Hierarchical α-Fe2O3/SnO2 semiconductor composites: Hydrothermal synthesis and gas sensing properties

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Abstract

Hierarchical α-Fe2O3/SnO2 composites were synthesized by a low-cost and environmentally friendly hydrothermal strategy. The structure and morphology of composites were investigated by X-ray diffraction (XRD), field-emission electron scanning microscopy (FESEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). The results revealed that the α-Fe2O3 nanorods grew epitaxially on the surface of SnO2 nanosheets. The diameter and length of the α-Fe2O3 nanorods were about 10 and 80 nm, respectively, and the thickness of the SnO2 nanosheets was about 15 nm. The acetone sensing properties of the pure SnO2 and α-Fe2O3/SnO2 composites were tested. The results indicated that such hierarchical α-Fe2O3/SnO2 nanostructures exhibited an enhanced acetone sensing properties compared with the primary SnO2 nanostructures. For example, at an acetone concentration of 100 ppm, the response of the α-Fe2O3/SnO2 composites was about 17, which was about 2.5 times higher than that of the primary SnO2 nanostructures. The response time of the sensor to 60 ppm acetone was shorter than 3 s at the operating temperature of 250 °C.

Introduction

Oxide semiconductor composite, which consists of chemically distinct components, has demonstrated great potential applications in gas sensor [1], [2], [3], catalysis [4], and lithium-ion battery [5]. In recent years, many efforts have been devoted to the synthesis of oxide semiconductor composites by various vapor-phase strategies [6], [7], [8]. However, this approach usually involves costly equipment for high vacuum and heating temperature, which possibly result in the increased cost and further limit the potential applications. Moreover, due to the instant reaction in a sealed system, the vapor-phase routes generally lack flexibility in structural control on the products. Compared with the above method, the solution-phase route is regarded as an economic alternative way to fabricate composites with the advantages of being low-cost, mild, and more controllable in the reaction process. Many oxide semiconductor composites with core/shell or hierarchical architectures have been successfully constructed via a surfactant controlled growth in a hot organic solvent [9], [10].

Metal oxides, as the basis of functional materials, have tunable properties and important technological applications. Among them, α-Fe2O3 and SnO2, as two kinds of important n-type semiconductors with band gaps of ~2.2 eV and ~3.6 eV, respectively, have been intensively investigated as gas sensors [11], [12], [13], electrodes [14], [15], and photocatalysts [16], [17]. Recently, many studies have demonstrated that the performance of Fe2O3 or SnO2 in gas sensing, photocatalytic degradation and lithium ion batteries can be significantly improved by formation of Fe2O3/SnO2 composites [5], [18], [19], [20], [21]. Therefore, various Fe2O3/SnO2 composites with hierarchical architectures have been prepared by introducing different α-Fe2O3 nanocrystals (such as nanobute, nanospindle, and nanocube) into the solution, which is originally designed for synthesizing pure SnO2. It is found that the performances of these composites are closely related to their composite structure. For this reason, design and synthesis Fe2O3/SnO2 composites with novel architectures still have importantly scientific and practical significance.

In this paper, we successfully prepared the novel α-Fe2O3/SnO2 semiconductor composites based on hierarchical SnO2 nanosheets by two-step hydrothermal reaction. First, hierarchical SnO2 nanostructures composed of two-dimensional (2D) nanosheets with the thickness of about 15 nm were synthesized. Subsequently, one-dimensional (1D) α-Fe2O3 nanorods with the diameter and length of about 10 and 80 nm, respectively, were assembled epitaxially on the surface of SnO2 nanosheets by a hydrothermal process. In order to demonstrate the potential applications, the resulting composite was used to fabricate gas sensor. It was found that the gas sensor based on as-prepared α-Fe2O3/SnO2 semiconductor composites showed a high response to acetone at 250 °C, superior to the pure SnO2 hierarchical nanostructures. The enhanced performance may be attributed to the synergetic effect exerted by α-Fe2O3 and SnO2 as well as the change of heterojuction barrier at the different gas atmosphere.

Section snippets

Synthesis of hierarchical SnO2 nanosheets

SnO2 hierarchical nanostructures were synthesized via a facile hydrothermal process according to our previous publication [22]. Briefly, SnSO4 (1.073 g), Na3C6H5O7·2H2O (2.94 g), were added into a basic alcohol–water (1:4, v:v) solution with vigorous stirring. The reaction mixture was transferred into a Teflon-lined stainless-steel autoclave and kept at 180 °C for 12 h. The autoclave was cooled down naturally after reaction. The precipitate was collected and washed by centrifugation several times

Structural and morphological characteristics

The phase and composition of the final product were identified by powder X-ray diffraction (XRD). The typical XRD pattern is shown in Fig. 1, from which the crystal phase of the product was the mixed oxide of SnO2 and α-Fe2O3, most of the diffraction peaks could be indexed to the tetragonal rutile structure of SnO2, which agreed well with the reported values from the Joint Committee on Powder Diffraction Standards card (JCPDS, 41-1445). The residual peaks were indexed to the rhombohedral

Conclusions

In summary, we reported the synthesis of novel hierarchical composites composed of SnO2 nanosheet stem and α-Fe2O3 nanorod branches by hydrothermal method. As a proof-of-concept demonstration of the function, such α-Fe2O3/SnO2 composite material was used as the sensing material of gas sensor. An enhanced sensing property to acetone was demonstrated, in comparison to the pure SnO2 nanostructures. The synergetic effect exerted by α-Fe2O3 and SnO2 as well as the change of heterojuction barrier at

Acknowledgments

This work is supported by the National Nature Science Foundation of China (Nos. 61074172, 61134010, 60906036, and 61104203) and Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT1017). Project (20121105) Supported by Graduate Innovation Fund of Jilin University.

Peng Sun received his MS degree from State Key Laboratory of Superhard Materials, Jilin University, China in 2009. He entered the Ph.D. course in 2010, majoring in microelectronics and solid state electronics. Now, he is engaged in the synthesis and characterization of the semiconducting functional materials and gas sensors.

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Peng Sun received his MS degree from State Key Laboratory of Superhard Materials, Jilin University, China in 2009. He entered the Ph.D. course in 2010, majoring in microelectronics and solid state electronics. Now, he is engaged in the synthesis and characterization of the semiconducting functional materials and gas sensors.

Yaxin Cai received her BS degree from the Electronics Information and Engineering department, Jilin University, China in 2012. Now, she is a graduate student and interested in the field of functional materials and gas sensors.

Sisi Du received her BS degree from the Electronics Information and Engineering department, Jilin University, China in 2012. Now, she is a graduate student and interested in the field of dye sensitized solar cell.

Xiumei Xu received her MS degree from college of chemistry, Jilin University, China. She entered the Ph.D. course in 2011, majoring in microelectronics and solid state electronics. Now, she is engaged in the synthesis and characterization of the semiconducting functional materials and gas sensors.

Lu You received his BS degree from the Electronics Science and Engineering department, Jilin University, China in 2010. Presently, he is a graduate student, majored in microelectronics and solid state electronics.

Jian Ma received his MS in 2009 from Jilin University at the Electronics Science and Engineering department. Presently, he is working as Technical Assistant in Electronics Science and Engineering department. His current research interests are gas sensor, the design and fabrication of micro-hot plates.

Fengmin Liu received the BE degree in Department of Electronic Science and Technology in 2000. She received his Doctor's degree in College of Electronic Science and Engineering at Jilin University in 2005. Now she is an associate professor in Jilin University, China. Her current research is preparation and application of semiconductor oxide, especial in gas sensor and solar cell.

Xishuang Liang obtained his Ph.D. from Jilin University of China in 2009. Presently, he is working as lecturer in Electronics Science and Engineering Department of Jilin University. His current research is solid electrolyte gas sensor.

Yanfeng Sun obtained his Ph.D. from Jilin University of China in 2007. Presently, he is working as an associate professor Jilin University. His current research interests are nanoscience and gas sensors.

Geyu Lu received his BS and MS degree in electronic sciences from Jilin University, China in 1985 and 1988, respectively, and Ph.D. degree in 1998 from Kyushu University in Japan. Now he is a professor of Jilin University, China. Presently, he is interested in the development of functional materials and chemical sensors.

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