TiO2 Multiply Coated Microspheres

A new type of multiply coated Ni/PSt/TiO2 composite particles with response to external electric and magnetic fields were presented in the paper. The ...
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Articles Synthesis of Ni/Polystyrene/TiO2 Multiply Coated Microspheres Hong-xia Guo, Xiao-peng Zhao,* Guang-hui Ning, and Gen-qi Liu Institute of Electrorheological Technology, Department of Applied Physics, Northwestern Polytechnical University, Xi’an, 710072, People’s Republic of China Received September 17, 2002. In Final Form: April 1, 2003 A new type of multiply coated Ni/PSt/TiO2 composite particles with response to external electric and magnetic fields were presented in the paper. The multilayer complex particles consisted of a metallic nickel (Ni) core encapsulated with a polymeric polystyrene (PSt) and coated with an outlayer of dielectric TiO2. The morphology and structure of the particles were characterized by transmission electron microscopy, scanning electron microscopy, thermogravimetric-differential thermal analysis, X-ray diffraction, and Fourier transform infrared spectroscopy. The resulting composite particles showed good responses to electric and magnetic fields, which was reflected by the formation of a network structure under electric and magnetic fields superimposed perpendicularly.

Introduction Composite particles (or core-shell materials) often exhibit improved physical and chemical properties over their single-component counterparts and hence are very useful in a broader range of applications.1-8 Research in this area has been largely spurred by the importance of core-shell materials in modern material science and technology. Composite colloids are utilized in the areas of coatings, electronics, catalysis, and separations.3-8 Also, they can be used as building blocks for photonic crystals.9-11 The creation of core-shell particles is also of interest from a fundamental and academic viewpoint, especially in the area of colloid and interface science. They can be utilized as model systems to investigate factors governing colloidal interactions and stabilization.5 The techniques to produce uniformly composite particles in solution have relied on coating of core particles with organic or inorganic layers.12-16 Synthetic routes include * To whom correspondence should be addressed. E-mail: [email protected]. (1) Hall, S, R.; Davis, S. A.; Mann, S. Langmuir 2000, 16, 1454. (2) Hofman-Caris, C. H. M. New J. Chem. 1994, 18, 1087. (3) Caruso, R. A.; Antonietti, M. Chem. Mater. 2001, 13, 3272. (4) Antonietti, M.; Berton, B.; Go¨ltner, C.; et al. Adv. Mater. 1998, 10, 154. (5) Caruso, F. Adv. Mater. 2001, 13, 11. (6) Davies, R.; Schurr, G. A.; Meenan, P.; et al. Adv. Mater. 1998, 10, 1264. (7) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (8) Caruso, F. Chem.sEur. J. 2000, 6, 413. (9) Rogach, A.; Susha, A.; Caruso, F.; et al. Adv. Mater. 2000, 12, 333. (10) Breen, M. L; Dinsmore, A. D.; Pink, R. H.; et al. Langmuir 2001, 17, 903. (11) Velikov, K. P.; Moroz, A.; Blaaderen, A v. Appl. Phys. Lett. 2002, 80, 49-51. (12) Zaiter, V. S.; Filimonov, D. S.; Presnyakor, I. A.; et al. J. Colloid Interface Sci. 1999, 212, 49. (13) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1998, 197, 293. (14) Averitt, R. D.; Sarkar, D.; Halas, N. J. Phys. Rev. Lett. 1997, 78, 4217. (15) Henglein, A.; Glersig, M. J. Phys. Chem. B 2000, 104, 5056.

monomer adsorption onto particles followed by subsequent polymerization, heterocoagulation-polymerization, and emulsion polymerization.1 Inorganic and hybrid coatings (or shells) on particles have been generally prepared by precipitation and surface reaction that make use of specific functional groups on the cores to induce coating.3 Alternatively, particles can be coated by direct adsorption of polymer or by adsorption of preformed inorganic nanoparticles onto larger particles utilizing electrostatic interactions.8,17 A host of new strategies have been developed for the modification of particle surfaces. However, earlier methods have primarily focused on achieving single-component coatings on particles. Multicomposition coatings with remarkably controlled properties are still in their infancy. The polarizable particles have been assembled into ordered structures under certain conditions when electric fields have been applied.18,19 Also, magnetic colloidal suspensions have been used as building blocks for the formation of ordered patterns via manipulation using magnetic fields.20,21 At the same time, multiply coated core-shell particles with response to external electric and magnetic fields can be assembled in a controlled manner under adjustable electric and magnetic fields.22,23 However, the larger particles are undoubtedly an obstacle in the formation and utilization of the ordered structures.23 In this paper, a novel approach to preparation of the composite particles with responses to electric and magnetic fields was presented. The technique was based on encapsulation of nickel (Ni) cores with polystyrene (PSt) to afford a magnetic polymer particle, Ni/PSt, and then the (16) Szabo´, D. V.; Vollath, D. Adv. Mater. 1999, 11, 1313. (17) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (18) Dassanayake, U.; Fradern, S.; Blaaderen, A. van J. Chem. Phys. 2000, 112, 3851. (19) Gu, Z.-Z.; Meng, Q.-B.; Hayami, S.; et al. J. Appl. Phys. 2001, 90, 2042. (20) Caruso, F.; Susha, A. S.; Gersig, M.; et al. Adv. Mater. 1999, 11, 950. (21) Wirtz, D.; Fermigier, M. Phys. Rev. Lett. 1994, 72, 2294. (22) Zhao, X. P.; Luo, C. R.; Zhang, Z. D. Opt. Eng. 1998, 37, 1598. (23) Wen, W.; Wang, N.; Ma, H.; et al. Phys. Rev. Lett. 1999, 82, 4284.

10.1021/la0265631 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/10/2003

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Figure 1. TEM images of particles for (a) Ni, (b) Ni/PSt, and (c) Ni/PSt/TiO2.

outermost titania coating on Ni/PSt composite particles. These Ni/PSt/TiO2 multicomposite particles manifested a strong response to external electric and magnetic fields, which was reflected by the formation of a network structure under electric and magnetic fields superimposed perpendicularly. The advantage of this method is that the shape of the polymer-encapsulated metal cores can be modified by multiple coating and the particles exhibit a strong resistance toward etching. Also, the outermost titania coating imparted a high dielectric index to the particles, thus making the multiply coated particles useful as building blocks to form three-dimensional ordered structures. Moreover, this study showed the successful combination of multiple methods to form multicomposite particles. Experimental Section 1. Procedures. A. Synthesis of Modified Nickel (Ni) Particles. The modified nickel (Ni) particles were prepared through the reduction of nickel chloride (NiCl2) with hydrated hydrazine (N2H4‚H2O) in an ethanol solution using poly(ethylene glycol) (PEG, Mw ) 400) as a stabilizer. In a typical reaction, 2.0 g of PEG and 1.0 g of NiCl2 were dissolved in 50 mL of absolute ethanol to form a homogeneous solution with green color (solution a). In addition, 10 mL of 4.0 M NaOH aqueous solution was added to 2.3 g of N2H4‚H2O (80 wt %) to form another clear homogeneous solution (solution b). Then, the two solutions were mixed together with a molar composition for NiCl2 and N2H4‚ H2O of 1:6. After a while, a black solid precipitated. The solvent was completely removed by placing a magnet under the beaker. Then the solid black sample was repeatedly washed with deionized water. B. Synthesis of Polystyrene-Encapsulated Nickel (Ni/PSt) Composite Particles. The Ni/PSt composite particles were synthesized by the route that the monomer was adsorbed onto the modified nickel particles followed by dispersion polymerization. A practice example is as follows: 0.6 g of the prepared nickel particles was dispersed into 100 mL of ethanol solution of water (20 wt %) in a 500 mL three-necked flask, submerged in a thermostated water bath. The flask vessel was equipped with a stirrer, a flux condenser, and a nitrogen gas inlet. To this mixture, 0.2 g of poly(vinylpyrrolidone) (PVP), 0.6 g of 2,2-azobis(isobutyronitrile) (AIBN), and 8 mL of styrene were added at room temperature with stirring. The mixture was allowed to swell for 20 h at 25 °C under stirring. Then, the temperature of the water bath was elevated to 65 °C for polymerization, while purging with nitrogen. After polymerization under nitrogen for 12 h, the gray precipitate was recovered by centrifugation, washed with ethanol repeatedly, and dried in ambient air. C. Titania Coating of Ni/PSt Particles. Titania was coated on Ni/PSt spheres by hydrolysis of a titanium butoxide precursor. In one experiment, 0.1 g of as-synthesized Ni/PSt composite particles were dispersed into 100 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 nitride acid water solution (2 wt %) were added under stirring. Then, 10 mL of 0.15 M titanium tetrabutyloxide in n-butanol was added dropwise into the mixture at room temperature with a controlled rhythm with stirring. When the dropwise addition was over, the mixture was refluxed

and stirred for 8 h. The resulting titania-coated spheres were separated centrifugally. The supernatant was removed, and the precipitate was washed repeatedly with anhydrous ethanol. Then the precipitate was dried in air at room temperature. 2. Characterization. Transmission electron microscopy (TEM) of the specimens was carried out on a JEM-200 CX (Japan) instrument using standard techniques. Scanning electron microscopy (SEM) images were obtained on a Hitachi-570 scanning electron microscope operating at 15 kV with Au sprayed prior to examination. Thermogravimetric and differential thermal analyses (TG-DTA) of the specimens were measured on WCT-2 instruments (Beijing, China) with a heating rate of 10 °C‚min-1 in air. The X-ray diffraction (XRD) patterns of the samples were performed at room temperature with a Cu KR X-ray source using a D/MAX-γA instrument (Japan). The Fourier transform infrared (FTIR) spectra of the samples were measured by an Equinox-55 Fourier IR instrument (Bruker, Germany) in the wavenumber range of 400-4000 cm-1. All samples were prepared for analysis using a KBr pellet.

Results and Discussion When the nickel chloride solution of absolute ethanol was mixed with hydrated hydrazine homogeneously, the black nickel particles were produced after a while. The obtained black precipitate can be easily attracted by a magnet and exhibited a strong magnetic property. The TEM images shown in Figure 1a indicated that the metallic nickel particles were close to spherical with an uneven surface. The average diameter of the particles was 0.61 µm. After encapsulation by polystyrene, the formation of Ni/PSt composite particles was reflected by the color change from dark black to gray and by attraction with the magnet. The TEM and the SEM images of the formed Ni/PSt particles are shown in Figure 1b and Figure 2a, respectively. All images showed that the particles were round and smooth on the surface. Direct measurement of each sphere in the SEM images gave a mean diameter of 1.04 µm, with the polydispersity of 0.041. The corresponding distribution in diameters shown in Figure 3a indicated that the particles were of uniform size. The outmost layer of titania was coated on the Ni/PSt composite particles based on the hydrolysis of a titanium butoxide precursor. The titania layer deposited on the Ni/PSt composite particles was determined by both the amount of precursor alkoxide used and the amount of water added before reaction. The aggregation and formation of homogeneous titania spheres can be minimized with n-butanol and ethanol (10:1 in volume) as the medium and less than 0.32 M amount of water. Multilayer coatings can be obtained by increasing the amount of alkoxide precursor while keeping the amount of water constant. As shown in the TEM and SEM images in Figure 1c and Figure 2b, the titania-coated Ni/PSt composite particles were rough on the surface and showed a little aggregation. And the magnification of the particles in the inset of Figure 2 showed that the titania coating layer was less uniform

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Figure 2. The SEM images of particles for (a) Ni/PSt and (b) Ni/PSt/TiO2.

Figure 3. The distribution in diameter of (a) Ni/PSt and (b) Ni/PSt/TiO2 particles.

and less even on the surface. Particle size distributions measured in the SEM images are shown in Figure 3b. The average diameter of the Ni/PSt/TiO2 composite particles was 1.33 µm. The diameters of the particles had a little shift to larger ones, but with a little wider distribution, compared with that of Figure 3a. The polydispersity was 0.076. This may be due to aggregation doublets of spheres, from which the aggregation extent was estimated to be 28.7%. When the prepared nickel particles were mixed with concentrated HCl solution, a redox reaction took place immediately, accompanied by an obvious green color of the solution and hydrogen bubbles outlet from it. In contrast, there was no reaction phenomenon on the encapsulated Ni/PSt particles immersed into the same HCl solution for a few hours. Moreover, application of an external electric field to the suspension of nickel powder in silicone oil failed due to the conduction phenomenon. However, there was no conduction when applying an external electric field to the Ni/PSt particles in the same suspension. And the particles were aligned into a chain structure at 1.5 kV/mm electric field strength. These results indicated that the formed polymer-encapsulated nickel particles showed enhanced resistance toward chemical etching and a different conductive property from that of the uncoated particles. This result is consistent with that of the literature,24 which reports on the preparation of polymethacrylate-encapsulated nickel composite particles, used for a new kind of electrorheological fluid, a smart material. In addition, the dielectric constant (24) Luo, C. R.; Tang, H.; Zhao, X. P. Int. J. Mod. Phys. B 2001, 15, 672.

Figure 4. The TG-DTA curves of (a,a′) Ni, (b,b′) Ni/PSt, and (c,c′) Ni/PSt/TiO2. (Curves a, b, and c are for TG; curves a′, b′, and c′ are for DTA).

(measured by an Automatic LCR Meter 4225, Tianjing, China) of the Ni/PSt/TiO2 composite particles was measured to be 21.8 at 10 kHz, close to that of titania at the same frequency. Moreover, the alignment of the Ni/PSt/ TiO2 composite particles under the external 0.5 kV/mm electric field strength indicated the higher dielectric property imparted by the titania coating layer. Figure 4 shows the TG-DTA curves of the samples. The DTA curves of as-made nickel particles in Figure 4a′

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Figure 6. FTIR spectroscopy of particles of (a) Ni, (b) Ni/PSt, and (c) Ni/PSt/TiO2.

Figure 5. XRD patterns of particles of (a) Ni, (b) Ni/PSt, and (c) Ni/PSt/TiO2.

showed the tiny endothermic peak below 160 °C, resulting from decomposition of the organic stabilizer, and the larger exothermic peak above 360 °C, related to the oxidation of nickel. And the TG curves in Figure 4a also showed a corresponding weight alteration: the weight loss below 160 °C and the weight increase above 360 °C. In contrast, from Figure 4b,b′, the TG and DTA curves of Ni/PSt particles showed three loss slopes: below 160 °C, 200430 °C, and above 510 °C. The endothermic weight loss below 160 °C was observed as that of nickel particles. The endothermic weight loss at 200-430 °C was assigned to the desorption and decomposition of the organic polystyrene component in the particles. The exothermic weight increase, however, elevated to above 510 °C resulting from oxidation of nickel. This suggested the PSt polymer coating layer can greatly enhance metal core resistance to thermal oxidation. Figure 4c,c′ showed the TG and DTA curves of the Ni/PSt/TiO2 composite particles. Below 520 °C, the weight loss slopes and peaks were similar to those of Ni/ PSt particles, however, with a little difference that the temperature for endothermic weight loss of the PSt component was lowered from 430 to 400 °C. This was due to the effect of the decomposition of the organic component in the coated TiO2 outlayer. Above 520 °C, the exothermic weight loss of titania between 520 and 600 °C was assigned to the crystallization of amorphous titania into anatase. And above 780 °C, the endothermic weight loss was related to the crystalline transformation of anatase to rutile. As a result, the TG-DTA curves in Figure 4 provide another determination on the structure of Ni/PST/TiO2 composite particles. Figure 5a shows the XRD patterns of the nickel particles. The peak values at 2θ of 44.36 and 51.59° were assigned as characteristic diffractions of the crystalline phase of nickel. After PSt-encapsulated nickel powder was accomplished, the XRD pattern shown in Figure 5b was similar to that of the nickel particles, but the intensity of the peaks lowered, resulting from the encapsulated PSt layer on the metal cores. From Figure 5c, it can be seen that there were two phases in the curve: an amorphous one, emanating from the amorphous titania phase, and

Figure 7. The alignment of particles under an electric (or magnetic) field.

Figure 8. The formed network structure of particles under electric and magnetic fields superimposed perpendicularly.

a crystalline one, corresponding to that of the nickel component in the sphere. Moreover, the intensity of the peaks for the nickel component was lowered greatly, owing to the effect of the outlayer coating of titania. The FTIR spectra in Figure 6 provided additional evidence for the formation of multiply coated particles. The spectra of nickel particles shown in Figure 6a only showed C-O-C bonds of PEG at 1105 cm-1 without any peaks due to the effect of no transmission of nickel in the infrared range. The Ni/PSt spectra in Figure 6b revealed bonds at 755 and 698 cm-1, which corresponded to the phenyl C-H out-of-plane bending and benzene out-ofplane ring bend, respectively. Aliphatic C-H stretching resonance of PSt in 2922.56 cm-1 can be seen in the spectrum. However, the transmittance of the Ni/PSt spectra was much lowered owing to the influence of the

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nickel cores in Ni/PSt particles. The spectra of Ni/PSt/ TiO2 composite particles shown in Figure 6c indicated that the bonds of PSt have been blurred by the TiO2 component, which revealed bonds at 1125-1035 and 650450 cm-1 corresponding to Ti-O-C bending and Ti-OTi bending, respectively. The synthesized Ni/PSt/TiO2 composite particles were dispersed into silicon oil. The particles were aligned into a chain structure as shown in Figure 7 at a 0.5 kV/mm external electric field. Also, an aligned chain structure similar to that in Figure 7 was formed under a 1200 Gs magnetic field. This illustrated that the composite particles were responsive to the external electric and magnetic field. Furthermore, the network patterns shown as Figure 8 were formed under electric and magnetic fields superimposed perpendicularly, and the formed patterns can be adjusted by the value of the electric and magnetic field strength. These provide a useful method to control the behavior of particles suspended in fluids. In conclusion, the multistep coating methods involving organic polymerization and inorganic heterocoagulation-

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precipitation can be successfully utilized to produce multiply coated Ni/PSt/TiO2 microspheres. The synthesized composite particles showed good responses to the external electric and magnetic fields by the reflection of an aligned structure. Network patterns formed when external electric and magnetic fields superimposed perpendicularly were applied to the suspension of the composite particles in silicone oil. The behavior and alignment of the particles under external fields are to be further investigated. Acknowledgment. The National Natural Science Foundation Project under Grant No. 90101005 and the National Science Foundation of China for Distinguished Young Scholars under Grant No. 50025207 as well as the Doctorate Foundation of Northwestern Polytechical University (200129) are all gratefully acknowledged. LA0265631