Synthesis and Characterization of Macroporous Photonic Structure

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Synthesis and Characterization of Macroporous Photonic Structure that Consists of Azimuthally Shifted Double-Diamond Silica Frameworks Lu Han,†,‡ Dongpo Xu,†,‡ Ye Liu,†,§ Tetsu Ohsuna,*,⊥ Yuan Yao,‡ Chun Jiang,*,§ Yiyong Mai,‡ Yuanyuan Cao,‡ Yingying Duan,‡ and Shunai Che*,‡ ‡

School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China § School of Electronic Information and Electrical Engineering, State Key Laboratory of Advanced Optical Communication System and Network, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China ⊥ Green Mobility Collaborative Research Center, Nagoya University, Nagoya 464-8601, Japan S Supporting Information *

ABSTRACT: A macroporous silica with azimuthally shifted double-diamond frameworks has been synthesized by the self-assembly of an amphiphilic ABC triblock terpolymer poly(tert-butyl acrylate)-b-polystyrene-b-poly(ethylene oxide) and silica source in a mixture of tetrahydrofuran and water. The structure of the macroporous silica consists of a porous system separated by two sets of hollow double-diamond frameworks shifted 0.25c along ⟨001⟩ and adhered to each other crystallographically due to the loss of the mutual support in the unique synthesis, forming a tetragonal structure (space group I41/amd). The unit cell parameter was changed from a = 168 to ∼240 nm with c = √2a by tuning the synthesis condition and the wide edge of the macropore size was ∼100 to ∼140 nm. Electron crystallography was applied to solve the structure. Our studies demonstrate electron crystallography is the only way to solve the complex structure in such length scale. Besides, this structure exhibits structural color that ranged from violet to blue from different directions with the bandgap in the visible wavelength range, which is attributed to the structural feature of the adhered frameworks that have lower symmetry. Calculations demonstrate that this is a new type of photonic structure. A complete gap can be obtained with a minimum dielectric contrast of 4.6, which is inferior to the single diamond but superior to the single gyroid structure. A multilayer core−shell bicontinuous microphase templating route was speculated for the formation of the unique macroporous structure, in which common solvent tetrahydrofuran in hydrophobic shell and selective solvent water in hydrophilic core to enlarge each microphase sizes.

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INTRODUCTION Ordered porous materials with micro-, meso-, and macroscales are predicted to be highly useful in application as catalyst and supports, adsorbents, electronics and optics.1−3 Specially, ordered macroporous materials with the pore diameters comparable to optical wavelength are expected to have unique optical properties such as photonic bandgaps and optical stopbands.3 Although several methods, such as single molecules, self-assembled molecular aggregates or supramolecular assemblies templating routes, hard templating routes and lithography exist for producing ordered porous materials with micro-, mesoand macroscales and their complexes, there is still no general method for producing ordered porous structures at larger length scales.4−8 The liquid-crystal-templating route provides promising possibilities for the fabrication of artificial, inorganic, porous materials that have mechanically and thermally stable structures.9,10 Mesostructured inorganic solids with a variety © 2014 American Chemical Society

of highly ordered structures and morphologies have been synthesized via the self-assembly of an amphiphilic molecule with an inorganic source.5 Among these structures, bicontinuous and tricontinuous structures have attracted great attention because of their complex and highly symmetric structures, which two or three identical, interwoven but disconnected porous networks are divided by a silica wall grow along a continuously curved surface. This surface is analogous to the minimal surface, which is defined as surfaces with zero mean curvature everywhere.11 In particular, three types of periodic minimal surfaces with inverse bicontinuous cubic structures have been discovered, namely, gyroid surface (G) with space group (SG) Ia3̅d,9,12 diamond surface (D) with SG Pn3̅m13,14 and primitive (P) Received: September 8, 2014 Revised: October 19, 2014 Published: October 20, 2014 7020

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surface with SG Im3̅m.15 Recently, a new tricontinuous hexagonal surface of three interwoven 3etc(187) nets with SG P63/mcm was also discovered.16,17 However, the use of the liquid-crystal templating route for producing these structures in larger length scale is unfortunately severely limited because the size of surfactant micelles is always too small. To drive the self-assembly process into an advanced length scales, microphases formed via the self-assembly of block copolymers have attracted increasing attention for the synthesis of various supermolecular structures. Several structures, including bilayer, bicontinuous, cylindrical, and spherical-type structures have been fabricated via the self-assembly of block copolymers.18−25 Besides, several mesoporous and macroporous structures have been reported.26−29 The combination of the liquid-crystal-templating route and the microphase separation provides us new possibilities for fabricating macroporous materials with unique structures and outstanding properties. Herein, we synthesized a delicate macroporous silica (MS) that consists of azimuthally shifted double-diamond frameworks. Our strategy was to use the multilayer core−shell bicontinuous microphase-templating route through the selfassembly of amphiphilic ABC triblock terpolymer (TBTP) with larger hydrophobic domains and smaller hydrophilic corona chains and a silica source in a mixture of tetrahydrofuran (THF) and water. A linear TBTP poly(tert-butyl acrylate)191-bpolystyrene57-b-poly(ethylene oxide)117 (PtBA191-b-PS57-bPEO117, ASO) (see Figure S1 in the Supporting Information) with hydrophobic−hydrophobic−hydrophilic sequences was designed. The ASO has a total molar mass of 35 700 g/mol, a polydispersity index of 1.46, and volume fractions of 69.0, 17.3, and 13.7% for the A, S, and O blocks, respectively (see the Supporting Information). A hydrophilic O volume fraction of 13.7% (in the range of 4−14.1 vol %)29 would provide a wide phase composition window of bicontinuous structures. The Flory−Huggins interaction parameters for this ASO copolymer are approximately χASN = 10.4, χSON = 50.6, and χAON = 106.8 as estimated using the approximation of Flory−Huggins interaction parameter30 χN =

and pore size. We have proved that THF/H2O ratio produces a dominating effect on the pore size of the MS. The structures were characterized by small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) analysis, optical micrographs (OM), photonic bandgap calculation, and finally, the mechanism of the structural formation was speculated.

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EXPERIMENTAL SECTION

Synthesis of Template. The PtBA191-b-PS57-b-PEO117 TBTP template was obtained via a two-step synthesis using atom-transfer radical polymerization (ATRP). The detail synthesis is shown in Supporting Information. Synthesis of MS. Two types of MS were synthesized with different mass ratios of ASO:THF:HCl/H2O(2 M):TEOS = x:80x:20:6, where x = 1 and 2. The sample names were denoted as MS-1 and MS-2 synthesized with THF/H2O mass ratios of 4 and 8, respectively. In a typical synthesis, the template, THF, HCl/H2O (2 M), and TEOS were mixed together and stirred for approximately 2 h, and the mixture was subsequently allowed to completely evaporate at room temperature. The final products were filtered, washed with water, and freezedried; they were then calcined at 550 °C in air for 10 h to remove the template. MS with different macropore size was obtained by varying the mass ratio of THF/H2O from 4 to 8. Details of the synthesis are described in the Supporting Information. Characterizations. The nuclear magnetic resonance (NMR) spectra were measured on a Varian Mercury Plus 400 MHz NMR spectrometer using tetramethylsilane (TMS) as the internal reference. The polymers were dissolved in deuterated chloroform. The molecular weights and molecular weight distributions of the polymers were determined on a HLC-8320GPC (TOSOH Corp.) gel permeation chromatography (GPC) apparatus, and the measurements were conducted using DMF as the eluent at a flow rate of 10−2000 μL/min. The SAXS experiments were recorded by synchrotron radiation XRD at beamline BL16B1, provided by the Shanghai Synchrotron Radiation Facility (SSRF). The microscopic features of the samples were observed using SEM, which was performed on a JEOL JSM-7401F. The samples were observed without any metal coating. A low accelerating voltage (1 kV with a point resolution of 1.4 nm) was employed. TEM observations were performed using a JEOL JEM-2100 microscope that was equipped with a LaB6 gun operated at 200 kV (Cs 1.0 mm, point resolution of 2.3 Å). Images were recorded using a TENGRA CCD camera (2304 × 2304 pixels with a 2:1 fiber-optical taper and an effective pixel size of 18 μm2). OM was recorded using an OLYMPUS BX51 microscope. For these observations, the bulk samples were lightly crushed into pieces, and the pieces of the samples were dispersed onto adhesive conductive tape, which was adhered to a quartz slide. The clear optical micrographs were obtained under different magnifications.

∑ N × Vref (δ1 − δ2)2 /RT

where Vref is the segment reference volume (100 cm3/mol) and δi is the Hildebrand solubility parameter for polymer i (δA = 18.5 (J/cm3)1/2, δS = 19.3 (J/cm3)1/2, and δO = 21.2 (J/ cm3)1/2). The segregation product, χtotalN, is 92.1 as estimated based on the solubility parameters (see the Supporting Information 3 for detail). This ASO with χAON ≫ χSON ≫ χASN tends to form microphases with a continuous S phase and without a direct interface between the A and O segregations.29,31 Thus, the ASO TBTP with high χN value is expected to form multilayer core−shell bicontinuous structures after microphase separation under appropriate conditions. The synthesis of MS was performed using tetraethyl orthosilicate (TEOS) as a silica source and ASO as a template under acidic conditions, in a mixture of THF and water with different THF/H2O ratios. The silicate species would be condensed in the hydrophilic O regions through the cointeraction of hydrogen bonding between ethylene oxide and silanol and the electrostatic interaction under acidic condition to form a silica wall. The alternation of the inorganic gel/hydrophobic part and selective solvent/common solvent ratio has been an effective strategy for controlling the structure

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RESULTS AND DISCUSSION Synthesis and Characterizations of MS-1 Synthesized with THF/H2O Mass Ratios of 4. Figure 1 presents the SAXS pattern of calcined MS-1, which exhibits the narrow first-order reflection and clear high-order reflections, suggesting highly ordered structures. As indicated by the black tick marks, a few reflections can be identified with the q2 ratio of 3:8:11:12:16:19, which can be normally indexed to 111, 220, 311, 222, 400, 331 reflections of the face centered cubic structure (all even or all odd). However, the structure was later determined to be tetragonal with the space group I41/amd and c = √2a via electron microscopy observations (vide post). It is interesting to note that the cubic and the tetragonal structure with c = √2a cannot be distinguished due to the particular geometrical relationship. Then the reflections are indexed to 101, 112 7021

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Figure 1. SAXS characterization of calcined MS-1. Black and red tick marks distinguish reflections between observed in the SAXS pattern and from the FDs, respectively. The scattering vector q is defined as q = 4π sin θ/λ, with scattering angle 2θ and wavelength of λ = 1.04 Å).

(200), 103 (121), 202, 004 (220) and 123 (301) reflections with the unit-cell dimension of a = 168.0 nm and c = 237.6 nm. The structure is so complex and many high order reflections appeared in the Fourier diffractograms (FDs) of the TEM images. The Bragg positions of the strong reflections obtained by FDs are labeled in Figure 1. It can be seen that a great number of reflections fall in the region with small q value and are overlapped with each other because of the extremely large unit cell and the complex structure. No individual high-order reflection can be identified by SAXS. Electron crystallography, which uses electrons as a probe through microscopy and/or diffractometry, has proven to be a powerful tool for the structural investigations of a variety of porous solids, showing the advantages of allowing not only the reciprocal space information by diffraction but also the direct visualization of the real space of structures and of retaining the crystal structure phases information.17,32−34 Herein this method was proven to be the only way to solve the structure. Figure 2 presents the SEM images of the calcined MS-1. The sample was crushed into pieces and attached to SEM sample stage covered with conductive tape. Interestingly, the SEM images reveal two sets of continuous silica frameworks, as indicated by red and blue arrows. Each silica framework contains tetrahedral arrangements at their nodes and may be identified as diamond structure; both frameworks are hollow with a thin silica wall. Thus, a hollow double-diamond structure can be speculated. However, the two sets of frameworks are closely adhered to each other, forming large vacancies. The wide edge of the macropore is ∼100 nm and the smallest entrance of the hollow tube (in the middle of the nodes) is 10− 15 nm. Normally the shift of the frameworks comes due to the networks are free to move once the intervening material is removed, which is determined by the interlocking geometry of the frameworks. However, as revealed by the SEM images (Figure 2 and Figure S3 in the Supporting Information), both as-synthesized and calcined samples take the same shift direction, suggesting the shift may be crystallographic. The TEM images of the calcined MS-1 recorded along the [010], [120], [230], [110], [001], [101], [201], and [111] directions are shown in Figure 3. The structure is highly ordered and a thin silica wall with ∼12 nm thickness is observed. The corresponding FDs shown in the insets indicate

Figure 2. SEM images of calcined MS-1 from different orientations, showing two sets of hollow diamond frameworks adhering to each other, overlapping at their node sites.

the extinction conditions for the reflections [hkl: h + k + l = even, hk0: h, k = even; 0kl: k+l = even; hhl: 2h + l = 4n, l = even; 00l: l = 4n; 0k0: k = even; hh0: h = even], suggesting the unique space group I41/amd (141). Interestingly, judging from the TEM images, the shift of two frameworks is a constant value of ∼0.25c. From the thin part, the contrast corresponding to the hollow diamond structure can be identified. However, due to the shifted structure and the overlapping of the hollow framework, the TEM images taken from different zone axes become very difficult to be interpreted. Moreover, the TEM image of the as-synthesized MS shows the formation of hollow diamond structure with the contrast corresponding to the SiO2 layers with a ∼12 nm wall thickness and the ASO layers attached to the SiO2; indicating the formation of the microphase structure with the hydrophobic region of the ASO outside of the hollow framework (see Figure S4 in the Supporting Information). It is worthy to note that the single diamond structure has the SG of Fd3̅m (227) (Figure 4a), while the SG of the doublediamond cubic structure is Pn3̅m (224) (Figure 4b), which is a subgroup of Fd3m ̅ . In consideration of the diamond structure, the frameworks can be expected to shift in two ways that the nodes will be joint (i) along the ⟨001⟩cubic axis to form a tetragonal structure with SG I41/amd (141) (Figure 4c) or (ii) along ⟨111⟩cubic axis to form the trigonal R3m ̅ (166) (Figure 4d). Notably, both I41/amd and R3̅m are the subgroups of Fd3̅m. From the SEM and TEM observations, the sample only takes the unique ⟨001⟩cubic shift to form a tetragonal lattice. The different projections using the stick model of the doublediamond cubic structure (Pn3̅m) and the shifted tetragonal structure (I41/amd) are shown in Figure S5 in the Supporting Information. To solve the complex hollow structure, the three-dimensional (3D) electrostatic potential map of MS-1 is obtained from the Fourier synthesis of the crystal structure factors.32,35 The amplitudes and phases (with amplitudes >2% of the largest 7022

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Figure 3. TEM images of MS-1 recorded from (a) [010], (b) [120], (c) [230], (d) [110], (e) [001], (f) [101], (g) [201], and (h) [111] directions, respectively. Images a−d were recorded from the tilting series of the same particle by tilting along the [001] axis. The insets show TEM images simulated using a 3-term nodal approximation, and stick models are overlaid on both the TEM images and on the simulated images to indicate the arrangement of the two sets of frameworks.

Figure 4. Schematic drawing of the relationship of the (a) diamond structure (SG Fd3̅m), (b) normal bicontinuous double-diamond cubic structure (SG Pn3̅m, 2 × 2 × 2 unit cells), shifted double-diamond structure along (c) ⟨001⟩cubic and (d) ⟨111⟩cubic.

calculated averaged phase error (phase residual ΦRES) of symmetry-related reflections (Tables S3−1 to S3−8). Origin choice 2 at the site with symmetry 2/m for the SG I41/amd is chosen so that the phases can be only 0° or 180°. The structure factors of reflections from different projections were merged into a 3D data set by adjusting the common origin and a normalization process by scaling the amplitudes with common reflections (see Table S4 in the Supporting Information).12,32 The 160 unique reflections with resolution limit to 15.0 nm

amplitude) were extracted from the Fourier transforms of the TEM images along the [010], [120], [230], [110], [001], [101], [201], and [111] directions using the crystallographic image-processing software CRISP.36 From the averaged TEM images of different projections, plane groups of c2mm, c2mm, c2mm, c2mm, p4mm, p2mm, c2mm, and c2mm along the eight zone axes were assigned, respectively (see Figure S6 in the Supporting Information). The amplitudes and phases from each projection (p1 symmetry) are listed along with the 7023

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Figure 5. Representations of the 3D reconstruction and the structural model of MS. (a) 3D structure (1 unit cell) reconstructed from the HRTEM images along eight directions. (b, c) 3D porous system with the stick model superimposed in the hollow channels (1.5 × 1.5 × 1.5 unit cells), illustrating the connectivity and the topology of the porous structure. (d) The cubic double-diamond model with SG Pn3̅m created using a 3-term nodal surface approximation by setting the threshold to 3.20−4.10 and −4.10 to −3.20 to simulate the silica wall. (e) Frameworks shifted 0.25c along ⟨001⟩cubic and adhered to each other, and the unit cell changed to tetragonal. (f) Single unit cell represented with a stick model.

Figure 6. Optical properties and corresponding structure of MS-2 with large unit cell parameter. (a) Optical micrograph of the calcined MS-2 showing the violet to blue colors over the particles. (b) Low-magnification SEM image with the OM image overlaid with 50% opacity, showing the color distributions. (c, d) High-resolution SEM images from different part shown in Figure 6b, showing two sets of hollow double-diamond frameworks adhering to each other. (e−h) TEM images recorded from [010], [001], [101], and [201] directions, respectively. Images f−h were recorded from the same particle by tilting along the [010] axis. The insets show simulated TEM images and the stick models are overlaid on both the TEM images and the simulation.

were taken into account for calculating the 3D electrostatic potential map φ(x,y,z) using the software VESTA.37 A threshold for separating the pore from the silica wall was

directly determined from the TEM images as the unit cell is so large and the silica wall can be directly recognized. 7024

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Figure 7. Gap map of the adhered double-diamond structure with 0.25c shift, (a) with 13:1 dielectric contrast. The maximum gap occurs for the radius of 0.90c. (b) with the rod radius of 0.10c.

expected, MS-2 with so large unit cell shows the structural color,40−44 which can be directly seen by naked eyes. The sample was crushed into pieces and attached to SEM sample stage covered with conductive tape, which is suitable for SEM observation and it can be transferred to the optical microscope conveniently to compare the same regions. Figure 6a presents an optical micrograph (OM) of several pieces of the calcined MS-2, the OM shows that the colors of the particles range from violet to blue, albeit in different regions. From SEM observations, the pore size is increased and most of the structures still possess the shift along ⟨001⟩cubic. The combination of the OM and SEM images (Figure 6b−d) show that the blue color only corresponds to the [201] direction of the tetrahedral unit cell and a blue shift is observed when the orientation deviates from [201], and the other orientations primarily show a violet color and the nonuniform color distributions in the OM are due to the domain structures with different orientations (see Figure S8 in the Supporting Information). In this case, the macropore can be as large as ∼140 nm and the smallest entrance of the hollow tube (in the middle of the nodes) becomes 38−45 nm. The TEM images of MS-2 recorded along the [010], [001], [101], and [201] directions are shown in Figure 6e-h. Similar TEM contrasts with wall thickness ∼12 nm were observed, which agrees well with the simulated TEM images (plot region of 2.95−3.55 and −3.55 to −2.95). However, increasing the unit cell size leads to much flexible wall with lower ordering. The unit-cell parameter varies from a = 230 to 250 nm in different particles with c/a ratio ≈ √2a. Photonic Bandgap Calculation. It is worth noting that the thermodynamically favored double-diamond and doubleprimitive structures exhibit no photonic bandgap and the double-gyroid only exhibits a pseudogap, while the single networks show the complete bandgap and the single diamond is well-known as the “champion” photonic structure.45−47 It has been shown that the increase in the symmetry of these structures has been shown to abolish the photonic crystal property.48 Herein, the bandgap was originated from the structural feature of the adhered frameworks, which has lower symmetry and exhibits similar to the single framework. The band structure diagram was calculated using the MIT PhotonicBands package49 on the basis of the TEM analysis. The band

Figure 5a presents the reconstructed 3D map of one unit cell, clearly showing that the material is composed of interwoven but disconnected tetrahedrally linked hollow frameworks divided by two sets of silica walls grown along a surface similar to the diamond minimal surface (D surface).11 The two frameworks are closely adhering and shifted along ⟨001⟩cubic direction by a distance of 0.25c while the 41 screw axis along the ⟨001⟩cubic is maintained, becoming tetragonal. The hollow double-diamond frameworks can be clearly observed with stick model superimposed in the hollow channels (Figure 5b, c). It is worth noting that the structure can be also described as interpenetrated-diamond frameworks. The reconstructed structures in different projections are shown in Figure S7 in the Supporting Information. The 3-term nodal surface approximation formula38 (see Table S4 in the Supporting Information) was used as the structure analogy. The plot region was set to 3.20−4.10 and −4.10 to −3.20, respectively, which provides a surface and wall thickness similar to the actual structure. Figure 5d shows the double-diamond cubic structure with the SG Pn3̅m (origin choice 2) obtained using the nodal equation, where the two frameworks are equally distributed in the space. With a 0.25c shift, the silica walls are closely adhered to each other, the origin was chosen at the inversion center, and the new tetragonal unit cell is shown in red (Figure 5e). Figure 5f shows the tetragonal unit cell using a stick model, revealing the connectivity of the channels. To verify the structure, we simulated TEM images with the software MesoPoreImage39 using the nodal approximation, which calculates the projected potential and the TEM images from a 3D continuum model of a crystal structure from the 3D density distribution. The TEM images simulated from different directions are shown as insets in the TEM images in Figure 3, and they are consistent with the contrast of the observed TEM images; however, the surface of nodal equation is not completely consistent with the actual surface, and the small differences between the real TEM images and the simulations can be observed. Synthesis and Characterizations of MS-2 Synthesized with Large THF/H2O Mass Ratio of 8. With increasing THF/H2O mass ratio, MS-2 with similar structure but much larger unit-cell parameters has been obtained (Figure 6). As 7025

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Figure 8. Schematic representation of the formation of MS. (a) Chemical structure of the linear ASO TBTP. (b) Uniform ASO polymer solution can be formed in the common solvent THF because of the similar solubility parameters of the terpolymer A, S, O, and THF. (c) With the addition of the selective solvent water, the multilayer core−shell bicontinuous double-diamond structure is formed because of the microphase separation. The continuous THF-rich core and water-rich phase are formed in the hydrophobic and hydrophilic regions, respectively, and the silicate species are simultaneously condensed in the O-layer of the hydrophobic−hydrophilic interface to form a silica wall, which leads to the formation of the hollow double-diamond structure. (d) When the THF solvent is evaporated from the hydrophobic region, the double-diamond frameworks lose the mutual support and adhere to each other to form a tetragonal structure. (e) After calcination, the ASO is removed and the adhered hollow double-diamond structure is formed.

Speculation of Solvents Incorporated Multilayer Core−Shell Bicontinuous Microphase Templating for MS. The unique phase behavior of above porous silica with large pore and adhered double-diamond framework can be speculated in terms of molecular structure of ASO (Figure 8a). As mentioned in the results (Figure S4), the MS has been synthesized with large amount of THF and the organic segments are in the macropore side. Therefore, the incorporation of solvent into microphase would be the key factor for the formation of our MS with large pore and adhered double-diamond framework. ASO is highly soluble in the common solvent THF because of the solubility parameters of PtBA (δ = 18.5 [J/cm3]1/2), PS (δ = 19.3 [J/cm3]1/2), PEO (δ = 21.2 [J/cm3]1/2), and THF (δ = 18.6 [J/cm3]1/2)31 are similar (Figure 8b). When the selective solvent water (δ = 23.4 [J/ cm3]1/2) is added, microphase separation occurs. The hydrophobic block (AS) is present in the THF-rich phase, whereas water is driven out of the hydrophobic segments of ASO and mainly present in the hydrophilic segment (O) region.52 Because the hydrophobic block (AS) is considerably larger than the hydrophilic segment (O), the two water-rich cores (the Cambridge blue domains in Figure 8c) surrounded by the O domain (in red) form the two independent, interwoven doublediamond frameworks. The AS shells (in green and dark blue) surrounding each core add two more distinct subdivisions, and THF (in gray) fills the remaining space. Thus, with a large THF/water ratio, the core−shell structure of H2O-rich/ O(TEOS)/S/A/THF-rich/A/S/O(TEOS)/H 2 O-rich is formed. The structure contains two equivalent labyrinths with multilayer core−shell bicontinuous double-diamond pattern. Because of the small interaction parameter χASN = 10.4, the microphase separation between A and S may not be favored. Simultaneously, under acidic conditions, the silicate species would be condensed in the hydrophilic regions through the cointeraction of hydrogen bonding between ethylene oxide and silanol as well as the electrostatic interaction between

structures were determined using a dielectric structure defined by a tube model and the calculation was confined to the irreducible Brillouin zone using a face-centered cubic lattice with a unit-cell parameter of 340 nm with the refractive indices of silica (1.5)50 see the Supporting Information Figure S9 for detail). The angular frequency was converted to the corresponding wavelength of visible light. The important feature of the band diagram is the presence of a bandgap in the wavelength range of 350−440 nm in the L-Γ [201], Γ-K [100], Y-Γ [110], Γ-Z [001], and K1-Γ [111] directions. In particular, blue color can be observed from the [201] direction, while the violet color can be observed from other directions, which is consistent with the OM and SEM observations. Inspired by this point, we performed the systematic calculation for the minimum dielectric contrast using the 0.25c shifted structure with the solid rod radius to enable a complete gap. Figure 7a shows the width of the bandgap with respect to the rod radius. The results show that the 0.25c (c is the unit cell parameter) shifted double-diamond structure exhibits a complete gap for rod radius from 0.04c to 0.125c (the rods will be overlapped when radius >0.125c). The maximum value appears approximately for radius = 0.090c. The gap decreases when the dielectric contrast between the media is decreased. By systematic searching for the minimum dielectric contrast by using different rod radius to enable a complete gap, a minimum dielectric contrast of 4.6 is needed to obtain a complete gap for the sample with the rod diameter of 0.10c (Figure 7b). The dielectric contrast for amorphous silica is ∼3.9.51 It is interesting to note that a complete gap that opens at a dielectric contrast of 3.6 and at a precise volume fraction of 0.34 for the “champion” single diamond, whereas the complete gap opens at a dielectric contrast of 5.2 with the volume fraction of 0.17 for single gyroid.48 Our calculation indicates that the shifted double-diamond structure is inferior to the single diamond but superior to the single gyroid structure. 7026

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EOm‑y[(EO)•H3O+]y and [yCl−•Si−OH2+]53 to form a silica wall in the O layer of the hydrophobic−hydrophilic interface (Figure 8c). Through evaporation of both THF and water from the hydrophobic and hydrophilic regions, the macroporous space of the frameworks was directed by the large hydrophobic segment and THF. In contrast, the mesopores inside the hydrophilic region were generated by the water-rich core and by aggregation of the silicate in the O-layer segment. Subsequently, the double-diamond frameworks lost the mutual support and could not maintain the original cubic symmetry; they shifted to adhere to each other (Figure 8d). The adhered double-diamond structure was formed when ASO was removed via calcination (Figure 8e). Because of the further condensation, the adhered double-diamond frameworks connected to form a stable structure. We believe the highly ordered structure and the synthesis method is the key to the crystallographic shift, which satisfies the shift value of 0.25c. It can be imagined that the macropore size would be enlarged with increasing mass ratio of THF/H2O and with the same ASO/THF ratio. Under the synthesis condition of MS-2, the vacancy of the macroporous space is remarkably enlarged due to the volume of THF-rich phase is increased, whereas the size of the hollow channels also increased correspondingly to stabilize the formation of the surface. Further studies for the formation mechanism are underway.

Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program (2013CB934101), the National Natural Science Foundation of China (21201120, 21101106), the Youth Natural Science Foundation of Shanghai (12ZR1445100), the “Chenguang” Program of the Shanghai Educational Development Foundation (12CG10), and Evonik Industries. The authors thank the Shanghai Synchrotron Radiation Facility (SSRF) for providing BL16B1 beamline for collecting synchrotron SAXS data.

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ABBREVIATIONS MS, macroporous silica; TBTP, triblock terpolymer; EC, electron crystallography; OM, optical micrograph; SEM, scanning electron microscopy; TEM, transmission electron microscopy; SG, space group; FD, Fourier diffractogram

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CONCLUSIONS To the best of our knowledge, this work represents the first report of the direct synthesis of a marcoporous inorganic material with minimum surface-type structure via the selfassembly of TBTP and an inorganic source. The hollow double-diamond framework and the shifted feature makes the MS one of the most complex porous structures ever solved. We have demonstrated such large structure can be only solved by electron crystallography. In addition, our calculation shows that the shifted structure is inferior to the single diamond while being superior to the single gyroid structure. This route provides the possibility of creating novel materials with large unit-cell parameters, different symmetries and unique performance (e.g., the photonic bandgap). Although the low refractive index of SiO2 and the domain structure at the current stage limited its application, the microphase-templated formation process is very promising for providing a new structural concept generating new structural colored materials with various low symmetry structures in the future. Therefore, this work could provide significant opportunities in various research areas and open new horizons in developing new materials in chemistry and materials science.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the synthesis, more SEM and TEM images, details of the 3D reconstruction and the nodal approximation are presented in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.C.). *E-mail: [email protected] (T.O.). *E-mail: [email protected] (C.J.). 7027

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