Hydrothermal synthesis of monodisperse porous cube, cake and spheroid-like α-Fe2O3 particles and their high gas-sensing properties
Introduction
Recently, much attention has been focused on the fabrication of monodisperse and porous materials [1], [2]. The unique structures with large specific surface area, low density and well dispersing property, enable the monodisperse porous materials to application in catalysis, microreactors and sensors [3], [4].
In recent decades, metal-oxide-semiconductors (MOS) such as TiO2, V2O5, α-Fe2O3, ZnO and SnO2 have been widely investigated as sensing materials due to their high response to the target gases and simplicity in synthesis [5], [6], [7], [8], [9]. Among them, Alpha-iron oxide (α-Fe2O3) as a typical n-type transition MOS with a band gap of 2.1 eV, has received considerable attention owe to its potential application in many technological areas such as lithium rechargeable batteries, magnetic materials, photocatalytic degradation, electrode materials and gas sensors [10], [11], [12], [13], [14], [15], [16], [17]. Especially, in the field of gas sensors, α-Fe2O3 has been proved to be the highly sensitive materials to detect various toxic and explosive gases [18], [19]. However, they usually suffer from several shortcomings, such as limited at high working temperatures, lack of long-term stability, poor selectivity and so on.
It is well-known that the size, morphology and porosity of MOS have a significant influence on its gas sensing performance. According to a paper [20], a reduction in crystalline size can significantly increase the sensitivity of sensors. Consequently, many investigations have been carried out in order to improve the properties of sensors by reducing the size of crystallites. However, the aggregation between the crystallites becomes very strong due to the van der Waals attraction when the size of crystallites is too small, which will block the diffusion of test gas [21], [22], [23]. Therefore, a high gas response can’t be achieved owing to low utilization rate of sensing layer. Up to now, significant efforts have been made to overcome these limitations and improve the performance by doping with either metal oxides or noble metals (Pd, Au, Pt and Ag) because of the formation of p–n junctions or catalytic activity of metals [24]. It can result in beneficial effect on sensitivity, response time and operating temperature [25], [26], [27]. Compare with the noble metals, the modifiers of non noble metals present potential advantages in cost and preparation produce. A variety of methods for synthesizing the α-Fe2O3 micro-/nanostructures have been reported, including vapor–solid growth [28], template aimed synthesis [29], sol–gel process [30] and hydrothermal synthesis [31], [32], [33]. However, it still remains a challenge for developing simple, low cost and versatile approaches in synthesizing 3D microstructures of α-Fe2O3. Currently, 3D microstructures have attracted much attention due to their unique properties and potential applications in catalysis, electrical, optical, drug delivery, medical diagnostics and sensors [17], [34]. Some literatures [35], [36], [37] show that compare to the pure α-Fe2O3 sensor, the doped α-Fe2O3 sensor with transition metal ions has a remarkably enhance on gas sensing performance to several volatile organic compounds (VOCs) including methanol, ethanol and acetone.
In this paper, we synthesize monodisperse porous pure and Cu-doped α-Fe2O3 microcubes, microcakes and microspheroids by combining hydrothermal and calcinations methods. The gas sensing properties of the gas sensors based on these α-Fe2O3 microparticles are mainly investigated, and the results show that the gas sensing properties can be significantly enhanced by doping Cu ions.
Section snippets
Experimental
All the reagents in the experiment were analytical grade (Tianjin Baishidai Chemical Reagent Co. Ltd.) and used as received without further purification.
XRD analysis
The crystallinity of the as-synthesized pure and Cu-doped α-Fe2O3 microparticles are examined by XRD analysis. Fig. 2(a) shows that all the diffraction peaks are indexed to a pure rhombohedral phase of α-Fe2O3 (JCPDS Card No. 33-0664) with ao = 5.035 Å, co = 13.74 Å. No characteristic peaks are observed for other impurities such as ferroferric oxide or ferrous iron oxide, indicating the high purity of the final products. Moreover, there is also no characteristic reflection peaks from Cu species can
Conclusions
In summary, monodisperse porous α-Fe2O3 microstructures have been synthesized by a template-free hydrothermal process and followed by annealing. The images of SEM and TEM indicate that these structures are microcubes, microcakes and microspheroids with the diameter about 2–3 μm when the reaction time of hydrothermal is 10, 15 and 20 h, respectively. In addition, the possible formation mechanism of monodisperse porous α-Fe2O3 microstructures and the gas-sensing mechanism of sensors are proposed.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 10874140), the College Basic Scientific Research Operation Cost of Gansu province (the manufacture and characteristic research of the optical gas sensing film and the Y series superconducting materials), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and Natural Science Foundational of Gansu province (Grant No. 1308RJZA258 and 1308RJZA216). The Basic
W.X. Jin is a Master degree candidate at Northwest Normal University. His research interest is focused on microstructure and gas sensing properties of semiconductor functional nanomaterials.
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W.X. Jin is a Master degree candidate at Northwest Normal University. His research interest is focused on microstructure and gas sensing properties of semiconductor functional nanomaterials.
S.Y. Ma received her M.S. degree in Semiconductor Physics from Lanzhou University in China in 1990. She received her Ph.D. degree in Condensed Matter Physics from Peking University in China 1997. She was a Visiting Professor at Duke University from September 2008 to September 2009. Now, she is a professor of Physics at Northwest Normal University in China. She is majored in preparation of various functional nanomaterials, such as ultraviolet light and gas sensing materials.
Z.Z. Tie is a Master degree candidate at Northwest Normal University. Her research interest is focused on environmental analysis, photoelectron catalysis and electrochemical analysis of biological and drug molecules.
X.H. Jiang is a Master degree candidate at Northwest Normal University. Her research interest is focused on microstructure and gas sensing properties of semiconductor functional nanomaterials.
W.Q. Li is a Master degree candidate at Northwest Normal University. His research interest is focused on microstructure, optical and gas sensing properties of semiconductor functional nanomaterials.
J. Luo is a Master degree candidate at Northwest Normal University. Her research interest is focused on microstructure and gas sensing properties of semiconductor functional nanomaterials.
X.L. Xu is a Ph.D. degree candidate in Key Laboratory of Atomic and Molecular Physics and Functional Materials of Gansu Province. Her research interest is focused on microstructure, optical and gas sensing properties of semiconductor functional nanomaterials.
T.T. Wang is a Master degree candidate at Northwest Normal University. Her research interest is focused on microstructure and gas sensing properties of semiconductor functional nanomaterials.