Enhanced gas sensing properties of V₂O₅ nanowires decorated with SnO₂ nanoparticles to ethanol at room temperature

Abstract

V₂O₅ nanowires decorated with SnO₂ nanoparticles are prepared by two-step mild hydrothermal reaction, and the gas sensor device is fabricated by coating the nanowires as a thick film on the alumina tube. The pure V₂O₅ nanowires have almost no response to ethanol at room temperature, however, the sensitivity of V₂O₅ nanowires decorated with SnO₂ nanoparticles is 1.46 upon exposure to 1000 ppm ethanol gas. The highest sensitivity of gas sensor based on V₂O₅ nanowires decorated with SnO₂ nanoparticles to 1000 ppm ethanol is about 14, which is 2–3 times of pure V₂O₅ nanowires. The improved sensing performance of the composite is due to the increased depletion width and active sites along the nanowires, the energy gap between V₂O₅ nanowires and SnO₂ nanoparticles promotes the electrons transport. Moreover, the gas sensor based on V₂O₅ nanowires decorated with SnO₂ nanoparticles possesses a good selectivity to ethanol compared with other gases, such as CO₂, H₂O and NH₃, and the stability of gas sensing performance is quite good, which implies that it would be a good candidate in the potential application.

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1. Introduction

With the increasing awareness of human health and security, alcohol sensors have been in great demand in biomedical, chemical, wine quality monitoring and breath analysis applications. Nowadays, resistive gas sensors based on metal oxide semiconductor materials have become a hot research area and occupied more than 60% of the market share due to their ease of use, real-time detection, high sensitivity and low cost.^1–7

At present, semiconductors including SnO₂, ZnO and Fe₂O₃ are mostly used for the detection of ethanol vapor. However, the poor selectivity and high working temperature limit their application and further exploration of new materials is needed.^2,3 Vanadium pentoxide (V₂O₅) has attracted considerable interest owing to its unique layered structure and various valence states like V⁵⁺, V⁴⁺ and V³⁺.^4–6 Having the large specific surface area and character of electrons transmission along axis direction, one-dimensional (1-D) structured V₂O₅ may have an even promising future in the application of gas sensor. Raible et al. fabricated gas sensors by depositing V₂O₅ nanofibres from aqueous suspension onto the silicon substrates and tested the gas sensing properties to amines.⁷ The sensors could be operated at room temperature, and the extremely high sensitivity for 1-butylamine (limit of detection (LOD) below 30 ppb) and moderate sensitivity for ammonia were obtained. However, very little sensitivity was observed for toluene and 1-propanol vapors. Yu et al. prepared gas nanodevices based on V₂O₅ nanowires to helium gas and environmental pressures,⁸ and the electrical response to helium was due to the physical adsorption of the helium atoms into the interlayer of V₂O₅ nanowires. Liu et al. prepared curly V₂O₅ nanobelts via a simple mild hydrothermal method and tested the gas sensing properties to ethanol, H₂S, NH₃, H₂, C₃H₈, CO and NO_x,⁹ and the sensors based on V₂O₅ nanobelts had a good response to ethanol in a wide range of 150–400 °C, and it was found that the layered structure would allow gas molecules to insert and approach the active position easily. Modafferi et al. reported a kind of highly sensitive resistive sensor based on electrospun V₂O₅ fibers to ammonia, it could response to ammonia at the temperature range from 50 °C to 300 °C, and the detection limit was about 100 ppb at 200 °C.¹⁰

Although V₂O₅ nanowires have excellent gas sensing properties according to the reports, which may make them an ideal candidate for gas sensor. However, there are few reports focused on V₂O₅ nanowires based gas sensor in recent years. The limited maximum sensitivity performance and high operating temperature might be the reasons that limit their real application.¹¹ The solutions to improve the sensitivity as well as decrease operating temperature in literature focus on controlling the morphology,¹² element doping¹³ and decorating with different semiconductor metal oxides.^14–16 Decorating with different semiconductor metal oxides is an effective choice due to the advantage of the simple progress, low cost, high yield, and so forth.

Among them, tin dioxide (SnO₂) is an attractive sensing material due to its high electrical conductivity, wide-band gap and excellent chemical stability.^17–20 SnO₂ nanoparticles are one of the most applied sensing materials, which have been used to fabricate gas sensors to C₂H₅OH, NH₃, H₂, CH₄, H₂O and so on. In particular, SnO₂ nanoparticles are widely used for an additive as well, and they show excellent performance at improving the gas sensing properties of the base materials.^21,22 Chen reported the significant enhancement of CuO nanowire gas sensing performance at room temperature through the surface functionalization with SnO₂ nanocrystals,²³ and the sensitivity enhancement could be as high as 300% for detecting 1% NH₃ diluted in air. Zhang prepared SnO₂ nanoparticles–reduced graphene oxide (SnO₂–RGO) nanocomposites via hydrothermal treatment.²⁴ It was found that SnO₂–RGO nanocomposites exhibited high response of 3.31 to 5 ppm NO₂ at 50 °C, which was much higher than that of RGO (1.13). The SnO₂–RGO nanocomposites also improved the sensing properties like rapid response, good selectivity and reproducibility. Therefore, it is still needed to enrich the works on the enhanced gas sensitivity by decorating with SnO₂ nanoparticles.

In this study, V₂O₅ nanowires are prepared via hydrothermal reaction and then decorated with SnO₂ nanoparticles. The gas sensing behavior to ethanol gas is tested at the temperature ranges from room temperature to 380 °C, and the influence of the decorated SnO₂ nanoparticles on the improved gas sensing behavior is also discussed.

2. Experimental section

2.1 Materials synthesis

Two steps were employed to prepare the V₂O₅ nanowires decorated with SnO₂ nanoparticles. Firstly, the V₂O₅ sols were prepared by melt quenching method.²⁵ In the typical process, V₂O₅ powder was melted at 800 °C for 30 min and then poured into deionized water under vigorous stirring. The above obtained sols were transferred into an autoclave and kept at 200 °C for 4 days. After the hydrothermal reaction, an orange and viscous solution of the V₂O₅ nanowires was obtained. The viscous solution was washed with ethanol and distilled water for several times. Then the nanowires were dried at 80 °C in vacuum for 12 h for future use. Secondly, the nanowires were dispersed uniformly in 40 mL deionized water to form a 2.5 mmol L⁻¹ solution (solution A). Then SnCl₄ powder was dissolved in deionized water, and the concentration of the solution was fixed to be 0.1 mmol L⁻¹ (solution B). 1 mL solution B was dropwise added into solution A and kept stirring for 3 hours. After that, a second hydrothermal reaction was carried out at 100 °C for 6, 12 and 18 hours to obtain V₂O₅ nanowires decorated with SnO₂ nanoparticles.

2.2 Characterization

The morphology and structure of as-prepared samples were characterized with an X-ray diffractometer (D/MAX-III) using a Cu Kα radiation and graphite monochromator, a field emission scanning electron microscope (FESEM) at 5 kV (Hitachi S-4800), a transmission electron microscope (TEM) at 200 kV (JEOL, JEM-2100 TEM), and the element analysis is characterized by energy-dispersive X-ray (EDAX) (EDAX, EDX Genesis XM) equipped in the field emission scanning electron microscopy.

2.3 Sensor fabrication and sensing response measurement

Sensors based on V₂O₅ nanowires decorated with SnO₂ nanoparticles are fabricated by using alumina tubes. The schematic of alumina tube and its operating mechanism is shown in Fig. 1. Its length, internal diameter and external diameter are 4.0 mm, 1.0 mm and 1.4 mm, respectively, and the Au electrodes with Pt lead wires are about 2.0 mm in distance. When fabricating a sensor, nanowires are firstly coated onto the alumina tube until it is wholly covered on the surface to form a sensing layer, and then the sensing layer is dried at 60 °C. In order to compare among different sensors, the weight of sensing layer should be controlled as the same (0.2 mg). Finally, a Ni–Cr heating wire is inserted into the tube for heating and the sensor is aged at 400 °C for 2 h. The gas sensing properties are characterized using a commercial gas sensing measurement system of WS-30A (ZhengZhou WeiSheng Corp.). The target gases are ethanol, CO₂, H₂O and NH₃ in dry air, and the ethanol gas is obtained by evaporating a certain volume of ethanol liquid on the heater plate of the machine. The response of sensor to testing gases (S) is determined by the relative resistance. For n-type gas sensors, the sensitivity is defined as S = R_a/R_g when the target gas is reducing gas, and S = R_g/R_a when the target gas is oxidizing gas, where R_a is the resistance of a sensor in air and R_g is the resistance of a sensor in a testing gas.

Fig. 1 The schematic of gas sensor fabricated by using alumina tube.

3. Results and discussion

The length of V₂O₅ nanowires is more than dozens of micron meters, the V₂O₅ nanowires are 50–100 nm in diameter in Fig. 2. Fig. 3 shows the FESEM images of pure V₂O₅ nanowires and V₂O₅ nanowires decorated with SnO₂ nanoparticles, which are synthesized after different hydrothermal times. As one can see, the shape of nanowires is totally remained and the surfaces of the nanowires are rough after a second hydrothermal reaction. The sizes of the SnO₂ nanoparticles on V₂O₅ nanowires increase with the increasing of hydrothermal time. The XRD patterns of the V₂O₅ nanowires and V₂O₅ nanowires decorated with SnO₂ nanoparticles are shown in Fig. 4, and it is observed that all the diffraction peaks of pure V₂O₅ nanowires are in good agreement with the orthorhombic V₂O₅ phase (JCPDS 089-2482). The diffraction peaks from SnO₂ phase are not found in XRD pattern because of the low amount in the product, as the amounts of V₂O₅ and SnCl₄ in the precursor are 0.1 mmol V₂O₅ and 0.1 × 10⁻³ mmol SnCl₄ in our case, respectively, and the existence of the SnO₂ nanoparticles is needed to be further proved by EDS spectrum and HRTEM image.

Fig. 2 FESEM images of V₂O₅ nanowires (a) low-resolution image, (b) high-resolution image.

Fig. 3 FESEM images of various V₂O₅ nanowires decorated with SnO₂ nanoparticles synthesized under various hydrothermal times: (a) pure V₂O₅ nanowires, (b) 6 h, (c) 12 h, (d) 18 h.

Fig. 4 XRD patterns of pure V₂O₅ nanowires and the V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after various hydrothermal times.

Fig. 5(a) shows the EDS spectrum of V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal time of 12 h, and it shows that the Sn content in the sample is about 0.08 at%, which is almost match with the ratio of the precursor of SnCl₄. Combined with the second electron image in Fig. 5(b) and EDS elemental mappings (Fig. 6(c)–(e)), the current result reveals that the sample is composed of three elements of O, V and Sn, which confirms the formation of SnO₂ phase in the V₂O₅ nanowires decorated with SnO₂ nanoparticles.

Fig. 5 (a) EDS spectrum, (b) SEM image, EDS elemental mappings of (c) Sn element, (d) V element, (e) O element for the V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal times of 12 h.

Fig. 6 TEM images of (a) pure V₂O₅ nanowires, and V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal times of (b) 6 h, (c) 12 h, (d) 18 h, (e) HRTEM images of SnO₂ decorated V₂O₅ nanowires prepared after hydrothermal time of 12 h.

Fig. 6(a)–(d) show the TEM images of the pure V₂O₅ nanowires and V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after different hydrothermal times. It shows that the surface of pure V₂O₅ nanowires before decoration is smooth without anything grown on it. However, the TEM images of V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal times of 6 h, 12 h and 18 h show that there are nanoparticles on the surface of the nanowires. The particles are randomly dispersed on the surface when the hydrothermal time is 6 h, and the particles are uniformly dispersed on the surface when the hydrothermal time is 12 h. When the hydrothermal time is 18 h, there are too much particles on the surface that they begin to be agglomerated. The HRTEM image of the sample prepared after hydrothermal time of 12 h is shown in Fig. 6(e), the marked values of 0.35 nm and 0.37 nm correspond to inter-planar d-spacing of the (210) and (110) lattice planes of the orthorhombic V₂O₅ and SnO₂, respectively, which further approves that the good crystallization of the SnO₂ nanoparticles on the V₂O₅ nanowires, and it is accord with the EDS results in Fig. 5.

Fig. 7(a)–(h) shows the sensitivity of the sensors based on V₂O₅ nanowires and samples prepared after hydrothermal times of 6, 12 and 18 h to different concentrations of ethanol gas, and the operating temperature ranges from 60 to 380 °C. It is observed that all the sensors have a response to ethanol gas in the range of 10–1000 ppm and operate a good reversibility. Sensors based on V₂O₅ nanowires decorated with SnO₂ nanoparticles show higher sensitivity compared with that based on pure V₂O₅ nanowires and V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal time of 12 h show the highest sensitivity in the whole range of operating temperature.

Fig. 7 Sensitivities of pure V₂O₅ nanowires and V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal times of 6 h, 12 h and 18 h to 10–1000 ppm ethanol at various operating temperatures: (a) 60 °C, (b) 100 °C, (c) 150 °C, (d) 190 °C, (e) 240 °C, (f) 273 °C, (g) 332 °C, (h) 380 °C.

Fig. 8(a) shows the sensitivity of sensors based on SnO₂ nanoparticles, V₂O₅ nanowires and V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal times of 6, 12 and 18 h to 1000 ppm ethanol at various temperatures. V₂O₅ nanowires decorated with SnO₂ nanoparticles show the best response, however, the highest sensitivity appears at the same temperature of 332 °C. The result shows that the formation of the composite does not change its optimum working temperature of 332 °C, which is accord with some others' work.^26,27 As the decoration of SnO₂ nanoparticle could favor to increase the depletion layer and exposure more active sites to react with the target gas, which may increase the sensitivity as shown in Fig. 7. However, the operating temperature is related with the reaction energy and activation energy between ethanol molecules and the sensing layer,²⁸ and they are influenced by the kinds, morphologies, surface states and the energy structures of the sensing materials.^29,30 Therefore, we think that the reason lies in the fact that the content of SnO₂ nanoparticles in V₂O₅ nanowires decorated with SnO₂ nanoparticles is too low, and it would not have an obvious change for the operating temperature.

Fig. 8 (a) Sensitivity of the sensors based on SnO₂ nanoparticles, V₂O₅ nanowires and V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal times of 6, 12 h and 18 h to 1000 ppm concentration of ethanol gas at various temperature, (b) sensitivity of SnO₂ nanoparticles, pure V₂O₅ nanowires and V₂O₅ nanowires decorated with SnO₂ nanoparticles to 10–1000 ppm ethanol at 60 °C, (c) sensitivity of the sensors based on V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal time of 12 h to different concentration of ethanol gas at room temperature.

Fig. 8(b) shows the value of sensitivity to ethanol at 60 °C among sensors based on SnO₂ nanoparticles, pure V₂O₅ nanowires and V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal times of 6, 12 and 18 h. It is obvious to see that sensor based on pure V₂O₅ nanowires has almost no response to ethanol at 60 °C, but sensors based on V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal times of 6, 12 and 18 h could have a great improvement in sensitivity. When the ethanol concentration is 1000 ppm, the sensitivity of V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal time of 12 h to ethanol is about 2.7, which is 1.8 times of the sensitivity of SnO₂ particles. The operating temperature of V₂O₅ gas sensors are commonly in the range of 200–400 °C in reported works, which may be a potential problem for the lifetime and safety of the gas sensor devices. In our case, the excellent ability of gas sensing response at low operating temperature suggests that the present gas sensor may possess a longer device lifetime, better device safety and lower energy consumption. Therefore, the above special advantages may favor for their further applications.

V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal time of 12 h even have a response to 10–1000 ppm ethanol gas at room temperature (Fig. 8(c)). In the concentration range of 10–500 ppm, the sensitivity value increases with the ethanol concentration. However, the sensitivity value shows a decreasing when the concentration is 1000 ppm, which may suggest that the sensor based on V₂O₅ nanowires decorated SnO₂ nanoparticles prepared after the hydrothermal time of 12 h has a maximum detection limit of 500 ppm at room temperature.³¹ As ethanol could not be desorbed easily at room temperature, when the concentration of testing gas is too high, it may fail to be desorbed absolutely from the surface and the undesorbed ethanol molecules may occupy the active sites.³² Therefore, the amount of ethanol molecules is too high to be absolutely desorbed on the surface at room temperature after testing 500 ppm ethanol. When testing 1000 ppm ethanol gas, although the concentration of target gas increases, the sites that have absorbed ethanol molecules could not absorb more ethanol molecules. Therefore, the total amount of gas adsorption is decreased actually. In this case, we suggest that the poor desorb ability in high gas concentration is the reason that the sensitivity value to 1000 ppm ethanol is lower than that of 500 ppm at room temperature, and study of the absorb/desorb mechanism is to be carried out in future work. The selectivity and stability properties are important for a gas sensor.³³ In our work, the selectivity and stability properties of sensor based on V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal time of 12 h are also tested, which are shown in Fig. 9. When the sensor based on V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal time of 12 h is exposed in C₂H₅OH, CO₂, H₂O and NH₃, it shows good response to C₂H₅OH and almost no response towards other target gases. The stability of sensors in 6 days shows that the values of sensitivity stay at the value of about 13, which shows that the present gas sensor has a good stability under long working time.

Fig. 9 (a) Selectivity of sensors based on V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal time of 12 h to 1000 ppm C₂H₅OH, CO₂, H₂O and NH₃ at different temperatures, (b) stability of sensors based on V₂O₅ nanowires decorated with SnO₂ nanoparticles prepared after hydrothermal time of 12 h to 1000 ppm ethanol at 332 °C.

There might be three points that lead to the improvement of gas sensing properties of V₂O₅ nanowires decorated with SnO₂nanoparticles. Firstly, the decorated SnO₂ nanoparticles would favor for the increasing of the depletion layer and exposure more active sites to react with the target gas (Fig. 10).³⁴ Therefore, V₂O₅ nanowires decorated with SnO₂ nanoparticles would have a larger change in resistance when exposed to the target gas, which finally results in the improvement of gas sensing response.³⁵ Secondly, the energy level of SnO₂ conduction band (−4.5 eV) is a little higher than that of V₂O₅ (−4.6 eV). When the ethanol molecule is captured on the surface of SnO₂ nanoparticles, electrons are generated in the conduction band, and then the electrons will quickly transport to the conduction band of V₂O₅ (Fig. 11).³⁶ Thirdly, in a simplistic approximation, inter-contacts between the nanowires are normally described as a high resistive barriers, which represents the main contribution to the overall resistance of the gas sensor device. As the special geometric structure of the nanowires, it could provide a direct path for electron transfer along the nanowire axis to shorten the inter-contacts between the nanowires (Fig. 12). Thus, after the decoration of SnO₂ nanoparticles, the existence of SnO₂ nanoparticles would firstly favor to absorb more oxygen molecules and form thicker depletion layer. Then more electrons could be released to the conduction band of V₂O₅ nanowires when the sensor is exposed to the reducing gas. Finally, the electrons transport along the nanowire axis efficiently due to the special advantage of ultralong V₂O₅ nanowires. As a result, V₂O₅ nanowires decorated with SnO₂ nanoparticles show a better gas sensing response than that of pure V₂O₅ nanowires and SnO₂ nanoparticles.

Fig. 10 Proposed ideal situation of oxygen adsorption depletion layer (a) before, (b) after SnO₂ decoration.

Fig. 11 Proposed ideal band structure (a) before, (b) after contact between SnO₂ nanoparticles and V₂O₅ nanowires.

Fig. 12 Proposed ideal path of electron transmission for (a) normal nanowires, (b) ultralong nanowires.

4. Conclusions

In summary, V₂O₅ nanowires decorated with SnO₂ nanoparticles are prepared and their gas sensing characteristics to ethanol are examined. The FESEM and TEM images suggest that SnO₂ nanoparticles evenly dispersed on the surface could be obtained after hydrothermal time of 12 h. The gas responses is significantly improved after the decoration with SnO₂ nanoparticles, the sensitivity of V₂O₅ nanowires decorated with SnO₂ nanoparticles at the hydrothermal time of 12 h to 1000 ppm ethanol is about 2–3 times higher than that of pure V₂O₅ nanowires. V₂O₅ nanowires decorated with SnO₂ nanoparticles would even have a response to 10 ppm ethanol at room temperature. The improved gas sensing performance of the composite is due to the increased depletion width and active sites along the nanowires, the formation of heterojunction and the reducing of contact effects between the nanowires.

Acknowledgements

This work is supported by International Science & Technology Cooperation Program of China (no. 2013DFR50710), Equipment pre-research project (no. 625010402), Science and Technology Support Program of Hubei Province (no. 2014BAA096), the National Nature Science Foundation of Hubei Province (no. 2014CFB165), Ministry of Education and Science of Russia (no. 14.613.21.0002), and the Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province (no. GD201402).

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Footnote

† These authors contributed equally to this study and share first authorship.

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