Hydrothermal Synthesis and Visible-light Photocatalytic Activity of

Hydrothermal Synthesis and Visible-light Photocatalytic Activity of Novel Cage-like Ferric Oxide Hollow Spheres Jiaguo Yu,*,† Xiaoxiao Yu,† Baibiao Huang,‡ Xiaoyang Zhang,‡ and Ying Dai‡ State Key Laboratory of AdVanced Technology for Materials Synthesis and Processing, Wuhan UniVersity of Technology, Wuhan 430070, P. R. China, State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan, 250100, P. R. China

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 3 1474–1480

ReceiVed August 26, 2008; ReVised Manuscript ReceiVed NoVember 13, 2008

ABSTRACT: Fe2O3 hollow spheres with novel cage-like architectures and porous crystalline shells were successfully fabricated by a controlled hydrothermal precipitation reaction using urea as a precipitating agent and carbonaceous polysaccharide spheres as templates in a mixed solvent of water and ethanol, and then calcined at 500 °C for 4 h. The as-prepared samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption-desorption isotherms, and UV-visible diffuse reflectance spectroscopy. The visible-light photocatalytic activity of the as-prepared samples was evaluated by photocatalytic decolorization of rhodamine B aqueous solution at ambient temperature under visible-light illumination in the presence of H2O2. The results indicated that the diameter, shell thickness, average crystallite size, specific surface areas, pore structures, and photocatalytic activity of Fe2O3 hollow spheres could be easily controlled by changing the concentration of FeCl3 and size of carbon spheres, respectively. With increasing FeCl3 concentration, the average crystallite size, shell thickness, and pore size increase. In contrast, specific surface areas and photocatalytic activity decrease. Further results show that other experimental conditions such as hydrothermal time and solvent composition also obviously influence the formation and morphology of hollow spheres. The samples can be more readily separated from the slurry system by filtration or sedimentation after photocatalytic reaction and reused compared to conventional nanosized powder photocatalysts. The prepared Fe2O3 hollow spheres are also of great interest in sensor, lithium secondary batteries, solar cell, catalysis, separation technology, biomedical engineering, and nanotechnology.

1. Introduction In recent years, systematic control over the desired morphologies and architectures of inorganic nanoparticles into higher order superstructures at micro- and nanoscale levels has attracted more and more interest due to their strong influence on materials properties.1,2 Among the different morphologies of nanomaterials, hollow structures of nanometer to micrometer dimensions have received much attention because of their widespread potential applications in catalysis, drug delivery, chromatography separation, chemical reactors, controlled release of various substances, protection of environmentally sensitive biological molecules, and lightweight filler materials.3-5 Numerous chemical and physicochemical methods, such as the Kirkendall effect, Ostwald ripening, self-assembly techniques, template-sacrificial techniques, and chemically induced self-transformation,6-11 have been developed to fabricate various hollow structures and spheres. Among them, the template-directed synthetic route has proved to be the most effective and multifunctional strategy to fabricate inorganic hollow superstructures, and various templates, such as hard templates (e.g., polymer latex, carbon, and anodic aluminum oxide templates) and soft templates (e.g., supramolecular, ionic liquids, surfactant, and organogel), have been extensively employed.12 Oxide semiconductor-mediated photocatalytic purification of polluted air and wastewater is a promising environmental remediation technology, especially for low levels of organic contaminants. Among various oxide semiconductor photocatalysts, TiO2 has been recognized as one of the excellent materials for its biological and chemical inertness, strong oxidizing power, * To whom correspondence should be addressed. Tel: 0086-27-87871029. Fax: 0086-27-87879468. E-mail: [email protected]. † Wuhan University of Technology. ‡ Shandong University.

cost effectiveness, and long-term stability against photo- and chemical corrosion.13 Although TiO2 is universally considered as the most important photocatalyst, other oxides such as ZnO, Fe2O3 are also a suitable alternative to TiO2 due to their similar bandgap energy and its lower cost. Moreover, in certain cases, larger quantum efficiency and higher photocatalytic activity than TiO2 have been reported.14 R-Fe2O3 (hematite), an n-type semiconductor (Eg ) 2.1 eV), is a very important multifunctional material because of its peculiar and fascinating physicochemical properties, low processing cost and high resistance to corrosion, and wide variety of potential uses in diverse fields such as catalysis, gas sensors, magnetic devices, photoelectrodes, and pigments.15 Usually, the properties of materials depend on their size, morphology, dimensionality, and architecture. Various approaches have been employed for the preparation of various morphological Fe2O3 nanostructures. These methods include hydrothermal precipitation, template-directed processes, solution-phase synthesis, and so on. Until now, well-defined Fe2O3 nanostructures with various morphologies such as 0D (nanoparticles), 1D (nanorods, nanowires, nanocables, and nanotubes), 2D/3D (disks, dendrites, flowers, and complicated hierarchical Fe2O3 nanostructures), and hybrids have been fabricated.16-19 Furthermore, fabrication of Fe2O3 hollow spheres has also attracted a great deal of attention over the past several years because of their low density, high surface area, good surface permeability, and hollow textures.20 And, it is expected that high photocatalytic activity and large light-harvesting efficiency could be achieved using Fe2O3 hollow spheres as photocatalysts. However, preparation of wellcrystallized Fe2O3 hollow microspheres with controllable surface morphology, architecture, shell thickness, and high photocatalytic activity is still a great challenge.21 In this study, Fe2O3 hollow spheres with novel cage-like structures and crystalline nanoporous shells are fabricated using carbonaceous polysac-

10.1021/cg800941d CCC: $40.75  2009 American Chemical Society Published on Web 01/16/2009

Cage-like Ferric Oxide Hollow Spheres

Crystal Growth & Design, Vol. 9, No. 3, 2009 1475

Table 1. Effects of FeCl3 Concentration and Carbon-Sphere Size on the Diameter, Shell Thickness, Crystallite Size, BET Surface Area and Pore Size of the Samples no. FeCl3 conc (mmol) carbon sphere size (nm) hollow spheres size (nm) shell thickness (nm) crystallite size (nm) SBET (m2 g-1) pore size (nm) 1 2 3 4

3 6 3 6

1000 800 500 600

600 400 250 300

charide spheres (or carbon spheres) prepared from saccharide solution as templates, and their visible-light photocatalytic activity and environment application are investigated. This kind of novel superstructure is also expected to have potential applications in catalyst support, catalysis, magnetic devices, drug delivery, separation technology, biomedical engineering, and nanotechnology owing to its unique cage-like structure.

2. Experimental Section 2.1. Preparation of Carbon Spheres. Glucose (analytical grade) was purchased from Shanghai Chemical Reagent Industrial Company. Carbonaceous polysaccharide spheres (or carbon spheres) were fabricated by the hydrothermal approach as reported previously.22 Briefly, glucose (12.0 g, 60.6 mmol) was dissolved in 120 mL of distilled water under stirring. Then the aqueous solution was transferred to a 200 mL Teflon-lined stainless steel autoclave, maintained at 180 °C for different hydrothermal times. The black or puce precipitates were washed with distilled water and ethanol three times and dried at 60 °C for 8 h. Carbon spheres with different sizes were prepared by changing the hydrothermal time (Table 1). Usually, a longer hydrothermal reaction time results in the formation of larger carbonaceous polysaccharide spheres. 2.2. Synthesis of Fe2O3 Cage-like Hollow Spheres. All the reagents used in the experiments were of analytical grade (purchased from Shanghai Chemical Reagent Industrial Company) and used without further purification. In a typical synthesis, 3 mmol (0.81 g) or 6 mmol (1.62 g) of FeCl3 · 6H2O was dissolved in a mixed solvent of 48 mL of ethanol and 8 mL of water under stirring, followed by dissolution of 30 mmol (1.8 g) of urea to form a clear solution. Then the as-prepared carbon spheres (300 mg) were added and well dispersed into the above solution with the assistance of sonication for 10 min. Finally, the mixture was kept at 60 °C for 48 h without stirring. After hydrothermal reaction, the products were washed with distilled water and ethanol three times, respectively. The washed precipitates were dried in a vacuum oven at 60 °C for 8 h and finally were calcined in air at 500 °C for 4 h. The experimental conditions are listed in Table 1. 2.3. Characterization. Powder X-ray diffraction (XRD) patterns were obtained on a D/Max-RB X-ray diffractometer (Rigaku, Japan) using Cu KR radiation at a scan rate of 0.05° 2θ S-1. The average crystallite sizes were determined according to the Scherrer equation using the full-width half-maximum data after correcting the instrumental broadening. Scanning electron microscopy (SEM) was performed with a S4800 field emission SEM (FESEM, Hitachi, Japan) at an accelerating voltage of 5 kV and linked with an Oxford Instruments X-ray analysis system. Transmission electron microscopy (TEM) analysis and selected area electron diffraction (SAED) were conducted using a JEM-2010 (HT) microscope at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were done on a VG ESCALAB MKII XPS system with Mg KR source and a charge neutralizer. All the binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. The Brunauer-EmmettTeller (BET) surface area of the powders was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). All the samples were degassed at 180 °C prior to nitrogen adsorption measurements. The BET surface area was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05-0.3. A desorption isotherm was used to determine the pore size distribution by the Barret-Joyner-Halender (BJH) method, assuming a cylindrical pore model.23 The nitrogen adsorption volume at the relative pressure (P/P0) of 0.994 was used to determine the pore volume and average pore size. UV-visible diffused reflectance spectra of Fe2O3 powders were obtained for the dry-pressed disk samples using a UV-visible spectrophotometer (UV-2550, Shimadzu, Japan). BaSO4 was used as a reflectance standard in a UV-visible diffuse reflectance experiment. The magnetization curves and hysteresis

33.7 36.8 32.2 36.6

33.7 36.8 32.2 36.6

30.5 22.1 33.3 26.4

7.6 10.9 6.3 10.4

loop of the sample was characterized with a model 4HF vibrating sample magnetometer (ADE, USA) with a maximum field of 18 kOe at room temperature. 2.4. Measurement of Photocatalytic Activity. The evaluation of photocatalytic activity of the prepared samples for the photocatalytic decolorization of RhB aqueous solution was performed at ambient temperature, as reported in our previous studies.13e Experiments were as follows: 0.05 g of the prepared Fe2O3 powder was dispersed in a 20 mL of RhB aqueous solution with a concentration of 1 × 10-5 M in a rectangle cell (52 W × 155 L × 30 H mm), followed by the addition of 0.1 mL of hydrogen peroxide solution (H2O2, 30 wt%). The solution was allowed to reach an adsorption-desorption equilibrium among the photocatalyst, RhB, H2O2, and water before visible-light irradiation. An 18-W daylight lamp (3 cm above the dish) was used as a light source. The integrated daylight intensity was 0.46 ( 0.01 mW cm-2, as measured by a UV radiometer (made in the photoelectric instrument factory of Beijing Normal University) with the peak intensity of 420 nm. The concentration of RhB was determined by an UV-visible spectrophotometer (UV-2550, Shimadzu, Japan). After irradiation for some time (20 min), the reaction solution was filtrated to measure the concentration change of RhB.

3. Results and Discussion 3.1. Phase Structures and Compositions. Fe2O3 hollow spheres were fabricated by a controlled hydrothermal precipitation reaction using urea as a precipitating agent and carbon spheres as templates in a mixed solvent of water and ethanol, and then calcined at 500 °C for 4 h (see Experimental Section and Table 1). X-ray diffraction (XRD) is used to investigate the changes of phase structure and crystallite size of the asprepared Fe2O3 samples before and after calcination. The asprepared composite spheres are amorphous before calcination (not shown here). On the contrary, the calcined sample at 500 °C is crystalline Fe2O3. Figure 1 presents XRD patterns of the final products obtained with varying FeCl3 concentrations and carbon sphere sizes and calcined at 500 °C for 4 h (samples 1-4). All the diffraction peaks can be easily indexed to the pure hexagonal phase of R-Fe2O3 with calculated lattice parameters a ) 5.035 Å and c ) 13.746 Å, which are in good agreement with the literature values (JCPDS 33-0664). No other diffraction peaks from FeOOH, Fe3O4, and γ-Fe2O3 are found, indicating that the products are pure R-Fe2O3. With increasing

Figure 1. XRD patterns of the samples prepared at different conditions, (a) sample 1, (b) sample 2, (c) sample 3, and (d) sample 4.

1476 Crystal Growth & Design, Vol. 9, No. 3, 2009

Yu et al.

Figure 2. SEM images of the samples prepared at different conditions, (a) sample 1, (b) sample 2, (c) sample 3, and (d) sample 4.

FeCl3 concentration, the intensities of diffraction peaks slightly increase, indicating that crystallization enhances and crystallite grows. According to the Debye-Scherrer formula, the average crystallite size (Table 1) of the samples 1, 2, 3, and 4 are 33.7, 36.8, 32.2, and 36.6 nm, respectively. These results indicate that the concentration of FeCl3 obviously influences the crystallization and crystallite size of Fe2O3 hollow spheres, whereas the diameters of carbon spheres have little effect on them, and the higher the concentration of FeCl3, the larger the average crystallite size of Fe2O3. This is because more FeCl3 precursors result in more Fe(OH)3 deposited on the surface of the carbon spheres and enhance the growth of Fe2O3 crystallites. The surface element composition and chemical status of the as-prepared hollow spheres were studied by X-ray photoelectron spectroscopy (XPS) analysis. The XPS survey spectrum (Supporting Information, Figure S1a) of the sample 1 indicates that no peaks of other elements except Fe, O, and C are observed. The carbon peak is due to the residual carbon from the sample and adventitious hydrocarbon from XPS instrument itself. The Fe2p3/2 peak at 706 eV and the O1s peak at 531 eV are from Fe and O elements in Fe2O3, respectively. The high-resolution XPS spectrum of O1s region (Figure S1b, Supporting Information) exhibits that the O1s region can be fitted into two small peaks. The main peak located at 529.7 eV is ascribed to the Fe-O in Fe2O3 and the other peak at 531.8 eV can be attributed to the OH on the surface of Fe2O3. Their atomic ratios are about 70 and 30%, respectively. The high hydroxyl content may be related to the destruction of Fe-O-Fe and the formation of Fe-OH on the surface of Fe2O3 hollow spheres. Both XRD and XPS analysis indicate that the as-prepared hollow spheres are pure R-Fe2O3 phase. 3.2. SEM and TEM Images. Figure 2 shows scanning electron microscopy (SEM) images of the samples obtained at different experimental conditions, indicating that the obtained

hollow spheres are relatively uniform and appear monodisperse. High-magnification images (inset in Figure 2a,b) clearly show that the individual spheres are porous cage-like hollow spheres, and that shell walls consist of crystallite 30-40 nm in size. Further observation (from Table 1 and Figure 2) reveals that the average diameter of hollow spheres is approximately 250-600 nm. The sizes of samples 1, 2, 3, and 4 are 600, 400, 250, and 300 nm, respectively. With increasing carbon sphere size, the diameters of hollow spheres increase. To get more information about the hollow structures, the samples were characterized by transmission electron microscopy (TEM). Figure 3a exhibits a typical TEM image of Fe2O3 hollow spheres. The obvious electron-density difference between the dark edge and pale center further confirms the hollow interiors clearly. The hollow sphere is composed of randomly aggregated nanocrystal particles with sizes of about 30-40 nm, which is in accord with the results of XRD (Table 1). The shell has a thickness of about 30-40 nm, the same range of the crystallite size. These results suggest that the shells are built with single layer of Fe2O3 nanoparticles of 30-40 nm in size, and consequently are high porous. The corresponding selected area electron diffraction (SAED) pattern on an individual hollow sphere (Figure 3b) reveals that the hollow spheres are polycrystalline. Therefore, it can be inferred from the above results that the thickness of shell walls of Fe2O3 hollow spheres could be easily controlled by changing concentration of FeCl3. The higher the concentration of FeCl3, the thicker the shell walls of hollow spheres. 3.3. Nitrogen Sorption and UV-vis Diffuse Reflectance Spectra. Nitrogen adsorption/desorption isotherms are measured to determine the specific surface area and pore size of Fe2O3 hollow spheres, and the corresponding results of sample 1 are presented in Figure 4. The sample 1 exhibits a type H3 hysteresis loop according to Brunauer-Deming-Deming-Teller (BDDT)



Figure 5. UV-visible absorption spectra of the samples 1, 2, 3, and 4.

Figure 3. TEM image (a) and SAED pattern (b) of the sample 1.

nm).14c,24 The pore size distribution of Fe2O3 hollow spheres (inset in Figure 4) indicates bimodal pore size distribution in the mesoporous and macroporous region with a maximum peak pore diameter of ca. 90 nm. The Brunauer-Emmett-Teller (BET) specific surface areas (Table 1) of samples 1, 2, 3, and 4 are 30.5, 22.1, 33.3, and 26.4 m2/g, respectively. According to our previous reports,14c,24 these mesopores and macropores presumably arise from the interstices among the different-sized nanoparticles within the shells of Fe2O3 hollow spheres. The UV-visible diffuse reflectance spectra (Figure 5) of the samples display that a significant increase in the absorption at wavelengths shorter than 600 nm can be assigned to the intrinsic bandgap absorption of Fe2O3 due to the electron transitions from the valence band to conduction band (O2p f Fe3d). With decreasing crystallite size (Table 1), the samples show an obvious blue-shift in the bandgap transition. The shift of absorption edge toward shorter wavelengths with decreasing FeCl3 concentration clearly indicates an increase in the bandgap energy of Fe2O3. This shift is caused by strengthening of the quantum confinement of charge carriers at a small Fe2O3 nanocrystalline particles.25 The Kubelka-Munk method based on the diffuse reflectance spectra was employed to determine the bandgap of the samples.25b,c For the direct bandgap semiconductor, the relation between the absorption coefficient (R) and photon energy (hV) can be written as

R ) Bd(hV - Eg)1/2 ⁄ hV

Figure 4. Nitrogen adsorption-desorption isotherms and pore size distribution curves (inset) of the sample 1.

classification,23 indicating the presence of mesopores (2-50 nm). The observed hysteresis loops shifts to a high relative pressure P/P0 ≈ 1, suggesting the presence of large pores (>50

(1)

where Bd is absorption constants for direct transitions. R can be determined from the scattering and reflectance spectra according to Kubelka-Munk theory. Plots of the (RhV)2 versus photon energy (hV) (not shown here) indicate that the direct bandap energies of samples 1, 2, 3, and 4 are 2.11, 2.09, 2.12, and 2.10 eV, respectively. 3.4. Formation Mechanism. The formation of cage-like Fe2O3 hollow spheres probably involves three steps (Figure 6A). The first step is the embedding and deposition of the Fe(OH)3 into the hydrophilic shell of microspheres because the surface of carbon spheres is hydrophilic and contains a large amount of OH and CdO groups, which can bind Fe(OH)3 through hydrogen-bonding or electrostatic interactions. After saturation absorption, more Fe(OH)3 can also directly deposit on the surface of carbon spheres via a heterogeneous nucleation growth (Figure 6B). Then, the removal of carbon cores, and densification, cross-link, and phase transformation of Fe(OH)3 in the layer via calcination result in the formation of Fe2O3 hollow spheres. Finally, with increasing calcination time, growth and


Yu et al.

Figure 6. (A) Schematic illustration of the formation of cage-like Fe2O3 hollow sphere. (B) SEM images of carbon sphere, composite sphere, and Fe2O3 hollow spheres at 500 °C for 5 min and 4 h. The scale bar is 200 nm.

restructure of crystallite lead to the formation of cage-like hollow superstructures. Therefore, it is not surprising that the thickness of shell can be easily controlled by changing FeCl3 concentration. The formation of cage-like Fe2O3 hollow superstructures is strongly dependent on experimental factors such as hydrothermal time, FeCl3 concentration, solvent composition, and the presence of templates. For example, in the current work when the hydrothermal time was reduced to 6 h, only Fe2O3 nanoparticles instead of hollow spheres are observed (Figure S2a, Supporting Information). This is attributed to the collapse of the hollow structure due to the incompleteness of shell walls or thin shell. When the aging times increase to 12 h, hollow spheres with some broken and shriveled ones were obtained (Figure S2b, Supporting Information), suggesting the increase of strength and shell-wall thickness of the hollow structures. From these results, we think that the following reactions take place in the ethanol/ water mixed solution:

CO(NH2)2+ H2O S CO2+ 2NH3

(2)

NH3+ H2O S NH+ 4 + OH

(3)

Fe3++ 3OH- f Fe(OH)3

(4)

2Fe(OH)3 f Fe2O3+ 3H2O

(5)

During hydrothermal precipitation, urea decomposes gradually at a mild temperature (60 °C), which generates an alkaline environment. Therefore, Fe(OH)3 deposits preferentially on the surface of carbon spheres to form a condensed Fe(OH)3 phase via adsorption or subsequently heterogeneous nucleation growth. This synthesis route is distinctively different from the surfaceadsorption method of metal ions,8 since the precipitation process is not limited by the adsorption capacity of the surface. In addition, it is well-known that the presence of abundant ethanol in the reaction system should reduce the polarity of the solvent and readily lead to amorphous layers on the surface of the templates. During the calcination step, reaction 5 occurs and results in the decomposition of the amorphous Fe(OH)3 layers while the carbon templates are eliminated by releasing gaseous carbon oxides. In this process, we suggest that some Fe2O3 primary particles first form, which act as nuclei for the crystallization of Fe2O3 nanoparticles. When the Fe2O3 primary particles grow into larger ones, aggregation or probably the

epitaxial growth occurs, which leads to the interconnecting of the Fe2O3 nanoparticles to form Fe2O3 nanoparticle chains. Finally, these Fe2O3 nanoparticles and Fe2O3 nanoparticle chains connect one another to form a cage-like superstructure web. The proposed formation mechanism is schematically illustrated in Figure 6. Further investigation shows that the addition of urea and ethanol is essential to obtain hollow spheres with unique cagelike superstructure. In the absence of urea, intact cage-like Fe2O3 hollow superstructures cannot be obtained (Figure S3a, Supporting Information). This can be explained by considering decreasing amount of ferric ions in the shells of as-prepared carbon-ferric composite microspheres for the limit of adsorption capability of Fe3+ ions on the hydrophilic surface of carbon spheres. So, the shell walls of hollow spheres are so thin that they easily collapse during calcination. This result also further proves that the synthesis route using urea as the precipitating agent is the precipitation strategy instead of the surfaceadsorption route. The role of ethanol in the reaction system is also studied using water as the solvent. In pure water, only Fe2O3 nanoparticles instead of hollow spheres are obtained (Figure S3b, Supporting Information). This result indicates that the presence of ethanol can enhance the ferric ions to deposit on the surface of the templates and form compact Fe(OH)3 amorphous layers. This is because ethanol reduces the deposited rate of Fe3+ ions and prevents homogeneous nucleation growth. Therefore, it is also not surprising that hollow superstructures cannot be obtained in the absence of carbon spheres or in the presence of a lower FeCl3 concentration (such as 1 mmol). 3.5. Photocatalytic Activity and Magnetic Property. The photocatalytic activity of the samples was evaluated by photocatalytic decolorization of rhodamine B (RhB) aqueous solution in the presence of H2O2. Figure 7 shows the comparison of photocatalytic activities of the samples prepared at different conditions. In the absence of H2O2, the prepared Fe2O3 hollow spheres show very weak photocatalytic activity under visiblelight illumination (not shown here). No photocatalytic activity is observed in the presence of H2O2 in the dark. H2O2 under visible-light illumination exhibits a weak activity for the decolorization of RhB aqueous solution (Figure 7), whereas in the presence of H2O2 and visible-light illumination, the prepared Fe2O3 hollow spheres show obviously visible-light photocatalytic



Figure 7. Comparision of the photocatalytic activity of samples 1, 2, 3 and 4, Ct and C0 denote the reaction and initial concentration of RhB in the system, respectively.

activity (Figure 7). This can be understood by the following mechanism suggested. With visible-light illumination, photogenerated electron-hole pairs are formed in Fe2O3 hollow spheres. The fate of photogenerated electrons is determined by the following two factors:25 (a) They are easily trapped by the H2O2 forming OH radicals. The beneficial effect of the added H2O2 under light illumination is due to its electron acceptor properties: + Fe2O3 f Fe2O3(ecb, hVB)

(6)

H2O2+ ecb f OH + ·OH

(7)

Or (b) they are trapped by the surface Fe3+ leading to Fe2+ in the following reaction: 2+ Fe3++ ecb f Fe

(8)

Fe2++ H2O2 f Fe3+ + OH- + ·OH

(9)

The formation of · OH in reactions 7 and 9 trigger the photocatalytic reaction. As expected, reaction 9 may play the key role in photocatalytic reaction due to the formation of Fenton’s reagent. The Fenton reaction is one of the most effective advanced oxidation processes (AOPs) for wastewater treatment, which is well-known as the production of hydroxyl radicals by reaction between Fe2+ and H2O2.27 It can be seen from Figure 7 that the photocatalytic activity of sample 3 is the highest. This can be ascribed to its high specific surface areas and small crystallite size and big bandgap. Furthermore, destroying the hollow superstructure by grinding the hollow spheres reduced the photocatalytic activity by about 5-10%, suggesting that the unusual hierarchically cage-like superstructure can promote the photocatalytic activity. First, it allows more efficient transport for the reactant molecules to get to the active sites on the framework walls and enhance the efficiency of photocatalysis.28 Second, the hollow spheres allow multiple reflections of visible light within the interior that facilitates more efficient use of the light source.29 We also perform the control experiments. Under dark conditions, the concentration of RhB does not change. This result indicates that the degradation decolorization of RhB aqueous solution is caused by photocatalytic reactions instead of adsorption on the particles. After five recycles for the photodegradation of RhB, the catalyst does not exhibit any significant loss of activity, confirming that Fe2O3 hollow spheres are not photocorroded during the photocatalytic oxidation of the pollutant molecules. Usually, Fe2O3 can be

Figure 8. Hysteresis loop of sample 1 measured at 298 K.

photodecomposed under light illumination. Why does Fe2O3 exhibit a good stability for photocatalytic reaction in this system? This is because once Fe2+ is generated, it reacts either with H2O2 generating OH radicals or it detaches from the oxide surface leaving a vacancy on the oxide surface (surface dissolution). The above results strongly suggest that the reaction between surface Fe2+ and H2O2 is kinetically faster than the Fe2+ dissolution reaction in the Fe2O3 catalyst surface, thus preventing occurrence of catalyst photocorrosion.25 The prepared cage-like hollow spheres can be regarded as an ideal photocatalyst for environmental purification at the industrial scale because they can be more readily separated from the slurry system by filtration or sedimentation after photocatalytic reaction and reused compared to conventional nanosized powder photocatalytic materials due to their larger weight, weaker Brownian motion, and better mobility. Magnetic hysteresis measurement for Fe2O3 hollow spheres is carried out in applied magnetic field at room temperature, with the field sweeping from -18 to 18 kOe. The hysteresis loop (Figure 8) of the sample does not reach saturation up to the maximum applied magnetic field. The magnetization measurement of the sample exhibits a hysteretic feature with the remanent magnetization (Mr) and coercivity (Hc) being determined to be 0.08 emu/g and 1162.2 Oe, respectively, suggesting that the cage-like Fe2O3 hollow spheres exhibit ferromagnetic behaviors at room temperature.30a It is wellknown that the magnetization of ferromagnetic materials is very sensitive to the microstructure of a particular sample.30b The smaller remanent magnetization for as-obtained R-Fe2O3 hollow spheres compared to that of commercial R-Fe2O3 (0.6 emu/g)30c is probably associated with the fine spherical shape of the hematite nanoparticles, since the remanent magnetization is strongly dependent upon the particle shape.30d However, the coercivity is relatively high, which may be caused by the polycrystalline nature of the R-Fe2O3 hollow spheres consisting of large numbers of smaller subparticles.

4. Conclusions Fe2O3 hollow spheres with novel cage-like architectures could be repeatedly fabricated on a large scale by a controlled hydrothermal precipitation reaction of FeCl3 in the presence of urea, ethanol, and carbon spheres, and then calcined at 500 °C. The diameters of hollow spheres are well controlled by carbon-sphere size. Moreover, the shell thickness, average crystallite size, specific surface areas, pore structures, and photocatalytic activity of Fe2O3 hollow spheres could be slightly tuned by changing the concentration of FeCl3. By


increasing the FeCl3 concentration, the average crystallite size, shell thickness, and pore size slightly increase; in contrast, specific surface areas and photocatalytic activity slightly decrease. The obtained Fe2O3 hollow spheres exhibit a good visible-light photocatalytic activity for the photocatalytic decolorization of RhB aqueous solution under the visible-light illumination in the presence of H2O2. Magnetic investigation shows that the cage-like R-Fe2O3 hollow spheres exhibit a ferromagnetic property at room temperature. Considering the unique cage-like hollow structures, the prepared Fe2O3 hollow spheres are also of great interest in sensor, lithium secondary batteries, solar cell, catalysis, separation technology, biomedical engineering, and nanotechnology. Acknowledgment. This work was partially supported by the National Natural Science Foundation of China (50625208, 20773097, and 20877061). This work was also financially supported by National Basic Research Program of China (2007CB613302 and 2009CB939704) and PCSIRT (No. IRT0547). Supporting Information Available: Figures S1-S3 show XPS and SEM results of the samples. This information is available free of charge via the Internet at http://pubs.acs.org.

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CG800941D

Hydrothermal Synthesis and Visible-light Photocatalytic Activity of

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