Hydrothermal synthesis of 3D urchin-like α-Fe2O3 nanostructure for gas sensor

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Abstract

Urchin-like α-Fe2O3 nanostructures were obtained by a facile chemical solution route combined with subsequent calcination process. The images of field emission scanning electron microscopy and transmission electron microscopy indicated that these nanostructures were composed of one-dimensional nanorods with the diameter of about 40–50 nm and the length of about 800 nm. When tested as sensing materials for gas sensors, these α-Fe2O3 nanostructures exhibited high response, good response and recovery characteristics to ethanol at the operating temperature of 260 °C. The response time was about 2 s to 100 ppm ethanol.

Introduction

The performance of devices based on nanomaterials is known to be affected by not only the electronic structure but also the shape of the nanomaterials. Thus, considerable attention has been focused on the morphology-controlled synthesis of multifunctional nanomaterials with complex micro/nanostructures, especially to three-dimensional (3D) hierarchical architectures that are assembled by one-dimensional (1D) and two-dimensional (2D) nanoscale building blocks [1], [2]. The hierarchical assembly of nanoscale building blocks represents a significant challenge in the field of nanoscale science and is a crucial step to realize the functional nanosystems. Compared with low dimension structures, the 3D hierarchical architectures can provide more opportunities for exploring novel properties and superior device performances, due to their higher specific area and peculiar characteristics. To synthesize hierarchical architectures, various strategies have been employed successfully [3], [4], [5]. However, it still remains a great challenge to develop a facile and novel preparation approach for controlled building blocks hierarchically self-assembled complex architectures with various morphologies.

Alpha-iron oxide (α-Fe2O3), an n-type semiconductor with an energy gap of 2.1 eV, is the most thermodynamically stable phase among all iron oxides under ambient conditions. Owing to its environmental friendliness, low preparation cost and high stability, it is already widely applied in a variety of fields such as gas sensors [9], lithium batteries [10], catalysts [11], pigments [12], and magnetic devices [13]. For most of these applications, α-Fe2O3 material with hierarchical nanostructures could be expected to provide improved performance. Moreover, research on the intrinsic relationship between morphology/size and property has engendered an urgent need for adjustable synthetic strategies, where the size and morphology of the α-Fe2O3 can be controlled with designed functionalities. Accordingly, various techniques have been employed to synthesize α-Fe2O3 nanostructures with diverse morphologies, such as nanoparticles [14], nanorods [15], nanotubes [16], hollow spheres [17], nanobelts [18], nanoplates [19], and complex hierarchical structures [20], [21]. Among these strategies, hydrothermal method with structure-directing agent has been proved to be effective for preparing α-Fe2O3 hierarchical nanostructures [22], [23].

In this paper, we present a simple hydrothermal process for the synthesis of α-Fe2O3 nanostructures. The synthesis was performed in aqueous solution with the aid of inorganic salt (ammonium sulfate, (NH4)2SO4). The results of characterization displayed that the products were composed of urchin-like particles, which exhibited a hierarchical structure composed of rod-like subunits with the diameter of 40–50 nm and the length of about 800 nm. The resulting sample was used to fabricate gas sensor devices which were then tested for sensitivity to a variety of gases. The sensors showed high response, good response and recovery characteristics to ethanol at the operating temperature of 260 °C.

Section snippets

Synthesis and characterization of α-Fe2O3

All the reagents in the experiment were purchased from Beijing Chemical Reagent and used as received without further purification. In a typical experiment, FeCl3·6H2O (0.015 mol), and (NH4)2SO4 (0.76 mmol) were added to a 30 mL of deionized water under magnetic stirring vigorously until a homogeneous solution was formed. Then the mixture was transferred into a Teflon-lined stainless-steel autoclave and kept at 120 °C for 12 h. After the hydrothermal procedure, the autoclave cooled naturally down to

Morphological and structural characteristics

Fig. 1 shows the X-ray diffraction (XRD) patterns of the precipitates prepared by hydrothermal process and the products obtained by calcining the precipitates. As can be seen in Fig. 1a, the diffraction peaks were quite similar to those of bulk β-FeOOH, which could be indexed as the tetragonal structure of β-FeOOH with lattice constants of a = 10.48 Å and c = 3.023 Å. It was consistent with the standard data file (JCPDS file no. 75-1594). After calcining at 600 °C, the β-FeOOH precursors were

Conclusions

In summary, 3D urchin-like α-Fe2O3 architectures were synthesized by a simple one-step hydrothermal route combined with a subsequent calcination process. The results of characterization showed that the hierarchical nanostructures were composed of many 1D nanorods. The obtained urchin-like α-Fe2O3 nanostructures possessed high surface areas. Moreover, gas-sensing properties of our α-Fe2O3-based sensor exhibited high response and fast response–recovery to ethanol and acetone.

Acknowledgments

This work was supported by NSFC (Nos. 61074172, 61134010, 61006055) and Program for Chang jiang Scholars and Innovative Research Team in University (No. IRT1017) and Jilin province science and technology development plan program (20106002). 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 PhD 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 PhD 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.

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

    Yinping Liu received her BS degree from the Electronics Information and Engineering department, Jilin University, China in 2011. Now, she is a graduate student and interested in the field of functional materials and gas sensors.

    Yanfeng Sun obtained his PhD from Jilin University of China in 2007. Presently, he is working as Lecturer in Electronics Science and Engineering department of Jilin University. His current research interests are nanoscience and gas sensors.

    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.

    Geyu Lu received his BS and MS degree in electronic sciences from Jilin University, China in 1985 and 1988, respectively, and PhD 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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