TiO2 Multicoated Microspheres

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Preparation of Porous SiO2/Ni/TiO2 Multicoated Microspheres Responsive to Electric and Magnetic Fields Hong-xia Guo, Xiao-peng Zhao,* Hui-lin Guo, and Qian Zhao Institute of Electrorheological Technology, Department of Applied Physics, Northwestern Polytechnical University, Xi’an, 710072, P R China Received May 30, 2003. In Final Form: July 12, 2003 A new type of multicoated porous SiO2/Ni/TiO2 composite particles with response to external electric and magnetic fields is presented in this paper. Porous silica spheres are used as cores instead of solid silica beads, which give a much lower density. Then, the magnetic nickel component is introduced to the porous silica core by using electroless plating, while the titania component is coated on the outerlayer. The morphology and structure of the particles were characterized by SEM, TEM, and XRD. The properties as well as the behavior of the synthesized multiplayer-coated spheres under external electric and magnetic fields have been investigated. The resulting composite particles showed good responses to the electric and magnetic fields by the reflection of being ordered into chains structures under an external electric or magnetic field, and the network patterns under the adjusted electric and magnetic fields superimposed perpendicularly. The introduction of a magnetic and dielectric function to porous silica particles may be of importance in various technologies.

Introduction Well-organized colloidal arrays have become a subject of intensive research, because of their potential applications in areas such as photonic crystals,1,2 wavelength division multiplexing technology,3 and sensing materials.4,5 Photonic crystals have recently been studied extensively because they offer unique ways of tailoring the propagation of light.6-11 A number of methods have been developed to create the periodic structures possessing photonic band gap (PBG).12-18 Some successes have been achieved through microfabrication techniques such as mechanically drilling holes within a dielectric slab,12 layer by layer growth on the length scales,13-17 and holographic pattering using multiple laser beams.18 However, the pursuit of a microscopically ordered PBG in the visible range is still a challenging endeavor. * To whom correspondence should be addressed. E-mail: [email protected]. (1) Tarhan, I. I.; Watson, G. H. Phys. Rev. Lett. 1996, 76, 315. (2) Younan, X.; Byron, G.; Zhi-Yuan, L. Adv. Mater. 2001, 13 (6), 409. (3) Avrutsky, I.; Kochergin, V.; Zhao, Y. IEEE Photonics Technol. Lett. 2000, 12, 1647. (4) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829. (5) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959. (6) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (7) John, S. Phys Rev. Lett. 1987, 58, 2486-2489. (8) Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. H. Nature 1997, 386, 143. (9) van-Blaaderen, A. Science 1998, 282, 887. (10) Pendry, J. Science 1999, 285, 1687. (11) Wijnhoven, J. G. G. J.; Zevenhuizen, S. J. M.; Hendriks, M. A. et al. Adv. Mater. 2000, 12, 888. (12) Yablonovitch, E.; Gmitter, T. J.; Leung, K. M. Phys. Rev. Lett. 1991, 67, 2295. (13) Ho, K.; Chan, C.; Soukoulis, C. et al. Solid State Commun. 1994, 89, 413. (14) So¨zu¨er, H.; Dowling, J. J Mod. Opt. 1994, 41, 231. (15) Lin, S. Y.; Fleming, J. G.; Hetherington, D. L. et al. Nature 1998, 394, 251. (16) Noda, S.; Yamamoto, N.; Kobayashi, H. et al. Appl. Phys. Lett. 1999, 75, 905. (17) O ¨ zbay, E.; Abeyta, A.; Tuttle, G. et al. Phys. Rev. B 1994, 50, 1945. (18) Campbell, M.; Sharo, D. N.; Harrison, M. T. et al. Nature 2000, 404, 53.

Colloidal self-assembly has been utilized as a process to form 3D periodic structures because of its simplicity.1,19,20 Thanks to many years of efforts from various groups, various techniques based on the use of colloids have been developed to construct these solid arrays. The best known methods are based on gravity sedimentation,21,22 repulsive electrostatic interaction,23 attractive capillary forces,24,25 and other methods.26,27 New ways to obtain large enough domains are continuously developed. The practical use of these self-assembled “opals”, however, has been inhibited for two major reasons: the assembly process is slow, often requiring as long as days to complete, and the resulting samples are typically conglomerates of many crystalline domains with random orientations.28 The self-assembly of strongly interacting particles might be a promising approach to optimize and accelerate the formation of ordered structures.29,30 Recent advances in control of the growth habit of colloidal crystals by applying an external electrical, magnetic, or optical field have allowed the fabrication of ordered structures in one, two, and three dimensions.31-34 The electrorheological (ER) or (19) Bogomolov, V. N.; Gapoenko, N. V.; Prokofiev, A. V. et al. Phys. Rev. E 1997, 55, 7619. (20) Zakhidov, A. A.; Baughman, R. H.; Iqbal, Z. et al. Science 1998, 282, 897. (21) Davis, K. E.; Russel, W. B.; Glantschnig, W. J. Science 1989, 245, 507. (22) Mayoral, R.; Requena, J.; Moya, J. S.; Lo´pez, C. et al. Adv. Mater. 1997, 9, 257. (23) Sunkara, H. B.; Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1994, 6, 362. (24) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303. (25) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (26) Velev, O. D.; Lenhoff, A. M.; Kaler, E. W. Nature 2000, 287, 2240. (27) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Lenhoff, A. M.; Rabolt, J. F.; Kaler, E. W. Adv. Mater. 2001, 13, 396. (28) Holgado, M.; Garcı´a-Santamarı´a, F.; Blanco, A.; Ibisate, M. et al. Langmuir 1999, 15, 4701. (29) Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M. Science 1997, 276, 233. (30) Kusner, R. E.; Mann, J. A.; Dahm, A. J. Phys. Rev. B 1995, 51, 5746. (31) Yeh, S. R.; Shraiman, B. I. Nature 1997, 386, 57. (32) Hayward, R. C.; Saville, D. A.; Aksay, I. A. Nature 2000, 404, 57.

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magnetorheological (MR) effect offers a unique technique whereby the suspended particles can be self-assembled into body-centered-tetragonal (bct) mesocrystallites through the application of an external field.35,36 When ER and MR effects are combined, the network structure has been formed by X. P. Zhao et al.,37 and the transition of the formed bct or fcc structure can be achieved by the ratio of crossed magnetic and electric fields.38 This provided a useful method to control the behavior of particles suspended in fluid.39 In this respect, the synthesis of colloidal particles with tailored shape and controlled chemical composition and physical properties may be one prerequisite for particles assembled under external fields. Routes to produce uniformly composite particles in solution have relied on coating of core particles with organic or inorganic layers by various techniques.40-43 Synthetic routes developed to produce polymer-coated particles include polymerization, heterocogulation-polymerization, and emulsion polymerization.40 Inorganic and hybrid coatings (or shells) on particles have been commonly prepared by precipitation and surface reaction that make use of specific functional groups on the cores to induce coating.41 In addition, particles can be coated by the direct adsorption of polymer or by adsorption of performed inorganic nanoparticles onto larger particles utilizing electrostatic interactions.42,43 Although a host of new strategies have been developed for the modification of particle surfaces, earlier methods have primarily focused on achieving single-component coatings on particles. Multicomposition coatings with remarkably controlled properties are still in their infancy.44 The multilayer particles with responses to electric and magnetic fields have been generated by our group based on the combination of multiple methods. The method relied on encapsulation of magnetic cores with polystyrene (PSt) followed by coating with an outer titania layer, in which Fe3O4/PSt/TiO2 and Ni/PSt/TiO2 micrometer composite particles were obtained.45,46 However, this method might require much effort to tailor the size and properties of the particles. For example, it often needs stricter conditions and longer times to encapsulate magnetic cores with polystyrene (PSt) so as to get magnetic polymer particles of uniform size. Porous materials are of significant interest because they can possess an attractive and unique set of properties.47,48 The large pore size, high surface area, and thermostability of these materials instantly created applications as catalysts, chromatography, sensors, pigments, microelectronics, and electrooptics.49-52 In further utilizing these ordered uniform size mesopores, several groups have (33) Grzybowski, B. A.; Stone, H. A.; Whitesides, G. M. Nature 2000, 405, 1033. (34) Wei, Q. H.; Bechinger, C.; Rudhardt, D.; Leiderer, P. Phys. Rev. Lett. 1998, 81, 2602. (35) Tao, R.; Sun, J. M. Phys. Rev. Lett. 1991, 67, 398. (36) Ma, H.; Wen, W.; Tam, W. Y. et al. Phys. Rev. Lett. 1996, 77, 2499. (37) Zhao, X.; Luo, C.; Zhang, Z. Opt. Eng. 1998, 37, 1589. (38) Wen, W.; Wang, N.; Ma, H. et al. Phys Rev. Lett. 1999, 82, 4248. (39) Tomlin, S. Nature 1999, 399, 637. (40) Hofman-Caris, C. H. M. New J. Chem. 1994, 18, 1087. (41) Caruso, R. A.; Antonietti, M. Chem. Mater. 2001, 13, 3272. (42) Caruso, F. Chem Eur. J. 2000, 6, 413. (43) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (44) Caruso, F. Adv. Mater. 2001, 13, 11. (45) Guo, H.-x.; Zhao, X.-p. Opt. Mater. 2003, 22, 39. (46) Guo, H.-x.; Zhao, X.-p.; Ning, G.-h.; Liu, G.-q. Langmuir 2003, 19, 4884. (47) Huo, Q.; Feng, J.; Schuth, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14-17. (48) Qi, L.; Ma, J.; Cheng, H.; Zhao, Z. Chem. Mater. 1998, 10, 16231626. (49) Ozin, G. A. Chem. Commun. 2000, 419.

Guo et al. Scheme 1. Schematic Diagram of (a) Porous Spheres Deposited with Nickel by Means of Electroless Plating, then (b) Covered by Titania Coating Layer

developed a number of techniques for depositing various components in the mesoporous materials.53-57 Of these techniques, electroless plating has been used to deposit Au within the pores of organic membranes.56,57 Inspired by this route, here we present a novel strategy to induce electric and magnetic properties onto porous spheres to produce composite particles responsive to electric and magnetic fields. As is illustrated in Scheme 1, in this paper the porous spheres are used as colloidal cores. The magnetic property is imparted by introduction of metallic nickel component into porous silica cores using electroless plating, while the dielectric property is offered by nanometer titania outerlayer. The properties as well as the responses to the electric and magnetic fields of the synthesized multiplayer-coated spheres have been investigated. This method would be a promising route to modify the properties of porous materials. Moreover, the introduction of a magnetic and dielectric function to porous silica particles may be of importance in various technologies besides as building blocks for photonic applications. Experimental Section Synthesis of Porous Silica Spheres. The porous silica spheres were synthesized by using poly(ethylene glycol) (PEG, Mw 6000) and cetyltrimethylammonium bromide (CTAB) as a template under acidic condition. In a typical preparation in aqueous solution, 1.0 g of PEG and 0.1 g of CTAB were dissolved in 30 g of HCl (2M) and then 2.6 g of tetraethyl orthosilicate (TEOS) was added. The solution was stirred for 20 h and kept at 100 °C in an oven for another 24 h. The solid products were recovered, washed, and dried at room temperature. Then it was calcined at 600 °C for 4 h. Synthesis of the Magnetically Modified Porous Spheres. Nickel was introduced onto porous silica spheres by electroless plating. Preactivation of SiO2 spheres was accomplished by ultrasonically dispersing the spheres in a solution of 0.1 M SnCl2/ 0.1M HCl for 40 min. This resulted in deposition of the “sensitizer” (Sn2+) onto the porous beads. Then the mixture was washed and centrifuged, and the supernant was discarded. Subsequently, the Sn2+-sensitized silica beads were further activated in an aqueous solution of 1.4 × 10-3 M PdCl2/0.25 M HCl for another 40 min. This resulted in the deposition of Pd catalyst onto the porous spheres. The beads were cleaned again using centrifugation and introduced into an electroless-plating bath. The composition of the plating solution and the reaction condition are given in Table 1. After 20 min reaction under stirring, the product was separated by placing a magnet under the bottom of the bath and washed twice with water. Surface Modification of the Magnetic SiO2/Ni Spheres by Titania. Titania was coated on SiO2/Ni spheres by hydrolysis of titanium butoxide precursor. In one experiment, 0.1 g of assynthesized SiO2/Ni composite particles was dispersed into 100 (50) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (51) Lee, T.; Yao, N.; Aksay, I. A. Langmuir 1997, 13, 3886. (52) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (53) Srdanov, V. I.; Alxneit, I.; Stucky, G. D.; Reaves, C. M.; DenBaars, S. P. J Phys. Chem. B 1998, 102, 3341. (54) Hirai, T.; Okubo, H.; Komasawa, I. J Phys. Chem. B 1999, 103, 4228. (55) Tolbert, S. H.; Firouzi, A.; Stucky, G. D.; Chmelka, B. F. Science 1997, 278, 264. (56) Jirage, K. B.; Hulteen, J. C.; Martin, C. R. Anal. Chem. 1999, 71, 4913. (57) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920.

Porous SiO2/Ni/TiO2 Multicoated Microspheres

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Table 1. Bath Composition and Operating Conditions of Electroless Nickel Coating chemicals

concentration (mol‚L-1)

NiSO4‚6H2O Na3C6H5O7‚1.5H2O NaH2PO2‚2H2O NH4Ac Pb(NO3)2 pH at 25 °C adjusted by NH4OH Bath temperature

0.114 0.054 0.240 0.32 7.5 10-3 7.2 45 °C

Scheme 2. Schematic Diagram of Applying Electric and Magnetic Fields

Figure 1. TEM micrograph of (a) the porous silica spheres, and (b) the magnification of panel a.

Figure 2. SEM micrograph the nickel-modified spheres. mL of n-butanol and anhydrous ethanol (10:1 in volume) solution. After sonication for 20 min, to this mixture a few of drops of nitric acid-water solution (2 wt %) were added under stirring. Then, 0.15 M titanium tetrabutyloxide in n-butanol was added dropwise into the mixture at room temperature with controlled rhythm under stirring. After the dropwise addition was over, the mixture was refluxed and stirred for 8 h. The resulting titaniacoated spheres were separated centrifugally. The supernatant was discarded, and the precipitate was washed repeatedly with anhydrous ethanol. Then the precipitate was dried in air at room temperature. Measurements. Transmission electron microscopy (TEM) of the specimens was carried out on a JEM-200 CX (Japan) instrument. Samples for TEM were prepared by depositing aqueous solution of the spheres upon copper grid. The mixtures were allowed to air-dry for 1 min, and the extra solution was then blotted off. Scanning electron microscopy (SEM) images were obtained on a Hitachi-570 scanning electron microscope operating at 20 kV with Au sprayed prior to examination. The particle size and polydispersity of all samples were analyzed by WD-5 system. The X-ray diffraction (XRD) patterns of the samples were collected at room temperature with Cu KR X-ray sources using a D/MAX-γA instrument (Japan). The thermal analysis measurement of the specimen was carried out using a Gera¨tebau GmbH Thermal Analysis instrument (NETZSCH, Germany) with a heating rate of 10 °C‚min-1 in air. The sedimentation of the specimens was directly suggested by the density of the particles, which was measured by using a 10 mL standard pycnometer. The behavior of the composite particles responsive to electric and magnetic fields was observed by an optical microscope (Alphaphot-2 YS2-H, Nikon). The particles were ultrasonically dispersed into a 10 mm × 5 mm × 5 mm sample cell containing silicone oil. The electric and magnetic fields were applied as in Scheme 2. And the structure was recorded by a CCD (Fijitsu) and computer.

Results and Discussion The calcined porous silica spheres can be visualized by transmission electron microscopy (TEM) in Figure 1. The porous silica particles shown in Figure 1a are spherical in shape and are a little transparent due to the porosity. Direct measurement of each sphere in the images gave a mean diameter of 1.71 µm, with the polydispersity of 0.24. The corresponding magnification image of the porous silica

Figure 3. SEM micrograph of SiO2/Ni/TiO2 composite particles.

particles shown in Figure 1b indicated that the pores were disorder and 3-5 nm in diameter. The presence of the loaded nickel nanoparticles on the silica particles clearly resulted in the dark black appearance of the particles and increased surface roughness as shown in Figure 2. Direct measurement of each sphere in the images gave a mean diameter of 1.92 µm. As can be seen from Figure 2, the nickel nanoparticles distributed on the surface of the silica beads were somewhat uneven and less than manolayer coverage. This resulted in polydisperse diameter. The distribution in diameter shown in Figure 4a gave the 0.67 of polydispersity in diameter. Aggregation of the nanoparticles on the surface may be ascribed to nonuniformity of the active sites on the surface of the porous beads. Due to the curved surface on the porous silica beads, the deposited nickel layer tends to form as the discrete grains, if the normal growth rate is higher than lateral growth rate. The further modification of magnetically SiO2/Ni particles surface was obtained by deposition of titania using titania butoxide as precursor. This was evidenced by an increase in the diameter of particles. A smoother and more regular titania layer can be observed in Figure 3. Direct measurement of each sphere in the images gave a mean diameter of 2.18 µm. Compared with that of Figure 4a, the distribution in diameter shown in Figure 4b shifted to lager diameters, which gave the polydispersity of 0.33

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Figure 4. Distribution in diameter of (a) porous SiO2/Ni and (b) porous SiO2/Ni/TiO2. Figure 6. TG-DSC curves of porous SiO2/Ni/TiO2 composite particles. Table 2. Specific Gravity of the Samples

Figure 5. XRD patterns of the particles: (a) porous silica and (b) nickel-modified spheres.

in diameter. This suggested that deposition of titania onto the particle surfaces occurred in the form of oligomers or very small particles. The coatings of hundreds nanometers were possible by controlling the concentration of the precursor, which cover the whole sphere surface and result in a smoother one and less polydispersity. The XRD pattern of the calcined porous silica in curve a of Figure 5 showed the diffraction peak at 2.03° of 2θ and the broad peak of amorphous silica around 20°. This pattern is characteristic of a mesophase with a pore system lacking long-range order, which is reminiscent of the disordered mesoporous silica. In curve b of Figure 5, there was another broad peak around 2θ of 40-60° besides that of porous silica, which corresponded to the nickelphosphorus alloy phases, different from that of pure nickel component, which showed two discrete sharp peaks at the same range of 2θ. This was reminiscent of the presence of nickel-phosphorus component on the porous silica particles. The XRD patterns of SiO2/Ni/TiO2 were similar to that shown in curve b of Figure 5 but with lower peak intensities related to the amorphous titania outer coating layer. When the dark black porous SiO2/Ni particles were dispersed in water, no coloration of the supernatant liquid or phase separation was observed. However, when the same particles were dispersed in dilute nitric acid (0.2 M), the dark black color of the particles disappeared and the matrix porous silica beads exposed. After the porous SiO2/Ni particles were coated by titania outerlayer, the formed porous SiO2/Ni/TiO2 composite particles, however, were stable in the same nitric acid. These reflected a strong

samples

density (g‚mL-1)

samples

density (g‚mL-1)

porous SiO2 spheres solid SiO2 spheres

1.23 1.82

porous SiO2/Ni/TiO2 solid SiO2/Ni/TiO2

3.60 4.22

adhesion of the nickel particles onto the silica surface and the formation of compact titania coating layers. The thermostability of the synthesized porous SiO2/ Ni/TiO2 was determined by the TG and DSC curves shown in Figure 6. The exothermic weight loss below 200 °C is related to combustion of the organic component. The exothermic weight increase above 500 °C is assigned to oxidation of the nickel-phosphorus alloy component of the spheres. The crystalline transformation of anatase to rutile can be seen from the corresponding DSC curve. This shows that the porous SiO2/Ni/TiO2 particles are thermally stable in the range of 200-500 °C. The measured densities of all samples are listed in Table 2. As can be seen from Table 2, the 1.23 g‚mL-1 density of the porous silica spheres is smaller than 1.82 g‚mL-1 of the nonporous solid one, which was prepared by Sto¨ber method.58 Also, the density of the prepared SiO2/Ni/TiO2 composite spheres with porous silica as cores is 3.60 g‚mL-1. The density of the similar composite spheres but with the nonporous solid silica as core is 4.22 g‚mL-1. This suggested that the as-prepared SiO2/Ni/TiO2 composite particles with porous silica spheres as core showed less density than that with no-porous solid silica matrix spheres as core. Thus, gravity may have a little influence on manipulating the particles by external fields. The behavior of the prepared particles was examined under the external fields. The porous SiO2/Ni particles lacking a titania coating can only form chain structure under external magnetic field. While the porous SiO2/ Ni/TiO2 composite particles can be ordered into long chain structures under either a 1400Gs magnetic field or a 0.7 kV/mm electric filed, shown in Figure 7a. Moreover, this chain structure changed with the strength of voltage. However, in the absence of external magnetic or electric field, the particles were randomly dispersed in the suspension as in Figure 7b. This demonstrated electric or magnetic ordering of the composite particles. Furthermore, when the suspension of the composite particles in silicone oil was placed under the adjusted external electric and magnetic field strength superimposed perpendicularly, the three-dimensional network pattern was formed as (58) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26.

Porous SiO2/Ni/TiO2 Multicoated Microspheres

Figure 7. Optical microscopy images of the particles structure under (a) electric or magnetic field, (b) without external field, (c) external electric and magnetic field strength superimposed perpendicularly.

shown in Figure 7c. This suggested the production of threedimensional structures by means of electric and magnetic fields. In conclusion, the multicoated porous SiO2/Ni/TiO2 composite particles were prepared by a new method using the porous silica spheres as cores, then introducing the magnetic nickel component to the porous silica core by electroless plating, and coating the titania component on the outerlayer. The resulting composite particles are of

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2.18 µm average diameter. Compared to the SiO2/Ni/TiO2 multilayer particles but with a dense and nonporous silica core, the prepared composite particles have smaller specific gravity. Also, the as-prepared composite particles showed modified properties and good response to the external electric or magnetic field by the reflection of being ordered into chain structures under an external electric or magnetic field and the network patterns under the adjusted electric and magnetic fields superimposed perpendicularly. This provided a new route to exploit the individual function of each component in the core-shell multilayer composite particles. Acknowledgment. The National Science Foundation of China for Distinguished Young Scholars under Grant No.50025207 and the National Natural Science Foundation Project under Grant No. 90101005 as well as Doctorate Foundation of Northwestern Polytechnical University (200129) are all gratefully acknowledged. LA034948T