Synthesis and enhanced ethanol sensing properties of α-Fe2O3/ZnO heteronanostructures
Introduction
Chemical sensors have played important roles in industrial, medical and domestic applications in detecting pollutant, toxic and combustible gases. Key requirements for chemical sensors contain not only high sensitivity and good selectivity to a trace targeted gas, but also the abilities of working in a continuous mode at room temperature. One-dimensional (1D) metal oxide nanostructures with a high surface-to-volume ratio have attracted much attention because their morphologies/structures can be controlled and the surface/bulk doping is readily realized [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Other advantage over dense films is that 1D nanostructures enable the formation of more empty space in the sensing layers, resulting in the faster sensor response and recovery.
Despite encouraging development of 1D nanostructures, the challenge in improvement of sensor sensitivity and selectivity to the target gases still exists. 1D heteronanostructures offer a possibility of overcoming the limit owing to the presence of two kinds of sensing materials and the formation of heterojunctions at their interfaces. Up till now, 1D heteronanostructures have shown exciting physical properties, and thus aroused one's wide research interests. Recently, important development of 1D heteronanostructures has been made based on silicon nanowires, carbon nanotubes (CNTs) and 1D semiconducting oxides. CNT/CdS, CNT/ZnO and CNT/SnO2 heterostructures have been synthesized by different groups [16], [17], [18], [19]. Zhu et al. synthesized CNT/ZnO nanocomposites and fabricated ultrafast nonlinear optical switch based on the hybrid system [18]. Liu and Lee reported enhanced optical absorption of CNT/CdS core–shell structures in the UV–vis region [17]. Recently SWNT/SnO2 core–shell nanostructures were synthesized by our group via a simple method [20]. The composites exhibited significant improvement in electrochemical properties. Using different methods, 1D metal oxide heteronanostructures were also successfully prepared [21], [22], [23], [24]. Kuang et al. reported the epitaxial growth of t-ZnO/SnO2 core–shell nanostructures, and observed the new luminescence properties induced by the epitaxial interfaces [21]. By a thermal evaporation method, Kim et al. prepared SnO2/InO3 core–shell nanowires which could be used as Li ion battery electrodes [23]. It is believed that the new and improved physical properties are related to the intrinsic properties of both core and shell materials and the heterojunction barrier formed at their interface [25]. Therefore, the physical properties of the heterostructures can be modulated by controlling the thicknesses of core and shell materials.
In this work, we report on synthesis of α-Fe2O3/ZnO heteronanostructures, in which the largest thickness of ZnO shell is comparable to the Debye length of bulk ZnO, by a three-step process. A dramatic improvement in ethanol sensing characteristics was found based on the heteronanostructures.
Section snippets
Experimental details
α-Fe2O3/ZnO heteronanostructures were fabricated by a three-step process. First, β-FeOOH nanorods were synthesized by a hydrothermal route at 110 °C. The detailed procedure was described elsewhere [24]. Then 0.1 g of the as-synthesized β-FeOOH nanorods was dispersed in 150 ml of distilled water by ultrasonication. The suspension solution was then mixed with 50 ml of 0.002 mol/L Zn (AC)2·2H2O aqueous solution, and heated to 40 °C under stirring. The 20 ml of 5% NH3·H2O was added dropwise into the mixed
Results and discussion
Fig. 1(a) is a typical SEM image of the as-synthesized β-FeOOH nanorods at 110 °C. It is found that the surface of the nanorods is very smooth and their lengths are 0.35–1.2 μm. The bottom pattern in Fig. 1(c) shows the corresponding XRD pattern from the nanorods. Compared with the data in JCPDs card (no. 75-1594), all peaks in the pattern can be indexed to β-FeOOH. Fig. 1(b) displays a SEM image of the final products. No obvious changes between the final products and the β-FeOOH nanorods in the
Conclusions
In summary, we successfully prepared 1D α-Fe2O3/ZnO heteronanostructures via a three-step process. The diameters and lengths of the obtained heterostructures were about 40 nm and 0.35–1.2 μm, respectively, and the largest thickness of the ZnO shell was about 10 nm. The heteronanostructures exhibited a dramatic improvement in ethanol sensing characteristics including sensor response and selectivity. According to the space-charge model, the enhanced sensing characteristics are attributed to smaller
Acknowledgements
This work is supported by the National Natural Science Foundation of China (grant no. 50772025), the Natural Science Foundation of Heilongjiang Province, China (grant no. F200828 and E200839), the Specialized Research Fund for the Doctoral Program of Higher Education of China (grant no.20070217002), China Postdoctoral Science Foundation (grant nos. 20060400042 and 200801044) and also the Innovation Foundation of Harbin City (grant no. RC2006QN017016).
C.L. Zhu has been a lecturer at Harbin Engineering University since 2005. She received her MS degree in 2005 from Lanzhou University of Technology. She is currently studying for PhD at Harbin Engineering University. Her main research interest is in the development of nanostructured materials for their applications.
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C.L. Zhu has been a lecturer at Harbin Engineering University since 2005. She received her MS degree in 2005 from Lanzhou University of Technology. She is currently studying for PhD at Harbin Engineering University. Her main research interest is in the development of nanostructured materials for their applications.
Y.J. Chen has been a professor at Harbin Engineering University since 2007. He received his MS degree in 2003 from Harbin Engineering University and PhD degree in 2005 from Chinese Academy of Science. His research interests focus on the nanostructured materials for the nanodevice applications.
R.X. Wang is currently studying for MS degree at Harbin Engineering University. His research interest is in the development of nanostructured materials for their applications.
L.J. Wang is currently studying for MS degree at Harbin Engineering University. Her main research interest is in the development of nanostructured materials for their applications.
M.S. Cao has been a professor at Beijing institute of Technology since 2004. His main research interest is in the development of nanostructured materials for their applications.
X.L. Shi is currently studying for PhD at Beijing Institute of Technology. Her main research interest is in the development of nanostructured materials for their applications.