Monodisperse WO₃ hierarchical spheres synthesized via a microwave assisted hydrothermal method: time dependent morphologies and gas sensing characterization

Abstract

Monodisperse WO₃ hierarchical spheres were successfully synthesized via a microwave assisted hydrothermal method and a subsequent annealing process. The synthesis was performed using peroxopolytungstic acid as a precursor in the presence of sodium sulfate. The effects of hydrothermal reaction time on the microstructure were also investigated and discussed. It is found that the reaction time influences the morphologies in terms of particle sizes and dispersities. A gas sensor based on the as-prepared WO₃ was fabricated and tested. The results revealed that the sensor showed relatively good selectivity and repeatability to acetone vapour. When the acetone concentration was in the range of 100 to 1000 ppm, the relationship between the response and the acetone concentration exhibited a good linearity.

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Introduction

Over the past decades, metal oxides have attracted significant attention due to their tunable properties and important technological applications. Among them, tungsten oxide (WO₃), an important n-type semiconductor with a band gap of about 2.5 eV, has attracted a lot of attention due to its unique and interesting properties which enable its application in many aspects. For instance, gas sensors,^1–4 photocatalysts,^5,6 electro/gas/photo chromic devices,^7–9 field-emission devices,^10,11 and solar-energy devices.^12,13 In order to satisfy the different requirements of such applications, much effort has been made to synthesize tungsten oxide with various nanostructures. Recently, various WO₃ nano/microstructures have been successfully synthesized, including zero dimensional (0D) nanoparticles,^14,15 one-dimensional (1D) nanorods,¹⁶ nanowires,¹⁷ nanofiber¹⁸ and nanotubes,¹⁹ two-dimensional (2D) nanobelts and nanosheets,^20–22 and three-dimensional (3D) brush-like and urchin-like,²³ flower-like,²⁴ hollow or porous architectures,^25,26 and so forth.

Compared to solid structures, functional materials with 3D hierarchical architectures which are assembled from 1D or 2D nanosized building blocks might have some unique physical and chemical properties. So they will provide more opportunities for exploring novel properties and superior device performances, such as catalysis,²⁷ adsorption,²⁸ separation,²⁹ self-cleaning,³⁰ and so on. Thus, recently, people are motivated to synthesize WO₃ materials with hierarchical structures for their electronic, chemical and optoelectronic applications. The precondition to obtain WO₃ hierarchical structures is to assemble functional nanoscale building blocks into appropriate superstructures. In order to obtain those desired structures, a lot of efficient methods were utilized, including template directed synthesis,³¹ solution-based approach,³² and chemical vapor deposition³³ and hydrothermal reactions.³⁴ Significantly, Yin and co-workers³⁵ reported the self-assembly of WO₃ nanorods into snowflake-like WO₃ hierarchical nanostructures by a hydrothermal process in the presence of glycine acid and also investigated their super-hydrophobicity, photocatalytic activity, and enhanced anode performances. Xiao et al.³⁶ described a hydrothermal approach to the synthesis of flowerlike WO₃ hierarchical nanostructures assembled with nanosheets. It was found that the film exhibited applicable electrochromic and adsorptive properties. By a template-free hydrothermal method, Cai et al.³⁷ prepared a hierarchical structure Ti-doped WO₃ thin film and tested its electrochromic properties in visible-infrared region. However, there was little report about the synthesis and gas sensing characterization of monodisperse WO₃ hierarchical spheres. In this work, monodisperse WO₃ hierarchical spheres were synthesized via a microwave assisted hydrothermal method. The dependence of the morphologies on the hydrothermal reaction time was investigated. A gas sensor using the sintered spheres was fabricated and tested, the results showed that the sensor had good selectivity and repeatability to acetone.

Experimental

Material synthesis

All the reagents were analytical grade (Beijing Chemical Reagent Company) and used as received without further purification. In a typical process to synthesize monodisperse WO₃ hierarchical spheres, 0.1 g tungsten powder (200 mesh) was dissolved into 10 mL hydrogen peroxide (H₂O₂, 30%) to obtain the peroxopolytungstic acid (PTA) precursor. The PTA precursor was then entirely added into a beaker containing 20 mL aqueous solution of 0.25 g Na₂SO₄·2H₂O. After stirring for 30 min, the obtained solution was transferred into a Teflon-liner and then put into a microwave digestion system (Ethos One, Milestone Inc, Italy). The reactions were conducted at 180 °C for 60 min. After cooling to room temperature, the precipitates were collected and washed with deionized water and absolute ethanol by centrifugation for several times. After that, the as-prepared products were dried at 80 °C for 10 h in air. In order to investigate the effect of hydrothermal reaction time, other three materials were prepared with almost same processes, just changing the hydrothermal reaction time to 10, 30 and 240 min.

Characterization

X-ray powder diffraction (XRD) analysis was conducted on a Rigaku D/max-2500 X-ray diffractometer with Cu-Kα₁ radiation (λ = 0.15406 nm), and the substrate was a quartz plate. Field emission scanning electron microscopy (FESEM) images were recorded on a JEOL JSM-7500F microscope operating at 15 kV. Transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL JEM-200EX microscope with accelerating voltage of 200 kV and a JEOL JEM-3010 microscope operated at 200 kV, respectively.

Fabrication and measurement of gas sensor

A sensor device was fabricated as follows: proper amount of the as-prepared WO₃ powder was mixed with deionized water to obtain a paste, and then the paste was coated onto the outside of an Al₂O₃ ceramic tube (4 mm in length, 1.2 mm in external diameter, and 0.8 mm in internal diameter, attached with a pair of gold electrodes, each electrode connected with a Pt wire). After drying in air for 1 h, the sensor was sintered at 400 °C for 2 h to improve its stability and repeatability. To keep the sensor working at elevated temperatures, a Ni–Cr alloy coil was inserted into the ceramic tube as a heater. The structure of the sensor was schematically shown in Scheme 1. The gas-sensing properties of the sensor were measured with a static gas-sensing characterization system, as also diagrammed in Scheme 1, environmental air was used as both a reference gas and a diluting gas to obtain desired concentrations of target gases.

Scheme 1 Schematic diagram of the sensor and the gas-sensing characterization system.

A typical testing procedure was as follows. First, fresh air (atmospheric air) was introduced into the chamber, and then the calculated amount of the target gas or liquid was injected into the chamber. The fans were used to mix the target gas with air uniformly. Finally, the sensor was put into the chamber to test its gas sensitive performance. When the response reached a constant value, the target gas was removed by a pump and fresh air was introduced into the testing chamber to let the sensor recover. For the target gases obtained from liquid, the concentration of target gas was calculated by the following formula,

C = (22.4 × ρ × φ × V₁ × 1000)/(M × V₂)

where C (ppm) is the target gas concentration, ρ (g mL⁻¹) is the density of the liquid, φ is the required gas volume fraction, V₁ (μL) is the volume of liquid, V₂ (L) is the volume of the chamber, and M (g mol⁻¹) is the molecular weight of the liquid. The response of the sensor was defined as R_g/R_a and R_a/R_g for oxidizing gas and reducing gas, respectively, where R_a and R_g stand for the resistances of the sensor in air and in the target gas, respectively.

Results and discussion

Materials characterization

Fig. 1 shows the XRD patterns of the obtained materials. As can be seen, the diffraction peaks could be indexed to the standard WO₃ with the hexagonal structure and matched well with the reported data (JCPDS: 33-1387). It is also can be seen that, with the increase of the hydrothermal reaction time, the intensities of the diffraction peaks are enhanced. It indicates that the extension of reaction time can improve the crystallinities of the precipitates. Introducing the Scherrer equation, the crystallite sizes of the four samples are calculated to be 38.2 nm 45.5 nm 48.3 nm 50.2 nm. This also demonstrates that the hydrothermal reaction time plays an important role for the WO₃ growth.

Fig. 1 XRD patterns of the materials obtained with different reaction time.

Fig. 2(a)–(d) corresponds to the SEM images of the products of different hydrothermal reaction time. It is obvious that the size and dispersity of the products are variable with reaction time. As be seen, when the reaction time increased from 10 to 240 min, the diameters of the spheres gradually increased from about 2 to 3 μm.

Fig. 2 SEM images of the products synthesized with different hydrothermal reaction time: (a) 10, (b) 30, (c) 60 and (d) 240 min.

Moreover, when the reaction time increased from 10 to 60 min, the dispersity of the products can be improved significantly. The products obtained with the reaction time of 60 min showed monodisperse and uniform morphologies. When the reaction time was further prolonged to 240 min, the sizes of the spheres had a wide distribution range, showed fairly uneven morphologies. So we can conclude that the morphologies of the spheres are very sensitive to reaction time and 60 min is the most suitable time to synthesize monodisperse WO₃ hierarchical spheres. Thus, the product obtained with the reaction time of 60 min was chosen as the sensing material for fabricating the gas sensors. A typical SEM image with higher magnification of the products obtained with the reaction time of 60 min is shown in Fig. S1 in ESI.† As can be observed, the spheres are composed of many nanorods.

The typical TEM image in Fig. 3(a) shows that the size and shape of WO₃ were corresponding well to those of the FESEM observations. Fig. 3(b) and (c) were the corresponding HRTEM image and the SAED pattern taken from a random nanorod in the white circle area on the WO₃ spheres, respectively. Both of them confirmed that the nanorods building up the sphere-like architecture were single-crystalline structures. From the HRTEM image we can calculate the lattice spacing was 0.316 nm, which is corresponding to the d spacing of (200) crystal planes of hexagonal WO₃.

Fig. 3 (a) TEM image of a WO₃ sphere, (b) HRTEM, and (c) SAED image of a randomly chosen nanorod on the sphere.

In order to show the influence of hydrothermal time more clearly, we further reduced the reaction time to 1 min. A SEM image of the obtained sample is shown in Fig. S2 in ESI.† As can be seen, the product is amorphous.

The formation of WO₃ hierarchical spheres can be ascribed to the adding of sodium sulphate. Similar to those in the literatures,^38–40 sodium sulphate plays an important role during the hydrothermal process.

Gas sensing properties test

In order to demonstrate the potential application, a gas sensor based on the as-prepared monodisperse WO₃ hierarchical spheres was fabricated and tested. For the sensor using the product obtained with the reaction time of 60 min, firstly, the optimum operating temperature of the sensor to 300 ppm acetone was examined. The response as a function of operating temperature is shown in Fig. 4(a). It is obvious that the response depends on the operating temperature. The response to acetone first increased with the rise of temperature, reached a peak at 350 °C, and then gradually decreased. Therefore, 350 °C was believed to be the optimum operating temperature for the detection of acetone, which is applied to further investigate the performances of the gas sensor.

Fig. 4 (a) Response of the sensor to 300 ppm acetone as a function of the operating temperature, (b) response of the sensor to 300 ppm various testing gases at the operating temperature of 350 °C.

As well known, selectivity is one of the important parameters to evaluate gas sensors' performances. In order to test the selectivity of the sensor based on the as-prepared WO₃, the responses of the sensor to 300 ppm various testing gases at the operating temperature of 350 °C were tested, the results were shown in Fig. 4(b). As can be seen, the sensor exhibited scarcely any response to CO, C₂H₄, H₂S and NH₃. The response of the sensor to ethanol is about half of its response to acetone. Therefore, it can be concluded that the sensor showed an acceptable selectivity towards acetone against other examined gases.

The relationship between response and acetone concentration for the sensor at operating temperature of 350 °C is displayed in Fig. 5(a), which shows that the response almost linearly increased with the acetone concentration increasing in the range of 10 to 1000 ppm for the sensor, it is corresponding to the pursuit of gas sensors design in recent years. The red straight line is the fitting line of the response–concentration curve, which can be expressed as R = 3.53719 + 0.00795 × C, where R and C stands for the response of the sensor and the concentration (ppm) of the target gas, respectively.

Fig. 5 (a) Response of the sensor to various acetone concentrations at 350 °C, the red line is the corresponding fitting curve. (b) Response transient of the sensor to 300 ppm acetone at 350 °C. The inset displays five periods of response curve.

Fig. 5(b) shows the response transient of the sensor to 300 ppm acetone at 350 °C, and the results indicate that the sensor's response process is much faster than the recover process. The five periods of response curve indicated that the sensor had good repeatability and stability, as shown in the inset of Fig. 5(b).

A comparison the gas sensing properties of WO₃ materials in the present work and the literatures is shown in Table 1. As can be seen, compared with WO₃ materials with other morphologies, our hierarchical spheres have some advantages in terms of working temperature and gas response. Moreover, the relationship between the response and the target gas concentration is linear.

Table 1 Gas sensing properties of WO₃ materials in the present work and the literatures

Materials	Target gas	Concentration (ppm)	Response	Temperature (°C)	Linear^a	Reference
a Linear means the relationship between the response and the target gas concentration exhibited a linearity.
Fe-doped film	CO	1000	∼1.2	150	Not	41
Nanoflakes	Alcohol	100	∼4.0	250	Not	42
Hollow spheres	Acetone	100	∼4.6	400	Not	43
Octahedrons	Benzene	44.1	∼5.0	400	Not	44
Solid spheres	Hydrogen	500	∼3.0	250	Yes	45
Nanorods	Alcohol	100	∼3.2	500	Not	46
Hierarchical spheres	Acetone	100	∼4.5	350	Yes	This work

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Gas sensing mechanism

The sensing mechanism can be explained by the change in resistance of the sensor upon exposure to different gas atmospheres.⁴⁷ When the sensor based on as-prepared WO₃ spheres is exposed to air, oxygen molecules will adsorb onto the surfaces of spheres, and form chemisorbed oxygen species by capturing electrons from the conductance band of WO₃. The decrease of the electron concentration in the conduction band leads to stabilization of high surface resistance. When the sensor is exposed to reductive gas atmospheres (such as ethanol, acetone and CO) at a moderate temperature, these gas molecules will react with the adsorbed oxygen species on its surfaces. This process releases the trapped electrons back to the conductance band of WO₃ and results in an increase in the electron concentration. This effect eventually increases the conductivity of the WO₃ spheres.

Conclusion

Monodisperse WO₃ hierarchical spheres were synthesized by a microwave assisted hydrothermal method. The effect of hydrothermal reaction time was investigated and discussed. A gas sensor based on the as-prepared WO₃ spheres showed good selectivity and repeatability to acetone. Moreover, the responses were proportional to the increasing concentrations of acetone, when the gas concentrations were in the range of 100 to 1000 ppm.

Acknowledgements

This work is supported by Application and Basic Research of Jilin Province (20130102010JC), the National Nature Science Foundation of China (no. 61304242, 61327804 and 61134010), Program for Chang Jiang Scholars and Innovative Research Team in University (no. IRT13018) and “863” High Technology Project (2013AA030902).

Notes and references

1. S. Vallejos, T. Stoycheva, F. E. Annanouch, E. Llobet, P. Umek, E. Figueras, C. Cane, I. Gracia and C. Blackman, RSC Adv., 2014, 4, 1489.
2. Y. S. Shim, H. G. Moon, D. H. Kim, L. H. Zhang, S. J. Yoon, Y. S. Yoon, C. Y. Kang and H. W. Jang, RSC Adv., 2013, 3, 10452.
3. T. Kida, A. Nishiyama, Z. Hua, K. Suematsu, M. Yuasa and K. Shimanoe, Langmuir, 2014, 30, 2571.
4. Z. Wang, P. Sun, T. Yang, Y. Gao, X. Li, G. Lu and Y. Du, Sens. Actuators, B, 2013, 186, 734.
5. D. Chen and J. Ye, Adv. Funct. Mater., 2008, 18, 1922.
6. M. Miyauchi, Y. Nukui, D. Atarashi and E. Sakai, ACS Appl. Mater. Interfaces, 2013, 5, 9770.
7. N. Tahmasebi Garavand, S. M. Mahdavi and A. Irajizad, Appl. Surf. Sci., 2013, 273, 261.
8. C. Yan, W. Kang, J. Wang, M. Cui, X. Wang, C. Y. Foo, K. J. Chee and P. S. Lee, ACS Nano, 2013, 8, 316.
9. Y. Djaoued, S. Balaji and N. Beaudoin, J. Sol-Gel Sci. Technol., 2013, 65, 374.
10. Y. Baek and K. Yong, J. Phys. Chem. C, 2007, 111, 1213.
11. M. Trapatseli, D. Vernardou, P. Tzanetakis and E. Spanakis, ACS Appl. Mater. Interfaces, 2011, 3, 2726.
12. B. M. Klepser and B. M. Bartlett, J. Am. Chem. Soc., 2014, 136, 1694.
13. D. Su, J. Wang, Y. Tang, C. Liu, L. Liu and X. Han, Chem. Commun., 2011, 47, 4231.
14. W. Morales, M. Cason, O. Aina, N. R. de Tacconi and K. Rajeshwar, J. Am. Chem. Soc., 2008, 130, 6318.
15. Z. Wang, P. Sun, T. Yang, X. Li, Y. Du, X. Liang, J. Zhao, Y. Liu and G. Lu, Sens. Lett., 2013, 11, 423.
16. F. Zheng, M. Guo and M. Zhang, CrystEngComm, 2013, 15, 277.
17. L. Gao, X. Wang, Z. Xie, W. Song, L. Wang, X. Wu, F. Qu, D. Chen and G. Shen, J. Mater. Chem. A, 2013, 1, 7167.
18. N. H. Kim, S. J. Choi, D. J. Yang, J. Bae, J. Park and I. D. Kim, Sens. Actuators, B, 2014, 193, 574.
19. Z. G. Zhao and M. Miyauchi, Angew. Chem., 2008, 120, 7159.
20. P. Q. Wang, Y. Bai, P. Y. Luo and J. Y. Liu, Catal. Commun., 2013, 38, 82.
21. M. R. Waller, T. K. Townsend, J. Zhao, E. M. Sabio, R. L. Chamousis, N. D. Browning and F. E. Osterloh, Chem. Mater., 2012, 24, 698.
22. S. B. Upadhyay, R. K. Mishra and P. P. Sahay, Sens. Actuators, B, 2014, 193, 19.
23. A. Yella, U. K. Gautam, E. Mugnaioli, M. Panthofer, Y. Bando, D. Golberg, U. Kolb and W. Tremel, CrystEngComm, 2011, 13, 4074.
24. Y. Qiu, G. L. Xu, Q. Kuang, S. G. Sun and S. Yang, Nano Res., 2012, 5, 826.
25. H. Ishihara, G. K. Kannarpady, K. R. Khedir, J. Woo, S. Trigwell and A. S. Biris, Phys. Chem. Chem. Phys., 2011, 13, 19553.
26. C. M. Sim, Y. J. Hong and Y. C. Kang, ChemSusChem, 2013, 6, 1320.
27. M. O. Coppens, J. Sun and T. Maschmeyer, Catal. Today, 2001, 69, 331.
28. W. Cai, J. Yu and M. Jaroniec, J. Mater. Chem., 2010, 20, 4587.
29. F. Xu, Y. Wang, X. Wang, Y. Zhang, Y. Tang and P. Yang, Adv. Mater., 2003, 15, 1751.
30. B. Bhushan, K. Koch and Y. C. Jung, Appl. Phys. Lett., 2008, 93, 093101.
31. C. Bae, H. Yoo, S. Kim, K. Lee, J. Kim, M. M. Sung and H. Shin, Chem. Mater., 2008, 20, 756.
32. K. Sivula, R. Zboril, F. Le Formal, R. Robert, A. Weidenkaff, J. Tucek, J. Frydrych and M. Grätzel, J. Am. Chem. Soc., 2010, 132, 7436.
33. Z. S. Houweling, J. W. Geus and R. E. I. Schropp, Chem. Vap. Deposition, 2010, 16, 179.
34. X. Su, F. Xiao, Y. Li, J. Jian, Q. Sun and J. Wang, Mater. Lett., 2010, 64, 1232.
35. J. Yin, H. Cao, J. Zhang, M. Qu and Z. Zhou, Cryst. Growth Des., 2012, 13, 759.
36. W. Xiao, W. Liu, X. Mao, H. Zhu and D. Wang, J. Mater. Chem. A, 2013, 1, 1261.
37. G. F. Cai, X. L. Wang, D. Zhou, J. H. Zhang, Q. Q. Xiong, C. D. Gu and J. p. Tu, RSC Adv., 2013, 3, 6896.
38. W. Cai, J. Yu, B. Cheng, B. L. Su and M. Jaroniec, J. Phys. Chem. C, 2009, 113, 14739.
39. Y. Xia, L. Zhang, X. Jiao and D. Chen, Phys. Chem. Chem. Phys., 2013, 15, 18290.
40. W. Cai, J. Yu, S. Gu and M. Jaroniec, Cryst. Growth Des., 2010, 10, 3977.
41. M. Ahsan, T. Tesfamichael, M. Ionescu, J. Bell and N. Motta, Sens. Actuators, B, 2012, 162, 14.
42. J. Xiao, P. Liu, Y. Liang, H. B. Li and G. W. Yang, Nanoscale, 2012, 4, 7078.
43. X. L. Li, T. J. Lou, X. M. Sun and Y. D. Li, Inorg. Chem., 2004, 43, 5442.
44. H. Zhang, M. Yao, L. Bai, W. Xiang, H. Jin, J. Li and F. Yuan, CrystEngComm, 2013, 15, 1432.
45. T. Yang, Y. Zhang and C. Li, Ceram. Int., 2014, 40, 1765.
46. Y. Li, X. Su, J. Jian and J. Wang, Ceram. Int., 2010, 36, 1917.
47. N. Yamazoe, G. Sakai and K. Shimanoe, Catal. Surv. Asia, 2003, 7, 63.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra01946f

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