DOI: 10.1002/chem.201001846

Self-Assembled 3D Flowerlike Hierarchical Fe3O4@Bi2O3 Core–ShellArchitectures and Their Enhanced Photocatalytic Activity under Visible

Light

Yang Wang,[a] Shikuo Li,[a] Xianran Xing,[b] Fangzhi Huang,[a] Yuhua Shen,*[a]

Anjian Xie,*[a] Xiufang Wang,[a] and Jian Zhang[a]

Introduction

Photocatalysts are expected to play an important role inhelping to solve many serious environmental and pollutionchallenges. An environmentally friendly efficient photocata-lyst should possess a good separation of electron–hole pairs;a fair response to visible light, to maximally utilize sunlightenergy; and suitable band potentials to produce hydroxylradicals.[1] To date, most of these investigations have focusedon titania (TiO2) on account of its peculiarities of chemicalinertness, resistance to photocorrosion, low cost, and non-toxicity.[2] However, TiO2 is a wide-bandgap semiconductor(3.2 eV for anatase) and can only absorb about 3–5 % ofsunlight in the ultraviolet region, which greatly limits itspractical applications.[3] Therefore, the development of visi-ble-light-driven photocatalysts has recently become a veryimportant topic of research.

In this respect, bismuth oxide (Bi2O3) is an attractive ma-terial because of its good electrical conductivity, thermalproperties, and narrow band-gap (2.8 eV).[2] Bi2O3 has been

extensively investigated for various applications in photoca-talysts,[2,4] photovoltaic cells,[5] optical coatings,[6] fuel cells,[5]

supercapacitors,[7] gas sensors,[8] and so on. Recently, manyresearch efforts have been focused on developing variousmorphologies of Bi2O3 and researching its optical properties.For example, Qiu et al. have synthesized ultrathin Bi2O3

nanowires by using an oxidative metal-vapor transport dep-osition technique.[9] Yu et al. have fabricated a Bi2O3 nano-plate by a hydrothermal method.[10] Reddy et al. have pre-pared a Sm3+-doped Bi2O3 photocatalyst by means of a hy-drothermal method and researched its photocatalytic prop-erty under solar light.[1] Very recently, Zhou et al. have de-veloped a facile template-free approach to fabricatehierarchical Bi2O3 nanostructures and controlled its mor-phology by varying the amount of VO3� and reaction tem-perature.[11] However, the use of Bi2O3 nanoparticles withhigh surface area for photocatalysis is often limited, becausethe suspended particulate catalysts are easily lost in the pro-cess of photocatalytic reaction and separation, which mayimprove the cost of industrial applications and pollute thetreated water again.

The composite materials of the magnetic core and catalyt-ic shell can possibly resolve the above problem. Such mate-rials could combine the advantages of activity of catalystswith the merits of an easy separation due to the incorpora-tion of magnetic nanoparticles. Recently, plenty of researchworks have been carried out with this in mind. For instance,Song et al. have fabricated Fe3O4/SiO2 microspheres througha sol–gel approach, and then synthesized the Fe3O4/SiO2/TiO2 photocatalyst by alcoholysis of titanium tetrabutoxidein ethanol solution.[12] Guo et al. and Wang et al. have con-

Abstract: Three-dimensional (3D)flowerlike hierarchical Fe3O4@Bi2O3

core–shell architectures were synthe-sized by a simple and direct solvother-mal route without any linker shell. Theresults indicated that the size of the 3Dflowerlike hierarchical microsphereswas about 420 nm and the shell wascomposed of several nanosheets with athickness of 4–10 nm and a width of100–140 nm. The saturation magnetiza-

tion of the superparamagnetic compo-site microspheres was about 41 emu g�1

at room temperature. Moreover, theFe3O4@Bi2O3 composite microspheresshowed much higher (7–10 times) pho-tocatalytic activity than commercial

Bi2O3 particles under visible-light irra-diation. The possible formation mecha-nism was proposed for Ostwald ripen-ing and the self-assembled process.This novel composite material mayhave potential applications in watertreatment, degradation of dye pollu-tants, and environmental cleaning, forexample.

Keywords: bismuth · compositematerials · core–shell structures ·magnetic properties · photocatalysts

[a] Y. Wang, S. Li, F. Huang, Prof. Y. Shen, A. Xie, X. Wang, J. ZhangSchool of Chemistry and Chemical EngineeringAnhui Univerity, Hefei 230039 (P.R. China)Fax: (+86) 551-5108702E-mail : anjx@163.com

[b] Prof. X. XingSchool of Metallurgical and Ecological EngineeringUniversity of Science and Technology BeijingBeijing 100083 (P.R. China)

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201001846.

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structed multifunctional Fe3O4/Au hybrid nanostructures byusing polymers as a linker to coat magnetic particles.[13]

However, to our knowledge, the synthesis of these magneticcomposite materials should require multiple steps as well assome linker shells (e.g., silica, polymers), which make thesynthesis route complicated and render the catalysts too ex-pensive for widespread industrial use; they also decrease thesaturation magnetization (Ms) of composite materials byadding a linker shell.

Herein, for the first time, we report a simple and directmethod to construct 3D flowerlike Fe3O4@Bi2O3 core–shellarchitectures without any linker shell. The size of theseflowerlike hierarchical microspheres is about 420 nm, andthe shells are composed of several nanosheets with a thick-ness of 4–10 nm and a width of 100–140 nm. These compo-site microspheres are superparamagnetic at room tempera-ture. It is worth noting that the obtained composite photoca-talysts can not only be easily recycled by applying an exter-nal magnetic field, but also exhibit powerful visible-lightphotocatalytic activity for the degradation of rhodamine B(RhB).

Results and Discussion

The phase and composition of the as-prepared Fe3O4 nano-particles and Fe3O4@Bi2O3 composite microspheres werecharacterized by X-ray diffraction (XRD; Figure 1). Fivemajor reflections that appeared in Figure 1a located at

about 30.4, 35.4, 43.2, 57.2, and 62.78 can be assigned to thediffraction of Fe3O4 crystal with inverse spinel structurefrom the (220), (311), (400), (511), and (440) planes (JCPDScard no. 19-0629), respectively. No other peaks are observedin Figure 1a, thereby indicating that the products are pureFe3O4 crystalline phase. Compared with Figure 1a, Figure 1bpresents other diffraction peaks at 27.82, 32.31, 46.33, and54.628, which correspond to the (111), (200), (220), and(311) planes of d-Bi2O3 crystal with a cubic phase (JCPDScard no. 27-0052).[11] The broadening of the peaks indicates

that the crystallites are small in size. The results suggest thatin the present case, the Fe3O4@Bi2O3 composite micro-spheres were successfully synthesized by a simple and directsolvothermal method.

The morphologies of the as-synthesized products were in-vestigated by field-emission scanning electron microscopy(FESEM). The low-magnification SEM image in Figure 2a

shows that the Fe3O4@Bi2O3 composites are composed ofmany uniform, spherelike architectures with an average di-ameter of 420 nm and present a rough surface. The detailedmorphologies of the as-synthesized products are shown inFigure 2b and c, which reveal that all the microspheres have3D flowerlike hierarchical morphology. Those 3D flowerlikearchitectures are built from several dozens of nanosheetswith a thickness of 4–10 nm and a width of 100–140 nm. Thesurface of the sheets assembled into the hierarchical micro-architectures is very smooth, probably due to Ostwald ripen-ing.[14] The spectrum of electron dispersive spectroscopy(EDS) of the spherical particles shown in Figure 2d presentsthe peaks of Bi, Fe, and O elements, further confirming thatthe products are Fe3O4@Bi2O3 composites.

The morphologies and structures of as-synthesized sam-ples were further characterized by transmission electron mi-croscopy (TEM). The as-prepared Fe3O4 nanospheres havea relatively smooth surface and an average diameter ofabout 300 nm (Figure 3a). It can be seen that the nano-sphere is composed of small primary nanoparticles with asize of 15–20 nm (inset in Figure 3a). In Figure 3b, the prod-ucts of Fe3O4@Bi2O3 with a rough surface and an average di-ameter of about 420 nm are presented. Diffraction contrastis observed in Figure 3c, in which the dark regions representthe Fe3O4 nanosphere and the bright regions represent the

Figure 1. XRD patterns of a) Fe3O4 and b) Fe3O4@Bi2O3 composite.

Figure 2. a)–c) SEM images with different magnifications of the as-ob-tained Fe3O4@Bi2O3 composite. d) EDX analysis of the Fe3O4@Bi2O3 mi-crospheres.

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Bi2O3 shell. It is clearly displayed that the Fe3O4 core is wellwrapped by the coating layer, and the average thickness ofthe coating shell is about 60 nm. TEM observations demon-strate that the products are flowerlike structures similar tothe SEM observation. To further confirm the result, thehigh-resolution TEM (HRTEM) image presented in Fig-ure 3d was taken. In the HRTEM image taken from theedge of composite spheres (in Figure 3c), the lattice fringesare clearly visible with a spacing of 0.32 nm, which is ingood agreement with the spacing of the (111) planes of d-Bi2O3 (JCPDS card no. 27-0052). The above results indicatethat the Fe3O4 core can be well wrapped by the Bi2O3 coat-ing layer by such a simple and direct solvothermal method.

To reveal the formation process of the 3D flowerlike ar-chitectures in more detail, time-dependent experimentswere carried out and the resulting products were analyzedby TEM (Figure 4). Under thepresent synthetic conditions,nanoparticles and some ultra-thin nanosheets adhered to thesurface of the Fe3O4 spheres asa result of aggregation andgrowth after reaction for 3–3.5 h (Figures 4a and b). Whenthe reaction time was pro-longed to 4 h, some underdevel-oped flowerlike architecturesexisted on the surface of theFe3O4 spheres, thereby indicating that oriented attachmentis still occurring (Figure 4c). After a reaction time of 5 h,fully developed 3D flowerlike Fe3O4@Bi2O3 hierarchical mi-crospheres were formed; they consisted of several nano-sheets (Figure 4d and the corresponding SEM images shownin Figure S1 of the Supporting Information).

Based on the results of time-dependent experiments, theformation of 3D flowerlike hierarchical architectures mayresult from the combined roles of ethylene glycol (EG)under the appropriate reaction conditions. The chemical re-action in the process to obtain Bi2O3 particles could be for-mulated as follows [Eqs. (1) and (2)]:

2 BiðNO3Þ3 � 5 H2Oþ 3 HOCH2CH2OHþ 6 C2H5OH

! Bi2ðOCH2CH2OÞ3 þ 6 C2H5ONO2 þ 16 H2Oð1Þ

Bi2ðOCH2CH2OÞ3 þ 3 H2O! Bi2O3 # þ3 HOCH2CH2OH

ð2Þ

A possible formation process is schematically illustrated inFigure 5. First, the as-prepared magnetic Fe3O4 spheres arehydrophilic and the EG is easily adsorbed onto the surfaceof Fe3O4 spheres by hydrogen bonding. Meanwhile, EG in

solution adsorbs Bi3+ to form a relatively stable complex,Bi2ACHTUNGTRENNUNG(OCH2CH2O)3, because of its strong coordination withBi3+ . Second, Bi2O3 nanoparticles are obtained by means ofthe hydrolyzation of Bi2ACHTUNGTRENNUNG(OCH2CH2O)3 in solution. Thenthese nanoparticles grow along the 2D direction, thereby re-sulting in the formation of nanosheets. Third, as the mass

Figure 3. a) TEM image of the Fe3O4 nanoparticles. b) Low-magnificationTEM image of Fe3O4@Bi2O3 composite microspheres; c) partial high-magnification image thereof. d) HRTEM image taken from the edge ofcomposite spheres.

Figure 4. TEM images of the obtained Fe3O4@Bi2O3 composite micro-spheres at various reaction stages: a) 3, b) 3.5, c) 4, and d) 5 h.

Figure 5. Schematic illustration of the proposed formation mechanism of Fe3O4@Bi2O3 hierarchical micro-spheres.

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Y. Shen, A. Xie et al.

diffusion and Ostwald ripening process proceed, the nano-sheets grow until all the nanoparticles are consumed, accom-panied by their self-organization into the flowery structureon the surfaces of Fe3O4 nanospheres. Finally, the 3D flower-like Fe3O4@Bi2O3 hierarchical microspheres are obtained. Inthe control experiment, the morphology of the as-synthe-sized Bi2O3 was a porous microsphere composed of manynanosheets in the absence of Fe3O4 nanospheres (see theSEM images in Figure S2 of the Supporting Information).The thickness and width of these sheets is different fromthose in Figure 2c. This result indicated that the Fe3O4 nano-sphere also plays an important role in formation of the flow-erlike architectures.

Surface analysis of the prepared Fe3O4@Bi2O3 compositeswas carried out using X-ray photoelectron spectroscopy(XPS). The binding energy for the C (1s) peak (284.6 eV)was used as an internal reference.[15] Figure 6a shows theXPS spectra in different spectral regions that correspond todifferent elements for the Fe3O4@Bi2O3 composite spheres.It displays the binding energies for Bi (4s), Fe (2p), O (1s),Bi (4d), C (1s), Bi (4f), and Bi (5d) of the compositespheres. And it is easy to find that the intensity of the Bipeaks is higher than the Fe peak, which possibly confirmsthat the composite spheres have a core–shell structure. Fig-ure 6b depicts two intense bands with binding energies of710.3 and 723.9 eV that are assigned to Fe (2p3=2

) and Fe(2p1=2

), respectively. Both these bands consist of Fe2+ (ofFeO) and Fe3+ (of Fe2O3) peaks and are typical characteris-tics of the Fe3O4 structure.[15a, 16] Figure 6c demonstrates thatelemental Bi is present in the form of Bi2O3, which corre-sponds to the binding energies of 159.9 and 164.8 eV in Bi4f7=2

and Bi 4f5=2levels, respectively.[2]

The magnetic properties of the as-prepared Fe3O4 nano-particles and the Fe3O4@Bi2O3 composite spheres were stud-ied by using a superconducting quantum interference device(SQUID) magnetometer at room temperature (300 K). Asshown in Figure 7, for the two samples the coercivity forceis almost negligible at 300 K, which indicates that the Fe3O4

nanoparticles and the Fe3O4@Bi2O3 composite spheres aresuperparamagnetic at room temperature. It can be calculat-ed that the Ms of Fe3O4 nanoparticles is about 75 emu g�1

(curve a), and the Ms of the Fe3O4@Bi2O3 composite spheresdecreases to 41 emug�1 (curve b). The decrease in magneti-zation reveals the contribution of the Bi2O3 shell on the sur-face of Fe3O4 nanospheres, which reduces the magnetitefraction in each microsphere.[17] The strong magnetization ofthe Fe3O4@Bi2O3 composite spheres means they can bequickly and conveniently separated from solution by an ex-ternal magnetic field.

The N2 adsorption–desorption isotherms for the theFe3O4@Bi2O3 composite microspheres and commercialBi2O3 product are presented in Figure 8. The hysteresis isobserved during the N2 adsorption–desorption cycles, there-by indicating that the surfaces of the prepared compositemicrospheres are porous (curve a). Given the observed mor-phology of the product, the smaller pores with a sharp peakat about 2.2 nm may be generated during the crystal-growth

process, whereas the larger pores with a wide pore-size dis-tribution can be attributed to the space between the inter-crossed Bi2O3 nanosheets (Figure 8, inset).[18] The Brunauer–Emmett–Teller (BET) surface area and total pore volume ofthe Fe3O4@Bi2O3 composite microspheres are 73.8 m2 g�1

and 0.292 cm3 g�1, respectively. But the surface area of thecommercial Bi2O3 product is 0.368 m2 g�1, which is clearlysmaller than the as-prepared composite (curve b) due to dis-orderly morphology such as sphere, sheet, and irregular par-ticles and a wide size range of 50–400 nm (Figure S3 in theSupporting Information). The higher specific surface area ofthe Fe3O4@Bi2O3 composite photocatalyst is beneficial toimproving the photocatalytic efficiency.

Figure 6. XPS spectra of a) as-obtained Fe3O4@Bi2O3 composite spheres,b) Fe 2p, and c) Bi 4f.

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Diffuse reflectance spectroscopy is a useful tool for char-acterizing the electronic states in optical materials. Figure 9shows the UV/Vis diffuse reflectance spectrum of the com-mercial Bi2O3 product and the as-prepared Fe3O4@Bi2O3

composite microspheres. It can be seen that both sampleshave strong absorption in the visible region. Comparing

curve a with curve b, a redshift is detected for Fe3O4@Bi2O3

composite material. This may be caused by the influence ofthe Fe3O4 core. The photocatalytic activities of theFe3O4@Bi2O3 composite microspheres were evaluated by thedegradation of RhB dye in water under visible-light irradia-tion (l>420 nm) at room temperature. The characteristicabsorption of RhB at l=553 nm was used to monitor thephotocatalytic degradation process. As seen from Fig-ure 10a, the as-prepared composite microspheres show a sur-

prising degradation of RhB under visible-light irradiation(curve 1). For comparison, the same photodegradation ex-periment using commercial Bi2O3 particles was also carriedout (curve 2). It is clear that the photodegradation rates ofthe as-synthesized Fe3O4@Bi2O3 hierarchitectures are muchhigher (7–10 times) than the commercial Bi2O3 particles.Moreover, it is important to point out that the RhB degra-dation reaches about 100 % for the as-preparedFe3O4@Bi2O3 composite microspheres under visible-light ir-radiation after 50 min. This result is also consistent with thecolor change of the suspension from pink to colorless as theirradiation time is gradually increased (shown inset of Fig-ure 10a). In general, the catalytic process is related to theadsorption and desorption of molecules on the surface ofcatalysts. Thus, the higher specific surface area of the photo-

Figure 7. Room-temperature magnetization curves of a) Fe3O4 andb) Fe3O4@Bi2O3

Figure 8. a) Typical N2 gas adsorption–desorption isotherm and pore-sizedistribution curve of the Fe3O4@Bi2O3 composite microspheres. b) N2 gasadsorption–desorption isotherm of commercial Bi2O3 product.

Figure 9. UV/Vis diffuse reflectance spectra of a) the as-preparedFe3O4@Bi2O3 composite microspheres and b) commercial Bi2O3 product.

Figure 10. a) Degradation rate of RhB under visible light: (curve 1) theas-prepared Fe3O4@Bi2O3 composite microspheres, (curve 2) commercialBi2O3 product. b) Six cycles of the photocatalytic degradation ofFe3O4@Bi2O3 core–shell structures.

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Y. Shen, A. Xie et al.

catalyst results in more unsaturated surface coordinationsites exposed to the solution, and the open porous structureallows more efficient transportation of the reactant mole-cules that move into the active sites, thereby enhancing theefficiency of photocatalysis. The pore diameters also cover arelatively wide range from several to dozens of nanometers.Therefore, the 3D flowerlike Fe3O4@Bi2O3 hierarchical mi-crospheres can raise the efficiency of the photocatalytic re-action of RhB under visible light. We have also monitoredthe stability of the magnetic photocatalysts by monitoringthe photocatalytic activity over six cycles of use. As shownin Figure 9b, one can see that the photocatalytic activity ofFe3O4@Bi2O3 composite microspheres does not exhibit anysignificant loss for the photodegradation of RhB after six re-cycles. This fact implies that the obtained hierarchicalFe3O4@Bi2O3 composite microspheres have high stabilityand do not photocorrode during the photocatalytic oxida-tion of the model pollutant molecules, which is especiallyimportant for its practical applications. Furthermore, thelarger diameter of the obtained microspheres is also favora-ble for separation and recycling of such catalysts by using amagnet.

Conclusion

The 3D flowerlike Fe3O4@Bi2O3 core–shell architectureshave been synthesized by a facile and direct solvothermalroute. The 3D flowerlike hierarchical architectures with asize of several hundred nanometers are composed of nano-sheets with a thickness of 4–10 nm and a width of 100–140 nm. Moreover, it is emphasized that the obtainedFe3O4@Bi2O3 hierarchical microspheres exhibit powerfulvisible-light photocatalytic activity for the degradation ofrhodamine B (RhB) under 500 W (Xe lamp) light irradia-tion. This simple strategy may provide a general way to fab-ricate other composite materials. In addition, due to theirunique architectures, the as-obtained products may have po-tential applications in water treatment, sensors, microelec-tronics, energy storage, and other related micro- or nano-scale devices.

Experimental Section

Materials : Bismuth nitrate pentahydrate (Bi ACHTUNGTRENNUNG(NO3)3·5H2O), sodium ace-tate (NaAc), iron ACHTUNGTRENNUNG(III) chloride hexahydrate (FeCl3·6 H2O), ethyleneglycol (EG), polyethylene glycol (PEG), dibismuth trioxide (Bi2O3), rho-damine B (RhB), and ethanol were obtained from Shanghai ChemicalReagent Co. Ltd. (China). All chemicals were analytical reagent gradeand used without further purification. Double-distilled water was used inour experiments.

Synthesis of 3D flowerlike hierarchical Fe3O4@Bi2O3 core–shell architec-tures : The Fe3O4 nanoparticles with high saturation magnetization weresynthesized by a solvothermal method in polyol medium according to theprocedure published by Li and co-workers.[19] In a typical process, Bi-ACHTUNGTRENNUNG(NO3)3·5 H2O (0.97 g, 2 mmol) was dissolved in a mixed solution that con-tained ethylene glycol (17 mL) and ethanol (34 mL), followed by vigo-rous stirring for 40 min. Then the as-prepared magnetic Fe3O4 nanoparti-

cles (50 mg) were added in mixed solution and sonicated for 30 min. Theabove mixed solution was sealed in a Teflon-lined stainless-steel auto-clave (50 mL capacity). The autoclave was heated to and maintained at160 8C for 5 h, then allowed to cool to room temperature. In the last step,the products were separated magnetically and thoroughly washed withdeionized water and ethanol several times to eliminate organic and re-dundant Bi2O3 impurities, then dried in vacuum at 60 8C for 24 h.

Characterization : UV/Vis spectra were measured using a TU-1901 modelUV/Vis double-beam spectrophotometer (Beijing Purkinje General In-strument Co. Ltd, China). Field-emission scanning electron microscopy(FESEM) measurements were taken using a Hitachi S-4800 scanningelectron microscopy. Transmission electron microscopy (TEM) measure-ments were performed using a JEM-2100 electron microscope (JapanElectron Co.) operated at an accelerating voltage at 80 kV. HRTEMimages were obtained using the same transmission electron microscope.XPS measurements on a film of silver nanocomplex were carried outusing a VG ESCALAB MK II instrument at a pressure greater than10�6 Pa. The general scan and C1s, Ag3d, W4f, and N1s core-level spectrawere recorded with non-monochromatized MgKa radiation (photonenergy=1253.6 eV). The core-level binding energies (BEs) were alignedwith respect to the C1s binding energy (BE) of 285.0 eV. The phase struc-ture and phase purity of the as-synthesized products were examined byX-ray diffraction (XRD) using a MAP18XAHF instrument, with the X-ray diffractometer using CuKa radiation (l =1.5 �) at a scan rate of 0.0482q s�1. The accelerating voltage and applied current were 36 kV and20 mA, respectively (MAC Science, Japan). Nitrogen adsorption–desorp-tion isotherms at the temperature of liquid nitrogen were measured byusing a Micromeritics ASAP 2010 Analyzer (USA) with nitrogen. Thesamples were degassed for 12 h at 77.38 K before the measurements.Pore sizes were calculated by the Barrett–Joyner–Halenda (BJH)method. The magnetic properties were measured using a QuantumDesign Magnetic Properties Measurement System (MPMS) XL-7 Super-conducting Quantum Interference Device (SQUID).

Photocatalytic tests : The photocatalytic activities of the Fe3O4@Bi2O3

composite were evaluated by the photocatalytic decolorization of amodel pollutant (RhB) under visible light. A 500 W Xe lamp was used asa light source to provide visible-light irradiation. The experiments wereperformed at room temperature as follows: In each run, Fe3O4@Bi2O3

catalyst (or commercial Bi2O3; 50 mg) was added into RhB solution(50 mL, 10�5 mol L

�1) and sonicated for 10 min. Before illumination, thesuspension was stirred for 30 min in the dark to reach the adsorption–de-sorption equilibrium between the RhB and the photocatalyst. Then thesuspension was stirred and exposed to visible-light irradiation. The con-centrations of RhB were monitored by checking the absorbance at553 nm during the photodegradation process by using a TU-1901 modelUV/Vis spectrophotometer.

Acknowledgements

This work is supported by the National Science Foundation of China(grant nos. 20871001, 50973001, 20731001), the Major Program of AnhuiProvincial Education Department (grant no. ZD2007004-1), the ResearchFund for the Doctoral Program of Higher Education of China(20070357002), the 211 Project of Anhui University (grant no.2009QN012A), and the Foundation of Key Laboratory of the FunctionalMaterial of Inorganic Chemistry of Anhui Province.

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Received: June 30, 2010Published online: March 8, 2011

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Y. Shen, A. Xie et al.