Ice-Assisted Assembly of Liquid Crystalline ... - ACS Publications

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Ice-Assisted Assembly of Liquid Crystalline Cellulose Nanocrystals for Preparing Anisotropic Aerogels with Ordered Structures Guang Chu,† Dan Qu,‡ Eyal Zussman,*,† and Yan Xu*,‡ †

NanoEngineering Group, Faculty of Mechanical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China

‡

S Supporting Information *

ABSTRACT: Imparting aerogels with structural anisotropy endows these porous materials with direction-dependent properties and promising novel applicability in material science. However, facile fabrication of such anisotropic aerogels under mild conditions still remains a major challenge. Here, we report on the fabrication of anisotropic aerogels from liquid crystalline ordered cellulose nanocrystals (CNCs) by a directional freezecasting process. The resulting CNCs aerogels were millimetersized with lamellar macrostructural features and optical anisotropy, exhibiting hierarchical structures perpendicular to the direction of freezing. We show that the birefringence of CNCs aerogels arose from the alignment of CNC liquid crystals, rather than from the shape of rod-like nanoparticles. Moreover, the versatility of the directional freezing approach was expanded to a CNC−silica system, resulting in anisotropic silica aerogels with an ordered mesopore-imprinted structure template by the liquid crystalline ordered CNCs. The highly directional alignment, optical anisotropy, hierarchical porosity, and large internal surface area of the CNC-based aerogels exhibit considerable potential for future fire-resistant, direction-dependent mechanical and electrical insulator applications.

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and sisal, etc.)23 and holds great promise as an exceptional building block for construction of macroscopic materials, such as hydrogels, porous monoliths, and aerogels.24 CNC contains abundant hydroxyl groups and is characterized by low density (1.6 g·cm−3), high mechanical strength (2−3 GPa), large surface area (up to 700 m2·g−1), and high elastic modulus (110−140 GPa), which make it suitable for construction of aerogel skeletons.25 Currently available CNC-based aerogels are mechanically tough and flexible, highly porous, and optically transparent.26 Unlike natural cellulose-based materials (e.g., woods, plant leaves, and bacterial shelter), the CNCs aerogels are fully amorphous, with randomly connected skeletal microstructures, which fail to demonstrate any significant structure−property relationships. Yet, it is well-known that, at or above a critical concentration, the dispersed CNCs can selfassemble in aqueous suspension into a chiral nematic or nematic organization, exhibiting anisotropic liquid crystalline structural properties, as well as fluidity and long-range order.27,28 Of note, the ordered CNCs assembly can be preserved in solid films, yielding a free-standing composite with intense birefringence and brilliant iridescence colors.29−31 This

INTRODUCTION Aerogels are a diverse class of ultralow density, highly porous solids with large surface areas, rendering them promising candidates for various advanced applications in modern industries.1,2 Based on composition, aerogels can be classified as inorganic,3−6 organic,7−9 hybrid organic−inorganic,10,11 and carbon-based aerogels.12,13 Aerogels are commonly fabricated using sol−gel chemistry techniques, including chemical gelation or deposition methods followed by subsequent solvent removal through supercritical drying to form monoliths.14−20 The assembled aerogels are notably weak and fragile in monolithic form, consisting of randomly interconnected three-dimensional backbone network skeletons and well-accessible pores yet exhibit outstanding performance when integrated in catalyst supports, electrocatalysts, thermal insulators, dust collectors, piezoelectrics, and sensors.21 Unlike synthetic aerogels, natural lightweight and porous materials such as woods, bones, and sea sponges are composed of hierarchical long-range ordered structures and show enhanced mechanical and optical properties.22 Imprinting similar ordered structures into synthetic aerogels may significantly improve their properties, rendering them as candidates for potential applications in superelastic monoliths and optical transparent insulator. Cellulose nanocrystal (CNC) is a sustainable and renewable nanomaterial that can be produced by controlled sulfuric acidassisted hydrolysis of lignocellulosic biomass (wood, cotton, © 2017 American Chemical Society

Received: January 26, 2017 Revised: April 12, 2017 Published: April 13, 2017 3980

DOI: 10.1021/acs.chemmater.7b00361 Chem. Mater. 2017, 29, 3980−3988

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Chemistry of Materials

Figure 1. (a) TEM image shows individual CNCs, prepared by fast evaporation of a dilute CNCs suspension (about 0.05 wt %). (b) Calculated ratio of the anisotropic phase for varying CNCs volume fractions. The specific CNCs concentrations chosen for freeze-casting are indicated by colored squares. (c) POM image of a 3.0 wt % CNCs suspension exhibiting the isotropic nature of a colloidal suspension. (d, e) POM images of the anisotropic phase of a 10 wt % CNCs suspension, which indicates the liquid crystalline ordering in focal conic texture and fingerprint texture. All the images were taken under crossed polarized light. (f−h) Illustrations of the alignment in CNCs suspension for their corresponding texture.

liquid crystalline CNCs interact with the growth of the anisotropic ice crystal, without the interference of additives. When there is a large-scale ordered arrangement in the liquid crystalline phase, and the system is rapidly quenched to ultralow temperatures, such that reorganization can be prevented, the liquid crystalline order can be frozen in solid phase.43 Herein, we demonstrate that hierarchical long-range structured aerogels can be readily fabricated by unidirectional freeze-casting of liquid crystalline CNCs. The resulting CNCsbased aerogels exhibited a self-supporting, anisotropic, lowdensity, porous, and multiscale structure. In addition, the directional freezing induced interconnected CNCs networks, resulting in strong hydrogels with a distinct macrostructure and birefringence, via a direct thawing process. The growth of ice crystals was accompanied by the formation of fluidic chiral nematic ordering in liquid crystalline CNCs. In particular, we systematically investigated the origin of optical anisotropy in CNCs aerogel and confirmed that it arises from the alignment of CNCs, rather than from the shape-persistent rod-like CNCs itself. In the end, we demonstrate a way to capture the ordered structure of CNCs aerogel through an ice-assisted sol−gel process and solidify it in silica matrix as a “fossil record”. The presented freezing-induced self-assembly of liquid crystalline CNCs in confined ice matrix expands the technique for fabrication of hierarchically structured free-standing anisotropic aerogels with bioderived nanoparticle building blocks.

structure can also be found in some plants and jewel beetles and is termed as Bouligand-type structure.32,33 Directional freeze-casting (also known as ice-segregationinduced self-assembly) is a convenient method to produce lowdensity aerogels with ordered structure by unidirectional freezing of colloidal nanoparticle suspension combined with ice template removing.34,35 It has demonstrated great potential in fabrication of materials with highly sophisticated structures. In this method, the precursor suspensions are transferred into a mold that is frozen in liquid nitrogen with unidirectional temperature gradient. As a consequence, periodic arrays of ice crystals grow parallel to the freezing direction and exclude the solute from the ice front, forcing them into the intercrystalline domain boundaries of the ice crystals, leading to an ordered structure of the solute phase.36 Upon selection of an appropriate solute precursor, previous works have shown a wide range of structurally ordered porous materials with aligned nanoparticles in the pore walls, which exhibited desired functionality.37−40 Note that hydrophilicity of the nanoparticles is required in order to obtain a homogeneous suspension of the solute phase. Recently, Munier et al.41 and Chau et al.42 independently reported closely related freeze-casting research on CNCs. In their studies they utilized low-concentration CNCs suspension (lacking liquid crystalline order) and polymer as precursors. The resulting freeze-casted products revealed one-dimensional ordering, with lamellar or columnar structure dictated by the ice template matrix. Yet, it still remains to be determined how 3981


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Figure 2. (a) Schematic illustration of the directional freeze-casting for CNCs suspension. (b) Photographs of the CNCs−ice monolith and CNCs aerogel. Note that the shape and dimensions of the aerogel can be controlled by use of a mold. POM images of the CNC−ice monolith before sublimation: directional freezing of 3.0 wt % CNCs suspension (c) and directional freezing of 10 wt % CNCs liquid crystal with the view directions parallel (d) and perpendicular (e) to the freezing directions.

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critical value of the transition is much lower than predicted.48 In order to experimentally characterize the lyotropic property of the liquid crystalline CNCs suspension, the initial pulp slurry of CNCs was diluted to give a series of concentrations ranging from 1.0 to 12.7 wt %, with the corresponding volume fraction ranging from 0.006 to 0.081 (see the Supporting Information for details). In general, a suspension of anisotropic colloids shows an isotropic phase at low concentrations but transits into a biphasic mixture with the coexisting isotropic and anisotropic phases, as particle concentrations increase. Figure 1b is the phase diagram of the concentration-dependent CNCs suspension, which enables determination of the critical values of CNC concentration at which the transition from isotropic phase to anisotropic phase occurs, i.e., a top isotropic phase with a bottom liquid crystalline anisotropic phase. Note that reaching equilibrium for CNC liquid crystals can last a couple of hours. It can be concluded from the diagram that as the concentration increases, the overlap of excluded volumes causes the colloidal CNCs to be fully aligned into a liquid crystalline order, which is in agreement with previous reports.48 We termed this anisotropic phase in CNCs suspension as chiral nematic liquid crystals. Panels c and d of Figure 1 are the polarized optical microscopy (POM) images (bright field, transmission mode) of the CNCs suspension with the concentrations of 3 and 10 wt %, respectively. Under crossed polarizer, the image of lower CNCs concentration appeared completely dark (Figure 1c), which indicated the totally isotropic nature of the CNCs suspension and exhibits nonorientational ordering (Figure 1f). Increasing the CNCs concentration could induce a transition from isotropic state to anisotropic state with network-like defects, leading to bright views in the POM image with a typical focal conic texture (Figure 1d,g). Further, the observed color was apparently arising from the birefringence and optical rotation, instead of selective reflection (Supporting Information Figure S1). It is worth mentioning that the POM image of a 10 wt % CNCs suspension showed a fingerprint texture when confined in a liquid crystal cell (Figure 1e), suggesting that CNCs were arranged in a chiral nematic liquid crystalline order with the helical axis running parallel to the cell plane and the

EXPERIMENTAL SECTION

The initial CNCs suspension was prepared by controlled sulfuric acidcatalyzed degradation of the bulk cellulose fibers that is based on modification of previously reported procedures,44,45 with the experiment details given in the Supporting Information. Typically, 10 mL of CNCs suspension (10 wt %) was injected into a 25 mL Teflon beaker and equilibrated for 6 h. Then unidirectional freeze-casting was performed on a coldfinger setup, as previously described.34 The beaker was put into a Cu cylinder disk, half of which was dipped into liquid nitrogen. The freezing process was visually assessed. Consequently, the CNCs suspension was directionally frozen from the bottom Cu/ suspension interface to the top area until the water in suspension was totally frozen. Subsequently, the frozen composite was transferred to a freeze drier, followed by sublimation of the ice at 0.6 mbar and room temperature for 24 h to obtain the CNCs aerogel. The obtained monolith was termed as LROC-1 aerogel. In order to study the concentration-dependent self-assembly of CNCs nanoparticles within the ice crystal matrix, 10 mL of a 3 wt % CNCs suspension was employed as the initial precursor for unidirectional freeze-casting, performed using the same process. The obtained aerogel was labeled as LROC-2.

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RESULTS AND DISCUSSION The starting CNCs aqueous suspension was prepared as described in the Supporting Information. Based on the transmission electron microscopy (TEM) analysis, the obtained CNCs exhibit rod-like morphology with an average length (L) and diameter (D) of 220 and 15 nm, respectively (Figure 1a). The ζ-potential of CNCs suspension is measured as −57.6 mV, which indicated colloidal stability and electrostatic repulsion between the CNCs nanorods. As mentioned above, CNCs is a kind of lyotropic liquid crystals; thus, at critical concentrations, the CNCs nanorods tend to orient parallel to each other, to simultaneously minimize the excluded volumes and maximize the packing entropy.46 In this case, the estimated critical volume fraction (φ) of the liquid crystalline CNCs based on Onsager theory is φ = 4D/L.47 Thus, the theoretical value of φ for CNCs extrapolated from the TEM results is 0.27. While the Onsager theory for excluded volume provides an intuitive way to describe the isotropic to anisotropic transition of the CNCs lyotropic liquid crystals, it has been found that the 3982


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Chemistry of Materials director winding anticlockwise (Figure 1h).49 The helical pitch of the chiral nematic CNCs liquid crystals was measured as 7.5 ± 0.3 μm, 2-fold higher than the periodicity of striped patterns. The 10 and 3 wt % CNCs suspensions were then subjected to unidirectional freeze-casting in a cylindrical mold on a coldfinger, with liquid nitrogen as the cooling source (Figure 2a). During the freezing process,35 the CNCs nanoparticles were accumulated between ice fronts, forming a continuous 3D network only when its volume fraction was higher than the percolation threshold. Figure 2b presents the photographs of the CNCs−ice block obtained following unidirectional freezecasting (left), and of the CNCs aerogel (LROC-1) after freezedrying (right). Sublimation of ice templates led to the formation of an aerogel-based material. Notably, the volume of LROC-1 was comparable to its corresponding CNCs−ice block, suggesting that ice sublimation during the freeze-drying process caused little shrinkage. A clear collapse of the macroscopic structure was observed, when the volume fraction of the initial CNCs was below 0.006 (with the critical CNCs concentration of 1.0 wt %, Figure S2). The self-supported network of CNCs monolith indicated that the colloidal CNCs were directly transferred into dry aerogel-like superstructures without a conventional hydrogelation process, as was the case for previously described aerogel fabrication methods.2,35 In order to track the ice-assisted self-assembly process of CNCs, we characterized the anisotropy of a series of frozen colloidal CNCs blocks. It is well-known that liquid water can freeze into solid ice with a crystalline phase and anisotropic properties.50 However, freezing of a multicomponent aqueous suspension causes phase separation that drives nonfreezing components to the space between ice fronts forming cellular structures, while the mesostructural features depend on the nature of aqueous suspensions and freezing dynamics.35 Thus, the growth of ice fronts in CNCs suspension can promote a phase separation of CNCs liquid crystals. The frozen colloid droplets of both 3 and 10 wt % CNCs proved to be birefringent (Figure 2c,d), which can be ascribed to their anisotropic nature. Judging from the fact that the 3 wt % CNCs suspension is isotropic without liquid crystalline textures (Figure 1c), it is reasonable to assume that the birefringence observed in frozen droplets arises from the anisotropic ice crystals or the alignment of rod-like CNCs. The POM image of the 10 wt % frozen droplets showed distinct birefringence colors (Figure 2d, similar to Figure 1d). Characteristics of fingerprint textures of the chiral nematic ordering of the CNCs were observed in certain areas of the 10 wt % frozen blocks (Figure 2e). This may suggest that the CNCs nanorods between adjacent ice fronts are arranged in a chiral nematic liquid crystalline order. By contrast, the ice crystal of pure water was examined using polarized light microscopy (Figure S3); domains of textures that were similar to that of the frozen droplets of 3 wt % CNCs suspension were observed, implying the origin of anisotropy for the 3 wt % CNC−ice block was mainly from the ice crystals. Above all, we believe that the optical anisotropy observed in the frozen droplets of CNCs−ice arises from the synergistic effect between the crystal structure of ice and the liquid crystalline alignment of CNCs. Of particular interest was the fact that the freeze−thaw process induced the reorganization of CNCs into a highly anisotropic order, with a hydrogel-like character. Unidirectional freezing of the 10 wt % CNCs aqueous suspension, followed by thawing, yielded hydrogel-like CNCs, lacking fluidity, which indicated the formation of an interconnected network of CNCs

(Figure 3). POM analysis revealed that the CNCs in the thawing hydrogel were well-oriented with anisotropy and

Figure 3. Photographs and POM images of the 10 wt % CNCs liquid crystal before and after a freeze−thaw cycle, demonstrating the formation of liquid crystalline CNCs networks after freezing.

showed different kinds of birefringent textures compared with the initial ones. These observations confirm the fact that freezing could induce phase solidification of liquid crystalline CNCs networks and that ordering can be preserved after thawing. In contrast, the freeze−thaw process has no effect on the fluidity and texture of the 3 wt % CNCs suspension (Figure S4). Further examination of the highly ordered aerogels demonstrated that the LROC-1 aerogel samples displayed brilliant birefringence colors, similar to those observed for 10 wt % CNCs liquid crystals (Figure 4a). In addition, fingerprint textures of the aerogel were apparent (Figure 4a, inset), which served as a confirmation for long-range ordered structures.51 Generally, when normal light passes through a nematic ordered sample sandwiched between two crossed polarizers, the intensity profile of the transmitted light can be expressed in the form of I = I0 sin 2(2θ ) sin 2(π ΔnL /λ)

where I0 is the intensity of light passed through from the polarizer, θ is the angle between the polarizer and the director of the long axis of the sample, Δn and L represent the birefringence and thickness of the sample, respectively, and λ is the wavelength of incident light.52 From this model, we can infer that when the director is parallel (θ = 0°) or perpendicular (θ = 90°) to the polarization of the incident light, the corresponding light intensity is at its minimum value, which leads to complete light extinction, whereas the maximum intensity is only obtained in the case of θ = 45°. POM images of two angular positions of the LROC-1 aerogel with high magnification are shown in Figure 4c,d, which correspond to rotating the orientation of the sample from 0° to 45°, respectively. Notably, no light extinction occurred and the intensities in both figures were comparable with different birefringence colors, which implied the polarization plane of the incident light had been rotated by the aerogel. This is a classic signature of twist,53 indicating that the LROC-1 CNCs aerogel may feature a twisted ordering in its structure. However, in the 3983


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Figure 4. Anisotropic properties of the LROC-1 and LROC-2 aerogels. (a) Low-magnification POM image of the LROC-1 aerogel, demonstrating the typical birefringence texture of this material. The inset shows a fingerprint texture of the aerogel. (b, c) High-magnification POM images of the LROC-1 at different orientation of the sample. (d) Low-magnification POM image of the LROC-2 aerogel indicating the anisotropy of the obtained material. (e, f) POM images of the LROC-2 with high magnification and different orientation of the sample, implying that the CNCs inside are highly aligned.

throughout the thickness of the surface (Figure 5a). In comparison, the side view SEM image of LROC-1 showed a typical layered structure with macropores, which was a consequence of the voids left between CNC colloids packaged at the boundaries of adjacent ice crystals (Figure 5b). SEM images of LROC-1 with oblique view and high magnifications revealed much more elaborate structures, as depicted in Figure 5c,d, respectively. The interlamellar spacing between each layer was about a few micrometers, which was in the same size of the grown ice crystals (Figure 5c). During freezing, the CNC colloids were squeezed into a two-dimensional laminar structure, in a layered long-range ordering. The highmagnification view showed that the CNCs self-assembled into a twisted order, with structures similar to those previously reported for chiral nematic cellulose-based materials (Figure 5d).54 This indicated that the liquid crystalline ordering of CNC had been preserved in solid CNC monolith. The corresponding TEM image of LROC-1 also indicated that the CNCs within were highly locally aligned (Figure S5). However, the LROC-2 sample contained large domains of periodic columnar structures, of several micrometers in diameter with typically nematic ordering of the CNCs inside (Figure S6). It means that there were significant differences in the ice-assisted self-assembly process between the two types of aerogels subjected to the ice-assisted self-assembly process, probably due to the influence of initial CNCs concentrations. A hypothetical mechanism of the ice-assisted assembly of CNC can be inferred from the basic physics of ice crystal growth, the interactions between CNC nanoparticles, and the liquid crystalline ordering of CNC (Figure 6). Before the directional freezing process, the CNC in water can form stable and homogeneous aqueous suspensions, which, due to the interparticle electrostatic repulsion between individual negatively charged CNC particles, generate isotropic and anisotropic phases with different concentrations. When the CNC suspension is frozen, ice crystals gradually grow in the same direction with the temperature gradient and create a layered microstructure oriented in a direction that is parallel to the movement of the freezing front. We speculate that during the directional freezing process, the rod-like CNCs in suspension are subjected to the shear flow field due to the

case of LROC-2 aerogel, it also exhibited anisotropic properties when viewed under crossed polarizers (Figure 4d). In this configuration, the images corresponding to Figure 4e,f were different from each other; one of them showed extinction of light and decreased in intensity. This implies that the plane of polarization for the incident light is either parallel or perpendicular to the long-range arrangement of the LROC-2 aerogel, indicating nematic ordering of CNCs along the growth direction of the ice fronts. Scanning electron microscopy (SEM) analysis was conducted to confirm continuous long-range ordering throughout the LROC-1 monolith, and all the samples were directly observed in different view angles without further damage (Figure 5). As expected, LROC-1 displayed distinct periodic, layered structures comprised of pure CNCs only (Figure 5a,b, low magnification). LROC-1 with top view had shown long-range ordered structure with a repeat distance of several micrometers

Figure 5. SEM morphology of the directional freeze-casted LROC-1 aerogel. Top view (a) and oblique view (b) of the LROC-1 with low magnification, which exhibit a laminar structure of the aerogel. (c) Side view of LROC-1 with high magnification of the aggregation induced CNCs layer. (d) Magnified SEM image of the layer in panel c, showing the periodic ordering of CNCs. 3984


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Figure 6. Schematic illustration of the ice-assisted hierarchical self-assembly approach of CNCs in isotropic and anisotropic phases, respectively.

Figure 7. Optical characterization of CNCs templated aerogel and its corresponding adsorptive properties. (a) POM image of the long-range ordered silica aerogel which exhibits strong birefringence. Inset is the photograph of the obtained silica aerogel. (b) SEM image of the obtained silica aerogel at low magnification with top view. (c) TEM image of the silica aerogel. The arrow indicates the direction of pore orientation. Inset is the magnified TEM image of the silica aerogel with pore diameter of about 8−10 nm. (d) Nitrogen adsorption isotherm for the silica aerogels.

structure as well as the different orientation of the CNCs can be preserved. To further explore the feasibility of this fabrication approach, attempts were made to fabricate silica aerogels using the chiral nematic liquid crystalline CNCs as template. The preparation method was similar to that used to prepare the CNCs aerogel (see Supporting Information for details). The CNCs template was removed by calcination, resulting in a pure free-standing silica aerogel, with affordable collapse. The conversion yield of hybrid aerogel into silica aerogel was approximately 20%, as determined by thermogravimetric analysis (TGA, Figure S7). The POM image showed strong birefringence, indicative of the anisotropic property of the silica aerogel (Figure 7a). The longrange orientational ordering of CNCs was essentially retained in silica aerogel (Figure 7b and Figure S8), which featured smooth surfaces combined with repeating layered structures perpendicular to its surface, as observed in pure CNCs aerogel. The presence of mesoporosity in the silica aerogel was evidenced by the high-magnification TEM image (Figure 7c)

difference in growth speed of ice crystals in length and thickness. Consequently, the CNC particles are expelled from the growing ice crystals and squeezed into the space between ice fronts. This causes a mechanical deformation of ice segregated CNCs networks.55 For the isotropic CNCs suspension, the shear pressure developed in the squeezed film can effectively align and trap the rod-like CNCs into a nematic orientation with an ordered state. Moreover, the formation of CNCs layers between ice crystals could bring the nanoparticles into an arrested state, preventing them from losing their orientation and relaxing back to an isotropic state. For the dense liquid crystalline CNCs, before directional freezing, the CNCs are in an anisotropic state with chiral nematic ordering. Once the freezing occurs, the resulting shear flow can align the fluid CNCs liquid crystals into a lamellar arrangement, with the formation of ice crystals between them. For both of the selfassembly processes, when the ice template is completely removed during the sublimation process, their hierarchical 3985


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Chemistry of Materials ORCID

and further confirmed by the nitrogen adsorption−desorption isotherms (Figure 7d). In Figure 7c, the mesopores in silica aerogel were locally aligned, which was consistent with the local nematic organization of the CNCs template. The silica aerogel also showed a type IV adsorption−desorption isotherm with a H2 hysteresis loop in the range of P/P0 = 0.4−0.7 (Figure 7d). The corresponding Brunauer−Emmett−Teller (BET) surface area and Barrett−Joyner−Halenda (BJH) average pore diameter of the aerogel were 300 m2·g−1 and 7.2 nm, respectively. The t-plot external surface area was approximately 480 m2·g−1, indicating both micro- and mesoporosity in the silica aerogel.56 In contrast, the nitrogen adsorption isotherm for LORC-1 was presented in Figure S9 with a negative slope, which indicated the sample outgases during the measurement. On the other hand, we learned from the SEM images (Figure 5) that the CNC monolith showed macroporous structure with the pore size ranges between 5 and 10 μm. Based on the mercury porosimetry data, the surface area was 45 m2·g−1 with the macropore profile of micrometers (Figure S10). The density, surface area, and porosity for the LROC-1 and the responding silica aerogel were summarized in Table S1. The density for LROC-1 is quite low. We think the reason for the lack of BET surface area in LORC-1 may be due to the shrinkage during the freeze-casting process which results in the mesopores inside being clogged. Indeed, most of the conventional aerogels are produced from hydrogel through CO2 supercritical drying which prevents shrinkage. For the silica aerogel, the resulting mesoporosity can be ascribed to the voids left after calcination of CNC. In addition, the fire-resistant performance of silica aerogel was further verified by the combustion process (Figure S11), which showed that the silica aerogel could effectively avoid heat transfer.

Guang Chu: 0000-0003-1538-5276 Yan Xu: 0000-0003-4590-660X Author Contributions

G.C. and D.Q. synthesized the aerogel and conducted the tests. G.C. designed and led the project. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. G.C and D.Q. contributed equally to this work. Funding

Russell Berrie Nanotechnology Institute (RBNI), the Israel Science Foundation (ISF Grant No. 286/15). E.Z. acknowledges the financial support of the Winograd Chair of Fluid Mechanics and Heat Transfer at Technion. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.C. acknowledges Dr. Ke Mo, Dr. Shengwei Deng, and Dr. Xuesi Wang for their valuable assistance in making this work possible.

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ABBREVIATIONS BET, Brunauer−Emmett−Teller; BJH, Barrett−Joyner−Halenda; CNC, cellulose nanocrystal; LROC, long-range ordered cellulose; POM, polarized optical microscopy; SEM, scanning electron microscopy; TEM, transmission electron microscopy; TGA, thermogravimetric analysis

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(1) Wu, Z. S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Müllen, K. 3D Nitrogen-Doped Graphene Aerogel-Supported Fe3O4 Nanoparticles as Efficient Electrocatalysts for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134, 9082−9085. (2) Pierre, A. C.; Pajonk, G. M. Chemistry of Aerogels and Their Applications. Chem. Rev. 2002, 102, 4243−4266. (3) Freytag, A.; Sánchez-Paradinas, S.; Naskar, S.; Wendt, N.; Colombo, M.; Pugliese, G.; Poppe, J.; Demirci, C.; Kretschmer, I.; Bahnemann, D. W.; Behrens, P.; Bigall, N. C. Versatile Aerogel Fabrication by Freezing and Subsequent Freeze-Drying of Colloidal Nanoparticle Solutions. Angew. Chem., Int. Ed. 2016, 55, 1200−1203. (4) Suh, D. J.; Park, T. J. Sol-Gel Strategies for Pore Size Control of High-Surface-Area Transition-Metal Oxide Aerogels. Chem. Mater. 1996, 8, 509−513. (5) Dai, S.; Ju, Y.; Gao, H.; Lin, J.; Pennycook, S.; Barnes, C. Preparation of Silica Aerogel Using Ionic Liquids as Solvents. Chem. Commun. 2000, 243−244. (6) Leventis, N.; Sadekar, A.; Chandrasekaran, N.; Sotiriou-Leventis, C. Click Synthesis of Monolithic Silicon Carbide Aerogels from Polyacrylonitrile-Coated 3D Silica Networks. Chem. Mater. 2010, 22, 2790−2803. (7) Leventis, N.; Chandrasekaran, N.; Sadekar, A. G.; SotiriouLeventis, C.; Lu, H. One-Pot Synthesis of Interpenetrating Inorganic/ Organic Networks of CuO/Resorcinol Formaldehyde Aerogels: Nanostructured Energetic Materials. J. Am. Chem. Soc. 2009, 131, 4576−4577. (8) Rigacci, A.; Marechal, J.; Repoux, M.; Moreno, M.; Achard, P. Preparation of Polyurethane-Based Aerogels and Xerogels for Thermal Superinsulation. J. Non-Cryst. Solids 2004, 350, 372−378. (9) Zhang, X.; Liu, J.; Xu, B.; Su, Y.; Luo, Y. Ultralight Conducting Polymer/Carbon Nanotube Composite Aerogels. Carbon 2011, 49, 1884−1893. (10) Leventis, N.; Sotiriou-Leventis, C.; Zhang, G.; Rawashdeh, A. M. M. Nanoengineering Strong Silica Aerogels. Nano Lett. 2002, 2, 957− 960.

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CONCLUSION In summary, this work demonstrated that directional freezecasting of liquid crystalline CNCs is an ideal method for preparation of novel anisotropic aerogel that features hierarchical structures and oriented long-range ordering. During the freeze-casting process, the isotropic CNCs suspension can be aligned and trapped by the shear flow in the vicinity of ice crystals, whose orientations can be preserved by sublimation of the ice template. In the case of anisotropic CNCs liquid crystals, CNCs is assembled with the assistance of ice crystals, with their liquid crystalline ordering then transferred into the subsequent aerogel. These new bio-based aerogels exhibit oriented ordering, anisotropic optical property and high porosity that may find applications in highperformance insulators, tissue engineering, catalyst supports, and adsorption media.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00361. Additional experimental details and figures (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(E.Z.) E-mail: [email protected]. *(Y.X.) E-mail: [email protected]. 3986


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