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Langmuir 2006, 22, 2444-2446
Reversible Control of Light Reflection of a Colloidal Crystal Film Fabricated from Monodisperse Mesoporous Silica Spheres Yuri Yamada,* Tadashi Nakamura, Masahiko Ishi, and Kazuhisa Yano Toyota Central Research & DeVelopment Laboratories, Inc., Nagakute, Aichi 480-1192, Japan ReceiVed NoVember 23, 2005. In Final Form: January 18, 2006 In this letter, we report a novel method for controlling the light reflection of a colloidal crystal. Highly monodisperse mesoporous silica spheres have been successfully organized into a hexagonally close-packed colloidal crystal film. Just by introducing water vapor into the fabricated colloidal film, the structural color and reflection spectra were changed dramatically because of water vapor adsorption occurring in the mesoporous channels. This phenomenon can be observed reversibly over five cycles. We are convinced that this is the first report on controlling the light reflection of a colloidal crystal film dynamically by taking advantage of adsorption properties inherent to mesoporous silica spheres.
Introduction Since Sto¨ber first synthesized monodisperse silica spheres in 1968,1 much attention has been paid to monodisperse colloidal spheres for use in separating molecules,2 chemical and biochemical sensors,3,4 colloidal crystals,5-8 and drug delivery.9 Among these applications, colloidal crystals have attracted much attention because they are considered to act as photonic crystals. The concept of photonic crystals was first proposed by Yablonovich10 and John11 in 1987. Photonic crystals are spacially periodic structures constructed from dielectric materials with different refractive indices; therefore, unique properties relating to the propagation of light are produced. To realize photonic crystals, much research has been conducted with respect to fabrication and simulation and the measurement of the optical properties of the colloidal crystals.12 So far, many kinds of monodisperse colloidal spheres have been proposed to achieve these goals.13-15 Recently, we described the synthesis of highly monodisperse mesoporous silica spheres having ordered starburst mesopores with hexagonal regularity.16 Many researchers also reported the synthesis of mesoporous silica spheres.17-20 Some of them mentioned fabricating mesoporous spheres into colloidal crystals.21-23 However, none of them reported the control of optical properties, such as a reflection spectrum. Furthermore, * Corresponding author. E-mail:
[email protected]. (1) Sto¨ber, W.; Fink, A. J. Colloid Interface Sci. 1968, 26, 62. (2) Newton, M. R.; Bohaty, K. H.; White, S.; Zharov, I. J. Am. Chem. Soc. 2005, 127, 7268. (3) Zhang, P.; Guo, J. H.; Wang, Y.; Pang, W. Q. Mater. Lett. 2002, 3, 400. (4) Velev, O. D.; Kaler, E. W. Langmuir 1999, 15, 3693. (5) Luck, W.; Kiler, M.; Wesslau, H.; Bunsenges, B. Phys. Chem. 1963, 67, 75. (6) Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. H. Nature 1997, 386, 143. (7) Pendry, J. Science 1999, 285, 1687. (8) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. AdV. Mater. 2000, 12, 693. (9) Zahr, A. S.; Villers, M.; Pishko, M. V. Langmuir 2005, 21, 403. (10) Yablonovich, E. Phys. ReV. Lett. 1987, 58, 2059. (11) John, S. Phys. ReV. Lett. 1987, 58, 2486. (12) AdV. Mater. 2001, 13 (special issue on photonic crystals). (13) Lin, Y.; Zhang, J.; Sargent, E. H. Appl. Phys. Lett. 2002, 81, 3134. (14) Liddell, C. M.; Summers, C. J. AdV. Mater. 2003, 15, 1715. (15) Jeong, U.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 3099. (16) Yano, K.; Fukushima, Y. J. Mater. Chem. 2003, 13, 2577. (17) Gru¨n, M.; Bu¨chel, G.; Kumar, D.; Schumacher, K.; Bidlingmajer, B.; Unger, K. K. Stud. Surf. Sci. Catal. 2000, 128, 155. (18) Schumacher, K.; Renker, S.; Unger, K. K.; Ulrich, R.; Chesne, A. D.; Spiess H. W.; Wiesner, U. Stud. Surf. Sci. Catal. 2000, 129, 1. (19) Luo, Q.; Li, L.; Xue, Z.; Zhao, D. Stud. Surf. Sci. Catal. 2000, 129, 37. (20) Gru¨n, M.; Lauer, I.; Unger, K. K. AdV. Mater. 1997, 9, 254. (21) Wang, YJ.; Caruso, F. AdV. Funct. Mater. 2004, 14, 1012. (22) Chen, C. N.; Lin, H. P.; Tsai, C. P.; Tang, C. Y. Chem. Lett. 2004, 33, 838. (23) Yang, S. M.; Coombs, N.; Ozin, G. A. AdV. Mater. 2000, 12, 1940.
the superiority of mesoporous does not seem to be well utilized with respect to the characteristics of a colloidal crystal. Many potential applications for photonic crystals require the tuning of a reflection spectrum, which mainly depends on the interplanar spacing and the refractive index of the fabricated film. There have been some attempts to control the reflection spectra by changing the interplanar spacing. By using elastic spheres, the reflection spectrum has been changed by applying an external mechanical force.24,25 Interplaner spacings have been filled with hydrogels, and the reflection spectra have been tuned by changing the temperature, the pH, and the ionic state due to the volume phase transition of the hydrogels.26 Some methods to control the refractive index have also been studied. For example, appropriate dyes have been used as a medium for changing the refractive index.27 Other candidates for the medium are liquid crystals. The refractive index can be changed by controlling the orientation of molecules with optical anisotropy.28 However, controlling the reflection spectrum by alternating the refractive index of the silica spheres themselves is still a challenge. Here, we describe a novel strategy to control the reflection spectrum of a colloidal crystal film just by utilizing the adsorption properties of monodisperse mesoporous silica spheres. Experimental Section Highly monodisperse mesoporous silica spheres were prepared from hexadecyltrimethylammonium bromide and tetramethoxysilane in accordance with our previous report.16 We represent the monodisperse mesoporous silica spheres by the acronym MMSS hereafter. To fabricate colloidal crystals from MMSS, we adopted the fluidic cell.29 The cell was composed of two flat glass substrates placed 30 µm apart. Glass microscope slides (Matsunami, S-1112, 76 × 26 × 1 mm) were used as glass substrates and cleaned in a UV/ozone cleaner (Nippon Laser) for 20 min. To obtain a wellordered colloidal crystal, we prepared a 10 wt % aqueous dispersion of MMSS. Before injection into the cell, continuous sonication was maintained for at least 3 h. A sufficiently disperse solution was fed into the cell by capillary forces and formed into a colloidal crystal by evaporating water gradually at room temperature. (24) Iwayama, Y.; Yamanaka, J.; Takiguchi, Y.; Takasaka, M.; Ito, K.; Shinohara, T.; Sawada, T.; Yonese, M. Langmuir 2003, 19, 977. (25) Yoshino, K.; Kawagishi, Y.; Ozaki, M.; Kose, A. Jpn. J. Appl. Phys. 1999, 38, L786. (26) Lee, K.; Asher, S. A. J. Am. Chem. Soc. 2000, 122, 9534. (27) Gu, Z.-Z.; Iyoda, T.; Fujishima, A.; Sato, O. AdV. Mater. 2001, 13, 1295. (28) Kubo, S.; Gu, Z.-Z.; Takahashi, K.; Fujishima, A.; Segawa, H.; Sato, O. J. Am. Chem. Soc. 2004, 126, 8314. (29) Ishii, M.; Nakamura, H.; Nakano, H.; Tsukigase, A.; Harada, M. Langmuir 2005, 21, 5367.
10.1021/la0531695 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/14/2006
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Langmuir, Vol. 22, No. 6, 2006 2445
Figure 1. SEM images of MMSS: (a) high-magnification image; (b) low -magnification image. Scanning electron microscope (SEM) images were taken with an S-3600N (Hitachi High-Technologies Corporation) at an acceleration voltage of 10 kV. The surface of the sample was coated with gold prior to the observation. The average particle size was calculated from the diameter over 300 particles in SEM images. Because only parts of SEMs are shown in the Figures; particles not appearing in the Figures were also examined. The nitrogen adsorption isotherm was measured with a Quantachrome Autosorb-1 at 77 K. The sample was evacuated at 423 K for 2 h before the measurement. The pore size distribution was calculated by the Barrett-Joyner-Halenda (BJH) method.30 Adsorption/desorption isotherms of water were also studied with a Belsorp-18 at 298 K in a static water vapor system. Prior to the measurement, the sample was immersed in water for 24 h and then evacuated at room temperature. To ensure the regularity of the mesopores of MMSS, X-ray diffraction measurement was carried out with a Rigaku Rint-2200 X-ray diffractometer using Cu KR radiation. To determine the refractive index of the fabricated crystalline array based on light scattering,31 angle-resolved reflection spectra were measured. To remove the water that exists in the interplanar spacing and in the mesopores, the film was evacuated for 5 h at 323 K. Then, the colloidal film was placed in a glass container, into which nitrogen containing water vapor was introduced. Reflection spectra were measured at three different relative water vapor pressures (P/P0 ) 0.2, 0.4, and 0.8). To increase the measurement accuracy, the incident angle between the beam and the normal to the surface of the film was varied from 12 to 34° (Supporting Information). By fitting each reflection peak to the Gaussian curve, λpeak was determined. For comparison, reflection spectra were also collected for the solid spheres (Nippon Shokubai Co., Ltd, 460 nm in diameter). We denote these nonporous silica spheres as NSS.
Results and Discussion Figure 1 shows SEM images of MMSS. The particles were spherical, and the average size was determined to be 473 nm with a standard deviation of 4.0%, indicating a high monodispersion characteristic. The pore size distribution and pore diameter of MMSS were calculated by the nitrogen adsorption measurement. As shown in Figure 2, the adsorption isotherm was a typical type IV.32 A steep increase in nitrogen adsorption indicated a narrow pore size distribution (inset). The pore diameter calculated from the isotherm was 20 Å by the BJH method.30 The specific surface area and pore volume were 1010 m2 g-1 and 0.6 mL g-1, respectively. Figure 3 shows the adsorption/desorption isotherms of water vapor. The adsorption isotherm is identified as type V,32 which is typical for mesoporous materials. The amount of adsorbed water vapor increases slightly on the low-pressure side and shows a sharp increase at around P/P0 ) 0.4 due to capillary condensation. The pressure at which capillary condensation occurs (30) Barrett, E. P.; Joyner, L. G.; Halenda, P. H. J. Am. Chem. Soc. 1951, 73, 373. (31) Alfrey, T.; Bradford, E. B.; Vanderhoff, J. W.; Oster, G. J. Opt. Soc. Am. 1954, 44, 603. (32) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pirotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem 1985, 57, 603.
Figure 2. Nitrogen adsorption isotherm in MMSS.
Figure 3. Adsorption/desorption isotherms of water vapor in MMSS.
Figure 4. SEM images of a colloidal crystal film from MMSS: (a) high-magnification image; (b) low-magnification image.
relates to the size of the mesopores. When adsorption occurs on porous materials, pore spaces are preferentially filled with adsorbed molecules. Therefore, in this study, water vapor is considered to be introduced into mesopores, not the spaces between spheres. The results of the two adsorption measurements agree well, and both of the results suggest that the silica spheres have an ordered mesoporous structure. The regularity of the mesopores was also studied by the X-ray diffraction measurement (Supporting Information). The XRD pattern exhibits low-angle diffraction peaks for the (100), (110), and (200) planes, confirming that the material has highly ordered hexagonal symmetry. These analyses reveal that MMSS has a large pore volume based on the highly ordered hexagonal mesopores. Different from that of MMSS, the water adsorption isotherm of NSS (Supporting Information) does not exhibit a sharp increase, and the amount of adsorbed water vapor is low. This adsorption property corresponds to the fact that NSS does not have ordered mesopores. Figure 4 shows SEM images of a colloidal crystal film fabricated from MMSS. The images of the film display a high degree of order and a close-packed array. It must be mentioned that some voids and cracks are observed in these films. They are caused by both capillary forces in existence during the drying process and mechanical forces encountered when peeling off the glass substrates to prepare the SEM sample. The optical properties of the colloidal crystal film fabricated from MMSS are shown in Figure 5 and Table 1. In Figure 5, λpeak is plotted against the incident angle θ at P/P0 ) 0.2 and 0.8. When P/P0 increased from 0.2 to 0.8, λpeak shifted to a longer
2446 Langmuir, Vol. 22, No. 6, 2006
Letters
Figure 5. λpeak of the reflection spectra vs θ at (A) P/P0 ) 0.2 and (B) P/P0 ) 0.8 and corresponding photographs showing the structural color of colloidal crystal films fabricated from MMSS. Table 1. Comparison of the Refractive Indices neff (A) Calculated from the Adsorption Properties and neff (B) Determined by Angle-Reflection Spectroscopy at Different P/P0 Values for MMSS P/P0
neff (A)
neff (B)
0.2 0.4 0.8
1.19 1.23 1.29
1.18 1.24 1.29
wavelength. The corresponding structural color of the colloidal film also changed in accordance with the secondary diffraction occurring in the visible region. At P/P0 ) 0.2, the colloidal film was pale blue, whereas at P/P0 ) 0.8, it turned pale pink as shown in Figure 4. This corresponds to a shift of λpeak, indicating that light reflection of the colloidal film can be changed during water vapor adsorption. In addition, it was confirmed that this spectrum change could be reversed over five cycles by alternating P/P0 from 0.2 to 0.8 and vice versa (Supporting Information). It was found that the reflection spectrum of the mesoporous colloidal film could be reversibly altered in a very simple and smart way by the adsorption and desorption of water vapor. This feature is advantageous in creating a useful system that will perform not only as a photonic crystal but also as a sensor or indicator. In the following paragraph, we try to make clear that the reflection spectra were changed by the water vapor adsorbing in the mesopores. From Bragg’s law (eq 1), the particle diameter d and effective refractive index neff were determined.
λpeak ) 1.633d(neff2 - sin2 θ)1/2
(1)
The diameter d calculated from the measurement was 470 nm, almost matching the mean diameter of MMSS obtained from the SEM images (473 nm). This result confirmed that the spectra were collected accurately. On the basis of the assumption of a close-packed structure (26% air), the effective refractive index neff can be calculated from eqs 2 and 3,33,34 where nm is the refractive index of the medium (air) and ni and Vi are the refractive indices and volume fractions, respectively, of the components within MMSS, in this case, silica, water, and air.
neff2 ) 0.26nm2 + 0.74nMMSS2 nMMSS2 )
∑i ni2Vi
(33) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266. (34) Yamasaki, T.; Tsutsui, T. Appl. Phys. Lett. 1998, 72, 1957.
(2) (3)
From eqs 2 and 3 and the water vapor adsorption properties, the effective refractive index neff (A) was calculated. We used the known refractive index values, nair ) 1.00, nwater ) 1.33, nsilica ) 1.46,29,35 and the density of MMSS, 1.8 cm3 g-1. The refractive indices, neff (B), determined by angle reflection spectroscopy are compared with the calculated ones, neff (A), in Table 1. The calculated values of neff (A) are 1.19, 1.23, and 1.29 for P/P0 ) 0.2, 0.4, and 0.8, respectively, and are in good agreement with the measured ones given as neff (B), which are 1.18, 1.24, and 1.29. This agreement proves the validity of eq 3. At the same time, it was revealed that the reflection spectra were controlled by water vapor adsorbing in the mesoporous channels. To clarify that the water vapor adsorption occurs in the mesopores of MMSS, we conducted the same measurement of reflection spectra on the colloidal crystal film fabricated from NSS. As shown in Table 2, the values of neff (A) and neff (B) Table 2. Comparison of the Refractive Indices neff (A) and neff (B) for NSS P/P0
neff (A)
neff (B)
0.2 0.8
1.27 1.31
1.27 1.31
agree well at P/P0 ) 0.2 and 0.8, which is the same as the result for MMSS. However, compared to the values of neff (B) between P/P0 ) 0.2 and 0.8, a change in MMSS shows an 8.5% increase, which is more than twice that of NSS (3%). This is considered to be caused by the difference in the adsorption capacity between MMSS and NSS. This comparison strongly supports the following idea: the water vapor adsorbing in the mesopores of MMSS control the light reflection extensively on the basis of its high adsorption properties. The results also suggest that the mesopores of MMSS adsorb water vapor dramatically even when they are fabricated in a colloidal film.
Conclusions In this letter, we fabricated a colloidal crystal film from monodisperse mesoporous silica spheres by using a fluidic cell. Just by controlling the amount of water vapor, the reflection spectrum from the colloidal film was changed dramatically, and this spectrum change was reversed over five cycles. From an estimation of the refractive index of silica spheres, it was clarified that the reflection spectra changed because of water vapor adsorption occurring in the mesoporous channels. To the best of our knowledge, this is the first report controlling the light reflection of a colloidal crystal film fabricated from monodisperse silica spheres with ordered mesopores by utilizing adsorption properties. It can be expected that the reflection spectra of colloidal films can easily be tuned just by changing the type and amount of adsorbant with the appropriate refractive index. We believe that this material can provide a novel strategy for controlling the propagation of light very simply. Studies are underway to explore this system by using different types of adsorbants with various refractive indices. Supporting Information Available: X-ray diffraction pattern, angle-resolved reflection spectra of MMSS, and water adsorption isotherm of NSS. This material is available free of charge via the Internet at http://pubs.acs.org LA0531695 (35) McComb, D. W.; Treble, B. M.; Smith, C. J.; De La Rue, R. M.; Johnson, N. P. J. Mater. Chem. 2001, 11, 143.
