Xylene sensor based on α-MoO₃ nanobelts with fast response and low operating temperature

Abstract

A hydrothermal treatment strategy was used to synthesize α-MoO₃ nanobelts. X-ray diffraction (XRD) and scanning electron microscopy (SEM) was used to characterize the phase and the morphology of the samples, respectively. The results show the length and the width of the α-MoO₃ nanobelts were about 6 μm and 200 nm, respectively. The sensing properties towards various types of gases were tested and heater-type sensors coated with α-MoO₃ nanobelts showed excellent performance towards xylene. The sensors achieved a response of 3 to 100 ppm xylene at an operating temperature of 206 °C. The response and recovery time were 7 s and 87 s, respectively.

---

Introduction

Xylene mainly comes from chemical engineering materials such as painting and adhesive in indoor environments. Even a microscale of xylene has tremendous damage to human beings. In addition, careless oral ingestion of xylene can cause acute pneumonia and cancer. Thus, it is significant to detect and measure the existence and content of xylene. Hitherto, various effective methods, such as solid-state chemical technology,¹ micro-fabricated preconcentrator technology,² and double-layered metal–oxide thin film technology,³ have been used to detect xylene. However, these methods have disadvantages of slow response speed, the use of bulky instruments and are dangerous to the body. Metal oxide semiconductors demonstrate the advantages of low cost, facile method, easy fabrication, fast response and recovery time.

Various materials, such as Co₃O₄ and In_2−xNi_xO₃, have been used to detect xylene and exhibited excellent performance.^1,3–5 However, they are not satisfactory in response time towards xylene. Therefore, an improvement in response time is urgently needed in the detecting xylene.

α-MoO₃ has been investigated and applied in solar cells,^6–9 catalysis,^10–13 capacitors,^14–16 light-emitting diodes,^17,18 gas sensors and optical detectors. In recent years, α-MoO₃ has been one of the most newly-developed n-type metal oxide semiconductors, which can detect toxic gas. α-MoO₃ is an n-type semiconductor with a wide band gap of 3.2 eV and has been investigated for NO₂,¹⁹ ethanol,²⁰ TMA,²¹ H₂S,²² H₂,²³ and C₂H₅OH²⁴ detection. However, to date, there is no report for the detection of xylene using α-MoO₃ nano structures.

One dimensional (1D) nanostructure oxide semiconductors, such as nanotubes, nanowires, nanofibers and nanobelts have received considerable attention due to the high ratio of surface to volume. Plenty of methods, such as electrospinning, vapor–solid and hydrothermal synthesis, have been used to prepare 1D nanostructured materials during the past few decades. One-dimension nano-products play an important role in gas sensing owing to their advantages of high surface area. A high ratio of surface to volume can achieve higher response when compared with that of a low surface to volume ratio.

In this study, optimal experimental conditions for synthesizing α-MoO₃ nanobelts were researched using a facile hydrothermal method at a low temperature of 180 °C. The gas sensing properties based on α-MoO₃ nanobelts materials were investigated for detecting xylene. Heater-type sensors based on α-MoO₃ nanobelts performed excellent response, as well as fast response time to xylene at a low operating temperature of 206 °C, which is lower than that of most other sensors.^1,5,23,25

Experimental

Synthesis of α-MoO₃ nanobelts

All of the chemical reagents were of analytical grade and used without any further purification. In a typical experimental procedure, 0.618 g of ammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O) was dissolved into 25 mL of deionized water with continuous stirring to achieve an aqueous solution (0.02 M). Then, 2.5 mL of nitric acid (HNO₃) was slowly added dropwise into the aqueous solution. After stirring, the solution was transferred into a Teflon-lined stainless steel autoclave with a 50 mL capacity and underwent hydrothermal treatment for 12/24/36/48 h at 120 °C and 12/24/36/48 h at 180 °C to explore the optimum conditions of the reaction, respectively. After the autoclave was cooled down to room temperature, the white precipitate was separated by centrifugation 5 times using deionized water and ethanol. Then, the precipitate was dried at 60 °C for 12 h. Finally the products were annealed at 300 °C for 2 h at the heating rate of 1 °C min⁻¹ to obtain the light yellow α-MoO₃ nanobelts.

Characterization

X-ray diffraction patterns were achieved to measure the phase identification and the crystal size (Rigaku TTRIII X-ray diffractometer with Cu Kα 1 radiation (λ = 1.5406 Å) in the range of 20–80°). The morphologies of the samples were investigated using scanning electron microscopy (JEOL JSM-7500F microscope operating at 15 kV).

Fabrication and measurement of gas sensors

Sensor devices were fabricated using a similar method used in our previous work.^4,26 The prepared products were mixed with deionized water at a weight ratio of 1 : 4 and ground for a while. Then, the specimens were coated on the surface of the ceramic tube, which was attached with a pair of parallel golden electrodes using a small brush. The Ni–Cr alloy heating coil was inserted through the ceramic tube. The structure of the sensors is shown in Fig. 1.

Fig. 1 Structure of a heater-type gas sensor after coating with the sensing film.

The working temperature was measured by the electric current, which were produced by a CGS-8 Intelligent Gas Sensing Analysis System (Beijing Elite Tech Co., Ltd., China). The test gases were injected using micro-injections from air bags. The response of the gas sensors was expressed and measured as R_a (the resistance of the sensors in air)/R_g (the resistance of the sensor in the test gas). The response time was defined as the period in which the resistance of the sensors reached 90% from 10% of the steady-state value when exposed to the test gases, while the recovery time was defined as the period in which the resistance of sensors reached 10% from 90% of the steady-state value after exposure to oxygen.²⁷

Results and discussion

Materials characterizations

Fig. 2(A–H) shows the morphologies of the samples, which were calcined at 300 °C for 2 h after hydrothermal treatment for 12/24/36/48 h at 180 °C and 12/24/36/48 h at 120 °C, respectively. As shown in Fig. 2(E–H), the morphologies of the samples were needle-like with hydrothermal treatment for 12/24/36/48 h at 120 °C. The morphologies in Fig. 2(A–D) show that nanobelts were achieved during hydrothermal treatment for 12/24/36/48 h at 180 °C. Fig. 2(D) shows that the nanobelts formed the phenomenon of agglomeration and the uniformity of Fig. 2(C) are better than Fig. 2(A and B), thus the following discussions are with reference to the hydrothermal treatment for 36 h at 180 °C. The width of the α-MoO₃ nanobelt was about 200 nm and the length was about 6 μm.

Fig. 2 (A–H) SEM images of the pure α-MoO₃ nanostructure for 12/24/36/48 h at 180 °C and for 12/24/36/48 h at 120 °C, respectively.

The phase identification and the crystal sizes of the prepared materials were investigated by X-ray diffraction. Fig. 3 shows the XRD patterns of the specimens, which were calcined at 300 °C. The sample was assigned to pure α-MoO₃ (JCPDS 05-0508) by verifying the patterns of the sample.

Fig. 3 XRD patterns for the pure α-MoO₃ nanostructure after calcination for 2 h at 300 °C.

Gas sensing properties

The sensors, which were coated with α-MoO₃ nanobelts had a good performance on xylene. In order to find the optimum temperature of the sensors, the responses of the sensors to 100 ppm xylene at different temperatures were collected. As shown in Fig. 4, the response increased along with the working temperature from 136 °C to 206 °C. It was obvious that the temperature improved the chemical reaction between the sensor and test gas because it increased the speed of molecular vibration. However, further increase in temperature resulted in the response of the sensors to decrease. The reason for this is illustrated in Fig. 5, which shows that chemical reaction between xylene and α-MoO₃ nanobelts was so fast that the penetration was less relative to the entire sensing film. Therefore, the response of the sensor decreased along with temperature. Therefore, the response reached a maximum value of 3 when the temperature was 206 °C.

Fig. 4 Response of sensors based on α-MoO₃ nanobelts to 100 ppm xylene at different temperatures.

Fig. 5 Illustration of the optimum temperature of sensors based on α-MoO₃ nanobelts.

Fig. 6 shows the response of the sensors towards different concentrations of the test gas (xylene) at 206 °C. The response increased nearly linearly with the concentration of xylene over the range of 5 to 100 ppm and came to saturation when the concentration was over 1000 ppm. Significantly, the response of the sensor could reach 1.89 towards xylene at 5 ppm.

Fig. 6 Response of sensors based on α-MoO₃ nanobelts at 206 °C versus concentration of xylene.

Selectivity is an important property of gas sensors in their practical applications. The responses of the sensors towards different test gases such as methylbenzene, formaldehyde and carbon monoxide are shown in Fig. 7. The gases were tested at 206 °C with a concentration of 100 ppm. The results show that the response of the sensors based on α-MoO₃ nanobelts to 100 ppm of xylene could reach 3 and to other test gases were less than 1.6. The sensors also showed a low response towards other test gases at different temperature including toluene. Therefore, α-MoO₃ has a good selectivity for xylene. However, it is still difficult for us to distinguish the three forms of xylene (o, m, p) at present because of their similar chemical properties.

Fig. 7 Response of sensors based on α-MoO₃ nanobelts at 206 °C towards various test gases.

Rapid response and recovery time are significant parameters in designing metal oxide semiconductor gas sensors. Fig. 8 shows the response and recovery characteristics of the sensors towards xylene from 20 ppm to 100 ppm at 206 °C. Fig. 8 indicates that the resistance of the sensors decreased rapidly at first, and then changed slowly due to the reaction slowing down gradually when the sensors were transferred from air to xylene. The resistance of the sensors resumed slowly when sensors were exposed to the air. The time of response and recovery were 7 s and 87 s, respectively. The fast response may be due to the high ratio of surface to volume in the 1D nanostructure.

Fig. 8 Response of α-MoO₃ nanobelts to different concentrations of xylene at 206 °C.

Sensing mechanism of α-MoO₃ nanobelts to xylene

As an n-type semiconductor, the sensing performance of α-MoO₃ nanobelts maybe related to the mechanism of adsorption and desorption process of gas molecules on the surface of the oxide because of its crystal structure.²⁸ As shown in Fig. 9, the formation of defects in the α-MoO₃ nanobelts is as follows: oxygen is attached on the α-MoO₃ nanobelts when the sensor is exposed to air. The oxygen captures electrons from α-MoO₃ nanobelts and transforms into O⁻ (420 K–670 K), O²⁻ (above 670 K) and O₂⁻ (below 420 K) on the surface of sensing layer, which would lead the holes spread all over the surface of α-MoO₃ nanobelts and the increasing resistance.²⁸ When the α-MoO₃ nanobelts are transformed to the test gas (xylene), the interaction between xylene and the adsorbed oxygen ions can be explained as follows:

C₈H₁₀ + 21O⁻ → 8CO₂ + 5H₂O + 21e⁻ (420 K–670 K) (1)

C₈H₁₀ + 21O²⁻ → 8CO₂ + 5H₂O + 42e⁻ (above 670 K) (2)

2C₈H₁₀ + 21O₂⁻ → 16CO₂ + 10H₂O + 21e⁻ (below 420 K) (3)

e⁻ + h⁰ → Null (4)

Fig. 9 Illustration of sensors based on α-MoO₃ nanobelts used for sensing performance for xylene.

Because the optimum temperature of the sensors based on α-MoO₃ nanobelts was 206 °C (479 K), the reaction, including O⁻, as shown in eqn (1), was the dominative form in the process. The oxygen ion would release CO₂ and electrons when the reaction progressed. These electrons neutralize the holes in the α-MoO₃ nanobelts and cause a decrease in the resistance of the sensor. The excellent sensing performance makes α-MoO₃ nanobelts a new potential candidate for the detection of xylene.

The fast response of the sensors based on α-MoO₃ nanobelts materials could be attributed to their 1D nanostructure. The ratio of length to width of the α-MoO₃ nanobelts could reach as high as 30/1 and the nanobelts were pretty thin, which leads the adsorption and desorption of O₂ and xylene faster at the surface of the sensing layer. Consequently, the reaction occurring at the sensing surface will be greatly accelerated, which results in the faster response of the sensors based on α-MoO₃ nanobelts.

Conclusions

In conclusion, the optimal conditions of synthesizing α-MoO₃ nanobelts were explored successfully using a hydrothermal method. The α-MoO₃ nanobelts were homogeneous and thin with a width of 200 nm and length of 6 μm. The sensors based on α-MoO₃ nanobelts exhibited a fast response of 7 s and recovery time of 87 s. The sensors showed a high response for xylene at a low operating temperature of 206 °C. The results show that the α-MoO₃ nanostructure is a potential candidate for xylene gas sensing.

Acknowledgements

The study was supported by National Natural Science Foundation of China (Grant no. 61404058, 51303061), National High Technology Research and Development Program of China (Grant no. 2013AA030902), Project of Science and Technology Plan of Changchun City (Grant no. 13KG49), and Post doctoral Sustentation Fund of Jilin Province (Grant no. RB201371).

Notes and references

1. J. Hue, M. Dupoy, T. Bordy, R. Rousier, S. Vignoud, B. Schaerer, T. H. Tran-Thi, C. Rivron, L. Mugherli and P. Karpe, Sens. Actuators, B, 2013, 189, 194–198.
2. C. E. Davis, C. K. Ho, R. C. Hughes and M. L. Thomas, Sens. Actuators, B, 2005, 104, 207–216.
3. T. Akiyama, Y. Ishikawa and K. Hara, Sens. Actuators, B, 2013, 181, 348–352.
4. Y. Chen, L. Zhu, C. Feng, J. Liu, C. Li, S. Wen and S. Ruan, J. Alloys Compd., 2013, 581, 653–658.
5. H.-M. Jeong, H.-J. Kim, P. Rai, J.-W. Yoon and J.-H. Lee, Sens. Actuators, B, 2014, 201, 482–489.
6. J. Park, G. Park, H.-J. Ko and J.-S. Ha, Ceram. Int., 2014, 40, 16281–16285.
7. Y. Ma, X. Zhang, M. Yang and Y. Qi, Mater. Lett., 2014, 136, 146–149.
8. N. Datta, N. S. Ramgir, S. Kumar, P. Veerender, M. Kaur, S. Kailasaganapathi, A. K. Debnath, D. K. Aswal and S. K. Gupta, Sens. Actuators, B, 2014, 202, 1270–1280.
9. M. Maniruzzaman, M. A. Rahman, K. Jeong, H.-s. Nam and J. Lee, Renewable Energy, 2014, 71, 193–199.
10. Y. Meng, T. Wang, S. Chen, Y. Zhao, X. Ma and J. Gong, Appl. Catal., B, 2014, 160–161, 161–172.
11. Z. Zhang, R. Yang, Y. Gao, Y. Zhao, J. Wang, L. Huang, J. Guo, T. Zhou, P. Lu, Z. Guo and Q. Wang, Sci. Rep., 2014, 4, 6797.
12. Y. Wang, X. Zhang, Z. Luo, X. Huang, C. Tan, H. Li, B. Zheng, B. Li, Y. Huang, J. Yang, Y. Zong, Y. Ying and H. Zhang, Nanoscale, 2014, 6, 12340–12344.
13. D. Wang, N. Liu, J. Zhang, X. Zhao, W. Zhang and M. Zhang, J. Mol. Catal. A: Chem., 2014, 393, 47–55.
14. F. Jiang, W. Li, R. Zou, Q. Liu, K. Xu, L. An and J. Hu, Nano Energy, 2014, 7, 72–79.
15. X. Zhang, X. Zeng, M. Yang and Y. Qi, ACS Appl. Mater. Interfaces, 2014, 6, 1125–1130.
16. C. Liu, Z. Li and Z. Zhang, Electrochim. Acta, 2014, 134, 84–91.
17. Z. Hongmei, X. Jianjian, Z. wenjin and H. Wei, Displays, 2014, 35, 171–175.
18. Z. Yin, X. Zhang, Y. Cai, J. Chen, J. I. Wong, Y. Y. Tay, J. Chai, J. Wu, Z. Zeng, B. Zheng, H. Y. Yang and H. Zhang, Angew. Chem., 2014, 53, 12560–12565.
19. S. Bai, S. Chen, L. Chen, K. Zhang, R. Luo, D. Li and C. C. Liu, Sens. Actuators, B, 2012, 174, 51–58.
20. A. D. Rushi, K. P. Datta, P. S. Ghosh, A. Mulchandani and M. D. Shirsat, J. Phys. Chem. C, 2014, 118, 24034–24041.
21. Y. H. Cho, Y. N. Ko, Y. C. Kang, I.-D. Kim and J.-H. Lee, Sens. Actuators, B, 2014, 195, 189–196.
22. W.-S. Kim, H.-C. Kim and S.-H. Hong, J. Nanopart. Res., 2009, 12, 1889–1896.
23. M. B. Rahmani, S. H. Keshmiri, J. Yu, A. Z. Sadek, L. Al-Mashat, A. Moafi, K. Latham, Y. X. Li, W. Wlodarski and K. Kalantar-zadeh, Sens. Actuators, B, 2010, 145, 13–19.
24. L. Wang, P. Gao, D. Bao, Y. Wang, Y. Chen, C. Chang, G. Li and P. Yang, Cryst. Growth Des., 2014, 14, 569–575.
25. L. Brigo, M. Cittadini, L. Artiglia, G. A. Rizzi, G. Granozzi, M. Guglielmi, A. Martucci and G. Brusatin, J. Mater. Chem. C, 2013, 1, 4252.
26. C. Feng, W. Li, C. Li, L. Zhu, H. Zhang, Y. Zhang, S. Ruan, W. Chen and L. Yu, Sens. Actuators, B, 2012, 166–167, 83–88.
27. L. Li, P. Gao, M. Baumgarten, K. Mullen, N. Lu, H. Fuchs and L. Chi, Adv. Mater., 2013, 25, 3419–3425.
28. F. Qu, C. Feng, C. Li, W. Li, S. Wen, S. Ruan and H. Zhang, Int. J. Appl. Ceram. Technol., 2014, 11, 619–625.

---