Nanospace-Mediated Self-Organization of Nanoparticles in Flexible

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Nanospace-Mediated Self-Organization of Nanoparticles in Flexible Porous Polymer Templates Yoshiyuki Kuroda, Itaru Muto, Atsushi Shimojima, Hiroaki Wada, and Kazuyuki Kuroda Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02344 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 26, 2017

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Nanospace-Mediated Self-Organization of Nanoparticles in Flexible Porous Polymer Templates Yoshiyuki Kuroda,*,†,ǁ Itaru Muto,‡ Atsushi Shimojima,‡ Hiroaki Wada,‡ Kazuyuki Kuroda*,‡,§ †

Waseda Institute for Advanced Study, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo 169-8050, Japan

‡

Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan

§

Kagami Memorial Research Institute for Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan ABSTRACT: Self-organization is a fundamental process for the construction of complex hierarchically ordered nanostructures, which are widespread in biological systems. However, precise control of size, shape, and surface properties are required for self-organization of nanoparticles. Here, we demonstrate a novel self-organization phenomenon mediated by flexible nanospaces in templates. Inorganic nanoparticles (e.g., silica, zirconia, and titania) are deposited in porous polymer thin films with randomly distributed pores on the surface, leaving a partially filled nanospace in each pore. Heating at temperatures beyond the glass transition temperature of the template leads to self-organization of the inorganic nanoparticles into one-dimensional chain-like networks. The self-organization is mediated by the deformation and fusion of the residual nanospaces, and it can be rationally controlled by sequential heat treatments. These results show that a nanospace, defined by the nonexistence of matter, interacts indirectly with matter and can be used as a component of selforganization systems.

Introduction Self-organization of biomolecules such as lipids, nucleic acids, and proteins allows for the development of complex but ordered biological systems and is critical for living organisms.1 A new ordered structure is generated from disordered and/or ordered structures through the collective motion of locally interacting biomolecules. Selforganized structures in biological systems show ordering as well as some degree of modulation and hierarchy. Artificial control of such modulated and hierarchical structures is one of the greatest challenges in nanotechnology.2 The development of biomimetic materials, using nanoparticles, for various structures comprising globular proteins has attracted much interest in the past few decades.3 Many functional nanomaterials whose synergistic properties are superior to those of the constituent biological and inorganic materials have been developed so far.4–6 However, self-organization of nanoparticles requires precise control of their size, shape, and surface properties, and hence finds limited application. Templating synthesis, which involves the use of physical confinement by molecular assemblies, colloidal particles, or porous materials to restrict the formation of porous frameworks, is a promising method to design nanostructures based on various materials.7,8 Such physical confinement is applicable to various compositions but offers a narrower range of nanostructural variations as compared to direct self-organization.

To overcome the aforementioned limitations of templating synthesis, we designed a novel approach, flexible templating, which can be extended to the formation of diverse nanostructures comprising inorganic particles of different compositions.10–12 In our previous research, we successfully deposited gold particles in the void spaces of three-dimensional (3D) colloidal crystals self-assembled from silica nanospheres. The large stress resulting from the overgrowth of gold within the voids caused cleavage of the colloidal crystals along specific crystallographic planes, leading to two-dimensional (2D) replication of the crystals into gold nanoplates on the cleaved surfaces. The combination of templating synthesis and flexible nanostructural modification of the template affords diverse nanostructures that are not accessible by conventional processes. To the best of our knowledge, this concept has been demonstrated only in a pioneering study by Li et al. using an iron nanoparticle as a self-destructive template13 and by our research group using colloidal crystals as flexible templates.10,11 In those studies, however, nanostructural changes in the templates occurred on a limited scale (cleavage of colloidal crystals) because of the use of hard materials (iron nanoparticles and colloidal crystals). The use of soft materials (e.g., polymers) as flexible templates is expected to facilitate significant nanostructural changes, similar to self-organization processes (e.g., generation of modulated and hierarchical nanostructures). Such a system is expected to incorporate the advantages of both templating and self-organization processes.

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at the lifting speed of 3 mm/s, at 25 °C under 50%RH, and dried under air. The composite films were calcined under air at 550 °C for 3 h (ramp rate 3 °C/min) to remove the templates. To examine the effect of thermal treatment, the composite films were heated under air at 80 °C or 120 °C. For replication with zirconia or titania, sol-gel precursor solutions with the following compositions were used: ZrOCl2∙8H2O/EtOH = 1:22 (zirconia); Ti(OiPr)4/EtOH/HCl/H2O = 1:73:63:15 (titania).

Figure 1. Self-organization of nanoparticles in a flexible porous polymer template through (a) condition 1 and (b) condition 2.

Here, we demonstrate a new class of self-organization phenomena in a flexible templating system, wherein nanoparticles (silica, zirconia, and titania) and nanospaces are cooperatively self-organized into one-dimensional (1D) networks, enabling dimensionality transformation (Figure 1). Our initial investigation on the use of porous polymer thin films with a novel bimodal structure as a softer flexible template than those reported in previous studies10–13 unexpectedly revealed that nanospaces in the template mediate the self-organization of replicated metal oxide nanoparticles. Our discovery implies that a nanospace, defined by the nonexistence of matter, interacts indirectly with matter. This concept would provide new insights into self-organization processes and enable the design of nanostructured materials with various compositions.

Figure 2. (a) Surface, (b) tilted, and (c,d) cross-sectional SEM images of the bimodal macroporous HPS film. (c) MP-L with a remaining void. (e) Schematic illustration of the formation of bimodal macroporous HPS film by the breath figure method.

Experimental Section

Characterization. SEM images of the samples were recorded on a Hitachi S-5500 microscope at an accelerating voltage of 2–5 kV. The samples were observed without metal coating. To observe the effects of heating, each sample was placed in a pre-heated oven at the exact time and temperature; then, the sample was naturally cooled to room temperature and subjected to SEM analysis.

Materials. Hyperbranched polystyrene (HPS) was kindly provided by Nissan Chemical Industries, Ltd. Dehydrated tetrahydrofurane (THF), conc HCl, and tetraethoxysilane were purchased from Wako Pure Chemical Ind. Ltd. Ethanol was purchased from Junsei Chemical Co., Ltd. Zirconium oxychloride hexahydrate and titanium tetraisopropxide were purchased from Sigma–Aldrich Co. LLC. All chemicals were used as received.

Results and Discussion

Preparation of porous polymer thin films by the breath figure method. The Si substrate was washed under sonication, first with detergent and alkaline solution, and then with deionized water. Hyperbranched polystyrene (HPS; Nissan Chemical Industries, Ltd.) was dissolved in dehydrated THF in a Schlenk flask under N2 flow to exclude the contamination with water from the atmosphere. The HPS concentration was 30 mg/mL. The Si substrate was spin-coated (3,000 rpm; 30 s) with the HPS solution at 25 °C under 50%RH, and dried under air.

Preparation and Characterization of the Porous Polymer Template. Porous polymer thin films used as templates were prepared by the breath figure method,14 wherein water droplets are used as templates of pores on a polymer thin film when a substrate is coated with an organic solution of the polymer. The droplets are formed by the condensation of water vapor upon the evaporation of organic solvents under humid conditions. Hyperbranched polystyrene15 was used as the polymer component in this study because it is commercially available, with a low molecular weight (Mw = 23,000) and relatively low glass transition temperature (Tg = 57 °C). Star-shaped polymers are reported to have higher ability to form porous structures by the breath figure method than linear ones.16 HPS is expected to be useful as a flexible template that can be softened by heating.

Replication of templates with metal oxide nanoparticles. Sol-gel precursor solutions of silica were prepared with the following compositions: TEOS/EtOH/HCl/H2O = 1:12.0:0.004:4.0 (condition 1); and TEOS/EtOH/HCl/H2O = 1:10.8:0.004:7.2 (condition 2). TEOS was hydrolyzed for 3.5 h. The porous polymer thin films were dip-coated with the sol-gel precursor solutions 2


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A clean Si substrate was spin-coated with a solution of HPS in tetrahydrofuran (THF) to obtain an HPS film with a unique bimodal porous structure (Figures 2a–d). Small macropores (size: approximately 200 nm; denoted as MPS) were randomly distributed on the surface, along with large depressions (size: 0.5–1.5 μm; denoted as MP-L). To the best of our knowledge, this is the first report on the formation of such a bimodal structure. The hydrophobic HPS did not wet the hydrophilic Si surface completely; consequently, dewetting17 resulted in the formation of voids between the HPS film and the substrate (Figure 2c). The voids eventually disappeared upon depression of the HPS film to form MP-L (Figures 2d and e). The pore sizes were sensitive even to slight changes in the preparation conditions (e.g., temperature and relative humidity). However, such pore size variations had no notable influence on the overall results, as demonstrated by the following experiments. In addition, such a structure was not obtained under the same conditions by using linear polystyrene, though it is reproducibly obtained by using HPS.

On the other hand, under condition 2, MP-S was partially filled with an ellipsoidal silica nanoparticle (composite 2; Figures 3c and d). Each MP-S was filled with a single silica nanoparticle, but only a part of the MP-L was filled with a large silica nanoparticle (Figure 3d). The sol-gel solution with composition 2 had a higher water content than that with the composition 1 and was thus less hydrophobic. MP-S was partially filled with silica because of its lower wettability with the HPS film.

Figure 3. (a) Surface and (b) tilted SEM images of the composite 1. (c) Surface and (d) tilted SEM images of the composite 2. Surface SEM images of (e) composite 1 and (f) composite 2 after template removal. Insets in (a) and (c) indicate the cross-sections of MP-S filled with silica nanoparticles.

Figure 4. Surface SEM images of composite 2 heated at 80 °C for 24 h (a) before and (b) after template removal, and composite 2 heated at 120 °C for (c) 0.5 h and (d) 24 h. Inset in (a) indicates the cross-section of a silica nanoparticle in the polymer matrix.

Replication of the Porous Polymer Template by Flexible Templating. The porous polymer thin film was dip-coated with a sol-gel precursor solution of silica, and silica nanoparticles were deposited within the pores. Two sol-gel precursor solutions (conditions 1 and 2) were used. For conditions 1 and 2, the EtOH/H2O ratios were 3.0 and 1.5, respectively, while the TEOS concentration was 25 wt%. Under condition 1, the MP-S was fully filled with a silica nanoparticle and MP-L was partially filled with a large silica nanoparticle (composite 1; Figures 3a and b).

Mechanism of the Self-Organization. To elucidate the mechanism underlying the nanostructural change, the composites were heated at temperatures above the glass transition temperature of HPS (57 °C). When composite 2 was heated at 80 °C for 24 h, both MP-S and MP-L closed because of the deformation of the softened HPS around them (Figure 4a). Non-crosslinked polymers are liquefied above glass transition temperature and their viscosity decreases with increasing temperature,18 which greatly affects its nanostructural deformation. It is also reported

After removal of the templates by calcination at 550 °C, silica nanopatterns were formed on the substrates. In the case of composite 1, 2D isolated circular silica nanoparticles, similar to MP-S in the original template, were observed (Figure 3e). The average size of the nanopatterns (181 nm) was similar to that of MP-S, showing that they are direct replicas of MP-S. Clustered nanoparticles were observed in some areas (Figure S1a). On the other hand, from composite 2, a 1D chain-like network of ellipsoidal silica nanoparticles was observed (Figure 3f, see also Figure S2a for a low-magnification image). Most nanoparticles in the 1D chain are not connected each other as shown in the cross-sectional image (Figure S3). Thus, silica nanoparticle arrays with different dimensionalities (2D and 1D) could be distinctly formed by simply changing the degree of infiltration within the pores. The flexible nanostructural changes in the polymer templates upon heating probably caused this difference.

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the silica nanoparticles are assembled into a 1D chain-like structure.

that porous thin films comprising conventional noncrosslinked polymers, which have low thermal stability, undergo deformation and become flat upon heating.19 However, the arrangement of silica nanoparticles remained unchanged in composite 2 upon heating. Interestingly, the 1D assembly of silica nanoparticles was not observed when composite 2 was further calcined at 550 °C to remove the template (Figure 4b, see also Figure S2b for a low-magnification image). In other words, the residual nanospaces in MP-S and MP-L are essential to promote the 1D assembly, implying that the self-organization of silica nanoparticles is mediated by the nanospaces in the polymer matrix.

The softened polymer matrix acts as a viscous fluid and affect the silica nanoparticles on it.18 Because of its ellipsoidal shape, a silica nanoparticle is subjected to a torque and rotates until its long axis is parallel to the direction of flow (Figure 5d).20 Thus, neighboring silica nanoparticles are forced to form a tip-to-tip arrangement. These effects explain the formation of the 1D chain-like network from ellipsoidal silica nanoparticles. SEM observation of the particles after heating at 120 °C for 0.5 h and 24 h (Figures 4c and d) reveals that this process proceeds with the clustering and growth of ellipsoidal silica nanoparticles.

The 1D assembly was observed when composite 2 was heated at 120 °C for 0.5 h. Relatively short (1–2 μm) chainlike clusters consisting of 2–10 ellipsoidal nanoparticles were formed on the polymer film with nearly closed pores (Figure 4c). After heating for 24 h, the chain-like clusters were linked to form a continuous 1D network (Figure 4d), which was almost identical to that obtained after template removal (Figure 3f), indicating that the 1D assembly is promoted upon heating the polymer template. These results strongly suggest that the deformation of the porous polymer template is the driving force for the selforganization of silica nanoparticles. The proposed mechanism for the 1D assembly is as follows. When composite 2 is heated above the glass transition temperature, the polymer framework is softened and deformed via two different modes: 1) macroscale deformation, i.e., shrinkage of the film and 2) microscale deformation, i.e., deformation of the macropores (MP-S and MP-L) to afford a nonporous structure. Judging from the number density of the replicated silica nanoparticles, the surface of the polymer template shrunk by ~20% during heating, causing the silica nanoparticles to move closer to one another. This process leads to self-organization of the silica nanoparticles, although it is not effective enough to promote 1D assembly. The MP-S macropores are deformed and become shallow upon heating, and each pore is elongated toward its neighbors (Figures 5a). Neighboring MP-S macropores fuse with each other to minimize their surface energy, as confirmed by SEM observation of the assembling silica nanoparticles (Figures 5b). On the basis of these results, it is derived that the 1D assembly is probably promoted by the deformation and fusion of the MP-S macropores.

Figure 5. SEM images of composite 2 heated at 120 °C for (a) 5 min and (b) 10 min. Insets in (a) and (b) are the corresponding surface images. Shallow MP-S is observed under silica nanoparticles in (a), and fused MP-S is observed under silica nanoparticles in (b). (c) A model of the 1D assembly of composite 2. (d) Model of the rotation of ellipsoidal silica nanoparticles by the flow of a polymer matrix. The light gray, dark gray, and yellow areas represent the polymer templates, substrates, and silica nanoparticles, respectively. Red and blue arrows represent the directions of flow and torque vectors, respectively.

The dominant capillary force between closely located silica nanoparticles on a softened polymer matrix strengthens their interactions, thereby stabilizing the 1D chain-like structure. Anisotropic nanoparticles are subjected to an anisotropic capillary force,21,22 which stabilizes the tip-to-tip and side-by-side configurations. Although the side-by-side configuration is thermodynamically more stable than the tip-to-tip configuration,21 the latter is predominantly observed because of the pronounced effect of the torque due to the viscosity resistance from polymer matrix.

We considered two neighboring MP-S macropores partially filled with silica nanoparticles (Figure 5c-1). These macropores fuse together upon heating to form a dumbbell-shaped pore (Figure 5c-2), which corresponds to the scenario depicted in Figures 5a. The neck of the pore is enlarged, and the pore shrinks along the long axis concomitant with the progress of fusion (Figure 5c-3), which corresponds to the scenario in Figures 5b. Finally, the macropores disappear and the silica nanoparticles come close to each other (Figure 5c-4). The silica nanoparticles are transferred by the deformation of the polymer matrix, and the deformation direction is determined by the shapes and arrangement of nanospaces. Consequently, 4


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Figure 6. SEM images of heat-treated composite 2, showing nanostructural changes. (a) Before heating; heated at 120 °C for 0.5 h (b) before and (c) after calcination; heated at 80 °C for 24 h (d) before and (e) after calcination; heated at 120 °C for 0.5 h and subsequently 80 °C for 24 h (f) before and (g) after calcination.

films is summarized as follows. (1) Heating at a temperature sufficiently higher than the glass transition temperature of the constituent polymer induces deformation of the polymer matrix, fusion of nanospaces, and movement of replicated nanoparticles. The fusion of neighboring nanospaces leads to the 1D chain-like arrangement. (2) The anisotropic shape of the replicated nanoparticles governs their orientation because of the torques and the capillary force. (3) Nanospaces with different sizes exert their unique influence on the self-organization of the replicated nanoparticles. The partial void space within MP-S and MP-L promotes 1D assembly of the replicated nanoparticles and radial movement of the replicated nanoparticles, respectively. In addition, the unique bimodal porous structure is an important factor influencing the formation of unique nanostructures. The present method is not only a templating process that affords nanoparticles with definite shapes within a porous template, but also a self-organization process for assembling replicated nanoparticles by the flexible nanostructural transformation of the template. We found a new characteristic of a nanospace that can mediate the movement of other materials

The MP-L also affects the arrangement of the silica nanoparticles on a larger scale. In the fully loaded composites prepared under condition 1, the circular silica nanoparticles hardly show any movement upon heating. Because the MP-S in composite 1 has almost no void space, the flow of the polymer framework to close the pores should not occur. However, clustered nanoparticles are observed around branched silica nanoparticles that are 200–500 nm in size (Figure S1a). The branched silica nanoparticles are attributable to the particles deposited in MP-L before template removal. Because MP-L was incompletely filled/not filled with silica, it could trigger the movement of the silica nanoparticles. The circular silica nanoparticles located around an MP-L are drawn to the center of the MP-L, which proceeded before the decomposition of the porous polymer template (Figure S4). On the other hand, when composite 1 is heated at 80 °C for 24 h, clustering of the nanoparticles is not observed after template removal (Figure S1b), confirming that clustering is promoted by the void space in MP-L. The mechanism underlying the self-organization of the nanoparticles formed as replicas of porous polymer thin 5


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when this method was applied to zirconia and titania (Figures 7a and b). In the case of titania, the replicated particles adopted the side-by-side configuration. The number density of titania nanoparticles after calcination was 27.6 particles/μm2, while that of silica nanoparticles in the composite 2 after the calcination was 7.1 particles/μm2. Two or more titania nanoparticles were possibly formed in a pore. The much higher density of titania nanoparticles is probably the reason for the side-by-side configuration. Furthermore, when a porous polymer thin film with more closely packed MP-S was used as a template (Figure 7c), another unique assembly of ellipsoidal silica nanoparticles (ladder-like double-chain structure) was observed (Figure 7d), implying that various types of structures can be controlled by using this concept. Only HPS was used as a polymer template in this study because of the difficulty in the formation of desired porous structures by the breath figure method. Further studies on the application of this method for various polymer components will be needed to control the nanostructures and self-organization behaviors.

around it by inducing deformation of the surrounding matrix (i.e., polymer); thus, the nanospace can be used as a new component of a self-organization system. This concept facilitates a deeper understanding of selforganization phenomena; in addition, it can be adopted to prepare a wide variety of self-organized nanostructures and develop novel stimuli-responsive self-organization systems. For example, nanoparticles are initially isolated in a porous polymer template. The nanoparticles form percolated 1D network via a heat treatment, which enables energy transfer and electronic/thermal conduction. While one may compare the self-organization of silica nanoparticles to that of particles at the interface of liquids,21,22 these processes are essentially different. Selforganization of particles on an interface is solely driven by the capillary forces between the particles; therefore, freely movable particles are assembled by local interactions. Self-organization of particles on a flexible template is driven by the structural deformation of the template, and the direction of this deformation is guided by the arrangement of nanospaces (MP-S and MP-L). As a result, the self-organization can be programmed by adjusting the location of the nanospaces. Introduction of interactions on different length scales by MP-S and MP-L is nearly impossible in a self-organization system of particles at the interface of liquids. These differences originate from the solid–liquid transition and very high viscosity of polymer above its glass transition temperature. Thermophoretic forces for colloidal nanoparticles23 are also negligible in the flexible templating system because the composites used here were uniformly heated.

Nanofabrication techniques such as lithography24 and nanomanipulation25 will significantly broaden the scope of this nanospace-mediated self-organization concept with stimuli-responsive motion. Accordingly, the present concept can be generalized for various nanostructures as well as various compositions.

Variety of Controlled Nanostructures. The clear understanding of the self-organization mechanism allows us to rationally control the self-organized structures (Figure 6). Upon heating composite 2 at 120 °C, followed by calcination, the ellipsoidal silica nanoparticles were organized into a 1D network with a very long chain-like structure (Figures 6b and c). 1D clusters were observed only after heating at 120 °C, and they grew into a 1D network during calcination. The intermediate 1D cluster can be “frozen” by closing the remaining nanospace in the polymer matrix. It is implied from the following experiment that the ellipsoidal silica nanoparticles do not move but adopt a 2D isolated distribution upon heating composite 2 at 80 °C for 24 h, prior to calcination (Figures 6d and e). Accordingly, when composite 2 was heated initially at 120 °C for 0.5 h to promote 1D clustering of the ellipsoidal silica nanoparticles, and subsequently at 80 °C for 24 h to deform the remaining MP-S nanospaces (Figure 6f, see also Figure S5a for a low-magnification image), 1D clusters (length: 1–2 μm) were successfully obtained after the removal of the template (Figure 6g, see also Figure S5b for a low-magnification image). Thus, multistep selforganization processes are expected to enable precise control of complex structures.

Figure 7. Surface SEM images of 1D replicas consisting of (a) zirconia and (b) titania. (c) A template with more closely packed MP-S partially filled with silica and (d) its selforganized silica replica. Arrows in (d) indicate ladder-like double chains.

Discussion on the Uniqueness of the Flexible Templating Synthesis. 1D structures are frequently observed in nature and play very important roles in living tissues. For example, nerve tissues and blood vessels contribute to information propagation and transport of nutrients, respectively. Nonetheless, fabrication of 1D structures by self-organization processes remains a great challenge.26 1D arrays of nanoparticles have been obtained by exploiting the 1D interactions between magnetic nanoparticles,27 anisotropic shape of nanoparticles,21 partial surface modi-

The present method is basically a templating process and hence can be extended to various materials other than silica. For example, 1D networks of ellipsoidal zirconia or titania nanoparticles were successfully formed 6


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fications28–30 and interactions between organic molecules attached on specific facets of nanocrystals.31 However, these strategies are applicable to a limited range of nanoparticles and/or allow for minimal surface modification. The present flexible templating method is applicable to various materials, and precise control of 1D structure is expected by using nanofabrication techniques; thus, it allows for potential control of 1D nanostructures.

*E-mail: [email protected] (Y. K.), [email protected] (K. K.).

Present Addresses ǁ

Green Hydrogen Research Center, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan.

Author Contributions

Because of their static nature, nanospaces in solid materials have been used for storage, transport, separation, reaction fields, and physical confinement of substances.32– 34 Previous research has demonstrated dynamic control of porous structures and new functions.10–12,35,36 In those studies, the nanospaces were found to be largely deformed, but their positions remained unchanged. In contrast, the flexible templates used in our study induce change in both the shapes and positions of nanospaces. This positional flexibility would lead to novel dynamic functions of nanospaces, such as stimuli-responsive molecular transport. Although the breath figure method has been applied for the assembly of inorganic nanoparticles,37–39 it provides porous structures similar to those of porous polymer thin films or replicas. We believe that the use of nanospaces as the components of a selforganization system will open new avenues for the novel design of ordered materials that mimic natural and biological systems.

The manuscript was written through contributions of all authors.

ACKNOWLEDGMENT We thank Mr. Shintaro Hara (Waseda University) for fruitful discussions. This work was supported in part by Grants-inAid for Scientific Research (No. 26810118 and No. 26248060), and Futaba Electronics Memorial Foundation.

REFERENCES (1) Eigen, M. Selforganization of Matter and the Evolution of Biological Macromolecules. Naturwissenschaften 1971, 58, 465– 523. (2) Pelesko, J. A. Self-Organization: The Science of Things That Putt Themselves Together (Abingdon: Chapman and Hall/CRC, (2007). (3) Kotov, N. A. Inorganic Nanoparticles as Protein Mimics. Science 2010, 330, 188–189. (4) Tang, Z.; Kotov, N. A.; Giersig, M. Spontaneous Organization of Single CdTe Nanoparticles into Luminescent Nanowires. Science 2002, 297, 237–240. (5) Manea, F.; Houillon, F. B.; Pasquato, L.; Scrimin, P. Nanozymes: Gold-Nanoparticle-Based Transphosphorylation Catalysts. Angew. Chem. Int. Ed. 2004, 43, 6165–6169. (6) Dong, A.; Chen, J.; Vora, P. M.; Kikkawa, J. M.; Murray, C. B. Binary Nanocrystal Superlattice Membranes Self-Organized at the Liquid–Air Interface. Nature 2010, 466, 474–477. (7) Davis, S. A.; Breulmann, M.; Rhodes, K. H.; Zhang, B.; Mann, S. Template-Directed Assembly Using Nanoparticle Building Blocks: A Nanotectonic Approach to Organized Materials. Chem. Mater. 2001, 13, 3218–3226. (8) Lu, A.-H.; Schüth, F. Nanocasting: A Versatile Strategy for Creating Nanostructured Porous Materials. Adv. Mater. 2006, 18, 1793–1805. (9) Wan, Y.; Zhao, D. On the Controllable Soft-Templating Approach to Mesoporous Silicates. Chem. Rev. 2007, 107, 2821– 2860. (10) Kuroda, Y.; Kuroda, K. Morphosynthesis of Nanostructured Gold Crystals by Utilizing Interstices in Periodically Arranged Silica Nanoparticles as a Flexible Reaction Field. Angew. Chem. Int. Ed. 2010, 49, 6993–6997. (11) Kuroda, Y.; Sakamoto, Y.; Kuroda, K. Selective Cleavage of Periodic Mesoscale Structures: Two-Dimensional Replication of Binary Colloidal Crystals into Dimpled Gold Nanoplates. J. Am. Chem. Soc. 2012, 134, 8684–8692. (12) Kuroda, Y. Effective Use of Flexible Low-Dimensional Colloidal Particles and Colloidal Crystals for the Control of Hierarchically Porous Materials. J. Ceram. Soc. Jpn. 2015, 123, 853–861. (13) Li, Z.; Li, W.; Camargo, P. H. C.; Xia, Y. Facile Synthesis of Branched Au Nanostructures by Templating Against a SelfDestructive Lattice of Magnetic Fe Nanoparticles. Angew. Chem. Int. Ed. 2008, 47, 9653–9656. (14) Bai, H.; Du, C.; Zhang, A.; Li, L. Breath Figure Arrays: Unconventional Fabrications, Functionalizations, and Applications. Angew. Chem. Int. Ed. 2013, 52, 12240–12255.

Conclusion In conclusion, nanospaces mediate the selforganization of nanoparticles in a flexible template such as a porous polymer thin film. Unique 2D and 1D arrays of nanoparticles can be separately controlled by using a novel bimodal porous polymer thin film made of HPS. The movement of metal oxide nanoparticles deposited in the pores yields a 1D chain-like structure via the deformation and fusion of the polymer template upon heating. We have unveiled that the nanostructural transformation of the polymer template is governed by the deformation of nanospaces; thus, a nanospace (i.e., nonexistence of matter) indirectly interacts with nanoparticles (i.e., existing matter). A polymer acts as a matrix that can simultaneously incorporate nanospaces and nanoparticles. The nanospaces influence nanostructural transformation at different length scales, depending on their sizes; thus, hierarchical structures can be controlled by combining differently sized nanospaces. The proposed flexible templating can be extended to various nanostructures consisting not of only matter but also nanospaces, thereby providing access to unique nanostructured materials with dynamic functions

ASSOCIATED CONTENT Supporting Information. SEM images of the composites and replicas at low magnifications. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author 7


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Assembly of Silica Nanospheres Mediated by Block Copolymer in Liquid Phase. J. Am. Chem. Soc. 2009, 131, 16344–16345. (30) Wang, J.; Yabe, H.; Shimojima, A.; Sekiguchi, T.; Sugawara-Narutaki, A.; Okubo, T. Amino Acid-Assisted OneDimensional Assembly of Semiconducting Metal Oxide Nanoparticles in Aqueous Alcohol Media. Chem. Lett. 2014, 43, 934– 935. (31) Taniguchi, Y.; Takishita, T.; Kawai, T.; Nakashima T. EndTo-End Self-Organization of Semiconductor Nanorods in Water by Using an Amphiphilic Surface Design. Angew. Chem. Int. Ed. 2016, 55, 2083–2086. (32) Davis, M. E. Ordered Porous Materials for Emerging Applications. Nature 2002, 417, 813–821. (33) Ferey, G. Hybrid Porous Solids: Past, Present, Future. Chem. Soc. Rev. 2008, 37, 191–214. (34) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Synthesis and Applications of Supramolecular-Templated Mesoporous Materials. Angew. Chem. Int. Ed. 1999, 38, 56–77. (35) Horike, S.; Shimomura, S.; Kitagawa, S. Soft Porous Crystals. Nat. Chem. 2009, 1, 695–704. (36) Nishihara, H.; Simura, T.; Kobayashi, S.; Nomura, K.; Berenguer, R.; Ito, M.; Uchimura, M.; Iden, H.; Arihara, K.; Ohma, A.; Hayasaka, Y.; Kyotani, T. Oxidation-Resistant and Elastic Mesoporous Carbon with Single-Layer Graphene Walls. Adv. Funct. Mater. 2016, 26, 6418–6427. (37) Böker, A.; Lin, Y.; Chiapperini, K.; Horowitz, R.; Thompson, M.; Carreon, V.; Xu, T.; Abetz, C.; Skaff, H.; Dinsmore, A. D.; Emrick, T.; Russell, T. P. Hierarchical Nanoparticle Assemblies Formed by Decorating Breath Figures. Nat. Mater. 2004, 3, 302– 306. (38) Böker, A.; He, J.; Emrick, T.; Russell, T. P. Self-Assembly of Nanoparticles at Interfaces. Soft Matter 2007, 3, 1231–1248. (39) Li, X.; Zhang, L.; Wang, Y.; Yang, X.; Zhao, N.; Zhang, X.; Xu, J. A Bottom-Up Approach to Fabricate Patterned Surfaces with Asymmetrical TiO2 Microparticles Trapped in the Holes of Honeycomblike Polymer Film. J. Am. Chem. Soc. 2011, 133, 3736– 3739.

(15) Ishizu, K.; Mori, A. Synthesis of Hyperbranched Polymers by Self-Addition Free Radical Vinyl Polymerization of Photo Functional Styrene. Macromol. Rapid Commun. 2000, 21, 665– 668. (16) Widawski, G.; Rawiso, M.; François, B. Self-Organized Honeycomb Morphology of Star-Polymer Polystyrene Films, Nature 1994, 369, 387–389. (17) Reiter, G. Dewetting of Thin Polymer Films. Phys. Rev. Lett. 1992, 68, 75–78. (18) Brandrup, J.; Immergut, E. H.; Grulke, E. A., Eds.; Polymer Handbook 4th ed.; Wiley: New York, 1999. (19) Yabu, H.; Tanaka, M.; Ijiro, K.; Shimomura, M. Preparation of Honeycomb-Patterned Polyimide Films by SelfOrganization. Langmuir 2003, 19, 6297–6300. (20) Jeffery, G. B. The Motion of Ellipsoidal Particles Immersed in a Viscous Fluid. Proc. R. Soc. Lond. Ser. A 1922, 102, 161–179. (21) Loudet, J. C.; Alsayed, A. M.; Zhang, J.; Yodh, A. G. Capillary Interactions between Anisotropic Colloidal Particles. Phys. Rev. Lett. 2005, 94, 018301. (22) Madivala, B.; Fransaer, J.; Vermant, J. Self-Organization and Rheology of Ellipsoidal Particles at Interfaces. Langmuir 2009, 25, 2718–2728. (23) Piazza, R. Thermophoresis: Moving Particles with Thermal Gradients. Soft Matter 2008, 4, 1740–1744. (24) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. New Approaches to Nanofabrication: Molding, Printing, and Other Techniques. Chem. Rev. 2005, 105, 1171–1196. (25) Junno, T.; Deppert, K.; Montelius, L.; Samuelson, L. Controlled Manipulation of Nanoparticles with an Atomic Force Microscope. Appl. Phys. Lett. 1995, 66, 3627–3629. (26) Tang, Z.; Kotov, N. A. One-Dimensional Assemblies of Nanoparticles: Preparation, Properties, and Promise. Adv. Mater. 2005, 17, 951–962. (27) Pileni, M. P. Nanocrystal Self-Assemblies: Fabrication and Collective Properties. J. Phys. Chem. B 2001, 105, 3358–3371. (28) Lin, S.; Li, M.; Dujardin, E.; Girard, C.; Mann, S. OneDimensional Plasmon Coupling by Facile Self-Assembly of Gold Nanoparticles into Branched Chain Networks. Adv. Mater. 2005, 17, 2553–2559. (29) Fukao, M.; Sugawara, A.; Shimojima, A.; Fan, W.; Arunagirinathan, M. A.; Tsapatsis, M.; Okubo, T. One-Dimensional

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Nanospace-Mediated Self-Organization of Nanoparticles in Flexible

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