Hydrothermal synthesis and NH3 gas sensing property of WO3 nanorods at low temperature

One-dimensional self-assembled single-crystalline hexagonal tungsten trioxide (WO3) nanostructures were synthesized by wet chemical-assisted hydrothermal processing at 120 °C for 24 h using sodium tungstate and hydrochloric acid. Urchin-like hierarchical nanorods (petal size: ∼16 nm diameter and 110 nm length) were obtained. The samples were characterized by field emission scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy and x-ray diffraction. Sensors based on WO3 nanorods were fabricated by coating them on SiO2/Si substrate attached with Pt interdigitated electrodes. NH3 gas-sensing properties of WO3 nanorods were measured at different temperatures ranging from 50 °C to 350 °C and the response was evaluated as a function of ammonia gas concentration. The gas-sensing results reveal that WO3 nanorods sensor exhibits high sensitivity and selectivity to NH3 at low operating temperature (50 °C). The maximum response reached at 50 °C was 192 for 250 ppm NH3, with response and recovery times of 10 min and 2 min, respectively.


Introduction
In recent years one dimension (1D) nanostructures, such as tungsten oxide nanowires (NWs) or nanorods (NRs) have attracted special attention due to their specific properties which are expected to be linked to their anisotropic shape related to a large surface to volume ratio. Gas sensors based on 1D nanostructures have advantages of higher sensitivity, superior spatial resolution and rapid response owing to the high surface to volume ratio compared to thin film gas sensors. Tungsten oxide NRs have shown interesting properties as NO 2 [1][2][3], CO [4], H 2 [5,6], C 2 H 5 OH [7,8], NO [9], O 3 [10], H 2 S [11] sensors. On the other hand, tungsten oxide (WO 3 ) is an important n-type semiconducting material with a band gap of 2.7 eV, finding applications in gas sensing, photocatalysis and electrochromic devices [12]. WO 3 NRs with hexagonal structure were synthesized by hydrothermal methods [13][14][15][16] for their advantages such as simple operation, low-cost, potential for large scale production and forming crystalline structure with good stability. Simple and low cost methods for well-ordered nanostructure construction are preferred in gas-sensing and other commercial applications. In the present paper, we deal with detailed NH 3 sensing properties of WO 3 NRs synthesized by hydrothermal process. Ammonia (NH 3 ) is a typical reducing gas. As a toxic but colorless gas with a special odor, it is a major air pollutant emitted from agricultural practices. The need to detect low ammonia concentrations has greatly increased in many fields of technological importance, such as food technology, chemical engineering, medical diagnosis, environmental protection, monitoring of car interiors and industrial processes. The electrical properties of the NRs can change upon exposure to gas and can be restored upon reexposure to air at low temperature. Such low temperatureprocesses are very desirable in detecting toxic gases safely at room temperature.

Experimental
All the chemical reagents were purchased from commercial sources of analytical grade and used without further purification.
Sodium tungstate dihydrated powder (Na 2 WO 4 .2H 2 O) was used as tungsten source. In a typical experiment, 4.125 g Na 2 WO 4 .2H 2 O was dissolved into 12.5 ml of bi-distilled water and stirred for 30 min to form a translucent 1 M Na 2 WO 4 solution. Then, a 3 M HCl solution was added drop wise under stirring in succession to acidify the Na 2 WO 4 solution to a pH of 1.8 to 2.2. The mixed reaction system was stirred for 4 h before introducing the obtained solution in a 20 ml teflon-lined stainless steel autoclave and the temperature was set at 120°C for 24 h under autogenous pressure. The pH of the solution remained close to 2 during the whole synthesis. The obtained powder was washed several times with distilled water and ethanol to remove possibly residual ions, and then dried at 80°C for 24 h in air. Finally, the dried WO 3 powders were subjected to heat treatment at 400°C for 1 h.
Morphology, composition and phase of the samples were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDX) and x-ray diffraction (XRD).
The obtained powders were dispersed in mixture of ethanol and polyethylene glycol solution using sonication to make a slurry. After that the slurry was coated onto Ptinterdigitated electrodes with the gap between the fingers of about 30 μm by the spin-coating method and then dried in air at 80°C for 24 h. The sensor sample was annealed at 400°C for 2 h to evaporate organic species and to stabilize the materials structure. The electrical resistance of the samples at different temperatures was measured as a function of the NH 3 concentration ranging from 25 ppm to 250 ppm. The sensor was placed in a glass test chamber and heated by an external source through the sample holder. The operating temperature of the sensor was determined with a thermocouple attached near the sensor element and varied from 50°C to 350°C. The response of the sample to ammonia gas was thus measured in a static system. The resistance between the two electrodes was measured at an applied voltage of 5 V. Herein, the sensor response to NH 3 gas was defined according to the standard definition of reducing gas as the ratio S = R g /R a for operating temperature below 150°C and S = R a /R g for operating temperature above 150°C, where R a is the sample resistance in air and R g is its resistances in the presence of NH 3 gas. The sensor response dynamics were evaluated through the response time (τ res ), calculated as the time duration for sensor resistance to reach 90% of its steady-state value during the exposure, and recovery time (τ rec ) as the time needed to recover 90% of the original baseline resistance.

Results and discussion
3.1. Crystal structure and surface morphology Figure 1 presents a typical SEM image of the NRs grown by hydrothermal process. Most of the nanostructures are single NRs, several NRs grown side by side. Besides the rod-like nanostructures, no other products can be found. The WO 3 NRs are straight and uneven surface with uniform dimension along their axial direction over a large area. TEM image shown in figure 2 exhibits the rough surface of the NRs with large specific surface area, which is consistent with the SEM image in figure 1. The average diameter and length of asprepared WO 3 nanorods are estimated to be ∼16 nm and 110 nm, corresponding to an aspect ratio of ∼7. Figure 3 shows the XRD pattern of WO 3 NRs. All the diffraction peaks of pre-annealed WO 3 can be indexed to hexagonal structure WO 3 (h-WO 3 ) with lattice constants of a = b = 0.7298 nm, c = 0.3899 nm, α = β = 90°, γ = 120°( JCPDS Card No. 75-2187), space group of primitive cell is P6/mmm [17]. No peaks of any other phase or impurities were observed from the XRD patterns. Strong and sharp diffraction peaks also indicate a good crystallinity of the sample. The crystallite size of the sample calculated from Debye-Scherrer's equation equals to about 20 nm. This result is roughly in agreement with the TEM measurement (figure 2).

Chemical composition
The EDX spectrum, acquired from individual NRs as shown in figure 4, indicates that tungsten and oxygen are the major elements present in the NRs with a 3:1 molar ratio for oxygen and tungsten elements, which solely constitute the composition of the h-WO 3 NRs. The C signal shown in the spectrum is the graphite layer deposited onto WO 3 powder before EDX analysis. The Na content is small and comes from traces of the precursor remaining in the product. Figure 5 shows the dynamic sensing characteristic of bare WO 3 NRs gas sensor towards different NH 3 gas concentrations at operating temperature of 50°C. As observed, the sensor resistance increased gradually when NH 3 gas was introduced into the chamber and then increased slowly to reach a stable state. The response of the sensor increases with increasing ammonia concentration from 25 ppm to 250 ppm. When the gas was turned off, the resistance dropped sharply and then reached its initial value. This increase of the resistance upon exposure to a reducing gas indicated p-type conduction of WO 3 at 50°C. The p-type behaviour of h-WO 3 at low temperature and the conversion of p-type to n-type conductivity in WO 3 by changing the operating temperature were also reported by other authors [18,19]. The origin of the n-to p-type transition is related to the formation of an inversion layer at the surface of the WO3 NRs at room temperature. When the WO 3 NRs sensor is exposed to air, the adsorption of a large number of oxygen and water molecules creates a p-type surface inversion layer. With increasing temperature the semiconductor surface becomes first intrinsic because of water and oxygen desorption, and then n-type depleted surface at high temperature due to further desorption. Figure 6 shows enlarged part of the data of figure 5 measured at 250 ppm NH 3 to reveal the moments of gas input and gas stop. The bare WO 3 NRs showed sensitivity of 192 at a NH 3 concentration of 250 ppm. The response and recovery times to 250 ppm NH 3 are estimated to be 10 min and 2 min, respectively. The response rate was slow due to low working temperature.

NH 3 sensing property
The relationship between WO 3 NRs sensor response and NH 3 concentration was determined at the operating temperature of 50°C. The sensitivity was defined as the ratio of the sensor resistance at various given ammonia concentrations to that in ambient atmosphere. The high sensitivity, low        detection level and large range of ammonia concentration of this sensor are very interesting points for its development. The resistance and response change linearly with NH 3 concentration ( figure 7). Linear dependence of response on gas concentration is an advantage for designing readout signal circuits. The detection limit of NH 3 for bare WO 3 NRs sensor is estimated to be 25 ppm at the operating temperature of 50°C. It is thus useful for low-power gas-sensing application, and low concentration ammonia detection is actually desired for the development of highly sensitive sensor. In addition, the WO 3 NRs sensor prepared by this method can detect NH 3 at relatively low operating temperature with high gas response compared to some other methods. To the best of our knowledge, there have been few reports on the NH 3 sensing at near room temperature of WO 3 nanomaterials [20]. The nanowire-like structure of tungsten oxide synthesized by the deposition of tungsten metal on the substrate of porous singlewall carbon nanotubes film, followed by thermal oxidation process, has a maximum response of 2.39 toward 300 ppm NH 3 at 250°C [21].
We also measured the sensor response to ammonia gas at different temperatures of 75, 100, 200, 250, 300, 350°C to examine the effect of operating temperature on the sensitivity to NH 3 gas. Figure 8 shows the response of WO 3 NRs based sensor as a function of ammonia concentration operating at different temperatures. The response of the sensor to NH 3 increased monotonously with increasing gas concentration. The sensor response increases with increasing operating temperatures from 100°C to 250°C and then decrease as the operating temperature increases further. At 250°C, the sensor response in the presence of 50 ppm NH 3 gas was 7, while it increased to 22 for NH 3 concentration of 250 ppm. At low operating temperatures (100°C -250°C), sufficient thermal energy is essential to overcome the activation energy barrier, however, when the operating temperature is too high (above 250°C), the desorption process becomes dominant. Furthermore, the diffusion depth becomes lower at a high temperature [22]. Thus, the optimum operating temperature of WO 3 NRs gas sensor in the low temperature region and in the high temperature region are around 50°C and 250°C, respectively (figure 9). So, nanorod-like WO 3 nanostructures were particularly suitable for ammonia detection at these temperatures and concentrations.
In order to investigate the selectivity of bare WO 3 NRs gas sensor, the sensor was exposed to other reducing gases such as C 2 H 5 OH (ethanol) and LPG (liquefied petroleum gas) at various concentrations. Ethanol gas response of bare WO 3 NRs sensor at the operating temperature of 50°C is displayed in figure 10. It can be seen from figures 5 and 10 that pure WO 3 NRs sensor exhibits a very impressive response toward NH 3 but is almost insensitive to ethanol (50 to 100 ppm) and low sensitivity to LPG at concentrations ranging from 2500 ppm to 1.25% at the same conditions. These results indicated fairly good ammonia selectivity of the WO 3 NRs thin films.

Conclusion
In summary, WO 3 NRs were synthesized through soft chemical process assisted by hydrothermal technique at 120°C for 24 h with the pH value of reaction solution of 1.8 to 2.2. The obtained urchin-like products consisted of many WO 3 NRs which are rather uniform with diameters in the range of 15-20 nm and lengths of 100-150 nm. As a potential gas sensor, the WO 3 NRs exhibit high response and selectivity toward NH 3 gas at near room temperature. The sensitivity of the WO 3 NRs to 250 ppm NH 3 gas is about 192 at an operating temperature of 50°C.