Superstructured Assembly of Nanocarbons: Fullerenes, Nanotubes,and GrapheneZheng Li, Zheng Liu, Haiyan Sun, and Chao Gao*
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, ZhejiangUniversity, Hangzhou 310007, China
CONTENTS
1. Introduction 70462. Fundamentals 7048
2.1. Characteristics of Fullerenes, CNTs, andGraphene 7048
2.2. Principles of Nanocarbon Assembly 70493. Assemblies of Fullerenes 7049
3.1. Fullerene Liquid Crystals 70503.2. 2D Fullerene Films 7050
1. INTRODUCTIONCarbon is one of the basic elements for life on Earth. It exists indiverse allotropic forms while being different in chemical andphysical properties. The best well-known natural allotropes ofcarbon are graphite and diamond. Either for recording or thesymbol of wealth, status, and love, the two carbon allotropeshave left indelible marks on the long river of human history. In1985, the advent of fullerenes1 opened the door to a novelgroup of carbon allotropes in the nanoscale. After that, theobsession of carbon nanotubes (CNTs) and graphenecontinued pushing materials development to an exciting climaxonce again. The so-called nanocarbons refer to graphiticmaterials with at least one dimension below 100 nm. There areseveral members in the growing nanocarbon family, includingfullerenes, nanodiamonds, nano-onions, CNTs, nanofibers,graphene, graphene quantum dots, graphene nanoribbons,etc., among which the sp2 hybridized nanocarbons have beenthe center of research attention for nearly 30 years owing totheir outstanding features. Speaking of which, fullerenes, CNTs,and graphene, the three most famous sp2 nanocarbons, have
Special Issue: 2015 Supramolecular Chemistry
Received: February 22, 2015Published: July 13, 2015
triggered tremendous interests in both scientific and techno-logical communities.2−5 In light of extensive studies on thesefantastic nanocarbons, which show great promise in a widevariety of applications ranging from high-performancecomposites, to electronic and energy storage devices, tobiological materials, and so on,6−12 researchers never stoptheir forward progress on the way to the new era ofnanocarbons.Publications on nanocarbons are on a continuously rising
trend in the past decade, as illustrated in Figure 1. The firstrevealed fullerenes show a slight yet continuous increase witheach passing year from 2005 to 2014. On the contrary, theother two nanocarbons apparently enjoy strong growth,especially for the youngest one, graphene. Published papersabout graphene numbered only a few hundred in the first threeyears; afterward, it became more and more popular. Acomparative amount of publications with CNTs were achievedfor graphene in 2013 which soon grew drastically above that in2014, demonstrating the exploding research interest ingraphene. As regards the regional distribution of publicationsabout nanocarbons, U.S.A. and China are the two countrieswho are the most active in this research area, contributing morethan half the total amount, especially for the currently hottestCNTs and graphene.Among the abundant research works devoted to nano-
carbons, the big challenge of extending their excellentproperties into the macroscopic world has long beenrecognized as the core issue for practical applications.13,14
Exactly as the polymeric materials suggest, molecules are muchmore functional when they are getting together; mostimportantly, their utilization at the macroscale is of greatsignificance to people. Despite the uses as reinforcements forcomposites, where the content of nanocarbons is normally at a
low level (below 5 wt %) and the integrated performancesremain based on the matrix materials,15−19 assemblingnanocarbons into macroscopic superstructures is anothereffective strategy to fulfill the realistic demands for multifunc-tional materials.20−23 The macroscopic assemblies of nano-carbons cover all three dimensions in this world, in the forms ofone-dimensional (1D) fibers, two-dimensional (2D) films, andthree-dimensional (3D) monoliths, as shown in Scheme 1.Besides, in the purpose of expanding their functionalities, theseassemblies sometimes go to hybrid structures with multi-components. For a specific superstructure, one should note thatit is always composed of delicate microstructures and organizedwith precise control at the molecular level and thus exhibitsunexpected functionalities which are absent in individual
Figure 1. (a) Timeline of scientific publications relating to nanocarbons in the past decade (2005−2014). (b) Regional distribution of publicationson each topic. Publication analysis was completed using Web of Science.
Scheme 1. Superstructured Assemblies of Nanocarbons inthe Macroscopic Scale
nanocarbons. The assembly of nanocarbons has shownsupramolecular behavior since they are firmly connectedthrough intermolecular forces, or, even better, they are largeand rigid building blocks that are easy to manipulate. Overall, inthe interest of designing and fabricating nanocarbon super-structures, the essential considerations are not only theindividual properties of nanocarbons but also the elaboratemicrostructures as well as the interactions in between.Although previous reviews concerning the assembly of
fullerenes, CNTs, or graphene were established, which,however, only focused on one kind of nanocarbon orassemblies at one dimension,24−32 there is still a lack ofintegral comprehension upon the ascending subject. In fact, thestructural relevance of nanocarbons allows them to getinspiration from each other, and what’s more, their distinctionsare expected to be drawn. In this review, we will systemicallysummarize state-of-the-art progress in fabrication and applica-tion of the macroscopic assemblies of the three typicalnanocarbonsfullerenes, CNTs, and graphenewith partic-ular emphasis on 1D fibers/yarns, 2D films/papers, and 3Daerogels/hydrogels. The major assembly methods andtechniques are reviewed here in order to give helpful guidancefor future research.
2. FUNDAMENTALS
2.1. Characteristics of Fullerenes, CNTs, and Graphene
The explosive chasing of nanocarbons began with the discoveryof the first fullerene molecule (C60) in 1985 by Kroto andSmalley et al.1 It is composed of 60 carbon atoms organizedinto 12 five membered pentagons linking with 20 six memberedhexagons and has the geometry of a hollow sphere, resemblingthe structure of a soccer ball; thus, it is also called thebuckyball.33−35 Being viewed as a zero-dimensional (0D)nanocarbon, the diameter of a C60 sphere is 7.1 Å,36 meaningthat the three dimensions of which are all below 1 nm.Although fullerenes include a wide range of carbon cages withvarious carbon atoms and symmetries, the readily available C60is recognized as the most stable and the dominant member inthe fullerene family and, therefore, attracts particularattention.37 The preparation of C60 in macroscopic quantitieswas reported since 1990,38 causing intensive investigation fordecades. In brief, fullerenes have displayed photosensitizing andelectron-acceptor features, electrical transport properties, andmany other splendid physical and chemical properties, whichallow the applications of fullerenes covering multiple areas ofphotovoltaics, catalysis, medicinal chemistry, biological uses,etc.39−43 Notably, the room temperature solubility of fullerenesin a variety of solvents enables straightforward processing ofsuch nanoscale carbon allotropes.44
Iijima’s report in 1991 brought CNTs into the awareness ofthe scientific community.45 CNTs are 1D nano graphiticmaterials presenting a seamless cylindrical morphology andextremely high aspect ratio (i.e., length to diameter ratio, 102−107), with diameters ranging from several to hundreds ofnanometers and lengths up to centimeters.46,47 With respect tothe wall number, CNTs might be classified into several types,such as single-walled CNTs (SWCNTs), double-walled CNTs(DWCNTs), and multiwalled CNTs (MWCNTs). In 1993, thesuccess in producing SWCNTs gave a huge boost to the field ofCNT research.46 Both theoretical and experimental studieshave demonstrated that the major properties of CNTs wouldvary in association with their wall numbers. The tensile strength
and Young’s modulus of a SWCNT is typically in the range of13−52 GPa and 320−1740 GPa, respectively,48,49 while being11−63 GPa and 270−950 GPa of a MWCNT.50 Besides, theirmetallic characteristics are highly dependent on the chirality ofthe graphitic hexagonal lattice, which means the electronicproperties are expected to be metallic or semiconducting.51
Nevertheless, the superior mechanical properties, conductiv-ities, and many other fantastic features have let CNTs stand atthe forefront of academic research in the past decades.52−54
Graphene is a planar monolayer of carbon atoms packed intoa 2D honeycomb lattice. Although the theory of graphene hasbeen around for a long time, which can be traced back to theyear of 1947,55 the first well-known experimental discovery andinvestigation of graphene in 2004 is a milestone for thegraphene research.56,57 By using a simple mechanical exfoliationof graphite, a single layer graphene was peeled and found tohave a thickness of ∼0.34 nm.58 After substantial measurementson graphene,59−65 it has been revealed with extremely highmechanical stiffness (tensile strength up to 130 GPa, modulusof 1000 GPa)66 and excellent thermal conductivity (4840−5300 W m−1 K−1),67 especially ultrahigh electron mobilitiesexceeding 2 × 105 cm2 V−1 s−1.68,69 However, the lack ofmassive production of perfectly structured graphene is still theprincipal limitation for its practical application. Solution-exfoliation of graphite in certain solvents or with surfactants,either by sonication70,71 or high-shear mixing,72 is one protocolfor large-scale production of graphene with high-quality.Meanwhile, chemists have found an alternative way initiatingfrom the heavily functionalized graphene derivative-grapheneoxide (GO), which could be easily obtained through a chemicaloxidation and exfoliation route upon natural graphite and is richin oxidative groups such as epoxy, hydroxyl, carbonyl, andcarboxyl groups.73−75 The preparation of GO has gone throughdecades of development, the KClO3-based Brodie−Stauden-maier method76−78 and the KMnO4-based Hummers method79
in combination with their modified forms are currently themost popular ways. Lately, an iron-based green and fastapproach for single-layer GO, known as the “Gao-Fe” method,was disclosed, able to avoid polluting heavy metals and toxicgases in the products, presenting industrial significance.80
Although the introduction of functional groups and defects onthe GO platelets during preparation has degraded theirperformances to a certain extent, the subsequently achievedprocessability in polar solvents, especially the stable dispersi-bility in water, is very exciting in application fields. Moreover,the elimination of functional groups and recovery of thegraphitic lattice could be realized through chemical reduction,thermal treatment, or irradiation on GO, resulting in a muchmore graphene-like structure and performances, known as thereduced GO (RGO).81,82 Therefore, assembling GO andfollowed by reduction has become the most commonly utilizedprotocol for realization of high performance grapheneassemblies on the macroscopic scale.83−86
Furthermore, there are subtle connections between thesenanocarbons. The atomic 2D graphene layer is considered as abasic building block for graphitic carbons of other dimension-alities; it can either be wrapped up into 0D fullerenes or berolled into 1D nanotubes.87 In fact, the structural relativity isnot simply true in the theoretical model; researchers havealready made breakthroughs to experimentally accomplish themorphological transition. To name a few examples, CNTs wereable to be longitudinally unzipped to form graphene nanorib-bons through a solution-based oxidative process and subse-
quent reduction,88,89 while graphene quantum dots weresynthesized from C60 molecules via the ruthenium-catalyzedcage-opening.90 Reversely, the transformation of flat graphenesheets into fullerene cages was observed under high energyelectron beam exposure;91 meanwhile, rolling up graphenesheets into CNTs was achievable by means of ultra-sonication,92,93 electron beam irradiation,94 or thermallyinduced self-intertwining.95 By the way, graphene nanoscrollswith an open tubular structure, which are distinct from CNTswith sealed edges, have received growing concerns. Theircharacteristic open topology allows foreign molecules readyaccess into the massive interlayer galleries.96,97
2.2. Principles of Nanocarbon Assembly
The assembly of individual nanocarbons into ordered super-structures is a thermodynamic process where the interactionsbetween building blocks play a decisive role. We are about tointerpret some basic principles before getting started onintroducing the well-defined assemblies of nanocarbons.First of all, the possible interactions existing between
nanocarbons might be complicated because they are attributedto the result of many forces, including van der Waal’sattractions, π−π stacking, and, some times, electrostaticinteraction, hydrogen bonding, and hydrophobic interaction.These interactions have provided essential opportunities forspontaneous assembling. As we know, nanoparticles alwayshold a strong tendency of agglomeration to lower their freeenergy due to the large specific surface area. When theseparticles are getting close enough, the above-mentionedinteractions will take effect to draw them together, although,in some circumstances, the electrostatic repulsion should beovercome first. However, the uncontrolled self-aggregationprocess generally causes a simple stacking which is sometimesundesirable and hard to become the specific structures we need.After all, the elaborate construction of the microstructure isexactly the magic of nanoparticle assemblies what makes themfunctional. Therefore, it is necessary to well control theassembly process, by means of preventing severe aggregationwhile making full use of the interactions to stabilize theassembled hierarchical architectures. In some cases when themechanical strength of a structure is under consideration, itsinternal interactions should be evidently increased to resiststructural damage caused by, basically, slippage between thebuilding blocks. π−π stacking between sp2-hybridized carbonatom domains and other π-conjugated materials is the mainattracting force between graphitic materials. Hydrogen bondingis another strong interaction that appears in betweennanocarbons with functional groups, such as hydroxyl groups.Above all, the most effective way is ascribed to the introductionof much stronger covalent bonding and the formation of across-linked structure, through modification on nanocarbons. Inother cases demanding a porous structure, it is a technologicalchallenge to sustain the loose framework. Actually, no matterhow the assembly process is carried out initially, the as-formedideal microstructure needs opposite effects against aggregationto neutralize the dense packing trend which causes collapse ofthe structure. The opposite effects usually come from thestructural rigidity of nanocarbons, in combination with theirsurface curvature and different orientation.Second, from a geometrical point of view, the morphology of
different nanocarbons has great influence on interparticleinteraction by affecting their way of contact. More specifically,the interaction between nanocarbons depends on their contact
area. That is, when other conditions are equal, a larger contactarea gives stronger interaction between the building blocks,because more carbon atoms interact with each other. For 0DC60 with a spherical shape, their contact mode can be seen aspoint contact, and for 1D rod-like CNTs, line contact. Theircurved surfaces cause incomplete contact, leading to relativelyweak interaction. For 2D graphene platelets, on the other hand,their planar surface makes them much easier to be denselypacked in a face-to-face manner on the basis of stronger van derWaal’s interactions than the other two, as well as thecontribution originated from the π-conjugated domains.98
The different contact modes for the three nanocarbons showinggeometric dependence are illustrated in Scheme 2. But it is
always more complicated in practice. For example, CNTs withhigh aspect ratio and flexibility tangle easily which greatlyincreases their resistance to intertube sliding while beingstretched. The case for graphene is even more complex since awavy morphology with wrinkles is quite common for graphenesheets. These wrinkles sometimes degrade the contact level dueto steric hindrance and at other times provide an interlockingeffect to impede interlayer sliding, depending on the stackingmode of graphene sheets and the applied load. In a word,figuring out interactions between nanocarbons is reallymeaningful for investigating the behavior in their assembledarchitectures; it is worthwhile to take the geometrical effectsinto account which may help us comprehend the interactingmechanism.
3. ASSEMBLIES OF FULLERENESEarly research about fullerenes mainly focused on the synthesisof fullerene family compounds and their basic physicochemicalproperties, with special attention to the implementation oforganic functionalization.99−102 Lately, the assembly of full-erenes and their derivatives into ordered structures based onnoncovalent or covalent interactions has been a hot topic fortheir enormous contributions to the design and fabrication oforganic electronic devices.103−106 While being reported indifferent forms, including liquid crystals, needles/nanofibers,films, etc., such assemblies generally took place in solutions, onsurfaces, as well as at interfaces.107−109
Pristine fullerenes lack dipolar interactions, and there is littlepossibility to assemble them in the solid state. Thus, the solventplays an important role in the solution-based assembly offullerene aggregations. For example, a water-soluble C60-terminating ammonium amphiphile revealed the formation ofboth long fibrous and disk-like aggregates,110 whereas excep-tionally long crystalline C60 nanowires with a length to widthaspect ratio as large as 3000 were grown using 1,2,4-trimethlbenzene as the solvent.111 Li et al. discovered a dropletreceding method for the solution-grown aligned C60 single-
crystal needles and ribbons, performing well in theirapplications of field-effect transistors (FETs).112−114 Besides,the interface between two solvents with/without C60 moleculescould act as nucleation sites for C60 crystals, generating needle-like crystalline precipitates at the interfaces.115−120
However, the solid state assemblies of fullerenes were mostlyin the molecular level and hardly get into the macroscale,despite several thin films supported by substrates, probablybecause the unique spherical geometry of fullerenes is lesseffective for a massive stacking with stable structure. Based onthese circumstances, the following sections will present aschematic introduction of some preliminary regular structuresof fullerenes in relatively large scale, namely the liquid crystalsand films.
3.1. Fullerene Liquid Crystals
Fullerene liquid crystals combine both advantages of excellentphotoelectric properties of fullerene and the ordered structureof liquid crystal, showing potential in research and applicationsimultaneously. Generally speaking, in order to form liquidcrystal, the aspect ratio in excess of 3 is essential for a specificmolecule.121 However, as a spherical molecule, fullerene itselfcannot meet this requirement. Thus, the introduction ofextremely long liquid crystal units is helpful for the realizationof its liquid crystallinity. Various morphologies of fullerene-containing liquid crystals, such as nematic, smectic, cholesteric,or columnar phases, have been reported in the past years.122 In1996, the first fullerene-containing thermotropic liquid crystalwas observed by Chuard and Deschenaux through thefunctionalization on C60 with a framework containing twocholesterol derivatives.123 Hexaaddition on the C60 spherecaused the successful preparation of room-temperatureenantiotropic nematic material.124 Another protocol for regularassemblies of fullerenes is the formation of inclusion complexes.Cyclotriveratrylene (CTV) derivatives substituted with 18 longalkyl chain displayed a nematic phase at room temperature, andtheir supramolecular complexes with C60 also revealed a fluidbirefringent phase as expected. But when heated above 70 °C,the birefringence of the texture under the microscopedisappeared while a cubic phase was formed. In this case, it isnoteworthy that the liquid crystalline behavior mainly comesfrom the microcyclic derivative subunit.125 With covalentlylinked liquid crystalline dendrimers, the functionalized C60 wasable to achieve thermotropic liquid crystals, where the orderedstructures provide opportunities for their photovoltaic andmolecular switching applications.126
Molecular assembly is an effective method to form liquidcrystals. The obtained liquid crystals usually show a hierarchicalsupramolecular structure which does not exist in commonliquid crystal systems. In 2002, Sawamura et al. first used themolecular symmetry of C60 to assemble anisotropic “nano-shuttlecocks” that were afterward stacked into a one-dimen-sional columnar supramolecular structure with liquid crystallinebehavior.127 Later, Nakanishi et al. prepared simple fullerenederivatives bearing long alkyl chains to format a long-rangeordered lamellar mesophase (Figure 2). The mesomorphicmaterials were of high C60 content (up to 50%), and a highelectron mobility of ∼3 × 10−3 cm2 V−1 s−1 was obtained.128
3.2. 2D Fullerene Films
The deposition of fullerene films, either in monolayer ormultilayer, is the way to transfer the unique fullerene propertiesto bulk materials and frequently by surface coating or interfaceassembly, while the self-assembled monolayers (SAMs)
attracted particular attention in this area.129 These 2D fullerenestructures have shown potential applications as n-type semi-conductors in organic FETs and ogranic photovoltaic cells.130
Generally, the assembly of fullerene films is mainly based onvan der Waals interaction or covalent bonding. Except for thesolution-based fabrication, physical vapor deposition is such aprotocol for making thin films of fullerenes directly while theas-formed highly ordered morphology can be nanostructuredby electrochemical reduction with well-defined surfacestructures.131−134
Starting from a well-dispersed solution, solvent evaporation isdefinitely one of the most straightforward ways to fullerenefilms. Evaporation of thin C60 films on Pt electrodes facilitatedthe first electrochemical study on C60.
135 Functionalized C60bearing three eicosyloxy aliphatic chains could be easily packedinto thin films with thickness of approximately 20 μm onvarious substrates by slow evaporation of a dilute 1,4-dioxanedispersion. The resulted films featured water-repellent super-hydrophobicity with a water contact angle of 152° (Figure3a,b). In contrast, spin-coating of the same raw material inchloroform solution resulted in a smooth film which presenteda contact angle of only 103.5°.136 By the way, even a hugeneedle-like C60 with size in the centimeter range was obtainedthrough a very slow evaporation of a supersaturated solution.137
The Langmuir−Blodgett (LB) assembly is another optionaltechnique to obtain supported fullerene films. In such a process,Langmuir films are first formed by spreading amphiphilicmolecules on a liquid surface, like water. The molecules arrangein a manner of the hydrophilic heads contact with water whilethe hydrophobic ends (usually the fullerene cage) stick out intothe air. LB films are then prepared after transferring theLangmuir films onto a solid substrate, with the conveniencethat the layer number could be easily adjusted from monolayerto multilayer depending on the number of times one runs thedipping process.138,139 For instance, by adding a C60 solution ofbenzene140 or CS2
141 to the air−water interface, Langmuir filmscontaining C60 molecules would form after volatilization of thesolvent. Then the LB films were deposited onto differentsubstrates, such as glass, quartz and silicon. For more detailedworks, one can refer to several published reviews summarizingthe LB assembly of fullerene films.139,142
To our knowledge, the first example of C60 SAM wasreported by Mirkin and co-workers; they showed the covalentlybonded fullerene structures onto the (MeO)3Si(CH2)3NH2modified indium tin oxide (ITO) surfaces.143 In fact, the
Figure 2. (a) Polarized optical microscopic texture indicating theordered mesomorphic domains of C60 derivatives. (b) Illustration forthe lamellar mesophase. Reprinted with permission from ref 128.Copyright 2008 American Chemical Society.
introduction of covalent linking between fullerene particles orwith the substrate is able to overcome the shortcoming of lowstability caused by weak interconnection. With this consid-eration, C60+Si films from the deposition of (C60)mSin clusterswere produced in a sophisticated double-target laser vapor-ization source, where the Si atom bonded with two C60molecules and acted as a bridge. The binding energy washigh enough to synthesize C60-based materials throughpolymerization of C60-Si clusters.
144 Apart from the in situcross-linking generated during the synthesis of C60 derivatives,when branched polyethylenimine (PEI), with a distribution ofprimary, secondary, and tertiary amino-groups was adsorbedonto the hydroxylated silicon wafer surfaces, the free primaryand secondary amino groups may help to catch the C60molecules, through N−H addition reactions across the CCbonds in C60. The obtained C60 films possessed good adhesiveresistance, as well as good friction reduction, load-carryingcapacity, and antiwear ability, owing to the unique surfacenature and mechanical properties of C60 units.145 Moleculeswith both HS and NH2 groups served as good cross-linkingagents to bind the Au substrate and C60, where the aminogroups fulfill the attachment of C60 while the thiol groups bindto the Au surface, as illustrated in Figure 3c.146 Similarly, HSand NH2 groups have done a good job in the preparation of ahybrid film containing both C60 and Au nanoparitcles. Amine-functionalized gold nanoparticles took part in the aminationreaction of C60, driving the formation of hybrid films via layer-by-layer (LBL) assembly or in situ cross-linking assembly. Thefilms prepared by the later method had smoother and morefeatureless surfaces than those generated from the LBL process.The in situ cross-linking assembled films were sensitive to thepresence of UV lights with recovery changes on the current,
which provided a tremendous opportunity for applications inthe optic and electronic fields.147
4. ASSEMBLIES OF CNTS
Although the rod-like CNTs were thought to be closely relatedto spherical fullerenes that CNTs were initially seen as theelongated fullerenes, their distinct geometries have broughttotally different assembly behavior. Owing to the large aspectratio of CNTs, it is quite easy for them to be tangled withstrong interactions, which will benefit the assembly process intothe macroscale. As a consequence, there is a far larger notice ofmacroscopic assemblies of CNTs in contrast to full-erenes.148−151
4.1. Synthesized CNT Arrays
The vertically aligned CNT arrays, also known as the CNTforests, which give a large-scale well-aligned morphology ofCNTs parallel to each other, have found use in manyapplications.152 The first achieved CNT array was grown onnickel-coated glass using a plasma-enhanced hot filamentchemical vapor deposition method below 666 °C (Figure4a,b).153 Afterward, numerous efforts have been devoted tocontrollable assembly of the highly ordered and vertical CNTarchitectures with varied CNT diameters and lengths, coveringareas,152 and even well-designed patterns.154,155 In this section,however, we are concerned with the aligned structures becausethey are important intermediate products for subsequent CNTassemblies, especially for CNT yarns and films. During a typicaldrawing procedure, CNT bundles are connected end-to-endcontinuously, implementing the fabrication of CNT yarns orfilms (Figure 4c). According to the model of the drawingprocess proposed by Kuznetsov et al., the interconnections
Figure 3. (a) Scanning electron microscope (SEM) image of a thin film of C60 made by solvent evaporation on a Si substrate; the inset is aphotograph presenting a water droplet on the surface with a contact angle of 152°. (b) SEM image of the cross-section of the film. Reprinted withpermission from ref 136. Copyright 2008 Wiley-VCH. (c) Deposition of C60 film on an Au substrate with the help of molecules possessing bothamino and thiol groups. Reprinted with permission from ref 146. Copyright 2003 Elsevier.
between CNT bundles should be the main factor inducing thedrawability of a CNT forest. Simultaneously, the highly alignedCNTs are necessary, and the forest height can only vary in alimited range for drawable CNT forests.156 Detaileddescriptions are included in the following sections.4.2. 1D CNT Fibers/Yarns
The large-scale production of continuous CNT fibers is highlydesirable for many applications relying on the superiorproperties of individual CNTs. It is believed to be a closecompetitor to conventional fiber materials in various areas dueto its unique characteristics, for being lightweight, strong,flexible, and highly conductive. Since the macroscopic assemblyof CNT fibers is the first step to fully realize the potential ofCNTs in fiber form, tremendous efforts have been paid to theirfabrication and consolidation. After several years of rapiddevelopment on producing mechanically strong CNT fibers,the research hotspot has gradually changed into exploringmultifunctional applications of these prospective fiber materials,with abundant achievements accomplished.157,158
4.2.1. Fabrication of CNT Fibers. The fabrication of CNTfibers/yarns includes solution based (wet-spinning) and solidbased protocols. The first macroscopic, meter-long CNT fiberswere presented by Vigolo et al, through a simple spinningprocess from the SDS dispersed SWCNT solution.159 Sincethen the fascination of continuous CNT fibers were recognizedby the researchers. While it is a promising candidate for theupgraded version of carbon fibers, there are still bottlenecks tounclog. The integrate properties of the assembled CNT fibersare far lower than the individual CNTs, which encouragespeople to keep searching for solutions. Fabrication is anessential part of the final performance since it builds the well-defined construction and microstructures. To date, severalmethods have been discovered for the fabrication of CNTfibers. In the meanwhile, multiple parameters should be takencare of within each method.
4.2.1.1. Solution Based Wet-Spinning. Unlike some of thethermoplastic polymers for fiber preparation, melt spinning isnot an option for CNTs because of the extremely high thermalstability of carbon allotropes. On the contrary, the wet-spinningstrategy is more affordable for CNTs based on their conditionalsolubility. Thus, the solution based wet-spinning method hasbecome one of the major routes for CNT fibers. In theindustrial field, wet-spinning is a well-established fabricationprocess for commercial high-performance polymer fibers, suchas Kevlar. Vigolo et al. extended this technique for makingCNT fibers by regarding CNT as a linear supramolecule. Theaqueous dispersion of SWCNT was injected into a poly(vinylalcohol) (PVA) solution which served as the coagulation bath.After partial substitution of the surfactant SDS which resists thevan der Waals-induced aggregation, the CNTs were stucktogether to form long and stable ribbons. Finally, CNT fiberswere obtained from collapse of the ribbons when dry (Figure5a−c). Although the as-prepared CNT fibers were of lowstrength (∼125 MPa), they showed high flexibility andresistance to torsion, which is hardly realized for carbonfibers.159 Meanwhile, the wet-spinning method is readilycontrollable since fibers with a wide range of diameters wereattained by varying several parameters, such as injection rate,
Figure 4. (a and b) SEM images of CNT arrays. Reprinted withpermission from ref 153. Copyright 1998 American Association for theAdvancement of Science. (c) Schematic drawing for the preparingprocess of CNT array-derived architectures.
Figure 5. (a) Typical wet-spinning process for CNT fibers. SEM image of (b) a CNT fiber and (c) a ribbon deposited on a substrate, the black arrowindicates the main axis of the ribbon. Reprinted with permission from ref 159. Copyright 2000 American Association for Advancement of Science.CNT fibers with different cross sections: (d) hollow, (e) folded, and (f) solid. Reprinted with permission from ref 163. Copyright 2005 Wiley-VCH.
flow conditions, and dimensions of the capillary tube. Based onthe wet-spinning strategy, Dalton and Baughman et al. modifiedthe process; thus, very strong CNT/PVA composite fiberscontaining 60 wt % SWCNT have been generated in 100-mlengths. The tensile strength of the composite fibers was as highas 1.8 GPa, while the Young’s modulus was 80 GPa. Especially,the toughness reflecting the absorbed energy before breakreached 570 J g−1. Such high toughness was achieved by coatingof amorphous PVA on the nanotubes.160 The PVA moleculesacted as binders for CNTs to enhance the intertubeinteractions; however, the electrical conductivity of the fiberswas reduced simultaneously due to the presence of insulatingsubstances on the path for electron transfer. To face thisshortcoming brought by PVA, SWCNT/PEI composite fibersspun with PEI coagulant exhibited significantly higher electricalconductivity (100−200 S cm−1) than the SWCNT/PVA fibers(0.01−2.5 S cm−1).161 Biomolecules could be involved in CNTfibers through the wet-spinning process and served asdispersant and coagulant, in order to generate biocompatibleconducting fibers. Among a series of SWCNT-biopolymer-containing fibers, the best electrical conductivity showed up forthe hyaluronic acid (HA) incorporated ones, with valuesaround 135 S cm−1.162 Moreover, a polymer-free spinningprocess was discovered to eliminate the influence brought by
polymers thoroughly, with a conductivity of 140 S cm−1 afterannealing at 1000 °C.163 It is interesting that the as-formedCNT fibers probably show hollow, folded, and solid structures(Figure 5d−f), depending on the preparation conditions, andcould be applied for different purposes.161,163
4.2.1.2. Drawing CNT Arrays. As is stated above, the directlygrown CNT arrays are perfect precursor structures forsubsequent fiber spinning. In Fan et al.’s creative work, CNTyarns could be generated continuously by drawing one end ofthe vertically aligned CNT arrays (Figure 6a).164 The fibersmade from CNT arrays seemed favorable for CNT alignment;hence, their conductivity should be pretty good.165 In addition,a twist technique is usually employed during the pulling-outprocess to further densify the as-formed fibrous structure(Figure 6b,c).166 Most of the related research was in agreementwith one opinion that the CNT yarns were only achievablefrom superaligned arrays with highly ordered struc-tures.164,167,168 Kuznetsov and co-workers explained thisrequirement by the reason that wavy arrays may causenonuniform interconnections between adjacent CNT bundleswhich ruptures the continuity of the pulling-out process.156
Besides, longer nanotube length in the arrays is favorable forfiber performances since longer CNTs provide higher frictionforces between the nanotubes and fewer mechanical defects as
Figure 6. (a) Photograph of a free-standing CNT array being drawn to make a CNT yarn. Reprinted with permission from ref 164. Copyright 2002Nature Publishing Group. (b) SEM image of a CNT yarn being pulled out from a CNT array and twisted. (c) Surface morphology of a CNT yarnshowing CNTs aligned at an angle with the yarn axis. Reprinted with permission from ref 166. Copyright 2004 American Association for theAdvancement of Science. (d) Optical images indicating comparison between a human hair and two SWCNT ropes grown by FCCVD method. (e)SEM image of a broken CNT strand. Reprinted with permission from ref 172. Copyright 2002 American Association for the Advancement ofScience. (f) Schematic of the drawing and winding process on the as-grown CNT aerogel. Reprinted with permission from ref 173. Copyright 2004American Association for the Advancement of Science. (g) Products of CNT yarns on the spool. Reprinted with permission from ref 177. Copyright2010 Wiley-VCH.
well (the ends of CNTs should be regarded as defects). Forexample, the CNT fibers spun from 1 mm high arrays showed atensile strength of 3.3 GPa,169 nearly double the value of fibersfrom 0.65 mm high arrays, which was only 1.91 GPa.170
Specifically, based on their series work upon CNT arrays andfibers, Zhu et al. claimed that the optimal CNT length for thespinnable arrays was restricted in the range of 0.5−1.5 mm. Theupper limit of 1.5 mm was set because the content ofamorphous carbon in longer CNT arrays was too high toconduct fiber spinning.171
4.2.1.3. Direct Spinning Based on CVD Technique. TheCVD technique is not only used to synthesize CNTs separatelybut is also possible to assemble them into connected structuresright at the time they are synthesized. The directly grownSWCNT strands were first reported by Wu and Ajayan et al.and prepared via a floating catalyst chemical vapor deposition(FCCVD) method in a vertical furnace, with tens ofcentimeters in length and a diameter of around 0.3 mm(Figure 6d,e). Taking care of the temperature and the hydrogenflow rates, continuous fabrication of long SWCNT strands inlarge yields were achieved. The macroscopic strands consistedof nanotubes in parallel orientation and separated by interstitialspace, showing superior mechanical and electrical properties.172
A more popular synthetic strategy for CNT fiber wasperformed by drawing and winding the CVD-grown CNTaerogel onto a rotating rod, as developed by Windle’s group,see Figure 6f.173 Several process conditions, such as carbonsource, the catalyst concentration, and the winding rate, werefound to cause significant influence on mechanical performanceof the as-grown CNT fibers.174,175 The CVD based “bottomup” route for CNT fibers makes it capable of engineering fiberproperties from the root, namely the nanotube structure, andthen their assembly into fibers.176 It is also a continuousprocess for large-scale manufacture of CNT fibers, as shown inFigure 6g.177 In comparison with the coagulation spun CNT
fibers, the directly spun ones were able to perform much betteron their fiber strength. The strongest CNT fiber until now wasreported by Windle and co-workers with a tensile strength of8.8 GPa, in conjunction with high elastic modulus (357 GPa)and toughness (121 J g−1), which was spun from a CNTaerogel, followed by densification through an acetone vaporstream. These values were comparable or even greater thanthose reported for carbon fibers.175
4.2.1.4. Twisting CNT Films. CNT thin films with mesh-likestructure are easily turned into fibers through a twisting orrolling process (Figure 7a). Although the film-derived spinningstrategy is not suitable for continuous production of CNTfibers, it is quite convenient to control the diameter andmicrostructure of the as-formed fibers for specific applicationpurposes.178−180 Furthermore, the twisted fibers possessedhigher mechanical performances, including both the strengthand modules, than the original CNT films, as shown in Figure7b, owing to the enhanced internal friction force to preventintertube sliding. In contrast, the fracture strain was significantlylowered for the twisted fibers in comparison with the originalfilm, which was also attributed to the confined free movementof the bundles.178 Beese et al. compared the structures andmechanical properties of the directly spun CNT yarns fromaerogels and the yarns fabricated by twisted CNT mats, andtheir conclusions showed that the directly spun ones havebetter aligned CNTs, however, with higher porosity before afurther densification process. The twisting speed was alsoexpected to affect the yarn structures where the slowly spunyarns exhibited increased alignment and decreased porosityover the quickly spun yarns; therefore, they have bettermechanical properties.181 What is more interesting is thatShang et al. accomplished a series of distinctive work wherethey overtwisted a CNT film to obtain a spring-like CNT ropewith superstretchability up to 285% of strain (Figure 7c).180
Through controlling the fashion and level of overtwisting, the
Figure 7. (a) SEM image of a CNT fiber being twisted from a CNT film. (b) The stress−strain curves of CNT films and fibers. Reprinted withpermission from ref 178. Copyright 2009 Wiley-VCH. (c) SEM image of an overtwisted spring-like CNT rope. Reprinted with permission from ref180. Copyright 2012 Wiley-VCH. (d)-(f) Three different types of scrolls twisted from CNT sheets. Reprinted with permission from ref 187.Copyright 2011 American Association for the Advancement of Science.
configuration of a twisted film could even vary among double-helical,182 partial-helical,183 and highly entangled structures.184
Otherwise, by preinfiltrating CNT films with polymers, theinteractions between nanotubes were markedly improved, andan evident decrease of porosity was seen in the obtained fibers,making it a possible approach for strong and tough compositefibers.185,186 Remarkably, by means of scrolling CNT sheetswith other functional materials, Lima and Baughman et al.incorporated more than 95 wt % of powders or nanofibers intoscrolled CNT yarns, while maintaining the guest functionalityin the meantime (Figure 7d−f). The good processability andflexibility of the biscrolled yarns thus presented attractiveprospects for applications of wearable electronic devices.187
4.2.1.5. Other Methods. The early try for CNT fibersincluded the electrophoretic assembly reported by Gommans etal., where a carbon fiber tip was withdrawn slowly from adispersion of SWCNT under an electric field, with CNTsgathering around and assembling one-by-one at the positivelycharged carbon fiber tip. The alignment of CNT was observed;however, only a maximum of 5 cm in length could beachieved.188 Starting from the CNT cotton produced by theFCCVD method, Ci and Ajayan et al. developed a drawing−drying process to fabricate CNT fibers with a relatively largescale (Figure 8a). The as-spun fibers were found to be capable
of electron-emitting and electrochemical applications.189
Similarly, CNT cotton made of millimeter to centimeter longindividual CNTs was able to be spun into fibers through the oldcotton-based spinning technology, which was performed simplyby pulling and rotating.190 Liu et al. reported a step-by-step wayto prepare CNT yarns by drawing the CVD-grown filmsthrough a series of diamond wire drawing dies (Figure 8b). Thepore diameters of the series dies were decreasing in sequencefrom 1.2 to 0.2 mm through 18 dies; therefore, the CNTnetworks were highly aligned and densified into 1D yarns,accompanying with excellent electrical properties at the sametime.191,192
4.2.2. Strengthening Protocols. Although the reportedCNT fibers have displayed outstanding mechanical perform-
ance with strength above several GPa and Young’s modulusreached hundreds GPa, these values are still far below thetheoretical value of individual CNT. The ultimate goal for CNTassemblies is introducing the perfect properties of CNT innanoscale into the macroscale, with minimal losses to theirperformance. Based on the impending demand for highperformance CNT fibers, researchers have discovered severalmeans to face the great challenge through optimization of thefiber structures. Generally speaking, the main factor thatdecides the performances of the assembled fibers is attributedto the interaction between CNT bundles, while the primaryfailure mechanism for a CNT fiber is the sliding betweenbundles rather than the breakage of an individual CNT. So thefundamental protocols to get improvement on the mechanicalstrength of a CNT fiber are basically focusing on increasing thepacking density/decreasing the porosity, improving the align-ment, as well as enhancing the intertube connection. Acommon understanding is currently realized that the fiberstrength should increase with decreasing diameter, since moredefects are included in larger diameter fibers, making thembroken easily. The most popular strategies in the currentliterature are in the forms of (1) introducing twist and/orstretch on the as-formed fibers, (2) passing through volatileliquids to consolidate the CNT fibers, (3) annealing at a hightemperature, and (4) infiltrating polymers as bonding agents. Infact, the above treatments on CNT fibers are usually performedin combination to achieve a better result. Apart from theuniversal strategies, liquid crystal spinning technique is a greatupdate for the wet-spinning process to facilitate the formationof a more ordered structure.
4.2.2.1. Liquid Crystal Spinning of CNT Fibers. CNTs,viewed as high aspect ratio, rigid rod supermolecules, arecapable of forming a lyotropic liquid crystalline phase in varioussolvents.193−195 Using the dispersion with liquid crystallinedomains as spinning dope, large-scale alignment during thefiber spinning process becomes a typical feature, which isbeneficial to conductive properties. The substructure of alignedsuperropes with 200−500 nm in diameter was revealed for awet-spun SWCNT fiber and thought to be linked to the startingliquid crystalline phase in the spinning dope.196 In one case ofMWCNT-water dispersion, the critical concentration for thetransition from isotropic to a Schlieren texture typical ofnematic liquid crystal is 4.3 vol % (Figure 9a,b).197
Profiting from the liquid crystal behavior of CNTs, the neatCNT fibers become more affordable during wet-spinning,because of the improved processability. Therefore, the liquidcrystal spinning was considered as the promising solutions formacroscopic assembly of CNT fibers.194 Since super acids aregood dispersants for CNTs, Ericson et al. dispersed SWCNT in102% sulfuric acid at a high concentration (8 wt %), with theformation of nematic liquid crystalline domains, and thedispersion was then extruded and coagulated in water to obtainneat SWCNT fibers. The continuous spinning process is shownin Figure 9c−e. As a result, the derived electrical and thermalconductivities were substantially enhanced than those of theconventional wet-spun SWCNT fibers with polymers inparticipation.198 Similarly, Windle et al. formed lyrotropicliquid crystalline phase of MWCNT in ethylene glycol solventto carry out fiber spinning, resulting in highly aligned CNTfibers. Furthermore, the nitrogen-doped MWCNTs with muchstraighter morphology induced better mechanical and electricalproperties.195 In a recent work done by Behabtu et al., CNTswith length of 5 μm were dissolved in chlorosulfonic acid at a
Figure 8. (a) Drawing-drying process to fabricate DWCNT fibersfrom the as-grown CNT cotton. Reprinted with permission from ref189. Copyright 2007 Wiley-VCH. (b) Schematic of drawing CNTfilms through a series of diamond wire drawing dies. Reprinted withpermission from ref 191. Copyright 2008 American Chemical Society.
concentration of 2−6 wt % to form a spinnable liquid crystaldope for further production of CNT fibers. The 5 μm length ofthe CNTs in their work was longer than the earlier wet-spinning process could handle. Additionally, the scalablefilament spinning was actualized by using a multihole spinneret(Figure 9f−h). The achieved CNT fibers exhibited not onlyremarkable electrical and thermal conductivities but alsocomparable mechanical performance even with solid-stateCNT fibers.199
4.2.2.2. Post Treatments on As-Spun CNT Fibers. Accordingto the curvature of their profile, and intertube entanglement,CNTs are not easy to realize a highly compact packing. Thelarge amount of interval space existing in CNT fiber is knownas one of the most serious defects that degrades fiberperformance. Simultaneously, the fiber conductivity wasfound decrease dramatically along with the increase ofporosity.200 The other key issue that accounts for fiberperformance is attributed to the alignment of CNTs to thefiber axis. Given that, the most frequently used protocols inquest for improvements focus on the consolidation andalignment in CNT fibers.Similar to the post treatment on conventional polymer fibers,
stretching/drawing on the as-produced CNT fibers leads toenhanced nanotube alignment along the fiber axis.201 Inaddition, the reduction of fiber diameter was observed afterthe stretching procedure, due to the condensing effect comingfrom straightening of tortuous CNTs, alignment of therandomly packed CNT bundles, and the generated radialcompression as well. The reduction in diameter also indicates amore densely packing, which means stretching is an efficientway to lessen the porosity in CNT fibers, plus increasing thecontact length between CNT bundles.202,203 Miaudet et al.treated wet-spun CNT/PVA composite fibers by hot-drawingtreatments, which yielded not only the alignment of CNTs but
also a crystallinity increase of the PVA. Structure of the
composite fibers was characterized by X-ray diffraction (XRD),
as shown in Figure 10a,b, being an indication of the alignment
Figure 9. (a) Polarized micrograph of an aqueous dispersion with 4.8 vol % MWCNT; the Schlieren texture shows the appearance of nematic liquidcrystalline phases. (b) Image of the sample in (a) after being dried. Reprinted with permission from ref 197. Copyright 2003 American Associationfor the Advancement of Science. The spinning process for 102% sulfuric acid dispersed SWCNTs: (c) the apparatus used for spinning, (d) extrusionof spinning dope, and (e) collection of as-spun fibers on a spool. Reprinted with permission from ref 198. Copyright 2004 American Association forthe Advancement of Science. (f) Polarized micrograph of a spinning dope of 3 wt % CNT in chlorosulfonic acid. The single and multihole spinneretsperforming (g) single- and (h) 19-filament spinning. Reprinted with permission from ref 199. Copyright 2013 American Association for theAdvancement of Science.
Figure 10. Two-dimensional detector image X-ray data of CNT/PVAcomposite fiber (a) before and (b) after hot-stretching, indicating thealignment of the PVA chains of ±27° in (a) and ±4.3° in (b).Reprinted with permission from ref 202. Copyright 2005 AmericanChemical Society. SEM images of (c) and (d) a twisted CNT yarnbefore acetone shrinking, (e) and (f) after acetone shrinking withreduced yarn diameter and porosity. Reprinted with permission fromref 207. Copyright 2010 IOP Publishing Ltd. (g) Schematic of acontinuous fabrication and densification process for CNT yarns.Reprinted with permission from ref 177. Copyright 2010 Wiley-VCH.
of PVA chains after hot-drawing. The resulting fibers weretough and displayed improved energy absorption at low strain,with a strain-to-failure of ∼11% and a toughness of ∼55 J g−1,much higher than that of Kevlar (33 J g−1).202
Twisting is another important and commonly seen processfor CNT fibers, especially for dry-spun ones to attain improvedmechanical properties. The twist angle which determines thefinal affect can be easily controlled by varying the rotation ofthe spindle, like the speed and number of turns. The role oftwisting is to put tension on the relaxed CNTs and forceindividual CNTs arranging in a certain angle to the fiber axis,thus the intertube distance is reduced, accompany withincreasing the friction force between CNT bundles.170,204
There is a generic theory suggests that the tensile strength of atwisted yarn can be depicted as
σ σ α α≈ − k/ cos [1 ( cosec )]y f2
(1)
where σy and σf correspond to the tensile strength of the yarnand CNT, respectively. α is the twist angle and k = (dQ/μ)1/2/3L. d is the diameter of CNT, μ is the friction coefficientbetween CNTs, L is the CNT length, and Q is the CNTmigration length. According to eq 1, apparently, the twist angleis critical to the yarn strength, which is also affected by CNTlength, CNT diameter, and intertube friction simultaneously.166
Meanwhile, the experimental results demonstrated that thetensile properties of CNT yarns do not grow consistently withthe twist angle. In another words, the maximum propertiesshowed at an intermediate twist level, to be specific, 20° angleto the yarn axis for strength and 10° for elastic modulus, asclaimed by some researchers.205,206
A third strategy refers to the surface tension-baseddensification, namely, by passing through or spray-coatingvolatile solvents, such as ethanol and acetone. It is the capillaryforce during evaporation from the intertube space that causescollapse and shrinkage of the CNT fibers (Figure 10c−g).167,175,177,207,208
Miao once applied a rubbing roller system to densify atwistless CNT yarn with straight and parallel aligned CNTs,and a unique structure was obtained as a consequence,appearing as a high packing density sheath and a low densitycore.209 Very recently, Wang et al. discovered a method formaximizing fiber performance through a stepwise densificationprocess. It combined condensation of a CNT aerogel in wateror alcohol to form a fiber-like structure and a pressurized rollingprocess to enable further densification. The resulting CNTribbon had excellent performances with a combination of highstrength (4.34 GPa), high ductility (10%), and high
conductivity (2 × 104 S cm−1),210 proving the great significanceof eliminating porosity in CNT fibers.Besides, there are still several post treatments performed in
different ways, like thermal annealing which removes thepossible impurities while performing the so-called weldingeffect164,167 and infiltration of polymeric binders which areutilized to bridge individual CNTs.166,208,211 For instance,polydopamine (PDA) infiltrated into dry-spun CNT fiberscould behave like adhesive agent effectively, and the followingpyrolysis thermally converted PDA into a conductive binder;thus, a simultaneous improvement in btoh conductivity andmechanical strength was realized.212 Interestingly, a recent workrevealed that passing an electric current through a CNT fiberwill cause mechanical and thermal responses. Not only themechanical behaviors of the fiber were changed but alsohomogeneous electrothermal heating was induced. Theelectrothermal responses allowed the strengthening of CNTfibers simply by fast curing thermosetting polymers orstructural reordering thermoplastic polymers which have beenpreviously infiltrated inside CNT fibers.213 Similar to thereinforcing mechanism of densification, the improvementsderived from the above methods are attributed to the increasedattractions between CNTs. Moreover, the oxygen-containingfunctional groups obtained from the oxidation on CNTsthrough either gamma-irradiation214 or acid treatment215 werealso found beneficial for the intertube interaction and, hence,led to stronger CNT fibers.
4.2.3. Mechanical Behaviors of CNT Fibers. Thecharacteristics of individual CNTs, combined with their fashionof assembly, determine the behavior of a CNT fiber. Since thetensile and conductive properties of CNT fibers have beenwidely studied for years, only a small amount of research isaware of their special behaviors which are helpful for us to gaina deeper understanding of the acting mechanism of CNT fibersand thus should not be ignored.216−218
Some fundamental research work relevant to such aninteresting aspect was conducted by Chou’s group. The highstrain rate characterization of CNT fibers revealed that theirdynamic mechanical behavior showed gage length dependencyand rate dependency. The ultimate tensile strength decreasedwith an increase in gage length; however, it increased withapplied strain rate. The piezoresistivity during dynamic tensileloading was also demonstrated, which may be used for sensingapplications in dynamically loaded composites.216 Zu et al.investigated the compressive properties of both pure and epoxyinfiltrated CNT fibers using the tensile recoil measurement.The infiltration of epoxy resin among CNTs gave rise to theintertube bonding and load transfer, and not only increased the
Figure 11. Micrographs of compressive damages after recoil test on (a) pure CNT fiber and (b) CNT/epoxy composite fiber. Reprinted withpermission from ref 217. Copyright 2012 American Chemical Society. (c) Stress relaxation behavior of pure CNT fiber, CNT/epoxy composite fiberand carbon fiber. Reprinted with permission from ref 218. Copyright 2013 Elsevier.
compressive fiber strength from 416 to 573 MPa but alsochanged their failure mode under compression. As suggested bythe microscopic analysis of the fiber surface morphologies,kinking was the primary compressive failure mode for pureCNT fibers, while composite fibers with higher brittlenessshowed the bending failure mode (Figure 11a,b).217 Later, thesame group delivered a successive study on the stress relaxationbehavior of the two CNT-based fibers, where stress decay wasobserved in both pure and CNT/epoxy composite fibers whenthey were held under constant strain (Figure 11c). The time-dependent relaxation behavior in CNT fibers, yet not evidentlyseen in carbon fibers, was supposed to originate from thesliding between CNT bundles and affected by the initial strainlevel, strain rate, and gauge length. In addition, the composite
fibers possessed a higher relaxation rate than that of the purefibers, due to the extra interfacial sliding at the CNT/epoxyinterface.218
4.2.4. Applications. The production of continuous CNTfibers has paved the way for applications on the macroscopicscale. Despite numerous applications in various areas, forinstance, composite reinforcements, electrochemical actuators,flexible supercapacitors, electrical wiring, biological scaffolds,electron and ion emitters, etc., the original intention for CNTfibers is still the pursuit of high strength, as a competitor totraditional carbon fibers. Although there remains a long way togo, great progress has been made in recent years, assummarized in Table 1. It appears that stronger fibers aremore inclined to be fabricated via dry methods; however, the
Table 1. Summary of the Preparation Methods, Strengthening Protocols, and Mechanical Properties of the Most Relevant CNTFibers
ref raw material preparation method treatment tensile strength Young’s modulus
simplicity and energy saving of wet methods should not bedisregarded. Furthermore, the employed treatments are quiteeffective in strengthening the as-prepared fibers.4.2.4.1. Composite Reinforcements. Inspired by the primary
uses of conventional high-performance fibrous materials, suchas carbon fibers, the utilization as reinforcements for polymericcomposites is one of the most prospective applications of CNTfibers. In contrast to brittle carbon fibers, CNT fibers generallyexhibited better flexibility, which is favorable for large-straindeformation uses. Gao et al. studied the axial compression oftwisted CNT fiber embedded in an epoxy matrix with the helpof in situ Raman spectroscopy. The results indicated that thereinforcing efficiency of hierarchically structured CNT fiber washigher than high-modulus carbon fiber, while it was able to beara load under large-strain compression without permanentdeformation and fracture.219 Theoretically, the interfacebetween reinforcements and the matrix which enables loadtransfer plays a critical role in the final performance of acomposite material. The principal failure mode belongs tosliding at the fiber/matrix interface. Both single-fiber compositefragmentation tests and microdroplet tests were used tomeasure the fiber/matrix interfacial shear strength for a systemof dry-spun CNT fiber in epoxy matrix. The attained valueswere 17 and 14.4 MPa, respectively, both comparable to that ofcommercial nonsized carbon fiber/epoxy composites.220,221
Therefore, the capacity of CNT fibers for entering the field offiber reinforced polymer materials has been testified.4.2.4.2. Yarn Actuators. Baughman’s group has contributed
a series of pioneering work concerning CNT yarn actuators,which hold great potential for uses as artificial muscles.Referring to the actuation of CNT assemblies in the sheet
form,222 Mirfakhrai et al. presented the first investigation on theelectrochemical actuation behavior of twist spun MWCNTyarns. Actuation strains up to 0.5% were reported at an appliedvoltage of 2.5 V.223 Afterward, the electrochemical actuationbehavior was extended to a promising application of torsionalartificial muscles, which were operated by electrochemicaldouble-layer charge injection, producing a reversible 15 000°rotation (more than 41 turns) and 590 rpm. The resultedlengthwise contraction and torsional rotation were explained bya hydrostatic actuation mechanism.224 Since then, the researchon torsional actuation has drawn particular interest. The samegroup subsequently discovered electrolyte-free torsional andtensile actuation on the basis of dimensional changes of guestactuating materials within the twisted yarns, triggered byelectrical, chemical or photonic excitation (Figure 12a).Notably, an average torsional actuation at 115 000 rotationsper minute and tensile actuation at 1200 cycles per minute and3% stroke were demonstrated.225 By combining paraffin waxand polystyrene-poly(ethylene-butylene)- polystyrene as mix-tures for guest actuating materials, rapid and precise positionalcontrol was realized with a peak rotation speed of 9800rotations per minute, which is of great importance for practicaluses.226 In a much simpler way, electromechanical torsion ofCNT fibers was derived by directly passing a low current alongthem, according to a different actuation mechanism based onAmpere’s law at the nanometer scale. The authors Guo et al.explained that the contraction and rotation of CNT yarns weremade by electromagnetic forces among helically aligned CNTs.Therefore, such type of actuation could occur in almost allavailable environmental media such as air, water, and organic
Figure 12. (a) Schematic of the yarn structures for tensile and torsional actuation. Reprinted with permission from ref 225. Copyright 2012American Association for the Advancement of Science. (b) Photos of CNT yarn supercapacitors arranged in parallel (up) and series (down).Reprinted with permission from ref 229. Copyright 2014 Wiley-VCH. (c) Structure of a wire-shaped lithium-ion battery. (d) Flexible textile wovenfrom wire-shaped batteries. Reprinted with permission from ref 240. Copyright 2014 Wiley-VCH. (e−h) Twisted DWCNT cables anddemonstration to light a bulb with the cable being a part in the circuit. Reprinted with permission from ref 242. Copyright 2011 Nature PublishingGroup. (i) A continuous and meters long Cu coated CNT fiber collected on a winder. Reprinted with permission from ref 247. Copyright 2011Royal Society of Chemistry.
solvents, without the need for electrolyte or guest actuatingmaterials.227
4.2.4.3. Fiber-Based Supercapacitors and Electronics. Thehigh electrical conductivity and voids containing structure ofCNT fibers are expected to be utilized in high performancefiber supercapacitors as well as fiber-based wearable electronicdevices.228 Dalton et al. first dip-coated CNT fibers withelectrolyte, twisted two fibers together and then recoated withelectrolyte to build a fiber supercapactior, which provided acapacitance of 5 F g−1 and energy storage density of 0.6 Whkg−1 at 1 V. Moreover, a certain amount of fiber supercapacitorscan be woven into textiles which facilitate their electronic-textile applications.160 Zhong et al. wove their aerogel derivedCNT yarns into a single jersey fabric. Electrochemicalmeasurements in an electrolyte of NaCl solution revealedthat the CNT fabric supercapacitor showed a capacitance of79.8 F g−1,177 much higher than the previously reported values.Very recently, Meng et al. fabricated SWCNT and chitosan(CHI) composite yarns using a wet-spinning method. Aftercarbonizing CHI constituent under high temperature treat-ment, a composite yarn electrode containing SWCNT andactive carbon was produced and with preferable mesopores forthe transport and storage of charges. Being assembled with aPVA/H2SO4 gel electrolyte (Figure 12b), the measuredcapacitance was 74.6 F g−1 (48.5 F cm−3) at a scan rate of 2mV s−1, and 65.2 F g−1 (42.4 F cm−3) at a current density of0.05 mA cm−2. The yarn microsupercapacitors also presentedsubstantial cycling stability after 10000 charge−discharge cyclesand high energy and power density (3.7 mWh cm−3 and 45.7mW cm−3, respectively).229 Differently, composite yarn electro-des consisting of SWCNT and conducting polyanilinenanowires (PAniNWs), and with a PVA outer sheath werefabricated by the same group via a one-step wet-spinningprocess. The composite yarns provided obviously improvedelectrochemical performances as compared with those of thesupercapacitors based on pure SWCNT yarns, owing to thepseudocapacitance coming with PAniNWs. Typically, amaximum areal capacitance of 6.23 mF cm−2 was reached forthe composite yarn-based supercapacitors.230 Peng et al.accomplished a number of researches upon the topic of novelfiber-based electronics,231−233 such as supercapacitors,234−236
solar cells237,238 and batteries.239,240 Taking their recent designas an example, a stretchable wire-shaped lithium-ion battery wasassembled by pairing two composite MWCNT yarns as anodeand cathode, modified with Li4Ti5O12 (LTO) and LiMn2O4(LMO) nanoparticles, respectively. The wire-shaped batteriesdisplayed extraordinary electrochemical performances, such asenergy densities of 27 Wh kg−1 or 17.7 mWh cm−3 and powerdensities of 880 W kg−1 or 0.56 W cm−3, as well as a highcapacity retention after 1000 bending cycles. Furthermore, theyalso can be woven into flexible textiles to serve as wearableelectronics (Figure 12c, d).240 Generally speaking, the CNTfibers built in such wire-shaped devices served as electrodes, onwhich active components were usually embedded to realize orenhance the device performance. No matter the fiber electrodeswere assembled symmetrically or asymmetrically, the partic-ipation of CNT fibers has made them strong, lightweight,flexible and high efficient.4.2.4.4. Electrical Wiring. High mechanical strength, light-
weight, as well as prominent thermal and electrical properties,the amazing attributes of CNT fibers endow them with thepossibility of electrical wiring applications. The mechanicalperformances of CNT fibers have already exceeded those of
conventional conductive metals while the CNT fibers alwayshave very low density at the same time. However, theirelectrical conductivity (in the 102 S cm−1 range) is still belowaluminum and copper. Although the lower electrical con-ductivity originating from a limited morphology control overCNT fibers becomes the major obstacle for the wiringapplications of CNT fibers, there are still various superioritiesmaking CNT fibers the promising candidate for replacingconventional metal wires in certain cases. Efforts are beingmade constantly aiming to meet the challenge. For example,KAuBr4 incorporated CNT fibers might have a conductivity of1.3 × 106 S m−1.241 Iodine doped DWCNT cables werereported to have electrical resistivity of ∼10−7 Ω·m and highcurrent-carrying capacity of 104−105 A cm−2, as well as muchbetter high temperature stability than metal wires (Figure 12e−h).242 Acid-doped CNT fibers possessed similar failure currentdensity (FCD) ranging from 103 to 105 A cm−2. These valuesfor doped CNT fibers were higher than those for previouslyreported carbon fibers and undoped CNT fibers. But themeasured FCD and continuous current rating (CCR) valueswere still lower than copper wires. Nevertheless, after beingnormalized by the mass density, both the specific failure current(SFC) and specific CCR of the CNT fibers were higher thanthose of copper wires as expected, which were highly preferredfor weight-critical applications.243 As a comparison, the iodinedoped CNT fibers showed better enhancement on conductivitythan acid doped ones, which coincides with the results obtainedby Behabtu et al. via performing doping on wet-spun fibers.199
Another way to improve the electrical conductivity of CNTfibers is the introduction of metal particles.244,245 Theelectrodeposition of Au or Cu particles onto CNT fiber surfacewould significantly increase their conductivity to 2−3 × 105 Scm−1.246 By giving overall consideration to conductivity, massdensity, strength and productivity of the anodized CNT fibers,Zhang and Li et al. developed a continuous electrodepositionmethod for fabrication of CNT-Cu composite fibers (Figure12i). The obtained fibers exhibited a metal-like conductivityfrom 4.08 × 104 to 1.84 × 105 S cm−1 and a mass density of1.87−3.08 g cm−3, depending on the thickness of Cu layer.Furthermore, little damage was caused to the strength of theCu-CNT composite fiber because of their strong interfacialbonding.247 On the other hand, the electrical insulation isequally important for the uses of CNT wires in every-dayelectrical circuits and devices. A good insulating layer for CNTfibers was supposed not to infiltrate or lead to any deteriorationof electrical or mechanical properties of CNT fibers. Thus, apolymer with high wetting angle on the fiber surface or highviscosity should be the right answer to the demand.248
Recently, Lekawa-Raus et al. provided an overview of studiesin this area and expressed a positive attitude on the applicationof CNT fibers in electrical wiring.249
4.2.4.5. Other Applications. Researchers have found thatCNT yarns show piezoresistive behavior whose electricalresistance increased linearly with increasing tensile strain,indicating their potential application as strain sensors.250−252
While exhibiting excellent repeatability and stability, CNT yarnsare suitable for in situ health monitoring after being embeddedin a composite structure.250 Otherwise, through a simpleprestraining-then-buckling approach, stretchable conductorswith buckled CNT fibers incorporated into a polydimethylsi-loxane (PDMS) substrate were fabricated. The CNT fiber/PDMS conductors demonstrated outstanding resistanceretention (∼1% variation) under multiple stretching-and-
releasing cycles up to a prestrain level of 40%.253 In addition,CNT yarns could be employed for the application of emitters,showing excellent field emission properties with highefficiency.254−258 Biological applications of CNT yarns werealso reported in the literature, as an example, mechanicallystrong scaffolds based upon a knitted structure made frommultiply MWCNT yarns were found suitable for tissueengineering.259
4.3. 2D CNT Films
CNT films are 2D structures assembled from CNTs. Benefitingfrom the properties of CNTs and configuration of the films,they exhibit mechanical flexibility, chemical stability, uniqueelectrical and thermal properties, and optical transparency forthe thin CNT films. Thus, CNT films have shown greatpotential for flexible and stretchable electronic and optoelec-tronic devices, while much attention has been paid to thefabrication, characterization, and application of these filmmaterials. From the view of configuration, CNT films can bedivided into randomly stacked films and networks with highlyaligned structures. Most of the reported CNT films are withrandom configuration while the later bearing oriented CNTswith anisotropic properties are chased for specific applications,such as highly electrical and thermal conductive films. Tofabricate the two-dimensional assemblies of CNTs, there areseveral strategies that mainly belong to two categories, whichare known as suspension-based wet method and CVD growth
approaches. Both of them have advantages and disadvantages.Here, the commonly used methods to afford CNT films will bereviewed with comparison and the emerging applications of theas-prepared CNT films will also be outlined at the end of thissection.
4.3.1. Wet Methods Assembled CNT Films. The wetmethods for CNT films include filtration-transfer process, dip-coating, spray-coating, spin-coating, bar-coating, drop-casting,electrophoretic deposition (EPD), and layer-by-layer (LBL)assembly. They are low-cost, highly efficient, and some of themare scalable. At the very beginning of a typical process, thehydrophobic CNTs need to be dispersed in a certain solvent,usually with the help of surfactants or functionalization. Themost commonly used surfactants are anionic surfactant sodiumdodecyl sulfate (SDS) and sodium dodecylbenzyl sulfonate(SDBS), or the nonionic surfactant Triton X-100. Afterdeposition and formation of the CNT films, the preaddedsurfactants are supposed to be eliminated to improve theconductivities within the films. The detailed preparationprocesses are discussed below.
4.3.1.1. Vacuum-Assisted Filtration. The first free-standingmacroscopic SWCNT film, named as “bucky paper”, wasfabricated by the vacuum-filtration method.260 In such a widelyemployed protocol, the well dispersed CNT suspension isfiltered under vacuum to form a randomly packed film (shownin Figure 13a−c). The density and thickness of the as-prepared
Figure 13. (a) Schematic of the vacuum-assisted filtration process for CNT films. (b) A large scale SWCNT thin film on a sapphire substrate. (c)Atomic force microscope (AFM) image of the film surface. Reprinted with permission from ref 261. Copyright 2004 American Association for theAdvancement of Science. (d) Illustration of the dip-coating process. (e) Samples of CNT films coated for 1, 3, 5, and 10 times on PET substrates.Reprinted with permission from ref 271. Copyright 2008 IOP Publishing Ltd. (f) Schematic of the spray-coating process. (g) Photograph of atransparent SWCNT film spray-coated on a glass substrate. (h) AFM image of the thin film, inset is the image under higher magnification. Reprintedwith permission from ref 273. Copyright 2009 Wiley-VCH.
film, which are critical to the sheet conductance and opticaltransparency, are highly controllable through adjusting thefiltration time, the CNT concentration, and the volume of thefiltered suspension.261,262 Afterward, the supported film can betransferred to another substrate263,264 or come to a free-standing CNT film by dissolving the filter paper,261 acquired byspecific needs for application. Many efforts have been made toimprove the properties of the as-formed CNT films, includingthe postdeposition strategy264 as well as optimization andselection of high quality CNT raw materials.265−267 Notably,Hone et al. applied strong magnetic fields during the filtrationof SWCNT suspension to gain films with aligned SWCNTs.The anisotropic films showed high electrical and thermalconductivity along the alignment axis.268 However, there arestill drawbacks for the filtration process. The size of the filmsobtained via filtration is limited by the filtration apparatus, moreparticularly the filter paper, which means it is difficult to work ata large scale. Besides, the irregular morphologies and significantroughness were also found for the filtration derived CNT films.4.3.1.2. Dip-Coating. Dip-coating is such a technique that is
simple and applicable to large areas or substrates with nomatter flat or curved surfaces. It comprises a dip-coatingprocess and a drying process (Figure 13d,e), during which threeparameters are playing an essential role in determining thequality of the obtained films, including CNT concentration,withdrawal velocity, and number of dip-coatings.269 Comparedwith other wet methods, the dip-coating process produces arelatively aligned CNT network since the configuration relieson the interplay between shear forces and Brownian motion ofCNTs during coating and drying. The former tends toalignment of CNTs, whereas the later acts as a randomizingeffect.270 On the other hand, the adhesion between hydro-phobic CNTs and hydrophilic substrate is weak and thusdegrades the quality of CNT coatings. To solve the problem,several studies have utilized surfactants, such as silanes, to
promote adhesion between the coatings and the sub-strates.271,272 Although the dip-coating technique is simpleand scalable, multiple coating-drying cycles are needed to reacha certain thickness with low sheet resistance, which makes italso time-consuming.
4.3.1.3. Spray-Coating. Spray-coating, also known as the airbrush technique, is another simple method to realize the roll-to-roll fabrication of CNT films (Figure 13f−h). Tenent et al. havereported the utilization of spray-coating for achieving large-areaSWCNT electrodes.273 Normally, the spray-coating methodinvolves three steps during the process which are dropletgeneration, deposition of the droplets onto the substrate by airflow, and drying of the droplets on a heated substrate to form afilm.274 It is noteworthy that heating on the substrate isnecessary for the spray-coating fabrication to accelerate thedrying of the droplet, in order to retain homogeneity in theformed films,275−277 which is really a big issue for the spray-coating technique. Also the spraying time is a critical parameterto control the quantity and thus density of the CNT films.268
4.3.1.4. Spin-Coating. As one of the most promisingprocesses for mass production of CNT films (Figure 14a,b),spin-coating shows several advantages such as high uniformityin the as-prepared films, easy and precise control on thethickness, short coating time, low-temperature fabrication, andhigh reproducibility.279−282 However, the features of such atechnique make it difficult for fabricating thick CNT films.
4.3.1.5. Bar-Coating. Bar-coating is suitable for large-scaleCNT films (Figure 14c,d). While using a wire-wound Mayerrod as the operating tool during coating, the diameter of thewound wire determines the size of the grooves and thus controlthe thickness of the final CNT films.283 Furthermore, theconductivity and transparency of the CNT films are easilytunable depending not only on the wire diameter but also onthe CNT concentration in the dispersion.284
Figure 14. (a) Schematic of the spin-coating process. (b) CNT films of different thicknesses spin-coated on PET (left) and glass (right). Reprintedwith permission from ref 279. Copyright 2009 American Chemical Society. (c) Schematic of the bar-coating process. (d) A uniform bar-coatedSWCNT thin film. Reprinted with permission from ref 283. Copyright 2009 American Chemical Society. Schematic illustration of (e)electrophoretic deposition and (f) layer-by-layer assembly.
4.3.1.6. Electrophoretic Deposition. Boccacini et al. havereviewed the electrophoretic deposition (EPD) of CNTs in2006,285 demonstrating it is an economical and versatileprocessing technique for the production of CNT films orcoatings. The EPD fabrication is a two-step process (Figure14e) including electrophoresis and deposition, and the finalmorphology highly depends on the gap between the twoelectrodes, strength of the applied electric filed, depositiontime, and lengths of CNTs.286−289 In addition, a conductivesubstrate is always necessary for satisfying the EPD process,which limits the widespread use of this technique. Although theas-prepared CNT films can be transferred onto anothersubstrate by utilizing a subsequent step such as hot-pressingtransfer, the additional procedure makes the fabrication processbecome much more complicate.290
4.3.1.7. Layer-by-Layer Assembly. The layer-by-layer (LBL)technique allows control of the structure of coatings withnanometer scale precision,291 which is performed by alternatelydepositing negatively and positively charged materials onto asubstrate (Figure 14f).292,293 Since the assembly is forced byelectrostatic and van der Waals interactions, the interconnec-tivity of the structural components is apparently strong.Usually, the LBL protocol is combined with other depositionmethods such as dip-coating and spray-coating. Mamedov et al.reported the LBL assembly of SWCNT and polyelectrolytethrough sequential dip-coating of negatively charged SWCNTin aqueous solution and positively charged polyelectrolyte. Thefreestanding SWCNT/polyelectrolyte LBL films showed highhomogeneity and minimized phase segregation of the twostructural components and therefore exhibited evidentenhancement on strength compared with the neat polymerfilms.294 Later, Kim et al. improved this technique with fasterfabrication by using vacuum-assisted spray-coating to fulfill theLBL assembly. In their work, high performance MWCNTelectrodes for lithium-ion batteries (LIB) with thicknesses oftens of microns were produced on porous carbon substrates,295
and the assembled components were two types of modifiedMWCNTs carrying opposite charges.296
4.3.1.8. Other Wet Methods. Apart from the above listedcommonly used wet methods to obtain CNT films, there areseveral exceptional solution based techniques which have beenreported. The Langmuir−Blodgett (LB) technique is consid-ered as a precise method for CNT films to readily control theirthicknesses and tube-orientation, which is based on the uniformsurface spreading of CNTs that contribute to the formation ofstable Langmuir monolayers on the solution surface. By usingthe LB technique, either multilayer-thick SWCNT films297 ormonolayer SWCNT films with aligned structures298 werefabricated.Sreekumar et al. drop-casted the solution of SWCNT in
oleum and obtained isotropic film with tensile modulus,strength, and fracture strain of 8 GPa, 30 MPa, and 0.5%,respectively, while the in-plane electrical conductivity isrelatively high, with the value of 1 × 105 S m−1.299
The evaporation-driven self-assembly (EDSA) technique,which capitalizes the “coffee ring phenomenon”, was shown tobe capable of attaining aligned SWCNT coatings ranging from0.5 μm wide stripes to continuous films with tunabletransparency in a recent work.300
Another novel method for the assembly of SWCNT films onglass substrates is described by Simmons et al. With the help ofshear mixing, SWCNTs could be dispersed into roomtemperature ionic liquid (RTIL). When the dispersion bead
(1−3 mm wide) was introduced to water, a SWCNT thin film(15 × 30 mm area) was quickly created by spreading on thewater surface and could be transferred to a clean glass substratelater. Although the obtained SWCNT films did not show veryhigh conductivity, the preparation process did pave another wayfor the roll-to-roll fabrication of CNT thin films.301 Speaking tomass production, Fischer et al. reported a mass printingprocess, namely the flexographic printing, for CNT structureson textile and paper substrates. The as-prepared MWCNT filmsalso exhibited good electrothermal properties.302
The advantages and disadvantages of the most popular wetmethods for preparing CNT films are summarized in Table 2.
4.3.2. CVD-Grown CNT Films. The previously introducedwet methods for CNT films are facile and energy saving;however, they still have disadvantages in common. Generally,the wet methods are based on the uniform dispersion of CNTsin specific solvents, which is important for the homogeneity ofthe as-prepared films yet quite difficult to realize due to thepoor surface activity of CNTs. The typical strategies toovercome such issue are usually the application of surfactants orthe introduction of functionalization on CNTs to improve theirdispersibility, accompanied by ultrasonication during mixing.Unfortunately, the addition of surfactants leaves residues whichare hard to be eliminated completely in the film products andthus significantly increase the sheet resistance, while thefunctionalization process disrupts the sp2 structure of CNTsand yielding low film conductivity. Moreover, the violentultrasonication treatment shortens individual CNTs, causing adramatic increase in the quantity of intertube junctions, whichalso inhibits transportation in the films. In one word, althoughthey are simple, efficient, and cost-effective ways for large-scaleCNT films, the solvent-based wet methods could hardly put an
Table 2. Comparison among the Wet Methods for CNTFilms
methods advantages disadvantages
vacuum-assistedfiltration
controllable thickness time consuminglimited sizeirregular morphology on theupper surface
dip-coating simple and scalable multiple coating-drying cyclesare needed to obtain a thickfilm
relatively aligned CNTnetwork
suitable for curvedsurfaces
spray-coating suitable for massproduction
heating on the substrate isnecessary to improve thehomogeneity
spin-coating suitable for massproduction
difficult for fabricating thickfilms
fast and high uniformityeasy and precise controlon the thickness
bar-coating suitable for large-scalefilms
low reproducibility
electrophoreticdeposition
economical and versatileprocess
conductive substrates arenecessary
layer-by-layerassembly
high-precision structurecontrol
multiple steps are included
Langmuir−Blodgettassembly
easy control on filmthickness and tube-orientation
end to certain limitations when it comes to higher perform-ances.On the other hand, nonsolvent techniques based on the
CVD process are able to avoid the issues faced by the wetmethods. The major merits of the CVD derived CNT films aretheir high purity, controllable tube lengths, and lessagglomerate bundles. Even though the CVD process isconsidered to be energy consuming, numerous interests havebeen attracted to the direct synthesis of tough, freestandingCNT films due to their irreplaceable performances.The direct synthesis process reported by Li and Windle et al.
could not only be used for spinning CNT fibers but also becapable of gathering CNT films while the spindle was rotatednormal to the furnace axis outside the hot zone (Figure15a,b).173 For arc-discharge growth of SWCNTs, theproductions are normally randomly oriented with an exceptionof Wang et al.’s work. They introduced a magnetic field duringnanotube growth in order to induce alignment to the as-prepared SWCNTs, and the well-aligned SWCNT films weredeposited onto various substrates including plastic ones.
According to its feasibility of orientation control, this approachis meaningful for scalable fabrication of flexible SWCNT-basedelectronic and optoelectronic devices.303
Xie’s group has developed the floating chemical vapordeposition (FCCVD) technique for nonwoven SWCNT films.The obtained thin films with areas up to several tens of squarecentimeters consisted of entangled SWCNT bundles withdiameters of ∼30 nm.304 Subsequently, slightly alignedSWCNT films with controllable growth rates were discoveredthrough precise control on the sublimation rates of thecatalysts. The directly synthesized films possessed superiorelectrical and mechanical properties over the films made fromsolution based filtration, owing to the oriented distribution andstrong interbundle connections (Figure 15c−g).305 A repeatedstretching and pressing treatment was able to fabricate asuperaligned and highly dense CNT sheet, performing 221%increase on tensile strength than the as-prepared one.306 It isnoteworthy that the enhanced interbundle connection mostlycomes from the as-formed Y-type junctions generating at thesame time with the SWCNT growth rather than the simpleoverlap between CNTs which occurs in the solution-derivedfilms. Furthermore, the efficiently obtained, large-scale, andfree-standing SWCNT films can be easily handled for furtherstudies, such as being twisted into macroscale CNT fibers,178
reinforcing and conducting scaffolds for polymers,307,308 andelectrodes for flexible supercapacitors309−311 or actuators.312
Besides, in a similar work reported by Liu et al., the FCCVDmethod was applied to assemble novel multisheeted book-likemacrostructures with uniform and tunable thickness, named asthe “buckybooks”. As demonstrated in the paper, thebuckybooks showed potential applications as binder-freeelectrodes for supercapacitors and filters with high molecularseparation efficiency.313
4.3.3. Array-Derived CNT Films. The aforementionedpreparation protocols generally lead to randomly packed CNTfilms or slightly oriented ones. Indeed, the well aligned CNTfilms are more promising for conductive uses. With thisconsideration, there are alternative routes toward the massproduction of highly aligned CNT films using verticallyoriented CNT arrays as the starting materials.In one way, meter-long transparent sheets were directly
drawn from a sidewall of CNT forests which were synthesizedby CVD method, seen in Figure 16a-d. The continuous CNTsheets with anisotropic features were generated via end-to-endconnections of CNTs along the draw direction.314 Based on thetunable fabrication of CVD, the tube diameter, number of CNTwalls and the length of sheets are well controlled for the desiredphysical properties.315,316 Especially, the long and nondefectivenanotubes are highly demanded for optimal CNT sheets withexcellent electrical and thermal conductivities and mechanicalproperties.314 In combination with the unique characteristics ofCNTs, the array-drawn CNT sheets with unidirectionalgeometry are capable of various applications, such as trans-parent electrodes, planar sources of polarized broadbandradiation, microwave welding of plastics, flexible organic light-emitting diodes,314 loudspeakers,317 electrical reinforcement forpolymers,318 as well as artificial muscles with charge-inducedactuation behavior.319
In another way, the well-aligned architectures could berealized by the so-called “domino pushing” above the CNTarrays. In particular, the vertical CNTs are forced down to onedirection by pushing a cylinder upon the array withcompression.320,321 The CNTs in the as-formed buckypapers
Figure 15. (a) Schematic of the direct synthesis process and (b)photograph of the synthesized CNT film after infiltration withpolyvinyl chloride. Reprinted with permission from ref 173. Copyright2004 American Association for the Advancement of Science. (c)Picture of an SWCNT film grown by the FCCVD method. (d) and (e)Samples of transparent and homogeneous/inhomogeneous films. (f)SEM image of the SWCNT film, the inset is at higher magnification.(g) SEM image of a single layer SWCNT network, the white arrowsindicate the Y-type junctions and the flow direction during the filmgrowth. Reprinted with permission from ref 305. Copyright 2007American Chemical Society.
were interconnected with strong van der Waals attractions,leading to mechanical stability, which enabled the formedpapers to go to large scale (Figure 16e−g). In addition, theimproved thermal and electrical conductance was attributed tothe well maintained straight morphology of individual CNTswithin the papers.321
4.3.4. Applications. CNT films as tempting 2D materialshave drawn considerable attention in both research andindustry areas. A wide range of applications concerning CNTfilms have been explored, and one may get hints from thesubstantial number of reviews and published research works.Herein, several representative applications will be highlighted.4.3.4.1. Transparent Conductive Films (TCFs). CNT-
transparent conductive films (CNT-TCFs) are well suited forapplications with requirements of high transparency while highconductivity is also in demand, such as touch panels,276
electrodes of solar cells273,277,278 and organic light emittingdiodes (OLEDs).263 In comparison with ITO which has beenwidely used in the electronic area, although the emergingcarbon-based films can hardly surpass ITO on optoelectronicperformances, their low-cost, environmental friendly, highlystable and flexible features, especially the outstanding capacityin theory, encourage researchers to keep chasing the excellenttheoretical value of individual CNT in its assembled macro-scopic 2D forms. To accompany the continuous struggle forfuture applications, CNT-TCFs are regarded as the prospectivereplacement for traditional ITO films. To this purpose, thereare two major issues that need to be paid attention to forachieving high performance CNT-TCFs: one is the intertubejunction resistance which causes principal degradation on theintegral conductivity of CNT films and the other is theinhomogeneity of the CNT networks which influences theuniform transparency. The first negative effect could becrippled at a certain extent via various strategies, such asenhancing the bonding between CNTs or choosing long CNTs
for the TCFs.322,323 The longer the unit CNT is, the lesscontact junctions exist within the film. As for the second issue,the homogeneity highly depended on the fabrication process,so it is important to optimize the preparation technique.Most of the TCFs are made from SWCNTs rather than
MWCNTs, and in general, better transparency and conductiveperformances were found in the SWCNT films.275 Althoughthe DWCNTs were revealed to provide a better transmittance-conductance performance on films than SWCNTs andMWCNTs, because semiconducting tubes are usually inevitablein SWCNTs, and MWCNTs are thought to absorb much morephotons.324 But even so, the research hotspots are still focusedon SWCNT-TCFs, probably due to the comprehensiveconsideration of their performances and the cost of synthesis.To face the polydispersity problem of SWCNTs, involvingmetallicity and diameter distribution, Green and co-workersemployed a density gradient ultracentrifugation (DGU)technique to produce transparent conductors consisting ofprimarily metallic SWCNTs with small diameter distributions,where enhanced conductivity over several times was obtainedas a consequence.265
The conductivity of a CNT film is an important performanceindex for its use in the field of electronics, and there are manyways to provide improvement on such a point. A related workdecreased the sheet resistance of SWCNT films by a factor of 5via a postdeposition in nitric acid (HNO3) and thionyl chloride(SOCl2) bath. Both of the exposures helped improve thestability of the functionalized SWCNT films, while the contactwith SOCl2 also caused formation of acyl chloride functionalgroups which firmly bonded SWCNTs, resulting in significantlyimproved transport properties.264 Meanwhile, post treatmentwith HNO3 is a commonly seen procedure for the CNT filmscoming by wet methods.271,272 The effect of HNO3 treatmentwas thought to remove the residual surfactants and densify theobtained films, both of which could improve the conductivity
Figure 16. (a) Photograph of an array-drawn MWCNT sheet with width of centimeters and length of meters. (b) and (c) SEM images of a CNTarray being drawn into a sheet from a different angle of view. (d) SEM micrograph of a structure made by stacked MWCNT sheets. Reprinted withpermission from ref 314. Copyright 2005 American Association for the Advancement of Science. (e) Schematic of the “domino pushing” method toprepare the aligned CNT paper. Images illustrate (f) the as-made buckypaper in large-scale and (g) folded paper. Reprinted with permission from ref321. Copyright 2008 IOP Publishing Ltd.
by enhancing the connectivity between CNTs,325 as well as p-type doping.326
Andrade et al. has compared the electrical properties of theCNT networks made from various solution-based techniques,including dip-coating, spray-coating, vacuum filtration, andelectrophoretic deposition. They found that SWCNT filmsobtained by dip-coating showed the lowest sheet resistance(186 Ω sq−1) for a given transparency (86%) at 550 nmwavelength, probably owing to their relatively smooth andaligned geometries.327 The CNT array derived films usuallydeliver inferior conducting performance in comparison withwet-assembled CNT films, for example, showing performanceof 1.6 kΩ sq−1 at 86.5% transparency. After laser trimming anddeposition of Ni and Au metal, Jiang et al. obtained CNT filmswith significantly improved conductivity. Two typical values ofsheet resistances and transmittances of the modified films wereindicated by 208 Ω sq−1 at 90% and 24 Ω sq−1 at 83.4%, whichwere comparable to those of ITO films and meet therequirements of touch screens, LCDs, and solar cells.328
As we’ve already known that the surfactants used to helpdispersing CNTs into solvents will have negative effects on thefilm conductivity simultaneously, there are some surfactant-freestrategies to thoroughly avoid such influences. Chlorosulfonicacid (CSA) was supposed to generate uniform and highlydebundled CNT dispersions for further deposition.323,329 Theuse of superacid CSA has a number of benefits over surfactants,especially for the exfoliation of CNT bundles and p-typedoping in the films. An extremely low sheet resistance of 60 Ωsq−1 was reported by Hecht et al. at 90.9% transparency (at 550nm) through filtrating the CSA dispersion of SWCNTs.266
Besides, a two-step shearing process initiating from CNT arrayscould also realize the surfactant-free dispersion of individualCNTs in benzyl alcohol, yet with relatively high resistance inthe formed films.330
Since TCF is one of the most promising applications forCNT thin films attracting intensive studies, we present acomparison of the main achievements reported in recentresearch to get a global overview upon the subject, and theresults are displayed in Figure 17. As is shown in the figure, thesolution-based wet methods are the most common ways toCNT-TCFs, which have proved their possibility for commercial
demands (e.g., touch screen and LCD); however, they are stillinferior to the best records made by ITO-based films.
4.3.4.2. Supercapacitors. The 2D macroscopic CNTnetworks are very popular materials for the electrodes ofsupercapacitors, thanks to their outstanding storage andtransport capabilities.One of the charming features of CNT film-based super-
capacitors is their flexibility. Niu et al. fabricated compact-designed supercapacitors by rolling up free-standing SWCNTfilms with separators (Figure 18a,b). The SWCNT films servedas both binder-free electrodes and current collectors, makingthe energy storage device really lightweight. Significantly, highenergy and powder densities (43.7 Wh kg−1 and 197.3 kWkg−1) were achieved due to the small equivalent seriesresistance.309 Later on they developed a “repeated halving”approach to transfer ultrathin SWCNT films onto PETsubstrates via electrostatic adsorption. After being assembledwith separators and electrolytes, the transparent and flexiblesupercapacitors were obtained.311 Cui et al. have made effortson large-scale flexible supercapacitors. For example, highperformance supercapacitors with a specific capacitance of200 F g−1 and a stable cycling life were realized based oncommercial paper. Certainly, the commercial paper was highlyconductive by conformal coating of SWCNT and Ag nanowiresthrough the rod-coating method. Furthermore, this low-costconductive paper can also be applied in lithium-ion batteries aslightweight current collector.331 Interestingly, the paper-basedsupercapacitor could even be integrated onto a single sheet ofpaper while SWCNT films acted as electrodes and currentcollectors and the paper as substrates and separator.332 Exceptfor paper, porous textiles such as cellulose or polyester werealso employed as the stretchable substrates following a similarprotocol to make them conductive. This coating method hasbeen demonstrated as a feasible approach for wearableelectronics and energy storage devices.333
Since the specific surface area and transportation paths inelectrodes are highly dependent on their porosity, theintroduction of sacrificial nanoparticles during the formationof CNT films was reported to enhance porosity in the filmelectrodes. 200 nm diameter polystyrene (PS) nanosphereswere used during the filtration process, and the performance ofthe supercapacitors was much better at a higher PSconcentration.334
In addition to the utilization in electrical double-layercapacitors (EDLCs), CNT films showing both large surfaceareas and good conductivities are frequently used inpseudocapacitors with much higher pseudocapacitance andenergy densities, as ideal scaffolds for electroactive materials,such as MnO2,
333 RuO2,334 or PANI.335,336
4.3.4.3. Other Applications. Besides the applications asTCFs and electrodes for energy storage devices, 2D CNTnetworks are also popular in diverse areas due to their superiorphysical and chemical properties. The possible uses of CNTthin films for electronics and sensors have been well proved(Figure 18c).337−339 CNT film heaters were developed basedon the sensitive and evident electrothermal response.302,340
Biomedical applications were also revealed where CNT filmsperformed as the substrates for growth of cells.341 The advancesof CNT membranes for water purification and gas separationwere developed based on their porous nature.342,343 Stacking ofaligned CNT sheets was used for lightweight microwaveabsorption film, with the absorption frequency being accuratelycontrolled by varying the intersectional angles of the CNT
Figure 17. Summary of the opto-electrical properties of CNT-TCFsand comparison with those of ITO and touch screen/LCDrequirements.
sheets.344 At last, the strong, flexible, and conductive CNT filmsare able to be incorporated with polymers by a simpleinfiltration process.345−347 The composites with integratedCNT network being the skeleton show tunable mechanical andelectrical properties,307 and have found possible applications asflexible/stretchable conductors (Figure 18d−g).308,318
4.4. 3D CNT Architectures
The 3D macroporous structures based on CNTs generallyexhibit large specific surface area and structural stability/elasticity under compression, as a result of the loose spongystacking of CNT building blocks. Unlike the ordinary interestpoints upon their fiber or film form, the pore structures in the3D assemblies of CNT receive intense investigation as thegreatest structural merit, rather than defects which areundesirable in CNT fibers and films. The cavities existed inthe 3D architectures provide numerous interspaces for massstorage and transport while keeping the porous monolithslightweight. In order to form and stabilize the pore structuresthrough the overlapping of CNTs, unique fabrication processand structure reinforcement are both in high demand.4.4.1. Wet Gels Initiated CNT Aerogels. CNT gels, either
hydrogels or organogels, are the most common precursors forCNT aerogels. In a typical process, the CNT gels are formedprimarily, followed by supercritical/freeze-drying which re-moves the solvents trapped in the pores while preserving theinterconnected porous structures simultaneously. Therefore,the two critical factors for the gel formation and architecturestabilization are (i) good dispersion of CNT in suspensions and(ii) strong interactions between the CNT units, which causesassembling into overlapped framework and retain the formedstructure during and after removal of the liquid phase.CNT foams can be produced from the relatively accessible
gelatin-SWCNT composite aerogels after the gelatin compoundwas thermally removed.348 Alternatively, the introduction of afoaming and drying process to the stabilized CNT suspensionsmay also create CNT foams with low specific surface area
(around 35 m2 g−1).349 However, these methods are not widelyapplied since their high cost, low yield, and unsatisfied qualityfor practical applications. On the other hand, the self-gelationbehavior of SWCNTs was studied after functionalizing thenanotubes by a technique similar to the oxidization of graphiteto graphite oxide. The oxidized SWCNTs (SWCNTox) wereable to generate a uniform suspension in water, owing to theGO-like functional groups. Furthermore, the major interactionsbetween SWCNTox came from hydrogen bonds involvingwater molecules, and viscous hydrogels could form slowly atlow concentration (0.3 wt %).350 Bryning et al. reported theCNT aerogels derived from aqueous gel precursors followed bycritical point drying (CPD) or lyophilization (freeze-drying;Figure 19). The obtained aerogels are light (with densitiesranging from 10 to 30 mg cm−3), robust, and conductive (withelectrical conductivity as high as 1 S cm−1), whose structure canbe further reinforced by incorporating small amounts of PVA.However, the conductivity decreased dramatically to 10−5 Scm−1 because of the PVA incorporation. Interestingly, they alsofound that the freeze-drying process induced more damages onthe CNT network than the CPD process and led to lessconductive samples.351 Using similar strategy and taking PVAas the reinforcement for CNT aerogels, another work fabricatedMWCNT aerogels by the suspension derived process withCNT content ranging from 25 to 100 wt %. The absorptioncapacity was found to increase with CNT content because thespecific surface area considered as the critical parameter for theabsorption behavior performed with this trend.352
It is well-known that the functionalization of CNTs could notonly help dispersing them in various solvents, which is theprincipal requirement for the gelation of CNTs, but also act ascross-linkers to enhance the interactions between individualCNTs, therefore facilitating the assembly process. In addition,the integrated behavior of the assemblies may possibly changeaccording to the unique character of the modifiers. MWCNTsgrafted with hyperbranched poly(amido amine) were able toassemble in DMF with the presence of linear poly(amido
Figure 18. (a) and (b) Photographs of the compact-designed supercapacitor made by rolling up SWCNT films with the separator. Reprinted withpermission from ref 309. Copyright 2011 Royal Society of Chemistry. (c) Optical image of the flexible SWCNT transistors integrated circuits on apolyimide substrate. Reprinted with permission from ref 339. Copyright 2008 Nature Publishing Group. (d) and (e) Demonstration of the flexibilityof an SWCNT/PDMS stretchable conductor. (f) A transparent SWCNT/PDMS film and (g) the transmittance spectrum of the sample in (f).Reprinted with permission from ref 308. Copyright 2012 Wiley-VCH.
amine) under sonication at 20 °C. The CNT gels were formedvia hydrogen bonds and performed a switchable featurebetween sols and gels, the sol−gel switching was easily realizedby heating and ultrasonicating.353 The functionalization withpoly(3-(trimethoxysilyl) prolyl methacrylate) (PTMSPMA)may provide intense chemical bonding between MWCNTs,leading to ultralight free-standing MWCNT aerogels with adensity of 4 mg cm−3. The resulting MWCNT aerogelspossessed a hierarchically porous structure with anisotropic andmacroporous honeycomb channels and mesoporous honey-comb walls (seen in Figure 20), which were also compressionrecoverable and conductive, thus with the potential to be usedas pressure and chemical vapor sensors.354 Similarly, ferrocene-grafted poly(p-phenyleneethynylene) (Fc-PPE) cross-linkedCNTs through noncovalent interactions. It is worth notingthat the thermal annealing process (350 °C in air) can further
improve the mechanical and electrical properties of theobtained CNT aerogels, as well as increase their surface areaand porosity. The authors ascribed this to reopening of theoriginally blocked micropores and small mesopores in the as-prepared CNT aerogels.355 In another protocol to fabricateMWCNT scaffolds, the radical initiated thermal cross-linkingby benzoyl peroxide (BP) was employed through the reactionbetween radicals and the double bond network on theMWCNT structure. Thereafter, this method was demonstratedas a general way for constructing scaffolds with othernanocarbons such as fullerenes, SWCNTs, and graphene.356
Some biomaterials, for instance, cross-shaped streptavidin-DNA(SA/DNA) complex can also be used to assist the assembly ofCNT aerogels. The resulting SWCNT aerogels possessed avery low density of less than 0.75 mg cm−3. Most interestingly,a completely new photon-assisted photoluminescence emissionat 1.3 eV was found in such SWCNT aerogels.357
As can be seen in the above depiction, although thepolymeric cross-linkers would bind the CNTs together, thepresence of the binders also limits the electrical and thermalproperties of the CNT assemblies. To prevent the degradationon such properties which are quite important to practicalapplications, organic sol−gel chemistry is an effective solutionto the problem. The process was first developed by Pekala,358
where the polymerization of organic precursors producedhighly cross-linked organic gels which can be further dried andpyrolyzed to yield porous carbon structures. Worsley and co-workers improved such a process to make it accommodate forcross-linking carbon nano materials, and several works havebeen reported on CNT and graphene 3D assemblies. In atypical process, CNTs are dispersed in deionized water withresorcinol and formaldehyde with sodium carbonate as thecatalyst. After reaction upon heating, the resulting gels weredried with supercritical CO2 and pyrolyzed at 1050 °C. It wasthe pyrolysis that transferred organic binders into carbonaceousones and thus allowed the electrical and thermal properties tobe maintained.359 Actually, the CNT loading played a criticalrole on the properties of the formed aerogels (Figure 21). AtSWCNT loading of 55 wt %, the original size and shape of theCNT network can be retained after supercritical drying andpyrolysis, which indicated the antishrinkage effect of CNTs.Moreover, the electrical conductivity was enhanced obviouslyonly when the SWCNT loading reached 20 wt %.360 To furtherextend their application, the SWCNT aerogels were coatedwith oxides (SiO2, SnO2, or TiO2) through sol−gel deposition.Owing to the mechanical robustness of the organic sol−gelchemistry derived aerogels, they were stable enough to sufferanother sol−gel process with the oxide deposition. As a result,the composite aerogels showed no degradation on theconductivity and bore significant reinforcement on themechanical properties. Although it is not investigated in thepaper, the authors did propose the potential use of the oxide/SWCNT aerogels as battery electrodes, sensing devices, andcatalysts.361
4.4.2. Template-Directed CNT Architectures. Icetemplating method is a facile and environment-friendly routeto the porous network of CNTs. The ice templates can beeasily removed and are controllable by adjusting some of thefabrication parameters. Kwon et al. created MWCNT cryogelswith aligned and nonaligned porous structures, which wereprepared through ice-templating or sol−gel gelation method,respectively. MWCNTs were first dispersed in the silk fibroinaqueous solution, nonaligned MWCNT cryogels were obtained
Figure 19. (a) Images of pristine CNT aerogels (left) and PVAincorporated composite aerogels. (b) Three pieces of the PVA-reinforced CNT aerogels are able to support 100 g. (c) SEM image ofa CNT aerogel reinforced in 0.5 wt % PVA solution. (d) Transmissionelectron microscopy (TEM) image of a pristine CNT aerogel.Reprinted with permission from ref 351. Copyright 2007 Wiley-VCH.
Figure 20. SEM images of MWCNT aerogels showing a porousstructure and mesoporous honeycomb walls. Reprinted withpermission from ref 354. Copyright 2010 American Chemical Society.
following the normal lyophilization process after the gelation ofsilk fibroin was completed, whereas the aligned cryogels wererealized by unidirectional freezing the MWCNT-silk fibroindispersion and drying. It is worth mentioning that the silkfibroin may act as structural binder for the aligned cryogels orcompound of solid structure in the nonaligned cryogels.Therefore, the aligned porous structures showed higher thermalstability and electrical conductivity than the randomly porousstructures due to preferable interconnections betweenMWCNTs.362
In fact, the mostly reported ice-templating method forordered 3D CNT porous structures is named as the icesegregation induced self-assembly (ISISA) process, which wasintensively studied by Monte and co-workers.363 Briefly, theISISA process was performed by unidirectional freezing andfreeze-drying of the well dispersed CNT-CHI aqueoussuspension. The unidirectional freezing allowed the micro-channeled structures to align well in the freezing direction witha well patterned morphology (Figure 22). It is a simple andversatile approach to achieve 3D CNT assemblies with orderedporous architectures, where the morphology of the CNTarchitectures is highly dependent on the CNT content and thefreezing rate.364 Furthermore, the long-range microchanneledstructure makes the as-prepared CNT 3D architecturesaffordable for various applications, especially as biocompatiblescaffolds,365−369 as well as supports for catalytic nano-particles.364
Sponges are the most widely used cleaning tools in our dailylife with a hierarchical macroporous nature. Benefiting fromtheir high porosity, sponges are strong absorbing media withsignificant inner surface area. That is to say, the readily availablesponges provide high possibility to absorb CNT suspensionsand allow the CNTs conformal coating onto their skeletons,achieving 3D CNT porous architectures with sponge templates.By employing a commercial cellulose sponge, which shows a
high water absorption capacity and with pore sizes in the rangeof 100−500 μm, CNTs were coated onto the template using asimple “dipping and drying” process by immersing the spongeinto CNT ink suspension several times. The relatively low massof CNT coating (0.24 mg cm−2) not only improved theelectrical conductivity of the sponge (with a sheet resistance of1 Ω sq−1) but also maintained a hierarchical macroporousmorphology with open pores. It is meaningful since the openpores are crucial for the accessibility of electrolyte in theapplication for energy storage.370 In another work, low cost andrecyclable kitchen sponges were taken through the sameprocedure to make the MnO2−CNT-sponge supercapacitorelectrodes yet with organic electrolyte. In comparison withaqueous electrolyte, energy density of the supercapacitors wassignificantly improved by several times in organic electrolyte.371
The polyurethane (PU) templated composite sponge achieveda conductance of 1 S cm−1 with a CNT coating of only 200 nmthick, and simultaneously, the coated sponge was bothstretchable and compressible as the mechanical properties ofthe original uncoated sponge were well preserved (Figure 23).Moreover, the CNT-sponges also showed advantages aselectrodes for microbial fuel cells (MFCs), showing a maximumareal power density of 1.24 W m−2 and a maximum volumetricpower density of 182 W cm−3.372 The sponge templates grantCNT assemblies hierarchical macroporous structures, andconversely, the coating of CNTs usually changes the surfacecharacters of the coated sponges. For example, improvedbiocompatibility was found in collagen sponges with MWCNTcoatings.373,374 Besides, the enhancement on electrical con-ductivity of polymer scaffolds was demonstrated with very littleamount of CNT coatings based on the similar principle.375,376
4.4.3. CVD-Grown CNT Sponges. The CVD path way is afeasible approach to achieve the growth of CNT and foam-likeCNT architectures at one time, allowing the latter with eitherwell-arranged or randomly arranged pores. In the case of CNT
Figure 21. SEM images of the organic sol−gel chemistry-derived CNTfoams at CNT loadings of (a) 30 wt % and (b) 55 wt %. TEMmicrographs of a 30 wt % CNT foam at (c) low and (d) highmagnifications. Reprinted with permission from ref 359. Copyright2009 American Institute of Physics.
Figure 22. SEM images of the morphology of MWCNT/CHI 3Dmonoliths with MWCNT content of (a) 66 wt %, (b) 80 wt %, (c) 85wt %, and (d) 89 wt %. The inset in (d) shows the walls of thearchitecture are constructed by interconnected MWCNTs. Reprintedwith permission from ref 364. Copyright 2007 American ChemicalSociety.
arrays/forests, CNTs are vertically aligned with their growthsimultaneously yet the third dimension is in strictly limitedrange according to the length of individual CNTs. However,some behaviors, especially mechanical performances of thevertically grown CNT arrays are even higher than the foam-likematerials, since the anisotropic structure prefers to exhibit theexcellent properties of CNT along the tube axis. Cao et al.fabricated CNT arrays with supercompressible behavior. Whilethe constructed architectures present an open-cell foamstructure, the nanotube struts can be squeezed intensively bybuckling and folding themselves like springs (Figure 24). As a
result, the high compressive strength, fast recovery rate, andstructural stability after thousands of compression-release cyclesindicated such CNT arrays a potential application as theenergy-absorbing coatings.377
On the contrary to the aligned structures of CNT network,Xu et al. prepared randomly arranged CNT foams using theCVD method, with a height of 4.5 mm for the bulk material.Their CNT foams were assembled from interconnected longCNTs and exhibited invariant viscoelasticity over a widetemperature range from −196 to 1000 °C.378 In another CVDroute to achieve the randomly packed CNT structures, Cao and
Wu et al. built centimeter-thick sponges with short and straightCNTs as building blocks, which were MWCNTs withdiameters of 30−50 nm and lengths of tens to hundreds ofmicrometers. The as-prepared CNT sponges had a specificsurface area of 300−400 m2 g−1 and average pore size around80 nm. Interestingly, while the wet-gel derived CNT aerogelsare always brittle and need reinforcement by polymer binders,the directly grown pristine CNT sponges are much more robustand flexible, revealing the strong interconnections betweenCNTs in a three dimensionally isotropic configuration (Figure25).379 Later, the authors found the bulk densities of the CNTsponges could be adjusted from 5.8 to 25.5 mg cm−3 bycontrolling the source injection rate from 0.10 to 0.25 mLmin−1. The variation made the sponges transferring from soft tohard along with the increase of density. The soft spongesshowed high compressibility of up to 90% volume reduction,while the hard sponges can merely recover to 93% of theiroriginal volume after compression.380
In addition, the CVD-grown CNT sponges could be easilydecorated with dopants or particles to modify their perform-ances for different purposes. The boron-doping during CVDsynthesis of CNT sponges led to the formation of atomic-scale“elbow” junctions and covalent interconnections betweenCNTs, thus making the materials robust and elastic.381
Meanwhile, the microwave decoration of Pt nanoparticles onan entangled CNT scaffold can be employed as a cathode for ahigh power proton-exchange membrane fuel cell (PEMFC).382
In a serial work, porous CNT sponges were decorated withamorphous Si383 or atomic layer V2O5,
384 correspondingly, inorder to be applied as anode or cathode for high performanceLi-ion batteries.
4.4.4. Applications. Because of the highly conductiveproperties of individual CNT and the porous configuration ofCNT 3D architectures which provide large specific surface areawith activity, as well as pathways for possible mass transport,the CNT assemblies in 3D construction have foundapplications in various areas.
4.4.4.1. Supercapacitors, Lithium-Ion Batteries, and BioFuel Cells. Aligned MWCNT arrays grown on a metallic alloycan be used directly as electrodes for a double-layercapacitor.385 Related researches further showed that 3D
Figure 23. (a) Schematic of the CNT-sponge. (b) SEM micrograph of the CNT-sponge. (c) A PU sponge before (left) and after (right) CNTcoating. Photographs demonstrating the CNT-sponge is (d) stretchable and (e) compressible. Reprinted with permission from ref 372. Copyright2012 Royal Society of Chemistry.
Figure 24. (a) SEM image of an original free-standing CNT foam(up) and a compressed foam after 1000 compression cycles with strainof 85% for each compression (bottom). (b) Magnified image showsthe wavelike buckles at the bottom of the compressed foam. Reprintedwith permission from ref 377. Copyright 2005 American Associationfor the Advancement of Science.
Figure 25. (a) Optical image, (b) cross-sectional SEM image, and (c) schematic of the CNT sponge. Photographs show (d) a stripe-like CNTsponge could endure severe twisting without breaking. (e) densified CNT sponges are able to full recovery to original shape upon ethanolabsorption. Reprinted with permission from ref 379. Copyright 2010 Wiley-VCH.
Figure 26. SEM images of cell cultivation of human Saos-2 osteoblasts on (a) an MWCNT-CHI (NTC) scaffold and (b) an MWCNT-CHIglutaraldehyde mineralized (NTCGM) scaffold. Reprinted with permission from ref 369. Copyright 2012 Wiley-VCH. (c) and (d) Optical imagesshowing the engine oil absorption of MWCNT sponge. (e) and (f) the sponge can be reused after burning or squeezing and tracked. Reprinted withpermission from ref 381. Copyright 2012 Nature Publishing Group. The resistance change of MWCNT aerogel (g) upon exposure to chloroformvapor and (h) with loadings, the arrows indicate the moments of applying and releasing of loading. Reprinted with permission from ref 354.Copyright 2010 American Chemical Society.
architectures with hierarchical structures were able to providehigher specific capacitances while being utilized as electrodes.With the help of 3D templates, such as sponges and textiles,porous electrodes were fabricated through a simple dip-coatingprocess, and the modification of MnO2 with pseudocapacitancefurther improved the performance of the integrated super-capacitors.370,371,386 In particular, a remarkable specificcapacitance of 1230 F g−1 was achieved at a scan rate of 1mV s−1, based on the MnO2−CNT-sponge hybrid electrodesand applying an aqueous electrolyte.370 However, organicelectrolytes permit a larger voltage window than aqueouselectrolytes; therefore, the energy density of supercapacitorswas reported tripled in Et4NBF4 electrolyte, and increased bysix times in LiClO4 electrolyte.
371
The CVD-grown CNT sponge-like structure was consideredas a perfect conductive backbone for electrodes of lithium-ionbatteries, either cathode or anode, depending on the activematerial incorporated. For instance, the Si deposited CNTsponge was applied as anode with large areal capacity,383
whereas the atomic layer deposition (ALD) V2O5 coatedMWCNT sponge served as a high performance cathode.384
CNT scaffolds exhibited outstanding biocompatibility whichallowed strong cell adhesion, protein adsorption, and bacteriaimmobilization. Both the collagen sponge template-assisted andISISA protocol derived 3D CNT monoliths were deeplyinvestigated in their applications for tissue engineering and cellcultivation (Figure 26a,b).365−367,369,373,374 Furthermore, theirmicrochannelled architectures showed high suitability for theuses as electrodes for microbial fuel cells.368,372,387
4.4.4.2. Absorbers. As for environmental applications, CNTsponges revealed high sorption capacity and high sorption ratefor a wide range of solvents, oils, and organics. The massuptaken by the unique CNT based sorbent materials wereusually over 100 times their own weight. After sorption, theflexible and stable structures grant CNT sponges goodrecylcability upon burning or squeezing (Figure 26c−f).379,381,388 In another way, filters made from CNT spongeswere used to remove nanoparticles (Au and CdS) and dyemolecules from water with high filter capacities, simply on thebasis of a physical trapping mechanism.389,390
4.4.4.3. Energy Adsorption. Interestingly, Cao et al.developed the design of 3D CNT architectures throughtailoring the combination of aligned CNT arrays and randomlyentangled sponges in various configurations, such as series,parallel, package, and sandwich structures. The resulted novelstructures differed in mechanical behavior depending on thearrangement of the two components, owing to their distinctdeformation mechanisms. The integral behavior is a combina-tion of the response from each component. The composite 3Darchitectures also performed differently from individual arraysor sponges according to the synergistic deformation process. Asa result, the well controlled and easily synthesized composite3D structures showed their advantage in energy absorption andcushioning under mechanical compression.391,392
4.4.4.4. Other Applications. The high specific surface areaand porosity also makes CNT scaffolds ideal supports fornanoparticles with catalytic activities (Pt, CuO, and CdS) andthus exhibited potential applications for catalytic purpo-ses.352,364,393
As CNTs form a conductive and compression recoverablenetwork, electrical resistivities were found to change linearlyand reversibly after 300 cycles of large-strain compression up to60%.380 Thus, the sensing capabilities of MWCNT aerogels to
pressure and chemical vapor were demonstrated with highsensitivity (Figure 26g,h).354
5. ASSEMBLIES OF GRAPHENE
Because of the structural correlation between graphene andCNTs, it is convenient to draw on the experience of maturetechnologies for CNTs, which has truly promoted the rapidgrowth of graphene research. However, the distinct character-istics of graphene suggest requirement to explore particularstrategies for graphene assemblies. Because of the much betterdispersibility, stability and processability in polar solvents,especially in water, the oxidized derivative of graphene−graphene oxide (GO) has become the most popular substitutefor graphene assemblies, with great convenience for wet-processing based on homogeneous solutions. More impor-tantly, these oxidized graphene assemblies could be reducedsubsequently to recover most of their graphene-like properties.Therefore, such indirect yet easy to access synthesis path fromGO to RGO has turned to a general protocol for the realizationof graphene architectures.
5.1. 1D Graphene Fibers
The topic of graphene fibers has seen recent favor in theresearch field. Unlike the traditional strategies for continuousfibers built up from 1D rod-like molecules, such as CNTs, theabout to be discussed graphene fibers are assembled by the 2Dsheet-like graphene platelets. Although the currently achievedmechanical properties of the later ones are usually inferior tothe CNT fibers, the relative simplicity and low cost of massiveproduction still attracted researchers’ passion on strong,flexible, and multifunctional graphene fibers. Theoreticallyspeaking, the 2D graphene sheets actually have higher contactarea with each other than CNTs with curved outer surfaces,which means better interactions and less gaps will be performedin the assembled fibers.
5.1.1. Wet-Spun Graphene Fibers. The development ofwet-spinning of graphene fibers started with a similar synthesisprocedure to the preparation of SWCNT fibers proposed byVigolo et al.,159 through injecting well-dispersed GO solutionsinto a coagulation bath containing PVA solution. The tensilestrength of the as-prepared GO/PVA composite fibers was 240MPa, a little higher than the value of SWCNT fibers (210 MPa)prepared previously. Moreover, the hybrid fibers combiningGO and CNTs exhibited improvements on both mechanicalproperties and electrical conductivities, taking advantage of theso-called synergetic effect.394 The first continuous and neatgraphene fibers, reported by Xu and Gao, were produced vialiquid crystal-based wet-spinning.395 The wet-spinning processis regarded as a facile way which enables tailoring both structureand property of the resulting graphene fibers. The reportedfiber configurations achieved by wet-spinning include solid,hollow, porous, and ribbon-like. Therefore, the accessibility anddiversity of wet-spinning have made it the most commontechnique for graphene fibers.
5.1.1.1. Liquid Crystal Behaviors of Graphene andGraphene Oxide. The discovery and studies on the liquidcrystal (LC) behaviors of graphene and its derivates hasmotivated the solution-based liquid crystal phase spinning ofgraphene fibers, since higher performances are derived from theprealignment of graphene sheets in the LC liquids. Single-layergraphene, which was exfoliated from graphite, was found toexhibit dissolution in superacid of CSA due to protonation ofthe graphene sheets. The mechanism of dissolution was similar
to that of SWCNTs in superacids. When the concentration ofgraphene dispersions reached 2 mg mL−1, the isotropic/liquid-crystalline transition occurred with formation of nematic phasewhich is typical for discotic materials.396 With more practicalsignificance, Kim et al. reported the liquid crystallinity of GOaqueous dispersions. The graphite source showed influence onthe liquid crystal formation by varying the shapes and sizes ofthe dispersed GO platelets. In their work, the GO sheets withaverage aspect ratio of 1600 completed nematic phaseformation at 0.53 wt %, whereas the smaller GO sheets withaspect ratio of 700 kept intermediate phase until 0.75 wt %.Moreover, the alignment of the GO liquid crystals were foundto be induced by mechanical shear or magnetic field.397 Similarresults were disclosed by Gao’s group, of which the well-dispersed GO aqueous solutions showed the isotropic−nematic
phase transition at a concentration of 0.025 wt % (0.25 mgmL−1), and a stable nematic phase formed at 0.5 wt % (5 mgmL−1) (Figure 27a−c).398 Afterward, the same groupdemonstrated the first chiral liquid crystal phase of GO sheetsin aqueous dispersions with a twist-grain-boundary phase-likefeature, holding lamellar and helical structural orderingssimultaneously (Figure 27d,e). In that case, the transition toa typical birefringent texture of nematic LCs showed up at avolume fraction of 0.23%.395 Then continuous and neatgraphene fibers were spun from high concentrated (5.7 vol%) LC dispersions, indicating the liquid crystalline phase ispromising for the fabrication of macroscopic ordering materials,especially for graphene fibers. In addition, the alignment of LCscould be utilized for more functional applications. To take oneexample: GO LCs were expected to induce partial orientation
Figure 27. (a) Typical AFM image of GO sheets. Polarized-light optical microscopy (POM) observations between crossed polarizers of GO aqueousdispersions indicating the formation of (b) nematic and (c) lamellar liquid crystalline phases. (b and c) Reprinted with permission from ref 398.Copyright 2011 American Chemical Society. (d) POM image of GO chiral liquid crystals. (e) Twist-lamellar-block model for GO chiral liquidcrystals. (a, d, and e) Reprinted with permission from ref 395. Copyright 2011 Nature Publishing Group.
Figure 28. (a) General apparatus for wet-spinning GO fibers, the inset shows GO aqueous dispersion as the spinning dope. (b) Typical fracturesurface of GO fibers made from giant GO sheets. Reprinted with permission from ref 405. Copyright 2013 Wiley-VCH. (c) SEM image of GO fiberwith a tighten knot. (d) Meters-long continuous GO fiber wound on a Teflon drum. (e) Graphene fibers woven with cotton threads. Reprinted withpermission from ref 395. Copyright 2011 Nature Publishing Group. (f) GO-Ag nanowires fibers collected on a plastic drum. (g) Demonstration ofRGO-Ag fibers lighting up LEDs in stretched (top) and relaxed (bottom) states. Reprinted with permission from ref 406. Copyright 2013 Wiley-VCH. (h) MMT-graphene fiber keeps conductive while being heated to glowing red, the inset shows the wet-spun protofibers. Reprinted withpermission from ref 408. Copyright 2015 American Chemical Society.
of organic molecules for residual dipolar coupling (RDC)measurements.399
5.1.1.2. Liquid Crystal Based Wet-Spinning of GrapheneFibers. The formation of liquid crystals facilitated efficientalignment of GO in the spinning dope and the subsequentlyprepared fibers.400,401 The first meters-long, polymer-free GOfibers (Figure 28a−e) reported by Xu et al. showed moderatestrength of 102 MPa and Young’s modulus of 5.4 GPa,however, the fracture elongations of 6.8−10.1% were greaterthan most of the CNT fibers. Importantly, after reduction inhydroiodic acid, the chemically reduced graphene fiberspresented a high electrical conductivity (2.5 × 104 S m−1)and even higher mechanical performance (140 MPa strengthand 7.7 GPa modulus) while reserving the remarkable fractureelongation (5.8%). The enhancement on fiber strength wasascribed to the increased intersheet interactions of graphene,originating from the decreased interlayer distance within thereduced fibers.395 Cong and Yu et al. obtained GO fibers from acoagulation bath of the hexadecyltrimethylammonium bromide(CTAB) solution, having comparative mechanical properties.Moreover, polymers or MWCNTs were combined within theirfibers through in situ or postsynthesis strategy, which couldenhance the properties of the macroscopic fibers further.402
Generally speaking, there are several aspects that significantlyinfluence the final properties of the fabricated graphene fibers,for instance, the alignment of graphene sheets, the intersheetinteractions, and defects in the fibers.403 The highly orderedalignment could be achieved by wet-drawing protocol asapplied for traditional polymeric fibers,404,405 while the defectswere restricted through controlling the quality of graphene andthe voids in fibers. Most of all, the improvement on intersheetinteractions was considered as a critical breakthrough point forachieving high performance fibers. Aside from introduction of
binders, one of the feasible ways to solve this problem from theroot was choosing large size graphene sheets as the buildingblocks, whose viability has been confirmed by a number ofstudies.403−407 In such fibers made up from large flakegraphene, the stress transfer efficiency among graphene sheetswas evidently improved, mainly coming from their increasedface-to-face contact area. Besides, the lessening of sheetboundaries helped promoting conductive properties at thesame time.406,407 For example, Xu et al. used giant GO sheetswhich were 18.5 μm in average lateral size to fabricatemacroscopic fibers, combining wet drawing and ion cross-linking to further increase the intersheet interactions. Afterchemical reduction, the fibers showed a record tensile strengthof 501.5 MPa for polymer free graphene fibers.405 Afterward,Ag nanowires were incorporated to attain improved electricalconductivity up to 9.3 × 104 S m−1 and current capacity of 7.1× 103 A cm−2 (Figure 28f,g).406 Montmorillonite (MMT)-graphene fibers for fire-resistant conductors were also derivedbased on the similar wet-spinning protocol (Figure 28h).408
5.1.1.3. Nacre-Mimetic Graphene/Polymer Fibers. Thenacre-mimetic composites with the so-called “brick-and-motar” layered structure are of particular interests to scientistsfor decades. Inspired by natural nacre with extraordinarymechanical properties, nacre-mimetic structures have beenintensively studied. Different from the ordinary attentions paidon 2D films or papers in limited scale but readily accessible,Gao and co-workers have done some innovative research workon continuous nacre-mimetic fibers taking advantage of thegood dispersibility and processability of graphene whichsatisfied the wet-spinning strategy.409−414 The biomimeticmacroscopic fibers were generally constructed by thesandwich-like building blocks composed of rigid grapheneplatelets and grafted polymers on both sides, acting as elastic
Figure 29. (a) Schematic of a typical process for making nacre-mimetic graphene fibers: (i) synthesis of sandwich-like building blocks by graftingpolymers (hyperbranched polyglycerol (HPG) for the current example) on graphene platelets. (ii) Prealignment of the building blocks in the LCspinning dope. (iii) Wet-spinning of continuous fibers possessing “brick-and-motar” architecture. (b) AFM image of the HPG grafted graphenesheets. Reprinted with permission from ref 409. Copyright 2012 Nature Publishing Group. (c) and (d) Cross-sectional SEM images of chemicallyreduced graphene (CRG)/PVA fiber. The inset in (d) shows the small-angle X-ray scattering (SAXS) 2D pattern of the fiber. (e) A reel ofkilometers-long CRG/PVA fiber. Reprinted with permission from ref 413. Copyright 2013 Royal Society of Chemistry. (f) Tensile properties of neatGO fibers and Polyacrylonitrile (PAN)-grafted-GO fibers. Reprinted with permission from ref 412. Copyright 2013 American Chemical Society. (g)Photographs showing the chemical resistance of polymer-grafted GO (PgG) fibers: PgG fiber (left) and Kevlar fiber (right) immersed in 98% sulfuricacid for 1 month and 3 min, respectively. Reprinted with permission from ref 411. Copyright 2013 Nature Publishing Group.
glue to firmly bind them together (Figure 29a−e). The liquidcrystallity of graphene sheets enables that the functionalizedgraphene building blocks could also form the LC spinning dopeat high concentrations, implying the suitability of a LC basedwet-spinning process. Characterization on the resultant fibersindicated that, although the nonconductive polymers loweredthe electrical conductivity in biomimetic fibers as comparedwith the neat graphene fibers, the nacre-mimetic fibers wereuniversally enhanced in fiber strength and toughness due totheir improved interactions between graphene sheets (Figure29f). Moreover, unique attributes were simultaneously granted,such as the remarkable anticorrosion ability against chemicals(Figure 29g).409,411
5.1.1.4. Coaxial Fibers. Another superiority for the solutionbased wet-spinning technique is the ready control on themorphology of cross section of the as-prepared fibers, in orderto fulfill different requirements. Through a coaxial two-capillaryspinning strategy, the design of graphene fibers with tunablecoaxial structures becomes realizable. Zhao and Qu et al.employed a coaxial two-capillary spinneret which containedinner and outer channels to continuously spin GO hollowfibers. The as-spun fibers were even of comparable strengthwith solid GO fibers. Interestingly, the morphology of theprepared fibers were well controlled via changing the spinningdope passing through the two channels, for instance, byadjusting the type and ejection mode of the inner fluid duringspinning, the obtained fibers will display a necklace-likestructure with a string of microspheres along the axis.415
Beyond the way of making hollow fibers, Kou et al. utilized thesimilar form of spinneret to produce polyelectrolyte (sodiumcarboxymethyl cellulose (CMC)) wrapped graphene core−sheath fibers (Figure 30a−d), which were subsequently used toassemble two-ply yarn supercapacitors.416 Differently, a face-to-face coaxial structure was revealed later with a graphene fibercore and a cylinder graphene sheath, by operating the wet-spinning and dip-coating of GO solutions successively.417 The
simple and scalable fabrication strategy for high performancesupercapacitors made a further step to accomplish the demandfor wearable electronics.
5.1.1.5. Porous Fibers. Fibers having a porous structure incombination with outstanding characters of graphene providedpossibilities for particular uses. As an extension of the LC basedwet-spinning technique, Xu et al. chose liquid nitrogen as thecoagulation bath for the concentrated GO LC dope andfollowed by freeze-drying. Since the lamellar structure of GOLC gels was perfectly sustained, porous fibers having alignedpores were prepared and showed unique “porous core-denseshell” morphology (Figure 30e−g). As a result, the fibers withlamellar ordering exhibited high mechanical strength and fineelectrical conductivity for the reduced ones. More importantly,the porous graphene fibers had high specific surface area up to884 m2 g−1, which is meaningful for various applications, suchas energy storage and catalyst beds.418 In another way,Aboutalebi et al. made the highly porous fibers from thecombination of large GO sheets, acidic GO LC dopants (pH 3)and a pure acetone bath. The resultant fibers possessedextremely high specific surface area of 2210 m2 g−1 afterreduction, laying the foundation for an electrochemicalcapacitance as high as 409 F g−1.419
5.1.1.6. Graphene Ribbons. The fabrication of grapheneribbons was reported by Sun et al. via a simple way of placing aglass rod in the coagulation solution during wet-spinning.Morphology control could be achieved by collecting GOribbons on the rod and drying (Figure 30h,i). It was the shearstress introduced during spinning that formed flat morphologyon the ribbons rather than circular morphology. Notably, theribbons were highly flexible, in terms of remarkably large failurestrains up to 14%, in connection with the orientation ofwrinkles.420
5.1.1.7. Graphene Nanoribbon Fibers. Graphene oxidenanoribbons (GONRs) derived from the oxidative unzipping ofMWCNTs should have certain advantage over GO platelets in
Figure 30. (a) Schematic of the coaxial spinning process. (b) POM image of a wet GO@CMC fiber showing the core−sheath structure. Cross-sectional SEM images of (c) a GO@CMC fiber and (d) a two-ply yarn supercapacitor. Reprinted with permission from ref 416. Copyright 2014Nature Publishing Group. (e) The core−shell structural model for porous graphene fibers. (f) and (g) Morphology of the fracture surface of reducedgaphene porous fiber (RGPF). Reprinted with permission from ref 418. Copyright 2012 American Chemical Society. (h) Setup for the spinning ofgraphene ribbons. (i) Surface morphology of a ribbon. Reprinted with permission from ref 420. Copyright 2013 American Chemical Society.
some aspects, especially the applicability for fiber spinning,probably due to their large length/width ratio. Actually, theutilization of such ribbon-like carbons should benefit from theirhigh aspect ratio while avoiding the curved surface as presentedin CNTs and thus makes it possible to attain a dense packingand strong interconnections for the prepared fibers. Researchhas confirmed that both oxidized and reduced graphenenanoribbons are entirely soluble in chlorosulfonic acid andable to form a liquid crystal phase in superacid.396,421
Therefore, macroscopic fibers made up from GONRs wereaffordable using the similar wet-spinning apparatus. It is worthmentioning that highly aligned GONRs were observed in theobtained fibers, while subsequent thermal treatment furtherimproved the alignment of ribbons, appearing as a considerablyhigh Young’s modulus (36.2 GPa) in the treated GNR fibers.Thus, the readily processable graphene nanoribbons wereregarded as new precursors for carbon fibers.421
5.1.2. Hydrothermally Fabricated Graphene Fibers.Except for wet-spinning, a handier method named as thedimensionally confined hydrothermal fabrication was devel-oped by Qu et al. Briefly, such one-step fabrication procedurewas operated via baking a sealed glass pipeline containingaqueous GO dispersion. The drying process caused evidentlyshrinkage of the fiber diameter due to water loss, leading to acompact structure. Meanwhile, the capillary-induced shear forceand surface tension-induced tensile force were found to beresponsible for the alignment of graphene sheets along the fiberaxis. Accordingly, the tensile strength for the hydrothermallyconverted graphene fibers was 180 MPa, comparable to theinitial values for CNT fibers and GO fibers, and further thermaltreatment increased the fiber strength significantly up to 420MPa.422 Although the continuity of the as-made fibers is not asgood as that of the wet-spun fibers, the strategy is moreadaptive for integrating graphene fibers with additionalfunctionality. As a demonstration, Fe3O4 and TiO2 nano-particles were incorporated into graphene fibers respectively inan in situ or postsynthesis manner, leading the functionalizedfibers magnetic or photo responsive, respectively.422 On theother hand, if Cu wires were embedded in the glass pipelinewhile the assembly process and removed afterward, graphenemicrotubings (μGTs) with single or multichannels wereafforded in meter-long scale (Figure 31). The μGTs wouldbe selectively functionalized at specific position, either inside oroutside of the wall, benefiting from the high controllability ofthe fabrication strategy. This is a good start for further design ofsmart devices, such as a self-powered micromotor, as presentedby the authors.423,424
Besides the above introduced methods with intensive studyfor graphene fibers, there are a couple ways less common yetalso reported in the literature, nevertheless, the producedgraphene fibers were in limited length. For example, graphenefibers can be achieved directly through drawing CVD-grownfilms from volatile liquid,425,426 or fabricated by electrophoreticself-assembly at a charged graphitic tip.427 Additionally,graphene nanoribbon yarns were derived through a sequentialstep process, including the chemical unzipping of alignedMWCNT sheets which realized graphene oxide nanoribbonsheets and the densification process via withdrawing the sheetsfrom the liquid-phase and drying.428
For comparison, we summarized some of the mechanical andelectrical properties of the reported graphene fibers in Table 3.Reduced graphene fibers usually possess good electricalconductivities up to 104 S m−1, and these values are close to
CNT fibers. Meanwhile, the enhancement on fiber strength andconductivity through the utilization of giant graphene sheets issignificant.
5.1.3. Applications. 5.1.3.1. Supercapacitors. The mostpromising and common applications for graphene fibers wereattributed to fiber/woven fabric supercapacitors, owing to theirconductivity and mechanical strength, along with the raisingdemands for wearable energy storage devices. In the relatedstudies, specific capacitance was evaluated by gravimetric,length, areal, or volumetric capacitance. No matter which kindof measurement was employed, the advantage of graphenearchitectures was apparently revealed. Huang et al. producedall-solid-state fiber supercapacitors from wet-spun graphenefibers, which combined fine specific capancitance of 3.3 mFcm−2 with good flexibility and cycling stability. Decoration ofpolyaniline endowed the fiber capacitor with additionalpseudocapacitance, demonstrating a high capacitance of 66.6mF cm−2.429 The highly porous rGO fibers reported byAboutalebi et al. revealed gravimetric specific capacitance of409 F g−1 at a scan rate of 1 A g−1, benefiting from the openchannels which offered pathways for ion transport.419 The two-ply yarn supercapacitors assembled from coaxial wet-spun fiberswith CMC shell and nano carbon core showed impressivecapacitance and energy density of 177 mF cm−2 and 3.84 μWhcm−2, respectively, using solid electrolyte and 269 mF cm−2 and5.91 μWh cm−2 using liquid electrolyte (Figure 32a-e).416 Later,the capacitance value was surpassed (205 mF cm−2) by anothercoaxial design with a graphene fiber core and a coated graphenesheath, due to the decreased solution resistance between theface-to-face electrodes.417 Alternatively, the two ply fibersupercapacitors could be assembled asymmetrically with twodifferent graphene fiber-based electrodes. For example, withMnO2 decorated graphene fiber and graphene-CNT hybridfiber acting as the two electrodes, the asmmetric deviceexhibited high energy density up to 11.9 μWh cm−2 (equivalentto 11.9 mWh cm−3).430
5.1.3.2. Actuators. A graphene-based actuator is known as astimulus-responsive system which normally performs mechan-ical movement in response to environmental stimulation,
Figure 31. (a) Photos of a spring-like μGT. (b) SEM image of a helicalμGT, inset shows the twisted Cu wires used for fabrication. SEMimages of the multichannel μGTs with channel number of (c) 2 and(d) 4. Reprinted with permission from ref 423. Copyright 2012American Chemical Society.
making it affordable for diverse applications ranging fromrobots and sensors to memory chips.431 In essence, it is anenergy conversion process in which the applied energies, suchas electrical, chemical, and thermal energies are converted tomechanical energy. The unique attributes of graphene fibers incombination with their flexibility and robustness enable a noveltype of actuator in fiber form. Qu’s group has contributed many
efforts on such research field with several fascinating resultsachieved.424,431,432 An electrochemical fiber actuator was firstdeveloped based on a bilayer graphene fiber/polypyrroleasymmetric structure, which can be further utilized for thefabrication of multiarmed tweezers and net actuators.433 Later,using positioned laser reduction on one side of a GO fiber, thegraphene/GO fiber became an actuator acting responsive
Table 3. Mechanical and Electrical Properties of Graphene Fibers
ref materialtensile strength
(MPa)Young’s modulus
(GPa) fracture strain electrical conductivity (S m−1)
Figure 32. (a) SEM image of a two-ply yarn supercapacitor (YSC), inset shows the configuration of a supercapacitor. (b) Galvanostatic charge−discharge (GCD) curves of single, two, and three YSCs connected in series. (c) Cyclic voltammetry (CV) curves of single, two, and three YSCsconnected in parallel, with scan rates of 10 mV s−1. (d) Photo of cloth woven by two coaxial graphene fibers (indicated by i and ii, respectively). (e)GCD curves of the cloth supercapacitor under different bending conditions, as indicated by the illustration on the right side. Reprinted withpermission from ref 416. Copyright 2014 Nature Publishing Group. (f) Schematic for the rotation of twisted GO fiber (TGF) driven by moisturechanges. Design of (g) the humidity switch and (h) generator based on the responsive TGFs. The inset in (g) shows the LED is turned on in theconductive electric circle. Reprinted with permission from ref 435. Copyright 2014 Wiley-VCH.
movement to humidity changes.434 Recently, a novel twistingstrategy was developed to afford moisture-driven rotationalmotor via rotary processing on a GO fiber hydrogel. The as-formed helical configuration enabled reversible rotation underthe alternation of humidity and thereby allows the developmentof humidity switches and moisture triggered generators (Figure32f−h).435
5.2. 2D Graphene Films
A macroscopic 2D film is one of the most applicable forms ofgraphene due to its extraordinary availability along the lateraldirection. Because of their extremely high aspect ratio (lateralsize/thickness), the 2D graphene sheets tend to assemble intolarger 2D structures spontaneously, like thin films, membranes,papers, and coatings with orderly packed lamellar structures.Different from the randomly packed CNT films with networkstructure, graphene films exhibit much regular and layeredstructure that endows them with superior specific opticaltransparency, mechanical, electrical, and thermal properties,showing great potential in practical applications. Typically,graphene and GO films have found their position in manyemerging fields, such as electrodes for energy storage devices,heat spreaders, and membranes for gas separation and waterdesalination. Except for those ultrathin graphene films directlysynthesized from CVD method, other graphene films areassembled essentially by a solution-based process learning fromthe wet-processing of nanoparticles and polymers, especially theclose relative of graphene-CNTs. With special concern on theassembling behavior, this context will only focus on the latterindirect approach for graphene films. Here, we classify the wetprocesses into two categories: (1) the conventional wetmethods, which are also commonly seen for other nanomaterials and (2) several protocols fit for graphene in particularaccording to its unique characters.5.2.1. Wet Methods Assembled Graphene Films. The
conventional wet methods for 2D graphene films includevacuum-assisted filtration, solution-casting, layer-by-layer as-sembly, bar-coating, dip-coating, spin-coating, spray-coating,electrophoretic deposition, and so on, all of which are highly
controllable during the fabrication, resulting in graphene filmswith varied thickness, dimension, and structure.
5.2.1.1. Vacuum-Assisted Filtration. Filtration is the mostlyused approach for paper like GO, graphene, and graphenebased composite films due to its easy manipulation.83 Theassembly of graphene sheets during the filtration processfollows a semiordered accumulation mechanism that the 2Dsheets first form loosely aggregated, semiordered lamellae andsubsequently endure contraction into the final highly orientedstructure through the removal of solvent.436 In contrast withother carbonaceous materials, graphene platelets are mucheasier to generate free-standing or self-standing films thanks totheir 2D sheet-like geometry and strong interlayer connections,accompanied by regular stacking. Correspondingly, the strongfilm-forming tendency of graphene also makes the filtrationmethod consume enormous amounts of time and energybecause of the blocking effect coming from the earlier packedlayers to the remaining dispersion.Early in 2007, Ruoff et al. achieved free-standing GO paper
via filtration and explored its mechanical performances,revealing an average modulus of 32 GPa and the highestvalue reached ∼42 GPa, which were much higher than thosereported for CNT based bucky paper (Figure 33a−c).437 TheGO paper was then reinforced with divalent ions of Mg2+ andCa2+ (less than 1 wt %) which resulted in enhancement onmechanical stiffness (10−200%) and fracture strength (∼50%),through chemical cross-linking between the functional groupson GO and the metal ions.438
As for pristine or chemically converted graphene (CCG)which are recognized with low solubility, the filtration methodis also applicable if only the well-dispersed solutions were takeninto account. Li and Wallace et al. prepared homogeneousaqueous solution of hydrazine converted graphene which wasfiltered into free-standing graphene paper with a shiny metallicluster (Figure 33d−f), further thermal treatment at 220 °Cimproved its mechanical properties, and a high electricalconductivity of 351 S cm−1 was achieved when annealed at 500°C.439,440 Interestingly, by transferring the freshly filtered CCGfiltrate cake in water immediately, a highly conductive and
Figure 33. Photos of (a) flat and (b) folded GO paper. (c) SEM image of the cross-section of GO paper. Reprinted with permission from ref 437.Copyright 2007 Nature Publishing Group. (d) Photo of two pieces of filtered free-standing CCG paper. SEM images of graphene paper in (e) top-view and (f) side-view. Reprinted with permission from ref 440. Copyright 2008 Wiley-VCH.
anisotropic hydrogel film was obtained via trapping the waterwithin the film, and the gel structure was stable while it waskept wet. This is an indication that the sol−gel transitionoccurred at the liquid−solid interface during filtration.441 Anextension work was built in order to optimize the volumetricelectrochemical performance of the obtained CCG hydrogelfilms, and the same group increased their packing density andpore interconnectivity by capillary compression duringcontrolled removal of the trapped volatile liquid which waspreviously exchanged into the gel films.442 In contrast, Zhao etal. reported the controlled chemical oxidation approach toincrease the porosity of graphene films, with the purpose ofproviding more ion diffusion channels, for their use as graphiticelectrodes for energy storage applications.443
Composite graphene films with expanded functionality canbe obtained by filtration of either chemically modified graphenesheets or mixtures of graphene and guest materials. Based onsuch a principle, nafion, octadecylamine, and ionic liquidfunctionalized graphene,444−446 as well as sulfonated GO447
were assembled into films. Another strategy to modify graphenefilms is the direct modification on the formed films, whichavoids the precipitate of graphene sheets during chemicalreactions and is able to reserve the layered structure. Forinstance, Compton et al. modified the filtered wet GO paperwith hexylamine in methanol to prepare conductive “alkylated”graphene paper.448
Polymers are apparently popular additives for graphene films.Putz and co-workers prepared GO/PVA and GO/PMMAcomposite films with different polymer loadings by filtratingGO/PVA aqueous solutions or GO/PMMA DMF dispersions,respectively. Significantly for the polymers, the filler content(GO) could reach over 50 wt %, resulting in remarkableenhancement on mechanical properties of the compositematerials.449 The combination of multilayered graphene andnanofibrillated cellulose (NFC) was realized using a sonicationprocess. In this case, the NFC served as the dispersing agentduring graphene exfoliation in water. The filtered graphene/NFC nanocomposite papers with 1.25 wt % graphene in thepresence exhibited excellent tensile mechanical propertiescombining both high strength and high toughness.450
Besides, hybrid films of graphene and nanoparticles withoutany organic or polymer binder are also accessible with thefiltration method. Zhao et al. obtained FeF3/graphenecomposite paper for lithium-ion batteries through thesuccessive process of spontaneous assembly, filtration, andphotothermal reduction.451 Zhang et al. prepared aqueouscolloidal dispersion of RGO and exfoliated montmorillonite(MMT) nanoplatelets by direct reduction of GO with thepresence of exfoliated MMT nanoplatelets, which was thenfiltered into films. The highly oriented graphene/MMT hybridfilms showed excellent flexibility, electrical conductivity, and fireretardant properties as expected.452 Other plate-like nano-particles, such as Co3O4 and MoS2, were found to becompatible with graphene as well, holding perfectly orderedlayer structures in the hybrid films.453,454
5.2.1.2. Drop-Casting. Drop-casting is the method toprepare thin films with nanometer-scale-thickness by solventevaporation of diluted polymers or nanoparticle dispersions. Itenables the exploration of ultrathin graphene film in single orfew layers. The assembly process is controlled by temperature,solvent evaporation, and size and concentration of graphene.Furthermore, the drop-casting process could also be used toprepare graphene based composite films, such as nanocrystal-
line cellulose/GO,455 peanut shaped α-Fe2O3/graphene,456 and
PU/graphene457 composite films. Although it is facile to formlarge pieces of films,458 the obvious disadvantages of suchuniversal approach are the poor uniformity and repeatabilityrestricted by the drying process.The supporting substrate usually serves as an important tool
to realize an efficient control upon the film fabrication process.Drop-casting has shown its suitability on various supports, evena suspended GO membrane over an orifice could be madethrough this method.459 Zhang and Li et al. achieved a singlelayer of CCG in an edge-to-edge manner by drop-casting thesheets on negatively charged substrates (Figure 34), and the
surface properties of CCG and the electrostatic repulsion fromthe substrate have combined effects on flattening and self-assembly process of CCG sheets.460 By site selective depositionof GO using substrates of prepatterned monolayer ofoctadecylphosphonic acid, Wu et al. obtained regularlypatterned GO and graphene.461 When deposited on glossypapers, the graphene films made by drop-casting could betransferred to the synthesized PU substrate by hot press.462
5.2.1.3. Spin-Coating. Spin-coating is a widely used methodshowing good control over morphology and microstructure ofthe prepared films with thickness of 10 nm or less, andnoteworthy, good reproducibility. The thickness of the finalfilm is dependent on many factors like the solid content,solution viscosity and spinning conditions. The downside ofthis approach is a lack of control over ordering due to the fastevaporation and being not suitable for large area production.Becerril et al. obtained single and few layer graphene films
with admiring thickness and transparency by spin-coating,which were adequate for the utilization of transparentconductors after thermal annealling.463 Through wettabilitymodulation on the substrate, Guo and co-workers conductedpatterned graphene films on SiO2 and PET surface incombination with spin-coating.464 Composite films withmultilayer laminate structure were affordable via the spin-assisted layer-by-layer assembly of GO and poly(methylmethacrylate) (PMMA), where the two layers of substances,both in the submicrometer range, were deposited alternately. Itwas found that the crack propagation mechanism was altered inthe free-standing biomimic GO/PMMA composite films,revealing dramatically enhanced tensile strength and defecttolerance.465 Spin-coating also possesses feasibility of incorpo-
Figure 34. Schematic of the drop-casting process for CCG onnegatively charged substrate. Reprinted with permission from ref 460.Copyright 2011 Royal Society of Chemistry.
rating guests into graphene films including small molecules,polymers, and nanoparticles. Based on the excellent electricalconductivity and high transparency of spin-coated graphenethin films, they have been used as transparent electrodes forultrathin devices such as solar cells and organic light-emittingdiodes.466,467
5.2.1.4. Dip-Coating. The dip-coating of graphene filmsconsists of multistages including immersion, deposition whilebeing pulled up, drainage of excess liquid, and evaporation. Thethickness, morphology and microstructure of the films aredetermined by the balance of forces at the liquid−substrateinterface and the solid content of GO suspension.468 Amodified horizontal-dip (H-dip) coating method for thin andhomogeneous GO films has been developed recently (Figure35). By imposing precise control on the coating speed U andgap height h0, the as-prepared GO films exhibited a highpacking density of GO sheets and a low surface roughness.469
5.2.1.5. Spray-Coating. Gilje et al. prepared thin layers ofGO by spraying aqueous GO solution onto preheated SiO2substrates. The spray-coating process resulted in a highlyuniform deposition, better than the standard drop-casting anddip-coating techniques, from the given point of view.470 Thecoverage density of GO sheets was controlled by varying theconcentration and the spray duration of the GO dispersion.When RGO sheets were well-dispersed in volatile ethanolmedium, they were readily spray-coated onto various substrateseven at room temperature, with the purpose of fabricatingRGO-TCFs.471 Spraying a GO and hydrazine mixture onto apreheated substrate at a high temperature of 240 °C allowedthe occurrence of deposition and reduction of graphene filmssimultaneously. A uniform CCG thin film was achieved forelectronic applications in a short time without gaseousbyproducts.472 Patterned graphene films were created facilelyby applying a patterned template to the target substrate duringthe spray-coating process,473 which was meaningful for thefabrication of graphene based devices.474 Similar to other wetmethods, composite films composed of PVA/graphene,475
CNTs/graphene,476 and MoS2/graphene477 have been ob-
tained via spray-coating the corresponding hybrid mixtures.In the meantime, there are extended approaches based on the
spray-coating technique, with greatly improved quality in thefinal films. For instance, Kim et al. developed a supersonickinetic spray method which used the supersonic accelerationinduced shearing to produce very small RGO droplets, andstretching of the RGO sheets in the formed films resulted inhealing of their defects.478 Another modified method used forgrpahene thin films is the electro-spray deposition (ESD),
which has already got intensive studies. In a typical process, anelectric field is applied between the injection nozzle andsubstrate, and monodispersed fine droplets composed aerosolare created due to repulsion forces originated from chargescarried in the droplets. Ju et al. deposited nitrogen-dopedgraphene nanoplatelets on fluorine doped SnO2 (FTO)/glasssubstrates by ESD, showing good electrocatalytic performancesfor the Co complexed redox couple.479 In order to achievelarger deposition coverage, Wang et al. exploited the super-hydrophilic-assisted ESD method, with superhydrophilic treat-ment on the glass substrates, and the resulted coverage wasreported to increase by more than 6 times.480 Notably, throughcombination of ESD with a continuous roll-to-roll process, Xinet al. prepared large area free-standing graphene films with highelectrical and thermal conductivities of ∼1238.3 W m−1 K−1
and ∼1.57 × 105 S m−1, respectively.481
5.2.1.6. Layer-by-Layer Assembly. Although layer-by-layer(LBL) assembly normally acquires the cooperation with otherwet processing strategies, its alternate and gradual assemblyprocedure performs precise control on the microstructure of agraphene film. Zhu et al. systematically compared the twodistinct protocols of vacuum-assisted flocculation (VAF) andLBL assembly for reduced graphene/PVA composite film.Their conclusions showed that the mechanical properties werenearly the same for the separately derived films but theirelectrical conductivities displayed evident distinction, where itwas more than 10 times higher for LBL composites than theother ones.482 The driving force for LBL assembly can be theelectrostatic interactions, hydrophobic attractions, or covalentattachments. For example, negatively charged GO was directlyassembled with cationic materials like cationic polyacrylamidecopolymer,483 polyelectrolyte-poly(diallyldimethylammoniumchloride) (PDDA),484,485 PDDA decorated Ag nanopar-ticles,486 and cationic molecule decorated graphene sheets.487
Zou and Kim developed an LBL assembly method for thefabrication of GO films by spreading GO suspension dropwiseto the surface of a chitosan solution, which is a kind ofpositively charged polyelectrolyte. The obtained thin films atthe air−liquid interface were of enough strength to bemanipulated with tweezers and even drawn into fibers.488
Subsequently, the utilization of branched polyethylenimine (b-PEI) instead of chitosan was demonstrated to extend thetechnique for creating thicker macrostructures displayingmillimeter range thickness and foam-like porous features.489
Zeng et al. prepared pyrene-grafted poly(acrylic acid) (PAA)modified graphene sheets in aqueous solution and thenalternately assembled with PEI for the detection of maltose.490
Figure 35. (a) Schematic of the horizontal-dip (H-dip) coating process for GO dispersion. (b) The relationship between film thickness h and coatingspeed U for two gap height h0, the solid curves show the theoretical predictions. Reprinted with permission from ref 469. Copyright 2014 RoyalSociety of Chemistry.
Park et al. fabricated thin films constructed by oppositelycharged graphene sheets of amine-functionalized graphene andpartly reduced carboxylic acid functionalized GO, followed bythermal treatment to achieve the recovered and purifiedgraphene films.491 Similarly, covalent modification on RGOsheets with negatively charged poly(acrylic acid) and positivelycharged poly(acrylamide) allows the operation of LBLassembly.492 As a complement, Ou et al. performed non-electrostatic LBL assembly for PDA and GO on the surface of asilicon substrate with the help of a series of reactions betweenthe functional groups of PDA and GO.493
5.2.1.7. Electrophoretic Deposition. When it comes to theassembly of 2D graphene films, the widely used EPD techniqueshows advantages of efficient thickness and uniformity control,strong coating-substrate adhesion, and large-scale accessibilityover other coating methods.494 The morphology and structureof EPD graphene films are affected by several conditions,including working voltage, deposition time, solvent type, andthe concentration of graphene. Ren and Cheng et al. fabricatedhomogeneous single-layer graphene films under the appliedvoltage of 100−160 V from a stable graphene suspension inisopropyl alcohol. The resulted graphene films displayedsuperior field-emission properties, benefiting from their highdensity, uniform thickness, numerous edges normal to the filmsurface, and good contact and adhesion with the substrate.495
RGO films also could be EPD assembled on conductivesubstrates, either before or after the chemical reduction uponGO nanosheets. Measurements indicated that reduction withhydrazine prior to electrophoretic deposition yielded smootherand better aligned RGO films, compared with the other onesobtained from the postdeposition reduction approach.496 Thereduced EPD graphene coatings were demonstrated to performexcellent corrosion resistance for ion diffusion and oxidizingenvironment.497 It is worth mentioning that partial electro-chemical reduction of GO was observed on the positiveelectrode during the EPD process,498,499 while the utilization oflow voltages <5 V would effectively prevent the GOreduction.500 Moreover, composite graphene films mixed withnanoparticles such as CNTs, MoS2, and ZnO were achieved bythe EPD method and found applicable for supercapacitors, dyesensitized solar cells, and antibacterial materials.501
5.2.1.8. Langmuir−Blodgett Assembly. The Langmuir−Blodgett (LB) method is used to prepare monolayer ormultilayer graphene assemblies, which could be controlled bythe degree of applied compression. To be specific, a typical LBassembly process originates from monolayer spreading ofgraphene sheets at liquid surface, and compression on themonolayer graphene sheets is then performed using a movingbarrier. The separate sheets are thus pushed together, an edge-to-edge manner can be fulfilled in the floating film if moderatecompression is applied, whereas the face-to-face stacking isinduced with overcompression. Finally, the LB graphene filmscan be gathered by dip-coating onto a solid substrate (Figure36a).502 Along with the elimination of agglomeration andoverlapping of GO sheets, the LB method is able to produceGO films with very accurate thickness and uniformity due tothe amphiphilic character of GO, controlled transfer of GOmonolayer sheets at the air/water interface, and the strongelectrostatic repulsion between the 2D confined layers. Inaddition, the edge-to-edge sheet spacing in single-layered GOfilms could be tuned by reversible monolayer compression−expansion cycles (Figure 36b−d).98 Likewise, Li and Dai et al.achieved multilayered films made of exfoliated graphene sheets
also using the LB technique, the resulting films in a layer-by-layer manner exhibited low sheet resistance and hightransparency.503 Furthermore, with high research significance,thin graphene films assembled by the LB method could be usedto study the FET performance,504 conductivities,505 protontransfer behaviors,506 and reduction processes507−509 ofmonolayer graphene.
5.2.2. Assembled Graphene Films at Interfaces.Comparatively speaking, the LB method for single/multilayeredthin films is carried out at the liquid/air interface and collectedby a substrate, whereas the protocol about to introduce enablesfilm assembling at 2D interfaces, either liquid/air or liquid/liquid, and normally experiences enrichment and stackingprocesses that free-standing films with certain thickness can berealized.510 In their original work, Chen et al. preparedmacroscopic free-standing GO membranes at a liquid/airinterface by evaporating the hydrosol of GO. The assembledmembranes with 5−10 μm thickness were shown comparabletensile strength (average value of ∼67.7 MPa) with thefiltration derived GO membranes. A large-area production wasalso demonstrated (Figure 37).511 Huang et al. claimed that thesurface activity of GO ensured the enrichment of GO sheets atthe air/water interface, which was promoted by a flotationprocess using gas bubbles as carriers to fulfill the transportationof GO, creating large-scale 2D assemblies at the watersurface.512 Afterward, Niu and co-workers prepared theevaporation induced GO films onto glass substrates withimproved orientation of GO sheets. The assembled GO filmssubsequently underwent low-temperature thermal reduction at200 °C, resulting in higher transmittance in the visibleregion.513 Further annealing at 800 °C endowed such graphenefilms with good electrical conductivity of 272.3 S cm−1.514
Alternatively, Zhu et al. combined the chemical reduction ofGO platelets and their self-assembly at the air/water interfaceinto one step, simply by adding hydrazine monohydrate intoaqueous GO dispersion and followed by heating at 80 °C. Theirintelligent work provided a facile route for thin, transparent,and conductive RGO films.515
Figure 36. (a) Schematic illustration of the LB assembly process forgraphene films. Reprinted with permission from ref 502. Copyright2013 Elsevier. SEM images of (b) a single layer of GO monolayer, (c)double layers with loosely packed top layer, and (d) double layers withhigh-density top layer. Reprinted with permission from ref 98.Copyright 2008 American Chemical Society.
Yang’s group has carried out intensive studies about thestructure and property control of the evaporation inducedgraphene films. With the hybridization of low temperatureexfoliated graphene nanosheets (LGNs) and graphene oxidenanosheets (GONs) in aqueous solution, the formation ofgraphene menbranes was achieved at the liquid/air interface. Inthe authors’ opinion, the GONs served as a stacking templateand sticking component for the assembly of LGNs. Theconductivity of the hybrid graphene membranes was thereforetunable according to the fraction of GONs.516 The trans-mittance and optical properties of GO membranes were alsoinfluenced by the pH value of the parent GO hydrosol. Thetransmittance of GO membrane was revealed to decreasecontinuously with the increase of pH value, since it affected thesheet size and surface chemistry of GONs, which wereconsidered as the two critical factors related to the final opticalproperties.517 Recently, the microstructure of the graphene-based membrane was tailored through the controlled removal
of trapped water within the wet GO membrane. By applying atwo-stage drying process with reduced pressure, the tightlystacked layered structure of the GO membrane was changedinto an open and graded microstructure.518 Based on similarassembly strategies, the same group also obtained PVA/graphene519 and CNTs/graphene520 hybrid films. Other thanthe commonly used approach performed at the liquid/airinterface, the interfacial assembly of graphene films wasobserved at liquid/liquid interface as well, driven by theminimization of interfacial energy, as depicted at the chloro-form/water,521 oil/water,522,523 and pentane/water524 interface.On account of the principle that metal materials can act asreducing agents to convert GO into a reduced form of CCG,Cao et al. developed the synchronously reducing anddepositing process to assemble CCG films onto metalsubstrates, simply by immersing metal foils into GO solution.This process is literally attributed to a novel kind of interfacial
Figure 37. (a) Evaporation induced self-assembly of GO membrane at liquid/air interface. Photographs of (b) a stable GO suspension, (c) self-assembled GO film at liquid surface after heating at 80 °C for 15 min, (d) a flexible and transparent GO membrane, and (e) a large-scale and free-standing GO film. Reprinted with permission from ref 511. Copyright 2009 Wiley-VCH.
Figure 38. (a) Schematic of the continuous wet spinning of GO films. (b) Photo showing a meters-long GO film wound on a glass reel. (c) Bamboo-mat-like fabrics woven by the wet-spun GO films. (d) Reduced wet-spun graphene film showing no break while being sharply folded. (e) Thegraphene film used as bendable conductor on PET substrate and (f) in the bent working state after 100 bending cycles. Reprinted with permissionfrom ref 526. Copyright 2014 American Chemical Society.
assembly since it actually happened at the liquid/solidinterface.525
5.2.3. Continuously Wet-Spun Graphene Films. Wetspinning is an efficient method to access high performancegraphene and graphene based composite fibers but never beused to make films. Recently, Gao’s group has developed thewet-spinning assembly methodology to produce continuousgraphene films for the first time.526 GO films withhomogeneous thickness were produced at a high speed of 1m min−1 by squeezing the GO dispersion through a thin andwide spinning channel into a coagulation bath and followed bydehydration (Figure 38a). The shear-induced orientation ofGO sheets and negligible in-plane contraction of coagulatedGO sheets resulted in a long-range highly ordered lamellarstructure. The as-made continuous GO films were flexible andtough enough to be woven into large-area bamboo-mat-likefabrics, and the reduced films were capable of bendableconductors, as shown in Figure 38b−f. Moreover, this strategycan be extended to make graphene based hybrid films bymixing guest materials with GO dispersion. Consequently,multifunctional composite films of GO-PVA with PVA contentof 33 wt % and GO/Fe3O4 with 50 wt % of Fe3O4 have beenachieved. Even more meaningful is that such continuousfabrication would be employed to produce electrodes forsupercapacitors.527,528 The participation of CaCO3 templateduring film spinning which was removed by acid-etchingafterward resulted in wrinkled graphene film (WGF) electrodes,showing excellent rate capability in the assembled devices.527
5.2.4. Applications. The 2D graphene assemblies frommono-, few-, to multilayer films hold great promise in variousapplications. With different viewpoints upon CNT films, thereare tremendous efforts devoted to pursuit of graphene filmshaving high mechanical performances, since the overlapping ofgraphene sheets produces strong interconnections whichestablish a basis for mechanical strength. Recent achievementswith this target are depicted in Figure 39, together with the
results in graphene fibers, CNT fibers, and CNT films forcomparison. Apparently, graphene and CNTs both exhibitbetter mechanical performance in their fiber form, probably as aconsequence of the higher alignment. Graphene films areshown to be much tougher than CNT films as being discussedabove. Not like the big difference between CNT films and
fibers, graphene films illustrate comparative modulus withgraphene fibers, which is thought to be related to their similararrangement of a layered packing in the formed structures. Inaddition, the gap between graphene fibers and CNT fibers isobvious, which means there is still significant scope for theyounger graphene fibers to rise further.As for the practical application of graphene films, generally
speaking, the thin films with single or few layer graphene havebeen used as transparent conductive films, separationmembranes, sensors, anticorrosion coatings, etc. Besides, thehigh electrical and thermal conductivity accompany with highsurface area allow graphene films to become ideal componentsfor energy-related devices like supercapacitors, lithium-ionbatteries, and so forth.
5.2.4.1. Transparent Conductive Films. The popularapplication of graphene films as TCFs is based on the meritsof high conductivity and transparency of graphene. A big classof graphene TCFs is originated from GO which enjoys betterprocessability and can be readily reduced to partially recover itsconductivity while causing little degradation on transpar-ency.529 The two major performance parameters for TCFs, interms of conductivity and transparency, own the convenienceof being tuned by control of film thickness and reductiondegree. In a typical process reported by Eda et al., filtered GOfilms ranging from single layer to multilayers were transferredto glass and plastic substrates, through a simple pressing andmembrane dissolving procedure (Figure 40a). A minimumsheet resistance of ∼43 kΩ sq−1 and transmittance of ∼65% atλ = 550 nm were obtained for reduced GO thin films, enablingtheir further uses as thin-film transistors (TFTs).530 De et al.transferred filtered, oxide-free, thin graphene films to polymersubstrates to give TCFs with lowest sheet resistance of ∼103 Ωsq−1 and corresponding thickness of 88 nm, while atransmittance of ∼35% after dissolving the cellulose filtermembrane. The utilization of graphene flakes avoided theadditional reduction process and was supposed to provide ahigher conductivity, however, the large aggregation within thefilm will contribute to roughness and limit on the ultimateconductivity.531 With the help of highly conducting 1Dnanofillers, hybrid thin films of RGO/SWNT prepared bylayer-by-layer LB assembly and subsequent thermal reductionshowed sheet resistance of 560 Ω sq−1 at high transmittance of77%.532
Although not addressed in this review, it is also worthmentioning that the CVD-grown large-scale graphene films arefavorable in TCF applications according to their betteroptoelectronic performances. For instance, Bae et al. achieved30-in. graphene films grown by CVD and displayed sheetresistance of 30 Ω sq−1 at 90% transparency in the HNO3doped films, which was superior to common ITO and CNTfilms.533 Generally speaking, the GO based wet processing ofgraphene films must be fully reduced for their use as TCFswhile incomplete reduction will cause low electrical con-ductivity. This is not necessary for CNT films and CVD-growngraphene films. As regards the grown graphene films, the CVDprocess and etching of the metal substrates are considered to behigh cost, which may be a manufacturing disadvantage againstthe GO based films. For those who care for more about CNTand graphene TCFs, one can refer to several reviews on thisaspect.534,535
On the basis of their outstanding optoelectronic properties,graphene TCFs are expected to serve as large-area transparent
Figure 39. Comparison of the mechanical properties among CNTfibers, CNT films, graphene fibers, and graphene films.
conductive electrodes to replace brittle and expensive ITO inflexible devices.For its use as anodes for OLEDs,467,536 graphene usually
exhibit relatively high sheet resistance and low work functioncompared with conventional ITO anodes. To this end, Kim etal. doped graphene films with bis(trifluoromethanesulfonyl)-amide (TFSA, [CF3SO2]2NH) which significantly improvedtheir optoelectronic performances. The TFSA doped five-layergraphene samples showed very low sheet resistance of 90 Ωsq−1 and high transmittance of 88%. As a result, the obtainedcurrent efficiency (9.6 cd A−1) and power efficiency (10.5 lmW−1) are comparable to those of polymer light-emitting diodes(PLEDs) based on ITO anodes.537
GO thin films with thickness of 2 nm were able to be utilizeddirectly as the effective hole transport and electron blockinglayer in organic photovoltaics (OPVs).466 Alternatively, chemi-cally derived graphene can be used as the junction emitter, thepassivation layer and antireflective layer in solar cells (Figure40b).538 In addition, MWCNTs/RGO,539 MoS2/graphene
540
composite films and N-doped graphene films479 were exploitedas counter electrodes for dye-sensitized solar cells (DSSCs) andperformed photovoltaic conversion efficiency (PCE) of 6.17%,5.81%, and 9.01%, respectively. The low-cost metal-free counterelectrode of N-doped graphene films even outperformed thePCE of Pt photoanodes by a factor of 1.07.479
Single-layer graphene films for FETs were first fabricatedusing the EPD method, which showed excellent field-emissionproperties of low turn-on electric field (Eto = 2.3 V μm−1) andthreshold field (Ethr = 5.2 V μm−1), high field-enhancementfactor of ∼3700, and good emission stability and uniformity.These performances were much better than those of agraphene-powder coating and even surpassed those ofCNTs.495 Few-layer RGO films were reported with well-controlled band gap ranging from 2.2 to 0.5 eV. The as-madeRGO film FET showed a photoresponsibility of ∼0.86 A W−1,which was 3 orders of magnitude higher than that of the FETmade with mechanically exfoliated graphene (0.1−0.5 mAW−1).541 Besides, organic FETs using RGO thin films aselectrodes displayed higher mobility than the Au electrode
devices.542 Furthermore, the RGO thin film FETs were capableof serving as biosensors with high sensitivity.543−545
5.2.4.2. Supercapacitors and Lithium-Ion Batteries. Theexcellent conductivity and high surface area of graphene make itan ideal electrode material for supercapacitors and batteries. Inorder to increase the electrode efficiency, based on the radicalprinciple of offering channels for ion diffusion, one of the mostconcerned issues is how to maintain the porosity of graphenefilms and avoid restacking. Besides, graphene films used in suchenergy storage devices are usually combined with othernanoparticles which act as both nanoscale spacers and activesites.Li and co-workers prepared self-stacked, solvated graphene
(SSG) films that used water as the spacer to separate CCGsheets against densely packing. After exchanging the water withan ionic liquid (IL), the IL-exchanged SSG film basedsupercapacitor exhibited superb electrochemical performanceof high specific capacitance up to 273.1 F g−1, energy density of150.9 Wh kg−1, and maximum power density of 776.8 kWkg−1.441 Afterward, the mixture of volatile and nonvolatileliquids was employed to replace water during film formation,while the controlled removal of volatile solvent enabledregulation of packing density. Therefore, the achieved graphenefilms with fixed pore sizes and continuous ion transportnetwork showed volumetric energy densities approaching 60Wh L−1. Highly compact CCG films with densities of 1.25−1.33 g cm−3 delivered a volumetric capacitance of 261.3 F cm−3
in organic electrolyte at 0.1 A g−1. Moreover, these electro-chemical capacitors showed dramatic recyclability anddurability that over 95% of the initial capacitance was retainedafter a 300-h constant voltage holding at 3.5 V.442 On the otherhand, ordered sulfonated graphene films prepared by LBmethod exhibited ultrahigh specific capacitance of 287 F g−1 ata scan rate of 0.5 mV s−1 in aqueous electrolyte and excellentcyclic stability.502 Hybrid asymmetric supercapacitor con-structed by graphene/MnO2 and graphene/Ag films as thepositive and negative electrodes, respectively, was found anoperating potential window of 1.8 V in an aqueous electrolyte,resulting in a maximum energy density of 50.8 Wh kg−1 (Figure40c,d).546
Figure 40. (a) Photograph of a GO thin film on plastic substrate. Reprinted with permission from ref 530. Copyright 2008 Nature Publishing Group.(b) Schematic of the heterojunction device with a RGO film coated on p-Si. Reprinted with permission from ref 538. Copyright 2014 Elsevier. (c)Graphene/MnO2∥graphene/Ag asymmetric supercapacitor packaged between two PET slices. (d) Photograph showing the supercapacitor couldlight up an LED at bending state. Reprinted with permission from ref 546. Copyright 2013 Royal Society of Chemistry.
As an important member in the supercapacitor family, all-solid-state flexible supercapacitors have triggered tremendousinterests. With such consideration, Choi et al. used the filteredNafion functionalized RGO paper as electrodes and solvent-cast Nafion membrane as electrolyte and separator. As a result,high specific capacitance (118.5 F g−1 at 1 A g−1) and ratecapability (90% retention at 30 A g−1) with recyclability anddurability up to 1000 cycles charging and discharging wereobtained due to the steady and good interpenetrating networkstructures between electrode and electrolyte.444 In anotherwork, ionic liquid functionalized graphene (IL-CMG) paperwas utilized as the negative electrode and RuO2−IL-CMGcomposite paper as the positive electrode. The solid-stateasymmetric supercapacitor could work under a maximum cellvoltage up to 1.8 V, and exhibited high energy density (19.7 Whkg−1), high power density (6.8 kW g−1), and good cyclingperformance over 2000 cycles even under normal, twisted, andbent states.547
With great similarities, the applications in Li-ion batteriesacquire the same characters from graphene. Wang et al. studiedthe electrochemical properties of graphene paper electrodes,and a discharge capacity of 582 mA h g−1 with a cutoff voltageof 2.0 V was observed, indicating the graphene paper was apromising cathode material in lithium batteries.548 Filteredpapers consisting of porous graphene exhibited much enhancedLi-ion storage capacity and transport properties than conven-tional graphene papers, while possessing comparable electricalconductivity and ductility. These features relying on theporosity on the basal plane of graphene sheets make the film-like materials suitable for high-performance energy storagedevices.443 Co3O4/graphene composite film can be useddirectly as a free-standing binder-less electrodes for lithium-ion batteries. Benefiting from the strong interfacial interactionsbetween the sheet-like Co3O4 and graphene, the interfacialelectron and lithium ion transport was improved and thusprovided a high specific capacity of 1400 mA h g−1 at 100 mAg−1, enhanced rate capability, and excellent cyclic stability(namely 1200 mA h g−1 at 200 mA g−1 after 100 cycles).453
Recently, composite films of MoS2/RGO were performed as
the first counter electrode in Na-ion batteries which can beoperated under normal conditions and exhibited a stable chargecapacity of approximately 230 mA h g−1 with a high Coulombicefficiency approaching 99%.454
5.2.4.3. Molecular Sieving. Assembled graphene, GO, andCCG films can form ordered lamellar structures withnanochannels through the films, making them permeable toliquids and gases. Li’s group fabricated a kind of wet graphenemembranes using wrinkled CCG as building blocks and for thefirst time investigated their performance in nanofiltration fornanoparticles and dyes.549,550 Han et al. achieved ultrathin(22−53 nm thick) graphene nanofiltration membranes(uGNMs) on microporous substrates using CCG. Owning tothe rejection mechanism of physical sieving and electrostaticinteraction (Figure 41), such uGNMs showed a relatively highpure water flux (21.8 L m−2 h−1 bar−1) and excellent retention(>99%) for organic dyes, especially for the charged dyes.551
Nair and Geim et al. reported the phenomenon insubmicrometer-thick GO films that they were completelyimpermeable to liquids, vapors, and gases (even helium) butunimpeded permeable to H2O. The authors attributed thesefindings to the low frictional water flow through the 2Dgraphene nanocapillaries. After thermal reduction, the interlayerdistance decreased, along with an evident lessening of the waterpermeability.552 Later, Joshi et al. studied the permeationthrough micrometer-thick GO membranes, which served asmolecular sieves while being immersed in water. Interestingly,the GO laminates blocked all solutes with hydrated radiusabove 4.5 Å, and unexpected fast transport was observed forsmaller ions, driven by a large capillary force.553 Hence,graphene membranes are very likely to become the nextgeneration nanofiltration membranes for water purification.
5.2.4.4. Heat Spreaders. Graphene films are able to use asheat spreaders due to the extremely high thermal conductivityof graphene (5300 W m−1 K−1), in combination withmechanical strength and lightweight. Technically, graphenefilms will be graphitized after being annealed at a hightemperature (>1000 °C) thus perform higher in-plane thermalconductivity than conventional heat spreading materials. Kong
Figure 41. (a) Optical images of a supported uGNM on an anodic aluminum oxide (AAO) disk (left) and a bent uGNM on a polyvinylidenefluoride (PVDF) membrane (right). (b) Structure of base-refluxing reduced GO (brGO) used for membrane preparation, with a large amount ofholes and oxidized groups. (c) Proposed route for water molecules permeation: through the nanochannels of the membrane and holes on thegraphene sheets, the uGNM is negatively charged. The color change of the solution of (d) methyl blue, (e) methyl orange, and (f) direct red before(left) after filtration (right). Reprinted with permission from ref 551. Copyright 2013 Wiley-VCH.
et al. prepared graphene/carbon fiber composite films with ahigh in-plane thermal conductivity of 977 W m−1 K−1 afterthermal annealing at 1000 °C in Ar atmosphere for 1 h.554 Shenet al. obtained large-area graphene films from the evaporationinduced assembly of GO suspension and subsequentgraphitization at 2000 °C, which displayed a high in-planethermal conductivity of ∼1100 W m−1 K−1 and excellentelectromagnetic interface (EMI) shielding efficiency of ∼20dB.555 By using a higher graphitization temperature of 2850 °C,Xin et al. improved both of the thermal and electricalconductivities of graphene films to a great extent, which were1434 W m−1 K−1 and 1.83 × 105 S m−1, respectively.481 Liu etal. used the reduced wet-spinning graphene films as fast-response electrothermal films with continuous heat dissipation,holding great promise of ultrafast deicing applications foraircrafts (Figure 42).526
5.2.4.5. Sensors. Because of their large detection area, as wellas high carrier mobility and ambipolar field effect, the 2Dgraphene films could be used as biosensors and detectors. Forexample, Choi et al. obtained free-standing RGO/Nafionhybrid films, which were subsequently employed as electro-chemical biosensors for organophosphate detection. Thegraphene-based high performance biosensor delivered asensitivity of 10.7 nA μM−1, detection limit of 1.37 × 10−7
M, and response time of <3 s.556 Besides, numerous researchwith regard to the sensing behavior of graphene films has beendisclosed, aiming for diverse target substances includinghumidity,557,558 hydrogen,559,560 NO2,
561 and temperature562
as well. In competing with CNTs, the accessible surface area ofplanar graphene is theoretically larger, since the inner face of aCNT is relatively inaccessible, which may differentiate thesensing properties of the two nanocarbons from the geo-metrical point of view. For biological and chemical sensing,graphene has already demonstrated advantages over CNTs inseveral aspects, including sensitivity, response time, cost,etc.559,563
5.2.4.6. Other Applications. Apart from the aforementionedpopular applications for graphene films, there are a few studiesfocused on different aspects which are of equal significance,demonstrating the omnipotence of graphene. As an example,graphene thin film has been utilized in photothermal ablationtherapeutic application due to its strong absorption of near-
infrared light.480 Meanwhile, the thin films of graphene and GOfacilitate research on the biocompatibility of their surfacefunctional groups. Akhavan et al. found that GO films expressedhigher proliferation of human neural stem cells because of theirbetter hydrophilicity, whereas RGO thin films showed moredifferentiation of the cells into neurons by pulsed laserstimulation due to the better thermal conductivity.564 Similarly,RGO films given by increased hydrophilicity through N2plasma treatment gained significant improvement on bio-compatibility.565
5.3. 3D Graphene Architectures
The 3D graphene architectures normally refer to the looselypacked networks of graphene. In order to achieve a realgraphene assembly in bulk form, the thickness of the buildingblocks, or chamber walls in another words, should be betterdown to single or few layers. As a consequence, the ultralowdensity and extremely high specific surface area enableconvenient access to the attracting characters of grapheneand most excitingly within a macroscopic architecture.According to the major strategies for nanocarbon assemblies,the preparation of 3D graphene architectures could be dividedinto two different ways, wet (solution derived) or dry (based onCVD).
5.3.1. Self-Assembled Graphene Hydrogels. Since thewet methods always start with graphene dispersions, GO isusually taken as the beginning material. The homogeneouslydistributed GO suspension is a balanced system which is inequilibrium of electrostatic repulsion, hydrophobic effect,hydrogen bonding, etc. among the dispersed GO sheets. Ifthe balance state is disturbed by, for example, changing thechemical environment of the system or initiating the reductionof GO, the gelation of graphene can be promoted by differentsupramolecular interactions, including hydrogen bonding, π-stacking, electrostatic interaction, and coordination,566 or to bemore specific, the gelation is triggered by either increasing theattractions or weakening the repulsions.Shi’s group has conducted some pioneering work on the self-
assembled graphene hydrogels (SGH) via a facile hydrothermalmethod.27,567 Such a hydrothermal process led to the reductionof GO and, simultaneously, the partial overlapping orcoalescing of the reduced GO sheets. Therefore, 3D grapehenearchitectures were formed through π−π stacking-induced cross-
Figure 42. (a) Temperature profiles of a reduced wet-spun graphene film heater working under different dc voltages. Real time infrared thermalimages of (b) the graphene film heater under an applied voltage of 25 V for 30 s, and (c) two graphene film heaters attached on the wings of an A380plane model applying a voltage of 25 V. Reprinted with permission from ref 526. Copyright 2014 American Chemical Society.
linking of GO sheets (Figure 43a−c). Actually, these π−πstacking interactions were strong bindings to make the SGHmechanically strong, thermally stable, and electrically con-ductive. GO concentration and hydrothermal reaction timewere thought to be the two key factors which determinedproperties of the SGH.568
Furthermore, thanks to the wide application of thehydrothermal process in fabricating nano materials, it isconvenient to assemble and modify the graphene hydrogelswith doping atoms or nanoparticles at the same time, simplythrough hydrothermal treatment on the blends of GO andcertain precursors.569−572 For instance, nitrogen dopedgraphene hydrogels were obtained in the presence of pyrrole573
or organic amine,574 while nitrogen and boron codopedframeworks were achieved by adding ammonia borontrifluoride (NH3BF3) into the reaction system.575 Similarly,graphene assemblies decorated with Fe2O3 were also fabricatedfrom the aqueous mixture of GO and FeCl3.
576 In a recentstudy, several nanomaterials, including CNTs, InN nanowires,Zn2SnO4 nanowires, Au nanoparticles, TiO2 nanoparticles,polyaniline nanofibers, and MnO2 nanowires were decoratedonto graphene 3D assemblies through a universal hydrothermalcoassembly strategy. GO sheets were supposed to capture andstabilize these nanoparticles in the aqueous suspension first andthen coassemble with them during the hydothermal processthrough either adsorbing or wrapping, depending on the shapeof the particles. Moreover, it is capable of combining severaldifferent nanoparticles in one hybrid framework at the sametime, which is quite meaningful for realizing multifunctionalmaterials.577 In addition, the hydrothermal approach can beextended to solvothermal process if an organic solvent is used.The solvothermal process is usually demanded by severalnanoparticle-embedding or chemical modification protocols,
since organic solvents are sometimes more compatible withspecific nanoparticles and chemicals.578−581
Shi and co-workers’ following work further revealed that GOgelation was highly depended on the pH value of thesolution582 and the lateral size of GO sheets.566 It is believedthat the negative charges on GO surfaces mainly originate fromthe carboxyl groups, which provide electrostatic repulsion (ER)to prevent aggregation.439 The basic environment (pH >7)caused ionization of carboxyl groups on GO sheets thusincreased the ER forces, while in the acidic environment (pH<7), the ER forces were weakened and the hydrogen bondingwas enhanced due to the protonation of carboxyl groups,leading to a compact GO framework, which was even strongerthan its neutral counterpart.582 In fact, a balance betweenrepulsion and bonding forces should be established in GOhydrogel to maintain the stability of its structure. That is to say,excessive change on any side will lead to collapse of theframework, such as overacidification and reducing the size ofGO sheets.566 The smaller GO sheets (<1 μm) get highermobility in the solution and aggregate more easily than largerones, making them inclined to form precipitation rather thangelation since the acidification has increased their interactions.However, if the balance is carefully controlled, GO hydrogelconstituted by small sheets is also affordable. Compton et al.reported a sonication-induced gelation with small size of GOsheets ranging from 80 to 250 nm, depending on the extent ofultrasonic treatment. The ultrasonication process may fracturethe GO nanosheets into smaller fragments, and the freshlyexposed sheet edges did not possess carboxyl functional groupswhich help stabilizing the sheets, so the GO aqueous solutionscan be readily converted into hydrogels (Figure 43d). Incomparison with Shi’s work,566 the critical gelation concen-tration (CGC) in their paper was quite low in the range of0.05−0.125 mg mL−1, which was more than 20 times lower
Figure 43. (a) Photos showing homogeneous GO aqueous dispersion (2 mg mL−1) before and after hydrothermal reduction. (b) The SGH is strongenough to be handled and support weight. (c) SEM image of the microstructures of the SGH. Reprinted with permission from ref 568. Copyright2010 American Chemical Society. (d) Schematic illustration of the sonication induced gelation of GO, inset shows the conversion of GO dispersioninto hydrogel. Reprinted with permission from ref 583. Copyright 2012 Elsevier. (e) Synthesis of GO sponges and GO films through a vacuumcentrifugal evaporation process. Reprinted with permission from ref 584. Copyright 2010 Wiley-VCH.
than that in the former work. It is the dilute concentration thatseparated GO sheets well and minimized their possibility toform severe aggregation.583 Another work was introduced usinga vacuum centrifugal evaporation process. Either GO spongesor free-standing GO films can be fabricated through controllingthe operating temperature. The combined effect of the outwardcentrifugal force and the upward evaporating rate should be thedeciding factor to the assembling dynamics of GO sheets incolloidal suspensions. At a relatively low temperature of 40 °C,the outward centrifugal force surpassed the upward evaporationforce, and a randomly oriented GO network was generated viavan der Waals force during water evaporation (Figure 43e).584
What’s more, the as-made GO hydrogels can be furtherreduced with hydrazine or hydroiodic acid to improve theirconductivity and expand their applications in the field ofelectrochemistry as promising electrodes.585
5.3.2. Reduction-Induced Graphene Hydrogels. Apartfrom these additive-free assemblies of GO structures, in situreduction of GO sheets is another commonly used strategy toachieve GO assemblies from its suspensions. Various reducingagents have been studied for the preparation procedure, forinstance, acids,586−590 hydrazine,591 metals,592,593 and reducingsalts.594 The reduction upon GO will eliminate the functionalgroups on the sheets and significantly decrease the electrostaticrepulsion between them and thus initiate the gelation of RGOsheets. Technically, the hydrothermal method can also beregarded as a kind of reduction-induced assembly method, andonly just the reducing agent in the hydrothermal process is notchemicals but heat and pressure.The typical reduction-induced assembly process begins with
adding certain reducing agent into GO aqueous suspension; thereduction of GO then occurs during heating without stirring,and self-assembly is initiated in term of overlapping the reducedGO sheets through hydrophobic and π−π interactions.Actually, the absence of stirring is a critical factor for formingthe assemblies; otherwise, aggregation and precipitation willgenerate instead of the loosely stacked network, due to thestrong interlayer interactions between graphene sheets. L-Ascorbic acid (vitamin C) is a mild reducing agent for GO andno gaseous products beyond reduction; thus, it was chosen tobuild GO hydrogel with uniform structure.587,590 Afterconverting the as-formed hydrogel precursors into aerogels bysupercritical CO2 drying or freeze-drying, they can be used aselectrodes with high rate performance for electrochemicalapplications.587 Although hydrazine was suggested to be anunsuitable reducing agents for the reduction-induced assemblyprocess,587 the following work has actualized such a course inthe presence of KMnO4, which is an typical oxidant in thepreparation of GO.591 It is supposed that KMnO4 may affectthe evaporation rate of water around macroassemblies, forminga core−shell structure with MnO2 nanoparticles synthesizedsimultaneously and distributed mainly on the outside shell.Chen and co-workers investigated the different behaviors of a
series of graphene hydrogels and aerogels derived from variousreducing agents, such as NaHSO3, Na2S, vitamin C, HI, andhydroquinone.586 They all exhibit high mechanical strength,low density, thermal stability, high electrical conductivity, andhigh specific capacitance, while the graphene aerogels preparedusing HI shows the highest conductivity (110 S m−1). Similarly,a graphene aerogel with superior conductivity (500 S m−1) wasachieved by using hypophosphorous acid-iodine mixture (HPA-I) as the reducing agent, owing to the high degree of reductionwhich was revealed by the high C/O atomic ratio (14.7).595
Another iodine-based halogenation agent, which is thecombination of oxalic acid (OA) and sodium iodine (NaI),was employed to replace the toxic HI while maintaining a highGO reduction efficiency.596 The resulting graphene assemblywith high conductivity and low density was further infiltrated byPDMS to form an electrically conductive composite, which iscapable of sensing organic solvents with different polarity.The formation of 3D GO assemblies constructs the scaffolds
with porous structure and pore sizes ranging from submi-crometers to several micrometers, making it possible to embednanoparticles.597 A normal way is to capture the foreignnanoparticles simultaneously with formation of the network, solong as the nanoparticles or their precursors are homoge-neously dispersed in GO suspension during the reduction-induced self-assembly process. For example, magnetic 3Dgraphene architecture can be obtained by introducing Fe3O4nanoparticles in the meantime when GO was reduced byNaHSO3.
598 Thanks to the unique properties of Fe3O4nanoparticles, the obtained gel was not only superparamagneticbut also exhibited good electrochemical performance which canbe used as the anode material for lithium-ion batteries.Metal (ion) is another kind of reported reducing agent
utilized in the reduction-induced assembly process. Such aprocess is usually accompanied by incorporating metal oxidenanoparticles in the formed macroscopic materials. Forexample, α-FeOOH nanorods and Fe3O4 nanoparticles canbe imparted to the graphene network by the control of pHduring reduction and assembly of graphene sheets using ferrousions as the reducing agent.592 The obtained graphene/metaloxide composite hydrogels and aerogels were capable ofefficient adsorbents for removal of heavy metal ions and oilsfrom water. By mixing Cu nanoparticles with GO aqueoussuspensions and heating, graphene hydrogels were obtainedand the Cu nanoparticles were oxidized to Cu2O nanoparticlesand incorporated to the interconnected network at the sametime.593
5.3.3. Cross-Linked Graphene Assemblies. In order toimprove the interactions among the building blocks thus keepthe as-formed architectures stable, the most effective strategyattributes to the utilization of chemical/physical cross-linkers tobind them together. The reported binders/cross-linkers includepolymers,599−602 organic/inorganic compounds,588,603,604 metalions605,606 and biomolecules.607−609 Interestingly, the strengthof a graphene aerogel can be improved greatly by simplysoaking ammonia solution before its graphene hydrogelprecursor was freeze-dried. The ammonia solution wassuggested to play two roles in the fabrication process, includingsupporting the porous structure during freeze-drying andproviding covalent bonding between graphene sheets.610
5.3.3.1. Cross-Linked with Polymers. PVA was considered asan efficient cross-linker for the graphene architectures.582,611,612
Since a strong interaction between the PVA component andGO sheets exists in term of hydrogen bonding,613 the GOnetwork can be well cross-linked. Pluronic copolymer (poly-(ethylene oxide)-block-poly(propylene oxide)-block-poly-(ethylene oxide) triblock copolymer, PEO-b-PPO-b-PEO)was able to stabilize reduced GO sheets in aqueous solution.In the homogeneously dispersed system, the hydrophobic PPOsegments of the triblock copolymer may attach onto thehydrophobic RGO surfaces, while the hydrophilic PEO chainsextended into water. After α-cyclodextrin (α-CD) wasintroduced, the PEO chains penetrated into the α-CD cavities,forming a supramolecular hydrogel (Figure 44a).599 Sub-
sequently, other kinds of amphiphilic molecules or polymerswere used as gelators to promote the gelation of GO sheets inboth aqueous and organic solutions.614,615
In situ polymerization within GO suspension is a one-stepprotocol to achieve the GO/polymer composite hydrogel, and anumber of reviews have brilliantly summarized the polymercomposites with graphene as the cross-linker for thematrix.616,617 However, our target in this section will focus onthe graphene assemblies cross-linked by polymers. In view ofthis, only selected works are presented. For instance, a thermaland pH responsive hydrogel was attained by covalently bondingGO sheets with poly(N-isopropylacrylamine) (PNIPAM)hydrogel via the reaction between epichlorohydrin (ECH)and carboxyl groups.601 On the other hand, the hydroxyl groupson GO sheets are supposed to interact with glutaraldehyde(GAD), with the help of polycondensation of GAD andresorcinol (Res), and the functionalized GO sheets were able toperform polymerization between each other, resulting in across-linked 3D network.618 Although the polymer cross-linkersmay significantly improve the stability and mechanical proper-ties of the composite hydrogels, their conductivity is alwayslimited since most of the adopted polymers are insulated. Toovercome such limitation on conductivity, one of the feasibleways is employing conductive polymers as the cross-linkers. Baiet al. studied the gelation behavior of GO with three conductivepolymers, namely polypyrrole (PPy), poly(3,4-ethylenediox-ythiophene) (PEDOT), and polyaniline (PANi). Throughhydrogen bonding and π−π and electrostatic interactions, thesepolymers strongly interacted with GO sheets, and the resultinghydrogels showed high conductivity, electrochemical activity, aswell as ammonia gas sensitivity.600
Following a similar organic sol−gel technique they’ve used toprepare CNT aerogels,359 Worsley and co-workers conducted
several studies on highly conductive graphene aerogels. In theirpreparation route, resorcinol (R) and formaldehyde (F) withsodium carbonate as a catalyst were mixed and polymerized inthe GO aqueous suspension, forming a polymer cross-linkedGO hydrogel. After being dried by supercritical CO2 andpyrolysis, graphene aerogel with carbon binders was obtained.In comparison with other graphene networks bonded withphysical interactions, the carbonized polymers at the junctionsprovided conductive interconnections between graphenesheets.619 Later on, their further work revealed that thedecrease of RF content during gelation may produce agraphene assembly with a higher degree of exfoliation andless C−H bonding. The RF content could be as low as 0%;nevertheless, the high specific surface area approaching 1200 m2
g−1 and the high conductivity were maintained.620 Based onsuch RF sol−gel method, a “top-down” strategy was reported tofabricate a graphene monolith. The monomers R and F werepolymerized without the presence of GO suspension, followedby etching away amorphous carbon and multilayer graphitecomponents after pyrolysis (Figure 44b). Amazingly, the finalgraphene foam possessed extremely high specific surface area of3000 m2 g−1, because the building blocks composed thenetwork were almost single-layer graphene nanoplates.621 Inaddition, a report claimed that by thermally reducing the RF-derived GO aerogel in H2, graphene aerogel with unprece-dented high C/O molar ratio of 69.9 was achieved.622 Besides,Worsley et al. also developed the direct cross-linking for GOassemblies other than using the multistep sol−gel chemistryapproach. In such a method, these 3D graphene architectureswere cross-linked on the native epoxide and hydroxyl groups onGO sheets under base conditions (NH4OH). It is believed thatthese cross-links were converted to sp2 carbon after thermalannealing, which contributed to the mechanical and electrical
Figure 44. (a) Structure of the PEO-b-PPO-b-PEO copolymer coated graphene. Reprinted with permission from ref 599. Copyright 2009 AmericanChemical Society. (b) Schematic of the RF sol−gel approach. Reprinted with permission from ref 621. Copyright 2012 Wiley-VCH. (c) Formationof RGO gel with divalent ion linkage, M2+ represents the divalent ion of Ca2+, Ni2+, or Co2+. Reprinted with permission from ref 606. Copyright 2010American Chemical Society. (d) Preparation of GO/DNA hydrogel. (e) Photographs demonstrating the self-healing behavior of the GO/DNAhydrogel. Reprinted with permission from ref 607. Copyright 2010 American Chemical Society.
properties of the assemblies. The final assembled materialspossessed comparable surface area to those of the RF-derivedones (1314 m2 g−1), plus they were stronger and moreconductive than most other 3D graphene assemblies withphysical cross-linking.623
5.3.3.2. Cross-Linked with (Metal) Ions. Apart frompolymeric binders/cross-linkers, GO hydrogels were obtainedthrough a hydrothermal process in the presence of glucose andnoble-metal salts, such as PdCl2, HAuCl4, RhCl3, etc. The insitu synthesized noble metals were revealed to promote theassembly process of GO sheets and influence the formation ofthe porous structure.624 Afterward, the Pd decorated GOassemblies were used as catalyst for the Heck reaction. Sincethe improved cross-linking of GO sheets by adding divalentions (Ca2+ and Mg2+) has already been reported on GOfilms,438 the concept of using metal ion linkages (Ca2+, Ni2+, orCo2+) was also borrowed into the 3D GO assemblies. As aresult, linkage was formed among the water molecules, thedivalent ions, and the functional groups on GO sheets, whichstabilized the porous structure (Figure 44c).606 Very recently,rigid tetracationic cyclophanes containing diazapyrenium units(CBDAP4+) were reported as the gelators for GO gelation inDMF, the gelation was triggered and stabilized by noncovalentdonor−acceptor, π−π stacking, and cation−π interactions
between the gelators and GO sheets.603 On the other hand,ferrocene (Fc) which possesses two cyclopentadienyl rings wasreported as an interlayer cross-linker through the π−πinteractions with the aromatic structure of GO sheets, and ahydrogel could be formed even at room temperature.605
The hydrolysis of glucono-δ-lactone (GDL) to gluconic acidwas utilized to promote the gelation process of multivalent ions(La3+, CO2+, and Ni2+) or polyamine (polyethylenimine,melamine, and polyamidoamine) cross-linked GO hydro-gels.625,626 Since the additives had a strong electrostaticinteraction with carboxyl groups at the edge of GO sheets,the GO hydrogels were supposed to be assembled edge-to-edge.626 Most interestingly, the La3+ cross-linked GO hydrogelshowed a reversible gel−sol transition when ethylenediamine-tetraacetic acid disodium salt (EDTA) solution was added,according to higher coordination capability between EDTA andthe metal ions.625
5.3.3.3. Cross-Linked with Organic/Inorganic Compounds.It is noteworthy that there are some molecules that couldpenetrate into the interlayer space between GO sheets, throughinteractions with the in-plane oxygen-containing groups or thearomatic structure of GO. Therefore, these molecules could notonly bind the interconnected framework, but also separate theGO layers for more porosity. The linear boronic acids have
Figure 45. (a) SEM images of the cork-like graphene monolith. (b) Formation mechanism of the cork-like structure by freeze casting. Reprintedwith permission from ref 629. Copyright 2012 Nature Publishing Group. (c) Schematic for the preparation of a macroporous graphene monolith(MGM). (d) SEM image of the morphology of an MGM. Reprinted with permission from ref 633. Copyright 2014 Wiley-VCH. (e) Photograph of ahydrophobic and oleophilic graphene-based sponge; the inset shows a water droplet at a contact angle (CA) of 162°. SEM images of (f) the puresponge and (g) sponge with 7.3% graphene coating. Reprinted with permission from ref 635. Copyright 2012 Royal Society of Chemistry.
been found as the pillaring units in the interlayer space of GO,offering the cross-linked and pillared GO frameworks withinteresting potential for gas storage.579,580 In another work,mercaptoacetic acid was shown able to reduce GO andcovalently bond to the GO sheets simultaneously through thereaction between −SH groups and hydroxyl or epoxy groups ofGO. The authors tried different types of compounds withsimilar structures, but only mercaptoacetic acid and mercaptoe-thanol resulted in the assembly to 3D architectures.588
Very recently, the porous GO assemblies were reinforced byconformal hexagonal boron nitride (h-BN) platelets. Since h-BN is an isomorph of graphene, they are structurallycompatible with a good lattice matching. The hybridization ofh-BN with GO prevented the crumbling of GO sheets andbound them together, forming a stable framework.627
5.3.3.4. Cross-Linked with Biomolecules. Biomolecules havealso been discovered as the cross-linkers for grapehene 3Darchitectures. Three kinds of biologically active molecules ofpolyamines including tris(aminoethyl) amine, spermine, andspermidine were introduced into GO network via the acid−base type electrostatic interaction.604 By heating the aqueousdispersion of GO and double-stranded DNA (dsDNA), thedsDNA was unwound to single-stranded DNA (ssDNA) whilebridging adjacent GO sheets by means of π−π interactions atthe same time (Figure 44d,e).607 In acidic environment, thepositively charged hemoglobin (Hb) molecules may attach tothe negatively charged GO sheets via the electrostaticattraction, promoting the formation of a composite hydrogel.609
Later on, chitosan (CS) chains were found cross-linking GOhydrogels through electrostatic and hydrogen bonds.628
5.3.4. Template-Directed Graphene Architectures.Freeze casting is a widely used strategy to attain porousassemblies from suspensions with solid particles. It is a phaseseparation process during freezing where the particles arerepelled from the forming ice, and accumulated at the icecrystal boundaries to form a continuous network whosemorphology is determined by the ice template. Li and co-workers provided detailed work relating the generation of cork-like 3D graphene assemblies through freeze casting without anypolymers or surfactants. By carefully controlling the reductionlevel of the original dispersed GO and freeze conditions, a cork-like graphene cellular structure with highly ordered morphologywas obtained (Figure 45a,b). The purpose of using partiallyreduced GO sheets is to enhance the π−π interactions betweenthe building blocks. Finally, the ice-templated graphenemonolith with anisotropic porous structure showed manyadvantages especially the superelasticity with a high recoveryrate up to 80% strain.629 By the way, the concept of cork-likepore structure has inspired several studies for seeking highlycompressive architectures.630,631 One of them realized ahierarchical 3D porous structure with cell size in the samerange with natural cork, through a functionalization-lyophiliza-tion-microwave strategy using ethylenediamine (EDA) as thefunctionalizing and reducing agent during the gelation of GOsheets. The resulted graphene monolith was ultralight (density3−5 mg cm−3) and able to recover from strain as high as90%.631 Baughman and Chen et al. in their very recent worksynthesized graphene sponge materials by a solvothermalprocess in alcohol, followed by freeze-drying and thermalannealing. The additive-free graphene sponges exhibitedcombined properties of super compressive elasticity and near-zero Poisson’s ratio in all directions.632
Emulsion can also be utilized as the template for theformation of 3D GO network. Li et al. introduced a modifiedhydrothermal method to prepare macroporous graphenemonoliths. Hexane droplets in the GO aqueous suspensionserved as the soft templates for the formation of interconnectednetwork during hydrothermal process (Figure 45c,d). Similar tothose studies just mentioned, the resulting macroporousgraphene monoliths showed good elasticity with a rapid rateof recovery.633
In a word, despite the fact that most of the ordinary grapheneaerogels are brittle, these highly elastic monoliths have severalpoints in common. First, they possess a closed-cell cellularstructure and the microstructure is better to be regular. Second,the pore sizes are large in the range of several hundredmicrometers, which provides plenty of room to satisfy thevolume contraction while being squeezed and also lead to a lowdensity for the bulk materials (<10 mg cm−3). At last, the cellwalls are composed of a multilayer of stacked graphene sheetsin parallel to make them strong enough and able to regain theirinitial shape after deformation.In addition to these works concerning a delicate freeze
casting process, the template assisted formation of the graphenenetwork can also be realized in a much simpler way, using aporous bulk template and GO solution. Xie et al. introducedthe preparation process of dipping synthetic PU sponge intoGO aqueous solution and drying, GO sheets were subsequentlyassembled on the surface of the sponge and formed a networkwith an open porous structure. In combination with a stainless-steel current collector, the composite sponge was used as ananode for a microbial fuel cell.634 Similarly, a melamine spongewith superhydrophilic surface was used as the scaffold for dip-coating graphene sheets in ethanol solvent as well (Figure 45e−g). The graphene sheets employed in their work were derivedfrom exfoliation of expanded graphite rather than GO. Theanchor of graphene turned the sponge surface into hydrophobicand oleophilic. After further coating of PDMS, the graphene-based sponge was applied as absorbents of oils and organicsolvents from water.635 Recently, GO framework wasassembled after soaking GO solution into a PU sponge,however, followed by annealing under 900 °C, which causedboth reduction of GO and pyrolysis of the PU sponge. Thus,the obtained graphene foam was expected to have betterelectrical conductivity, facilitating its use as an electrodematerial.636
5.3.5. CVD-Grown Graphene Foams. CVD is a typicalprotocol with great importance to achieve graphene. In such amethod, a metallic substrate is usually needed. Cheng’s groupreported a direct synthesis of graphene 3D architectures grownby the Ni foam template-directed CVD method (Figure 46).637
However, the Ni etching process by hot HCl or FeCl3 solutionis too violent and may cause collapse of the as-formed graphenenetwork. Therefore, the authors applied a thin layer of PMMAto support the carbon structure, which was carefully removedby hot acetone afterward. Different from the macroscopic 3Dassemblies obtained from chemically derived graphene sheets,the as-prepared foam from CVD-grown graphene exhibitedsuperior electrical conductivity due to the high quality ofgraphene sheets and their perfect connection. The free-standinggraphene foam was further infiltrated by PDMS to make elasticand flexible conductors. In their later work, the graphene foamfound applications of reversible chemical sensors for NH3 andNO2,
638 as well as the skeleton for superhydrophobic foamswith Teflon coating.639 Ji et al. employed a similar protocol to
fabricate free-standing and ultrathin graphite foam (UGF) butwith a slower cooling procedure during the CVD process,
leading to a thicker strut wall thickness of more than 10graphene layers. Consequently, the UGF possessed a muchhigher electrical conductivity of ∼1.3 × 105 S m−1.640 Thenthey further investigated the relationship between the synthesisconditions and the thermal conductivity of the as-made grahenefoam. Six samples with three kinds of Ni etchants and differentCVD growth time were prepared for comparison. The resultsshowed that the replacement of the aggressive Ni etchant ofHCl into a gentle one should provide a significant improvementon the thermal conductivity of the free-standing graphenefoam.641
The template-directed assembly method is a facile way toobtain bulk materials with regular morphology of a porousstructure, depending on the applied templates. In an alternativeway, 3D amorphous carbon structure was utilized as the parentscaffold and then converted to graphene with the help ofconformal Ni coating.642 Such an approach was inspired by thestudies accomplished by Tour and co-workers,643,644 wherecarbon atoms diffused into Ni coating during annealing andsubsequent removal of Ni leaving multilayered graphene. Theresulting porous graphene structure was hollow and withregularly dispersed pores of around 500 nm in size. Besides, thecombination of Ni and polystyrene (PS) spheres as thetemplates may also generate a hierarchically porous architec-ture.645
In an alternative way, Peng et al. implemented the conversionfrom CVD-grown MWCNT sponges to graphene nanoribbon(GNR) aerogels via a chemical unzipping process whichunzipped MWCNTs into multilayered GNRs while maintainingtheir original 3D network. The obtained GNR aerogelsdisplayed completely different compressive behavior and
Figure 46. (a) Preparation process for CVD-grown graphene foamand infiltration with PDMS. (b) Optical image of a large-scale free-standing graphene foam. (c) SEM image of the foam. Reprinted withpermission from ref 637. Copyright 2011 Nature Publishing Group.
Figure 47. (a)−(f) Photographs of densely packed graphene monoliths in different shapes before drying. (g) Schematic diagram of the procedure forcompact graphene products with certain shapes. Reprinted with permission from ref 647. Copyright 2012 Wiley-VCH.
increased specific surface area, making them suitable forapplications as supercapacitor electrodes and polymer re-inforcements.646
5.3.6. High-Density Graphene Monoliths. The above-mentioned 3D graphene assemblies, no matter hydrogels,aerogels, or foams, are all interconnected graphene frameworkswith high porosity, which limits their mechanical strength andconductive properties. Conversely, rising demands for thedensely stacked graphene-based materials emerge in recentyears, since the compact structures with densities close tographite exhibit several orders of magnitude higher mechanicaland electrical properties than the highly porous structures. Tothis end, Sun and Ruoff et al. took colloidal GO suspension intohydrothermal treatment while using ammonia or NaOH toadjust the pH value of the system. After the pH-mediatedhydrothermal reduction process, the obtained graphene gelwith controllable shape was dried near room temperature (32°C), rather than supercritical drying or freeze-drying which wascritically useful for preserving the porous structure in thenormal process for graphene aerogels. Therefore, the capillaryforce-induced shrinkage led to the formation of a compactstructure (Figure 47). As a result, when the pH was 10, thestacked graphene assemblies showed a density of 1.6 g cm−3,comparable to conventional graphite products (1.54−1.78 gcm−3); meanwhile, the compressive strength and electricalconductivity also reached their maximum values, which were361 MPa and 7.6 S cm−1 after annealing, respectively.647 Asimilar work without pH adjustment during the hydrothermalprocess also obtained a highly dense monolith (density up to1.58 g cm−3) but displayed a porous structure with numberless
micropores (<2 nm). Although mechanical properties were notincluded in the paper, the combination of high density, porousstructure and good electrical conductivity (0.16 S cm−1)rendered such material an ultrahigh volumetric capacitance of376 F cm−3.648 The newly discovered study by Worsely et al.focused on the highly stacked graphene structure (density of∼1 g cm−3) as well. Remarkably, the bulk electrical conductivityof their graphene monolith could reach as high as 1750 S cm−1
after thermal annealing, which was the highest value for 3Dgraphene assemblies. They also presented the superlineardependences of electrical and mechanical properties on themonolith density for porous materials in the paper.649
Consequently, it is demonstrated that the highly densegraphene-based materials have offered an alternative way topursue the excellent properties of graphene.
5.3.7. Applications. 5.3.7.1. Supercapacitors and Lith-ium-Ion Batteries. The highly porous structure and electricallyconductive feature in the reduced form are the main merits ofgraphene aerogels/hydrogels while being used as electrodes forenergy storage devices. The 3D interconnected frameworkswith macroporous architectures are favorable for ion diffusionand electron transport in bulk electrodes. The electrochemicalperformance of early graphene aerogel electrodes was limitedby their pore structure and surface chemistry of graphenesheets, with specific capacitance of 120 F g−1.612 Afterward, theself-assembled graphene hydrogel derived by the hydrothermalmethod was directly applied as a 3D supercapacitor electrode,exhibiting specific capacitance of 175 F g−1 in an aqueouselectrolyte.568 The conductivity of electrodes not onlyinfluences their capacitance values but also plays an important
Figure 48. (a) Schematic of the formation of graphene-S hybrid monolith and the preparation of an electrode. Main performance of the G-S63cathode for its application in LIB. (b) CV curves at 0.1 mV s−1, (c) first galvanostatic charge−discharge curves at 0.3 A g−1, (d) capacity at variouscurrent densities, and (e) cyclic performance and Coulombic efficiency. Reprinted with permission from ref 572. Copyright 2013 American ChemicalSociety.
role in their rate capabilities.622 Further reduction withhydrazine improved the conductivity of graphene hydrogels,thus increasing their specific capacitance to 220 F g−1 at 1 Ag−1, and a high rate capability was obtained at the same timewhere the capacitance can be maintained for 74% at anextraordinary fast discharge rate of 100 A g−1.585 The chemicalreduction induced graphene hydrogels/aerogels showedcomparable specific capacitance up to 166 F g−1 at a potentialscan rate of 10 mV s−1.586,587,623 Interestingly, a bacteriapromoted hierarchical carbon material with rich porosity (0.74cm3 g−1) and multilevel architecture, including both mesopores(3−5 nm) and micropores (<2 nm), led to a relatively highspecific capacitance of 327 F g−1.650 In order to achievesupercapacitors with large volumetric capacitance, graphenehydrogels were dried through an evaporation-induced dryingprocess. As a consequence, the obtained dense yet porousmaterials contributed to a volumetric capacitance as high as 376F cm−3.648
Gao et al. exploited another way to realize an asymmetricsupercapacitor which could work efficiently in a wide potentialwindow of 0−2.0 V and with remarkable energy and powerdensity. In such a novel design, a graphene hydrogel with 3Dinterconnected pores was utilized as the negative electrode, andvertically aligned MnO2 nanoplates on a nickel foam was usedas the positive electrode. Thus, the achieved energy densitycould reach a value of 23.2 Wh kg−1.651
Incorporation of PANI with pseudocapacitance intographene 3D architectures may increase the capacitance up to475 F g−1.577 The nitrogen and/or boron doping in carbonnetworks would facilitate charge transfer between carbon atomsand thus enhance the electrochemical performance of carbon-based materials. Chen et al. developed a supercapacitor withhigh performance at ultrafast charge/discharge rates due to theutilization of nitrogen-doped graphene hydrogel electrodes. Apower density of 205 kW kg−1 can be obtained at 185 A g−1,and 92.5% of its capacitance was retained for 4000 cycles at acurrent density of 100 A g−1.574 Wu et al. built all-solid-state
supercapacitors (ASSSs) in which B and N doped 3Dmonolithic graphene aerogels served as electrodes and aPVA/H2SO4 gel as solid-state electrolyte and separator.Although the resulted specific capacitance (62 F g−1) was notas high as the above-mentioned supercapacitor with liquidelectrolyte, the ASSSs provided a possible solution forminimized energy storage devices.575
With the same role as CNT frameworks played, where theinterconnected carbon architectures served as conductive andporous platforms for nanoparticles with electrochemicalactivity, graphene based 3D architectures were also intensivelystudied for their potential application as electrodes for LIBs.For anode materials, Fe3O4,
598 Fe2O3,576,578 MnO2,
591 SnO2,615
and SnS2,569 which are efficient at Li+ insertion and extraction,
were embedded in graphene frameworks. For cathodematerials, as an example, a graphene−sulfur hybrid structurewas utilized. In such a lithium−sulfur system, polysulfidesconverted from sulfur bound to the oxygen containing groupson reduced GO, preventing their dissolution into theelectrolyte, while resulting in an improved cyclic stability over100 cycles (Figure 48).572 It is worth mentioning that, throughideally combining hierarchically porous structure, highlyconductive network, and heteroatom doping in one grapheneelectrode, Wang et al. achieved an extremely high storage levelfor graphene based LIBs, showing a supercapacitor-like powerdensity (116 kW kg−1) and a battery-like energy density (322Wh kg−1).645
5.3.7.2. Absorbers. Due to the large specific surface area andaffinity with different kinds of substances, graphene monolithic3D structures were widely applied as absorbing materials fordyes, oils, organics, metal ions, and gases. Therefore, graphenemacroassemblies are expected for removing pollutants fromcontaminated water, as well as for storing gases. GO hydrogelsshowed high dye-loading ability, which came from the strongelectrostatic interactions between negatively charged GO andpositively charged dye molecules. The absorption efficiency wasnormally above 80% after 12 h and approaching 100% after 24
Figure 49. (a) Photograph of the kerosene adsorption using a gallic acid based graphene aerogel (GaA-GA). Reprinted with permission from ref 589.Copyright 2012 Royal Society of Chemistry. (b) Ammonia gas sensing performance of three devices with the one based on lyophilized GO/PPylhydrogel being the best. Reprinted with permission from ref 600. Copyright 2011 Royal Society of Chemistry. (c) The resistance variation of agraphene foam-based stretchable conductor as a function of uniaxial tensile strain; the inset shows the stretching process. Reprinted with permissionfrom ref 637. Copyright 2011 Nature Publishing Group.
h.607,614 The absorption process of heavy metal ions was alsobased on electrostatic interactions, thus it was influenced bysolution pH, and more alkaline conditions were relativelyfavorable for the performance of the absorbents.588,592,652 Incontrast, the quick and strong absorption capability of reducedgraphene aerogels for oils and organics were mainly attributedto the hydrophobic nature of graphene sheets. Owing to theultralow density of graphene sorbents, their oil-uptakecapacities could reach hundreds of times their ownweight,589,593,653,654 605 g g−1 for motor oil,654 as an example(Figure 49a). As for gas sorption, the porous structure of 3Dgraphene architectures offers plenty of accessible surface areawhich could interact with H2 or CO2 molecules, and theirabsorption capacity is comparable to or even higher than that ofother carbon based macroporous materials.579,580,618
5.3.7.3. Catalytic Applications. The good absorptioncapability, highly porous structure, and low density makegrapehene 3D assemblies prospective candidates for catalystsupports for various reactions; meanwhile, their mechanicalrobustness provides reusability. Even the neat GO aerogel wasshown to effectively catalyze the reaction of S → O acetylmigration for the synthesis of thiol compounds, while themicrocells made up of GO sheets acted not only as reactors butalso as catalysts.655 Compared with CNT based 3Darchitectures, graphene based ones are more accessible interms of hydrogels and aerogels, subsequently, a series ofcatalysts could be readily incorporated during the in situformation process. For instance, Pd containing GO 3Dmacroassemblies showed high catalytic activity and selectivityin the Heck reaction.624 Au nanoparticles distributed graphenehydrogel exhibited excellent catalytic performance toward thereduction of nitroaromatic compounds.597,604 In addition, 3Dgraphene architectures with the decoration of Pt nanoparticlesand iron nitrides were proposed as electrocatalysts in fuelcells,581,656 while the photocatalytic activity of the hybridnetworks were also reported.571,593
5.3.7.4. Other Applications. Several modified graphenemonoliths were demonstrated as high performance sensors.The GO areogel with ultrathin PPy layers showed highsensitivity toward ammonia (Figure 49b).600 After infiltrating aresponsive polymer of PDMS, the composite became a sensingdevice for differentiating organic solvents with differentpolarity.596 In addition, the unique transition behavior of
graphene hydrogels induced by changing temperature or pHhas the potential for drug releasing applications.582,599 At last,their outstanding electrical and mechanical properties providegraphene networks the opportunity of serving as a conductiveskeleton for flexible conductors (Figure 49c)602,637 andstructural reinforcement for composites.630
6. HYBRID ASSEMBLIES OF NANOCARBONSThe three major characters in this review, fullerenes, CNTs,and graphene, are known to possess various charming featuresand so do their assembled architectures from 1D fibers to 3Dstructures. Since the assembly strategies are supposed totranslate the excellent properties of individual nanocarbons tolarger aggregates, the hybridization of two or three constituentsusually delivers more advanced outcomes than singlecomponents. It is called the synergetic effect. Consequently,the simultaneous assembly of nanocarbons has become anemerging interest in this research area. The fundamentalprinciples and applications of nanocarbon hybrids have alreadybeen well-reviewed,657,658 however, with the main focus beingon the modified nanocarbons with functional components. Thehybridization between nanocarbons has so far occupied toolittle attention, especially for their macroscopic assembledstructures, which is highly regarded in this context.6.1. 1D Hybrid Fibers
Most of the 1D hybrid nanocarbon fibers were prepared fromCNTs and graphene or their derivatives. As we know,nanocarbon fibers are fabricated through wet or dry methods.These methods apparently could well apply to hybrid fibers.For instance, RGO and SWNTs were mixed to form a stabledispersion with the existence of surfactant SDBS, followed bywet-spinning into PVA coagulation bath and continuous,strong, tough fibers with gravimetric toughness as high as1000 J g−1 were produced. The outstanding toughness wasexplained by strong interactions between graphene and CNTs,in conjunction with the alignment during fiber spinning (Figure50a,b).659 In the case of a GO−CNT dispersion, GO is a goodsurfactant to disperse CNTs because of its amphiphilic natureand interaction with CNTs, while simultaneously, CNTs couldhinder the restacking of GO.660 Nitric-acid-treated SWNTs andGO were combined in the presence of ethylenediamine (EDA),resulting in a homogeneous aqueous suspension. After ahydrothermal process being applied to the mixture, hybrid
Figure 50. (a) Schematic of the formation of the oriented interconnected network of RGO and SWCNT. (b) Cross-sectional SEM image of aRGOF/SWCNT/PVA composite fiber. Reprinted with permission from ref 659. Copyright 2012 Nature Publishing Group. SEM images of hybridCNT−graphene yarn surface at (c) low and (d) high magnification. (e) Cross-section of the hybrid yarn. Reprinted with permission from ref 663.Copyright 2014 Wiley-VCH.
fibers made of SWCNTs and nitrogen doped RGO wereformed with an interconnected network architecture. Themesoporous morphology, large specific surface area (396 m2
g−1), and high electrical conductivity (102 S cm−1) offered thehierarchically structured fibers specific volumetric capacitanceas high as 305 F cm−3 in sulfuric acid or 300 F cm−3 in PVA/H3PO4 electrolyte.661 As a modification on graphene fibers,along which Qu and Dai et al. directly grew CNTs withpreintercalated Fe3O4 nanoparticles as catalysts for the CVDprocess during the hydrothermal fabrication of graphene fibers.The preparation of CNT/G hybrid fibers was further appliedinto textile electrodes for flexible supercapacitors showing finedurability under repeated flat-to-bending cycles.662
Hybrid fibers were also achieved based on the dry-spunmethod. By taking the as-drawn CNT sheet as a host andgraphene solution as a guest during the fabrication, CNT array-derived graphene/CNT composite fibers were obtained afterthe subsequent twisting process. Because of the incorporationof graphene sheets, the morphology of the composite fibers wasevidently different from the pristine CNT fibers, with graphenesheets presenting on the surface and penetrating among CNTs(Figure 50c−e).663,664 Theoretically, the addition of graphenemay bridge neighboring CNTs to improve the charge transportdue to strong π−π interaction. In a CCG/MWCNT system,graphene caused a porous structure in the composite yarns.Thus, the elastic modulus was reduced a lot while the fracturestrain was 100% higher in the graphene/CNT yarns ascompared with the pristine CNT yarns. Most significantly,the electrical conductivity of the composite yarns (900 S cm−1)was found to be 400% and 1250% higher than that of pristineMWCNT yarns and graphene paper, respectively. Meanwhile,the synergy of CNTs and graphene endowed the hybrid yarnswith a strong electrochemical response with substantialimprovement on specific capacitance, which was 425% higher
than capacitance of pristine MWCNT yarns.663 When thegraphene/CNT composite fibers were taken into applicationsof wire-shaped dye-sensitized solar cells and supercapacitors, ahigh energy conversion efficiency of 8.50% was obtained, whilethe specific capacitance (31.5 F g−1) was much higher than thatof the bare CNT fibers (5.83 F g−1).664 Without a need forusing GO solution, dry-spun CNT fibers were treated by anoxidizing solution. Such a process led to a core−sheathstructure with GO nanoribbons at the outer surface. As a result,the CNT core was responsible for the high flexibility, tensilestrength and electrical conductivity, while the graphenenanoribbon-contained carbonaceous sheath enabled a highcatalytic activity.665 Alternatively, wet-spun RGO fiberswrapped with superaligned dry-spun CNT film showedenhanced specific strength and conductivity by 22% and 49%,respectively.666
6.2. 2D Hybrid Films
The simplest and most direct way to prepare a hybrid film is thedrop-casting protocol, simply by mixing the components intoslurry and then coating on a substrate. It is easy to be used forthe fabrication of electrodes for electrochemical devices.667,668
As an example, GO-MWCNT aqueous dispersion was cast ontoa glassy carbon electrode and further coated with horseradishperoxidase (HRP) to form a HRP/GO-MWCNT/glass carbonelectrode via electrostatic interaction. While the GO-MWCNTlayer may enhance the direct electron transfer between HRPand the glassy carbon electrode, the entire electrode exhibitedgood electrocatalytic performance.668 It is quite meaningful thatWallace et al. reported the ability of GO lyotropic LC phase toorganize and align SWCNTs, their casted composite filmsshowed a layer-by-layer morphology (shown in Figure 51a,b)and evidently enhanced mechanical performances (tensilestrength 505 MPa, modulus 51 GPa, elongation at break9.8%).669 Hydrothermal treatment with ammonia on a formed
Figure 51. SEM images of (a) surface and (b) cross-section of a layer-by-layer GO/SWCNT hybrid paper. Reprinted with permission from ref 669.Copyright 2013 American Chemical Society. (c) Schematic of the graphene-C60 hybrid film-fabrication process. Reprinted with permission from ref687. Copyright 2014 Royal Society of Chemistry.
GO/CNT film containing Co2+ will give rise to thesimultaneous formation of Co3O4 nanoparticles, reductionand nitrogen doping of GO, realizing the fabrication of flexiblepaper catalyst.670
Once the homogeneous dispersion of GO and CNTs wasformed, vacuum-assisted filtration,671,672 spin-coating,673 orheating-driven liquid/air interfacial self-assembly520 was carriedout to produce GO/CNTs hybrid films. Because of the layeredconfiguration of graphene in its 2D film structures, theincorporated CNT components are always sandwichedbetween the graphene sheets, with the role of bridging thedefects for electron transfer and increasing the space betweengraphene sheets. When they were used as nanofiltrationmembranes, the CNT “nanowedges” made the water fluxdoubled as compared with the neat graphene membranewithout degrading the rejection ratios to organic dyes.674
Besides, the facile solution-based protocols also allow variousfunctional components to be uniformly hybridized in the filmsand with tunable fractions. After thermal reduction, the GOcomponent changed into RGO thus the composite films with30 wt % MWCNTs delivered a specific capacitance of 379 F g−1
at 0.1 A g−1 in aqueous electrolyte.672 Based on sequentialfiltration of COOH-functionalized MWCNTs and GO, abilayer graphene film actuator which could be actuated by therelative humidity at room temperature was fabricated.675 Inorder to combine the profits from graphene, MnO2, and CNTs,graphene sheets were functionalized with MnO2 first and thenfiltered with CNTs. In that case, graphene served as a high-surface-area substrate for MnO2, and CNTs constructed theconductive network while keeping the whole structure solid.The flexible hybrid films displayed a tensile strength of 48 MPaand specific capacitance of 372 F g−1 with excellent ratecapability for supercapacitors.676 Other than performingmodification on graphene, hybrid films with coaxial PPy/CNT nanocables sandwiched by graphene sheets werefabricated. The composite particles were supposed to enlargethe intersheet space of graphene as well as providepseudocapacitance to achieve higher electrochemical perform-ance.677 In a sequential procedure, the filtrated CNT paper waselectrochemically decorated with MnO2, and GO was finallysoaked with the purpose of preventing MnO2 from detaching.Consequently, in combination with improved surface con-ductivity, the initial electrochemical capacitance of thecomposite paper reached 486.6 F g−1, as well as the stabilityof the capacitance after a long period of charge/discharge cycleswas ensured.678 Recently, the N and O dual doped graphene−CNT hydrogel films were reported having numerous catalyticcenters for oxygen evolution reaction (OER), in terms of thedual active sites, C−N and C−O−C.679Strong adhesion would be realized if charged components are
employed. With an electrophoretic deposition method, acid-treated MWCNTs and graphene nanosheets were electro-phoretically deposited on Ni foil and directly used as an anodefor Li-ion batteries.680 Another efficient pathway based oncharged nanoparticles is the layer-by-layer technique. Thisapproach involved the electrostatic interactions between twooppositely charged suspensions with RGO nanosheets (RGO−COO−) and functionalized CNTs (CNTs-NH3
+) sepa-rately.681−683 The obtained hybrid films were of well controlover optical and electrical properties through adjustment on thethickness. If acid treated CNTs normally with negative chargewere utilized, the substrate or graphene sheets should undergopreprocess to be positively charged.684,685 For example, the
adsorption of cationic PEI chains rendered graphene sheets tobe water-soluble and possible for sequential self-assembly withnegatively charged CNTs (acid-oxidized CNTs). The resultedhybrid films possessed interconnected network carbonstructures with well-defined nanopores, showing a specificcapacitance of 120 F g−1 at a high scan rate of 1 V s−1.685 Inaddition, the negatively charged nanocarbons (GO sheets andacid treated CNTs) also could be assembled via a layer-by-layerLB process. In such a related research, the optoelectronicproperties of GO/SWCNT hybrid thin films indicated a greatimprovement compared with those of single component GOfilms.686
Except for electrostatic interactions, the covalent interactionsare taken into account under some circumstances. For thefabrication of graphene−C60 hybrid films, GO was firstcovalently attached onto the (3-aminopropyl)triethoxysilane(APTES) modified silicon surface through reactions betweenepoxy-carbonxyl and amine groups. After being amine-functionalized on the graphene surface, C60 was immobilizedsubsequently via N−H addition reactions across the CCbonds in C60 (Figure 51c). As the authors claimed that thegraphene layer performed as the load-carrying and wearresistance phase and the C60 outer layer as friction reducer.Such synergistic effects between graphene and C60 significantlyenhanced the lubricating and antiwear properties of the hybridfilms.687 Speaking of which, macroscopic hybrid assembliescontaining fullerenes are not common in the literature as far aswe know and only appear in the form of hybrid films withgraphene or CNTs. When C60 or C70 molecules were arrangedon the external surface of SWCNTs and deposited on a SnO2electrode, the photocurrent generation efficiency of thephotoelectrochemical devices increased to a large degree,thanks to the more favorable heterointeractions betweenfullerene molecules and the SWCNT nanoscaffolds, as well asenhanced electron transportation.688−690 Based on the findingsthat fullerodendron and C60(OH)n disperse SWCNTseffectively in aqueous and nonaqueous solutions, the mixtureof such fullerene derivatives and SWCNTs is applicable forgenetic wet assembly process accordingly.284 Self-assembly ofC60, SWCNTs and few-layer graphene was realized at thetoluene-water interface and so did the composite films formedby pairs of nanocarbons. Through experimental investigations,the occurrence of charge-transfer interaction between C60 andSWCNTs, and between C60 and few-layer graphene weredemonstrated, due to the strong acceptor (electron-with-drawing) behavior of C60.
691
The CVD approach was also applied for the fabrication ofnanocarbon hybrid films. SWCNTs were first spin-coated ontoCu foils, followed by the synthesis of graphene films on theSWNTs/Cu foils using a thermal CVD method. The resultinghybrid films exhibited a sheet resistance of 300 Ω sq−1 with96.4% transparency, thus providing potentials for possibleapplications of high-performance flexible electrodes andFETs.692 Through the investigation on the combination of asingle-layer CVD-grown graphene and a MWCNT sheet, thedependence of the optical, electrical, and electromechanicalproperties on its configuration was revealed. Better responseswere found when graphene was on the top, since the graphenelayer could cover the external surface of the CNT film andhence made evident changes on its morphology.693 Thesefindings emphasized the importance of structure design whilechasing the superior functionalities of hybrid materials.
Up to now, several achievements in fabricating randomlyoriented CNTs/graphene sandwiches have been reported. Forinstance, pristine or acid treated MWCNTs were mixed withGO suspension and then subjected to a reduction-inducedassembly process to form hydrogel precursors. The graphene/MWCNT hybrid aerogels obtained after supercritical CO2drying resulted in high desalination capacity (633.3 mg g−1)with the NaCl concentration up to 35 g L−1.694 In a muchsimpler process, the GO−CNT aqueous mixture was
cryodesiccated directly avoiding the formation of hydrogels.The template-free “sol-cryo” protocol reported by Sun et al.enabled fabrication of ultraflyweight hybrid aerogels withcontrolled densities, depending on the weight fraction ofCNTs.695 It is the unique configuration of giant graphene sheetcell walls with CNT ribs as reinforcements that guaranteed suchan ultralight and stable all-carbon structure.696 The ultrafly-weight aerogels were found with outstanding elasticity, thermalstability, and adsorption capacities (Figure 52). When CNTswere entirely absent in the aerogels, the density could be
Figure 52. (a) Photograph showing the ultraflyweight aerogel standing on a flower like dog’s tail. (b) SEM images of the CNT-coated graphene cellwalls. (c) Schematic illustrating the cell walls of the hybrid aerogel made by synergistic assembly of graphene and CNTs. (d) The graphene−CNThybrid aerogel is recoverable after repeated compression. Reprinted with permission from ref 695. Copyright 2013 Wiley-VCH.
Figure 53. Growth ring of superstructured assembly of sp2 nanocarbons.
extremely low, however, with evident loss of elasticity.695
Notably, the achieved minimum density of the neat grapheneaerogels (0.16 mg cm−3) was once seen as the lowest value forultralight materials until the record was broken very recently bya lower value of 0.12 mg cm−3.697 Similarly, the singlecomponent CNT aerogels appeared to have less elasticity,whereas coating with graphene had altered their elastic behaviorfrom inelastic to superelastic, by means of strengthening thenodes and the struts in the aerogels. It is noteworthy that thecyclic compression performance, Young’s modulus, energystorage modulus, and loss modulus were all significantlyimproved in the graphene-coated CNT aerogels.698
Taking GO as the substrate to load catalysts for growingCNTs by the CVD method, 3D CNT/graphene hybridstructures were prepared with CNT pillars grown in betweenthe graphene layers. The sandwich structures had high specificsurface area of 612 m2 g−1, exhibiting a specific capacitance of385 F g−1 at 10 mV s−1 in 6 M KOH aqueous solution.699 Thecomposite materials with RGO layers pillared by CNTs,correspondingly, showed excellent visible light photocatalyticperformance in degrading dye rhodamine B, not only becauseof the porous structure, but also the splendid electron transferproperty of graphene.700 Furthermore, the intercalated growthof vertically aligned carbon nanotubes (VACNTs) intothermally expanded highly ordered pyrolytic graphite(HOPG) resulted in CNT/graphene architectures with tunablepillar length, which were promising for supercapacitorapplications.701 In a word, all the above CVD methods weredevoted to the preparation of 3D architectures with parallelgraphene sheets and vertically pillared CNTs, enjoying themerits of high porosity, remarkable conductivity, and structuraladjustability.
7. CONCLUSIONS AND PERSPECTIVESThe development on fullerenes, CNTs and graphene iscurrently enjoying a rapid pace, with enormous researchachievements emerging every day. In this review, we focused onthe tailored assembly of these sp2-hybridized nanocarbons intovarious macroscopic superstructures covering 1D fibers/yarns,2D fims/papers, and 3D porous/dense monoliths. Therepresentative events with influential effects in this area arepointed out on the timeline in Figure 53. The tempting natureof nanocarbons endows their assembled structures with variousfunctionalities and superiorities over other macroscopicmaterials, such as high strength and high conductivity, incombination with lightweight. Hitherto, materials scientistshave devoted a great deal of efforts in fully realizing theirpotential applications in areas like energy, electronics,composite materials and environmental remediation. For 1Dnanocarbon fibers, their flexibility and knittability, as well ashigh conductivities show prospects for wearable electronics. For2D films, the chasing of high quality TCFs draws muchattention, while sensing and filtrating applications are also ofenormous potential. For 3D architectures, their high specificsurface area facilitates uses with various purposes, particularly asabsorbers, catalytic scaffolds and electrode materials forenergetic devices.Fullerenes triggered the huge rush for nanocarbons and
afterward, the obsession with CNTs lasted for decades andaccumulated a wealth of experience; therefore, the newlyemerging graphene could avoid detours during its development.Furthermore, a range of superior macroscopic materials are alsorelevant to the nanocarbon assemblies. Taking carbon fibers as
an example, it is convenient for either CNT fibers or graphenefibers to draw lessons from the mature technology of carbonfibers, such as the common stratergies for fiber preparation,modification, and performance enhancement, as well asmultiple applications. Seeing that carbon fibers have alreadybeen widely used in many high-end products and become oneof the most important materials in industry and military fieldsafter more than 55 years of development, the rising CNT fibersand graphene fibers, together with other forms of nanocarbonsuperstructures, will face great opportunities and challenges.The universal concept for designing conventional materials,from building blocks in microscopic scale to assembledstructures and then to the ultimate macroscopic materials, isalso instructive for nanocarbon assemblies.Because huge successes have been made for superstructured
assembly of nanocarbons in the macroscopic scale, several vitalissues still need to be addressed on the way forward. First, highquality and mass production of raw nanocarbons is the basicrequirement for high performance graphitic assemblies, yet it isnot perfectly realized. For instance, research progress on thepreparation of perfect monolayer grephene sheets lags farbehind the great demands on their practical applications. Therecent exfoliation method can hardly provide uniform and cleansingle layer graphene, while the commonly used oxidation−reduction pathway is bothered with the introduction of defectsand decrease in transport properties. As a consequence, the lackof an effective way to ideal graphene has gradually become oneof the biggest bottlenecks for making further improvement onthe performances of assembled structures. Second, it is of greatimportance to implement precise control upon structures anddefects during the assembly process, since the microstructuresand defects, voids for instance, will significantly influence thefinal behaviors of the resulting materials. At last, along with theadvancement of research and technique, hybridization turnsinto a new trend for obtaining higher performance andmultifunctional materials, benefiting from the so-calledsynergetic effect. This hybridization is developed based onnanocarbon assemblies, not only with functional nanoparticlesor substances, but also among these sp2 nanocarbonsthemselves. Especially, the latter holds greater promise forexcited results getting one plus one more than two. Of course,composition and construction of the hybrid nanocarbonstructures need delicate control to realize their optimalperformance. In conclusion, although nanocarbons possesscharming characteristics and have been under extensive studyfor decades, the performances of their superstructuredassemblies in the macroscopic world are still far from theirexpectations, impelling scientists and technologists to delivercontinuing efforts toward the bright future that lies ahead.
Zheng Li received his Ph.D. degree in Materials Science in 2013 fromHarbin Institute of Technology, China. He is currently a postdoctoralfellow in the group of Prof. Chao Gao at Zhejiang University, China.His current research interest is mainly focused on the design andmacroscopic assembly of graphene-based superstructures, as well astheir applications.
Zheng Liu received his B.Mat. in 2007 and M.Sc. in 2010 from Collegeof Chemistry, Xiangtan University, China. Currently he is pursuing hisPh.D. degree at the Department of Polymer Science and Engineeringof Zhejiang University, under the supervision of Prof. Chao Gao. Hiscurrent research interest is mainly focused on the macroscopicassembly and application of graphene-based materials.
Haiyan Sun recently received her Ph.D. degree in Polymer Chemistryand Physics at Zhejiang University under the direction of Prof. ChaoGao. Now she is a research scientist in Zhejiang C6G6 MaterialsTechnology Co., Ltd. (www.c6g6.com). Her research interests include
the synthesis and processing of graphene aerogels and theirapplications for energy storage and oil adsorption.
Chao Gao is a professor in the Department of Polymer Science andEngineering at Zhejiang University, China. He earned his Ph.D. degreein Polymer Materials from Shanghai Jiao Tong University in 2001. Hesuccessively became a Lecturer and Associate Professor at ShanghaiJiao Tong University in 2001 and 2002, respectively. He then spentseveral years (2003−2006) at the University of Sussex, U.K. andBayreuth University, Germany, as a postdoctoral fellow and Alexandervon Humboldt research fellow. He joined the Department of PolymerScience and Engineering at Zhejiang University in 2008 and waspromoted as a full Professor. He has received several awards, includingNational Science Fund for Distinguished Young Scholars, NationalExcellent Doctoral Dissertation of China, etc. His current researchinterests include graphene chemistry, liquid crystal and macroscopicself-assembly, and hyperbranched polymers.
ACKNOWLEDGMENTS
This work was supported by the National Natural ScienceFoundation of China (Nos. 21325417 and 51173162) and StateKey Laboratory for Modification of Chemical Fibers andPolymer Materials, Donghua University (No. LK1403).
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