One-step synthesis of flower-shaped WO₃ nanostructures for a high-sensitivity room-temperature NO_x gas sensor

Abstract

In this work, we report a one-step synthesis of spherical flower-shaped tungsten oxide (WO₃) nanostructures by using polyethylene glycol (PEG) as a surfactant. The PEG played a crucial role in controlling the final morphology of the WO₃ product. The structure and morphology of the product were characterized by carrying out XRD and SEM experiments. These characterizations indicated these three-dimensional flower-shaped nanostructures to be well crystallized, to have a uniform morphology with an average diameter of 3 μm, and to consist of many two-dimensional nanosheets. The flower-shaped WO₃ particles exhibited excellent room-temperature NO_x gas-sensing performance, including a sensitivity as high as 80%, a short response time of 4.5 s to 97 ppm NO_x, and a low detection limit of 970 ppb. The excellent performance was due to the structure and morphology of the WO₃ product.

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Introduction

The development of industry, while having improved living standards, has also resulted in the continuous emission of various pollutants, including gaseous pollutants, into the environment.^1–4 Therefore, it has become imperative to effectively detect toxic and hazardous gases such as NO_x (NO_x: NO₂, NO),^5,6 CO,^7,8 H₂S,^9–11 and NH₃,^12–14 and gas sensors play an important role in detecting such gases in the environment. NO_x gas is the most dangerous air pollutant, as it influences the environment by inducing the formation of ozone and acid rain.^15,16 The most effective NO_x-sensing materials are semiconductor materials because of their high sensitivity levels and fast response times; and examples of these materials include SnO₂,¹⁷ In₂O₃,¹⁸ ZnO,¹⁹ TiO₂,²⁰ Cu_xO,²¹ CeO₂,²² and two-dimensional semiconducting layered materials like MoS₂,²³ MoSe₂,²⁴ black phosphorus²⁵ and graphene.²⁶ These metal oxides have led to extensive research in gas sensors. In most of the relevant investigations, use of the semiconductor metal oxides has required high temperatures,^27–29 and the metal oxides being decorated with noble metals³⁰ (Au,³¹ Ag,^32,33 Pd³⁴ and Pt³⁵) to improve the sensing properties. These requirements can pose difficulties, and have hence prompted researchers to explore gas-sensing materials that can be effectively used at low temperatures and without noble metals. While introducing noble metals can effectively lower operating temperatures and increase sensitivity, their high costs have nevertheless limited their further applications. An ideal sensor requires high sensitivity and selectivity, short response and recovery times and low cost. It is necessary and pivotal to choose an appropriate material with an appropriate microstructure to meet these requirements.

Most studies on tungsten oxide (WO₃) have concentrated on controlling its microstructure for special materials. Preparation of inorganic tungsten oxide semiconductor materials, with their superior properties, has attracted strong interest,³⁶ and tungsten oxide plays an important role in electrode materials,³⁷ electrochromic devices,^38,39 photocatalysis^40,41 and gas sensing.^41–43 As a semiconductor gas sensor, WO₃ offers several advantages, such as low cost and ultrasensitivity, and can sense trace amounts of NO_x at room temperature without the introduction of a noble metal. Also, Nayak et al.⁴⁴ have studied the three-dimensional (3D) nanostructure of WO₃–SnO₂ for gas sensing. Zhao et al.⁴⁵ have reported the use of flower-shaped tungsten oxide particles as gas sensors. Isaac et al.⁴⁶ have also recently reported using tungsten oxide thin films for sensing NO₂ gas. While tungsten oxide has been used to detect NO_x at high temperatures,⁴⁷ the room-temperature use of WO₃ for detecting NO_x has hardly been reported.

In this paper, we report a one-step synthesis of flower-shaped WO₃ nanostructures with polyethylene glycol (PEG) as a newly used surfactant. PEG is an excellent surfactant for the growth of WO₃ sheet structures.^48,49 The 3D flower-shaped WO₃ structures were found to efficiently facilitate gas diffusion, adsorption, desorption and electrical transport on the surface of the nanomaterials. The flower-shaped WO₃ gas sensors displayed enhanced electrical conductivity, which resulted in an improved sensing for NO_x gas at room temperature. A possible sensing mechanism was also derived.

Experimental

Sample preparation

Materials. Sodium tungstate hydrate (Na₂WO₄·2H₂O), polyethylene glycol (PEG, MW = 2000), about 36% hydrochloric acid (HCl), anhydrous ethanol, and deionized water were used for our experiments. The deionized water was made in the laboratory, and the other reagents were purchased, were of analytical grade, and were directly used without further purification.

Synthesis of the flower-shaped WO₃ nanoparticles. Flower-shaped WO₃ nanoparticles were synthesized by using a facile hydrothermal method. In a typical synthesis, 1.2 g Na₂WO₄·2H₂O and 0.4 g PEG were dissolved in 30 mL of deionized water. Then, concentrated HCl was slowly dropped into the transparent solution while stirring until the pH value of the solution reached 1.45. Subsequently, the solution was transferred to a 50 mL Teflon-lined stainless steel autoclave, which was kept at 160 °C for 20 h in an oven. After the autoclave cooled to room temperature naturally, the product was washed with deionized water and anhydrous ethanol, and finally dried at 60 °C for 12 h. A white powder product was obtained.

Characterization

The flower-shaped WO₃ particles were characterized by using X-ray powder diffraction (XRD, Bruker, D8 FOCUS) with Cu-Kα radiation. The operating voltage and current were kept at 40 kV and 30 mA, respectively. The morphology and structure of the flower-shaped WO₃ particles were observed by using a field-emission scanning electron microscope (FESEM, JSM-7610F, JEOL). The elemental analysis of these particles was performed by using an energy-dispersive spectrometer (EDS, Oxford X-MaxN80) attached to the FESEM. All of the measurements were taken at room temperature.

Measurements of the gas sensor

The synthesized product was made into thin film on an interdigitated Au electrode to obtain the gas sensor, then it would be measured using the gas delivery system.^50,51 The gas sensor was fabricated using the thin film technique with a typical process including dispersion, painting and drying. First, the sensing materials were ultrasonically dispersed in ethanol for 30 min. A paste of the materials was laid uniformly on an interdigitated Au electrode of the sensor, and then dried at 60 °C for 5 h. The loaded mass of WO₃ on the interdigitated electrode was 1.0 mg. And the thickness of the sensing film was about 3 μm. The interdigitated Au electrode (7 × 5 × 0.254 mm, in Fig. S1†) was selected for the gas sensing detection and the electrode spacing was 20 μm.

The electrical resistance measurements of the sensor were taken at room temperature (22 °C) and a relative humidity (RH) range of 26–28%. A diagram of the gas delivery system for the gas sensing process is shown in ref. 21. First, the interdigitated Au electrode sensor was installed into the test chamber with inlet valve 4 and outlet valve 2 (labeled in the diagram) exposed to the external environment. Valve 1 was kept open. The test chamber was flushed with air for 2 min to remove any contaminants from the chamber and glove box. Then, inlet valve 4 and outlet valve 2 were shut. A temperature and humidity meter was placed in the glove box to monitor the environment. Finally, valve 4 was opened, and a certain volume of the NO_x gas was injected. When the resistance became balanced, valve 2 was opened, and then the chamber was purged with air by using an oil pump to recover the sensor resistance. After one response–recovery cycle was finished, the previous three steps were repeated and then the second response–recovery cycle was carried out. The sensitivity values of the thin film sensor were determined by measuring the relative change in the ratio of the resistance of the sample in the NO_x gas atmosphere to its resistance in air. Specifically, the sensitivity (S) values were calculated according to the equation^52–54

where R_g and R_a are the thin film resistances measured in NO_x and air atmosphere, respectively. The response time is defined as the time it took for the sensor to attain 85% of the maximum change in resistance on exposure to the target gas.

Results and discussion

Structure and morphology

The typical morphology and structure of the obtained product were observed by using field emission scanning electron microscopy (FESEM). A panoramic SEM image of the obtained product is shown in Fig. 1a, from which flower-shaped nanostructures with an average diameter of 3 μm were clearly observed. No other morphology could be detected, indicating that these 3D nanostructures formed a fairly uniform morphology. The flower-shaped nanostructures were observed, in the SEM images of Fig. 1b and c, to be composed of many two-dimensional (2D) nanosheets. The high-magnification SEM image in Fig. 1d shows detailed morphological information of the nanosheets, and the sheets were observed here to be formed by many rectangular rods. Moreover, the nanorods were determined, according to statistical analyses of their sizes in Fig. 2a, to have similar thicknesses, in the range of 14–20 nm. The chemical composition of the as-collected product was further checked by using an energy-dispersive spectrometer. The EDS spectrum of this product (Fig. 2b) indicated that the product contained only W and O, as no impurity peaks were observed (the Zn, Cu, Zr and C elements were from the substrate).

Fig. 1 (a–d) SEM images of flower-shaped WO₃ particles at various magnifications.

Fig. 2 (a) Histograms of the diameters of the WO₃ nanorods. (b) EDS spectrum of the flower-shaped WO₃ particle.

In addition, the phase purity and crystal structure of the flower-shaped WO₃ particle were examined by using X-ray diffraction (XRD). Fig. 3 shows the XRD pattern of a flower-shaped WO₃ particle. Here, diffraction peaks were observed at 14.09°, 22.90°, 24.37°, 27.00°, 28.31°, 33.67°, 36.65°, 43.25°, 44.60°, 46.79°, 49.21°, 50.08°, 55.59°, 57.64°, 58.36°, and 63.54°, and were assigned to the (100), (001), (110), (101), (200), (111), (201), (300), (211), (002), (301), (220), (221), (311), (400) and (401) planes of the hexagonal phase (JCPDS card no: 35-1001)⁵⁵ of WO₃·0.33H₂O with lattice parameters a = b = 7.285 Å and c = 3.883 Å. No impurity peaks were detected from the pattern, and the sharp and strong diffraction peaks indicated a well-crystallized product. Therefore, both XRD and EDS analyses revealed that pure hexagonal tungsten oxide was successfully synthesized by the current hydrothermal method.

Fig. 3 XRD pattern of the flower-shaped WO₃ particle.

Growth of the flower-shaped WO₃ particle

In order to study the growth process of the flower-shaped WO₃ particle, a series of time-dependent experiments were carried out. The WO₃ samples were synthesized at 160 °C for various reaction times. Fig. 4a–d display the SEM images of the synthesized WO₃ samples. A reaction time of 12 h yielded relatively uniform microclusters formed by strips (Fig. 4a). When the reaction time was increased to 15 h, the strips gradually became longer, and the microclusters became larger and somewhat closer together (Fig. 4b). As the hydrothermal reaction time was increased to 18 h, the 3D WO₃ nanostructures gradually grew, and at the same time the microclusters were even closer together, as shown in Fig. 4c.

Fig. 4 SEM images of the WO₃ sample synthesized at 160 °C for various reaction times: (a) 12 h, (b) 15 h, (c) 18 h, and (d) 20 h.

Finally, 3D flower-shaped nanostructures were obtained for a reaction time of 20 h (Fig. 4d).

SEM images of the WO₃ product synthesized without PEG, using the hydrothermal method at 160 °C for 20 h, are shown in Fig. 5. PEG had a concrete effect on the morphology. Fig. S2† shows the SEM images of the products synthesized using different dosages of PEG. The self-assembled spherical morphology was still observed when PEG was added. When a mass of 0.2 g of PEG was added to the solution, the spherical morphology was observed to consist of many lamellar structures. With the amount of PEG used was increased to 0.4 g, the spherical flower-shaped WO₃ particle was obtained. As the amount of PEG included was increased, the agglomeration of lamellar structures became more compact and the uniformity of structure became disturbed. Too much PEG obviously changed the structure, as the spherical flower-shaped structure was no longer observed. Thus, taken together, the reaction time and PEG played critical roles in controlling the final morphology of the WO₃ product. Furthermore, the structure and morphology had consequences on the performance of the product.

Fig. 5 (a and b) SEM images of the WO₃ sample synthesized without PEG.

Gas-sensing performance of the flower-shaped WO₃ particle

The 3D structures of semiconductor materials might be advantageous for gas-sensing applications. Therefore, we investigated the gas-sensing performances of the fabricated samples towards NO_x in air at room temperature. Fig. 6a shows the typical response and recovery curves of a gas sensor, fabricated from flower-shaped WO₃ particles, that was exposed to various concentrations of NO_x ranging from 97 ppm down to 0.97 ppm at room temperature in air. The gas sensitivity was obviously highly dependent on the gas concentration. As shown in Fig. 6a, the resistance of the WO₃ sensor was observed, under the present operating conditions, to undergo a drastic decline when NO_x gas was injected, but then an increase to approximately its initial value after the sensor was exposed to air for some time. The decline was due to NO_x, as oxidizing gases, being able to react with the chemisorbed oxygen and withdraw electrons from WO₃, thus increasing the hole conductance. In general, when p-type semiconductors are exposed to NO_x gas molecules,²¹ the concentration of hole carriers on the surface of the semiconductor increases due to the loss of an electron, as NO_x has a higher electron affinity, which results in a decrease in resistance in the semiconductor. Thus the synthesised WO₃ acted as a p-type semiconductor. The decrease in electrons and increase in hole carriers on the surface of the semiconductor. The sensitivity of WO₃ to 97 ppm NO_x was observed to be 80%. Previous reported studies on NO₂/NO sensing by WO₃ nanostructures indicated that good sensitivity could only be achieved at high temperatures.^56–58 The current WO₃ sensor, however, showed high sensitivity to NO_x and a short response time even at room temperature.

Fig. 6 The results of the gas response of the WO₃ sensor to 97.0–0.97 ppm NO_x at room temperature in air. (a) Response recovery curves. (b) Corresponding sensitivity and response time.

The relationship between the sensitivity and the response time of the flower-shaped WO₃ particle at various NO_x concentrations is shown in Fig. 6b. When the concentration of NO_x used was 97 ppm, the response time was only 4.5 s and the highest sensitivity, at 80%, was observed. As the concentration of NO_x was decreased from 97.0 ppm to 0.97 ppm, the response time of the WO₃ sensor was found to increase, and its sensitivity decreased. This result was associated with the gas concentration, and gas diffusion and adsorption on the surface of the nanomaterials. It should be noted that the lowest concentration of NO_x that we tested was 0.97 ppm. Using the current conditions, the sensitivity and response time of our WO₃ sensor to this concentration of NO_x were observed to be 9% and 39 s, respectively. These results suggest that 0.97 ppm NO_x approximately represents the limit of detection of the WO₃ sensor for NO_x at room temperature. The gas sensitivity and response time results are also listed in Table 1. Moreover, results of the previously published studies of WO₃–based devices that detect NO_x (NO or NO₂) gases are listed in Table S1. † The gas sensor based on flower-shaped WO₃ particle was unique in showing a high response and low detection limit at a low working temperature (i.e., room temperature).

Table 1 The sensitivity and response time of the flower-shaped WO₃ particle sensor to 97.0–0.97 ppm NO_x at room temperature in air

NOx (ppm)	97.0	48.5	29.1	9.70	4.85	2.81	0.97
Sensitivity (%)	80	76	62	40	22	12	9
Response time (s)	4.5	7.0	8.0	12.5	18.5	26.0	39.0

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Moreover, the relationship between the gas sensitivity and Ln[NO_x] for the current WO₃ sensor was found to be relatively linear in the investigated NO_x concentration range, with a correlation coefficient (R²) of 0.9488 for the equation y = 17.82x − 0.4459 (Fig. 7a). It is thus possible to use this relationship and the current sensor to determine the concentration of an unknown sample of NO_x gas. In addition, to determine the selectivity of the WO₃ sensor, we tested its ability to detect O₂, NH₃, H₂, CO and C₂H₂ as well as NO_x, all at the same 97 ppm concentration and at room temperature. The results, as shown in Fig. 7b, revealed the WO₃ sensor to be quite selective for NO_x.

Fig. 7 (a) Gas sensitivity as a function of the natural logarithm of the NO_x concentration for the WO₃ sensor. A linear relationship was observed. (b) Sensitivity of the WO₃ sensor to 97.0 ppm of various gases at room temperature in air.

These results taken together suggest that the WO₃ sensor is suitable for being commercially applied to NO_x detection.

Gas-sensing mechanism of the WO₃ sensor

We expect the gas-sensing mechanism of the WO₃ sensor to follow the surface charge model, which may be explained by the change in the resistance of the sensor upon exposure to different gas atmospheres.^16,21,22 Oxygen adsorption plays an important role in electrical transport properties of WO₃. According to the literature, the semiconductor reacts with O₂, resulting in the formation of several oxygen adsorbates (such as O⁻, O²⁻ and O₂⁻) at its surface and grain boundaries under an air atmosphere.^59–61 When WO₃ is exposed to air, oxygen molecules can easily trap the electrons to form oxygen adsorbates, by capturing free electrons from the conduction band or donor level of WO₃.

A schematic of the proposed gas-sensing mechanism of the WO₃ sensor is shown in Fig. 8. When the sensor film is exposed to an oxidizing gas such as NO_x, the gas molecules could attract the electrons from WO₃ because of the high electron affinity of the NO_x molecules, which would lead to electron transfer from the flower-shaped WO₃ particle to surface adsorbates of NO_x. The adsorption of NO₂ on WO₃ would lead to the formation of NO₂⁻ and the adsorption of electrons from the conduction band or donor level of WO₃, which would lead to an increase in the hole density. Finally a rapid decrease of the resistance would result (as was observed, see Fig. 6a). The target gas molecules (NO₂) would directly be adsorbed onto WO₃ and react with O⁻ and generate bidentate NO₃⁻.^21,22 The target gas molecules (NO) can also be adsorbed onto WO₃ and react with O⁻ and generate NO₂⁻.

Fig. 8 Schematic of a proposed mechanism of the detection of NO_x by the WO₃ sensor.

Conclusions

Here, we have demonstrated a facile hydrothermal method to synthesize flower-shaped WO₃ particles with dimensions of 3 μm. These WO₃ particles showed a terrific gas-sensing performance for NO_x as a p-type semiconductor material, with a low detection limit of 0.97 ppm, a short response time of 4.5 s to 97 ppm NO_x, and high sensitivity. The good gas-sensing performance is attributed to the thinness of the WO₃ nanosheets and the 3D structures of the flower-shaped WO₃ particles, which can efficiently facilitate gas diffusion, adsorption, desorption and electrical transport on the surface of these nanomaterials. These 3D flower-shaped nanostructures should provide other nanomaterials with places to synthesize nanocomposite materials.

Acknowledgements

We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (No. 51572034, 21501104, 201401012, 201501014, 201601018), the Natural Science Foundation of Heilongjiang Province (B2015014), Science and technology project of Jilin Province (20140520079), the Natural Science Foundation of Changchun University of Science and Technology (No. XQNJJ-2015-07, XQNJJ-2013-10).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21322g

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