Effects of sintering temperature on sensing properties of WO3 and Ag-WO3 electrode for NO2 sensor

Pure WO3 and Ag-WO3 (mixed solid solutions Ag with WO3) have been successfully synthesized by sol-gel method and the influences of calcination temperature on the particle size, morphology of the WO3 and Ag-WO3 nanoparticles were investigated. Powder X-ray diffraction results show that the hexagonal to monoclinic phase transition occurs at calcination temperature varying from 300°C to 500°C. SEM images show that calcination temperature plays an important role in controlling the particle size and morphology of the as-prepared WO3 and Ag-WO3 nanoparticles. The NO2 gas sensing properties of the sensors based on WO3 and Ag-WO3 nanoparticles calcined at different temperatures were investigated and the experimental results exhibit that the gas sensing properties of the Ag-WO3 sensors were superior to those of the pure WO3. Especially, the sensor based on Ag-WO3 calcined at 500°C possessed larger response, better selectivity, faster response/recovery and better longer-term stability to NO2 than the others at relatively low operating temperature (150°C).


RL, 0000-0002-2465-7459
Pure WO 3 and Ag-WO 3 (mixed solid solutions Ag with WO 3 ) have been successfully synthesized by sol-gel method and the influences of calcination temperature on the particle size, morphology of the WO 3 and Ag-WO 3 nanoparticles were investigated. Powder X-ray diffraction results show that the hexagonal to monoclinic phase transition occurs at calcination temperature varying from 3008C to 5008C. SEM images show that calcination temperature plays an important role in controlling the particle size and morphology of the asprepared WO 3 and Ag-WO 3 nanoparticles. The NO 2 gas sensing properties of the sensors based on WO 3 and Ag-WO 3 nanoparticles calcined at different temperatures were investigated and the experimental results exhibit that the gas sensing properties of the Ag-WO 3 sensors were superior to those of the pure WO 3 . Especially, the sensor based on Ag-WO 3 calcined at 5008C possessed larger response, better selectivity, faster response/recovery and better longer-term stability to NO 2 than the others at relatively low operating temperature (1508C).
2.5-2.8 eV, has received much interest for applications including photocatalysis [2][3][4][5], electrochromic devices [6][7][8], solar energy conversion [9] and gas sensors [10], due to excellent catalytic, optical and dielectric properties, good physical and chemical stability. For gas sensing applications, WO 3 has attracted great attention for its distinctive sensing properties, and has been regarded as a promising material for detecting various gases, including CO [11], H 2 [12][13], SO 2 [14], NO x [15 -19], H 2 S [20] and organic vapours [21]. In addition, tungsten trioxide (WO 3 ) has been considered as a promising sensing material of solid-state semiconductor gas sensors for NO 2 monitoring because of its excellent sensitivity and selectivity. WO 3 nanoparticles can be fabricated by various techniques such as chemical vapour deposition [22], hydrothermal method [23 -26], microwave irradiation method [27] and sol-gel process [28]. Sol-gel technique is the most common method of fabricating WO 3 nanoparticles because of the advantages of simple, quick process and easy control of particle size, crystal structure and morphology. In recent years, many attempts have been made to enhance the gas sensitivity of semiconductor gas sensors [29], one of which involved the doping of noble metal in the materials. It has been shown that the sensing performance of WO 3 can be substantially improved by loading particular elements [30 -33]. Wang et al. [34] have shown that a high sensitivity was achieved when noble metals such as Pt, Au and Pd were deposited as activator layers on WO 3 films. Najim et al. [35] have mixed SnO 2 with WO 3 and prepared to synthesize nanostructured thin films by pulsed laser deposition as gas sensor. The sensors showed high sensitivity. However, there is still room for improvement in stability, selectivity and working temperature of the gas sensors [36].
In this work, WO 3 nanoparticles and Ag-WO 3 were prepared by a simple sol-gel method. The influences of calcination temperature on the particle size, morphology of the WO 3 and Ag-WO 3 nanoparticles were extensively investigated and NO 2 gas sensing properties of WO 3 and Ag-WO 3 nanoparticles were discussed.

Preparation and characterization
The WO 3 and Ag-WO 3 nanoparticles were prepared by sol-gel method. All of the chemical reagents were of analytical grade and used as received without further purification. In a typical synthesis, 3.5 g tungsten (W) powder was dissolved into 200 ml of deionized water under constant stirring for 30 min. Then, 100 ml of hydrogen peroxide (H 2 O 2 ) were added into the above solution and stirred for 20 min. A homogeneous WO 3 precursor solution was produced by slowly dropping 30 ml of alcohol and stirred in the thermostatic water bath at 808C until an opaque gel was formed. Ag-WO 3 precursor solution was also fabricated by dropping 4 ml of 0.1 mol l 21 AgNO 3 aqueous solution into the WO 3 precursor solution. The pH value of the solution was fixed at 4.5 which was adjusted by using nitric acid (HNO 3 ) solution in the reaction process. After that, the transparent gel was transferred into a crucible and baked at 808C for 12 h. Finally, the obtained products were calcined at 3008C, 5008C and 7008C for 2 h. A series of pure WO 3 and Ag-WO 3 powders were obtained.

Fabrication and measurement of gas sensor
The products' crystallographic structures information were observed with small angle X-ray diffraction in Shimadzu diffractometer (XRD-6000, Japan) using Cu Ka line radiation at 40 kV and 40 mA. The XRD patterns were collected at 2 angles of 10 -808 at a scan rate of 18 min 21 . A field emission scanning electron microscope (FESEM, JSM-6700F, Japan) was used to measure the particle size and morphology of WO 3 nanoparticles. Brunauer -Emmett-Teller (BET) method was used to determine the pore size distributions and surface areas. The gas sensing property of Ag-WO 3 nanoparticle film was measured by semiconductor characterization system (Keithley, 4200-SCS) with interdigital electrode structures. The concentration of Ag and W were measured by inductively coupled plasmaatomic emission spectroscopy (ICP-AES, Vista).
Side-heating gas sensors were built to measure the gas sensing properties of WO 3 and Ag-WO 3 -based nanoparticles. Figure 1a shows the structure of thick film sensor, a commercial Si substrate with dimensions of 15 Â 9 Â 0.76 mm, and built-in Ag electrodes at 0.5 mm intervals. The WO 3 and Ag-WO 3 nanoparticles paste were coated in a 35 mm thickness via the screen-printing method and then sintered at 3008C for 15 days in air, in order to improve their stability and repeatability. The measuring electric circuit of gas sensing properties is shown in figure 1b. The operating voltage (V heat ) rsos.royalsocietypublishing.org R. Soc. open sci. 5: 171691 was supplied to heat the sensor with A Ni-Cr heater, which can control the operating temperature from 100 to 5008C, and a test voltage (V test ) was supplied across. A load resistor R L was connected in series with the sensor, that resistance was measured and used for calculating and outputting the corresponding sensor resistance.
A sensor performance testing apparatus is shown in figure 2. The sensor was installed at a distance of 70 mm from the bottom of a 20 l (500 Â 200 Â 200 mm) chamber. The sensing electrodes were connected to the test circuit by copper probes. After the target gas was injected into the chamber with the fan on, the resistance was measured with an electrometer after the equilibrium concentration was reached. The sensitivity (S) of the sensing electrodes was defined as: S ¼ R a /R g for reducing gases or S ¼ R g /R a for oxidizing gases, where R a and R g represent the resistances of the sensing electrodes in air and in a target gas, respectively. Furthermore, the response and recovery times were defined as the times at which a total resistance change of 90% was achieved.    The effect of the calcination temperature and mixing Ag on the crystallite dimensions of WO 3 was also detected by XRD. The average crystal size of WO 3 was estimated by using the Scherrer equation: where D is the crystalline size, k is the so-called shape factor and usually taken as 0.89, l and u are the radiation wavelength (0.154056 nm) and Bragg's angle, respectively, b is the full width at half maximum (FWHM) of the diffraction peak. The average grain sizes of pure WO 3 and Ag-WO 3 particles calcined at 3008C, 5008C and 7008C were about 235, 343 and 414 nm, 98, 129 and 314 nm, respectively. To measure the precise amount of Ag in the Ag-WO 3 , ICP-AES was used. The results show that the mass percentage of Ag in Ag-WO 3 is 0.9 wt%. Indicating that the grain size of as-prepared WO 3 grew with increasing calcination temperature. Moreover, the average grain sizes decreased slightly by mixing Ag, which may be due to a small amount of Ag loaded in the mesoporous WO 3 or a homogeneous distribution of Ag particles [37]. Figure 4 shows the SEM micrographs of pure WO 3 (a-c) and Ag-WO 3 (d-f ) nanoparticles samples heated at 3008C, 500 o C and 7008C, respectively. As illustrated in figure 4a-c the irregular mixture of sliced or granular structure at 3008C and 5008C, and mainly spherical particles of diameter 50-800 nm with irregular fringe are observed. When the calcination temperature increases to 7008C as shown in figure 4c, the powder presents three-dimensional (3D) irregular microspheres and some of them are interconnected with each other. Furthermore, larger size particles appear and tend to be of irregular  shape with straight edges, which can be attributed to the thermally promoted crystallite growth. Figure 4d-f shows the SEM images of Ag-WO 3 at different calcination temperatures. The images of the Ag-WO 3 powder calcined at 3008C is shown in figure 4d. Ag-WO 3 particles tend to form large agglomerates due to physical attraction between the particles with small sizes and irregular shapes. As the temperatures raise to 5008C, figure 4e, the particles showed good homogeneity and discreteness. Figure 4f shows the irregular mixture of sliced or granular structure with significant agglomeration at 7008C. The experiment results indicate that Ag-WO 3 of the sol-gel system for the fabrication of uniform nanoparticles of hexagonal and monoclinic WO 3 from condensed WO 3 gel is a key factor for controlling the final particle size and shape of the product.

Field emission scanning electron microscopy
Nitrogen gas sorption analyses were carried out to study the porosity of the composites. Figure 5a shows nitrogen adsorption -desorption isotherms and figure 5b pore diameter distribution curves of WO 3 and Ag-WO 3 calcined at 5008C. More details about nitrogen adsorption -desorption isotherms and pore diameter distribution curves can be found in the electronic supplementary material.

Gas sensing characteristics
Both of the nitrogen adsorption -desorption isotherms are type IV curves, characteristic of mesoporous materials [38,39]. The average values of the specific surface area and pore sizes calculated by the Barrett -Joyner -Halenda (BJH) method are illustrated in table 1. The specific surface area of the pure WO 3 powders decreased sharply with the increasing sintering temperature more than 5008C due to the grain sintering, phase transformation and growing up. The pore size of Ag-WO 3 is larger than the diameter of the WO 3. The surface area increases slightly by mixing Ag at the same temperature, which may be due to the incorporation of Ag attached to the WO 3 attached to the WO 3 framework affects the integrity and mesostructure [40]. Futuremore, the biggest specific surface area and pore size can be obtained with Ag-WO 3 calcined at 5008C.
The gas sensing properties of WO 3 and Ag-WO 3 at different calcination temperatures to 10 ppm NO 2 were measured at various operating temperatures, as shown in figure 6a. It is obvious that the response of these sensors to 10 ppm NO 2 varies with not only the operating temperature but also mixing Ag. It can be seen that all the sensitivity change shows a sharp upward trend at first and decreased rapidly with an increase in operating temperature. For all the Ag-WO 3 sensors, there is a maximum value at 1508C, while  all the pure WO 3 sensors have the maximum gas response at 2008C. The operating working temperature of all the Ag-WO 3 sensors is lower than pure WO 3 sensors. In addition, the sensitivity of Ag-WO 3 sensors exhibits much higher response than the pure WO 3 sensors, compared with previous reports about NO 2 sensors [41][42][43]. Especially, the Ag-WO 3 sensor calcined at 5008C presents the largest response to NO 2 at 1508C, which indicates that the sensitivities of the WO 3 sensors are much enhanced by mixing Ag. Responses of pure WO 3 and Ag-WO 3 sensors calcined at different temperatures to different concentrations of NO 2 (0.25-20 ppm) were measured at the same operating temperature (1508C) and are shown in figure 6b. All the responses have the same trend that they increased with the increase in the concentration of NO 2 . Furthermore, the gas response of all the Ag-WO 3 sensors is higher than that of the pure WO 3 sensors at the same condition. It can also be observed that Ag-WO 3 calcined at 5008C exhibits the highest gas response, which may be due to the good crystallization and biggest specific surface area. The response of all sensors has the biggest increasing rate in the range of 5-10 ppm NO 2 , which indicated that the sensors have an excellent performance in monitoring NO 2 gas, especially in low concentrations.
To highlight the highest performance of Ag-WO 3 calcined at 5008C, Ag and WO 3 components were separately synthesized by the sol-gel method and calcined at 5008C and then a control sample of the physical mixture of Ag and WO 3 was tested for sensing NO 2 . Ag particles can be prepared by sol-gel method through Si(OC 2 H 5 ) 4 , AgNO 3 and HNO 3 [44]. Two grams of silver particles was fully mixed with 200 g WO 3 and its sensor performance is shown in figure 7. Physical mixture of Ag and WO 3 has the same performance as Ag-WO 3 and its operating working temperature is lower than pure WO 3 sensors.
It is well known that semiconductor gas sensors are surely affected by the presence of ambient moisture [45]. When chemisorbed on material, water molecules influence the conductivity. The effect of humidity on semiconductor sensor is also related to the temperature and gas composition of the sensor. So all the experiments were measured in the same presence of ambient moisture. The gas sensing mechanism of n-type semiconductor oxide is based on the change in resistance, which is primarily caused by the chemical adsorption and reaction of the gas on the surface of the sensing materials. When WO 3 is exposed in the atmosphere, oxygen molecules are adsorbed on the surface, and changed into chemisorbed oxygen species (O 22 , O 2 ) by capturing free electrons from conduction band. Presence of these oxygen species is decided by the operating temperature.
The Depletion region is formed on the surface of WO 3 , leading to a decrease of carrier concentration and electron mobility [46]. Exposure to NO 2 gas results in a further decrease of the carrier concentration, for the electrons of WO 3 are captured [47], as represented in equations (3.4)-(3.6), and the depletion width further increases, which eventually decreases the conductivity of the sensor. When the Ag-WO 3 are exposed to NO 2 , NO 2 as a polar molecule with positive charge localizes on the nitrogen NO 2 (gas) þ e À $ NO À 2 (ads), ð3:4Þ NO 2 (gas) þ e À $ NO(gas) þ O À (ads) ð3:5Þ and NO 2 (gas) þ O À 2 (ads) þ 2e ! NO À 2 (ads) þ 2O À (ads): ð3:6Þ and negative charge on the oxygen atoms, and electron interaction with the Ag will repel the negatively charged oxygen and attract the positively charged nitrogen [48]. The sensing properties of Ag-WO 3 materials are enhanced compared with pure WO 3 material due to the catalytic activity of Ag nanoparticles. The Ag additive serving as an active catalyst plays an important role in enhancing sensitivity, which can create more active sites [49]. Furthermore, the underneath areas of Ag particles will be less depleted by the electron flow from Ag to WO 3 , for the work function of Ag (4.26 eV) is smaller than that of WO 3 (5.05 eV) [50].
As is known to all, response and recovery characteristics are important for estimating the performance of a sensor. The resistance changes of Ag-WO 3 powder calcined at 5008C were repeatable for three successive measures in 250 ppb NO 2 . The response and recovery times in a single cycle were about 47 s and 103 s, respectively, as shown in figure 8a. Figure 8b shows relative responses of pure WO 3 and Ag-WO 3 power sensors calcined at 5008C on exposure to different gases (ethanol, CO, H 2 , and NH 3 at 1000 ppm and NO 2 at 10 ppb) at 1508C. The responses of the two sensors exhibited a high performance of NO 2 , while they are a little sensitive to four gases. Moreover, compared to the pure WO 3 sensor, the Ag-WO 3 sensor exhibited higher responses to all the testing gas, especially to NO 2 . As a result, the Ag-WO 3 sensor can be a very promising sensor to monitor NO 2 at relatively low temperature; both sensitivity and selectivity are taken into consideration. The long-term stability is important to ensure the accuracy of detection for gas sensors. Consequently, a test for the long-term stability of sensor calcined at 5008C to 10 ppm NO 2 was measured for three months. As shown in figure 9, it can be observed that the sensor still showed excellent response performance to NO 2 gas even after three months, and the response values were just floating around 400, which indicated that the sensors based on Ag-WO 3 have enough stability to detect NO 2 gas for a relatively long period. The sensing properties (working temperature, response to a certain NO 2 concentration,) of several NO 2 sensors are compared in table 2.

Conclusion
In summary, WO 3 and Ag-WO 3 (mixed solid solutions Ag with WO 3 ) nanoparticles were successfully fabricated by sol-gel method. The XRD results show that the hexagonal to monoclinic phase transition takes place in the temperature range from 3008C to 5008C. The crystalline size of WO 3 nanoparticles increases with increasing calcination temperature and decreased slightly by mixing Ag. The gas sensing properties of Ag-WO 3 nanoparticles were measured and the experimental results exhibit that the gas sensor based on Ag-WO 3 nanoparticle film has excellent selectivity and long-term stability to NO 2 gas. The operating temperature and the amounts of additives play an important role in the  response of the sensors. The optimum performance was obtained at 1508C for the Ag-WO 3 sensor calcined at 5008C and can be suitable for detecting NO 2 at relatively low operating temperature.
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