Structural Study of Hexagonal Close-Packed Silica Mesoporous

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Structural Study of Hexagonal Close-packed Silica Mesopo-rous Crystal Yanhang Ma, Lu Han, Keiichi Miyasaka, Peter Oleynikov, Shunai Che, and Osamu Terasaki Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm401294j • Publication Date (Web): 27 Apr 2013 Downloaded from http://pubs.acs.org on May 2, 2013

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Chemistry of Materials

Structural Study of Hexagonal Close-packed Silica Mesoporous Crystal Yanhang Ma,†,⊥ Lu Han*,‡,⊥ Keiichi Miyasaka,§ Peter Oleynikov,† Shunai Che,*,‡ and Osamu Terasaki*,†,§ †

Department of Materials and Environmental Chemistry, Bezelii Center EXSELENT on Porous Materials, Stockholm University, S-‐10691 Stockholm, Sweden. ‡

School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China. §

Graduate School of EEWS, WCU, KAIST, 335 Gwahangno Yuseong-‐Gu, Daejeon 305-‐701, Republic of Korea.

KEYWORDS: Silica mesoporous crystals, surfactant, co-‐structure directing agent, hexagonal close-‐packing, electron microscopy ABSTRACT: Close-‐packed spheres can be stacked into two crystalline structures: cubic close-‐packed (ccp) and hexagonal close-‐packed (hcp). Both of these structures were found in silica mesoporous crystals (SMCs). Herein, pure hcp mesostructure with P63/mmc symmetry of silica mesoporous crystals (SMCs) has been obtained in the synthetic system of cationic gemini surfactant as template and the N-‐[(3-‐trimethoxysilyl)propyl]ethylenediamine triacetix acid trisodium salt (EDTA-‐silyl) as the co-‐structure directing agent (CSDA), which gives rise to the three-‐dimensional (3D) hexagonal structure and hexagonal plate morphology. The formation of the pure hcp structure was controlled by organic/inorganic interface curvature induced by charge matching between carboxylate groups of the CSDA and quaternary ammonium head-‐groups of surfactant. Electrostatic potential distribution 3D map was reconstructed using Fourier analysis of HRTEM images based on electron crystallography, which showed characteristic features of the shape and connectivity of mesopores in the hcp structure. Small windows for connecting cages can be found only between layers, which determine the symmetry and local curvature of structures. As a result, the point group symmetry of mesopores becomes 6𝑚2, in-‐ stead of the 𝑚 3𝑚 symmetry observed for perfect spheres in the ccp. The mechanism of stabilization and favorable growth of the pure hcp structure in meso-‐scale has been proposed based on synthesis strategy and symmetry support. This work provides people a better understanding of the priority of two sphere close-‐packed forms by comparing hcp and ccp struc-‐ tures.

INTRODUCTION 2D-‐triangular lattice with lattice constant 1/ 3 can be divided into three equal hexagonal sublattices A, B and C with lattice constant 1. The closest packed arrays of hard spheres with diameter 1 on a plane form a layer with a hexagonal sphere arrangement. The layer is called A or B or C, as the centers of the spheres form a p6mm lattice, with lattice constant 1 corresponding to the A or B or C sublattice. The spheres of the second layer on the first layer, A, is either the B or C layer. When we extend the idea of closely packing spheres with uniform size to three dimensions, a simple structural description can be gener-‐ ated through a stacking sequence of the layers perpendic-‐ ular to the plane using layers A, B and C. The resultant three-‐dimensional lattices can be described by a stacking sequence of layers, such as ABABCB… or ABACBA…, etc. and coordination numbers for all sequences are 12, i.e., 6 in the plane, 3 above the plane and 3 below the plane. Two extreme cases are the ABCABC… and ABABAB… stacking sequences. The point group symmetries of the centers of the spheres in the ABCABC… and ABABAB… stackings are m3m and 6m2; therefore, the stackings are called cubic close-‐packed (ccp) and hexagonal close-‐ packed (hcp) structures, respectively. Both ccp and hcp structures have identical space occupation, coordination

number and similar energy, and they show very similar equations of state.1 A planar defect in ccp such as …ABCABCBACBA… is called a twin plane, which is frequently observed in the ccp structure. As early as the year 1611, hcp and ccp structures have at-‐ tracted scientists’ interest; Johannes Kepler asserted that no arrangement of equally sized spheres filling space had a greater average density than that of ccp or hcp. This is known as Kepler’s conjecture and has been recently proved by Thomas Hales using a computer-‐based solu-‐ tion.2 In nature, a large number of metals, alloys and in-‐ organic compounds take a close-‐packed structure held together by interatomic forces. Approximately 25% of the elements crystallize in the ccp structure, and 20% do so in the hcp structure.1 This brings an intense interest to the field and makes it attractive to discover the priority of each structure in crystal growth. Many studies have been devoted to solving this straightforward but difficult prob-‐ lem. Some studies have employed dynamics methods3-‐7 or Monte Carlo simulations8,9 to study the differences in stability for the two structures by comparing entropies; these studies have concluded that ccp is more stable than hcp. Other researchers have claimed that the deformation of hard sphere crystals or the motion of spheres would

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make hcp more stable.10,11 However, there is a lack of di-‐ rect experimental proof for either case. Silica mesoporous crystals (SMCs), which can be con-‐ sidered “cavity crystals”, were discovered in the early 1990s.12,13 Generally, they are synthesized with surfactant or block copolymers as templates for the subsequent and/or simultaneous condensation of inorganic materials. The self-‐assembly of micelles and charge density match-‐ ing between surfactants and inorganic precursors are essential for the formation of various mesostructures.12 SMCs with different structures (hereafter mesostructures) can be formed according to the packing behavior of the surfactants. SMCs can be considered good models for crystallographic research for the following reasons: (i) various micelle types and shapes can be easily formed by amphiphilic molecules in the water; (ii) mesostructures can be controlled by changing synthetic conditions; (iii) defects and intergrowth are commonly observed in SMCs, revealing the structural relationship and the crystal growth; and (iv) the structure can be kept as a replica after removing the surfactant for further observation and characterization. Transmission electron microscopy (TEM) can be used especially to observe the arrangement of mesopores directly through electron crystallography. Generally, micelle types can be described by a surfac-‐ tant packing parameter, g = V/a0l, where V refers to the micelle volume, a0 represents the effective hydropho-‐ bic/hydrophilic interfacial area and l is the length of chain.15 As parameter g decreases, a mesostructure can be formed in an order of bilayer, bi-‐/tri-‐continuous, cylindri-‐ cal and cage-‐type structures, corresponding, respectively, to the g values 1, 2/3, 1/2 and 1/3. Cage-‐type mesostruc-‐ tures are formed by the regular/disordered packing of spherical/ellipsoidal micelles. There are several typical cage-‐type structures observed in SMCs, including Im3m (SBA-‐1616,17), Fm 3 m (SBA-‐1217, FDU-‐118,19and FDU-‐1220), P63/mmc (SBA-‐217,21 and SBA-‐1217,22-‐25), Pm3n (SBA-‐116,26 , SBA-‐1117 and SBA-‐616,18), Fd3m (FDU-‐227 and AMS-‐828,29), P42/mnm (AMS-‐930), Pmmm (FDU-‐1331), and P4/mmm (FDU-‐1131), etc. Among these, the Fm3m and P63/mmc structures are analogous to ccp and hcp, respectively. This finding provides us with information about the differ-‐ ences and stabilities between the two structures in the meso-‐scale. However, the intergrowth of ccp with hcp is normally observed in SMCs such as in SBA-‐217,21 and SBA-‐ 1217,22-‐25. In a few cases, pure ccp has been observed in some SMCs, and its structure has been solved using high-‐ resolution TEM (HRTEM).32,33 As far as we know, there is no report on the structural features or growth mechanism of an SMC with a pure hcp structure, neither of the study on priority of two close-‐packed structures using SMCs. Herein, we present a successful synthesis of SMCs with pure three-‐dimensional hexagonal (SP: P63/mmc) struc-‐ ture and hexagonal plate morphology using a gemini cationic surfactant (C18-‐3-‐1) in the presence of N-‐[(3-‐ trimethoxysilyl)propyl] ethylenediamine triacetix acid

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trisodium salt (EDTA-‐silyl) as the co-‐structure directing agent (CSDA). The structure solution was determined by the combination of powder X-‐ray diffraction pattern and EM observations, including scanning electron microscopy (SEM) and HRTEM. 3D electrostatic potential distribution map was reconstructed to elucidate characteristic features, such as the shape and connectivity of mesopores in the hcp structure. The stabilization mechanism and favorable growth of a pure hcp structure in the meso-‐scale has been discussed from the viewpoint of organic/inorganic inter-‐ face curvature and symmetry support as well as through a comparison with the ccp structure. This research will provide a novel idea and better understanding on the study of growth mechanism for two close-‐packed struc-‐ tures. EXPERIMENTAL SECTION 1. Chemical Agents All materials, including tetraethyl orthosilicate (TEOS; SCRC, China), N,N-‐dimethyl-‐n-‐tetradecylamine (TCI, Japan), (3-‐bromopropyl) trimethylammonium bromide (Aldrich, USA), and N-‐[(3-‐ trimethoxysilyl)propyl]ethylenediamine triacetix acid trisodium salt (EDTA-‐silyl, 25% water solution; Gelest, UK), were used as purchased without further purification. 2. Surfactant preparation The gemini surfactant C18-‐3-‐1 was synthesized according to reference 32. 3. Synthesis of SMC with pure P63/mmc structure SMCs were synthesized with C18-‐3-‐1 as template. EDTA-‐ silyl was used as CSDA and TEOS was used as silica source under various conditions. Typically, C18-‐3-‐1 was added into the mixture of deionized water and a specific amount of NaOH/HCl (1 M) at 80 °C. After the surfactant was dissolved, EDTA-‐silyl and TEOS were added together, stirred for 1 h and aged for 2 days at 80 °C. The resultant white precipitate powder was filtered and dried overnight. Surfactant-‐free samples for HRTEM analyses and gas adsorption were obtained by calcination at 550 °C in air for 6 h. 4. Characterization of materials Powder X-‐ray diffraction (XRD) patterns were col-‐ lected on a PANalytical X’Pert Pro equipped with Cu Kα radiation (45 kV, 40 mA), fixed divergence slit of 1/4o, anti-‐scattering slit of 1/8o, a program-‐controlled receiving slit (with constant irradiated length of 10 mm) and a step size of 0.0042° in transmission mode to reduce errors at small scattering angles in the Bragg-‐Bretano configura-‐ tion. Scanning electron microscope images were obtained using a JEOL JSM-‐7401F and a JEOL JSM-‐7600F under different equipment conditions. High-‐resolution trans-‐ mission electron microscopy was performed using a JEOL JEM-‐2100 microscope that was equipped with a LaB6 gun


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Figure 1. Schematic representation for the synthesis strategy of 3D hexagonal SMCs.

operating at 200 kV (Cs 1.0 mm, point resolution 2.3 Å). Images were recorded using a KeenView CCD camera (resolution 1376 × 1032 pixels, pixel size 6.45 × 6.45 μm) at 50,000−120,000 times magnification under low-‐dose con-‐ ditions. The structure factor amplitudes and phases were extracted from Fourier transforms of the HRTEM images using the crystallographic image processing software Crisp34. The reconstructed model was built in a program written by Mathematica 8.0 and visualized in the Vesta software35. TEM image simulations of the idealized meso-‐ porous structures were performed using MesoPoreImage36. The nitrogen adsorption/desorption isotherms were measured at 77 K with Quantachrome Nova 4200E. RESULTS AND DISCUSSION The designation of this synthetic strategy comes from the co-‐structure directing method for synthesizing SMCs proposed by Che et al.37 Structurally, a CSDA contains two parts: an alkoxysilane site that is capable of condensing with the silica source TEOS and an organic site that can form diverse interactions (e.g., electrostatic, covalent, hydrogen bonding or π–π interactions) with surfactant head groups to provide the driving force for the formation of highly ordered mesostructures. The mesostructure can be well controlled by tuning the curvature of the organ-‐ ic/inorganic interface through the ionization of either the surfactant head group or the CSDA part. Herein, the SMC was synthesized with C18-‐3-‐1 as tem-‐ plate and EDTA-‐silyl as CSDA (Figure 1). The carboxylate organic site of EDTA-‐silyl is co-‐condensed with the silica source TEOS to be subsequently assembled to form the silica network (Figure 1). The synthesis was performed under various experimental conditions by systematically changing concentration (Figure S1), temperature (Figure S2), amount of CSDA (Figure S3) and pH values (Figure S4). At low concentration, preferred growth orientations along a and c axes were observed. Enough amount of CSDAs (> 1 CSDA/1C18-‐3-‐1) and high temperature (>50 oC) were necessary to form well crystallized SMCs with hcp structure. The change of pH values adjusted the ioniza-‐

tion of CSDA and controlled the electrostatic interactions between carboxylate groups of EDTA-‐silyl and quaternary ammonium groups of the cationic surfactant. In this way, with the addition of HCl, the phase transformation from pure hcp to hcp&ccp occurs, and crystallinity declines. Typically, an extremely low surfactant concentration of 0.1~0.4 wt% was used to form homogeneously spherical micelles in aqueous solution at 80 oC. Molar ratios of C18-‐3-‐1: EDTA-‐silyl: TEOS: H2O =1: 1: 15: 30,000 were verified as the proper choice to obtain an hcp phase with or without the addition of NaOH (Figure S4). SMCs with pure hcp structure and uniformly hexagonal plate shape were ob-‐ tained with the condition in Figure S4b, which was used for the following characterization and discussion. Figure 2 shows X-‐ray diffraction profile of as-‐made and calcined samples, both of which reveal six clearly distinc-‐ tive reflections in the range of 1-‐4o. Lattice constants and peak indexing are simultaneously determined to explain all reflections only based on crystal system, in this case hexagonal. The five low-‐angle reflections can be indexed properly as [1010] , [0002] , [1011] , [1120] , [1013] and [1122] using four digital Miller-‐Bravais indices hkil (h+k+i=0). The corresponding unit cell parameters for as-‐ made SMCs were a = 6.06 nm, c = 9.81 nm and c/a ratio of 1.62. The calcined sample also gave similar results of a = 5.58 nm, c = 9.12 nm and c/a = 1.63. The calcined sample had slightly smaller unit cell parameters due to the fur-‐ ther silica condensation and lattice contraction after cal-‐ cinations. The theoretical c/a value of 1.633 for the hexag-‐ onal spherical close-‐packed model is slightly larger than that of the as-‐made SMC, which was regarded as a close-‐ packed mesocage. The small contraction along the c axis might contribute to distinguish hcp from ccp.



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growth of crystal by the stacking of layers along the c axis. The side surface of the plates also shows the information of crystal growth. Figures 4c and 4f show high magnifica-‐ tion side surfaces of the as-‐made and calcined SMCs, respectively, where slim streaks with ~9 nm width can be clearly observed, corresponding to the c parameter of one unit cell that results from layer packing. The dihedral angle of 90° between the top/bottom and side planes and the indistinct streaks along c axis are prominent signs of the pure hcp structure. Moreover, in crystallography, the morphology of a crystal is normally related to the corre-‐ sponding point group symmetry of its structure. In a hex-‐ agonal crystal system, there are seven possible point groups: 6, 6, 6/m, 622, 6mm, 62m (or 6m2) and 6/mmm. The characteristic morphology observed is consistent with the point group 6/mmm.

Figure 2. Powder XRD pattern of the (a) as-‐made and (b) calcined SMCs collected in the region of 2θ = 1-‐7°. The chem-‐ ical molar compositions of mixtures are as follows: C18-‐3-‐1: EDTA-‐silyl: TEOS: NaOH: H2O=1:1:15:5:30,000.

The crystal structure was further characterized using TEM. HRTEM images and the corresponding Fourier diffractograms (FDs) of both as-‐made and calcined sam-‐ ples were taken along the [0001], [2113] and [1100] di-‐ rections from thin areas, as shown in Figure 5. Especially in the image along [2113], the white contrast in the im-‐ ages corresponds to a row of low electron-‐scattering den-‐ sity formed by the projection of the mesocages. Uniform mesocages were arranged in a plane to form a hexagonal close-‐packed layer (ab plane, hereafter called the “layer”). Notably, the zigzag stacking sequence (i.e., ABAB…) of mesopores was observed, indicating a pure hcp structure in the crystal. The extinction conditions of reflections were obtained from the FDs of HRTEM images, which are summarized as {hh-‐2hl: l = 2n; 000l: l = 2n} (Figure S5) and suggest the following three possible space groups: P63mc, P 6 2c and P63/mmc. These three space groups correspond, respectively, to different point groups: 6mm, 62m and 6/mmm. Moreover, from three averaged HRTEM images of different projections, plane group of p6mm, p2gm and p2mm along three zone axis was assigned and verified the space group of the crystal to be P63/mmc. This result was consistent with the point group of 6/mmm observed in the SEM images. (See also Table S1 for the plane groups projected along three zone axes of the three possible space groups)

The nitrogen adsorption-‐desorption experiment was performed to study properties of mesopores, including pore volume, surface area and pore size distribution, which shows a typical type IV isotherm with an evident H2 hysteresis loop in the range of P/P0 = 0.35–0.6, indi-‐ cating a cage-‐type mesopore structure (Figure 3). The total mesopore volume was calculated to be 0.657 cm3/g using the t-‐Plot method, surface area of 769 m2/g in the Brunauer-‐Emmett-‐Teller (BET) method and the average pore size of 4.2 nm was determined using the Barret-‐ Joyner-‐Halenda (BJH) method with adsorption branch.

Figure 3. N2 adsorption/desorption isotherm and the corre-‐ sponding pore size distribution curve of calcined sample.

Figure 4 shows the SEM images of both the as-‐made and calcined samples. Most of the crystals display a char-‐ acteristic morphology of hexagonal plates, which are quite uniform in size and shape, with approximately 2 μm in the ab plane and 0.5 μm along the c axis. This result indicates that crystal grows more quickly in the ab plane than along the c axis. The surface growth steps of layer stacking in the ab plane can be clearly observed, as indi-‐ cated by the arrows in Figure 4b, which reveals the

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Fourier analysis of HRTEM images was performed to elucidate the structural details of the crystal based on electron crystallography. Normalized structure factors after applying symmetry and correcting the contrast transfer function (CTF) are tabulated in Tables S2 and S3. A reconstruction of 3D structural model was built to show the electrostatic potential distribution v(x,y,z) in one unit cell. The threshold value of the isosurface was determined to be 0.48 for calcined SMC and 0.37 for as-‐made SMC. These values were determined using a self-‐consistent structure approach in which the curvature elastic energy was minimized and an averaged mean curvature was thus taken as a spontaneous curvature, leading to a constant mean curvature scheme (Figure S6)33. With the isosurface to be the mesopore/wall boundary, the corresponding porosities are 42.3% and 23.3%. Except for in a self-‐ consistent method, pore fraction can also be


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Figure 4. SEM images of (a-‐c) as-‐made and (d-‐f) calcined sample taken at low and high magnification under different equipment settings.

Figure 5. HRTEM images of as-‐made SMCs along the a) [0001], b) [2113] and c) [1100] directions, and calcined SMCs along the d) [0001], e) [2113] and f) [1100] directions. Inserts depict Fourier diffractograms (FDs). Projected potential maps obtained by 33 crystallographic image processing using CRISP were also inserted in the HRTEM images.



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Figure 6. 3D reconstruction model of the electrostatic potential distribution of (a-‐c) as-‐made and (d-‐f) calcined samples along different directions with threshold values of 0.48 and 0.37, respectively.

calculated by analyzing the gas adsorption/desorption result. This calculation is only feasible for calcined mate-‐ rials that lack a template. Gas adsorption analysis reveals that the total mesopore volume is 0.657 cm3/g. Assuming the density of the silica wall to be 2.2 g/cm3, the pore fraction turns out to be 59%. From the plot of the fraction versus threshold value, we learned that 59% porosity corresponds to a threshold value of ~0.57 (Figure S7); this deviates by 0.09 from the value determined using a self-‐ consistent method. The 3D model with a threshold value equal to 0.57 (Figure S8) presents larger open windows and pore volumes in comparison to the model in Figure 6. The pore volumes determined from the electron potential map do not exactly agree with the corresponding data measured from gas adsorption isotherm. This incon-‐ sistency might arise from the existence of particle aggre-‐ gation and roughness of the silica wall. Moreover, the curvature energy fluctuates slightly, with threshold values in the range of 0.43-‐0.57, making it difficult to determine the actual value based on a reconstructed 3D model with resolution in the nanometer scale. Figure 6 shows the 3D model of as-‐made and calcined SMCs with v(x,y,z) values equal to 0.48 and 0.37, respec-‐ tively. The model was viewed from different directions to display the shape and connectivity of mesocages within the crystal. In these reconstructed 3D maps, the yellow surface represents the boundary between the silica wall and the mesopores, and it defines the shape and connec-‐ tions of the pores in the structure. Unlike perfectly spher-‐ ical cages in mesoporous silica with a pure ccp structure31, mesocages in this crystal are aspherical but expand or compress from different orientations. Details of this structure can be observed in Figure S9. Typically, the mesocages are expanded along the c axis and are not connected to the adjacent ones in the same layer, and windows can be only observed between two layers. When

we consider symmetry, the morphology and connectivity of cages fits the requirements of the hcp structure. Com-‐ pared with the infinite symmetries of perfect spheres, mesocages here present the point group symmetry of 6m2 (Wyckoff 2c or 2d site of P63/mmc). However, in SMCs with ccp structures, close packing is formed in four equiv-‐ alent planes of {111}, and the point symmetry at the center of the pore is m3m. This is a characteristic structural difference between the features of SMCs with hcp and ccp structures. Furthermore, the comparison of reconstructed models between as-‐made and calcined samples shows fine changes in pores through calcination. In the as-‐made material, cages are close to spherical shape, and the pore-‐ openings are smaller than those observed in the calcined sample, as shown in Figure 6b. This result agrees with the change in porosity after removing templates. Very rarely, ABC stacking sequences of the layers were observed in a few of the crystals. A very small amount of ccp intergrowth produced streaks parallel to the c* axis in the FD, as shown in the insets of Figure 7 (indicated by the white arrowheads). Interestingly, the external surfaces of the crystal were also observed to the arrangement of the cages, which kink at the point of ccp stacking, which is shown in Figure 7 (black arrowheads). The existence of even a very thin layer (one ABC stack-‐ ing sequence) can complicate the contrast of HRTEM image along [0001] axes, as shown in Figure 8. The TEM images and averaged TEM image shows very different contrast from the image taken from [0001] of hcp (Figure 5d).


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Chemistry of Materials tration condition is the key to exhibiting pure ccp struc-‐ ture.

Figure 7. HRTEM image and FD taken from the [2113] direc-‐ tion of layer stacking. Defects can be observed in the sample. Three parts (marked by black squares) are magnified to show stacking sequences.

The diffraction intensities also changed in FD 1120 and became much stronger than in 2020. The ED patterns and TEM images of the hcp structure with the ccp con-‐ nection and the pure hcp have been simulated in the dedicated software MesoPoreImage36, which provides images calculated from a 3D continuum model of meso-‐ porous crystal structures. The simulated hcp-‐ccp connec-‐ tion structure showed a similar ED pattern and TEM im-‐ ages (Figure 8b) as the experimental results, but its re-‐ sults differed from those of the pure hcp structure (Figure 8c). Figures 8d and 8e show the structural model with a BABCB stacking sequence, where two hcp structures have been connected by a ccp layer. The void of the hcp struc-‐ ture (C site) is occupied by the stacking of ccp and new hcp layers. Additionally, the coexistence of pure hcp and defects can make very complex structures. When the nucleation and growth rates are comparable, the crystal growth occurs at many different places, and domain structures are often created. Figure S10 shows a TEM image taken from the [0001] axes, suggesting three dif-‐ ferent domains (divided by dotted lines). The typical contrast from both pure hcp and hcp connected by ccp can be observed, suggesting that the inhomogeneous growth of the crystal occurs with layer-‐by-‐layer stacking. When the domains did not fit each other well, some de-‐ fects and Morie patterns were created. In our previous work32, by using C18-‐3-‐1 as template and carboxyethylsilanetriol sodium salt (CES) as CSDA, re-‐ markably pure ccp have been synthesized with an ex-‐ tremely low surfactant concentration of 0.1~0.4 wt%. This finding agrees with another report that the SMC with pure fcc was synthesized also by C18-‐3-‐1 under similarly low concentration conditions but in strong acid and without CSDA33. These data indicate that a low synthesis concen-‐

Figure 8. (a) HRTEM image, FD and simulated image along the [0001] area with stacking defects; Electron diffraction (ED) pattern and simulated TEM image of hcp; (b) hcp with ccp connection; (c) pure hcp structure; d, e) structure model of BABCB layer stacking sequence.

The 3D reconstruction result shows that the mesocages are highly spherical. At low concentration, spherical mi-‐ celles can be formed homogeneously in aqueous solution, and the kinetics of cage growth can be well controlled. The structural transformation from pure ccp to the mixture of ccp and hcp structures occurs with increasing amounts of co-‐structure directing agents (CSDA with one carboxylate group) and lower concentrations of surfactant; both of these factors decrease the repulsion of surfactant headgroups and the interface curvature of micelles. These results imply a possible requirement of micelles with larger g values for hcp compared to ccp. Moreover, these data suggest that the mesocages in the hcp structure should be aspherical. Miyata reported on the preparation of mesoporous silica films with 3D hexagonal structures by inducing a gradual phase transition of aligned tube-‐ like micelles into spherical ones38. However, a deviation of 39.4% from the 3D hexagonal structure was also reported after calcination, which seems to be out of acceptable error. And the morphology of mesocages was strangely considered spherical without any direct evidence. Herein, SMCs with pure hcp structure were also syn-‐ thesized with extremely low concentrations of approxi-‐ mately 0.1 wt%. However, in the presence of EDTA-‐silyl as CSDA, the high negative charge density due to three car-‐ boxylate groups would bind strongly with the quaternary ammonium groups of the cationic surfactants through electrostatic interaction, thus decreasing repulsion be-‐ tween the head groups of C18-‐3-‐1. The increase of pH values also furthered the increase in charge density of CSDA, resulting in a decreased repulsion of cationic surfactant head groups. In other words, to maintain the charge matching at the interface between surfactant and CSDA,



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the head-‐groups of surfactant shortened the mutual dis-‐ tance and packed to form a structure with lower curva-‐ ture. In this case, micelles prefer to stay ellipsoid and are easily distorted. Densely packed micelles thus form a different packing from the ccp structure, and the struc-‐ tural transformation from hcp&ccp to hcp occurs (Figure S4). In this case, it is possible to distinguish and characteri-‐ ze the hcp and ccp structures separately. The ccp is pre-‐ ferred in the stacking of spherical cages, while hcp be-‐ comes favorable in the deformation of spheres with a lower curvature. As a result, hcp and ccp give different structural features. CONCLUSIONS Hexagonal close-‐packed SMCs with 3D hexagonal mesostructures have been successfully synthesized using a special CSDA (EDTA-‐silyl) in a simple way. The struc-‐ ture was determined to be P63/mmc by combination of powder XRD, SEM and HRTEM images. We believe that this is the first report to obtain pure hcp structure in a large mesoporous crystal. Furthermore, the 3D recon-‐ struction model of electrostatic potential distribution was built using Fourier synthesis of crystal potentials obtained from HRTEM images to illustrate the shape and connec-‐ tion of mesopores. The mechanism of stability in the hcp structure during crystal growth is discussed in considera-‐ tion of layer stacking and symmetry support. Occasional stacking defects were observed in a few crystals and stud-‐ ied by TEM image and simulation. This subject will be of interest to researchers in diverse areas of chemistry, par-‐ ticularly those in inorganic, colloid, physical, and materi-‐ als chemistry.

ASSOCIATED CONTENT Supporting Information. Powder XRD patterns of SMCs synthesized and selected under different pH values, selected electron diffraction patterns along three zone axes and HRTEM images of three different domains were provided as images. These data were used to evaluate pore fraction versus threshold value, F/A versus the threshold value, several 2D EPM slices, 3D model with threshold value of 0.57. Tables of plane groups for different space groups and structure factors for as-made and calcined samples were tabulated. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Email: [email protected]; [email protected]；luhan@

Funding Sources This work is supported by VR, Knut and Alice Wallenberg Foundation, Berzelii EXSELENT (Sweden), WCU (R-‐31-‐2008-‐ 000-‐10055-‐0, Korea) and the National Natural Science Foun-‐ dation of China (Grant No. 21201120) and 973 project of China (2009CB930403, 2013CB934101) and Evonik industry.

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ABBREVIATIONS CCR2, CC chemokine receptor 2; CCL2, CC chemokine ligand 2; CCR5, CC chemokine receptor 5; TLC, thin layer chroma-‐ tography.

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SYNOPSIS TOC

3D electrostatic potential maps depicting a hexagonal, close-‐packed structure in silica mesoporous crystal.


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Structural Study of Hexagonal Close-Packed Silica Mesoporous

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