Nanowires-assembled WO3 nanomesh for fast detection of ppb-level NO2 at low temperature

Hierarchical WO3 nanomesh, assembled from single-crystalline WO3 nanowires, is prepared via a hydrothermal method using thiourea (Tu) as the morphology-controlling agent. Formation of the hierarchical architecture comprising of WO3 nanowires takes place via Ostwald ripening mechanism with the growth orientation. The sensor based on WO3 nanomesh has good electrical conductivity and is therefore suitable as NO2 sensing material. The WO3 nanomesh sensor exhibited high response, short response and recovery time, and excellent selectivity towards ppb-level NO2 at low temperature of 160 ℃. The superior gas performance of the sensor was attributed to the high-purity hexagonal WO3 with high specific surface area, which gives rise to enhanced surface adsorption sites for gas adsorption. The electron depletion theory was used for explaining the NO2-sensing mechanism by the gas adsorption/desorption and charge transfer happened on the surface of WO3 nanomesh.


Introduction
NO 2 , as one of the most toxic and harmful gases in the atmosphere, is the precursor of acid rain and photochemical smog. It is generated by the fuel burning and has caused a seriously harmful effect to the living environment [1,2]. Thus, the development of NO 2 gas sensors with high sensitivity and low detect limitation is urgently necessary. At present, metal oxide semiconductors with various nanostructures, such as SnO 2 [3][4][5], ZnO [6,7], WO 3 [8,9], and TiO 2 [10], have been well investigated to detect NO 2 owing to their low cost, simple operation, and nontoxic nature. Among them, tungsten oxide, a n-type semiconductor with wide bandgap (2.6-3.2 eV), has been considered as a promising sensing material for the detection of NO 2 because of its low cost, high sensitivity, and good repeatability [11][12][13]. However, the operating temperature of WO 3 gas sensors is usually above 200 ℃ due to the poor electrical conductivity of some WO 3 nanostructures [14][15][16]. For example, WO 3 nanoparticles synthesized by spark ablation were required to detect NO 2 gas at a high operating temperature of 200 ℃ due to their high electrical resistance [15]. Triple-shelled WO 3 spheres synthesized by ultrasonic spray pyrolysis showed a large resistance (>1 GΩ) when exposed to NO 2 gas at low operating temperature [16]. A general strategy to solve this problem is surface functionalization with noble metals (Au, Ag, and Pt) [8,[17][18][19][20]. Gao's group reported Ag-loaded mesoporous WO 3 showed excellent response to NO 2 at 75 ℃ [17]. Recently, Zhao et al. [8] reported Au@WO 3 core-shell nanospheres showed an improvement in NO 2 sensing performance in terms of sensitivity, response/ recovery rate, and detection limit at 100 ℃. However, the method is not always effective, because the noble metal particles tend to catalytic poisoning and noble metal modification increases the fabricating cost [21]. Another way to achieve this is to increase their surfaceto-volume ratio by preparing hierarchical nanostructures. Hierarchical WO 3 nanostructures can increase the active adsorption sites for NO 2 molecules and these thin structural units can provide efficient electron transport pathways [22]. Many techniques have been used to synthesize WO 3 in various forms and investigate the effect of morphology on their gas-sensing performance. Various morphologies of WO 3 , such as nanofibers, nanoflowers, and nanoplates, have been prepared by a variety of methods including electrospinning, thermal evaporation, and hydrothermal and plasma spraying [23][24][25][26][27]. Compared to other synthetic techniques, hydrothermal method is more advantageous due to low-temperature synthesis and ease of mass production [28][29][30][31][32].
In this work, hierarchical tungsten trioxide nanomesh assembled by single-crystalline nanowires was fabricated via a facile hydrothermal method using thiourea (Tu) as the morphology-directing agent. Hexagonal WO 3 nanomesh has good electrical conductivity owing to the assembled nanowires promoting electron transport. The as-prepared WO 3 nanomesh exhibited high sensibility, short response and recovery time, and good sensing selectivity to NO 2 at a relatively low working temperature of 160 ℃. Such excellent performance can be attributed to the highly exposed surface area, which propels the interaction between NO 2 gas and sensing element. The sensing mechanism is explained in terms of the gas adsorption/desorption and electron transport between gas molecules and WO 3 nanomesh.

2 Synthesis of WO 3 nanomesh
In a typical procedure, 1.25 g tungstic acid was dissolved in 25 mL hydrogen peroxide solution heated in water bath at 90 ℃ with magnetic stirring. After stirring for 1 h and 20 min, the above solution was diluted to 60 mL with deionized water, then taking 42 mL of diluted solution and adding 168 mL deionized water and 14 mL HCl (3 mol/L) into above diluted solution. Next, Tu with different weight was added to above solution to achieve different concentration (0, 0.05, 0.1, and 0.2 M), followed by magnetic stirring for 20 min forming a suspension solution. Next, 30 mL of above solution was taken to 50 mL teflon-lined stainless steel autoclave. After heating in an electric oven at 180 ℃ for 12 h, the autoclave was naturally cooled down. The product was obtained by filtration, washed with enough deionized water and absolute ethanol, and dried under vacuum at 60 ℃ overnight. The as-prepared product was subsequently annealed in air at 300 ℃ for 2 h to improve the crystallinity.

3 Characterizations
The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-V2500 diffractometer with Cu Kα radiation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained with scanning electron microscope (SEM, Zeiss, Germany) and transmission electron microscope (TEM, JEM-2100F, Japan), respectively. X-ray photoelectron spectroscopy (XPS) was recorded with an ESCALAB 250Xi instrument (Thermo Fisher Scientific, USA). Raman spectra were carried out on a LabRAM HR800 Raman spectrometer with 632.8 nm laser radiation. The Brunauer-Emmett-Teller (BET) surface area and Barrett-Joyner-Halenda (BJH) pore sizeand pore distribution were analyzed by nitrogen adsorption using an American Contador QuadraSorb SI-MP instrument. The current-voltage (I-V) characteristics of the sensors were measured using an electrochemical workstation (CHI-660E, China).

4 Fabrication and measurement of gas sensors
The annealed powder sample was placed in an agate mortar and thoroughly ground after adding an appropriate amount of ethanol. The obtained sensing slurry was deposited on the cleaned Au interdigital electrodes by a thin brush. Then, the electrodes with sensing film were dried at 70 ℃ in air for 12 h. The gas-sensing properties www.springer.com/journal/40145 of the electrodes were measured in a computer-controlled gas-sensing testing system consisting of mass flow controllers (MFC, Beijing Qixing Co., Ltd., China), a test chamber with a heating plate, and a data-acquisition system. The electrical resistances of the sensors were recorded by a digital multimeter (Agilent 34465A), and the upper limit of the digital multimeter is 1.2 GΩ. The test NO 2 gas was mixed by 10 ppm NO 2 (Beijing AP BAIF Gases Industry Co., Ltd., China) and background gas. The flow rates of 10 ppm NO 2 and background gas were controlled using mass flow controllers. The total gas flow rate was fixed at 200 mL/min. The desired gas concentration is determined by the multiple of the NO 2 gas diluted by background gas. The sensor response was defined as R g /R a for oxidizing gas or R a /R g for reducing gas, where R g and R a are the resistances of the sensor in targeted gas and in air, respectively. The response and recovery time were defined as the time taken by the sensor to achieve 90% of the total resistance change.

1 Material characteristics
The crystal structure of the samples synthesized in precursor with different concentration Tu after heating treatment at 300 ℃ for 2 h was confirmed by XRD as illustrated in Fig. 1. The main diffraction peaks of the sample synthesized without adding Tu can be well indexed to the orthorhombic phase WO 3 ·0.33H 2 O (JCPDS No. 54-1012). With the increase of precursor Tu concentration, the diffraction intensity of the (111) peak for the orthorhombic WO 3 ·0.33H 2 O gradually decreases. When the sample is prepared in precursor with a Tu concentration of 0.2 M, the (111) peak for the orthorhombic WO 3 ·0.33H 2 O disappears and the main diffraction peaks can well correspond to the hexagonal phase WO 3 (JCPDS No. 85-2460). It is worth noting that the crystalline phase of the sample can be adjusted by the concentration of Tu in the precursor from the XRD results.
The morphology and microstructure of the samples after heating treatment at 300 ℃ for 2 h were examined by SEM measurement. As shown in Fig. 2(a), the sample  www.springer.com/journal/40145 synthesized in precursor without Tu consists of micronlevel flowers assembled with rectangular blocks. As shown in Fig. 2(b), the sample synthesized in precursor with 0.05 M Tu consists of micron-level flowers assembled with rectangular sheets. As shown in Fig. 2(c), the sample synthesized in precursor with 0.1 M Tu consists of urchin-shaped spheres assembled with needlelike units. As shown in Fig. 2(d), the sample synthesized in precursor with 0.2 M Tu consists of nanomesh assembled with nanowires. It is worth noting that the morphologies and microstructure of the samples can be adjusted by the concentrations of Tu in the precursor from the SEM results. Therefore, Tu, an anionic surfactant, acted as a structure-directing agent in the process of aggregation as well as a morphologydirecting agent by confining crystal growth in certain directions.
The I-V curves of the sensing devices based on annealed samples that were synthesized in precursor with 0.05, 0.1, and 0.2 M Tu were recorded by an electrochemical workstation. Figure 3 shows the I-V curves of the sensors measured from -5 to +5 V to investigate the electrical contact between the sensing material and the Au interdigital electrodes in air at room temperature (25±5 ℃). When the voltage increases, the current also increases linearly suggesting good electrical contact between the sensing material and electrodes. As shown in Fig. 3, the resistances of sensors based on annealed samples that were synthesized in precursor with 0.05, 0.1, and 0.2 M Tu could be calculated to be 424, 28, and 3 MΩ, respectively. The resistances of the sensing devices based on annealed samples decrease sharply with the increase of the concentration of Tu in precursor. The sensing device based on the annealed sample synthesized in precursor with 0.2 M Tu exhibits linear ohmic behavior and the best electrical conductivity. As a result, the annealed sample synthesized in precursor with 0.2 M Tu was selected as sensing material in the following characterization and gas sensing tests.
The high-magnification SEM and TEM images of the annealed sample synthesized in precursor with 0.2 M Tu are shown in Fig. 4. As shown in Figs. 4(a) and 4(b),   Figure 4(c) shows the TEM image of interlaced nanowires, whose diameters are tens of nanometers and lengths are over several micrometres. The corresponding selected area electron diffraction (SAED) pattern (the inset in Fig. 4(c)) exhibits the diffraction rings, corresponding to the (100), (002), and (111) planes of hexagonal WO 3 . Figure 4(d) shows that the fringe interval of 0.383 nm agrees well with the d-spacing of (002) lattice planes perpendicular to the long axis of the nanowire.
The surface chemical state of the WO 3 nanomesh was determined by XPS spectra. The C 1s peak of 284.8 eV was used to calibrate all binding energies. Figure 5(a) shows the high-resolution XPS spectrum of the O 1s. The intense peak at 530.4 eV can be assigned to lattice oxygen in WO 3 . Figure 5(b) shows the highresolution XPS spectrum of the W 4f. The small peak at 41.5 eV can be attributed to the W 5p 3/2 of WO 3 [17,33]. The two evident peaks at 35.5 and 37.7 eV, belonging to W 4f 7/2 and W 4f 5/2 of WO 3 , respectively, suggest that the tungsten oxide nanostructure exists as W 6+ [34][35][36].
The structural and electronic properties of the WO 3 nanomesh were further characterized by Raman spectroscopy. As shown in Fig. 5(c), Raman peaks centered at about 262, 326, 708, and 802 cm -1 can be observed clearly, which can be assigned to the fundamental modes of hexagonal WO 3 [37]. The sharp peaks located at about 262 and 326 cm -1 are assigned to the W-O-W bending mode; the evident peaks at about 708 and 802 cm -1 are attributed to the O-W-O stretching mode [38,39]. No peaks associated to impurities were observed indicating the high purity of the annealed WO 3 nanomesh. The specific surface area and pore-size distribution of the WO 3 nanomesh were measured by the nitrogen adsorption/desorption isotherms, as shown in Fig. 5(d). The BJH pore size distribution (the inset in Fig. 5(d)) is widely distributed in the range of 1-135 nm. The pores are mainly centered at 12.5 nm, which arise from the space between nanowires. The BET surface area of the WO 3 nanomesh is 104.3 m 2 /g. In view of its large pore size and high exposed surface area, the WO 3 nanomesh has great potential to show high gas-sensing performance.

2 Gas sensing properties
First, the response curves of the WO 3 nanomesh sensor at different temperatures were measured. For the temperatures less than 160 ℃, the resistances of the sensor in NO 2 gas was too large to measure by the multimeter. Therefore, the operating temperature of 160 ℃ or higher temperature was used in the gas-sensing tests.
The response of the WO 3 nanomesh-based sensor to 500 ppb NO 2 was measured at various operating temperatures. As shown in Fig. 6(a), the sensor response decreases as the temperature increases. Therefore, the optimal operating temperature of the WO 3 nanomesh sensor is 160 ℃. Figure 6(b) shows the response and recovery time of the WO 3 nanomesh sensor to 500 ppb NO 2 as a function of operating temperature. As the operating temperature increases, the response and recovery time of the sensor becomes shorter. These results indicate that the increase in temperature increases the adsorption and desorption rate of NO 2 gas on the surface of WO 3 namomesh. Figure 6(c) shows a typical dynamic response curve for WO 3 nanomeshbased sensor to NO 2 at a concentration ranging from 50 to 500 ppb at an optimum operating temperature of 160 ℃. When the sensor is exposed to the oxidizing gas of NO 2 , its resistance increases; when the air is introduced, the resistance of the sensor returns to its original value. Figure 6(d) shows the relationship between the response of the WO 3 nanomesh sensor and the NO 2 concentration at 160 ℃. As the NO 2 concentration increases from 50 to 500 ppb, the response of the sensor increases. Moreover, there is a relatively good linear relationship between response and NO 2 concentration. This indicates that the WO 3 nanomesh sensor has the advantage of detecting low concentrations of NO 2 . It is worth mentioning that the detection limit (LOD) of the WO 3 nanomesh sensor is as low as 50 ppb. Table 1 summarizes the comparison of NO 2 sensing performance between WO 3 nanomesh-based sensors fabricated in this work and other reported WO 3 nanostructures. It is clear from Table 1 that the WO 3 nanomesh material in this work exhibits the lowest detection limit at low operating temperature compared to other materials from Refs. [40][41][42][43][44]. In the absence of noble metal modification and ion doping, the intrinsic WO 3 nanomesh material exhibits high response and very low detection limit at low operating temperature. There is no doubt that the WO 3 nanomesh material prepared in this work is a competitive sensing material with good application prospects.
In order to evaluate the selectivity of the WO 3 nanomesh-based sensor towards NO 2 gas, sensing tests were also performed in other gases at the operating Fig. 6 (a) Response of WO 3 nanomesh to 500 ppb NO 2 as a function of operating temperature; (b) response and recovery time of WO 3 nanomesh to 500 ppb NO 2 as a function of operating temperature; (c) dynamic response of WO 3 nanomesh toward NO 2 gas at concentration ranging from 50 to 500 ppb at 160 ℃; (d) correlation between the sensor response and NO 2 concentration. temperature of 160 ℃. Figure 7 shows the response of WO 3 nanomesh-based sensor to NO 2 (500 ppb), C 2 H 6 O (10 ppm), NH 3 (10 ppm), CO (50 ppm), and H 2 S (5 ppm). It indicates that the WO 3 nanomesh sensor has a significant response to NO 2 compared to other gases, although the concentrations of other gases are higher. Therefore, it can be concluded that the prepared WO 3 nanomesh sensor exhibits excellent selectivity to NO 2 . Repeatability and long-term stability are also important indicators for evaluating gas sensor performance. As shown in Fig. 8(a), the WO 3 nanomeh-based sensor after 30 days of storage was used to test its cyclic response curve to 500 ppb NO 2 at 160 ℃. It can be observed that the response curve of the sensor was excellent in repeatability, indicating good long-term stability of the sensor based on the WO 3 nanomesh. The effect of humidity on the sensor response was investigated by comparing the NO 2 sensing performance in dry air and humid air (produced by bubbling water). The response curve of the WO 3 nanomesh-based sensor to 500 ppb NO 2 under dry and humid conditions at 160 ℃ is shown in Fig. 8(b). Considering the margin of error, the response of the WO 3 nanomesh sensor to NO 2 is slightly reduced in a humid atmosphere. Therefore, it can be concluded that humidity has a small effect on the NO 2 sensing characteristics of the WO 3 nanomesh sensor.  (b) response curves of WO 3 sensor to NO 2 (500 ppb) at 160 ℃ in dry and humid condition.

3 Gas sensing mechanism
The electron depletion theory is well known to explain gas-sensing mechanisms by charge transfer between gas molecules and sensing materials [36,45]. Figure 9 shows the schematic diagram of NO 2 -sensing mechanism for WO 3 nanomesh sensor. In air, oxygen molecules are adsorbed on the surface of WO 3 nanomesh forming the adsorbed oxygen at low temperature (< 200 ℃) as described in the following reaction: 2 2 O (gas) e O (ads) − − + → (1) Thus, an electron depletion layer is formed by reducing the free charge carrier consequently increasing the resistance of the sensor. When WO 3 nanomesh is exposed to oxidizing gas of NO 2 , NO 2 molecule can easily capture electron from the surface of sensing material forming 2 NO − as described in the following reaction: 2 2 NO (gas) e NO (ads) − − + → (2) Thus, a wider electron depletion layer is formed when NO 2 is introduced and the resistance of the WO 3 nanomesh increases. The WO 3 nanomesh composed of WO 3 nanowires can provide a higher specific surface area than conventional bulk materials, thereby providing more surface active sites for gas adsorption and improving NO 2 sensing performance. Moreover, the high-purity single crystal nanowire assemblies promote rapid electron transfer and thus improve the gas sensing performance of WO 3 .
In order to verify the NO 2 -sensing mechanism proposed above, the responses of the WO 3 nanomesh sensor to NO 2 (500 ppb) in different backgrounds at 160 ℃ were measured. It can be shown in Fig. 10 that the Fig. 10 Response curves of the WO 3 nanomesh sensor to NO 2 (500 ppb) in nitrogen balance and air balance, and 80% O 2 +20% N 2 background at 160 ℃. response of the gas sensor decreases as the oxygen concentration in the background atmosphere increases. According to the above NO 2 -sensing mechanism, oxygen is not required when the WO 3 nanomesh sensor detects NO 2 gas. The WO 3 nanomesh sensor has the same resistance when exposed to NO 2 in different backgrounds. This means oxygen may not compete with NO 2 gas for active sites on the WO 3 nanomesh surface and therefore cannot affect the concentration of adsorbed NO 2 . This is why the sensor response (R g /R a ) decreases as the oxygen in the background gas increases.

Conclusions
Hierarchical WO 3 nanomesh assembled from singlecrystalline WO 3 nanowires was prepared via a hydrothermal method using Tu as a structure-directing agent and a morphology-controlling agent in the nucleation crystallization and assembly process. The interdigitated single crystal nanowires increase electron transfer so that the WO 3 nanomesh has good conductivity and is suitable as a NO 2 -sensing material. The sensor based on WO 3 nanomesh exhibits high response, fast response and recovery behavior, and excellent selectivity towards ppb-level NO 2 at low temperature of 160 ℃. The excellent NO 2 -sensing performance may be primarily attributed to the large specific surface area of WO 3 nanomesh and high purity nanowire assemblies improving gas absorption/desorption and electron transfer process. The prepared WO 3 nanomesh material is a high-performance NO 2 gas sensing material capable of working at low temperature, and therefore has a good application prospect. Our synthetic strategy towards hierarchical WO 3 nanomesh with high gas sensitivity provides a way for developing high-performance gas sensor based on other wide-bandgap metal oxides.