Tin oxide thin films prepared by aerosol-assisted chemical vapor deposition and the characteristics on gas detection

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Abstract

Pure and Cu-doped SnO2 thin films are prepared by aerosol-assisted chemical vapor deposition for H2S detection. Three doping routes are used to prepare thin films with different structures. The response and recovery characteristics are analyzed. The thin film is able to completely recover between testing cycles at room temperature and has the response and recovery times of 60 and 90 sec, respectively. The selectivity test illustrates that the film is only sensitive to H2S and acts no response to CO and CH4 at all. The relationships between response and gas concentration as well as operating temperature are also discussed.

Introduction

Since the first commercial SnO2 gas sensor (Tagushi gas sensor) was successfully produced in 1968 [1], this type of gas sensor were widely used in the field of mine and oil exploitation [2], environment monitoring [3], automobile industry [4], etc. In the later decades, a great mount of researches were carried out and SnO2 gas sensors in form of pellet [5], thick film [6], [7], thin film [8], [9], [10] and sensor array [11] were prepared. Comparatively, the SnO2 thin film gas sensor is the most prospective one due to its high response, short responding time and well compatibility to microelectronic technique. There are several methods to prepare SnO2 thin films, such as evaporation [12], [13], sputtering [14], [15], and sol–gel route [16], [17], [18], [19]. Various elements of Cu [20], Pd [3], In [21] are doped into the films to enhance the characteristics of thin films. The response, selectivity, recovery ability and operating temperature are concerned as the most important factors. However, the gas sensing performances of these thin film sensors are still not satisfactory enough for practical use yet.

Hydrogen sulfide (H2S) is an important industrial gas used in production of inorganic compounds, alkali metal sulfides and analytical chemistry. It is colorless, highly toxic and easily flammable. It is able to paralyse one's sense of smell and the potential victims may be unaware of its leakage until it is too late. Therefore, the detection of the H2S gas, especially at low concentration, is urgently expected in above application areas. For SnO2 semiconducting gas sensors, the Cu element is proved to be an efficient additive to enhance the sensor response and other sensor performances [22]. The promoted performances are ascribed to the p–n junction between p-type CuO and n-type SnO2 and the transformation from semiconductive CuO to metallic CuS when exposed to the H2S gas [23].

Aerosol-assisted chemical vapor deposition (AACVD) is a newly developed technique for thin film preparation. It has been used for preparation of WO3 thin films [24], SrTiFeO3 films [25], indium tin oxide (ITO) thin films [26] and ZnO thin films [27]. This deposition method is interested for its low cost, uniformity of doping and easy deposition of large scale thin films. These advantages benefit the practical use of the method.

In the present work, AACVD is employed for the preparation of SnO2 thin films for H2S gas detection and the prepared films are found to be recoverable at room temperature. Cu is selected as the additive to enhance the response and different doping routes are used. Several parameters that may influence the response of thin films to the H2S gas are investigated.

Section snippets

Experimental

The facility of aerosol-assisted chemical vapor deposition in the present work was shown in Fig. 1. SnCl2·2H2O (analytical reagent) was dissolved into 40 ml absolute ethyl alcohol and the solution, with Sn2+ concentration of 0.1 mol/L, was stirred in a magnetic stirring apparatus for 1 h. N2 was employed as carrier gas and the flux is controlled at a level of 80 sccm for a sustained depositing rate as well as a depositing time dependent film thickness. The precursor solution was atomized by an

Film morphology and XRD analysis

Fig. 2(a) shows the film morphology of the pure SnO2 thin film prepared by AACVD with the depositing time of 2 h. There are a great mount of pores among grains. The porous structure benefits the diffusion of reducing gas as well as the reaction between gas molecules and grains. The grains are in a uniform shape and the average grain size of the film is about 55 nm. The morphology of the film with depositing time of 5 h is shown in Fig. 2(b). Comparatively, it appears to be much denser and this

Conclusion

Aerosol-assisted chemical vapor deposition is employed to prepare pure and Cu-doped SnO2 thin films. Influence of depositing time and different doping routes are investigated. The Sn–Cu thin film shows the response of 85–50 ppm H2S gas and has the ability of complete recovery between testing cycles at room temperature, comparing with the thin films prepared by routes (a) and (b). The response and recovery times are 60 and 90 sec, respectively. Selectivity investigation shows the film is only

Acknowledgements

The authors acknowledge the assistance by the Analytical and Testing Center of Huazhong University of Science and Technology. This work was financially supported by Specialized Research Fund for the Doctoral Program of Higher Education of China (20070487182).

Jun Zhao acquired his ME degree in 2003 and PhD degree in 2007, respectively, in Huazhong University of Science and Technology. He is now working as post-doctor and his research interests include electronic circuit and material physics and chemistry.

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    Jun Zhao acquired his ME degree in 2003 and PhD degree in 2007, respectively, in Huazhong University of Science and Technology. He is now working as post-doctor and his research interests include electronic circuit and material physics and chemistry.

    Shuxi Wu acquired his ME degree in 2008 in HUST.

    Jianqiao Liu is pursuing his PhD degree in Huazhong University of Science and Technology (HUST) after acquiring his Bachelor's degree in 2007. His research interests include preparation of nanocrystalline SnO2 thin film sensors and investigation of sensing mechanism.

    Huan Liu acquired her PhD degree in HUST in 2008 and now is an instructor in Department of Electronic Science and Technology. Her research interests concentrate on the semiconducting sensing functional ceramics.

    Shuping Gong received her Bachelor's degree in electronic materials and components from Huazhong Institute of Technology in 1970. After the graduation, she became a teacher in the Department of solid-state electronics. For many years, her research program has concentrated on functional ceramics, sensor technology and its applications. At present, Prof. Gong assumes the chief engineer in Engineering and Researching Center of Functional Ceramics of MOE.

    Dongxiang Zhou is a seasoned expert in the studies of solid-state electronics and micro-electronics. He acquired his Master Degree in Huazhong Institute of Technology and used to engage in researching information functional ceramic in Shonan Institute of Technology in Japan. He has continuously conducted systematic research into theories and application technologies of semiconductor ceramics physics, functional materials design and virtual instruments, etc. His publications include “PTC Materials & Applications”, “Semiconductor Ceramics and Applications”, “Electronic Materials and Components Testing Technology”.

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