Synthesis of Novel Tantalum Oxide Sub-micrometer Hollow Spheres

Jan 3, 2008 - Mukesh Agrawal , Smrati Gupta , Andrij Pich , Nikolaos E. Zafeiropoulos , Jorge Rubio-Retama , Dieter Jehnichen , and Manfred Stamm...
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Langmuir 2008, 24, 1013-1018

1013

Synthesis of Novel Tantalum Oxide Sub-micrometer Hollow Spheres with Tailored Shell Thickness Mukesh Agrawal,*,† Andrij Pich,*,‡ Smrati Gupta,† Nikolaos E. Zafeiropoulos,† Paul Simon,§ and Manfred Stamm† Leibniz-Institut fu¨r Polymerforschung Dresden e.V., Hohe Strasse 6, 01069 Dresden, Germany, Technische UniVersita¨t Dresden, Institut fu¨r Makromolekulare Chemie und Textilchemie, Mommsenstrasse 4, 01062 Dresden, Germany, Max-Planck-Institut fu¨r Chemische Physik Fester Stoffe, No¨thnitzer Strasse 40, 01187 Dresden, Germany ReceiVed August 13, 2007. In Final Form: October 25, 2007 Sub-micrometer-sized hollow tantalum oxide (Ta2O5) spheres with tunable shell thickness and void size have been fabricated exploiting β-diketone-functionalized polystyrene (PS) beads as sacrificial templates in a sol-gel process. First, a controlled precipitation of Ta2O5 nanoparticles was carried out on the template surface by hydrolyzing tantalum ethoxide (Ta(OEt)5) at room temperature, and subsequently, the polymer core was removed either via chemical treatment with toluene or calcination at 650 °C. The thickness of the tantala shell precipitated on the PS core during the coating process was tuned between 100 and 142 nm by varying the concentration of tantala precursor in the reaction media. The obtained Ta2O5-coated PS particles and hollow microspheres were characterized by scanning electron microscopy, transmission electron microscopy, infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis. Due to the unique optical and dielectric properties, these nanostructured materials are envisaged to be used in applications such as novel building blocks for the fabrication of advanced materials, surface coatings, catalysts, and drug delivery systems.

Introduction In recent years, there has been an increasing fascination with the fabrication of hollow spheres with nanometer-to-micrometer dimensions because of their potential use in a wide range of applications.1 For example, they can be effectively used as extremely small containers for encapsulationsa well-known process that is actively explored for applications in catalysis, delivery of drugs, development of artificial cells, or protection of biologically active agents (such as proteins, enzymes, or DNA). When used as fillers, pigments, or coatings, hollow spheres may also provide some immediate advantages over their bulk counterparts because of their relatively low densities.2 Traditionally, hollow spheres are fabricated by coating a sacrificial template with a shell of desired materials and subsequently removing it via selective etching with a solvent or calcination in air. Due to the simplicity and ease of control over particle size and shell thickness, sol-gel3 and layer-by-layer (LBL) approaches4 have been frequently employed for the coating of colloidal templates. The inner diameter of hollow spheres is solely determined by the dimensions of the template. A variety of sacrificial templates * To whom correspondence should be addressed. E-mail: agrawal@ ipfdd.de (M.A.); [email protected] (A.P.). † Leibniz-Institut fu ¨ r Polymerforschung Dresden e. V. ‡ Technische Universita ¨ t Dresden. § Max-Planck-Institut fu ¨ r Chemische Physik Fester Stoffe. (1) (a) 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, Vol. 372, Materials Research Society: Pittsburgh, 1995 and references therein. (b) Jeong, U.; Wang, Y.; Ibisate, M.; Xia, Y. AdV. Funct. Mater. 2005, 15, 19071921. (c) Mathlowitz, E.; Jacob, J. S.; Jong, Y. S.; Carino, G. P.; Chickering, D. E.; Chaturvedl, P.; Santos, C. A.; Vijayaraghavan, K.; Montgomery, S.; Bassett, M.; Morrell, C. Nature 1997, 386, 410-414. (d) Mandal, T. K.; Fleming, M. S.; Walt, D. R. Chem. Mater. 2000, 12, 3481. (e) Huang, H.; Remsen, E. E. J. Am. Chem. Soc. 1999, 121, 3805-3806. (2) Ohmori, M.; Matijevic, E. J. Colloid Interface Sci. 1992, 150, 594-598. (3) Tissot, I.; Reymond, J. P.; Lefebvre, F.; Bourgeat-lami, E. Chem. Mater. 2002, 14, 1325. (4) (a) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (b) Caruso, F. AdV. Mater. 2001, 13, 740.

including silica spheres,5,6 polystyrene (PS) latex spheres,7-9 resin spheres,10 vesicles,11 liquid droplets,12 and miniemulsion13 or microemulsion14 droplets has been exploited to achieve the ceramic or polymer hollow spheres. Apart from colloidal templating, there are a number of approaches including nozzlereactor systems,15 emulsion or interfacial polymerization/chemical reaction strategies,16 surface living polymerization process,17 and self-assembly techniques,18 which have also been successfully used to fabricate the hollow spheres. So far a variety of metal oxide hollow spheres such as SiO2, TiO2, ZnO, Ga2O3, etc. has been prepared.4a,19 To the best of our knowledge, investigations (5) Kim, S.-W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (6) Velikov, K. P.; van Blaaderen, A. Langmuir 2001, 17, 4779. (7) Rhodes, K. H.; Davis, S. A.; Caruso, F.; Zhang, B.; Mann, S. Chem. Mater. 2000, 12, 2832. (8) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400. (9) Breen, M. L.; Donsmore, A. D.; Pink, R. H.; Qadri, S. Q.; Ratna, B. R. Langmuir 2001, 17, 903. (10) Bourlinos, A. B.; Karakassides, M. A.; Petridis, D. Chem. Commun. 2001, 1518. (11) Schmidt, H. T.; Ostafin, A. E. AdV. Mater. 2002, 14, 532. (12) Huang, J.; Xie, Y.; Li, B.; Liu, Y.; Qian, Y.; Zhang, S. AdV. Mater. 2000, 12, 808. (13) Putlitz, B.; Landfester, K.; Fischer, H.; Antonietti, M. AdV. Mater. 2001, 13, 500. (14) Walsh, D.; Lebeau, B.; Mann, S. AdV. Mater. 1999, 11, 324. (15) (a) Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Nature 1999, 398, 223. (b) Iida, M.; Sasaki, T.; Watanable, M. Chem. Mater. 1998, 10, 3780. (16) (a) Bruinsma, P. J.; Kim, A. Y.; Liu, J.; Baskaran, S. Chem. Mater. 1997, 9, 2507. (b) Rana, R. K.; Mastai, Y.; Gedanken, A. AdV. Mater. 2002, 14, 1414. (17) (a) Zhou, Q.; Wang, S.; Fan, X.; Advincula, R. C. Langmuir 2002, 18, 3324. (b) Perruchot, C.; Khan, M. A.; Kamitsi, A.; Armes, S. P.; von Werne, T.; Patten, T. E. Langmuir 2001, 17, 4479. (c) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 2001, 123, 7497. (18) (a) Discher, B. M.; Won, Y. Y.; Ege, D. S.; Lee, J. C. M.; Battes, F. S.; Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143. (b) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (c) Wendland, M. S.; Zimmerman, S. C. J. Am. Chem. Soc. 1999, 121, 1389. (19) (a) Zhong, Z.; Yin, Y.; Gates, B.; Xia, Y. AdV. Mater. 2000, 12, 206-209. (b) Yan, C.; Xue, D. J. Phys. Chem. B 2006, 110, 7102-7106. (c) Sun, X.; Li, Y. Angew. Chem. 2004, 116, 3915-3919; Angew. Chem. Int. Ed. 2004, 43, 38273831.

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on the preparation and characterization of closed hollow Ta2O5 spheres have never been reported. Tantalum oxide has many outstanding properties such as wide band gap (∼3.9 eV), photoactivity in near-UV areas,20 chemical resistance (except to hydrofluoric acid), good conductivity, a high melting point, ductility, mechanical strength, biocompatibility,21 etc. Therefore, it is being widely studied and used in the construction of furnaces, chemical reactors, surgical instruments, capacitors,22 barrier layers in integrated circuits,23 and more. It is presently thought to be the most promising capacitor material to be used in the near future in dynamic random access memories (DRAM).24 Moreover, tantalum oxide is a very good catalyst for a variety of chemical reactions.25 Because of such interesting properties and its growing importance in industries, it is desirable to explore the fabrication of hollow Ta2O5 spheres. Herein, we report on a simple, fast, and facile route to the mesoscale hollow Ta2O5 spheres with tunable void size and shell thickness. In this protocol, the first step involves coating of the functionalized PS latex particles with Ta2O5 nanoparticles, exploiting the well-known colloidal templating approach. Subsequently obtained PS-Ta2O5 composite particles are turned into spherical hollow structures via either calcination or the core dissolution method. The obtained composite particles and hollow spheres can be used for a wide range of applications such as building blocks for the fabrication of tantalum-based sensors, UV detectors, electronic filters, and photonic crystals because of their simple synthesis, nanoscopic dimensions, narrow particle size distribution, and flexible adjustment of the size and Ta2O5 content. Biocompatibility of tantala makes these hollow structures use as drug delivery carriers and bioreactors. In addition, hydroxyl groups present on the surface of these nanostructures provide an ideal anchorage for covalent bonding of specific ligands (e.g., streptavidin, antibodies, etc.) Moreover, these nanostructured materials with fascinating morphology and architecture are very promising for applications in catalysis because they enable a fine dispersion and stabilization of small nanoparticles and provide access to a larger number of active sites than the corresponding bulk components.

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Materials. Acetoacetoxyethyl methacrylate (AAEM) (97%) and sodium peroxydisulphate (SPDS) (97%) were purchased from Aldrich. Styrene (ST) was obtained from Fluka. Monomers were passed through an inhibitor removal column and then vacuumdistilled under nitrogen before use. Tantalum ethoxide (99.98%) and ethanol (99%) were obtained from Aldrich and used as received. Glacial acetic acid and toluene (99.9%) were purchased from Merck and Acros, respectively, and were used without additional purification. Millipore water was employed as the polymerization medium. Synthesis of PS Particles. Monodisperse and β-diketonefunctionalized PS particles with an average diameter of 540 nm were synthesized by surfactant-free emulsion polymerization as reported earlier by our group.26 In a typical process, 170 g of water and appropriate amounts of ST (19 g) and AAEM (1 g) were

introduced into a double-wall glass reactor equipped with mechanical stirrer, reflux condenser, nitrogen inlet, and temperature controller. After deoxygenating the reaction mixture via bubbling the nitrogen gas for 30 min, the temperature was increased to 70 °C and aq SPDS solution (0.3 g in 10 g water) was added into it to start the polymerization process. The reaction was allowed to proceed for another 24 h, and PS latex particles were obtained as a stable dispersion in water with ca. 10% solid content. Synthesis of PS-Tantala Composite Particles. Tantala-coated PS particles were prepared by hydrolyzing tantalum ethoxide in ethanol at 28 °C in presence of functionalized PS beads. In a typical process, the tantala sol was prepared by mixing the variable amounts of tantalum ethoxide (0.08 to 0.20 g) into 5 g of extra pure ethanol followed by addition of 0.05 g of glacial acetic acid. The overall concentration of Ta(OEt)5 was varied from 0.2 to 0.5 mM. The reaction mixture was thoroughly stirred at room temperature for 5 min under nitrogen blanket. Subsequently, 1 g of latex (containing 10 wt % PS particles) mixed with 5 g of ethanol was added dropwise into the reaction media. After reacting for 6 h at 28 °C, PS-Ta2O5 composite particles were cleaned by two centrifugation/redispersion cycles in each ethanol and water, respectively. Synthesis of Hollow Tantala Spheres. Once the core shell particles were analyzed and satisfactory coverage of template surface was confirmed, the samples were either treated with toluene or calcinated at elevated temperature to produce the hollow tantala spheres. In the core dissolution method, a dispersion of composite particles in ethanol was added into an excess of toluene.32 After 10 h, the obtained hollow spheres were centrifuged two times at 8000 rpm for 8 min by redispersing in toluene in order to ensure the complete removal of the PS core. Finally, the obtained hollow tantala spheres were transferred into ethanol via one more centrifugation/ redispersion cycle for further characterization. For calcination, a few milligrams of PS-tantala powder was placed into a porcelain cup and heated inside a furnace up to 650 °C under air at a 5 K/min heating rate. After keeping for 5 h at this temperature, the sample was cooled down to room temperature at a cooling rate of 10 K/min.33 Particle Characterization and Instrumentation. 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 a few drops of water dispersion on an 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. 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 carbon-coated copper grid. The high-resolution TEM (HRTEM) images were taken with a field emission microscope from FEI (Eindhoven, NL) CM200 FEG/ST-Lorentz, and resulting images were analyzed with Digital Micrograph software (Gatan, USA). 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. Thermogravimetric analysis (TGA) was performed on a TGA 7 (Perkin-Elmer) analyzer. Before the measurement, samples were dried under vacuum for ca. 48 h. Subsequently, the samples were heated in platinum crucibles in a 25-700 °C temperature range with nitrogen as carrier gas at 5 K/min heating rate. XRD spectra were taken by analyzing the powdery samples on a HZG 4/A-2 (Seifert FPM) X-ray diffractometer using Cu KR monochromatic beam (1.54 Å).

(20) Wang, C.; Geng, A.; Guo, Y.; Jiang, S.; Qu, X.; Li, L. J. Colloid Interface Sci. 2006 301, 236-247. (21) Findlay, D. M.; Welldon, K.; Atkins, G. J.; Howie, D. W.; Zannettino, A. C. W.; Bobyn, D. Biomaterials 2004, 25, 2215-2227. (22) Chaneliere, C.; Autran, J. L.; Devine, R. A. B.; Balland, B. Mater. Sci. Eng. R 1998, 22, 269-322. (23) Kuiry, S. C.; Seal, S.; Fei, W.; Ramsdell, J.; Desai, V. H.; Li, Y.; Babu, S. V.; Wood, B. J. Electrochem. Soc. 2003, 150, C36-C43. (24) Ezhilvavan, S.; Tseng, T. Appl. Phys. Lett. 1999, 74, 2477. (25) (a) Chen, Y.; Fierro, J. L. G.; Tanaka, T.; Wachs, I. E. J. Phys. Chem. B 2003, 107, 5243-5250. (b) Budoace, S.; Cimpeanu, V.; Parvulescu, V.; Centeno, M. A.; Grange, P.; Parvulescu, V. I. Catal. Today 2004, 91-92, 219-223. (c) Yue, C.; Trudeau, M.; Antonelli, D. Chem. Commun. 2006, 1918-1920. (26) Pich, A.; Bhattacharya, S.; Adler, H.-J. P. Polymer, 2005, 46, 1077.

PS Core Particles. PS particles functionalized with β-diketone groups have been prepared via surfactant-free copolymerization of ST and AAEM (described in the Experimental Section).26 Due to its hydrophilic character, AAEM locates predominantly on the surface of emulsion droplets during the copolymerization process and hence stabilizes the obtained colloidal system, as well as provides the functionality to PS beads. In addition, this system offers an effective control over the size of PS beads by varying the amount of AAEM co-monomer in the reaction

Experimental Section

Results and Discussion

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Figure 1. SEM images of (a) PS particles and PS-Ta2O5 composite particles prepared at different Ta(OEt)5 concentrations: (b) 0.2 mM (31.45 wt % Ta2O5); (c) 0.3 mM (41.87 wt % Ta2O5); (d) 0.4 mM (48.98 wt % Ta2O5); and (e) 0.5 mM (55.52 wt % Ta2O5).

media.26 Figure 1a shows SEM image of PS beads used in the present study, indicating that particles are monodisperse in size with quite smooth surface. The average diameter of the PS particles used in present study was 540 nm. The same PS beads have been successfully employed in our previous investigations for the deposition of zinc oxide,27 titanium dioxide28 maghemite,29 and zinc sulfide30 nanoparticles. Tantala-Coated PS Spheres. Figure 1b-e shows scanning electron micrographs of tantala-coated PS spheres produced at different concentrations of Ta(OEt)5. These images illustrate presence of a thick, homogeneous and complete tantala shell on the PS beads. A continuous increase in size of PS-Ta2O5 composite particles was observed as the concentration of Ta(OEt)5 was increased in the reaction media. An average increment of 0.1 mM in concentration of Ta(OEt)5 was found to cause around 30 nm increase in overall diameter of these composite particles. Moreover, as evident by TGA analysis (described later), a linear increase in tantala content of composite particles was also observed with increase in employed Ta(OEt)5 concentration. As shown in Figure 2, when the coating reaction was performed at 0.2 mM concentration, the size of PS beads increased from 540 (neat PS) to 740 nm, as determined by TEM, yielding the 100 nm thick tantala shell (the thickness of the deposited tantala shell is half of the increment in total diameter of the particles after coating process). When the concentration of tantalum ethoxide was gradually increased further from 0.3 to 0.5 mM in three different reaction sets, an increase of the size of coated particles from 770 to 825 nm was observed. Figure 2 indicates that thickness of the deposited tantala shell gradually increases with increase of the Ta2O5 content. These results demonstrate that an effective control over the thickness of precipitated tantala shell can be achieved. Interestingly, one can observe from Figure 1 that even at the highest concentration of tantalum ethoxide, the obtained coated particles remained spherical in shape and showed a narrow particle size distribution similar to the PS template particles. Moreover, (27) Agrawal, M.; Pich, A.; Zafeiropoulos, N. E.; Gupta, S.; Pionteck, J.; Simon, F.; Stamm, M. Chem. Mater. 2007, 19, 1845-1852. (28) Agrawal, M.; Pich, A.; Gupta, S.; Zafeiropoulos, N. E.; Stamm, M. Polym. Prepr. 2007, 48, 534-535. (29) Choi, H.-J.; Jang, L. B.; Lee, J. Y.; Pich, A.; Bhattacharya, S.; Adler, H.-J. P. IEEE Trans. Magn. 2005, 41, 3448. (30) Pich, A.; Hain, J.; Prots, Y.; Adler, H.-J. P. Polymer, 2005, 46, 7931.

Figure 2. Variation in the size of composite particles with Ta(OEt)5 concentration (inset shows dependency of the Ta2O5 shell thickness on the Ta2O5 content of composite particles).

no formation of uncoated tantala particles was observed. It is worth mentioning that the described system enabled us to achieve a gradual increase in thickness of tantala shell up to 140 nm or even more. In contrast, Cheng et al.31a and Wang et al.31b observed that increase in titania shell thickness beyond even 80 nm causes the formation of secondary nanoparticles and/or deviation of composite particles from spherical shape. It is also noteworthy that surface of the all PS-tantala composite particles is remarkably rough as compared to those of previously reported core-shell organic-inorganic composite particles.1b It can be attributed to the relatively high rate of the hydrolysis of Ta(OEt)5 and hence the rapid precipitation of tantala nanoparticles on the PS core. Formation of the tantala shell on the surface of PS beads is thought to be composed of nucleation and growth processes. During the nucleation, the tantala precursors interact with the β-diketone groups and provoke the nucleation of the tantala nanoparticles on the core surface (the detailed mechanism is described later). Afterward, these nuclei grow in the size and finally form a tantala shell on the polymeric core. The final morphology of composite particles depends on the relative rate of the nucleation and growth processes. In a previous study,27 we observed that deposition of the ZnO nanoparticles (31) (a) Cheng, X.; Chen, M.; Wu, L.; Gu, G. Langmuir 2006, 22, 3858-3863. (b) Wang, P.; Chen, D.; Tang, F.-Q. Langmuir 2006, 22, 4832-4835.

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Figure 3. SEM images of hollow Ta2O5 spheres obtained after chemical dissolution of PS core from composite particles with (a) 31.45 wt % Ta2O5; (b) 41.87 wt % Ta2O5; (c) 55.52 wt % Ta2O5, and (d) calcinations of composite particles with 55.52 wt % Ta2O5.

on the PS beads can lead to the core-shell (smooth shell) and raspberrylike (rough shell) morphology of composite particles, depending on the relative rate of these processes which are controlled by different reaction parameters. We believe that, in the case of tantala, the employed reaction conditions are favorable for the growth process. Consequently, tantala nuclei grow at fast rate and finally fuse with each other forming the rough and porous tantala shell on PS beads. Hollow Tantala Spheres. Hollow tantala spheres were prepared either by chemical treatment of PS-Ta2O5 composite particles with toluene or their calcination at elevated temperature. It has been reported in the literature that, after dispersing the composite particles in toluene, all the PS is expected to come out into solution leaving behind porous and hollow spheres.32 On the other hand, calcination of these composite particles in a furnace at 650 °C for 3 h also turned them into hollow spheres.33 Figure 3 shows SEM micrographs of hollow tantala spheres obtained from different composite samples. Figure 3a reveals that dissolution of PS core from sample prepared at lowest concentration of Ta(OEt)5 (0.2 mM) resulted in broken tantala hollow spheres. Similar results (not shown here) were obtained after calcination of this sample at 650 °C. This suggests that the shell thickness of obtained hollow spheres is not sufficient to maintain the initial spherical structure of the PS lattices upon removal of the colloidal core.32b,c,27 In contrast, calcination and core dissolution of other samples prepared at higher Ta(OEt)5 concentrations resulted in preparation of spherical, intact, and complete hollow spheres, as shown in Figure 3b-d. Moreover, no apparent creases and folds were observed on the shell wall. This may be attributed to the high thickness of tantala shell (115-140 nm) which provides good mechanical strength to the ceramic wall of hollow spheres and thus maintains the spherical shape of hollow spheres even after the removal of the PS core. Few broken spheres seen in these images were obtained by deliberately crushing them prior to analysis in order to confirm their hollow nature. The successful formation of hollow tantala spheres confirms the high uniformity of tantala coating on PS templates. Compared to previously reported studies on hollow spheres,1b the surface of hollow tantala spheres obtained in this study is quite rough. This high roughness offers relatively high surface area as compared to those of smooth ones, which is suitable for

Figure 4. TEM images of PS-Ta2O5 composite particles with (a) 41.87 wt % Ta2O5 (b) 55.52 wt % Ta2O5 and hollow Ta2O5 spheres (c, d) obtained after their calcination at 650 °C. (e) HR-TEM image and (f) fast Fourier transform (FFT) of the selected area of HR-TEM image of hollow Ta2O5 spheres obtained after calcination of PSTa2O5 composite particles with 55.52 wt % Ta2O5.

the use of these particles in catalytic applications. Similarly, it makes these particles effective adsorbents in separation and purification processes. Additionally, we believe that the tantala shell is remarkably porous and hence exhibits the high permeability and diffusivity of the foreign materials through it, which make these hollow structures suitable for the controlled delivery of relatively large molecules. Figure 4a-d shows TEM images of PS-Ta2O5 composite particles and hollow tantala spheres obtained via calcination at 650 °C. In agreement with SEM data, in the case of composite particles, a uniform coating of tantala layer on PS beads was observed. A noticeable difference in contrast of TEM images of tantala-coated PS particles and hollow tantala spheres (due to the difference in electron density), confirms that the produced tantala spheres are hollow in nature. A close inspection of the TEM images of hollow tantala spheres reveals the presence of individual tantala nanoparticles forming a thick and continuous shell. The diameter of the obtained hollow spheres was found 10-15% less than the ones for uncalcinated tantala coated PS particles.32a,b,34 This can be attributed to the sintering contraction of nanoparticles or further condensation/polymerization of (32) (a) Wang, D.; Caruso, F. Chem. Mater. 2002, 14, 1909-1913. (b) Caruso, F.; Caruso, R. A.; Mohwald, H. Chem. Mater. 1999, 11, 3309-3314. (c) Imhof, A. Langmuir 2001, 17, 3579. (33) Zhang, M.; Gao, G.; Li, C.-Q.; Liu, F.-Q. Langmuir 2004, 20, 14201424.

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Figure 5. (a) FTIR spectra of (1) tantala-coated PS particles (48.98 wt % Ta2O5) and hollow tantala spheres produced after their (2) chemical treatment with toluene and (3) calcination at 650 °C (b) XRD patterns of (1) tantala-coated PS particles (55.52 wt % Ta2O5) and (2) hollow tantala spheres obtained by calcination of same composite particles at 650 °C.

molecular precursors upon calcination. HR-TEM analysis provides further insight into the nanostructures of the of hollow tantala spheres. Figure 4e shows a typical HR-TEM image of an individual nanocrystal from the shell of hollow tantala sphere, obtained after the calcination of PS-Ta2O5 composite particles with 55.52% Ta2O5 content. This micrograph clearly illustrates the lattice images from the tantala nanocrystal. The distance between two lattice fringes was found to be 3.09 Å, which is consistent with the separation of (2.0.0) planes of the lowtemperature form L-Ta2O5 crystalline phase.35 In addition, the fast Fourier transform (FFT) image (shown in Figure 4f) of the selected area of HR-TEM micrograph, indicates the electron diffraction patterns attributable to the L-Ta2O5 crystalline phase. Apparently, reflections from (0.7.0) and (2.0.0) lattice planes corresponding to the 5.75 and 3.09 Å d-spacings can be observed. The FTIR spectra of PS-Ta2O5 composite particles and hollow tantala spheres obtained from core dissolution, as well as calcination methods, are shown in Figure 5a. In the case of composite particles (spectrum 1), one can observe the presence of the C-H stretching band at around 3000 cm-1, the aromatic C-C stretching band at around 1470 cm-1, the C-H out-ofplane band at 765 cm-1, and the aromatic C-C out-of-plane band at 700 cm-1. Aromatic overtones are visible in the range of 1700-2000 cm-1. All these peaks are characteristic of PS. In addition, the Ta-O-Ta and Ta-OH stretching bands at 664 and 3420 cm-1 are also visible, which indicates the presence of Ta2O5 in these composites particles.36 After chemical treatment with toluene, all the characteristic peaks of PS are barely visible (Figure 5a, spectrum 2), indicating that only traces of PS are left in these hollow spheres. It is likely that some of the dissolved PS has been absorbed on the outer surface of the particles. Similar results have been reported by Imhof32c during the fabrication of hollow TiO2 spheres. On the other hand, calcination of composite particles leaves no traces of PS at all, and resulting spectra resemble that of pure Ta2O5 (Figure 5a, spectra 3).36 Figure 5b shows the X-ray diffraction patterns of Ta2O5-coated PS particles and calcinated hollow tantala spheres. XRD pattern 1 indicates that the amorphous phase of tantala is deposited on the PS core during the coating process. During calcination at 650 °C, the amorphous phase transforms into the crystalline phase, as evident by XRD pattern 2. In agreement with the HRTEM analysis, all the peaks of this pattern can be indexed to a (34) Wang, L.; Sasaki, T.; Ebina, Y.; Kurashima, K.; Watanabe, M. Chem. Mater. 2002, 14, 4827-4832. (35) Stephenson, N. C.; Roth, R. S. Acta Crystallogr. 1971, B27, 1037. (36) Sun, Y.; Sermon, P. A.; Vong, M. S. W. Thin Solid Films 1996, 278, 135-139.

Figure 6. Thermogravimetric traces of (a) uncoated PS particles and (b-e) tantala-coated PS particles produced at different Ta(OEt)5 concentrations.

pure L-Ta2O5 orthorhombic crystalline structure with calculated cell constants of a ) 6.19 Å, b ) 40.29 Å, and c ) 3.88 Å. The peak intensities and peak positions are in good agreement with expected literature values.35 Moreover, it can be observed that the XRD pattern of hollow tantala spheres (pattern 2) has also a noticeable background, which reveals the presence of the amorphous tantala phase still present in the sample.33 This can be attributed to the presence of organic residue in the composite particles which may have obstructed the coagulation of Ta2O5 nanoparticles, thus resulting in a more restricted/imperfect crystallization of Ta2O5 in the composite particles. Figure 6 presents TGA scans of uncoated and tantala-coated PS particles. In all cases, the weight loss stage below 300 °C is the result of the evaporation of physically absorbed water and residual solvent in samples. The major weight loss between 325 and 460 °C can be attributed to the loss of PS. The weight loss which is visible only in the case of composite particles in the range of 500-650 °C, can be due to the decomposition of tantalabonded groups such as -OH and/or unhydrolyzed -OR groups. Moreover, in agreement with SEM data, one can observe a continuous increase in final residue, i.e., Ta2O5 contents of composite particles from 31% to 55.5%, with an increase in Ta(OEt)5 concentration employed during their preparation. We believe that hydrogen bonding between β-diketone groups, located on the surface of colloidal templates and tantala precursors Ta(OH)x(OR)y (formed in reaction media) acts the as the driving force for the precipitation of tantala layer on PS beads. A schematic presentation of the preparation of tantala-coated PS particles and hollow tantala spheres is shown in Scheme 1. According to the

1018 Langmuir, Vol. 24, No. 3, 2008 Scheme 1. Schematic Presentation of the Synthesis of PS-Ta2O5 Composite Particles and Hollow Ta2O5 Spheres

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nucleating sites and induce the precipitation of Ta2O5 nanoparticles on the template surface. Subsequently, the condensationpolymerization process of adsorbed Ta(OH)x(OR)y species takes place resulting in precipitation of a tantala layer on the PS core. Similar observations have been made by Fu et al.38 and Hanprasopwattana et al.39 for deposition of TiO2 nanoparticles on silica spheres and Ocana et al.40 for deposition of TiO2 nanoparticles on ZnO.

Conclusions

literature,36 preparation of Ta2O5 nanoparticles from tantalum alkoxide, i.e., Ta(OR)5, is thought to be composed of two stages. The first stage is a hydrolysis reaction, in which alkoxide groups are replaced with hydroxide groups via the formation of an intermediate of expanded coordination number and tantala precursors Ta(OH)x(OR)y; x and y are the number of hydroxyl and alkoxy groups, respectively, present on tantalum atom in hydrolyzed product of tantalum alkoxide. The second stage is a condensation-polymerization reaction where Ta-O-Ta bonds are formed from Ta(OH)x(OR)y, resulting in creation of a 3D structure, which precipitates out of the reaction solution. It has been reported in the literature37 that if metal alkoxides are hydrolyzed in the presence of functionalized templates then the precipitation of metal oxide nanoparticles occurs preferentially on template surface (heterogeneous precipitation) rather than in solution (homogeneous precipitation), and it is driven by the interaction between hydrolyzed monomer M(OH)x(OR)y and active groups present on the template. As mentioned above, colloidal cores used in the present study are functionalized with β-diketone groups on their surfaces. Hence, as soon as the tantala precursors, i.e., Ta(OH)x(OR)y, are formed from Ta(OEt)5 in reaction media, they are thought to be captured by templates via their interaction with β-diketone groups. These groups act as the (37) (a) Zhang, D.; Yang, D.; Zhang, H.; Lu, C.; Qi, L. Chem. Mater. 2006, 18, 3477-3485. (b) Cheng, H.; Ma, J.; Zhao, Z.; Qi, L. Chem. Mater. 1995, 7, 663.

In summary, we have demonstrated a novel, fast, and facile approach to fabricate the mesoscale hollow tantala spheres. The obtained hollow spheres have a well-defined void size which can be effectively modulated by varying the size of PS template and a homogeneous shell whose thickness can be controlled by the concentration of Ta(OEt)5 in the reaction media. Although we only demonstrated this procedure with 540 nm PS beads as an example, we believe that this method should be extendible to PS colloidal templates with smaller dimensions and to hollow spheres made of other materials. The only requirement seems to be that the intermediate formed during the hydrolysis of the metal salt can effectively interact with β-diketone groups so that precipitation of a metal oxide layer can be induced on the surface of the template. This procedure can also be applied to the fabrication of monodisperse, composite particles consisting of cores covered with shells of different chemical compositions. The sizes and compositions of these core-shell particles can be changed in a controllable way to tailor their properties, such as optical, electrical, or magnetic responses. Acknowledgment. The authors are thankful to Dr. Rudiger Ha¨ssler, Mr. Axel Mensch, and Mrs. Ellen Kern for helping us out with TGA, TEM, and SEM analysis, respectively. In addition, we acknowledge Prof. H. Lichte for providing us the highresolution TEM facility at the Special Electron Microscopy Laboratory, Technical University of Dresden. LA702509J (38) Fu, X.; Qutubuddin, S. Colloids Surf. A: Physicochem. Eng. Aspects 2001, 178, 151-156. (39) Hanprasopwattana, A.; Srinivasan, S.; Sault, A. G.; Datye, A. K. Langmuir 1996, 12, 3173-3179. (40) Ocana, M.; Hsu, W. P.; Matijevic, E. Langmuir 1991, 7, 2911.