Template-Free Synthesis and Photocatalytic Properties of Novel

Novel Fe2O3 hollow spheres with mesopores on the surface were first synthesized on a large scale by a facile and efficient hydrothermal process, witho...
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J. Phys. Chem. C 2007, 111, 2123-2127

2123

Template-Free Synthesis and Photocatalytic Properties of Novel Fe2O3 Hollow Spheres Lili Li, Ying Chu,* Yang Liu, and Lihong Dong Department of Chemistry, Northeast Normal UniVersity, Changchun, Jilin 130024, People’s Republic of China ReceiVed: October 10, 2006; In Final Form: NoVember 21, 2006

Novel Fe2O3 hollow spheres with mesopores on the surface were first synthesized on a large scale by a facile and efficient hydrothermal process, without templates in the system. The samples were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and N2 adsorption-desorption. When the amorphous Fe2O3 hollow spheres were used as the photocatalytic material, they performed better than the nanocrystal samples. This synthetic procedure is straightforward and thus facilitates mass production of Fe2O3 hollow spheres.

Introduction Hollow nanostructures have attracted tremendous interest as a special class of materials compared to other solid counterparts, owing to their higher specific surface area, lower density and better permeation, and widespread potential applications in chemical reactors, drug delivery, catalysis, sensors, lightweight materials, and various new application fields,1 Generally, manipulation of hollow materials is performed by templatedirected synthesis. Representative examples are the layer-bylayer deposition of nanoparticles onto spherical colloids (e.g., polystyrene beads and silica sol)2-7 and sacrificial substitutions of metal nanoparticles by those with a higher standard reduction potential.8,9 Following these procedures, the templates of colloidal spheres or emulsion droplets are removed via a timeconsuming treatment (e.g., calcination or dissolution with solvent) to form hollow structure.10 In some cases, the yield of hollow spheres prepared via template routes is low, or their shells are not intact, which usually leads to poor mechanical performance. It will be a main task to explore other wetchemical means, aiming at a simple “one-pot” synthetic approach for hollow structures. So, the utilization of some physical phenomena, such as the Kirkendall effect or Ostwald ripening, provides new opportunities for the template-free fabrication of hollow spheres.11,12 To date, silver cages and ZnO dandelion structures were synthesized through a Kirkendall effect.13,14 Hollow Sb2S3, TiO2, SnO2, and MnO2 structures were prepared through Ostwald ripening.15-18 Our group has prepared hollow Cu nano/microstructures via a mild hydrothermal process.19 Metal oxide structures20,21 have great natural abundance and excellent environmental compatibility.In particular, Fe2O3, the most stable iron oxide under ambient conditions, is widely used in catalysts,22 photoelectrodes,23 and sensors.24 Because of their excellent properties, much attention has been directed to the controlled synthesis of hematite particles. Shuttle, wires, rods, and urchinlike25 Fe2O3 have been fabricated. Recently, many groups have synthesized Fe2O3 hollow structures through various methods. Chen and co-workers26 described a facile route for preparation of submicrometer ferrite/block copolymer hollow spheres. Fe2O3 with ordered mesoporous structure and crystalline walls, was synthesized by use of mesoporous silica as template.27 Upon calcination, Thomas and co-workers28 used carbohydrates * Corresponding author: e-mail [email protected].

and metal salts as reaction resources to synthesized various metal oxides sphere. It is still a challenge to synthesize Fe2O3 hollow spheres via a one-step process without hard templates in solution. Herein, we first synthesized novel Fe2O3 hollow spheres via a single-step process without any hard templates. In particular, there are mesoporous structures on the shells of Fe2O3 hollow spheres. The hollow Fe2O3 sphere possessed higher photocatalytic properties than other Fe2O3 crystals. Through controlling the concentration of H2PO4-, we could get dendritic and hollow Fe2O3 spheres. Experimental Section In a typical experiment, 0.42 g of K4Fe(CN)6, 0.02 g of hexadecyltrimethyl ammonium bromide (CTAB), and a certain amount of (NH4)2S2O8 and NaH2PO4 were dissolved in deionized water under magnetic stirring at room temperature until a yellow transparent solution was obtained. The solution was then sealed into a Teflon-lined autoclave, followed by hydrothermal treatment at 180 °C for 8 h. After the treatment, carmine Fe2O3 products were collected by filtration, washed several times with deionized water and ethanol, and dried at room temperature for 24 h. The product is Fe2O3 hollow spheres. The size and morphology of R-Fe2O3 nanostructures were examined by field emission scanning electron microscopy (FESEM, JEOL, 7500B) equipped with energy-dispersive spectroscopy (EDX) and by transmission electron microscopy (TEM; JEOL 2010). The X-ray powder diffraction (XRD) pattern was obtained on a Rigaku D/max 2500V PC diffractometer with Cu KR radiation. X-ray photoelectron spectroscopy (XPS) measurements were employed to gain further insight into the chemical composition of the Fe2O3 hollow spheres. N2 adsorption-desorption isotherms were performed on a Micromeritics NOVA-1000 apparatus with nitrogen as the analysis gas. The pore diameter and the pore size distribution at the shell were determined by the Barret-Joyner-Halenda (BJH) method. The photocatalytic property of Fe2O3 was determined by fluorescence spectroscopy with a Hitachi F-4500 fluorophotometer. Results and Discussion The XRD spectra are given in Figure 1. It can be seen that the as-prepared sample is an amorphous material, although it

10.1021/jp066664y CCC: $37.00 © 2007 American Chemical Society Published on Web 01/12/2007

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Li et al. hollow spheres (Figure 3d). It indicates that hollow Fe2O3 spheres can be obtained under the present experimental conditions. Moreover, the results of many experiments show that this approach has excellent reproducibility, and the resulted structures are highly stable, without morphological or compositional change over several months when stored in air. We think the hollow structure with mesopores at the surface will be widely used in catalysis and biotechnology. And the hollow structure is more efficient than other solid structures. The chemical reaction for synthesizing hollow Fe2O3 spheres can be expressed as

Fe(CN)64- f Fe2+ + 6CN-

Figure 1. XRD pattern of Fe2O3 hollow spheres powder (a) as prepared and (b) after calcination at 700 °C.

matches the XRD spectrum of hematite (JCPDS file 86-0550, Figure 1a). However, after calcination at 700 °C, the structure changes to R-Fe2O3 (hematite) (JCPDS file 86-0550), with sharp X-ray diffraction lines showing increased crystallinity (Figure 1b). Figure 2 shows the XPS spectrum of as-prepared hollow Fe2O3 sphere. The Fe2p3/2 binding energy of 710.6 eV and the corresponding satellite peak at 718.4 eV, a result of chargetransfer screening, can be solely attributed to the presence of Fe3+ ions of Fe2O3. The ratio of atomic concentrations calculated from the O1s and Fe2p peak areas deviates slightly from the theoretical value of 1.5, but it has been reported that quantitative analysis of the XPS data of iron oxides involves certain difficulties (such as OH groups on the surface and photoreduction of Fe3+).29 Figure 3a is a typical SEM image of the hollow Fe2O3 spheres. Examining numerous SEM images of the sample, we found that almost all the products are hollow spheres with 150200 nm diameter. From the broken sphere, we could conclude these spheres are hollow. The hollow sphere could have predominance in drug delivery and chemical reactors owing to the large inside space with 135-180 nm inside diameter. As can be seen from a higher magnification SEM image (Figure 3b), the entire hollow sphere shell is built up of numerous Fe2O3 nanoparticles with 15-20 nm diameter. Interestingly, these nanoparticles connected together side by side to form sphere structures. The hollow nature of the sphere was also observed in the contrast between the dark edge and pale center in transmission electron microscopy (TEM) observations, as shown in Figure 3c, in which almost all the spherical particles have a hollow cavity inside. The thickness of the shell (15-20 nm) is the same as the diameter of nanoparticles at the surface of

(1)

2Fe2+ + S2O82- + 12H2O f 2[Fe(H2O)6]3+ + 2SO42(2) [Fe(H2O)6]3+ f FeOOH + 4H2O + 3H+

(3)

2FeOOH f Fe2O3 + H2O

(4)

To understand the formation mechanism of the Fe2O3 hollow spheres, time-dependent experiments were carried out at 180 °C, and the resultant products were analyzed by TEM investigations. As shown in Figure 4, three obvious evolution stages could be clearly identified. In the first stage (1 h), the final product is solid spheres 50-60 nm in diameter (entirely dark; Figure 4a). With increasing reaction time (2.5 h), the nanospheres continuously swelled in size, and the surface become looser. The hollowing effect is observed, which is the typical character of the second stage (Figure 4b). The second stage lasts for several hours. Upon prolonging reaction time to 8 h, the product consists predominantly of hollow spheres, with their cores disappearing completely (Figure 4c). As indicated, the solid Fe2O3 spheres are composed of numerous smaller particles. Compared to those in the outer surfaces, the particles located in the inner cores have high surface energies, because they can also be visualized as a smaller sphere having a higher curvature (i.e., higher surface energies and thus easily dissolved). Consequently, they were merged into particles in the outer surface, resulting in the formation of hollow interiors. In this reaction, surfactant CTAB is used not as template but as dispersant. Without CTAB in the reaction, we could get Fe2O3 hollow spheres with poor dispersed state. Replacing CTAB by sodium dodecyl sulfate (SDS), poly(ethylene glycol) (PEG), or poly(vinylpyrrolidone) (PVP) had no effect on the morphology of the final product. On the other hand, the concentration of H2PO4- is crucial in ensuring the formation of hollow Fe2O3 spheres. Figure 5a shows the typical TEM images of Fe2O3

Figure 2. XPS survey spectrum of Fe2O3 hollow sphere: (a) Fe2p and (b) O1s.

Synthesis and Properties of Fe2O3 Hollow Spheres

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Figure 3. (a, b) SEM images and (c, d) TEM images of Fe2O3 hollow sphere observed under different magnifications.

Figure 4. Schematic representations and TEM images of formation of Fe2O3 hollow sphere with different reaction times under hydrothermal conditions: (a) 1 h; (b) 2.5 h; (c) 8 h.

Figure 5. TEM images of Fe2O3 observed at different concentrations of H2PO4-: (a) 0.006 M; (b) 0.008 M; (c) 0.01 M.

micropine dendrites, when the concentration of H2PO4- was kept at 0.006 M, and other reaction conditions were kept similar. It reveals a clear and well-defined dendritic structure with a pronounced trunk and orderly branches distributed on both sides of the trunk. The lengths of the dendrite trunk are 4-6 µm. Figure 5b shows a TEM image of the final product, when the concentration of H2PO4- was increased to 0.008 M. This sample is the mixture of feathers and hollow spheres. The lengths of the feather structure are 1-2 µm. When the concentration of H2PO4- was 0.01 M, we got Fe2O3 hollow spheres (Figure 5c). On the basis of the weak dissociation of [Fe(CN)6]3- ions under hydrothermal conditions, Hu and co-workers30 had synthesized

dendritic Fe2O3 and think the structure of the Fe(CN)63complex is important to form dendritic-like Fe2O3. From above discussion, we can deduce that the final morphology will be dendritic structure if Fe(CN)64- is slowly dissociated. In neutral solution, the ionization of Fe(CN)64- is weak, but in acidic solution, the ionization of Fe(CN)64- is strong. Higher concentrations of H2PO4- will result in increased acidity, which could accelerate the ionization of Fe(CN)64-, and destroy the structure of Fe(CN)64-. So the final morphology was not dendritic structure after fast dissociation of Fe(CN)64- complex. The concentration of H2PO4- is important to synthesize Fe2O3 hollow structures. Through controlling the concentration of

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Figure 6. TEM images of Fe2O3 hollow cube at 160 °C at different magnifications.

Figure 7. N2 adsorption-desorption isotherm and BJH pore-size distribution plot (inset) of Fe2O3 hollow spheres.

H2PO4- to adjust the different speeds of Fe(CN)64- ionization, we could get dendritic and hollow Fe2O3 spheres. Surprisingly, we found temperature is another important factor for product morphology when other reaction conditions were kept similar. The Fe2O3 hollow polyhedrons with 130-150 nm edge length were formed at 160 °C (Figure 6) Almost all the products are hollow polyhedrons with 20-30 nm wall thickness and 120-130 nm inside edge length. However, at temperatures lower than 160 °C under hydrothermal conditions for 8 h, the hollow structures could not be obtained. For example, the product obtained at 120 °C was solid spheres. The comparative experiments indicate that the appropriate reaction temperature is important for the preparation of Fe2O3 hollow structures. The high porosity of the shell wall was confirmed by measurement of the Brunauer-Emmett-Teller (BET) surface area and corresponding nitrogen adsorption and desorption isotherms. The isotherm can be categorized as type IV, with a distinct hysteresis loop observed in the range of 0.45-1.0 p/p0

(Figure 7). The measurement shows that these spheres have pores with diameters of ca. 8.8 nm (Figure 7, inset) and that the BET surface area is 42.6 m2/g. To evaluate the photocatalytic activity of our product, we compared it with other nanocrystals. A solution of 50 mL containing 20 mg of Fe2O3 hollow spheres and 5 mg of salicylic acid was placed in a 100 mL beaker and irradiated with UV light at a distance of 25 cm. The solution was stirred with a magnetic stirrer. At different time intervals, 5 mL samples were withdrawn for analysis from the beaker and the salicylic acid concentration was measured. Salicylic acid analysis was performed by measuring the fluorescence intensity of salicylic acid at 412 nm upon excitation at 296 nm. Fluorescence intensities were measured on a fluorometer. Figure 8a shows the fluorescence spectrum of salicylic acid solution after 25-W ultraviolet (UV) lamp irradiation for different times, from which we can see that the concentration of salicylic acid decreased rapidly. We also used R-Fe2O3 nanoparticles (80 nm in diameter, ringlike structure) as references to evaluate the photocatalytic performance of our product. As illustrated in Figure 8b, we plotted the conversion of salicylic acid (K) versus irradiation time for two different Fe2O3 structures used as photocatalyzer. The conversion of salicylic acid (K) can be expressed as K ) (I0 It)/I0, where I0 represents the fluorescence intensity of salicylic acid at the original reaction (t ) 0), and It is the fluorescence intensity at a certain time t. It is obvious that our amorphous Fe2O3 hollow spheres exhibited superior photoactivities compared with nanocrystals, indicating that the generation of hierarchical hollow structures could improve their photocatalytic performance. It is generally accepted that the catalytic process is mainly related to the adsorption and desorption of molecules on the surface of the catalyst.31 The high specific surface area of the nanocatalysts results in more unsaturated surface coordination sites exposed to the solution. In addition, the interconnected hollow pores in the catalyst enable storage of more

Figure 8. (a) Change in fluorescence intensity of salicylic acid in the presence of 20 mg of Fe2O3 hollow spheres. (b) Plot of conversion of salicylic acid versus irradiation time for different photocatalyzer solutions.

Synthesis and Properties of Fe2O3 Hollow Spheres molecules. Therefore, the enhancement of catalytic activity by high specific surface area and hollow structure is reasonable. Conclusions In summary, we find an economical and efficient process for large-scale synthesis of hollow spheres via a one-step solutionbased route. The key process does not use hard templates. Thanks to the mesoporous structures on the shell, the photocatalytic performance has been significantly improved. We believe that the present work will open up ways to systematically explore fabrication of hollow nanostructures. Although the growth mechanism of Fe2O3 hollow sphere is unclear, the method is a good way to synthesize other metal oxide hollow spheres. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (20573017), the Science and Technology Development Project of Jilin province (20040566), Science Foundation for Young Teachers of Northeast Normal University (20060306), and Analysis and Testing Foundation of Northeast Normal University. References and Notes (1) (a) Li, J.; Zeng, H. C. Angew. Chem., Int. Ed. 2005, 44, 4342. (b) Shchukin, D. G.; Shchukin, G. B.; Mo¨wald, H. Angew. Chem., Int. Ed. 2003, 42, 4472. (c) Sun, Y.; Xia, Y. Anal. Chem. 2002, 74, 5297. (d) Li, X. L.; Lou, T. J.; Sun, X. M.; Li, Y. D. Inorg. Chem. 2004, 43, 5442. (e) Jiang, Z. Y.; Xie, Z. X.; Zhang, X. H.; Lin, S. C.; Xu, T.; Xie, S. Y.; Huang, R. B.; Zheng, L. S. AdV. Mater. 2004, 16, 904. (2) Cochran, J. K. Curr. Opin. Solid State Mater. Sci. 1998, 3, 474. (3) Go¨ltner, C. G. Angew. Chem., Int. Ed. 1999, 38, 3155. (4) Bourlinos, B.; Karakassides, M. A.; Petridis, D. Chem. Commun. 2001, 16, 1518. (5) Imholf, A. Langmuir 2001, 17, 3579. (6) (a) Mandal, T. K.; Fleming, M. S.; Walt, D. R. Chem. Mater. 2000, 12, 3481. (b) Yang, F. Y.; Chu, Y.; Ma S. Y.; Liu, J. L. J. Colloid Interface Sci. 2006, 301, 470. (7) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002,124, 7642.

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