Close-Packed Colloidal Crystalline Arrays Composed of Silica

Titania coated monodisperse silica spheres have been synthesized and fabricated as a close-packed colloidal crystalline array. We have demonstrated th...
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Langmuir 2006, 22, 1268-1272

Close-Packed Colloidal Crystalline Arrays Composed of Silica Spheres Coated with Titania Hiroshi Nakamura,* Masahiko Ishii, Azusa Tsukigase, Masashi Harada, and Hideyuki Nakano Toyota Central Research & DeVelopment Laboratories, Inc., Aichi, Japan ReceiVed July 27, 2005. In Final Form: October 4, 2005 Titania coated monodisperse silica spheres have been synthesized and fabricated as a close-packed colloidal crystalline array. We have demonstrated that the coated colloidal sphere can be used to control the peak position of the optical stop band through variation of the coating thickness. The titania coated silica spheres were prepared by the layerby-layer assembly coating process, which reciprocally laminates the cationic polyelectrolyte and the anionic titania nanosheets on a monodisperse silica spheres, and were sintered to change the titania nanosheets to anatase. The Bragg diffraction peak of the colloidal crystalline array shifted to the long wavelength region with an increase of thickness of the titania layer. Angle-resolved reflection spectra measurements clarified that the red shift was caused by increasing of the refractive index with increase of the thickness of the layer. The current work suggests new possibilities for the creation of advanced colloidal crystalline arrays with 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 photonic crystals.1 The majority of these studies, however, have been limited to polystyrene latex spheres and silica spheres because of the ease of processing these materials as colloidal spheres with extremely monodisperse sizes and the ease of controlling surface properties. Utilization of composite colloids, in particular core-shell or coated spheres, represents an interesting alternative method for the formation of novel photonic crystals.2 Structures fabricated from core-shell colloids are expected to exhibit unique optical properties. Recent experimental studies have focused on the preparation of, for example, zinc sulfide-coated polystyrene3 or silica spheres,4 core-shell latex spheres with semiconductors5 or dyes incorporated into the shell,6 and silica-core gold-shell spheres,7 suggesting that such spheres could have potential as photonic crystals. These investigations highlight the increasing interest in the development of new synthetic routes for producing homogeneously coated spheres with sufficient colloidal stability for use in colloidal crystal fabrication. One of the main challenges for the fabrication of core-shell composites using molecular precursors and nanoparticles as inorganic shell building blocks is to obtain uniform and controllable structures of shells.8 Sasaki et al. have demonstrated the layer-by-layer (LBL) assembly of titania “nanosheet” * Corresponding author. Phone: +81-561-63-6599. Fax: +81-561-636507. E-mail: [email protected] (1) (a) A special issue on photonic crystals: AdV. Mater. 2001, 13, 369. (b) A special issue on materials science aspects of photonic crystals: 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) Moroz A. Phys. ReV. Lett. 1999, 83, 5274. (3) Breen, M. L.; Dinsmore, A. D.; Pink, R. H.; Qadri, S. B.; Ratna, B. R. Langmuir 2001, 17, 903. (4) Velikov, K. P.; van Blaaderen, A. Langmuir 2001, 17, 4779. (5) Rogach A. L.; Sucha A.; Carso F.; Sukhorukov G.; Kornowshi A.; Kershaw S.; Mohwald H.; Eychmuller A.; Weller H. AdV. Mater. 2000, 12, 333. (6) Kalinina, O.; Kumacheva, E. Chem. Mater. 2001, 13, 35. (7) Graf, C.; van Blaaderen, A. Langmuir 2002, 18, 524. (8) (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.

crystallites onto the surface of colloidal polymer templates to produce core-shell structures.9 The flexibility of the nanosheets enables the reliable replication of the original template morphology and properties, e.g., monodispersity. The lamellar nanosheets are advantageous as building blocks for several reasons. Their uniform thickness in the sub-nanometer range and highly crystalline nature may lead to a well-defined shell and precise control of its thickness. In our previous works, we synthesized a titania-nanosheetscoated polystyrene latex by LBL assembly coating and subsequently attempted to fabricate the spheres into a colloidal crystalline array.10 However, the array was not sufficiently crystallinity because of delamination of the titania nanosheets layer and low surface charge of the titania-nanosheets-coated spheres. And we could not estimate the effect of the titania layer thickness on the optical properties of the colloidal crystalline array. In this paper, we demonstrate the synthesis of silica spheres coated with titania nanosheets by the LBL assembly coating method, the sintering for fixation of the titania layer, and the fabrication of the spheres into a close-packed colloidal crystalline array by use of a fluidic cell composed of two flat glass substrates and two spacers.11 Additionally, we report on the optical properties and microstructures of the colloidal crystalline array estimated by measurements of angle-resolved reflection spectra. We have investigated the possibility of using a coating of nanosheets to tune the optical properties of colloidal crystalline arrays for photonic crystal applications. 2. Experimental Section 2.1. Synthesis of the Titania Coated Monodisperse Silica Spheres. Monodisperse silica spheres were coated with titania by the LBL assembly of a cationic polyelectrolyte and anionic titania (9) (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. (c) Wang, L.; Ebina, Y.; Takada, K.; Sasaki, T. J. Phys. Chem. B 2004, 108, 4283. (d) Wang, L.; Sasaki, T.; Ebina, Y.; Kurashima, K.; Watanabe, M. Chem. Mater. 2002, 14, 4827. (10) Nakamura, H.; Ishii, M.,; Tsukigase, A.; Harada, M.; Nanano, H. Langmuir 2005, 21, 8918. (11) Ishii, M.; Nakamura, H.; Nanano, H.; Tsukigase, A.; Harada, M. Langmuir 2005, 21, 5347.

10.1021/la052034w CCC: $33.50 © 2006 American Chemical Society Published on Web 01/04/2006

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Figure 1. Process of synthesis with LBL coating and assembling of titania-coated silica spheres.

Figure 2. XRD patterns of dispersions of (a) silica spheres, (b) silica spheres coated with five layers of titania nanosheets, (c) silica spheres coated with 10 layers of titania nanosheets, and (d) sintered titania-nanosheets-coated silica spheres.

Figure 3. XRD profiles of dispersions of sintered titania-nanosheetscoated silica spheres. nanosheets onto monodisperse silica spheres and sintering after LBL coating (Figure 1). Silica spheres with an average diameter of 280 nm were purchased from Nihon Shokuibai Co. Ltd. Poly (diallyldimethylammonium chloride) (PDADMAC), a cationic polyelectrolyte, was purchased from Aldrich Co. and used as received. The titania nanosheets of composition Ti1-δO24δ- (δ ∼ 0.09) were produced by soft chemical delamination of a layered titanate. The LBL coating process of titania nanosheets/PDADMAC was performed as reported.10 The pairs of titania nanosheets/PDADMAC layers were repeatedly deposited from 3 to 10 times. After LBL coating, the coated spheres were sintered at 450 °C for 1 h, for the purpose of crystallization and fixation of titania layers on the silica spheres. That a layered structure was obtained by the repeated LBL assembly of PDADMAC and titania nanosheets was demonstrated by X-ray diffraction (XRD) data. XRD spectra were collected using a Rigaku Rint2200 with Cu KR radiation at acceleration current and voltage of 30 mA and 40 kV, respectively. The surface electrical potential (ζ-potential) of the colloidal spheres was measured by the microscope electrophoresis method using a Microtec Nichion ZEECOM2000, and scanning electron microscope (SEM) images were taken with a conventional SEM (Hitachi High Technologies S-3600N) at an acceleration voltage of 15 kV. The crystal structure

Figure 4. ζ-potential versus pH of aqueous dispersions of (a) silica spheres, (b) silica spheres coated with titania nanosheets, and (c) sintered titania-nanosheets-coated silica spheres. of the titania layers of the sintered coated spheres was examined by X-ray diffraction (XRD). 2.2. Fabrication and Characterization of the Colloidal Crystalline Array. Close-packed colloidal crystalline arrays were 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, with two openings of the same thickness as the spacer.11 A fluid reservoir was connected to one of the openings. A dispersion of the colloidal spheres was injected into the reservoir and then allowed to penetrate into the space of the cell by capillary forces. The close-packed colloidal crystalline array grew gradually from the vicinity of the other opening, and the sample was dried by solvent evaporation at room temperature. Optical properties of the arrays were evaluated by measuring their reflection spectra at normal incidence, using a multichannel spectrometer (Soma Optics, Fastevert S-2650). Structural analysis was performed by angle-resolved reflection spectroscopy. Angleresolved reflection spectra were measured by changing the angle of incidence θ between the beam and the normal to the sample surface from 9° to 34° and by collecting the light scattered in the Bragg configuration. The wavelength λpeak of each reflection peak was plotted against θ, and the interplanar spacing d111 and neff were determined by fitting the diffraction eq 1 to the plotted data. mλpeak ) 2d111(neff2 - sin2 θ)1/2

(1)

3. Results and Discussion 3.1. Titania Coated Monodisperse Silica Spheres. Figure 2 shows the XRD profiles of silica spheres coated with titania nanosheets/PDADMAC. The suspensions of silica spheres did not display a diffraction peak, but those of the synthesized spheres had diffraction peaks at around 5°, showing peak intensity increasing with the number of added titania nanosheets layers. From the Bragg diffraction conditions of the X-rays, the 5° peak corresponds to an interplanar spacing of about 1.9 nm, which can be attributed to the repeated deposition of pairs of inorganicorganic layers consisting of titania nanosheets and cationic polyelectrolyte, respectively. The thickness of a titania nanosheet is ca. 0.7-0.8 nm,8 and the thickness of a cationic polyelectrolyte layer is ca. 0.8-1.0 nm. The XRD data are in reasonable

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Figure 5. SEM images of close-packed colloidal crystalline arrays of (a) silica spheres, (b) titania-nanosheets-coated silica spheres at pH 7, (c) titania-nanosheets-coated silica spheres at pH 5, and (d) titania-nanosheets-coated silica spheres at pH 9. The scale bar in the inset corresponds to 200 nm.

agreement with this model of inorganic-organic layer pairs.11 Upon sintering of the spheres, the previously observed diffraction peak disappeared. This behavior was caused by the polymer layers being burned off, and titania being fixed and crystallized to anatase titania. The XRD profile of the sintered spheres was shown in Figure 3. The peak positions of the profile correspond with the peaks of anatase titania and supports the above assumption. Figure 4 shows the ζ-potential profiles of titania-nanosheetscoated silica spheres. The ζ-potential of the spheres depends on pH, that is, they possess a positive charge at low pH and a negative charge at high pH. The isoelectric point of silica spheres is about pH 4, and that of silica spheres coated with titania nanosheets/ PDADMAC is about pH 7. However, the isoelectric point shifted to about pH 5 for sintered spheres. This behavior was assumed to be caused by a transformation titania nanosheets/PDADMACcoated silica into anatase-titania-coated silica. The LBL assembly of nanosheets on silica spheres and subsequent sintering of the spheres are effective for fixation of the titania nanosheets to, and tuning the surface charge of, the colloidal spheres. 3.2. Fabrication and Characterization of Colloidal Crystalline Arrays. Figure 5 shows scanning electron microscopy (SEM) images of close-packed colloidal crystalline arrays made of bare silica spheres (template) and of those coated with titania nanosheets/PDADMAC. Silica spheres form hexagonal closepacked structures, but the coated silica spheres do not. Decreasing the pH of the suspension (to pH 5) or increasing its pH (to 9) caused the structure to become more regular. This behavior was caused by interparticle interactions. At pH 7, the surface charge of the spheres was very small (nearly 0) and electrostatic repulsion very weak. At pH 5 or 9, the surface charge of the spheres was large (positive or negative), and electrostatic repulsion was consequently strong. However, the crystallinity of the structures made from suspensions at pH 5 or 9 was not high enough to exhibit Bragg diffraction peaks, because of counterions shielding the electrostatic repulsion between electrical double layers. We

used sintered spheres, which were strongly positively charged at pH 7, for forming colloidal crystalline arrays. Figure 6 shows SEM images of the close-packed colloidal crystalline arrays made of anatase-titania-coated silica spheres. The crystallinity of all colloidal arrays composed of sintered spheres with different coating layers was higher than that of arrays made from non-sintered spheres. Low surface charge makes it difficult to form non-close-packed colloidal crystalline arrays, as the thickness of the electric double layer of lower surface charge is thinner. The above result supports the contention that surface charge is significant for forming close-packed colloidal crystalline arrays for not a non-closed-packed one but a closedpacked one with the drying process. Figure 7 shows the reflection spectra of the close-packed colloidal crystalline arrays made of silica spheres (template) and silica spheres coated with different thickness of anatase titania layers. 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 peak widths in the reflection spectra were narrow for the colloidal crystalline array of silica template, but increasingly broader with increasing thickness of titania layers. These changes can probably be attributed to partial destruction of the crystalline lattice (Figure 6). Peak positions in the reflection spectra were shifted to higher wavelength with increasing thickness of titania layers, and may have been caused by either increasing lattice constant or increasing refractive index. To clarify the factor responsible for this shift, angle-resolved reflection spectra measurements were performed. The interplanar spacing between (111) planes, d111, and the mean effective refractive index of this crystalline lattice, neff, were derived from the angle dependence of the reflection spectra (Figure 8A). The spectrum evolved gradually with increasing angle θ. The peak wavelength of the reflection corresponds to the resonance on a set of (111) planes, and this is a function of the angle. When the incident light was rotated toward the (111)

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Figure 8. (A) Angle-resolved reflection spectra of close-packed colloidal crystalline arrays made of silica spheres coated with five layers of titania nanosheets and sintering: Angle-dependent Bragg stop bands were observed, with the peak position shifting toward lower wavelengths as the angle was increased to 34°. (B) The wavelength of the reflection peak plotted against the incident angle of light for close-packed colloidal crystalline arrays. The solid line is the curve fit from the Bragg law, eq 1. Table 1. Interplanar Spacing (d111) and Effective Refractive Index (neff) of the Colloidal Crystalline Arrays Derived from Angle-Resolved Diffraction Spectraa

Figure 6. SEM images of close-packed colloidal crystalline arrays of silica spheres with various numbers of layers of titania nanosheets coatings and sintering: (a) 3 layers, (b) 5 layers, and (c) 10 layers. The scale bar in the inset corresponds to 200 nm.

Figure 7. Reflection spectra of close-packed colloidal crystalline arrays of silica spheres with various numbers of layers of titania nanosheets coatings and sintering: (a) silica spheres, (b) 3 layers of titania nanosheets, (c) 5 layers of titania nanosheets, and (d) 10 layers of titania nanosheets. Spectra were collected at an angle of incidence of 9°.

surface of the crystal within the zx- and zy-planes, the position of the diffraction peak shifted to shorter wavelengths. As both spectra indicated a shift to shorter wavelengths, it could be concluded that these colloidal crystalline arrays were face-centered cubic (fcc) structures.12 The wavelength λpeak of each reflection

TiO2 TiO2 TiO2

template (SiO2) 3 layers 5 layers 10 layers

d (nm)

neff

234.6 233.4 232.7 228.4

1.298 1.317 1.347 1.381

a The refractive index of the colloidal crystalline array increased with increasing thickness of titania layers without changing interplanar spacing.

peak is plotted against θ. The interplanar spacing d111 and neff were determined by fitting the Bragg diffraction eq 1. Figure 8B shows the relationship between incident angle and peak of reflection spectra of the close-packed colloidal crystals. The very close fit also indicates that these structures diffract as predicted by the Bragg law. From the fitting of eq 1, neff and d111 of the colloidal crystalline array were found to be 1.347 and 232.7 nm, respectively. In the same way, neff and d111 of colloidal crystalline arrays of silica spheres with titania layers of different thickness were derived from angle-resolved diffraction spectra (Table 1). The refractive index of the colloidal crystalline array increased with increasing thickness of titania layers without changing interplanar spacing. As the refractive index of titania (2.4-2.9) is larger than that of silica (1.45), we suppose that the refractive index of the spheres increased with application of the titania coating. The Bragg diffraction equation can be applied to these systems together with the effective medium approximation, neff ) nparticleφ + nair(1 - φ), where φ is the filling fraction of the volume occupied by the particles (φ ) 0.74 for the closepacked fcc structure). nparticle and nair are the refractive indices (12) (a) Miguez, H.; Lopez, C.; Meseguer, F.; Blanco, A.; Vazquez, L.; Mayoral, R.; Ocana, M.; Fornes, V.; Mifsud, A. Appl. Phys. Lettr. 1997, 71, 1148. (b) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266.

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Table 2. Relationship between Number of Coated Layers and Volume Ratio of Titania to the Spherea no. of coated layers

TiO2 volume ratio (%)

3 5 10

2.2 3.6 7.3

a The volume fraction of titania increased in proportion to the number of titania nanosheets coatings applied by 0.7% of total volume fraction with each coating.

of particle and air, respectively. Table 1 shows the nparticle of silica and titania-coated silica spheres in the case of φ ) 0.74. The results for nparticle of silica almost agree with 1.45. This suggests that the colloidal crystalline array forms a close-packed fcc structure. The volume fraction of titania derived from neff ) nsilicaφsilica + ntitaniaφtitania is shown in Table 2. The volume fraction of titania increased in proportion to the number of titania coatings applied by 0.7% of the total volume fraction with each coating. Overall, monodisperse silica spheres could be coated with anatase-titania by the LBL assembly method, with sintering of the spheres after coating, to fabricate close-packed colloidal crystalline arrays. The refractive indices of the particles and colloidal crystalline arrays increased in proportion to the number of titania nanosheets coatings applied.

4. Conclusions We have demonstrated the synthesis of monodisperse silica spheres coated with titania and the fabrication of these spheres into close-packed colloidal crystalline arrays. We have also

reported on the optical properties and microstructures of the colloidal crystalline array estimated by angle-resolved reflection spectra measurements. The titania nanosheets were synthesized by delamination of layered titanate crystallites. The anatasetitania-coated silica spheres were prepared by the LBL assembly process, which consisted of alternately laminating cationic polyelectrolyte and anionic titania nanosheets on monodisperse silica spheres and sintering of the spheres after lamination. The Bragg diffraction peak of the colloidal crystalline array shifted to longer wavelengths with increasing thickness of the titania layers. Angle-resolved reflection spectra measurements showed that this red shift was caused by increasing refractive index with increasing thickness of titania layers. 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 on monodisperse colloidal spheres, the current work suggests new possibilities for the creation of advanced colloidal crystalline arrays with tunable optical properties from tailored colloidal spheres. Acknowledgment. We thank Shizuyo Takashimada for her help with synthesis of the titania-nanosheets-coated silica spheres and Meiko Kato for her help with angle-resolved reflection spectra measurements. Supporting Information Available: UV absorption spectra of dispersions of silica spheres, titania nanosheets, and silica spheres coated with titania nanosheets. This material is available free of charge via the Internet at http://pubs.acs.org. LA052034W