Hydrothermal synthesis and NH3 gas sensing property of WO3 nano particles

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2015-0034 Mathematical and Physical Sci., 2015, Vol. 60, No. 7, pp. 68-74 This paper is available online at HYDROTHERMAL SYNTHESIS AND NH3 GAS SENSING PROPERTY OFWO3 NANO PARTICLES Nguyen Dac Dien, Luong Huu Phuoc, Do Duc Tho, Nguyen Anh Phuc Duc, Nguyen Duc Chien and Dang Duc Vuong School of Engineering Physics, Hanoi University of Science and Technology Abstract. In this paper, a very simple procedure was presented for the synthesis of WO3 nanoparticles by hydrothermal treatment at 180 ◦C for 48 h using sodium tungstate and hydrochloric acid as starting materials. As-obtainedWO3 nanoparticles were characterized by field emission electron microscopy (FE-SEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The prepared WO3 nanoparticles have a size of 20 - 30 nm. The gas sensing properties of film derived from WO3 nanoparticles are tested in NH3 at concentrations in the range of 25 - 300 ppm at working temperatures in the range of 250 - 400 ◦C. The nano-sizeWO3 film exhibits the highest response to NH3 at the optimum operating temperature of 350 ◦C. In addition, the selectivity of the WO3 nanoparticles film to NH3 in the present of other gases LPG, ethanol and acetone was also investigated. Keywords: NH3, WO3, hydrothermal method, gas sensor. 1. Introduction Being a strong toxic gas, ammonia (NH3) is one of the most harmful gases to the human body and also an air pollutant. The detection of NH3 is crucial for monitoring environmental pollution to safeguard public health and is required in many applications such as leak-detection in air-conditioning systems, the sensing of trace amounts of ambient NH3 in the air for environmental protection, breath analysis for medical diagnoses, animal housing, food processing, fertilizers, and chemical plants. The attention level for NH3 in ambient air is 25 ppm. Solid state NH3 detectors that are low cost, small and reliable are in demand and have been widely used by measuring the resistance change in surface phenomena in terms of gas adsorption and desorption [1]. There have been various proposals for an NH3 sensor that uses different sensing materials such as a carbon nanotube [2], polymers [3], SnO2 [4], TiO2 [5], ZnO [6] and WO3 [7]. Among them, tungsten trioxide (WO3) is considered to be one of the most interesting materials that could be used as a component of a gas sensor based on metal oxide semiconductors [8]. Porous WO3 materials were prepared by anodization of sputtered tungsten thin films deposited on insulating substrates [9], tungsten oxide nanowires were synthesized by solvothermal method [10] and nanocrystalline tungsten oxide were obtained by the acid precipitation method [11] or sol-gel process [12]. Compared with other techniques, the aqueous chemical process is attractive because Received September 11, 2015. Accepted October 16, 2015. Contact Nguyen Dac Dien, e-mail address: nddien1980@yahoo.ca 68 Hydrothermal synthesis and NH3 gas sensing property of WO3 nanoparticles of its cost effectiveness and ease of material preparation. Recently, the hydrothermal treatment method has been used to prepare nano-size WO3 powder with particle size in the range of 39 - 60 nm. NO2 sensing properties were also investigated [12]. In this work, we also synthesized WO3 nanoparticles using the hydrothermal method and particle size was estimated to be 20 - 30 nm. The film derived from obtained WO3 nanoparticles showed high performance to ppm levels of NH3 and selectivity to NH3 compared to other gases such as ethanol (C2H5OH), LPG (liquefied petroleum gas) and acetone (CH3COCH3). 2. Content 2.1. Experiment Tungsten oxide nanoparticles were synthesized using the hydrothermal method with sodium tungstate dihydrate (Na2WO4.2H2O) as precursor and distilled water as solvent. A certain amount of Na2WO4.2H2O was dissolved in 25 mL of distilled water in a beaker to obtain a solution. The concentration of the final solution was maintained at 1 M. Then, the 3 M HCl solution was added dropwise under constant stirring to acidify the Na2WO4 solution to a pH of 1. The mixed reaction system was stirred for 4 h and became a homogeneous and stable solution. The prepared solution was subsequently transferred to and sealed in a 20 mL Teflon-lined stainless steel autoclave and the hydrothermal reaction was conducted at 180 ◦C for 48 h in an electric oven. After that, the autoclave was allowed to cool to room temperature. The final products were washed sequentially with deionized water and ethanol several times and the obtained powder was dried at 80 ◦C for 24 h in air. The morphology and crystalline structure of the tungsten oxides were characterized by using field emission scanning electron microscopy (FESEM, Hitachi S4800 Japan) and an X-ray diffractometer (XRD, D8 Advance Bruker Germany, Cu-Kα radiation λ = 0.15406 nm and settings of 40 mA and 40 kV at a scanning rate of 0.03 ◦/s ranging from 20 to 70 ◦ for the 2 θ with a step time of 1 s). The crystalline size of the sample was estimated from the XRD pattern using the Debye-Scherrer’s equation: D = kλβ cos θ , where k = 0.893 is the Scherrer constant, D is the crystalline size, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity and θ is the Bragg diffraction angle. The composition of the product was analyzed using an energy dispersive X-ray detector (EDS, OXFORD JEOL 5410 LV, Japan) operating at 15 kV. In order to fabricate the thick-film sensing device, a paste prepared from the mixture ofWO3 powder, deionized water and polyethylene glycol was coated on SiO2/Si substrate attached with Pt – interdigitated electrodes. The coated sensing film was dried in air and subsequently annealed at 80 ◦C for 24 h and sintered at 400 ◦C for 2 h at ambient atmosphere, respectively. The gas-sensing characteristics were measured in a static gas sensing system. The system consisted of a movable glass test chamber (volume 20 l), a flat heating plate with a temperature controller fixed in the test chamber, a professional digital multimeter and a data acquisition system. The sensor was placed on the heating plate and the working temperature of the film was determined with a thermocouple attached to the heating plate. The temperature was kept constant during each measurement and the pure target gas was introduced into the test chamber by static volumetric method using a syringe. A predetermined amount of gas was injected into the closed chamber directly by a micro-injector to get the desired concentration. After each measurement, the sensor was exposed to air again by opening the chamber. In our measurement system, the NH3 concentration was checked using a NH3 detector (GasAlert Extreme NH3 - BW Technologies, Canada). The operating temperature of the sensing film changed from 250 ◦C to 400 ◦C and the NH3 concentration varied from 25 69 N. D. Dien, L. H. Phuoc, D. D. Tho, N. A. P. Duc, N. D. Chien and D. D. Vuong ppm to 300 ppm. In measuring the voltage, a load resistor (Rl = 20 kΩ) was connected in series with the gas sensor. The circuit voltage was set at 5 V and the output signal (Uv) was the terminal voltage of the load resistor. The resistance between the two electrodes was calculated using the following formula: Rs = 20(5/Uv - 1) (kΩ). The gas response was defined by Ra/Rg, where Ra and Rg were the resistances of a sensitive film in clean air and in the measuring gas, respectively. The time taken by the sensor to achieve 90% of the total resistance change was defined as the response and recovery time. 2.2. Results and discussion Figure 1 shows the SEM image of the as-synthesized tungsten oxide nanoparticles. It can be seen that the hydrothermally synthesized nanoparticles exhibited diameters in the range of 20 - 30 nm. Besides the particle-like nanostructure, no other products can be found. The WO3 nanoparticles are of uneven surface with a uniform dimension over a large area. The rough surface exhibits the large specific surface area. Figure 1. SEM images of the WO3 nanoparticles with different magnifications of 50,000 (a) and 300,000 (b) Figure 2. XRD pattern (left) and EDS spectrum (right) of the as-synthesized WO3 nanoparticles XRD analysis was used to investigate the crystalline structure of the as-synthesized WO3 nanoparticles. Figure 2 (left) illustrates the XRD pattern of the sample. All the diffraction peaks of WO3 can be indexed to hexagonal structure WO3 (h-WO3) with lattice constants of a = b = 7.298 A˚, c = 3.899 A˚, α = β = 90◦ and γ = 120◦ (JCPDS card 01-075-2187). The space group of the 70 Hydrothermal synthesis and NH3 gas sensing property of WO3 nanoparticles primitive cell is P6/mmm. No peaks of any other phase or impurities were observed from the XRD patterns, indicating that the products were of quite high purity. Strong and sharp diffraction peaks also indicate good crystallinity of the sample. The crystallite size of the sample calculated from the Debye-Scherrer’s equation is about 23 nm. The EDS spectrum of WO3 nanoparticles shown in Figure 2 (right) indicates that tungsten and oxygen elements belonged to the sample and the calculated composition for W:O was a molar ratio of 1:2.72 which was consistent with the W18O49 molecular formula. The C signal shown in the spectrum is the graphite layer deposited onto WO3 powder before EDS analysis. The Na content is small and comes from traces of the precursor remaining in the product. Figure 3. Real-time response curves of the sensor device upon exposure to different concentrations of ammonia (NH3) at several working temperatures (a), sensor response as a function of operating temperature (b) The gas-sensing properties of the sensors based on tungsten oxide nanoparticles towards 25 - 300 ppm NH3 were tested at operating temperature ranging from 250 - 400 ◦C. Figure 3 indicates the dynamic responses of the sensor based on the tungsten oxide nanoparticles at temperatures of 250 ◦C, 300 ◦C, 350 ◦C and 400 ◦C, respectively. The measured resistances for all four operating temperatures of sensors decrease upon exposure to NH3 gas, as expected, and after purging of ammonia from the gas chamber, the resistance was quickly recovered to the initial value. It is well known that sensor response is strongly dependent on the operating temperature and the maximum response value is achieved at optimal operating temperature. The maximum response value to 300 ppm NH3 is as high as 14.9 at 350 ◦C while it is 7, 8.6 and 11.4 at the operating temperatures of 250 ◦C, 300 ◦C and 400 ◦C, respectively. Figure 4a shows the profile of the sensor response as a function of NH3 gas concentration. The responses of the sensor at different operating temperatures increase when increasing NH3 gas concentration in the range of 25 - 300 ppm. The linear correlation lines are quite good for such broad ammonia concentrations. The dynamic response performance of the sensor was investigated at optimum operating temperature. Figure 4b displays the response transient curve of the sensor to NH3 gas at the optimum operating temperature 350 ◦C. The resistance of the sensor is reproducible for repeated testing cycle. The response time and recovery time to 300 ppm NH3 are about 30 s and 20 s, respectively. It is thought that, differing from fully oxidized WO3, the non-stoichiometric crystalline structure of W18O49 is favorable for high gas response due to the large amounts of oxygen 71 N. D. Dien, L. H. Phuoc, D. D. Tho, N. A. P. Duc, N. D. Chien and D. D. Vuong vacancies in the crystal structure which can serve as adsorption sites of gas molecules [13]. Microstructure parameters such as effective surface area, grain size, crystallinity and porosity are the main factors determining the gas-sensing properties of oxide semiconductor films [14]. The conductivity of a gas-sensitive metal oxide film is dominated by the Schottky-type potential barrier that develops at the intergrain boundaries of the film in the presence of oxygen atoms. In the case of n-type tungsten oxide, upon exposure to NH3 gas, the NH3 gas molecules can be directly absorbed onto the surface by releasing electrons into the conduction band or they can interact with the chemisorbed oxygen on the surface, leading to a decrease of thickness of the electron-depleted layer. As a result, the Schottky barrier heights at the boundaries of the nanostructures decrease resulting in a decrease in the resistance. On the other hand, the gas diffusion is one of the key factors that determine the sensor response and recovery characteristics. The relatively higher NH3 response for the 350 ◦C operating temperature in comparison with the 250 - 300 ◦C samples can be attributed to the relatively looser bundle of aggregates which can act as gas diffusion channels making the diffusion much easier. An operating temperature higher than 350 ◦C hampers the diffusion of NH3 toward the sensing surface resulting in reduced gas response. Figure 4. Relationship between the sensor responses and NH3 concentration at different operating temperatures (a), the response transient curve of the sensor to NH3 gas at optimum operating temperature 350 ◦C (b) In addition, a sensor based on WO3 nanoparticles was reproducible on repeat testing to 300 ppm NH3 at optimum operating temperature 350 ◦C as depicted in Figure 5a. This indicates that the sensor has good reproducibility, long-term stability and good repeatability. It is well known that tungsten oxide sensors can respond to various gases, including NO2, NH3, ethanol (C2H5OH), H2S, LPG (liquefied petroleum gas) and acetone (CH3COCH3). To study the selectivity of the samples, the gas responses of the sensor to the other gases were also examined. Figure 5b shows the response values of the WO3 sensor to NH3, ethanol, acetone and LPGwith the same concentration of 300 ppm at the operating temperature of 350 ◦C. We can see that the sensor exhibits selectivity to NH3 gas. It shows a significant change in the relative degree of response to NH3 when compared to hydrocarbon gases, ethanol or acetone vapors. The response of the sample to 300 ppm C2H5OH, 300 ppm LPG and 300 ppm CH3COCH3 was quite low compared to that of ammonia. The good selectivity of WO3 nanoparticles to NH3 when compared to other gases may be related to the affinity of WO3 to the target gases at operating temperature. A more detailed reason and qualitative explanation requires further study. 72 Hydrothermal synthesis and NH3 gas sensing property of WO3 nanoparticles Figure 5. Sensor responses of sample to 300 ppm NH3 at operating temperature 350 ◦C for repeated testing cycle (a), the sensing of WO3 nanoparticles to different gases at 350 ◦C (b) 3. Conclusion In summary, we have demonstrated that n-type semiconducting WO3 nanoparticles can be successfully prepared using a simple hydrothermal technique at a temperature of 180 ◦C for 48 h with pH value of precursor solution of 1. The obtained WO3 nanoparticles exhibited uniform diameters of ca. 20 - 30 nm. The as-synthesized nanoparticles were used to fabricate the gas sensor using the drop-coating method. The sensor device derived from WO3 nanoparticles shows the highest response to ammonia gas at an operating temperature of 350 ◦C with the response to 300 ppm NH3 being 15. 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