Highly sensitive and selective toluene sensor based on Ce-doped coral-like SnO₂

Abstract

Coral-like SnO₂ nanostructure was synthesized via a simple hydrothermal treatment in the presence of glucose. The characterization results showed that the size of coral-like SnO₂ powders is about 13 nm with porous structure. The special surface area and the average size of pores are about 46.9 m² g⁻¹ and 7.61 nm, respectively. Subsequently, we investigated the gas-sensing properties of the sensors based on the coral-like SnO₂. We chose CeO₂ as a dopant and made three CeO₂ doped sensors with 0.5%, 1.5%, and 2.5% for the mole ratio of Ce to Sn. The gas sensing tests indicated that CeO₂ as the dopant can significantly enhance the gas selectivity toward toluene, and the sensor with 1.5% mole ratio showed the highest sensitivity toward toluene at 190 °C, which is much higher than that toward other VOC gases such as ethanol, acetone, and formaldehyde. The efforts in the research have proved that the CeO₂ doped SnO₂ could be a potential candidate of highly sensitive and selective gas sensors for toluene.

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Introduction

Toluene has a potential to cause severe neurological harm as it is easily inhaled, ingested and absorbed by the skins of human beings.¹ At present, toluene sensitive sensors can be categorized into organic semiconductor sensors,² metal oxide semiconductor sensors,^3,4 solid electrolyte type sensors,⁵ optical sensors,⁶ and bacterial biosensors.⁷ Among them, metal oxide semiconductor sensors could detect the concentration at ppm level,⁸ and they are considered as the most potential materials due to their low-cost and reliability.

Among these metal oxides semiconductors, SnO₂, a nontoxic and inexpensive n-type semiconductor, has been proved to be an excellent gas sensing material for the detection of both toxic and combustible gases such as CO,⁹ H₂,¹⁰ acetone,¹¹ and ethanol.¹² Although numerous efforts have been made to improve the gas-sensing performance of a single semiconductor, their properties are still limited by some shortcomings such as high operating temperature, low sensitivity, poor selectivity and stability. Extensive studies have found many basic factors concerning the sensing properties of semiconductor gas sensors: (1) grain size of particles,¹³ (2) microstructure of the sensing body^14,15 and (3) surface modification of particles.^16,17

Recently, some researchers revealed that SnO₂ is also a potential candidate for sensing toluene. Qi et al. found a type of SnO₂ nanofiber, which is very selective toward toluene at a quite high temperature at 350 °C.¹⁸ Huang et al. synthesized porous flower-like SnO₂ and porous SnO₂ microcubes. They investigated their gas-sensing properties toward volatile organic compounds (VOCs)^19,20 and found that both materials exhibited good responses to toluene. However, the selectivity was not satisfied. Doping as an efficient method to improve the selectivity of the gas sensor was widely used. Ceria as a dopant has received great attention due to its peculiar properties arising from the availability of the 4f shell. For example, Ce-doped SnO₂ nanomaterials have been used to improve ethanol response selectivity in the presence of CO, LPG and CH₄.²¹ Based on Ce-doped SnO₂ thin films, Fang et al.²² prepared a high sensitivity H₂S sensor, which can be used at room temperature. Nevertheless, there are few reports on the sensors based on Ce-doped SnO₂ for detecting toluene.

This study focuses on the investigation of Ce-doped SnO₂ sensors for detecting toluene. A coral-like SnO₂ with a hollow architecture was prepared by using a facile wet-chemical approach integrated in an annealing process.²³ It is reported that nano coral-like architecture can provide many quick passages to absorb and desorb gas, which enhances gas sensing, thus showing high response to several VOCs gases at low operating temperature. As expected, Ce dopant can improve response to VOCs gas. We made some sensors by doping CeO₂ to the coral-like SnO₂ and tested their sensitivity and selectivity toward toluene; moreover, the results showed that CeO₂ dopant improved selectivity toward toluene.

Experimental

The coral-like SnO₂ was prepared by hydrothermal treatment combined with an annealing process as we have reported in a previous research paper.²⁴ The crystal structure and morphology of SnO₂ were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The specific surface area and the average size of pores were obtained upon the multi-point Brunauer–Emmett–Teller (BET) analysis of N₂ sorption isotherms recorded at 77 K with a Nova 2000e surface area and pore size analyzer.

A gas sensor was produced as a side heated structure. Ce(NO₃)₃·6H₂O ethanol solution was added to the as-prepared coral-like SnO₂ powder with CeO₂ molar ratio of 0.5%, 1.5% and 2.5%, and then ground until a printable paste was obtained. The paste was transferred onto an alumina ceramic tube (4 mm in length, 1.2 mm in external diameter and 0.8 mm in internal diameter), on which Au electrodes and Pt wires have been fixed at both ends. After being sintered at 400 °C for 2 h in a muffle furnace, a Ni–Cr heater strip with resistance of about 35 Ω was placed across the ceramic tube centre and welded to the element tubes to control the operating temperature. Finally, the element was aged at an operating temperature of 400 °C for 24 h to improve its stability.

Gas sensing properties were measured under room conditions (humidity range 30–60%) using a static test system made by Henan Hanwei Electronics Co. Ltd., which included a test chamber (about 18 L in volume) and a data acquisition/processing system. The resistances of sensors were measured via direct current bridge method. The operating temperature of a gas sensor was adjusted by varying the heating voltage, because each heating voltage has its corresponding operating temperature. The magnitude of the response in this study was defined as S = R_a/R_t, where R_a and R_t are the resistance of the sensor in air and in tested gas, respectively.

Results and discussions

Structure and morphology

Fig. 1(a) is the XRD patterns of the coral-like SnO₂, which indicate that all peaks correspond well to the tetragonal rutile structure of SnO₂ (JCPDS card no. 41-1445). No other crystal phase is detected, indicating the high purity of the final products. The average particle size of coral-like SnO₂ is about 13 nm, which is calculated by Topas Software package.²⁵

Fig. 1 XRD (a), TEM (b), SEM (c), and nitrogen adsorption isotherms and corresponding pore size distribution (inset) (d) of the prepared coral-like SnO₂.

The TEM and SEM images show that the product is of a coral-like morphology as shown in Fig. 1(b) and (c). The formation of the coral-like porous SnO₂ should be due to the Sn(OH)₄ colloid obtained by the hydrolyzed Sn⁴⁺ on the surface of a carbonaceous sphere, which is conglomerated from small carbonaceous spheres by OH groups.²⁶

The nitrogen adsorption–desorption isotherms and the pore size distribution curves of porous coral-like SnO₂ are shown in Fig. 1(d). The isotherm of the SnO₂ sample exhibits a hysteresis loop at the P/P₀ ranges of 0.69–0.98, which is associated with the filling and emptying of microspores by capillary condensation. The result clearly indicates that the coral-like SnO₂ sample exhibits a large textural porosity. The pore size distribution of the coral-like SnO₂ sample shows a peak in pore size region of 7–12 nm. The calculated pore-size distribution indicates that the material contains an average pore size of 7.61 nm. The BET surface area of the porous coral-like SnO₂ sample is calculated to be 46.9 m² g⁻¹.

Gas sensing studies

The responses toward 50 ppm toluene of the pure and CeO₂ doped SnO₂ sensors at different operating temperatures are shown in Fig. 2(a). It is evident that the response of the pure SnO₂ sensor decreases with increasing operating temperature. The CeO₂ doped sensors exhibit high responses to toluene under temperatures of 190 °C, and the corresponding responses are much higher than that of the pure SnO₂ sensor. The response value of the sensor based on 1.5% CeO₂ doped SnO₂ was calculated to be 365 at 190 °C, which is 28 times higher than the pure SnO₂. Therefore, 190 °C was chosen as the optimum operating temperature for the following tests.

Fig. 2 Responses of the sensors to 50 ppm toluene at different operating temperatures (a); responses of the sensors to different concentrations of toluene at 190 °C (b); responses of the sensors to 50 ppm different test gases at 190 °C (c).

Fig. 2(b) shows the response curves of the pure and CeO₂ doped SnO₂ sensors toward toluene with concentrations from 5 to 100 ppm. It can be seen that the CeO₂ can significantly improve the sensor response toward toluene. Clearly, the dopant content has a great impact on the sensitivity of the sensors; however, 1.5% CeO₂ doped SnO₂ shows the best gas sensing performance.

To further investigate the practicability of these sensors, the selectivity of the sensors was tested by exposing them to various 50 ppm gases, as shown in Fig. 2(c). Apparently, the responses of pure SnO₂ and 0.5% CeO₂ doped SnO₂ sensors followed this order: C₂H₅OH > HCHO > CH₃COCH₃ > C₆H₅CH₃. The only difference between them is that the sensitivity of the 0.5% CeO₂ doped sensor was higher than that of pure SnO₂, especially for toluene. When the dopant content further increased to 1.5%, the responses of the sensors toward ethanol, acetone and formaldehyde decreased remarkably; however, the response toward toluene was enhanced. The responses toward all the test gases decreased when the CeO₂ dopant increased to 2.5%. Namely, the pure SnO₂ and 0.5% CeO₂ doped SnO₂ exhibited the low response and selectivity toward toluene, the 1.5% and 2.5% CeO₂ doped SnO₂ showed high selectivity toward toluene, and the responses ratio of toluene to other interference gases were over 4. It is evident that CeO₂ can catalyze the reaction of toluene with adsorbed oxygen. This has been confirmed by Barakat et al.²⁷ The sensor based on 1.5% CeO₂ doped SnO₂ exhibits the highest response toward toluene.

The sensing mechanism of the CeO₂ doped SnO₂ sensor is based on the surface chemical reaction. The response of the sensor derives from the change of electrical conductivity when the gas molecules react with adsorbed oxygen ions on the surface of SnO₂. CeO₂ dopant can improve the capability of oxygen adsorption.¹⁸ When SnO₂ is doped by CeO₂, the number of adsorbed oxygen ions on the SnO₂ surface increases, therefore the gas sensing reaction of gas molecules with adsorbed oxygen ions enhances, which results in the increase of response value. However, as the amount of CeO₂ increases further, the adsorbed oxygen ions become excess and occupy the limited active sites at the SnO₂ surface, which prevents the test gas molecules such as ethanol and toluene from contacting with the active sites to react with oxygen ions, so the response value decreases. Owing to the two aspects of influence, there is an optimum doping amount. Because the oxygen consumption of toluene is larger than ethanol, acetone and formaldehyde, the optimum doping amount toward toluene should be more than that toward ethanol, acetone and formaldehyde. The maximum response to ethanol, acetone and formaldehyde appears in the sensor based on 0.5% CeO₂ doping SnO₂, and the maximum response to toluene appears in 1.5% CeO₂ doping SnO₂.

In addition, the sensors of pure and Ce-doped SnO₂ exhibited good repeatability (Fig. S1†), reproducibility and long-term stability. The gas responses of the sensors were similar to that examined in two months, as shown in Fig. 3.

Fig. 3 Gas responses of pure and Ce doped SnO₂ sensors change as time changes at 190 °C toward 50 ppm toluene.

Conclusion

The sensors based on CeO₂ doped SnO₂ were fabricated with different doping content of CeO₂ and exposed to various gases. The results indicated that CeO₂ dopant can significantly enhance the toluene gas performances, and CeO₂ seemed to be a potential addictive in fabricating a toluene sensor as well as toluene catalyst. The results also revealed that CeO₂ content has a great effect in sensor properties. The 1.5% CeO₂ doped sensors showed the best gas sensing properties toward toluene with relatively high sensitivity and selectivity, which is a potential toluene sensor.

Acknowledgements

The research was financially supported by the National Natural Science Foundation of China (21307035).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12315h

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