Fast response thin film SnO2 gas sensors operating at room temperature

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Abstract

SnO2 thin film gas sensors operating at room temperature were prepared with SnCl4 as the starting material. The gas sensing characteristics of the sensors towards methyl alcohol vapor were investigated. The sensors exhibit ultra-fast and reversible electrical response (t90% ∼5 s for response and ∼1 s for recovery) at room temperature. In addition, the electrical responses are reproducible during cycling tests. The particle size of the hydrolyzed SnCl4 affects the sensitivity of the sensors, but does not have much effect on their response time. The sensors show different electrical response in the detection of the vapors of methyl, ethyl, isopropyl and butyl alcohol at room temperature, which is proposed to be related to the variation in the reducing ability of the alcohols with a different number of methyl groups.

Introduction

Gas sensors have found wide applications in industrial production, environmental monitoring and protection, etc. [1], [2], [3], [4], [5]. Among the sensors investigated and developed, SnO2 based sensors received much attention since they can detect a wide variety of gases with high sensitivity, good stability and also low production cost [5], [6], [7], [8], [9], [10], [11]. However, like other semiconductor type gas sensors, SnO2 sensors should be operated at high temperatures, which brings about much inconvenience for practical applications and sometimes is even unsafe for detecting combustion gases [12], [13], [14], [15], [16], [17], [18], [19], [20]. In recent years, efforts have been done to realize the room temperature detection by using nanosized SnO2, applying the dopants in SnO2 films, etc. Law et al. [18] developed individual single-crystalline SnO2 based photochemical NO2 sensors that worked at room temperature. Tadeev et al. [17] found that the addition of noble metals such as Pd and Pt in SnO2 could decrease the working temperature of the sensor and the detection of CO at near room temperature was realized by optimizing the microstruture of the film and the concentration of the doping additives. Selim [19] fabricated a SnO2 thick film sensor doped with ZrO2 for the detection of H2S at room temperature, and Patel et al. [20] applied copper as a catalytic layer over thin films of indium tin oxide (In2O3 + SnO2) to improve the sensitivity of the sensor to methanol at room temperature. Srivastava et al. [21] found that annealing the SnO2 sensor in oxygen plasma reduced its barrier height and improved the sensitivity to enable the sensor to work at room temperature. Wei et al. [16] also revealed that doping of even a very small amount of single-wall carbon nanotubes (SWNTs) could effectively improve the room temperature sensitivity of SnO2 sensors towards NO2. However, in general, the researches on the SnO2 sensors operating at room temperature are still quite limited and much more work needs to be done to understand the sensing mechanism and the relationship between the composition and microstructure of SnO2 and its sensing properties at room temperature. In this paper, we report our preliminary work on the preparation of thin film SnO2 gas sensors featured with low temperature annealing of hydrolyzed SnCl4 and on their gas sensing characteristics at room temperature.

Section snippets

Experimental

All the chemicals used in the work are of analytical grade and used as received. The precursor solution for the sensor fabrication was obtained by the hydrolysis of SnCl4 in the existence of urea. In a typical procedure, a flask containing 25 mL of an aqueous solution of SnCl4 (0.1 mol/L) was heated to 90°C. The urea solution containing 1.2 g urea and 25 mL of deionized water was slowly dropped into the SnCl4 solution with magnetic stirring in 20 min. The obtained mixture was then heated for another

Results and discussions

The preparation of SnO2 gas sensors usually involves annealing at a high temperature of 400–950 °C [4], [5], [7], [8], [9], [10], [11], [16]. In this paper, the fabrication of the sensitive film is featured with the annealing of the SnCl4 hydrolyzed with urea at a temperature as low as only 150 °C. The XRD pattern of the SnO2 film so prepared is presented in Fig. 1. It can be seen that the annealed film shows a strong peak corresponding to (1 0 1) plane with narrow width, which was attributed to a

Conclusions

Thin film SnO2 gas sensors can be prepared by the hydrolysis and low temperature annealing of SnCl4. The sensor is featured with fast, reversible and reproducible response to methyl alcohol vapors at room temperature.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (Contract no. 50403020) and Zhejiang Provincial Natural Science Foundation of China (Grant no. M203093).

Hui-cai Wang is now a PhD student in the department of polymer science and engineering, Zhejiang University, China. His research interests are polymer and composite materials for gas sensors.

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Hui-cai Wang is now a PhD student in the department of polymer science and engineering, Zhejiang University, China. His research interests are polymer and composite materials for gas sensors.

Yang Li received his PhD degree in polymer chemistry and physics from Zhejiang University in 2000. He has been working in Department of Polymer Science and Engineering, Zhejiang University since 2000 and was appointed associate professor in polymer science in 2002. His research interests include polymer materials and organic/inorganic composites for chemical sensors.

Mu-jie Yang graduated from Zhongshan University, China in 1963. She has been working in Zhejiang University since 1963. She was promoted to full professor in polymer science in 1992. Her research interests are functional polymers with optical and electrical characteristics.

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