外文文献：为双粒子电泳显示Fe3O4／SiO2粒子的制备和表征

Synthetic Metals 162 (2012) 89 94 Contents lists available at SciVerse ScienceDirect Synthetic Metals journa l h o me page: /locate/synmet Preparation and characterization of Fe3O4/SiO2particles for dual-particle electrophoretic display Sheng Liu, Gang Wu, Hong-Zheng Chen, Mang Wang MOE Key Laboratory of Macromolecule Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, State Key Lab of Silicon Materials, Hangzhou 310027, Peoples Republic of China a r t i c l e i n f o Article history: Received 30 August 2011 Received in revised form 14 November 2011 Accepted 17 November 2011 Available online 14 December 2011 Keywords: Electrophoretic display Electrophoretic mobility Coreshell structure a b s t r a c t In order to prepare metal-oxide-pigment based electrophoretic particles with good suspension stabil- ity in the low dielectric medium for electrophoretic display application, ferroferric oxide/silicon oxide (Fe3O4/SiO2) composite particles were prepared via in situ solgel method. The transmission electron microscopy (TEM)/scanning electron microscope (SEM) images present the multi coreshell morphology for the Fe3O4/SiO2particles and the average particle size is 250350 nm. It was found that the Fe3O4/SiO2 composite particles own lower density than that of Fe3O4because of SiO2encapsulation and can suspend stably in tetrachloroethylene after OLOA treatment. Zeta potential and electrophoretic mobility of the OLOA treated Fe3O4/SiO2composite particles in tetrachloroethylene were measured to be 24.2 mV and 3.80 1010m2/V s, respectively. The dual-particles electrophoretic dispersion containing OLOA treated Fe3O4/SiO2composite particles and TiO2grafted with polymer can show white/black image under low DC fi eld. 2011 Elsevier B.V. All rights reserved. 1. Introduction The electrophoretic image display (EPID), owing to advantages including paper-like high contrast appearance, ultra-low power consumption, thinness, fl exibility, etc., has attracted great interest in recent years. EPID is a refl ective display based on the movement of charged pigment particles in a low dielectric media under the effect of an electric fi eld 1,2. Efforts to make full color display with rapid response never stopped. Most of the studies focused to improve the property of white pigment particles which was applied in the single particle electrophoretic system while using dyes as the background 310. To improve the contrast of the electrophoretic display, dual-particles electrophoretic systems can be introduced 11,12, which is also essential for the development of color EPID. Thus, more attention was paid to the fabrication of the color electrophoretic particles. Yu et al. fabricated negatively charged polymer particles containing a black dye for EPID usage 13. Kim et al. prepared magenta, yellow and cyan polymer ball for the dual-particle electrophoretic display 14. Oh enwrapped organic dyes such as Acid Blue 25, Acid Red 8 and Acid Yellow 76 Corresponding author. Tel.: +86 571 8795 2557; fax: +86 571 8795 3733. E-mail address: (G. Wu). by styrene/4-vinylpyridine copolymer nanoparticles through free- radical polymerization method 15. Commonly, the optical properties and the chemical stability of the organic particles are not as good as inorganic particles, which restrict its practical application. In particular, the dissolution and swelling of organic material in the organic media could hardly be totally avoided. Although density of the inorganic pigments is too high to match that of the medium, they are still more attractive due to their strong durability in environments such as solvent, air, sun- light 16. The density of the inorganic pigment can be decreased by two routes. One is preparing composite particles 17,18. The other is selecting a suitable dispersing agent that can be absorbed on the particle surface to provide steric stabilization and electrostatic repulsion to stabilize the suspension 1921. Metal oxide including cobalt blue/green, cobalt titanate green, and nickel titanate yellow were important inorganic pigment widely applied in various areas. However, up to now, there are rarely reports about the research on the metal-oxide based color electrophoretic particles except for white pigment like TiO2, ZnO2, ZrO, Al2O310,2226. In this study, nano Fe3O4was chosen to act as an example of metal oxide. Fe3O4/SiO2composite particle was prepared by encapsulating nano Fe3O4into SiO2shell. The existence of SiO2 shell not only decreases the density of the pigment, but also facil- itates the interaction between the composite particles and the dispersing agent due to the abundant hydroxyl group on the sur- face of SiO27,2628. The Fe3O4/SiO2composite particles were 0379-6779/$ see front matter 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.11.016 90S. Liu et al. / Synthetic Metals 162 (2012) 89 94 Fig. 1. XRD pattern of Fe3O4nanoparticles and Fe3O4/SiO2composite particles. treated by commercial dispersing agent OLOA1200, which can be easily attached on the SiO2surface by its polar group. Then the treated composite particles, acting as black electrophoretic particle, were applied in the dual-particles electrophoretic system together with the polymer-grafted TiO2particles. This method to prepare the electrophoretic particle can be easily applied to modify other color oxide particles by substituting nano Fe3O4with the desired one. 2. Experimental 2.1. Material FeCl36H2O, oleic acid, ethanol, cyclohexane, tetrachloroethy- lene (C2Cl4), NH3H2O and tetraethylorthosilicate (TEOS) were pur- chased from Sinopharm Group Chemical Reagent Co. FeCl24H2O was from Shanghai Shanhai Chemical Co. Alkylphenols poly- oxyethylene (OP-10) was from Wenzhou Dongshen Chemical Co. Polyisobutylene succinimide (OLOA1200) was supplied by Jiangyin Yongda Chemical Co. All chemicals were used as received. 2.2. Preparation 2.2.1. Preparation of hydrophobic Fe3O4nanoparticles as pigment particle The Fe3O4nanoparticles were prepared by alkaline co- precipitation of FeCl36H2O and FeCl24H2O. FeCl24H2O (5.62 g) and FeCl36H2O (9.74 g) were dissolved in 80 mL of deionized water and bubbled with nitrogen for 1 h to purge oxygen under stirring. Then the solution was heated to 90C and 24 mL of ammonium hydroxide was added into the solution. After 3 min, 0.8 g oleic acid was added into the solution and the temperature was kept for another 4 h. The Fe3O4nanoparticles were washed with deionized water for times and dried at 80C. 2.2.2. Encapsulation of Fe3O4nanoparticles with SiO2 Fe3O4nanoparticles were encapsulated by SiO2through solgel method. First, 0.5 g Fe3O4nanoparticles, 4 mL TEOS and 0.2 g OP-10 Fig. 2. TEM photographs of Fe3O4nanoparticles (a) and Fe3O4/SiO2composite particles (b and c). S. Liu et al. / Synthetic Metals 162 (2012) 89 9491 Fig. 3. Particles size distribution of Fe3O4nanoparticles and Fe3O4/SiO2composite particles. were added into 40 mL deionized water, and the mixture was ultra- sonicated to become a stable emulsion. After 1 h, the emulsion was added into a mixture of 50 mL ethanol and 12 mL NH3H2O. The reaction solution was stirred at a rate of 400 rpm at room tem- perature for 4 h. The obtained products were magnetic separated, washed by ethanol for times and dried at 80C. 2.3. Characterization Morphology of the composite particles was observed by scan- ning electron microscope (SEM, JEOL100 LX) and transmission electron microscope (TEM, JEOLJEM-1230). Crystal structure of the particles was measured by X-ray power diffraction (XRD, Rigaku D/Max-2550/PC) with monochromatic Cu K? radiation (? = 1.5406A). The particle size, particle size distribution together with the zeta potential, and electrophoretic mobility were detected by Dynamic Light Scattering measurement (DLS, Zeta PALS, Brookhaven Instruments Corp.). The magnetic property was exam- ined by Vibrating Sample Magnetometer (MPMS XL-7, Quantum Design Corp.). The component of composite particles was detected by energy dispersive X-ray spectrometry (EDX, Oxford Inca Energy300). Fourier transform infrared (FT-IR) spectrum was mea- sured on Nicolet 6700 Fourier transform infrared spectrometer. Thermogravimetric analysis (TGA) was carried out on a Pyris ? Thermogravimetric Analyzer (PerkinElmer) with a heating rate of 20 K/min ramp rate in a stream of nitrogen. The UVvis refl ectance spectrum was recorded on a Shimadzu UV-2450 spectrophotome- ter. 3. Result and discussion 3.1. Synthesis and characterization of the Fe3O4/SiO2composite particles Fig. 1 shows the X-ray diffraction patterns of the iron oxide nanoparticles before and after SiO2encapsulation. Bragg refl ec- tions of the as prepared iron oxide nanoparticles can be indexed to a pure crystal phase of Fe3O4(JCPDS no. 19-0629). A broad band near 1624was observed when the iron oxide nanoparticles were coated with SiO2, demonstrating the existence of amorphous sil- ica shell. The peaks attributed to Fe3O4in both lines are similar to each other, which indicated that the crystalline of the core material is retained after encapsulation. The TEM images of the Fe3O4nanoparticles and Fe3O4/SiO2 composite particles are shown in Fig. 2a and b respectively. Because of the hydrophobicity for the oleic acid treated Fe3O4nanoparticles, the particles tended to be trapped in the oil droplets. When the emulsion was added into the mixture of ammonia and ethanol, the SiO2shell was formed by the hydrolyzation of TEOS and the Fe3O4 nanoparticles were enwrapped in. Obvious contrast between the core and shell can be detected in Fig. 2c, indicating the formation of multi coreshell structure. The diagram in Fig. 3 shows the par- ticle size and particle size distribution of the Fe3O4and Fe3O4/SiO2 particles. When the Fe3O4nanoparticles with size less than 20 nm were encapsulated into SiO2, the diameter of composite particles increases up to 250350 nm. The result is in good agreement with the size of particles shown in TEM images. To further examine the morphology of composite particles, the SEM image of the Fe3O4/SiO2composite particle is shown in Fig. 4a. It is obvious that the particle is made up of several small parti- cles. In other words, the Fe3O4/SiO2composite particle has a multi coreshell structure, which is good for the anchoring of the dispers- ing agent by increasing the surface area of the composite particles each. The component of the composite particles was confi rmed by EDX spectrometry given in Fig. 4b, in which the signal of Al ele- ment comes from the substrate. From the content of Si (16.2%) and Fe (9.00%), the approximate density of composite particle can be calculated by the following equation: ?composite= 1 (WSiO2/?SiO2) + (WFe3O4/?Fe3O4) in which W is the mass fraction of the component. By assum- ing the densities of 5.18 g/cm3for Fe3O4and 2.32 g/cm3for SiO2, Fig. 4. SEM photograph (a) and EDX spectrometry (b) of Fe3O4/SiO2composite particles. 92S. Liu et al. / Synthetic Metals 162 (2012) 89 94 Fig. 5. Magnetization vs. applied magnetic fi eld for Fe3O4/SiO2composite particles at room temperature. the approximate density of composite particle is 2.99 g/cm3, much lower than that of the oxide pigment. The superparamagnetic property of the Fe3O4/SiO2composite particles is essential for the application in electrophoretic dis- play, which prevents composite particles from aggregation. Fig. 5 shows the magnetization curve of the Fe3O4/SiO2composite par- ticles. Almost no remanence was observed when the magnetic fi eld was removed, revealing superparamagnetism of the par- ticles. Thus, when the Fe3O4/SiO2composite particles acted as electrophoretic particles, the agglomeration caused by inter- particle magnetic force can be ignored. 3.2. Application of Fe3O4/SiO2composite particles in dual-particle electrophoretic system Fig. 6 shows the FT-IR spectra of the Fe3O4/SiO2composite par- ticles with/without OLOA1200 treatment. Owing to the existence of abundant hydroxyl groups on the SiO2surface, the Fe3O4/SiO2 composite particle can easily absorb OLOA 1200 after ultrasonic treatment. The anchoring of OLOA 1200 on the particles surface could be proved by the appearance of the CH stretching vibration peak at 2920 cm1, 2829 cm1from OLOA 1200. From the TGA analysis shown in Fig. 7, for the OLOA treated composite particles, when the temperature is beyond 200C, the weight loss of 8.63% could be found, which can be attributed to the Fig. 6. FT-IR spectra of Fe3O4nanoparticles, Fe3O4/SiO2composite particles with/without OLOA1200 treatment. Fig. 7. TGA curves of OLOA1200 treated Fe3O4/SiO2composite particles. Fig. 8. Suspending behavior of the Fe3O4/SiO2composite particles in tetra- chloroethylene, before and after OLOA1200 treatment. thermal degradation of OLOA1200. The weight loss below 200C could be ascribed to the evaporation of physically absorbed water and residual solvent in the samples. As shown in Fig. 8, because of the existence of OLOA1200, the suspension stability of the Fe3O4/SiO2composite particles in the Fig. 9. UVvis refl ectance spectra of dual-particles electrophoretic dispersion. S. Liu et al. / Synthetic Metals 162 (2012) 89 9493 Fig. 10. Images of the white and black stated of the electrophoretic display prototype. media was greatly enhanced, which can be attributed the steric hindrance effect coming from the long chain of OLOA1200 extend- ing in the media. Moreover, even after longtime ultrasonication and repeating washing, the composite particles can still suspend in the media stably. Owing to the functional group on the chain of OLOA1200, the particles were positively charged in the media. The zeta potential and electrophoretic mobility of the composite parti- cles in tetrachloroethylene were 24.2 mV and 3.80 1010m2/V s respectively. The OLOA treated SiO2/Fe3O4composite particles were applied as black electrophoretic particle in the dual-particles elec- trophoretic system together with the negative charged polymer- grafted TiO2particles. An EPID prototype was fabricated by inject- ing electrophoretic suspension into a cell constructed by two ITO glass. Under the DC fi eld of 0.075 V/?m, as shown in Fig. 9, the pos- itive/negative charged electrophoretic particles can move towards opposite direction and white/black image can display. The refl ec- tivity of the white/black image in the visible regions of 400750 nm determined by UVvis spectroscopy in a refl ectance mode is shown in Fig. 10. The contrast of the EPID can be calculated by the formula applied in the fi eld of refl ective liquid crystal display 29,30. C?= ? ?W(?)d? ? ?BK(?)d? in which C?is the refl ectance contrast ratio, ?W(?) and ?BK(?) is the refl ectance of white and black state at different wavelength, ? is the wavelength of incident light. ? ?W(?)d? and ? ?BK(?)d? is can be measured by Shimadzu UV-2450 spectrophotometer. The refl ectance contrast ratio of the dual-particles electrophoretic suspension is about 15.4. 4. Conclusion Fe3O4/SiO2multi coreshell composite particles with narrow particle-size distribution from 250 to 350 nm were prepared via solgel method by encapsulating Fe3O4nanoparticles into silica. SiO2enwrapping makes the density of the Fe3O4/SiO2composite particles much lower than that of the Fe3O4pigment. The existence of SiO2shell facilitates the anchoring of the charge control agent OLOA1200 on to the particle surface, which help the Fe3O4/SiO2 composite particles suspend stably in tetrachloroethylene. The zeta potential and electrophoretic mobility of the composite par- ticles in tetrachloroethylene are 24.2 mV and 3.80 1010m2/V s respectively. The composite particles can were applied in the dual- particles electrophoretic system with TiO2grafted with polymer. The dual-particles electrophoretic dispersion shows a high contrast ratio of about 15.4 under an electric fi eld of 0.075 V/?m. Color elec- trophoretic particles can be prepared through analogous method by substituting Fe3O4with any other metal oxide with desired color. Acknowledgments This work was fi nancially supported by the National High Technology Research and Development Program of China (863 Pro- gram) (Grant No. 2008AA03A331) and the developing program of Changjiang Scholar and Innovation Team from Ministry of Educa- tion of China under Grant No. IRT0651. References 1 B. Comiskey, J.D. Albert, H. Yoshizawa, J. Jacobson, Nature 253 (1998) 394. 2 Y. Chen, J. Au, P. Kazlas, A. Ritenour, H. Gates, M. 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