Glycine-assisted hydrothermal synthesis of peculiar porous α-Fe2O3 nanospheres with excellent gas-sensing properties
Introduction
Nanomaterials of well-defined size and morphology have attracted considerable research efforts because of their size- and morphology-dependent properties. Among them are porous metal oxides. A variety of porous metal oxides including those of TiO2[1], [2], SnO2[3], ZnO [4], [5], [6], [7], [8], [9] and In2O3[10] have been synthesized and explored for applications in catalysis, lithium-ion batteries, gas sensing, and so on.
Hematite (α-Fe2O3), a stable phase of iron oxide, is an n-type semiconductor (Eg = 2.1 eV). It is non-toxic, highly resistant to corrosion, and environment friendly. It has been intensively investigated for applications in catalysts [11], [12], [13], lithium-ion batteries [14], [15], water treatment [16], [17] and gas sensors [18], [19], [20], [21], [22], [23], [24]. Hematite of varied morphologies have been fabricated, including nanocrystals [25], [26], particles [27], [28], [29], cubes [30], rods [31], [32], wires [33], tubes [34], flute-like [18], hollow spheres [35] and nanocups [36], and ordered mesoporous Fe2O3[37].
Glycine-assisted hydrothermal synthesis is frequently used to prepare specific structures [38], [39]. For example, Prevot and co-workers prepared flower-like NiAl-layered double hydroxides using Ni (II) glycinate complex as precursor [38]. Zhang and co-workers prepared hollow Ni(OH)2 microspheres using Ni (II) glycinate complex under strong basic conditions. Glycine played an important role in the formation of the specific morphologies and architectures [39].
In the current work, glycine-assisted hydrothermal synthesis was adopted to prepare α-Fe2O3 nanomaterials. The obtained nanomaterials were analysed by X-ray powder diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, and selected area electron diffraction. The results showed that the obtained nanomaterials consist of spherical particles of pure α-Fe2O3 phase. The spherical particles have pores of dual sizes in the core and shell, respectively. The gas-sensing properties of the porous α-Fe2O3 nanospheres were evaluated, and better sensing performance was observed compared with that using α-Fe2O3 nanoparticles of 30 nm in size [40].
Section snippets
Materials and synthesis
All chemicals used were analytic grade, and used as received without further purification. Ultrapure water used in experiments had a resistivity of 18.2 MΩ cm, and was obtained from a Millipore Milli-Q system (Academic).
In a typical synthesis, 0.9 g of urea, 1 g of FeCl3·6H2O, and 2 g of glycine were dissolved in 30 mL of pure water, and stirred for 20 min to form a purple-black solution. The mixture was transferred to a 50 mL Teflon-lined stainless steel autoclave, sealed, and maintained at 160 °C for
Structure and morphology of porous α-Fe2O3 nanospheres
Fig. 1 shows XRD patterns of the Fe2O3 product before and after calcination in air. All the diffraction peaks were readily indexed to a pure rhombohedral phase [space group: R−3c (167)] of α-Fe2O3 (JSPDS card No. 89-0597) [25]. No other impurities could be detected by XRD. The increases in peak intensities after calcination indicate that the annealing at 450 °C for 3 h enhanced the crystallinity of the α-Fe2O3 product.
The annealed α-Fe2O3 product was observed by SEM and TEM. Fig. 2a shows its
Conclusions
In summary, peculiar porous α-Fe2O3 nanospheres were obtained by glycine-assisted synthesis. They have large mesopores (ca. 10 nm) in the core and small mesopores (<4 nm) in the shell. To our best knowledge, there have been so far no reports on the synthesis of such peculiar porous α-Fe2O3 nanospheres. The time and temperature of hydrothermal treatment influence the structure and morphology of product. The α-Fe2O3 crystal phase forms at the early stage, and extended reaction time results in
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grants No. 10776034), and the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant Nos. KGCX2-YW-111-5 and KSCX2-YW-G-059), National Basic Research Program of China (No. 2006CB933000), and Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 112307.
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