Creating Quasi Two-Dimensional Cluster-Assembled Materials

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Creating Quasi Two-Dimensional Cluster-Assembled Materials through Self-Assembly of a Janus Polyoxometalate-Silsesquioxane Co-Cluster Han Wu, Yu-Qi Zhang, Min-Biao Hu, Li-Jun Ren, Yue Lin, and Wei Wang Langmuir, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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Creating Quasi Two-Dimensional Cluster-Assembled Materials through Self-Assembly of a Janus Polyoxometalate-Silsesquioxane Co-Cluster †

†

†

†

Han Wu, Yu-Qi Zhang, Min-Biao Hu, Li-Jun Ren, Yue Lin *,ǁ and Wei Wang*,

†,‡,

†

Center for Synthetic Soft Materials, Key Laboratory of Functional Polymer Materials of Ministry of Education and Institute of Polymer Chemistry, College of Chemistry, Nankai University, Tianjin, 300071, P. R. China

‡

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin, 300071, P. R. China

ǁ

Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui Province 230026, P. R. China Keywords: cluster-assembled silsesquioxanes

materials,

self-assembly,

1

ACS Paragon Plus Environment

clusters,

polyoxometalates,

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Abstract: Clusters are an important class of nanoscale molecules or superatoms that exhibit an amazing diversity in structure, chemical composition, shape, and functionality. Assembling two types of clusters is creating emerging cluster-assembled materials (CAMs). In this paper, we report an effective approach to produce quasi two-dimensional (2D) CAMs of two types of sphere-like clusters, polyhedral oligomeric silsesquioxanes (POSS) and polyoxometalates (POM). To avoid macrophase separation between the two clusters, they are covalently linked to form a POM-POSS co-cluster with Janus characteristics and a dumbbell shape. This Janus characteristics enables the co-cluster to self-assemble into diverse nano-aggregates, as conventional amphiphilic molecules and macromolecules do, in selective solvents. In our study we obtained micelles, vesicles, nanosheets and nanoribbons by tuning the n-hexane content in mixed solvents of acetone and n-hexane. Ordered packing of clusters in the nanosheets and nanoribbons were directly visualized using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) technique. We infer that the increase of packing order results in the vesicle-to-sheet transition and the change in packing mode causes the sheet-to-ribbon transitions. Our findings have verified the effectivity of creating quasi 2D cluster-assembled materials though the co-cluster self-assembly as a new approach to produce novel CAMs.

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INTRODUCTION When a group of atoms or molecules are held together by interactions ranging from weak van der Waals contacts to strong covalent bonds they can form clusters that have intermediate size between a molecule and a bulk solid and thus exhibit an amazing diversity in structure, chemical composition, shape, and functionality.1,2 The studies on clusters over the past few decades have become an interdisciplinary field encompassing chemistry, physics, materials science, and biology. Meanwhile, assembling two types of clusters is an additional approach to further create so-called cluster-assembled materials (CAMs) with desired property and functionality.3−7 But, examples in the successful outcomes of the CAM preparation by simply mixing two kinds of clusters are still limited mainly due to the immiscibility between various types of clusters.6,7 In chemistry, it is well known that covalently linking two types of molecules or macromolecules possessing different properties, for instance, one is hydrophilic and another lipophilic together creates a large number of compounds possessing both hydrophilic and lipophilic properties.8,9 They are called amphiphilic surfactants or amphiphilic block copolymers. These molecular or macromolecular amphiphiles have a primeval instinct that they self-assemble into various aggregates, such as micelles, vesicles in solution and diverse microphase-separated morphologies, such as spheres, cylinders and lamellae in bulk.8,9 These aggregates and morphologies have been utilized to fabricate novel materials with a variety of nanostructures directly or as templates. The structure diversity and compositional flexibility of amphiphiles enable the properties, functions and hierarchical structures of materials to be manipulated widely. Whether consciously or unconsciously, it is natural to imitate the amphiphilic structure of surfactants or block copolymers to design and synthesize co-clusters composed of two types of clusters using a tether.10−17 So far, the clusters frequently used are polyhedral oligomeric silsesquioxanes (POSS), fullerenes (C60) and polyoxometalates (POM). Several co-clusters 3


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with a dumbbell-shape and Janus characteristics have been successfully constructed, by covalently

linking

two

types

of

clusters

or

the

same

cluster

with

different

surface-functionalizations together using an organic tether. They self-assembled into bilayer sheets, micelles and vesicles in solution, suggesting their behavior like surfactants or block copolymers. Thus, they are anisotropic tethered nanoparticle shape amphiphiles.18,19 An important characteristic of the shape amphiphiles is that the nanoscale size and specified shape of the individual clusters will synergistically define the self-assembly process of the co-clusters as well as self-assembled nanostructures. In one of our previous works, we are convinced of the importance of cluster packing in the formation of ordered nanostructures after visualized individual clusters in real space.16 In the present work, we continue to study the self-assembly of a POM-POSS co-cluster in mixed solvents of acetone and n-hexane. We observed diverse aggregates, like micelles, vesicles, nanosheets and nanoribbons as well as changes from vesicles via nanosheets to nanoribbons, depending on the n-hexane content using transmission electron microscopy (TEM). Nanoscale size of the POM clusters and difference in atomic number of the elements in POM and POSS clusters allows us to directly visualize individual POM clusters in the nanosheets and nanoribbons using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) technique.20,21 Based on the fine structure characterizations we revealed the arrangement of the POM cluster in the nanosheets and nanoribbons. We further infer that the increase of packing order results in an increase in the bilayer rigidity, thus the size increase of vesicles and final morphology evolution from vesicles to nanosheets. We also infer that various packing modes of the POM cluster in the nanosheets and nanoribbons causes the sheet-to-ribbon transitions. We believe that these fundamental understanding is beneficial for development of quasi two-dimensional (2D) CAMs via self-assembly of co-clusters.

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EXPERIMENTAL SECTION Materials. The synthetic routes and proofs of the co-cluster can be found in our previous publications.11,16 The (Bu4N)6H3(P2W15V3O62) complex was synthesized according to the literature.22 1-Aminopropyl-3,5,7,9,11,13,15-heptaisobutyl-POSS was purchased from Hybrid Plastics

Inc.

and

used

tris(hydroxymethy1)-aminomethane

as

received.

(Tris)

were

Succinic purchased

anhydride from

Aldrich,

and and

2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ) was purchased from Alfa Aesar. Other reagents and solvents were purchased from major chemical supplies in China. All solvents were purified by redistillation before use. Self-Assembling Process. The co-cluster was well dissolved in acetone (ACT) to give a clear solution with concentration of 2 mg/mL. A 300 μL ACT solution of the co-cluster was pipetted to a 2 mL glass vial, then n-hexane (HEX) was injected into the solution by a syringe pump (Harvard Pump 11 Elite) at a constant rate of 100 μL/min to reach different solvent ratios. After the completion of injecting process, the solutions with different solvent ratios were sealed and placed in 25 °C oven for 12 h. Characterization. The transmittance characterization was carried out on a Hitachi U-3900H spectrophotometer using a quartz cell with a path length of 10 mm. The incident wavelength was set as 700 nm. The solutions for the transmittance detection were prepared by adding pre-calculated amount of HEX into an ACT solution of the co-cluster with an initial concentration of 2 mg/mL. After adding HEX, the solutions were stood for several minutes to stabilize. Transmission electron microscopic characterization in bright field (BF) mode was performed using a field emission transmission electron microscope (TEM) (FEI Tecnai G2 F20) operating at an acceleration voltage of 200 kV. HAADF-STEM images were obtained using a TEM (FEI Talos F200X), operated at 200 kV in scanning transmission electron 5


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microscopy (STEM) mode using a high-angle annular dark field (HAADF) detector. The TEM samples were prepared by dropping 10 μL of each solution onto carbon-coated copper grids, then washed with 10 μL HEX for 3 times. The solvent was rapidly removed by using a piece of filter paper and then dried under vacuum at 25.0 °C for 2 hours. Atomic force microscopy (AFM) images were recorded using a multi-model atomic force microscope (Digital Instrumental Nanoscope IV) in tapping mode. The samples were prepared by dropping a drop of the solution on a freshly-cleaved mica surface. The mixed solvent was rapidly removed by using a piece of filter paper and then dried under vacuum at 25.0 °C for 24 hours. The post-processing and the measurement of AFM images were carried out by the Nanoscope Analysis.

Figure 1. Molecular models of a phospholipid, a block copolymer and the co-cluster (only one TBA+ cation was displayed for concision). RESULTS AND DISCUSSION Structure of Co-Cluster. In this study we continue to use the A-B co-cluster composed by covalently tethering two organo-functionalized inorganic clusters together by a short organic tether (Figure 1). The core of cluster A is a 3V-capped Wells-Dawson-type POM anion (P2W15V3O62)9− in which three vanadium atoms on the one end are the sites for covalent organo-functionalization (Figure 1).22 This functionalized anion is covered by six 6


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tetrabutylammonium (TBA, Bu4N+) cations. Hereafter, this TBA-covered POM cluster is referred as POM for simplification. The anionic core maintains a truncated ellipsoidal shape with a long axis of ca. 1.2 nm and a short axis of ca. 1.0 nm.23 Because its electrostatic interaction is mediated by TBA counterions, the TBA-covered POM dissolves well in solvents with strong polarity like N,N-dimethylformide, acetonitrile and acetone. Cluster B is a POSS core organo-functionalized by isobutyl groups attached to seven corners and an aminopropyl group to the last corner. Hereafter, this isobutyl- and aminopropyl-functionalized POSS cluster is referred as POSS for simplification. With eight isobutyl groups on a cubic core, the POSS occupies a spherical space with a freely-extended diameter of approximately 1.3 nm.23 The POSS cluster dissolves more easily in solvents with weak polarity like chloroform, tetrahydrofuran and hexane. In short, the POM and POSS clusters studied here have similarities in rigidity and specified shapes in their cores and difference in solvent solubility after organo-functionalization. Therefore, they are incompatible in the conditions used in this study. The co-cluster, referred as POM-POSS, is formed by tethering the two unlike clusters together using a short organic tether (ca. 1.0 nm) and thus will exhibit new structural features and unique properties. First, it has an asymmetric dumbbell shape. Overall length of the fully extended co-cluster is ca. 3.5 nm, and has an atomically precise molecular weight of 6445 Da.11,16 The two values indicate that its length is close to phospholipids (or conventional surfactants) and its molecular weight is close to some amphiphilic block copolymers. Unlike phospholipids or block copolymers, however, the intrinsic rigidity of the two clusters and the flexibility of the short tether afford the co-cluster a rigid structure with limited degree of freedom. Second, the co-cluster has all characteristics of amphiphiles due to the incompatibility of the two clusters. Therefore, it is a giant and dumbbell-shaped amphiphile that will self-assemble into nano-objects as natural or synthetic amphiphiles do in solution.

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Solubility of Co-Cluster. We have carefully investigated the self-assembling process and the self-assembled structure of this co-cluster in a mixed solvent of acetonitrile (εACN = 37.5 at room temperature) and water (εwater = 80.4).16 The subtle balance between crystallization and nanoscale phase separation defined the self-assembly of the co-cluster in the polar mixed solvent into nanocrystals with a honeycomb superstructure. In the present work, we focus on revealing the self-assembling process of the co-cluster toward nonpolar solvent environment. Here, ACT (εACT = 20.7) and HEX (εHEX = 1.6) were chose to constitute the solvent systems for carefully manipulating the self-assembly of the co-cluster. Our observation shows that ACT is a good solvent for the POM cluster but a precipitator for the POSS cluster, while HEX is precisely the opposite. This coincides the molecular essences of two distinct clusters as we described before.

Figure 2. Plot of T (red) and εMS (blue) versus fHEX showing the evolution of the solution transmittance upon the amount of HEX added into the ACT solution of the co-cluster. Transmittance of Co-Cluster Solution in Mixed Solvents. In our experiment we observed that the co-cluster dissolve well in ACT to form a transparent solution at a concentration c = 2 mg/mL but do not dissolve in HEX. This is because the counterion-mediated electrostatic interaction between the POM clusters is stronger than the van der Walls interaction between the POSS clusters. When we prepared its solutions of 8


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HEX/ACT mixed solvents by adding HEX into the ACT solution of the co-cluster at an initial concentration c = 2 mg/mL, we observed changes in turbidity with increasing volume fraction, fHEX, of HEX (Figure 2). The dielectric constant, εMS, of the mixed solvents varies in the range between εACT = 20.7 and εHEX = 1.6. We obtained or estimated εMS values from the relation of εMS and fACT that is reported in ref. 24. The solutions are transparent when fHEX < 0.45 at which εMS > 10.8. Upon fHEX > 0.45 the solution turned turbid and the transmittance (T, %) evidently decreases in 0.50 ≤ fHEX ≤ 0.60 (9.7 ≥ εMS ≥ 7.9). Upon fHEX > 0.60 (εMS < 7. 9) the solution gradually reaches lowest light transmittance. However, the solution becomes partially clear after fHEX > 0.70 (εMS < 6.1). In this case, we saw some visible precipitates at the bottom of glass vial.

Figure 3. BF-TEM images showing vesicle assemblies of the co-cluster formed at fHEX = 0.63 (a), 0.67 (b) and 0.70 (c). (d) Proposed model for the vesicle assemblies that are formed by a hybrid bilayer of the co-clusters.

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Vesicle Assemblies. The decrease of the transmittance should be related to the formation of assemblies in the solution. Thus, we examined the self-assembled structures formed in five solutions of fHEX = 0.63, 0.67, 0.70, 0.73, and 0.75 by TEM and AFM. In the solution of fHEX = 0.63 (εMS ≈ 7.3), we found small vesicles with diameter of 22.2 ± 3.0 nm and a wall thickness of ca. 5.5~6.0 nm (Figure 3a and Figure S1a). Besides, some micelles with diameter around 10 nm were also found in this solution, which may be the former stage of the small vesicles (Figure S1b). Larger vesicles with diameter of 105.9 ± 26.9 nm were observed in the solution of fHEX = 0.67 (εMS ≈ 6.6), while the corresponding membrane thickness slightly shrank to ca. 5.0 nm (Figure 3b and Figure S1c). Figure 3c shows irregular-shaped and broken large vesicles in the solution of fHEX = 0.70 (εMS = 6.1). The average diameter is over 500 nm. In Figure 3d, we proposed a model to describe the vesicle assemblies of the co-cluster. As the fully-extended molecular length of the co-cluster is estimated to be 3.5 nm, it is reasonable to speculate vesicles are formed by a hybrid bilayer of the co-clusters in which a POM layer is sandwiched between the two POSS layers. The size increase of the vesicles with increasing volume fraction of HEX indicates an increase of the bilayer rigidity. Due to stronger counterion-mediated electrostatic interaction, it is possible that more ordered packing of the POM clusters results in the bilayer becoming more rigid with increasing volume fraction of HEX.

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Figure 4. (a) BF-TEM image showing nanosheets of the co-cluster formed at fHEX = 0.73. Inset is a corresponding SAED pattern of a single-layer nanosheet. (b) HAADF-STEM image displaying bright dots and Fast Fourier transform (FFT) pattern (inset) indicating hexagonal close-packed structure of the bright dots. (c) Further FFT-filtered image of a HAADF-STEM image and close-up image (inset) showing hexagonal close-packed structure of bright POM clusters. (d) Proposed model for a single-bilayer nanosheet. Nanosheet Assemblies. When we slightly increased the volume fraction of HEX to fHEX = 0.73 (εMS ≈ 5.6), we obtained 2D nanosheet assemblies with a shape approximate to a rectangle of which diagonal length larger than 500 nm (Figure 4a). The SAED pattern shows a set of hexagonal diffraction spots with a periodicity of 1.41 nm (inset in Figure 4a). This pattern indicates a hexagonal-close packing of POM clusters. Moreover, we directly visualized the hexagonal packing of POM clusters with the aid of HAADF-STEM. Because atomic numbers, Z, of tungsten and vanadium in the POM cluster are ZW = 74 and ZV = 23, respectively, much higher than ZC = 6 for carbon, ZO = 8 for oxygen and ZN = 7 nitrogen existing in the TBA+ cation and bright contrast in HAADF-STEM is proportional to the 11


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square of the atomic number, this Z-contrast enables us to see bright individual POM clusters when viewed under HAADF-STEM conditions (Figure 4b). The FFT pattern (inset in Figure 4b) also indicates hexagonal close-packed structure of the POM clusters. A further FFT-filtered image (Figure 4c) and a close-up image (inset Figure 4c) definitely show a hexagonal close-packed structure of individual POM clusters in real space. Of course, the TBA+ cations stay in the dark gaps. The nanosheet thickness, determined by AFM, is 5.2 nm (Figure S2), which is similar to the wall thickness of the vesicles. In Figure 4e we proposed a model for a single-bilayer nanosheet. Actually, this model is similar to the bilayer model of the vesicles in Figure 3e, that is, a hybrid bilayer in which a POM layer is sandwiched between the two POSS layers. The ordered packing of the POM clusters enables the hybrid bilayers becoming so rigid that they no longer form vesicles. It is worth mentioning that hexagonal-close packing of the POM clusters means the formation of 2D crystals of single POM layer in the nanosheets. Nanoribbon Assemblies and Fine Structures. When the solvent polarity decreased further to εMS ≈ 5.3 (fHEX = 0.75), we see the formation of large precipitates in the solution with the naked eye. When viewed under TEM the precipitates are large strap-like aggregates with length over 10 μm and width over 1 μm. In the BF-TEM image (Figure S3) we also see that the aggregates are made by intertwisting a few of twisted thin nanoribbon assemblies. In the BF-TEM image (Figure 5a) we amplified one part of nanoribbon, which reveals a strap-like structure composed of dark-bright stripes within the nanoribbon.

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Figure 5. (a) BF-TEM image showing strap-like structures composed of alternatively arranged bright and dark stripes in a nanoribbon formed at fHEX = 0.75. (b) FFT-filtered HAADF-STEM image showing fine stripes within the bright and dark stripes. (c) Intensity line profile along the horizontal direction of the HAADF-STEM image with integration width ca. 50 nm, which is indicated by the dotted rectangular box in b. (d) FFT pattern corresponding to the image in b. All spots that reflect the ordering structures are highlighted using color circles. (e) FFT-filtered and close-up HAADF-STEM image showing orderly packed dots at least in the bright stripes. (e) Proposed fishbone-shape model for the packing of the POM and POSS cores of the co-clusters in the nanoribbons. We also used HAADF-STEM technique to characterize the fine structures within the nanoribbon. An average background filtered HAADF-STEM image is shown in Figure 5b (the raw image can be seen in Figure S4 in Supporting Information), where we can see alternatively arranged bright and dark stripes. The stripes are further split into finer bright and gray stripes, separated by dark pinstripes. To limpidly represent these fine structures, an intensity line profile along the horizontal direction of the image (integration width ca. 50 nm) is shown in Figure 5c. It is clear that, along the direction perpendicular to the stripes, the 13


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bright stripes are split into two fine stripes, while the gray ones are composed of two fine gray stripes and one dark stripes. Periodicity of the bright and gray stripes is 5.7 ± 0.13 nm. Widths of the bright and dark stripes are 2.2 ± 0.06 nm and 3.5 ± 0.09 nm, respectively. The widths of these fine stripes are 1.0 ± 0.11 nm for bright fine stripes, 0.9 ± 0.13 nm for gray fine stripes and 1.2 ± 0.13 nm for dark fine stripes. To reveal the arrangements of the POM and POSS clusters within the fine stripes in detail, an FFT pattern of the HAADF-STEM image is obtained (Figure 5d), in which the x- and y-axes are parallel to the radial directions of the stripes. Spots, highlighted by the color circles, reflect on the ordered packing of these clusters. The four sets of spots on the x-axis correspond to the ordered packing of the clusters along the radial direction of the stripes. Their d-spacing, obtained from measured intensity profile of the spots, are 5.73, 2.87, 1.91 and 1.45 nm, corresponding to the 1st to 4th reflections. The multi reflections indicate a good periodicity of the stripes and fine stripes. Notice that the fourth spot is strong in intensity and large in size in comparison with the other ones, because it is also the reflection of the fine stripes. There are also two sets of reflections off x-axis, highlighted by the blue and red circles in Figure 5d. The angles between the x-axis and the connecting lines between the origin with the spots in the first quadrant are 71.9° and 57.0°, respectively. The d-spacings are 1.58 nm for the spots highlighted by blue circles and 1.80 nm for the spots highlighted by red circles. With all these highlighted spots masked, we did further filtered inverse fast Fourier transform (iFFT). A close-up image after filtered iFFT is displayed in Figure 5e in which we see individual bright, gray and dark dots on corresponding stripes. In the light of highest atomic number of the tungsten (ZW = 74) we infer that the bright dots correspond to the tungsten-containing POM clusters and the dark gaps around them are the TBA+ cations. Obviously, these POM clusters arrange into bilayer in the zigzag way to form the bright stripes as shown by a proposed a fishbone shape model in Figure 5f. The angles between long 14


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axes of the ellipsoidal POM core and the stripe radial directions are ±33.0°, determined by the blue circles highlighted reflections. The periodicity of the POM clusters arranged in a parallel way is 1.58 nm. In this way, the POM clusters pack densely. In Figure 5f it is difficult to identify individual POSS clusters clearly. We infer that the POSS cores stay in the gray fine stripes as the atomic number of silicon is ZSi = 14. The dark fine stripes are the isobutyl groups attached to seven corners of the POSS cores. Finally, the spots highlighted by red circles in Figure 5d correspond to the arrangement of the whole POM-POSS co-clusters in the way that angles between long axes of the whole co-clusters and the stripe radial directions are ±18.1° and the periodicity between them is 1.80 nm.

Figure 6. Schematic demonstration of the transformation of the key self-assembled nano-objects constructed by the co-cluster with increasing HEX volume fraction in the HEX/ACT mixed solvents. Discussion on Transformation of Self-Assembled Nano-Objects. In our experiment we have demonstrated that increasing the HEX fraction in the HEX/ACT mixed solvents induced transformations of self-assembled aggregates from vesicles, to nanosheets and finally to nanoribbons, as schematized in Figure 6. As mentioned above, the POM-POSS co-cluster, formed by tethering the two unlike clusters together using a short organic tether, possesses the following key factors including the dumbbell shape composed of two nanosized and rigid 15


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clusters, limited flexibility of the short tether and different interactions between the two clusters. They will synergistically define the self-assembled nano-objects as well as the transformations. First, we found that the co-cluster preferred to self-assemble into the key nanostructure―hybrid bilayer in which a POM layer was sandwiched between the two POSS layers (referred as POSS-2POM-POSS sandwich nanostructure). It is well known that the formation of the bilayer is common sense for natural and synthetic amphiphiles, including shape amphiphiles. Normally, the formation of the sandwich nanostructure is mainly due to a change in solubility from a good solvent for one block to mixed solvents that might be not good for one or both blocks. Adding a precipitator for one block into a solution of an amphiphile results in an intermediate and selective solubility. Intermediate characteristic means a reduction of dissolving capacity thus the co-cluster aggregation is triggered. Selectivity will promote specific nanostructure formation. In our experiment we observed that the POM cluster hardly dissolved into the solvents with dielectric constant lower than that of ACT. In this case, adding nonpolar HEX into ACT will lead to a gradual decrease of the dielectric constant of the HEX/ACT mixed solvents with increasing the HEX content (see Figure 2). At the critical point at which fHEX = 0.5 and εMS ≈ 9.7, the POM-POSS co-cluster no longer dissolves in the mixed solvent and thus the start to self-assemble into the hybrid bilayer. Thus the polar POM layers selectively stay in the middle of the hybrid bilayer to face the solvent. In the way, the POSS-2POM-POSS sandwich nanostructure is formed. Second, the rigidity of the hybrid bilayer plays the key role in defining the morphology of the self-assembled aggregates. It is well known that flexible and rigid bilayers will form vesicles and flat bilayers or nanosheets, respectively. Note that the TBA-counterion-mediated electrostatic interaction is dominant, thus, POM packing will mostly control the morphology and size of the self-assembled aggregates. Formation of the vesicles within 0.63 ≤ fHEX ≤ 0.70 indicates the hybrid bilayers being flexible. When fHEX increases from 0.63 to 0.70, the vesicle 16


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size increases rapidly, indicating an increasing rigidity. Observation of the nanosheets at fHEX = 0.73 presents the formation of very rigid bilayers (Figure 4a). This rigidity comes from the ordered packing or crystallization of the POM clusters as evidenced by the hexagonal diffraction spots in the SAED pattern in Figure 4a. Therefore, we conclude that increase of the vesicle size from fHEX = 0.63 to 0.70 is actually due to the gradual increase of the ordered-packed POM clusters. Furthermore, a partial crystallization of the POM clusters at fHEX = 0.70 possibly leads to the formation of the irregular-shaped and broken large vesicles and a full crystallization of the POM clusters at fHEX = 0.73 causes the formation of the flat bilayers. Third, the formation of the 2D nanoribbons at fHEX ≥ 0.75 reflects a change in the self-assembly process of the co-cluster. When fHEX = 0.75 εMS ≈ 5.3 of the mixed solvent is close to the dielectric constant of tetrahydrofuran (εTHF = 7.6), a solvent of the POSS cluster. At this moment, the POSS block will be more active in the self-assembly process and the POSS layers or stripes in the self-assembled aggregates prefer to directly face the solvent. In this case, the co-clusters should still self-assemble into nanosheets with the sandwich nanostructure―POSS-2POM-POSS. However, we found a fishbone-shaped arrangement of the POM and POSS clusters in the nanoribbons (Figure 5f). Possible reasons are that the fishbone-shaped arrangement of the POM-POSS co-cluster is denser than that in the sandwich nanostructure, thus dominant electrostatic interaction between the POM blocks is maximized and the difference between POM and POSS sizes is minimized. Finally, we see the fact that the bilayer thickness (ca. 5.5 nm) is always smaller than twice the length of the co-cluster (ca. 7.0 nm). Possible reasons are that the organic tether is more or less compressed and the two clusters prefer a full or partially interdigitated and/or tilted arrangement for denser packing. We also see the fact that and the stripe periodicity (5.7 nm) is smaller than twice the length of the co-cluster (ca. 7.0 nm). Geometric relationship

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between 5.7 nm stripe periodicity and 7.0 nm co-cluster length give an angle of ~±35.5°. This value is very close to ~33° obtained in analysis of the fishbone-shaped arrangement of the POM-POM. CONCLUSIONS In summary, we have demonstrated successful manipulation of the solution self-assembly process and thus self-assembled aggregates and/or nanostructures of the dumbbell-shape co-cluster, i.e. POM-POSS by adjusting polarity of the organic solvents. Experimentally, solvent polarity was adjusted by changing the HEX content (fHEX) in the mixed solvents of ACT/HEX in which polar ACT (dielectric constant ε = 20.7) is a solvent of the POM block and nonpolar HEX (ε = 1.6) is a solvent of the POSS block. Upon fHEX = 0.63 (ε ≈ 7.3) the co-cluster start to self-assemble into small vesicles with basic bilayer structure in which a POM layer is sandwiched by the two POSS layers. Then, the vesicle size increases with increasing fHEX, the vesicles become broken at fHEX = 0.70 (ε ≈ 6.1) and finally convert into flat bilayer nanosheets at fHEX = 0.73 (ε ≈ 5.6). These changes are because increasing packing order of the POM clusters increases the bilayer rigidity. Further increasing fHEX to 0.75 (ε ≈ 5.3) or higher value the co-cluster self-assembles long nanoribbons in which the POM-POSS co-clusters arrange according to a fishbone-shape model. In this denser arrangement dominant electrostatic interaction between the POM blocks is maximized. Our fundamental study has clearly demonstrated a new bottom-up strategy that we can fabricate 2D cluster-assembled materials with the various nanostructures through designing new co-clusters and then manipulating their self-assembly process. ASSOCIATED CONTENT Supporting Information Additional figures S1 to S5 (PDF) . AUTHOR INFORMATION 18


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Corresponding Authors *(W.W.) E-mail: [email protected]. *(Y.L.) E-mail: [email protected]. ORCID Wei Wang: 0000-0002-0608-4749 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Nankai group appreciate the financial support of the National Natural Science Foundation of China for grants (21334003 and 21674052). Yue Lin thanks the support of the Young Scientists Fund of the National Natural Science Foundation of China (11404314), Anhui Provincial Natural Science Foundation (1708085MA06) and the Fundamental Research Funds for the Central Universities (WK2340000055). REFERENCES and NOTES (1) Mingos, D. M. P.; Wales, D. J. Introduction to Cluster Chemistry, Englewood Cliffs, N.J: Prentice Hall, 1990. (2) González-Moraga, G. Cluster Chemistry, Springer-Verlag Berlin Heidelberg, 1993. (3) Perez, A.; Melinon, P.; Dupuis, V.; Jensen, P.; Prevel, B.; Tuaillon, J.; Bardotti, L.; Martet, C. Cluster Assembled Materials: A Novel Class of Nanostructured Solids with Original Structures and Properties. J. Phys. D: Appl. Phys. 1997, 30, 709–721. (4) Castleman, Jr. A. W.; Khanna, S. N. Clusters, Superatoms, and Building Blocks of New Materials. J. Phys. Chem. C 2009, 113, 2664–2675 (5) Claridge, S. A.; Castleman, Jr. A. W.; Murray, C. B.; Sen, A.; Weiss, P. S. Cluster-Assembled Materials. ACS Nano 2009, 3, 244–255. (6) Roy, X.; Lee, C. H.; Crowther, A. C.; Schenck, C. L.; Besara, T.; Lalancette, R. A.; Siegrist, T.; Stephens, P. W.; Brus, L. E.; Kim, P.; Steigerwald, M. L.; Nuckolls, C. Nanoscale Atoms in Solid-State Chemistry. Science 2013, 341, 157–160. (7) Turkiewicz, A.; Paley, D. W.; Besara, T.; Elbaz, G.; Pinkard, A.; Siegrist, T.; Roy, X. Assembling Hierarchical Cluster Solids with Atomic Precision. J. Am. Chem. Soc. 2014, 136, 15873–15876. (8) Marques, E. F.; B. F. B. Silva. Surfactant Self-Assembly, in Encyclopedia of Colloid and Interface Science, 1202–1241, Springer-Verlag Berlin Heidelberg, 2013. (9) Smart, T.; Lomas, H.; Massignani, M.; Flores-Merino, M. V.; Perez, L. R.; Battaglia, G. Block Copolymer Nanostructures. Nano Today 2008, 3, 38–46. (10) Li, Y. W.; Zhang, W. B.; Hsieh, I. F.; Zhang, G. L.; Cao, Y.; Li, X. P.; Wesdemiotis, C.; Lotz, B.; Xiong, H. M.; Cheng, S. Z. D. Breaking Symmetry toward Nonspherical Janus Particles Based on Polyhedral Oligomeric Silsesquioxanes: Molecular Design, “Click” Synthesis, and Hierarchical Structure. J. Am. Chem. Soc. 2011, 133, 10712−10715. (11) Hu, M.-B.; Hou, Z.-Y.; Xiao, Y.; Yu, W.; Ma, C.; Ren, L.-J.; Zheng, P.; Wang, W. POM−Organic−POSS Cocluster: Creating A Dumbbell-Shaped Hybrid Molecule for Programming Hierarchical Supramolecular Nanostructures. Langmuir 2013, 29, 5714−5722. 19


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(12) Liu, H.; Hsu, C. H.; Lin, Z. W.; Shan, W. P.; Wang, J.; Jiang, J.; Huang, M. J.; Lotz, B.; Yu, X. F.; Zhang, W. B.; Yue, K.; Cheng, S. Z. D. Two-dimensional Nanocrystals of Molecular Janus Particles. J. Am. Chem. Soc. 2014, 136, 10691−10699. (13) Lin, Z.-W.; Lu, P. T.; Hsu, C.H.; Yue, K.; Dong, X.-H.; Liu, H.; Guo, K.; Wesdemiotis, C.; Zhang, W.-B.; Yu, X.-F.; Cheng, S. Z. D. Self-Assembly of Fullerene-Based Janus Particles in Solution: Effects of Molecular Architecture and Solvent. Chem. Eur. J. 2014, 20, 11630−11635. (14) Hou, Z.-Y.; Hu, M.-B.; Wang, W. Synthesis and Self-Assembled Structure of A Cluster-Cluster Hybrid Molecule Composed of POM and POSS Clusters. Acta Chim. Sinica 2014, 72, 61−68. (15) Zhang, B.-B.; Ma, C.; Wang, X.-G.; Hu, M.-B.; Wang, X.-L.; Wang, W. Langmuir and of Langmuir-Blodgett Films of Two Dumbbell-shaped Hybrids Composed of A Polyoxometallate and Two Polyhedral Oligosilsesquioxanes. Acta Chim. Sinica 2015, 73, 441−449. (16) Ma, C.; Wu, H.; Huang, Z.-H.; Guo, R.-H.; Hu, M.-B.; Kübel, C.; Yan, L.-T.; Wang, W. A Filled-Honeycomb-Structured Crystal Formed by Self-Assembly of a Janus Polyoxometalate-Silsesquioxane (POM–POSS) Co-Cluster. Angew. Chem. Int. Ed. 2015, 54, 15699–15704. (17) Liu, H.; Luo, J.; Shan, W.; Guo, D.; Wang, J.; Hsu, C.-H.; Huang, M.; Zhang, W.; Lotz, B.; Zhang, W.-B.; Liu, T.; Yue, K.; Cheng, S. Z. D. Manipulation of Self-Assembled Nanostructure Dimensions in Molecular Janus Particles. ACS Nano, 2016, 10, 6585– 6596. (18) Zhang, Z.-L.; Horsch, M. A.; Lamm, M. H.; Glotzer, S. C. Tethered Nano Building Blocks: Toward a Conceptual Framework for Nanoparticle Self-Assembly. Nano Lett. 2003, 3, 1341–1346. (19) Iacovella, C. R.; Glotzer, S. C. Complex Crystal Structures Formed by the Self-Assembly of Ditethered Nanospheres. Nano Lett. 2009, 9, 1206–1211. (20) Pennycook, S.J. Z-contrast STEM for Materials Science. Ultramicroscopy 1989, 30, 58– 69. (21) Kaiser, U.; Muller, D.A.; Grazul, J.L.; Chuvilin, A. Kawasaki, M. Direct Observation of Defect-Mediated Cluster Nucleation. Nat. Mater. 2002, 1, 102–105. (22) Hou, Y.; Hill, C. L. Hydrolytically Stable Organic Triester Capped Polyoxometalates with Catalytic Oxygenation Activity of Formula [RC(CH2O)3V3P2W15O59]6− (R = CH3, NO2, CH2OH). J. Am. Chem. Soc. 1993, 115, 11823−11830. (23) The lenght was estimated using the single-crystal data of Tris-derived Wells-Dawson POM, octa-isobutyl-POSS and succinic acid with the aid of Chem-Bio3D. CCDC 675452 (Wells–Dawson POM), 206050 (octa-isobutyl-POSS), and 154423 (succinic acid) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre. (24) Sen, A. D.; Anicich, V.G.; Arakelian, T. Dielectric Constant of Liquid Alkanes and Hydrocarbon Mixtures. J. Phys. D Appl. Phys. 1992, 25, 516–521.

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