Template-Assisted Fabrication of Magnetically Responsive Hollow

Oct 15, 2010 - E-mail: [email protected] (M.A.); [email protected] (M.S.)., §. Current address: .... Weiwei Wu. Materials Research Bulletin 2015 72, 176-...
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Template-Assisted Fabrication of Magnetically Responsive Hollow Titania Capsules Mukesh Agrawal,*,† Smrati Gupta,†,§ Andrij Pich,‡, Nikolaos E. Zafeiropoulos,†,^ Jorge Rubio-Retama,†,# Dieter Jehnichen,† and Manfred Stamm*,† † Leibniz-Institut f€ ur Polymerforschung Dresden e.V., Hohe Strasse 6, 01069 Dresden, Germany, and Institut f€ ur Makromolekulare Chemie, Technische Universit€ at Dresden, Zellescher Weg 19, 01069 Dresden, Germany. § Current address: Institut f€ ur Makromolekulare Chemie, Technische Universit€ at Dresden, Zellescher Weg 19, 01069 Dresden, Germany. Current address: Functional and Interactive Polymers, DWI RWTH Aachen University, Pauwelsstr. 8, 52056 Aachen, Germany. ^Current address: Department of Materials Science & Engineering, University of Ioannina, Greece. #Current address: Physical Chemistry II Department, Pharmacy Faculty, Complutense University of Madrid, Ram ony Cajal s/n, 28040 Madrid, Spain )

‡

Received September 2, 2010. Revised Manuscript Received September 30, 2010 This study reports on the fabrication of magnetically responsive hollow titania capsules by confining the superparamagnetic Fe3O4 nanoparticles within a hollow and porous titania (TiO2) shell. The employed protocol involves precipitation of titania shell on the magnetite (Fe3O4) encapsulated polystyrene beads followed by the calcination of resulting composite particles at elevated temperature. Scanning electron microscopy and transmission electron microscopy reveal the presence of a thick, complete but irregular titania shell on the magnetic polystyrene beads after the templating process. Electron energy loss mapping image analysis has been employed to investigate the spatial distribution of titania and magnetite phases of magnetic hollow titania capsules (MHTCs). Magnetic characterization indicates that both titania-coated magnetic polystyrene beads (TMPBs) and MHTCs are superparamagnetic in nature with the saturated magnetizations of 5.6 and 8.1 emu/g, respectively. X-ray diffraction (XRD) analysis reveals that titania shell of these capsules is composed of photoactive anatase phase. Nitrogen adsorption-desorption analysis has been employed to estimate the specific surface area and the average pore diameter of the fabricated hollow structures. Photocatalytic activity of the fabricated MHTCs for the photodegradation of rhodamine 6G dye has been demonstrated and compared with that of bulk titania nanoparticles.

Introduction In recent years, fabrication of hierarchical meso- and nanostructures of functional materials such as metal oxides has attracted considerable interest due to their potential applications in advanced devices and systems.1 A great deal of the research work has been dedicated to the preparation of hierarchical metal oxide nanocomposites in order to improve their properties including mechanical, chemical, structural, optical, and electrical/magnetic ones.2 For example, recently a lot of research efforts have been made on the controlled synthesis of ZnO-TiO2,3 ZnO-SnO24 TiO2-SnO2,5 WO3-TiO2,6 and semiconductor oxides doped with metal ions such as Zn-doped SnO27 and Sn-doped Fe2O3.8 Nevertheless, in most of the cases complicated experimental procedures *To whom correspondence should be addressed. E-mail: agrawal@ ipfdd.de (M.A.); [email protected] (M.S.). (1) Fierro, J. L. G. In Metal Oxides: Chemistry and Applications; CRC Press, Taylor & Francis Group: Boca Raton, FL, 2006; pp 33487-2742. (2) (a) Polarz, S.; Neues, F.; Van den Berg, M. W. E.; Gr€unert, W.; Khodeir, L. J. Am. Chem. Soc. 2005, 127, 12028. (b) Wang, N.; Sun, C.; Zhao, Y.; Zhou, S.; Chen, P.; Jiang, L. J. Mater. Chem. 2008, 33, 3909. (c) Lao, J. Y.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F. J. Mater. Chem. 2004, 4, 770. (d) Agrawal, M.; Gupta, S.; Stamm, M. J. Mater. Chem. DOI: 10.1039/C0JM02631J. (3) Agrawal, M.; Gupta, S.; Pich, A.; Zafeiropoulos, N. E.; Stamm, M. Chem. Mater. 2009, 21, 5343. (4) Cun, W.; Wang, X. M.; Xu, B. Q.; Zhao, J. C.; Mai, B. X.; Peng, P.; Sheng, G. Y.; Fu, H. M. J. Photochem. Photobiol. A 2004, 168, 47. (5) Vinodgopal, K.; Bedja, I.; Kamat, P. V. Chem. Mater. 1996, 8, 2180. (6) Kwon, Y. T.; Song, K. Y.; Lee, W. I.; Choi, G. J.; Do, Y. R. J. Catal. 2000, 191, 192. (7) Saadeddin, I.; Hilal, H. S.; Pecquenard, B.; Marcus, J.; Mansouri, A.; Labrugere, C.; Subramanian, M. A.; Campet, G. Solid State Sci. 2006, 8, 7. (8) Sorescu, M.; Diamandescu, L.; Tarabasanu-Mihaila, D. Mater. Lett. 2005, 59, 22.

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and critical conditions, e.g., high temperature and high pressure, were usually required to obtain these fantastic hierarchical materials. Among various semiconductor oxides, TiO2 and Fe3O4 are well-known and technologically important materials due to excellent electronic, chemical, and optical properties.9 Therefore, incorporating two of these materials into an integrated structure is of great significance because the resulting products may possess improved physical and chemical properties, which should find applications in biomedical fields, in microelectronics, in the production of electrochemistry electrodes, capacitors, solar cells, gas sensors, catalysis, and photocatalysis. Hollow structures (spheres or capsules) represent a special class of materials, which offers interesting prospects in a wide spectrum of potential applications such as protection of lightsensitive components and biologically active agents (proteins, enzymes, or DNAs), chromatography, catalysis, coatings, composites and fillers, waste removal, nanoreactors, and large biomolecular release systems.10 A number of studies are reported into the literature on the fabrication of nanomicrocapsules with customized physicochemical properties by incorporation of one or more functional species such as biomacromolecules,11 lipids,12 (9) (a) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62. (b) Lao, C. S.; Liu, J.; Gao, P. X.; Zhang, L. Y.; Davidovic, D.; Tummala, R.; Wang, Z. L. Nano Lett. 2006, 6, 263. (10) (a) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (b) Hollow and Solid Spheres and Microspheres: Science and Technology Associated With Their Fabrication and Application ; Wilcox, D. L., Berg, M., Bernat, T., Kellerman, D., Cochran, J. K., Eds.; MRS Symposium Proceedings; Materials Research Society: Pittsburgh, 1995; Vol. 372, and references therein. (11) Schuler, C.; Caruso, F. Biomacromolecules 2001, 2, 921.

Published on Web 10/15/2010

DOI: 10.1021/la103504e

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Article Scheme 1. Schematic Diagram of the Fabrication of Magnetic Hollow Titania Capsules

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a high surface area for photocatalytic reactions but also can easily be recovered from reaction media by applying an external magnetic field. Unlike the previously reported metal oxide hollow spheres,22 fabricated MHTCs can also be envisioned to use for biomedical applications including controlled and targeted drug delivery and protection of light-sensitive biologically active agents, owing to the biocompatibility of Fe3O4 and TiO2 phases.23

Experimental Section

photoactive dyes,13 nanoparticles,14 and multivalent ions15 onto the capsule walls or into the capsule interiors. In the present study, we report on fabrication of magnetic hollow titania capsules (MHTCs) by exploiting the well-known template-assisted protocol.16 The synthetic process involves three steps (as shown in Scheme 1): (1) encapsulation of magnetite nanoparticles into polystyrene (PS) beads by miniemulsion polymerization, (2) templating of the titania nanoparticles against magnetic PS beads, and (3) removal of the PS core from the titania-coated magnetic PS beads (TMPBs) by heat treatment. The encapsulated Fe3O4 nanoparticles render magnetic properties to these hollow capsules, making them sensitive to an external magnetic field, whereas the porous anatase titania shell can be exploited for any of the above-mentioned applications. However, a number of studies have been reported on the preparation of titania-coated magnetic particles,17-19 but fabrication of MHTCs encapsulated with superparamagnetic Fe3O4 nanoparticles has rarely been explored. It has been realized that both the size and morphology have an influence on the properties of the semiconductor oxides.20 The unique structures may show new properties and promising applications in many fields.21 In the context of photocatalytic application, the most vital problem associated with titania catalyst is recovery from the reaction media after the photocatalysis process. Owing to the magnetic responsiveness and porous and hollow nature, fabricated MHTCs offer not only (12) Moya, S.; Donath, E.; Sukhorukov, G. B.; Auch, M.; Baumler, H.; Lichtenfeld, H.; M€ohwald, H. Macromolecules 2000, 33, 4538. (13) Sukhorukov, G. B.; D€ahne, L.; Hartmann, J.; Donath, E.; M€ohwald, H. Adv. Mater. 2000, 12, 112. (14) Dai, Z. F.; D€ahne, L.; M€ohwald, H.; Tiersch, B. Angew. Chem., Int. Ed. 2002, 41, 4019. (15) Radtchenko, I. L.; Sukhorukov, G. B.; Leporatti, S.; Khomutov, G. B.; Donath, E.; M€ohwald, H. J. Colloid Interface Sci. 2000, 230, 272. (16) (a) Caruso, F.; Caruso, R. A.; M€ohwald, H. Science 1998, 282, 1111. (b) Agrawal, M.; Pich, A.; Zafeiropoulos, N. E.; Gupta, S.; Pionteck, J.; Simon, F.; Stamm, M. Chem. Mater. 2007, 19, 1845. (c) Agrawal, M.; Pich, A.; Gupta, S.; Zafeiropoulos, N. E.; Simon, P.; Stamm, M. Langmuir 2008, 24, 1013. (d) Agrawal, M.; Pich, A.; Gupta, S.; Zafeiropoulos, N. E.; Formanek, P.; Jehnichen, D.; Stamm, M. Langmuir 2010, 26, 526. (17) Beydoun, D.; Amal, R.; Low, G. K.-C.; McEvoy, S. J. Phys. Chem. B 2000, 104, 4387. (18) Lee, S.; Drwiega, J.; Wu, C. Y.; Mazyck, D.; Sigmund, W. M. Chem. Mater. 2004, 16, 1160. (b) Litter, M. I.; Navio, J. A. J. Photochem. Photobiol. A 1994, 84, 183. (19) (a) Atsuya, T.; Mutsuo, S. U.S. Pat. 703550, 1997. (b) Hiroshi, F.; Yukiko, H.; Michichiro, Y.; Shoichi, A. JP 06154620, 1994. (20) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (21) (a) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (b) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (c) Caruso, F. Adv. Mater. 2001, 13, 11. (22) (a) Wang, W. W.; Zhu, Y. J.; Yang, L. X. Adv. Funct. Mater. 2007, 17, 59. (b) Liu, X. M.; Yin, W. D.; Miao, S. B.; Ji, B. M. Mater. Chem. Phys. 2009, 113, 518. (c) Zhang, X. L.; Qiao, R.; Kim, J. C.; Kang, Y. S. Cryst. Growth Des. 2008, 8, 2609. (d) Sun, X.; Li, Y. Angew. Chem., Int. Ed. 2004, 43, 3827.

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Materials. Ferric chloride (FeCl3) (97%), ferrous chloride tetrahydrate (FeCl2 3 4H2O) (99%), and potassium peroxodisulfate (KPS) (99%) were purchased from Fluka and used as received. Titanium ethoxide [Ti(OEt)4] (85%) and ammonium hydroxide (28-30% NH3 in water) were obtained from Acros and used without additional purification. Acetic acid (CH3COOH) (100%) was purchased from Merck and used as received. Octane (98%), hexadecane (99%), sodium dodecyl sulfate (SDS) (97%), oleic acid (90%), and rhodamine 6G (99%) were obtained from Aldrich and used without additional purification. Acrylic acid and styrene were purchased from Aldrich and distilled under reduced pressure before use. Ultrapure Millipore water was used as the solvent throughout. Synthesis of Magnetite Nanoparticles. Magnetite nanoparticles were synthesized by coprecipitation method from an aqueous Fe3þ/Fe2þ solution (ratio 3:2) and subsequently coated with oleic acid to render them hydrophobicity as described elsewhere.24 In a typical process, 14.6 g of FeCl3 and 12.0 g of FeCl2 3 4H2O were dissolved in 50 mL of distilled water and subsequently added with 40 mL of ammonium hydroxide rapidly. After coprecipitation of magnetite nanoparticles, oleic acid (22.2 wt% of magnetite) was added into the reaction media, and the resulting suspension was heated to 70 C for 30 min. The reaction temperature was increased to 110 C to evaporate water and excess of ammonium hydroxide from reaction media. Finally, the resulting black lumplike residue was cooled to room temperature, washed with distilled water several times, and dried in a vacuum oven. Synthesis of Magnetic Polystyrene Beads. Magnetic nanoparticles synthesized as above were encapsulated into the polystyrene (PS) beads by miniemulsion polymerization as described by Ramirez et al.24 In the first step, a water-based dispersion of magnetite nanoparticles was prepared as follows: 2.5 g of magnetite nanoparticles dispersed into 15 g of octane was added into the aqueous solution of 0.175 g of SDS in 60 g of water. After stirring for 1 h, the mixture was subjected to the sonication twice for 2 min at 90% amplitude with a Branson sonifier W450 digital in an ice-cooled bath. After carefully evaporating the octane at 80 C for 8 h, while adding 2 mL of water at every 30 min, a stable waterbased ferrofluid was obtained. In second step, a styrene miniemulsion was prepared by adding 6.0 g of styrene and 250 mg of hexadecane into the 72 mg of SDS dissolved in 24 g of water. After stirring for 1 h for pre-emulsification, the miniemulsion was obtained by sonication for 2 min at 90% amplitude with a Branson sonifier W450 digital in an ice-cooled bath. Finally, to encapsulate the Fe3O4 nanoparticles into PS beads, styrene miniemulsion and the water-based magnetite dispersions as obtained from above two steps were mixed with each other in a 1:2 weight ratio of magnetite to monomer. The resulting mixture was cosonified twice for 1 min at 50% amplitude in an ice-cooled bath and placed into the double-wall glass reactor, equipped with magnetic stirrer, temperature controller, and water condenser. To start the polymerization process, 20 mg of KPS dissolved in 2 g of water was added into reaction media, and the temperature was increased (23) (a) Han, W.; Allio, B. A.; Foster, D. G.; King, M. R. ACS Nano 2010, 4(1), 174. (b) Shokuhfar, T.; Arumugam, G. K.; Heiden, P. A.; Yassar, R. S.; Friedrich, C. ACS Nano 2009, 3, 3098. (c) Eshed, M.; Irzh, A.; Gedanken, A. Inorg. Chem. 2009, 48, 7066. (d) Mahmoudi, M.; Simchi, A.; Imani, M.; Milani, A. S.; Stroeve, P. J. Phys. Chem. B 2008, 112, 14470. (24) Ramirez, L. P.; Landfester, K. Macromol. Chem. Phys. 2003, 204, 22.

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Agrawal et al. to 80 C. After 40 min, 100 mg of acrylic acid was added into the reaction media to functionalize the PS beads. Finally, magnetic PS beads were obtained after stirring the reaction mixture for 24 h. Fabrication of Magnetic Hollow Titania Capsules. To prepare MHTCs, PS beads as prepared above have been coated with titania nanoparticles and subsequently heat-treated at elevated temperature. In a typical process, 0.4 g of titanium ethoxide was mixed into 10 g of extra pure ethanol, and the mixture was refluxed at 70 C for 2 h. Subsequently, 1 g of aqueous dispersion of magnetic PS beads (18% solid content) as obtained above was injected into the reaction media followed by the addition of 4-5 drops of glacial acetic acid. After reacting for 20 h, TMPBs were cleaned by three cycles of centrifugation/redispersion in each ethanol and water and dried at room temperature in a vacuum oven. Finally, PS cores have been removed from these particles by the two-step calcination process.25 The first step involved the heat treatment at 700 C for 2 h in an inert atmosphere followed by the second one at 450 C for 2 h in air. Photocatalytic Activity. In the photocatalytic experiments, 60 mg of MHTCs powder was added into 70 mL of the aqueous solution of rhodamine 6G (1  10-5 M), and the reaction mixture was stirred in the dark for 1 h to ensure the adsorption/desorption equilibrium of dye with the hollow capsules. Then, solution was exposed to UV radiations from a high-pressure Hg [Xe] lamp at room temperature. The analytic samples exposed to the UV light for different time intervals were taken out from the reaction suspension and filtered off to remove the MHTCs and analyzed by a UV-vis spectrophotometer. Characterization Methods. Scanning electron microscopy (SEM) images were taken on a Gemini microscope (Zeiss, Germany) at an accelerating voltage of 4 kV. Samples were prepared by drying few drops of water dispersion on aluminum support at room temperature. In order to increase the contrast and quality of images, the samples were coated with a thin Au/Pd layer prior to analysis. SEM images were taken by using the secondary electron detector. Transmission electron microscopy (TEM) images were recorded on a Zeiss Omega 912 microscope at 160 kV. Samples were prepared via drying a drop of water dispersion on a carboncoated copper grid. The electron mapping images of “Ti” and “Fe” elements have been acquired by means of TEM via visualizing the inelastically scattered electrons in the ranges of energy loss (ΔEloss) of 390-464 and 660-718 eV, respectively. These energy loss windows cover the L2.3 edges of the respective elements. Thermogravimetric analysis (TGA) was performed on a TGA 7 (Perkin-Elmer) analyzer. Before the measurements, samples were dried under vacuum for ca. 48 h. Subsequently, the samples were heated in platinum crucibles from 25 to 700 C under nitrogen at 5 K/min heating rate. X-ray scattering patterns were scanned by analyzing the powdery samples on a Seifert XRD 3003 T/T diffractometer using Cu KR monochromatic beam (1.54 A˚). IR spectra were recorded with Mattson Instruments Research Series 1 FTIR spectrometer. Prior to analysis, dried samples were mixed with KBr and pressed to form a tablet. For particle size measurements, dynamic light scattering (DLS) analyses were carried out on Zetasizer 2000, Malvern instruments. The magnetic properties were measured with a Quantum Design SQUID magnetometer MPMS-XL at 298 K as a function of the external magnetic field in the range of -10 000 to 10 000 Oe. The magnetometer was calibrated prior to the measurements using metallic Pd as a standard.

Results and Discussion Magnetic Polystyrene Beads. Magnetic PS beads functionalized with acrylic acid have been prepared by the miniemulsion polymerization of styrene (ST) in the presence of water-based dispersion of magnetite nanoparticles. First, water-based dispersion of hydrophobic magnetite aggregates was prepared using (25) Stefik, M.; Lee, J.; Wiesner, U. Chem. Commun. 2009, 2532.

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Figure 1. (a) SEM and (b) TEM images of magnetic PS beads with 30% magnetite content. In (b), dark dots confirm presence of the 5-7 nm size Fe3O4 nanoparticles encapsulated into a PS bead. (c) SEM image of TMPBs. Inset shows a TMPB (scale bar 100 nm). (d) TEM image of a TMPB revealing the presence of a thick and rough titania shell on PS bead.

SDS as surfactant. Subsequently, this dispersion was mixed with the styrene miniemulsion, and polymerization was carried out as discussed in the Experimental Section. Figure 1a shows SEM image of magnetic PS beads, indicating that particles are spherical in shape with 60-70 nm size and own quite smooth surfaces. Supporting Information 1a shows DLS analysis of magnetic PS beads revealing the average size of 70 nm. The apparent aggregation of magnetic PS particles can be ascribed to the drying of their aqueous dispersion during SEM sample preparation and/or physical interaction among themselves, as these particles are functionalized with acrylic acid. The TEM image, shown in Figure 1b, confirms the presence of magnetite nanoparticles inside the PS beads. The dark dots, in light background of the PS, indicate the encapsulated magnetite nanoparticles with 5-7 nm size range. Titania-Coated Magnetic Polystyrene Beads (TMPBs). In order to prepare the titania shell on magnetic PS beads, Ti(OEt)4 salt has been hydrolyzed in the presence of glacial acetic acid. Figure 1c,d illustrates SEM and TEM images of TMPBs. These results reveal the presence of a thick and complete but irregular titania shell on magnetic PS core. After the coating process, particles are no more spherical in shape and have developed the rough surfaces. It is worth mentioning here that for proposed photocatalytic applications the shape of the resulting hollow capsules is not a critical factor. However, they should be porous in nature with an enhanced surface area. TEM results illustrate that particle size has increased from the 60 nm (uncoated magnetic PS beads) to 135 nm after the coating process. It should be noted that size has been calculated from several microscopic images taken from different parts areas of the sample. From Figure 1c, one can observe that TMPBs are aggregated similar to the titania-coated magnetic particles as reported by Beydoun et al.17 However, some nonaggregated particles have also been observed as shown in the inset of Figure 1c. The drying of aqueous dispersion of TMPBs on glass substrate during SEM sample preparation might have caused their apparent aggregation as appearance the single peak around 123 nm in DLS analysis of TMPBs (shown in Supporting Information 1b) reveals their DOI: 10.1021/la103504e

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Figure 2. (a) SEM, (b) TEM, and electron energy loss (c) “Ti” and (d) “Fe” element mapping images of MHTCs. In (a) arrows indicate some open hollow capsules revealing their cavities. In (c) and (d), red color indicates “Ti” and “Fe” enriched areas of MHTCs, respectively.

monomodal size distribution in aqueous media. An increase in average size of magnetic PS beads from 70 to 123 nm after titania coating reveals the thickness of precipitated titania shell as 26.5 nm (which is estimated as half of the increment in total diameter of the particles after the coating process). Deviation from the spherical shape and aggregation of these particles can also be attributed to the small size (60-70 nm) of the employed template particles (magnetic PS beads) and a strong physical interaction among them (as they are functionalized with -COOH group), which caused slight aggregation of magnetic PS beads before the precipitation of titania shell. Earlier studies26 reported on the preparation of hollow titania spheres employ relatively bigger size of the template particles (more than 200 nm). For example, in our previous study,27 we used the PS beads with 540 nm size as template for the deposition of the titania shell, and we could nicely control the spherical shape of obtained hollow structures and shell thickness of the precipitated ceramic shell. In addition, it can also be related to the as high as 40 wt % (as estimated by TGA, described later) titania content of the TMPBs. It is well-known that high reactivity of titanium alkoxide leads to the uncontrolled precipitation of titania nanoparticles on PS beads, especially when deposited titania content is higher than 20%.28 Magnetic Hollow Titania Capsules (MHTCs). In order to turn TMPBs into the MHTCs, samples were calcinated at elevated temperature. Heat treatment of these samples not only transforms the amorphous titania shell into a crystalline one but also removes the PS cores from TMPBs. Figure 2a,b illustrates SEM and TEM images of MHTCs obtained after the heat treatment of TMPBs. SEM image (Figure 2a) reveals that complete, closed, and intact MHTCs have been fabricated after the calcination process. The inset in Figure 2a shows SEM image of a single, (26) (a) Cheng, X.; Chen, M.; Wu, L.; Gu, G. Langmuir 2006, 22, 3858. (b) Wang, P.; Chen, D.; Tang, F. Q. Langmuir 2006, 22, 4832. (27) Agrawal, M.; Pich, A.; Zafeiropoulos, N. E.; Stamm, M. Colloid Polym. Sci. 2008, 286, 593. (28) Hanprasopwattana, A.; Srinivasan, S.; Sault, A. G.; Datye, A. K. Langmuir 1996, 12, 3173.

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close, and intact hollow capsule. However a small fraction of open capsules can also be observed (shown by arrow in Figure 2a), revealing their cavities and thus confirming their hollow structure. Formation of these broken or open particles can be attributed to the evolution of degradation gases during calcination process and to the application of a high vacuum in SEM chamber during their analysis.29A plot of the intensity as a function of the size of MHTCs in DLS analysis (shown in Supporting Information 1c) reveals a single peak around 97.5 nm, suggesting their monomodal size distribution in aqueous media. A decrease in average size from TMPBs to MHTCs after the calcination process can be attributed to the sintering contraction of the amorphous titania into the crystalline one. It should be noted here that owing to the polycrystalline nature of the precipitated thick titania shell, one cannot see the encapsulated magnetic nanoparticles in the TEM image of MHTCs shown in Figure 2b. However, in order to confirm the presence of magnetite particles in these MHTCs, samples have been analyzed by electron energy loss mapping analysis, and results are shown in Figure 2c, d. In this analysis, when the electron beam is incident into a specimen, a part of the electrons is inelastically scattered and loses a fraction of the energy. The distribution of element in specimen is clarified by selecting and imaging the electrons with a specific energy loss. A detailed description of the phenomena involved is reported in the literature.30 The red areas shown in Figure 2c,d indicate “Ti” and “Fe” enriched areas of MHTCs. The appearance of the red dots in the “Fe” mapping image confirms the presence of magnetite particles encapsulated into the titania shell. Similarly, the “Ti” mapping image proves presence of a continuous titania shell of MHTCs. Figure 3a displays the X-ray diffraction pattern of MHTCs obtained after the calcination of TMPBs. The diffraction peaks appeared in this XRD pattern can be indexed to a hexagonal anatase phase of TiO2 with calculated cell constants of a = b = 3.785 A˚ and c = 9.514 A˚. The peak intensities and peak positions are in good agreement with expected literature values (JCPDS File No. 21-1272).31 The indices and spacings for the diffraction rings are (101), 0.353 nm; (004), 0.236 nm; (200), 0.189 nm; (105 þ 211), 0.167 nm; (204), 0.151 nm; and (116), 0.134 nm, respectively. In addition, reflection peaks at the 2θ values of 30.09, 35.42, 43.05, 56.94, and 62.51 can be indexed with the characteristic peaks from magnetite (Fe3O4) phase. These results are consistent with the X-ray database JCPDS-IC 72-2303.32 The absence of characteristic peaks from pseudobrookite (Fe3TiO5) and Fe2O3 phases in XRD pattern of MHTCs confirms that employed calcination conditions do not only limit the unwanted chemical interactions between these phases as reported in earlier studies17,18 but also has the potential to protect the encapsulated magnetite nanoparticles against oxidation. Recently, Xuan et al.33 reported fabrication of Fe3O4/TiO2 spheres by solvothermal process of polymer/Fe3O4-TiO2 composite particles. They deposited Fe3O4 and TiO2 layers consecutively on polymer core, which were in direct contact with each other and converted these core-shell particles into the hollow Fe3O4-TiO2 sphere by dissolution of organic core in the presence of tetrahydrofuran followed by the solvothermal treatment at 160 C in ethanol. In comparison to (29) Wang, L.; Sasaki, T.; Ebina, Y.; Kurashima, K.; Watanabe, M. Chem. Mater. 2002, 14, 4827. (30) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1999, 15, 4447– 4453. (31) Bryan, J. D.; Heald, S. M.; Chambers, S. A.; Gamelin, D. R. J. Am. Chem. Soc. 2004, 126, 11640. (32) Turgeman, R.; Gedanken, A. Cryst. Growth Des. 2006, 6, 2260. (33) Xuan, S.; Jiang, W.; Gong, X.; Hu, Y.; Chen, Z. J. Phys. Chem. C 2009, 113, 553.

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Figure 3. (a) XRD pattern of MHTCs (where M, A, and R indicate reflection peaks from magnetite, anatase, and rutile phases) and (b) magnetization curves for TMPBs and MHTCs measured at 298 K. Inset shows that synthesized MHTCs can easily be attracted from their water dispersion by applying an external magnetic field.

this method, the described approach exploits encapsulation of magnetic nanoparticles into the polymer core before coating with TiO2 rather than coating the template particles with both the metal oxides consecutively. It avoids possibilities of unwanted chemical interactions between Fe3O4 and titania phases as well as diffusion of Fe ions into the titania at higher temperature as both metal oxides are not in contact with each other. In presented study, employed approach allows to heat treat the particles up to 700 C and does not need extra core dissolution process of coreshell particles. It is well-known that better quality of anatase phase can be produced from amorphous titania by calcinating at relatively higher temperature.1 In addition, it should also be noted that size of fabricated magnetic titania capsules is significantly lower than Fe3O4/TiO2 hollow spheres reported in the literature.33 The lower the size of capsules, the higher will be the surface area. A close inspection of XRD pattern reveals that a small fraction of amorphous titania has been transformed into the rutile phase also as one prominent reflection peak of this crystalline structure at 27.42 can be observed.17 Above 500 C, this phase transformation can be expected due to the metastability of anatase phase.34 Moreover, the presence of the Fe3O4 nanoparticles has also been reported to induce anatase to rutile transformation.17 The rutile phase of the titania is commonly thought to have low photocatalytic activity or even to be inactive at all.35 However, titania photocatalyst consisting of a mixture of anatase (34) Campbell, L. K.; Na, B. K.; Ko, E. I. Chem. Mater. 1992, 4, 1329. (35) Agrios, A. G.; Gray, K. A.; Weitz, E. Langmuir 2003, 19, 1402. (36) (a) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (b) Deng, X. Y.; Yue, Y. H.; Gao, Z. Appl. Catal., B 2002, 39, 135.

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and rutile phases has been found to be more active than either of the two pure crystalline phases for many kinds of photocatalytic reactions.36 Thus, the presence of a small amount of the rutile phase in the shell of MHTCs may help in improving the photocatalytic activities of these nanostructures. The appearance of a noticeable background in this XRD pattern reveals the presence of amorphous titania phase still in this sample.37 This could be attributed to the fact that the presence of PS beads obstructed the coagulation of TiO2 nanoparticles, thus resulting in a more restricted/imperfect crystallization of TiO2 shell during the calcination process. Figure 3b illustrates magnetization curves for the TMPBs (curve “a”) and MHTCs (curve “b”), taken at 298 K. One can observe that both the samples show hysteresis with very low coercivity values (12.61 Oe for TMPBs and 13.79 Oe for MHTCs) and hence exhibit superparamagnetic behavior, desirable for their easy and fast manipulation by an external magnetic field. It is important to point out that the magnetization values are given in emu/g, i.e., considering the total weight of PS core, titania shell, and magnetite nanoparticles. The magnetization saturation values for TMPBs and MHTCs have been estimated as 5.6 and 8.1 emu/g, respectively, which are significantly below the 92 emu/g value of pure Fe3O4 particles.38 The low magnetization is most likely attributed to the presence of nonmagnetic components in the samples. In addition, it can also be correlated with the small size (5-7 nm) of encapsulated Fe3O4 nanoparticles as it is well-known that magnetic behavior of Fe3O4 is very sensitive to the particle size.39 Recently, similar results have been reported by Wu et al.40 for magnetic hollow silica particles and Cheng et al.41 for the Fe3O4 and Fe3C (iron carbide) nanoparticles embedded into the carbon nanotubes matrix. The increase in saturation magnetization value after the heat treatment can be attributed to the weight loss of the particles due to elimination of PS core and condensation of the amorphous titania shell. In comparison to the magnetic hollow silica spheres reported by Zheng et al.42 and Arruebo et al.,43 these MHTCs are superparamagnetic in nature and possess a 2-3-fold higher saturation magnetization value. The superparamagnetic nature of obtained MHTCs suggests that they would not tend to be agglomerated heavily at least by magnetic interactions among themselves in the absence of external magnetic field and hence would offer a relatively large surface area. The inset of Figure 3b illustrates that MHTCs possess enough magnetic activity to be manipulated in water solution by an external magnetic field. The MHTCs required a few seconds to be aggregated on the wall of a glass bottle under the magnetic field of 1 T. Supporting Information 2 presents thermogravimetric analysis (TGA) scans of uncoated PS beads and TMPBs. These data reveal encapsulation of 29.57 wt % magnetite nanoparticles in PS beads during the miniemulsion polymerization (scan 1). In addition, scan 2 illustrates that fabricated TMPBs have titania and magnetite contents as 39.9 and 17.8 wt %, respectively. In both cases, the weight loss stage below 350 C is the result of the evaporation of physically absorbed water and residual solvent in samples. The major weight loss between 360 and 475 C can be (37) Zhang, M.; Gao, G.; Li, C.-Q.; Liu, F.-Q. Langmuir 2004, 20, 1420. (38) Han, D. H.; Wang, J. P.; Luo, H. L. J. Magn. Magn. Mater. 1994, 136, 176. (39) (a) Bica, I. Mater. Sci. Eng., B 1999, 68, 5. (b) Goya, G. F.; Berquo, T. S.; Fonseca, F. C.; Morales, M. P. J. Appl. Phys. 2003, 94, 3520. (40) Wu, W.; Caruntu, D.; Martin, A.; Yu, M. H.; O’Connor, C. J.; Zhou, W. L.; Chen, J.-F. J. Magn. Magn. Mater. 2007, 311, 578. (41) Cheng, J. P.; Zhang, X. B.; Yi, G. F.; Ye, Y.; Xia, M. S. J. Alloys Compd. 2008, 455, 5. (42) Zheng, T.; Pang, J.; Tan, G.; He, J.; McPherson, G. L.; Lu, Y.; John, V. T.; Zhan, J. Langmuir 2007, 23, 5143. (43) Arruebo, M.; Galan, M.; Navascues, N.; Tellez, C.; Marquina, C.; Ibarra, M. R.; Santamaria, J. Chem. Mater. 2006, 18, 1911.

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Figure 4. IR spectra of magnetic PS beads before (solid line) and after (dotted line) the titania coating and MHTCs (dashed line).

attributed to the decomposition of PS. It is noteworthy that both titania and magnetic contents of these capsules are significantly higher than Fe3O4/TiO2 hollow spheres reported in the literature.33 Needless to mention that titania and magnetic contents are critical to the performance of these nanostructures. The shell wall of the hollow capsules with a high titania content would be thick enough to render the mechanical robustness to MHTCs. We have observed this effect in our previous study on fabrication of hollow tantala spheres.16c When employed concentration of the tantalum oxide precursor was lower than a critical value, broken hollow tantala spheres have been obtained. On the other hand, MHTCs with higher magnetite content would be relatively fast in response to the external magnetic field and may be manipulated even with a relatively weaker magnetic strength. To ensure the complete removal of PS core from MHTCs after the calcination process of TMPBs, samples were analyzed by IR spectroscopy. Figure 4 illustrates IR spectra of magnetic PS beads before and after the titania coating as well as of MHTCs. In the case of magnetic PS beads (solid line), one can observe the presence of a C-H stretching band at 3000 cm-1, aromatic C-C stretching band at 1470 cm-1, C-H out-of-plane band at 765 cm-1, and aromatic C-C out-of-plane band at 700 cm-1. Additionally, aromatic overtones can be observed in the range of 1700-2000 cm-1. All these peaks are characteristic peaks of the PS core. A strong band at 620 cm-1 further confirms the presence of Fe3O4 nanoparticles in PS beads as it can be attributed to the vibrations from crystalline lattice of nanocrystalline magnetite.44 After the titania coating on magnetic PS beads (dotted line), the appearance of a broad band in the range of 2800-3500 cm-1 can be associated with the stretching vibrations of the hydrogenbonded -OH groups of titanium hydroxide and adsorbed water molecules.45 In the case of MHTCs (dashed line), one can observe that all the characteristic peaks from PS have disappeared, suggesting the complete removal of PS core from TMPBs during heat treatment. A strong band at 520-720 nm in the same spectrum can be ascribed to the vibrations from the crystalline lattices of magnetite as well as anatase and rutile phases.46 We believe that the physical interaction between -COOH groups, located on the surface of colloidal templates and titania (44) (a) Yu, L.; Zheng, L.; Yang, J. Mater. Chem. Phys. 2000, 66, 6. (b) Grzeta, B.; Ristic, M.; Nowik, I.; Music, S. J. Alloys Compd. 2002, 334, 304. (45) (a) Morterra, C. J. Chem. Soc., Faraday Trans. 1988, 84, 1617. (b) Primet, M.; Pichat, P.; Mathieu, M. V. J. Phys. Chem. 1971, 75, 1216. (46) Bobovich, Y. S.; Arkhipenko, D. K. Opt. Spektrosk. 1964, 17, 755.

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precursors (formed in reaction media), is the driving force for precipitation of titania layer on magnetic polystyrene beads. As reported by Park et al.,47 preparation of TiO2 nanoparticles from titanium alkoxides, i.e. Ti(OR)4, is composed of two stages. The first stage involves hydrolysis of alkoxide groups leading to the formation of titania precursors, i.e., Ti(OH)x(OR)y, where x and y are the number of hydroxyl and alkoxy groups present on titanium atom in hydrolyzed product of titanium alkoxide and (x þ y = 4). The second stage is a condensation-polymerization reaction where Ti-O-Ti bonds are formed from Ti(OH)x(OR)y, resulting in creation of a three-dimensional titania shell, which precipitates out of the reaction solution. It has been reported in the literature48 that hydrolysis of titanium alkoxides in the presence of functionalized templates leads to the precipitation of titania nanoparticles preferentially on the template surface because of the interaction between hydrolyzed monomer Ti(OH)x(OR)y and functionality of the template particles. Since magnetic PS beads employed as template for precipitation of titania shell are functionalized with acrylic acid, therefore it can be presumed that the titania precursor Ti(OH)x(OR)y is captured by template particles from reaction media; subsequently, the condensation-polymerization process of adsorbed Ti(OH)x(OR)y species leads to the preparation of a titania layer on the PS core. Similar observations have been made by Fu et al.49 and Hanprasopwattana et al.28 while deposition of titania nanoparticles on silica spheres and Ocana et al.50 on ZnO. Surface area and pore size distribution are important attributes of the hollow structures. These attributes are measured by the use of nitrogen adsorption/desorption isotherms at liquid nitrogen temperature and relative pressures (P/P0) ranging from 0.00 to 1.00.51 Supporting Information 3a reveals that MHTCs follow a typical IV type physisorption isotherm.52 An increase in nitrogen uptake at high relative pressure P/P0 = 0.9-1.0 and a wide hysteresis loop can be observed, which suggest that fabricated particles are mesoporous in nature.53 The pore diameter distribution curve and Brunauer-Emmett-Teller (BET) plot are shown in Supporting Information 3b and 3c, respectively. Following the BET method,51 the specific surface area and the pore diameter for these hollow structures were estimated as 22.95 m2/g and 4-5 nm, respectively. The presence of the small peak at 20 nm in Supporting Information 3b may be ascribed to the holes in titania shell of broken hollow spheres. The breakage of a small fraction of the MHTCs might have occurred while handling the samples during analysis. It has been observed in previous studies as well on fabrication of ceramic hollow spheres.29 To demonstrate the photocatalytic activity of MHTCs, a timedependent photodegradation of Rhodamine 6G dye has been investigated in the presence of fabricated MHTCs. The progress in photodegradation with irradiation time was monitored by analyzing the UV-vis spectra of reaction solutions, taken out at different time intervals, and the results are shown in Figure 5. The intensity of the characteristic absorption peak has been found to decrease gradually with increasing the exposure time, indicating the photocatalytic degradation of the dye (Figure 5a). These (47) Park, J.; Myoung, J.; Kyong, J.; Kim, H. Bull. Korean Chem. Soc. 2003, 24, 671. (48) Zhang, D.; Yang, D.; Zhang, H.; Lu, C.; Qi, L. Chem. Mater. 2006, 18, 3477. (49) Fu, X.; Qutubuddin, S. Colloids Surf., A 2001, 178, 151. (50) Ocana, M.; Hsu, W. P.; Matijevic, E. Langmuir 1991, 7, 2911. (51) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (52) Sing, K. S.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pieritti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (53) Wu, X.; Tian, Y.; Cui, Y.; Wei, L.; Wang, Q.; Chen, Y. J. Phys. Chem. C 2007, 111, 9704.

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for the precipitation of titania shell on magnetic PS beads and fabrication of MHTCs, respectively. Moreover, it is worth mentioning here that rate of the photodegradation of Rhodamine 6G, observed in the presence of fabricated MHTCs, is significantly higher than previously reported ones in the literature with hollow titania spheres and carbon-modified TiO2 nanoparticles.33,54 A high rate of the photodegradation in the presence of these MHTCs can be ascribed to their porous nature and higher surface area, offered to the reactants. These features of hollow capsules allow the reactants to access the large number of active sites of the catalyst, leading to the higher rate of the degradation. In addition, carefully employed fabrication and calcination conditions allowed us to avoid the formation of the mixed compounds from titania and magnetic oxides like pseudobrookite (Fe3TiO5) (as evident from XRD analysis). It has been reported that presence of these mixed compounds acts as impurities and leads to the decreases in photocatalytic activities.18

Conclusions

Figure 5. (a) UV-vis spectra of the aqueous solutions of Rhodamine 6G dye after UV irradiation for different time periods in the presence of MHTCs. (b) Photodegradation of the rhodamine 6G dye in the absence of the catalyst and the presence of TiO2 nanoparticles and MHTCs.

results succinctly demonstrate that MHTCs can potentially be used as photocatalyst for the degradation of organic dyes. In order to evidence the effect of morphology and porosity, progress of the photodegradation of rhodamine 6G in the presence of bulk TiO2 powder and MHTCs have been compared, and the results are shown in Figure 5b. One can observe that MHTCs offer a higher rate of photodegradation as compared to the TiO2 nanoparticles. In order to have a fair comparison, bulk TiO2 nanoparticles employed in these experiments have been synthesized and calcinated under the same reaction conditions as used (54) Shi, X.; Wang, S.; Dong, X.; Zhang, Q. J. Hazard. Mater. 2009, 167, 692.

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We demonstrated the fabrication of MHTCs exploiting the template-assisted protocol. Magnetite nanoparticles have been encapsulated into the PS colloidal particles by miniemulsion polymerization of styrene in the presence of water-based dispersion of hydrophobic magnetite nanoparticles. Resulting magnetic PS beads have been used as a template to achieve the titaniacoated core-shell particles. Calcination of these particles at elevated temperature resulted in the fabrication of MHTCs composed of superparamagnetic magnetite nanoparticles confined within a thick and porous anatase titania shell. These MHTCs can readily be used in a wide range of potential applications including as photocatalyst. The described approach is versatile in nature and can be exploited for fabrication of a number of metal oxide nanocomposites. Acknowledgment. The authors are thankful to Mrs. Ellen Kern, Mr. Alex Mensch, Dr. Petr Formanek, Dr. R€udiger H€assler, and Dr. Victoria Albrecht for helping out in characterization of the samples. Prof. Lopez E. Cabarcos (University of Madrid) is acknowledged for providing the facilities for magnetic measurements. We are also thankful to Georgi Paschew (Technical University, Dresden) for helping in photocatalytic measurements. Supporting Information Available: Dynamic light scattering (DLS), thermogravimetric (TG), and Brunauer-EmmettTeller (BET) analyses. This material is available free of charge via the Internet at http://pubs.acs.org.

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