Mesoscale Graphene-like Honeycomb Mono- and Multilayers

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Mesoscale Graphene-Like Honeycomb Mono- and MultiLayers Constructed via Self-Assembly of Co-Clusters Xue-Sen Hou, Guo-Long Zhu, Li-Jun Ren, Zihan Huang, RuiBin Zhang, Goran Ungar, Li-Tang Yan, and Wei Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11324 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

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Journal of the American Chemical Society

Mesoscale Graphene-Like Honeycomb Mono- and Multi-layers Constructed via Self-Assembly of Co-Clusters Xue-Sen Hou,†,⊗ Guo-Long Zhu,§,⊗ Li-Jun Ren,† Zi-Han Huang,§ Rui-Bin Zhang,⊥,# Goran Ungar,*,⊥,# Li-Tang Yan,*, § and Wei Wang*,†,‡ †

Center for Synthetic Soft Materials, Key Laboratory of Functional Polymer Materials of Ministry of Education and Institute of Polymer Chemistry, Nankai University, Tianjin 300071, China. ‡ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin, 300071, China § Key Laboratory of Advanced Materials (MOE), Department of Chemical Engineering, Tsinghua University Beijing, 100084 (P. R. China) ⊥

Physics, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou 310018, China. Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, U.K. KEYWORDS. Artificial graphene; self-assembly; honeycomb structure; nanocluster; stacking superstructure; hierarchical kinetics process. #

Supporting Information Placeholder

ABSTRACT: Honeycomb structure endows graphene with extraordinary properties. But could a honeycomb monolayer superlattice also be generated via self-assembly of colloids or nanoparticles? Here we report the construction of mono- and multilayer molecular films with honeycomb structure that can be regarded as self-assembled artificial graphene (SAAG). We construct fanshaped molecular building blocks by covalently connecting two kinds of clusters, one polyoxometalate (POM) and four polyhedral oligomeric silsesquioxanes (POSS). The precise shape control enables these complex molecules to self-assemble into a monolayer 2D honeycomb superlattice that mirrors that of graphene but on mesoscale. The self-assembly of the SAAG was also reproduced via coarse-grained molecular simulations of a fan-shaped building block. It revealed a hierarchical process, and the key role of intermediate states in determining the honeycomb structure. Experimental images also show a diversity of bi- and trilayer stacking modes. The successful creation of SAAG and its stacks opens up prospects for the preparation of novel self-assembled nanomaterials with unique properties.

INTRODUCTION Two dimensional (2D) materials led by graphene and transition-metal dichalcogenides exhibit unique physical properties dominated by their 2D honeycomb structures. They have attracted intense interest because of their significance for both basic science and potential applications.1 Inspired by the success of these systems, designing new artificial honeycomb lattices is a rapidly expanding area of research aimed at developing novel 2D materials with properties beyond those of existing systems.2 The challenge is how to construct such structures, particularly on lengthscales beyond the covalent bonds. Colloidal crystal lattices formed by self-assembly of nano- or microparticles, reported so far, have covered almost all lattices that have been found in atomic or ionic crystals,3 except the

2D honeycomb lattice. This is because their spherical shape directs them toward close-packed structures.4 Early simulations pointed out the role of specific interparticle potentials as key for their self-assembly into a 2D honeycomb lattice.5a A subsequent simulation predicted that optimal interactions could be achieved by patchy colloidal particles obtained by precise surface decoration.5a More specifically, three-patch particles with C3h symmetry should self-assemble into a stable 2D superlattice with honeycomb symmetry via attractive patch–patch interactions. So far there have been only two successful constructions of a honeycomb structure in singlecrystalline sheets through 2D oriented attachment, reported in certain semiconductor nanocrystals.6 Evidently, construction of a 2D honeycomb superlattice via self-assembly of colloids is still a challenge.

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cluster. The organic tether is a second generation benzamide dendron based on 5-aminoisophthalic acid; it is a rigid framework due to the aromatic amide bonds between the benzene rings (Figure S1). Energy minimization of the single molecule gave a fan-shaped conformation shown in Figure 1b and c.

Figure 1. (a) Structure of the POM-4POSS molecule. (b and c) Front and side views of the single molecule after energy minimization using ChemBio3D. Note that the six TBAs that cover the POM core are ignored.

Here we report a 2D honeycomb superlattice via self-assembly of a planar building block with well-controlled shape, containing two types of covalently linked organofunctionalized inorganic clusters. Electron microscopy has revealed single- and multilayer selfassembled sheets with a diversity of graphene-like stacking modes, albeit on an order-of-magnitude larger lengthscale.

Synthesis of the building block. The general synthetic route of the POM-4POSS molecule, as shown in Figure 1, includes a seven-step reaction procedure. The detailed information is available in the Supporting Information (SI). Amidation between 5-nitroisophthaloyl dichloride10 and aminopropyl-isobutyl-POSS produces 2POSS-NO2, a key step in introduction of POSS clusters into the building block. The most important step is step III in which amidation between 5nitroisophthaloyl dichloride and phenylamine in 2POSS-NH2 creates the rigid framework. The reaction with the lowest yield (47 %) is step IV in which nitro group of 4POSS-NO2 is

RESULTS AND DISCUSSION Design of the molecule (building block). Covalently linking two different types of clusters forming Janus particles7 or co-clusters8 is a relatively recent development in the construction of novel building blocks. In the present work a Wells-Dawson-type polyoxometalate (POM) cluster and four polyhedral oligomeric silsesquioxane (POSS) clusters are covalently linked via organic tethers to give a wedgeshaped building block referred to below as POM4POSS (Figure 1a). The anionic POM core (P2W15V3O62)9− is organo-functionalized with nine counterions: six tetrabutylammonium (TBA, Bu4N+) surfactant cations and three protons (H+), which makes it a (Bu4N)6H3(P2W15V3O62) complex. Its molecular weight is 5422.2 Da. Hereafter, the POMcounterion complex is referred to as “POM cluster”. The POSS cluster is 1-aminopropyl3,5,7,9,11,13,15-heptaisobutyl-POSS with a cubic cage in which seven isobutyl groups are attached to seven of its corners and an aminopropyl group to the eighth. Its molecular weight is 874.6 Da. Hereafter, the aminopropyl-isobutyl-POSS is referred to as “POSS cluster”. In designing the compound in this study, among Figure 2. The POM-4POSS molecule was prepared by a seven-step reaction the points considered were the incompatibility of the procedure from the following raw materials: 5-nitroisophthaloyl dichloride, two cluster types reflected in their solubility aminopropyl-isobutyl-POSS, succinic anhydride, tris(hydroxymethy1) aminodifference, and the difference in their interactions. methane (Tris) and (Bu4N)6H3(P2W15V3O62) complex. The POM cluster with counterion-mediated electrostatic interaction dissolves well in some polar reduced to a phenylamine to prepare 4POSS-NH2. The yield of aprotic solvents like dimethylformamide and the POSS cluster subsequent reactions (steps V-VII) is below 60% possibly due with van der Waals interaction dissolves well in weak polar to the size effect of the four POSS clusters. Finally, the three solvents like tetrahydrofuran and toluene. From single-crystal hydroxyl groups in 4POSS-Tris were reacted with the data,9 we obtain the unit cell volumes of 2550 Å3 for the (Bu4N)6H3(P2W15V3O62) complex to prepare the targeted POM-6TBA complex and 580 Å3 for the octa-isobutyl-POSS


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product, POM-4POSS.11 The total yield of the synthesis of the POM-4POSS molecule is about 3.5%. Self-assembled honeycomb. Self-assembly of POM-4POSS was achieved by slowly pumping its THF solution into water under stirring. The building block aggregated in the THFwater solvent, the suspended particles causing Tyndall scattering (Figure S2). The suspension was left at ambient temperature for different periods, ta, ranging from 8 to 35 days. Then a drop of solution was spread on a copper grid covered with a carbon film. The grid was dried under vacuum at 25 °C for 8 hours to remove residual solvent. The self-assembled structures of the POM-4POSS were mainly observed by transmission electron microscopy (TEM) in the bright field (BF) mode. More ordered structures were observed in precipitates of suspensions that were annealed at ambient temperature at ta = 8 to 35 days as indicated in the figure captions. The fact that the honeycomb layers grow larger and become more ordered upon annealing in the suspension indicates that the honeycomb mono-, bi- and tri-layers, micrometers in size, indeed form in the solution.

pattern (inset in Figure 3a). The FFT pattern is dominated by the strong inner {10} and outer {11} spots, and showing additional weaker {20}, {21} and {30} spots. The unit cell parameter is a = 7.9 ± 0.3 nm, thus the distance between two neighboring dark dots is a/√3 = 4.6 nm. Thus we obtain the first self-assembled artificial graphene (SAAG) from solution. The filtered image in Figure 3c shows a nearly perfect hexagonal honeycomb structure of uniform dots, two dots (shown red and blue) per unit cell (green). Meanwhile, we determined a grayscale intensity profile along the zigzag-direction of the honeycomb structure (Figure S6) and obtained a diameter d = 3.1 ± 0.4 nm of the dark dot from the full width at half maximum (FWHM) of the peaks. We find a gap of ca. 1.5 nm existing between two neighboring dark dots. This gap is attributed to the POSS and the organic tether.12 Diverse superstructures. In our TEM images we frequently observed patterns that seemingly differ from the honeycomb of SAAG (Figure 3). Figure 4a is a Fourier-filtered BF-TEM image showing diverse superstructures still composed of dots with different gray levels. Three close-up images (Figures 4b to d) are selected from areas b’ to d’ in Figure 4a. In Figure 4b we see a hexagonal lattice with a unit cell containing one black and two gray dots of approximately equal density. Meanwhile, in c and d the hexagonal symmetry is broken. In c the 6-fold axis is replaced by a 3-fold axis and the plane group is reduced from p6mm to p3m1. In d the hexagonal unit cell is replaced by a rectangular one with twice the area, plane group c2mm. In C there are spots with three grey levels, while in d for each light-grey there are two black spots.

Figure 3. (a) BF-TEM image showing a honeycomb lattice of dark dots. Six yellow dots highlight a honeycomb hexagon. Inset is a corresponding FFT pattern with hexagonal symmetry. ta = 15 days. (b) HAADF-STEM image showing the same honeycomb lattice composed of bright dots. ta = 15 days. (c) Fourier-filtered TEM image highlighting a unit cell.

The solid suspended in the solution is in the form of sheets, as shown by the dark platelet with a size of a few tens of square micrometers (Figure S3) and an average thickness of 2.4 nm, as measured by atomic force microscopy (Figure S4). As the short axis of the POM core is between 1.0 and 1.5 nm, depending on the arrangement of the surrounding TBA cations, the majority of the sample seems to be double-layered. However single and triple layers are also observed. The highmagnification BF-TEM image (Figure 3a) shows a selfassembled sheet nanostructure containing dark dots on a hexagonal honeycomb grid. These dots are the POM-units, having high electron density contributed by tungsten and vanadium (see EDX spectrum in Figure S5). When this nanostructure is viewed using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) the dots become bright, as expected (Figure 3b). In these TEM images we see a well-ordered hexagonal honeycomb (see the six yellow dots in Figure 3a), each dark dot having three neighbors. Hexagonal symmetry is also reflected in the fast Fourier transform (FFT)

Figure 4. (a) Fourier-filtered BF-TEM image showing different periodic arrangements of dots with different grey levels. (b to d) Three close-up images, selected from areas b’ to d’ in a, highlighting the three types of patterns. ta = 35 days.

Polycrystalline nature and grain boundaries. As in graphene,13 the honeycombs are polycrystalline. This can be seen in the samples with more ordered structures. The Fourierfiltered BF-TEM image in Figure 5 shows an example in which we see a grain boundary zone existing in-between two



honeycomb monolayers. This zone is highlighted by the two yellow curves and have a width of ca. 40 nm. The honeycomb structure is distorted within the zone, but nearly perfect in the two grains outside the zone. The red and blue arrow lines that are parallel to the airchair edges of the honeycombs and the 162° angle between them indicates the grain orientation. The arrows point out the growth directions of the honeycombs. The red and blue clusters of four honeycomb cells present a perfect honeycombs in the areas that they cover.

three elementary building blocks, i.e. the POM-4POSS molecules (Figure 6b). Thus, we can conclude that counterionmediated electrostatic interaction between POM blocks drives three building blocks to form a trimer and then van der Waals interaction between POSS shells of the trimers drives the trimers to form the cells.

Figure 6. (a) Typical snapshot of the honeycomb superstructure self-assembled from coarse-grained model POM-4POSS units shown in the inset. (b) Hierarchical structure of a honeycomb cell where the red truncated triangle and the pentagon highlight the structural units at different levels. (c to g) Snapshots showing evolution of the self-assembly. The times of recordings are: 0τ (c); 1×104τ (d); 4×104τ (e); 6×104τ (f); and 4×105τ (g). The circles and arrows in d to g point out the formation of the structural units at different levels during the hierarchical self-assembly.

Figure 5. This BF-TEM shows polycrystalline nature of the honeycomb. ta = 15 days. The two yellow curves delineate a grain boundary zone in which the honeycomb structure is distorted. The red and blue arrow lines are parallel to the (110) planes and the arrows indicate the growth direction of the honeycomb domains. The red and blue clusters of four honeycomb cells are examples of perfect honeycombs.

Coarse-grained molecular simulations of the honeycomb lattice. To identify the detailed molecular organization in the experimentally observed honeycomb superstructure as well as its formation mechanism, we performed systematic coarsegrained (CG) molecular simulations of the self-assembly of POM-4POSS constrained to a 2D plane (see SI for simulation details). As shown in the inset in Figure 6a, our CG model of the asymmetric building block is directly mapped from the molecular structure of POM-4POSS, where the size ratio of the two kinds of beads is determined from their single-crystal data and the beads are frozen into the desired geometry, i.e., fan shape (Figure S7). Figure 6a shows a representative structure self-assembled by such asymmetric building blocks. Despite the mild fluctuation of some local structures, we definitely observe hexagonal honeycomb lattices on a large scale. As indicated by the blue circles and the red hexagon, the unit cell is porous and contains three POM clusters arranged around one its apices. The good agreement with TEM observation corroborates our simulation model of the SAAG. This allows us to identify the detailed molecular organization of the hexagonal honeycomb structure. A close examination of a typical lattice reveals its hierarchical nature: six truncated-triangle entities organize into a hexagonal array and each truncated triangle is composed of

Fundamentally, the building-block shape plays a significant role in shaping such a unique structure because the geometry of the building block contributes directional entropic forces driving the assembly.14 As to the present system, this however raises an important question: what are the individual contributions of the shape of the elementary building block and of the truncated triangle entity? To address this issue, we consider the detailed self-assembling kinetics from a disordered staring configuration to the honeycomb (Figures 6c to 6g). Interestingly, we find two characteristic stages during the whole process. That is, the building block with fan shape firstly self-organize into dispersed truncated triangle entities (Figure 6d), and then these entities assemble into the hexagonal honeycomb lattice (Figures 6e to 6f). In the second stage, each entity behaves as an integral building block whose shape determines the final honeycomb structure (Figure 6e and Figure S8). This finding may point to a novel mechanism regarding shape-entropy-driven self-assembly: the geometry of the intermediate entity formed by the elementary building blocks directly shapes the self-assembled superstructure, instead of the geometry of the elementary building block itself, as proposed previously.14a,14c,14d The force due to the shape entropy is typically of the order of a few kT (where k is the Boltzmann’s constant and T is temperature), making it comparable to van der Waals.14a Thus, the entropic forces due to the building-block geometry can contribute to the bonding between the building blocks and thereby facilitate the stabilization of the structures. Fundamentally, the kinetic pathway plays a critical role in determining the final assembly mode. Here we report a 2D honeycomb superlattice via self-assembly of a planar building block with well-controlled shape, containing two types of covalently linked organo-functionalized inorganic clusters. Electron microscopy has revealed single- and multilayer self-assembled sheets with a diversity of graphene-


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like stacking modes, albeit on an order-of-magnitude larger lengthscale. Understanding the superstructures. The diverse stacking superstructures appearing in the same system are an intriguing feature. We have summarized them in Figure 7. Five different types of patterns are shown in the Fourier-filtered BF-TEM images (a-1 to a-5). We have recorded relative intensity profiles of the dots along the red lines to estimate the amount of POM in the dots by assuming it to be proportional to the optical density (grey level, GL). The five normalized intensity profiles are shown in Figures 7b-1 to b-5. In Figure 7b-1 all the peaks have the same height, hence all the dots in a-1 contain the same number of POM clusters. In b-2 two peaks with GL ≈ 0.57 are found between each two GL ≈ 1 peaks, hence the grey dots in A-2 contain about half the number of POM clusters as those in the black dots. The two peaks with GL ≈ 0.39 and 0.67, found between each two GL ≈ 1 peaks in b-3 mean that the numbers of POMs in the three types of dots in a-3 are in the ratio 1:2:3. Finally, in b-4 and b-5 all the peaks have the same GL, hence all the dots in a-4 and a-5 contain the same number of POM clusters (note that the very weak dots in a-1 and a-5 are artifacts of Fourier truncation).

ized 2D projection is shown in Figure 7d-1. Applying this configuration we simulated the TEM image (Figure 7e-1) and the corresponding normalized grayscale scan (f-1). These should be compared with Figures 7a-1 and b-1. In c-2 to c-4 three different stacking models for shifted layers are suggested, pertaining to the different areas of the BF-TEM image. In an AB (or Bernal) stacked honeycomb bilayer (Figure 7c-2) the blue monolayer A is overlapped by a yellow monolayer B, shifted by 1/3 of the unit cell parallel to the (110) plane.16a Thus, for each overlapped black node with GL = 1 (Figure 7d-2), there are two non-overlapped grey nodes with GL = 1/2 (cf. Figures 7d-2 and e-2). In Figures 7c-3 and c-4 we propose, respectively, an ABA16a model and an ABC (rhombohedral)16b trilayer stacking model. In the ABA trilayer, the red network is exactly above the blue bottom network, while the intercalated yellow layer is shifted by 1/3 of the unit cell along the (110) plane. In this case, ABA overlapped dots have GL = 1, AA overlapped dots GL = 2/3 and the non-overlapped dots at B GL = 1/3. Accordingly along a horizontal line (Figures 7d-3 and e-3) we see a light grey, medium grey and a black dot sequence, with GL profiles 1/3, 2/3 and 1 (Figures 7b-3 and f-3). The projection has a hexagonal unit cell with plane group symmetry p3m1. In the ABC-stacked trilayer (Figure 7c-4) the B-C shift of the top red layer is again by 1/3 but now in the direction 120° to that of the A→B translation. In other words, the nodes of the third monolayer cover the non-overlapped nodes of the AB bilayer. Thus all nodes of the trilayer are covered only once giving all the dots the same grey level and the TEM pattern the appearance of a triangular lattice (Figure 7d-4). The 3D unit cell is rhombohedral, space group R m. Unlike the hexagonal and trigonal “supercells” (Figures 7d-2 and d-3), the projection (Figure 7d-4) has a small hexagonal p6mm unit cell. These findings confirm the existence of ABA- and ABC-stacked trilayers in our samples. The three distinct layers are actually discernible at the edge of the film (Figure S9).

Figure 7. (a-1 to a-5) Fourier-filtered BF-TEM images showing five superstructures: 1: monolayer or AA… multilayer; 2: AB bilayer; 3: ABA trilayer; 4: ABC trilayer; 5: AB bilayer with irrational shift. (b-1 to b-5) Normalized grayscale scans along the red lines in a. (c-1 to c-5) Stacking models of the honeycomb network layers. (d-1 to d-5) Idealized images of the stacking models in c. (e-1 to e-5) Simulated images obtained from the corresponding models in c. (f-1 to f-5) Normalized grayscale profiles along the red lines in e.

In Figures 7c-1 to c-5 we propose models of honeycomb layer stacking, four of which (c-2 to c-5) with layer shift. Similar models have been utilized in interpreting BF-TEM images of monolayer and stacked multilayer graphene.15 The basic model in Figure 7c-1 depicts a single honeycomb layer or an eclipsed AA… stack (layers precisely on top of each other). The ideal-

In the above model the shift between the layers was always a rational vector in the xy plane, in fact in all cases by 1/3 of the unit cell. We refer to these as rational superstructures. There are also, of course, many intermediate stacking modes where the shift is irrational. An intermediate displacement is shown in Figure 7a-5, where the two layers are shifted parallel to the (110) plane by a small fraction of the unit cell. The model and the simulation (Figures 7c-5 and f-5) correspond to the experimental observation in Figures 7a-5 and b-5. The variable shift between stacked layers indicates weak interlayer interaction, and the changing stacking modes suggest a slight mismatch in expansion or contraction of the adjacent soft layers. It appears that the “rational” stacking modes are slightly favored, indicating that a weak lock-in force exists between adjacent layers. Moreover, even when the shift vector is irrational, it is in most cases directed parallel to a {100} plane, i.e. along the grooves of the honeycombs. The attractive interlayer force responsible is believed to be predominantly van der Waals. Significantly, the above findings are the same as those in monolayer and few-layer graphene,16 in which the diversity is also closely related to interactions between monolayers. In



few-layer graphene as well as in graphite16 AA- and intermediate-stacking structures are energetically unfavourable as compared with AB-stacking. CONCLUSION In conclusion, we have successfully constructed, through selfassembly, a mesoscale version of graphene-like nanohoneycomb monolayers. Like graphene, they were found to form bi- and trilayers via different stacking modes. The reconstruction of the honeycomb structure via coarse-grained molecular simulations demonstrates that the key to success lies in precise shape control of the building block. The successful generation of a self-assembled mesoscale graphene-like structure promises further developments of novel 2D materials with unique properties. The huge specific area combined with porosity of such structures can ensure highly accessible active sites and generate extremely high chemical affinity. The structure presented here can be regarded as a truly 2D cluster-based metal-organic framework, an emerging class of clusterassembled materials17 with considerable potential for applications such as in catalysis, sensors, electrochemistry, membranes, 2D heterojunctions, templated synthesis, as complex nanoreactors, etc.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications Experiments, characterization and Simulations; Figures S1−S24 (PDF). AUTHOR INFORMATION

Corresponding Authors [email protected] [email protected] [email protected] Author Contributions ⊗

X.-S.H. and G.-L.Z. contributed equally.

.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The Nankai and Tsinghua groups are grateful for the financial support given by the National Natural Science Foundation of China for grants (21274069, 51273105, 21334003, and 21422403, 21674052, 51633003). G.U. acknowledges support from the State Specially Recruited Experts program of the Government of China and the joint NSF-EPSRC grant “RENEW” (EP-K034308). L. T. Y. acknowledges financial support from Ministry of Science and Technology of China (Grant No. 2016YFA0202500). Finally, W.W. thanks Ms. Lan-Lan Zhang and Mr. Hong-Kai Liu for their assistance in the synthesis of the POM-4POSS molecule.

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Table of Contents Entry

Co-clusters of silsesquioxane and polyoxymetalate self-assemble in solution forming single- and multi-layer honeycomb nano-sheets.


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Mesoscale Graphene-like Honeycomb Mono- and Multilayers

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