Artificial Black Opal Fabricated from Nanoporous Carbon Spheres

Photonic crystals have attracted great interest since the initial proposals(1, 2) ... (12, 13) The beautiful iridescent color of natural opals is appr...
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Artificial Black Opal Fabricated from Nanoporous Carbon Spheres Yuri Yamada,* Masahiko Ishii, Tadashi Nakamura, and Kazuhisa Yano Toyota Central Research & Development Laboratories., Inc., Nagakute, Aichi 480-1192, Japan Received January 13, 2010. Revised Manuscript Received April 9, 2010 A nanocasting method via chemical vapor deposition of acetonitrile was successfully employed to fabricate porous carbon colloidal crystal using colloidal crystal from monodispersed mesoporous silica spheres (MMSS) as a sacrificial scaffold. The mesostructure as well as periodic arrays within (111) plane of MMSS were replicated for the carbon colloidal crystal (black opal) with the length scale in the centimeter range. Brilliant iridescent colors were clearly observed for the first time on the black carbon colloidal crystal fabricated from porous carbon spheres, and they changed dramatically in accordance with the observation angle, like natural black opals. Reflection spectra measurements based on 2D surface diffraction and Bragg diffraction in the mirror mode were conducted for the fabricated carbon periodic arrays. The periodicity in the (111) plane as well as in the direction perpendicular to the (111) plane of the colloidal crystal was evaluated by comparing the results obtained from these two measurements. It was found that the periodicity in the direction perpendicular to the (111) surface is not high for the obtained black carbon opal. On the other hand, the relationship between the incident angles and the peak wavelengths of the reflection spectra, collected in the condition where the incident light and the reflected light pass through in the same direction, is governed by an approximation based on 2D surface diffraction. The results imply that the origin of the iridescent colors on the fabricated black carbon opal is derived from the periodicity not in the direction perpendicular to the (111) plane but within the (111) plane.

1. Introduction Photonic crystals have attracted great interest since the initial proposals1,2 suggested the inhibition of light emission in periodic structures by a photonic band gap (PBG). The formation of a complete PBG requires large refractive index contrast between the different components, and to realize this system, many efforts have been conducted using lithographic or microfabrication (i.e., top-down fabrication).3,4 For optoelectronic applications, PBG in the visible wavelength range is of great importance. However, the top-down fabrication method involves difficulty because of the limitation of the optical resolution or costly procedure. One type of photonic crystals is the colloidal crystal, containing periodic arrays of monodispersed colloidal spheres formed through self-assembly (i.e., bottom-up fabrication). Since there are large varieties of available colloidal particles and the fabrication process is economically friendly, a number of studies on colloidal crystals have been conducted, including investigations on fabrication, simulation, and measurement of optical properties.5-8 In the case where the periodic arrays have length scales in the submicrometer range, the structure can reflect light according to the Bragg equation in the visible region, and brilliant colors generated by diffraction are observed. These colors produced by the microstructure, and not with dyes or pigments, are *Corresponding author. E-mail: [email protected]. (1) Yablonovich, E. Phys. Rev. Lett. 1987, 58, 2059. (2) John, S. Phys. Rev. Lett. 1987, 58, 2486. (3) Lin, S. Y.; Fleming, J. G.; Hetherington, D. L.; Smith, B. K.; Biswas, R.; Ho, K. M.; Sigalas, M. M.; Zubrzycki, W.; Kurtz, S. R.; Bur, J. Nature 1998, 394, 251. (4) Campbell, M.; Sharp, D. N.; Harrison, M. T.; Denning, R. G.; Turberfield, A. J. Nature 2000, 404, 53. (5) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693. (6) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (7) Wijnhoven, J. E. G. L.; Vos, W. L. Science 1998, 281, 802. (8) Harland, J. L.; VanMegen, W. Phys. Rev. E 1997, 55, 3054. (9) Takeoka, Y.; Watanabe, M. Langmuir 2003, 19, 9104. (10) Marlow, F.; Muldarisnur.; Sharifi, P.; Brinkmann, R.; Mendive, C. Angew. Chem. 2009, 48, 6212.

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called “structural colors”.9-11 Structural color in nature arises mostly from optical interference of multiple light paths reflected inside periodically textured materials. The tail of a peacock and the wing of a Morpho butterfly yield beautiful colors generated by their microstructure.12,13 The beautiful iridescent color of natural opals is appreciated and favored all over the world and is known to be caused by a periodic arrangement of silica particles.14,15 Natural opals are considered to be one of the most beautiful gems owing to its play of color (or iridescent color). One of the terms used to classify natural opals is their background color (or body tone); white opals and black opals are representatives of this classification scheme.16 The white Opal is the most common type having a light body tone where the incident light is apt to be scattered, resulted in spreading of the brilliant opalescence. In contrast, the dark background color of the black opal absorbs white color, which plays an effective role in emphasizing the iridescent colors observed on the surface. Therefore, the black opal is more favored and consequently more precious. Many researchers have attempted to fabricate colloidal arrays (so-called artificial opals) and utilize the resulting iridescent colors, to replicate the appearance of natural opals. For example, Egen et al. have applied artificial opals of polymer particles as effective pigments in transparent coatings without using dyes.17 Periodic arrays of hydrogel microspheres have been studied as a novel ink or thermoresponsive sensor, yielding brilliant colors without pigments.18,19 (11) Barry, R. A.; Wiltzius, P. Langmuir 2006, 22, 1369. (12) Potyrailo, R.; Ghiradella, H.; Wertiatchikh, A.; Dovidenko, K.; Cournoyer, J.; Olson, E. Nat. Photonics 2007, 1, 123. (13) Li, Y. Z.; Lu, Z. H.; Yin, H. W.; Yu, X. D.; Liu, X. H.; Zi, J. Phys. Rev. E 2005, 72, 010902. (14) Okrusch, M.; Matthes, S. Mineralogie, 7th ed.; Springer: Berlin, 2005. (15) Weise, C. Opal. Extra-Lapis No. 10; C. Weise-Verlag: M€unchen, 1996. (16) Eckert, A. W. The World of Opals; Wiley: New York, 1997. (17) Egen, M.; Braun, L.; Zentel, R.; T€annert, K.; Frese, P.; Reis, O.; Wulf, M. Macromol. Mater. Eng. 2004, 289, 158. (18) Tsuji, S.; Kawaguchi, H. Langmuir 2005, 21, 8439. (19) Debord, J. D.; Lyon, L. A. J. Phys. Chem. B 2000, 104, 6327.

Published on Web 05/07/2010

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Schroden et al. have fabricated inverse opals with inorganic materials (e.g., titania, zirconia) and succeeded in demonstrating the structural colors based on Bragg reflection.20 The periodic arrays of the particles can yield the structural color, even though there are some voids or cracks within the structure. There are a number of studies on artificial opals reporting iridescent colors, however, most of the research has focused on polymer or silica artificial opals,21,22 which somewhat resemble white opals. It should be mentioned that, by adding subtle amount of carbon nanoparticles into the face-centered cubic (fcc) lattice fabricated from polymer opal, the structural color changes remarkably from milky white to intense green.18 Therefore, the ordered arrays of carbon colloidal particles are expected to realize more iridescent structural colors. Mimicking the brilliant opalescence characteristics of black opals is more desirable and useful, though there have only been a few reports published on this until now. For instance, Zakhidov et al. converted silica opals to carbon inverse opals and succeeded in duplicating the impressive opalescence.23 Lee and co-workers also observed colloidal opalescence on the inverse opal carbon fabricated from PMMA colloidal crystal template.24 However, on the basis of the assumption of a close-packed structure, the occupied volume of colloidal spheres is higher than that of the macropores between spheres, 74% for the former and 26% for the latter. There is the possibility that the color would be more brilliant and iridescent if carbon colloidal crystals are fabricated from periodic arrays of carbon spheres. Therefore, evoking the colloidal opalescence inherent to black opals on periodic arrays of colloidal carbon spheres is still a challenge. We have succeeded in synthesizing monodispersed mesoporous silica spheres, by the acronym MMSS hereafter, that have highly monodisperse particle diameters and pore sizes.25 A nanocasting method, which is widely conducted to obtain microporous or mesoporous carbons26-29 using ordered mesoporous silica or zeolites as sacrificial templates, enables us to fabricate monodispersed nanoporous carbon colloidal spheres. The particle morphology and the porous structure of MMSS were retained for the nanocast carbon spheres, and threedimensionally ordered arrays were also fabricated.30,31 We have also previously described a nanocasting synthesis method, in which colloidal arrays of MMSS were directly used as a sacrificial scaffold, to prepare carbon opal structures.32 In these previous reports, furfuryl alcohol was used as the carbon source and infiltrated into the mesopores of MMSS until incipient wetness was achieved. After that, the impregnated carbon precursor was polymerized, followed by carbonization at higher temperature under nitrogen atmosphere. The process was repeated a number of times to replicate the morphology and porous structure of the MMSS completely, implying that one may feel this process a little (20) Schroden, R. C.; Al-Daous, M.; Blanford, C. F.; Stein, A. Chem. Mater. 2002, 14, 3305. (21) Pursiainen, O. L.; Baumberg, J. J.; Winkler, H.; Viel, B.; Spahn, P.; Ruhl, T. Opt. Express 2007, 15, 9553. (22) Yamada, Y.; Nakamura, T.; Ishii, M.; Yano, K. Langmuir 2006, 22, 2444. (23) Zakhidov, A. A.; Baughman, R. H.; Iqbal, Z.; Cui, C.; Khayrullin, I.; Dantas, S. O.; Marti, J.; Ralchenko, V. G. Science 1998, 282, 897. (24) Lee, K. T.; lytle, J. C.; Ergang, N. S.; Oh, S. M.; Stein, A. Adv. Funct. Mater. 2005, 15, 547. (25) Yano, K.; Fukushima, Y. J. Mater. Chem. 2003, 13, 2577. (26) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1993, 103, 7743. (27) Lee, J.; Yoon, S.; Hyeon, T.; Oh, S. M.; Kim, K. B. Chem. Commun. 1999, 2177. (28) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, N.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2002, 122, 10712. (29) Kyotani, T.; Nagai, T.; Inoue, S.; Tomita, A. Chem. Mater. 1997, 9, 609. (30) Nakamura, T.; Yamada, Y.; Yano, K. Chem. Lett. 2006, 35, 1436. (31) Nakamura, T.; Yamada, Y.; Yano, K. Microporous Mesoporous Mater. 2009, 117, 478. (32) Yamada, Y.; Nakamura, T.; Yano, K. Chem. Lett. 2008, 37, 378.

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cumbersome. Furthermore, we have observed SEM images of periodic arrays fabricated from the porous carbon spheres; however, neither optical characterization nor the colloidal opalescence could be obtained. The reason for this was that the area of the ordered arrays of the porous carbon spheres was not sufficiently wide for observations of optical properties. Here, we present a versatile route involving chemical vapor deposition (CVD) to fabricate opaline arrays from porous carbon spheres. CVD is one of the common strategies to prepare carbon materials, since it permits easy penetration of volatile carbon precursors into the pores of a hard template, whether they be small micropores (e.g., zeolite templates)33,34 or larger mesopores (e.g., silica opal templates).35,36 Wang and co-workers used acetonitrile as a carbon precursor to obtain an inverse opal carbon with hierarchical porous structure from polymer opal templates. They have demonstrated that acetonitrile vapor was efficiently introduced to the pores of the carbon walls by CVD treatment.37 They have also suggested that CVD is a better choice over liquid impregnation methods because acetonitrile vapor deposits mostly inside mesopores, while macropores are left unaffected. This alternative method could be the possible solution to fabricate large-scale porous carbon colloidal crystals using MMSS as a sacrificial template. We also measured reflection spectra, mainly based on two-dimensional (2D) surface diffraction, for the obtained porous carbon arrays. Bragg diffraction in the mirror reflection mode is the typical diffraction procedure for analyzing the structure of colloidal crystals, and most research until now has revealed information from such a diffraction on the lattice constants and/or the refractive indexes of opaline crystals.38,39 Recently, some research groups have presented methods for measuring 2D surface diffraction, which is derived from the lattice rows within the (111) plane of the colloidal crystals. By comparison of the results obtained from the measurements of 2D surface diffraction and the normal Bragg diffraction, the refractive index could be determined accurately40 or the crystallinity of the opaline arrays could be evaluated.41 The application of 2D surface diffraction for carbon colloidal materials, to our knowledge, has not been previously reported thus far.

2. Experimental Section 2.1. Chemicals. Tetramethyl orthosilicate (TMOS) and cetyltrimethylammonium chlorides [C16H33(CH3)3NCl, CTMACl] were obtained from Tokyo Kasei Kogyo. Methanol, 1 M sodium hydroxide solution, 48% hydrofluoric acid solution, sulfuric acid, and acetonitrile were obtained from Wako Pure Chemical Industries. These chemicals were used without further purification.

2.2. Fabrication of Opaline Crystal Composed of Porous Carbon Spheres. In this study, an opaline colloidal crystal of MMSS was used as a sacrificial scaffold to prepare porous carbon colloid crystal. We denote these colloidal crystals as MMSS (synthetic) opal and black (carbon) opal, respectively. It should be noted that natural black opals are not composed of carbon materials. The synthesis procedure for MMSS has been described in detail elsewhere.25 Typically, 7.04 g of CTMACl and 6.84 g of 1 M (33) Ma, Z. X.; Kyotani, T.; Tomita, A. Carbon 2002, 40, 2367. (34) Xia, Y.; Mokaya, R. J. Phys. Chem. C 2007, 111, 10035. (35) Su, F. B.; Zhao, X. S.; Wang., Y.; Zeng, J. H.; Zhou, Z. C.; Lee, J. Y. J. Phys. Chem. B 2005, 109, 20200. (36) Chai, G. S.; Shin, I. S.; Yu, J. S. Adv. Mater. 2004, 16, 2057. (37) Wang, Z.; Li, F.; Ergang, N. S.; Stein, A. Chem. Mater. 2006, 18, 5543. (38) Vlasov, Y. A.; Astratov, V. N.; Baryshev, A. V.; Kaplyanskii, A. A.; Karimov, O. Z.; Limonov, M. F. Phys. Rev. E 2000, 61, 5784. (39) Huang, J.; Eradat, N.; Raikh, M. E.; Vardeny, Z. V.; Zakhidov, A. A.; Baughman, R. H. Phys. Rev. Lett. 2001, 86, 4815. (40) Kanai, T.; Sawada, T.; Kitamura, K. Langmuir 2003, 19, 1984. (41) Ishii, M.; Harada, M.; Tsukigase, A.; Nakamura, H. J. Opt. A 2007, 9, S372.

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sodium hydroxide solution were dissolved in 800 g of methanol/ water (50/50 = w/w) solution. Then, 5.28 g of TMOS was added to the solution with vigorous stirring at 298 K. The clear solution turned opaque within several minutes after the addition of the silica source, yielding a white precipitate. After 8 h of continuous stirring, the mixture was aged overnight. The white precipitate was then filtered off, washed with distilled water three times, and dried at 318 K for 72 h. The powder was calcined in air at 823 K for 6 h to remove the surfactant. In this study, we used MMSS with a diameter of 550 nm. The obtained MMSS was dispersed in water (10 wt %) and subjected to continuous sonication for more than 3 h. We used a fluidic cell42 to fabricate colloidal opal. The colloidal opal is formed between two flat quartz glass substrates (38  28  1 mm3, 20  28  1 mm3) placed 30 μm apart by double-stick tape (Nitto Denko Corporation). The cell is equipped two openings and one of them is connected with a fluid reservoir (Figure S1 in the Supporting Information). A sufficiently dispersed solution was fed into the reservoir, followed by introduction between the two quartz glasses by capillary forces. The fabricated crystals of MMSS within the fluidic cell were annealed at 1073 K to peel off the quartz substrates and to create necks between MMSS, which means the MMSS template was solidified. The MMSS synthetic opal on one side of the substrates was placed in a quartz tube equipped with a furnace to introduce a carbon precursor using chemical vapor deposition (CVD). First, the sample was heated to 1173 K under nitrogen flow; subsequently, nitrogen gas containing 10 wt % acetonitrile vapor was introduced into the quartz tube for 3 h. The carbon/silica composite was then cooled in pure nitrogen gas, followed by immersion into a 5% hydrofluoric acid solution to dissolve the silica templates. The quartz substrates were immersed in a concentrated sulfuric acid beforehand to increase hydrophilicity. 2.3. Analysis Procedures. In this study, Bragg diffraction analysis and 2D surface diffraction analysis were conducted on the black opal fabricated from porous carbon spheres. The analysis procedures are described below. Opaline crystals, fabricated by drying a colloidal suspension on a substrate, usually have a (111)-plane-oriented face-centered cubic (fcc) structure. Bragg diffraction measured in the mirror reflection mode is usually conducted to analyze the features of fcc colloidal crystals. The position of the reflection peak is expressed by a modified Bragg’s law, combined with Snell’s law:42,43 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1Þ mλ ¼ 2d111 n2eff - sin2 θ where λ is the reflection peak wavelength, θ is the incident angle of the light, d111 is the interplanar spacing of the (111) planes, and neff is the effective refractive index of the opaline crystal. An illustration of Bragg diffraction in the mirror reflection was shown in Figure 1a, which indicates that the lattice rows are derived from the periodicity in the direction perpendicular to the (111) plane. On the other hand, the periodicity within the (111) plane is the key in the case of 2D surface diffraction. A side view of the (111) surface viewed along the [110] direction is shown in Figure 1b. When light is irradiated at an angle in a plane perpendicular to the [110] direction and is scattered in the backward direction along paths S1 and S2, the path difference Δ is given by Δ ¼ 2p sin θ

ð2Þ

where p is the distance between adjacent lattice rows parallel to the [110] direction. The relation between p and the lattice constant a2D is defined by eq 3. pffiffiffi 6 p ¼ a2D ð3Þ 4 (42) Ishii, M.; Nakamura, H.; Nakano, H.; Tsukigase, A.; Harada, M. Langmuir 2005, 21, 5367. (43) Takeoka, Y.; Watanabe, M. Adv. Mater. 2003, 15, 199.

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Figure 1. Side view of the (111) surface viewed along the [110] direction. Illustrations of Bragg diffraction in (a) mirror Bragg diffraction and (b) 2D surface diffraction mode. The details for relating p and a2D has been described in the literature.40 Constructive interference occurs if the path difference is equal to an integer number of wavelengths. Therefore, the condition for 2D surface diffraction is given by the following expression: mλ ¼

pffiffiffi 6 a2D sin θ 2

ð4Þ

As can be seen from eq 4, the reflection, which originates from the 2D surface diffraction, shifts continuously in accordance with the incident angle θ. To evaluate the reflection peak wavelength λ, the following relation between a2D and D (the particle diameter) is used in this study, assuming that the opaline colloidal crystal has an fcc structure. a2D ¼

pffiffiffi 23D

ð5Þ

Here, D is estimated from an SEM image. 2.4. Characterizations. Scanning electron microscope (SEM) images were taken with an S-3600N (Hitachi HighTechnologies) instrument at an acceleration voltage of 10 kV. On the observation of the MMSS synthetic opal, the arrays attached on one side of the substrates (without CVD treatment) was used, and it was coated with gold prior to SEM observation. The average particle size was calculated from the diameter of over 50 particles in the SEM images. It is to be noted that only portions of SEM micrographs are shown in the figures, and particles not shown in the figures were also examined. The elemental composition of the obtained black opal was clarified with an energydispersive X-ray (EDX) attached to the S-3600N. Nitrogen adsorption isotherm was measured with a Quantachrome Autosorb-1 instrument at 77 K. The sample was evacuated at 423 K for 2 h before the measurement. Powder X-ray diffraction (XRD) measurements were performed with a Rigaku RINT-2200 X-ray diffractometer using Cu KR radiation. Angle-resolved reflection measurements were conducted in both Bragg diffraction in the mirror mode and 2D surface diffraction mode. Schematic drawings of the experimental setups are shown in Figure 2. Bragg reflection spectra in the mirror reflection mode were measured by changing the incident angle of light normal to the sample surface. In the case of the 2D surface diffraction mode, the incident light was introduced through a coaxial optical fiber and collimated using an optical lens and a slit. Backscattered light from the sample was collected by the same fiber head through the other branch to the detector. The spectra were obtained by changing the incident angle from 36° to 60°. It should be noted that the incident angle is changed by rotating the sample, while the incident light and the detected fiber are fixed. Thus, by setting the Langmuir 2010, 26(12), 10044–10049

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Figure 3. SEM image of black opal fabricated from porous carbon spheres. Figure 2. Schematic diagram of the experimental setups for measuring reflection spectra in (a) the mirror Bragg diffraction mode and (b) the 2D surface diffraction mode. sample at different angles with respect to the direction of the incident light, angle-resolved emission spectra can be collected. For both measurements, the sample was irradiated with white light from a halogen lamp and the reflection spectra were collected using a multichannel spectrometer (Fastevert S-2600, Soma Optics).

3. Results and Discussion 3.1. Physical Properties. SEM images of MMSS synthetic opals are shown in the Supporting Information (Figure S2). A close-packed structure was observed with the closest-packing plane oriented parallel to the substrate. The particle size of MMSS decreased from 550 to 440 nm owing to heat treatment at 1073 K. The shrinkage of the particle diameter causes cracks partially within the colloidal structure (data not shown), although the close-packed structure of MMSS was maintained within the domains. Figure 3 shows an SEM image of black opal fabricated from porous carbon spheres. The particles are well-ordered and the particle size of MMSS (440 nm) is inherited by the black carbon opal. It should be mentioned that some voids and cracks are observed in the SEM image. The black opal was obtained by peeling off the glass substrates, followed by CVD treatment and immersion in the hydrofluoric acid solution. The SEM image is taken for the sample surface where the effect of mechanical strength is likely. The regularity of the black carbon opal is evaluated in the next section closely. The ratio of C/Si of the opal on the quartz substrate was estimated to be 95.2/4.8 at % by the EDX analysis. We also conducted EDX analysis for the carbon spheres stripped from the quartz substrate on immersion in the hydrofluoric acid solution, revealing that the content of Si was below 1 at %. Therefore, the Si signal of the carbon opal is thought to arise from the quartz substrate mainly, implying that almost all the silica templates were removed. The porosity of the carbon spheres is clarified by XRD patterns and nitrogen adsorption isotherm (Figures S3 and S4 in the Supporting Information). The small-angle XRD pattern for MMSS exhibits diffraction peaks corresponding to (100), (110), and (200) planes (Figure S3a in the Supporting Information). After introduction of the carbon source by the CVD treatment, the peak intensity of the (100) diffraction decreases (Figure S3b in the Supporting Information). This result is associated with a porefilling effect44 due to the incorporation of carbon into the (44) Gierszal, K. P.; Yoon, S. B.; Yu, J. S.; Jaroniec, M. J. Mater. Chem. C 2006, 16, 2819.

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Figure 4. Structural colors of the black opal fabricated from porous carbon spheres observed from the same direction of the incident light. The images (from (a) to (d)) were taken with decreasing incident angle normal to the sample surface.

mesopores of MMSS, which provides conclusive evidence that the introduction of the carbon source by CVD is effective, even when a sacrificial template (MMSS) is fabricated as colloidal crystal arrays. The shape of the adsorption isotherm for the obtained carbon spheres (Figure S4 in the Supporting Information) suggests the development of microporosity. However, the volume of nitrogen adsorbed was small compared to our previously obtained samples30-32 in which the incipient wetness of furfuryl alcohol was repeated until the mesopores of MMSS were sufficiently filled. These results indicate that a single-cycle CVD treatment was not sufficient to impregnate all of the mesopores of MMSS with carbon. The XRD patterns shown in Figure S3 support such an insufficient introduction of carbon. Although the introduction of carbon might not be sufficient, the mesostructure of MMSS was well-replicated, even when they were in the form of colloidal arrays. 3.2. Iridescent Colors. Color of the obtained carbon opal is black, but it displays colloidal opalescence under illumination through the entirely area. The size of the fabricated black opal is estimated to be approximately 4 cm2 (Figure S1 in the Supporting Information), and by increasing the size of the quartz substrate, black carbon opal with large area can be obtained. Figure 4 shows the structural color of the black carbon opal observed from the same direction as the incident light by rotating the sample (see Figure 2b). The color of the black carbon opal changes dramatically in accordance with the observation angle. When the observed direction is close to parallel to the sample surface (i.e., when θ in Figure 2b is close to 90°), the black opal displays an iridescent red (a). The opalescence structural color changes continuously from red to deep blue (d) when the sample is observed near the normal to the surface (i.e., when θ in Figure 2b is close to zero). It is worth mentioning that a black opal appearing red in body tone, as observed in Figure 4a, is the most precious and valuable among natural opals.16 DOI: 10.1021/la1001732

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Figure 5. (a) Angle-resolved Bragg reflection spectra for the MMSS synthetic opal. The incident angle (θ) changes from 9° to 38°. (b) Relationship between the reflection peak wavelengths and the incident angles.

Figure 6. Angle-resolved Bragg reflection spectra for the black carbon opal. Spectra were collected at the incident angle from 9° to 40°.

In spite of the structural defect of the black carbon opal observed in the SEM image (Figure 3), the colloidal opalescence is clearly observed through the entire area. It implies that the fabricated black opal has some structural regularity or periodic arrays of particles, which induces the colloidal iridescence. Here, we investigate the origin of such colloidal opalescence emerging from the fabricated black carbon opal below. First, Bragg reflection measurements were conducted. Angle-resolved reflection spectra measured in the Bragg mirror reflection mode were collected for the MMSS synthetic opal (prior to the anneal procedure), and the resultant peak wavelengths were plotted as a function of the incident angle (Figure 5). The peak in the reflection spectrum shifts steadily to shorter wavelengths with an increase in the angle of incidence from 9° to 38°, as indicated by the arrow. The solid line shown in (b) is the curve obtained by fitting the plotted data to eq 1. The fitted curve matched the data well, indicating that the MMSS synthetic opal consists of wellordered face-centered cubic (fcc) structure with the periodicity in the direction perpendicular to the (111) plane. Figure 6 shows the angle-resolved Bragg reflection spectra for the black carbon opal. With increasing incident angle θ, the peak shifts toward the shorter wavelengths. However, being different from the results obtained for the MMSS synthetic opal, the Bragg reflection spectra of the black carbon opal exhibit broad shapes. In addition, the peaks of the spectra are not fitted well to the relations defined by eq 1 (Figure S5 in the Supporting Information). The particle diameter based on the Bragg reflection is approximated to be 485 nm from the fitted curve,41 and the lattice constant a2D is evaluated to be 686 nm by applying eq 5. The obtained values are not in good agreement with those estimated from the SEM images (440 nm for the particle diameter and 622 nm for the lattice constant). As we have mentioned in Experimental section, the Bragg reflection in the mirror mode is derived from the periodicity in the direction perpendicular to the 10048 DOI: 10.1021/la1001732

(111) plane. Therefore, the result from Figure 6 indicates that the ordering in the thickness direction of the black carbon opal is relatively low. The SEM image of the black opal reveals some voids and cracks, resulting in the disorder of the fcc structure (Figure 3). It is implied that the disagreement between the theory and the measured data is derived from the partially disordered fcc structure of the black opal. The nanocasting route from the MMSS opal to the black carbon opal is accompanied by many steps, including immersion in a hydrofluoric acid solution. As we have mentioned in the previous report,30 it is surprising that colloidal structure is retained during silica template dissolution. It is assumed that the hydrophobic interaction among carbon spheres in water is very strong. The possibility cannot be denied that such a strong interaction among carbon spheres causes structural distortion. In addition, several papers41,45 have suggested the subtle shrinkage of polystyrene (PS) spheres during the drying process of the colloidal suspensions, and it is observed that PS opaline crystal is compressed especially in the direction perpendicular to the (111) plane because of the shrinkage. Judging from the size of the carbon spheres and the thickness of the spacers, the black carbon opal is expected to have approximately 70 layers perpendicular to the (111) plane. We cannot mention the precise number of layers that affects the Bragg reflection spectra in the Figure 6. However, it is speculated that the nanocasting process may cause structural distortion especially in the thickness direction. For the above-mentioned reasons, it is plausible to consider that the iridescent colors on the black carbon opal are not derived from the Bragg reflection peaks, which are based on the ordering of the (111) plane. Reflection spectra based on 2D surface diffraction measured for the black opal and the relationship between the peak wavelengths and the incident angles are shown in Figure 7. The dotted line in Figure 7b is the fitted curve obtained from the plotted data applied to eq 4. It is to be noted that the incident angle of light is defined in the experimental setup shown in Figure 2b and the angle changes in accordance with rotating the sample. The approximation based on 2D surface diffraction displays good agreement with the plotted data. The lattice constant a2D is estimated to be 636 nm from the fitted curve, and the particle diameter is evaluated to be 450 nm by applying eq 5. The obtained number is not perfectly matched to the estimated value from the SEM observation (440 nm), but it is close. This result indicates that the arrays of the fabricated black carbon opal display good regularity within the (111) plane. As discussed in relation to Figure 6, the black carbon opal is thought to have structural distortion, especially in the thickness direction. Such a partially distorted structure may have an adverse affect on the subtle (45) Nam, H. J.; Jung, D.-Y.; Yi, G.-R.; Choi, H. Langmuir 2006, 22, 7358.

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clarifies the origin of this effect. Black carbon opals are thought to be useful, not only for practical applications, but also for basic research on optical investigations. Unfortunately, we cannot discuss the results obtained from Bragg reflection spectra in the mirror mode because of low ordering along the thickness direction. Studies are currently underway in basic as well as applied areas using the black carbon opals reported here.

4. Conclusions

Figure 7. (a) Reflection spectra based on the 2D surface diffraction measured for the black opal fabricated from porous carbon spheres. The numbers in the figure on the right side are the incident angles of light. (b) Relationship between the reflection peak wavelengths and the incident angles. The dotted line represents a fitted curve using eq 4.

A nanocasting method using chemical vapor deposition of acetonitrile, in which an MMSS colloidal crystal (MMSS synthetic opal) was used as a sacrificial scaffold, has been successfully employed to prepare porous carbon colloidal crystal (black opal) with the length scale in the centimeter range. Even though they are in the form of a colloidal crystal, the mesostructure of the MMSS synthetic opal was replicated well in the carbon opal. Though the color of the carbon opal is black, it exhibits colloidal opalescence clearly and changes dramatically in accordance with the observation angle. To investigate the origin of the iridescent colors, reflection spectra based on 2D surface diffraction and Bragg diffraction in the mirror mode were measured for the black opal composed of porous carbon spheres. The periodicity in the (111) plane and in the direction perpendicular to the (111) plane of the colloidal crystal was evaluated by comparing the results obtained from these two measurements. Poorly resolved Bragg reflection peaks were obtained for the black carbon opal, indicating that the periodicity in the direction perpendicular to the (111) surface is low. On the other hand, the relationship between the incident angles and the peak wavelengths of the reflection spectra are explained well for 2D surface diffraction. These results imply that the origin of the iridescent colors on the fabricated black carbon opal is derived from the periodicity not in the direction perpendicular to the (111) plane but within the (111) plane. This is the first report that observed the structural colors of a colloidal crystal fabricated from porous carbon spheres and clarifies the origin of this effect.

mismatch in the analysis based on 2D surface diffraction. Judging from the analyses conducted for the Bragg diffraction and the 2D surface diffraction, the iridescent colors on the black carbon opal are considered to be derived from the latter diffraction spectra. To the best of our knowledge, this is the first report that observed the structural colors of a colloidal crystal fabricated from porous carbon spheres, and furthermore the first that

Supporting Information Available: Schematic drawing of the fluidic cell and photo image of the black carbon opal, SEM images of MMSS opal, XRD patterns, nitrogen adsorption isotherm, and the analysis of angle-resolved Bragg reflection spectra for the black carbon opal. This material is available free of charge via the Internet at http://pubs. acs.org.

Langmuir 2010, 26(12), 10044–10049

DOI: 10.1021/la1001732

10049