2014-carbon IF=6.33 Synthesis and failure behavior of super-aligned

Synthesis and failure behavior of super-aligned carbon nanotube ?lm wrapped graphene

?bers

Fancheng Meng a ,Ru Li b ,Qingwen Li b ,Weibang Lu b ,Tsu-Wei Chou

a ,*

a Department of Mechanical Engineering,University of Delaware,Newark,DE 19716,USA

b

Suzhou Institute of Nano-Tech and Nano-Bionics,Chinese Academy of Sciences,Ruoshui Road 398,Suzhou 215123,China

A R T I C L E I N F O

Article history:

Received 4December 2013Accepted 31January 2014Available online 7February 2014

A B S T R A C T

Signi?cant progress has been made in recent years in research on wet spinning and hydro-thermal synthesis of graphene ?bers.In this paper,we report the relationship between the mechanical performance of graphene oxide (GO)?bers and their processing parameters in wet spinning.With super-aligned carbon nanotube (CNT)?lm wrapping,the speci?c strength and electrical conductivity of reduced GO (rGO)?bers were simultaneously enhanced by 22%and 49%,respectively.Thicker CNT ?lm wrapping of the rGO ?ber induces a core–sheath structure;the resulting interfacial debonding and slippage under ?ber axial loading were examined.The ?ndings of this study provide guidelines for optimization of high-performance graphene/CNT hybrid ?bers.

Published by Elsevier Ltd.

1.Introduction

Graphene has recently attracted signi?cant research interest in areas including nanoelectronics,sensors,energy conver-sion and storage,and transparent conducting ?lms [1–5].To transfer the excellent mechanical and physical properties of individual graphene to its macrostructures for practical appli-cation,numerous efforts have been made to assemble graph-ene sheets into ?bers,?lms,aerogels,etc.[6–11].Although one-dimensional graphene ?ber has the potential to retain the superb properties of individual graphene,it is extremely dif?cult to ?nd an appropriate solvent for dispersing the highly concentrated graphite ?akes for ?ber spinning.It is only very recently that graphene oxide (GO)liquid crystalline (LC)solutions in water and other polar organic solvents have been used to facilitate graphene ?ber synthesis [12–17].By employing LC GO,a wet spinning method using various coag-ulation baths and a solvothermal approach with long capil-lary tubes was developed to fabricate graphene-based ?bers,and remarkable achievements have been made in terms of

their mechanical and functional properties [12–21].However,to synthesize graphene ?bers with high mechanical and elec-trical properties,not only are large graphene sheet size and high GO concentration in the precursor essential,but strict high temperature reducing conditions,acid and post-stretch treatments should also be applied [12,13,22].

Super-aligned carbon nanotube (CNT)?lms have been extensively studied and recognized by researchers as ?exible,lightweight,and highly transparent conductive materials,demonstrating their potential applications in numerous areas including ultrastrong and foldable papers,electrothermal heaters/acoustics,double-layered capacitors,and multifunc-tional composites,to name a few [23–27].CNT and graphene sheet can be easily assembled together with good synergistic performance [28,29].For example,graphene-wrapped CNT ?ber electrodes [30]and CNT decorated graphene-?ber supercapacitors [31]were recently investigated,and excellent mechanical and energy conversion/storage properties were demonstrated.Three-dimensional CNT and graphene hybrid networks have also been fabricated and applied in

https://www.wendangku.net/doc/211506633.html,/10.1016/j.carbon.2014.01.0730008-6223/Published by Elsevier Ltd.

*Corresponding author .

E-mail address:chou@https://www.wendangku.net/doc/211506633.html, (T.-W .Chou).

energy-related?elds[32–34].However,CNT/graphene hybrid structures reported so far are mostly based on randomly dispersed CNTs.The development of super-aligned CNT ?lm/graphene hybrid?bers,which is essential for developing high-performance CNT/graphene hierarchical structures,has yet to be explored.

In this paper,we report the design and synthesis of super-aligned CNT?lm wrapped graphene?ber(rGO?ber)and its failure behaviors under?ber axial tensile loading.The rGO?-ber was synthesized in a chitosan coagulation bath with a GO dope precursor,followed by chemical reduction using hydroi-odic acid(HI).The relationship between the?ber processing parameters and their mechanical performance was exam-ined.With CNT?lm wrapping,the speci?c strength and elec-trical conductivity of rGO?ber were greatly enhanced.In addition,the?ber failure behavior showed signi?cant correla-tions with the CNT?lm thickness and the interfacial nano/ micro structures.The results obtained should contribute to the future production of stronger and more conductive graph-ene-based?bers.

2.Experimental

2.1.Synthesis of graphene?ber

The graphene?bers used in this study were synthesized by a wet spinning method followed by a chemical reduction treat-ment.Brie?y,chitosan(molecular weight:600,000–800,000, Acros Organics)solution in de-ionized water(1wt.%,with 1vol.%acetic acid)was prepared as the coagulation bath. The dish containing the bath was rotated at a constant speed of V=10rpm.GO dope(3wt.%in water,Angstron Materials) was injected into the rotating bath via a blunt-tipped24-gage needle(ori?ce diameter d=305l m).The injection radius r (the distance between the needle ori?ce and the dish center) was set at5cm.After soaking in the bath for20min,the GO ?ber was rinsed with de-ionized water for1min and then laid on a water-absorbing paper to dry in air for30min.After-wards,the?ber was densi?ed with ethanol drops and verti-cally hung at room temperature overnight under an applied load of3.1g.Finally,graphene?bers were obtained by chem-ically reducing the GO?bers in a30wt.%HI solution at100°C for3h and then drying at100°C for12h.

https://www.wendangku.net/doc/211506633.html,T?lm wrapping

The CNT?lm used for wrapping was directly drawn from a spinnable CNT array.The details of the CNT?lm preparation can be found in Ref.[35–38].To wrap the graphene?bers with the desired number of CNT?lm layers,the width of the?lm was carefully controlled.The drawn CNT?lm was?rst sus-pended between two rigid supports,and then a graphene?ber was gently placed at one side of the?lm with the alignment direction of CNT?lm parallel to the longitudinal direction of the graphene?ber.The?lm was wrapped around the?ber layer by layer.After wrapping,drops of ethanol were applied to densify the CNT layers and promote interaction between the CNT?lm and the graphene?ber.Finally,the CNT?lm wrapped?bers were dried at60°C for2h.2.3.Characterization

High-resolution scanning electron microscope(SEM,JSM-7400F)images were taken at an acceleration voltage of3kV. The cross-section images of the?ber were examined by Tabletop Microscope(TM-1000,Hitachi),and the net cross-sectional area was analyzed using the software Adobe Acro-bat XI Pro.The nominal diameter of the?ber was determined as the diameter of an equal area circle.For a net cross-sec-tional area A,the nominal?ber diameter D can be calculated by D?

????????????

4A=p

p

.The weight of the?ber was measured by MX5 Micro Balance(precision of0.1l g,Mettler Toledo).A?ber specimen for electrical and mechanical characterization was mounted on a stiff paper with a rectangular opening 20mm in length.Copper electrodes were secured to each end of the specimen using conductive adhesive(40-3900sil-ver-?lled epoxy resin,Epoxies,Etc.).An Instron5848Micro Tester equipped with micropneumatic grips and a5N load cell was used for the?ber mechanical property test.A four-point method using a Keithley2182A Nanovoltmeter and a Keithley6430Sub Femtoamp Remote Sourcemeter was ap-plied to measure the?ber electrical resistance variation dur-ing loading.

3.Results and discussion

Measuring the sizes of120GO sheets in25SEM images showed that the GO sheets of the as-received GO precursor have an average nominal diameter of4l m(Fig.1a and b). Fig.1c and d show the experimental set-up for GO?ber syn-thesis and an enlarged syringe needle.GO dope was extruded into the bath,with the extrusion rate(ER)varied from0.1to 1.0mL/min.The GO injection speed v at the outlet of the nee-dle ori?ce was calculated as v=4ER/p d2(d:ori?ce diameter). Except for ER=0.1mL/min,which resulted in disconnected GO segments,all other ERs gave rise to continuous GO?bers.

As shown in Table1,the GO dope injection speed v in-creased linearly with ER,which ranged from0.2to1.0mL/ min.Fibers spun at larger ERs had higher linear density(tex, with the unit of g/km).In contrast,low GO injection speed gave rise to poor?ow-induced GO orientation and fewer load-bearing GO sheets per unit length,and consequently, low tensile speci?c strength and fracture load.On the other hand,at higher injection speed,the GO stream was hindered by the coagulation bath,and GO sheets tended to be squeezed and over-layered in the?ow,which is also undesirable.

Fig.2a shows that the speci?c strength of the GO?ber in-creases rapidly from ER=0.2mL/min(38mN/tex)to ER=0.4mL/min(67mN/tex).The?ber strength is reduced upon further increases in ER.At the high ER of1.0mL/min, the resulting GO?ber shows an obviously wrinkled surface (Fig.2d),indicating that the GO sheets were squeezed and loosely packed and accounting for the poorer?ber speci?c strength(36mN/tex).Therefore,the optimal GO?ber speci?c strength occurs at ER=0.4mL/min.The unit of N/tex for speci?c strength is preferable when dealing with?bers of irregular cross-sectional shape[12,39].The GO?ber fracture load increases initially with ER,up to534mN(ER=0.8mL/min), and then decreases,as shown in Table1and Fig.2b.

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The dependence of GO?ber speci?c strength on draw ratio (DR)in the present study is different from the result reported by Xiang et al.[12],who observed that DR=1.45resulted in the GO?ber with the highest speci?c strength.In contrast, in this research,the DR of?bers spun from ER=0.4mL/min is0.48(calculated as the ratio of?nally obtained?ber length l1to theoretically injected?ber length l2.l2=vt,v and t are GO injection speed and time,respectively.).The reason for the difference lies in the GO sheet size and the coagulation medium.In Ref.[12],the average diameter of the large GO ?ake was22l m,more than?ve times that of our samples; thus,higher DR is necessary to stretch the folded GO?akes. In the case of small GO sheets(4l m),high DR resulted in dis-connected GO segments,as discussed previously.Further, low-viscosity ethyl acetate was employed as the coagulate bath in Ref.[12].In our experiment,however,a more viscous chitosan bath was used,which requires higher GO injection speed(i.e.,lower DR)to overcome the strong shear resistance during spinning.Consequently,the highest speci?c strength GO?ber was achieved at a DR of0.48,which is much lower than the1.45reported in Ref.[12].

Fig.2c shows the stress–strain curves of GO?bers spun at ER=0.8,0.4and0.2mL/min,and their speci?c strengths are 55,67and38mN/tex,respectively.To compare our result with previously reported works,the unit N/tex was converted to Pa.The cross-section images of the?bers were obtained using a Tabletop Microscope and given in Fig.2e–g.The nominal?ber diameters D were estimated to be82,43and29l m,and the corresponding tensile strengths were101,212,and145MPa, respectively.The highest as-spun GO?ber strength obtained in the present study is comparable to those obtained from CTAB coagulated GO?bers(ca.145MPa)[19],LC GO ?bers(ca.102MPa)[6],and other GO based papers(ca.43–130MPa)[40,41].

The speci?c tensile stress–strain curve of GO?ber spun at ER=0.8mL/min(Fig.2c)shows a higher elongation to failure than the other two?bers.At strain larger than1.2%,the?ber speci?c stress rose slowly with strain.This trend of variation was also observed in the yielding/necking stage of some poly-mer materials associated with macromolecular chain slipping [42,43].For the GO?ber,this phenomenon may be attributed to the sliding of GO sheets with respect to each other,as well as the unfolding of GO sheets in the?ber.To further demon-strate this phenomenon,a cyclic loading experiment was per-formed,and the results are shown in Fig.3.Here,the GO ?bers(ER=0.8mL/min)were loaded with peak cyclic forces of200mN(a)and400mN(b).Fig.2c shows that for maximum speci?c stress of20.6mN/tex(corresponding to200mN),the ?ber deformation is elastic,and hence the strain is recover-able(Fig.3a).At the peak stress of41.2mN/tex(corresponding

of GO sheets(a)and the GO sheet normalized diameter distribution(b).(c)The experimental

which is composed of a pump,an injection syringe and a rotating dish.(d)Enlarged

version of this?gure can be viewed online.’’

Table1–Processing parameters and properties of GO?ber.

ER(mL/min)0.20.250.30.350.40.50.60.70.80.9 1.0 v(cm/s) 4.56 5.71 6.857.999.1311.4113.6915.9718.2620.5422.82 Linear density(tex) 2.52 2.86 3.42 4.08 4.61 5.687.168.039.7110.7912.41 Load(mN)96129177254307362437454534529445 Strength(mN/tex)3845526267646157554936

SEM images,which show an irregular ?ber outline.The

Fig.3–Time variations of strain of GO ?ber (spun at

ER =0.8mL/min)under cyclic stresses with peak values of:(a)20.6mN/tex (200mN)and,(b)41.2mN/tex (400mN).The dotted line in (b)shows the level of residual strain.

thickness of each layer of CNT?lm was30–50nm after den-si?cation with ethanol[44].Therefore,a CNT?lm1.57mm wide can wrap the rGO?ber with a CNT layer thickness of218–363nm.It can be seen from Fig.4b that some areas of the rGO?ber surface were not covered by CNT,especially the rugged spots.This is mainly due to void space between

illustration of CNT?lm wrapping an rGO?ber with CNT alignment direction parallel image of the aligned CNT?lm.(b–e)SEM images of rGO?ber wrapped with CNT

mm(f3)and9.42mm(f4),respectively.All CNT?lm wrapped?bers were densi?ed

(a)and50l m in(b–e).‘‘A color version of this?gure can be viewed online.’’

stress–strain curves(a)and the corresponding electrical resistance-strain curves wrapped?bers f1,f2,f3and f4.Failure of core rGO?ber(c)and CNT?lm sheath(d).‘‘A color online.’’

CNT bundles as shown in Fig.4a.Fig.4c shows the3.14mm wide CNT?lm wrapped?ber.In the case of6.28mm?lm width,the?ber surface was better covered.Ethanol densi?-cation was effective in densifying the layers and enabling them to be tightly packed around the core?ber(Fig.4d). Fig.4e shows that thick CNT?lm wrapping can completely cover the?ber.The super-aligned CNT?lm is known to ad-here tightly to many kinds of substrate materials including glass,plastics,silicon,and metals mainly by van der Waals interaction and surface tension due to ethanol evaporation [44–46].Previous simulation results also showed that it would take only$0.5ns for a graphene nanosheet to form

a full layer around a CNT via large van der Waals coupling

[47],and pàp interaction was also demonstrated to contrib-ute much to the assembly process[48].

Fig.5demonstrates the failure behaviors of the rGO?bers. Fig.5a shows that the rGO?ber(f0)has a speci?c tensile strength of73mN/tex,which is33%higher than that of the parent GO?ber.The speci?c strengths of rGO/CNT?lm wrapped?bers were further increased to77,82,89and 92mN/tex for?bers f1,f2,f3,and f4,respectively.Here,in cal-culating the speci?c strength,the weight of CNTs was not ta-ken into account considering that their weight percentage is only0.014–0.056%of the resulting?ber.

The ultimate failure behavior of these?bers can be corre-lated with the?ber nano/micro structures.Fig.5a shows that their fracture strains decreased from the original3.41%(f0)to 2.18%(f1),1.96%(f2)and1.75%(f3)after more CNT?lms were wrapped around the?bers.It is interesting to note that a load drop of4mN/tex occurred at1.33%strain in the stress–strain curve of?ber f3.This phenomenon can be attributed to the sudden debonding and slippage between the CNT?lm sheath and the rGO?ber core,which?nally led to the core?ber being extracted from the CNT?lm sheath,as can be seen in Fig.5c and d.A similar load drop occurred in the stress–strain curve of the f4?ber.The above observations suggest that the core–sheath?ber structure had a signi?cant in?u-ence on the?ber failure strain;as the thickness of the wrap-ping?lm increased(f0–f1–f2–f3),the?ber became more brittle.Then at a certain threshold?lm thickness,the?lm sheath no longer maintained its adhesion to the core?ber and slippage occurred(f3).As?lm thickness further in-creased,the load drop was as large as8mN/tex,and the interfacial slippage resulted in a signi?cantly enhanced strain to failure(f4).

The?ber electrical performance was also characterized during tensile loading.Fig.5b shows that the initial electrical resistances(R)of?bers f0–f4are378,280,262,238and234O, respectively,corresponding to electrical conductivities(r)of 142,187,196,212,and211s/cm,calculated by r=4l/p RD2(l denotes the specimen length of20mm).The conductivities of the present?bers are higher than those of CTAB coagulated rGO?ber($35s/cm)[13]and hydrothermal synthesized rGO ?ber($10s/cm)[7].The results also indicate that thicker CNT?lm wrapping in?ber f4was less effective in enhancing r compared with other wrapping thicknesses.Fig.5b also demonstrates that the electrical resistances of the rGO?bers with CNT?lm wrapping were more stable under increased loading and elongation.4.Conclusions

In this research,graphene oxide(GO)?bers were synthesized using a wet spinning method,and the?ber processing param-eters/property relations were investigated.At ER=0.4mL/ min,the strongest GO?ber with speci?c strength of67mN/ tex was obtained,while the highest load-bearing GO?ber was synthesized at ER=0.8mL/min.Wrapping of CNT?lm with width of6.28mm simultaneously enhanced the speci?c strength and electrical conductivity of rGO?ber by22%and 49%(to89mN/tex and212s/cm),respectively.The failure behavior of reduced GO(rGO)?bers wrapped with super-aligned CNT?lms under tensile loading was characterized. The rGO?bers with thicker CNT?lm layer wrapping were more susceptible to interfacial debonding and slippage in the core–sheath structure,resulting in higher ultimate failure strain.The electrical and mechanical performance enhance-ment of rGO?ber by CNT?lm wrapping may expand their applications in electronic devices,such as?ber supercapaci-tors,sensors,and functional textiles.

Acknowledgments

Fancheng Meng and Tsu-Wei Chou acknowledge the support of the US.Air Force Of?ce of Scienti?c Research(Dr.Byung Lip Lee,Program Director).

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