Iron oxide hollow spheres: Microwave–hydrothermal ionic liquid preparation, formation mechanism, crystal phase and morphology control and properties

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Acta Materialia 57 (2009) 2154–2165 www.elsevier.com/locate/actamat

Iron oxide hollow spheres: Microwave–hydrothermal ionic liquid preparation, formation mechanism, crystal phase and morphology control and properties Shao-Wen Cao, Ying-Jie Zhu * State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China Graduate School of Chinese Academy of Sciences, China Received 10 October 2008; received in revised form 8 January 2009; accepted 11 January 2009 Available online 21 February 2009

Abstract This paper reports on the microwave–hydrothermal ionic liquid method for the synthesis of a variety of iron oxide nanostructures such as a-FeOOH hollow spheres, b-FeOOH architectures and a-Fe2O3 nanoparticles. The formation mechanism for a-FeOOH hollow spheres is discussed. The effects of the reaction parameters on the morphology and crystal phase of the final product are studied. The relationship between the morphology and crystal phase of the product is discussed. A general thermal transformation strategy is designed to prepare a-Fe2O3 hollow spheres using a-FeOOH hollow spheres as the precursor and template. By thermal treatment of the as-prepared a-FeOOH hollow spheres, a-Fe2O3 hollow spheres showing good photocatalytic activity are obtained. And by autocatalysis of the adsorbed available Fe(II) on the a-FeOOH surfaces, Fe3O4 hollow spheres are also obtained. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ionic liquid; Nanostructure; Oxides; SEM; TEM

1. Introduction Room temperature ionic liquids (RTIL) have been widely studied as new types of environmentally friendly reaction media, owing to their unique properties such as extremely low volatility, wide temperature range in liquid state, good dissolving ability, high thermal stability, excellent microwave absorbing ability, high ionic conductivity, wide electrochemical window, non-flammability, etc. [1,2]. The application of RTIL in inorganic nanomaterial synthesis has received increasing attention in recent years. A variety of inorganic nanostructures have been fabricated via various RTIL-involved processes, including Te nanowires [1], hollow TiO2 spheres [3] and nanocrystals [4], Bi2S3 and Sb2S3 nanorods [5], Bi2Se3 nanosheets [6], PbCrO4 *

Corresponding author. Tel.: +86 2152412616; fax: +86 2152413122. E-mail address: [email protected] (Y.-J. Zhu).

and Pb2CrO5 rods [7], gold nanoparticles [8] and nanosheets [9], and recyclable catalysts of palladium [10], iridium [11] and platinum [12] nanoparticles. Electrochemical deposition of nanocrystalline metals such as Al, Fe, and Al–Mn alloys has also been reported [13]. However, in contrast to their application in organic chemistry, the use of RTIL in inorganic synthesis is still in its primary stage, and more exploration is necessary to utilize their advantages fully. In recent years, the microwave–hydrothermal (MH) method has been rapidly developed as a result of rapid heating, faster kinetics, homogeneity, higher yield, better reproducibility and energy saving compared with the conventional hydrothermal (CH) method [14–16]. Moreover, the MH method also enables chemical reactions to occur in an elevated-temperature and pressurized closed system similar to a hydrothermal condition in a short preparation time of minutes rather than days. RTIL have excellent microwave absorbing ability, thus leading to a very high

1359-6454/$36.00 Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2009.01.009

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heating rate. However, the application of microwave chemistry to materials synthesis in conjunction with RTIL is focused mostly on the open reaction system. The use of RTIL in a closed MH system has hardly been exploited except in a recent report on SnO2 microspheres [17]. Iron oxyhydroxides and iron oxides are functional materials of great industrial and scientific importance. a-FeOOH and b-FeOOH are used as catalysts [18], electrode materials [19–22], adsorbents of arsenite [23,24] and gallium (III) [25]. They are also important precursors in the synthesis of iron oxides such as a-Fe2O3 and c-Fe2O3. a-Fe2O3, an environmentally friendly n-type semiconductor (Eg = 2.1 eV), is the most stable iron oxide under ambient conditions. a-Fe2O3 is used extensively in catalysts, pigments, sensors [26–28], the anode material for Li-ion batteries [29] and the raw material for the synthesis of magnetic c-Fe2O3. Fe3O4 is a very important functional material owing to its magnetic properties, chemical stability, biocompatibility and low toxicity. It is widely used in magnetic storage media [30], clinical diagnosis and treatment [31,32], etc. Among iron oxyhydroxides and iron oxides, synthetic a-FeOOH is typically acicular and often aggregates into bundles or rafts [33–36], though a-FeOOH nanocrystals have also been synthesized [37]. However, to the best of the authors’ knowledge, the synthesis of a-FeOOH hollow spheres whose shell is constructed by self-assembly of nanoparticles has not been reported. One of the difficulties in synthesizing iron oxyhydroxides and iron oxides is control over the crystal phase of the final product. The crystal phases of the products are sensitive to the reaction conditions, such as the pH of the solution, the concentration of iron ions, the polarity of the solution, etc. The final products are usually a mixture of two or three phases, and it is hard to obtain pure products. In addition, the direct relationship between the morphology and the crystal phase of the final product is hardly understood. In order to address these issues in the synthesis of iron oxyhydroxides and iron oxides, there is a need to develop synthetic routes which can be adopted for simple inorganic ferric chemical species, while gaining a relative general understanding of the rules underlying the phase transitions and the connection between the morphology and crystal phase of the final product. The present paper reports on the first successful selective synthesis of a-Fe2O3 nanoparticles, b-FeOOH architectures assembled by bricks and a-FeOOH hollow spheres whose shell is constructed by self-assembly of nanoparticles via a microwave–hydrothermal ionic liquid (MHIL) method. The formation mechanism for a-FeOOH hollow spheres is discussed. The effects of the reaction parameters on the morphology and crystal phase control of a-Fe2O3 nanoparticles, b-FeOOH architectures assembled by bricks and a-FeOOH hollow spheres are systematically investigated. a-Fe2O3 hollow spheres are obtained by thermal treatment of as-prepared a-FeOOH hollow spheres. And magnetic Fe3O4 hollow spheres are also obtained by autocatalysis of the adsorbed Fe2+ ions on the a-FeOOH surface. It is demonstrated that

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the MHIL method is a fast, template-free, energy-saving and environmentally friendly green route. This simple method does not need the subsequent complicated workup procedure required for the removal of the template. 2. Experimental section 2.1. Preparation of a-FeOOH hollow spheres In a typical synthetic procedure, 0.202 g Fe(NO3)3 9H2O, 0.08 g urea and 0.5 ml 1-n-butyl-3-methyl imidazolium tetrafluoroborate ([BMIM][BF4]) were added into 25 ml deionized water under magnetic stirring. The resultant solution was loaded into a 60 ml Teflon autoclave, sealed, microwave-heated to 160 °C and kept at this temperature for 30 min. The microwave oven used for sample preparation was a MH synthesis system (MDS-6, Sineo, Shanghai, China). After being cooled to room temperature, the a-FeOOH product was collected and washed by centrifugation–redispersion cycles with deionized water. Refer to Table 1 for the detailed preparation conditions for typical samples. 2.2. Preparation of a-Fe2O3 hollow spheres The dried as-prepared a-FeOOH powder was heated in air to 275 °C at a rate of 5 °C min-1, and maintained at this temperature for 1 h, and then cooled down naturally. Red a-Fe2O3 powder was obtained. 2.3. Preparation of Fe3O4 hollow spheres To 30 ml deionized water were added 0.036 g as-prepared a-FeOOH powder and 0.056 g FeSO47H2O. The mixture was dispersed in an ultrasonic oscillator for 10 min. Then 2 ml NH3H2O (28 wt.%) was added to the mixture. Black Fe3O4 powder was obtained by continuing sonication for another 5 min. The product was collected and washed by centrifugation–redispersion cycles with deionized water. 2.4. Photocatalytic activity measurements The photocatalytic reactor consisted of two parts: a 70 ml quartz tube and a high-pressure Hg lamp. The Hg lamp was positioned parallel to the quartz tube. In all the experiments, the photocatalytic reaction temperature was kept at 35 °C. The reaction suspension was prepared by adding the powder sample (20 mg) to 50 ml of a salicylic acid solution with a concentration of 20 mg l1. The suspension was sonicated for 15 min and then stirred in the dark for 30 min to ensure adsorption/desorption equilibrium prior to UV irradiation. The suspension was then irradiated using UV light under continuous stirring. Analytical samples were taken from the reaction suspension after various reaction times and centrifuged at 10,000 rpm for 5 min to remove the particles for analysis.

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Table 1 Experimental parameters for the synthesis of typical samples. Sample no.

Solution

Heating method

T [°C]

t [min]

Phase of product

Morphology of product

1

MH

160

30

a-FeOOH

Large hollow spheres

MH

160

30

a-Fe2O3

Nanoparticles

3

0.202 g Fe(NO3)39H2O + 0.08 g CO(NH2)2 + 25 ml H2O + 0.5 ml [BMIM]BF4 0.202 g Fe(NO3)39H2O + 0.08 g CO(NH2)2 + 25 ml H2O Same as sample 1

MH

160

5

b-FeOOH

4

Same as sample 1

MH

160

10

5

Same as sample 1

MH

160

20

6

CO(NH2)2 + 25 ml

MH

160

30

CO(NH2)2 + 25 ml

MH

160

30

b-FeOOH

Small architectures

CO(NH2)2 + 25 ml

MH

160

30

a-FeOOH

Large hollow spheres

CO(NH2)2 + 25 ml

MH

160

30

b-FeOOH

Large spheres and large architectures

CO(NH2)2 + 25 ml

MH

160

30

b-FeOOH

Large architectures

11 12 13 14

0.202 g Fe(NO3)39H2O + 0.08 g H2O + 0.1 ml [BMIM]BF4 0.202 g Fe(NO3)39H2O + 0.08 g H2O + 0.3 ml [BMIM]BF4 0.202 g Fe(NO3)39H2O + 0.08 g H2O + 1 ml [BMIM]BF4 0.202 g Fe(NO3)39H2O + 0.08 g H2O + 5 ml [BMIM]BF4 0.202 g Fe(NO3)39H2O + 0.08 g H2O + 10 ml [BMIM]BF4 Same as sample 1 Same as sample 1 Same as sample 1 Same as sample 1

b-FeOOH aFeOOH a-FeOOH bFeOOH b-FeOOH

Small spheres + Dispersed bricks + Small architectures Small architectures

MH MH MH MH

120 140 180 200

30 30 30 30

b-FeOOH b-FeOOH b-FeOOH a-FeOOH

15

Same as sample 1

CH

160

30

b-FeOOH

Middling architectures Middling architectures Small architectures Large hollow spheres and hollow spindles Middling architectures

2

7 8 9 10

Large hollow spheres + Large solid spheres Small architectures

2.5. Characterization of samples The prepared samples were characterized using X-ray powder diffraction (XRD) (Rigaku D/max 2550 V, Cu Ka ˚ ), scanning electron microscopy radiation, k = 1.54178 A (SEM) (JEOL JSM-6700F) and transmission electron microscopy (TEM) (JEOL JEM-2100F). A physical property measurement system (PPMS) (USA) was used to evaluate the magnetic property at room temperature. Brunauer–Emmett–Teller (BET) surface area measurements were performed on a Micrometitics Tristar 3000 system. The salicylic acid concentrations were analyzed using a UV–vis spectrophotometer (UV-2300, Techcomp) at a wavelength of 297 nm. 3. Results and discussion The detailed preparation conditions and procedures for the samples are described in Section 2, and the preparation conditions for some typical samples are listed in Table 1. Fig. 1a shows the XRD pattern of sample 1 prepared by the MHIL method at 160 °C for 30 min, from which one can see that the product is a single phase of well-crystallized a-FeOOH with an orthorhombic structure (JCPDS No. 81-0464). However, in the absence of ionic liquid ([BMIM][BF4]), a different product (sample 2) was obtained, as shown in Fig. 1b, which can be indexed to a single phase of well-crystallized a-Fe2O3 with a hexagonal structure (JCPDS No. 80-2377). These results show that

Fig. 1. XRD patterns of samples: (a) a-FeOOH hollow spheres (sample 1); (b) a-Fe2O3 nanoparticles (sample 2); (c) a-Fe2O3 hollow spheres; (d) Fe3O4 hollow spheres ( stands for a-FeOOH).

[BMIM][BF4] has a significant effect on the crystal phase of the product, and [BMIM][BF4] facilitates the formation of a-FeOOH. The morphology and microstructure of the samples prepared by the MHIL method were investigated with SEM and TEM. Fig. 2a–d shows the SEM micrographs of a-FeOOH (sample 1), from which one can see hollow spheres with diameters 1 lm. The detailed morphology

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Fig. 2. SEM and TEM micrographs of a-FeOOH hollow spheres (sample 1): (a–d) SEM micrographs; (e and f) TEM micrographs, inset of (e) is SAED pattern of an individual hollow sphere; (g and h) HRTEM images of areas 1 and 2 marked in (f), respectively.

of the hollow spheres is shown in Fig. 2b–d. The SEM micrographs in Fig. 2b and c show the broken spheres, indicating the hollow structure. The high-magnification SEM micrograph of the sphere surface (Fig. 2d) shows that the shell of a-FeOOH hollow sphere was constructed by self-assembly of nanoparticles. Fig. 2e shows the typical TEM micrograph of sample 1, which confirms the hollow structure of the spheres. The selected-area electron diffraction (SAED) pattern of an individual sphere (inset

in Fig. 2e) reveals the single-crystal-like feature of the hollow sphere. The corresponding nearest three spots in the SAED pattern can be indexed to (0 2 0), (0 0 1) and (0 2 1) planes of a-FeOOH. Fig. 2f shows a single a-FeOOH hollow sphere. High-resolution TEM (HRTEM) micrographs taken on areas 1 and 2 are shown in Fig. 2g and h, respectively, from which one can see that the lattice fringes of a-FeOOH nanocrystals have the same orientation, indicating an oriented aggregation of

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a-FeOOH nanoparticles in the shell of the hollow sphere. For comparison, the morphology of a-Fe2O3 (sample 2) synthesized in the absence of [BMIM][BF4] was also investigated. As shown in Fig. 3a–c, well-dispersed nanoparticles with sizes of 20 nm instead of hollow spheres were obtained. This experimental result indicates the significant effect of [BMIM][BF4] on the formation of aFeOOH hollow spheres. The ionic liquid [BMIM][BF4] influences not only the crystal phase, but also the morphology of the product, which will be discussed further below. In order to understand the formation mechanism of aFeOOH hollow spheres, time-dependent experiments were carried out, and the characterization of the samples is shown in Fig. 3d–k. Sample 3 was obtained by the MHIL

method for 5 min (Fig. 3d–g), and one can see solid and hollow spheres with diameters 500 nm, the brick aggregated by solid and hollow spheres, the brick aggregated by several solid spheres, and architectures with diameters 1.5 lm assembled by bricks. Fig. 4a shows the XRD pattern of sample 3, the diffraction peaks can be indexed to bFeOOH (JCPDS No. 75-1594). When the MH time was increased to 10 min (sample 4), architectures with diameters 1.5 lm assembled by bricks were obtained (Fig. 3h and i). The XRD pattern in Fig. 4b indicates that sample 4 consisted of a mixture of b-FeOOH (main phase) and a-FeOOH (minor phase). However, both solid and hollow spheres with diameters 1 lm were obtained when the MH time was increased to 20 min (sample 5), as shown in Fig. 3j and k. The XRD pattern in Fig. 4c is similar to that of sam-

Fig. 3. TEM micrographs of (a–c) sample 2, (d–g) sample 3, (h and i) sample 4, (j and k) sample 5.

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Fig. 4. XRD patterns of (a) sample 3, (b) sample 4, (c) sample 5 ( stands for a-FeOOH).

ple 4, whereas the relative peak intensities of a-FeOOH increased, indicating that the ratio of a-FeOOH in the mixture increased. On the basis of the above experimental results, a possible formation mechanism that combines the aggregation at the initial stage, self-assembly at the subsequent stage and Ostwald ripening at the last stage is proposed. As illustrated in Scheme 1, at steps A and B, several small solid spheres aggregated together to form a single solid brick. At step C, architectures formed by self-assembly of the bricks. For the formation of the architectures with hollow structures, b-FeOOH bricks with hollow structures first formed by aggregation of both solid and hollow spheres (step A0 and B0 ), then b-FeOOH bricks with hollow structures self-assembled to form the architectures with hollow structures (step C0 ). In these three steps, the phase of the product was b-FeOOH. Then the Ostwald ripening occurred. For the architectures with hollow structures, bFeOOH started to dissolve from the outer ends of the bricks, and recrystallized in the void close to the center of the architectures among the bricks, thus to form the hollow spheres, as shown in step D0 . This proposal is also consistent with the result that the diameters of a-FeOOH hollow spheres (1 lm) were smaller than those of architectures (1.5 lm). In this step, b-FeOOH dissolved and

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transformed to a-FeOOH by recrystallization. For the architectures without hollow structures, b-FeOOH started to dissolve from the exterior of the architectures, and recrystallized in the void close to the center of the architectures among the bricks, thus to form the solid spheres composed of a-FeOOH in the exterior and b-FeOOH in the interior (step D). Then, the b-FeOOH in the interior of the solid spheres continued to dissolve and recrystallize and finally form the a-FeOOH hollow spheres, as shown in step E. It is well known that the Ostwald ripening process involves ‘‘the growth of larger crystals at the expense of smaller ones which have a higher solubility than the larger ones” [38,39]. For three-dimensional architectures, it is comprehensible that they can transform to spheres via the Ostwald ripening process, as larger crystals are essentially immobile while the smaller ones are undergoing mass transport through dissolution and re-growth. The ability to form the spheres can be attributed to the existence of intrinsic density variations inside the starting solid architectures. Driven by the minimization of the total energy of the system, the bricks aggregate and organize to form threedimensional architectures, which exhibit different packing densities along the radial direction. The exterior of the architectures with smaller packing densities serves as the starting growth sites for the subsequent recrystallization. As the mass dissolves and recrystallizes in the void among the bricks, a sphere is generated through the Ostwald ripening. By controlling the dosage of [BMIM][BF4] added to the reaction solution, the morphology and phase of the product can be controlled. When the dosage of [BMIM][BF4] was below 0.5 ml (Table 1, samples 6 and 7), b-FeOOH architectures with diameters 1 lm assembled by small bricks were observed (Figs. 5a–f and 6a and b). When 1 ml of [BMIM][BF4] was added (Table 1, sample 8), the product still consisted of a-FeOOH hollow spheres with diameters 1 lm (Fig. 5g and h, Fig. 6c), similar to sample 1 prepared using 0.5 ml [BMIM][BF4]. However, when the dosage of [BMIM][BF4] increased to 5 ml (Table 1, sample 9), b-FeOOH spheres with diameters 1 lm as well as bFeOOH architectures with sizes 5 lm assembled by large bricks were obtained (Figs. 5i and j and 6d). And b-FeOOH architectures with diameters 5 lm assembled by large bricks were observed as a major morphology in the presence of 10 ml [BMIM][BF4] (Table 1, sample 10, Figs. 5k

Scheme 1. The proposed formation mechanism of a-FeOOH hollow spheres.

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Fig. 5. (a–c) SEM micrographs and (d) TEM micrograph of sample 6; (e and f) TEM micrographs of sample 7; (g and h) TEM micrographs of sample 8; (i and j) SEM micrographs of sample 9; (k and l) SEM micrographs of sample 10.

and l and 6e). Therefore, the optimum dosage of [BMIM][BF4] for the production of a-FeOOH hollow spheres is in the range 0.5–1 ml. The MH temperature had a significant influence on the morphology and phase of the final product. No product was obtained at 100 °C. At 120 °C (Table 1, sample 11), b-FeOOH architectures with diameters 2 lm assembled by bricks were observed (Figs. 7a–c and 8a). At 140 °C (Table 1, sample 12), b-FeOOH architectures with diameters from 1 to 2 lm assembled by bricks were observed (Figs. 7d–f and 8b). b-FeOOH architectures with diameters 1 lm assembled by bricks were observed at 180 °C (Table 1, sample 13, Figs. 7g–i and 8c). One can also see that the bricks became longer and narrower as the temperature increased. However, when the temperature was at 200 °C (Table 1, sample 14), a-FeOOH hollow spheres with diameters 1 lm and hollow spindles were

Fig. 6. XRD patterns of (a) sample 6, (b) sample 7, (c) sample 8, (d) sample 9 and (e) sample 10.

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Fig. 7. TEM micrographs of (a–c) sample 11, (d–f) sample 12, (g–i) sample 13, (j–l) sample 14 and (m–o) sample 15.

observed (Figs. 7j–l and 8d). Therefore, the optimum temperature for the production of a-FeOOH hollow spheres is 160 or 200 °C. The morphology and phase of the final product obtained by the CH method were also investigated (Table 1, sample 15). b-FeOOH architectures with diameters

>2 lm assembled by bricks instead of a-FeOOH hollow spheres were obtained by the CH method (Fig. 7m–o and 8e). This demonstrates the advantages of the MHIL method in the synthesis of a-FeOOH hollow spheres. It was found that there was a direct connection between the morphology and phase of the final product. When the

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Fig. 8. XRD patterns of (a) sample 11, (b) sample 12, (c) sample 13, (d) sample 14 and (e) sample 15.

final product was hollow structures (samples 1, 8 and 14), the crystal phase was a-FeOOH. When the final product was architectures assembled by bricks (samples 6, 7, 10, 11, 12, 13, 15), the crystal phase was b-FeOOH.

On the basis of the preparation of a-FeOOH hollow spheres, a general thermal transformation strategy was designed to prepare a-Fe2O3 hollow spheres using a-FeOOH nanostructured hollow spheres as the precursor and template. There has been a report on properties of aFe2O3 synthesized by the thermal treatment of a-FeOOH as well as hematite activation for catalysis and adsorption [40]. The dried powder of as-prepared a-FeOOH hollow spheres (sample 1) was heated to 275 °C in air at a rate of 5 °C min1, maintained at this temperature for 1 h, cooled down naturally, and then the red a-Fe2O3 powder was obtained. Fig. 1c shows the XRD pattern of a-Fe2O3 powder. One can see that the diffraction peak intensities of (1 0 4) and (2 1 4) were much weaker than (1 1 0) and (3 0 0), respectively, which is different from the XRD pattern of sample 2 (Fig. 1b). These differences may be partly responsible for the different photocatalytic properties of the two a-Fe2O3 samples, which will be discussed below. Fig. 9a–c shows the SEM micrographs of a-Fe2O3 hollow spheres, from which one can see that the morphology of hollow spheres was well preserved during the thermal transformation from a-FeOOH to a-Fe2O3. The diameters of a-Fe2O3 hollow spheres were also similar to those of a-FeOOH hollow spheres.

Fig. 9. (a–c) SEM micrographs of a-Fe2O3 hollow spheres; (d–f) SEM micrographs of Fe3O4 hollow spheres; (g and h) TEM micrographs of Fe3O4 hollow spheres.

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Fig. 10. UV–vis absorption spectra of salicylic acid solution in the presence of a-Fe2O3 hollow spheres at different UV-irradiation time: (a) 0 min; (b) 120 min. The degradation rate of salicylic acid over different asprepared photocatalysts: (c) a-Fe2O3 hollow spheres; (d) a-Fe2O3 nanoparticles (sample 2).

To evaluate the photocatalytic activity of a-Fe2O3 hollow spheres assembled by nanoparticles, comparison experiments were performed. Fig. 10a and b shows the UV–vis absorption spectra of salicylic acid solution in the presence of a-Fe2O3 hollow spheres at 0 min and 120 min after UV irradiation, from which one can see that the concentration of salicylic acid decreased rapidly after UV irradiation. Fig. 10c shows the degradation rate of salicylic acid over a-Fe2O3 hollow spheres, from which one can see that the degradation percentage of salicylic acid increased rapidly with increasing time, and reached 60% in 120 min. The photocatalytic activity of a-Fe2O3 nanoparticles (sample 2) was also investigated as a reference, and it was found that a-Fe2O3 nanoparticles show very weak photocatalytic activity, as illustrated in Fig. 10d. It is obvious that the a-Fe2O3 hollow spheres assembled by nanoparticles exhibited superior photocatalytic activity to dispersed nanoparticles, indicating that the hollow structures assembled by nanoparticles are

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favorable for the enhancement of photocatalytic performance. The BET surface areas of a-Fe2O3 hollow spheres and dispersed nanoparticles were measured, and it was found that a-Fe2O3 hollow spheres have a higher specific surface area (50.7 m2 g1) than dispersed nanoparticles (42.1 m2 g1). The higher specific surface area of aFe2O3 hollow spheres can be explained by the fact that they are assembled by nanoparticles. Generally, a larger BET surface area leads to higher photocatalytic activity. In addition, the differences in the XRD patterns (Fig. 1b and c) of the two a-Fe2O3 samples indicate that crystal planes (1 0 4) and (2 1 4) of a-Fe2O3 hollow spheres are less exposed than those of a-Fe2O3 nanoparticles, which may augment the photocatalytic activity of aFe2O3 hollow spheres. A solid–liquid strategy was also designed to prepare Fe3O4 hollow spheres using a-FeOOH hollow spheres as the precursor. It was proposed that the Fe3O4 phase was formed by c- or a-FeOOH transformation by adsorption of Fe(OH)2 or Fe(OH)+ species onto the oxyhydroxide nanoparticle surface followed by an autocatalytic oxidation reaction [41–43]. Fe3O4 formation involves the reaction between FeOOH and iron (II) ions proceeding in two steps: (a) adsorption step of iron (II) ions on the FeOOH resulting in an intermediate species; and (b) transformation step of the intermediate to Fe3O4 [44]. Fig. 1d shows the XRD pattern of the product, from which one can see that the major peaks could be indexed to Fe3O4 (JCPDS No. 85-1436), although weak diffractions of a-FeOOH could also be observed because of its incomplete transformation due to the inadequate adsorption of iron (II) ions. Fig. 9d–f shows the SEM micrographs of Fe3O4 hollow spheres, from which one can see that the morphology of hollow spheres was mostly preserved during the transformation from a-FeOOH to Fe3O4, which is confirmed by the TEM micrograph (Fig. 9g). Fig. 9h shows a representative HRTEM micrograph taken from the edge of the hollow sphere, which indicates that the hollow sphere is assembled by nanoparticles. One can see that the sizes of the primary nanoparticles are less than 25 nm. This strategy may also be extended to the synthesis of Fe3O4 with other morphologies by the transformation of a-FeOOH. Fig. 11 shows the magnetization curve of the Fe3O4 hollow spheres measured at room temperature. The curve presents a very small hysteresis loop that is hardly observable at the scale shown in Fig. 11a, but can be seen in the close-up view of Fig. 11b. It can be read that the Ms (magnetization saturation) value is 70.9 emu g1 (Fig. 11a). The remnant Mr is 2.3 emu g1 (defined as the magnetization at H = 0), and the coercivity Hc is 15 Oe (defined as the field magnitude necessary to obtain M = 0) (Fig. 11b). The Fe3O4 hollow spheres exhibit a relatively high Ms value with relatively low Hc and remnant Mr values, which can be attributed to both the small primary particle size and the assembly of the nanoparticles.

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Fig. 11. (a) Magnetization curve measured at room temperature for Fe3O4 hollow spheres. (b) Detailed view of curve (a) in the region around the origin.

4. Conclusion

References

In summary, a-FeOOH hollow spheres, a-Fe2O3 nanoparticles, b-FeOOH architectures assembled by bricks were successfully synthesized via the MHIL method. The ionic liquid [BMIM][BF4] influences not only the crystal phase but also the morphology of the final product. By controlling the dosage of [BMIM][BF4], a-FeOOH hollow spheres and b-FeOOH architectures assembled by bricks can be obtained. The optimum dosage of [BMIM][BF4] for the production of a-FeOOH hollow spheres is in the range 0.5–1 ml. The MH temperature has a significant influence on the morphology and phase of the final product. a-FeOOH hollow structures and bFeOOH architectures assembled by bricks can be obtained at different temperatures. The optimum temperature for the production of a-FeOOH hollow spheres is 160 or 200 °C. The hollow structures and architectures of the final product are directly connected to a-FeOOH and b-FeOOH, respectively. By thermal treatment of the as-prepared a-FeOOH hollow spheres, a-Fe2O3 hollow spheres showing good photocatalytic activity are obtained. And by autocatalysis of the adsorbed Fe (II) on the a-FeOOH surface, Fe3O4 hollow spheres are also obtained. This method may also be extended to the synthesis of a variety of other elemental and compound nanostructures.

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Acknowledgments Financial support from the Program of Shanghai Subject Chief Scientist (07XD14031), the Fund for NanoScience and Technology from Science and Technology Commission of Shanghai (0852nm05800), the National Natural Science Foundation of China (50772124, the Fund for Innovative Research Groups (50821004)), CAS International Partnership Program for Innovative Research Team, the Key Project for Innovative Research (SCX0606) and Director Fund of Biomaterials Research Center from Shanghai Institute of Ceramics, and the fund of State Key Laboratory of High Performance Ceramics and Superfine Microstructure is gratefully acknowledged.

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