a review on graphene oxide two-dimensional macromolecules

https://doi.org/10.1007/s10118-021-2515-1

Chinese J. Polym. Sci.

A Review on Graphene Oxide Two-dimensional Macromolecules: fromSingle Molecules to Macro-assembly

Wen-Zhang Fang, Li Peng, Ying-Jun Liu, Fang Wang, Zhen Xu, and Chao Gao*

Ministry of Education Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University,Hangzhou 310027, China

Abstract Graphene oxide (GO), which consists of two-dimensional (2D) sp2 carbon hexagonal networks and oxygen-contained functional

groups, has laid the foundation of mass production and applications of graphene materials. Made by chemical oxidation of graphite, GO is highly

dispersible or even solubilized in water and polar organic solvents, which resolves the hard problem of graphene processing and opens a door to

wet-processing of graphene. Despite its defects, GO is easy to functionalize, dope, punch holes, cut into pieces, conduct chemical reduction, form

lyotropic liquid crystal, and assemble into macroscopic materials with tunable structures and properties as a living building block. GO sheet has

been viewed as a single molecule, a particle, as well as a soft polymer material. An overview on GO as a 2D macromolecule is essential for studying

its intrinsic properties and guiding the development of relevant subjects. This review mainly focuses on recent advances of GO sheets, from single

macromolecular behavior to macro-assembled graphene material properties. The first part of this review offers a brief introduction to the

synthesis of GO molecules. Then the chemical structure and physical properties of GO are presented, as well as its polarity in solvent and rheology

behavior. Several key parameters governing the ultimate stability of GO colloidal behavior, including size, pH and the presence of cation in

aqueous dispersions, are highlighted. Furthermore, the discovery of GO liquid crystal and functionalization of GO molecules have built solid new

foundations of preparing highly ordered, architecture-tunable, macro-assembled graphene materials, including 1D graphene fibers, 2D

graphene films, and 3D graphene architectures. The GO-based composites are also viewed and the interactions between these target materials

and GO are carefully discussed. Finally, an outlook is provided in this field, where GO is regarded as macromolecules, pointing out the challenges

and opportunities that exist in the field. We hope that this review will be beneficial to the understanding of GO in terms of chemical structure,

molecular properties, macro-assembly and potential applications, and encourage further development to extend its investigations from basic

research to practical applications.

Keywords Graphene; 2D macromolecules; Macro-assembly

Citation: Fang, W. Z.; Peng, L.; Liu, Y. J.; Wang, F.; Xu, Z.; Gao, C. A review on graphene oxide two-dimensional macromolecules: from single moleculesto macro-assembly. Chinese J. Polym. Sci. https://doi.org/10.1007/s10118-021-2515-1

1. INTRODUCTION

Graphene is a single layer two-dimensional (2D) matter with str-ong covalently bonded sp2 carbon hexagonal networks. Basedon its remarkable properties such as high carrier mobility, highthermal conductivity, outstanding mechanical performance, andunique optical and magnetic properties, graphene has emerged

* Corresponding author, E-mail: [email protected]

Invited reviewReceived August 28, 2020; Accepted October 5, 2020; Published onlineNovember 26, 2020

1. Introduction……………………………………………………… 12. Preparation of GO………………………………………………… 2 2.1 The Development of GO Fabrication Methods……………… 2 2.2 Size of GO sheets……………………………………………… 33. Molecular Properties of Graphene Oxide……………………… 5 3.1 Chemical Structure………………………………………… 5 3.2 Polarity and Solvation……………………………………… 6 3.2.1 Conformational phase map of 2D GO macromolecule… 8 3.2.2 pH-dependent behavior………………………………… 10 3.2.3 Interlayer interactions by cations………………………… 11 3.3 Viscosity and Rheology Behavior…………………………… 12 3.4 Reduction…………………………………………………… 144. Atomic and Molecular Functionalization of GO………………… 16 4.1 Atomic Chemical Doping…………………………………… 16 4.2 Molecular Modification……………………………………… 185. Liquid Crystals of GO……………………………………………… 18

5.1 Introduction to GO Liquid Crystals…………………………… 18 5.2 Key Parameters……………………………………………… 20 5.3 Applications………………………………………………… 216. Macroscopic Assembly of GO…………………………………… 22 6.1 1D Graphene Fibers………………………………………… 22 6.1.1 Mechanical strength and flexibility…………………… 22 6.1.2 Electrical and thermal transport……………………… 25 6.2 2D Graphene Films…………………………………………… 26 6.3 3D Graphene Architectures…………………………………… 277. GO-based Composites………………………………………… 29 7.1 Incorporation with Organics……………………………… 30 7.2 Growth of Metal, Metal Oxide/Hydroxide………………… 338. Conclusions and Outlook……………………………………… 34

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as a promising material for effective photovoltaic, high-perform-ance energy storage system, durable and flexible conductiveelectrodes, micro- and macro-mechanics, sensitive chemicalsensors and photodetector, memory devices, twistronics, andcarbon-based next-generation electronic devices.[1−6]

Typically made from chemical exfoliation of graphite,graphene oxide (GO) possesses similar cyclohexane-like net-work of carbon units in chair configuration like graphene,however, with carbon hexagonal networks decorated by oxy-gen functional groups.[7,8] Each 2D carbon layer of GO mo-lecule consists of sp3- and sp2-bonded carbon atoms. As a 2Dtopology polymer, single molecule of GO is easy to be detec-ted and viewed by optical microscopy due to its large lateralsize up to tens to hundreds of microns compared to conven-tional polymers, and it is facile to form nematic and lamellarliquid crystals (LCs) in water and polar organic solvents, re-lated to the functional groups on the surface and its high as-pect ratio.[9,10] Therefore, GO appears to be a very importantmaterial apart from a precursor of graphene, because of itshigh dispersibility, tunable functionalization and its easy ac-cessibility to graphene.

Generally, GO can be regarded as either a particle, a mo-lecule or a soft polymer material. The domain size, pH andionic strength are key parameters determining the colloidalbehavior of GO dispersion, which are crucial for guiding thefabricating process of GO-based materials in various applica-tions. Furthermore, atomic or molecular functionalization isan efficient way to achieve the best performance out ofgraphene and GO by modifying its intrinsic properties andsurface chemistry. Probing and quantifying the chemical het-eroatom doping and molecular interactions between GO andthe target materials (chemical bonding or physical interac-tion) are crucial for improving performances of graphene-based materials.

Numerous papers have been published to understand thestructure and properties of graphene-based materials and ex-plore their applications. Graphene architectures from 0D to3D including 0D graphene quantum dots (QDs), 1D graphenefibers, 2D graphene films or membranes, and 3D grapheneaerogels, foams or fabrics levels have been established. Thesegraphene architectures, as well as their composite materials,possess superior mechanical strength, high electrical trans-port efficiency, excellent thermal conductivity and unique op-tical and magnetic properties, etc.

Here, we present a review on GO 2D macromolecules, fromsingle molecular properties to their mass-production scaleapplications. The development of GO molecular synthesis isdiscussed, with an emphasis on the size distribution of GOsheets by tuning oxidation conditions. This review articlehighlights the molecular properties of GO sheets, includingcolloidal behavior in solvents, viscosity and rheology behavi-ors, as well as the related affecting factors (e.g., size, pH andcation interaction). Atomic and molecular functionalization ofGO sheets can be used to improve the intrinsic properties ofgraphene and realize designed characters. We then presentsome examples of macroscopic assembled GO materials, suchas fibers, films, foams, and fabrics. GO-based composite ma-terials are also presented in details. We discuss in-depthgraphene fibers and their mechanical properties and func-

tional applications, which is one of the most promising can-didates for carbon-based materials. We hope that this re-view can give new views of GO from the point of 2D macro-molecules.

2. PREPARATION OF GO

2.1 The Development of GO Fabrication MethodsThe development of scalable and low-cost production of high-quality GO is crucial to its commercialization and large-scaleapplications. Presently, the most common chemical methodo-logy for the preparation of GO is based on strong oxidantsunder acidic conditions. The preparation of GO by chemicalexfoliation usually consists of three key steps: the oxidation ofnatural graphite, the exfoliation of graphite oxide by sonicationto obtain individual sheets, and centrifugation to achievepurified GO. Chemical oxidation of graphite into GO sheetscreates functional oxygen-containing groups at the surface suchas hydroxyl, epoxy, carbonyl and carboxyl groups. Supersonic orthermal treatment promotes the exfoliation of GO sheets whichare previously held together by van der Waals force.[11]

Brodie firstly reported the synthesis of GO starting fromgraphite, potassium chlorate and fuming nitric acid.[12] Thisapproach requires 4 cycles of oxidation process, washing anddrying of graphite, and leads to acid fog evolution from thefuming HNO3. Then the optimized Staudenmaier-Hofmann-Hamdi method was promoted, where potassium chlorate isadded to the mixture of concentrated sulfuric acid, nitric acidand graphite.[13] However, this method was still limited by thelong reaction time and the release of ClO2 gas from the reac-tion of chlorate and strong acid. In 1958, Hummers andOffeman oxidized graphite with a mixture of concentratedsulfuric acid, sodium nitrate and potassium permanganate,which is regarded as the most wildly used method for prepar-ing GO so far.[8] The GO synthesized by Hummers’ methodshows a bright yellow color, and has a carbon to oxygenatomic ratio between 2.1 and 2.9. The use of H2SO4 andKMnO4 ensures the complete oxidation of graphite in severalhours, and the replacement of HNO3 with NaNO3 prevents theacid fog generation. However, Hummers’ method is not an ul-timate choice, for it cannot eliminate the NO2/N2O4 toxicgases evolution, and the removal of Na+ and NO3

– remains tobe solved. Chen et al. found that GO synthesized without us-ing NaNO3 in Hummers’ method is nearly the same to thatprepared by conventional Hummers’ method, without any ef-fects of the yield or oxidation degree of GO.[14]

Benzoyl peroxide (BPO) is a strong oxidizer and it can re-duce the oxidation time to 10 min while reacting with graph-ite for the preparation of GO.[15] No additional solvent is re-quired due to the low melting point of BPO and it can be usedas the solvent directly at the reaction temperature of 110 °C.The intercalated BPO in graphite sheets decomposed uponheating and subsequently released volatile gaseous species,which promoted the exfoliation of graphite and the forma-tion of GO.

A complete conversion of graphite to GO and eliminatingthe separation process of GO and unoxidized graphite are re-quired in industrial production. Because the degree of oxidiz-ation increases faster for small graphite flakes than that forlarge flakes,[16] the filtration of graphite powder to similar

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flake size is beneficial to the complete oxidization of graphitein a certain time. During the chemical oxidation, the obtainedGO products are largely affected by the degree of oxidationand chemical impurities coming from the source regents andthe reactions in the multistep process. A new electrochemicaloxidative approach provides an alternative choice for the syn-thesis of GO, and it prevents the use of toxic chemicals and isfree from complicated steps. It shows advantages of low cost,easy operation, environmental friendliness and controllableproducts. Oxidation and exfoliation of graphite take placesimultaneously in an electrochemical cell containing 0.2mol/L sodium citrate, resulting in GO with carbon to oxygenratio of 7.6.[17] The degree of oxidation of the produced GOcan be controlled by adjusting the electrochemical paramet-ers of the electrochemical process. Recently, Gao’s group es-tablished a high-efficiency strategy to convert the polyimide-pyrolyzed industrial carbon wastes into graphene QDs withtunable size through the electrochemical method, whichprovides a new way for the industrial production of grapheneQDs.[18]

To avoid the use of aggressive concentrated acids, a drymixture of graphite and an oxidant KMnO4 or (NH4)2S2O8 wasmechanochemically treated by planetary ball milling to getGO with different oxidations degrees.[19] This process viasolvent-free method leads to the decrease of graphene sizecompared to the parent graphite caused by the smearing ef-fect of the graphite layers and the oxidant particles. In addi-tion, it is reported that even chemical-free preparation ofgraphene oxide can be realized by this dry planetary ballmilling process from natural graphite.[20] The atomic concen-tration ratio of C/O can be controlled by the ball milling timeduration.

Significantly, monolayer GO, with the first and the only IG-CC (International Graphene Product Certificate Center) certi-ficate, has been manufactured in 10-ton scale by GaoxiTech.The reagent-level products with different lateral sizes havebeen widely sold in more than ten counties and regions forresearch and development, which will pave the way to large-scale applications of GO in various industrial and civilianareas.

2.2 Size of GO SheetsThe GO sheets are usually cut into pieces with wide sizedistribution from tens of nanometers to several micrometersafter the oxidation and sonication treatment in the preparationprocess. The inherent properties of GO vary a lot with its size. Ingeneral, lager-size GO (LGO) is believed to be highly desirablefor electrical, thermal and mechanical behaviors of the finalassembled graphene-based materials because of the highaspect ratio,[21] while molecular-size graphene sheets contributeto the formation of biocompatible functionalized surface in theapplications of biosensing and drug delivery.[22] Therefore, thecontrollable size fractionation of GO is highly desirable for itsapplications.

The factors that affect the synthesis of LGO include thetreatment of parent graphite, oxidation conditions, centrifu-gation process and pH value.[23−25] Generally, the breaking ofGO sheets occurs unavoidably in the vigorous oxidation andexfoliation processes. The size of obtained GO flakes is usu-ally limited to tens of micrometers regardless of the size of

parent graphite if the oxidation and exfoliation processes arenot well controlled. By modifying the oxidation and exfoli-ation in the Hummers’ method, the synthesis of LGO can berealized by choosing large-size parent graphite, mild oxida-tion condition, and low centrifugation speed in the oxidationprocess (Fig. 1a).[24]

Enhanced oxidation such as high temperature and extraamount of oxidant means increased hydroxyl and epoxidegroups in the carbon networks of GO. These oxygen-contain-ing groups increase the interlayer space and weaken the vander Waals interactions between GO sheets, which makes GOsheets easy to cleave and results in the decrease of the size ofGO sheets. Therefore, the reaction conditions should be de-signed less vigorous to minimize the cracking of GO sheets.By utilizing a density-functional theory (DFT) approach, Liet al. proposed GO unzipping model initiated by the forma-tion of epoxy groups on the surface of graphite oxide.[26]

Once two epoxides are formed on the opposite ends of a car-bon hexagon, more epoxide rings along a straight line arepromoted to generate, thus generating strain by the cooper-ative alignment of epoxy groups and yielding the cracking ofGO sheets (Figs. 1b and 1c). Pan et al. ascribed the crackingsize of GO sheets to the balance between the crack formationenergy, the elastic strain energy due to both the hydroxyl andepoxy sites, and the interaction energy between graphenelayers.[25] LGO sheets are maintained only if the core of thegraphite is fully oxidized by the penetrating solution beforethe crack reaches the middle of the particle. In order to pre-vent the excess usage of oxidants and multiple oxidation pro-cesses, expanded graphite (EG) was demonstrated to be be-neficial to the preparation of LGO at low oxidant dosage, be-cause of its high solvent-accessible surface areas.[27]

In addition, strong ultrasonic treatment is not suggestedfor the synthesis of LGO, because the sonochemical effectsand generated ultrahot gas bubbles cause breaks of C―C andC―O―C bonds and the cracking of GO sheets. The size of GOsheets decreased with the increase of sonication time.[28] Qiet al. found that the post-ultrasonication treatment of LGOalso results in the decrease of the size, which has a Gaussiandistribution depending on the ultrasonication time.[29] Thehigh temperature, rapid cooling and high pressure providedby ultrasonication treatment ensure the breakage and exfoli-ation GO sheets, as well as the enhanced oxidation degree ofGO sheets. Furthermore, GO sheet with narrow size distribu-tion can be efficiently controlled by varying the centrifuga-tion speed.[30,31] Small-size GO (SGO) with a size of severalhundreds of nanometers is obtained in the GO supernatant ata high centrifugation speed. LGO remains in the precipitate atthe bottom of the centrifuge tube, and it can be collected byrepeated centrifugation-dispersion steps.

Because the properties and functions of GO sheets greatlydepend on their lateral size and size distribution, it is import-ant to control the size of GO sheets in the preparation pro-cess and develop the methods for size fractionation of GO.The size fractionation can be realized by pH adjustment,[23]

controlled directional freezing method,[32,33] centrifugationtechnique,[30,31] filtration treatment through membranes,[34]

polar-solvent-selective natural deposition method,[35] non-gelelectrophoresis technique,[36] circular-flow-assisted fractiona-

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tion,[37] etc.Pristine GO sheets were mainly stabilized by the polar and

charged ionized functional groups through electrostatic re-pulsion, which prevent the aggregation in aqueous medium.Here, the size fractionation of GO can be realized by adjust-ing the pH value of GO aqueous solutions.[23] On the onehand, SGO sheets have a higher solubility than LGO becauseof the higher density of ionized carboxyl groups of the SGOsheets than the large ones as a result of more edges. On theother hand, the solubility of GO sheets in water decreaseswith the decrease of pH value because the electrostatic repul-sion force is weakened by the protonation of the ionizedcarboxyl groups. Therefore, the GO colloidal dispersion canbe separated into two portions by adjusting the pH to a prop-er value among the “pH window” (3.34−4.24) (Fig. 1d). Wanget al. showed the efficient separation of this method, with GO

sheets mostly larger than 40 μm and smaller than 50 μm intwo parts with narrow size distribution (Fig. 1e).[23]

A controlled directional freezing method of GO aqueousdispersions was developed for the size fractionation of GOnanosheets both experimentally and theoretically.[32,33] In thiscase, GO sheets are adsorbed on the surface of ice crystals viahydrogen bond interactions, and then stratification of GOsheet occurs at the ice growth front. Therefore, the size distri-bution of GO was controlled by the growth rate of the freez-ing front. Besides, Chen et al. reported a cheap and efficientfiltration method based on the exclusive passing throughtrack-etched filter membrane with a given pore size.[34] Fur-thermore, based on the different dispersibility and stability ofGO with various sizes in polar solvent, the separation of varis-ized GO sheets can also be obtained.[35]

Quantum confinement occurs when decreasing the GO sp2

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Fig. 1 (a) Scanning electron microscopy (SEM) image of the LGO sheet. (Reprinted with permission from Ref. [24]; Copyright (2010) AmericanChemical Society). (b) A planar view of partially oxidized graphene sheets containing epoxy lines, and (c) the undulation and crack formation onGO sheets. (Reprinted with permission from Ref. [25]; Copyright (2011) American Chemical Society). (d) Zeta (ζ) potential of large (f1) or small (f2)lateral dimension aqueous dispersion as a function of pH. (e) GO size distribution and Gaussian fit curve of crude GO (cGO), f1 and f2 portions.(Reprinted with permission from Ref. [23]; Copyright (2011) American Chemical Society). (f) High-resolution transmission electron microscopy(TEM) image of GOQDs and corresponding 2D FFT images. (g) PL spectra of GQDs (0.01 mg/mL in an ethanol/H2O mixture) and GOQDs (0.1mg/mL in water). Inset: digital images indicating blue and green PL emissions of GQDs and GOQDs, respectively. (Reprinted with permission fromRef. [38]; Copyright (2013) Wiley). (h) High-resolution TEM image of nitrogen-doped GO quantum dots. (Reprinted with permission from Ref. [40];Copyright (2014) Wiley).


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domain size to less than 10 nm. Based on the moderate pho-toluminescence (PL) signal, non-toxicity, cell permeability,and tunable band gap, GO QDs extend their application incellular imaging, biosensors, drug delivery, catalyst, photo-and electronic-related fields. The nano-size graphene can befabricated by various physical and chemical methods, as sum-marized in the article by Liu et al.[38] They employed graphitenanoparticles with a diameter of 4 nm as starting materials toprepare graphene oxide quantum dots (GOQDs) and re-duced GOQDs (rGOQDs). GOQDs and rGOQDs possess greenand blue luminescence, due to defect states with oxygenousfunctional groups and intrinsic states in the high-crystallinestructure, respectively (Figs. 1f and 1g).[38] The photolumines-cent GOQDs linked with DNA aptamer can capture targetPb2+ for trace lead detection associated with the fluores-cence quenching of the GOQD caused by the electron trans-fer from GOQD to Pb2+ upon UV irradiation.[39] Furthermore,the doping of nitrogen atoms in the GOQD (NGOQD) framechanges the p-conductivity type to the n-conductivity type ofNGOQD, which makes partial nitrogen-doped GOQDs p-ntype photochemical diodes. The created band gap of theNGOQDs was measured to be approximately 2.2 eV, a typicalvalue for semiconductor, and was capable of absorbing vis-ible light to generate excitons. Yeh et al. showed thatNGOQDs presents stable overall water-splitting performanceunder visible light illumination, with p-type and n-type do-mains (Fig. 1h) at the semiconductor-water interfaces func-tioning as electron injection to produce H2 and hole injectionto produce O2, respectively.[40]

3. MOLECULAR PROPERTIES OF GRAPHENEOXIDE

3.1 Chemical StructureGO can be regarded as a kind of 2D macromolecule whichconsists of graphene-like carbon panel and oxygen functio-nalized groups (Fig. 2). The oxygen-containing groups arecovalently bonded to carbon frameworks and convert themfrom sp2-hybridized state to sp3-hybridized state. Graphitepresents perfect carbon hexagonal networks on a 2D atom-thinplane. After the aggressive oxidation and sheet exfoliation, GOsheets are obtained with oxygen functional groups and holes, asevidenced by TEM images (Figs. 3a–3c).[41] The oxygenfunctional groups consist of hydroxyl and epoxide species onthe carbon planes, and carbonyl, carboxyl, and lactol groups atthe defects, holes, and edges. Among them, carboxyl and

carbonyl groups, which are formed via the intense oxidationcleavage of C―C bonds, are usually accompanied by theformation of structural defects. C―O related functional groupscan be removed upon chemical reduction, leading to arecovered graphitic structure. The functional groups of GOsheets can be controlled by tuning the oxidation conditions inthe preparation processes. For instance, Chen et al. reportedthat the GO fabricated by an optimized recipe by skipping theaddition of water during the oxidation steps possessed a lowerC＝O defect region compared to the GO sample prepared bymodified Hummer’s method, and it also had a higher C/Oratio and lower fraction of un-oxidized C＝C skeletoncompared to the GO obtained from a low-temperature reaction(~5 °C).[42] GO sheets treated with concentrated nitric acidsolution possess nanopores with diameters from several tohundreds of nanometers. It was found that a large number ofcarboxyl groups were located at the edges of the nanopores.[43]

GO presents holes and defects compared to a perfectgraphene sheet, which provides GO with special intrinsicproperties. The rich functional groups endow GO with am-phiphilic behavior and make it reactive such as easy-doping.Its high solubility, liquid crystal behavior and easy accessibil-ity compared to graphene offer GO competitive advantagesto practical applications.

C 1s X-ray photoelectron spectroscopy (XPS) spectrum ofGO molecule prepared by modified Hummer’s method can bedivided into three peak profiles, carbonyl, carboxyl, and lactolgroups centered at ~289.2 eV, hydroxyl and epoxide speciesat 286.5 eV, and nonoxidized C species at ~284.8 eV. The O 1sspectrum consists of peaks at ~532−533 eV related to hy-droxyl, epoxide and H2O species, and peaks at 529–531 eVcorresponding to carbonyls, carboxyls, and lactols (Fig. 3e).[44]

Polarized near-edge X-ray absorption fine structure (NEXAFS)provides an evaluation of electronic corrugations in graphenebased on the directionality of the graphene π cloud. Three ad-ditional spectral features centered at ~286.7, 288.7, and 289.8eV are detected between π* (~285.2 eV) and σ* (~293.2 eV)resonances along with a shoulder at ~287.5 eV (Fig. 3f).[45] Theresonances in the intermediate region (between the π* and σ*

peaks) are attributed to functional groups of GO molecules.DFT calculations also indicated a homogeneous distributionof epoxide and hydroxyl species on GO sheets, and the pres-ence of small amounts of intercalated water molecules, ethergroups, and doubly oxidized carbon species (Fig. 3g).[44] Be-cause of the presence of oxygen-containing groups and thesp3-hybridized carbon which is slightly above and below the

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Fig. 2 Schematic illustration showing the structure of GO sheet, and its promising properties.


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original graphene atoms, the monolayer GO sheet is thickerthan monolayer graphene sheet, with a thickness of ~1.0 nm.

Chemical reduction by hydrazine and subsequent anneal-ing under 550 °C leads to the reduction of oxygen functional-ities and restoration of the original sp2 bonding characterwhich has been lost in the oxidation process. However, thearea of holes of rGO increases compared to GO due to the re-lease of CO and CO2 during annealing (Fig. 3b).[41] NEXAFS re-veals the increase of the relative intensities of the π* and σ*

resonances and integrated π* intensity after chemical reduc-tion of GO, indicating its significant recovery of π-conjuga-tion.[45] Minimizing the oxidation-induced defects/holes andrestoring the structure of graphene are crucial to realizing thefull potential of graphene materials assembled by GO sheets.Therefore, we have employed an ultrahigh temperaturethermal treatment on GO sheets and found that the voids anddefects can be healed, and the obtained graphene films showan almost perfect graphite-like lattice structure in scanningtunneling microscope (STM) image (Fig. 3d).

3.2 Polarity and SolvationGO sheets consist of a basal plane which is essentially a networkof hydrophobic polyaromatic islands of unoxidized benzenerings and hydrophilic acidic groups at the edge. The polarsurface properties of GO are usually determined by the oxygen-containing groups at GO surface, which contributes to the basicand acidic character of pristine GO molecules. Therefore, GO can

be dispersed easily in polar organic solvents because of thefunctional groups on the surface, such as carboxylic acid,hydroxyl and epoxide groups.

GO is verified to be dispersed in solvents such as N,N-di-methylformamide (DMF), N-methyl-2-pyrrolidone (NMP), tetra-hydrofuran (THF), ethylene glycol and their mixture solventssuch as ethylene glycol/water mixtures with long-term stabil-ity.[46,47] Besides, GO is not stable in ethanol or isopropanoldue to the strong hydrogen bonding between GO interlayersinduced by the oxygen functional groups and its two-dimen-sional morphology.[48] The dispersion behavior of GO is re-lated to parameters such as dipole moment, surface tension,Hansen and Hildebrand solubility parameters.[49] Park et al.obtained homogeneous colloidal suspensions of rGO in awide variety of organic solvent systems, and found thatsolvents with δP+δH (polarity and hydrogen bonding cohe-sion) in a range of 13–29 show good rGO dispersion.[50] Fur-thermore, Neklyudov et al. demonstrated that the solubilityof GO and the stability of the as-formed solutions dependnot just on the solute and solvent cohesion parameters, butmainly on the strong hydrogen bonding interactions estab-lished between GO functional groups and solventmolecules.[51] By the DFT and quantum theory of atoms inmolecules (QTAIM) theoretical calculations, the hydrogenbond energy between GO functional groups and solvent mo-lecules follows the order of water>methanol>ethanol, in ac-cordance with the experimental trend.

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Fig. 3 (a–c) TEM images of a single suspended sheet of (a) graphene, (b) GO, (c) reduced and annealed GO. Holes are indicated in blue, graphiticregions are indicated in yellow, and high contrast disordered regions are indicated in red. (Reprinted with permission from Ref. [41]; Copyright(2010) Wiley). (d) STM image of rGO film after ultrahigh thermal treatment at 2800 °C for 2 h. The STM experiments were performed using a low-temperature (~77 K) STM system (UNISOKU USM-1500S) equipped with an ultrahigh vacuum (UHV). The pristine GO sheets were synthesized bymodified Hummer’s method and chemically reduced by hydroiodic acid (HI) before annealing. (e) Experimental C 1s (top panel) and O 1s (bottompanel) XPS spectra of a graphene oxide film obtained by Hummer’s oxidation of epitaxial graphene grown on SiC. (Reprinted with permissionfrom Ref. [44]; Copyright (2013) American Chemical Society). (f) Magic angle (54.7° incidence) C K-edge NEXAFS spectrum acquired for GO film.(Reprinted with permission from Ref. [45]; Copyright (2012) American Chemical Society). (g) Scheme showing selected configurations andenergies of two hydroxyl groups adsorbed on a pristine graphene layer. (Reprinted with permission from Ref. [44]; Copyright (2013) AmericanChemical Society).


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GO sheets usually appear as negatively charged entitiesdue to the acidic groups, and the water dispersity and hydro-philicity of GO have been mainly attributed to the ionizableedge ―COOH groups.[52] Therefore, GO can be regarded asan amphiphilic regent with a hydrophobic basal plane andhydrophilic edges. Kim et al. found that GO behaves like botha molecular and a colloidal surfactant. GO shows an ability toadsorb on interfaces and lower the surface or interfacial ten-sion, creating highly stable Pickering emulsions of organicsolvents like solid particles. It can also process insoluble ma-terials such as graphite and CNTs in water through π-π attrac-tions like a molecular dispersing agent (Figs. 4a–4c).[52,53] They

also employed Brewster-angle microscopy (BAM) surfaceimaging technique to investigate the enrichment of GO at li-quid/liquid interfaces.[54] No scattering points were observedat the surface of GO suspension, indicating that no GO waspresent at the interface. After chloroform was applied andevaporated, bright spots appeared in the BAM image, con-firming the enrichment of GO at liquid/liquid interfaces (Figs.4d and 4e). Similarly, BAM images also show the GO enrich-ment at liquid/air interfaces. Based on the surface activity ofGO suspension, it can be utilized for the fabrication of surfact-ant-free Langmuir-Blodgett (LB) GO monolayers, which har-vests the full potential for large-scale device fabrication. Be-

f

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Methanol Ethanol 2-Propanol THF DMF NMP n-Hexane DCM CF Toluene CB oDCB CNAcetylaceton Diethylether

Ethyleneglycol

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Fig. 4 (a) Schematic illustration showing the flotation of GO in carbonated water. Due to their surface-active property, GO sheets were trappedat the air-water interface to minimize the surface energy, and then transfer to the water surface by the rising CO2 bubbles. (b) GO sheets canpromote the formation of Pickering emulsions by stabilizing oil droplets in water via adsorbing at the oil-water interface. (c) CNTs form a colloidaldispersion in GO water (left vial) while they rapidly aggregate in DI water (right vial). (Reprinted with permission from Ref. [52]; Copyright (2010)American Chemical Society). (d, e) BAM images of the surface of GO-dispersion before and after adding chloroform, respectively. (Reprinted withpermission from Ref. [54]; Copyright (2010) Wiley). (f) The Tyndall effect of CCG shows the stability of colloidal CCG dispersion, and (g) the salteffect confirms the colloidal nature of the CCG dispersions. Adding sodium chloride into a CCG dispersion leads to immediate coagulation.(Reprinted with permission from Ref. [55]; Copyright (2008) Nature Publishing Group). (h) Digital picture of GO and rGO dispersions in varioussolvents after 2 weeks. (Reprinted with permission from Ref. [49]; Copyright (2014) Elsevier).


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sides, smaller GO sheets should be more hydrophilic due totheir higher charge density resulting from the edge ―COOHgroups, which was proved by the increasing potential of GOdispersion after sonication.

Generally, it is difficult to disperse hydrophobic graphite orgraphene sheets in water without the assistance of surfactant.However, Li et al. demonstrated that chemically convertedgraphene (CCG) can also readily disperse in aqueous solutionto generate stable graphene colloids, without the need foreither polymeric or surfactant stabilizers.[55] pH, the electro-lyte concentration, and the content of dispersed particleshave been considered in this experiment. Ammonia is addedinto the reducing solution to increase the pH to around 10, toobtain maximal charge density on the as-prepared graphenesheets. Similar to the original GO dispersion, the obtainedCCG sheets remain separated and well dispersed in theaqueous solution. The Tyndall effect test of CCG in which alaser beam passes through CCG colloidal solution shows adiscernible track as a result of light scattering, proving the sta-bility of colloidal CCG dispersion (Fig. 4f). In addition, both GOand rGO dispersion show long-term stability in different solu-tions (Fig. 4h).[49] Compared to GO, rGO presents greater in-teraction with non-polar solvents such as chloroform, tolu-ene, and chlorobenzene. Besides, metal salts and acids shouldbe completely removed because the residual electrolytesdestabilize the resulting dispersions by neutralizing thecharges on the GO sheets. Adding an electrolyte solution toCCG dispersion causes destabilization of the dispersion, andimmediate coagulation occurs if sodium chloride is added in-to a CCG dispersion (Fig. 4g). The influence of adding cationsto GO dispersion will be discussed in the next part.

Pristine graphene is generally difficult to disperse in com-mon solvents like water due to the hydrophobic interaction.Although surfactant-free single-layer graphene can be dis-persed in water efficiently by mixing graphenide solution intetrahydrofuran (THF) with degassed water and sub-sequently evaporating THF,[56] the complicated and time-con-suming process hinder the industrial application of graphene.Benefitting from the oxygen functional groups on the surface,functionalization of GO with other groups can be achieved torealize designed properties, especially the dispersal of othermaterials in organic solvents, and miscibility with polymermatrices. Recently, the solvation behavior of graphene oxidenanosheets in water is employed for probing the critical nuc-leus size for ice formation (Fig. 5a).[57] The size of GOnanosheets contained in the water droplets has a significantimpact on ice nucleation, which gives experimental evidenceon the existence of the critical ice nucleus. Koltonow et al.proposed that GO sheets in water are neither crumpled norcollapsed, with or without poor solvent.[58] Planar GO sheetscan align to form a nematic liquid crystalline phase due totheir high aspect ratios and excellent dispersity in water, asfirst reported by Xu et al.[9] and Kim et al.[10] Under poorsolvent conditions such as acetone, ethanol, 2-propanol, andethylene glycol, the GO dispersions still exhibited a strong bi-refringence under cross polarized configuration (Figs. 5b–5d).The observation through cross-polarizers of a piece of sedi-ment obtained from the slow addition of poor solvent clearlyshows that the alignment of the GO sheets in the sediment is

parallel to the wrinkles (Figs. 5e and 5f). It is suggested thatface-to-face restacking is preferred and it provides an effi-cient way for sheets to hide their surfaces from poor solvents.

In addition, Tang et al. reported the formation of GO nano-scrolls (Fig. 5g) in both aprotic solvents (pyridine and DMF)and protic solvents (methanol, ethanol, isopropyl alcohol,acetic acid, and isobutyric acid), but the yields vary with thesolvents.[59−61] The formation mainly occurs in the sedimentsafter sonication for hours. The scrolling behavior is controlledby ζ potential, dipole moment, and Hansen solubility para-meters of the solvents. Relatively large GO nanoscrolls are fa-cilitated to form in the solvents with the relatively small di-pole moment and less negative ζ potential. Besides, the sedi-mentation rate of the GO sheets from the suspension is con-trolled by the dipole moment for the aprotic solvents andHansen solubility parameters for the protic solvents, whichdetermine the average number of the included sheets perscroll.[59]

GO can maintain its morphology and structure in solventwith long-term stability. However, most dried GO powders onthe market are insoluble, or difficult to recover it original mor-phology in the solvent, indicating that the structure of GO haschanged in the drying process. Gao’s group revealed a “swell-ing-dissociation-stretching” behavior of flower-shaped driedGO powder while redispersing the GO particle in solvents ona molecular level (Figs. 5h−5j): (1) the interaction between GOlayers is weakened since water permeates into GO particles,and GO particles expand significantly; (2) the GO particleskeep on swelling until the interlayer distance is large enough,and then the highly crumpled GO layers start to peel off; (3)the single GO sheets with wrinkles and ridges graduallystretch to a plane in order to lower the surface energy.[62] Thefluctuant morphology (e.g., swelled particles, crumpled flakes,flat sheets, as well as their intermediates) of GO powder iscontrollable by adjusting solvent type or regulating agitationtime, which endows macroscopic graphene materials withdifferent properties.

3.2.1 Conformational phase map of 2D GO macromoleculeThe atomically-thick 2D GO macromolecules display richconformational phases that can be flat, wrinkled and crumpled,which in turn determines the properties of single GO molecules.Understanding the conformation of graphene is important fordesigning macroscopic assembled graphene materials. Twostrategies have been specifically developed to control theconformation of graphene, i.e. flow-induced filtration guidanceto form alignment, and post-buckling of graphene materials onprestrained or responsive substrates.[63−66] GO sheets in goodsolvents can collapse to hierarchically wrinkled conformationstriggered by poor solvents, which has been proved to be anefficient way to control the structure and performance of theirmacroscopically assembled materials. Xiao et al. tracked the real-time structural evolution of collapsing behavior in the gelationand drying process of GO film by POM and optical microscopy(Fig. 6a).[67] The stretching of wrinkles and their intertwinednetwork in collapsed GO and graphene papers account for theirrubber-like mechanical behavior.

According to Landau-Ginsburg theory of phase transitions,rich conformations were predicted including flat, crumpled,folded, compact, anisotropic tubular and globular phas-


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es.[68−70] Previous experimental investigations were limitedbecause graphite oxide with large thickness was usually takenas the model material. Recently, Wang et al. established a con-formational phase map of 2D GO macromolecules in solutionusing single-layer GO sheet as the experimental model, basedon theoretical simulation and experimental observations.[71]

The conformational phase map was summarized in the po-tential energy landscape predicted by coarse-graining mo-lecular dynamics (CGMD) simulations (Fig. 6b). In this phasemap, GO macromolecules exhibited rich conformations, in-cluding anisotropic folds (II) and isotropic crumples (III) as in-tramolecular phases which were transformed from the flat (I)phase, as well as intermolecular phases of flat stacks (IV), mul-tiple folds (V) and multiple crumples (VI). Experimental obser-

vations were carried on by tuning the intramolecular and in-termolecular interactions in three modes: (1) the self-avoid-ance mode (the introduction of poor solvents), (2) the short-range attraction mode (Ca2+-crosslinking), (3) the long-rangeattraction mode (mild chemical reduction with hydrazine). Bycombining the theory with experiments, Wang et al. foundthat self-avoidance and short-range attraction tend to induceanisotropic folding transitions, while long-range attractionleads to globular crumpling transitions. The self-avoidancemode (poor solvent introduction) and the short-range attrac-tion mode (Ca2+-crosslinking) make the flat phase of GO (la-belled as I) transit into an anisotropic fold (phase II), while thelong-range attraction mode via hydrazine reduction leads toa transit to isotropic globally-crumpled phase (phase III). At

h

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Acetone

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a

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−17.7 °C −17.8 °C−17.6 °C −17.9 °C

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−27.5 °C

f

i

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Fig. 5 (a) Optical microscopic images showing typical freezing processes of GO-containing water droplets, with average lateral sizes of 8 nm(upper row) and 11 nm (lower row). (Reprinted with permission from Ref. [57]; Copyright (2019) Nature Publishing Group). (b) Slow addition ofaqueous GO dispersion into an acetone bath. (c, d) The obtained GO dispersions exhibiting a strong birefringence when viewing through a pair ofcross-polarizers. (e, f) Optical and corresponding polarized optical microscopy images of a piece of sediment collected from the slow addition ofGO sheets into acetone. (Reprinted with permission from Ref. [58]; Copyright (2017) American Chemical Society). (g) TEM image of the GO scrollsformed from GO in the pyridine suspensions. (Reprinted with permission from Ref. [59]; Copyright (2018) American Chemical Society). (h) Digitaland (i) SEM images of flower-shaped GO powder. (j) Polarized-light optical microscopy (POM) image of flower-shaped GO dispersion. (Reprintedwith permission from Ref. [62]; Copyright (2016) American Chemical Society).


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high concentrations (above 1 mg/mL), three intermolecular ph-ases were observed, i.e. flat stacks (IV), multiple folds (V) andmultiple crumples (VI), which were induced by the self-avoid-ance mode, the short-range attraction mode and the long-range attraction mode, respectively (Fig. 6c). The conforma-tional phase map of 2D GO macromolecule offers a guidanceto precisely control multiscale condensed conformations ofother 2D macromolecules and their macroscopic materials.

3.2.2 pH-dependent behaviorThe amphiphilicity of GO can be tuned by pH via adjusting the

degree of ionization of ―COOH groups at the edge of GOsheets.[52] Molecular dynamics (MD) simulation shows that pH-dependent behavior of GO aqueous solution is related to thedegree of deprotonation of the carboxyl groups at the edgesof GO sheets.[72] GO does not behave like conventionalsurfactants in pH 1 and 14 aqueous solutions. As shown inFig. 7(a), the surface tension continues to decrease graduallywith increasing GO concentration at pH 1. GO aggregates arenon-micelle-like. It is suggested that GO sheets tend to beconcentrated at the water-air interface instead of staying in thebulk aqueous phase. At low pH, the carboxyl groups of GO

a

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Fig. 6 (a) Track of conformation change of GO film in the gelation and drying process. (Reprinted from permission from Ref. [67];Copyright (2017) American Chemical Society). (b) The conformation phase map of GO in solution accompanying with the potentialenergy landscape. GO exhibited rich conformations including anisotropic folds (II) and isotropic crumples (III) as intramolecular phasestransformed from the flat (I) phase, as well as intermolecular phases of flat stacks (IV), multiple folds (V) and multiple crumples (VI). (c) Thecorresponding AFM and SEM images of these phases. (Reprinted from permission from Ref. [71]; Copyright (2020) Elsevier).


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flakes are protonated so that the GO sheets become lesshydrophilic and form suspended aggregates, while at high pH,the deprotonated carboxyl groups are hydrophilic, makingindividual GO sheets prefer to dissolve in bulk water like aregular salt, stabilized by electrostatic repulsions between theionized carboxyl groups. Wu et al. demonstrated that both thedegree of deprotonation of the carboxyl groups at the edgesand the electrostatic repulsion between the ionized carboxylgroups are responsible for the increased stability of GO solutionat high pH (Fig. 7c).[73] As a result, GO aqueous solution isvisually homogeneous with a dark-brown color at pH 14 butpresents a precipitate which remains dispersed and stable in thesolution at pH 1 (Fig. 7a inset). Besides, Bouchard et al.demonstrated that pH did not have a notable influence on thestability of GO at pH from 4 to 10.[74] Because the pH observedfrom aquatic environment is usually 5–9, pH has minor effectson the fate and transport of GO suspensions.

As we mentioned in the previous part, rGO sheets can alsobe well dispersed in water due to its highly negativelycharged properties that result from ionization of the carboxyl-ic acid and phenolic hydroxyl groups on GO sheets. Similarly,the stabilization of rGO sheets is attributed to electrostatic re-pulsion. Therefore, as the ionization of carboxylic acid groupsis strongly related to pH, the zeta potential of the rGO disper-sion is pH dependent. Li et al. reported that the zeta potentialof CCG aqueous solution is below –30 mV when the pH isgreater than 6.1 and it reaches –43 mV when the pH ap-proaches 10 (Fig. 7b).[55] CGG solution with potential valuesmore negative than –30 mV are generally considered to yieldsufficient mutual repulsion to maintain the stability of disper-sion by electrostatic repulsion. Residual acid originated fromthe preparation of GO is harmful to GO and rGO dispersion,and it should be completely removed.

3.2.3 Interlayer interactions by cationsAdding cations to GO can cause the destabilization of GOdispersion. Immediate coagulation occurs if sodium chloridewas added into a chemically converted graphene dispersion.[55]

Yang et al. investigated colloidal behavior of GO in the prese-nce of nine different cations, and they found that the desta-bilizing ability of cations follows the order of Cr3+> >Pb2+>Cu2+>Cd2+>Ca2+>Mg2+> >Ag+>K+>Na+.[75] The destabilizing ability ofmetal cations is consistent with their adsorption affinity with

GO determined by their electronegativity and hydration shellthickness.[74] Based on the Schulze-Hardy rule, cations withhigher valence possess higher charge screening capability, andthe same valent cations should produce similar chargescreening effects. However, it is found that heavy metal cationsdestabilized GO suspension more aggressively than commoncations: Pb2+, Cu2+, Cd2+>Ca2+, Mg2+, and Ag+>K+, Na+.Therefore, Schulze-Hardy rule is not the single factor thatcontributes to the colloidal behavior of GO. Attachmentefficiency (α) and adsorption isotherms of cations, whichoriginate from hydration shell thickness of the metal cationsand electronegativity, are used to normalize the aggregationkinetics and adsorption behavior of GO respectively (Fig. 8a),both of them determining the colloidal behavior of GO sheets.GO aggregation and stability in the aquatic environment fol-lows Derjaguin-Landau-Verwey-Overbeek (DLVO) theory andSchulze-Hardy rule.[74,76]

Based on the colloidal behavior of GO affected by addingsalt, Yeh et al. found that multivalent aluminum ions facilit-ated the crosslinking of GO assembled films prepared by fil-tration using AAO, making the membranes stable and intactin water.[77] Neat GO membranes do disintegrate in water be-cause GO sheets become negatively charged on hydrationand the membrane should disintegrate due to electrostaticrepulsion (Fig. 8d). However, during filtration, Al3+ releasesfrom AAO, and efficiently crosslinks the sheets in the driedstate, especially in the dispersion with low pH value (Fig. 8e).Calcium ions are also reported as gelators to enable readilydirect writing monolithic lattices under ambient surround-ings.[78] Three types of cross-linking interactions were pro-posed by Wu et al. that may cause the aggregation of GOsheets by multivalent cations (Fig. 9a): (1) the edges of the GOsheets being bridged through chelating carboxylate groups;(2) intercalating between the GO basal planes through eitherweak alkoxide or dative bonds; (3) cross-linking hydrogenbonds being formed among the surface oxygen functionalgroups and the interlamellar water molecules.[73] Wu et al.summarized two fundamental interacting modes betweenGO sheets for their colloidal behavior in the conditions ofadding cations and tuning pH (adding H+): edge-to-edge andface-to-face modes (Fig. 9b). The aggregation of GO sheets byadding multivalent cations is induced by the strong cross-

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Protonated +H+

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c

Fig. 7 (a) Measured surface tension as a function of GO concentration at pH 14 (red dots) and pH 1 (blue dots). Photographs of a 1 mg/mL GOdispersion at pH 14 and pH 1. (Reprinted with permission from Ref. [72]; Copyright (2012) American Chemical Society). (b) Zeta potential of GOand hydrazine reduced graphene as a function of pH, in aqueous dispersions at a concentration of ~0.05 mg/mL. (Reprinted with permissionfrom Ref. [55]; Copyright (2008) Nature Publishing Group). (c) Deprotonation of the carboxyl groups at the GO edges when pH increases.(Reprinted with permission from Ref. [73]; Copyright (2013) American Chemical Society).


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linking of GO sheets, therefore, edge-to-edge is the domin-ant interaction mode of GO aggregation in this case. Whileadding H+ to GO dispersion, ―COOH groups at the edgewere protonated, leading to a decrease of electrostatic repul-sion. van der Waals interaction and the residual π-conjugateddomains in GO sheets are the main factors that contribute tothe aggregation via face-to-face mode.[73]

Recently, Gao’s group investigated the dynamic dispersionstability of GO suspension with ions, and they observed a re-dispersion behavior of GO in concentrated Fe3+ solutionrather than permanent aggregation.[79] The redispersion phe-nomenon is caused by the specifically strong electrostaticshielding effect and the coordination attraction between Fe3+

and functional oxygen groups (Figs. 9c and 9d): (1) in the ab-sence of Fe3+, GO dispersion remains stable because the re-pulsion potential energy (Er) is larger than the adsorption po-tential energy (Ea); (2) as Fe3+ was loaded on the denser layer,the zeta (ζ) potential and Er decreased, leading to the floccula-tion of GO sheets; (3) increasing Fe3+ continuously occupiesthe coordinated sites on GO sheets until the GO sheets arecovered by positive charges, leading to the increase of Er. As aresult, driven by repulsion of surface positive charges, theprecipitated GO sheets are isolated to individual sheets andredisperse in concentrated Fe3+ solution. The dynamic disper-sion stability of GO suspension with ions can be utilized forthe fabrication of graphene-metal hybrid materials.

3.3 Viscosity and Rheology BehaviorThe rheology behavior of GO dispersion is intimately related tothe characteristics of material, processing of the system, andstructure design of GO-based composites. Probing and quanti-fying the rheology behavior of GO sheets is crucial to guidingthe fabricating methods of GO-based materials, such as electro-

spraying, spray coating, filler, ink-printing techniques, etc.In this part, we report recent progress on the shear rhe-

ology of both aqueous GO dispersions and GO-based com-posite materials, as well as the external factors which affectthe rheology behavior such as pH and adding salt. We alsorefer to the review of Del Giudice et al.[80] for the shear rhe-ology of both GO and GO-based materials. The rheologicalbehavior of GO aqueous dispersion at different volume con-centrations can be classified as follows: (1) Oscillatory linearflow properties. When ϕ<ϕc (ϕc is the critical volume or massconcentration), viscous response prevails over the elastic re-sponse; when ϕ>ϕc, elastic response prevails over the viscousresponse. (2) Steady shear flow properties. When ϕ<ϕc and atPeclet number Pe = γa2/D0 < 1, GO sheets are randomly ori-ented; at Pe>1, GO sheets are oriented along the flow direc-tion. When ϕ>ϕc, GO sheets are arranged in randomly ori-ented clusters at Pe<1 and broken down at Pe>1. (3) Transi-ent shear flow properties. Only the case ϕ<ϕc is considered.The dispersion is arranged randomly oriented and breaksdown while applying a flow at Pe>1. Then GO sheets start toself-arrange when the flow is arrested (Pe=0), and recover theinitial cluster configuration after sufficient resting time.

Moreover, Naficy et al. found that the rheological behaviorof GO sheets varies considerably with dispersion concentrat-ion.[81] Ultra-large GO sheets (with a high aspect ratio of ~4.5×104) were employed as the model material to study the vis-coelastic properties of GO sheets. Four distinct regions aresummarized: viscoelastic liquid (with GO concentration:~0.05–0.25 mg/mL), in which considerable elastic compon-ent still exists, and this behavior enables electrospray andspray-coating; transition to viscoelastic soft solid (with GOconcentration: ~0.25–0.75 mg/mL), in which GO sheet disper-

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Fig. 8 (a–c) Attachment efficiency (α) of GO as a function of monovalent, divalent and trivalent cation concentration. (Reprinted withpermission from Ref. [75]; Copyright (2016) American Chemical Society). (d) The stability of GO membranes prepared by filtration usingAAO or Teflon in water. (e) Concentration of released Al from GO membranes in water as a function of pH after one day. (Reprinted withpermission from Ref. [77]; Copyright (2015) Nature Publishing Group).


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sions show biphasic behavior of both isotropic and nematicphases, making them ideal for inkjet printing; viscoelastic softsolid (with GO concentration: ~0.75–2.5 mg/mL), at which GOsheet dispersions form single-phase nematic liquid crystals,providing the possibility for processing weak gel-like materi-als such as the wet-spinning approach; viscoelastic gel (withGO concentration up to 13.3 mg/mL), where GO sheets showa gel-like behavior with high elastic modulus, suitable for pro-cessing gel-extrusion printing and dry spinning (Fig. 10a).

Tesfai et al. investigated the rheological properties and in-trinsic viscosity of diluted GO aqueous suspension (0.05mg/mL to 0.5 mg/mL).[82] The dispersions were well fitted bythe Einstein-Stokes equation because the concentration is

sufficiently low that they did not interact. Dilute GO suspen-sion exhibits a shear thinning behavior at low shear rates, andshear-independent region starts at a shear rate between 5and 100 s–1 depending on the concentration. While shearrates reach above 200 s–1, the shear-dependent viscosity ofthe GO suspension reveals a Newtonian behavior (Fig. 10b).Besides, the shear-thinning behavior is more pronounced atthe higher GO concentrations due to the stronger sheet-sheetand multi-sheet interactions with the increase in concentra-tions (Fig. 10c). Vallés et al. studied GO sheets with high con-centrations which are more relevant to bulk applications.[83]

At high concentrations of GO, the dispersions were thixo-tropic, and the structure is recovered after 60 min rest. In ad-

GO suspension

Aggregates Aggregates

Face-to-facedominant

Edge-to-edgedominant

Increasing [H +]In

crea

sing [R

2+ ]

c

d

b

x

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ϕ0

x x

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Denserlayer

c(Fe3+)=0.3 mmol/L−30 mV<ζ<0

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Ea<ErStable

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ζ

ϕ0 ϕ0

x

ϕ0

GO sheets Fe3+ Charges

Strong Weak

Weak

Bridging the edges

H-bonds with water molecules

Intercalating the basal planesM2+

a

ζ

Fig. 9 (a) GO aggregation caused by cross-linking of GO sheets by divalent cations (Mg2+ and Ca2+ were selected as examples) and (b)proposed aggregation modes of GO nanosheets. (Reprinted with permission from Ref. [73]; Copyright (2013) American ChemicalSociety). (c) Schematic showing the evolution of electric double layer (EDL) on the surface of GO sheets in Fe3+ solution and (d) thecorresponding diagram of GO-Fe3+ dispersion. (Reprinted with permission from Ref. [79]; Copyright (2020) Elsevier).


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dition, Vallés et al. demonstrated the importance of interac-tions between graphene and polymer and the dispersion ofthe flakes in the matrix in the graphene-polymer system. Theviscosities and percolation thresholds of the composite sys-tem can be tuned by changing either the surface chemistry ofthe flakes or the nature of the polymer.

The rheological behaviors of GO inks are crucial for directink writing (DIW), especially 3D printing technology. To real-ize DIW printing with GO inks, three criteria have beendemonstrated by Gao’s group: (1) the ink has to keep homo-geneous to prevent its clogging in the nozzle; (2) the elasticshear modulus of GO ink should be relatively low under highshear stress, in order to pass through the nozzle stably; (3) thestatic elastic modulus of the GO ink should be large enoughto maintain the shape of the extruded filament with depos-ited features, and defy the collapse caused by the self-weightand surface tension.[84] Jiang et al. used trace Ca2+ ions as thegelators to convert aqueous GO sol to printable gel ink, andverified the same shear-thinning non-Newtonian fluid behavi-or of both neat GO dispersion and GO hydrogel ink with addi-tion of Ca2+ ions (cCa=15×10−3 mol/L) (Fig. 10d).[84] The stor-age modulus (G′) and the yield stress (τ) also increase fromaround 3500 and 40 Pa to near 6.0×104 and 160 Pa, respect-ively (Fig. 10e). In addition, the G′ of both the neat GO solu-tion and the Ca2+-crosslinked GO hydrogel ink are approxim-ately an order of magnitude higher than the loss modulus (G″)in the range of 0.01–100 rad/s (Fig. 10f). These rheological be-

haviors of GO hydrogel inks maintain the stable flow of ink inthe extrusion process and retain the printed 3D architectureswhen leaving the nozzle.

3.4 ReductionAn ideal reduction process should fulfil the requirement of bothoxygen functional group removal and the recovery of sp2

conjugation. Until now, a variety of reduction approacheshave been developed, including chemical reduction, thermaltreatment, Joule heating, microwave, photoreduction, electroc-hemical reduction, hot press reduction, and so on.

Chemical reduction using hydrazine hydrate is a conven-tional method for synthesizing reduced graphene oxide inlarge quantities. This method is efficient to remove the oxy-gen functional groups on the surface of GO. Epoxide openingreacts with hydrazine and further reacts via the formation ofan aminoaziridine moiety, and finally forms a double bondthrough thermal elimination of diimide.[85] It is also revealedthat strong alkaline solutions contribute to fast deoxygena-tion of GO and result in stable aqueous rGO suspensions.[86]

Hydrazine reduction greatly enhances the electrical perform-ance of GO films. However, immersing GO films in hotaqueous hydrazine solutions (80 °C) usually results in filmfragmentation and delamination. Hydrazine vapor reductionprovides an alternative scheme,[87] but the thickness of hy-drazine reduced GO films expand more than 10 times, andthe films are too rigid to bend. Stiffening and disintegrationoccur after chemical reduction by hydrazine or NaBH4. Hy-

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Fig. 10 (a) Ratio of elastic and storage moduli against various GO concentrations (ultra-large GO sheet, with an aspect ratio of ~4.5×104),measured over a range of testing frequencies. (Reprinted with permission from Ref. [81]; Copyright (2014) Royal Society of Chemistry). (b) Relativeviscosity (ηr) of GO suspension versus shear rate for different GO concentrations. (Reprinted with permission from Ref. [82]; Copyright (2013)Springer). (c) Steady shear rate flow behavior of the GO dispersions at different concentrations. (Reprinted with permission from Ref. [83];Copyright (2014) Springer). (d) Rheological behaviors of the GO neat solution and GO gel ink with addition of Ca2+ ions (cCa=15×10−3 mol/L). Thestorage modulus and loss modulus as functions of (e) shear stress and (f) angle frequency. (Reprinted with permission from Ref. [84]; Copyright(2018) Wiley).


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droiodic acid (HI) provides an alternative to hydrazine hy-drate for GO reduction. The obtained rGO films reduced byHI show good flexibility and improved tensile strength. Hy-drohalic acids including HI and HBr can effectively catalyzethe ring-opening reaction of epoxy groups on GO and con-vert them into hydroxyl groups, and the halogen elementscan be easily removed from the carbon basal plane becauseof the low binding energy between a carbon and a halogenatom (Br or I).[88]

The TGA curves of pristine graphene, hydrazine reducedgraphene, thermally reduced graphene and GO are shown inFig. 11(a). The weight loss up to 150 °C is caused by the evap-oration of water, and that from 150–310 °C is due to decom-position of carboxyl groups, and that above 310 °C is mainlyattributed to carbonyl groups.[89] It suggests that after chem-ical or thermal reduction, the amount of adsorbed water de-creases and the obtained rGO is more thermally stable thanGO. XPS provides the information about the composition, de-gree of oxidation and kind of oxygen species of GO and rGO

sheets. The oxygen content of GO decreases after thermal orchemical reduction. For instance, the sp2 peak increases,while C―OH and C＝O peaks decrease after thermal reduc-tion (Figs. 11b and 11c).[89] Yang et al. investigated theO1s/C1s ratio obtained from XPS of rGO after thermal treat-ment under argon, a mixture of argon and hydrogen and ul-tra-high vacuum, and found that GO underwent a partial re-duction at temperature even as low as 200 °C.[90] C K-edgeNEXAFS spectra at magic angle (54.7°) provide the informa-tion of electronic structure upon chemical defunctionaliza-tion of GO. The collected relative intensities of the π* and σ*

resonances (Iπ*/Iσ*) enhanced upon chemical reduction of GO,indicating significant recovery of π-conjugation (Fig. 11d).[45]

Raman spectroscopy is a tool to characterize the physicaland chemical properties of graphene, such as the identifica-tion of graphene, GO and rGO, checking the number ofgraphene layers, and the investigation of graphene-basedheterostructures. D peak can be used to identify the defect ofgraphene-based materials including disorder and holes, as

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Fig. 11 (a) TGA of pristine graphene (curve1), hydrazine reduced graphene (curve 2), thermally reduced graphene (curve 3) and GO (curve 4).High-resolution XPS C 1s spectra of (b) GO and (c) thermally reduced GO. (Reprinted with permission from Ref. [89]; Copyright (2010) Wiley). (d) CK-edge NEXAFS spectra at magic angle (54.7° incidence) for eight GO and rGO films, variously reduced using NaBH4, hydrazine, andphenylhydrazine. (Reprinted with permission from Ref. [45]; Copyright (2012) American Chemical Society). (e) Polarized Raman spectra ofgraphene fiber annealed at different temperatures. (f) Crystallite size in perpendicular and parallel directions to the fiber axis after thermaltreatment at different temperatures. (Reprinted with permission from Ref. [94]; Copyright (2015) American Association for the Advancement ofScience). The (g) thermal and electrical conductivity, (h) XRD patterns and (i) Raman spectra of graphene films annealed at different temperatures(from 20 °C to 3000 °C). (Reprinted with permission from Ref. [95]; Copyright (2017) Wiley).


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well as the degree of oxidation. D band (around 1350 cm−1) isassigned to the breathing mode of κ-point phonons of A1gsymmetry. G peak (around 1582 cm−1) is the E2g phonon ofsp2 carbon atoms, and the 2D peak (around 2723 cm−1) is re-lated to the stacking order of graphene layers. In addition,other external factors such as strain, stress, temperature andmagnetic fields also show significant influence on the elec-tronic and vibration properties, which can also be shown byRaman spectroscopy.[91]

Thermal conductivity is usually determined by phonontransport in the strong covalent sp2 lattice vibrations,[92] andelectrical conductivity is dominated by the delocalized π-bond. Therefore, highly aligned sp2 graphene sheets are re-quired for superior electrical and thermal transport proper-ties, while functional groups and sp3 bonds usually act aselectron- and phonon-scattering centers, greatly hinderingthe electrical and thermal transport of graphene materials.The defects and chemical residue are the keys to determiningwhether GO is well reduced. The phonon- and electron-scat-tering centers caused by lattice vacancies and residual func-tional groups after thermal treatment at a relatively low tem-perature significantly deteriorate the thermal and electricalproperties. Besides, the chemical reduction of GO solutiontends to introduce defects, vacancies and dislocations to theobtained rGO material, as well as the functionalization of itssurface, edge, and basal plane. High-temperature thermaltreatment is beneficial to the healing of defects and the re-moval of chemicals in the basal plane of GO sheets. With theincrease of temperature, oxygen-containing functionalgroups are removed by a simultaneous conversion from sp3

to sp2 carbon in the graphitic lattice. The rGO films canachieve a high thermal conductivity of 1043.5 W/(m·K) as thetemperature reached 1200 °C.[93] Xin et al. reported graphenefibers composed of large-size graphene sheets and small-sizegraphene sheets through high-temperature treatment (2850°C), with excellent electrical and thermal conductivities up to2.21×105 S/m and 1290 W/(m·K), respectively.[94]

High-temperature thermal treatment contributes to theformation of the submicrometer crystallite domain on gra-phene sheets. The increase of thermal treatment temperat-ure leads to the removal of defects (oxygen functionalgroups, nanocavities, and sp3 carbon bonds) and crystalliza-tion, as evidenced by the diminishment of D band and ch-ange of 2D band.[94,95] After thermal treatment over 2000 °C,the D peaks are usually undetectable (Fig. 11e), suggesting adefect-free structure of graphene. High-temperature carboni-zation allows the growth of crystalline graphitic domains(Fig. 11f), harvesting the simultaneous improvement ofthermal and electrical conductivities (Fig. 11g). Annealing at2800 °C with sufficient time leads to the construction of ABstacking structure of graphene (Figs. 11h and 11i). AB stack-ing is the most common and stable stacking type ofgraphene-based materials, e.g. graphite. It means that theempty centers of the hexagonal carbon network are occu-pied by the carbon atoms of the adjacent layers.

Besides, GO can also be reduced by photochemical reac-tions with the assistance of photocatalysts such as TiO2,[96]

ZnO[97] and BiVO4[98]. For example, charge separation occurs

on the surface of TiO2 under UV irradiation, and holes are

scavenged to produce ethoxy radicals in the presence of eth-anol, while electrons are accumulated and reduce GO sheetsto rGO.

To avoid the use of toxic chemicals and massive energy con-sumption, other unconventional heating approaches havebeen utilized to realize thermal reduction including Jouleheating,[99] laser pulse[100] and microwave irradiation[101]. Thenature of the reduction of GO by laser pulse is photochemicalreduction. The oxidation degree of as-reduced graphene hasstrong dependences on the output power of femtosecondlaser.[100] Joule heating generates high temperature at thejunction point where the high resistance located, while tradi-tional thermal treatment heats the whole space of the fur-nace. It can efficiently raise the temperature to 2750 K in lessthan 1 min of annealing time, providing the graphene withcrystalline structure and ultrahigh electrical conductivity ofup to 3112 S/cm.[102]

Reducing GO using 1- to 2-s-long microwave pulses is asimple and rapid method for preparing high-qualitygraphene.[101] GO possesses a good ability of microwave ab-sorption due to its functional groups. The absorption of mi-crowaves and fast quenching lead to rapid heating of the GO.Oxygen functional groups are almost entirely removed aftermicrowave reduction, which results in the formation of highlyordered structure. Raman spectrum reveals graphene-like fea-tures of GO after microwave treatment with sharp G and 2Dpeaks and a nearly absent D peak. In other words, microwave-induced reduction of GO causes desorption of oxygen func-tional groups and reordering of the carbon basal plane.

Different from approaches based on thermal treatment, ele-ctrochemical reduction of GO relies on the direct electrontransfer between the electrode and GO sheets, and this meth-od avoids the addition of harsh and toxic chemicals.[103−105]

The oxygen functional groups of GO sheets can be success-fully removed by the electrochemical reduction process, mak-ing electrochemical reduction a low-cost, rapid, high-yield,and environmentally friendly approach for GO reduction.

4. ATOMIC AND MOLECULARFUNCTIONALIZATION OF GO

4.1 Atomic Chemical DopingChemical oxidation of graphite into GO sheet creates functionaloxygen-containing groups at the surface (hydroxyl, epoxy,carbonyl, and carboxyl groups) and corresponding polar surfaceproperties. To achieve designed properties, the reactive groupsare usually used to functionalize GO sheets with various groups,such as atoms, small molecules, and polymer branches. Atomicand molecular functionalization provide efficient ways toachieve the best performance out of graphene or GO. Actually,graphene oxide itself can be regarded as the covalentfunctionalization of graphene by oxygen atoms. In this part, wemainly focus the atomic and molecular functionalization of GOsheets, through the reconstruction of carbon lattice andmodification of surface chemistry of GO, while details ofcovalent or noncovalent functionalization of graphene/GO withpolymer matrix will be discussed in Part 7, where the interactionforces inside the composites are presented.

Heteroatom doping can efficiently reconstruct the sp2-bon-


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ded carbon atoms, induce polarization of electron density atthe carbon-heteroatom bonds, and tailor the surface chem-istry interaction, leading to the modification of electricalproperties and chemical activity of graphene. The chemicaldoping of heteroatoms induces defects into the grapheneplane and provides additional active sites for chemical reac-tions, changes the conductivity of GO, and also benefits thecontrol of the n- and p-type semiconducting electronic prop-erties, as well as engineering its bandgap and magnetic ef-fect. Atomic radicals provide sufficient energy to overcomethe thermodynamic and kinetic barriers to covalently bond-ing with the basal plane of carbon networks but lack the en-ergy required to break the C―C bonds that will destroy thecarbon lattice.[106] For instance, n-type electrical doping canbe obtained by replacing carbon atoms with nitrogen atomsin graphene frameworks.

Three forms of N doping were proposed in graphene car-bon frameworks, i.e. sp2 hybridized graphitic N, pyridinic N,and sp3 hybridized pyrrolic N (Fig. 12a).[107] Graphitic N in-duces the replacement of carbon atoms in the hexagonalrings, and benefits the conductivity of graphene, whilepyridinic and pyrrolic N provide active defect sites in favor ofthe chemical or electrochemical process. XPS N1s spectrum ofnitrogen-doped graphene prepared by annealing at 800 °C ofGO with melamine reveals mainly four peaks (Fig. 12b), withlower binding energy located at about 398.2 and 399.5 eVcorresponding to pyridine-like and pyrrole-like nitrogen, peakat 400.8–401.3 eV resulting from graphitic nitrogen substi-tuted of carbon atoms, and high energy peak at 402.3–402.9eV attributed to oxidized nitrogen.

Various nitrogen sources have been used for N doping ofGO/graphene, including melamine,[108] ammonia,[107,109] and

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Fig. 12 (a) Schematic structure of nitrogen-doped graphene: pyridinic N, pyrrolic N, and graphitic N. (b) XPS N1s spectrum of nitrogen-dopedgraphene prepared by annealing at 800 °C of GO with melamine. (Reprinted with permission from Ref. [108]; Copyright (2011) American ChemicalSociety). (c) Schematic illustration of cationic nitrogen doping of graphene: (1) planar zigzag-edged N, (2) planar armchair N, (3) nonplanar cove-edged N (doubly charged aza[4]helicenes), (4) nonplanar helical N (singly charged aza[5]helicene). (Reprinted with permission from Ref. [113];Copyright (2017) Wiley). (d) The configuration and energy diagram for the N-doped GOQD photochemical diode. (Reprinted with permissionfrom Ref. [40]; Copyright (2014) Wiley). (e) GO molecular with quaterthiophene as antennas. (Reprinted with permission from Ref. [116]; Copyright(2010) Royal Society of Chemistry). (f) The rGO functionalized with polymerization of dopamine exhibiting a high capacity to acceleratehydroxyapatite mineralization. (Reprinted with permission from Ref. [117] Copyright (2012) American Chemical Society). (g) GO sheets covalentlygrafted with polyhedral oligomeric silsesquioxane via the amide formation between amine-functionalized POSS and oxygen-containing groupsof GO sheets. (Reprinted with permission from Ref. [118]; Copyright (2012) American Chemical Society).


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(NH4)2CO3.[110] During thermal annealing of GO in an ammo-nia atmosphere, oxygen groups in GO were responsible forC―N bond formation,[107] and the reduction, exfoliation ofGO and nitrogen doping occurred simultaneously in this pro-cess.[110] Compared to high-temperature thermal treatment,plasma-assisted downstream microwave technique enablesthe reaction at near room temperature, and provides a high-level amount of nitrogen doping.[109] Thiourea can be used asboth the N and the S source, and directly annealing GO withthiourea in argon leads to N and S co-doping, which makesgraphene an efficient metal-free oxygen reduction reaction(ORR) electrocatalysts for energy conversion and storagedevices.[111] Moreover, co-doping of cobalt and nitrogen of-fers graphene excellent electrocatalytic activity for ORR bothin alkaline and acidic solution.[112]

Nitrogen doping of rGO provides this material with impro-ved electrical properties, along with enhanced chemical re-activity. The doped nitrogen atoms change the local densityof states around the Fermi level, providing graphene with en-ergetically low-lying frontier molecular orbitals as well as apronounced stabilization of the lowest unoccupied molecu-lar orbital (LUMO) (Fig. 12c).[113] The formation of a delocal-ized conjugated system with nitrogen atoms and the sp2-hy-bridized carbon frameworks is favorable for its electrocatalyt-ic performance.[108] The heterostructure of p-doped pristineGO and n-type N-doped GO triggers effective exciton separa-tion and charge transfer under solar light irradiation, achiev-ing high photosynthetic H2 generation performance from ove-rall water-splitting (Fig. 12d).[40] In addition, the doped nitro-gen atoms can serve as coordination and reduction sites ofmetal ions, such as gold ion aqueous solution (HAuCl4).[114]

Due to the terminal carboxylates, gold nanoparticles are be-lieved to coordinate with the nitrogen site by electrostaticinteraction.

4.2 Molecular ModificationExcept for atomic chemical doping of graphene frameworks,surface modification of GO with small molecules and polymerbranches is promoted. The electronic structure of graphene iscrucial to its applications, especially the development of carbon-based electronics. The direct interaction of GO with electron-donors and electron-acceptors induces modifications of theelectronic structure of the obtained graphene. Chemical dopingof graphene surface by attachment of gas molecules has provedto endow graphene with gas detecting function based on thechanges of electrical conductivity, because the adsorbedmolecules act as donors (NH3, CO, and ethanol) or acceptors(NO2, H2O, and iodine), thus changing the local carrierconcentration in graphene.[115]

The abundant oxygen functional groups in GO make it pos-sible to assemble GO sheets on other materials. The oxygen-containing groups act as two roles, dispersion of GO in theaqueous solution and modification of bonding groups in cer-tain chemical reactions. Functionalization of GO with quater-thiophene molecules,[116] dopamine,[117] amine-functional-ized polyhedral oligomeric silsesquioxane,[118] polymerizednorepinephrine,[119] and gamma-aminopropyltriethoxysila-ne[120] has been reported (Figs. 12e–12g). For example, Ouet al. assembled GO sheets onto silicon wafers via covalentbonding, through chemical adsorption and subsequent

thermal reduction progress.[121] GO can be easily bonded with(3-aminopropyl)triethoxysilane (APTES)-covered silicon waferthrough the chemical reaction between oxygenous (epoxyand carboxyl groups) and amine-terminated groups via nuc-leophilic substitution and amidation, respectively.

5. LIQUID CRYSTALS OF GO

5.1 Introduction to GO Liquid CrystalsDue to the poor dispersibility of pristine graphene in commonsolvents, it is hard to process graphene in industrial applications.But GO can disperse well in water and other polar solventsowing to its functional groups. Therefore, GO appears as a veryimportant precursor of graphene because of its highdispersibility and it can be easily reduced to graphene. Besides,a great investigation has been attempted to tailor the orderingand orientation of GO and GO-assembling materials. In the longjourney of 2D colloids, the phenomenon of flow birefringencehas been usually observed in their dispersions,[122,123] andproven as a flow induced mesophasic behavior, includingclay[124] and graphite oxide.[125] As early as 1948, Thiele hadobserved the flow-birefringence of graphite acid (the old nameof graphite oxide) when studying its viscosity property,[125]

which was considered as the first observation of flow-inducedLC of graphite oxide.[122] Till 2011, the stable LC of grapheneoxide was re-observed and re-investigated systematically byseveral groups of Kim,[10] Gao,[9] and Pasquali and Smalyukh.[126]

Afterwards, the liquid crystalline behavior of GO and rGO sheetswere also reported by other research teams.[127−129]

According to Onsager's theory, if the volume fraction of 2Dsheets is higher than a critical value (Φ≈4T/W), LCs tend toform in dispersions.[130] Especially, graphene and GO sheetspossess a high aspect ratio and anisotropy, with monatomicthickness (T) and a micrometer-scale lateral width (W), result-ing in the formation of nematic and lamellar liquid crystalsin water and polar organic solvents. The LC properties ofGO dispersion were directly evidenced by POM, with thestable birefringence spreading through the whole disper-sions and displaying vivid Schlieren texture, as well as mac-roscopic colorful textures observed by the naked eye whenthe GO dispersions are placed between crossed polarizers(Figs. 13a and 13b), indicating the formation of a uniformnematic phase of GO.[9] Kim et al. monitored the local orienta-tion of the GO LCs by SEM after quickly quenching concen-trated liquid-crystalline dispersion in liquid nitrogen.[10] GOplatelets were left oriented in the nematic phase and ±1/2disclinations were dominantly identified after subsequentlyfreeze-drying treatment (Figs. 13c–13e), consistent with thetypical disclination morphology. Upon the addition of the ul-tralarge graphene oxide sheets, lyotropic nematic liquid crys-tals were obtained, as reported by Aboutalebi et al.[131] GOLCs can also be obtained in a wide range of organic solventsexcept water, such as ethanol, acetone, tetrahydrofuran, N-di-methylformamide, N-cyclohexyl-2-pyrrolidone, etc.[128] In ad-dition, GO LCs dispersions exhibit typical shear thinning andtypical colloidal isotropic-nematic phase transition behaviors,which have been described by power law or simple Curreaumodel.[132]

Gao’s group discovered a new form of chiral liquid crystal(CLC) of GO suspension when the concentration of narrow


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size distribution GO increases to 0.38 vol%.[133] Cryo-SEM wasemployed to visualize the helical structural details of GO CLC,and it confirmed the annual ring-like structures of GO CLCcomprised of undulating bands with distinct boundaries. Fur-thermore, POM for the confined GO CLCs in a cavity also re-veals highly regular fingerprint-like and focal conic textures(Figs. 13f and 13g). They proposed the structure mode of CLCof 2D colloidal GO sheets as both continuous lamellar andhelical features (Fig. 13h). The electrostatic repulsion betweenGO sheets is the dominant interaction in GO CLCs. Therefore,the neighboring blocks repel each other due to the negativecharges at boundaries and on surfaces of GO sheets. In orderto minimize the free energy, rotated conformation was like toform by turning to a certain angle.

Meta-periodicity exists in solids but tends to fade away influid states after a short relaxation time because of energeticinstability. The achievement of stable meta-periodicity forlyotropic LCs remained a challenge. Jiang et al. fabricated

lyotropic liquid metacrystals (LMCs) in colloidal GO LCs byshearing microlithography (SML) (Figs. 14a and 14b).[134]

This technology allows the design of crystalline cells byprogramming the probe movement, covering five Bravaislattices (with translational and rotational symmetry) andquasi-crystals (with rotational symmetry but not translationalsymmetry), as shown in Figs. 14(c)–14(i). The programmableGO LMCs allow us to construct arbitrary lattices and patternsin liquid 2D space, making it possible for programming solidmaterials with designed behavior and performance.

f ∝ e− t

τ1⧸2 S ∝ e− t

τ1⧸2

LMCs exhibit special relaxation behavior compared to con-ventional 1D macromolecules. Gao’s group has tracked thefraction of reorientation area (f) and the translational orderparameter (S) over time in a p4mm LMC. The relaxation dy-

namics follows a relationship of and .

The fraction of reorientation area and the translational orderparameter over time at various concentration are shown in

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Fig. 13 (a) POM images between crossed polarizers of GO aqueous dispersions with fm's of 5×10−4, 1×10−3, 3×10−3, 5×10−3,8×10−3, and 1.0×10−2 (from 1 to 6). The green arrows indicate the disclinations. (b) Macroscopic photographs on crossedpolarizers of GO aqueous dispersions with fm's of 1.0×10−4, 2.5×10−4, 5×10−4, 1.0×10−3, 5×10−3, 1.0×10−2, and 2.0×10−2 (from 1 to7). The stable birefringence of GO aqueous solution indicates a typical nematic phase. (Reprinted with permission from Ref. [9];Copyright (2011) American Chemical Society. (c, d) Typical nematic schlieren texture of GO LCs dispersion with ±1/2disclinations and a +1 disclination. (e) SEM of GO LCs in a freeze-dried sample, with blue and red symbols indicating +1/2 and–1/2 disclinations, respectively. (Reprinted with permission from Ref. [10]; Copyright (2011) Wiley). (f) The annual-ringundulating fracture morphology of GO CLC and (g) POM images between crossed polarizers of GO CLCs in lateral domains. (h)Proposed model for one pitch of GO chiral liquid crystals. GO sheets are indicated as pink nets, and the semitransparent blueblocks indicate interlayer water. (Reprinted with permission from Ref. [133]; Copyright (2011) Nature Publishing Group).


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Figs. 15(a) and 15(c). For instance, the radial autocorrelationfunction (ACF) of p4mm GO LMC at 0.09 wt% gradually lostcharacteristic peaks within 4 h (Fig. 15b). Furthermore, the au-thors proposed an anisotropic melting behavior of GO LMCs,where the perpendicular vibration against π walls possessed anarrower melting range and higher melting frequency thanthose of parallel vibration, which reflected the anisotropicstrength of crystalline planes.[134]

Pristine GO LCs were mainly stabilized by polar andcharged functional groups through electrostatic repulsion.However, in the case of polyacrylonitrile (PAN) grafted GOLCs, the surface charge of GO slightly decreased due to thepartial reduction of GO in the polymerization process. There-fore, the delicate balance of GO-g-PAN LCs is mainly attrib-uted to the remaining charge and the free-volume entropicrepulsion of grafted polymers. Liu et al. grafted PAN on GOsurface for assembling continuous strong nacre-mimetic

fibers.[135] The mixed GO-g-PAN sheets were highly dispers-ible in polar organic solvents. At low concentration, the GO-g-PAN sheets form nematic LCs, and evolve into lamellar LCphase with increasing concentration. The increase of heightof the sheets after grafting indicates the covalent bondingbetween PAN and GO (Fig. 14j).

5.2 Key ParametersHigh aspect ratio is essential for the formation of GO LCs. In thecase of normal-sized GO sheets, a high concentration is usuallyrequired for highly oriented LC. Increasing GO LCs concentra-tion leads to the phase transitions from isotropic to biphasicsystem and then to a discotic nematic liquid crystal of GOsheets.[126] Besides, increasing the pH value of GO suspensionprovides a possibility for LC formation at relatively low concen-tration.[136] Yao et al. observed a well-organized rod-like textureif the suspension contained 0.132 mol/L KOH at GO concen-tration of 0.5 mg/mL, while the neat GO dispersion showed

2D

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1 1/μm 1 1/μm 1 1/μm 1 1/μm 1 1/μm 1 1/μm Fig. 14 (a) Schematic of shearing microlithography. (b) A 3D structural model of a π wall created by shearing microlithography. (c–i) POMimages and FFT diffraction patterns of 2D Bravais LMCs and quasi-crystalline LMCs. (Reprinted with permission from Ref. [134]; Copyright (2019)Nature Publishing Group). (j) Schematic of helical lamellar phase for GO-g-PAN dispersions in DMF. (Reprinted with permission from Ref. [135];Copyright (2013) American Chemical Society).


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disordered phase at this concentration. At GO concentration of3.5 mg/mL, GO containing KOH showed much more regularordered laminar texture than the neat GO (Figs. 16a–16c). K ionwas excluded for this phenomenon because K+ weakens of theelectrostatic repulsion between GO sheets and makes zetapotential of GO sheets positively shifted. In contrast, increasingpH value decreases the zeta potential of GO sheets evenconsidering the existence of K+, which was caused by thedeprotonation of the hydroxyl groups and additional carboxylgroups formation.

As we discussed in Part 3.2, pH and ionic strength are keyfactors that determine the colloidal behavior of GO disper-sion. Therefore, inducing ionic impurities or decreasing thepH value of GO suspension deteriorates the electrostatic re-pulsion and causes the coagulation of GO suspension. Re-moval of ionic impurities is crucial for the stability of LCphase. Gao’s group has established the isotropic-nematic sol-id-phase diagram versus mass fraction and the adding NaClconcentration.[9] The pristine GO dispersions consist of threephases (Fig. 16e): isotropic (I, mass fraction (fm)<2.5×10−4),nematic (N, fm>5×10−3), and biphasic coexistence (I+N).However, with the addition of NaCl salt, the biphasic phaseturns into an I phase initially and solids (S) phase eventually,and the concentration of the transition to I phase by addingsalt scales with fm of GO. The phase evolution is caused by thedecreased electrostatic repulsive forces induced by addingsalt, which is confirmed by tracking the zeta potentials of thedispersions along with the concentration of salt (Fig. 16f).

CaCl2 added in the GO LCs dispersion can act as an efficientcross-linker, which turns GO sol into hydrogel to prevent thecollapse of GO-based 3D structures, while being applied intodirect 3D printing (Fig. 16f).[84]

5.3 ApplicationsGO LCs were applied in electro-optic devices due to theirtunable degree of ordering and direction of alinement.[137] Inaddition to electric field, magnetic field and mechanicaldeformation can also be used for controlling the macroscopicalignment of GO LCs.[10] The formation of GO LCs has beenutilized to develop high-performance supercapacitors.[138] Thecomponent, defect, structure and interface of GO sheets shouldbe considered during preparation. Guo et al. utilized liquidcrystal-derived GO phases as smart stimuli-response materialsbased on unique anisotropic swelling behaviors of GO LCs.[139]

Besides, GO aqueous dispersions possess typical shear flows andshow decreased viscosity upon shearing. The prealignedorientation of GO LCs can be directly used for the orderedassembly of macroscopic architectures or composite structuresunder shear flow.[136,140−143] GO LCs facilitate the design of long-range and ordered structures of graphene-based functionalmaterials and provide these materials with different topologiesby controlling the alignment of graphene sheets in the fluidphase. It can be concluded that GO is a living building block inthe 21st century (Fig. 17). Until now, the most widely usedapplication of GO LCs is the ordered assembly for the design ofordered materials with optimal performances, which will bediscussed in Part 6.

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Fig. 15 (a) The fraction of reoriented domains (f) as a function of time (t). Insets are POM tracking images of LMC (0.09 wt%) in therelaxation process. Scale bars, 500 μm. (b) RACFs for LMC (0.09 wt%) in the relaxation process. (c) The translational order parameter (S) asa function of t and c. Insets are FFT spectra of LMC (0.09 wt%) in different periods in relaxation. Scale bars, 0.1 μm–1. (d) Lattice integrity asa function of vibration frequency. Insets are POM images for LMCs with reoriented domains arranging perpendicular (orange), parallel(green) to the vibration direction and GO LC without meta periodicity (blue). Scale bars, 500 μm. (Reprinted with permission from Ref.[134]; Copyright (2019) Nature Publishing Group).


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6. MACROSCOPIC ASSEMBLY OF GO

The discovery of GO LCs has built solid new foundations forhighly ordered, macro-assembled graphene materials. Theassembly is mainly realized through π-π stacking and hydrogenbonding of GO sheets. Moreover, wet-fusing assembly occurson two GO fibers because of the swelling of fibers and hydrogenbonding at the interface. Until now, a lot of graphenearchitectures from 1D to 3D levels have been established suchas graphene fibers, films, foams (or aerogels), fabrics, as well astheir composites with other materials (Fig. 18).

6.1 1D Graphene FibersGraphene fibers possess promising applications compared withconventional polymer fibers and metal wires, benefiting fromtheir excellent properties including high electrical conducti-vity, chemical stability, lightweight and strong mechanicalproperties.[144] In 2011, Gao and Xu firstly created graphenefibers by the wet spinning of GO LCs in the lab, and opened anew field for high-performance carbonaceous fiber with richfunctionalities.[133] Then many reports have been published to

understand the structure and properties of GFs and exploretheir applications. In this part, mechanical strength andflexibility of graphene fibers are demonstrated and their relatedelectrical and thermal transport properties are discussed.Detailed discussions on graphene fibers such as preparation,morphology, properties, and applications are beyond the scopeof this review, and an excellent review article on graphene fibersis suggested to follow for interested readers.[145]

6.1.1 Mechanical strength and flexibilityThe significant differences between conventional polymericfibers and 2D graphene-sheets-based fibers lie in the structu-re of basic constituent units and the interaction mode thereof(Fig. 19).[146] In general, the mechanical performance ofpolymeric fibers is dominated by the molecular weight, chain-by-chain interaction, defect-free engineering and entanglem-ent, while that of graphene fibers is usually determined by (1)the crystalline graphitic domain size, (2) defect-free engineeringof graphene sheets, (3) alignment of graphene sheets and (4)the compactness of graphene layers, as well as (5) hierarchicaland multiscale interactions. The interactions inside graphene

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Fig. 16 (a–c) POM images of neat GO, GO+KOH, and GO+KCl suspensions, respectively. (Reprinted with permission from Ref. [136]; Copyright(2016) Wiley). (d) Phase diagram of GO aqueous dispersions versus fm and NaCl concentration. (e) Zeta potential of GO dispersions (fm=0.005) as afunction of NaCl concentration. (Reprinted with permission from Ref. [9]; Copyright (2011) American Chemical Society). (f) The elastic modulusand yield stress of GO dispersions as a function of added Ca2+ concentration. (Reprinted with permission from Ref. [84]; Copyright (2018) Wiley).

Interface

Structure

Defect

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Graphene oxide: a living building block

Raw material Materials Devices & applications

Graphene oxide (GO) GO liquid crystalOrdered graphene

macroscopic material

Fig. 17 The route map of GO design from raw materials to applications using GO as a living building block.


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fibers include π-π interaction of graphene sheets on atomicscale, nanoscale interlocked structure caused by wrinkles, andmicroscale sheet-by-sheet friction. Although the interactionof polymer chains is strengthened by the chain-by-chaininteraction and entanglement, the polymeric fibers still sufferfrom chain sliding under uniaxial tension. In terms of 2Dgraphene sheet, it can be considered as intermolecular cross-linking of individual “carbon chain”, and the chain-sliding canbe effectively inhibited. Meanwhile, the interlocked wrinklesof 2D graphene sheets also block the slippage within thegraphene fibers.

Although the mechanical strength of single graphenesheet reaches an ultrahigh value of ~130 GPa, the limitedstrength of macroscopic graphene materials is still far frombeing achieved. Efficient strategies are required to minimizethe large mechanical performance gap between graphenefibers and individual graphene sheets. Giant GO sheets withextremely high aspect ratio are usually selected as the build-ing blocks for graphene fiber fabrication to reduce defectiveedges. Xu et al. hold the view that LGO sheets contribute tothe improvement of the mechanical performance of the ob-tained graphene fibers because of the high aspect ratio.[21]

Among the process of wet-spinning of neat GFs, GO LCs canform regular alignments along the directional flowing field,

which is helpful to get continuously assembled GO fibers withhighly ordered micrometer-sized structures (Figs. 20a–20d).More recent studies by Xin et al. reveal that the combinationof LGO and SGO is beneficial to the fabrication of mechanic-ally strong graphene fibers, with LGO sheets acting as ahighly ordered arrangement and SGO sheets filling the spaceand microvoids (Fig. 20e).[94]

Afterwards, a conventional high-temperature thermal treat-ment is introduced to improve the performance of GFs.[147]

High-temperature annealing contributes to the formationof fine graphene crystallite domain, as well as the removal ofdefects such as oxygen functional groups, nanocavities andsp3 carbon bonds, resulting in the enhanced mechanical per-formance of graphene fibers. However, this method may benot suitable for graphene films, because of the formation of“micro-gasbag” inside the GO layers.[95]

Furthermore, a well-stacked microstructure assembled bydirectional fluids is the key to outstanding mechanical per-formance.[148,149] A uniform alignment of graphene sheets fa-vors the mechanical strength of assembled graphene fibersin the axial direction. In 2012, Xu et al. presented neat GOaerogel fibers with unique “porous core-dense shell” struc-ture originated from lamellar-ordered GO LCs via spinning as-sembly strategy and ice-templating strategy, offering

GO liquid crystals and self-fusing/healing of GO fibers

Graphenefiber

Graphenefilm

Graphenefoam

Graphenefabric

(F4)

Fig. 18 Diagram of GO liquid crystals and their macroscopic assembly to get grapheneF4 materials, i.e. fiber, film, foam and fabric.

Polymeric fibers Graphene fibers

The factors for the enhancement of mechanical strength

(1) Molecular weight

(2) Defect-free engineering

(3) Chain-by-chain interaction

(4) Entanglement

Chain endVoid

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(2) Defect-free engineering

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(5) Hierarchical and multiscale interactions

Wrinkle

Sheetboundary

Inter-lock

Fig. 19 Structural models of polymeric fibers and graphene fibers and the factors for the enhancement of their mechanical strength.


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graphene fibers high specific tensile strength (188 kN·m/kg)and high compression modulus (3.3 MPa).[150] The structureretained during the phase transformation from ordered fluidsto ordered solids.[21] To fine the fiber dimensions simultan-eously, Xu et al. further induced a stretching strategy in thewhole wet-spinning process to improve the alignment ofgraphene and the compactness of fibers (Figs. 20f–20h).[151]

Therefore, a full-scale synergetic defect engineering methodhas been proposed to minimize the possible defects of

graphene fibers at all levels ranging from atomic to macro-scale scales, offering the obtained graphene fibers ultrahighstiffness of 282 GPa and record mechanical tensile strength of1.45 GPa in 2016. Recently, Gao and co-workers improved themechanical strength of graphene fibers to 3.4 GPa via a plasti-cization spinning strategy.[152] The intercalated plasticizersenable GO fibers to achieve a 580% enhanced deformableplasticity, followed by the flattening of random graphenewrinkles and formation of large crystallite domains.

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50 µm Fig. 20 (a) Schematic apparatus for wet-spinning GO fibers. (b) A five-meter long GO fiber wound on a ceramic reel. (c) Photograph ofthe process to twist graphene fibers to yarns. (d) SEM images of the twisted yarn. (Reprinted with permission from Ref. [21]; Copyright(2013) Wiley). (e) Schematics of graphene fibers consisting of LGO and SGO. (Reprinted with permission from Ref. [94]; Copyright (2015)American Association for the Advancement of Science). (f) Schematic illustration of wet-spinning of GO fibers with continuous stretchingin two stages, followed by the transformation of GO fibers into graphene fibers through graphitization up to 3000 °C. (g) A close-upsnapshot of 50-filament GO fibers extruded from the spinneret in the coagulation bath. (h) A roll of 500-m long 50-filament graphenefibers with a metallic luster. (Reprinted with permission from Ref. [151]; Copyright (2016) Wiley). (i) Photograph of a CNT/rGO hybrid fibersewn into a rubber band and an attached 3.5-cm long needle. (Reprinted with permission from Ref. [156]; Copyright (2012) NaturePublishing Group). (j) Photo of four-meter-long GO fiber wound on a Teflon drum. (k) SEM image of a tightening knot of graphene fiber.(Reprinted with permission from Ref. [133]; Copyright (2011) Nature Publishing Group).


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Besides, the improvement of the mechanical performanceof GO fibers or graphene fibers can be achieved by the ioncross-linking bond introduction between the layers. Previousexperimental and theoretical investigations have proposedthat adding cation promotes cross-linking bridges betweenthe oxygen-containing groups, and thus leads to the mechan-ical enhancement of graphene fibers/films.[21,153] The intro-duction of coordinating cross-linking enhances the interlayerinteraction of graphene sheets, leading to the improvementof graphene fiber strength. The wet-spinning technology canbe extended to polymer-grafted graphene LCs to obtain hier-archically assembled, nacre-mimetic fibers.[135,154,155] In 2013,Gao and coworkers reported the nacre-mimetic fibers with“brick-and-mortar” microstructure starting from the mixtureof GO and PAN, showing excellent mechanical property ofgraphene fibers with a tensile strength of 452 MPa at thattime.[135] Moreover, combining carbon nanotubes and rGOflakes promotes an interconnected network of partiallyaligned rGO flakes and carbon nanotubes during solutionspinning, forming super-tough nanostructured fibers bystrong interaction between these two types of carbon com-ponents. The assembled CNT/rGO hybrid fiber can be sewninto a rubber band with an 3.5-cm long needle (Fig. 20i).[156]

Furthermore, nacre-inspired approaches are also suitable forfabricating graphene composite films. Ni et al. applied a longlinear chain form 1-aminopyrene (AP) and disuccinimidylsuberate (DSS) to make cross-link adjacent betweengraphene sheets via π-π interfacial interactions, aiming to en-hance the tensile strength of graphene films.[157]

Flexibility is regarded as the one of the most important fig-ures of merit for graphene fiber. It can be twisted together toobtain continuous threads and yarns, which makes flexibleand wearable devices such as electronic skins, smart clothes,wearable heater, bendable display, and flexible fiber-shapedsupercapacitors, etc.[144] Fig. 20(k) shows a knot made ofgraphene fiber and it does not break as the knot is tightened.It is suggested that the flexibility results from the locallycrumbled structures of individual graphene sheets.[133]

6.1.2 Electrical and thermal transportExcept for improving the mechanical strength of graphene fib-ers, the large and fine crystalline graphitic domain, defect-freeengineering of graphene sheets and the alignment of graphenelayers also contribute to the superior electrical and thermaltransport properties. Fabricating graphene fibers with highstrength and electrical/thermal conductivity simultaneouslyremains a great challenge. For superior electrical and thermaltransport properties, highly aligned sp2 graphene sheets arerequired, while sp3 bonds and functional groups usually act aselectron- and phonon-scattering centers, which greatly hinderthe electrical and thermal transport of graphene.[94,158]

The electrical conductivity of graphene films is dominatedby the contact resistance between the graphene sheets. LGOsheets reduce the number of graphene-graphene contacts fora given number of conducting paths. As a result, the large sizeof GO sheets reduces the total contribution of contact resist-ance and enhances the conductivity.[159] We believed thatLGO sheets contribute to the improvement of the mechanic-al performance of the obtained graphene fibers, as well astheir electrical performance due to the high aspect ratio.[21]

Moreover, thermal annealing heals structural defects in thelattice structure and removes functional groups, greatlyimproving the electrical and thermal properties.[160] Withthe increase of annealing temperature, the D band at neat1350 cm−1 in Raman spectra is depressed, suggesting the re-moval of defects. It is reported that the D band can be nolonger evidenced after thermal treatment at 2200 °C orabove, meaning the complete removal of defects ingraphene.[148] The D band becomes narrow and the 2D bandrecovers after high-temperature treatment, indicating thecrystallization of graphene sheets. As well as its influence onmechanical strength, fine alignment of graphene sheets alsocontributes to the high conductivity of the assembledgraphene structure.[150]

The high flexibility and electrical conductivity of graphenefibers enable various applications among wearable electro-nic devices and flexible power sources (Fig. 21a).[147,161−169]

Much of the research in wearable energy storage devices inthe last two decades has examined to meet growing de-mands in the field of communication, biomedical devices,and entertainment systems.[170] Qu et al. fabricated fiber-shaped supercapacitors using hollow graphene fiber as theelectrodes, by incorporating conducting polymer into graph-ene fibers (Figs. 21b and 21c).[171] Ma et al. introduced extern-al conductive connection “bridges” polydopamine (PDA) intographene sheets and employed thermal treatment at 1200 °Cunder inert atmosphere, providing the assembled graphenefiber with high electrical conductivity of 6.6×104 S/m.[154] Oth-er polymers such as polyaniline were used by electrodepos-ition to fabricate superelastic wet-spun hybrid carbon nan-otube graphene fibers for high-performance fiber-based su-percapacitor.[172] Gao’s group proposed a coaxial wet-spin-ning assembly approach to make polyelectrolyte-wrappedgraphene/carbon nanotube core-sheath fibers for yarn super-capacitors (Fig. 21d).[173] An ultra-high capacitance of 269mF/cm2 and energy densities of 5.91 mWh/cm2 were ob-tained. Meng et al. reported a hierarchical all-graphene hy-brid structure with a highly conductive core and 3D graphenenetwork with a highly-exposed surface, offering the great ad-vantages as flexible electrodes for electrochemical supercapa-citor (Fig. 21e).[174] Chen et al. introduced cellulose nanocrys-tal into GO via non-liquid-crystal spinning followed by chem-ical reduction to fabricate hybrid fibers.[175] Moreover, thehigh stretchability and high electrical conductivity ofgraphene fiber make it favorable wearable heater with an ul-trafast electrothermal response and very low operationvoltage to access high temperature.[176]

Neat graphene fibers reduced by chemical reduction stillshowed limited electrical conductivity (σ) only about0.2×104–4.1×104 S/m, because of the inherent structural de-fects and residual oxygen-containing groups, as well as thelower carrier density compared to metal (up to 1022 cm−3).Therefore, increasing the carrier density by doping providesan option to improve the electrical conductivity of GFs. Liuet al. employed a facile two-zone vapor transport method(Fig. 21f) to dope GFs chemically with superior electrical con-ductivity with various elements, reaching 0.77×107 S/m (GF-FeCl3), 1.5×107 S/m (GF-Br2), and 2.24×107 S/m (GF-K).[177] Thedopants were sublimated in the lower temperature zone and


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diffused to the higher temperature zone where the dopingreaction occurred. Moreover, Ca intercalation of graphenefiber provides it with superconducting property, showing asuperconducting transition at ~11 K, which is comparable tothat of commercial superconducting NbTi wire (Fig. 21g).[178]

6.2 2D Graphene FilmsGraphene/GO films have been applied in numerous fields suchas electrode of electrochemical capacitors,[179−182] battery separ-ators,[183] cathodes for Al ion batteries,[184−186] ion sieving,[187−189]

liquid handing,[190] gas separation,[191] thermal managem-ent,[192−194] electromagnetic interference shielding,[195,196] andso on. The mechanical properties of graphene films are qui-te critical for many applications such as human-motion sen-sors, flexible electronics and stretchable energy-harvestingdevices. A variety of structures are designed to endowgraphene films with excellent behaviors such as high tensilestrength, favorable Young’s modulus, long durability andrapid recovery while retaining its high conductivity.[63] In thispart, we would like to present mainly the mechanical strength,flexibility and thermal transport of graphene films.

The issue of mechanical strength of graphene/GO films hasreceived considerable critical attention in the past decade.Factors influencing the mechanical strength have been ex-plored in several studies, such as denser grain boundary,[197]

cross-linking network,[198] modulated plasticization stretch-ing,[199] etc. The efficient cross-linking of neighboring GOsheets is essential for improving the mechanical strength ofgraphene films,[200] which is also one of the key factors in thecase of graphene fibers. Liu et al. have documented that Fe3+-tannic acid complex network served as the “mortar” to stick

the GO “bricks” together, which induced the cross-linking ofGO sheets and greatly improved the fracture strength andYoung′s modulus of GO papers.[198] Recently, Gao and co-workers have achieved graphene paper with superior mech-anical properties, with tensile strength of 1.1 GPa and stiff-ness of 62.8 GPa.[199] In this work, an intercalation modulatedplasticization strategy was applied to regulate the spontan-eous graphene wrinkles into crystalline orders. Besides, bene-fiting from the crystalline orders, superior conductive proper-ties were achieved, with high electrical conductivity of1.1×105 S/m and thermal conductivity of 109 W/(m·K).

Leaf spring is a good example of elastic property from dailylife, which is usually used in vehicles to support the axle andabsorb shocks based on their arch structure of the suspen-sion system. Other similar examples include honeycomb-likestructure,[201,202] microvoids,[203] and natural cork.[204] Finiteelement mechanical simulation suggests that the arch-shaped structure can sustain large geometric deformationwithout yielding and recover to its original shape immedi-ately because its true strain is small (Fig. 22a). Learning fromthe examples from daily life, researchers have developed aseries of structures for elastic materials, e.g. a super-elasticcarbon-graphene (C-G) monolith with hierarchical lamellar ar-chitecture.[205] Thousands of microscale arches serve as struc-tural and elastic units of the carbon-graphene composite. Thelink between adjacent lamellae is crucial for elastic perform-ance because it helps restrict lateral extension and slip of thelayers while undergoing vertical compression (Fig. 22b). Be-sides, the link or interlocking-tile is suggested to promote thethermal and electrical behaviors of graphene films, because itprovides a transmission tunnel for thermal and electrical con-

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Fig. 21 (a) Schematic illustration of fiber-based micro-supercapacitors integrated into a flexible fabric substrate to power electronics. (Reprintedwith permission from Ref. [169]; Copyright (2017) Wiley). Cross-sectional SEM images of the hollow rGO/conducting polymer composite at (b) lowand (c) high magnifications, respectively. (Reprinted with permission from Ref. [171]; Copyright (2016) Wiley). (d) Two intact coaxial fibers wovenwith cotton fibers. (Reprinted with permission from Ref. [173]; Copyright (2014) Nature Publishing Group). (e) SEM image of graphene fiber with acore covered with a sheath of 3D porous network-like graphene framework. (Reprinted with permission from Ref. [174]; Copyright (2013) Wiley).(f) Comparison of the specific conductivity of the doped GFs, the doped CNT fibers, the doped CF, and several commonly used metallicconductors. (Reprinted with permission from Ref. [177]; Copyright (2016) Wiley). (g) Temperature dependence of the electrical resistance of Caintercalation of graphene fiber, showing a superconducting transition at ~11 K. Inset: a photograph of Ca intercalation of graphene fiber with ayellow color. (Reprinted with permission from Ref. [178]; Copyright (2017) American Chemical Society).


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ductivity.[93] Yang et al. proposed Maxwell-Wiechert model todescribe the dynamic mechanical behavior of graphenefilms/papers and revealed that relaxation behavior can be re-lieved by the interlayer crosslinking between adjacent layers(Fig. 22c).[206] Based on the ternary Maxwell-Wiechert model,the stress-time curves of graphene films during stress relaxa-tion can be expressed by the following equation:

σ (t) = σ∞ + σ1e−t/τ1 + σ2e

−t/τ2 (1)where σ refers to the stress, σ∞ refers to the residual stress ofstandalone spring. σ1 and σ2, and τ1 and τ2 are the initial stressand relaxation time of each Maxwell unit.

The mechanically flexible property can be achieved by pro-ducing 3D macroporous structure inside graphene films via abreath-figure method.[114] Lee et al. disperses polymer-graf-ted GO platelets in an organic solvent and exposed the sus-pension to a stream of humid air. Endothermic evaporation ofthe volatile organic solvent leads to the packing of aqueousdroplets at the surface of the organic solvent. Finally, a spe-cial porous morphology of the graphene films forms after dry-ing (Fig. 22d). Huang et al. induced graphene film with micro-voids structure by coating hydrazine hydrate solution on GOfilm followed by confining the film between two aluminaplates in order to ensure the controlled expansion ofgraphene films in thickness.[203] Xu et al. directly reducedwavy GO films electrodes by in situ reducing wavy GO filmsprecoated on elastic tapes without involving film transfer orhigh-temperature annealing.[207] The rGO electrodes showedexcellent electrochemical stability and flexibility for solid-state electrochemical capacitors.

Moreover, the flexibility study of graphene can be exten-

ded from 2D films to 3D foams. Qiu et al. fabricated graphene-based cellular monoliths with similar cork-like structure by asimple freeze-casting method.[204] Upon freezing, rGO sheetswere rejected and entrapped between neighboring ice crys-tals to form a continuous network around the ice crystal. As aresult, the rGO sheets concentrated at the boundary of icecrystals and aligned along the ice growth direction due to thesqueezing effect.

Peng et al. reported super-flexible graphene films with ul-trahigh thermal conductivity (K), with the assistance of thehigh-temperature annealing technique. The obtained graph-ene films present a high thermal conductivity of about 1940W/(m·K), which is higher than that of copper foil (K = 390W/(m·K)) and graphitized polyimide film (GPI, with K = 1750W/(m·K)) (Figs. 22e and 22f).[95,208] The high thermal conduct-ivity of graphene film is mainly attributed to its high crystal-line structure and debris-free graphene sheets, because thedefects and grain boundaries of the graphene sheets arehealed after high-temperature thermal treatment. The highthermal transport property mainly relies on the large crystal-line graphitic domain, defect-free and aligned sp2 graphenesheets.[148] Besides, the high-temperature annealinggraphene film also maintains the highly flexible property, andit can endure 180°-seamless folding for over 6000 cycles and180°-bending for 1.0×105 cycles. This property is determinedby the microfolds generated by mechanical pressing. the ran-domly dispersed and compacted microfolds can be stretchedout and deformed to crumples under tension or folding.

6.3 3D Graphene ArchitecturesThe control of GO platelets into 3D structure functionalmaterials is a great issue. 3D graphene architectures such as

Compress

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24

b

20 µm

Fig. 22 (a) The true stain of the arch-shaped model via finite element simulation. (b) SEM image of the arch microstructure of carbon-graphenecomposite. (Reprinted with permission from Ref. [205]; Copyright (2016) Nature Publishing Group). (c) The schematic diagram of ternary Maxwell-Wiechert model of graphene laminated structure. (Reprinted with permission from Ref. [206]; Copyright (2018) Royal Society of Chemistry). (d)Procedure for the self-assembly of rGO into flexible macroporous carbon films via a breath-figure method. (Reprinted with permission from Ref.[114]; Copyright (2010) Wiley). (e) A piece of super flexible graphene film folded into a crane without breakages. (f) Photo of a commercialsmartphone on the back side, and its thermal dispersion under ready mode (1), using GPI (2) and graphene film (3) as the heat-dissipation film,respectively. (Reprinted with permission from Ref. [95]; Copyright (2017) Wiley).


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aerogels, fabrics and foams have been developed, which makesthem viable candidates for catalysis,[209−211] energy stora-ge,[212−216] gas/ion/dye/wave absorption,[217−221] water harvesterfrom air,[222] hygroelectric generator,[223] solar-thermal energyconversion,[224−226] solar-electron energy conversion,[227] andsensing applications.[228−230]

The fabrication of highly stretchable carbon aerogels (CAs)or foams is crucial to their uses for growing demands in wear-

able devices, stretchable electronics, and smart manufactur-ing. Jiang et al. fabricated graphene CAs from GO-based inkby a facile ion-induced gelation method. CaCl2 was added toGO sol as an efficient cross-linker to turn GO sol into printablegel ink (Figs. 23a–23c).[84] With trace addition of Ca2+ ions asgelators, aqueous GO sol converts to printable gel ink. Theions prevent the collapse of 3D structure and keep GO sus-pension a shear thinning behavior to flow, which provides the

220

2103DGC-1

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f

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Original 100% Tensile Recovered

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Fig. 23 (a) Photograph of as-printed woodpile structure of graphene CAs from a top view. (b) SEM image of the 3D graphene CAs architectureand its gradient porous structure. (c) The rate performance of supercapacitors for 3D GAs with different thicknesses. (Reprinted with permissionfrom Ref. [84]; Copyright (2018) Wiley). (d) The catalytic mechanism of S→O acetyl migration by neat graphene oxide aerogel in a confined cell.(Reprinted with permission from Ref. [231]; Copyright (2015) American Chemical Society). (e) Schematic illustration of the assembly of highlystretchable CAs through 3D ink-printing. (f) A photograph of ultralight CAs with hierarchical buckled structures (5.7 mg/cm3) floating on a flower.(g) In situ SEM tracking on the structural evolution of CAs. (Reprinted with permission from Ref. [78]; Copyright (2018) Nature Publishing Group).(h, i) Micrographs of re-dispersed GO fibers and the wet-fusing process of GO fibers under optical microscopy and POM. (Reprinted withpermission from Ref. [235]; Copyright (2016) Nature Publishing Group).


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availability of the DIW method for a neat GO system. The ob-tained graphene CAs present high gravimetric capacitance of213 F/g at 0.5 A/g and 183 F/g at 100 A/g while used as theelectrode of supercapacitors, and retain over 90% after5.0×104 cycles due to the rich hierarchical pores and highelectrical conductivity. Besides, this ion-induced gelationmethod is also available for preparing GO-based compositeinks, such as GO/CNT ink. Moreover, neat GO aerogel can beused as a catalyst in the reaction of S→O acetyl migration forthe synthesis of thiol compounds (Fig. 23d).[231]

However, CAs still face stability and performance issuesthat hinder their wide-spread applications. For example, CAsbehave severely brittle under tensile deformation, especiallyunder extreme temperatures due to frozen chains at low tem-perature and viscous behavior at high temperature. Besides,incorporation with polymer materials may weaken the favor-able functionalities of CAs like highly electrical conductivityand low density. To solve these problems, Guo et al. de-veloped a kind of highly stretchable carbon aerogels with aretractable 200% elongation through hierarchical synergisticassembly.[78] The CAs were fabricated by ink-printing homo-geneous aqueous mixtures of GO and purified multiwalledcarbon nanotubes (MWNTs) to program-controlled 3D struc-tures, followed by freeze-drying and chemical/thermal reduc-tion treatment (Figs. 23e–23g). The elastic strain energy toconform to large tensile deformations originates from the en-hanced van der Waals force between MWNTs and graphenecaused by restored conjugated domains after chemical reduc-tion. Therefore, the multi-order hierarchical structures andsynergistic reinforcement between graphene and MWNTs en-dow CAs with highly stretchable behavior and allow applica-tions such as strain sensors for precise logic identification ofcomplex shape conversions.

The intrinsic properties of graphene aerogel films (GAF)such as lightweight, chemically stable and high conductiveendow them with high electromagnetic interference (EMI)shielding performance. Xi et al. designed GAF with multilayerstructure, showing excellent shielding effectiveness (SE) of~135 dB in 0.1–3 GHz at GAF thickness of 1.4 mm, and anuptrend from 65 dB to 105 dB in 2−18 GHz at thickness of 120mm.[195] The high EMI shielding performance of GAF is re-lated to the expanded interlayer spacing in a multilayer struc-ture, as proposed as “expansion enhancement effect (EEE)”.The electro-magnetic waves are reflected or transmit at eachinterface of the layers, and the reflected wave will experiencereflection and transmission again on the other interface. As aresult, the incident wave would experience infinite reflec-tions between the interfaces, and the effective transmissivity(τe) can be calculated by the equation:

τe =2Z1Z2(Z1 + Z2) ⋅ sinh (γ1d) + (2Z1Z2) ⋅ cosh (γ1d) (2)

where Z1 and Z2 are wave impendence of free space (~377 Ω)and material, respectively, γ1 (m−1) refers to propagation cons-tant of free space, and d (m) is the thickness of space layer.Other carbon-based materials for EMI shielding and microwaveabsorption (MA) have also been reported, such as porousgraphene microflowers, graphene micro-popcorns with hollowstructure, porous biomass-pyrolized carbon (PBPC) based onnatural wood, graphene/carbon nanotube composite films,

etc.[196,232−234]

A new concept of non-woven graphene fiber fabrics (GFFs)was raised by Gao’s group.[235] These non-woven GFFs consistof randomly oriented graphene fibers with strong interfiberinteraction. In other words, GFFs present macroscopically pa-per-like structure with randomly cross-linked graphene fibersat the microscopic scale. The synthesis of GFFs is shown asfollows: firstly, GO staple fibers were prepared by the continu-ous spinning of GO/DMF solution into an ethyl acetate coagu-lation bath; then the collected GO fibers were re-dispersed inthe mixture of water and ethanol, and subsequently filtratedand dried to make interfused GO fiber fabrics; finally, GFFswere obtained after chemical reduction and high-temperat-ure thermal treatment of GO fiber fabrics. Particularly, themechanism of cross-linking of the graphene fibers was ex-plained as wet-fusing assembly. The dried GO fibers got wet-ted immediately during the re-dispersion in aqueoussolvents, and they gradually swelled and then turned into gelfibers by solation. Strong interfiber interaction could be real-ized among the gel state of fibers, and the gel fiber was ableto fuse at the contact part, connect with each other and be-come one crossing fiber in the subsequent drying process(Figs. 23h and 23i). We ascribe this wet-fusing assembly to theswelling of fibers and hydrogen bonding between GO sheetsat the interface. Benefitting from the well-connected frame-work and link transmission routes for electrons and phonons,the obtained binder-free GFFs possess record-breaking-spe-cific electrical (2.8×104 S/m) and thermal conductivities (301.5W/(m·K)), as well as low density (0.22 g/cm). GFFs can be usedin applications such as electrothermal heaters, organic ad-sorption, separator membranes, catalyst supports, and con-ductive electrode. While using as textile electrodes of super-capacitors, hydrothermally activated GFFs can achieve anareal capacitance of 1060 mF/cm2 at a thickness of 150 μmand 7398 mF/cm2 by overlaying several layers of GFFs.[236]

In conclusion, Gao and coworkers have established a com-mon method for the assembly of GO molecules from liquidcrystals to macroscopic materials, including aerogel ball, aero-gel fiber, non-woven fabric, short/long fiber, film, ribbon andtheir composite materials (Fig. 24). The properties of thesegraphene-based materials can be tuned by the successfulcontrolling of GO liquid crystal precursors.

7. GO-BASED COMPOSITES

Hybridization of the materials can bring more degrees offreedom to combine physical/chemical properties and achievemulti-functional applications. The synergistic effects of theindividual materials allow intriguing properties on optical,electrical, thermal, mechanical and electrochemical fields. Ingeneral, the interaction between GO and the target materialscould be maintained by π-π interaction, electrostatic attraction,van der Waals force, covalent bonding, hydrogen bonding, andcoordinate bonding. It is important to demonstrate that themanufacturing of graphene-based composites should meet thefollowing requirements, to harness the individual andsynergistic properties to applications. First, graphene sheetsshould be produced on a sufficient scale via a relatively simple,low-cost and energy-saving method. Secondly, graphene isproposed to incorporate into the target materials successfully.


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Thirdly, these materials should be distributed homogeneously.GO has proved its advantages over pristine graphene in thecompositing progress due to its scalable production and gooddispersion in organic/inorganic solvents. Moreover, based onthe rich groups on GO sheets, the hybridization of GO and otherorganic/inorganic materials gives new opportunities to producedesirable-performance materials in target applications.

7.1 Incorporation with OrganicsCompositing graphene/graphene oxide with polymers is animportant area which can usually improve the electrical,mechanical or barrier properties. In this part, we would like tosummarize the interactions between graphene and targetmaterials and demonstrate how these interactions work fordesigned applications. According to Kinloch’s opinion,[237]

four different chemical interactions were demonstrated forgraphene incorporated with polymer matrix, including chemicalbonding (e.g., covalent bonding, hydrogen bonding andcoordinate bonding), π-π interaction, electrostatic attraction,and van der Waals force.

Continuous neat graphene fibers have been achieved bywet-spinning due to the liquid crystal behavior of GO. There-fore, if the LCs of polymer-grafted GO could be stabilized insolvents, it would show great potential for the synthesis ofpolymer-functionalized graphene fibers. Liu et al. reportedthat PAN grafted GO was highly dispersible in polar organicsolvents such as DMF and dimethyl sulfoxide (DMSO), and itwas introduced to the continuous wet-spinning of nacre-mi-metic GO-g-PAN fibers.[135] PAN molecules were uniformlygrafted onto GO surface, and a classic “brick-and-mortar”(B&M) microstructure was formed. Due to the covalent bond-ing between PAN and GO, the obtained fiber showed superi-or enhanced mechanical performance compared to the neatGO fiber, neat PAN fiber and GO/PAN blending fiber.

Generally, the functionalized oxygen-containing groups ofGO sheets allow more possibilities for incorporation withother materials than graphene or rGO. For example, incorpor-ating rGO into polyaniline (PANI) can increase the specific sur-face and its electric conductivity, endowing the compositeswith excellent behavior as electrode materials.[238,239]

However, in situ polymerization of aniline on rGO matrixyields a non-uniform PANI coating, and it will cause un-desired concentration polarization in the composite elec-trode.[240] In contrast, the approach starting from GO aqueoussolution and PANI solution in N-methyl-2-pyrrolidinone (NMP)leads to uniform incorporation of GO into PANI at molecularlevel. Because of the π-π interaction between PANI and GO,the PANI on GO sheets are stable in the GO reduction processafter the PANI@GO sheets lose their dispersibility when theoxygen-containing groups of GO are removed by the reduct-ant ascorbic acid, and a uniform 3D network is finally as-sembled (Figs. 25a and 25b).[241] The supercapacitor usingPANI/rGO composite as the electrode shows a high specificcapacitance of 808 F/g (5717 mF/cm2) at a current density of53.33 A/g. Boland et al. reported viscoelastic graphene-silic-one polymer nanocomposites with electromechanical prop-erties.[242] The electrical conductivity of the composites in-creases strongly with graphene content, and the stiffness ofthe composite also increases. These special properties makethe obtained graphene-grafted silicon polymer high-perform-ance sensing materials which can monitor deformation andpressure with high sensitivity (Figs. 25c and 25d). The π-πstacking of rGO and commercially poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) is crucial tothe self-assembly of composites. The rGO/PEDOT:PSS com-posites work as high-performance flexible electrodes for elec-trochemical capacitors with high volumetric and areal specif-ic capacitance performance. PEDOT:PSS works as a binder tobridge graphene sheets together and provides highly con-ductive networks for electron/ion transport. Firstly, the one-dimensional PEODT:PSS chains crosslinked with rGO particlesto form a hierarchical architecture, and then PSS was proton-ated and partially removed after the adding of H2SO4, result-ing in the enhancement of π-π stacking of rGO and PEDOT(Figs. 25g–25i). Hence, the inherited hierarchical architectureensures the 3D interlinked network with sufficient ion trans-port, delivering ultrahigh areal (5365 mF/cm2) and volumetric(203 F/cm3) specific capacitances at 1 A/g.[243]

Chemical bonding interactions between GO and polymers

Long fiber

Aerogel ball

Loading

Unloading

Film

Ribbon

Nacre-like composite fiber

Aerogel fiber100 µmFolded

Short fiber andnon-woven fabric

500 nm

Wet assembly of liquid crystal

Fig. 24 A route map for GO assembly from liquid crystals to various macroscopic materials.


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can be realized by choosing proper pairing materials, e.g.gelatin. Gelatin is a natural protein with both hydrophobicand hydrophilic amino acid side chains, while GO possessesan amphiphilic structure with hydrophobic carbon backboneand hydrophilic functionalized groups on the basal plane andedges, thus facilitating the formation of multiple interactionsbetween GO and this polymer. The ultrafiltration hybridmembrane fabricated by layer-by-layer (LbL) self-assembly ofthe composites of GO, gelatin and hydrolyzed polyacrylo-nitrile (H-PAN) possesses multiple interactions: electrostaticattraction, hydrogen bond, and hydrophobic interaction(Fig. 26a):[244] (1) electrostatic attractions originated from theionized carboxyl groups on GO and the protonated aminogroups on gelatin; (2) hydrogen bonds arising from oxygen-containing groups (such as hydroxyl, epoxy, and carboxylgroups) on GO sheets and polar groups on gelatin (such asamino, carboxyl, and carbonyl groups); (3) hydrophobic inter-actions between the carbon backbone of GO and hydro-phobic amino acid side chains on gelatin (such as pyrrolidinering and aromatic ring). Shin et al. combined CNTs with rGO in

solution-spun polymer fibers, and assigned the toughness en-hancement to the hydrogen bonding interactions betweenCOOH of rGO and SO3

– of sodium dodecyl benzene sulfonateattached to SWNTs (Figs. 26b and 26c).[156]

The spin-coating method affords GO films with a well-stacked, highly-interlocked microstructure with in-plane ori-entated GO sheets, as well as its simplicity to employ. He et al.chose 1,4-phenylenediamine-2-sulfonic acid (PDASA) as acrosslinker for coating GO films onto a Nafion support, be-cause the double primary amine groups can react with theepoxy groups of GO during thermal treatment.[245] Stankovichet al. fabricated the graphene-polymer composites via themolecular-level dispersion of individual GO sheets with poly-styrene.[246] An isocyanate treatment of the surface of GOsheets promotes the formation of amide and carbamate es-ter bonds to the carboxyl and hydroxyl groups, making GOstable dispersions in polar aprotic solvents such as DMF. Sub-sequently, the isocyanate-derivatized GO flakes can be easilysolution-phase mixed with other organic polymers, which isfavorable for the synthesis of graphene-polymer composites.

Reductionself-assemblySelf-assembly

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Fig. 25 (a) Schematic illustration of the preparation of PANI/rGO composite materials by solution-based self-assembly method. (b) SEMimage of PANI/rGO composite with 81.3% PANI content. (Reprinted with permission from Ref. [241]; Copyright (2018) Royal Society ofChemistry). (c) Photograph of spheres of silicon polymer and graphene-grafted silicon polymer. (d−f) Resistance measured using thegraphene-grafted polymer in (d) fringer motion, (e) breathing and (f) pulse. (Reprinted with permission from Ref. [242]; Copyright (2016)American Association for the Advancement of Science). (g) Stability comparison of the graphene/PEDOT:PSS composite film treated with(right) and without (left) 1 mol/L H2SO4. Inset: photograph of the graphene/PEDOT:PSS composite film bent by finger. (h) Cross-sectionalSEM image of the graphene/PEDOT:PSS composite film. (i) The schematic mechanism of the interaction between rGO and PEDOT:PSSwithin 1 mol/L H2SO4 treatment. (Reprinted with permission from Ref. [243]; Copyright (2018) Royal Society of Chemistry).


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At the same time, the presence of polymer in the solutionprevents the agglomeration of GO sheets, facilitating its coat-ing on GO sheets and individual dispersion in solvents.

Apart from the chemical bonding interaction, electrostaticattraction is one of the main interactions between GO andpolymers. GO/rGO sheets can be integrated with other func-tional polymer materials at a molecular scale to create multi-functional graphene-based composites.[247] The formation ofelectrostatic attraction arises from the negatively chargedcarboxyl groups on GO and a positively charged polycationsolution, such as poly(allylamine hydrochloride) (PAH),[248]

PAN,[244] PEI,[249] PSS,[250] etc. For example, higher-order 3D ar-chitectures such as hollow micrometer-size spheres (Fig. 26dand 26e) or sponge-like macroporous scaffolds (Figs. 26f and26g) can be designed for graphene-polymer nanocomposi-tes using the stable aqueous suspension of GO/PSS.[250] Be-sides, Yun et al. wrapped GO on nylon-6 via electrostatic self-assembly with bovine serum albumin (BSA).[251] BSA mo-lecules were grafted via a dipping process which inducedpositive charges on the surface of the nylon-6 yarn, facilitat-ing the subsequent GO coating via electrostatic self-as-sembly (Figs. 26h–26j). Therefore, BSA serves as a universal

adhesive for enhancing the adsorption of GO sheets onto thetextile, thus leading to GO easily attached to textiles whichprovides a very promising platform for wearable electronicdevices applications.

In addition, van der Waals force should not be ignored es-pecially in the physically adhesive GO coating materials, for itmakes the interaction between GO and substrate muchstronger due to the large contact area of the interface in-duced at a nanoscale regime. For monolayer graphene, theadhesion energy is measured to be ~0.45 J/m2 while coatingon a silicon oxide substrate. Therefore, it is predicted thatlarge contact areas between GO and the substrate benefit thestrong adhesion of these two individual components. Hu et al.explained the interfacial behavior as the competitionbetween the binding energy of GO and adhesion energy ofGO-substrate, which determines the equilibrium morphologyof the membranes (Fig. 26k).[252] High deposition pressurewas employed to make conformal morphology, which facilit-ates the strong adhesion because of the strong van der Waalsinteraction resulting from the large contact areas.

Simply coating or wrapping of GO/rGO on the second com-ponent is a facial, scalable and economic approach for pre-

a HN

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Fig. 26 (a) Schematic illustration of multiple interfacial interactions between gelatin and GO, including electrostatic attraction, hydrogen bondand hydrophobic interaction. (Reprinted with permission from Ref. [244]; Copyright (2015) Elsevier). (b) TEM image showing the binding ofSWNTs with rGO fibers and (c) the schematic diagram of hydrogen bonding interactions between them. (Reprinted with permission from Ref.[156]; Copyright (2012) Nature Publishing Group). (d, e) SEM images of PSS-G-coated PAH-functionalized polystyrene beads and the sample afterthe treatment of toluene. (f) Longitudinal section and (g) cross-section of PSS-G/PVA freeze-dried monolith. (Reprinted with permission from Ref.[250]; Copyright (2009) Wiley). (h) Schematic illustration of the steps used to prepare the rGO/Nylon-6 yarns. SEM image of (i) knotted rGO/Nylon-6 yarns and (j) two-ply rGO/Nylon-6 yarns. (Reprinted with permission from Ref. [251]; Copyright (2013) Wiley-VCH). (k) Schematic illustrations ofthe conformal and nonconformal morphologies of GO coatings on microfiltration (MF) and reverse osmosis (RO) membranes, to explain thefactors determining the adhesion of graphene films and the substrate. (Reprinted with permission from Ref. [252]; Copyright (2018) AmericanChemical Society).


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paring the composites. Numerous graphene-based compos-ite materials such as reduced graphene oxide wrapped plantfiber sponges (PFS@rGO),[253] Nafion/GO film,[245] cotton fab-ric,[254] and GO/acrylic yarns[255] have been reported. GO/rGOprovides the conductive pathway for the incorporated com-posite materials. Conventional preparation of PANI/graphenecomposite used for electrode materials focuses on the uni-form structure of the composites, and great attention hasbeen paid to enhancing the electrical transport. However, auniform structure cannot guarantee the diffusion of the elec-trolyte which is the key factor determining the high rate per-formance. Wu et al. deposited PANI outside the graphenemonolith, forming a phase-separated PANI/graphene struc-ture.[240] It is distinct from the uniform-dispersion or LbL struc-ture reported. In this structure, PANI was electrochemicallydeposited onto the graphene matrix using cyclic voltam-metry (CV) technique. The phase-separated structure avoidsblocking of channels for electrolyte diffusion, thus facilitatingthe diffusion of electrolyte and improving the performance ofPANI/graphene composite at large current density.

7.2 Growth of Metal, Metal Oxide/HydroxideThe composite materials show enhanced electrochemical or cat-alytic performance compared to the original system by takingadvantage of the additional properties of graphene.[256,257] Avariety of graphene-metal, metal oxide or hydroxide materialshave been employed for preparing GO-base composites, suchas Au,[256] ZnO,[258] TiO2,[259,260] g-C3N4,[261] γ-Fe2O3,[262] Fe3O4,[263]

CuS (Fig. 27h),[264] etc. Tuning the surface chemistry of GO orrGO sheets offers a way to control the growth and morphologyof metal or metal oxides. GO sheets are able to capture the fun-ctional nanomaterials with multiple adhesion sites by variousinteractions due to their abundant surface chemistry, such asphysisorption, van der Waals, hydrophobic and π-π interactions.

A universal strategy was developed to incorporate func-tional nanomaterials into porous graphene structures to form3D porous graphene hybrid architectures, such as Au-NPs,TiO2-NPs, MnO2 nanowires (MnO2-NWs), InN-NWs, Zn2SnO4-NWs, polyaniline nanofibers and CNTs (Fig. 27a).[256] The func-tional porous GHAs were fabricated by solution-based ap-proaches based on mixing assisted with ultrasonication, hy-drothermal, and freeze-drying process. All these nanomateri-als can be well dispersed in GO suspensions and form stablemixed suspensions after ultrasonication treatment becausethe hydrophobic basal plane and hydrophilic edges of GOsheets make it act as a surfactant. The hydrothermal processinitiates the reduction of GO sheets, as well as embeds the ad-ditive nanomaterials in hydrogels. SEM images suggest the ef-ficient embedding of Au-NPs in the rGO walls of its continu-ously cross-linked 3D network, as well as other functionalnanomaterials (Figs. 27b–27e).[256] In addition, electrochemic-al treatment is an efficient approach to the reduction of GO,as well as incorporation of noble metal nanoparticles andmetal oxide. Chen et al. fabricated 3D porous graphene-basedcomposite materials by electrochemical reduction of GOdispersion, followed by electrochemical treatment again to

a

Mixedsolution

Au NP

Zn2SnO4 NW

35 nm

Mapping area

InN NWTiO2 NP

TiO2 NPGO Au NP CNTs PANI MnO2 InN Zn2SnO4TiO2

FeC

NiO

CuS

b c

d

f g h

e

500 nm 2 µm

5 µm

1 µm 1 µm 1 µm

100 nm

5 µm

Fig. 27 (a) Optical images of Au-NPs, TiO2-NPs, CNTs, PANI-NFs, MnO2-NWs, InN-NWs and Zn2SnO4-NWs aqueous solutions and theirmixture with GO aqueous suspension. (b−e) SEM images of rGO-based composites incorporated with (b) Au-NPs, (c) TiO2-NPs, (d)Zn2SnO4-NWs, and (e) InN-NWs, respectively. (Reprinted with permission from Ref. [256]; Copyright (2014) Wiley). Cross-section images ofelectrochemical reduced 3D graphene architectures deposited by (f) Pt and (g) ZnO nanoparticles, respectively. (Reprinted withpermission from Ref. [258]; Copyright (2012) Royal Society of Chemistry). (h) FESEM image of magnetically decorated (NiFe2O4)graphene@CuS with CTAB and the corresponding elemental mapping images. (Reprinted with permission from Ref. [264]; Copyright(2016) American Chemical Society).


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deposit the second component (Pt and ZnO nanoparticles,Figs. 27f and 27g).[258]

The interaction between GO and nanoparticles can be fur-ther enhanced by adding a surfactant in the solution. It is re-ported that glucose is able to serve as the linker, promotingthe in situ growth of TiO2 nanocrystals on the surface ofgraphene aerogel owing to its rich hydroxyl groups.[259] TiO2

nanocrystals are highly dispersed on the graphene aerogelsurface, affording the high surface area for active sites andframeworks for efficient carrier transport pathways. At thesame time, glucose molecules facilitate the growth of nano-size TiO2 seeds with exposed (001) facets, imparting thesecomposite materials excellent photocatalytic activities andoutstanding Li-storage performance. Apart from TiO2, two-di-mensional g-C3N4 is proven to be an efficient photocatalyst.GO sheets can inhibit the aggregation or stacking of subunits,thus exposing more active sites for catalytic reactions.Moreover, the electron-hole recombination of g-C3N4 is inhib-ited by the good electrical conductivity of GO, and the visible-light utilization is improved via the light multi-reflectionacross the 3D-connected porous open-framework.[261]

Moreover, the cationic polymers connected with GO sheetscould promote the growth of inorganic nanoparticles such asmetal and metal oxide particles on the surface or in the bulkof the composites without aggregation. For example, poly-ethyleneimine (PEI) is a kind of cationic polymer with highamine density, which can be easily physically adsorbed ontothe GO surface (hydrogen bonding, electrostatic interaction)and chemically bonded with the functional oxygen-contain-ing groups of GO sheets. These physical and chemical interac-tions play key roles in controlling the assembly of GO withpolymer, as is mentioned above. Besides, the free aminegroups of PEI cationic polymer act as mineralization inducers,offering a way to catalyze the nucleation and growth of a vari-ety of inorganic nanoparticles such as metal and metal oxidenanoparticles.[249] The obtained inorganic nanoparticles canbe well distributed on the surface of the composites. There-fore, the cationic groups of polymers have played the follow-ing critical roles during the synthesis of the composites: (1) as-sembling GO via physical and chemical interactions, (2) initi-ating chemical reduction of GO sheets, and (3) inducing themineralization of nanoparticles.

8. CONCLUSIONS AND OUTLOOK

In recent years, huge progress was registered in terms of thestudy of GO 2D macromolecules, including experimentalobservation and theoretical description of the chemicalstructure and physical properties of GO. The relative molecularmass of GO macromolecules is several orders of magnitudehigher than that of 1D polymers. For instance, the relativemolecular mass of GO single-layer sheet with a size of 100 μmcan reach 1013, while that of a 1D polymer usually locates at104–106. The conformation of 1D polymers includes stretching,fold, random and spiral chains due to its linear intrinsic property,while the conformation of 2D GO polymer varies fromanisotropic 1D folding to isotropic 3D crumpling phases(Table 1). Both 1D polymers and 2D GO macromolecules can beused for the assembly of macroscopic materials, such as fibers,films, foams, fabrics and aerogels. However, 1D polymermaterials are usually assembled by entanglement of polymerchains, which is different from the assembly process of 2D GOmacromolecules via the stacking of GO layers.

The easy dispersion of GO in polar solvents makes it pos-sible for its macro-assembly and modification. The amphi-philicity of GO can be tuned by domain size, pH, and cationintroduction. The formation of a uniform nematic phase ofGO molecule clusters enlightens the exploitation of the mac-roscopic ordering graphene materials. Probing and quantify-ing the rheology behavior of GO sheets is also crucial to guid-ing their assembly. In terms of GO molecular functionaliza-tion, heteroatom chemical doping and surface modificationwith small molecules and polymer branches are promoted byreconstructing the sp2-bonded carbon atoms and tailoringthe surface chemistry interaction, respectively.

Furthermore, graphene-based materials have exhibitedoutstanding mechanical, optical, thermal, magnetic and elec-tronic properties, harvesting great interest of scientific com-munity and huge potential for wide-spread industrial applica-tions. The fundamental parameters between the assemblyand mechanical/transport properties have been proposed, in-cluding crystalline graphitic domain size, defect-free engin-eering, alignment of graphene sheet and compactness ofgraphene layers, to further improve mechanical, electrical andthermal transport properties of graphene-based materials.

Table 1 The properties of 2D GO macromolecule and 1D polymer.

2D GO macromolecule 1D polymer

Structure

Relative molecular mass 108−1013 (Size: 1−100 μm) 104−106

Conformation

Flat, fold, crumple, flat stack,multiple folds, multiple crumples Stretch, fold, random, spiral

Liquid crystal behaviorShear shinning

AssemblyFiber, film, foam, fabric, aerogel

Stacking Entanglement


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For graphene-based composite materials, the interactionbetween GO and the target materials has been discussed,such as covalent bonding, hydrogen bonding, coordinatebonding, π-π interaction, electrostatic attraction, and van derWaals force.

Generally, we still need a deeper understanding of GO mac-romolecules, from single-molecule behavior, cluster interac-tions, macro-assembly to their applications. Despite thathuge achievement on GO has been made, there are still nu-merous challenges on GO macromolecules, as well asgraphene-based materials.

(1) The precise synthesis of graphene oxide with designedproperties remains a challenge, including tuning its structure,domain size, C/O ratio, holes and defects, designing theblocks, and multi-level control over morphology from microto macro.

(2) The study of GO single-molecule behavior in solvent(conformation, stretching, phase separation, dissolution kin-etics, thermodynamics, and the interface between liquid andother mediums) is still limited, and efficient observation ap-proaches are waiting to be developed. Subsequently, the sys-tem of GO two-dimensional macromolecular physics remainsto be established.

(3) Controllable and designable doping, intercalation, andfunctionalization have not been realized. For example, cova-lent functionalization generally compromises the integrity ofthe sp2 network of graphene lattices, resulting in defects andloss of the electronic properties. Besides, only specified ele-ments or compounds can be intercalated into graphene lay-ers, such as Au, Co, Na+, Li+, Ca2+, FeCl3, MoCl5, AlCl4–, etc. Oth-er intercalation compounds of graphene remain to be ex-plored.

(4) The out-of-plane thermal transport of graphene filmslimits the practical applications related to a high-efficiencyheat dissipation mobile phone. More researches should focuson the out-of-plane thermal transport of graphene films inthe future, rather than the in-plane thermal transport.

Up to now, graphene-based materials have gained hugeachievements since its discovery in 2004, and shown specificproperties and superior performance. They have alreadyshown promising applications in catalysis, energy storage,mechanical devices, chemical sensors, gas/ion/dye/wave ab-sorption, solar-thermal energy conversion, solar-electron en-ergy conversion, electronics, photonics, etc. In the light of thehuge progress we have registered, there is still a long way tographene-dominated “carbon age”. Therefore, disruptive andirreplaceable applications based on graphene materialsshould be fully explored. A predictable direction is construct-ing graphene fibers with super-strong, stiff, tough, light-weight, flexible and electrical transport performance. Sincecarbon fibers have already been widely used in industry, theindustrialization of graphene fibers should be promoted tomeet the demands in practical applications. Another direc-tion is developing carbon-based nano-electronics with dom-inant advantages compared with the current semiconductorsin “silicon age”. We expect that graphene-based electronicsand photonics will usher in a wave of a new round of devel-opment very soon, once the interface between graphene andtarget materials reaches atomic level, based on CVD-graphene membrane with uniform thickness by new techno-

logy or controllable macro-assembled graphene nano-films.We believe carbon materials represented by graphene areunveiling the dawn of a new era in the 21st century.

BIOGRAPHY

Chao Gao received his Ph.D. degree from Shanghai Jiao TongUniversity (SJTU) in 2001. He was appointed as an AssociateProfessor at SJTU in 2002. He did postdoctoral research at theUniversity of Sussex with Prof. Sir Harry Kroto and AvH researchat the Bayreuth University with Prof. Axel H. E. Müller. He joinedthe Department of Polymer Science and Engineering, ZhejiangUniversity, in 2008 and was promoted as a Qiushi DistinguishedProfessor in 2014. He leads a Nanopolymer group working ongraphene chemistry, macroscopic assembly, and energy storage.

ACKNOWLEDGMENTS

This work was financially supported by the National Key R&DProgram of China (No. 2016YFA0200200), the National NaturalScience Foundation of China (Nos. 51533008, 51703194,51873191, and 21805242), Hundred Talents Program ofZhejiang University (No. 188020*194231701/113), Key Researchand Development Plan of Zhejiang Province (No. 2018C01049),Fujian Provincial Science and Technology Major Projects (No.2018HZ0001-2), and Key Laboratory of Novel Adsorption andSeparation Materials and Application Technology of ZhejiangProvince (No. 512301-I21502).

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