J. Phys. Chem. C 2008, 112, 6253-6257
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Hierarchically Nanostructured r-Fe2O3 Hollow Spheres: Preparation, Growth Mechanism, Photocatalytic Property, and Application in Water Treatment Shao-Wen Cao and Ying-Jie Zhu* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: January 3, 2008; In Final Form: February 5, 2008
R-Fe2O3 hierarchically nanostructured hollow spheres assembled by nanosheets were prepared by thermal decomposition of a precursor which was obtained using FeCl3‚6H2O, NaOH, and sodium dodecylbenzenesulfonate in the solvent ethylene glycol by a facile microwave-assisted solvothermal method. The growth process of the precursor was investigated. The products were characterized by X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy. Because of the unique hierarchically nanostructured hollow structures composed of nanosheets, the photocatalytic property and potential application in water treatment of the R-Fe2O3 samples were also investigated.
Introduction Currently, there is growing interest in the fabrication of nanostructures with desired morphologies and properties. Among the various morphologies of nanostructures, nanostructured hollow spheres are drawing intense research interest not only for their unique properties, but also for their broad range of applications such as in catalysis, drug delivery, chemical storage, light fillers, photonic crystals, and low dielectric constant materials.1-6 A variety of hollow spheres such as nickel,7 titania,8 carbide,9 NiS,10 Bi2Te3,11 and ZnO/SnO212 have been successfully fabricated. The general approach for the preparation of such hollow spheres is based on the use of various templates including latex spheres, resin spheres, microemulsions, polymer micelles, block copolymers, and so on.13-21 Moreover, the hierarchical nanostructures are also promising candidates for new functional nanomaterials. So far, many hierarchical structures, including cubic PbS22 and noble metals,23 Fe2O324 and HgS,25 tetragonal tungstate26 and PbMoO4,27 and orthorhombic Bi2S3,28 Ni(OH)2 and NiO,29 and Co(OH)2 and Co3O4,30 have been reported. R-Fe2O3 (hematite), an environmentally friendly n-type semiconductor (Eg ) 2.1 eV), is the most stable iron oxide under ambient conditions. It is widely used in catalysts, pigments, and sensors31-33 and as the raw material for the synthesis of magnetic γ-Fe2O3. Up to now, a variety of R-Fe2O3 structures such as rhombohedra,34 particles,35 nanocubes,36 rings,37 wires,38 rods,39 tubes,40 fibers,41 flakes,42 cages,43 and hierarchical structures44 have been fabricated. Recently, some groups have synthesized R-Fe2O3 hollow structures through various methods.45 However, hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets have not been reported yet. In this paper, we report a facile and economical route to prepare hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets, taking advantage of the microwaveassisted hydrothermal method. Compared with the existing solution-phase synthetic methods using conventional heating, * To whom correspondence should be addressed. Phone: +86-2152412616. Fax: +86-21-52413122. E-mail:
[email protected].
microwave irradiation prompts a much faster reaction rate. Importantly, modern microwave synthetic systems offer the capability of temperature and time programming, allowing fast and easy optimization of reaction conditions. To the best of our knowledge, there has been no report on the preparation of hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets by a facile microwave-assisted solvothermal route combined with subsequent thermal decomposition. Experimental Section Preparation of r-Fe2O3 Hollow Spheres. In a typical synthetic procedure of the Fe-ethylene glycol (EG) precursor,46 solution A was prepared by dissolving 2 mmol of FeCl3‚6H2O and 1.72 mmol of sodium dodecylbenzenesulfonate (SDBS) in 20 mL of EG, and solution B was prepared by dissolving 4 mmol of NaOH in 10 mL of EG. SDBS was used as a surfactant and EG as both a solvent and a reductant. Then the two solutions were mixed together, and 20 mL of the resultant brown suspension was loaded into a 60 mL Teflon autoclave, which was sealed, microwave-heated to 200 °C, and kept at this temperature for 30 min. The microwave oven used for the sample preparation was a microwave-solvothermal synthesis system (MDS-6, Sineo, Shanghai, China). After cooled to room temperature, a jade-green precursor was obtained. The product was collected and washed by centrifugation-redispersion cycles with alcohol. The dried powder of the precursor was heated in air to 500 °C at a rate of 1 °C min-1 and then immediately cooled naturally. A red R-Fe2O3 powder was obtained. Photocatalytic Activity Measurements. The photocatalytic reactor consisted of two parts: a 70 mL quartz tube and a highpressure Hg lamp. The Hg lamp was positioned parallel to the quartz tube. In all experiments, the photocatalytic reaction temperature was kept at about 35 °C. The reaction suspension was prepared by adding the sample (20 mg) to 50 mL of a salicylic acid solution with a concentration of 20 mg L-1. The suspension was sonicated for 15 min and then stirred in the dark for 30 min to ensure an adsorption/desorption equilibrium prior to UV irradiation. The suspension was then irradiated using UV light under continuous stirring. Analytical samples were
10.1021/jp8000465 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/02/2008
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Figure 1. XRD patterns of the samples prepared for different microwave heating times: (a) 0 min, (b) 5 min, (c) 20 min, (d) 30 min.
taken from the reaction suspension after various reaction times and centrifuged at 10 000 rpm for 5 min to remove the particles for analysis. Organic Pollutant Adsorption Experiments. For the adsorption of organic pollutants, 50 mL of methyl orange (MO) solution (20 mg L-1) was mixed with 20 mg of R-Fe2O3 nanostructured hollow spheres. The suspension was then stirred continuously at about 25 °C. Analytical samples were taken from the suspension after various adsorption times and centrifuged at 10 000 rpm for 5 min to remove the particles. Characterization of the Samples. The prepared samples were characterized using X-ray powder diffraction (XRD) (Rigaku D/max 2550 V, Cu KR, λ ) 1.54178 Å), scanning electron microscopy (SEM) (JEOL JSM-6700F), and transmission electron microscopy (TEM) (JEOL JEM-2100F). The Brunauer-Emmett-Teller (BET) surface area and pore size distribution were measured with an accelerated surface area and porosimetry system (ASAP 2010). The photocatalytic reactions were carried out under irradiation of a 300 W high-pressure Hg lamp (GGZ300, Shanghai Yaming Lighting) with a maximum emission at about 365 nm. The salicylic acid concentrations were analyzed using a UV-vis spectrophotometer (UV2300, Techcomp) at a wavelength of 297 nm. The MO concentrations were analyzed using a UV-vis spectrophotometer (UV-2300, Techcomp) at a wavelength of 464 nm. Results and Discussion The XRD pattern in Figure 1d shows the crystalline, layered structure of the precursor that was obtained using FeCl3‚6H2O, NaOH, and SDBS in EG by microwave-assisted solvothermal treatment at 200 °C for 30 min. The most intense peak was located at about 10.88°, and its d spacing was 8.1245 Å. We found that the precursor could be oxidized in the aqueous solvent with the green color turning to brown, indicating that the ferrous ions were oxidized to form ferric ions. Figure 2i-k shows the typical SEM micrographs of the precursor. One can see that the precursor consisted of hierarchically nanostructured hollow spheres organized by nanosheets. Figure 2j shows the SEM micrograph of a broken hollow sphere, indicating the hollow structure of the sphere. The high-magnification SEM micrograph of the sphere surface (Figure 2k) shows that the nanosheets were organized to form three-dimensional networks in the wall of the hollow spheres. To investigate the growth process of hierarchically nanostructured hollow spheres of the precursor assembled by nanosheets, experiments were designed to investigate the intermediates at different microwave-assisted solvothermal times. One can see that irregularly shaped big blocks formed in the suspension before microwave heating (Figure 2a). Figure 1a shows the XRD pattern of the sample before microwave
Cao and Zhu heating, from which one cannot see any obvious peaks, indicating the amorphous phase. When the microwave-assisted solvothermal treatment was conducted for 5 min, a small number of hollow spheres were observed on the surface of irregularly shaped big blocks (Figure 2b,c). Figure 2e is the SEM micrograph of the amorphous block surface, suggesting that a dissolution process may occur. The high-magnification SEM micrograph of the surface (Figure 2f) shows that the nanosheet network structures were not constructed completely. The XRD pattern in Figure 1b indicates that the crystalline phase formed. When the time was increased to 20 min, hollow spheres were produced, leaving only little amorphous phase (Figure 2g). Figure 2h is the high-magnification SEM micrograph of the sphere surface, indicating that the nanosheet network structures were achieved. The XRD pattern in Figure 1c is similar to that in Figure 1b, except the higher intensities of the peaks. That means the phase did not change, but the crystallinity was improved. Combined with the XRD pattern and SEM micrographs of the precursor that was obtained by microwave treatment for 30 min, we propose that the formation of the hierarchically nanostructured hollow spheres of the precursor may follow reduction, dissolution, and recrystallization processes. In the reaction system, the EG molecules lose protons and the dianions are coordinated with ferric ions. During the microwave-assisted solvothermal process, the ferric ions are reduced to ferrous ions, and dissolution and recrystallization occur, leading to the formation of hierarchically nanostructured hollow spheres assembled by nanosheets with the help of the inducing action of SDBS. In a conventional solvothermal synthetic process, nucleation tends to occur on the container walls or dust particles and experiences a slow growth rate due to a small number of seeds and heating inhomogeneity.47 However, in the microwave-assisted solvothermal process, the rapid heating rate of the solution leads to faster nucleation and faster crystal growth throughout the bulk solution.48 Figure 3 shows the XRD pattern of the R-Fe2O3 sample that was obtained by thermal treatment of the precursor, which agrees well with the reported data (JCPDS no. 33-0664). R-Fe2O3 possesses a rhombohedrally centered hexagonal structure of the corundum type with a close-packed oxygen lattice in which twothirds of the octahedral sites are occupied by Fe(III) ions.49 The narrow sharp peaks of the XRD pattern indicate that the R-Fe2O3 product was well crystallized. No impurities were detected by XRD. Figure 4a-d shows SEM micrographs of the R-Fe2O3 sample, from which one can see hierarchically nanostructured hollow spheres consisting of organized R-Fe2O3 nanosheets, very similar to those of the precursor; that is to say, the morphology of the precursor was successfully maintained after thermal transformation of the precursor to R-Fe2O3. The SEM observation of broken spheres indicates the hollow structure of the spheres, which was also confirmed by the TEM observation (Figure 4e). The diameters of hierarchically nanostructured hollow spheres assembled by R-Fe2O3 nanosheets were also similar to those of the precursor. Figure 4f shows the high-resolution TEM (HRTEM) micrograph of the nanosheets building up the hollow spheres, from which one can see small pores. These results indicate that the simple method reported here is suitable for the synthesis of hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets. Figure 5 shows the N2 adsorption/desorption isotherm and the corresponding pore size distribution of hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets. The measurement shows that the BJH (Barett-Joyner-Halenda)
Hierarchically Nanostructured R-Fe2O3 Spheres
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Figure 2. SEM micrographs of the samples prepared for different microwave heating times: (a) 0 min, (b-f) 5 min, (g, h) 20 min, (i-k) 30 min.
Figure 3. XRD pattern of hierarchically nanostructured R-Fe2O3 hollow spheres.
desorption average pore size of these spheres was ca. 24.2 nm and that the BET surface area was 12.2 m2/g. To evaluate the photocatalytic activity of the hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets, comparison experiments were performed. A suspension composed of 20 mg of hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets and 50 mL of salicylic acid (20 mg L-1) was placed in a 70 mL quartz tube and irradiated with UV light under continuous stirring. At different time intervals, analytical samples were withdrawn from the quartz tube and the salicylic acid concentration was analyzed using a UV-vis spectrophotometer at a wavelength of 297 nm. Figure 6a shows the degradation rate of salicylic acid over hierarchically nanostructured R-Fe2O3 hollow spheres, from which one can see that the concentration of salicylic acid decreased rapidly with increasing time. We also investigated the photocatalytic activity of R-Fe2O3 nanoparticles synthesized according to the reported method (70 nm in diameter, ringlike structure),37 and the degradation rate of salicylic acid over R-Fe2O3 nanoparticles is illustrated in Figure 6b. It is obvious that the
Figure 4. (a-d) SEM micrographs of hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets. (e, f) TEM micrographs of hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets.
hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets exhibited superior photocatalytic activities, indicating that the hierarchical hollow structures assembled by
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Cao and Zhu recycling and the facile method of regeneration suggest that the product may be applicable in water treatment. However, because the surface area of the as-obtained hollow nanostructures is not very large, we believe that the adsorption performance of the hollow nanostructures will be greatly improved by enlarging the surface area. The surface area of the product may be tuned by controlling the size and porosity, which are closely related to the preparation method and the thermal treatment conditions.50 Detailed research is still under way. Conclusions
Figure 5. N2 adsorption/desorption isotherm of hierarchically nanostructured R-Fe2O3 hollow spheres and the corresponding pore size distribution.
Figure 6. Degradation rate of salicylic acid over different photocatalysts: (a) hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets, (b) R-Fe2O3 ringlike nanoparticles.
We have successfully prepared hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets by a facile microwave-assisted solvothermal method combined with subsequent thermal decomposition. Hierarchically nanostructured hollow spheres organized by nanosheets of an Fe-EG precursor were synthesized using FeCl3‚6H2O, NaOH, and SDBS in the solvent EG by a microwave-assisted solvothermal method. Then hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets were obtained by the thermal decomposition of the precursor. The formation of the precursor may follow the reduction, dissolution, and recrystallization process. The asobtained R-Fe2O3 hierarchically nanostructured hollow structures exhibit a better photocatalytic property than ringlike nanoparticles and also show a satisfactory removal capacity for MO in water treatment. We expect this synthetic strategy may also be extended to the preparation of hierarchical nanostructures of other metal oxides. Acknowledgment. Financial support from the National Natural Science Foundation of China (Grants 50772124 and 50472014), the Program of Shanghai Subject Chief Scientist (Grant 07XD14031), the Key Project for Innovative Research (Grant SCX 0606), and the Director Fund of the Biomaterials Research Center from the Shanghai Institute of Ceramics, Chinese Academy of Sciences, is gratefully acknowledged References and Notes
Figure 7. Adsorption rate of MO on hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets for the first (a), second (b), and third (c) cycles.
nanosheets are favorable for the enhancement of the photocatalytic performance. To evaluate the potential application in water treatment of hierarchically nanostructured R-Fe2O3 hollow spheres assembled by nanosheets, the adsorption capacities for the organic pollutant were investigated. Here, MO was chosen as the model organic pollutant. The initial concentration of the MO solution was set to be 20 mg L-1. Figure 7a shows the adsorption rate of MO on hierarchically nanostructured R-Fe2O3 hollow spheres versus time, which shows that the maximum removal capacity of the absorbent reached about 50% in a time period of 60 min. The removal of MO may be associated with the electrostatic attraction between the surface of R-Fe2O3 nanostructured hollow spheres and MO molecules. Furthermore, the used R-Fe2O3 nanostructured hollow spheres can be recycled by a simple thermal treatment in air at 400 °C for 2 h. Figure 7b,c shows the adsorption performance of the R-Fe2O3 hollow nanostructures for two and three cycles. The MO removal capacities for the regenerated absorbents reached about 45% and 43% in a time period of 60 min, respectively. The slight decrease in the removal capacity of the R-Fe2O3 hollow nanostructures for
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