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Langmuir 2005, 21, 8918-8922
Close-Packed Colloidal Crystalline Arrays Composed of Polystyrene Latex Coated with Titania Nanosheets Hiroshi Nakamura,* Masahiko Ishii, Azusa Tsukigase, Masashi Harada, and Hideyuki Nakano Toyota Central Research & Development Laboratories, Inc. Aichi, Japan Received March 27, 2005. In Final Form: June 8, 2005 We have demonstrated that polystyrene latex coated with titania nanosheets can be fabricated into a close-packed colloidal crystalline array, and that these coated colloidal spheres can be used to control the peak position of optical stop bands through the coating. The titania-nanosheets-coated polystyrene latex was prepared by the layer-by-layer (LBL) assembly coating process, involving alternating lamination of cationic polyelectrolytes and anionic titania nanosheets on monodisperse polystyrene latex particles. The Bragg diffraction peak of the colloidal crystalline array shifted to longer wavelengths with the coating of titania nanosheets. This red shift was caused by an increase in refractive index upon coating, as revealed by angle-resolved reflection spectra measurements. The current work suggests new possibilities for the creation of advanced colloidal crystals having tunable optical properties from tailored colloidal spheres.
1. Introduction Colloidal crystalline arrays, which are three-dimensionally periodic lattices of monodisperse colloidal spheres, have recently been examined by many research groups for application to photonic crystals.1 The majority of these studies, however, have been limited to polystyrene latex particles and silica spheres because of the ease of processing these materials as colloidal spheres with extremely monodisperse sizes and controlled surface properties. A few demonstrations have involved the synthesis and crystallization of colloidal spheres made of compound semiconductors, such as CdS and ZnS.2 Photonic band structures of these new systems have been shown to exhibit features different from those of conventional opals due to their higher refractive indices relative to those of polystyrene or silica. Among various inorganic semiconductors, titania has long been considered an ideal candidate for forming photonic crystals due to its low absorption in the visible and near-infrared regions and its relatively high refractive index. Unfortunately, it has been difficult to prepare titania as monodisperse colloidal spheres with size variations within 5%. Utilization of core-shell-structured colloidal spheres is an interesting alternative method for the formation of novel photonic crystals expected to exhibit unique optical properties.3 The layer-by-layer (LBL) templating strategy, the basis of which is primarily the electrostatic attraction between oppositely charged species deposited from solution onto colloidal spheres, lends itself well to the task of producing colloidally stable, homogeneously coated par-
ticles.4 This flexible and facile procedure permits the coating of colloids of various shapes and sizes with uniform layers of diverse composition. The advantages associated with the use of the LBL-constructed core-shell-structured colloidal spheres as building blocks for the formation of engineered photonic crystals are that the colloidal spheres can be designed to contain a core with a low refractive index surrounded by a shell of material with a high refractive index. Recently, titania “nanosheet” crystallites have been produced by soft-chemical delamination of layered titanate crystals.5 The nanosheets possess the various properties of layered titanate crystals and may behave as pieces of wrapping paper when covering the template colloidal spheres via the LBL assembly approach.6 Since the flexibility of the nanosheets enables reliable replication of the original template morphology, monodisperse colloidal spheres with titania shells can be synthesized by the LBL assembly coating of titania nanosheets onto monodisperse latex particles. In this study, we demonstrate the synthesis of titaniananosheets-coated polystyrene latex by LBL assembly coating and the fabrication of the particles into a closepacked colloidal crystalline array. We also report on the optical properties and microstructures of the colloidal crystalline array estimated by angle-resolved reflection spectra measurements. Finally, we explore the novel prospects of using the nanosheets coating to tune the optical properties of the colloidal crystalline array for use in photonic crystal applications. 2. Experimental Section
* To whom correspondence should be addressed. E-mail:
[email protected]. (1) (a) A special issue on photonic crystals in Adv. Mater. 2001, 13, 369. (b) A special issue on materials science aspects of photonic crystals in MRS Bull. 2001, 26, 608. (c) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693. (d) van Blaaderen, A.: Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (2) (a) Murphy-Wilhemy, D.; Matijevic, E. J. Chem. Soc., Faraday Trans. 1 1984, 80, 563. (b) Matijevic, E.; Murphy-Wilhemy, D. J. Colloid Interface Sci. 1982, 86, 476. (3) (a) Velikov, K. P.; van Blaaderen, A. Langmuir 2001, 17, 4779. (b) Breen, M. L.; Dinsmore, A. D.; Pink, R. H.; Qadri, S. B.; Ratna, B. R. Langmuir 2001, 17, 903. (c) Kalinina, O.; Kumacheva, E. Chem. Mater. 2001, 13, 35. (b) Graf, C.; van Blaaderen, A. Langmuir 2002, 18, 524.
2.1. Synthesis of the Titania-Nanosheets-Coated Polystyrene Latex. The titania-nanosheets-coated polystyrene latex was prepared by LBL assembly of cationic polyelectrolyte and (4) (a) Caruso, F.; Carso, R. A.; Mohwald, H. Science 1998, 282, 1111. (b) Caruso, F. Adv. Mater. 2001, 13, 11. (c) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400. (d) Caruso, F.; Shi, X.; Carso, R. A. Adv. Mater. 2001, 13, 740. (5) (a) Sasaki, T.; Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4682. (b) Sasaki, T.; Nakano, S.; Yamauchi, S.; Watanabe, M. Chem. Mater. 1997, 9, 602. (6) (a) Wang, L.; Ebina, Y.; Takada, K.; Sasaki, T J. Phys. Chem. B 2004, 108, 4283. (b) Wang, L.; Sasaki, T.; Ebina, Y.; Kurashima, K.; Watanabe, M. Chem. Mater. 2002, 14, 4827.
10.1021/la050805q CCC: $30.25 © 2005 American Chemical Society Published on Web 08/10/2005
Close-Packed Colloidal Crystalline Arrays anionic titania nanosheets on monodisperse polystyrene latex particles.5 The titania nanosheets having composition Ti1-δO24δ(δ ∼ 0.09) was produced by soft chemical delamination of a layered titanate, CsTi1-δO24δ- (δ ∼ 0.09), which was obtained by reaction of CsO2 with TiO2. Before the delamination, CsTi1-δO24δ- was substituted to HTi1-δO24δ- by addition of HCl solution and exfoliated by shaking with tetrabutylammonium oxide solution. Monodisperse polystyrene latex was obtained by soap-free emulsion polymerization.7 Styrene monomer was polymerized at 65 °C over a period of 6 h with sodium p-styrene sulfonate, which plays the role of emulsifier and stabilizer of emulsion particles, using potassium persulfate (K2S2O8) as the initiator. The average diameter of the synthesized latex particles was 260 nm, and deviation was below 5%. Prior to the adsorption of the negatively charged titania nanosheets, the surfaces of the polystyrene particles were modified by adsorbing a cationic polyelectrolyte, poly(diallyl dimethylammonium chloride) (PDADMAC), in aqueous solution. One gram of polystyrene particles was dispersed in 200 mL of H2O containing 0.1 g of PDADMAC and 0.5 mol/L of NaCl under stirring, and then the suspension was ultrasonically treated for 20 min. Excess PDADMAC was removed by twice repeating a cycle of centrifugation (17 000 rpm, 30 min) and water washing. Afterward, the PDADMAC-coated polystyrene particle was dispersed in 100 mL of H2O again; 100 mL of H2O containing 0.05 g of titania nanosheets was added to the turbid polystyrene suspension under stirring. Immediately after addition of the suspension, some flocculated aggregates formed in the mixture due to the electrostatic interaction of the oppositely charged nanosheets and polystyrene surfaces. Excess titania nanosheets were removed by twice repeating a cycle of centrifugation (17 000 rpm, 30 min) and water washing. The pairs of layers (of PDADMAC alternating with titania nanosheets) were repeatedly deposited 10 times. The polystyrene latex consisted of negatively charged particles. The coating process entailed the sequential deposition of oppositely charged nanosheets and polyelectrolytes onto colloidal spheres, exploiting primarily electrostatic interactions for the buildup of nanosheets and polyelectrolyte layers. Following deposition of these layers, excess nanosheets and polyelectrolytes were removed by centrifugation and filtration, with intermediate water washing. The key to the formation of the nanosheets and polyelectrolyte multilayers was that not all of the cationic (anionic) groups of the deposited polyelectrolytes (nanosheets) interacted with the particle surface. Hence, unutilized charged groups, which caused charge overcompensation, facilitated the electrostatic binding of the subsequently adsorbed layer. The presence of titania nanosheets was confirmed by absorption at 265 nm5 in diffuse reflection UV spectra recorded using a Hitachi UV-2100 spectrophotometer. That a layered structure was obtained by repeated LBL assembly of PDADMAC and titania nanosheets was demonstrated by X-ray diffraction (XRD) data. The XRD spectrum was collected on a Rigaku Rint2200 instrument using Cu KR radiation at an acceleration current and a voltage of 30 mA and 40 kV, respectively. The surface electrical potential (ζ-potential) of the colloidal spheres was measured by a microscope electrophoresis method using a Microtec Nichion ZEECOM2000 instrument. Scanning electron microscope (SEM) images were taken with a conventional SEM (Hitachi High Technologies S-3600N) at an acceleration voltage of 15 kV. 2.2. Fabrication and Characterization of the Colloidal Crystalline Array. The close-packed colloidal crystalline array was prepared by injecting a dispersion of the colloidal spheres into a fluidic cell composed of two flat glass substrates with two spacers sandwiched between them.8 The cell had two openings with the same thickness as the spacer. A fluid reservoir was connected to one of the openings. The dispersion of colloidal spheres was injected into the reservoir and allowed to penetrate into the space of the cell by capillary forces. The close-packed colloidal crystalline array grew gradually from the vicinity of (7) (a) Juang, M. S.-D.; Kriger, I. M. J. Polym. Sci. 1976, 14, 2089. (b) Chonde Y.; Kriger, I. M. J. Appl. Polym. Sci. 1981, 26, 1819. (8) Ishii, M.; Nakamura, H.; Nanano, H.; Tsukigase, A.; Harada, M. Langmuir 2005, 21, 5347.
Langmuir, Vol. 21, No. 19, 2005 8919 the other opening, and the sample was dried by solvent evaporation at room temperature. Optical properties of the colloidal crystalline array were evaluated by measuring their reflection spectra at normal incidence, using a multichannel spectrometer (Soma Optics, Fastevert S-2650). Structural analysis was performed by angleresolved reflection spectroscopy. Angle-resolved reflection spectra were measured by changing the angle of incidence θ between the beam and the normal of the sample surface from 9 to 40° and collecting the light scattered in the Bragg configuration. The Bragg equation is given by eq 1:
mλpeak ) 2 d111 (neff2 - sin2 θ)1/2
(1)
where m is the order of diffraction; λpeak is the wavelength of the Bragg diffraction peak; d111 is the spacing between (111) planes; θ is the angle between the incident light and the normal to the diffraction planes (at normal incidence, θ ) 0°); and neff is the mean effective refractive index of this crystalline lattice. The wavelength, λpeak, of each reflection peak was plotted against θ. The interplanar spacing, d111 and neff, were determined by fitting the Bragg equation to the plotted data.
3. Results and Discussion 3.1. Titania-Nanosheets-Coated Polystyrene Latex. Figure 1 shows TEM images of the synthesized titania-nanosheets-coated polystyrene latex. The polystyrene core/titania nanosheets shell particles prepared in this way existed as single, unaggregated particles in solution. The coated particles had a narrow size distribution (size variations within 5%) almost equal to that of the monodisperse polystyrene latex template. The coated particles had some sheet material on their surfaces, as seen from the slightly rough surfaces of the particles in the TEM image in Figure 1b. However, the particles were coated uniformly by titania nanosheets. Figure 1c shows that the titania nanosheets formed a multilayer structure on the particle surfaces. It is apparent that the surfaces of these core-shell particles are smoother than those with adsorbed nanoparticles because nanosheets are much thinner than nanoparticles. Figure 2 shows the UV absorption spectra of dispersions of the particles. The titania-nanosheets-coated polystyrene latex displayed dual peaks at 230 and 265 nm. The absorption band centered at around 230 nm with a shoulder is diagnostic of polystyrene polymers, and the absorbance at 265 nm is attributable to the titania nanosheets.5 The presence of both absorbances indicates coverage of the polystyrene latex surface with the titania nanosheets. XRD profiles of a dispersion of the colloidal spheres indicate the microstructure of the particle (Figure 3). A diffraction peak at around 5° was detected in the profile of the titania-nanosheets-coated polystyrene latex. From the Bragg diffraction equation, the 5° peak corresponds to an interplanar spacing of about 1.9 nm, which can be attributed to the repeated inorganic-organic layer pairs of titania nanosheets and PDADMAC. The thickness of a titania nanosheet is ca. 0.7-0.8 nm,5 and the thickness of a PDADMAC monolayer is ca. 0.8-1.0 nm. The inorganic-organic layer pairs model is in reasonable agreement with the XRD data. These results suggest the titania nanosheet/PDADMAC coating is highly uniform and regular, and that this method constitutes a suitable synthetic process for fabricating novel monodisperse colloidal spheres. Figure 4 shows the ζ-potential of the particles. The ζ-potential of polystyrene latex is about -40 mV, independent of dispersion pH. However, that of titaniananosheets-coated polystyrene latex depends on pH; that is, the particles carry positive charge at low pH and
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Figure 1. TEM images of (a) polystyrene latex, (b) titania-nanosheets-coated polystyrene latex prepared by the LBL assembly coating method, and (c) high-magnification image showing the nanosheets layers of titania-nanosheets-coated polystyrene latex. The scale bar in the inset corresponds to 100 nm.
Figure 2. UV absorption spectra of a dispersion of (a) titania nanosheets, (b) polystyrene latex, and (c) titania-nanosheetscoated polystyrene latex.
Figure 3. XRD patterns of a dispersion of particles of (a) polystyrene latex, (b) titania-nanosheets-coated polystyrene latex, and a schematic illustration of titania-nanosheets-coated polystyrene latex prepared by the LBL assembly coating method.
negative charge at high pH, with the isoelectric point at about pH 7. This behavior, which is almost identical to that of titania particles, confirms that the polystyrene latex particles have been coated uniformly with titania nanosheets. The LBL assembly of nanosheets is effective not only for maintaining monodispersity but also for tuning surface charge on the colloidal spheres. Thus, the self-assembly of nanosheets onto monodisperse colloidal spheres via the LBL technique allows the production of monodisperse core-shell-structured col-
Figure 4. Plots of ζ-potential versus pH for (a) polystyrene latex and (b) titania-nanosheets-coated polystyrene latex.
loidal spheres with tailored compositions and well-defined morphologies. 3.2. Fabrication and Characterization of Colloidal Crystalline Arrays. Figure 5 shows scanning electron microscopy (SEM) images of the close-packed colloidal crystalline arrays made of polystyrene latex (template) and of titania-nanosheets-coated polystyrene latex. Polystyrene latex particles form a hexagonal close-packed structure, but titania-nanosheets-coated polystyrene latex particles do not. This is due to two reasons. One is the contamination from nanosheets impurities during the LBL assembly coating process, and the other is the low surface charge (as shown in Figure 4) of the coated particles near pH 7, to which the dispersion of particles is exposed during drying (due to solvent evaporation). In general, a colloidal crystalline array is formed by electrostatic interactions between charged monodisperse spheres in aqueous dispersion. Low surface charge makes it difficult to form a non-close-packed colloidal crystalline array. At pH 7, surface charge of the particle was very small, nearly 0, and electrostatic repulsion was very weak. At pH 5 or 9, surface charge of the particle was large (positive or negative), and electrostatic repulsion was strong. However, the crystallinity of the structure of the suspension at pH 5 or 9 was not good enough for indicating Bragg diffraction peak because the counterions shielded the electrostatic repulsion between electric double layers. The above results support the fact that surface charge is significant for forming a close-packed colloidal crystalline array during
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Figure 7. Angle-resolved reflection spectra of close-packed colloidal crystalline arrays made of (a) polystyrene latex and (b) titania-nanosheets-coated polystyrene latex. Angle-dependent Bragg stop bands were observed, with the peak position shifting toward lower wavelengths as the angle was increased to 43°.
Figure 5. SEM images of close-packed colloidal crystalline arrays of (a) polystyrene latex and (b) titania-nanosheetscoated polystyrene latex. The scale bar in the inset corresponds to 200 nm.
Figure 6. Normalized reflection spectra of close-packed colloidal crystalline arrays made of (a) polystyrene latex and (b) titania-nanosheets-coated polystyrene latex. Spectra were collected at an angle of incidence of 9° with respect to the (111) crystal plane.
the drying process because negatively and positively charged particles aggregate with each other during the process. Figure 6 shows the reflection spectra of the close-packed colloidal crystalline arrays made of polystyrene latex (template) and titania-nanosheets-coated polystyrene latex. The (111) planes of the colloidal crystalline array were oriented parallel to the surface of the supporting
substrate. In the measurements, the incident light and detector were both oriented perpendicular to the (111) plane of this lattice. The width of the peak in the reflection spectrum of the colloidal crystalline array of the polystyrene template was narrow, but broadened with the coating of titania nanosheets. These changes can probably be attributed to partial destruction of the crystalline lattice (Figure 5) due to formation of nanosheets impurities and decrease in surface charge (Figure 4). The position of the peak in the reflection spectrum was shifted to higher wavelength after application of the coating and may have been caused by either increased lattice constant or increased refractive index. To identify the responsible factor, angle-resolved reflection spectra were measured. Figure 7 shows the angle dependence of the reflection spectra of the dispersions. The wavelength, λpeak, of each reflection peak is plotted against θ. Each spectrum evolved gradually with increasing θ. The peak wavelength of the reflection corresponds to the resonance on a set of (111) planes as a function of the angle. When the incident light is rotated toward the (111) surface of the crystal within the zx- and zy-planes, the position of the diffraction peak shifts to shorter wavelengths. As both spectra displayed a shift to shorter wavelengths, it could be concluded that these colloidal crystalline arrays had a face-centered cubic (fcc) structure. The interplanar spacings, d111 and neff, were determined by fitting the Bragg diffraction of eq 1 to the plotted data. Figure 8 shows the relationship between incident angle and the peak of reflection spectrum of the close-packed colloidal crystals. The very close fit confirms that these structures exhibit Bragg diffraction. From the fitting of eq 1, neff of the colloidal crystalline arrays of polystyrene latex and titania-nanosheets-coated polystyrene latex was found to be 1.512 and 1.566, respectively, and d111 values were 187.8 and 187.7 nm, respectively. The refractive index of the colloidal crystalline array increased with the coating of titania nanosheets without
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styrene particles were 1.65 and 1.73, respectively. As a result, the refractive index of the polystyrene particles shouldhave increasedupon coatingwith titaniananosheets. However, the above analysis indicates a discrepancy in the refractive index of polystyrene latex between theory and measurement. We submit that the volume fraction of the particles may not be 0.74 because of partial melting of polystyrene latex, resulting in increased volume fraction. 4. Conclusions
Figure 8. Wavelength of reflection peak versus incident angle of light for close-packed colloidal crystalline arrays made of (a) polystyrene latex and (b) titania-nanosheets-coated polystyrene latex. The solid lines are curves fitted using the Bragg law, eq 1.
changing the interplanar spacing. As the refractive index of titania (2.4-2.9) is larger than that of polystyrene (1.6), we suppose that the refractive index of the particle increased with application of the titania nanosheets coating. The Bragg diffraction equation can be combined with the effective medium approximation, namely, that neff ) nparticle φ + nair(1 - φ), where φ is the filling fraction of the volume occupied by the particles (φ ) 0.74 for the close-packed fcc structure), and nparticle and nair represent the refractive indices of particle and air, respectively. If φ ) 0.74 for both samples, the refractive indices of polystyrene particles and titania-nanosheets-coated poly-
We have demonstrated the synthesis of titaniananosheets-coated polystyrene latex and fabrication of the particles into close-packed colloidal crystalline arrays. We have also reported on the optical properties and microstructures of these arrays, estimated by angleresolved reflection spectra measurements. The titania nanosheets were synthesized by delamination of layered titanate crystallites. The titania-nanosheets-coated polystyrene latex was prepared by a layer-by-layer assembly process in which a cationic polyelectrolyte and anionic titania nanosheets were alternately deposited on a monodisperse polystyrene latex. The Bragg diffraction peak of the colloidal crystalline array shifted to a longer wavelength with the coating. This red shift was caused by an increase in refractive index with the coating, shown by angle-resolved reflection spectra. Since a wide range of coated colloids of different size, composition, and optical properties can be prepared via the LBL assembly coating of various nanosheets onto monodisperse colloidal spheres, the current work suggests new possibilities for the creation of advanced colloidal crystalline arrays from tailored colloidal spheres. Acknowledgment. We thank Yusuke Akimoto for his help with TEM observations. LA050805Q
