synthesis of hollow microspheres constructed with fe2o3 nanorods

Journal of Alloys and Compounds 477 (2009) 90–99 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Synthesis of hollow microspheres constructed with ␣-Fe 2 O 3 nanorods and their photocatalytic and magnetic properties Xiaoli Xie, Heqing Yang ∗ , Fenghua Zhang, Li Li, Junhu Ma, Hua Jiao, Jianying Zhang Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an 710062, China article info Article history: Received 18 June 2008 Received in revised form 22 October 2008 Accepted 29 October 2008 Available online 16 December 2008 Keywords: Nanostructures Chemical synthesis X-ray diffraction Magnetic measurements abstract Microspheres constructed with ␣-FeOOH nanorods were fabricated via a novel hydrothermal route using a mixture of FeCl 3 and Na 2 SO 4 or Fe 2 (SO 4 ) 3 as raw material, and can b e transformed into hol- low microspheres constructed with ␣-Fe 2 O 3 nanorods by calcining in air at 600 ◦ C for 2 h. The ␣-Fe 2 O 3 superstructures consist of well-aligned ␣-Fe 2 O 3 nanorods with an average length of about 1␮m growing radially from the centers of the superstructures. A possible mechanism was also proposed to account for the formation of the ␣-FeOOH and ␣-Fe 2 O 3 superstructures. Photocatalytic and magnetic properties of the ␣-Fe 2 O 3 superstructures were studied. The results indicate that the hollow microspheres constructed with ␣-Fe 2 O 3 nanorods are effective photocatalyst for the degradation of methyl orange and display weak ferromagneticbehaviorwith a remanence of 0.071 emu g −1 and a coercivityof 219 Oe at room temperature. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Recently, morphology controlled synthesis and the large-scale self-assembly of the nanoscale building blocks into complex struc- tures have been the focus of significant interests in materials chemistry and device fabrications. ␣-Fe 2 O 3 , as one of the most important transition magnetic metal oxides, has received increas- ing attention due to its extensive applications, such as magnetic recording media, catalysts [1], pigments [2], gas sensors [3,4] and optical devices. Therefore, many efforts have been directed toward the fabrication of ␣-Fe 2 O 3 with nanometer dimensions to enhance its performance in currently existing applications and find novel properties. Up to now, a number of ␣-Fe 2 O 3 nanos- tructured materials in various geometrical morphologies such as nanorods [5], nanobelts [6], nanowires [6,7], nanotubes [4,8,9], nanocubes [1,10], and nanoplates [11] were synthesized by various soft chemical and vapour-phase processes. However, the assem- bly of one-dimensional (1D) and two-dimensional (2D) ␣-Fe 2 O 3 nanoscale building blocks into three-dimensional (3D) superstruc- tures is still a challenge in the realization of advanced nanodevices. Recently, Zhu et al. [12,13] prepared 3D urchin- and cantaloupe- like superstructures constructed with ␣-Fe 2 O 3 nanorods by a hydrothermal route using FeSO 4 ·7H 2 O and NaClO 3 as reagents. Li et al. [14] synthesized airplane-like ␣-Fe 2 O 3 nanostructures by the hydrothermal reaction of FeCl 3 with NaOH and ethy- ∗ Corresponding author. E-mail address: hqyang@snnu.edu.cn (H. Yang). lene glycol and sequential annealing at 600 ◦ C in air. Zhong et al. [15] reported the synthesis of 3D flower-like nanostructures constructed with ␣-Fe 2 O 3 nanosheets by refluxing the mixed solu- tion of FeCl 3 , tetrabutylammonium bromide, urea and ethylene glycol at 195 ◦ C and sequential annealing at 450 ◦ C in air and their application in water treatment. Zeng et al. [16] synthesized porous hematite nanoflowers by thermal decomposition of pre- cursors synthesized by the solvothermal reaction of FeCl 3 ·6H 2 Oin ethanol. The ␣-Fe 2 O 3 micro-pine dendrites [17] were synthesized by the hydrothermal reaction of K 3 [Fe(CN) 6 ] in aqueous solu- tion. In addition, hollow nanostructures have widespread potential application in drug delivery, catalysts, photonic crystals, nano- reactors, etc., owing to their higher specific surface area, lower density. Bang and Suslick [18] reported sonochemical preparation of nanosized hollow Fe 2 O 3 particles by using carbon nanoparticles as a template. Li et al. [19] and Lian et al. [20] prepared hol- low hematite spheres through a hexadecyltrimethyl ammonium bromide (CTAB)-assisted hydrothermal or solvothermal process. Moreover, ␣-Fe 2 O 3 hierarchically nanostructured hollow spheres assembled by nanosheets [21] were prepared by thermal decompo- sition of a precursor which was obtained using FeCl 3 ·6H 2 O, NaOH, and sodium dodecylbenzenesulfonate in the solvent ethylene gly- col by a facile microwave-assisted solvothermal method. However, to our knowledge, hollow microspheres constructed with ␣-Fe 2 O 3 nanorods have not been reported yet. Herein, we synthesized ␣-FeOOH nanorod-based microspheres via a hydrothermal reaction of Fe 3+ in the presence of SO 4 2− ions. Hollow microspheres constructed with ␣-Fe 2 O 3 nanorods were obtained by thermaldecompositionof theas-synthesized␣-FeOOH 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.161 X. Xie et al. / Journal of Alloys and Compounds 477 (2009) 90–99 91 for the first time. This procedure is distinctly different from the reported methods of hollow nanostructures in literatures [18–21], and it does not require carbon particles or surfactants. In addition, the photocatalytic and magnetic behaviors of ␣-Fe 2 O 3 nanorod- based hollow microspheres were investigated and the formation mechanism of such ␣-FeOOH and␣-Fe 2 O 3 superstructureswas also discussed. 2. Experimental In a typical experiment, 10 mL of 0.1 mol/L Na 2 SO 4 aqueous solution was added to 2 mL of 0.5mol/L FeCl 3 solution understirring. After stirring about 10 min, 8mL of deionized water was added under continuous stirring to form a homogeneous solu- tion, the mixed solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and heated to 140 ◦ C and then maintained at the same temperature for 12h. After being cooled to room temperature, the yel- low product was isolated by centrifugation, and washed with deionized water and absolute ethanol several times. Finally, the products were dried in air at room tem- perature. In the next step, the as-prepared products were heated to 600 ◦ C at rate of 1 ◦ C/min and then maintained at 600 ◦ C for 2 h. The red powder was obtained, which was used for further analysis and characterization. The photocatalytic property of the ␣-Fe 2 O 3 products was determined by mea- suring the decoloration of methyl orange solution, which was selected as a test compound. 30 mg of hollow microspheres constructed with ␣-Fe 2 O 3 nanorods was added to 30 mL of 1.25 × 10 −5 mol/L aqueous methyl orange solution. Prior to irradi- ation, the suspensions were magnetically stirred in the dark for 30 min to establish the adsorption/desorption equilibrium between the dye and the as-prepared sam- ples. Subsequently, the 0.15mL of H 2 O 2 (3%) was added to the mixed solution. Then, the mixed solution was transferred into a 50 mL quartz test tube and irradiated with two 15 W ultraviolet (UV) lamps (365 nm) at a distance of about 25 cm. At given irradiation time intervals, 5mL samples were withdrawn from the test tube for analysis. The absorption spectra of these solutions were measured by using a 7U-1901 ultraviolet-visible spectrophotometer. The as-prepared␣-FeOOHand␣-Fe 2 O 3 nanorod-based microsphereswere char- acterized and analyzed by X-ray diffraction (XRD), infrared (IR) spectra, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The powder XRD was performed on a Rigaku DMX-2550/PC X-ray diffrac- tometer at a voltage of 40 kV and a current of 100 mA with Cu K␣ radiation ( = 1.5406 Å), employing a scanning rate of 8 ◦ /min in the 2Â rangefrom20to70 ◦ . SEM images were obtained using a FEI Quanta 200 scanning electron microscope at an accelerating voltage of 20 kV. HRTEM images and the corresponding selected area electron diffraction (SAED) patterns were taken on a JEOL JEM-3010 transmis- sion electron microscope at an accelerating voltage of 300 kV. The samples for TEM were prepared by dispersing ␣-FeOOH/␣-Fe 2 O 3 products on carbon-coated copper grids. The IR spectra were recorded using a Brucher EQUINX55 Fourier Transform IR spectrophotometer. The magnetic properties of the ␣-Fe 2 O 3 products were exam- ined using a Lake Shore 7307 vibrating sample magnetometer at room temperature. The Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution were performed by N 2 gas adsorption using an America Micromeritics ASAP 2020 surface analytical instrument. 3. Results and discussion Fig. 1 displays the XRD patterns of the iron oxide precursor syn- thesized by the hydrothermal reaction of FeCl 3 with H 2 Oat140 ◦ C for 12 h in the presence of Na 2 SO 4 and the products obtained by the calcination of the precursors at 600 ◦ C. In Fig. 1a, all the diffraction peaks can be indexed as orthorhombic phase of ␣-FeOOH, which are consistent with the values in the literature (Joint Committee on Powder Diffraction Standards (JCPDS) card No: 81-0462). Fig. 1b shows the XRD pattern of the products obtained by the further annealing of ␣-FeOOH nanostructures at600 ◦ C in air for 2 h. Allthe strong and sharp diffraction peaks can be indexed as the hexagonal phase of ␣-Fe 2 O 3 (JCPDS card No: 33-0664). No peaks from other phases are found, suggesting high purity of the as-synthesized ␣- Fe 2 O 3 . The morphology and structure of the as-synthesized ␣-FeOOH precursor were further studied by SEM and TEM, the results are shown in Fig. 2. SEM image at low magnification (Fig. 2a) shows that the products consist of a large quantity of microspheres with typical diameters in the range of 2–4.5 ␮m. The SEM image at high magnification shown in Fig. 2b reveals that the microsphere is con- Fig. 1. XRD patterns of (a) theprecursors prepared by thehydrothermal process and (b) the products obtained by the calcination of the precursors at 600 ◦ C. structed from 1D nanorods with the diameter of about 150 nm. Fig. 2c shows a typical TEM image of an isolated microsphere, which indicates that the microsphere is solid and has diameter of 2␮m. The basic nanorods are straight and have diameters in the range of 60–100 nm and lengths of 0.6–1.0 ␮m. Interestingly, an important phenomenon is found in the TEM observations: a sheaf of tiny nanorods with diameters of 3 nm are attached side- by-side into the external sharp end of the constituent “mother” nanorods, as shown in Fig. 2d. The high-resolution TEM image of an individual ␣-FeOOH nanorod is displayed in Fig. 2e. The fringe spacing is about 0.24 and 0.26 nm, corresponding to the (1 11) and (0 21) crystal planes of the orthorhombic ␣-FeOOH, respec- tively. The product obtained by annealing the as-synthesized ␣-FeOOH nanostructures at 600 ◦ C in air for 2 h was characterized by SEM and TEM, the results are shown in Figs. 3 and 4. SEM observations show that although ␣-FeOOH was transformed to ␣-Fe 2 O 3 after the annealing process, the spherical morphology of the products was almost maintained (Fig. 3a). In addition, we found that the solid microspheres became into the hollow ones from the half- baked microsphere shown in the top right corner of Fig. 3a and that in Fig. 3b. The hollow spheres were built from a single layer of radially oriented nanorods with a diameter of about 120nm, self- wrapping to form hollow interior 2 ␮m in diameter (Fig. 3b). This result was further proven by the following TEM analysis. Fig. 4ais the TEM image of an individual microsphere. The center portion of the superstructure is lighter than that of the edge, further confirm- ing the hollow interiors of the unique self-wrapped nanorod arrays. The constituent nanorods are in fact a sheaf of tiny nanorods with diameters of about 5nm (Fig. 4b and c). Fig. 4d is the corresponding SAED pattern, which clearly indicates thatthe constituent nanorods were formed by an oriented-growth. The HRTEM image is shown in Fig. 4e, the measured lattice spacings of 0.27 and 0.37 nm are con- sistent with the d values of the (1 04) and (01 2) planes of ␣-Fe 2 O 3 with a hexagonal structure, respectively. In order to understand the formation process of the ␣-FeOOH nanorod-based microspheres, time-dependent experiments were carried out and the resultant products were analyzed by SEM and XRD. The representative SEM images of the products prepared at 140 ◦ C for different reaction times are shown in Fig. 5. The SEM observations show that the products obtained for 15 min consist of nanorods with diameters of about 60 nm and lengths in the range of 300–350nm. These nanorods assemble together to form loose aggregates (Fig. 5b). With an increase on the reaction time, the 92 X. Xie et al. / Journal of Alloys and Compounds 477 (2009) 90–99 Fig. 2. Typical SEM (a, b), TEM (c, d) images and HRTEM (e) image of the ␣-FeOOH nanorod-based microspheres prepared by the hydrothermal process. constituent nanorods grew further, and thus their diameters and lengths increased. At the same time, the nanorods self-assembled further into 3D urchin-like congeries and sphere-like superstruc- tures as a result of oriented-attachment. When the reaction time was prolonged to 12 h, the products are entirely composed of the nanorod-based microspheres (Fig. 1a). Fig. 6 shows XRD patterns of the products prepared for different reaction times. The XRD pattern shown in Fig. 6a indicates that the products obtained for 15 min are not well crystallized, and may be amorphous struc- ture. When the reaction time increased to 8 h, the products are well crystallize d. All the diffraction peaks shown in Fig. 6b can be indexed as Fe 8 (OOH) 16 Cl 1.3 (Akaganeite-M) with a monoclinic structure [22], which are consistent with the values in the litera- ture (JCPDS No: 42-1315). When the reaction time increase d to 2 h, in addition to Fe 8 (OOH) 16 Cl 1.3 , ␣-FeOOH with a monoclinic struc- ture is also formed (Fig. 6c). The Fe 8 (OOH) 16 Cl 1.3 transforms into ␣-FeOOH with an increase on the reaction time (Fig. 6c–e). When the reaction time was prolonged to 8h, the Fe 8 (OOH) 16 Cl 1.3 are completely transformed into ␣-FeOOH. When the reaction time is 12 h, the products are composed of the ␣-FeOOH nanorod-based microspheres (Figs. 1a and 2a). Therefore, it is reasonable to con- clude that the chemical reactions to form ␣-FeOOH are formulated X. Xie et al. / Journal of Alloys and Compounds 477 (2009) 90–99 93 Fig. 3. Typical SEM images of the ␣-Fe 2 O 3 hollow microspheres obtained by calcination of ␣-FeOOH microspheres at 600 ◦ C. as follows: FeCl 3 + H 2 O → Fe 8 (OOH) 16 Cl 1.3 (1) Fe 8 (OOH) 16 Cl 1.3 → ␣-FeOOH + H 2 O + HCl (2) In addition, the yield also increases with increasing the reaction time, indicating that formation of Fe 8 (OOH) 16 Cl 1.3 is a slow process in the presence of Na 2 SO 4 . The as-prepared ␣-FeOOH nanostruc- tures were heated at 600 ◦ C in an air atmosphere to obtain hollow microspheres constructed with ␣-Fe 2 O 3 nanorods. As we know, an aggregation process involving the formation of larger crystals by greatly reducing the interfacial energy of small primary nanocrystals is energetically favored. However, the inter- action between unprotected building units with nanoscale size is generally not competent to form stable and uniform microstruc- tures [13], such as the sphere-like superstructures discussed here. Moreover, the building blocks would always randomly aggre- gate into disordered crystals rather than single crystals in the absence of sufficiently strong surface-protecting layers. Therefore, the presence of Na 2 SO 4 was believed to be crucial for the for- mation of the unique ␣-FeOOH microspheres constructed with nanorods. To illuminate the role of Na 2 SO 4 in the formation of ␣- FeOOH nanorod-based microstructures, the products obtained via a hydrothermal reaction of FeCl 3 with H 2 Oat140 ◦ C for 12 h in the absence of Na 2 SO 4 were characterized with SEM and XRD, and the results are presented in Fig. 7. The SEM image and XRD pattern indicated that FeCl 3 reacts with H 2 O without Na 2 SO 4 to form ␣- Fe 2 O 3 nanocubes with a hexagonal structure instead of ␣-FeOOH nanorod-based microspheres. Obviously, the advantage is that we can control the morphology of the products by adding or removing the sulfate ions. In addition, the IR spectra of ␣-FeOOH nanorod- based microspheres obtained at 140 ◦ C and Na 2 SO 4 powders were measured, and results are shown in Fig. 8. In the IR spectra the bands at 1125 and 619 cm −1 are attributed to SO 4 2− stretching and bending vibrations, the broad bands around 3402 and 1632 cm −1 is ascribed to H 2 O stretching and bending modes, the band at 3160 cm −1 can be assigned to O-H stretching mode in ␣-FeOOH, the bands at 893 and 794 cm −1 are attributed to O–H bending modes in ␣-FeOOH [23], and the band at 490 cm −1 was ascribed to Fe–O stretching vibrations [24]. The IR spectra demonstrate that there are SO 4 2− ions on the surface of the constituent ␣-FeOOH nanorod. In order to further understand the role of Na 2 SO 4 in the formation of ␣-FeOOH nanorod-based microstructures, the prod- ucts obtained via a hydrothermal reaction at 140 ◦ C for 2 and 12 h using Fe 2 (SO 4 ) 3 as raw material were characterized with SEM and XRD, and the results are given in Figs. 9 and 10. The SEM images and XRD patterns reveal that the products obtained for 2 and 12 h are all composed of ␣-FeOOH nanorod-based microspheres. When the reaction is increase d from 2 to 12 h, the diameters and lengths of constituent nanorods are increased, and the diameters of microspheres are increased from 2–2.7 to 4.6–6.8 ␮m. These results indicate that the formation of ␣-FeOOH nanorod-based microspheres is the result of SO 4 2− ions alone. The SO 4 2− ions sup- press the further dehydration of ␣-FeOOH and play an important role in the formation and self-assembly of Fe 8 (OOH) 16 Cl 1.3 and ␣- FeOOH nanorod into sphere-like superstructures in the aqueous solution. In this work, the sphere-like ␣-FeOOH and ␣-Fe 2 O 3 super- structures were synthesized without using any hard templates or surfactants. Thus, the formation of sphere-like ␣-FeOOH superstructures in hydrothermal conditions may be due to the “oriented-attachment” mechanism [25,26]. On the basis of the investigations described above, it is possible to interpret the formation process. The possible formation process of the sphere- like ␣-FeOOH and ␣-Fe 2 O 3 superstructures is schematically illustrated in Fig. 11. First, at an early reaction stage, the pri- mary Fe 8 (OOH) 16 Cl 1.3 nuclei with a amorphous structure were formed through conventional nucleation. Then, many neighbor- ing primary Fe 8 (OOH) 16 Cl 1.3 nuclei further grew into the rod-like nanocrystals with a monoclinic structure through oriented aggre- gation. Simultaneously, the rod-like nanocrystals gradually evolved into 3D urchin-like superstructures through oriented-attachment and self-assembly. Finally the urchin-like superstructures fur- ther assembled and became dispersed sphere-like superstructures driven by the minimization of surface energy. The SO 4 2− ions play an important role in the formation and self-assembly of Fe 8 (OOH) 16 Cl 1.3 nanorods into sphere-like superstructures. When nucleation and growth of Fe 8 (OOH) 16 Cl 1.3 nanocrystallites with an orthorhombic structure take place in the aqueous solution, SO 4 2− ions serve as ligand to Fe 3+ , and may adsorb on the facets parallel to the c-axis of Fe 8 (OOH) 16 Cl 1.3 nuclei by a monodentate structure (Fe–O–SO 3 ) to obtain Fe 8 (OOH) 16 Cl 1.3 nanorods [27]. The pres- ence of SO 4 2− ions not only induce formation of Fe 8 (OOH) 16 Cl 1.3 nanorods, but also urge the assembly of Fe 8 (OOH) 16 Cl 1.3 nanorods into sphere-like superstructures by oriented-attachment process. The nanorods gradually assemble into 3D urchin-like and spheri- cal congeries because that bidentate (Fe–O–SO 2 –O–Fe) structure is formed between Fe 8 (OOH) 16 Cl 1.3 nanorods [27]. During the growth 94 X. Xie et al. / Journal of Alloys and Compounds 477 (2009) 90–99 Fig. 4. Typical TEM images (a–c), SAED pattern (d) and HRTEM image (e) of the ␣-Fe 2 O 3 nanorod-based hollow microspheres. and self-assembly of Fe 8 (OOH) 16 Cl 1.3 nanorods into urchin-like and spherical superstructures, the Fe 8 (OOH) 16 Cl 1.3 is gradually decom- posed into ␣-FeOOH with a monoclinic structure. The ␣-FeOOH rod-based microspheres grow further into microspheres with a large size with increasing the reaction time. It is noted that the evo- lution of Fe 8 (OOH) 16 Cl 1.3 and ␣-FeOOH nanorods into sphere-like superstructures through oriented-attachment was almost simul- taneous with their formation and growth. After, the ␣-FeOOH microspheres are annealed at 600 ◦ C in air and transformed into ␣- Fe 2 O 3 nanorod-based hollow microspheres. During the annealing process, the ␣-FeOOH was first decomposed to Fe 2 O 3 and H 2 O, the Fe 2 O 3 nucleated and grew in situ to form ␣-Fe 2 O 3 nanorod-based hollow microspheres. When Fe 2 (SO 4 ) 3 is used as raw material, the formation of the sphere-like ␣-FeOOH is not from Fe 8 (OOH) 16 Cl 1.3 nanorod-based microspheres, but from directly hydrolysis of Fe 3+ ions in the presence of SO 4 2− ions. SO 4 2− ions adsorb on the facets parallel to the c-axis of the ␣-FeOOH nuclei [27], and their role in the formation and self-assembly of ␣-FeOOH nanorod into sphere- X. Xie et al. / Journal of Alloys and Compounds 477 (2009) 90–99 95 Fig. 5. SEM images of the products prepared at 140 ◦ C for different reaction times (a) 15 min, (b) 1 h, (c) 2 h, (d) 4 h, (e) 6 h, (f) 8 h. like superstructures in the aqueous solution is the same with that of Fe 8 (OOH) 16 Cl 1.3. Fig. 12 shows the N 2 adsorption/desorption isotherm and the corresponding pore size distribution of the ␣-Fe 2 O 3 nanorod- based hollow microspheres. The measurement shows that the BJH (Barett–Joyner–Halenda) desorption average pore size of these spheres was ca. 51.0 nm and that the BET surface area was 20.78 m 2 /g. Powdere d ␣-Fe 2 O 3 and colloidal ␣-Fe 2 O 3 particles have been found to be active for the water splitting [28] and photodecompo- sition of phenol and oxalic acid and salicylic acid [29,30]. However, their efficiencies are quite low. Recently, photocatalytic proper- ties of ␣-Fe 2 O 3 nanostructures with defined size and shape, such as ␣-Fe 2 O 3 hollow nanostructures were investigated. The results demonstrate that the ␣-Fe 2 O 3 hollow spheres were effective pho- tocatalysts for the degradation of salicylic acid [19] and diethyl phthalate [20]. Herein, the photocatalytic performance of the as-synthesized ␣-Fe 2 O 3 nanorod-based microspheres was inves- tigated in the degradation of methyl orange. We chose the methyl orange as a target pollutant based on the considerations as follows: (1) the methyl orange is a good model compound. It is stable under UV light irradiation and resistant to hydrogen peroxide oxidation. (2) Dye pollutants are becoming a major source of environmental contaminant. Azo dye accounts for over 60% of the total number of 96 X. Xie et al. / Journal of Alloys and Compounds 477 (2009) 90–99 Fig. 6. XRD patterns of the samples prepared at 140 ◦ C for different reaction times (a) 15 min, (b) 1 h, (c) 2 h, (d) 4 h, (e) 6 h, (f) 8 h. dye structures known. These dyes are non-biodegradable and show a relatively high persistence in soils and aquatic systems. To evaluate the photocatalytic activity of our products, the absorption spectra of methyl orange, methyl orange-H 2 O 2 and methyl orange-H 2 O 2 solutions containing ␣-Fe 2 O 3 hollow spheres after UV light irradiation for 55 min were measured, and results is shown in Fig. 13a. Methyl orange shows a maximum absorp- tion band at 465 nm (curve I in Fig. 13a). The absorption intensity of the peak was reduced by about 63% in the presence of H 2 O 2 (curve II in Fig. 13a). While the absorption peak disappeared almost completely in the presence of H 2 O 2 and our products (curve III in Fig. 13a). The time-dependent absorption spectra of methyl orange solution containing H 2 O 2 and ␣-Fe 2 O 3 nanostructures catalyst dur- ing the irradiation are illustrated in Fig. 13b. It can be seen that the maximum absorbanceat 465nm decreased rapidly withirradiation time. The decoloration of solution may be due to the destruction of the dye chromogen. Since no new absorption peak was observed, the methyl orange has been decomposed. It is obvious that the hollow ␣-Fe 2 O 3 nanorod-based microspheres were effective pho- tocatalysts for the direct degradation of methyl orange. The fitting of absorbance maximum plot versus time indicates an exponen- tial decay as shown in Fig. 13c. The normalized concentration of Fig. 8. IR spectra of: (a) Na 2 SO 4 powder and (b) ␣-FeOOH nanorod-based micro- spheres. the solution equals the normalized maximum absorbance, so we use C 0 /C to take place of A 0 /A. The photodegradation of methyl orange catalyzed by the hollow ␣-Fe 2 O 3 nanorod-based micro- spheres fits pseudo first-order reaction well, i.e., −dc/dt = Kt,or ln(C 0 /C)=Kt, where C 0 and C are the initialand actual concentration of methyl orange, respectively, K is the apparent rate constant of the degradation. In our experiment, K is found to be 0.030/min. The photocatalytic properties inthe degradation ofmethyl orange of the ␣-Fe 2 O 3 hollow nanostructures suggest that the products may have potential application in water treatment. It is generally accepted that the catalytic process is mainly related to the adsorption and desorption of molecules on the surface of the catalyst. The high specific surface area of the ␣-Fe 2 O 3 nanostructures results in more unsaturated surface coordination sites exposed to the solution. In addition, the hierarchical structures in the catalyst enable storage of more molecules. However, the surface area of the as-obtained ␣- Fe 2 O 3 hollow nanostructures is not very large. Therefore, the size and porosity of the ␣-Fe 2 O 3 nanostructures were tuned by chang- ing the preparation method and the thermal treatment conditions to further enlarge the surface area and photocatalytic performance of the hollow ␣-Fe 2 O 3 nanostructures. Detailed research is still under way. Bulk hematite is weakly ferromagnetic at 298 K witha Néel tem- perature of 955 K and undergoes a “spin-flop” (Morin) transition at 263 K, in which the magnetic moments change orientation [31]. Nanoparticles, however, often exhibit unusual magnetic behaviors Fig. 7. SEM image (a) and XRD pattern (b) of the products obtained via a hydrothermal reaction of FeCl 3 with H 2 Oat140 ◦ C for 12 h in the absence of Na 2 SO 4 . X. Xie et al. / Journal of Alloys and Compounds 477 (2009) 90–99 97 Fig. 9. SEM images of the products obtained using Fe 2 (SO 4 ) 3 as raw material at 140 ◦ C for 2 h (a) and 12 h (b). Fig. 10. XRD patterns of the products obtained using Fe 2 (SO 4 ) 3 as raw material at 140 ◦ C for 2 h (a) and 12 h (b). Fig. 12. Nitrogenadsorption–desorption isotherm and the BJH pore size distribution curve (inset) for the ␣-Fe 2 O 3 nanorod-based hollow microspheres. Fig. 11. Schematic illustration of the formation process and shape evolution of ␣-FeOOH and ␣-Fe 2 O 3 nanorod-based microspheres. 98 X. Xie et al. / Journal of Alloys and Compounds 477 (2009) 90–99 different from that of bulk samples, owing to finite size effects [32]. Herein, we investigated the magnetic property of the as-synthesized hollow microspheres constructed with ␣-Fe 2 O 3 nanorods. The magnetic hysteresis measurements of ␣-Fe 2 O 3 nanostructureswerecarried outat room temperaturein theapplied magnetic field from −10 to 10 kOe. Fig. 14 is the hysteresis loop of the ␣-Fe 2 O 3 superstructures. It can be seen that the satura- tion is not reached up to the maximum applied magnetic field, which is similar to the reported cases in literature [33]. The hys- teresis loop shows a weak ferromagnetic behavior with a remanent magnetization of 0.071emu g −1 and a coercivity of 219 Oe at room temperature. The coercive face of 219 Oe is larger than that of Fig. 13. (a) absorption spectra of methyl orange (I), methyl orange-H 2 O 2 (II) and methyl orange-H 2 O 2 solutions containing ␣-Fe 2 O 3 hollow spheres (III) after ultra- violet irradiation for 55 min (b) absorption spectra of methyl orange-H 2 O 2 solutions containing ␣-Fe 2 O 3 hollow spheres after UV irradiation with different time and (c) the fitting of absorbance maximum plot vs. time. Fig. 14. Hysteresis loop for the ␣-Fe 2 O 3 superstructure at room temperature. 110 Oe for spherical hematite with the diameters of 20–50 nm [33], and is lower than that of 1510 Oe for ␣-Fe 2 O 3 micro-pine dendrites [17] and 2279 Oe for cantaloupe-like␣-Fe 2 O 3 superstruc- tures [13]. It is well known that the magnetization of ferromagnetic materials is dependent on the morphology and structures of the samples [33]. Compared to sphericalnanoparticles, theas-prepared 1D nanorod-based microspheres have increased anisotropies in magnetocrystalline anisotropy, which exerts the influence on their magnetic properties. The enhanced anisotropy induces large mag- netic coercivity, where themagneticspins are preferentiallyaligned the long axis and their reversal to the opposite direction requires higher energy than that for spheres [5]. Therefore, coercivity of our products is larger than that of spherical hematite with the diameters of 20–50 nm. Compared with our samples, Fe 2 O 3 micro- pine dendrites and cantaloupe-like superstructures have increased anisotropies in the shape, and thus have higher coercivity. 4. Conclusions In conclusion, we have successfully obtained hollow micro- spheres constructed with ␣-Fe 2 O 3 nanorods by a two-step process, including hydrothermal synthesis of the ␣-FeOOH pre- cursor at low temperature and a subsequent heating treatment. SO 4 2− ions played an important role in the formation of ␣- FeOOH nanorod-based microstructures. During the synthesis process, no carbon particle or surfactant was used. This proce- dure has great advantages in large-scale industrial manufacturing for a simple hydrothermal process, such as inexpensive raw materials, high purity, and a high morphology yield of the products. The as-prepared iron oxide nanomaterials show indis- putable photocatalytic activity for degradation of methyl orange and weak ferromagnetic behavior with a remanent magneti- zation of 0.071emu g −1 and a coercivity of 219 Oe at room temperature, and are expected to be useful in many other appli- cations. Acknowledgements We would like to thank Liang Gongying from Xi’an Jiaotong Uni- versity and Chen Dichun from Xi’an University of Technology for their helps in measurement of magnetic properties and TEM anal- ysis, respectively. 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Solid State Chem. 178 (2005) 2798. . www.elsevier.com/locate/jallcom Synthesis of hollow microspheres constructed with ␣-Fe 2 O 3 nanorods and their photocatalytic and magnetic properties Xiaoli. reaction of Fe 3+ in the presence of SO 4 2− ions. Hollow microspheres constructed with ␣-Fe 2 O 3 nanorods were obtained by thermaldecompositionof theas-synthesized␣-FeOOH 0925-8388/$