Superstructured Assembly of Nanocarbons: Fullerenes, Nanotubes

Review pubs.acs.org/CR

Superstructured Assembly of Nanocarbons: Fullerenes, Nanotubes, and Graphene Zheng Li, Zheng Liu, Haiyan Sun, and Chao Gao* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310007, China 5.2.3. Continuously Wet-Spun Graphene Films 5.2.4. Applications 5.3. 3D Graphene Architectures 5.3.1. Self-Assembled Graphene Hydrogels 5.3.2. Reduction-Induced Graphene Hydrogels 5.3.3. Cross-Linked Graphene Assemblies 5.3.4. Template-Directed Graphene Architectures 5.3.5. CVD-Grown Graphene Foams 5.3.6. High-Density Graphene Monoliths 5.3.7. Applications 6. Hybrid Assemblies of Nanocarbons 6.1. 1D Hybrid Fibers 6.2. 2D Hybrid Films 6.3. 3D Hybrid Architectures 7. Conclusions and Perspectives Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Fundamentals 2.1. Characteristics of Fullerenes, CNTs, and Graphene 2.2. Principles of Nanocarbon Assembly 3. Assemblies of Fullerenes 3.1. Fullerene Liquid Crystals 3.2. 2D Fullerene Films 4. Assemblies of CNTs 4.1. Synthesized CNT Arrays 4.2. 1D CNT Fibers/Yarns 4.2.1. Fabrication of CNT Fibers 4.2.2. Strengthening Protocols 4.2.3. Mechanical Behaviors of CNT Fibers 4.2.4. Applications 4.3. 2D CNT Films 4.3.1. Wet Methods Assembled CNT Films 4.3.2. CVD-Grown CNT Films 4.3.3. Array-Derived CNT Films 4.3.4. Applications 4.4. 3D CNT Architectures 4.4.1. Wet Gels Initiated CNT Aerogels 4.4.2. Template-Directed CNT Architectures 4.4.3. CVD-Grown CNT Sponges 4.4.4. Applications 5. Assemblies of Graphene 5.1. 1D Graphene Fibers 5.1.1. Wet-Spun Graphene Fibers 5.1.2. Hydrothermally Fabricated Graphene Fibers 5.1.3. Applications 5.2. 2D Graphene Films 5.2.1. Wet Methods Assembled Graphene Films 5.2.2. Assembled Graphene Films at Interfaces © 2015 American Chemical Society

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1. INTRODUCTION Carbon is one of the basic elements for life on Earth. It exists in diverse allotropic forms while being different in chemical and physical properties. The best well-known natural allotropes of carbon are graphite and diamond. Either for recording or the symbol of wealth, status, and love, the two carbon allotropes have left indelible marks on the long river of human history. In 1985, the advent of fullerenes1 opened the door to a novel group of carbon allotropes in the nanoscale. After that, the obsession of carbon nanotubes (CNTs) and graphene continued pushing materials development to an exciting climax once again. The so-called nanocarbons refer to graphitic materials with at least one dimension below 100 nm. There are several members in the growing nanocarbon family, including fullerenes, nanodiamonds, nano-onions, CNTs, nanofibers, graphene, graphene quantum dots, graphene nanoribbons, etc., among which the sp2 hybridized nanocarbons have been the center of research attention for nearly 30 years owing to their outstanding features. Speaking of which, fullerenes, CNTs, and graphene, the three most famous sp2 nanocarbons, have

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Figure 1. (a) Timeline of scientific publications relating to nanocarbons in the past decade (2005−2014). (b) Regional distribution of publications on each topic. Publication analysis was completed using Web of Science.

low level (below 5 wt %) and the integrated performances remain based on the matrix materials,15−19 assembling nanocarbons into macroscopic superstructures is another effective strategy to fulfill the realistic demands for multifunctional materials.20−23 The macroscopic assemblies of nanocarbons cover all three dimensions in this world, in the forms of one-dimensional (1D) fibers, two-dimensional (2D) films, and three-dimensional (3D) monoliths, as shown in Scheme 1. Besides, in the purpose of expanding their functionalities, these assemblies sometimes go to hybrid structures with multicomponents. For a specific superstructure, one should note that it is always composed of delicate microstructures and organized with precise control at the molecular level and thus exhibits unexpected functionalities which are absent in individual

triggered tremendous interests in both scientific and technological communities.2−5 In light of extensive studies on these fantastic nanocarbons, which show great promise in a wide variety of applications ranging from high-performance composites, to electronic and energy storage devices, to biological materials, and so on,6−12 researchers never stop their forward progress on the way to the new era of nanocarbons. Publications on nanocarbons are on a continuously rising trend in the past decade, as illustrated in Figure 1. The first revealed fullerenes show a slight yet continuous increase with each passing year from 2005 to 2014. On the contrary, the other two nanocarbons apparently enjoy strong growth, especially for the youngest one, graphene. Published papers about graphene numbered only a few hundred in the first three years; afterward, it became more and more popular. A comparative amount of publications with CNTs were achieved for graphene in 2013 which soon grew drastically above that in 2014, demonstrating the exploding research interest in graphene. As regards the regional distribution of publications about nanocarbons, U.S.A. and China are the two countries who are the most active in this research area, contributing more than half the total amount, especially for the currently hottest CNTs and graphene. Among the abundant research works devoted to nanocarbons, the big challenge of extending their excellent properties into the macroscopic world has long been recognized as the core issue for practical applications.13,14 Exactly as the polymeric materials suggest, molecules are much more functional when they are getting together; most importantly, their utilization at the macroscale is of great significance to people. Despite the uses as reinforcements for composites, where the content of nanocarbons is normally at a

Scheme 1. Superstructured Assemblies of Nanocarbons in the Macroscopic Scale

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and Young’s modulus of a SWCNT is typically in the range of 13−52 GPa and 320−1740 GPa, respectively,48,49 while being 11−63 GPa and 270−950 GPa of a MWCNT.50 Besides, their metallic characteristics are highly dependent on the chirality of the graphitic hexagonal lattice, which means the electronic properties are expected to be metallic or semiconducting.51 Nevertheless, the superior mechanical properties, conductivities, and many other fantastic features have let CNTs stand at the forefront of academic research in the past decades.52−54 Graphene is a planar monolayer of carbon atoms packed into a 2D honeycomb lattice. Although the theory of graphene has been around for a long time, which can be traced back to the year of 1947,55 the first well-known experimental discovery and investigation of graphene in 2004 is a milestone for the graphene research.56,57 By using a simple mechanical exfoliation of graphite, a single layer graphene was peeled and found to have a thickness of ∼0.34 nm.58 After substantial measurements on graphene,59−65 it has been revealed with extremely high mechanical stiffness (tensile strength up to 130 GPa, modulus of 1000 GPa)66 and excellent thermal conductivity (4840− 5300 W m−1 K−1),67 especially ultrahigh electron mobilities exceeding 2 × 105 cm2 V−1 s−1.68,69 However, the lack of massive production of perfectly structured graphene is still the principal limitation for its practical application. Solutionexfoliation of graphite in certain solvents or with surfactants, either by sonication70,71 or high-shear mixing,72 is one protocol for large-scale production of graphene with high-quality. Meanwhile, chemists have found an alternative way initiating from the heavily functionalized graphene derivative-graphene oxide (GO), which could be easily obtained through a chemical oxidation and exfoliation route upon natural graphite and is rich in oxidative groups such as epoxy, hydroxyl, carbonyl, and carboxyl groups.73−75 The preparation of GO has gone through decades of development, the KClO3-based Brodie−Staudenmaier method76−78 and the KMnO4-based Hummers method79 in combination with their modified forms are currently the most popular ways. Lately, an iron-based green and fast approach for single-layer GO, known as the “Gao-Fe” method, was disclosed, able to avoid polluting heavy metals and toxic gases in the products, presenting industrial significance.80 Although the introduction of functional groups and defects on the GO platelets during preparation has degraded their performances to a certain extent, the subsequently achieved processability in polar solvents, especially the stable dispersibility in water, is very exciting in application fields. Moreover, the elimination of functional groups and recovery of the graphitic lattice could be realized through chemical reduction, thermal treatment, or irradiation on GO, resulting in a much more graphene-like structure and performances, known as the reduced GO (RGO).81,82 Therefore, assembling GO and followed by reduction has become the most commonly utilized protocol for realization of high performance graphene assemblies on the macroscopic scale.83−86 Furthermore, there are subtle connections between these nanocarbons. The atomic 2D graphene layer is considered as a basic building block for graphitic carbons of other dimensionalities; it can either be wrapped up into 0D fullerenes or be rolled into 1D nanotubes.87 In fact, the structural relativity is not simply true in the theoretical model; researchers have already made breakthroughs to experimentally accomplish the morphological transition. To name a few examples, CNTs were able to be longitudinally unzipped to form graphene nanoribbons through a solution-based oxidative process and subse-

nanocarbons. The assembly of nanocarbons has shown supramolecular behavior since they are firmly connected through intermolecular forces, or, even better, they are large and rigid building blocks that are easy to manipulate. Overall, in the interest of designing and fabricating nanocarbon superstructures, the essential considerations are not only the individual properties of nanocarbons but also the elaborate microstructures 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 or assemblies at one dimension,24−32 there is still a lack of integral comprehension upon the ascending subject. In fact, the structural relevance of nanocarbons allows them to get inspiration from each other, and what’s more, their distinctions are expected to be drawn. In this review, we will systemically summarize state-of-the-art progress in fabrication and application of the macroscopic assemblies of the three typical nanocarbonsfullerenes, CNTs, and graphenewith particular emphasis on 1D fibers/yarns, 2D films/papers, and 3D aerogels/hydrogels. The major assembly methods and techniques are reviewed here in order to give helpful guidance for future research.

2. FUNDAMENTALS 2.1. Characteristics of Fullerenes, CNTs, and Graphene

The explosive chasing of nanocarbons began with the discovery of the first fullerene molecule (C60) in 1985 by Kroto and Smalley et al.1 It is composed of 60 carbon atoms organized into 12 five membered pentagons linking with 20 six membered hexagons and has the geometry of a hollow sphere, resembling the structure of a soccer ball; thus, it is also called the buckyball.33−35 Being viewed as a zero-dimensional (0D) nanocarbon, the diameter of a C60 sphere is 7.1 Å,36 meaning that the three dimensions of which are all below 1 nm. Although fullerenes include a wide range of carbon cages with various carbon atoms and symmetries, the readily available C60 is recognized as the most stable and the dominant member in the fullerene family and, therefore, attracts particular attention.37 The preparation of C60 in macroscopic quantities was reported since 1990,38 causing intensive investigation for decades. In brief, fullerenes have displayed photosensitizing and electron-acceptor features, electrical transport properties, and many other splendid physical and chemical properties, which allow the applications of fullerenes covering multiple areas of photovoltaics, catalysis, medicinal chemistry, biological uses, etc.39−43 Notably, the room temperature solubility of fullerenes in a variety of solvents enables straightforward processing of such nanoscale carbon allotropes.44 Iijima’s report in 1991 brought CNTs into the awareness of the scientific community.45 CNTs are 1D nano graphitic materials presenting a seamless cylindrical morphology and extremely high aspect ratio (i.e., length to diameter ratio, 102− 107), with diameters ranging from several to hundreds of nanometers and lengths up to centimeters.46,47 With respect to the 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, the success in producing SWCNTs gave a huge boost to the field of CNT research.46 Both theoretical and experimental studies have demonstrated that the major properties of CNTs would vary in association with their wall numbers. The tensile strength 7048

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quent reduction,88,89 while graphene quantum dots were synthesized from C60 molecules via the ruthenium-catalyzed cage-opening.90 Reversely, the transformation of flat graphene sheets into fullerene cages was observed under high energy electron beam exposure;91 meanwhile, rolling up graphene sheets into CNTs was achievable by means of ultrasonication,92,93 electron beam irradiation,94 or thermally induced self-intertwining.95 By the way, graphene nanoscrolls with an open tubular structure, which are distinct from CNTs with sealed edges, have received growing concerns. Their characteristic open topology allows foreign molecules ready access into the massive interlayer galleries.96,97

area. That is, when other conditions are equal, a larger contact area gives stronger interaction between the building blocks, because more carbon atoms interact with each other. For 0D C60 with a spherical shape, their contact mode can be seen as point contact, and for 1D rod-like CNTs, line contact. Their curved surfaces cause incomplete contact, leading to relatively weak interaction. For 2D graphene platelets, on the other hand, their planar surface makes them much easier to be densely packed in a face-to-face manner on the basis of stronger van der Waal’s interactions than the other two, as well as the contribution originated from the π-conjugated domains.98 The different contact modes for the three nanocarbons showing geometric dependence are illustrated in Scheme 2. But it is

2.2. Principles of Nanocarbon Assembly

The assembly of individual nanocarbons into ordered superstructures is a thermodynamic process where the interactions between building blocks play a decisive role. We are about to interpret some basic principles before getting started on introducing the well-defined assemblies of nanocarbons. First of all, the possible interactions existing between nanocarbons might be complicated because they are attributed to the result of many forces, including van der Waal’s attractions, π−π stacking, and, some times, electrostatic interaction, hydrogen bonding, and hydrophobic interaction. These interactions have provided essential opportunities for spontaneous assembling. As we know, nanoparticles always hold a strong tendency of agglomeration to lower their free energy due to the large specific surface area. When these particles are getting close enough, the above-mentioned interactions will take effect to draw them together, although, in some circumstances, the electrostatic repulsion should be overcome first. However, the uncontrolled self-aggregation process generally causes a simple stacking which is sometimes undesirable and hard to become the specific structures we need. After all, the elaborate construction of the microstructure is exactly the magic of nanoparticle assemblies what makes them functional. Therefore, it is necessary to well control the assembly process, by means of preventing severe aggregation while making full use of the interactions to stabilize the assembled hierarchical architectures. In some cases when the mechanical strength of a structure is under consideration, its internal interactions should be evidently increased to resist structural damage caused by, basically, slippage between the building blocks. π−π stacking between sp2-hybridized carbon atom domains and other π-conjugated materials is the main attracting force between graphitic materials. Hydrogen bonding is another strong interaction that appears in between nanocarbons with functional groups, such as hydroxyl groups. Above all, the most effective way is ascribed to the introduction of much stronger covalent bonding and the formation of a cross-linked structure, through modification on nanocarbons. In other cases demanding a porous structure, it is a technological challenge to sustain the loose framework. Actually, no matter how the assembly process is carried out initially, the as-formed ideal microstructure needs opposite effects against aggregation to neutralize the dense packing trend which causes collapse of the structure. The opposite effects usually come from the structural rigidity of nanocarbons, in combination with their surface curvature and different orientation. Second, from a geometrical point of view, the morphology of different nanocarbons has great influence on interparticle interaction by affecting their way of contact. More specifically, the interaction between nanocarbons depends on their contact

Scheme 2. Contact Modes for Fullerene Spheres, CNT Rods, and Graphene Platelets

always more complicated in practice. For example, CNTs with high aspect ratio and flexibility tangle easily which greatly increases their resistance to intertube sliding while being stretched. The case for graphene is even more complex since a wavy morphology with wrinkles is quite common for graphene sheets. These wrinkles sometimes degrade the contact level due to steric hindrance and at other times provide an interlocking effect to impede interlayer sliding, depending on the stacking mode of graphene sheets and the applied load. In a word, figuring out interactions between nanocarbons is really meaningful for investigating the behavior in their assembled architectures; it is worthwhile to take the geometrical effects into account which may help us comprehend the interacting mechanism.

3. ASSEMBLIES OF FULLERENES Early research about fullerenes mainly focused on the synthesis of fullerene family compounds and their basic physicochemical properties, with special attention to the implementation of organic functionalization.99−102 Lately, the assembly of fullerenes and their derivatives into ordered structures based on noncovalent or covalent interactions has been a hot topic for their enormous contributions to the design and fabrication of organic electronic devices.103−106 While being reported in different forms, including liquid crystals, needles/nanofibers, films, etc., such assemblies generally took place in solutions, on surfaces, as well as at interfaces.107−109 Pristine fullerenes lack dipolar interactions, and there is little possibility to assemble them in the solid state. Thus, the solvent plays an important role in the solution-based assembly of fullerene aggregations. For example, a water-soluble C60terminating ammonium amphiphile revealed the formation of both long fibrous and disk-like aggregates,110 whereas exceptionally long crystalline C60 nanowires with a length to width aspect ratio as large as 3000 were grown using 1,2,4trimethlbenzene as the solvent.111 Li et al. discovered a droplet receding method for the solution-grown aligned C60 single7049

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crystal needles and ribbons, performing well in their applications of field-effect transistors (FETs).112−114 Besides, the interface between two solvents with/without C60 molecules could act as nucleation sites for C60 crystals, generating needlelike crystalline precipitates at the interfaces.115−120 However, the solid state assemblies of fullerenes were mostly in the molecular level and hardly get into the macroscale, despite several thin films supported by substrates, probably because the unique spherical geometry of fullerenes is less effective for a massive stacking with stable structure. Based on these circumstances, the following sections will present a schematic introduction of some preliminary regular structures of fullerenes in relatively large scale, namely the liquid crystals and films.

Figure 2. (a) Polarized optical microscopic texture indicating the ordered mesomorphic domains of C60 derivatives. (b) Illustration for the lamellar mesophase. Reprinted with permission from ref 128. Copyright 2008 American Chemical Society.

3.1. Fullerene Liquid Crystals

Fullerene liquid crystals combine both advantages of excellent photoelectric properties of fullerene and the ordered structure of liquid crystal, showing potential in research and application simultaneously. Generally speaking, in order to form liquid crystal, the aspect ratio in excess of 3 is essential for a specific molecule.121 However, as a spherical molecule, fullerene itself cannot meet this requirement. Thus, the introduction of extremely long liquid crystal units is helpful for the realization of its liquid crystallinity. Various morphologies of fullerenecontaining liquid crystals, such as nematic, smectic, cholesteric, or columnar phases, have been reported in the past years.122 In 1996, the first fullerene-containing thermotropic liquid crystal was observed by Chuard and Deschenaux through the functionalization on C60 with a framework containing two cholesterol derivatives.123 Hexaaddition on the C60 sphere caused the successful preparation of room-temperature enantiotropic nematic material.124 Another protocol for regular assemblies of fullerenes is the formation of inclusion complexes. Cyclotriveratrylene (CTV) derivatives substituted with 18 long alkyl chain displayed a nematic phase at room temperature, and their supramolecular complexes with C60 also revealed a fluid birefringent phase as expected. But when heated above 70 °C, the birefringence of the texture under the microscope disappeared while a cubic phase was formed. In this case, it is noteworthy that the liquid crystalline behavior mainly comes from the microcyclic derivative subunit.125 With covalently linked liquid crystalline dendrimers, the functionalized C60 was able to achieve thermotropic liquid crystals, where the ordered structures provide opportunities for their photovoltaic and molecular switching applications.126 Molecular assembly is an effective method to form liquid crystals. The obtained liquid crystals usually show a hierarchical supramolecular structure which does not exist in common liquid crystal systems. In 2002, Sawamura et al. first used the molecular symmetry of C60 to assemble anisotropic “nanoshuttlecocks” that were afterward stacked into a one-dimensional columnar supramolecular structure with liquid crystalline behavior.127 Later, Nakanishi et al. prepared simple fullerene derivatives bearing long alkyl chains to format a long-range ordered lamellar mesophase (Figure 2). The mesomorphic materials were of high C60 content (up to 50%), and a high electron mobility of ∼3 × 10−3 cm2 V−1 s−1 was obtained.128

attracted particular attention in this area.129 These 2D fullerene structures have shown potential applications as n-type semiconductors in organic FETs and ogranic photovoltaic cells.130 Generally, the assembly of fullerene films is mainly based on van der Waals interaction or covalent bonding. Except for the solution-based fabrication, physical vapor deposition is such a protocol for making thin films of fullerenes directly while the as-formed highly ordered morphology can be nanostructured by electrochemical reduction with well-defined surface structures.131−134 Starting from a well-dispersed solution, solvent evaporation is definitely one of the most straightforward ways to fullerene films. Evaporation of thin C60 films on Pt electrodes facilitated the first electrochemical study on C60.135 Functionalized C60 bearing three eicosyloxy aliphatic chains could be easily packed into thin films with thickness of approximately 20 μm on various substrates by slow evaporation of a dilute 1,4-dioxane dispersion. The resulted films featured water-repellent superhydrophobicity with a water contact angle of 152° (Figure 3a,b). In contrast, spin-coating of the same raw material in chloroform solution resulted in a smooth film which presented a contact angle of only 103.5°.136 By the way, even a huge needle-like C60 with size in the centimeter range was obtained through a very slow evaporation of a supersaturated solution.137 The Langmuir−Blodgett (LB) assembly is another optional technique to obtain supported fullerene films. In such a process, Langmuir films are first formed by spreading amphiphilic molecules on a liquid surface, like water. The molecules arrange in a manner of the hydrophilic heads contact with water while the hydrophobic ends (usually the fullerene cage) stick out into the air. LB films are then prepared after transferring the Langmuir films onto a solid substrate, with the convenience that the layer number could be easily adjusted from monolayer to multilayer depending on the number of times one runs the dipping process.138,139 For instance, by adding a C60 solution of benzene140 or CS2141 to the air−water interface, Langmuir films containing C60 molecules would form after volatilization of the solvent. Then the LB films were deposited onto different substrates, such as glass, quartz and silicon. For more detailed works, one can refer to several published reviews summarizing the LB assembly of fullerene films.139,142 To our knowledge, the first example of C60 SAM was reported by Mirkin and co-workers; they showed the covalently bonded fullerene structures onto the (MeO)3Si(CH2)3NH2 modified indium tin oxide (ITO) surfaces.143 In fact, the

3.2. 2D Fullerene Films

The deposition of fullerene films, either in monolayer or multilayer, is the way to transfer the unique fullerene properties to bulk materials and frequently by surface coating or interface assembly, while the self-assembled monolayers (SAMs) 7050

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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 a photograph presenting a water droplet on the surface with a contact angle of 152°. (b) SEM image of the cross-section of the film. Reprinted with permission from ref 136. Copyright 2008 Wiley-VCH. (c) Deposition of C60 film on an Au substrate with the help of molecules possessing both amino and thiol groups. Reprinted with permission from ref 146. Copyright 2003 Elsevier.

which provided a tremendous opportunity for applications in the optic and electronic fields.147

introduction of covalent linking between fullerene particles or with the substrate is able to overcome the shortcoming of low stability caused by weak interconnection. With this consideration, C60+Si films from the deposition of (C60)mSin clusters were produced in a sophisticated double-target laser vaporization source, where the Si atom bonded with two C60 molecules and acted as a bridge. The binding energy was high enough to synthesize C60-based materials through polymerization of C60-Si clusters.144 Apart from the in situ cross-linking generated during the synthesis of C60 derivatives, when branched polyethylenimine (PEI), with a distribution of primary, secondary, and tertiary amino-groups was adsorbed onto the hydroxylated silicon wafer surfaces, the free primary and secondary amino groups may help to catch the C60 molecules, through N−H addition reactions across the CC bonds in C60. The obtained C60 films possessed good adhesive resistance, as well as good friction reduction, load-carrying capacity, and antiwear ability, owing to the unique surface nature and mechanical properties of C60 units.145 Molecules with both HS and NH2 groups served as good cross-linking agents to bind the Au substrate and C60, where the amino groups fulfill the attachment of C60 while the thiol groups bind to the Au surface, as illustrated in Figure 3c.146 Similarly, HS and NH2 groups have done a good job in the preparation of a hybrid film containing both C60 and Au nanoparitcles. Aminefunctionalized gold nanoparticles took part in the amination reaction of C60, driving the formation of hybrid films via layerby-layer (LBL) assembly or in situ cross-linking assembly. The films prepared by the later method had smoother and more featureless surfaces than those generated from the LBL process. The in situ cross-linking assembled films were sensitive to the presence of UV lights with recovery changes on the current,

4. ASSEMBLIES OF CNTS Although the rod-like CNTs were thought to be closely related to spherical fullerenes that CNTs were initially seen as the elongated fullerenes, their distinct geometries have brought totally different assembly behavior. Owing to the large aspect ratio of CNTs, it is quite easy for them to be tangled with strong interactions, which will benefit the assembly process into the macroscale. As a consequence, there is a far larger notice of macroscopic assemblies of CNTs in contrast to fullerenes.148−151 4.1. Synthesized CNT Arrays

The vertically aligned CNT arrays, also known as the CNT forests, which give a large-scale well-aligned morphology of CNTs parallel to each other, have found use in many applications.152 The first achieved CNT array was grown on nickel-coated glass using a plasma-enhanced hot filament chemical vapor deposition method below 666 °C (Figure 4a,b).153 Afterward, numerous efforts have been devoted to controllable assembly of the highly ordered and vertical CNT architectures with varied CNT diameters and lengths, covering areas,152 and even well-designed patterns.154,155 In this section, however, we are concerned with the aligned structures because they are important intermediate products for subsequent CNT assemblies, especially for CNT yarns and films. During a typical drawing procedure, CNT bundles are connected end-to-end continuously, implementing the fabrication of CNT yarns or films (Figure 4c). According to the model of the drawing process proposed by Kuznetsov et al., the interconnections 7051

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4.2.1. Fabrication of CNT Fibers. The fabrication of CNT fibers/yarns includes solution based (wet-spinning) and solid based protocols. The first macroscopic, meter-long CNT fibers were presented by Vigolo et al, through a simple spinning process from the SDS dispersed SWCNT solution.159 Since then the fascination of continuous CNT fibers were recognized by the researchers. While it is a promising candidate for the upgraded version of carbon fibers, there are still bottlenecks to unclog. The integrate properties of the assembled CNT fibers are far lower than the individual CNTs, which encourages people to keep searching for solutions. Fabrication is an essential part of the final performance since it builds the welldefined construction and microstructures. To date, several methods have been discovered for the fabrication of CNT fibers. In the meanwhile, multiple parameters should be taken care of within each method. 4.2.1.1. Solution Based Wet-Spinning. Unlike some of the thermoplastic polymers for fiber preparation, melt spinning is not an option for CNTs because of the extremely high thermal stability of carbon allotropes. On the contrary, the wet-spinning strategy is more affordable for CNTs based on their conditional solubility. Thus, the solution based wet-spinning method has become one of the major routes for CNT fibers. In the industrial field, wet-spinning is a well-established fabrication process for commercial high-performance polymer fibers, such as Kevlar. Vigolo et al. extended this technique for making CNT fibers by regarding CNT as a linear supramolecule. The aqueous dispersion of SWCNT was injected into a poly(vinyl alcohol) (PVA) solution which served as the coagulation bath. After partial substitution of the surfactant SDS which resists the van der Waals-induced aggregation, the CNTs were stuck together to form long and stable ribbons. Finally, CNT fibers were obtained from collapse of the ribbons when dry (Figure 5a−c). Although the as-prepared CNT fibers were of low strength (∼125 MPa), they showed high flexibility and resistance to torsion, which is hardly realized for carbon fibers.159 Meanwhile, the wet-spinning method is readily controllable since fibers with a wide range of diameters were attained by varying several parameters, such as injection rate,

Figure 4. (a and b) SEM images of CNT arrays. Reprinted with permission from ref 153. Copyright 1998 American Association for the Advancement of Science. (c) Schematic drawing for the preparing process of CNT array-derived architectures.

between CNT bundles should be the main factor inducing the drawability of a CNT forest. Simultaneously, the highly aligned CNTs are necessary, and the forest height can only vary in a limited range for drawable CNT forests. 156 Detailed descriptions are included in the following sections. 4.2. 1D CNT Fibers/Yarns

The large-scale production of continuous CNT fibers is highly desirable for many applications relying on the superior properties of individual CNTs. It is believed to be a close competitor to conventional fiber materials in various areas due to its unique characteristics, for being lightweight, strong, flexible, and highly conductive. Since the macroscopic assembly of CNT fibers is the first step to fully realize the potential of CNTs in fiber form, tremendous efforts have been paid to their fabrication and consolidation. After several years of rapid development on producing mechanically strong CNT fibers, the research hotspot has gradually changed into exploring multifunctional applications of these prospective fiber materials, with abundant achievements accomplished.157,158

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 arrow indicates 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. 7052

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Figure 6. (a) Photograph of a free-standing CNT array being drawn to make a CNT yarn. Reprinted with permission from ref 164. Copyright 2002 Nature Publishing Group. (b) SEM image of a CNT yarn being pulled out from a CNT array and twisted. (c) Surface morphology of a CNT yarn showing CNTs aligned at an angle with the yarn axis. Reprinted with permission from ref 166. Copyright 2004 American Association for the Advancement 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 of Science. (f) Schematic of the drawing and winding process on the as-grown CNT aerogel. Reprinted with permission from ref 173. Copyright 2004 American Association for the Advancement of Science. (g) Products of CNT yarns on the spool. Reprinted with permission from ref 177. Copyright 2010 Wiley-VCH.

polymers thoroughly, with a conductivity of 140 S cm−1 after annealing at 1000 °C.163 It is interesting that the as-formed CNT fibers probably show hollow, folded, and solid structures (Figure 5d−f), depending on the preparation conditions, and could be applied for different purposes.161,163 4.2.1.2. Drawing CNT Arrays. As is stated above, the directly grown CNT arrays are perfect precursor structures for subsequent fiber spinning. In Fan et al.’s creative work, CNT yarns could be generated continuously by drawing one end of the vertically aligned CNT arrays (Figure 6a).164 The fibers made 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-out process to further densify the as-formed fibrous structure (Figure 6b,c).166 Most of the related research was in agreement with one opinion that the CNT yarns were only achievable from superaligned arrays with highly ordered structures.164,167,168 Kuznetsov and co-workers explained this requirement by the reason that wavy arrays may cause nonuniform interconnections between adjacent CNT bundles which ruptures the continuity of the pulling-out process.156 Besides, longer nanotube length in the arrays is favorable for fiber performances since longer CNTs provide higher friction forces between the nanotubes and fewer mechanical defects as

flow conditions, and dimensions of the capillary tube. Based on the wet-spinning strategy, Dalton and Baughman et al. modified the process; thus, very strong CNT/PVA composite fibers containing 60 wt % SWCNT have been generated in 100-m lengths. The tensile strength of the composite fibers was as high as 1.8 GPa, while the Young’s modulus was 80 GPa. Especially, the toughness reflecting the absorbed energy before break reached 570 J g−1. Such high toughness was achieved by coating of amorphous PVA on the nanotubes.160 The PVA molecules acted as binders for CNTs to enhance the intertube interactions; however, the electrical conductivity of the fibers was reduced simultaneously due to the presence of insulating substances on the path for electron transfer. To face this shortcoming brought by PVA, SWCNT/PEI composite fibers spun with PEI coagulant exhibited significantly higher electrical conductivity (100−200 S cm−1) than the SWCNT/PVA fibers (0.01−2.5 S cm−1).161 Biomolecules could be involved in CNT fibers through the wet-spinning process and served as dispersant and coagulant, in order to generate biocompatible conducting fibers. Among a series of SWCNT-biopolymercontaining fibers, the best electrical conductivity showed up for the hyaluronic acid (HA) incorporated ones, with values around 135 S cm−1.162 Moreover, a polymer-free spinning process was discovered to eliminate the influence brought by 7053

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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 with permission from ref 178. Copyright 2009 Wiley-VCH. (c) SEM image of an overtwisted spring-like CNT rope. Reprinted with permission from ref 180. 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.

fibers, the directly spun ones were able to perform much better on their fiber strength. The strongest CNT fiber until now was reported by Windle and co-workers with a tensile strength of 8.8 GPa, in conjunction with high elastic modulus (357 GPa) and toughness (121 J g−1), which was spun from a CNT aerogel, followed by densification through an acetone vapor stream. These values were comparable or even greater than those reported for carbon fibers.175 4.2.1.4. Twisting CNT Films. CNT thin films with mesh-like structure are easily turned into fibers through a twisting or rolling process (Figure 7a). Although the film-derived spinning strategy is not suitable for continuous production of CNT fibers, it is quite convenient to control the diameter and microstructure of the as-formed fibers for specific application purposes.178−180 Furthermore, the twisted fibers possessed higher mechanical performances, including both the strength and modules, than the original CNT films, as shown in Figure 7b, owing to the enhanced internal friction force to prevent intertube sliding. In contrast, the fracture strain was significantly lowered for the twisted fibers in comparison with the original film, which was also attributed to the confined free movement of the bundles.178 Beese et al. compared the structures and mechanical properties of the directly spun CNT yarns from aerogels and the yarns fabricated by twisted CNT mats, and their conclusions showed that the directly spun ones have better aligned CNTs, however, with higher porosity before a further densification process. The twisting speed was also expected to affect the yarn structures where the slowly spun yarns exhibited increased alignment and decreased porosity over the quickly spun yarns; therefore, they have better mechanical properties.181 What is more interesting is that Shang et al. accomplished a series of distinctive work where they overtwisted a CNT film to obtain a spring-like CNT rope with superstretchability up to 285% of strain (Figure 7c).180 Through controlling the fashion and level of overtwisting, the

well (the ends of CNTs should be regarded as defects). For example, the CNT fibers spun from 1 mm high arrays showed a tensile strength of 3.3 GPa,169 nearly double the value of fibers from 0.65 mm high arrays, which was only 1.91 GPa.170 Specifically, based on their series work upon CNT arrays and fibers, Zhu et al. claimed that the optimal CNT length for the spinnable arrays was restricted in the range of 0.5−1.5 mm. The upper limit of 1.5 mm was set because the content of amorphous carbon in longer CNT arrays was too high to conduct fiber spinning.171 4.2.1.3. Direct Spinning Based on CVD Technique. The CVD technique is not only used to synthesize CNTs separately but is also possible to assemble them into connected structures right at the time they are synthesized. The directly grown SWCNT 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 of centimeters in length and a diameter of around 0.3 mm (Figure 6d,e). Taking care of the temperature and the hydrogen flow rates, continuous fabrication of long SWCNT strands in large yields were achieved. The macroscopic strands consisted of nanotubes in parallel orientation and separated by interstitial space, showing superior mechanical and electrical properties.172 A more popular synthetic strategy for CNT fiber was performed by drawing and winding the CVD-grown CNT aerogel onto a rotating rod, as developed by Windle’s group, see Figure 6f.173 Several process conditions, such as carbon source, the catalyst concentration, and the winding rate, were found to cause significant influence on mechanical performance of the as-grown CNT fibers.174,175 The CVD based “bottom up” route for CNT fibers makes it capable of engineering fiber properties from the root, namely the nanotube structure, and then their assembly into fibers.176 It is also a continuous process for large-scale manufacture of CNT fibers, as shown in Figure 6g.177 In comparison with the coagulation spun CNT 7054

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configuration of a twisted film could even vary among doublehelical,182 partial-helical,183 and highly entangled structures.184 Otherwise, by preinfiltrating CNT films with polymers, the interactions between nanotubes were markedly improved, and an evident decrease of porosity was seen in the obtained fibers, making it a possible approach for strong and tough composite fibers.185,186 Remarkably, by means of scrolling CNT sheets with other functional materials, Lima and Baughman et al. incorporated more than 95 wt % of powders or nanofibers into scrolled CNT yarns, while maintaining the guest functionality in the meantime (Figure 7d−f). The good processability and flexibility of the biscrolled yarns thus presented attractive prospects for applications of wearable electronic devices.187 4.2.1.5. Other Methods. The early try for CNT fibers included the electrophoretic assembly reported by Gommans et al., where a carbon fiber tip was withdrawn slowly from a dispersion of SWCNT under an electric field, with CNTs gathering around and assembling one-by-one at the positively charged carbon fiber tip. The alignment of CNT was observed; however, only a maximum of 5 cm in length could be achieved.188 Starting from the CNT cotton produced by the FCCVD method, Ci and Ajayan et al. developed a drawing− drying process to fabricate CNT fibers with a relatively large scale (Figure 8a). The as-spun fibers were found to be capable

ance with strength above several GPa and Young’s modulus reached hundreds GPa, these values are still far below the theoretical value of individual CNT. The ultimate goal for CNT assemblies is introducing the perfect properties of CNT in nanoscale into the macroscale, with minimal losses to their performance. Based on the impending demand for high performance CNT fibers, researchers have discovered several means to face the great challenge through optimization of the fiber structures. Generally speaking, the main factor that decides the performances of the assembled fibers is attributed to the interaction between CNT bundles, while the primary failure mechanism for a CNT fiber is the sliding between bundles rather than the breakage of an individual CNT. So the fundamental protocols to get improvement on the mechanical strength of a CNT fiber are basically focusing on increasing the packing density/decreasing the porosity, improving the alignment, as well as enhancing the intertube connection. A common understanding is currently realized that the fiber strength should increase with decreasing diameter, since more defects are included in larger diameter fibers, making them broken easily. The most popular strategies in the current literature are in the forms of (1) introducing twist and/or stretch on the as-formed fibers, (2) passing through volatile liquids to consolidate the CNT fibers, (3) annealing at a high temperature, and (4) infiltrating polymers as bonding agents. In fact, the above treatments on CNT fibers are usually performed in combination to achieve a better result. Apart from the universal strategies, liquid crystal spinning technique is a great update for the wet-spinning process to facilitate the formation of a more ordered structure. 4.2.2.1. Liquid Crystal Spinning of CNT Fibers. CNTs, viewed as high aspect ratio, rigid rod supermolecules, are capable of forming a lyotropic liquid crystalline phase in various solvents.193−195 Using the dispersion with liquid crystalline domains as spinning dope, large-scale alignment during the fiber spinning process becomes a typical feature, which is beneficial to conductive properties. The substructure of aligned superropes with 200−500 nm in diameter was revealed for a wet-spun SWCNT fiber and thought to be linked to the starting liquid crystalline phase in the spinning dope.196 In one case of MWCNT-water dispersion, the critical concentration for the transition from isotropic to a Schlieren texture typical of nematic liquid crystal is 4.3 vol % (Figure 9a,b).197 Profiting from the liquid crystal behavior of CNTs, the neat CNT fibers become more affordable during wet-spinning, because of the improved processability. Therefore, the liquid crystal spinning was considered as the promising solutions for macroscopic assembly of CNT fibers.194 Since super acids are good dispersants for CNTs, Ericson et al. dispersed SWCNT in 102% sulfuric acid at a high concentration (8 wt %), with the formation of nematic liquid crystalline domains, and the dispersion was then extruded and coagulated in water to obtain neat SWCNT fibers. The continuous spinning process is shown in Figure 9c−e. As a result, the derived electrical and thermal conductivities were substantially enhanced than those of the conventional wet-spun SWCNT fibers with polymers in participation.198 Similarly, Windle et al. formed lyrotropic liquid crystalline phase of MWCNT in ethylene glycol solvent to carry out fiber spinning, resulting in highly aligned CNT fibers. Furthermore, the nitrogen-doped MWCNTs with much straighter morphology induced better mechanical and electrical properties.195 In a recent work done by Behabtu et al., CNTs with length of 5 μm were dissolved in chlorosulfonic acid at a

Figure 8. (a) Drawing-drying process to fabricate DWCNT fibers from the as-grown CNT cotton. Reprinted with permission from ref 189. Copyright 2007 Wiley-VCH. (b) Schematic of drawing CNT films through a series of diamond wire drawing dies. Reprinted with permission from ref 191. Copyright 2008 American Chemical Society.

of electron-emitting and electrochemical applications.189 Similarly, CNT cotton made of millimeter to centimeter long individual CNTs was able to be spun into fibers through the old cotton-based spinning technology, which was performed simply by pulling and rotating.190 Liu et al. reported a step-by-step way to prepare CNT yarns by drawing the CVD-grown films through a series of diamond wire drawing dies (Figure 8b). The pore diameters of the series dies were decreasing in sequence from 1.2 to 0.2 mm through 18 dies; therefore, the CNT networks were highly aligned and densified into 1D yarns, accompanying with excellent electrical properties at the same time.191,192 4.2.2. Strengthening Protocols. Although the reported CNT fibers have displayed outstanding mechanical perform7055

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Figure 9. (a) Polarized micrograph of an aqueous dispersion with 4.8 vol % MWCNT; the Schlieren texture shows the appearance of nematic liquid crystalline phases. (b) Image of the sample in (a) after being dried. Reprinted with permission from ref 197. Copyright 2003 American Association for the Advancement of Science. The spinning process for 102% sulfuric acid dispersed SWCNTs: (c) the apparatus used for spinning, (d) extrusion of spinning dope, and (e) collection of as-spun fibers on a spool. Reprinted with permission from ref 198. Copyright 2004 American Association for the Advancement of Science. (f) Polarized micrograph of a spinning dope of 3 wt % CNT in chlorosulfonic acid. The single and multihole spinnerets performing (g) single- and (h) 19-filament spinning. Reprinted with permission from ref 199. Copyright 2013 American Association for the Advancement of Science.

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

concentration of 2−6 wt % to form a spinnable liquid crystal dope for further production of CNT fibers. The 5 μm length of the CNTs in their work was longer than the earlier wetspinning process could handle. Additionally, the scalable filament spinning was actualized by using a multihole spinneret (Figure 9f−h). The achieved CNT fibers exhibited not only remarkable electrical and thermal conductivities but also comparable mechanical performance even with solid-state CNT fibers.199 4.2.2.2. Post Treatments on As-Spun CNT Fibers. According to the curvature of their profile, and intertube entanglement, CNTs are not easy to realize a highly compact packing. The large amount of interval space existing in CNT fiber is known as one of the most serious defects that degrades fiber performance. Simultaneously, the fiber conductivity was found decrease dramatically along with the increase of porosity.200 The other key issue that accounts for fiber performance is attributed to the alignment of CNTs to the fiber axis. Given that, the most frequently used protocols in quest for improvements focus on the consolidation and alignment in CNT fibers. Similar to the post treatment on conventional polymer fibers, stretching/drawing on the as-produced CNT fibers leads to enhanced nanotube alignment along the fiber axis.201 In addition, the reduction of fiber diameter was observed after the stretching procedure, due to the condensing effect coming from straightening of tortuous CNTs, alignment of the randomly packed CNT bundles, and the generated radial compression as well. The reduction in diameter also indicates a more densely packing, which means stretching is an efficient way to lessen the porosity in CNT fibers, plus increasing the contact length between CNT bundles.202,203 Miaudet et al. treated wet-spun CNT/PVA composite fibers by hot-drawing treatments, which yielded not only the alignment of CNTs but

Figure 10. Two-dimensional detector image X-ray data of CNT/PVA composite fiber (a) before and (b) after hot-stretching, indicating the alignment of the PVA chains of ±27° in (a) and ±4.3° in (b). Reprinted with permission from ref 202. Copyright 2005 American Chemical Society. SEM images of (c) and (d) a twisted CNT yarn before acetone shrinking, (e) and (f) after acetone shrinking with reduced yarn diameter and porosity. Reprinted with permission from ref 207. Copyright 2010 IOP Publishing Ltd. (g) Schematic of a continuous fabrication and densification process for CNT yarns. Reprinted with permission from ref 177. Copyright 2010 Wiley-VCH. 7056

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Figure 11. Micrographs of compressive damages after recoil test on (a) pure CNT fiber and (b) CNT/epoxy composite fiber. Reprinted with permission from ref 217. Copyright 2012 American Chemical Society. (c) Stress relaxation behavior of pure CNT fiber, CNT/epoxy composite fiber and carbon fiber. Reprinted with permission from ref 218. Copyright 2013 Elsevier.

conductivity (2 × 104 S cm−1),210 proving the great significance of eliminating porosity in CNT fibers. Besides, there are still several post treatments performed in different ways, like thermal annealing which removes the possible impurities while performing the so-called welding effect164,167 and infiltration of polymeric binders which are utilized to bridge individual CNTs.166,208,211 For instance, polydopamine (PDA) infiltrated into dry-spun CNT fibers could behave like adhesive agent effectively, and the following pyrolysis thermally converted PDA into a conductive binder; thus, a simultaneous improvement in btoh conductivity and mechanical strength was realized.212 Interestingly, a recent work revealed that passing an electric current through a CNT fiber will cause mechanical and thermal responses. Not only the mechanical behaviors of the fiber were changed but also homogeneous electrothermal heating was induced. The electrothermal responses allowed the strengthening of CNT fibers simply by fast curing thermosetting polymers or structural reordering thermoplastic polymers which have been previously infiltrated inside CNT fibers.213 Similar to the reinforcing mechanism of densification, the improvements derived from the above methods are attributed to the increased attractions between CNTs. Moreover, the oxygen-containing functional groups obtained from the oxidation on CNTs through either gamma-irradiation214 or acid treatment215 were also found beneficial for the intertube interaction and, hence, led to stronger CNT fibers. 4.2.3. Mechanical Behaviors of CNT Fibers. The characteristics of individual CNTs, combined with their fashion of assembly, determine the behavior of a CNT fiber. Since the tensile and conductive properties of CNT fibers have been widely studied for years, only a small amount of research is aware of their special behaviors which are helpful for us to gain a deeper understanding of the acting mechanism of CNT fibers and thus should not be ignored.216−218 Some fundamental research work relevant to such an interesting aspect was conducted by Chou’s group. The high strain rate characterization of CNT fibers revealed that their dynamic mechanical behavior showed gage length dependency and rate dependency. The ultimate tensile strength decreased with an increase in gage length; however, it increased with applied strain rate. The piezoresistivity during dynamic tensile loading was also demonstrated, which may be used for sensing applications in dynamically loaded composites.216 Zu et al. investigated the compressive properties of both pure and epoxy infiltrated CNT fibers using the tensile recoil measurement. The infiltration of epoxy resin among CNTs gave rise to the intertube bonding and load transfer, and not only increased the

of PVA chains after hot-drawing. The resulting fibers were tough 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 process for CNT fibers, especially for dry-spun ones to attain improved mechanical properties. The twist angle which determines the final affect can be easily controlled by varying the rotation of the spindle, like the speed and number of turns. The role of twisting is to put tension on the relaxed CNTs and force individual CNTs arranging in a certain angle to the fiber axis, thus the intertube distance is reduced, accompany with increasing the friction force between CNT bundles.170,204 There is a generic theory suggests that the tensile strength of a twisted yarn can be depicted as σy /σf ≈ cos2 α[1 − (k cosec α)]

(1)

where σy and σf correspond to the tensile strength of the yarn and CNT, respectively. α is the twist angle and k = (dQ/μ)1/2/ 3L. d is the diameter of CNT, μ is the friction coefficient between CNTs, L is the CNT length, and Q is the CNT migration length. According to eq 1, apparently, the twist angle is critical to the yarn strength, which is also affected by CNT length, CNT diameter, and intertube friction simultaneously.166 Meanwhile, the experimental results demonstrated that the tensile properties of CNT yarns do not grow consistently with the twist angle. In another words, the maximum properties showed at an intermediate twist level, to be specific, 20° angle to the yarn axis for strength and 10° for elastic modulus, as claimed by some researchers.205,206 A third strategy refers to the surface tension-based densification, namely, by passing through or spray-coating volatile solvents, such as ethanol and acetone. It is the capillary force during evaporation from the intertube space that causes collapse and shrinkage of the CNT fibers (Figure 10c− g).167,175,177,207,208 Miao once applied a rubbing roller system to densify a twistless 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 density core.209 Very recently, Wang et al. discovered a method for maximizing fiber performance through a stepwise densification process. It combined condensation of a CNT aerogel in water or alcohol to form a fiber-like structure and a pressurized rolling process to enable further densification. The resulting CNT ribbon had excellent performances with a combination of high strength (4.34 GPa), high ductility (10%), and high 7057

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Table 1. Summary of the Preparation Methods, Strengthening Protocols, and Mechanical Properties of the Most Relevant CNT Fibers ref

raw material

preparation method

159 201 160 163

SWCNT + PVA SWCNT + PVA 60 wt % SWCNT + PVA SWCNT

wet-spinning wet-spinning wet-spinning wet-spinning

202

wet-spinning

161 198 195 199 172 166

50 wt % CNT (SWCNT/ MWCNT) + PVA >75 wt % SWCNT + PEI SWCNT N doped MWCNT CNT SWCNT MWCNT

wet-spinning liquid crystal wet-spinning liquid crystal wet-spinning liquid crystal wet-spinning FCCVD drawing twisting CNT array

167

CNT

drawing CNT array

169 170 203

DWCNT MWCNT MWCNT

drawing twisting CNT array drawing CNT array drawing CNT array

207 206

MWCNT MWCNT

drawing CNT array drawing twisting CNT array

214 211

MWCNT CNT

drawing twisting CNT array drawing CNT array

215

MWCNT

drawing twisting CNT array

209 174 175 176 177

MWCNT SWCNT + DWCNT DWCNT CNT DWCNT

drawing drawing drawing drawing drawing

208

CNT

drawing CNT aerogel

189 185

DWCNT 30−50 vol % SWCNT + epoxy/PVA

drawing-drying spinning infiltration-twisting CNT mat

186

DWCNT + PVA

infiltration and twisting and stretching CNT mat

CNT CNT CNT CNT CNT

treatment stretching stretching + thermal annealing (1000 °C) hot-drawing

array aerogel aerogel aerogel aerogel

thermal annealing (850 °C)

ethanol densification

postspin twisting drawing + twisting + heat treatment (200 °C) twisting + shrinking liquid densification +PS infiltration gamma-irradiation twisting + PEI-C infiltration + solvent densification ethanol densification + HNO3 treatment rubbing roller densification acetone densification water densification + acetone densification acetone densification + HDE-UV infiltration heat treatment (150 °C)

compressive fiber strength from 416 to 573 MPa but also changed their failure mode under compression. As suggested by the microscopic analysis of the fiber surface morphologies, kinking was the primary compressive failure mode for pure CNT fibers, while composite fibers with higher brittleness showed the bending failure mode (Figure 11a,b).217 Later, the same group delivered a successive study on the stress relaxation behavior of the two CNT-based fibers, where stress decay was observed in both pure and CNT/epoxy composite fibers when they were held under constant strain (Figure 11c). The timedependent relaxation behavior in CNT fibers, yet not evidently seen in carbon fibers, was supposed to originate from the sliding between CNT bundles and affected by the initial strain level, strain rate, and gauge length. In addition, the composite

tensile strength

Young’s modulus

∼150 MPa 230 MPa 1800 MPa 65 MPa g−1 cm−3 770 MPa g−1 cm−3 (PVA -infiltrated) 1400−1800 MPa

9−15 GPa 40 GPa 80 GPa 12 GPa g−1 cm−3 8.9 GPa g−1 cm−3 (PVA -infiltrated) 35−45 GPa

70−100 MPa cm3 g−1 116 ± 10 MPa 170 ± 70 MPa 1000 ± 200 MPa 1200 MPa 150−300 MPa (singles yarns) 250−460 MPa (two-ply yarns) 850 MPa (PVA-infiltrated singles yarns) 600 MPa 564 MPa (heat-treated) 1350−3300 MPa 1910 MPa 970−1400 MPa

∼6 GPa cm3 g−1 120 ± 10 GPa 142 ± 70 GPa 120 ± 50 GPa 49−77 GPa

74 GPa (heat treated) 100−263 GPa 330 GPa

1100 MPa 1040 MPa (PS infiltrated)

56 GPa

731−846 MPa 2200 ± 150 MPa 2500 ± 310 MPa (Further Fe(III) treated) 1520 MPa

18.5−23.3 GPa 120 ± 23 GPa

650 MPa 1460 MPa 8800 MPa 2.2 N/tex 400−1250 MPa

41 GPa ∼30 GPa 357 GPa 160 N/tex

2.3 GPa SG−1

∼75 GPa SG−1

299 MPa 900−1600 MPa (epoxy) 700−1300 MPa (PVA) 540 MPa

8.3 GPa 30−50 GPa (epoxy) 20−35 GPa (PVA) 30.6 GPa

60 GPa

fibers possessed a higher relaxation rate than that of the pure fibers, due to the extra interfacial sliding at the CNT/epoxy interface.218 4.2.4. Applications. The production of continuous CNT fibers has paved the way for applications on the macroscopic scale. Despite numerous applications in various areas, for instance, composite reinforcements, electrochemical actuators, flexible supercapacitors, electrical wiring, biological scaffolds, electron and ion emitters, etc., the original intention for CNT fibers is still the pursuit of high strength, as a competitor to traditional carbon fibers. Although there remains a long way to go, great progress has been made in recent years, as summarized in Table 1. It appears that stronger fibers are more inclined to be fabricated via dry methods; however, the 7058

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Figure 12. (a) Schematic of the yarn structures for tensile and torsional actuation. Reprinted with permission from ref 225. Copyright 2012 American 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 woven from wire-shaped batteries. Reprinted with permission from ref 240. Copyright 2014 Wiley-VCH. (e−h) Twisted DWCNT cables and demonstration to light a bulb with the cable being a part in the circuit. Reprinted with permission from ref 242. Copyright 2011 Nature Publishing Group. (i) A continuous and meters long Cu coated CNT fiber collected on a winder. Reprinted with permission from ref 247. Copyright 2011 Royal Society of Chemistry.

form,222 Mirfakhrai et al. presented the first investigation on the electrochemical actuation behavior of twist spun MWCNT yarns. Actuation strains up to 0.5% were reported at an applied voltage of 2.5 V.223 Afterward, the electrochemical actuation behavior was extended to a promising application of torsional artificial muscles, which were operated by electrochemical double-layer charge injection, producing a reversible 15 000° rotation (more than 41 turns) and 590 rpm. The resulted lengthwise contraction and torsional rotation were explained by a hydrostatic actuation mechanism.224 Since then, the research on torsional actuation has drawn particular interest. The same group subsequently discovered electrolyte-free torsional and tensile actuation on the basis of dimensional changes of guest actuating materials within the twisted yarns, triggered by electrical, chemical or photonic excitation (Figure 12a). Notably, an average torsional actuation at 115 000 rotations per minute and tensile actuation at 1200 cycles per minute and 3% stroke were demonstrated.225 By combining paraffin wax and polystyrene-poly(ethylene-butylene)- polystyrene as mixtures for guest actuating materials, rapid and precise positional control was realized with a peak rotation speed of 9800 rotations per minute, which is of great importance for practical uses.226 In a much simpler way, electromechanical torsion of CNT fibers was derived by directly passing a low current along them, according to a different actuation mechanism based on Ampere’s law at the nanometer scale. The authors Guo et al. explained that the contraction and rotation of CNT yarns were made by electromagnetic forces among helically aligned CNTs. Therefore, such type of actuation could occur in almost all available environmental media such as air, water, and organic

simplicity and energy saving of wet methods should not be disregarded. Furthermore, the employed treatments are quite effective in strengthening the as-prepared fibers. 4.2.4.1. Composite Reinforcements. Inspired by the primary uses of conventional high-performance fibrous materials, such as carbon fibers, the utilization as reinforcements for polymeric composites is one of the most prospective applications of CNT fibers. In contrast to brittle carbon fibers, CNT fibers generally exhibited better flexibility, which is favorable for large-strain deformation uses. Gao et al. studied the axial compression of twisted CNT fiber embedded in an epoxy matrix with the help of in situ Raman spectroscopy. The results indicated that the reinforcing efficiency of hierarchically structured CNT fiber was higher than high-modulus carbon fiber, while it was able to bear a load under large-strain compression without permanent deformation and fracture.219 Theoretically, the interface between reinforcements and the matrix which enables load transfer plays a critical role in the final performance of a composite material. The principal failure mode belongs to sliding at the fiber/matrix interface. Both single-fiber composite fragmentation tests and microdroplet tests were used to measure the fiber/matrix interfacial shear strength for a system of dry-spun CNT fiber in epoxy matrix. The attained values were 17 and 14.4 MPa, respectively, both comparable to that of commercial nonsized carbon fiber/epoxy composites.220,221 Therefore, the capacity of CNT fibers for entering the field of fiber 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 7059

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conventional conductive metals while the CNT fibers always have very low density at the same time. However, their electrical conductivity (in the 102 S cm−1 range) is still below aluminum and copper. Although the lower electrical conductivity originating from a limited morphology control over CNT fibers becomes the major obstacle for the wiring applications of CNT fibers, there are still various superiorities making CNT fibers the promising candidate for replacing conventional metal wires in certain cases. Efforts are being made constantly aiming to meet the challenge. For example, KAuBr4 incorporated CNT fibers might have a conductivity of 1.3 × 106 S m−1.241 Iodine doped DWCNT cables were reported to have electrical resistivity of ∼10−7 Ω·m and high current-carrying capacity of 104−105 A cm−2, as well as much better high temperature stability than metal wires (Figure 12e− h).242 Acid-doped CNT fibers possessed similar failure current density (FCD) ranging from 103 to 105 A cm−2. These values for doped CNT fibers were higher than those for previously reported carbon fibers and undoped CNT fibers. But the measured FCD and continuous current rating (CCR) values were still lower than copper wires. Nevertheless, after being normalized by the mass density, both the specific failure current (SFC) and specific CCR of the CNT fibers were higher than those of copper wires as expected, which were highly preferred for weight-critical applications.243 As a comparison, the iodine doped CNT fibers showed better enhancement on conductivity than acid doped ones, which coincides with the results obtained by Behabtu et al. via performing doping on wet-spun fibers.199 Another way to improve the electrical conductivity of CNT fibers is the introduction of metal particles.244,245 The electrodeposition of Au or Cu particles onto CNT fiber surface would significantly increase their conductivity to 2−3 × 105 S cm−1.246 By giving overall consideration to conductivity, mass density, strength and productivity of the anodized CNT fibers, Zhang and Li et al. developed a continuous electrodeposition method for fabrication of CNT-Cu composite fibers (Figure 12i). The obtained fibers exhibited a metal-like conductivity from 4.08 × 104 to 1.84 × 105 S cm−1 and a mass density of 1.87−3.08 g cm−3, depending on the thickness of Cu layer. Furthermore, little damage was caused to the strength of the Cu-CNT composite fiber because of their strong interfacial bonding.247 On the other hand, the electrical insulation is equally important for the uses of CNT wires in every-day electrical circuits and devices. A good insulating layer for CNT fibers was supposed not to infiltrate or lead to any deterioration of electrical or mechanical properties of CNT fibers. Thus, a polymer with high wetting angle on the fiber surface or high viscosity should be the right answer to the demand.248 Recently, Lekawa-Raus et al. provided an overview of studies in this area and expressed a positive attitude on the application of CNT fibers in electrical wiring.249 4.2.4.5. Other Applications. Researchers have found that CNT yarns show piezoresistive behavior whose electrical resistance increased linearly with increasing tensile strain, indicating their potential application as strain sensors.250−252 While exhibiting excellent repeatability and stability, CNT yarns are suitable for in situ health monitoring after being embedded in a composite structure.250 Otherwise, through a simple prestraining-then-buckling approach, stretchable conductors with buckled CNT fibers incorporated into a polydimethylsiloxane (PDMS) substrate were fabricated. The CNT fiber/ PDMS conductors demonstrated outstanding resistance retention (∼1% variation) under multiple stretching-and-

solvents, without the need for electrolyte or guest actuating materials.227 4.2.4.3. Fiber-Based Supercapacitors and Electronics. The high electrical conductivity and voids containing structure of CNT fibers are expected to be utilized in high performance fiber supercapacitors as well as fiber-based wearable electronic devices.228 Dalton et al. first dip-coated CNT fibers with electrolyte, twisted two fibers together and then recoated with electrolyte to build a fiber supercapactior, which provided a capacitance of 5 F g−1 and energy storage density of 0.6 Wh kg−1 at 1 V. Moreover, a certain amount of fiber supercapacitors can be woven into textiles which facilitate their electronictextile applications.160 Zhong et al. wove their aerogel derived CNT yarns into a single jersey fabric. Electrochemical measurements in an electrolyte of NaCl solution revealed that the CNT fabric supercapacitor showed a capacitance of 79.8 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. After carbonizing CHI constituent under high temperature treatment, a composite yarn electrode containing SWCNT and active carbon was produced and with preferable mesopores for the transport and storage of charges. Being assembled with a PVA/H2SO4 gel electrolyte (Figure 12b), the measured capacitance was 74.6 F g−1 (48.5 F cm−3) at a scan rate of 2 mV s−1, and 65.2 F g−1 (42.4 F cm−3) at a current density of 0.05 mA cm−2. The yarn microsupercapacitors also presented substantial cycling stability after 10000 charge−discharge cycles and high energy and power density (3.7 mWh cm−3 and 45.7 mW cm−3, respectively).229 Differently, composite yarn electrodes consisting of SWCNT and conducting polyaniline nanowires (PAniNWs), and with a PVA outer sheath were fabricated by the same group via a one-step wet-spinning process. The composite yarns provided obviously improved electrochemical performances as compared with those of the supercapacitors based on pure SWCNT yarns, owing to the pseudocapacitance coming with PAniNWs. Typically, a maximum areal capacitance of 6.23 mF cm−2 was reached for the composite yarn-based supercapacitors.230 Peng et al. accomplished a number of researches upon the topic of novel fiber-based electronics,231−233 such as supercapacitors,234−236 solar cells237,238 and batteries.239,240 Taking their recent design as an example, a stretchable wire-shaped lithium-ion battery was assembled by pairing two composite MWCNT yarns as anode and cathode, modified with Li4Ti5O12 (LTO) and LiMn2O4 (LMO) nanoparticles, respectively. The wire-shaped batteries displayed extraordinary electrochemical performances, such as energy densities of 27 Wh kg−1 or 17.7 mWh cm−3 and power densities of 880 W kg−1 or 0.56 W cm−3, as well as a high capacity retention after 1000 bending cycles. Furthermore, they also can be woven into flexible textiles to serve as wearable electronics (Figure 12c, d).240 Generally speaking, the CNT fibers built in such wire-shaped devices served as electrodes, on which active components were usually embedded to realize or enhance the device performance. No matter the fiber electrodes were assembled symmetrically or asymmetrically, the participation of CNT fibers has made them strong, lightweight, flexible and high efficient. 4.2.4.4. Electrical Wiring. High mechanical strength, lightweight, as well as prominent thermal and electrical properties, the amazing attributes of CNT fibers endow them with the possibility of electrical wiring applications. The mechanical performances of CNT fibers have already exceeded those of 7060

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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 the Advancement 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 a transparent SWCNT film spray-coated on a glass substrate. (h) AFM image of the thin film, inset is the image under higher magnification. Reprinted with permission from ref 273. Copyright 2009 Wiley-VCH.

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 high efficiency.254−258 Biological applications of CNT yarns were also reported in the literature, as an example, mechanically strong scaffolds based upon a knitted structure made from multiply MWCNT yarns were found suitable for tissue engineering.259

approaches. Both of them have advantages and disadvantages. Here, the commonly used methods to afford CNT films will be reviewed with comparison and the emerging applications of the as-prepared CNT films will also be outlined at the end of this section. 4.3.1. Wet Methods Assembled CNT Films. The wet methods for CNT films include filtration-transfer process, dipcoating, 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 them are scalable. At the very beginning of a typical process, the hydrophobic CNTs need to be dispersed in a certain solvent, usually with the help of surfactants or functionalization. The most commonly used surfactants are anionic surfactant sodium dodecyl sulfate (SDS) and sodium dodecylbenzyl sulfonate (SDBS), or the nonionic surfactant Triton X-100. After deposition and formation of the CNT films, the preadded surfactants are supposed to be eliminated to improve the conductivities within the films. The detailed preparation processes are discussed below. 4.3.1.1. Vacuum-Assisted Filtration. The first free-standing macroscopic SWCNT film, named as “bucky paper”, was fabricated by the vacuum-filtration method.260 In such a widely employed protocol, the well dispersed CNT suspension is filtered under vacuum to form a randomly packed film (shown in Figure 13a−c). The density and thickness of the as-prepared

4.3. 2D CNT Films

CNT films are 2D structures assembled from CNTs. Benefiting from the properties of CNTs and configuration of the films, they exhibit mechanical flexibility, chemical stability, unique electrical and thermal properties, and optical transparency for the thin CNT films. Thus, CNT films have shown great potential for flexible and stretchable electronic and optoelectronic devices, while much attention has been paid to the fabrication, characterization, and application of these film materials. From the view of configuration, CNT films can be divided into randomly stacked films and networks with highly aligned structures. Most of the reported CNT films are with random configuration while the later bearing oriented CNTs with anisotropic properties are chased for specific applications, such as highly electrical and thermal conductive films. To fabricate the two-dimensional assemblies of CNTs, there are several strategies that mainly belong to two categories, which are known as suspension-based wet method and CVD growth 7061

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Figure 14. (a) Schematic of the spin-coating process. (b) CNT films of different thicknesses spin-coated on PET (left) and glass (right). Reprinted with permission from ref 279. Copyright 2009 American Chemical Society. (c) Schematic of the bar-coating process. (d) A uniform bar-coated SWCNT 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.

film, which are critical to the sheet conductance and optical transparency, are highly controllable through adjusting the filtration time, the CNT concentration, and the volume of the filtered suspension.261,262 Afterward, the supported film can be transferred to another substrate263,264 or come to a freestanding CNT film by dissolving the filter paper,261 acquired by specific needs for application. Many efforts have been made to improve the properties of the as-formed CNT films, including the postdeposition strategy264 as well as optimization and selection of high quality CNT raw materials.265−267 Notably, Hone et al. applied strong magnetic fields during the filtration of SWCNT suspension to gain films with aligned SWCNTs. The anisotropic films showed high electrical and thermal conductivity along the alignment axis.268 However, there are still drawbacks for the filtration process. The size of the films obtained via filtration is limited by the filtration apparatus, more particularly the filter paper, which means it is difficult to work at a large scale. Besides, the irregular morphologies and significant roughness 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 no matter flat or curved surfaces. It comprises a dip-coating process and a drying process (Figure 13d,e), during which three parameters are playing an essential role in determining the quality of the obtained films, including CNT concentration, withdrawal velocity, and number of dip-coatings.269 Compared with other wet methods, the dip-coating process produces a relatively aligned CNT network since the configuration relies on the interplay between shear forces and Brownian motion of CNTs during coating and drying. The former tends to alignment of CNTs, whereas the later acts as a randomizing effect.270 On the other hand, the adhesion between hydrophobic CNTs and hydrophilic substrate is weak and thus degrades 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 substrates.271,272 Although the dip-coating technique is simple and scalable, multiple coating-drying cycles are needed to reach a certain thickness with low sheet resistance, which makes it also time-consuming. 4.3.1.3. Spray-Coating. Spray-coating, also known as the air brush technique, is another simple method to realize the roll-toroll fabrication of CNT films (Figure 13f−h). Tenent et al. have reported the utilization of spray-coating for achieving large-area SWCNT electrodes.273 Normally, the spray-coating method involves three steps during the process which are droplet generation, deposition of the droplets onto the substrate by air flow, and drying of the droplets on a heated substrate to form a film.274 It is noteworthy that heating on the substrate is necessary for the spray-coating fabrication to accelerate the drying of the droplet, in order to retain homogeneity in the formed films,275−277 which is really a big issue for the spraycoating technique. Also the spraying time is a critical parameter to control the quantity and thus density of the CNT films.268 4.3.1.4. Spin-Coating. As one of the most promising processes for mass production of CNT films (Figure 14a,b), spin-coating shows several advantages such as high uniformity in the as-prepared films, easy and precise control on the thickness, short coating time, low-temperature fabrication, and high reproducibility.279−282 However, the features of such a technique make it difficult for fabricating thick CNT films. 4.3.1.5. Bar-Coating. Bar-coating is suitable for large-scale CNT films (Figure 14c,d). While using a wire-wound Mayer rod as the operating tool during coating, the diameter of the wound wire determines the size of the grooves and thus control the thickness of the final CNT films.283 Furthermore, the conductivity and transparency of the CNT films are easily tunable depending not only on the wire diameter but also on the CNT concentration in the dispersion.284 7062

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(1−3 mm wide) was introduced to water, a SWCNT thin film (15 × 30 mm area) was quickly created by spreading on the water surface and could be transferred to a clean glass substrate later. Although the obtained SWCNT films did not show very high conductivity, the preparation process did pave another way for the roll-to-roll fabrication of CNT thin films.301 Speaking to mass production, Fischer et al. reported a mass printing process, namely the flexographic printing, for CNT structures on textile and paper substrates. The as-prepared MWCNT films also exhibited good electrothermal properties.302 The advantages and disadvantages of the most popular wet methods for preparing CNT films are summarized in Table 2.

4.3.1.6. Electrophoretic Deposition. Boccacini et al. have reviewed the electrophoretic deposition (EPD) of CNTs in 2006,285 demonstrating it is an economical and versatile processing technique for the production of CNT films or coatings. The EPD fabrication is a two-step process (Figure 14e) including electrophoresis and deposition, and the final morphology highly depends on the gap between the two electrodes, strength of the applied electric filed, deposition time, and lengths of CNTs.286−289 In addition, a conductive substrate is always necessary for satisfying the EPD process, which limits the widespread use of this technique. Although the as-prepared CNT films can be transferred onto another substrate by utilizing a subsequent step such as hot-pressing transfer, the additional procedure makes the fabrication process become 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 with nanometer scale precision,291 which is performed by alternately depositing negatively and positively charged materials onto a substrate (Figure 14f).292,293 Since the assembly is forced by electrostatic and van der Waals interactions, the interconnectivity of the structural components is apparently strong. Usually, the LBL protocol is combined with other deposition methods such as dip-coating and spray-coating. Mamedov et al. reported the LBL assembly of SWCNT and polyelectrolyte through sequential dip-coating of negatively charged SWCNT in aqueous solution and positively charged polyelectrolyte. The freestanding SWCNT/polyelectrolyte LBL films showed high homogeneity and minimized phase segregation of the two structural components and therefore exhibited evident enhancement on strength compared with the neat polymer films.294 Later, Kim et al. improved this technique with faster fabrication by using vacuum-assisted spray-coating to fulfill the LBL assembly. In their work, high performance MWCNT electrodes for lithium-ion batteries (LIB) with thicknesses of tens of microns were produced on porous carbon substrates,295 and the assembled components were two types of modified MWCNTs carrying opposite charges.296 4.3.1.8. Other Wet Methods. Apart from the above listed commonly used wet methods to obtain CNT films, there are several exceptional solution based techniques which have been reported. The Langmuir−Blodgett (LB) technique is considered as a precise method for CNT films to readily control their thicknesses and tube-orientation, which is based on the uniform surface spreading of CNTs that contribute to the formation of stable Langmuir monolayers on the solution surface. By using the LB technique, either multilayer-thick SWCNT films297 or monolayer SWCNT films with aligned structures298 were fabricated. 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 is relatively 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 to be capable of attaining aligned SWCNT coatings ranging from 0.5 μm wide stripes to continuous films with tunable transparency in a recent work.300 Another novel method for the assembly of SWCNT films on glass substrates is described by Simmons et al. With the help of shear mixing, SWCNTs could be dispersed into room temperature ionic liquid (RTIL). When the dispersion bead

Table 2. Comparison among the Wet Methods for CNT Films methods

advantages

vacuum-assisted filtration

controllable thickness

dip-coating

simple and scalable relatively aligned CNT network suitable for curved surfaces suitable for mass production

spray-coating spin-coating

bar-coating electrophoretic deposition layer-by-layer assembly Langmuir− Blodgett assembly

suitable for mass production fast and high uniformity easy and precise control on the thickness suitable for large-scale films economical and versatile process high-precision structure control easy control on film thickness and tubeorientation monolayer CNT films are realizable

disadvantages time consuming limited size irregular morphology on the upper surface multiple coating-drying cycles are needed to obtain a thick film

heating on the substrate is necessary to improve the homogeneity difficult for fabricating thick films

low reproducibility conductive substrates are necessary multiple steps are included time consuming

4.3.2. CVD-Grown CNT Films. The previously introduced wet 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 CNTs in specific solvents, which is important for the homogeneity of the as-prepared films yet quite difficult to realize due to the poor surface activity of CNTs. The typical strategies to overcome such issue are usually the application of surfactants or the introduction of functionalization on CNTs to improve their dispersibility, accompanied by ultrasonication during mixing. Unfortunately, the addition of surfactants leaves residues which are hard to be eliminated completely in the film products and thus significantly increase the sheet resistance, while the functionalization process disrupts the sp2 structure of CNTs and yielding low film conductivity. Moreover, the violent ultrasonication treatment shortens individual CNTs, causing a dramatic increase in the quantity of intertube junctions, which also inhibits transportation in the films. In one word, although they are simple, efficient, and cost-effective ways for large-scale CNT films, the solvent-based wet methods could hardly put an 7063

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end to certain limitations when it comes to higher performances. On the other hand, nonsolvent techniques based on the CVD process are able to avoid the issues faced by the wet methods. The major merits of the CVD derived CNT films are their high purity, controllable tube lengths, and less agglomerate bundles. Even though the CVD process is considered to be energy consuming, numerous interests have been attracted to the direct synthesis of tough, freestanding CNT 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 be capable of gathering CNT films while the spindle was rotated normal to the furnace axis outside the hot zone (Figure 15a,b). 173 For arc-discharge growth of SWCNTs, the productions are normally randomly oriented with an exception of Wang et al.’s work. They introduced a magnetic field during nanotube growth in order to induce alignment to the asprepared SWCNTs, and the well-aligned SWCNT films were deposited onto various substrates including plastic ones.

According to its feasibility of orientation control, this approach is meaningful for scalable fabrication of flexible SWCNT-based electronic and optoelectronic devices.303 Xie’s group has developed the floating chemical vapor deposition (FCCVD) technique for nonwoven SWCNT films. The obtained thin films with areas up to several tens of square centimeters consisted of entangled SWCNT bundles with diameters of ∼30 nm.304 Subsequently, slightly aligned SWCNT films with controllable growth rates were discovered through precise control on the sublimation rates of the catalysts. The directly synthesized films possessed superior electrical and mechanical properties over the films made from solution based filtration, owing to the oriented distribution and strong interbundle connections (Figure 15c−g).305 A repeated stretching and pressing treatment was able to fabricate a superaligned and highly dense CNT sheet, performing 221% increase on tensile strength than the as-prepared one.306 It is noteworthy that the enhanced interbundle connection mostly comes from the as-formed Y-type junctions generating at the same time with the SWCNT growth rather than the simple overlap between CNTs which occurs in the solution-derived films. Furthermore, the efficiently obtained, large-scale, and free-standing SWCNT films can be easily handled for further studies, such as being twisted into macroscale CNT fibers,178 reinforcing and conducting scaffolds for polymers,307,308 and electrodes for flexible supercapacitors309−311 or actuators.312 Besides, in a similar work reported by Liu et al., the FCCVD method was applied to assemble novel multisheeted book-like macrostructures with uniform and tunable thickness, named as the “buckybooks”. As demonstrated in the paper, the buckybooks showed potential applications as binder-free electrodes for supercapacitors and filters with high molecular separation efficiency.313 4.3.3. Array-Derived CNT Films. The aforementioned preparation protocols generally lead to randomly packed CNT films or slightly oriented ones. Indeed, the well aligned CNT films are more promising for conductive uses. With this consideration, there are alternative routes toward the mass production of highly aligned CNT films using vertically oriented CNT arrays as the starting materials. In one way, meter-long transparent sheets were directly drawn from a sidewall of CNT forests which were synthesized by CVD method, seen in Figure 16a-d. The continuous CNT sheets with anisotropic features were generated via end-to-end connections of CNTs along the draw direction.314 Based on the tunable fabrication of CVD, the tube diameter, number of CNT walls and the length of sheets are well controlled for the desired physical properties.315,316 Especially, the long and nondefective nanotubes are highly demanded for optimal CNT sheets with excellent electrical and thermal conductivities and mechanical properties.314 In combination with the unique characteristics of CNTs, the array-drawn CNT sheets with unidirectional geometry are capable of various applications, such as transparent electrodes, planar sources of polarized broadband radiation, microwave welding of plastics, flexible organic lightemitting diodes,314 loudspeakers,317 electrical reinforcement for polymers,318 as well as artificial muscles with charge-induced actuation behavior.319 In another way, the well-aligned architectures could be realized by the so-called “domino pushing” above the CNT arrays. In particular, the vertical CNTs are forced down to one direction by pushing a cylinder upon the array with compression.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 with polyvinyl chloride. Reprinted with permission from ref 173. Copyright 2004 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 arrows indicate the Y-type junctions and the flow direction during the film growth. Reprinted with permission from ref 305. Copyright 2007 American Chemical Society. 7064

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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 CNT array being drawn into a sheet from a different angle of view. (d) SEM micrograph of a structure made by stacked MWCNT sheets. Reprinted with permission from ref 314. Copyright 2005 American Association for the Advancement of Science. (e) Schematic of the “domino pushing” method to prepare the aligned CNT paper. Images illustrate (f) the as-made buckypaper in large-scale and (g) folded paper. Reprinted with permission from ref 321. Copyright 2008 IOP Publishing Ltd.

for the TCFs.322,323 The longer the unit CNT is, the less contact 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 conductive performances were found in the SWCNT films.275 Although the DWCNTs were revealed to provide a better transmittanceconductance performance on films than SWCNTs and MWCNTs, because semiconducting tubes are usually inevitable in SWCNTs, and MWCNTs are thought to absorb much more photons.324 But even so, the research hotspots are still focused on SWCNT-TCFs, probably due to the comprehensive consideration of their performances and the cost of synthesis. To face the polydispersity problem of SWCNTs, involving metallicity and diameter distribution, Green and co-workers employed a density gradient ultracentrifugation (DGU) technique to produce transparent conductors consisting of primarily metallic SWCNTs with small diameter distributions, where enhanced conductivity over several times was obtained as a consequence.265 The conductivity of a CNT film is an important performance index for its use in the field of electronics, and there are many ways to provide improvement on such a point. A related work decreased the sheet resistance of SWCNT films by a factor of 5 via a postdeposition in nitric acid (HNO3) and thionyl chloride (SOCl2) bath. Both of the exposures helped improve the stability of the functionalized SWCNT films, while the contact with SOCl2 also caused formation of acyl chloride functional groups which firmly bonded SWCNTs, resulting in significantly improved transport properties.264 Meanwhile, post treatment with HNO3 is a commonly seen procedure for the CNT films coming by wet methods.271,272 The effect of HNO3 treatment was thought to remove the residual surfactants and densify the obtained films, both of which could improve the conductivity

were interconnected with strong van der Waals attractions, leading to mechanical stability, which enabled the formed papers to go to large scale (Figure 16e−g). In addition, the improved thermal and electrical conductance was attributed to the well maintained straight morphology of individual CNTs within the papers.321 4.3.4. Applications. CNT films as tempting 2D materials have drawn considerable attention in both research and industry areas. A wide range of applications concerning CNT films have been explored, and one may get hints from the substantial number of reviews and published research works. Herein, several representative applications will be highlighted. 4.3.4.1. Transparent Conductive Films (TCFs). CNTtransparent conductive films (CNT-TCFs) are well suited for applications with requirements of high transparency while high conductivity is also in demand, such as touch panels,276 electrodes of solar cells273,277,278 and organic light emitting diodes (OLEDs).263 In comparison with ITO which has been widely used in the electronic area, although the emerging carbon-based films can hardly surpass ITO on optoelectronic performances, their low-cost, environmental friendly, highly stable and flexible features, especially the outstanding capacity in theory, encourage researchers to keep chasing the excellent theoretical value of individual CNT in its assembled macroscopic 2D forms. To accompany the continuous struggle for future applications, CNT-TCFs are regarded as the prospective replacement for traditional ITO films. To this purpose, there are two major issues that need to be paid attention to for achieving high performance CNT-TCFs: one is the intertube junction resistance which causes principal degradation on the integral conductivity of CNT films and the other is the inhomogeneity of the CNT networks which influences the uniform transparency. The first negative effect could be crippled at a certain extent via various strategies, such as enhancing the bonding between CNTs or choosing long CNTs 7065

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by enhancing the connectivity between CNTs,325 as well as ptype doping.326 Andrade et al. has compared the electrical properties of the CNT networks made from various solution-based techniques, including dip-coating, spray-coating, vacuum filtration, and electrophoretic deposition. They found that SWCNT films obtained by dip-coating showed the lowest sheet resistance (186 Ω sq−1) for a given transparency (86%) at 550 nm wavelength, probably owing to their relatively smooth and aligned geometries.327 The CNT array derived films usually deliver inferior conducting performance in comparison with wet-assembled CNT films, for example, showing performance of 1.6 kΩ sq−1 at 86.5% transparency. After laser trimming and deposition of Ni and Au metal, Jiang et al. obtained CNT films with significantly improved conductivity. Two typical values of sheet resistances and transmittances of the modified films were indicated by 208 Ω sq−1 at 90% and 24 Ω sq−1 at 83.4%, which were comparable to those of ITO films and meet the requirements of touch screens, LCDs, and solar cells.328 As we’ve already known that the surfactants used to help dispersing CNTs into solvents will have negative effects on the film conductivity simultaneously, there are some surfactant-free strategies to thoroughly avoid such influences. Chlorosulfonic acid (CSA) was supposed to generate uniform and highly debundled CNT dispersions for further deposition.323,329 The use of superacid CSA has a number of benefits over surfactants, especially for the exfoliation of CNT bundles and p-type doping in the films. An extremely low sheet resistance of 60 Ω sq−1 was reported by Hecht et al. at 90.9% transparency (at 550 nm) through filtrating the CSA dispersion of SWCNTs.266 Besides, a two-step shearing process initiating from CNT arrays could also realize the surfactant-free dispersion of individual CNTs in benzyl alcohol, yet with relatively high resistance in the formed films.330 Since TCF is one of the most promising applications for CNT thin films attracting intensive studies, we present a comparison of the main achievements reported in recent research to get a global overview upon the subject, and the results are displayed in Figure 17. As is shown in the figure, the solution-based wet methods are the most common ways to CNT-TCFs, which have proved their possibility for commercial

demands (e.g., touch screen and LCD); however, they are still inferior to the best records made by ITO-based films. 4.3.4.2. Supercapacitors. The 2D macroscopic CNT networks are very popular materials for the electrodes of supercapacitors, thanks to their outstanding storage and transport capabilities. One of the charming features of CNT film-based supercapacitors is their flexibility. Niu et al. fabricated compactdesigned supercapacitors by rolling up free-standing SWCNT films with separators (Figure 18a,b). The SWCNT films served as both binder-free electrodes and current collectors, making the energy storage device really lightweight. Significantly, high energy and powder densities (43.7 Wh kg−1 and 197.3 kW kg−1) were achieved due to the small equivalent series resistance.309 Later on they developed a “repeated halving” approach to transfer ultrathin SWCNT films onto PET substrates via electrostatic adsorption. After being assembled with separators and electrolytes, the transparent and flexible supercapacitors were obtained.311 Cui et al. have made efforts on large-scale flexible supercapacitors. For example, high performance supercapacitors with a specific capacitance of 200 F g−1 and a stable cycling life were realized based on commercial paper. Certainly, the commercial paper was highly conductive by conformal coating of SWCNT and Ag nanowires through the rod-coating method. Furthermore, this low-cost conductive paper can also be applied in lithium-ion batteries as lightweight current collector.331 Interestingly, the paper-based supercapacitor could even be integrated onto a single sheet of paper while SWCNT films acted as electrodes and current collectors and the paper as substrates and separator.332 Except for paper, porous textiles such as cellulose or polyester were also employed as the stretchable substrates following a similar protocol to make them conductive. This coating method has been demonstrated as a feasible approach for wearable electronics and energy storage devices.333 Since the specific surface area and transportation paths in electrodes are highly dependent on their porosity, the introduction of sacrificial nanoparticles during the formation of CNT films was reported to enhance porosity in the film electrodes. 200 nm diameter polystyrene (PS) nanospheres were used during the filtration process, and the performance of the supercapacitors was much better at a higher PS concentration.334 In addition to the utilization in electrical double-layer capacitors (EDLCs), CNT films showing both large surface areas and good conductivities are frequently used in pseudocapacitors with much higher pseudocapacitance and energy 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 as TCFs and electrodes for energy storage devices, 2D CNT networks are also popular in diverse areas due to their superior physical and chemical properties. The possible uses of CNT thin films for electronics and sensors have been well proved (Figure 18c).337−339 CNT film heaters were developed based on the sensitive and evident electrothermal response.302,340 Biomedical applications were also revealed where CNT films performed as the substrates for growth of cells.341 The advances of CNT membranes for water purification and gas separation were developed based on their porous nature.342,343 Stacking of aligned CNT sheets was used for lightweight microwave absorption film, with the absorption frequency being accurately controlled by varying the intersectional angles of the CNT

Figure 17. Summary of the opto-electrical properties of CNT-TCFs and comparison with those of ITO and touch screen/LCD requirements. 7066

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Figure 18. (a) and (b) Photographs of the compact-designed supercapacitor made by rolling up SWCNT films with the separator. Reprinted with permission from ref 309. Copyright 2011 Royal Society of Chemistry. (c) Optical image of the flexible SWCNT transistors integrated circuits on a polyimide substrate. Reprinted with permission from ref 339. Copyright 2008 Nature Publishing Group. (d) and (e) Demonstration of the flexibility of 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.

(around 35 m2 g−1).349 However, these methods are not widely applied since their high cost, low yield, and unsatisfied quality for practical applications. On the other hand, the self-gelation behavior of SWCNTs was studied after functionalizing the nanotubes by a technique similar to the oxidization of graphite to graphite oxide. The oxidized SWCNTs (SWCNTox) were able to generate a uniform suspension in water, owing to the GO-like functional groups. Furthermore, the major interactions between SWCNTox came from hydrogen bonds involving water molecules, and viscous hydrogels could form slowly at low concentration (0.3 wt %).350 Bryning et al. reported the CNT aerogels derived from aqueous gel precursors followed by critical point drying (CPD) or lyophilization (freeze-drying; Figure 19). The obtained aerogels are light (with densities ranging from 10 to 30 mg cm−3), robust, and conductive (with electrical conductivity as high as 1 S cm−1), whose structure can be further reinforced by incorporating small amounts of PVA. However, the conductivity decreased dramatically to 10−5 S cm−1 because of the PVA incorporation. Interestingly, they also found that the freeze-drying process induced more damages on the CNT network than the CPD process and led to less conductive samples.351 Using similar strategy and taking PVA as the reinforcement for CNT aerogels, another work fabricated MWCNT aerogels by the suspension derived process with CNT content ranging from 25 to 100 wt %. The absorption capacity was found to increase with CNT content because the specific surface area considered as the critical parameter for the absorption behavior performed with this trend.352 It is well-known that the functionalization of CNTs could not only help dispersing them in various solvents, which is the principal requirement for the gelation of CNTs, but also act as cross-linkers to enhance the interactions between individual CNTs, therefore facilitating the assembly process. In addition, the integrated behavior of the assemblies may possibly change according to the unique character of the modifiers. MWCNTs grafted with hyperbranched poly(amido amine) were able to assemble in DMF with the presence of linear poly(amido

sheets.344 At last, the strong, flexible, and conductive CNT films are able to be incorporated with polymers by a simple infiltration process.345−347 The composites with integrated CNT network being the skeleton show tunable mechanical and electrical properties,307 and have found possible applications as flexible/stretchable conductors (Figure 18d−g).308,318 4.4. 3D CNT Architectures

The 3D macroporous structures based on CNTs generally exhibit large specific surface area and structural stability/ elasticity under compression, as a result of the loose spongy stacking of CNT building blocks. Unlike the ordinary interest points upon their fiber or film form, the pore structures in the 3D assemblies of CNT receive intense investigation as the greatest structural merit, rather than defects which are undesirable in CNT fibers and films. The cavities existed in the 3D architectures provide numerous interspaces for mass storage and transport while keeping the porous monoliths lightweight. In order to form and stabilize the pore structures through the overlapping of CNTs, unique fabrication process and 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 for CNT aerogels. In a typical process, the CNT gels are formed primarily, followed by supercritical/freeze-drying which removes the solvents trapped in the pores while preserving the interconnected porous structures simultaneously. Therefore, the two critical factors for the gel formation and architecture stabilization are (i) good dispersion of CNT in suspensions and (ii) strong interactions between the CNT units, which causes assembling into overlapped framework and retain the formed structure during and after removal of the liquid phase. CNT foams can be produced from the relatively accessible gelatin-SWCNT composite aerogels after the gelatin compound was thermally removed.348 Alternatively, the introduction of a foaming and drying process to the stabilized CNT suspensions may also create CNT foams with low specific surface area 7067

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improve the mechanical and electrical properties of the obtained CNT aerogels, as well as increase their surface area and porosity. The authors ascribed this to reopening of the originally blocked micropores and small mesopores in the asprepared CNT aerogels.355 In another protocol to fabricate MWCNT scaffolds, the radical initiated thermal cross-linking by benzoyl peroxide (BP) was employed through the reaction between radicals and the double bond network on the MWCNT structure. Thereafter, this method was demonstrated as a general way for constructing scaffolds with other nanocarbons 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 of CNT aerogels. The resulting SWCNT aerogels possessed a very low density of less than 0.75 mg cm−3. Most interestingly, a completely new photon-assisted photoluminescence emission at 1.3 eV was found in such SWCNT aerogels.357 As can be seen in the above depiction, although the polymeric cross-linkers would bind the CNTs together, the presence of the binders also limits the electrical and thermal properties of the CNT assemblies. To prevent the degradation on such properties which are quite important to practical applications, organic sol−gel chemistry is an effective solution to the problem. The process was first developed by Pekala,358 where the polymerization of organic precursors produced highly cross-linked organic gels which can be further dried and pyrolyzed to yield porous carbon structures. Worsley and coworkers improved such a process to make it accommodate for cross-linking carbon nano materials, and several works have been reported on CNT and graphene 3D assemblies. In a typical process, CNTs are dispersed in deionized water with resorcinol and formaldehyde with sodium carbonate as the catalyst. After reaction upon heating, the resulting gels were dried with supercritical CO2 and pyrolyzed at 1050 °C. It was the pyrolysis that transferred organic binders into carbonaceous ones and thus allowed the electrical and thermal properties to be maintained.359 Actually, the CNT loading played a critical role on the properties of the formed aerogels (Figure 21). At SWCNT loading of 55 wt %, the original size and shape of the CNT network can be retained after supercritical drying and pyrolysis, which indicated the antishrinkage effect of CNTs. Moreover, the electrical conductivity was enhanced obviously only when the SWCNT loading reached 20 wt %.360 To further extend their application, the SWCNT aerogels were coated with oxides (SiO2, SnO2, or TiO2) through sol−gel deposition. Owing to the mechanical robustness of the organic sol−gel chemistry derived aerogels, they were stable enough to suffer another sol−gel process with the oxide deposition. As a result, the composite aerogels showed no degradation on the conductivity and bore significant reinforcement on the mechanical properties. Although it is not investigated in the paper, the authors did propose the potential use of the oxide/ SWCNT aerogels as battery electrodes, sensing devices, and catalysts.361 4.4.2. Template-Directed CNT Architectures. Ice templating method is a facile and environment-friendly route to the porous network of CNTs. The ice templates can be easily removed and are controllable by adjusting some of the fabrication parameters. Kwon et al. created MWCNT cryogels with aligned and nonaligned porous structures, which were prepared through ice-templating or sol−gel gelation method, respectively. MWCNTs were first dispersed in the silk fibroin aqueous solution, nonaligned MWCNT cryogels were obtained

Figure 19. (a) Images of pristine CNT aerogels (left) and PVA incorporated composite aerogels. (b) Three pieces of the PVAreinforced CNT aerogels are able to support 100 g. (c) SEM image of a CNT aerogel reinforced in 0.5 wt % PVA solution. (d) Transmission electron microscopy (TEM) image of a pristine CNT aerogel. Reprinted with permission from ref 351. Copyright 2007 Wiley-VCH.

amine) under sonication at 20 °C. The CNT gels were formed via hydrogen bonds and performed a switchable feature between sols and gels, the sol−gel switching was easily realized by heating and ultrasonicating.353 The functionalization with poly(3-(trimethoxysilyl) prolyl methacrylate) (PTMSPMA) may provide intense chemical bonding between MWCNTs, leading to ultralight free-standing MWCNT aerogels with a density of 4 mg cm−3. The resulting MWCNT aerogels possessed a hierarchically porous structure with anisotropic and macroporous honeycomb channels and mesoporous honeycomb walls (seen in Figure 20), which were also compression recoverable and conductive, thus with the potential to be used as pressure and chemical vapor sensors.354 Similarly, ferrocenegrafted poly(p-phenyleneethynylene) (Fc-PPE) cross-linked CNTs through noncovalent interactions. It is worth noting that the thermal annealing process (350 °C in air) can further

Figure 20. SEM images of MWCNT aerogels showing a porous structure and mesoporous honeycomb walls. Reprinted with permission from ref 354. Copyright 2010 American Chemical Society. 7068

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Figure 21. SEM images of the organic sol−gel chemistry-derived CNT foams at CNT loadings of (a) 30 wt % and (b) 55 wt %. TEM micrographs of a 30 wt % CNT foam at (c) low and (d) high magnifications. Reprinted with permission from ref 359. Copyright 2009 American Institute of Physics.

Figure 22. SEM images of the morphology of MWCNT/CHI 3D monoliths with MWCNT content of (a) 66 wt %, (b) 80 wt %, (c) 85 wt %, and (d) 89 wt %. The inset in (d) shows the walls of the architecture are constructed by interconnected MWCNTs. Reprinted with permission from ref 364. Copyright 2007 American Chemical Society.

following the normal lyophilization process after the gelation of silk fibroin was completed, whereas the aligned cryogels were realized by unidirectional freezing the MWCNT-silk fibroin dispersion and drying. It is worth mentioning that the silk fibroin may act as structural binder for the aligned cryogels or compound of solid structure in the nonaligned cryogels. Therefore, the aligned porous structures showed higher thermal stability and electrical conductivity than the randomly porous structures due to preferable interconnections between MWCNTs.362 In fact, the mostly reported ice-templating method for ordered 3D CNT porous structures is named as the ice segregation induced self-assembly (ISISA) process, which was intensively studied by Monte and co-workers.363 Briefly, the ISISA process was performed by unidirectional freezing and freeze-drying of the well dispersed CNT-CHI aqueous suspension. The unidirectional freezing allowed the microchanneled structures to align well in the freezing direction with a well patterned morphology (Figure 22). It is a simple and versatile approach to achieve 3D CNT assemblies with ordered porous architectures, where the morphology of the CNT architectures is highly dependent on the CNT content and the freezing rate.364 Furthermore, the long-range microchanneled structure makes the as-prepared CNT 3D architectures affordable for various applications, especially as biocompatible scaffolds,365−369 as well as supports for catalytic nanoparticles.364 Sponges are the most widely used cleaning tools in our daily life with a hierarchical macroporous nature. Benefiting from their high porosity, sponges are strong absorbing media with significant inner surface area. That is to say, the readily available sponges provide high possibility to absorb CNT suspensions and 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 range of 100−500 μm, CNTs were coated onto the template using a simple “dipping and drying” process by immersing the sponge into CNT ink suspension several times. The relatively low mass of CNT coating (0.24 mg cm−2) not only improved the electrical conductivity of the sponge (with a sheet resistance of 1 Ω sq−1) but also maintained a hierarchical macroporous morphology with open pores. It is meaningful since the open pores are crucial for the accessibility of electrolyte in the application for energy storage.370 In another work, low cost and recyclable kitchen sponges were taken through the same procedure to make the MnO2−CNT-sponge supercapacitor electrodes yet with organic electrolyte. In comparison with aqueous electrolyte, energy density of the supercapacitors was significantly improved by several times in organic electrolyte.371 The polyurethane (PU) templated composite sponge achieved a conductance of 1 S cm−1 with a CNT coating of only 200 nm thick, and simultaneously, the coated sponge was both stretchable and compressible as the mechanical properties of the original uncoated sponge were well preserved (Figure 23). Moreover, the CNT-sponges also showed advantages as electrodes for microbial fuel cells (MFCs), showing a maximum areal power density of 1.24 W m−2 and a maximum volumetric power density of 182 W cm−3.372 The sponge templates grant CNT assemblies hierarchical macroporous structures, and conversely, the coating of CNTs usually changes the surface characters of the coated sponges. For example, improved biocompatibility was found in collagen sponges with MWCNT coatings.373,374 Besides, the enhancement on electrical conductivity of polymer scaffolds was demonstrated with very little amount of CNT coatings based on the similar principle.375,376 4.4.3. CVD-Grown CNT Sponges. The CVD path way is a feasible approach to achieve the growth of CNT and foam-like CNT architectures at one time, allowing the latter with either well-arranged or randomly arranged pores. In the case of CNT 7069

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Figure 23. (a) Schematic of the CNT-sponge. (b) SEM micrograph of the CNT-sponge. (c) A PU sponge before (left) and after (right) CNT coating. Photographs demonstrating the CNT-sponge is (d) stretchable and (e) compressible. Reprinted with permission from ref 372. Copyright 2012 Royal Society of Chemistry.

Wu et al. built centimeter-thick sponges with short and straight CNTs as building blocks, which were MWCNTs with diameters of 30−50 nm and lengths of tens to hundreds of micrometers. The as-prepared CNT sponges had a specific surface area of 300−400 m2 g−1 and average pore size around 80 nm. Interestingly, while the wet-gel derived CNT aerogels are always brittle and need reinforcement by polymer binders, the directly grown pristine CNT sponges are much more robust and flexible, revealing the strong interconnections between CNTs in a three dimensionally isotropic configuration (Figure 25).379 Later, the authors found the bulk densities of the CNT sponges could be adjusted from 5.8 to 25.5 mg cm−3 by controlling the source injection rate from 0.10 to 0.25 mL min−1. The variation made the sponges transferring from soft to hard along with the increase of density. The soft sponges showed high compressibility of up to 90% volume reduction, while the hard sponges can merely recover to 93% of their original volume after compression.380 In addition, the CVD-grown CNT sponges could be easily decorated with dopants or particles to modify their performances for different purposes. The boron-doping during CVD synthesis of CNT sponges led to the formation of atomic-scale “elbow” junctions and covalent interconnections between CNTs, thus making the materials robust and elastic.381 Meanwhile, the microwave decoration of Pt nanoparticles on an entangled CNT scaffold can be employed as a cathode for a high power proton-exchange membrane fuel cell (PEMFC).382 In a serial work, porous CNT sponges were decorated with amorphous Si383 or atomic layer V2O5,384 correspondingly, in order to be applied as anode or cathode for high performance Li-ion batteries. 4.4.4. Applications. Because of the highly conductive properties of individual CNT and the porous configuration of CNT 3D architectures which provide large specific surface area with activity, as well as pathways for possible mass transport, the CNT assemblies in 3D construction have found applications in various areas. 4.4.4.1. Supercapacitors, Lithium-Ion Batteries, and Bio Fuel Cells. Aligned MWCNT arrays grown on a metallic alloy can be used directly as electrodes for a double-layer capacitor.385 Related researches further showed that 3D

arrays/forests, CNTs are vertically aligned with their growth simultaneously yet the third dimension is in strictly limited range according to the length of individual CNTs. However, some behaviors, especially mechanical performances of the vertically grown CNT arrays are even higher than the foam-like materials, since the anisotropic structure prefers to exhibit the excellent properties of CNT along the tube axis. Cao et al. fabricated CNT arrays with supercompressible behavior. While the constructed architectures present an open-cell foam structure, the nanotube struts can be squeezed intensively by buckling and folding themselves like springs (Figure 24). As a

Figure 24. (a) SEM image of an original free-standing CNT foam (up) and a compressed foam after 1000 compression cycles with strain of 85% for each compression (bottom). (b) Magnified image shows the wavelike buckles at the bottom of the compressed foam. Reprinted with permission from ref 377. Copyright 2005 American Association for the Advancement of Science.

result, the high compressive strength, fast recovery rate, and structural stability after thousands of compression-release cycles indicated such CNT arrays a potential application as the energy-absorbing coatings.377 On the contrary to the aligned structures of CNT network, Xu et al. prepared randomly arranged CNT foams using the CVD method, with a height of 4.5 mm for the bulk material. Their CNT foams were assembled from interconnected long CNTs and exhibited invariant viscoelasticity over a wide temperature range from −196 to 1000 °C.378 In another CVD route to achieve the randomly packed CNT structures, Cao and 7070

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Figure 25. (a) Optical image, (b) cross-sectional SEM image, and (c) schematic of the CNT sponge. Photographs show (d) a stripe-like CNT sponge could endure severe twisting without breaking. (e) densified CNT sponges are able to full recovery to original shape upon ethanol absorption. 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-CHI glutaraldehyde mineralized (NTCGM) scaffold. Reprinted with permission from ref 369. Copyright 2012 Wiley-VCH. (c) and (d) Optical images showing the engine oil absorption of MWCNT sponge. (e) and (f) the sponge can be reused after burning or squeezing and tracked. Reprinted with permission from ref 381. Copyright 2012 Nature Publishing Group. The resistance change of MWCNT aerogel (g) upon exposure to chloroform vapor 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.

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pressure and chemical vapor were demonstrated with high sensitivity (Figure 26g,h).354

architectures with hierarchical structures were able to provide higher 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-coating process, and the modification of MnO2 with pseudocapacitance further improved the performance of the integrated supercapacitors.370,371,386 In particular, a remarkable specific capacitance of 1230 F g−1 was achieved at a scan rate of 1 mV s−1, based on the MnO2−CNT-sponge hybrid electrodes and applying an aqueous electrolyte.370 However, organic electrolytes permit a larger voltage window than aqueous electrolytes; therefore, the energy density of supercapacitors was reported tripled in Et4NBF4 electrolyte, and increased by six times in LiClO4 electrolyte.371 The CVD-grown CNT sponge-like structure was considered as a perfect conductive backbone for electrodes of lithium-ion batteries, either cathode or anode, depending on the active material incorporated. For instance, the Si deposited CNT sponge was applied as anode with large areal capacity,383 whereas the atomic layer deposition (ALD) V2O5 coated MWCNT sponge served as a high performance cathode.384 CNT scaffolds exhibited outstanding biocompatibility which allowed strong cell adhesion, protein adsorption, and bacteria immobilization. Both the collagen sponge template-assisted and ISISA protocol derived 3D CNT monoliths were deeply investigated in their applications for tissue engineering and cell cultivation (Figure 26a,b).365−367,369,373,374 Furthermore, their microchannelled architectures showed high suitability for the uses as electrodes for microbial fuel cells.368,372,387 4.4.4.2. Absorbers. As for environmental applications, CNT sponges revealed high sorption capacity and high sorption rate for a wide range of solvents, oils, and organics. The mass uptaken by the unique CNT based sorbent materials were usually over 100 times their own weight. After sorption, the flexible and stable structures grant CNT sponges good recylcability upon burning or squeezing (Figure 26c− f).379,381,388 In another way, filters made from CNT sponges were used to remove nanoparticles (Au and CdS) and dye molecules from water with high filter capacities, simply on the basis of a physical trapping mechanism.389,390 4.4.4.3. Energy Adsorption. Interestingly, Cao et al. developed the design of 3D CNT architectures through tailoring the combination of aligned CNT arrays and randomly entangled sponges in various configurations, such as series, parallel, package, and sandwich structures. The resulted novel structures differed in mechanical behavior depending on the arrangement of the two components, owing to their distinct deformation mechanisms. The integral behavior is a combination of the response from each component. The composite 3D architectures also performed differently from individual arrays or sponges according to the synergistic deformation process. As a result, the well controlled and easily synthesized composite 3D structures showed their advantage in energy absorption and cushioning under mechanical compression.391,392 4.4.4.4. Other Applications. The high specific surface area and porosity also makes CNT scaffolds ideal supports for nanoparticles with catalytic activities (Pt, CuO, and CdS) and thus exhibited potential applications for catalytic purposes.352,364,393 As CNTs form a conductive and compression recoverable network, electrical resistivities were found to change linearly and reversibly after 300 cycles of large-strain compression up to 60%.380 Thus, the sensing capabilities of MWCNT aerogels to

5. ASSEMBLIES OF GRAPHENE Because of the structural correlation between graphene and CNTs, it is convenient to draw on the experience of mature technologies for CNTs, which has truly promoted the rapid growth of graphene research. However, the distinct characteristics of graphene suggest requirement to explore particular strategies for graphene assemblies. Because of the much better dispersibility, stability and processability in polar solvents, especially in water, the oxidized derivative of graphene− graphene oxide (GO) has become the most popular substitute for graphene assemblies, with great convenience for wetprocessing based on homogeneous solutions. More importantly, these oxidized graphene assemblies could be reduced subsequently to recover most of their graphene-like properties. Therefore, such indirect yet easy to access synthesis path from GO to RGO has turned to a general protocol for the realization of graphene architectures. 5.1. 1D Graphene Fibers

The topic of graphene fibers has seen recent favor in the research field. Unlike the traditional strategies for continuous fibers built up from 1D rod-like molecules, such as CNTs, the about to be discussed graphene fibers are assembled by the 2D sheet-like graphene platelets. Although the currently achieved mechanical properties of the later ones are usually inferior to the CNT fibers, the relative simplicity and low cost of massive production still attracted researchers’ passion on strong, flexible, and multifunctional graphene fibers. Theoretically speaking, the 2D graphene sheets actually have higher contact area with each other than CNTs with curved outer surfaces, which means better interactions and less gaps will be performed in the assembled fibers. 5.1.1. Wet-Spun Graphene Fibers. The development of wet-spinning of graphene fibers started with a similar synthesis procedure to the preparation of SWCNT fibers proposed by Vigolo et al.,159 through injecting well-dispersed GO solutions into a coagulation bath containing PVA solution. The tensile strength of the as-prepared GO/PVA composite fibers was 240 MPa, a little higher than the value of SWCNT fibers (210 MPa) prepared previously. Moreover, the hybrid fibers combining GO and CNTs exhibited improvements on both mechanical properties and electrical conductivities, taking advantage of the so-called synergetic effect.394 The first continuous and neat graphene fibers, reported by Xu and Gao, were produced via liquid crystal-based wet-spinning.395 The wet-spinning process is regarded as a facile way which enables tailoring both structure and property of the resulting graphene fibers. The reported fiber configurations achieved by wet-spinning include solid, hollow, porous, and ribbon-like. Therefore, the accessibility and diversity of wet-spinning have made it the most common technique for graphene fibers. 5.1.1.1. Liquid Crystal Behaviors of Graphene and Graphene Oxide. The discovery and studies on the liquid crystal (LC) behaviors of graphene and its derivates has motivated the solution-based liquid crystal phase spinning of graphene fibers, since higher performances are derived from the prealignment of graphene sheets in the LC liquids. Single-layer graphene, which was exfoliated from graphite, was found to exhibit dissolution in superacid of CSA due to protonation of the graphene sheets. The mechanism of dissolution was similar 7072

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Figure 27. (a) Typical AFM image of GO sheets. Polarized-light optical microscopy (POM) observations between crossed polarizers of GO aqueous dispersions 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 liquid crystals. (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 fracture surface of GO fibers made from giant GO sheets. Reprinted with permission from ref 405. Copyright 2013 Wiley-VCH. (c) SEM image of GO fiber with a tighten knot. (d) Meters-long continuous GO fiber wound on a Teflon drum. (e) Graphene fibers woven with cotton threads. Reprinted with permission from ref 395. Copyright 2011 Nature Publishing Group. (f) GO-Ag nanowires fibers collected on a plastic drum. (g) Demonstration of RGO-Ag fibers lighting up LEDs in stretched (top) and relaxed (bottom) states. Reprinted with permission from ref 406. Copyright 2013 WileyVCH. (h) MMT-graphene fiber keeps conductive while being heated to glowing red, the inset shows the wet-spun protofibers. Reprinted with permission from ref 408. Copyright 2015 American Chemical Society.

phase transition at a concentration of 0.025 wt % (0.25 mg mL−1), and a stable nematic phase formed at 0.5 wt % (5 mg mL−1 ) (Figure 27a−c).398 Afterward, the same group demonstrated the first chiral liquid crystal phase of GO sheets in aqueous dispersions with a twist-grain-boundary phase-like feature, holding lamellar and helical structural orderings simultaneously (Figure 27d,e). In that case, the transition to a typical birefringent texture of nematic LCs showed up at a volume fraction of 0.23%.395 Then continuous and neat graphene fibers were spun from high concentrated (5.7 vol %) LC dispersions, indicating the liquid crystalline phase is promising for the fabrication of macroscopic ordering materials, especially for graphene fibers. In addition, the alignment of LCs could be utilized for more functional applications. To take one example: GO LCs were expected to induce partial orientation

to that of SWCNTs in superacids. When the concentration of graphene dispersions reached 2 mg mL−1, the isotropic/liquidcrystalline transition occurred with formation of nematic phase which is typical for discotic materials.396 With more practical significance, Kim et al. reported the liquid crystallinity of GO aqueous dispersions. The graphite source showed influence on the liquid crystal formation by varying the shapes and sizes of the dispersed GO platelets. In their work, the GO sheets with average aspect ratio of 1600 completed nematic phase formation at 0.53 wt %, whereas the smaller GO sheets with aspect ratio of 700 kept intermediate phase until 0.75 wt %. Moreover, the alignment of the GO liquid crystals were found to be induced by mechanical shear or magnetic field.397 Similar results were disclosed by Gao’s group, of which the welldispersed GO aqueous solutions showed the isotropic−nematic 7073

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Figure 29. (a) Schematic of a typical process for making nacre-mimetic graphene fibers: (i) synthesis of sandwich-like building blocks by grafting polymers (hyperbranched polyglycerol (HPG) for the current example) on graphene platelets. (ii) Prealignment of the building blocks in the LC spinning dope. (iii) Wet-spinning of continuous fibers possessing “brick-and-motar” architecture. (b) AFM image of the HPG grafted graphene sheets. Reprinted with permission from ref 409. Copyright 2012 Nature Publishing Group. (c) and (d) Cross-sectional SEM images of chemically reduced graphene (CRG)/PVA fiber. The inset in (d) shows the small-angle X-ray scattering (SAXS) 2D pattern of the fiber. (e) A reel of kilometers-long CRG/PVA fiber. Reprinted with permission from ref 413. Copyright 2013 Royal Society of Chemistry. (f) Tensile properties of neat GO 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% sulfuric acid for 1 month and 3 min, respectively. Reprinted with permission from ref 411. Copyright 2013 Nature Publishing Group.

binders, one of the feasible ways to solve this problem from the root was choosing large size graphene sheets as the building blocks, whose viability has been confirmed by a number of studies.403−407 In such fibers made up from large flake graphene, the stress transfer efficiency among graphene sheets was evidently improved, mainly coming from their increased face-to-face contact area. Besides, the lessening of sheet boundaries helped promoting conductive properties at the same time.406,407 For example, Xu et al. used giant GO sheets which were 18.5 μm in average lateral size to fabricate macroscopic fibers, combining wet drawing and ion crosslinking to further increase the intersheet interactions. After chemical reduction, the fibers showed a record tensile strength of 501.5 MPa for polymer free graphene fibers.405 Afterward, Ag nanowires were incorporated to attain improved electrical conductivity 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 derived based on the similar wet-spinning protocol (Figure 28h).408 5.1.1.3. Nacre-Mimetic Graphene/Polymer Fibers. The nacre-mimetic composites with the so-called “brick-andmotar” layered structure are of particular interests to scientists for decades. Inspired by natural nacre with extraordinary mechanical properties, nacre-mimetic structures have been intensively studied. Different from the ordinary attentions paid on 2D films or papers in limited scale but readily accessible, Gao and co-workers have done some innovative research work on continuous nacre-mimetic fibers taking advantage of the good dispersibility and processability of graphene which satisfied the wet-spinning strategy.409−414 The biomimetic macroscopic fibers were generally constructed by the sandwich-like building blocks composed of rigid graphene platelets and grafted polymers on both sides, acting as elastic

of organic molecules for residual dipolar coupling (RDC) measurements.399 5.1.1.2. Liquid Crystal Based Wet-Spinning of Graphene Fibers. The formation of liquid crystals facilitated efficient alignment of GO in the spinning dope and the subsequently prepared fibers.400,401 The first meters-long, polymer-free GO fibers (Figure 28a−e) reported by Xu et al. showed moderate strength of 102 MPa and Young’s modulus of 5.4 GPa, however, the fracture elongations of 6.8−10.1% were greater than most of the CNT fibers. Importantly, after reduction in hydroiodic acid, the chemically reduced graphene fibers presented a high electrical conductivity (2.5 × 104 S m−1) and even higher mechanical performance (140 MPa strength and 7.7 GPa modulus) while reserving the remarkable fracture elongation (5.8%). The enhancement on fiber strength was ascribed to the increased intersheet interactions of graphene, originating from the decreased interlayer distance within the reduced fibers.395 Cong and Yu et al. obtained GO fibers from a coagulation bath of the hexadecyltrimethylammonium bromide (CTAB) solution, having comparative mechanical properties. Moreover, polymers or MWCNTs were combined within their fibers through in situ or postsynthesis strategy, which could enhance the properties of the macroscopic fibers further.402 Generally speaking, there are several aspects that significantly influence the final properties of the fabricated graphene fibers, for instance, the alignment of graphene sheets, the intersheet interactions, and defects in the fibers.403 The highly ordered alignment could be achieved by wet-drawing protocol as applied for traditional polymeric fibers,404,405 while the defects were restricted through controlling the quality of graphene and the voids in fibers. Most of all, the improvement on intersheet interactions was considered as a critical breakthrough point for achieving high performance fibers. Aside from introduction of 7074

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Figure 30. (a) Schematic of the coaxial spinning process. (b) POM image of a wet GO@CMC fiber showing the core−sheath structure. Crosssectional SEM images of (c) a GO@CMC fiber and (d) a two-ply yarn supercapacitor. Reprinted with permission from ref 416. Copyright 2014 Nature Publishing Group. (e) The core−shell structural model for porous graphene fibers. (f) and (g) Morphology of the fracture surface of reduced gaphene porous fiber (RGPF). Reprinted with permission from ref 418. Copyright 2012 American Chemical Society. (h) Setup for the spinning of graphene ribbons. (i) Surface morphology of a ribbon. Reprinted with permission from ref 420. Copyright 2013 American Chemical Society.

glue to firmly bind them together (Figure 29a−e). The liquid crystallity of graphene sheets enables that the functionalized graphene building blocks could also form the LC spinning dope at high concentrations, implying the suitability of a LC based wet-spinning process. Characterization on the resultant fibers indicated that, although the nonconductive polymers lowered the electrical conductivity in biomimetic fibers as compared with the neat graphene fibers, the nacre-mimetic fibers were universally enhanced in fiber strength and toughness due to their improved interactions between graphene sheets (Figure 29f). 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 solution based wet-spinning technique is the ready control on the morphology of cross section of the as-prepared fibers, in order to fulfill different requirements. Through a coaxial two-capillary spinning strategy, the design of graphene fibers with tunable coaxial structures becomes realizable. Zhao and Qu et al. employed a coaxial two-capillary spinneret which contained inner and outer channels to continuously spin GO hollow fibers. The as-spun fibers were even of comparable strength with solid GO fibers. Interestingly, the morphology of the prepared fibers were well controlled via changing the spinning dope passing through the two channels, for instance, by adjusting the type and ejection mode of the inner fluid during spinning, the obtained fibers will display a necklace-like structure with a string of microspheres along the axis.415 Beyond the way of making hollow fibers, Kou et al. utilized the similar form of spinneret to produce polyelectrolyte (sodium carboxymethyl cellulose (CMC)) wrapped graphene core− sheath fibers (Figure 30a−d), which were subsequently used to assemble two-ply yarn supercapacitors.416 Differently, a face-toface coaxial structure was revealed later with a graphene fiber core and a cylinder graphene sheath, by operating the wetspinning and dip-coating of GO solutions successively.417 The

simple and scalable fabrication strategy for high performance supercapacitors made a further step to accomplish the demand for wearable electronics. 5.1.1.5. Porous Fibers. Fibers having a porous structure in combination with outstanding characters of graphene provided possibilities for particular uses. As an extension of the LC based wet-spinning technique, Xu et al. chose liquid nitrogen as the coagulation bath for the concentrated GO LC dope and followed by freeze-drying. Since the lamellar structure of GO LC gels was perfectly sustained, porous fibers having aligned pores were prepared and showed unique “porous core-dense shell” morphology (Figure 30e−g). As a result, the fibers with lamellar ordering exhibited high mechanical strength and fine electrical conductivity for the reduced ones. More importantly, the porous graphene fibers had high specific surface area up to 884 m2 g−1, which is meaningful for various applications, such as energy storage and catalyst beds.418 In another way, Aboutalebi et al. made the highly porous fibers from the combination of large GO sheets, acidic GO LC dopants (pH 3) and a pure acetone bath. The resultant fibers possessed extremely high specific surface area of 2210 m2 g−1 after reduction, laying the foundation for an electrochemical capacitance as high as 409 F g−1.419 5.1.1.6. Graphene Ribbons. The fabrication of graphene ribbons was reported by Sun et al. via a simple way of placing a glass rod in the coagulation solution during wet-spinning. Morphology control could be achieved by collecting GO ribbons on the rod and drying (Figure 30h,i). It was the shear stress introduced during spinning that formed flat morphology on the ribbons rather than circular morphology. Notably, the ribbons were highly flexible, in terms of remarkably large failure strains up to 14%, in connection with the orientation of wrinkles.420 5.1.1.7. Graphene Nanoribbon Fibers. Graphene oxide nanoribbons (GONRs) derived from the oxidative unzipping of MWCNTs should have certain advantage over GO platelets in 7075

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some aspects, especially the applicability for fiber spinning, probably due to their large length/width ratio. Actually, the utilization of such ribbon-like carbons should benefit from their high aspect ratio while avoiding the curved surface as presented in CNTs and thus makes it possible to attain a dense packing and strong interconnections for the prepared fibers. Research has confirmed that both oxidized and reduced graphene nanoribbons are entirely soluble in chlorosulfonic acid and able to form a liquid crystal phase in superacid.396,421 Therefore, macroscopic fibers made up from GONRs were affordable using the similar wet-spinning apparatus. It is worth mentioning that highly aligned GONRs were observed in the obtained fibers, while subsequent thermal treatment further improved the alignment of ribbons, appearing as a considerably high Young’s modulus (36.2 GPa) in the treated GNR fibers. Thus, the readily processable graphene nanoribbons were regarded as new precursors for carbon fibers.421 5.1.2. Hydrothermally Fabricated Graphene Fibers. Except for wet-spinning, a handier method named as the dimensionally confined hydrothermal fabrication was developed by Qu et al. Briefly, such one-step fabrication procedure was operated via baking a sealed glass pipeline containing aqueous GO dispersion. The drying process caused evidently shrinkage of the fiber diameter due to water loss, leading to a compact structure. Meanwhile, the capillary-induced shear force and surface tension-induced tensile force were found to be responsible for the alignment of graphene sheets along the fiber axis. Accordingly, the tensile strength for the hydrothermally converted graphene fibers was 180 MPa, comparable to the initial values for CNT fibers and GO fibers, and further thermal treatment increased the fiber strength significantly up to 420 MPa.422 Although the continuity of the as-made fibers is not as good as that of the wet-spun fibers, the strategy is more adaptive for integrating graphene fibers with additional functionality. As a demonstration, Fe3O4 and TiO2 nanoparticles were incorporated into graphene fibers respectively in an in situ or postsynthesis manner, leading the functionalized fibers magnetic or photo responsive, respectively.422 On the other hand, if Cu wires were embedded in the glass pipeline while the assembly process and removed afterward, graphene microtubings (μGTs) with single or multichannels were afforded in meter-long scale (Figure 31). The μGTs would be selectively functionalized at specific position, either inside or outside of the wall, benefiting from the high controllability of the fabrication strategy. This is a good start for further design of smart devices, such as a self-powered micromotor, as presented by the authors.423,424 Besides the above introduced methods with intensive study for graphene fibers, there are a couple ways less common yet also reported in the literature, nevertheless, the produced graphene fibers were in limited length. For example, graphene fibers can be achieved directly through drawing CVD-grown films from volatile liquid,425,426 or fabricated by electrophoretic self-assembly at a charged graphitic tip.427 Additionally, graphene nanoribbon yarns were derived through a sequential step process, including the chemical unzipping of aligned MWCNT sheets which realized graphene oxide nanoribbon sheets and the densification process via withdrawing the sheets from the liquid-phase and drying.428 For comparison, we summarized some of the mechanical and electrical properties of the reported graphene fibers in Table 3. Reduced graphene fibers usually possess good electrical conductivities up to 104 S m−1, and these values are close to

Figure 31. (a) Photos of a spring-like μGT. (b) SEM image of a helical μGT, inset shows the twisted Cu wires used for fabrication. SEM images of the multichannel μGTs with channel number of (c) 2 and (d) 4. Reprinted with permission from ref 423. Copyright 2012 American Chemical Society.

CNT fibers. Meanwhile, the enhancement on fiber strength and conductivity through the utilization of giant graphene sheets is significant. 5.1.3. Applications. 5.1.3.1. Supercapacitors. The most promising and common applications for graphene fibers were attributed to fiber/woven fabric supercapacitors, owing to their conductivity and mechanical strength, along with the raising demands for wearable energy storage devices. In the related studies, specific capacitance was evaluated by gravimetric, length, areal, or volumetric capacitance. No matter which kind of measurement was employed, the advantage of graphene architectures was apparently revealed. Huang et al. produced all-solid-state fiber supercapacitors from wet-spun graphene fibers, which combined fine specific capancitance of 3.3 mF cm−2 with good flexibility and cycling stability. Decoration of polyaniline endowed the fiber capacitor with additional pseudocapacitance, demonstrating a high capacitance of 66.6 mF cm−2.429 The highly porous rGO fibers reported by Aboutalebi et al. revealed gravimetric specific capacitance of 409 F g−1 at a scan rate of 1 A g−1, benefiting from the open channels which offered pathways for ion transport.419 The twoply yarn supercapacitors assembled from coaxial wet-spun fibers with CMC shell and nano carbon core showed impressive capacitance and energy density of 177 mF cm−2 and 3.84 μWh cm−2, respectively, using solid electrolyte and 269 mF cm−2 and 5.91 μWh cm−2 using liquid electrolyte (Figure 32a-e).416 Later, the capacitance value was surpassed (205 mF cm−2) by another coaxial design with a graphene fiber core and a coated graphene sheath, due to the decreased solution resistance between the face-to-face electrodes.417 Alternatively, the two ply fiber supercapacitors could be assembled asymmetrically with two different graphene fiber-based electrodes. For example, with MnO2 decorated graphene fiber and graphene-CNT hybrid fiber acting as the two electrodes, the asmmetric device exhibited high energy density up to 11.9 μWh cm−2 (equivalent to 11.9 mWh cm−3).430 5.1.3.2. Actuators. A graphene-based actuator is known as a stimulus-responsive system which normally performs mechanical movement in response to environmental stimulation, 7076

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Table 3. Mechanical and Electrical Properties of Graphene Fibers ref

material

tensile strength (MPa)

394 395

GO-PVA GO RGO GO RGO RGO GO Reduced giant GO Reduced giant GO +divalent ions cross-linking Reduced giant GO-Ag NW GO-HPG Giant GO-HPG-GA RGO-PVA GO-PAN GONR

240 102 140 145 182 420 442 ± 18 360.1 ± 12.7 501.5 305 125 ± 10 652 199 452 ± 24 378 ± 5.0

402 422 407 403 405 406 409 410 413 412 421

Young’s modulus (GPa) 5.4 7.7 4.2 8.7 22.6 ± 1.9 12.8 ± 0.8 11.2 8.2 ± 2.2 20.9 17.1 8.31 ± 0.56 36.2 ± 3.8

fracture strain

electrical conductivity (S m−1)

5−6% ∼6.8−10.1% ∼5.8% 4.0% ∼3.1% 3−6% ∼3.5% ∼2.5% 6.7% 5.5% ∼3.7% 4.1%

3.2 × 104 4.1 × 104 9.3 × 104 2.4 × 10−1 5261 (HI-AcOH reduced)

5.44 ± 0.34% 1.10 ± 0.13%

2.85 × 102 (1500 °C heat treatment)

∼2.5 × 104 ∼3.5 × 103 1 × 103

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 YSCs connected 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 with permission from ref 416. Copyright 2014 Nature Publishing Group. (f) Schematic for the rotation of twisted GO fiber (TGF) driven by moisture changes. 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 the conductive electric circle. Reprinted with permission from ref 435. Copyright 2014 Wiley-VCH.

efforts on such research field with several fascinating results achieved.424,431,432 An electrochemical fiber actuator was first developed based on a bilayer graphene fiber/polypyrrole asymmetric structure, which can be further utilized for the fabrication of multiarmed tweezers and net actuators.433 Later, using positioned laser reduction on one side of a GO fiber, the graphene/GO fiber became an actuator acting responsive

making it affordable for diverse applications ranging from robots and sensors to memory chips.431 In essence, it is an energy conversion process in which the applied energies, such as electrical, chemical, and thermal energies are converted to mechanical energy. The unique attributes of graphene fibers in combination with their flexibility and robustness enable a novel type of actuator in fiber form. Qu’s group has contributed many 7077

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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) topview and (f) side-view. Reprinted with permission from ref 440. Copyright 2008 Wiley-VCH.

controllable during the fabrication, resulting in graphene films with varied thickness, dimension, and structure. 5.2.1.1. Vacuum-Assisted Filtration. Filtration is the mostly used approach for paper like GO, graphene, and graphene based composite films due to its easy manipulation.83 The assembly of graphene sheets during the filtration process follows a semiordered accumulation mechanism that the 2D sheets first form loosely aggregated, semiordered lamellae and subsequently endure contraction into the final highly oriented structure through the removal of solvent.436 In contrast with other carbonaceous materials, graphene platelets are much easier to generate free-standing or self-standing films thanks to their 2D sheet-like geometry and strong interlayer connections, accompanied by regular stacking. Correspondingly, the strong film-forming tendency of graphene also makes the filtration method consume enormous amounts of time and energy because of the blocking effect coming from the earlier packed layers 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 highest value reached ∼42 GPa, which were much higher than those reported for CNT based bucky paper (Figure 33a−c).437 The GO paper was then reinforced with divalent ions of Mg2+ and Ca2+ (less than 1 wt %) which resulted in enhancement on mechanical stiffness (10−200%) and fracture strength (∼50%), through chemical cross-linking between the functional groups on GO and the metal ions.438 As for pristine or chemically converted graphene (CCG) which are recognized with low solubility, the filtration method is also applicable if only the well-dispersed solutions were taken into account. Li and Wallace et al. prepared homogeneous aqueous solution of hydrazine converted graphene which was filtered into free-standing graphene paper with a shiny metallic luster (Figure 33d−f), further thermal treatment at 220 °C improved its mechanical properties, and a high electrical conductivity of 351 S cm−1 was achieved when annealed at 500 °C.439,440 Interestingly, by transferring the freshly filtered CCG filtrate cake in water immediately, a highly conductive and

movement to humidity changes.434 Recently, a novel twisting strategy was developed to afford moisture-driven rotational motor via rotary processing on a GO fiber hydrogel. The asformed helical configuration enabled reversible rotation under the alternation of humidity and thereby allows the development of humidity switches and moisture triggered generators (Figure 32f−h).435 5.2. 2D Graphene Films

A macroscopic 2D film is one of the most applicable forms of graphene due to its extraordinary availability along the lateral direction. Because of their extremely high aspect ratio (lateral size/thickness), the 2D graphene sheets tend to assemble into larger 2D structures spontaneously, like thin films, membranes, papers, and coatings with orderly packed lamellar structures. Different from the randomly packed CNT films with network structure, graphene films exhibit much regular and layered structure that endows them with superior specific optical transparency, mechanical, electrical, and thermal properties, showing great potential in practical applications. Typically, graphene and GO films have found their position in many emerging fields, such as electrodes for energy storage devices, heat spreaders, and membranes for gas separation and water desalination. Except for those ultrathin graphene films directly synthesized from CVD method, other graphene films are assembled essentially by a solution-based process learning from the wet-processing of nanoparticles and polymers, especially the close relative of graphene-CNTs. With special concern on the assembling behavior, this context will only focus on the latter indirect approach for graphene films. Here, we classify the wet processes into two categories: (1) the conventional wet methods, which are also commonly seen for other nano materials and (2) several protocols fit for graphene in particular according to its unique characters. 5.2.1. Wet Methods Assembled Graphene Films. The conventional wet methods for 2D graphene films include vacuum-assisted filtration, solution-casting, layer-by-layer assembly, bar-coating, dip-coating, spin-coating, spray-coating, electrophoretic deposition, and so on, all of which are highly 7078

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anisotropic hydrogel film was obtained via trapping the water within the film, and the gel structure was stable while it was kept wet. This is an indication that the sol−gel transition occurred at the liquid−solid interface during filtration.441 An extension work was built in order to optimize the volumetric electrochemical performance of the obtained CCG hydrogel films, and the same group increased their packing density and pore interconnectivity by capillary compression during controlled removal of the trapped volatile liquid which was previously exchanged into the gel films.442 In contrast, Zhao et al. reported the controlled chemical oxidation approach to increase the porosity of graphene films, with the purpose of providing more ion diffusion channels, for their use as graphitic electrodes for energy storage applications.443 Composite graphene films with expanded functionality can be obtained by filtration of either chemically modified graphene sheets or mixtures of graphene and guest materials. Based on such a principle, nafion, octadecylamine, and ionic liquid functionalized graphene,444−446 as well as sulfonated GO447 were assembled into films. Another strategy to modify graphene films is the direct modification on the formed films, which avoids the precipitate of graphene sheets during chemical reactions and is able to reserve the layered structure. For instance, Compton et al. modified the filtered wet GO paper with 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/PMMA composite films with different polymer loadings by filtrating GO/PVA aqueous solutions or GO/PMMA DMF dispersions, respectively. Significantly for the polymers, the filler content (GO) could reach over 50 wt %, resulting in remarkable enhancement on mechanical properties of the composite materials.449 The combination of multilayered graphene and nanofibrillated cellulose (NFC) was realized using a sonication process. In this case, the NFC served as the dispersing agent during graphene exfoliation in water. The filtered graphene/ NFC nanocomposite papers with 1.25 wt % graphene in the presence exhibited excellent tensile mechanical properties combining both high strength and high toughness.450 Besides, hybrid films of graphene and nanoparticles without any organic or polymer binder are also accessible with the filtration method. Zhao et al. obtained FeF3/graphene composite paper for lithium-ion batteries through the successive process of spontaneous assembly, filtration, and photothermal reduction.451 Zhang et al. prepared aqueous colloidal dispersion of RGO and exfoliated montmorillonite (MMT) nanoplatelets by direct reduction of GO with the presence of exfoliated MMT nanoplatelets, which was then filtered into films. The highly oriented graphene/MMT hybrid films showed excellent flexibility, electrical conductivity, and fire retardant properties as expected.452 Other plate-like nanoparticles, such as Co3O4 and MoS2, were found to be compatible with graphene as well, holding perfectly ordered layer structures in the hybrid films.453,454 5.2.1.2. Drop-Casting. Drop-casting is the method to prepare thin films with nanometer-scale-thickness by solvent evaporation of diluted polymers or nanoparticle dispersions. It enables the exploration of ultrathin graphene film in single or few 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 to prepare 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 form large pieces of films,458 the obvious disadvantages of such universal approach are the poor uniformity and repeatability restricted 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, even a suspended GO membrane over an orifice could be made through this method.459 Zhang and Li et al. achieved a single layer of CCG in an edge-to-edge manner by drop-casting the sheets on negatively charged substrates (Figure 34), and the

Figure 34. Schematic of the drop-casting process for CCG on negatively charged substrate. Reprinted with permission from ref 460. Copyright 2011 Royal Society of Chemistry.

surface properties of CCG and the electrostatic repulsion from the substrate have combined effects on flattening and selfassembly process of CCG sheets.460 By site selective deposition of GO using substrates of prepatterned monolayer of octadecylphosphonic acid, Wu et al. obtained regularly patterned GO and graphene.461 When deposited on glossy papers, the graphene films made by drop-casting could be transferred to the synthesized PU substrate by hot press.462 5.2.1.3. Spin-Coating. Spin-coating is a widely used method showing good control over morphology and microstructure of the prepared films with thickness of 10 nm or less, and noteworthy, good reproducibility. The thickness of the final film is dependent on many factors like the solid content, solution viscosity and spinning conditions. The downside of this approach is a lack of control over ordering due to the fast evaporation 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 transparent conductors after thermal annealling.463 Through wettability modulation on the substrate, Guo and co-workers conducted patterned graphene films on SiO2 and PET surface in combination with spin-coating.464 Composite films with multilayer laminate structure were affordable via the spinassisted layer-by-layer assembly of GO and poly(methyl methacrylate) (PMMA), where the two layers of substances, both in the submicrometer range, were deposited alternately. It was found that the crack propagation mechanism was altered in the free-standing biomimic GO/PMMA composite films, revealing dramatically enhanced tensile strength and defect tolerance.465 Spin-coating also possesses feasibility of incorpo7079

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Figure 35. (a) Schematic of the horizontal-dip (H-dip) coating process for GO dispersion. (b) The relationship between film thickness h and coating speed U for two gap height h0, the solid curves show the theoretical predictions. Reprinted with permission from ref 469. Copyright 2014 Royal Society of Chemistry.

rating guests into graphene films including small molecules, polymers, and nanoparticles. Based on the excellent electrical conductivity and high transparency of spin-coated graphene thin films, they have been used as transparent electrodes for ultrathin devices such as solar cells and organic light-emitting diodes.466,467 5.2.1.4. Dip-Coating. The dip-coating of graphene films consists of multistages including immersion, deposition while being pulled up, drainage of excess liquid, and evaporation. The thickness, morphology and microstructure of the films are determined by the balance of forces at the liquid−substrate interface and the solid content of GO suspension.468 A modified horizontal-dip (H-dip) coating method for thin and homogeneous GO films has been developed recently (Figure 35). By imposing precise control on the coating speed U and gap height h0, the as-prepared GO films exhibited a high packing density of GO sheets and a low surface roughness.469 5.2.1.5. Spray-Coating. Gilje et al. prepared thin layers of GO by spraying aqueous GO solution onto preheated SiO2 substrates. The spray-coating process resulted in a highly uniform deposition, better than the standard drop-casting and dip-coating techniques, from the given point of view.470 The coverage density of GO sheets was controlled by varying the concentration and the spray duration of the GO dispersion. When RGO sheets were well-dispersed in volatile ethanol medium, they were readily spray-coated onto various substrates even at room temperature, with the purpose of fabricating RGO-TCFs.471 Spraying a GO and hydrazine mixture onto a preheated substrate at a high temperature of 240 °C allowed the occurrence of deposition and reduction of graphene films simultaneously. A uniform CCG thin film was achieved for electronic applications in a short time without gaseous byproducts.472 Patterned graphene films were created facilely by applying a patterned template to the target substrate during the spray-coating process,473 which was meaningful for the fabrication of graphene based devices.474 Similar to other wet methods, composite films composed of PVA/graphene,475 CNTs/graphene,476 and MoS2/graphene477 have been obtained 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 the final films. For instance, Kim et al. developed a supersonic kinetic spray method which used the supersonic acceleration induced shearing to produce very small RGO droplets, and stretching of the RGO sheets in the formed films resulted in healing of their defects.478 Another modified method used for grpahene thin films is the electro-spray deposition (ESD),

which has already got intensive studies. In a typical process, an electric field is applied between the injection nozzle and substrate, and monodispersed fine droplets composed aerosol are created due to repulsion forces originated from charges carried in the droplets. Ju et al. deposited nitrogen-doped graphene nanoplatelets on fluorine doped SnO2 (FTO)/glass substrates by ESD, showing good electrocatalytic performances for the Co complexed redox couple.479 In order to achieve larger deposition coverage, Wang et al. exploited the superhydrophilic-assisted ESD method, with superhydrophilic treatment on the glass substrates, and the resulted coverage was reported to increase by more than 6 times.480 Notably, through combination of ESD with a continuous roll-to-roll process, Xin et al. prepared large area free-standing graphene films with high electrical 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 other wet processing strategies, its alternate and gradual assembly procedure performs precise control on the microstructure of a graphene film. Zhu et al. systematically compared the two distinct protocols of vacuum-assisted flocculation (VAF) and LBL assembly for reduced graphene/PVA composite film. Their conclusions showed that the mechanical properties were nearly the same for the separately derived films but their electrical conductivities displayed evident distinction, where it was more than 10 times higher for LBL composites than the other ones.482 The driving force for LBL assembly can be the electrostatic interactions, hydrophobic attractions, or covalent attachments. For example, negatively charged GO was directly assembled with cationic materials like cationic polyacrylamide copolymer,483 polyelectrolyte-poly(diallyldimethylammonium chloride) (PDDA),484,485 PDDA decorated Ag nanoparticles,486 and cationic molecule decorated graphene sheets.487 Zou and Kim developed an LBL assembly method for the fabrication of GO films by spreading GO suspension dropwise to the surface of a chitosan solution, which is a kind of positively charged polyelectrolyte. The obtained thin films at the air−liquid interface were of enough strength to be manipulated with tweezers and even drawn into fibers.488 Subsequently, the utilization of branched polyethylenimine (bPEI) instead of chitosan was demonstrated to extend the technique for creating thicker macrostructures displaying millimeter 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 then alternately assembled with PEI for the detection of maltose.490 7080

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Park et al. fabricated thin films constructed by oppositely charged graphene sheets of amine-functionalized graphene and partly reduced carboxylic acid functionalized GO, followed by thermal treatment to achieve the recovered and purified graphene films.491 Similarly, covalent modification on RGO sheets with negatively charged poly(acrylic acid) and positively charged poly(acrylamide) allows the operation of LBL assembly.492 As a complement, Ou et al. performed nonelectrostatic LBL assembly for PDA and GO on the surface of a silicon substrate with the help of a series of reactions between the functional groups of PDA and GO.493 5.2.1.7. Electrophoretic Deposition. When it comes to the assembly of 2D graphene films, the widely used EPD technique shows advantages of efficient thickness and uniformity control, strong coating-substrate adhesion, and large-scale accessibility over other coating methods.494 The morphology and structure of EPD graphene films are affected by several conditions, including working voltage, deposition time, solvent type, and the concentration of graphene. Ren and Cheng et al. fabricated homogeneous single-layer graphene films under the applied voltage of 100−160 V from a stable graphene suspension in isopropyl alcohol. The resulted graphene films displayed superior field-emission properties, benefiting from their high density, uniform thickness, numerous edges normal to the film surface, and good contact and adhesion with the substrate.495 RGO films also could be EPD assembled on conductive substrates, either before or after the chemical reduction upon GO nanosheets. Measurements indicated that reduction with hydrazine prior to electrophoretic deposition yielded smoother and better aligned RGO films, compared with the other ones obtained from the postdeposition reduction approach.496 The reduced EPD graphene coatings were demonstrated to perform excellent corrosion resistance for ion diffusion and oxidizing environment.497 It is worth mentioning that partial electrochemical reduction of GO was observed on the positive electrode during the EPD process,498,499 while the utilization of low voltages 99%) for organic dyes, especially for the charged dyes.551 Nair and Geim et al. reported the phenomenon in submicrometer-thick GO films that they were completely impermeable to liquids, vapors, and gases (even helium) but unimpeded permeable to H2O. The authors attributed these findings to the low frictional water flow through the 2D graphene nanocapillaries. After thermal reduction, the interlayer distance decreased, along with an evident lessening of the water permeability.552 Later, Joshi et al. studied the permeation through micrometer-thick GO membranes, which served as molecular sieves while being immersed in water. Interestingly, the GO laminates blocked all solutes with hydrated radius above 4.5 Å, and unexpected fast transport was observed for smaller ions, driven by a large capillary force.553 Hence, graphene membranes are very likely to become the next generation nanofiltration membranes for water purification. 5.2.4.4. Heat Spreaders. Graphene films are able to use as heat spreaders due to the extremely high thermal conductivity of graphene (5300 W m−1 K−1), in combination with mechanical strength and lightweight. Technically, graphene films will be graphitized after being annealed at a high temperature (>1000 °C) thus perform higher in-plane thermal conductivity than conventional heat spreading materials. Kong

As an important member in the supercapacitor family, allsolid-state flexible supercapacitors have triggered tremendous interests. With such consideration, Choi et al. used the filtered Nafion functionalized RGO paper as electrodes and solventcast Nafion membrane as electrolyte and separator. As a result, high specific capacitance (118.5 F g−1 at 1 A g−1) and rate capability (90% retention at 30 A g−1) with recyclability and durability up to 1000 cycles charging and discharging were obtained due to the steady and good interpenetrating network structures between electrode and electrolyte.444 In another work, ionic liquid functionalized graphene (IL-CMG) paper was utilized as the negative electrode and RuO2−IL-CMG composite paper as the positive electrode. The solid-state asymmetric supercapacitor could work under a maximum cell voltage up to 1.8 V, and exhibited high energy density (19.7 Wh kg−1), high power density (6.8 kW g−1), and good cycling performance over 2000 cycles even under normal, twisted, and bent states.547 With great similarities, the applications in Li-ion batteries acquire the same characters from graphene. Wang et al. studied the electrochemical properties of graphene paper electrodes, and a discharge capacity of 582 mA h g−1 with a cutoff voltage of 2.0 V was observed, indicating the graphene paper was a promising cathode material in lithium batteries.548 Filtered papers consisting of porous graphene exhibited much enhanced Li-ion storage capacity and transport properties than conventional graphene papers, while possessing comparable electrical conductivity and ductility. These features relying on the porosity on the basal plane of graphene sheets make the filmlike materials suitable for high-performance energy storage devices.443 Co3O4/graphene composite film can be used directly as a free-standing binder-less electrodes for lithiumion batteries. Benefiting from the strong interfacial interactions between the sheet-like Co3O4 and graphene, the interfacial electron and lithium ion transport was improved and thus provided a high specific capacity of 1400 mA h g−1 at 100 mA g−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 7085

DOI: 10.1021/acs.chemrev.5b00102 Chem. Rev. 2015, 115, 7046−7117

Chemical Reviews

Review

Figure 42. (a) Temperature profiles of a reduced wet-spun graphene film heater working under different dc voltages. Real time infrared thermal images 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 A380 plane model applying a voltage of 25 V. Reprinted with permission from ref 526. Copyright 2014 American Chemical Society.

et al. prepared graphene/carbon fiber composite films with a high in-plane thermal conductivity of 977 W m−1 K−1 after thermal annealing at 1000 °C in Ar atmosphere for 1 h.554 Shen et al. obtained large-area graphene films from the evaporation induced assembly of GO suspension and subsequent graphitization at 2000 °C, which displayed a high in-plane thermal conductivity of ∼1100 W m−1 K−1 and excellent electromagnetic interface (EMI) shielding efficiency of ∼20 dB.555 By using a higher graphitization temperature of 2850 °C, Xin et al. improved both of the thermal and electrical conductivities of graphene films to a great extent, which were 1434 W m−1 K−1 and 1.83 × 105 S m−1, respectively.481 Liu et al. used the reduced wet-spinning graphene films as fastresponse electrothermal films with continuous heat dissipation, holding great promise of ultrafast deicing applications for aircrafts (Figure 42).526 5.2.4.5. Sensors. Because of their large detection area, as well as high carrier mobility and ambipolar field effect, the 2D graphene films could be used as biosensors and detectors. For example, Choi et al. obtained free-standing RGO/Nafion hybrid films, which were subsequently employed as electrochemical biosensors for organophosphate detection. The graphene-based high performance biosensor delivered a sensitivity of 10.7 nA μM−1, detection limit of 1.37 × 10−7 M, and response time of 7) caused ionization of carboxyl groups on GO sheets thus increased the ER forces, while in the acidic environment (pH