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Cite this: Nanoscale, 2020, 12, 1374

Received 18th October 2019,Accepted 3rd December 2019

DOI: 10.1039/c9nr08918g

rsc.li/nanoscale

Ultra-tough and highly ordered macroscopic fiberassembly from 2D functional metal oxidenanosheet liquid crystals and strong ionicinterlayer bridging†

Yalei Wang,a Yuanchuan Zheng,a Li Sheng, a Jiupeng Zhao *a and Yao Li *b

Macroscopic assembly of 2D nanomaterials, especially for the one-

dimensional macroscopic ordered fiber assembly from 2D liquid

crystals (LCs), is rising to an unprecedented height and will con-

tinue to be an important topic in materials. However, this case of

2D functional metal oxide nanosheets is quite challenging. For the

first time, the high-performance tungstate macroscopic fiber has

been realized through an LC wet-spinning process involving the

formation of LC colloid with spinnability and performance

improvement by interlayer bridging in macroscopic assembly. The

resultant macroscopic fiber yields record high tensile strength

(198.5 MPa) and fracture toughness (3.0 MJ m−3) owing to their

highly ordered structure and strong ionic interlayer bridging.

Despite the intrinsically weak mechanical strength of the

nanosheets, with only a few percent of graphene, the fibers mani-

fest mechanical properties comparable to that of graphene fibers.

Inspired by this concept, the possible macroscopic fibers

assembled from other 2D functional metal oxide nanosheets will

become a reality in the near future, holding great promise in aero-

space and wearable applications.

Introduction

Liquid crystals (LCs), a thermodynamically stable meso-morphic state of anisotropic nanomaterials, have been widelyemployed as building blocks for macroscopic materials due totheir liquid-like fluidity and crystal-like ordering.1–4 In thesematerials, the macroscopic ordered fibers are highlighted asthe optimal structure to translate the unique merits of individ-ual components from the nanoscopic to the macroscopic

scale, holding great promising prospects in practicalapplications.5–8 For example, the graphene fibers assembledfrom graphene oxide LCs have proved remarkable potential inmultifunctional materials and devices within a fairly short timesince their production.9–14 However, the case of 2D functionalmetal oxides remains a considerable challenge, especially forthe 2D tungstate nanosheets, which are running into abooming development by virtue of their fascinating photochro-mic, electrochromic, and energy storage functions.15–18

Realizing their macroscopic ordered fibers will be the masterkey to open the gate of the promising applications.

According to the research on graphene, the formation of LCcolloid with spinnability is supposed to be the first prerequi-site for macroscopic assembly, which requires the nanosheetsto have satisfactory structural anisotropy and high solvent dis-persibility under the influence of thermal fluctuation.19,20 Sofar, Sasaki et al. have prepared titania nanosheet LCs21,22 andthen Geng et al. assembled them into macroscopic fibers,23,24

which was inspiring in this context and had also triggered thedevelopment of MoS2 nanosheet LCs and their macroscopicfibers.25 As mentioned above, developing the 2D LC colloid isindeed of significant interest in scientific and technologicalfields, which will offer a practical route to design and fabricatethe desired macroscopic architecture. However, the formationof LC colloid with spinnability for such 2D functionalnanosheets suffers from a series of challenges: first, the poly-dispersity in the lateral size and large thickness may causeimpediments in the control of structural anisotropy; second,the low entropy increment for mixing will lead to poor solutiondispersibility; lastly and arguably the most important, theionic impurities introduced in the preparation will destroy thethermodynamic stability of the charge-bearingnanosheets.26–28 On the other hand, the mechanical propertiesof macroscopic fibers are also key in promoting their practicalapplications. It is noteworthy that the mechanical propertiesof the above-mentioned fibers are still far inferior to the ideallevel, which show moderate strength (70–160 MPa) and rela-tively lower toughness (0.5–1.5 MJ m−3), thus falling short of

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr08918g

aMIIT Key Laboratory of Critical Materials Technology for New Energy Conversion

and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of

Technology, Harbin 150001, China. E-mail: jpzhao@hit.edu.cnbCenter for Composite Materials and Structure, Harbin Institute of Technology,

Harbin 150001, China. E-mail: yaoli@hit.edu.cn

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the impressive performance in comparison with graphenefibers.23–25,29–32 The weak inherent mechanical strength of thenanosheets (only a few percent of that for graphene) and theinsufficient interlayer bridging (e.g., van der Waals attractionsand hydrogen bonding) should be responsible, which is also amajor challenge for macroscopic assembly.

Herein, we present the 2D tungstate nanosheets LCs andtheir macroscopic assembled fibers for the first time. Thestable LC colloid has been achieved by the exfoliation oflayered tungstate crystals via an osmotic swelling process,which ensures the determining factors of excellent dispersibil-ity and spinnability for macroscopic fiber assembly. The highlyordered macroscopic fibers have been assembled by the indus-trial wet-spinning process from the LC colloid that possesses along-range ordered structure. Considering the inferiormechanical properties, an improvement measure of ionicinterlayer bridging is also proposed in the meantime. Duringthe macroscopic assembly process, the ionic bond will be builtup at the interface of the surface oxygen atom and protonatedamino group based on the strong electrostatic interactions,which will fix the lamellar orientation structure of the LCcolloid while also enhancing the interlayer bridging. Asexpected, the macroscopic fibers manifest record high strength(198.5 MPa) and toughness (3.0 MJ m−3), which is comparableto that of graphene fibers despite the low inherent mechanicalstrength of the molecularly thin nanosheets. In this work, wehave shed light on the formation of LC colloid with spinnabil-ity as well as the mechanical properties’ improvement by ionicinterlayer bridging in macroscopic assembly, which will lay afoundation to realize macroscopic fibers from other 2D func-tional nanosheets. In the future, these emerging macroscopicfibers and their derived structures might possibly target aero-space and intelligent wearable devices.

Results and discussionSynthesis, structure, and stability of the 2D tungstate LCcolloid

The formation of LC colloid with spinnability is the first andforemost condition for macroscopic fiber assembly, which ishighly dependent on the colloid stability and structural an-isotropy of the nanosheets.20,33 For this system, the dispersionstability of the 2D charge-bearing nanosheets is critically con-trolled by electrostatic repulsive force; however, the contri-bution of electrostatic field is not strong enough to competewith the screening effect brought by the increasing ionicstrength and pH value, which will sharply decrease the Debyelength of the electrical double layer.34,35 In other words, theionic impurities will severely impact the dispersibility andeven lead to the flocculation of the nanosheets. Unfortunately,in the general production of 2D nanosheets, the strongly cor-rosive concentrated organic alkali is overused, such as tetra-methylammonium hydroxide (TMAOH) and tetrabutyl-ammonium hydroxide (TBAOH), which inevitably introducesexcessive ionic impurities that seriously affect the dispersion

stability of the nanosheets, thus failing in the formation of theLC phase.15,16 In this work, we employed an osmotic swellingprocess of water molecules to realize the stable and spinnableLC colloid, and the corresponding schematic illustration isprovided in Fig. 1a.

The layered tungstate crystals with the composition ofCs6+xW11O36 have been prepared by a high-temperature sinter-ing process,16,36,37 which consist of repeatedly stacked tung-state layers (Cs4W11O36

2−) and a slight excess of Cs+ ions in theinterlayer gallery. As shown in the scanning electron micro-scopic (SEM) images (Fig. S1†), the tungstate crystals possess aplate-type structure with the average lateral size of 50–75 μmand the thickness of 10–20 μm. As shown in Fig. S2a and b,†the X-ray photoelectron spectra (XPS) exhibit two signals ofCs–O at about 528.5 and 530.2 eV, which confirms that Cselement has two different forms in the crystals. Moreover, theinductively coupled plasma (ICP) analysis indicates that thetotal content of Cs in the formula is 6.27, which may becaused by the partial reduction of W6+. In the protonation, theCs+ ions in the interlayer gallery are replaced by H+ ions,which can be proved by XPS, as shown in Fig. S2c and d,†where the signal of Cs–O at 530.2 eV disappeared after thisprocess. On the other hand, the appearance color of the crys-tals changed from dark green to greyish white, which suggeststhat the reduced W6+ is oxidized completely in the protonationprocess. Therefore, the formula of the resultant crystals can becalculated as H2Cs4W11O36, namely the protonated products. Itis noteworthy that the protonated products exhibit a left shiftin the (006) diffraction peak in the X-ray diffraction (XRD)pattern (Fig. S3†) and the expansion of the interlayer distanceis estimated to be 4.0 Å. The expansion is also confirmed bySEM images (Fig. S4†), displaying obvious interlayer slippages.As the expansion is consistent with the size of H2O, the watermolecules intercalate into the gallery to expand the interlayerspacing during protonation, which significantly weakens theinteraction force between the layers and facilitates the follow-ing delamination. Then, the protonated products are trans-ferred into a TBAOH solution to react for as short as 30 min tounclench the ‘gate’ of the interlayer gallery, in contrast to theexceptionally long reaction time (over 10 days) in the generalproduction.15,16,38 After that, the excessive TBAOH is removedand the resulting crystals are re-dispersed in water. Owing tothe osmotic swelling, plenty of water molecules are introducedinto the interlayer gallery. Finally, accompanying the mechani-cal shaking, the crystals result in significant delamination andproduce the tungstate colloid with a milky appearance of an-isotropic shiny textures (Fig. S5†). Due to the dispersion oftungstate nanosheets in the colloid, a typical Tyndall scatter-ing of laser light can be clearly observed in Fig. 1b.Satisfyingly, the tungstate colloid exhibits excellent dispersionstability; no precipitate can be observed in the preparedcolloid even after being placed for 60 h (Fig. S6†). This resultsuggests the successful delamination of tungstate nanosheetsfrom the layered tungstate crystals and the feasibility foremploying the tungstate colloid as a stable LC spinning dope.As shown in Fig. S7,† the macroscopic photographs of the pre-

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pared tungstate colloid between crossed polarizers exhibitbright birefringence after being placed at room temperaturefor one week, which could suggest the formation of a lyotropicLCs phase.39

Furthermore, the geometric structure of the tungstatenanosheets was characterized by atomic force microscopy(AFM), as shown in Fig. 1c. The average thickness of thenanosheets is about 2.0 nm, which can be estimated as thesum value of the thickness of the Cs4W11O36

2− host layer(Fig. S8†) and the size of the surface adsorbed ions. As shownin Fig. S9,† the lateral size of the nanosheets is estimated to be800 nm, which suggests that the nanosheets possess a huge an-isotropy ratio (over 400). The transmission electron microscopic(TEM) image of the tungstate nanosheets reveal an almost trans-parent 2D structure, confirming their molecularly thin thick-ness (Fig. 1d). The corresponding selected area electron diffrac-tion (SAED) pattern displays only one set of diffraction spots,which can be indexed as h k reflections of a 2D hexagonal unitcell of a = 0.73 nm (the known bond length of Cs–Cs in tung-state), indicating that the in-plane crystallographic structure ismaintained completely during the delamination process(Fig. 1e). In other words, as the formation of the LC colloid withspinnability is primarily dependent upon the dispersion stabi-lity and structural anisotropy under sufficient density of thenanosheets,2,3,12 such a stable 2D tungstate colloid shouldsatisfy the prerequisite for macroscopic fiber assembly.

Formation and evolution of 2D tungstate LC colloid

Fig. 2a–h show the typical time sequence polarized opticalmicroscope (POM) images of the tungstate colloid under

crossed polarizers, which manifests the birefringent texturesof the LC phase that increase with time. Herein, the left brightportion represents the birefringent LCs phase, whereas theright dark portion represents the isotropic phase, whichdemonstrates the dynamic flip and assembly process of thenanosheets, thus proving successful formation of the tungstateLC colloid unambiguously (Movie S1†). Furthermore, the con-tinuous interaction and directed rotation of the anisotropicnanosheets is also observed in the high-resolution timesequence POM images and the movie of the dynamic self-assembly process (Fig. S10 and Movie S2†). Interestingly,Fig. S11† shows the POM image of tungstate dispersion pre-pared by the general method, where almost no textures can beobserved under the same conditions. The dark POM image isascribed to the optically isotropic states, meaning that thetungstate nanosheets are simply dispersed in the dissolventwithout any particular orientation or orderly self-assemblystructures. Both phenomena, i.e., the lyotropic LC phase andthe isotropic phase, could be possibly caused by the exfoliationprocess. For the general method, electrostatic repulsion is notenough to overcome the ionic screening effect upon theincrease of density and the very intense delamination byorganic alkali should severely reduce the structural anisotropyof the nanosheets, which will ultimately lead to the isotropicphase. In this work, the gentle swelling exfoliation by watermolecules has successfully prevented the influence of ionicscreening effect for the eventual stability of nanosheets whileguaranteeing their well-controlled shape, structure, and ulti-mate 2D anisotropy. In other words, the electrostatic repulsionwill balance the attraction force between the anisotropic 2D

Fig. 1 Preparation and characterization of the 2D tungstate LC colloid. (a) Schematic illustration for the preparation process of 2D tungstate LCcolloid. (b) Photograph of the nanosheet colloid showing the Tyndall scattering effect. (c) Representative AFM image of the nanosheets, the inset isthe corresponding thickness profile. (d) TEM image and (e) SAED pattern of the nanosheets.

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nanosheets, which is supposed to play a crucial role in the for-mation of LC colloid.

The evolution of the LC phases along with the volume frac-tion (φ) of the nanosheets was also investigated by POM. Asshown in Fig. 2i, the POM image displays isolated and thread-like birefringence textures, declaring the preliminary for-mation of the nematic phase, that is to say, the anisotropiccolloid starts to transit from the isotropic phase to the nematicphase at a volume fraction as low as 0.5 vol%. Upon increasingthe volume fraction to 1.0 vol% (Fig. 2j) and 1.5 vol% (Fig. 2k),the more orderly and vivid birefringence textures expand tomost regions of the LC colloid, which is the typical texture ofthe evolutive nematic phase. This growing biphasic mixture ofthe isotropic and nematic phases indicates that the degree oforientation in the LC colloid will increase with the increase inthe volume fraction of the nanosheets. Eventually, the bright-ness and homogeneous birefringence textures are observed inthe whole LC colloid at a volume fraction of 2.0 vol% (Fig. 2l),implying the unified orientation ordering in the LC colloidand the formation of the nematic phase. This evolutive behav-ior could be explained by the excluded volume effect.40 At highvolume fraction, the degree of freedom for the anisotropicnanosheets is restricted due to the overlap of excluded volume,which leads to the loss of orientation entropy. Synchronously,in order to maintain the thermodynamic stability, thenanosheets must be orientationally aligned to increase thetranslational freedom and total entropy of this system. Inother words, the formation of the nematic phase will introducea gain in the translational entropy to compensate for the lossof orientational entropy, which is reasonable in the second lawof thermodynamics. According to Onsager theory, the above

volume fraction could be estimated by the empirical equation(φ ≈ 4T/L), where φ is the critical volume fraction, and T and Ldenote the thickness and lateral size of the nanosheets,respectively.2,41 Therefore, the empirical value of φ is calcu-lated to be about 1.0 vol% for this system. Above this criticalvolume fraction, the overlap of excluded volumes will drive thenanosheets to orient parallelly to each other, minimize theexcluded volume, maximize the total entropy, and ultimatelyform the nematic phase. Moreover, the low structural an-isotropy will claim an unreachable critical volume fraction onthe premise of dispersion stability, which also explains why itis difficult to form a stable LC colloid through generalnanosheet dispersion and why it does not possess spinnabil-ity. Therefore, the fascinating 2D nematic LC colloid in thiswork should lay the foundation for macroscopic fiberassembly.

Macroscopic assembly and structure of tungstate fibers

The wet-spinning technique was employed to assemble themacroscopic fibers from 2D LC colloid, as illustrated inFig. 3a. The LC colloid as the spinning dope was preloadedinto a syringe pump and slowly injected into a cationic baththrough the uniaxial spinning channel. When the negativelycharged nanosheets contacted with the electropositive ionicbonding agents, the LC flow would transform into a gel fiberwithin a very short space of time. After the washing and dryingprocesses, the macroscopic fiber was obtained, which could beconvolved on a metal spool (Fig. 3f). The assembly process canbe divided into three stages: (I) orientation along the spinningchannel, (II) fixation of the long-range ordering structure byelectrostatic interactions, and (III) drying and compaction of

Fig. 2 POM observations of the tungstate LC colloid. (a–h) Time sequence POM images of the LC colloid with increasing time of 1, 3, 5, 7, 9, 11, 13,and 15 min, respectively. (i–l) POM images of the LC colloid with increasing φ of 0.5, 1.0, 1.5, and 2.0 vol%, respectively. P and A represent the direc-tions of the polarizer and the analyzer, respectively.

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the fibers by water evaporation. In the first stage (I), stable andhomogeneous birefringence of the tungstate LC colloid can beobserved between crossed polarizers regardless of the disclina-tions (Fig. 3b–e), which suggests that the tungstate nanosheetscould orient to a long-range ordered structure along the spin-ning channel, thus providing a powerful support for the fol-lowing macroscopic assembly.

Considering the high surface charge density of thenanosheets, the ionic bond base on electrostatic interaction isprompt to fix the ordered structure as well as to enhance theirinterlayer bridging within the macroscopic fibers. Thus, in thesecond stage (II), the LC flow will transform into a self-stand-ing gel fiber instantly due to charge neutralization between thenanosheets and ionic bonding agents. Various bonding agentsincluding metal cations with diverse valences (i.e., Na+, K+,Ca2+, Ni2+, Mg2+, and Fe3+) and macromolecules with diversegeometries (i.e., polyvinyl alcohol (PVA) and chitosan inaqueous acetic acid) were used to induce transition from thelamellar structure into the fiber (Fig. S12†). Among them, theprotonated chitosan will be the best fit candidate, whichshould be beneficial in achieving the optimum structure ofhighly ordered sheet stacking because of its particular straight-chain structure.23,24 In this work, we also propose that the pro-tonated amino (–NH3

+) group will contribute in enhancing theinterlayer bridging within the fibers simultaneously based onthe formation of the ionic bond. In the last stage (III), the gel-state fiber will shrink significantly during the drying processof solvent evaporation, according to previous studies.23,24

Therefore, the gel fiber will dry on a shelf under axial tensionto preserve the regular alignment of the nanosheets along thefiber axis. The continuous evaporation will allow shrinkingalong the radial direction of the fiber. Finally, the dried macro-scopic fibers will be obtained with a compact sheet-on-sheetstacking structure. As shown in Fig. 3g–i, the dried fiber exhi-bits strong and homogeneous birefringence at 0, 45, and 90degrees under the crossed polarizers, which suggests that thetungstate fiber possesses regular alignment structures byinheriting it from the long-range ordering of the LC colloid inthe macroscopic assembly process.

The typical SEM images of the fiber surface show awrinkled structure along the fiber axis, which is supposedlycaused by water evaporation during the drying process(Fig. 4a–c). The fiber cross-section presents a ribbon mor-phology with an aligned sheet-on-sheet stacking structure,which evidences the structural evolution from the LC colloidinto macroscopic fibers (Fig. 4d and e). Moreover, these resultsalso indicate that the fibers are more likely to form a flatribbon composed of parallelly stacked nanosheets in contrastwith the internal folded structure in graphene fibers due tothe inherent rigidity of the tungstate nanosheets. As shown inFig. 4f, the high-resolution transmission electron microscopic(HRTEM) image of the ultramicrocut for the fiber cross-section(details in Fig. S13†) shows a compact stacking structure witha periodic stacking thickness of about 2.0 nm, which indicatesthat the nanosheets are orderly stacked in the fiber. The TEMimage of the ultramicrocut along the fiber axis demonstrates a

Fig. 3 Schematic for the macroscopic assembly of tungstate fibers. (a) Illustration for the assembly process of macroscopic fibers. (b–e) POMimages of the LC colloid within a uniaxial channel. (f ) Photograph of the tungstate fibers. (g–i) POM images of the tungstate fibers at 0, 45, and 90degrees. P and A represent the directions of polarizer and analyzer, respectively.

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clear step profile (Fig. 4g), which also evidences the orderedstacking structure. Also, the corresponding SAED pattern(Fig. 4h) displays a suit of diffraction spots similar to that forthe nanosheet, which signifies that the nanosheets possiblyalign in a crystallographic assembly strategy, suggesting thatthe long-range ordered structure of the LC colloid should befixed completely within the fiber. In addition, the fiber cantwist with other fibers (a Cu line) without any noticeablecracks, from which we can speculate that the fibers maypossess excellent mechanical properties (Fig. 4i).

Strong ionic interlayer bridging within tungstate fibers

The XRD pattern of the macroscopic fibers shows the typicalbasal reflections of a lamellar structure, while all the peakscan be indexed to the c-axis of the tungstate nanosheet plane,which indicates that the nanosheets are highly stacked in thedirection of c-axis within the fiber (Fig. 5a). The basal spacingis estimated to be 2.73 nm, which is supposed to be the sumof the thickness of the nanosheets (2.0 nm) and the size ofchitosan (the maximum width and length of the chitosanmonomer are 0.492 and 0.773 nm, respectively).23 Accordingly,it is reasonable to assume that the straight-chain chitosan islaid flat in the stacking spacing between contiguousnanosheets and links with them through the protonatedamino group based on electrostatic interactions. The interlayerbridging in the fibers was examined by Fourier transforminfrared spectroscopy (FTIR). The absorption bands in the

range of 850–1250 cm−1 corresponding to saccharine ringstructure exhibit no significant difference between the fibersand chitosan (Fig. 5b), and the energy dispersive spectroscopy(EDS) element mapping analysis manifests that C and Nelements are uniformly distributed in the fibers (Fig. S14†).These results confirm that the fibers are composed of tung-state nanosheets and chitosan. The absorption peak of –NH2

that is located at 1597.5 cm−1 in the chitosan shifted to1557.5 cm−1 in the fibers, which suggests that protonated chit-osan interacts with the nanosheets via the –NH3

+ group. XPSwas also performed to investigate this interlayer bridging. Inthe survey spectrum, the signals corresponding to C and Nelements are detected in addition to Cs, W, and O elements,which verifies the existence of chitosan in the fibers (Fig. 5c).The N 1s spectrum shows a visible peak located at 401 eV,which can be assigned to the –NH3

+ group (Fig. 5d). Also, theO 1s spectrum shows a peak located at 532 eV, which isattributable to O–NH3

+ (Fig. 5e). This is a strong indicationthat the interlayer bridging of ionic bond is possibly derivedfrom the –NH3

+ group by electrostatic interaction. Thus, thefiber is more likely to be an ionic compound with a fixedformula rather than a simple composite with two individualcomponents.

As shown in Fig. 5f, the thermogravimetric (TG) curve ofthe fibers could be divided into two stages, including theremoval of water and decomposition of chitosan. Accordingly,the formula of the fiber can be estimated as (Cs4W11O36

2−)

Fig. 4 Morphology and structural characterization of the fibers. (a–c) SEM images for the fiber surface with different magnifications. (d and e) SEMimages for the fiber cross-section. (f ) HRTEM image for fiber cross-section. (g) TEM image and (h) SAED pattern of ultramicrocut along the fiberaxis. (i) SEM image of the twisted fibers.

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(C6H12NO4+)1.88(H

+)0.1212.45H2O, which indicates that nearlyall the negative charge of the nanosheets will be neutralized byprotonated chitosan through the formation of ionic bond,thus implying strong interlayer bridging within the fibers. Thecharge density difference was calculated to investigate thenature of interlayer bridging. As shown in Fig. 5g, the chargedensity difference suggests that the ionic bond will form at theinterface region between the nanosheets and protonated chito-san. The corresponding charge density difference 2D slice(Fig. S15†) shows two intensive charge centers, namely thenegative charge center around the surface oxygen atom whilethe positive charge center is around the –NH3

+ group, implyingthat the formation of ionic bond is derived from electrostaticinteraction between the surface oxygen atom and protonatedamino group. Moreover, the bonding situation between the

two contiguous nanosheets was also investigated, as shown inFig. 5h and i. The charge density difference and its corres-ponding 2D slice clearly indicate that the protonated chitosanwill link with the contiguous nanosheets by ionic bonds simul-taneously. As discussed above, the ionic bridging contributesto fix the highly ordered stacking structure and will improvethe mechanical properties of the fibers.

Mechanical properties of tungstate fibers

The typical tensile stress–strain curve of tungstate fiber isshown in Fig. 6a. The tungstate fiber provides the maximumtensile strength of 198.5 MPa with fracture elongation of 3.6%.Also, the toughness of the tungstate fiber is substantiallyimproved to 3.0 MJ m−3 (Fig. 6b). In terms of fracture mor-phology, the tungstate fiber also shows typical characteristics

Fig. 5 Interlayer bridging analysis. (a) XRD pattern of the fibers. (b) FTIR spectra for the fibers and chitosan. (c) Survey scan XPS profile of the fibers.(d and e) XPS spectra of N 1s and O 1s for the fibers. (f ) TG curves for the fibers and chitosan. (g) Charge density difference with iso-surface level of0.003 e Å−3 for the model of –NH3

+ group on a single tungstate nanosheet, and (h) charge density difference with iso-surface level of 0.005 e Å−3

for the model of protonated chitosan molecules between two contiguous nanosheets. The yellow region represents the charge accumulation andthe light blue region represents the charge depletion. (i) Corresponding charge density difference 2D slice for the model in (h), through the co-planeof the amino nitrogen and surface oxygen. The color bar represents the charge accumulation and depletion.

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of ductile fracture with full curled edges and irregular pulled-out structures (Fig. 6c). Moreover, the tungstate fiber can loadtwo weights (total of 30 g) without any cracks or breakage(Fig. 6d) and can be bent into an arbitrary angle as well as canbe weaved into a cotton network in an English abbreviation(“HIT”, Harbin Institute of Technology) and a Chinese charac-ter (‘ ’, Hagongda), as shown in Fig. S16.† Theseresults could likely be explained due to the highly orderedstacking alignment and strong interlayer bridging in fibers. Inaddition, we also compared the mechanical properties of thetungstate fibers and typical materials reported previously, as

shown in Fig. 6e. Although the inherent strength of the tung-state nanosheets is perhaps only a few percent due to gra-phene, the tungstate fibers exhibit comparable and even sur-passing mechanical properties to previously reportedmaterials, including nacre (80–135 MPa, 1.8 MJ m−3),42 carbonnanotube (CNT) fibers (150 MPa, 2.25 MJ m−3),5 grapheneoxides (GO) fibers (198 MPa, 2.4 MJ m−3),43 GO fibers (102MPa, 3.5 MJ m−3),12 GO fibers (260 MPa, 3.9 MJ m−3),44 andreduced GO (rGO) fibers (140 MPa, 4.0 MJ m−3).12

Based on their excellent mechanical properties, theimprovement mechanism can be proposed as follows. In the

Fig. 6 Mechanical properties of the fibers. (a) Typical stress–strain curve of the tungstate fiber and the circular dots denote the ultimate tensilestrength and corresponding elongation for the titania fiber (blue) and MoS2 fiber (yellow), respectively. (b) Comparisons of toughness and tensilestrength for the fibers, including the titania fiber (ref. 23) and MoS2 fiber (ref. 25), and the tungstate fiber (this work). (c) SEM image of the fracturedcross-section for the tungstate fiber. (d) Photograph of the tungstate fiber supporting two weights. (e) Comparisons of mechanical propertiesbetween the tungstate fiber and typical materials reported previously. (f ) Typical stress–strain curves of the tungstate fibers after structural stabilitytesting.

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beginning, the stretching stress will be loaded and distributedevenly on the highly stacked nanosheets, which avoids thebreak up arising from stress concentration. Then, the increas-ing stress will be transferred from the nanosheets into chito-san molecules through ionic bridging, thus retaining theirstructural stability. During this process, the bridging will effec-tively prevent the rotation and breakage of nanosheets untilthe chitosan molecules have been straightened completelyalong the tensile direction. Finally, upon increasing the stress,the ionic bridging and highly stacked structure will bedestroyed, thus ultimately leading to the fracture of fibers. Inother words, despite of the low inherent strength of the mole-cularly thin nanosheets, the highly ordered stacking structureand strong ionic bridging will endow their macroscopicassembled fibers with an impressive mechanical property.

Additionally, the structural stability of the tungstate fiberswas investigated under different conditions. As shown inFig. S17,† the morphologies and structures of the fibers showno obvious damages or cracks after treatments with sulfuricacid solution (1 mol L−1, soaking for 3 days), sodium hydrox-ide solution (1 mol L−1, soaking for 3 days), and ultra-sonication in water (100 W, 40 kHz, for 1 hour), respectively.Also, their corresponding tensile strengths also manifestnearly no change before and after these treatments (Fig. 6f),suggesting that the fibers possess outstanding resistance tostructural failure in corrosive solutions and ultrasonic dis-solution. Moreover, a burning test has also been carried outfor the tungstate fiber. As displayed in Movie S3 and Fig. S18,†the fiber possesses excellent flame retardation, in contrast tothe conventional fibers (polymer or cotton) that are easilyflammable in fire.

Conclusions

In this study, we have creatively realized the 2D tungstate LCcolloid and its macroscopic assembled fiber for the first time.The innovative exfoliation of osmotic swelling ensured the for-mation of LC colloid with spinnability, which can be employedas a stable spinning dope for the construction of highlyordered macroscopic fiber. During the assembly process bywet-spinning, strong ionic bonds were introduced to fix theregular alignment structure of the LC flow and also enhancedthe interlayer bridging between the nanosheets. As expected,the resultant macroscopic fibers manifest mechanical pro-perties comparable to that of graphene fibers, regardless oftheir weak inherent mechanical strength. After this enlighten-ing work, we believe that the possible macroscopic fibersassembled from other 2D functional metal oxide nanosheetswill be achieved, which can have remarkable potential appli-cations in aerospace and wearable devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (no. 51572058, 51761135123),National Key Research & Development Program(2016YFB0303903, 2016YFE0201600), the International Science& Technology Cooperation Program of China (2013DFR10630,2015DFE52770), and Foundation of Equipment DevelopmentDepartment (6220914010901).

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