A selective NH3 gas sensor based on Fe2O3–ZnO nanocomposites at room temperature
Introduction
Since many decades, the world awareness about environmental problems and human safety is increasing with the technological development. Therefore, sensors are required for many applications. Recently, the need to detect low ammonia concentrations has greatly increased in many fields of technological importance, such as food technology, chemical engineering, medical diagnosis, environmental protection, monitoring of car interiors and industrial processes.
Seiyama et al. proposed the gas sensors based on ZnO thin films for the first time [1]. ZnO is sensitive to many gases of interest, such as trimethylamine (TMA) [2], [3], [4], H2 [5], oxygen [6], [7], [8], H2O [9], [10], ethanol [11] and NH3 [12], etc. It also has a rapid response with a possibility of miniaturization. However, it has some drawbacks, such as high working temperature, normally between 400 and 500 °C, poor gas selectivity and relatively low gas sensitivity [13].
To overcome these disadvantages, considerable research and development are underway. There are various techniques to modify the sensing properties of the gas sensors. One critical approach is to modify the metal oxide surface by using noble metals (Au, Pt or Pd) [14], [15] or rare earth metals (La, Y and Ce) [16], [17]. ZnO(n)/CuO(p) heterocontact configuration also showed some possibility of improving the selectivity [18]. Nanto et al. have reported that a sensor based on a ZnO thin film doped with Al, In or Ga could detect the ammonia gas whose concentration was as low as 1 ppm [12]. But the working temperature was as high as 350 °C. Recently, Ivanovskaya et al. suggested that a sensor based on α-Fe2O3/In2O3 nanocomposites exhibited high sensitivity to NO2 [19].
The present work was undertaken to investigate the gas sensing behavior of ZnO nanoparticle thin films doped with α-Fe2O3 nanoparticles prepared by a sol–gel and spin-coating method. Morphological, structural and sensing properties at room temperature were studied. The ultimate objective of this study is to improve the gas selectivity and sensitivity of the nano-sized ZnO-based sensors at room temperature.
Section snippets
Experimental
ZnO nanoparticles doped with α-Fe2O3 nanoparticles were prepared in a similar manner to the literature procedure [20]. The α-Fe2O3 nanoparticles were synthesized by a hydrothermal method [21]. Some of the α-Fe2O3 nanoparticles were ultrasonically dispersed into methanol (200 ml) at about 60 °C. Subsequently, 0.01 M Zn(Ac)2 was dissolved into the above solution. Then, a 0.03 M solution of KOH (65 ml) in methanol was added dropwise. The reaction mixture was stirred for 2 h. The resulting solution was
Morphology of Fe2O3–ZnO nanocomposites
Fig. 1 shows TEM images of the Fe2O3–ZnO nanocomposites with different compositions of Fe:Zn. Morphology of the pure α-Fe2O3 prepared by a hydrothermal method was nanoparticles (Fig. 1a). Size of the nanoparticles was about 10 nm. When 1% Fe2O3 (molar ratio) nanoparticles (sample 1) were doped into the ZnO nanoparticles, the morphology of the resulting sample was similar to that of the pure α-Fe2O3 (Fig. 1b). It is obvious that the prepared nanoparticles were crystalline from the inset of Fig. 1
Discussion
Based on the above results, the reason for the enhanced NH3 sensitivity and selectivity of the Fe2O3–ZnO nanocomposite thin film gas sensors was put forward. Ivanovskaya et al. [19], [24] have suggested that ethanol detection is a multi-step process involving both reductive–oxidative and acid–base interactions with a sensor based on heterojunction oxide structures. The reactivity of oxides in acid–base reactions depends on the electronegativity of the metal cation. The electronegativity is the
Conclusion
Gas sensors based on Fe2O3–ZnO nanocomposites have been prepared with different compositions of Fe:Zn. The sensor with Fe:Zn = 2% exhibited fairly excellent sensitivity and selectivity to NH3 at room temperature. The response and recovery time of the sensor were about 20 s. The reproducibility of the ZnO gas sensor with Fe:Zn = 2% was good. So, the sensor could be used for many times. The increased sensitivity and selectivity to NH3 may largely be attributed to the addition of Fe2O3 nanoparticles,
Acknowledgments
The authors would like to appreciate the financial supports of the Natural Science Foundation of China (60225010) and 863 Project No. (2004AA513024). We also thank Prof. Youwen Wang for TEM and EDX measurement.
Huixiang Tang was born in 1978. Now, she is PhD candidate from State Key Lab of Silicon Materials at Zhejiang University, China. Her research project is gas sensors based on the ZnO nanoparticles.
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Huixiang Tang was born in 1978. Now, she is PhD candidate from State Key Lab of Silicon Materials at Zhejiang University, China. Her research project is gas sensors based on the ZnO nanoparticles.
Mi Yan was born in 1965. He has been a professor of materials science at Zhejiang University since 1998. He graduated from Department of Materials Science and Engineering, Southeast University in 1980. Professor Yan has been working in the fields of functional materials and surface treatment. His current research interests are magnetic materials and related functional materials.
Hui Zhang received his PhD degree from State Key Laboratory of Silicon Materials at Zhejiang University in 2004. Now, he is a teacher at Zhejiang University. His research interest is preparation and application of nano-sized compound semiconductor materials.
Shenzhong Li received his master degree from State Key Laboratory of Silicon Materials at Zhejiang University in 2005. His research interest is preparation and application of nano-sized compound semiconductor materials.
Xingfa Ma, Professor, executive director in chief of the Department of Adhesives and Coatings, vice director of the Department of Sealants and Rubber Composites of Shandong Research Institute of Non-metallic Materials, Committee Member of Chinese Standard of Adhesives and Sealants. Now, he is a PhD candidate of materials physics and chemistry of Zhejiang University. His research interests include polymer coating, interface, surface modifications, polymer-based composites and organic sensitive materials for sensor.
Mang Wang graduated from Department of Chemical Engineering of Zhejiang University in 1961. Now, he is a professor in materials physics and chemistry and polymer materials at Zhejiang University. He is currently the director of Reprographic Science and Engineering Society of China and a member of Specialist Group of Polymer Materials Division.
Deren Yang was born in Yangzhou, China in 1964. He received his bachelor degree from Department of Material Science and Engineering at Zhejiang University, and in 1991 PhD degree in the State Key Laboratory of Silicon Materials at Zhejiang University. Now, he is a Cheung Kong Professor, deputy director of the State Key Laboratory of Silicon Materials at Zhejiang University. His current research interests are semiconductor materials, including growth, process and defect engineering of Czochralski silicon used for ultra-large scale integrated circuits (ULSI); preparation and application of silicon nano-wires, nano-tubes and other one dimensional semiconductor materials.