Effects of SnO2 additives on nanostructure and gas-sensing properties of α-Fe2O3 nanotubes
Introduction
Metal-oxide semiconductor based sensing materials have obtained substantial attention over the past five decades [1], [2]. It is expected that materials with novel morphologies and structures could bring surprising performances. Driven by this, a number of efforts have been carried out to confirm the relationship between the observed gas-sensing properties and structural features of the binary oxide materials since they have greater potentiality to improve the gas-sensing properties than individual oxide. In general, the existing synthesis strategies for binary oxides can be classified into two major categories: one is a step-by-step process with a two-step hydrothermal and/or by combining other deposition techniques. Demonstrated materials include SnO2–ZnO [3], ZnO–In2O3 [4], Co3O4–ZnO [5], and CuO–SnO2 [6]. In this two-step growth procedure, a core-shell nanoheterostructure can be easily prepared and has been widely investigated. However, this method often suffers from complicated operating conditions because of the contents and distribution of shell oxide are greatly affected by reaction time, reaction temperature, and surface state of core oxide, etc. The other is a simple one-step method, such as coprecipitation [7], hydrothermal [8], electrospinning [9] and various approaches [10], [11], [12], [13], which shows some obvious advantages of easy operation, controllable components, special constructions, and economical costs over the above two-step processes. Among them, electrospinning has been considered as a facile, controllable, and effective method to prepare one-dimensional (1D) composite/heterostructure nanomaterials, which has significantly improved the properties of gas sensors [14], [15], [16].
Among numerous oxide semiconductors, α-Fe2O3 and SnO2 (Eg = 2.2 eV and 3.6 eV at 300 K, respectively) are two multifunctional n-type materials and possessing wide applications in photocatalysis, photovoltaic devices, Li-ions storage, and gas detection [17], [18], [19], [20], [21], [22]. It has been demonstrated that the core-shell and composite heterostructures, by combining α-Fe2O3 and SnO2, could enhance their performances at high levels [23], [24]. Due to the specific energy band structures between α-Fe2O3 and SnO2, the carrier transport behaviors in two oxides have been mainly discussed, particularly the effective electron-hole separation in photocatalytic reactions [25]. Recently, SnO2/α-Fe2O3 (SFO) nanotubes synthesized by electrospinning for photocatalysis and toluene detection were discussed, and received significant improvements compared to pristine α-Fe2O3 nanotubes [26], [27]. However, previous studies showed an unclear view of the influence of SnO2 in the formation process of SFO heterostructure nanotubes. In this regard, the systematically study of the growth mechanism and the sensing performance of SFO nanotubes may still have scientific values and be necessary for practical applications.
In this paper, we present a direct electrospinning method to prepare SFO-x (xSnO2/0.5(1 − x)α-Fe2O3) nanotubes with different SnO2 contents in α-Fe2O3 nanotubes. Upon analysis of electron microscopy images and X-ray diffraction patterns, a possible mechanism for the formation of the SFO nanotubes was proposed. Furthermore, comparing the sensing properties of the SFO nanotube based sensors under several reductive gases (ethanol, acetone, methanol, toluene, ammonia, methane, and hydrogen), the enhancement in gas-sensing properties was also discussed in detail. Our results revealed that even the mere existence of solid-solution or amorphous phase could play an important role in the sensing materials.
Section snippets
Synthesis of SFO nanotubes
The α-Fe2O3 nanotubes have been synthesized by a single-capillary electrospinning route, whose details have been described in our previous work [28]. Similarly, 1.5 mmol salts with different Sn/Fe molar ratios (listed in Table 1) were dissolved into a mixture of 5.0 ml of absolute ethanol and 5.0 ml of N, N-dimethylformamide (DMF). Meanwhile, 0.68 g poly (vinyl pyrrolidone) (PVP, Mw = 1,300,000) was added into the above solution and stirred constantly at room temperature for 6 h. In our experiment, a
Structural and morphological characteristics
The morphology and structure of the SFO-0.05 nanotubes calcined in air at 500 °C for 2 h were characterized by FESEM and HRTEM. The FESEM image in Fig. 1a indicates that the samples consist of uniform nanofibers, with a mean diameter of 65 nm and lengths up to micrometer scale. The magnified FESEM image (Fig. 1b) shows that the rough surfaces of nanofibers are built up of numerous grains. And from the cross section of the broken nanofibers, we can find that the fibers actually have a tubular-like
Conclusions
In summary, a simple electrospinning method was used to prepare SnO2/α-Fe2O3 (SFO) composite nanotubes. The changes in morphology, grain size, phase structure, and gas-sensing properties of SFO nanotubes are greatly affected by SnO2 contents. The results of sensing enhancement may attribute to the fine grain size, doping effect and heterostructures between two oxides. Among them, SFO-0.05 nanotube based sensor performed the best sensing performance, with a high response (27.45), rapid response
Acknowledgments
This work was predominantly supported by Hui-Chun Chin and Tsung-Dao Lee Chinese Undergraduate Research Endowment (CURE) at Lanzhou University. This research was partially supported by the National Natural Science Foundation of China (Grant No. 51302122and U1232121).
Changhui Zhao received his BS and MS degree from the school of physical science and technology, Lanzhou University, in 2011 and 2013, respectively. As a PhD student in condensed matter physics at the same university since 2013, he is interested in the field of oxide semiconductor nanomaterials and gas sensors.
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Cited by (0)
Changhui Zhao received his BS and MS degree from the school of physical science and technology, Lanzhou University, in 2011 and 2013, respectively. As a PhD student in condensed matter physics at the same university since 2013, he is interested in the field of oxide semiconductor nanomaterials and gas sensors.
Wenqi Hu is an undergraduate student in physics from the school of physical science and technology, Lanzhou University. His research interests mainly focus on one-dimensional nanomaterials.
Zhenxing Zhang received his PhD degree in semiconductor physics from the school of physical science and technology, Lanzhou University, in 2009. After completing his visiting research scholar at the department of electrical engineering and computer science in University of California-Berkeley from 2008 to 2009, he is working at Lanzhou University since 2009. His current research interests focus on nanomaterials, biomimetic materials, and photocatalysts for clean water.
Jinyuan Zhou received his PhD degree in condensed matter physics from Lanzhou University in 2010, and then joined in Lanzhou University as a lecturer. From 2010 to 2012, he worked at Nanyang technological University of Singapore as a postdoctoral fellow. He is engaged in the SiC based optoelectronic nanomaterials, carbon nanotube based sensors, and carbon nanotube intramolecular junctions.
Xiaojun Pan received his PhD degree in condensed matter physics from Lanzhou University in 2008. And he holded his postdoctoral position in Nanyang technological University of Singapore from 2007 to 2008. He became an associate professor of Lanzhou University in 2012. His current researches are centred on the GaN films and luminescent rare earth nanomaterials.
Erqing Xie is a professor at Lanzhou University of China since 2001. He received his PhD degree in condensed matter physics from Lanzhou University in 1995, and joined Lanzhou University as a lecturer. His group interests are in the fields of superhard films and their applications, such as DLC, SiC, SiCN, BN, and C3N4); key materials and thin film technology for solar energy applications, and thin film technology in optoelectronics, functional nanomaterials and their applications. His current researches mainly include dye-sensitized solar cells, supercapacitor electrode materials, oxide semiconductor based gas sensors, UV photodetectors, and photocatalytic technology.