Mesoporous WN/WO₃-Composite Nanosheets for the Chemiresistive Detection of NO₂ at 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 = neμ_n, where n is the concentration of electrons, e is electronic charge, and μ_n 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 R_a 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)

(1)

O₂ (ads) + 2e⁻ → O²⁻ (ads)

(2)

NO₂ (g) + e → NO₂ (ads)⁻

(3)

NO₂ (g) + O²⁻ (ads) → NO₂²⁻ (ads) + O₂ (g)

(4)

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 = (R_g/R_a − 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 = (|R_g − R_a|/R_a) × 100%, where R_a and R_g 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.