Fabrication and gas sensing properties of hollow core–shell SnO2/α-Fe2O3 heterogeneous structures
Introduction
Nowadays, environmental problems have become great challenges for human beings to keep the sustainable development [1]. With increasingly severe air pollution, gas sensors with high sensitivity, low detection limit as well as fast response and recovery behavior are urgently required for their applications in monitoring gas emission. Thus, sensor materials with fascinating structures and low price should be designed and fabricated. In the past two decades, nanostructured materials have attracted wide interests due to their small size and large specific surface area endowed by different dimensions of materials [2], [3], [4], [5], [6], [7]. A smart design of these nanostructures is very important for their performances. For example, hybrid materials with core–shell hetero-structure have appealed to many scientists for their multi-functional properties over any of their single-component, which are also widely used in many fields such as Li-ion batteries [8], [9], [10], [11], supercapacitors [12], [13], [14], solar cells [15], [16] and gas sensors [17], [18], [19], [20], [21]. Accordingly, it is of great importance for us to fabricate various core–shell nanostructures by facile methods.
SnO2 [22], [23] and α-Fe2O3 [24], [25] are both n-type semiconductors, and have been extensively applied in gas sensing field. However, the low sensitivity of these materials usually limits their application in gas sensing device. In order to improve their properties, enormous efforts have been taken. Decreasing the grain size is an effective method to obtain a moderate gas response [26], [27], [28]. Meanwhile, modifying the sensor materials with noble metal nanoparticles such as Pt [29], [30], Pd [29], [30], [31] and Au [32], [33], can offer more active sites and accelerate the reduction or oxidation reactions as catalysts during the sensing processes. However, in consideration of the high cost of noble metals, they are unlikely widely used in practical applications. With respect to SnO2 and α-Fe2O3, the advantages of low-cost and naturally abundant make them very suitable as sensor materials. Therefore, combining these two materials together to form a unique core–shell heterogeneous nanostructure is anticipated to be another route to improve sensing properties, resulting in a better performance than any single components. Recently, some core–shell nanostructures including SnO2/α-Fe2O3 [8], α-Fe2O3/ZnO [19], SnO2/ZnO [34] and TiO2/SnO2 [35], have been successfully synthesized. Chen et al. prepared SnO2/α-Fe2O3 hierarchical nanostructure and found its sensitivity about 6 toward 100 ppm ethanol [36]. Sun et al. fabricated hierarchical α-Fe2O3/SnO2 composite with α-Fe2O3 nanorods uniformly grown on SnO2 nanosheets, and it showed a high response to acetone at 250 °C [18]. Zhang and co-workers synthesized γ-Fe2O3/SnO2 core–shell hollow nanoparticles by a seed-mediated hydrothermal method, and their performances as gas sensors were also investigated [37]. The synthesis of α-Fe2O3/ZnO core–shell nanospindles with a high response to ethanol was reported by Zhang et al. [19]. All above reports inspire us to prepare a novel SnO2/α-Fe2O3 core–shell heterogeneous structure with an excellent performance in gas sensing, and to explore its growth mechanism and controllable fabrication.
Although some hierarchical SnO2/α-Fe2O3 structures have been fabricated, there are few reports about one-dimensional α-Fe2O3 nanorods directly grown on electrospun hollow SnO2 nanofibers, which are promising sensor materials due to the unique open-porous structure. In this paper, we fabricated hollow SnO2/α-Fe2O3 core–shell heterogeneous structure via three steps. First, one dimensional SnO2 hollow nanofibers were prepared by combination of electrospinning and appropriate calcination at 600 °C. The hollow SnO2 nanofibers were then used as substrates for the subsequent growth of α-FeOOH nanorods through hydrothermal method. It was found that glacial acetic acid could control the nucleation site of α-FeOOH nanorod and facilitate its growth on SnO2 fibers in the hydrothermal process. Finally, the as-prepared hierarchical SnO2/α-FeOOH product was calcined at 500 °C to obtain SnO2/α-Fe2O3 core–shell heterogeneous structures. Due to its unique structure, the SnO2/α-Fe2O3 product was tested in a gas sensing system as a sensor material. The results indicated that the performances of the heterogeneous structure, including gas response, response and recovery times, were much better than those of any single component.
Section snippets
Chemicals and materials
All chemicals here were analytical reagents and used without further purification. Polyvinylpyrrolidone (PVP, Mw = 1,300,000), tin (II) chloride dehydrate (SnCl2⋅2H2O, ⩾98.0%), iron (III) chloride hexahydrate (FeCl3⋅6H2O, ⩾99.0%), sodium sulfate anhydrous (Na2SO4, ⩾99.0%), N,N-dimethylformamide (DMF, ⩾99.5%), glacial acetic acid (CH3COOH, ⩾99.5%), and absolute ethanol (⩾99.7%) were obtained from Sinopharm Chemical Reagent Co., Ltd.
Preparation of hollow SnO2 nanofibers
Hollow SnO2 nanofibers (SNFs) were prepared as follow. 0.007 Mol
Structure and morphology
The general morphologies of the samples were observed by SEM. Fig. 1a shows the SEM image of electrospun hollow SnO2 nanofibers (sample SNFs), in which they are interweaved to form a porous network with diameter ranging from 100 to 200 nm and length up to several tens of micrometers. The SNFs have a rough surface, composed of a lot of tiny SnO2 nanocrystals, and its hollow feature can be easily observed from the broken tip which is pointed out by an arrow. Subsequently, α-FeOOH nanorods can
Conclusions
In summary, a facile multi-step method combined electrospinning with followed hydrothermal growth and calcination was developed for the fabrication of one dimensional hierarchical SnO2/α-Fe2O3 nanofibers. These hierarchical nanofibers were composed of α-Fe2O3 nanorods branches uniformly deposited on the external surface of hollow SnO2 nanofibers, exhibiting better performance than pure urchin-like α-Fe2O3 nanostructures and SnO2 nanofibers when it was applied to gas sensors. It showed a high
Acknowledgements
This work was financially supported by Zhejiang Provincial Natural Science foundation of China under Grant Nos. LY13E020002 and LY12A04010, Zhejiang Province Environmental Protection Science Research Plan (2011B14) and experimental research project of Zhejiang University.
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