Preparation and crystallization of hollow α-Fe2O3 microspheres following the gas-bubble template method
Introduction
The production of hollow microspheres is of current interest due to their promising applications in photonic crystals, encapsulation, drug delivery, catalysis, chemical storage, light fillers and low dielectric constant materials [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. A variety of hollow spheres such as carbide [15], Ni [16], TiO2 [17], NiS [18], Bi2Te3 [19] and ZnO/SnO2 [20] have been successfully fabricated. The most common techniques to produce hollow spheres are based on the use of core organic/inorganic hard templates such as monodispersed silica spheres [21], [22], [23], polymer latex colloids [11], [24], carbon spheres [25] and block copolymers [26], [27] or soft templates, such as emulsion droplets [28], [29], surfactants vesicles [30] and liposome [31]. In general, the template technique involves four major steps (as represented in Fig. 1) [1]: (1) Preparation of the templates; (2) functionalization/modification of the templates surface to achieve favourable surface properties; (3) coating the templates with desired materials or their precursors; and (4) selective removal of the templates in appropriate solvents or calcination to obtain the hollow structures.
The hard template technique is effective for controlling the morphology of the final product. Nevertheless, this technique requires tedious synthetic procedures such as a careful selection of an affine template and a lot of care to prevent the collapse to affecting the quality of the shell during template removal. Some other drawbacks include limited sphere size, quality, purity, cost of production, and low temperature capability of the produced hollow spheres.
Recently different free-template approaches have been developed to produce hollow spheres. Some of these methods are based on Oswald ripening [32], simultaneous blowing and melting hidrogels [33], [34], [35], Kirkendall Effect [36], [37], [38], among others. However the average size of the hollow spheres produced by these methods are usually larger than 10 μm. Furthermore, it is difficult to obtain small microspheres having a narrow particle size distribution, and high purity metal oxide composition. Another less explored method for the production of hollow spheres is ‘the gas-bubble template method’. This method involves the production of gas microbubbles during the chemical preparation of nanoparticles by using selected ligands. It is believed that the nanoparticles cover the surface and become hollow spheres after calcinations at high temperatures [39], [40], [41], [42], [43], [44], [45], [46]. However the exact mechanism for the bubble nucleation and grow is unclear.
Hematite is the most stable iron oxide. It is n-type semiconductor (Eg = 2.2 eV) under ambient conditions and it is easy to synthesize. Due to its magnetic properties, corrosion-resistance, low cost and low toxicity it is widely used in catalysis [47], [48], [49], [50], environmental protection [51], [52], [53], [54], [55], [56], [57], sensors [58], [59], [60], [61], magnetic storage materials [62] and clinic diagnosis and treatment [63]. To date, the preparation of a variety of hematite morphologies such as rhombohedra [64], particles [65], [66], [67], [68], nanocubes [69], [70], rings [71], wires [72], [73], rods [74], [75], fibbers [76], flakes [77], cages [78], airplane-like structures [79] and hierarchical structures [80], [81], [82] have been reported. Recently, some works have reported the production of crystalline hematite hollow spheres through various methods. Some of the approaches are listed in Table 1. Note that most of the existing methods for obtaining the hematite hollow spheres involve templates, surfactants, toxic organic solvents, or complex steps. Among them, the hydrothermal/solvothermal method has some advantage over the rest due to its fast reaction time, effective control of particle shape, and low incorporation of impurities into the products. However, this technique requires of steel pressure vessels or autoclaves during preparation to apply high pressure and thus to achieve the formation of the hollow spheres [88], [89], [90], [91], [92], [93], [94], [95], [96]. In contrast, in this work we report the preparation of hematite hollow spheres by the gas-bubble template technique in which no high pressure or any special conditions of atmosphere are required. Here, the hollow hematite microspheres are formed by annealing sol–gel iron oxide precursor in air. We propose a mechanism for the hollow formation based on the condensation, crystallization and oxidation of bubbles shells at high temperatures. This method is reproducible, simple, cheap, environmental friendly and it allows good control of the size, crystallization and oxidation of the product.
Section snippets
Experimental
Hollow hematite microspheres were produced by a modified gas-bubble template method following annealing in air an iron oxide precursor obtained by sol–gel [98]. For the precursor, 200 ml of colloidal ferric nitrate nine-hydrate (Fe(NO3)3·9H2O) particles and mono hydrated citric acid (C6H8O7·H2O, 0.2 M) were dissolved in 800 ml of de-ionized. The solution was vigorously agitated in a magnetic stirrer at 350 rpm (70 °C) for a period of 48 h to form Fe(OH)3. The citric acid was used as ligand, to
Results and discussion
Fig. 2 shows the X-ray diffraction patterns of the samples after annealing at different temperatures from 180 to 600 °C. Initially, after annealing at 180 °C, the sample consists of an amorphous solid with no preferred reflections in the XRD. After annealing at 250 °C, magnetite (Fe3O4) and maghemite (γ-Fe2O3) coexisting with a small amount of hematite (α-Fe2O3) were found. The first two phases were differentiated in the XRD by following the Kim's method [106], in which the (511) peak around
Conclusions
Hollow hematite microspheres were prepared by the gas-bubble template method. Boiling at high temperatures promotes bubble formation on which crystallites agglomerate, crystallize and oxidize to the hematite phase leading in the formation of hollow microspheres. The size and crystallization of the hematite hollow spheres increases with annealing temperature. After annealing at 550 °C, hollow spheres with mean diameter of 0.889 μm are obtained, whereas after annealing at 600 °C, hollow spheres
Acknowledgements
This work was supported by the Engineering and Physical Science Research Council (EPSRC No. EP/J003638/1). The work in Peru has been supported by the Ministry of Production of Peru through the program “Programa Nacional de Innovación para la Competitividad y Productividad” (Innóvate Perú project No. ECIP-1-P-069-14). The work in Brazil was supported by CNPq (307552/2012-8), CAPES (PNPD-230.007518/2011-11) and FACEPE (APQ-0589-1.05/08).
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