Cu-doped α-Fe2O3 hierarchical microcubes: Synthesis and gas sensing properties
Graphical abstract
Uniform Cu-doped α-Fe2O3 cubes piled up nanoparticles as secondary units have been prepared and tested for detecting ethanol with enhanced response and selectivity.
Introduction
Driven by growing concerns about air-quality, environmental monitoring, medical diagnosis, and detection of explosive and toxic gases, developing new sensor strategies for ever increasing sensitivity, selectivity, and reduction of cost represents one of the major scientific challenges [1], [2], [3]. In the past few decades, metal-oxide semiconductors such as SnO2, α-Fe2O3, ZnO, and In2O3 have been widely investigated as sensing materials, owing to their high response to the target gases and simplicity in synthesis [4], [5], [6], [7], [8], [9]. It is well known that the sensing mechanism can be explained by the change in resistance caused by the adsorption of oxygen and reaction with test gas molecules on the surface. Therefore, the gas sensing properties of oxide semiconductors are closely related with their composition, crystalline size, and surface morphology. According to the literature [10], a reduction in crystalline size can significantly increase the sensitivity of sensors. Consequently, many investigations have been carried out in order to improve the performance of sensors by using crystallites with reduced size. However, the aggregation between the crystallites becomes very strong due to the van der Waals attraction, which will block the diffusion of test gas [11], [12], [13]. Thereby, a high gas response can’t be achieved owing to low utilization rate of sensing layer. In recent years, the above-mentioned drawback has been alleviated by using unique morphology of oxides as sensing materials [14].
Alpha-iron oxide (α-Fe2O3), one of the most studied functional oxides with n-type semiconducting properties, is believed to be a promising sensing material due to its nontoxicity, stability, and low cost [15], [16], [17]. Many investigations about gas sensors have suggested that sensing materials with hierarchical structures can increase both sensitivity and response speed owing to their large surface area, low density, and surface permeability [18], [19], [20], [21]. Despite exciting results have been obtained, the development of more highly sensitive and markedly selective gas sensors based on α-Fe2O3 nanostructures remains a challenge. Nowadays, many efforts have been taken such as element doping, heterostructure constructing, and adding catalyst in order to meet the increasing demands for making sensors work in more complicated systems and under more harsh conditions [22], [23], [24], [25], [26], [27], [28], [29]. Among these methods, doping has been reported to be a very simple and feasible way to enhance the gas sensing properties of oxides.
In this paper, we present a one-step hydrothermal method for the synthesis of monodisperse pure and Cu-doped α-Fe2O3 “quasi-cubes”, which were piled up by nanoparticles. To demonstrate the potential applications, the gas-sensing properties of as-prepared α-Fe2O3 microstructures were investigated. A comparative gas sensing study between the Cu-doped α-Fe2O3 and pure α-Fe2O3 hierarchical architectures was performed to demonstrate the superior gas sensing properties of the doped samples. As expected, the sensor using 3.0 wt% Cu-doped α-Fe2O3 cubes displayed high response to ethanol. It was found that the sensor given a response of 19–100 ppm C2H5OH, which was about three times higher than that of the pure α-Fe2O3 cubes at the same operating temperature (225 °C).
Section snippets
Synthesis and characterization of cubic Cu-doped α-Fe2O3
All the chemical reagents were of analytical grade (Beijing Chemical Co., Ltd.) and used as received without further purification, FeCl3·6H2O and Cu(NO3) 2·3H2O were used as iron and copper sources, respectively. A series of hierarchical Cu-doped α-Fe2O3 (0.0 wt%, 1.0 wt%, and 3.0 wt%) cubes were synthesized by one-step hydrothermal reaction. In brief, FeCl3·6H2O (4.054 g) and hexamethylenetetramine (HMT 1.2 g) were directly dissolved in 30 mL mixture of ethanol and water (1:1 v/v) with vigorous
Results and discussion
The morphologies and microstructures of the obtained α-Fe2O3 products were characterized by FESEM. A panoramic FESEM image of pure α-Fe2O3 prepared by hydrothermal process at 160 °C for 12 h shows that the product was composed of uniformly dispersed particles (Fig. 1a). No other morphologies could be detected, which indicates that the high dispersity and uniformity were achieved using this route. It can be observed from the enlarged FESEM image (Fig. 1b) that the monodisperse particles had a
Conclusions
In summary, we have developed a simple one-step hydrothermal method for the synthesis of pure and Cu-doped α-Fe2O3 hierarchical microcubes. The results of X-ray diffraction indicated that Cu ions entered into the crystal lattice of α-Fe2O3 without deteriorating the original crystal structure. As a proof-of-concept demonstration of the function, such pure and doped α-Fe2O3 microcubes were used as the sensing materials of gas sensors. An enhanced sensing property to ethanol was demonstrated, in
Acknowledgment
This work is supported by the National Nature Science Foundation of China (Nos. 61074172, 61134010, 61374218 and 61327804) and Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT13018). National High-Tech Research and Development Program of China (863 Program, No. 2013AA030902).
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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Cited by (0)
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.
Chen Wang received his BS degree from the Electronics Science and Engineering department, Jilin University, China in 2013. Presently, he is a graduate student, majored in microelectronics and solid state electronics.
Xin Zhou received his BS degree from the Electronics Science and Engineering department, Jilin University, China in 2013. Now, he is a graduate student, majored in microelectronics and solid state electronics.
Pengfei Cheng obtained his MS degree from Jilin University, China. He entered the Ph.D. course in 2011, majoring in physical electronics. His current research is dye sensitized solar cell.
Kengo Shimanoe has been a professor at Kyushu University since 2005. He received a B. Eng. degree in applied chemistry in 1983 and a M. Eng. degree in 1985 from Kagoshima University and Kyushu University, respectively. He joined Nippon Steel Corp.in1985, and received a Dr. Eng. degree in1993 from Kyushu University. His current research interests include the development of gas sensors and other functional devices.
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.
Noboru Yamazoe had been a professor at Kyushu University since 1981 until he retired in 2004. He received his M. Eng. degree in applied chemistry in 1963 and his Dr. Eng. Degree in 1969 from Kyushu University. His research interests were directed mostly to the development and application of functional inorganic materials.