Preparation of Hierarchical Architectures of Silica Particles with Hollow

Apr 6, 2010 - This is a simple (one-step process), inexpensive approach (silica source is sodium silicate) to producing hollow silicas. The addition o...
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Preparation of Hierarchical Architectures of Silica Particles with Hollow Structure and Nanoparticle Shells: A Material for the High Reflectivity of UV and Visible Light Masahiro Fujiwara,* Kumi Shiokawa, Ikuko Sakakura, and Yoshiko Nakahara National Institute of Advanced Industrial Science and Technology, Kansai Center (Nanotechnology Research Institute), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Received September 1, 2009. Revised Manuscript Received February 7, 2010 Silica microcapsules (silica hollow particles) are readily prepared by a single step of the interfacial reaction method, where a W/O/W emulsion is employed effectively. This is a simple (one-step process), inexpensive approach (silica source is sodium silicate) to producing hollow silicas. The addition of NaCl to the sodium silicate solution as the inner water phase of the W/O/W emulsion plainly influenced the shell structure of the silica hollow particles. The increase of the addition of NaCl expanded the size of the mesopores in their silica shell, which reached to macropores (>50 nm). The nanoparticles in the shells of some silica hollow particles attained approximately 200-400 nm in size, which is comparable to the wavelengths of UV and visible light. According to the diffuse reflection spectra of the silica hollow particles in powder form, these particles showed the high reflection of UV and visible light, which increased with added NaCl in the preparation process of the interfacial reaction method. The reflectance of a silica hollow particle from 300 to 800 nm in wavelength was over 90%, which was significantly higher than a common solid (not hollow) silica gel. In addition, even the reflectance of UV light shorter than 300 nm in wavelength was greater than 60%. These characteristic reflections in a wide range of wavelengths were caused by both nanoparticle shells whose sizes are comparable with the wavelength of light and the hollow structures of the main micrometer-sized particles.

Introduction Small particles of silica materials are used in widespread applications because of their very stable, comparatively easy preparation and environmentally friendly nature.1,2 Recently, small silica hollow spheres and silica microcapsules have attracted much attention from materials chemists because of their high potential in various technologies.3-6 Their novel preparation procedures are still actively being studied.7-11 A recently exciting area of research on these materials is related to controlled-release and drug delivery systems because their hollow spaces are *Corresponding author. E-mail: [email protected]. (1) Otterstedt, J.-E.; Brandreth, D. A. Small Particles Technology; Plenum Press: New York, 1998. (2) Bergna, H. E., Roberts, W. O., Eds. Colloidal Silica: Fundamentals and Applications; CRC Press: Boca Raton, FL, 2006. (3) Yang, S. M.; Kim, S. H.; Lim, J. M.; Yi, G. R. J. Mater. Chem. 2008, 18, 2177–2190. (4) Asefa, T.; Shi, Y. L. J. Mater. Chem. 2008, 18, 5604–5614. (5) Jeong, U.; Wang, Y. L.; Ibisate, M.; Xia, Y. N. Adv. Funct. Mater. 2005, 15, 1907–1921. (6) Lin, H.-P.; Mou, C.-Y. Acc. Chem. Res. 2002, 35, 927–935. (7) Zhao, Y. J.; Zhang, J. L.; Li, W.; Zhang, C. X.; Han, B. X. Chem. Commun. 2009, 2365–2367. (8) Zhang, L.; D’Acunzi, M.; Kappl, M.; Auernhammer, G. K.; Vollmer, D.; van Kats, C. M.; van Blaaderen, A. Langmuir 2009, 25, 2711–2717. (9) Zhang, T. R.; Zhang, Q.; Ge, J. P.; Goebl, J.; Sun, M. W.; Yan, Y. S.; Liu, Y. S.; Chang, C. L.; Guo, J. H.; Yin, Y. D. J. Phys. Chem. C 2009, 113, 3168–3175. (10) Wang, J. G.; Li, F.; Zhou, H. J.; Sun, P. C.; Ding, D. T.; Chen, T. H. Chem. Mater. 2009, 21, 612–620. (11) Li, L.; Choo, E. S. G.; Tang, X. S.; Ding, J.; Xue, J. M. Chem. Commun. 2009, 938–940. (12) Hughes, G. A. DM, Dis.-Mon. 2005, 51, 342–361. (13) Wang, Y.; Angelatos, A. S.; Caruso, F. Chem. Mater. 2008, 20, 848–858. (14) Yang, J.; Lee, J.; Kang, J.; Lee, K.; Suh, J. S.; Yoon, H. G.; Huh, Y. M.; Haam, S. Langmuir 2008, 24, 3417–3421. (15) Liu, Y. Y.; Miyoshi, H.; Nakamura, M. Colloids Surf., B 2007, 58, 180–187. (16) Zhu, Y. F.; Shi, J. L.; Li, Y. S.; Chen, H. R.; Shen, W. H.; Dong, X. P. Microporous Mesoporous Mater. 2005, 85, 75–81. (17) Zhu, Y. F.; Shi, J. L.; Shen, W. H.; Dong, X. P.; Feng, J. W.; Ruan, M. L.; Li, Y. S. Angew. Chem., Int. Ed. 2005, 44, 5083–5087. (18) Chen, J. F.; Ding, H. M.; Wang, J. X.; Shao, L. Biomaterials 2004, 25, 723–727.

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advantageous for high doses of drugs.12-18 However, their optical applications are also important research topics.19,20 It is known that hollow silica or hollow glass materials have specific properties for controlling the inflection and reflection of various kinds of lights, which are often involved in light guiding and confinement technologies. These technologies are related to photonic materials as well.21-23 The high performance of photonic crystals and analogous components require very regular structure systems over wide regions.24-26 However, even in the moderate arrangements of material structures, some light with a wavelengths comparable to the size of material components will be scattered on the basis of the Mie scattering principle.27-29 Nonglare coating technology is using these phenomena, where small silica particles deposited on the surface of the substrates produce diffuse reflections and the scattering of visible light.30,31 (19) Douma, K.; Prinzen, L.; Slaaf, D. W.; Reutelingsperger, C. P. M.; Biessen, E. A. L.; Hackeng, T. M.; Post, M. J.; van Zandvoort, M. A. M. J. Small 2009, 5, 544–557. (20) Song, X. F.; Gao, L. Langmuir 2007, 23, 11850–11856. (21) Cregan, R. F.; Mangan, B. J.; Knight, J. C.; Birks, T. A.; Russell, P. S. J.; Roberts, P. J.; Allan, D. C. Science 1999, 285, 1537–1539. (22) Smith, C. M.; Venkataraman, N.; Gallagher, M. T.; M€uller, D.; West, J. A.; Borrelli, N. F.; Allan, D. C.; Koch, K. W. Nature 2003, 424, 657–659. (23) Rengarajan, R.; Jiang, P.; Colvin, V.; Mittleman, D. Appl. Phys. Lett. 2000, 77, 3517–3519. (24) Astratov, V. N.; Vlasov, Y. A.; Karimov, O. Z.; Kaplyanskii, A. A.; Musikhin, Y. G.; Bert, N. A.; Bogomolov, V. N.; Prokofiev, A. V. Superlattices Microstruct. 1997, 22, 393–397. (25) Abe, M. J. Ceram. Soc. Jpn. 2008, 116, 1063–1070. (26) Sun, H. B.; Xu, Y.; Matsuo, S.; Misawa, H. Opt. Rev. 1999, 6, 396–398. (27) Mishchenko, M. I.; Travis, L. D.; Lacis, A. A. Multiple Scattering of Light by Particles: Radiative Transfer and Coherent Backscattering; Cambridge University Press: New York, 2006. (28) Dienerowitz, M.; Mazilu, M.; Dholakia, K. J. Nanophotonics 2008, 2, 021875. (29) Johnson, P. Int. Rev. Phys. Chem. 1993, 12, 61–87. (30) Thomas, I. M. Appl. Opt. 1992, 31, 6145–6149. (31) Lohmuller, T.; Helgert, M.; Sundermann, M.; Brunner, R.; Spatz, J. P. Nano Lett. 2008, 8, 1429–1433.

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Fujiwara et al. Table 1. Preparation Profiles and Pore Properties of Silica Hollow Particles

samplea

ratio % (w/w)b

SSA (m2/g)c

PV (cm3/g)d

PA (m2/g)d

PPD (nm)d

SHP-0 0 508 0.782 552 7.05 SHP-NaCl-1 6.7 539 1.408 464 28.25 SHP-NaCl-2 13.3 472 1.079 355 59.00 SHP-NaCl-3 20.0 585 0.368 283 1.84 SHP-NaCl-4 21.7 282 0.295 225 5.41 SHP-KCl 37.1 381 1.019 269 44.14 silica gel 493 0.697 564 8.06 a Sample names are given in the additional components in the preparation process. SHP is the acronym for silica hollow particle. The first hyphenated part represents the inorganic salt added to the sodium silicate solution. SHP-0 indicates a silica hollow particle without additional components. The last part represents the number of samples. b Weight ratio of NaCl/sodium silicate (w/w). c Specific surface area estimated by a BET plot using nitrogen adsorption-desorption isotherms. d Calculated by the BJH method using nitrogen adsorption-desorption isotherms with the corresponding adsorption branches.

We have found and reported a unique preparation method for silica hollow particles, which we called “silica microcapsules”.32 There are various approaches to synthesizing silica hollow materials, and some representative methods are listed in ref 32. Our method is a simple, inexpensive approach for silica hollow materials in comparison to other procedures such as template methods. No template is required; the product is single-stage, and sodium silicate is the silica source. These silica particles (microcapsules) are readily produced by the reaction of sodium silicate with a silica precipitant such as NH4HCO3 using a W/O/W emulsion interface.32 A sodium silicate solution (such as an inner water phase, IWP) and an organic solution such as hexane (an oil phase, OP) are first emulsified, and the resulting emulsion is added in one portion to another aqueous solution of a silica precipitate (as the outer water phase, OWP). The precipitation of silica matrices occurring along the interface of a W/O/W emulsion fabricates the hollow structure of an amorphous silica material spontaneously.32 Additional components mixed with the solution of sodium silicate (IWP) can be encapsulated into the hollow particles. For example, the direct encapsulation of proteins such as BSA into some inorganic microcapsule materials is accomplished using this interfacial reaction method.33-35 However, when water-soluble polymers such as sodium polymethacrylate were added to the IWP, unique silica hollow particles bearing macropore shells were produced instead of encapsulated products. The additional polymers were not included in these particles. Because the hollow particles consist of only amorphous silica, they are similar to diatomic earth in morphology and composition. The macropores in their shells can be controlled by the kind and content of the polymers mixed with the sodium silicate solution.36 Mesopores with approximately 20 nm diameter observed in the shells of silica hollow particles prepared without polymers are expanded to macropores by their addition. Thus, it is found that the addition of other components to a sodium silicate solution effectively modifies the structure and properties of the shells of silica hollow particles (silica microcapsules). In this article, we report a simple, effective method of forming hierarchical architectures of silica spherical particles with a hollow structure and a nanoparticle shell. The key procedure is the addition of water-soluble inorganic salts to the sodium silicate solution as the IWP. These unique silica hollow particles had (32) Fujiwara, M.; Shiokawa, K.; Tanaka, Y.; Nakahara, Y. Chem. Mater. 2004, 16, 5420–5426. (33) Fujiwara, M.; Shiokawa, K.; Hayashi, K.; Morigaki, K.; Nakahara, Y. J. Biomed. Mater. Res., Part A 2007, 81, 103–112. (34) Fujiwara, M.; Shiokawa, K.; Morigaki, K.; Tatsu, Y.; Nakahara, Y. Mater. Sci. Eng., C 2008, 28, 280–288. (35) Fujiwara, M.; Shiokawa, K.; Morigaki, K.; Zhu, Y.; Nakahara, Y. Chem. Eng. J. 2008, 137, 14–22. (36) Fujiwara, M.; Shiokawa, K.; Sakakura, I.; Nakahara, Y. Nano Lett. 2006, 6, 2925–2928.

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specific light-reflecting properties of UV and visible light despite consisting only of silica.

Results and Discussion First, NaCl as a water-soluble inorganic salt was added to the sodium silicate solution used as the IWP of the interfacial reaction method. The added ratios of NaCl to sodium silicate varied (0, 6.7, 13.3, 20.0, and 21.7% (w/w)), and these samples are called SHPs (silica hollow particles) -0 (meaning no salts), SHP-NaCl-1, SHP-NaCl-2, SHP-NaCl-3, and SHP-NaCl-4, respectively. The preparation conditions and the pore properties of these silica hollow particles are summarized in Table 1. XRD patterns of the silica hollow particles prepared in this study were generally similar (an XRD pattern of SHP-NaCl-3 as a representative example is shown in Supporting Information, Figure S1-A) and had no clear peaks except a broad peak around 22° (2θ), indicating that these materials are amorphous. In infrared analyses, typical spectra of amorphous silica free from organic components were observed in all samples. A representative Si-OH absorption was detected at around 3441 cm-1, and a series of absorptions from Si-O-Si bonds were found at 1096, 801, and 469 cm-1. (The infrared spectrum of SHP-NaCl-3 is shown in Supporting Information, Figure S1-C). From these results, the materials obtained from the interfacial reaction method upon the addition of NaCl were common amorphous silica materials with respect to chemical composition. In Figure 1, the SEM images of these samples are summarized. It is obvious that samples SHP-0, SHP-NaCl-1, SHP-NaCl-2, and SHP-NaCl-3 are spherical particles. The particle size distributions of SHP-0 and SHP-NaCl-3 from the SEM images were in the range from 2 to 5 μm. These distributions measured by a laser diffraction particle analyzer (Figure S3) are in good agreement with the SEM observations shown in Figure 1. The surfaces of the shells of SHP-0 prepared without NaCl were generally smooth, but somewhat textured surfaces were also observed. Sample SHPNaCl-1 obtained with a small amount of NaCl (6.7 wt % with respect to sodium silicate) had more textured surfaces in the shells. Although the small nanoparticles were found in the shells of SHPNaCl-2 (13.3 wt %), they looked smaller than 100 nm from the SEM images (Figure 1E,F). However, nanoparticles from 200 to 400 nm in size were clearly observed in the shells of SHP-NaCl-3, which were prepared in the presence of 20 wt % NaCl to sodium silicate in IWP. Their hollow structure was ascertained from the images of SHP-NaCl-3 in Figure 1, especially from Figure 1I, where some spherical particles were fractured to show their hollow interiors. It is clear that the structures of the shells of SHP-NaCl samples changed with the amount of added NaCl up to 20 wt %. SHP-NaCl-3 was a silica spherical particle bearing hierarchical architecture with a hollow structure and a nanoparticle shell. However, further addition of NaCl was Langmuir 2010, 26(9), 6561–6567

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disadvantageous to the formation of spherical particles. For example, a considerable ratio of particles were not the spherical hollow particles in SHP-NaCl-4 prepared with 21.7 wt % NaCl to sodium silicate as shown in Figure 1J. In this case, moderate amounts of fibrous precipitate were formed in the IWP, which was not detected in the IWP solution with less than 20 wt % NaCl. When approximately 23 wt % NaCl was added to IWP, gelatinous precipitates were observed in the solution and no spherical hollow particles were obtained at all as illustrated in Figure 1K. The whole solution was gelated by the addition of more than 30 wt % NaCl in a few seconds. Therefore, 20 wt % NaCl was likely to be the maximum amount for the preparation of spherical particles. The gelation of the IWP solution was also observed in our previous research on the formation of diatom-like silica hollow particles.36 The addition of other related salts such as LiCl, NaBr, and NaI with a ratio similar to NaCl in the SHP-NaCl-3 preparation resulted in no formation of spherical particles. SEM images of samples prepared with LiCl, NaBr, or NaI (no spherical particles) are displayed in the Supporting Information (Figure S4). The spherical particles were obtained only in the case of the addition of KCl (SHP-KCl) as shown in Figure 2A. The use of fluoride salts was avoided because the fluorine ion generally dissolves silica materials. In cases without the formations of spherical particles, significant amounts of gelatinous precipitates were formed in sodium silicate solutions such as IWP, as in the case of excess amounts of NaCl as mentioned above.37 When the amounts of these salts were reduced by half, spherical particles were successfully obtained. SEM images of these particles with half quantities of LiCl (SHP-LiCl), NaBr (SHP-NaBr), and NaI (SHP-NaI) are also shown in Figure 2. Only the addition of a half equiv of NaI produced spherical particles with nanoparticle shells (Figure 2D), which were analogous to SHP-NaCl-3. Figure 3 show the nitrogen adsorption-desorption isotherms of some SHP-NaCl samples prepared with or without NaCl (Figure 3A) and the corresponding BJH plots (Figure 3B) from the adsorption branches. The calculated data are summarized in Table 1. These results clearly demonstrated the significant influence of NaCl on the porosity properties of SHP-NaCl samples. The relative pressure (P/P0) where the capillary condensation of nitrogen occurred was shifted toward higher values from 0.6 to 0.9 (from SHP-0 to SHP-NaCl-2), and finally no capillary condensation was detected in the isotherms of SHP-NaCl-3 and SHPNaCl-4. As shown in the SEM image of SHP-NaCl-4 (Figure 1J), considerable parts of particles are not spherical. From the results of the specific surface area in Table 1, the addition of more than 20 wt % NaCl decreased the surface area markedly (SHP-NaCl-4). The addition of excess amounts of NaCl was disadvantageous to (37) The situation of the sodium silicate solution as IWP changed with the addition of the aqueous solution of inorganic salts. With the addition of small amounts, no variation of the solution was observed in some cases. Further addition of an inorganic salt solution gradually changed the IWP solution. In the first stage, small amounts of fibrous precipitate were detected in the IWP solution. In the next stage, gelatinous precipitates were clearly found, which increase in volume with the addition of inorganic salt solutions. Finally, the whole IWP solution turns into a gel. In the following table, the required amounts of the respective inorganic salt solutions for the formation of a small, insoluble fibrous precipitate are compared. The concentration of the inorganic salt solutions was fixed to be approximately 3.2 M.

salt

pH value

NaCl NaBr NaI LiCl KCl

6.40 6.20 6.39 5.65 6,67

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added amount (mL) 18-19 1.5-2.0 ∼0.5 ∼0.4 49-50

Figure 1. SEM images of SHP samples. (A, B) SHP-0, (C, D) SHP-NaCl-1, (E, F) SHP-NaCl-2, (G-I) SHP-NaCl-3, (J) SHPNaCl-4, and (K) silica material with 23.3 wt % NaCl.

the formation of the porous silica species as well as the production of hollow particles. The shells of SHP-0 prepared without NaCl had mesopores from 5 to 20 nm in diameter according to the BJH plots in Figure 3B. The size of the mesopore in the shell was expanded to be about 30 nm in diameter by the addition of 6.7 wt % NaCl in sodium silicate solution (SHP-NaCl-1). With DOI: 10.1021/la9043396

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Figure 2. SEM images of SHP samples. (A) SHP-KCl, (B) SHPLiCl, (C) SHP-NaBr, and (D) SHP-NaI.

this expansion of the mesopore, the corresponding pore volume increased from 0.782 to 1.408 cm3/g. The addition of 13.3 wt % NaCl enlarged the mesopore much more; its mean size was estimated to be approximately 60-80 nm (SHP-NaCl-2). These mesopores disappeared when more than 20 wt % NaCl was added. In the SEM images of SHP-NaCl-3 and SHP-NaCl-4 (Figure 1G-J), macropore spaces that were greater than 100 nm were observed as interparticle voids of nanoparticles in their shells. It is likely that the mesopores found in SHP-0 were expanded to macropores as interspatial void spaces between nanoparticles in SHP-NaCl-3 and SHP-NaCl-4. These kinds of variations were also found in the case of the preparation of diatom-like silica.36 It is appropriate that the enlargement of mesopores to macropores reduced the volume (PV) and the surface area (PA) of mesopores as estimated by the BJH method. The decrease in mesopore volumes (PV) was observed with the addition of more than 13.3 wt % NaCl from SHP-NaCl-2 (1.079 cm3/g) to SHP-NaCl-4 (0.295 cm3/g). However, the mesopore area (PA) monotonically decreased from SHP-0 to SHP-NaCl-4. In conclusion, the mesopores originally found in the shells of silica hollow particles (SHP-0) were enlarged by the addition of NaCl and eventually formed nanoparticle shells and interparticle macropores. It is reported that the solubility of amorphous silica into aqueous solution decreases with the concentration of inorganic salts such as NaCl.38-40 This phenomenon also means that a great amount of inorganic salt promotes the deposition of silica species from sodium silicate solution. As mentioned before, the addition of more than 23 wt % NaCl to sodium silicate resulted in no formation of spherical hollow particles, although the sodium silica solution was still homogeneous. When more than 30 wt % NaCl was mixed with the sodium silicate solution, the entire solution turned into a gel immediately37 and was dissolved again by the extra addition of fresh deionized water. These observations mean that NaCl significantly depressed the solubility of sodium silicate in aqueous solution. To reveal the effects of NaCl on the reactions of sodium silicate with NH4HCO3 solution, an IWP (sodium silicate solution) used in the preparation of SHP-0 or (38) Marshall, W. L. Geochim. Cosmochim. Acta 1980, 44, 907–913. (39) Marshall, W. L.; Warakomski, J. M. Geochim. Cosmochim. Acta 1980, 44, 915–924. (40) Marshall, W. L. Geochim. Cosmochim. Acta 1980, 44, 925–931.

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SHP-NaCl-3 and the OWP (NH4HCO3 solution) were simply mixed without OP. The SEM images shown in Figure 4 indicate that these silicas were not spherical and unshaped. The silica material prepared in the presence of NaCl was composed of nanoparticles from 100 to 400 nm in size (Figure 4A,B). The aggregation of these nanoparticles fabricated some larger, shapeless bulk solids. Two kinds of nanoparticles observed in the shapeless bulk solids and in the shells of SHP-NaCl-3 were well resembled in shape. However, no nanoparticles were observed in the silica material produced from the starting solutions of SHP-0 preparation without NaCl (Figure 4C,D). It is thought that the addition of a large amount of NaCl to the sodium silicate solution is the main factor in the formation of the nanoparticles observed in the shells of SHP-NaCl-3 even by the reaction using the W/O/ W interfacial reaction. However, the interfacial reaction at the W/ O/W emulsion plays an essential role in the fabrication of the spherical hollow structure because of the formation of nonspherical silica materials by the above-mentioned direct reaction. The combination of these two effects produced the hierarchical architecture of the silica spherical particle with the hollow structure and the nanoparticle shell. The production of nanoparticles by the addition of NaCl to sodium silicate must be related to the salting-out effect.41 The interaction of water molecules with coexisting other substances produces the dissolution of the substances in an aqueous solution. When the concentration of other salts increases in the aqueous solution, water molecules are significantly attracted by the salt ions to decrease the number of water molecules interacting with the coexisting substances. Consequently, the intermolecular interaction among these substances becomes stronger to result in the coagulation of these substances.41 It is also reported that the addition of various salts including NaCl to sodium silicate solutions influences the solubility and the structure of sodium silicate.38-40,42,43 Furthermore, it is noted that the concentration of NaCl is an important factor in the salting-out effect of silicate materials.42,43 Therefore, the structure and reactivity of sodium silicate with NH4HCO3 are changed by the added inorganic salts. The variation of the shell structures of silica hollow particles with the amount of NaCl seems to be caused by the salting-out effect of NaCl on sodium silicate species during the reaction with NH4HCO3 solution. According to these observations, a pertinent mechanistic scheme for the formation of silica hollow microparticles with nanoparticle shells such as SHP-NaCl-3 is illustrated in Figure 5. The mixing of IWP and OWP along the interface of the W/O/W emulsion produces the silica matrices.32 When enough NaCl is present in the IWP solution, silica nanoparticles are instantly produced at the interface. The silica nanoparticles thus yielded are agglutinated along the W/O/W interface to form the shells, finally producing silica particles with hierarchical architecture. Because sodium polyacrylate and polymethacrylate are also regarded as salt materials similar to NaCl, there are strong correlations between the formation of the nanoparticle shell and the macroporous shell (diatom-like silica particle) in the previous case.36 Detailed investigations on the mechanism of the formation of the macroporous shells are now in progress in our group. Details of the effects of salt on the characteristics of silicate in aqueous solution are still under investigation as well. The size of nanoparticles in the shells of silica particles with hierarchical architecture is approximately from 200 to 400 nm, which is equivalent to the wavelengths of UV and visible light. It is (41) Grover, P. K.; Ryall, R. L. Chem. Rev. 2005, 105, 1–10. (42) Tanaka, M.; Takahashi, K. Fresenius J. Anal. Chem. 2000, 368, 786–790. (43) Tanaka, M.; Takahashi, K. J. Solution Chem. 2007, 36, 27–37.

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Figure 3. Nitrogen adsorption-desorption isotherms (A) and the corresponding BJH plots (B) using the adsorption branches of SHP samples.

Figure 4. SEM images of silica materials directly prepared from the IWP and OWP of SHP-NaCl-3 (A, B) and SHP-0 (C, D) without OP.

well known that light used to irradiate repeating structures of high and low refractive index materials undergo strong reflections and refractions, which are closely related to Mie scattering.27-29 Hence, silica particles with hierarchical architecture are expected to have unique properties with respect to scattering UV and visible light based on analogous principles because the different components with a higher refractive index (silica) and a lower one (air) are arranged at the several hundred nanometers level. Figure 6 is the diffuse reflection spectra (%R) of various silica hollow particle powders measured by DR-UV spectroscopy with an integrating sphere. For comparison, a reflection spectrum of a common silica gel (Merck silica gel 60) is also shown. This common silica gel is not composed of hollow particles from 100 to 300 μm in size (SEM images in Supporting Information, Figure S5). As shown in the spectrum of this silica gel in Figure 6, the reflectance of UV light shorter than 300 nm drastically decreased and was considerably low, and even the reflectance of UV light longer than 300 nm and visible light was less than 60%. Thus, silica species do not generally reflect UV and visible light, especially UV light shorter than 300 nm. However, the reflectance of UV and visible light by SHP-NaCl samples was much higher than that of silica gel. Even a silica hollow particle (SHP-0) Langmuir 2010, 26(9), 6561–6567

prepared without NaCl had a higher reflection than the silica gel. The hollow structure is predictably effective for the reflection. The reflectance of UV and visible light increased with the addition of NaCl to IWP, and the maximum reflection of UV and visible light was observed in SHP-NaCl-3, which was prepared with 20 wt % NaCl to sodium silicate. The addition of NaCl of more than 20 wt % decreased the reflectance of the light (21.7 wt % SHP-NaCl-4), and further addition of NaCl resulted in no formation of spherical hollow particles as mentioned before. In the Supporting Information (Figure S6), the relationships between the reflectance of UV light (λ = 316 and 250 nm) and the added amounts of NaCl are shown. The reflection of UV light of 250 nm wavelength was remarkably enhanced by the addition of NaCl. In the Supporting Information (Figure S7), the DR-UV spectra of silica hollow particles prepared with various kinds of inorganic salts are summarized. A silica particle prepared with KCl (equimolar to NaCl in SHP-NaCl-3), NaBr, and LiCl (half molar with respect to NaCl in SHP-NaCl-3) had a lower reflection of UV and visible light than did SHP-NaCl-3. The reflectance spectrum of SHPKCl was similar to that of SHP-0 obtained without NaCl. However, SHP-NaI prepared with half molar NaI to NaCl in SHP-NaCl-3 had a comparably high reflection of UV and visible light with SHP-NaCl-3. As shown in Figure 2D, the shell of this particle (SHP-NaI) consisted of silica nanoparticles from 200 to 400 nm in size, similar to SHP-NaCl-3. Therefore, these nanoparticle shells from 200 to 400 nm in size are essential for the high reflection of UV and visible light. It is likely that the nanoparticle shells of SHP-NaCl-3 are attributed to the high reflection of UV and visible light because two components with different refractive indices are arranged at the several hundred nanometers level. The two components in these shells are amorphous silica and air, whose refractive indices are around 1.4-1.530 and 1.0, respectively. The difference in the refractive index of amorphous silica and air is thought to induce the strong reflection and refraction based on the Mie scattering principle.27-29 Then, a simple experiment to reveal this effect was attempted as follows. Air in the silica particles was replaced with decane whose refractive index is about 1.41, analogous to that of silica,44 and the transmission spectra of these two samples were compared. In Figure 7, the two UV-visible transmission spectra of the SHP-NaCl-3/air sample and the SHP-NaCl-3/decane sample are shown. In the Supporting Information (Figure S8), (44) Aminabhavi, T. M.; Patil, V. B.; Aralaguppi, M. I.; Phayde, H. T. S. J. Chem. Eng. Data 1996, 41, 521–525.

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Figure 5. Pertinent mechanistic scheme for the formation of silica particles with hierarchical architecture.

Figure 6. Diffuse reflection spectra of silica hollow particles with the addition of NaCl from 200 to 800 nm in wavelength.

Figure 7. Transmission spectra of SHP-NaCl-3 in air or in decane.

pictures of the quartz cells filled with SHP NaCl-3/air and SHP NaCl-3/decane are given. No transmission of UV and visible light was observed in the spectrum of SHP-NaCl-3 in air. However, the considerable increase in the transmission of UV and visible light occurred in the case of SHP-NaCl-3 with decane. (As shown in Figure S8-A, SHP-NaCl-3 in air was completely opaque. The background of the cell was invisible. In contrast, SHP-NaCl-3 with decane (Figure S8-B) became slightly transparent and the yellow part of the background could be seen through the cell). The transmission of UV light at 300 nm was approximately 30% and increased with the wavelength. Decane has no absorption at wavelengths longer than 300 nm. These simple experiments 6566 DOI: 10.1021/la9043396

Figure 8. Reflectance spectra of various crushed SHP samples.

clearly indicated that the high reflectance of UV and visible light by silica particles with hierarchical architecture is caused by the strong reflection, refraction, and resulting scattering of light by silica nanoparticles and air arranged at the several hundred nanometers level. The involvement of the hollow structure of the silica particles in their reflection of UV and visible light is also an interesting feature of optical materials. Indeed, SHP-0 without a nanoparticle shell reflected light more than a solid (not hollow) silica gel as shown in Figure 6. Then SHP-NaCl-3, which had the best reflectance of UV and visible light, was crushed by two different procedures, and the reflectance properties were compared. The first crushing procedure was to pound SHP-NaCl-3 in a mortar by hand (assigned as SHP-NaCl-3M), and the second one was to press it under 6 tons of force using a common KBr-pellet pressing apparatus for IR measurements (assigned as SHP-NaCl-3P). For comparison, SHP-0 was also pressed under 6 tons of force to be named SHP-0P. The SEM images of these broken silica hollow particles are summarized in the Supporting Information (Figure S9). In the case of being ground with a motor (SHP-NaCl-3M), silica particles are partially destroyed according to the SEM image (Figure S9-A). When SHP-NaCl-3 is pressed under 6 tons of force (SHP-NaCl-3P, Figure S9-B), the spherical and hollow particles were completely collapsed and the nanoparticles of SHP-NaCl-3 retained their original forms (Figure S9-C). However, no observable nanoparticles were found in the pressed SHP-0 under 6 tons of force (SHP-0P, Figure S9-D). Figure 8 summarizes the diffuse reflectance spectra of these crushed samples. Whereas the decrease in the reflectance of SHP-NaCl-3 by being crushed with a motor (SHP-NaCl-3M) was insignificant in degree and was Langmuir 2010, 26(9), 6561–6567

Fujiwara et al.

Article

comparable over the total range from 200 to 800 nm (%R: from 93.4 to 88.5 at 700 nm and from 78.6 to 76.1 at 250 nm), the reflectance spectrum of the pressed SHP-NaCl-3 (SHP-NaCl-3P) remarkably changed compared to that observed before pressing. In particular, the reflectance in the UV light range crucially decreased. The reflectance at 250 and 700 nm decreased to 75% (%R: from 78.6 to 59.3) or 96% (%R: from 93.4 to 89.7). The change in reflectance from SHP-0 (before pressing) to SHP-0P (after pressing) was similar to that of SHP-NaCl-3. The decreases at 250 and at 700 nm were 79% (%R: from 54.7 to 43.2) and 92% (%R: from 92.0 to 84.7). From these results, it is thought that the high reflection of UV and visible light by SHP samples is caused not only by their nanoparticle shells but also by their hollow structure. The hierarchical architectures of the silica hollow particles both at the nano- and microlevels, the nanoparticle shell, and the micrometer-sized hollow structures are very important elements in the high reflection of UV and visible light.

Conclusions and Perspectives The addition of NaCl to the sodium silicate solution changed the shell structures of the silica hollow particles prepared using the W/O/W emulsion system. When appropriate amounts of NaCl or some other salts were added to the sodium silicate solution as IWP, the shells of the obtained silica particles were composed of nanoparticles. The addition of 20 wt % NaCl to sodium silicate formed nanoparticle shells whose diameters were analogous to the wavelengths of UV and visible light. The hierarchical architectures of the silica particles, the nanoparticle shells, and the micrometer-sized hollow structures are essential characteristics of the high reflection of UV and visible light, which is induced by the Mie scattering principle. Common representative inorganic materials for UV protection are TiO2 and ZnO, whose mechanism of UV protection is the absorption of UV light.45,46 However, the absorption of UV light often generates reactive oxygen species that damage skin and other organic components.45,46 However, the silica particles reported in this article do not absorb UV light because of the poor UV light absorption property of silica. It is expected that the utilization of these silica particles will extend the technology of controlling UV and visible light.

sodium silicate solution (approximately 90.4 mmol as silicon; water glass no. 3 was purchased from Kishida Chemical). Additional water was added to fix the total volume of this mixed solution at 36 mL. This solution as an IWP was added to the oil phase (OP) consisting of an n-hexane solution (72 mL) with Tween 85 (1.5 g), and the resulting two-phase solution was emulsified at 8000 rpm by a homogenizer (IKA-T25 digital ULTRA-TURRAX) with shaft generator S25N-25F for 1 min. This W/O emulsion was immediately poured into an aqueous solution (2 M; 250 mL) of NH4HCO3 as OWP at 40 °C with stirring via an agitating blade at 400 rpm. The mixed solution became clouded with time because of the formation of silica hollow particles. After 10 min of stirring at 40 °C, the precipitated solid was filtered, washed three times with deionized water and washed with methanol, and dried at 100 °C for 12 h. The crushing treatments for silica hollow particles were carried out by two methods. One procedure was pulverization in an agate mortar by hand for about 5 min. Another method was to compress the particles under 6 tons of force using a common KBr-pellet pressing apparatus for IR measurements. Sample Characterzation. Nitrogen adsorption-desorption isotherms were measured using a Belsorp Mini instrument (Bel Japan). Specific surface areas of samples were estimated by the BET method. The BJH calculation was used for mesopore analyses such as pore size distribution, pore volume (PV), and pore area (PA) using the adsorption branches. SEM images were obtained using a JEOL JSM-6390 scanning electron microscope (secondary electron image). The particle size distributions of the silica particles were measured by a laser diffraction particle analyzer (Shimadzu SALD-2000; wavelength of semiconductor laser, 680 nm; 3 mW). XRD patterns of materials were recorded by a Mac Science MXP3V diffraction meter with Cu KR radiation. Infrared spectra were obtained using a Perkin-Elmer Spectrum One spectrometer with the KBr-pellet method. Diffuse reflection UV-visible spectra were obtained with a JASCO V-550 spectrometer equipped with an integrating sphere (JASCO ISV469) using a dedicated power sample holder (JASCO PSH-001). In the cases of transmission spectra, quartz cells (length of light path = 5 mm) completely filled with silica hollow particles were used (Figure S8). The displacement of air by decane was carried out by the direct addition of decane to the cell with careful mixing for the complete removal of air bubbles, and these cells were utilized for spectrum measurements.

Methods Procedures. The preparation procedure is generally similar to the method for silica microcapsules that we reported previously, 32,33,36 except for the addition of inorganic salts such as NaCl. In a typical procedure, an aqueous solution of the defined amount of NaCl in 5 mL of ion-exchanged water was mixed with 18.8 g of a (45) DeBuys, H. V.; Levy, S. B.; Murray, J. C.; Madey, D, L.; Pinnell, S. R. Dermatol. Clin. 2000, 18, 577–590. (46) Gonzalez, S.; Fernandez-Lorente, M.; Gilaberte-Calzada, Y. Clin. Dermatol. 2008, 26, 614–626.

Langmuir 2010, 26(9), 6561–6567

Supporting Information Available: XRD pattern and infrared spectrum of a silica hollow particle (SHP-NaCl-3). SEM images of various materials, including silica hollow particles, crushed silica particles, and other silica materials. Particle size distributions measured by a laser diffraction particle analyzer. A nitrogen adsorption-desorption isotherm and magnified diffuse reflection UV-vis spectra of some materials. Photographs of some silica particles in a quartz cell with air or with decane. This material is available free of charge via the Internet at http:// pubs.acs.org.

DOI: 10.1021/la9043396

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