Rattle-Type Colloidal Crystals Composed of Spherical Hollow

Apr 28, 2015 - Controls over the position and orientation of anisotropic particles in their assemblies are intriguing issues for functional colloidal ...
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Rattle-Type Colloidal Crystals Composed of Spherical Hollow Particles Containing an Anisotropic, Movable Core Kanako Watanabe, Daisuke Nagao,* Haruyuki Ishii, and Mikio Konno* Department of Chemical Engineering, Tohoku University 6-6-07 Aoba, Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan S Supporting Information *

ABSTRACT: Controls over the position and orientation of anisotropic particles in their assemblies are intriguing issues for functional colloidal crystals that are switchable with external fields such as electric and magnetic fields. We propose a novel approach for the fabrication of rattle-type colloidal crystals containing an anisotropic, movable core surrounded by a void space that allows rearrangement of the anisotropic core in the assembly. In the fabrication, multilayered core−shell particles composed of a titania core, polystyrene shell, and silica shell were prepared and then heated at 500 °C for 4 h to selectively remove the middle layer of polystyrene. The heating treatment induced deformation of spherical titania cores in the compartment of silica shells, while the void space required for the orientation and relocation of anisotropic core was generated. The rattle particles fabricated were self-assembled by a simple dip-coating to form an arrangement of the spherical yolk/shell particles incorporating an anisotropic core. Brownian motion of the anisotropic cores observed with an optical microscope showed that the assembly of rattle-type particles had the potential to control location and orientation of the anisotropic cores in the shell compartment by application of external fields.

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INTRODUCTION Synthesis of monodisperse particles with anisotropic shapes has been extensively studied because of their intrinsic properties that are not observed for isotropic particles.1−4 Fabrication of colloidal crystals in which anisotropic particles are regularly arranged has also been studied5−7 to apply the intrinsic properties to optical, electronic, and magnetic devices. Since anisotropic particles exhibit phase behavior richer than that of spherical particles,8,9 colloidal crystals of the anisotropic particles are often fabricated with the aid of external fields such as electric and magnetic fields to orient them in the process of self-assembly.10−14 The anisotropic particles used as building blocks in the reports were, however, fixed with each other in the assemblies, which resulted in preventing them from being oriented and relocated. Our group has recently reported a new type of colloidal crystals in which magneto-responsive, spherical cores incorporated into yolk/shell particles could be relocated by application of a magnetic field under the confinement of shells that are 2D hexagonally arranged.15 Yolk/shell or rattle-type particles in this report are defined as core−shell particles with a void space between the core and the shell.16,17 Here we present for the first time novel colloidal crystals of yolk/shell particles containing an anisotropic core. The experimental procedure for fabrication of the colloidal crystals is presented in Figure 1, where multilayered particles incorporating a spherical core were first fabricated (Figure 1a− c) and then heated at 500 °C to selectively remove the middle © XXXX American Chemical Society

layer of the polymer shell from the multilayered particles (Figure 1d). Interestingly, the heat treatment deformed the spherical core in the shell confinement and created a void space that was required for the orientation and relocation of the anisotropic core. Previously reported anisotropic yolk/shell particles were fabricated with a combined technique of multiple coating of anisotropic cores and the selective removal of the middle layer,18−20 which caused anisotropies in both cores and shells of the yolk/shell particles. On the other hand, our synthetic method can create spherical yolk/shell particles containing an anisotropic core. The sphericity of the shells, as shown in Figure 1d, facilitates self-assembling of the particles without any assistance from the electric and magnetic fields, bringing about the rattle-type colloidal crystal containing an anisotropic core. According to our previous report15 on the relocation of cores incorporated into spherical shells by an external magnetic field, it is highly expected that orientation and position of the present anisotropic titania cores can be manipulated by adjusting an applied electric field to the crystal with a compartment of spherical shells.

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EXPERIMENTAL SECTION

Materials. Titanium tetraisopropoxide (95%), tetraethyl orthosilicate (TEOS, 95%), aqueous methylamine solution (40 wt %), aqueous Received: February 11, 2015 Revised: April 28, 2015

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Figure 1. Schematic procedure for the synthesis of yolk/shell particles. ammonia solution (25 wt %), acetonitrile (99.5%), ethanol (99.5%), styrene (St, 99%), p-styrenesulfonic acid sodium salt (NaSS, 80%), and potassium persulfate (KPS, 95.0%) were purchased from Wako Pure Chemical Industries (Osaka, Japan). 3-Methacryloxypropyltrimethoxysilane (MPTMS, 95.0%) was obtained from Shin-Etsu Chemical (Tokyo, Japan). The inhibitor for the St monomer was removed by an inhibitor removal column. Polyvinylpyrrolidone (PVP, MW = 360 000 g/mol) was purchased from Tokyo Chemical Industry. Poly(allylamine hydrochloride) (PAH, MW = 15 000 g/mol) was obtained from Aldrich. A silane coupling agent of 3-aminopropyltrimethoxysilane (95%) was purchased from Chisso Corp (Tokyo, Japan). Particle Synthesis. The yolk/shell particles were prepared with a hard template method in which multilayered particles composed of silica core/polystyrene (PSt) shell/silica shell were heated at 500 °C for 4 h to selectively remove the polystyrene middle layer. The detailed procedure is described as follows: sub-micrometer-sized titania cores shown in Figure 1a were prepared according to our method previously reported.21,22 The coating of the titania cores (Figure 1, a → b) was performed by a two-step polymerization. In the first polymerization to form a PSt layer, a suspension of titania cores was bubbled with nitrogen for 30 min. To the suspension was then added MPTMS at 35 °C. After being stirred for 1 h, KPS and NaSS were added and stirred for 1 h. Styrene monomer (50 mM) was added as an inner shell source to the suspension at 65 °C. The polymerization was conducted at [MPTMS] = 2 mM, [KPS] = 2 mM, [NaSS] = 0.25 mM, and a titania concentration of 0.065 vol %. To further increase the thickness of the PSt shell, the second polymerization of St (150 mM) was conducted with KPS initiator (2 mM) at a core/shell particle concentration of 0.05 vol %. In the second step (Figure 1, b → c), the doubly PSt-coated titania particles were coated with silica. A suspension of the PSt-coated particles was added to a solution containing PAH and NaCl.23 The concentrations of the doubly PSt-coated particles, PAH, and NaCl were 0.05 vol %, 0.71 kg/m3, and 36 mol/m3 in the mixture, respectively. After two centrifugation steps to remove non-adsorbed PAH and NaCl, the particles were redispersed into 30 mL of an ethanolic solution containing 0.168 g of PVP. Two more centrifugation steps were used to remove excess PVP, and particles were redispersed into 7.2 mL of ethanol. To the 14.4 mL ethanolic solution were added 7.95 mL of ammonia solution, silica sources of TEOS (444 μL), and aminopropyltriethoxysilane (28 μL).24,25 PSt particles coated with silica were dried, followed by heat treatment for 4 h in air in an oven at 500 °C (Figure 1, c → d). Preparation of Yolk/Shell Particle Assembly. Two-dimensional assemblies of the yolk/shell particles were prepared with a simple dipcoating method as follows: a suspension of the yolk/shell particles (20 μL) was dropped onto a glass substrate (Matsunami glass, ϕ = 18 mm) that was then inclined and dried for more than 12 h. Prior to this, the glass substrate was hydrophilized by UV-ozone irradiation for 1 h (Biosource Nanosciences, UV/ozone Procleaner, PC450). The particle concentration in the suspension was 0.1 wt %. A small droplet of water (20−100 μL) to mobilize the cores was dropped on the particle assembly on the hydrophilic substrate, and a cover glass was placed on the wet particle assembly in order to avoid quick drying of water during observation with an optical microscope.

Figure 2. TEM images of particles formed in the synthetic processes of yolk/shell particles: (a) TiO2 particles, (b) TiO2/PSt core/shell particles, (c) silica-coated core/shell particles, (d) particles obtained by calcination of the particles (c).

the Supporting Information. Spherical titania cores with an average size of 355 nm (CV = 9.4%) were prepared in a sol−gel method using a mixed solvent of ethanol and acetonitrile21 (Figure 2a). The titania cores were coated with a PSt shell in a two-step soap-free emulsion polymerization, which formed core−shell composite particles having an eccentric core,26,27 as shown in Figure 2b. The composite particles were further coated with silica in a modified sol−gel method and then heated at 500 °C for 4 h to remove the PSt shell from the multilayered particles. As prepared in other previous reports,16,17 spherical particles with a void space between the core and the shell were prepared by the combined technique of silica coating and heating. It should be noted in Figure 2d that the titania cores incorporated into yolk/shell particles were anisotropically shrunk in the heating process, although conventional oxide particles prepared in the sol−gel method were isotropically shrunk in their heatings.21,28,29 Long- and short-axis sizes of the anisotropic titania cores were measured for the particles Figure 2d, which are 356 nm (L) and 310 nm (D), respectively, corresponding to an aspect ratio of 1.14 (L/ D). The anisotropic shrink of titania cores was examined by heating experiments for spherical titania cores (bare cores) and PSt-coated titania cores. The titania particles formed by 500 °C heating the particles of Figure 2a,b are shown in Figure 3. Comparison of the particles before and after the heating indicated that the titania cores with and without a PSt shell were both isotropically shrunk in the heatings. The isotropic shrinkage of titania cores suggests that the inner PSt shell and

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RESULTS AND DISCUSSION Figure 2 shows transmission electron microscopy (TEM) images of particles formed in the fabrication process of yolk/ shell particles. Detailed synthetic conditions are described in B

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during sample preparation or observed in TEM under a reduced pressure. The composition of oxide particles synthesized in the sol−gel method was reported to change with the solvent composition used for the particle synthesis.21,30 According to the reports, PSt coatings of titania cores formed in different solvent compositions were conducted to understand the effect of oxide core composition on the anisotropic shrinkage behavior. Figure 4 shows TEM images of PSt-coated particles incorporating titania cores formed in different mixed solvents of acetonitrile and ethanol. In the TEM images, it seems that the positions of titania cores in the PSt-coated particles in Figure 4b, where 30 wt % acetonitrile was used in the core synthesis, are more eccentric than those in Figure 4a,c, corresponding to 20 and 42 wt %, respectively. The eccentricity of titania cores is discussed in the Supporting Information. The PSt-coated particles of Figure 4a,c were also used in the successive processes of silica coating and heating to prepare yolk/shell particles. Titania cores less eccentric than those in Figure 2d were, however, incorporated into spherical silica shells obtained by the heating process. For the yolk/shell particles of Figure 2d, a colloidal crystal was fabricated by their self-assembly. Figure 5a shows a wideview scanning electron microscopy (SEM) image of the assembly of the yolk/shell particles. The sample for the SEM image was prepared by dropping a small amount of yolk/shell suspension on a hydrophilic substrate and drying it at ambient temperature. A slight chemical etching with a basic solution (pH 11) to detach the cores from inner silica walls was applied to the yolk/shell suspension before the SEM sample preparation. The view of the image indicated that hexagonally close-packed yolk/shell particles were widely formed on approximately 400 μm2 on the substrate due to the selfassembly of monodisperse yolk/shell particles (CV = 3.2%). Figure 5b,c shows magnified SEM and TEM images for Figure 5a, respectively, showing that the anisotropic titania cores were successfully introduced into hollow particles that were hexagonally arranged. Movability of the titania cores in the arranged hollow particles was examined in a simple test where a tiny amount of a water droplet was dropped on the dry assembly of the yolk/ shell particles and the wet assembly was directly observed with an optical microscope. Brownian motions of the anisotropic cores could be observed under the shell compartment (see the movie in the Supporting Information), although some part of yolk/shell particles in the assembly exhibited no motions because the cores stuck strongly to the inner silica wall. The

Figure 3. TEM images (a,b) of TiO2 particles obtained by heating the TiO2 particles (Figure 2a) and the PSt-coated particles (Figure 2b), respectively. The average sizes of the particles in panels a and b were 273 and 344 nm, respectively. The insets are SEM images to show surface features of the TiO2 particles.

the outer silica shell were both required for the anisotropic deformation of titania cores. The measurements of particle sizes for Figure 3a,b showed that the shrinkage percentage in volume of the PSt-coated cores (43%) was lower than that of the bare cores (55%). The low percentage of shrinkage volume indicated that the PSt shell covering titania cores suppressed the core shrinkage probably due to suppression of heat transportation to the titania cores by coating with a PSt shell. The different heat transportation could be experimentally supported by the crystal-like surface of titania particles shown in Figure 3, where the surface feature of the particles in Figure 3a was rougher than that in Figure 3b. Based on the experimental results described above, one can speculate that the titania cores anisotropically covered with a PSt shell (again see Figure 2b,c) were inhomogeneously heated due to the locally thin PSt layer. Furthermore, during the heating, the inorganic shells (corresponding to the SiO2 shell in this case) scarcely shrank, keeping the shape of spheres in which the shrinkage of the titania core at the eccentric position might generate an anisotropic stress distribution. Since deformation of solid materials by stress generally depends on their elasticities, composition of oxide particles is an important factor to elucidate the anisotropic shrinkage of titania cores in the heating process. Comparing the TEM images of spherical titania cores in Figure 2a,b indicated that the sizes of bare titania cores were smaller than those of PStcoated particles, suggesting that the amount of solvent-derived organic components coming out of bare titania cores was larger than that of PSt-coated cores when the particles were dried

Figure 4. TEM images of TiO2/PSt core/shell particles obtained in polymerizations with different TiO2 cores. The TiO2 cores used in the polymerizations (a−c) were prepared in different mixed solvents of acetonitrile and ethanol at acetonitrile fractions of 20 (a), 30 (b), and 42 wt % (c). C

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Figure 5. SEM images of rattle-type colloidal crystals containing an anisotropic core. (a,b) Wide and magnified images of the colloidal crystals, respectively. (c) TEM image corresponding to the image (b).

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percentage of cores moving in the present silica shells can be increased by optimizing environmental conditions in the particle assembly process and by prolonging the chemical etching time. The Brownian motion of the cores in the movie indicated that the water that was added to the dried sample penetrated into the porous silica shell and mobilized the anisotropic cores in the shell compartment. The periodical confinement of anisotropic particles with a function of movability opens the door to multidimensional controls over the orientation of anisotropic particles by application of external fields to the rattle-type colloidal crystal.

Corresponding Authors

*E-mail: [email protected]. Tel: +81-22-7957239. Fax: +81-22-795-7241. *E-mail: [email protected]. Tel: +81-22-7957239. Fax: +81-22-795-7241. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was mainly supported by the Ministry of Education, Culture, Sports, Science and Technology (JSPS KAKENHI Grant Nos. 25600001 and 26286019).

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CONCLUSIONS Yolk/shell particles containing a movable, anisotropic core were successfully fabricated by using the deformation of spherical titania cores under the confinement of the silica shell in the heating process. The yolk/shell particles were spherical and sufficiently monodisperse to be self-assembled, resulting in the formation of a hexagonally close-packed structure of the yolk/ shell particles (rattle-type colloidal crystal) without any assistance from electric and magnetic fields. The observation of anisotropic particles in their assembly showed that the anisotropic cores randomly moved in the shell compartment. Based on the combination of the present result and our work previously reported,15 it is highly expected that electric fields applied to the assembly are able to control orientation of the anisotropic cores in each compartment. The switchability of orientation of the regularly arranged anisotropic particles is likely to be applied to optical and memory devices in the near future.

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AUTHOR INFORMATION

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

A movie on Brownian motion of cores in the rattle-type colloidal crystals to show the movability of anisotropic particles incorporated into the spherical silica shell. Results of the preparation of TiO2 cores and PSt-coated TiO2 cores to examine why the TiO2 cores were eccentric in the PSt-coated particles. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.langmuir.5b01148. D

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DOI: 10.1021/acs.langmuir.5b01148 Langmuir XXXX, XXX, XXX−XXX