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Highly Compressible, Anisotropic Aerogel with Aligned Cellulose Nanofibers Jianwei Song,†,§ Chaoji Chen,†,§ Zhi Yang,‡ Yudi Kuang,† Tian Li,† Yiju Li,† Hao Huang,‡ Iain Kierzewski,† Boyang Liu,† Shuaiming He,† Tingting Gao,† Sevket U. Yuruker,‡ Amy Gong,† Bao Yang,‡ and Liangbing Hu*,† †
Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742, United States
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‡
S Supporting Information *
ABSTRACT: Aerogels can be used in a broad range of applications such as bioscaffolds, energy storage devices, sensors, pollutant treatment, and thermal insulating materials due to their excellent properties including large surface area, low density, low thermal conductivity, and high porosity. Here we report a facile and effective top-down approach to fabricate an anisotropic wood aerogel directly from natural wood by a simple chemical treatment. The wood aerogel has a layered structure with anisotropic structural properties due to the destruction of cell walls by the removal of lignin and hemicellulose. The layered structure results in the anisotropic wood aerogel having good mechanical compressibility and fragility resistance, demonstrated by a high reversible compression of 60% and stress retention of ∼90% after 10 000 compression cycles. Moreover, the anisotropic structure of the wood aerogel with curved layers stacking layer-by-layer and aligned cellulose nanofibers inside each individual layer enables the wood aerogel to have an anisotropic thermal conductivity with an anisotropy factor of ∼4.3. An extremely low thermal conductivity of 0.028 W/m·K perpendicular to the cellulose alignment direction and a thermal conductivity of 0.12 W/m·K along the cellulose alignment direction can be achieved. The thermal conductivity is not only much lower than that of the natural wood material (by ∼3.6 times) but also lower than most of the commercial thermal insulation materials. The top-down approach is low-cost, scalable, simple, yet effective, representing a promising direction for the fabrication of high-quality aerogel materials. KEYWORDS: aerogel, compressible, cellulose nanofibers, layered structure, thermal conductivity, anisotropic
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mechanical properties, light weight, and abundant availability have been the most suitable and widely used building blocks for aerogels. However, the bottom-up fabrication process of nanocellulose-based aerogels generally involves multiple steps including chemical,25 enzymatic,26 and/or mechanical27 treatment extracting CNFs from plants, dispersing into solutions, and then reconstructing into aerogels by different manufacturing techniques. The consumption of both energy and time is significant in the bottom-up approaches, which would be one of the major limitations for the scalable applications of these approaches. Meanwhile, the reconstructed aerogels usually possess isotropic structures due to the random assembling of CNFs. It remains challenging to construct an anisotropic structure through such bottom-up approaches. Here we developed a facile yet effective top-down approach to fabricating an anisotropic wood aerogel with layered
erogels as a lightweight material with large surface area, low density, low thermal conductivity, and high porosity are promising for various applications, including energy storage devices,1−3 sensors,4−6 bioscaffolds,7 pollutant treatment,8−10 catalytic support,11 and thermal insulation materials.12−14 In the past few decades, many efforts have been dedicated to developing various aerogel materials including silica,15 graphene oxide,13,16,17 carbon nanotubes,9,10,18 and synthetic polymers19 with many different functionalities. However, all these materials either suffer poor mechanical properties or are made from nonrenewable materials. Biomasses, on the other hand, are widely distributed on earth with high source abundance and low cost, as well as high biocompatibility and biodegradability, positioning them as more attractive building blocks for a new generation of aerogels compared to graphene oxide and synthetic polymers. Recently, a number of bioaerogels have been successfully fabricated from biomasses such as bacteria cellulose,20,21 nanocellulose,2,8,13,22 watermelon,23 and pectin.24 Among them, cellulose nanofibers (CNFs) extracted from cell walls of plants with good © 2017 American Chemical Society
Received: June 17, 2017 Accepted: November 21, 2017 Published: December 19, 2017 140
DOI: 10.1021/acsnano.7b04246 ACS Nano 2018, 12, 140−147
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Figure 1. Schematic and photographic illustration of the wood aerogel. (a) Graphical illustration of the highly compressible, anisotropic wood aerogel directly fabricated from natural wood via a simple chemical treatment. (b) Photo images of the natural wood before compressing, under compressing, and after release, respectively. (c) Photo images of the wood aerogel before compressing, under compressing, and after recovery, respectively. (d) Photo image of the lightweight and biodegradable wood aerogel.
immersed into a boiling mixed solution of NaOH/Na2SO3 to partially remove lignin and hemicellulose, followed by treatment in H2O2 to totally remove the residual lignin, and then freeze-dried to obtain the wood aerogel (Figure S1). After delignification, the yellow natural balsa wood completely turns white. Meanwhile, the lattice structure of the original balsa wood evolves to a layered structure due to the destruction of the thin cell walls, whereas the aligned cellulose micro- and nanofibers within an individual layer are preserved, resulting in a curved layer stacking structure. Interestingly, the wood aerogel can be reversibly compressed and released, demonstrating an attractive compressibility that is totally distinct from the uncompressible original wood materials (Figure 1b and c). Moreover, the wood aerogel with an all-nanocellulose component is eco-friendly and biocompatible (Figure 1d). The morphology and structure of the natural wood and wood aerogel are characterized by scanning electron microscopy (SEM), as shown in Figure 2a−h. Figure 2a and d show the appearance of natural wood and a wood aerogel, confirming that the yellow natural wood becomes totally white after delignification. The weight percentage of natural wood decreases from 100% to 35% due to the partial removal of cellulose, hemicellulose, and lignin, resulting in an evolution of density from 135 to 55 mg cm−3 (Figures S2 and S3). By using a two-step sulfuric acid hydrolysis method,42 the evolution of
structure. The wood aerogel was prepared by a simple chemical treatment directly from natural wood. After chemical treatment, the lignin and hemicellulose of the natural wood are removed, while the alignment of cellulose nanofibers is preserved, leading to a layered structure of the wood aerogel. The layered structure endows the anisotropic wood aerogel with a good mechanical compressibility and fragility resistance. Moreover, the anisotropic structure of the wood aerogel with curved layers stacking layer-by-layer and aligned cellulose nanofibers inside each individual layer provides the wood aerogel with an anisotropic and super thermal insulation property, holding great promise for potential applications in the field of thermal management.28
RESULTS AND DISCUSSION Wood material as one of the most abundant resources on earth has been widely used in cellulose manufacturing,29 buildings,30,31 water purification,32−35 energy storage and conversion,36−38 and flexible electronics.39,40 Here, a straightforward “wood to aerogel” strategy is proposed via a top-down synthesis process, as illustrated in Figure 1a (see Methods for more synthesis details). Balsa wood as the lightest wood in the world was used as the starting material to build the wood aerogel block due to its low density, high porosity, and thin cell walls.41 Typically, a piece of precut balsa wood block was 141
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Figure 2. Structural characterization of the natural wood and wood aerogel. (a) Photo of the natural wood. (b, c) SEM images of the natural wood: (b) cross-sectional image showing the wood lumen structure (in the XZ plane); (c) longitudinal image showing the lumina along the growth direction (in the YZ plane). (d) Photo image of the wood aerogel. (e, f) SEM images of the wood aerogel. (e) Cross-sectional SEM image showing the layered structure in the XZ plane. (f) Longitudinal SEM image of the wood aerogel with destroyed lumina in the YZ plane. (g) 2D SAXS pattern of the wood aerogel. (h) Magnified cross-sectional SEM image of the wood aerogel, showing the cellulose nanofibers. (i) Magnified SEM image showing the aligned cellulose nanofibers.
adsorption−desorption isotherms further confirm that the nanopore volume and specific surface area of the wood aerogel are much higher than natural wood due to the removal of lignin and hemicellulose by chemical treatment (Figure S10). To obtain further insight into the structure and chemical composition evolutions of wood materials induced by chemical treatment, we performed various measurements including SEM, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and high-performance liquid chromatography (HPLC). As we know, wood contains three main parts: cellulose, hemicellulose, and lignin. In cell walls, lignin fills the spaces between cellulose and hemicellulose, which acts as an “adhesive” agent in the cell wall structure to covalently link the hemicellulose.43 The evolution of the wood microstructure after chemical treatment over time is shown in Figure 3a−d and Figure S11. After chemical treatment of process I by a mixed solution of NaOH/Na2SO3 for 5 h, the lumina are broken due to the partial removal of lignin and hemicellulose, while the structure has not been isolated (Figure 3b and Figure S11b). Following treatment in H2O2 (7 h), most of the lignin/ hemicelluose was removed, resulting in the destruction of the thin cell walls (Figure 3c and Figure S11c). After further chemical treatment for 9 h, the lumina were almost totally separated due to the almost complete removal of lignin and hemicellulose, resulting in a wood aerogel with a layered structure (Figure 3d and Figure S11d). FT-IR spectra show that
cellulose, hemicellulose, and lignin content decreased from 37.5% to 27.5%, 14.9% to 1.9%, and 26.2% to 0.2%, respectively (Figure S4 and Table S1). The almost total removal of lignin and hemicellulose suggests that the wood aerogel is composed primarily of cellulose. The natural wood possesses a threedimensional (3D) porous structure with lumina of 30−50 μm in diameter along the tree-growth direction (Figure 2b,c and Figures S5 and S6). After lignin and hemicellulose removal, the porous wood structure distinctly evolves from irregular hexagonal lumina to stacking curved layers (Figure 2e and Figure S7). A longitudinal SEM image of the wood aerogel shows that the cell lumina have almost disappeared (Figure 2f). The layered structure of the wood aerogel is expected to contribute to a high mechanical compressibility. In addition, the small-angle X-ray scattering (SAXS) image of the wood aerogel demonstrates a much stronger scattering along the parallel and vertical directions than the other directions, suggesting the alignment of cellulose nanofibers in the wood aerogel can be well preserved (Figure 2g). This is further confirmed by the magnified SEM image, which shows good alignment of cellulose nanofibers in the wood aerogel (Figure 2i and Figure S8). After chemical treatment, the cellulose nanofibers in the cell walls are isolated from each other (Figure 2h and Figure S9). Meanwhile, nanoscale pores were generated between the cellulose nanofibers due to the almost complete removal of lignin/hemicellulose (Figure 2h and Figure S9). Nitrogen 142
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Figure 3. Chemistry characterization of the natural wood and wood aerogel. (a−d) Cross-sectional SEM images showing the wood lumen structure change by chemical treatment for different times: (a) natural wood (0 h), (b) after treatment for 5 h, (c) after treatment for 7 h, (d) wood aerogel (9 h). (e) FT-IR spectra of various wood samples after chemical treatment for different times. (f) XRD pattern of various wood samples after chemical treatment for different times. (g) Cellulose, hemicellulose, and lignin content evolution from natural wood to wood aerogel.
Figure 4. Mechanical compression test of a wood aerogel. (a) Stress−strain curves of the wood aerogel with different maximum strains of 20%, 40%, and 60%, respectively. (b) Stress−strain curves of the wood aerogel at the maximum strain of 40% for four cycles. (c) Energy loss coefficient of the wood aerogel in different cycles derived from (b). (d) Elastic strength retentions during 10 000 compressing/releasing cycles at a constant strain of 40%.
the peaks at 1736 and 1235 cm−1 for carbonyl stretching and C−O stretching are significantly reduced, which reflects the removal of hemicellulose after NaOH/Na2SO3 treatment. Meanwhile, the disappearance of peaks at 1593 and 1505 cm−1 is indicative of an aromatic skeleton, indicating the removal of lignin during NaOH/Na2SO3 and H2O2 treatment (Figure 3e).44,45 This can be further evidenced by the carbohydrates and lignin content evolutions, which show that hemicellulose and lignin are almost completely removed by
chemical treatment (Figure 3g and Table S1).42 Note that the cellulose molecules and their arrangement did not change during the chemical treatment process (Figure 3f). Compressive stress (δ)−strain (ε) measurements were performed to evaluate the mechanical properties of the wood aerogel. The stress (δ)−strain (ε) curves with different compression strains (ε: 20%, 40%, and 60%) for the wood aerogel are shown in Figure 4a. The curves show three distinct regions.16,20 In the first region, compressive stress increases 143
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Figure 5. Thermal properties of natural wood and wood aerogel. (a) Schematic of the anisotropic thermally insulating properties of the wood aerogel. (b) IR images of natural wood and wood aerogel, showing the temperature distribution along the in-plane surface. (c) Thermal conductivities of natural wood and wood aerogel, perpendicular (radial) and parallel (axial) to the cellulose nanofiber alignment direction. (d) Schematic description of heat conduction in the wood aerogel along two different directions. (e) Thermal conductivity of the wood aerogel with different compressions. (f) Schematic description of heat conduction in the wood aerogel with different compressions. (g) Comparison of the anisotropic factor of natural wood and wood aerogel. (h) Comparison of thermal conductivity between the wood aerogel and other thermal insulating materials. Compared with other commercially available thermally insulating materials, the wood aerogel exhibits a lower thermal conductivity.
together by intermolecular hydrogen bonds (Figure 1). The excellent compressibility of the wood aerogel is due to the curved layered structure, where significant stress can be released by a small bending of each curved layer (Figure S12). One example with similar structure is the honeycomb, which comprises a network through a curved layer. This structure leads to its superb compressibility. In addition, the wood aerogel also shows an excellent fragility resistance, demonstrated by a stress retention of ∼90% and negligible plastic deformation even after repeatedly compressing/releasing for 10 000 cycles with a constant strain of 40% (Figure 4d). Note that achieving good mechanical properties, especially fragility resistance, remains highly challenging for aerogel materials. The excellent mechanical properties of the wood aerogel including the highly reversible compression strain of up to 60%, small energy loss coefficient, and superior fragility resistance render it highly promising for applications in thermal insulation, pollutant absorption, acoustic absorption, structural materials, and compressible energy storage devices. In addition to the mechanical properties, we have also characterized the thermal properties of the wood aerogel. As shown in Figure 5b, both the temperature of the natural wood and wood aerogel increase under the incident laser as a heat
linearly, suggesting an elastic property of the wood aerogel with an ε value below 20%, which reflects the cell walls bending in the wood aerogel. When the ε increases from 20% to 40%, a stress plateau stage can be observed due to the gradual compression of the layered structure in the wood aerogel. The compressive stress increases rapidly in the ε range of >40%, resulting from continuous deformation of the layered structure. Figure 4b shows the cyclic stress (δ)−strain (ε) curves at a constant strain of ε = 40%. Compared with the first cycle, the deformation of the second cycle is very small. Moreover, the stress−strain curves after the second cycle are almost overlapped, reflecting the excellent stability of the wood aerogel. The ε can totally decrease to zero when the stress is reduced to zero, suggesting that the wood aerogel can be completely recovered without plastic deformation. We have also calculated the energy loss coefficient per cycle of the wood aerogel. As shown in Figure 4c, the energy loss coefficient decreases from 0.42 (cycle 1) to 0.34 (cycle 4), which is relatively low compared with other aerogels,10,16,46 indicating the excellent compressibility of the wood aerogel. The wood aerogel has a curved layered structure, showing adjacent layers that are joined together in a localized region on the curved surface, while the cellulose fibers within the layer are held 144
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other applications, such as acoustic materials, pollution adsorption, sensors, energy storage devices, and electronics.
source, suggesting that heat accumulated around the hot spot due to insufficient heat dissipation for both natural wood and the wood aerogel. The maximum temperature of the wood aerogel is 49.3 °C, which is ∼3 °C higher than that of the natural wood, indicating the better insulating property of the wood aerogel. Thermal conductivity measurements were carried out to further characterize the thermal conductivity of two samples qualitatively. As shown in Figure 5c and d, thermal conductivity values of the wood aerogel perpendicular and parallel to the cellulose nanofiber alignment direction are 0.028 and 0.12 W/m·K, respectively. Both are lower than those of the natural wood (0.1 and 0.15 W/m·K), suggesting the superior insulating properties of the wood aerogel with regard to natural wood, which is commonly used as a thermal insulating material. The low thermal conductivity especially along the radial direction is due to the high porosity of the demonstrated wood aerogel. The aerogel exhibits a mass density of as low as 0.055 g cm−3, 2.45 times lower than that of natural balsa wood. A high porosity of >95% was obtained, estimated from the density of the dry wood cell wall.47 Meanwhile, the wood aerogel exhibits anisotropic thermal conductivity with an anisotropy factor of 4.3, suggesting the sample orientation can be altered based on different applications for better performance (Figure 5g). This anisotropy in thermal conductivity is mainly due to the anisotropic structure of the wood aerogel. A slight thermal conductivity enhancement was observed for the wood aerogel under compression pressure (Figure 5e,f). Under compression, the air gaps between layers is minimized and contact between layers increases, leading to an overall reduction in thermal resistance. The wood aerogel demonstrates one of the lowest thermal conductivities among reported and commercial thermal insulating materials,19,32,48−51 such as light concrete, polyamide aerogels, expanded polystyrene (EPS), and so on. For example, Williams et al.19 reported a highly porous polyamide polymer aerogel with a thermal conductivity of 0.05 W/m·K due to its high porosity and the low intrinsic thermal conductivity of the polyamide polymer. Mihlayanlar and co-workers51 demonstrated an EPS board with a low thermal conductivity of 0.039 W/m·K. The lower thermal conductivity of the wood aerogel renders it extremely promising for thermal insulation applications.
METHODS Materials and Chemicals. Balsa wood was used for the synthesis of the wood aerogel. Sodium hydroxide (>97%, Sigma-Aldrich), sodium sulfite (>98%, Sigma-Aldrich), and deionized (DI) water were used for processing the wood. Wood Aerogel Fabrication. Bleached white wood samples were prepared by cooking the wood blocks in a mixture solution of sodium hydroxide (NaOH) and sodium sulfite (Na2SO3) at 100 °C for 5 h, followed by boiling in the H2O2 solution to completely remove the residual lignin. Then, the obtained white wood samples were preserved in a freeze dryer and dried for 1 day to get the wood aerogel. Measurements and Characterizations. A scanning electron microscope (Hitachi SU-70) was used to characterize the morphology and structure of the wood. XRD patterns were collected using a Rigaku Ultima III equipped with a curved detector manufactured by Rigaku Americas Corp. (operating tube voltage at 40 kV, tube current at 30 mA, Cu Kα, λ = 1.5406 Å). Composition content of the wood was measured by two-step sulfuric acid hydrolysis as described previously.42 A Thermo Nicolet NEXUS 670 FT-IR was used to measure the FT-IR spectrum. The mechanical compression properties were measured using a dynamic mechanical analyzer. The nitrogen adsorption−desorption isotherms of wood samples were determined by CO2 absorption using a TriStar II 3020 analyzer (Micromeritics Instrument Corporation). Thermal Conductivity Measurement. The thermal conductivities of the samples were measured on a system consisting of a laser heat source, an infrared (IR) thermal camera, two standard aluminum blocks, and a temperature-controlled heat sink (Figure S13). The sample was placed between two highly thermal conductive aluminum blocks (206 W/m·K) for testing. A Coherent Highlight FAP-1000 (820 nm) laser was used as heat source to provide continuous and stable heat input power on the upper surface of the top aluminum block. The lower aluminum block was connected to a temperature controllable water bath (heat sink). The steady-state temperature distribution over the aluminum−sample−aluminum assembly was captured and recorded using an FLIR Merlin MID IR camera with a resolution of 320 × 256 pixels. A thin layer of graphite (ε = 0.9) was coated on the exposed surfaces of the assembly to ensure accurate temperature readings from the IR camera. The calibration experiment was carried out on a standard material with calibrated conductivity (Teflon K = 0.25 W/m·K) to accurately identify the effective average heat transfer coefficients of radiation and natural convection.
CONCLUSION In summary, we achieved a highly compressible, anisotropic wood aerogel by a simple top-down chemical treatment directly from natural balsa wood. The removal of lignin and hemicellulose partially destroys the thin cell walls, resulting in a layered structure with aligned cellulose nanofibers inside each individual layer. The wood aerogel becomes highly compressible and antifragile owning to the structure with stacking curved layers, demonstrated by a high reversible compression of 60% and stress retention of ∼90% after 10 000 compressing/ releasing cycles. An interesting anisotropic thermal property with an extremely low thermal conductivity of 0.028 W/m·K along the layer-stacking direction and 0.12 W/m·K along the cellulose-alignment direction is further disclosed, which is mainly attributed to the anisotropic structure of the wood aerogel. The top-down approach demonstrated here is low cost, scalable, simple, yet effective, representing one promising direction for developing high-quality aerogel materials with anisotropic and superinsulating thermal properties, excellent mechanical compressibility, and fragility resistance. The wood aerogel with aligned cellulose nanofibers can find a range of
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04246. Digital photo images of natural wood and the wood aerogel; weight loss of the wood samples; density comparison from natural wood to the wood aerogel; the cellulose, hemicellulose, and lignin content; SEM images of natural wood and the wood aerogel; N2 adsorption−desorption isotherms (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (L. Hu). ORCID
Yiju Li: 0000-0001-9240-5686 Liangbing Hu: 0000-0002-9456-9315 Author Contributions §
J. Song and C. Chen contributed equally to this work.
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L.H., J.S., and C.C. contributed the idea and experimental design. J.S. and C.C. contributed the wood treatment and mechanical measurements. T.L., I.K., H.H., S.H., T.G., A.G., and B.L. contributed the characterizations of the materials. Y.K. contributed the 3D illustrations. Y.L. and C.C. contributed the SEM characterization. Z.Y., S.Y., and B.Y. contributed the thermal conductivity measurements. L.H., J.S., and C.C. contributed to the paper writing. All authors commented on the final manuscript. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We acknowledge the support of the Maryland NanoCenter and its AIMLab. J.S. acknowledges financial support from the China Scholarship Council (CSC). We acknowledge the help of Dr. J. Y. Zhu (USFA Forest Product Lab) for compositional analysis of delignified wood. REFERENCES (1) Biener, J.; Stadermann, M.; Suss, M.; Worsley, M. A.; Biener, M. M.; Rose, K. A.; Baumann, T. F. Advanced Carbon Aerogels for Energy Applications. Energy Environ. Sci. 2011, 4, 656−667. (2) Hamedi, M.; Karabulut, E.; Marais, A.; Herland, A.; Nyström, G.; Wågberg, L. Nanocellulose Aerogels Functionalized by Rapid Layerby-Layer Assembly for High Charge Storage and Beyond. Angew. Chem., Int. Ed. 2013, 52, 12038−12042. (3) Zhao, Y.; Liu, J.; Hu, Y.; Cheng, H.; Hu, C.; Jiang, C.; Jiang, L.; Cao, A.; Qu, L. Highly Compression-Tolerant Supercapacitor Based on Polypyrrole-Mediated Graphene Foam Electrodes. Adv. Mater. 2013, 25, 591−595. (4) Si, Y.; Wang, X.; Yan, C.; Yang, L.; Yu, J.; Ding, B. Ultralight Biomass-Derived Carbonaceous Nanofibrous Aerogels with Superelasticity and High Pressure-Sensitivity. Adv. Mater. 2016, 28, 9512− 9518. (5) Xu, X.; Wang, R.; Nie, P.; Cheng, Y.; Lu, X.; Shi, L.; Sun, J. Copper Nanowire Based Aerogel with Tunable Pore Structure and Its Application as Flexible Pressure Sensor. ACS Appl. Mater. Interfaces 2017, 9, 14273−14280. (6) Chen, Z.; Ren, W.; Gao, L.; Liu, B.; Pei, S.; Cheng, H.-M. ThreeDimensional Flexible and Conductive Interconnected Graphene Networks Grown by Chemical Vapour Deposition. Nat. Mater. 2011, 10, 424−428. (7) Silva, S. S.; Duarte, A. R. C.; Carvalho, A. P.; Mano, J. F.; Reis, R. L. Green Processing of Porous Chitin Structures for Biomedical Applications Combining Ionic Liquids and Supercritical Fluid Technology. Acta Biomater. 2011, 7, 1166−1172. (8) Jiang, F.; Hsieh, Y.-L. Amphiphilic Superabsorbent Cellulose Nanofibril Aerogels. J. Mater. Chem. A 2014, 2, 6337−6342. (9) Gui, X.; Wei, J.; Wang, K.; Cao, A.; Zhu, H.; Jia, Y.; Shu, Q.; Wu, D. Carbon Nanotube Sponges. Adv. Mater. 2010, 22, 617−621. (10) Sun, H.; Xu, Z.; Gao, C. Multifunctional, Ultra-Flyweight, Synergistically Assembled Carbon Aerogels. Adv. Mater. 2013, 25, 2554−2560. (11) 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. (12) Si, Y.; Yu, J.; Tang, X.; Ge, J.; Ding, B. Ultralight NanofibreAssembled Cellular Aerogels with Superelasticity and Multifunctionality. Nat. Commun. 2014, 5, 5802. (13) Wicklein, B.; Kocjan, A.; Salazar-Alvarez, G.; Carosio, F.; Camino, G.; Antonietti, M.; Bergström, L. Thermally Insulating and Fire-Retardant Lightweight Anisotropic Foams Based on Nanocellulose and Graphene Oxide. Nat. Nanotechnol. 2015, 10, 277−283. 146
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