DOI: 10.1002/cssc.201300950

Comparative Study of Aerogels Obtained from DifferentlyPrepared Nanocellulose FibersWenshuai Chen,[a] Qing Li,[a] Youcheng Wang,[b] Xin Yi,[a] Jie Zeng,*[b] Haipeng Yu,*[a]

Yixing Liu,[a] and Jian Li[a]

Introduction

As a polysaccharide consisting of a linear chain of several hun-dred to over ten thousand ringed glucose molecules, celluloseis recognized as the most abundant and renewable biopoly-mer on earth.[1] During biosynthesis, van der Waals and inter-molecular hydrogen bonds promote the parallel stacking ofcellulose chains to form elementary fibrils that further organizeinto larger microfibrils with 5–50 nm in diameter.[2] Withinthese nanocellulose fibers (NCFs) there are regions where thecellulose chains are arranged in highly crystalline structures, aswell as regions containing amorphous structures.[2] The uniquestructure of NCFs, which leads to high Young’s modulus (esti-mated at ~138 GPa in the crystal region along the longitudinaldirection) and specific strength, makes them ideal buildingblocks for products with desirable mechanical properties. Asa result, enormous efforts have been made to isolate NCFs ina variety of forms, which gave birth to cellulose nanocrystals,

cellulose nanowhiskers, micro-/nanofibrillated cellulose, andcellulose nanofibers.[2–5]

Methods for NCF production involve the biological, physicaland chemical extraction of NCFs from plant or tunicate cellwalls. The NCFs acquired exhibit low density (~1.6 g cm�3),high specific strength and modulus, high surface area, and re-active surfaces containing �OH side groups.[2–5] Because ofthese distinctive properties, NCFs have been found useful ina wide range of applications such as optically transparent ma-terials,[6–8] reinforced polymer nanocomposites,[9, 10] biomimeticfoams,[11] multifunctional high-performance fibers,[12] templatesfor chiral nematic mesoporous materials,[13, 14] and conductivematerials.[15, 16]

Among them, fabricating aerogels from NCFs creates a newbulk material that integrates the remarkable performances ofNCFs with the unique structures and related properties of aer-ogels. Aerogels are highly porous materials that can have ex-tremely low densities, large open pores, and low thermal trans-port.[17, 18] They can be engineered for applications in cataly-sis,[19, 20] super adsorbents,[21] elastic conductors,[22] hydrogenand electrical energy storage,[20] as well as desalination.[20] Todate, most aerogels are fabricated from silica, metal oxides, py-rolyzed organic polymers, and carbon-based materials.[18] How-ever, the traditional aerogels suffer from poor mechanical ro-bustness, which has led to numerous attempts to overcomethis problem. Several nanomaterials including carbon nano-tubes,[23, 24] carbonaceous nanofibers,[25] and graphene,[26, 27]

have been recently developed as building blocks in the assem-bly of strong aerogels. Among them, NCF aerogels are ob-tained from globally abundant and renewable sources.[28–30]

The long and entangled NCFs of cellulose I crystal type can

This article describes the fabrication of nanocellulose fibers(NCFs) with different morphologies and surface propertiesfrom biomass resources as well as their self-aggregation intolightweight aerogels. By carefully modulating the nanofibrilla-tion process, four types of NCFs could be readily fabricated, in-cluding long aggregated nanofiber bundles, long individual-ized nanofibers with surface C6-carboxylate groups, short ag-gregated nanofibers, and short individualized nanofibers withsurface sulfate groups. Free-standing lightweight aerogelswere obtained from the corresponding aqueous NCF suspen-sions through freeze-drying. The structure of the aerogels

could be controlled by manipulating the type of NCFs and theconcentration of their suspensions. A possible mechanism forthe self-aggregation of NCFs into two- or three-dimensionalaerogel nanostructures was further proposed. Owing to web-like structure, high porosity, and high surface reactivity, theNCF aerogels exhibited high mechanical flexibility and ductility,and excellent properties for water uptake, removal of dye pol-lutants, and the use as thermal insulation materials. The aero-gels also displayed sound-adsorption capability at high fre-quencies.

[a] Dr. W. Chen, Q. Li, X. Yi, Prof. H. Yu, Prof. Y. Liu, Prof. J. LiKey Laboratory of Bio-based Material Science and TechnologyMinistry of EducationNortheast Forestry UniversityHarbin 150040 (P.R. China)E-mail : yuhaipeng20000@gmail.com

[b] Y. Wang, Prof. J. ZengHefei National Laboratory for Physical Sciences at the Microscaleand Department of Chemical PhysicsUniversity of Science and Technology of ChinaHefei, Anhui 230026 (P.R. China)E-mail : zengj@ustc.edu.cnHomepage: http ://zengnano.ustc.edu.cn/

Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cssc.201300950.

� 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7, 154 – 161 154

CHEMSUSCHEMFULL PAPERS

form a strong aerogel network because of their fibrillar mor-phology and strong mutual hydrogen bonds, which facilitatemechanical ductility and flexibility of the aerogels.[31, 32] TheNCF aerogels can then be used as templates for inorganichollow nanotubes[33] and cobalt-ferrite nanoparticles,[34] andfurther functionalized to achieve ideal electrical,[35] optical[36]

and fire-resistant[37] characteristics.The structures and properties of the NCF aerogels are

mainly determined by the morphologies and surface proper-ties of the NCF building blocks. Although NCF aerogels havebeen prepared from several types of NCFs,[31, 32, 34, 38–40] compara-tive studies on how characteristics of NCFs modulate structuresand properties of aerogels remain limited. Herein, we reportthe isolation of four types of NCFs with different morphologiesand surface properties. They self-aggregated into aerogelssimply through freeze-drying. The structure of aerogels couldbe easily tuned by varying the type of the NCFs or the concen-tration of the NCF suspensions. The as-prepared aerogels ex-hibited excellent properties in water uptake, organic dye ab-sorption, thermal insulation, and sound absorption at high fre-quencies.

Results and Discussion

Preparation of NCFs with different morphologies and sur-face properties

The hierarchical structures of plant cell walls are schematicallyshown in Figure 1 a. Four methods with different nanofibrilla-tion effects on the cellulose fibers were used to produce NCFs.High-intensity ultrasonication (HIUS) and strong hydrochloricacid (HCl) hydrolysis were chosen to prepare NCFs with origi-nal hydroxyl groups on the surfaces. After HIUS treatment (Fig-ure 1 b), interconnected bundles with long NCFs containingboth amorphous and crystalline phases were obtained (Fig-

ure 1 f). The bundles resulted from the hydrogen-bonding in-teractions between the hydroxyl groups on the NCF surfaces.Transmission electron microscopy (TEM) observations (Fig-ures 2 a and S1 in the Supporting Information) indicate thatthe bundles consisted of single fibers that were 2–6 nm wideand several micrometer long. In contrast, HCl hydrolysis re-moved the amorphous regions of the cellulose fibers, but leftthe crystalline regions intact (Figure 1 c), leading to short ag-gregated NCF bundles (Figure 1 g). As shown in Figures 2 band S2, the single fibers in the bundles were 100–400 nm longand 4–9 nm wide, with aspect ratios ranging from 11 to 100.

To reduce the amount of bundles, surface modification ofNCFs is necessary during the nanofibrillation process. 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO)-mediated oxidation(TMP) was used to regioselectively convert the primary surfacehydroxyl groups of NCFs to carboxylates, which thereforebecame negatively charged (Figure 1 d). The repulsive effect ofthese charges greatly assisted separation.[41] After ultrasonictreatment, individualized NCFs with high aspect ratios wereproduced (Figure 1 h). The TMP-NCFs were 2–5 nm wide andseveral micrometer long (Figures 2 c and S3). Individualized,highly dispersed, and highly crystalline NCFs were fabricatedby performing a strong sulfuric acid (H2SO4) hydrolysis (HSO)method. The use of H2SO4 not only removed the amorphousregions of cellulose fibers, but also introduced negativelycharged sulfate groups on the NCF surfaces (Figure 1 e), whichcaused the NCFs to disperse in water (Figure 1 i). The HSO-NCFs were 2–9 nm wide and 50–350 nm long (Figures 2 d andS4). The aspect ratios ranged from 5 to 175.

Characterization of the chemical composition and crystallini-ty of NCFs

The chemical composition of NCFs was investigated by usingFourier transform infrared (FTIR) spectra (Figure 3 a). Although

the carboxylate and sulfurgroups were introduced to theTMP- and HSO-NCF surfaces, re-spectively, their spectra re-mained similar. The peak at3340 cm�1 was attributed to theO�H stretching vibration. Thepeaks at 2900 and 1430 cm�1

corresponded to the C�Hstretching and bending of the �CH2 groups, respectively. Thepeaks at 1640 and 897 cm�1

were attributed to the H�O�Hstretching vibration of absorbedwater in the carbohydrate andthe C1�H deformation vibrationsof cellulose. These data indicatethat the main component ofeach isolated NCF was cellulose.

To understand the influence ofthe nanofibrillation process onthe NCF crystal structure, the

Figure 1. Schematic illustrations of a) the hierarchical structures of plant cell walls, b–e) the four methods of indi-vidualization of NCFs, and f–i) the corresponding as-prepared NCFs. Specifically, they represent b,f) HIUS methodand HIUS-NCFs, c,g) HCl hydrolysis method and HCl-NCFs, d,h) TMP method and TMP-NCFs, and e,i) H2SO4 hydroly-sis method and HSO-NCFs. The arrows in the illustration refer to the broken area of cellulose fibers during thenanofibrillation process, whereas the small red balls represent the negative charges introduced by the nanofibrilla-tion process.

� 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7, 154 – 161 155

CHEMSUSCHEMFULL PAPERS www.chemsuschem.org

changes in the X-ray diffraction (XRD) curves of the four typesof NCF were examined (Figure 3 b). All samples exhibited dif-fraction peaks at 14.68, 16.58, and 22.68, which correspondedto the (1-10), (110), and (200) planes, respectively. These planesare typical crystal patterns of cellulose I, indicating that thenative crystal structure of cellulose was preserved. However,the crystallinity of the NCFs changed dramatically. The removalof the amorphous areas using strong acid hydrolysis resultedin a higher crystallinity of HCl- and HSO-NCFs compared withthose of the HIUS- and TMP-NCFs (Figure 3 c).

Self-aggregation of NCFs into aerogels

Figure 4 shows scanning electron microscopy (SEM) images ofthe aerogels obtained from the 0.2 wt % NCF suspensions.

During the freeze-drying process, HIUS- and TMP-NCFs self-ag-gregated into long fibers in the longitudinal direction (Fig-ure 4 a, b, e and f). The diameter of the fiber of the HIUS-NCFaerogel (Figure 4 a, b) ranged from 150 to 900 nm and that ofthe TMP-NCF aerogel (Figure 4 e, f) ranged from 50 to 300 nm.The lengths of some of the fibers were nearly >1 mm (Fig-ure S5). The entangled fibers formed strong 3D network struc-tures with >99 % porosity. In comparison, the HCl- (Figure 4 c,d) and HSO-NCFs (Figure 4 g, h) aggregated into 2D sheet-likestructures, which were mutually connected. Micrometer-sizedpores appeared between the layered sheets, whereas nanome-ter-sized pores could be identified on sheet surfaces. Althoughthe basic sheet-like structures were similar, the diameters ofthe aggregated nanofibers in the sheets varied owing to thedifferences in the NCF morphologies and properties. As HCl-

Figure 2. TEM images (a–d) and diameter distributions (e) of the NCFs isolated with different nanofibrillation methods [A) HIUS-NCFs, B) HCl-NCFs, C) TMP-NCFs, and D) HSO-NCFs].

Figure 3. Characterization of the NCFs. a) FTIR spectra, b) XRD pattern and c) crystallinity of the NCFs.

� 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7, 154 – 161 156

CHEMSUSCHEMFULL PAPERS www.chemsuschem.org

NCFs exhibited bundle structures, they self-aggregated intonanofibers that were 50–80 nm wide and approximately 500–600 nm long (Figure 4 c, d). However, because HSO-NCFs couldbe dispersed more freely in water, they self-aggregated intothinner nanofibers with widths of 20–50 nm (Figure 4 g, h).

Control of structures of aerogels

The aerogel structures are influenced by the type of NCFs andthe concentration of their suspensions. Figure 5 a and e showsthat NCFs with high aspect ratios self-aggregated into 3D web-like structures when the suspension concentration was below0.2 wt %. They aggregated into 2D-sheet-like skeletons whenthe concentration exceeded 0.5 wt % (Figure 5 b, f). At low con-centrations, the long NCFs and their bundles were separatedby large amounts of water. When the suspension froze, the

NCFs became concentrated at the boundary of the ice crystalsand were then aligned along the growth direction of the icecrystals because of the squeezing effect. During the sublima-tion of the ice in the freeze-drying process, strong hydrogenbonds formed between the hydroxyl groups on adjacent NCFsurfaces. Thus, the NCFs were in close proximity to one anoth-er and eventually self-aggregated into 3D web-like structureswith long fibers. However, when the concentration reached0.5 wt %, the space for dispersion of the NCFs was insufficient.Therefore, the NCFs were tightly cross-linked with each otherand eventually evolved into 2D-sheet-like structures afterfreeze-drying (Figure 5 b, f). The short HCl- and HSO-NCFsfailed to form bulky aerogels with 3D-web-like structures whenthe suspension concentration was less than 0.2 wt % (Fig-ure 5 c, g). Although the NCFs were condensed at the boun-dary of the ice crystals and then partly aligned after freezing,

Figure 4. SEM images of aerogels obtained from 0.2 wt % NCF suspensions:a, b) HIUS-NCF aerogel, c, d) HCl-NCF aerogel, e, f) TMP-NCF aerogel, andg, h) HSO-NCF aerogel. a, c, e, g) Low-magnification and b, d, f, h) high-magnifi-cation images of the aerogels.

Figure 5. SEM images of the NCF aerogels : a, b) HIUS-NCF aerogel, c, d) HCl-NCF aerogel, e, f) TMP-NCF aerogel, and g, h) HSO-NCF aerogel. The sampleswere obtained through freeze-drying of the suspensions with different NCFcontent: a, c, e, g) 0.1 and b, d, f, h) 0.5 wt %. The scale bars in the insets of allimages correspond to 1 mm.

� 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7, 154 – 161 157

CHEMSUSCHEMFULL PAPERS www.chemsuschem.org

these materials did not self-aggregate into long fibers alongthe longitudinal direction through end-to-end connectionsduring freeze-drying. In addition, a significant shrinkage oc-curred because of the weak skeletons made of short NCFs.Thus, aerogels with 2D-sheet-like structures formed afterfreeze-drying. Because the short NCFs were tightly connectedwith each other in the suspension and the frozen samplewhen the suspension concentration reached 0.5 wt %, the 2D-sheet-like structures of the aerogels were retained after freezedrying (Figure 5 d, h).

Density, mechanical flexibility, water uptake, and dye ad-sorption capacities of aerogels

The HIUS- and TMP-NCF aerogels retained their volumes afterfreeze-drying. Their densities showed a similar trend of increas-ing (Figure 6 a) from ~1.1 � 10�3 to ~8.8 � 10�3 g cm�3 as con-centration of the suspension increased from 0.1 to 0.8 wt %.

Because HCl- and HSO-NCF aerogels shrank during freeze-drying, the densities of the HCl- and HSO-NCF aerogels werehigher than those of the high-aspect-ratio counterparts. Itshould be noted that the testing errors for the HCl- and HSO-NCF aerogels were significantly high (Table S1). Because of theentangled high-aspect-ratio nanofibers, the HIUS- and TMP-NCF aerogels exhibited high ductility and flexibility, which al-lowed for large deformations without fractures. The as-pre-pared 0.5 wt % HIUS- and TMP-NCF aerogels were highly flexi-

ble and could be reversibly bent and twisted without structuraldamage (Figure 6 b, c). The aerogels (Figure 6 d) could alsobear compression strain as high as 99 % (Figure 6 e) and recov-er most of its original volume upon immersion in water (Fig-ure 6 f) because of the robustly interconnected network andstable porous structures. Inspired by the unique porous struc-ture, we tested water uptake and adsorption capacities ofHIUS- and TMP-NCF aerogels. HCl- and HSO-NCF aerogels werenot tested because they are easily dispersed in water as shownin Figures S6 and S7. The water-uptake ratios of the HIUS-NCFaerogels ranged from 89.68 to 146.63, whereas those of theTMP-NCF aerogels ranged from 75.27 to 115.79 (Figure 6 g).The nanoscale networks with open pores in the NCF aerogelsallowed entry and fast diffusion of ions and molecules, whichendowed them with good performance as adsorbents. To ex-amine this performance, an as-prepared 0.5 wt % HIUS-NCFaerogel was immersed in a 10 mg L�1 methylene blue (MB) so-lution (Figure 6 h). After several hours, the MB-saturated aero-

gel (Figure 6 i) was removed andsignificant discoloration of thesolution was observed. Anotheridentical aerogel was then im-mersed in the solution and laterremoved after saturation withMB. The process was repeateduntil no peaks were observed inthe UV/Vis spectrum of the so-lution (Figures 6 j and S8). Theadsorption capacities of theHIUS- and TMP-NCF aerogels forMB can reach 2.90 and3.70 mg g�1, respectively, where-as those for toluidine blue (TB)can reach 3.02 and 4.16 mg g�1,respectively. These results indi-cate that the as-prepared high-aspect-ratio NCF aerogels canserve as absorbents for waterpurification.

Thermal stability, heat insula-tion, and sound absorptioncharacter of aerogels

The thermal stability of aerogelswas surveyed to assess theirvalue in high-temperature appli-cations. The thermogravimetric

analysis (TGA) curves of the aerogels illustrate apparent differ-ences (Figure 7 a). The HIUS- and HCl-NCF aerogels exhibitedhigh thermal degradation temperatures of 336.1 and 341.7 8C,respectively. However, the thermal degradation of the TMP-NCF aerogels started at approximately 210 8C because of theformation of sodium carboxylate groups on the NCF surfaces,which led to a decrease in the thermal stability.[42] The HSO-NCF aerogels exhibited the lowest thermal stability becausethe sulfate groups on the NCF surface promoted rapid thermal

Figure 6. a) Bulk densities of the NCF aerogels. b, c) The HIUS-NCF aerogel exhibited high flexibility and could bereversibly bended (b) and twisted (c) without structural damage. d–f) The TMP-NCF aerogel (d) could bear a com-pression strain as high as ~99 % (e) and almost recover its original volume after release of the compression andimmersion in water (f). g) Water-uptake capability of the NCF aerogels. h) A HIUS-NCF aerogel was prepared to im-merse in a 10 mg L�1 MB solution; i) after immersion in the MB solution for several hours, the aerogel was saturat-ed with MB; j) the transparent purified water after removing MB with HIUS-NCF aerogel from the solution.

� 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7, 154 – 161 158

CHEMSUSCHEMFULL PAPERS www.chemsuschem.org

degradation.[43] The degradation of HSO-NCF aerogels startedat ~120 8C and occurred over a broader temperature range.

NCF aerogels have attracted attention as heat insulators. Toimprove the size stability of the HCl- and HSO-NCF aerogels forthermal conductive and sound absorption experiments, chito-san was introduced as a glue to connect the short NCFs priorto freeze-drying. The thermal conductivity of the aerogels wasdetermined using the transient hot-wire method. Figure 7 bshowed that all aerogels exhibited low thermal conductivitiesof <0.016 W m�1 K�1, which was lower than those of Styrofoam(0.030 W m�1 K�1),[44] polyurethane foam (0.026 W m�1 K�1),[44]

and cellulose–silica nanocomposite aerogels (>0.020 W m�1 K�1).[30] The low values corresponding to NCF aero-

gels could be ascribed to their low density (approximately0.005 g cm�3) and high porosity. When the aerogels were com-pacted with 4.9 N to increase the density and decrease the po-rosity, the thermal conductivity increased, but still remainedbelow 0.025 W m�1 K�1, indicating a high thermal insulationcharacter.

As a consequence of porous structures, NCF aerogels are ex-pected to have high sound absorption ability. Figure 7 c showsthe sound-absorption ratio of the aerogels at various frequen-cies. The absorption was poor at low frequencies, but im-proved quickly as the frequency increased. At approximately1000 Hz, the sound-absorption ratio of all aerogels was below20 %. At 4000 Hz, the sound-absorption ratio of the HIUS- andTMP-NCF aerogels reached approximately 57.1 % and 54.1 %,respectively, which was larger than those of wood-basedpanels such as particleboard, fiberboard, and plywood.[45] Theresults clearly show the sound-absorption ability of the NCFaerogels at high frequencies.

Conclusions

Four types of NCFs with different morphologies and surfaceproperties were prepared. The modulation of NCF types andsuspension concentrations over structures and properties ofthe aerogels was then clarified. When the concentration of thesuspension was <0.2 wt %, HIUS- and TMP-NCFs could befreeze-dried into long fibers that formed 3D-web-like struc-tures. When the concentration of the suspension was>0.5 wt %, the fibers self-aggregated into 2D-sheet-like struc-tures. However, the short HCl- and HSO-NCFs were freeze-dried into 2D-sheet-like structures regardless of the concentra-tion of the suspension. The as-prepared HIUS- and TMP-NCFaerogels were mechanically flexible and could be compressedwithout losing structural integrity. They also showed excellentwater-uptake ability and the capability of removing dye pollu-tants. All aerogels exhibited low thermal conductivity and highsound absorption capability at high frequencies.

Experimental Section

Materials : Poplar wood and cotton were used as native cellulosefibers. Benzene, ethanol, sodium chlorite, acetic acid, potassium hy-droxide, TEMPO, hydrochloric acid, sulfuric acid, and other labora-tory grade chemicals were used without further purification.

NCF preparation: HIUS-NCFs were prepared according to previouslydescribed methods[46–48] but with minor modifications. The poplarwood fibers after benzene/ethanol extraction were first subjectedto chemical pretreatments using acidified (with acetic acid toaround pH 5) sodium chlorite (75 8C for 1 h, 5 times), 2 wt % potas-sium hydroxide (room temperature, overnight), acidified sodiumchlorite (75 8C for 1 h, once), 6 wt % potassium hydroxide (90 8C,2 h), and 1 wt % hydrochloric acid solution (80 8C, 1 h), to removemost of the lignin and hemicellulose from the wood cell walls.Subsequently, the chemically purified fibers were disintegratedusing high intensity ultrasonication and HIUS-NCFs were obtained.TMP-NCFs were prepared through the TEMPO-mediated oxida-tion[49, 50] of the chemically purified cellulose fibers and the subse-quent nanofibrillation using HIUS treatment for 5 min at 1000 W.

Figure 7. a) TG curves, b) thermal conductivity, and c) sound-absorption ratioof NCF aerogels.

� 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7, 154 – 161 159

CHEMSUSCHEMFULL PAPERS www.chemsuschem.org

HCl-NCFs were prepared by treating the cotton fibers with~20 wt % aqueous HCl solution at 80 8C for 2 h. After washing withdistilled water, the resultant sample suspension was subjected toHIUS treatment for 5 min at 1000 W. HSO-NCFs were produced bymixing the cotton fibers with ~65 wt % of aqueous H2SO4 solution.The mixture was stirred at 45 8C for 2 h. The suspension was thendiluted and later thoroughly dialyzed against distilled water forseveral days until the pH value of the solution was around 7.0. Thesuspension was finally ultrasonicated for 5 min at 1000 W to yieldthe HSO-NCFs.

Preparation of NCF aerogels by freeze-drying : The obtained NCF sus-pensions were poured into molds and then placed in a refrigeratorat �18 8C for more than 24 h. Afterwards, the samples were sub-jected to freeze-drying using a freeze-dryer (Scientz-10N, NingboScientz Biotechnology Co., Ltd, China) to allow the frozen water inthe samples to sublime directly from the solid phase to the gasphase. During freeze-drying, the cold-trap temperature was below�55 8C and the vacuum was below 15 Pa.

Characterization techniques : An FEI Tecnai G2 electron microscopewas used for TEM imaging to characterize the morphology of theNCFs. The SEM images of the NCF aerogels were acquired usinga scanning electron microscope (SEM, Quanta200, FEI, USA) anda field-emission scanning electron microscope (FE-SEM, Sirion 200,FEI, Netherlands). The sample preparation and diameter detectionwere described in detail elsewhere.[46–48] FTIR spectra were record-ed on a Fourier transform infrared instrument (Magna 560, Nicolet,Thermo Electron Corp. , USA) in the range of 400–4000 cm�1 witha resolution of 4 cm�1. The dried NCFs were ground into powderusing a fiber microtome and then blended with KBr before beingpressed into ultra-thin pellets. XRD patterns of the NCFs were ob-tained using an X-ray diffractometer (D/max 2200, Rigaku, Japan)with Ni-filtered CuKa radiation (l= 1.5406 �) at 40 kV and 30 mA.Scattered radiation was detected in the range of 2 q= 58–408 ata scan rate of 18 per min. TGA was performed to compare the deg-radation characteristics of the NCF aerogels. The thermal stabilityof each sample was determined by using a thermogravimetric ana-lyzer (Pyris 6, PerkinElmer, USA) in a nitrogen environment ata heating rate of 10 8C min�1.Bulk density measurements : The bulk densities of the NCF aerogelswere determined from the dimensions and weights of the sam-ples.

Porosity : The porosities (P) of the NCF aerogels were calculated byusing the density of the NCF aerogels (da) and the density of thecrystalline cellulose nanofibers (dn�1.6 g cm�3) using Equation (1),which was obtained from the simple mixing rule with a negligiblegas density.

Pð%Þ ¼ ð1� da

dnÞ � 100% ð1Þ

Water uptake capability : The water uptake ratio (WUR) of the NCFaerogels was measured in distilled water at room temperature. Theaerogels were immersed in distilled water and allowed to reachthe swelling equilibrium. The aerogel weight was then recorded.The WUR of the aerogels was calculated using Equation (2):

WUR ¼ Ww �Wd

Wdð2Þ

where Ww is the weight of the wet aerogel after reaching the swel-ling equilibrium, and Wd is the weight of the dry aerogel beforebeing immersed in water.

Dye adsorption capacities : The weight of the NCF aerogel was mea-sured before immerging it in a 10 mg L�1 MB or TB solution. Afterseveral hours, MB- or TB-saturated aerogel was squeezed intoa small ball and then removed. The water adsorbed by the aerogelwas released and subsequently returned to the dye solution. Next,a new aerogel was immersed in the solution and then removedafter saturation with the dye. The process was repeated until thesolution became transparent and no peaks were observed in theUV/Vis spectrum of the solution. The adsorption capacity (AC) ofthe aerogels for the organic dye was calculated by using Equa-tion (3):

AC ¼ Wod

Wað3Þ

where Wod is the weight of the organic dye, and Wa is the totalweights of the dry NCF aerogels used for testing.

Thermal conductivity : The thermal conductivity of the NCF aerogelswas measured through the transient hot-wire method using a ther-mal conductivity tester (TC3020, Xi’an Xiatech Electronic Technolo-gy Co., Ltd, China).

Sound-absorption : The sound-absorption ratio was determined byusing the standing-wave-tube method according to 1/3 octave. Cy-lindrical aerogels with an approximate radius of 9.6 cm anda height of 1 cm were used for experiment. The experiment wasconducted on a sound-absorption testing equipment (AWA6122A,Hangzhou Aihua Instrument Co., Ltd. , China). Each value repre-sents the average of three samples.

Acknowledgements

This work was supported in part by the Fundamental ResearchFunds for the Central Universities (DL12DB01), the Program forNew Century Excellent Talents in University (NCET-10-0313), andthe National Natural Science Foundation of China (Nos.31270590 and 21203173).

Keywords: aerogels · biomass · nanocellulose ·nanostructures · self-aggregation

[1] D. Klemm, B. Heublein, H.-P. Fink, A. Bohn, Angew. Chem. 2005, 117,3422; Angew. Chem. Int. Ed. 2005, 44, 3358.

[2] R. J. Moon, A. Martini, J. Nairn, J. Simonsen, J. Youngblood, Chem. Soc.Rev. 2011, 40, 3941.

[3] Y. Habibi, L. A. Lucia, O. J. Rojas, Chem. Rev. 2010, 110, 3479.[4] D. Klemm, F. Kramer, S. Moritz, T. Lindstrçm, M. Ankerfors, D. Gray, A.

Dorris, Angew. Chem. 2011, 123, 5550; Angew. Chem. Int. Ed. 2011, 50,5438.

[5] S. Eichhorn, A. Dufresne, M. Aranguren, N. Marcovich, J. Capadona, S.Rowan, C. Weder, W. Thielemans, M. Roman, S. Renneckar, W. Gindl, S.Veigel, J. Keckes, H. Yano, K. Abe, M. Nogi, A. Nakagaito, A. Mangalam,J. Simonsen, A. Benight, A. Bismarck, L. Berglund, T. Peijs, J. Mater. Sci.2010, 45, 1.

[6] M. Nogi, S. Iwamoto, A. N. Nakagaito, H. Yano, Adv. Mater. 2009, 21,1595.

[7] M. Nogi, H. Yano, Adv. Mater. 2008, 20, 1849.

� 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7, 154 – 161 160

CHEMSUSCHEMFULL PAPERS www.chemsuschem.org

[8] H. Yano, J. Sugiyama, A. N. Nakagaito, M. Nogi, T. Matsuura, M. Hikita, K.Handa, Adv. Mater. 2005, 17, 153.

[9] J. R. Capadona, K. Shanmuganathan, D. J. Tyler, S. J. Rowan, C. Weder,Science 2008, 319, 1370.

[10] J. R. Capadona, O. Van Den Berg, L. A. Capadona, M. Schroeter, S. J.Rowan, D. J. Tyler, C. Weder, Nat. Nanotechnol. 2007, 2, 765.

[11] A. J. Svagan, M. A. S. A. Samir, L. A. Berglund, Adv. Mater. 2008, 20, 1263.[12] A. Walther, J. V. I. Timonen, I. D�ez, A. Laukkanen, O. Ikkala, Adv. Mater.

2011, 23, 2924.[13] K. E. Shopsowitz, H. Qi, W. Y. Hamad, M. J. MacLachlan, Nature 2010,

468, 422.[14] K. E. Shopsowitz, W. Y. Hamad, M. J. MacLachlan, Angew. Chem. 2011,

123, 11183; Angew. Chem. Int. Ed. 2011, 50, 10991.[15] L. Hu, N. Liu, M. Eskilsson, G. Zheng, J. McDonough, L. W�gberg, Y. Cui,

Nano Energy 2013, 2, 138.[16] L. Hu, G. Zheng, J. Yao, N. Liu, B. Weil, M. Eskilsson, E. Karabulut, Z.

Ruan, S. Fan, J. T. Bloking, M. D. McGehee, L. W�gberg, Y. Cui, Energy En-viron. Sci. 2013, 6, 513.

[17] A. C. Pierre, G. M. Pajonk, Chem. Rev. 2002, 102, 4243.[18] N. H�sing, U. Schubert, Angew. Chem. 1998, 110, 22; Angew. Chem. Int.

Ed. 1998, 37, 23.[19] C. Moreno-Castilla, F. J. Maldonado-H�dar, Carbon 2005, 43, 455.[20] J. Biener, M. Stadermann, M. Suss, M. A. Worsley, M. M. Biener, K. A.

Rose, T. F. Baumann, Energy Environ. Sci. 2011, 4, 656.[21] X. Gui, J. Wei, K. Wang, A. Cao, H. Zhu, Y. Jia, Q. Shu, D. Wu, Adv. Mater.

2010, 22, 617.[22] K. H. Kim, M. Vural, M. F. Islam, Adv. Mater. 2011, 23, 2865.[23] J. Zou, J. Liu, A. S. Karakoti, A. Kumar, D. Joung, Q. Li, S. I. Khondaker, S.

Seal, L. Zhai, ACS Nano 2010, 4, 7293.[24] K. H. Kim, Y. Oh, M. F. Islam, Nat. Nanotechnol. 2012, 7, 562.[25] H. Liang, Q. Guan, L. Chen, Z. Zhu, W. Zhang, S. Yu, Angew. Chem. 2012,

124, 5191; Angew. Chem. Int. Ed. 2012, 51, 5101.[26] Z. Xu, Y. Zhang, P. Li, C. Gao, ACS Nano 2012, 6, 7103.[27] S. Yin, Z. Niu, X. Chen, Small 2012, 8, 2458.[28] C. Tan, B. M. Fung, J. K. Newman, C. Vu, Adv. Mater. 2001, 13, 644.[29] J. Cai, S. Kimura, M. Wada, S. Kuga, L. Zhang, ChemSusChem 2008, 1,

149.[30] J. Cai, S. Liu, J. Feng, S. Kimura, M. Wada, S. Kuga, L. Zhang, Angew.

Chem. 2012, 124, 2118; Angew. Chem. Int. Ed. 2012, 51, 2076.

[31] W. Chen, H. Yu, Q. Li, Y. Liu, J. Li, Soft Matter 2011, 7, 10360.[32] M. Pkkç, J. Vapaavuori, R. Silvennoinen, H. Kosonen, M. Ankerfors, T.

Lindstrçm, L. A. Berglund, O. Ikkala, Soft Matter 2008, 4, 2492.[33] J. T. Korhonen, P. Hiekkataipale, J. Malm, M. Karppinen, O. Ikkala, R. H. A.

Ras, ACS Nano 2011, 5, 1967.[34] R. T. Olsson, M. A. S. Azizi Samir, G. Salazar-Alvarez, L. Belova, V. Strçm,

L. A. Berglund, O. Ikkala, J. Nogus, U. W. Gedde, Nat. Nanotechnol.2010, 5, 584.

[35] H. Liang, Q. Guan, Z. Zhu, L. Song, H. Yao, X. Lei, S. Yu, NPG Asia Mater.2012, 4, e19.

[36] M. Kettunen, R. J. Silvennoinen, N. Houbenov, A. Nyknen, J. Ruokolai-nen, J. Sainio, V. Pore, M. Kemell, M. Ankerfors, T. Lindstrçm, M. Ritala,R. H. A. Ras, O. Ikkala, Adv. Funct. Mater. 2011, 21, 510.

[37] Z.-Y. Wu, C. Li, H. W. Liang, J. F. Chen, S. H. Yu, Angew. Chem. 2013, 125,2997; Angew. Chem. Int. Ed. 2013, 52, 2925.

[38] T. Saito, T. Uematsu, S. Kimura, T. Enomae, A. Isogai, Soft Matter 2011, 7,8804.

[39] M. D. Gawryla, O. van den Berg, C. Weder, D. A. Schiraldi, J. Mater. Chem.2009, 19, 2118.

[40] L. Heath, W. Thielemans, Green Chem. 2010, 12, 1448.[41] A. Isogai, T. Saito, H. Fukuzumi, Nanoscale 2011, 3, 71.[42] H. Fukuzumi, T. Saito, T. Iwata, Y. Kumamoto, A. Isogai, Biomacromole-

cules 2009, 10, 162.[43] M. Roman, W. T. Winter, Biomacromolecules 2004, 5, 1671.[44] D. R. Lide, CRC Handbook of Chemistry and Physics, 84th ed. , CRC Press,

Boca Raton, FL, 2003, pp. 12/226 – 12/227.[45] H. S. Yang, D. J. Kim, H. J. Kim, Bioresour. Technol. 2003, 86, 117.[46] W. Chen, H. Yu, Y. Liu, Carbohydr. Polym. 2011, 86, 453.[47] W. Chen, H. Yu, Y. Liu, P. Chen, M. Zhang, Y. Hai, Carbohydr. Polym. 2011,

83, 1804.[48] W. Chen, H. Yu, Y. Liu, Y. Hai, M. Zhang, P. Chen, Cellulose 2011, 18, 433.[49] T. Saito, S. Kimura, Y. Nishiyama, A. Isogai, Biomacromolecules 2007, 8,

2485.[50] T. Saito, Y. Nishiyama, J.-L. Putaux, M. Vignon, A. Isogai, Biomacromole-

cules 2006, 7, 1687.

Received: September 5, 2013Revised: October 28, 2013

� 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7, 154 – 161 161

CHEMSUSCHEMFULL PAPERS www.chemsuschem.org