that the sensor experiences a loss in response of less than 12.27% after 7 weeks of aging. The response of the sensor comes into saturation after 2 weeks. The results indicate that the WN/WO3 composites sensor exhibits good long-term stability.
Dynamic sensing transients of WN/WO3 composites and WO3 nanosheets to 100 ppb NO2, at room temperature and 300 °C (the optimum working temperature of WO3, data shown in Figure S2) respectively, are shown in Figure 6. It can be observed that the Ra of WN/WO3 composites is lower than that of WO3 nanosheets, which confirms that the introduction of WN into WO3 can decrease the resistance of the materials. In addition, the responses of WN/WO3 composites and WO3 nanosheets sensors to 100 ppb NO2 were 23.7 and 6.2, respectively, which suggests that gas sensing responses towards NO2 can be enhanced by partial nitrogenizing of WO3. Furthermore, the introduction of WN into WO3 apparently decreases the working temperature from 300 °C to room temperature.

2.3. Gas Sensing Mechanism

Typically, the gas sensing mechanism of WO3 belongs to surface-controlled type, where the adsorption and desorption of NO2 molecules on the surface of WO3 play a vital role [24]. In ambient air, the oxygen molecules will adsorb on the surface of WO3 and trap electrons from the conduction band of WO3 to form ionic oxygen species (O2−, O and O2−), decreasing the electron concentration of the surface of WO3, resulting in a high-resistance depletion layer at the surface [37]. When the NO2 gases come in, the NO2 molecules, which are highly electrophilic, will capture electrons from the conduction band of WO3, and will also react with the adsorbed ionic oxygen species. This will lead to a further decrease in electron concentration, increasing the resistance of WO3. The reactions taking place on the surface of WO3 are as follows.
O2 (g) → O2 (ads)
O2 (ads) + 2e → O2− (ads)
NO2 (g) + e → NO2 (ads)
NO2 (g) + O2− (ads) → NO22− (ads) + O2 (g)
The as-fabricated WN/WO3 composites-based sensor, when compared with WO3 sensor, exhibited enhanced sensing performances. The reasons why WN/WO3 composites-based sensor shows enhanced gas sensing characteristics are still unclear and worthy of further investigation. The following can be considered as a plausible explanation based on our experimental results. Firstly, WN exhibits metallic properties with a high electron concentration of about (5–6) × 1020 cm−3 and a resistivity of (1–2) × 10−1 Ω at room temperature [38]. However, WO3 is an n-type semiconductor, whose resistivity is far larger than that of WN. Thus, the electrons will transfer from the high concentration side to the low concentration side because of diffusion effect, resulting in a lower resistivity of the composites when compared with bare WO3. The lower baseline of resistivity of WN/WO3 composites will eventually increase the response (S = (Rg/Ra − 1) × 100%) of the sensor. In addition, WN and WO3 themselves did not show any sensing properties for NO2 at room temperature (Figure S3). However, the WN/WO3 composites exhibit excellent sensing characteristics to NO2 at room temperature. This may largely prove that there might be some synergetic effect in our WN/WO3 composites, leading to room temperature sensing for NO2.

3. Materials and Methods

3.1. Materials

Sodium tungstate dihydrate (Na2WO4·2H2O, AR) of ≥99.5% purity was purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). Hydrochloric acid (HCl) and oxalic dihydrate (H2C2O4·2H2O) of analytical reagent grade were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

3.2. Synthesis

Na2WO4·2H2O (3.2985 g, 0.01 mol) was dissolved in 50 mL deionized water with continuous stirring for 30 min at room temperature. 50 mL of HCl aqueous solution (2 M) was added dropwise into the above solution under continuous stirring. A certain amount of H2C2O4·2H2O, with a mole ratio of 0.25 Na2WO4·2H2O to H2C2O4·2H2O, was then added into the above system. The reaction vessel was then transferred to a 90 °C oil bath for 3 h. The yellow precipitate obtained was filtered and washed with deionized water and absolute ethanol several times, and subsequently dried at 60 °C in a vacuum oven for 24 h. Then, the yellow powder was nitrided at 700 °C at a ramp rate of 10 °C·min−1 under NH3 gas flow rate of 300 sccm (standard-state cubic centimeter per minute) for 3 h. Finally, the nitride was calcined at 420 °C in air for 3 h at a ramp rate of 10 °C·min−1.

3.3. Materials Characterizations

X-ray diffraction (XRD) analysis was conducted on a Rigaku MiniFlex 600 powder X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (λ = 1.5418 Å, accelerating voltage 40 kV, applied current 15 mA) at scanning rate of 1 °/min. Scanning electron microscopy (SEM) images were performed on a JSM-7800F (Japan Electron Optics Laboratory Co., Ltd., Tokyo, Japan) instrument (accelerating voltage 3.0 kV). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were conducted on an FEI TecnaiG2 F30 (Japan Electron Optics Laboratory Co., Ltd.). Differential Scanning Calorimetry (DSC) and thermal gravimetric analysis (TGA) were done on a NETZSCH/STA 449 F1 (Netzsch, Selb, Freistaat Bayern, Germany).

3.4. Fabrication and Measurement of Gas Sensor

Typically, 50 mg·mL−1 WN/WO3 composites-ethanol solution was deposited on the surface of the device and calcined at 100 °C for 3 h. Gas sensing performances were measured by a homemade sensor testing system (a cylindrical glass chamber with a volume of 100 mL). A gas mixing line equipped with mass flow controllers was designed to prepare target gases at specific concentrations in the testing chamber as shown in Figure S4. The resistance changes of sensor in air or tested gas were monitored by a high-resistance meter (Victor, 86E, Shenzhen, China). The response value (S) was defined as S = (|RgRa|/Ra) × 100%, where Ra and Rg denoted the resistance of the sensors in the absence and presence of the target gases (reducing gases), respectively. The time taken by the sensor to achieve 90% of the total resistance change was defined as the response time in the case of response (target gas adsorption) or the recovery time in the case of recovery (target gas desorption).

4. Conclusions

In conclusion, we successfully synthesized WN/WO3 composites through a simple strategy, and innovatively utilized them as gas sensing materials. The WN/WO3 composites-based sensor exhibited sensitivity and high selectivity to NO2 at room temperature. Overall, the excellent NO2 sensing performance of the WN/WO3 composites-based sensor supplies exciting opportunities for environmental monitoring and disease diagnosing. Furthermore, we expect our findings to bring up new promising gas sensing materials, and to inspire rational synthesis of other transition metal nitride and oxide hybrids for high-performance gas sensing.

Supplementary Materials

The following are available online at www.mdpi.com/2304-6740/4/3/24/s1, Figure S1: Response of WN/WO3 composites sensor at RT upon exposure to 200 ppb NO2 concentration at various relative humidities (RH), Figure S2: Response of the sensor based on WO3 nanosheets to 100 ppb NO2 as a function of the operating temperature, Figure S3: Resistances of the sensors based on WN and WO3 to 100 ppb NO2 at room temperature, Calculation of theoretical limit of detection using signal/noise ratio, Figure S4: The schematic illustration of (a) gas sensing analysis system and (b) gas mixing line equipment.

Acknowledgments

This work is supported by National Natural Science Foundation of China through grant 21471147 and Liaoning Provincial Natural Science Foundation through grant 2014020087. Minghui Yang would like to thank the National “Thousand Youth Talents” program of China.

Author Contributions

The preparation of the manuscript was made by all authors. Fengdong Qu and Minghui Yang conceived and designed the experiments; Fengdong Qu and Bo He performed the experiments; Fengdong Qu, Bo He and Rohiverth Guarecuco analyzed the data.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Powder X-ray diffraction (XRD) patterns of tungsten oxide hydrate (WO3·H2O) A, tungsten nitride (WN) B and hybrid tungsten nitride and oxide (WN/WO3) composites C; (b) differential scanning calorimetry-thermal gravimetric analysis (DSC-TGA) curves of WN/WO3 composites.
Figure 1. (a) Powder X-ray diffraction (XRD) patterns of tungsten oxide hydrate (WO3·H2O) A, tungsten nitride (WN) B and hybrid tungsten nitride and oxide (WN/WO3) composites C; (b) differential scanning calorimetry-thermal gravimetric analysis (DSC-TGA) curves of WN/WO3 composites.
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Figure 2. SEM images (a); top/side-view transmission electron microscopy (TEM) images (bc); and high-resolution transmission electron microscopy (HRTEM) images of WN/WO3 composites (d). Inset: selected area electron diffraction (SEAD) pattern of WN/WO3 composites.
Figure 2. SEM images (a); top/side-view transmission electron microscopy (TEM) images (bc); and high-resolution transmission electron microscopy (HRTEM) images of WN/WO3 composites (d). Inset: selected area electron diffraction (SEAD) pattern of WN/WO3 composites.
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Figure 3. (a) Dynamic sensing performance of WN/WO3 composites sensor to NO2 with concentration from 20 to 1000 ppb at room temperature (RT); (b) calibration curves of WN/WO3 composites sensor towards various concentrations of NO2 at RT.
Figure 3. (a) Dynamic sensing performance of WN/WO3 composites sensor to NO2 with concentration from 20 to 1000 ppb at room temperature (RT); (b) calibration curves of WN/WO3 composites sensor towards various concentrations of NO2 at RT.
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Figure 4. Cross-response of WN/WO3 composites sensor to various gases.
Figure 4. Cross-response of WN/WO3 composites sensor to various gases.
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Figure 5. (a) Response and recovery time of WN/WO3 composites sensor to various concentrations of NO2 at RT. (b) Stability measurement of the WN/WO3 composites sensor to 100 ppb NO2 at RT for 7 weeks.
Figure 5. (a) Response and recovery time of WN/WO3 composites sensor to various concentrations of NO2 at RT. (b) Stability measurement of the WN/WO3 composites sensor to 100 ppb NO2 at RT for 7 weeks.
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Figure 6. Dynamic sensing transients to 100 ppb NO2 of (a) WN/WO3 composite nanosheets sensors at room temperature; and (b) WO3 nanosheets sensors at 300 °C.
Figure 6. Dynamic sensing transients to 100 ppb NO2 of (a) WN/WO3 composite nanosheets sensors at room temperature; and (b) WO3 nanosheets sensors at 300 °C.
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