Self‐Assembled Graphene‐Based Architectures and Their Applications

Abstract Due to unique planar structures and remarkable thermal, electronic, and mechanical properties, chemically modified graphenes (CMGs) such as graphene oxides, reduced graphene oxides, and the related derivatives are recognized as the attractive building blocks for “bottom‐up” nanotechnology, while self‐assembly of CMGs has emerged as one of the most promising approaches to construct advanced functional materials/systems based on graphene. By virtue of a variety of noncovalent forces like hydrogen bonding, van der Waals interaction, metal‐to‐ligand bonds, electrostatic attraction, hydrophobic–hydrophilic interactions, and π–π interactions, the CMGs bearing various functional groups are highly desirable for the assemblies with themselves and a variety of organic and/or inorganic species which can yield various hierarchical nanostructures and macroscopic composites endowed with unique structures, properties, and functions for widespread technological applications such as electronics, optoelectronics, electrocatalysis/photocatalysis, environment, and energy storage and conversion. In this review, significant recent advances concerning the self‐assembly of CMGs are summarized, and the broad applications of self‐assembled graphene‐based materials as well as some future opportunities and challenges in this vibrant area are elucidated.

applications. [1][2][3][4][5][6][7] To date, graphene has been fabricated by using various methods including mechanical [1] or chemical exfoliation, [5] chemical vapor deposition (CVD), [7] and epitaxial growth. [8] Pristine defectfree graphene cannot be well dis persed in water or in a range of various organic solvent. [8] Because of graphene's poor processability, it is difficult to employ traditional processing methods to build these 2D building blocks into desirable structures and eventually into a func tional system that is of great importance to exploit useful properties of the indi vidual graphene nanosheets (NS) for practical macroscopic applications. In sharp contrast, graphene oxides (GOs), reduced graphene oxides (rGOs), and the related derivatives, generally defined as chemically modified graphenes (CMGs), are superior alternatives because of the following advantages: (i) the preparation for CMGs is usually performed by a com bined chemical oxidation and exfoliation process using pristine graphite as starting precursors without any particular instruments such as the CVD system or the tedious mechanical exfoliation process, and therefore is low cost and easy for the mass production; (ii) some CMGs such as GOs and chemically modified rGOs bear rich functional groups including hydroxyl, carboxyl, epoxy, and carbonyl groups, which could reduce the intersheet stacking and thereby render their good dispersion in solutions; [9] (iii) CMGs usually act like both molecules and colloid surfactants, making them unique 2D building blocks favorable for the assembly with themselves and other inorganic/organic components by noncovalent forces; [9] (iv) the multiple functional groups and conjugated carbon basal plane of CMGs afford a lot of opportunities for chemical func tionalization to endow them with improved properties and new functions. Therefore, CMGs hold great potential in a series of various applications including those in electronics, optoelec tronics, electrocatalysis/photocatalysis, energy conversion and storage systems, environment, and so on, and have attracted significant interest especially in the fields of materials and chemistry. [8][9][10] On the other hand, it has been recognized that trans lating individual graphene sheets into welldefined hierar chical architectures and ultimately into highperformance functional systems will pave the way to meet the practical demand of various applications that require bulk graphene materials. Selfassembly has been considered as one of the most promising approaches to construct advanced graphene based functional materials, which involves the organization of different building blocks into complex superstructures of various scales through noncovalent interactions. By virtue of a range of noncovalent forces such as hydrogen bonding, van der Waals interactions, metaltoligand bonds, electrostatic attraction, hydrophobic-hydrophilic interactions, and π-π interactions, CMGs with multiple functional groups are desir able for the assembly with themselves and a wide variety of organic and/or inorganic functional components which can yield hierarchical nanostructures and macroscopic compos ites endowed with unique structures, properties and func tions for broad practical applications. [9] Accordingly a variety of graphenebased hierarchical nanostructures and functional hybrid architectures have been produced via the selfassembly of CMG building blocks.
In this review, we provided a broad overview covering the latest assembly strategies that have been explored to build a variety of graphenebased nanostructures as well as macro scopic architectures including macroscopic fibers, thin films, membranes, papers, and porous networks. Furthermore, we also highlight some typical examples of the potential technical applications of selfassembled graphenebased materials. Lastly, we elucidate some potential opportunities and challenges in this ever growing field. We anticipate that this review and ongoing efforts in this emerging field will provide useful guide lines to scientists toward highly efficient production of self assembled CMGbased structures and their diverse realworld applications.

State-of-the-Art Self-Assembly Strategies of Graphene Architectures
The selfassembly for CMGs usually occurs in a solution or at an interface with π-π stacking, hydrophobic/hydrophilic interactions between each of their basal planes, or electro static interactions of the functional groups as the main driving forces. Because of their attractive 2D structural characteris tics including high aspect ratio and large specific surface area as well as their unique supermolecular properties, CMGs are favorable for the assembly into different graphene architec tures, such as largearea thin films, paperlike membranes, 3D porous network structures, 1D macroscopic fibers. To date, several effective approaches have been established to assemble these 2D building blocks either in solution or at the interface, including layerbylayer (LbL) assembly technique, Langmuir-Blodgett (LB) deposition, flowdirected, evaporationinduced, or interfaceinduced selfassembly, and templatedirected/space confined assembly. This section will present the stateoftheart of the strategies for the selfassembly of graphene nanostruc tures and architectures.

Langmuir-Blodgett Method
LB assembly is well known as a robust and powerful approach for translating 2D building blocks into highquality thin films with ordered microstructures and desired functionality at the molecular scale by promotion of lateral packing and wellcontrolled compression at the air/liquid interface. [11] It was suggested that the marginal electrostatic repulsion between single graphene layers inhibits their overlap when compressed at the air-liquid interface, thereby yielding an ordered mono layer film on hydrophilic substrate. CMGs are featured as planar macromolecules with large aspect ratios. To exploit prac tical applications of graphene in nanoscale electronics, the LB technique is an ideal approach to achieve flat, largearea, single or multilayered graphene films, since it is capable of achieving fine control of the monolayer thickness and homogeneous deposition as well as the formation of multilayers with various layer compositions.
Fine dispersion of the building blocks in solutions is a key prerequisite for LB assembly. Considering that amphi philic GO sheets are able to be readily dispersed in aqueous solution and certain organic solvents, GO has now been considered as a popular precursor for fabricating largearea graphene films. [12,13] Huang and coworkers demonstrated that stable monolayer GO films could be achieved without using any stabilizing agent by the LB technique, as shown in Figure 1. [14] The monolayers are easy to form a largearea, and flat GO film with adjustable density from dilute closepacked to overpacked monolayers. An ordered monolayer film con structed by GO decorated with octadecyl ester of rhodamine B was assembled onto a hydrophilic substrate by using the LB technique as well. [15] By varying the pH and the GO concen tration in the water subphase, the thinfilm architecture can be readily tuned. In addition to hydrophilic GO sheets, the LB technique was also employed to assemble hydrophobic rGO sheets produced by various methods into largearea con ducting film with fine control over the thickness on different substrates. [16][17][18] Traditionally, the LB method has often been used for the deposition of 2D structures. More recently, by exploring the conventional dipping mechanism in an LB trough, Vengadesh and coworkers have built an unconventional LB assembly method, which compresses the monolayer beyond the collapse pressure state to produce the 3D porous graphene films. [19] The high porosity and the tunable roughness of rGO films from nanoscale to macroscale can be well engineered during the deposition process.
Patterning of graphene films on different substrates has tremendous impact in a range of fields like nanoelectronics. Kumar and coworkers successfully utilized the LB deposi tion method in conjunction with selective N 2 plasmaengaged treatment of the SiO 2 /Si substrates to transform GO sheets with nonuniform size into a largearea and highly ordered array. [20] This method enables the formation of various pat terned graphene thin films with controllable dimensions.

LbL Assembly Method
The LbL method, which involves electrostatic interactions and hydrogen bonding as attractive forces for sequential adsorption of oppositely charged components, is an appealing approach for the fabrication of highly tunable and multilayered gra phene architectures, as it enables the nanoscalelevel controlled thickness and composition, as well as the tunable physico chemical properties of the multilayered films, by adjusting some parameters including the ionic strength, pH values, and concentration. [21] Up to now, the assembly of a series of different CMGs into the multilayered architectures by LbL assembly has been demonstrated, yielding highly controllable and con formal thin films on different substrates. [21][22][23][24][25] To achieve an effective LbL multilayered assembly, a critical requirement is that two oppositely charged components should be well dispersed in solution. Yoo and coworkers prepared well dispersed aqueous solutions of negatively charged graphene nanosheets (rGO-COO − ) by chemical reduction of GO and positively charged graphene nanosheets (rGO-NH 3 + ) by the modified thionyl chloride chemistry. Therefore, through alter nating LbL assembly between rGO-COO − and rGO-NH 3 + , homogeneous multilayered allgraphene thin films were cre ated based on electrostatic interaction (Figure 2). Precise control of the thinfilm thickness was achieved by tuning the stacking numbers. The LbLassembled rGO films showed easily modulated light transmittance and sheet resistance by varying the bilayer number, which is highly dependent on the multilayer thickness, via nanoscale engineering of the assembly. Further thermal treatment leads to enhanced elec trical conductivity. Considering that all processing steps are carried out in aqueous environments, the LbL method is envi ronmentally friendly and compatible with industrial scaleup processes. [26] Zhu and Tour also produced dispersions of the positively charged aminofunctionalized graphene nanoribbons (fGNRs) in N,Ndimethylformamide (DMF)/HCl and the negatively charged fGNRs with sulfonic group in water. Even though the LbL deposition was alternatively performed in water and DMF/HCl, uniform GNR films can also be produced on quartz and silicon wafer substrates with controllable thick nesses. When these GNR films were used for bottomgated fieldeffect transistors (FETs), the device exhibited mobilities of 0.1-0.5 cm 2 V −1 s −1 . [27] Grunlan and coworkers reported the fabrication of a multilayered thin film, in which polyvinylpyrrolidonestabi lized graphene (PVPG) platelets and poly(acrylic acid) (PAA) are alternately deposited using hydrogenbondingassisted LbL assembly. Such a multilayered thin film showed an increase of the elastic modulus (E) of a polymer multilayered thin film by 322% (from 1.41 to 4.81 GPa), while maintaining a visible light transmittance of ≈90%. [82] Furthermore, LbL assembly enables the preparation of not only conformal 2D thin films on a 2D surface but also 3D objects. By repeatedly assembling the components of rGO-NH 3  images showing the collected GO monolayers on a silicon wafer at different regions of the isotherm. The packing density was continuously tuned: a) dilute monolayer of isolated flat sheets, b) monolayer of closepacked GO, c) overpacked monolayer with sheets folded at interconnecting edges, and d) over packed monolayer with folded and partially overlapped sheets interlocking with each other. e) Isothermal surface pressure/area plot showing the corresponding regions (panels (a)-(d)) at which the monolayers were collected. Scale bars in panels (a)-(d) represent 20 µm. Reproduced with permission. [14] Copyright 2009, American Chemical Society. this approach, a new functionality such as gold nanoparticles (NPs) can be introduced into a hollow graphene capsule. The approach may open up new avenues for constructing hollow graphene structures with multiple functionalities. [24]

Flow-, Evaporation-, and Interface-Induced Self-Assembly
Flowdirected selfassembly, also called shearing fieldassisted alignment of suspensions, has been widely employed as a cheap yet efficient method to produce various foillike and inorganic paperlike films based on exfoliated vermiculite or mica platelets. Pioneering work to extend this method for the assembly of GO was performed by Ruoff and coworkers [28] In this respect, the aqueous GO solution consisting of indi vidual nanosheets was first prepared by chemical exfoliation from graphite powder followed by the ultrasonication in water, and the resultant aqueous GO was filtrated through a porous membrane filter, finally yielded GO paper with the thickness of 1-30 µm. Since vacuum filtration induces a directional flow, individual GO sheets were subjected to a nearparallel arrange ment, affording a freestanding membrane consisting of a wellpacked layered architecture after airdrying (Figure 3). The produced GO paper has good flexibility and excellent mechan ical performance possessing a tensile strength up to ≈80 MPa with Young's modulus up to ≈32 GPa, which outperforms many other paperlike or foillike materials. It is suggested that hydrogen bonding and strong van der Waals forces occurred within the GO nanosheets [28] are responsible for the remark able mechanical performance of the GO paper.
Though the GO papers often possessed great mechanical strength, they still suffered from low electric conductivity and poor thermal stability, constraining their broad applica tions. Following on the Ruoff's work, Li et al. prepared rGO papers by using similar vacuum filtration of readily dispersed aqueous rGO stabilized by electrostatic repulsion. The obtained layered rGO paper showed a shiny metallic luster while pos sessing a tensile strength of 35 GPa similar to that obtained for a GO paper. Moreover, the rGO paper exhibited signifi cantly enhanced electrical conductivity of up to 7200 S m −1 and thermal stability. By modulating the volume and concentra tion of the rGO dispersion, fine control over the thickness of the rGO paper could be achieved with flowinduced assembly technique. [29] This strategy were also applied to the preparation of transparent graphene films with nanoscale thicknesses. [30][31][32][33] For instance, Eda et al. fabricated largearea GO films with controllable thickness, varying from a monolayer to multiple layers through the vacuum filtration strategy using a diluted GO aqueous dispersion. [33] Combined chemical reduction and thermal treatment can transform the insulated GO thin films into highly conductive rGO films desirable for use in trans parent conductive electrodes and flexible electronics such as transistors.
In fact, flowinduced assembly method was also extended to other readily dispersed graphene sheets either in aqueous solution or in some organic solvents such as propylene carbonate. [34][35][36][37][38][39][40] For example, through vacuum filtration, 1pyrenebutyratemodified rGO aqueous disper sion could form CMG films with high conductivity and good mechanical performance. [34] rGO sheets produced by thermal Adv. Sci. 2018, 5, 1700626 Figure 2. Schematic illustration of a) synthesis of oppositely charged graphene and b) the generation of graphene thin films using an electrostatic layer-by-layer (LbL) assembly between oppositely charged graphene nanosheets and subsequent thermal treatment. Reproduced with permission. [26] Copyright 2011, American Chemical Society. reduction can be dispersed in propylene carbonate, which was utilized for assembly into paperlike membranes with good conductivity and thermal stability. [40] Further investigation on the assembly mechanism for the paperlike GO thin films indicated the occurrence of graphene gelation at the solution/filter membrane interface during the course of vacuum filtration.
Selfassembly at the phase interface is also regarded as a promising approach for the construction of paperlike gra phene films. In this regard, GO with an amphiphilic character is able to generate a packing in an oriented fashion at the air-liquid interface. Sequential heating led to the formation of a condensed film of GO at the air-liquid interface. After subjecting to drying and further reduction, a semitransparent and selfsupporting membrane with high mechanical flex ibility and a tunable thickness of 0.5-20 µm was obtained. [41] Within the presence of a handful of ethanol, adjustable trans mittance and resistivity of the GO films were readily realized at the water/pentane interface during the course of the pentane evaporation. [42] Evaporation has been employed to prepare GO paper, since it is capable of triggering the directional flow of the solvent. [41] Compared with the conventional vacuum filtration method, the evaporationtriggered selfassembly method can save time. A GO paper with the thickness as thin as several micrometers was achieved by selfassembly induced by the evaporation, which took only 10-40 min at 80 °C, while flowdirected self assembly requires 12-24 h for obtaining a GO paper with the identical thickness, yet it led to superior mechanical perfor mance relative to the evaporationinduced assembly. In con junction with in situ chemical reduction, evaporationtriggered assembly can be applicable to fabricate transparent conductive graphenebased films. [43] By controlling the evaporation for the GO suspension in a special vacuum concentrator, either 3D GO sponges or papers can be selectively produced. An rGO sponge can be obtained by subsequent thermal treatment at high temperature. [44]

Template-Directed Self-Assembly and Hydrothermal Processes
Selfassembly of graphene into monolithic macroscopic super structures with 3D networks can largely translate the admirable characteristics of individual graphene into resultant macro structures, thus enabling applications that require highconduc tivity, largesurfacearea, and selfstanding structures. To date, 3D selfassemblies for CMGbased architectures in the form of hydrogels, aerogels, and foams have been realized by template directed assembly and hydrothermal processes. [45,46] Pioneering work in this area was conducted by Shi and co workers, who effected the effective selfassembly of graphene hydrogels via a hydrothermal processing. In this regard, by a convenient onestep hydrothermal process of a highly con centrated GO aqueous dispersion using a sealed Teflonlined autoclave at 180 °C for 12 h, 3D porous graphene hydrogel was generated, which consisted of a highly interconnected 3D graphene network (≈2 wt%) filled with water (≈98 wt%) with various pore sizes from nanometers to several micrometers, and pore walls constructed by the graphene stacking. [46] As proposed by Shi and coworkers, when GO was subjected to hydrothermal reduction, the oxygenated functional groups on graphene sheets significantly decreased, and the πconjugation was largely restored. When the GO concentration was suffi ciently high, the π-π stacking interaction in conjunction with decreased hydrophilicity and electrostatic repulsion triggered flexible rGO sheets to partially overlap and interlock with each other to generate enough crosslinking sites for building a 3D porous framework with entrapped water by the residual oxygencarrying groups. By contrast, when the GO concentra tion was too low, the crosslinking became less likely to occur in time owing to relative few opportunities for the contact between the dispersed graphene sheets in the aqueous solu tion. Therefore, GO sheets were converted into graphene aggre gates, as shown in Figure 4. Qiu and coworkers developed an ultralow density and compressible aerogels of graphene by a hydrothermalinduced assembly in conjunction with function alization-lyophilization-microwave treatment. The resulting aerogels have shown an ultralow density of only 3 mg cm −3 , but the original structure can recover with no explicit fracture after 90% compression. [47] Hollow CMG structures can be produced through template directed assembly. For instance, by utilizing a soft template based on the water-oil interface from a water/oil emulsion, Guo et al. assembled amphiphilic GO sheets into hollow GO spheres with diameters ranging from 2 to 10 µm. [48] The oxi dation degree of GO affected its selfassembly behavior as well as the structures and sizes of the resulting microspheres. The vigorous oxidation can yield smaller and more hydrophilic GO nanosheets, thereby generating smaller graphene microspheres with hollow structures. In such a system, the flexible amphi philic GO nanosheets can reaggregate at the water-oil interface in the absence of extra surfactant, while the compact shell of graphene is achieved mainly by interactions between individual Adv. Sci. 2018, 5, 1700626 graphene sheets such as van der Waals forces and/or electro static attraction and/or hydrogen bonding interaction. Diphe nylalanine peptide nanowires were also employed as a solid template to incorporate GO sheets into core-shell hybrid archi tecture by virtue of electrostatic interaction. Thickness modu lation for the shell of GO was achieved by adjusting the pH during selfassembly processes and thereby tuning the inter action between GO and peptide templates. Further calcination treatment will remove the peptide core and generate hollow rGO structures with large surface area and excellent chemical/ thermal stability. [49]

Spinning and Space-Confinement Self-Assembly
In comparison with the 2D and 3D selfassemblies of CMG sheets discussed in the previous section, integrating CMG sheets into 1D macroscopic fiber is a challenging task due to the irregular shape and nonuniform size together with strong stacking tendency of graphene layers. Two major barriers for making macroscopic graphene fibers were the poor dispersion of graphene and the lack of efficient assembly strategy. [50][51][52] The 2D topology of graphene satisfies the basic asymmetrical structural factor for liquid crystals (LCs), [53][54][55][56][57][58][59] and the find ings of crystalline behavior of graphene and its derivatives offer a straightforward yet effective LCbased wetspinning method for making macroscopic graphene fibers. Xu and Gao first reported continuous and neat graphene fibers via a wet spinning process built on unique LC character of graphene ( Figure 5). By injecting a condensed GO dispersion into the coagulation bath, the first meterlong, polymerfree GO fibers, as demonstrated by Xu and Gao showed great mechanical per formance (102 MPa strength and 5.4 GPa modulus) and greater fracture elongations than most of the carbon nanotube (CNT) fibers. Importantly, after the reduction in hydroiodic acid, the rGO fibers presented a good conductivity of 2.5 × 10 4 S m −1 and even greater mechanical performance (140 MPa strength and 7.7 GPa modulus) while maintaining the remarkable frac ture elongation (5.8%). The strength enhancement of graphene fibers was attributed to the increased intersheet interactions of graphene, originating from the decreased interlayer distance within the reduced fibers (as shown in Figure 5). [50] Taking advantage of the solutionbased wetspinning tech nique, the morphology of the cross section of the fibers could be easily controlled to fulfill different requirements for multi functional applications. By modulating the coagulation pro cess and nozzle types in the spinning process, porous and coaxial structures were introduced into the graphene fiber structures. Gao and coworkers, employing liquid nitrogen as the coagulation bath with subsequent freezedrying, prepared fiberlike graphene aerogel with a core-shell structure (porous core and compact shell). The coaxial layered architecture with an oriented packing of GO sheets yielded a large specific ten sile strength of 188 kN m kg −1 together with a compression strength of 3.3 MPa, to a certain extent, compromising the strength and porosity. [52] Spaceconfined hydrothermal processing was found to be even a handier method compared to the wetspinning process. In short, the onestep hydrothermal fabrication procedure was achieved by baking a hydrothermal reactor with spaceconfine ment fillers containing cylindrical channels with the aqueous GO dispersion. It was found that the GO sheets aligned along the fiber axis, which was ascribed to the shear force induced by capillary and the tensile force due to surface tension. For   instance, Qu and coworkers utilized a glass microtube as the spaceconfined microreactor. The concentrated aqueous GO suspension was filled into the glass microtube, followed by subsequent thermal treatment in a twoendsealed microtube at 230 °C for 120 min. A neat graphene fiber was then produced, showing a tensile strength of 180 MPa and a much higher strength up to 420 MPa achieved after further vacuum thermal treatment of the fiber at 800 °C. This annealing effect could fur ther improve the strength and toughness of the CNT fiber by rendering extra covalent bonding at the interfaces. [51]

Self-Assembled Graphene-Based Hybrid Structures
As mentioned in the above section, selfassembly has proved an effective method to construct a variety of pure graphenebased architectures. Actually, CMGs can also be integrated with other inorganic/organic components to form functional hybrid archi tectures with desirable morphology, structure, and properties. CMGbuilt hybrid structures are of great significance due to intriguing features which not only inherited intrinsic charac teristics of individual component but also generated new prop erties and functions by the synergistic effect. This following section describes the builtup of CMGbased architectures including hybrid structures by using the abovementioned self assembly techniques.

Hybrid Structures with Carbon Nanomaterial
Carbon nanomaterials other than graphene, such as fullerenes and CNTs, possess many attractive features, and so does their assembled architecture with graphene in all three dimensions. Because of the strong interactions between assembled com ponents (strong π-π interaction, van der Waals force, large interaction surfaces between sheets, corrugation at atomic scale, and winkled morphology at the submicrometer scale) and the synergic effect of individual nanocarbon components, the hybridization of two or more constituents usually exhibits more advanced properties than a single component. Thus, such nanocarbon/graphene hybrids have attracted increasing interest during the past decades, especially for hybridization with assembled architectures from macroscopic 1D fibers to 3D structures. [60,61] 1D hybrid nanocarbon fibers can be prepared from CNTs and graphene or their derivatives via a similar wetspinning process for enhanced mechanical and electrical performance. For instance, Kim and coworkers synthesized continuous hybrid fibers by mixing rGO and singlewalled carbon nano tubes (SWCNTs) into a stable dispersion with the surfactant sodium dodecyl benzene sulfonate, followed by wet spinning into a poly(vinylalcohol) (PVA) coagulation bath. Such hybrid fibers showed admirable mechanical performance with gravi metric toughness as high as 1000 J g −1 . Such outstanding toughness was attributed to a framework consisting of partially aligned CNTs and rGO sheets during solution spinning. [62] In our earlier effort, we proposed a scalable and continuous hydrothermal method for the synthesis of GO/CNT hybrid fibers by integrating GO sheets and acidoxidized SWCNTs within the presence of ethylene diamine (EDA) to an uniform aqueous suspension with a subsequent in situ hydrothermal process (Figure 6). The hierarchically structured microfiber consisting of an interconnected aligned SWCNTs' network with interposed Ndoped rGO sheets yielded an ultralarge specific capacitance up to 305 F cm −3 in a fibershaped super capacitor (SC). [63] Foroughi et al. reported another CNT/gra phene composite yarn with the electrical conductivity as high as 900 S cm −1 considerably higher than that of pristine CNT yarns. [64] This value was 400% and 1250% larger than those achieved for graphene paper or pristine CNT yarns, respectively. The enhanced conductivity is credited to the increased density of states around the Fermi level by two orders of magnitude as well as the decreased hopping distance by an order of magni tude by introducing graphene into CNT yarns, which results in more delocalized charge carriers and significantly increased conductivity of the CNT/graphene yarns. [64] Recently, Filleter and coworkers reported an improvement in the mechanical performance of commercially available CNT yarns/fibers by the introduction of interlocking GO sheets by immersing CNT yarns into a GO solution with subsequent airdrying. It was found that GO could serve as an effective bridge that interlocks neighboring CNTs, as illustrated in Figure 6, and enhance load transfer within the whole yarn with further improvement in the macroscopic mechanical performance. [65] 2D macroscopic graphene hybrid films with excellent mechanical strength, superb electric conductivity, and excel lent chemical stability have been widely studied for transparent conductive films, [66][67][68][69] flexible SC electrodes, [70] sensors, [71] and other applications. [72] For instance, Gao and coworkers designed graphene/multiwalled carbon nanotube (MWCNT) membranes by assembling refluxed GO and MWCNTs on a porous substrate via simple filtrationassisted assembly [73] with an increased water flux compared to the previously reported neat graphene nanofiltration membrane. [74] In this composite film, MWCNTs were applied as "nanoedge" to expand the interlayer spacing between neighboring graphene sheets. LbL electrostatic selfassembly as a versatile fabrication method can also be utilized for graphenebased composite films. Our group also reported the selfassembly of positively charged watersoluble poly(ethylene imine) (PEI)modified graphene sheets with negatively charged acidoxidized MWCNTs. The resulted composite films were found to have an interconnected carbon network structures with desirable porosity, which holds promise for SC electrodes. [75] Besides CNTs, other nanocarbon materials can also form hybrid films with graphene with the filtration method. Zhang and coworkers reported mesoporous carbon nanosphere/gra phene hybrid films by the filtration of a mixture of a dispersion of mesoporous carbon nanosphere with a 200 nm porous poly tetrafluoroethylene (PTFE) membrane to produce a graphene/ mesoporous carbon sphere film, showing higher specific capac itance per surface area (0.36 F m −2 ) than that of a powdery gra phene/mesoporous carbon sphere composite (0.23 F m −2 ). [76] Selfassembly of graphene hybrids into monolithic macro scopic superstructures with 3D networks can largely translate the admirable features of the individual component into the resultant macrostructures and enriching applications that demand high conductivity, large surface area, and selfstanding structures. It has therefore attracted significant interest in the past years. To date, some examples regarding the fabrication of graphene/carbon compositebased 3D architectures have been demonstrated. For instance, Zhang and coworkers reported a green process for making graphene/MWCNT hybrid aero gels in a pioneering work. This process was established with the homogeneous mixing of predispersed GO sheets and MWCNTs, chemical reductioninduced hydrogel formation, and subsequent supercritical CO 2 drying, as demonstrated in Figure 7. [77] The resulting CNT-graphene hybrid aerogel showed high desalination capacity (633.3 mg g −1 ). In a much simpler process, a GO-CNT aqueous mixture was directly cryodesiccated, avoiding the formation of the hydrogel. This "sol-cryo" protocol developed by Sun et al. enabled the fab rication of ultralightweight hybrid aerogels with controlled densities, depending on the weight fraction of CNTs. [78] The unique configuration of graphene sheet cell walls with CNT ribs as reinforcements guaranteed such an ultralight and stable allcarbon structure. The ultralightweight aerogels were found to have outstanding elasticity, thermal stability, and adsorption capacity. When CNTs were entirely absent in the aerogels, the density could be extremely low. Interestingly, the achieved minimum density of the neat graphene aerogels (0.16 mg cm −3 ) was once seen as the lowest value for ultralight materials until the record was broken very recently by a lower value of 0.12 mg cm −3 . [79]

Hybrid Structures with Polymers
Various graphene/polymer hybrid structures have been pre pared via LbL assembly because of their functional groups for hydrogen bonding or electrostatic attraction. Such graphene/ polymer hybrids with outstanding electrical and mechanical properties have indicated great promise in various applications such as SCs, conductive electrodes, and gas barriers. Though tremendous efforts have been made in using graphene as a nanofiller in polymer matrices, in this section, we focus on the selfassemblies in which graphene and polymer are considered as individual building blocks.
For instance, the negatively charged GO and PEI with posi tively charged groups could form GO/PEI assemblies by virtue of apparent electrostatic interactions. Grunlan and coworkers prepared GO/PEI on a polyethylene terephthalate (PET) film via the LbL assembly method for gas barrier application. It was noticed that a graphene/poly mer assembly could provide a tightly packed architecture with insufficient gas diffusion channels. [80] Chen et al. employed similar methods to con struct gas barrier films using GO/PEI multilayered structures and demonstrated that both the pH for the GO suspension and the GO/PEI bilayer number had great influence on the oxygen barrier performance for the multilayered film, [81] implying the intercalated structures of tightly oriented and stacked GO/PEI system. Enhanced mechanical properties were also reported by Grunlan and coworkers for a PAA/PVPG system.  PAA x /PVPG x layered films can also be obtained by sequential dipping a substrate, coated with a PEI primer layer, into PAA and PVPG aqueous solutions. This asprepared film showed increased elastic modulus by 322% (from 1.41 to 4.81 GPa), with a high light transmittance of ≈90%. [82] Other polymers such as polyaniline (PANI), [83][84][85] poly (di methyldiallylammonium chloride) (PDDA), [86] and PVA [87] were also widely used in the LbL assembly of graphene/polymer hybrid films. For instance, Shi and coworkers demonstrated an LbL assembly of electroactive graphene/PANI multilayered films with good transparency and conductivity yet without any supporting conductive transparent electrodes. [88] By adjusting the deposition cycles of the multilayered films, one can readily modulate the film conductivity, the film thickness as well as the film transmittance. As for the 3D architecture of gra phene/polymer composite, the most straightforward method is templatedirected assembly. Vickery et al. demonstrated the preparation of graphene/ polymer hybrid structure with highly ordered 3D architectures by utilizing either ice or colloid particles as effective templates. A homogeneous aqueous solution of polysty rene sulfonatestabilized graphene (PSSG) sheets and PVA was injected into liquid nitrogen through an insulin syringe. The frozen sample can lock water inside the com posite network. The removal of the ice tem plate led to the formation of a PSSG/PVA aerogel. [89] When using colloid particles as templates, PS beads were first modified with positively charged poly(allylamine hydrochlo ride) (PAH). After that, the modified PS was employed as the template to anchor the neg atively charged PSSG onto the PS surfaces via electrostatic forces. The PS templates could be readily removed by toluene wash, ultimately generating hollow PSSG micro spheres as shown in Figure 8. [89] Polymers can also act as the crosslinker in the 3D graphene/polymer monolith to boost the mechanical properties of the com posite system. Zhao et al. demonstrated the fabrication of a compressible and conductive graphene aerogels with introduction of con ducting polymer polypyrrole (PPy), which functioned as an effective crosslinker to yield the uniform dispersion of GO in aqueous solution for compensating the decrease in conductivity from conventional polymer crosslinking. [90] Such graphene aerogels pre pared using the PPy as the crosslinker were highly compressible with high conductivity. Even after deformation to 80% of the fracture strain, they could recover their original state without any apparent network collapse while retaining the electrical conductivity as high as ≈30 S cm −1 .

Hybrid Structures with Metal or Metal Compounds
Selfassembly of graphene or CMG with various metal or metal compounds has been widely studied. [91][92][93][94][95] Inorganic nanoparticles can assemble on the CMG surfaces as particle nucleated/anchored on CMG substrates, with different char acters and morphologies, resulting in hybrid structures with unique features inherited not only from the individual compo nents but also emerged from the synergistic coupling of their components. [96][97][98][99][100] The inherent properties of graphene or chemically modified graphene make it a suitable substrate for nanomaterials in many various applications. [101,102] In addition, graphene and nanoparticles could complement each other, and both sides of the 2D structured graphene were exposed to the solution, resulting in a large exposure of active surface area and a homogeneous distribution of nanoparticles. Among them,  Reproduced with permission. [77] Copyright 2012, Royal Society of Chemistry.
Graphenebased inorganic nanoparticle hybrids synthesized by LbL assembly are very common, which exhibit improved performance with respect to that of simply mixed hybrids. For instance, Zhou and coworkers [112] synthesized sandwichlike hybrid nanosheets consisting of graphenewrapped SnO/SnO 2 nanocrystals anchored on graphene by a facile LbL selfassembly method. This unique double protection of graphene layers results in the hierarchical structure exhibiting good durability and rate performance when used in Liion batteries (LIB).
Liu and coworkers [110] demonstrated welldefined graphene-CdS quantum dots (GNs-CdS QDs) by sequential LbL self assembly. First, by in situ chemical reduc tion of GO sheets within the presence of cationic PAH, welldispersed polymerfunc tionalized graphene nanosheets in aqueous solution were prepared. Subsequently, the resultant positively charged PAHmodified GNs (GNsPAH) were assembled with nega tively charged QDs to produce welldefined composite films of GNsCdS QDs through a sequential LbL method. The alternating GNs− CdS QDs multilayered films, wherein the CdS QDs were uniformly spread over the 2D graphene nanosheets, showed great enhance ment of photocatalytic and photoelectro chemical activities upon visible light irradia tion with respect to the pristine CdS QDs and graphene nanosheet films. It was suggested that all the enhancement lies in the judi cious integration of CdS QDs with graphene nanosheets in an alternative fashion, thus maximizing the 2D structural merit of gra phene nanosheets in GNs-CdS QDs hybrid films. The LbLassembly method was also used to fabricate other graphenebased metal and metal compound nanostructures, such as Pd, [113] Au, [114] MoO 2 , [115] and MnO 2 . [116] Graphenebased composites can also be fabricated by solutionbased electrostatic self assembly, yielding a variety of readily dis persed and easily processable composites on a large scale by simply blending the aqueous dispersions of the assembling components, compared to the aforementioned LbL method. For example, Ping et al. [117] produced a novel 3D graphene networkbased CoAllayered double hydroxides (LDHs) (3D GN/CoAlNS) nanocomposite through electrostatic self assembly of CoAlLDH nanosheets (CoAl NSs) with a single layer on a 3D GN. As shown in Figure 9, the bulk CoAlLDH (CO 3 2− ) was first prepared by a hydrothermal treatment. Subsequently, the bulk CoAlLDH crystal was immersed into a solution bearing an excess of anions to increase the interlayer distance, which accelerated the subsequent exfolia tion of CoAlLDH. [118] Finally, the singlelayer CoAlNSs were self assembled on the acidtreated 3D graphene networks through the electrostatic adsorption. The achieved 3D GN/CoAlNS showed comparable or much superior performance and longterm sta bility for oxygen evolution reaction (OER) in alkaline medium in comparison with most previously reported LDHbased OER electrocatalysts. This method was also extended to produce CMGbased hybrid structures in combination with other metal NPs, [119,120] metal oxide/hydroxide NPs, [121][122][123] and QDs. [124,125] Adv. Sci. 2018, 5, 1700626 A GO or CMG aqueous solution can be assembled to form a 2D thin film by the orientation of the external force. Both vacuum filtration and flowinduced selfassembly approaches were based on this principle, and graphene/inorganic nano particle hybrids with ordered microstructures could also be prepared by the above methods, provided that the same solvent could be utilized for obtaining stable dispersion of assembled components. Recently, Li et al. [126] proposed a new and effec tive approach to synthesize graphene/Fe 3 O 4 composites with 3D structures by hydrothermal processing. First, the GO/Fe 3 O 4 mixture suspension was deposited on an Ni foam by vacuum filtration, which was followed by freezedrying. The GO/Fe 3 O 4 composites further underwent a plasma treatment to achieve simultaneous nitrogen doping and reduction. The asprepared 3D Ndoped graphene/Fe 3 O 4 composite was used for a high performance SC electrode, showing a 153% improvement in specific capacitance compared to 3D graphene/Fe 3 O 4 structures synthesized by the traditional hydrothermal method.
It is known that selfassembly of nanoparticles on the gra phene surface is a great challenge because of the lack of dan gling bonds in the graphene plane. Yu and coworkers [127] reported the synthesis of graphenesupported ultrafine MgH 2 NPs with both homogeneous distribution and high loading percent by a hydrogenationinduced solvothermal method. The proposed unique bottomup assembly strategy enabled strong coupling between graphene and MgH 2 NPs as well as the homogeneous distribution of nanoscale particles, resulting in admirable H 2 storage performance in graphene/MgH 2 compos ites. On the other hand, the concept of assembling MgH 2 NPs on graphene with high loading also represents a viable path for fabricating various nanostructured nanocomposites with poten tial applications in energyrelated fields. This strategy was uni versal and was also employed to fabricate other graphenebased metal or metal compound composites, such as TiO 2 , [121,128] Au nanoparticles NPs, [129] CdS QDs, [130] and GaN layers. [131] Another successful fabrication method for graphene/inor ganic particle hybrids is built on in situ growth and assembly of metal nanoparticles on functionalized graphene. How ever, metal nanoparticles are usually located randomly on the graphene surface. Takeuchi and coworkers [95] demonstrated that gold (I) cyanide (AuCN) could be grown on pristine GN as nanowires via a selforganized process in a water solution at ambient condition. The AuCN nanowires could align them selves along the zigzag lattice direction of graphene, which originated from the interaction with the gold atom and the lattice match suggested by the firstprinciples calculations. Dangling bonds of damaged graphene, [132,133] vaporphase deposition at high temperature, [134][135][136][137][138][139][140][141][142][143][144][145][146][147][148] and intermediate seed materials [132,[148][149][150] have been used to pro duce the hybrid of pristine graphene/inor ganic nanostructure; however, the formed inorganic nanostructures were randomly ori ented or poorly aligned on pristine graphene. This work paves a new way for precisely assembling inorganic nanomaterials on pris tine graphene. [93,[135][136][137][138][139] Zhang and coworkers [141] recently demon strated that the welldispersed mesoporous TiO 2 nanocrystals with (001) facets could be assembled onto a graphene aerogel surface (TiO 2 /GAs) by a onepot hydrothermal treatment. In this process, the glucose functioned as not only the linker but also the faceregulating agent to yield (001) facets as well as a mesoporous TiO 2 struc ture. The resulting TiO 2 /GAs composite showed high spe cific capacity and high recyclability of photocatalytic activity for methyl orange pollutant when used for photocatalysis and Liion battery. Other metal or metal oxides or hydroxides (e.g., TiO 2 , SnO 2 , NiOH, and MnO 2 ) have also been assembled on graphene, forming hybrids with wellcontrolled nanostructures by using a similar strategy. [142][143][144][145][146][147] These graphenebased hybrid nanostructures exhibit prom ising performance in various applications because of the strong interaction and synergistic effects between graphene and the metal/metal compounds. Moreover, graphene as an ideal sup port material can greatly enhance the durability and stability of the hybrid materials. To broaden practical applications of self assembled graphene/metal or metal hybrid structures, their size and morphology need to be further optimized.

Applications of Self-Assembled Graphene-Based Architectures
Selfassembly of graphenebased structures with unique mor phologies and tunable compositions has paved the way for a wide variety of potential applications such as electronic, opto electronics, energy storage and conversion, electrocatalysis, photocatalysis, and environment. Graphenebased architectures with the versatility of their selfassembly behavior are capable of obtaining desirable hybrids and macroscopic architectures aiming for particular functions and properties. Certainly, CMGs within these architectures or superstructures do not only func tion as a 2D scaffold or support featuring atomic thickness, high conductivity, and a large specific surface area but also fre quently provide a synergistic effect to the active components affording further performance improvement in various devices. In this section, we highlight some typical examples of the selfassembled graphenebased architectures applied in elec tronics, optoelectronics, energy storage and conversion, catal ysis, and environment to indicate the diverse directions of this research area.
flexible transparent graphenebased elec trodes to compete with conventional indium tin oxide electrodes. Various selfassembly methods such as flowinduced selfassembly, LbL, and LB techniques are effective in con structing transparent conductive electrode with low cost and tunable properties com pared with conventional metal/metal oxide electrode. For instance, by taking advantage of graphene over conventional metals for the anode, Wu et al. fabricated devices based on a graphene/P3HT/Si nanowire array, achieving a power conversion efficiency (PCE) of 9.94%, which is much higher than that of Cu and Au filmbased devices with 5.3% and 7.8% power conversion efficiency. The enhanced device performance is attributable to a much flatter transmittance curve of the graphene films in the whole spectrum relative to metallic films. Moreover, the high transmittance, along with high conductivity and adjustable work func tion, makes graphene an appealing choice for uses in transparent conductors, [148,149] photosensitizers, [150] and channels for charge transport [151] in solar cell applications. In addition, selfassembled CMG structures could function as electron acceptors or hole extraction layers (HEL) for use in polymer solar cells (PSCs). For example, the bulk heterojunction solar cells using the hybrid fiber assembled from dioctylperylenedicar boximide (dioctylPDI) and rGO as an elec tron acceptor and poly(3hexylthiophene) (P3HT) as an electron donor exhibited a PCE of 1.04%, supe rior to those using pure dioctylPDI and the dioctylPDI/rGO mixture. The improved performance arises from the efficient exciton dissociation at the donor/acceptor interface together with fast charge transport within the nanofibers. [152] Gao et al. demonstrated the use of GO thin films as the HEL in inverted PSCs. The homogeneous GO layer was assembled onto the P3HT: [6,6]phenylC61butyric acid methyl ester active layer. The resulting inverted PSC with desirable GO layer thicknesses of 2-3 nm yielded a PCE of 3.60%. This value is on a par with that of traditional poly(3,4ethylenedioxythiophene) doped with poly(styrene sulfonate)based inverted device. It was suggested that GO bearing oxygencarrying groups including enolic and carboxylic groups with a relatively high proton content could dope P3HT at the active layer surface, which favored the for mation of an Ohmic contact between the top metal electrode and the active layer, and thus results in significantly improved device performance. [153] Nakanishi and coworkers demonstrated an optoelec tronically active assembly of alkylatedC60 and graphene through direct exfoliation of graphite in organic solvents within the presence of 3,4,5tris(eicosyloxy)phenyl substi tuted Nmethyl [60]fulleropyrrolidine through noncovalent interactions (Figure 10). [154] The resulting graphene/alkylated C60 assembly yielded around 270fold higher photocurrent than that of the parent C60 derivative (Figure 10). This result implies that the graphene/alkylatedC60 assembly can function as excellent electron accepting/charge transport materials for highly efficient photovoltaic devices. [154] LbL assembly in conjunction with other wetchemical methods such as dielectrophoresis can be utilized to produce GO/rGO-nanoparticle hybrids with multilayered structures. For instance, Li and coworkers reported layered thin films assembled from CdS quantum dots and rGO sheets which involved the dielectrophoretic deposition of an rGO thin film on a targeted substrate with subsequent in situ growth of a CdS layer through a wetchemical method. The photovoltaic devices constructed using these hybrid films showed an opencircuit voltage of 0.68 V when the rGO/CdS bilayer number reached 2 or more. At eight bilayers, the device yielded a shortcircuit photocurrent density of 1.08 mA cm −2 and a high incident photocurrent efficiency of 16%. Such enhanced device per formance should arise form unique layered structures of the hybrid films. [155] By LbL assembly, Yao et al. prepared TiO/GO multilayered hybrid films. After reduction of GO, the resulting hybrid films exhibited improved photoresponse as well as excel lent reversibility and stability under ambient conditions. [156] Selfassembled graphene hybrid structure can also be used in highperformance FETs, which require a thin film with smaller toughness and higher reduction degree. Hybrid films of (PAH/GO/PAH/PW) n with a different number of layers (n) of GO were fabricated via the electrostatic LbL assembly, and  subsequent photoreduction under UV light converted GO to rGO in the composite film. [157] FET devices constructed using graphenebased hybrid films presented typical ambipolar char acteristics with good hole and electron transport properties. The holedominated transport feature was credited to the elec trontrapping effect of the cagetype structure of H 3 PW 12 O 40 (PW). The on/off ratios for these FET devices were calculated to be about 1.1-2.0 for the films with different GO layers. By selfassembly of graphene single crystals induced by a mutual electrostatic force between the neighboring crystals with the assistance of the airflowinduced hydrodynamic forces, Fu and coworkers produced a new graphene superordered struc ture (GSOS) with adjustable periodicity in the spacing and remarkable uniformity in size and orientation. Such a simple yet efficient strategy inherits the merit of the conventional selfassembly approach. A backgated FET array constructed using GSOS exhibited the device mobility of 3882 ± 896 cm 2 V −1 s −1 , which signifies high quality of the resulted GSOS. [158] Halik and coworkers assembled oxofunctionalized graphene/ polymer hybrid films for memory devices, which can be oper ated at a low voltage of 3 V. It was found that the coating layer of ≈5 nm is mandatory to afford the relative low opera tion voltage of the device. [159] More recently, selfassembled 3D freestanding, hollow, polyhedral graphene was reported, dem onstrating interesting optoelectronic properties. It was found that such a unique 3D graphene polyhedron induced uniform plasmon-plasmon couplings at each of the faces, while abun dant 3D plasmonic hybridization behavior triggered a high degree of volumetric light confinement. [160]

Photocatalysis
Compared with the conventional active materials, the CMG based hybrid structures have been proved to be efficient catalyst in many photocatalysis systems where CMG sheets with ani sotropic 2D structure, large specific surface area, and superb conductivity, are regarded as ideal host for anchoring catalytic materials. [161a-e] For instance, due to its strong oxidizing power, longterm thermodynamic stability, and low toxicity, TiO 2 was recognized as one of the most promising photocatalytic mate rials. As expected, the combination of TiO 2 and graphene sheets could also yield hybrid nanostructures with high photocatalytic activity. As a typical example, 2D hybrid structures of GO sheets and rodlike TiO 2 particles(TiO 2 /GO) were produced via the interfacial selfassembly. The photodegradation of methylene blue (MB) for TiO 2 /GO was evaluated by comparing with that of the blend of GO sheets and commercial TiO 2 particles. [161] By monitoring the UV-vis absorption spectrum of MB in aqueous solution, the complete photodegradation of MB (10 ppm) for TiO 2 /GO was achieved with 15 min of UVlight irradiation. In contrast, only 70% of MB was found to be decomposed in the case of oleic acid stabilized TiO 2 nanorods. Such good photocat alytic performance for TiO 2 /GO can be credited to the use other GO scaffold, which is able to inhibit charge recombination during the photocatalytic process. For the photoreduction of nitroaromatic compounds, Xiao et al. constructed an alternating GN-CdS QDs multilayered film through the LbL selfassembly of watersoluble cationic PAHmodified graphene sheets and ionic CdS QDs (Figure 11). The photocatalytic reduction of organic pollutant over GN-CdS QD composite films showed over 90% generation rate for 3 h under visiblelight irradiation (λ > 420 nm). Such performance was superior to that of CdS QD films (≈60% for 3 h). The good photocatalytic performance for GNsCdS QD hybrid films comes from the presence of the graphene sheets that not only act as an efficient electron col lector but also facilitate the transport for the photoexcited elec tron from CdS QDs, thus rendering an effective suppression of the photoexcited electron-hole recombination. [110] Apart from TiO 2 and CdS QDs, hybrid structures formed by graphene and layered transition metal dichalcogenides have also been given attention due to the fact that this combination showed inter esting physical properties suitable for potential applications in photocatalysis. [161c-e] For example, Rajamathi and coworkers reported a large freestanding MoS 2 -rGO hybrid papers via a simple exfoliation-costacking method involving evaporation under ambient conditions (Figure 12). Such hybrids exhibit good performance as photocatalysts for degradation of organic dyes (complete degradation of 10 mg L −1 MB in 30 min). [161e]

Electrochemical Energy Systems
Electrochemical energy storage conversion systems are of great significance in fulfilling the increasing demand for highperfor mance energy source throughout the world, with great potential in electric vehicles, portable electronics, and so on. The excel lent conductivity and high surface area of the graphene hybrid structure make it an ideal substitute of conventional metal elec trode for energy devices such as SCs, Li and Naion batteries, Li-S and Li-O 2 batteries.
The highly porous structure and electrical conductivity of the graphene hybrid superstructure enable better ion diffusion and electron transport in bulk electrodes, which are desirable for macroscale and microscale SC applications. In our previous work, we reported the sequential selfassembly of largearea hybrid films of functionalized CNT, and graphene and CNTs via electrostatic attraction. A rectangularlike cyclic voltammogram was achieved at a high scan rate of 1000 mV s −1 while yielding an specific capacitance up to 120 F g −1 , which signifies rapid charge/discharge behavior in formed hybrid film electrodes. Such good capacitive behavior of the hybrid film electrode was attributed to the conductive interconnected porous network carbon structures for rapid ion diffusion. [86] High electrical con ductivity is another essential requirement for SC electrodes. Pham et al. synthesized the CNT/graphene films via self assembly based on Coulombic interaction. The resulting flex ible graphene/CNT films exhibited high electric conductivity up to 394 S cm −1 . SCs fabricated with these films showed a specific energy density of 110.6 Wh kg −1 at 400 kW kg −1 .
[161a] The CMGs can also be utilized as a conductive scaffold to anchor various electrochemical active components such as conductive polymer, transition metal oxides and hydroxides, forming new hybrid structures for highperformance pseudocapacitors. [162][163][164][165] As a representative example, Ni(OH) 2 with an ultrahigh theoretical capacitance of 2600 F g −1 was assembled onto rGO sheets, which delivered an ultrahigh specific capacitance up to 1335 F g −1 at 2.8 A g −1 and a long cycling life. Such excellent  performance was ascribed to the synergistic effect of conductive graphene and highly crystalline Ni(OH) 2 nanoplates. [166] More recently, Gogotsi and coworkers produced a flexible and conductive MXene/rGO SC electrode. Through electro static assembly between negatively charged titanium carbide MXene sheets and positively charged rGO functionalized by PDDA, it was found that rGO sheets were inserted in between MXene layers, prohibiting the restacking of MXene sheets and resulting in significantly increased interlayer spacing. This will accelerate electrolyte ion diffusion and enable more accessible active sites. Thus, the produced selfsupporting MXene/rGO5 wt% SC electrode yielded a volumetric capacitance as high as 1040 F cm −3 at 2 mV s −1 , a high rate capability and a long cycle life. Furthermore, the constructed binderfree SC gave an ultra high volumetric energy density up to 32.6 Wh L −1 . [167] Selfassembled 3D CMGbased architectures were prom ising electrode materials for flexible or deformationtolerant SCs. Duan and coworkers produced a 3D graphenebased hydrogel by a hydrothermal process. The resulted hydrogel can be used for advanced flexible allsolidstate SCs. The optimized SCs exhibited remarkable capacitive performance, including a gravimetric specific capacitance of 186 F g −1 , an areal spe cific capacitance of 372 mF cm −2 , very small leakage current (10.6 µA), superb stability, and excellent mechanical flexibility. The extraordinary performance was attributed to a highly inter connected porous network structure with high electrical con ductivity and mechanical robustness. [168] Qu and coworkers explored a strategy to construct a CMGbased hydrogel that integrated 3D graphene with polypyrrole by premixing pyrrole monomer with GO sheets for hydrothermal processing, fol lowed by the electropolymerization. The produced composite foams can be utilized for compressiontolerant capacitors, which yielded large specific capacitances without significant change when undergoing longterm compressively loading/ unloading cycles. [169] Due to continued miniaturization of wearable/portable elec tronics, microsupercapacitors (microSCs) in the forms of either fibers or thin films are particularly attractive as minia ture energy storage units. Recently, we fabricated a new kind of 1D macroscopic rGO/CNT composite fiber by spaceconfined assembly. Such selfassembled hybrid fibers can be used to build fiberbased microSCs, exhibiting an energy density as high as ≈6.3 mWh cm −3 , on par with 4 V/500 µAh thinfilm lithium bat teries, together with a power density much higher than that of batteries as well as an ultralong cycle life and good flexibility. The remarkable performance was attributed to unique selfassembled fiber structure combining high conductivity, large surface area, and desirable porosity. Following on our work, a series of various selfassembled CMGbased fibers have been employed to con struct fiber SCs, demonstrating excellent electrochemical per formance and great potential of CMGbased fiber electrodes for advanced microscale power sources. In addition to fiber micro SCs, an inplane microSC was fabricated using stackedlayered heterostructure films from thiophene nanosheets and exfoli ated graphene reported by Feng and coworkers The optimized microSC can work at an ultrahigh rate of up to 1000 V s −1 with a landmark areal capacitance of 1.30 mF cm −2 and volumetric capacitance up to 123 F cm −3 at 100 V s −1 as well as excellent flexibility under various bending states. [163] Apart from their use in SCs, selfassembled CMGbased hybrid structures are found to be promising electrode mate rials for lithiumion battery (LIB), since it possesses control lable thickness, large aspect ratio, and high surface area, which favor the electrolyte access and fast ion/electron diffusion. Selfsupporting and flexible SnO 2 -rGO hybrid films were pre pared by in situ selfassembly which could be utilized as effi cient electrodes LIB. [145] The composites with 40 wt% rGO showed a specific capacity of up to 760 mA h g −1 at 0.008 A g −1 , which approaches the theoretical value of 780 mA h g −1 . SnO 2 is regarded as a promising highcapacity anode material for LIB, but further commercialization is hindered by the poor cycling stability. [170] By contrast, the reported SnO 2 -rGO com posites exhibited good stability at various current densities with no apparent drop in specific capacities. This suggests excellent spaceconfinement effect and electrical contact between rGO sheets and SnO 2 in formed layered hybrid structures. Recently, Zhao et al. explored an in situ reduction method for synthe sizing SnO 2 quantum dots@GO by the oxidation of Sn 2+ and the GO reduction. Such composites showed a capacity reten tion of 86% obtained even after 2000 cycles at 2 A g −1 . [170] Effec tive combination of ultrafine SnO 2 quantum dots on conductive graphene sheets provides abundant active sites for high lithium storage capacity, and the confinement of SnO 2 quantum dots in graphene sheets inhibits the aggregation of each particle for highrate cycling stability, which also verified the great potential of the graphene assemblies for the fabrication of advanced LIBs. Other active materials such as Mn 3 O 4 , [171] SnO 2 /Fe 2 O 3 , [172,173] CoO [174] and V 2 O 5 [175] could also be incorporated into graphene scaffold via simple selfassembly methods.
3D networks of graphene hybrids were also demonstrated as anode materials for LIBs. For example, Bai et al. demon strated the synthesis of a porous 3D graphene hydrogel with an embedded Si/SiO x core-shell structure (Si@SiO x /GH) through a solutionbased selfassembly process. [175] Such 3D hybrids afforded a stable capacity up to 1020 mA h g −1 at 4 A g −1 and 1640 mA h g −1 over 140 cycles at 0.1 A g −1 . The superb electrode performance for Si@SiO x /GH can be attributable to the fol lowing aspects: (1) the conductive graphene matrix is capable of accommodating the volume change of Si nanoparticles; (2) the porous 3D architecture with large specific surface area allows fast Liion diffusion as well as easy electrolyte penetration. [176] More recently, Wallace and coworkers fabricated a flexible, selfsupporting 3D porous MoS 2 -rGO film with a 3D porous structure via a onepot selfassembly, gelation, and subsequent reduction (Figure 13). The selfassembled MoS 2 -rGO film containing 75 wt% of MoS 2 showed a high capacity of up to 800 mA h g −1 at 100 mA g −1 with excellent rate performance, and superb cycle stability with no obvious capacity fading over 500 cycles at 0.4 A g −1 . [177] Through a facile hydrothermal coassembly route, Chen and coworkers prepared a 3D porous composite foam made up of graphene/Sb 2 S 5 nanoparticles. Such assembled 3D graphene based structures can serve as efficient anode materials for Naion batteries without using a binder or current collector, which yielded a high capacity of 845 mA h g −1 at 0.1 A g −1 , a long cycling life with a 91.6% capacity retention over 300 cycles at 0.2 A g −1 , and superb rate performance (525 mA h g −1 at 10 A g −1 ). The excellent electrochemical performance is credited to rapid Na + ion diffusion from the ultrasmallsized nanoparticles and excellent charge transport between the active component and 3D porous graphene foam structure. [178] By combining a tem plated method and a selfassembly process, a 3D porous struc ture consisting of rGO nanowire on 3D graphene foam was used as the anode material for Naion batteries, demonstrating a reversible capacity of more than 301 mA h g −1 without capacity drop over 1000 cycles. [179] Manthiram and coworkers produced 3D porous nitrogen and sulfurcodoped graphene sponges by a hydrothermal assembly and utilized them as selfstanding electrode for Li-S batteries, which exhibited a large specific capacity of 1200 mA h g −1 and a highrate capacity of 430 mA h g −1 at 0.2 C rate accordingly with superior cycling stability. The excellent electrode performance lies in the combination effect of the physical adsorption of lithium polysulfides onto 3D selfassem bled porous graphene network and the chemical adsorption of polysulfides over N and S sites of graphene. [180] Xu and co workers demonstrated the use of selfassembled graphene/poly (anthraquinonyl sulfide) composite aerogel as a selfsupporting flexible cathode for rechargeable Li and Naion batteries. The obtained 3D aerogels yielded a capacity of 156 mA h g −1 at 0.1 C with good rate performance (102 mA h g −1 at 20 C) for LIBs, and demonstrated a highrate capability (72 mA h g −1 at 5 C) and an ultralong stability (71.4% capacity retention over 1000 cycles at 0.5 C). The excellent cathode performance for both Li and Naion batteries was ascribed to the rapid redox kinetics and electron transport within 3D porous graphene network. [181] 3D graphene aerogels decorated with Ru particles (Ru GAs) showed the pore volumes of 2.8 and 14.1 cm 3 g −1 below and above a critical pore size of 100 nm, which could be used as a freestanding cathode in an Li-O 2 battery. The produced cathode yielded a high capacity of >12 000 mA h g −1 and superb cycling stability, which was ascribed to the 3D porous struc ture, rich active sites with Ru particles, and excellent chemical stability. [182]

Electrocatalysis
The electrochemical oxygen reduction reaction (ORR) is the pivotal step for various nextgeneration energy conversion systems including fuel cells, metal-air batteries, and cer tain electrolyzers. [183,184] Even though Ptbased catalysts have been commercially available, their high cost and poor durability still hampered the largescale application of such catalysts. Nonnoble metal/metal oxidedecorated graphenes have received much attention in pursuit of more cheap and efficient electrocatalysts bearing comparable or even superior catalytic performance relative to the conventional Ptbased catalyst. For instance, Co/CoO@graphene hybrid exhib ited superior performance for the ORR in a 0.1 m KOH by anchoring monodispersive Co/CoO nanoparticles onto a graphene surface by adopting a solutionbased selfassembly strategy. [185] Previous investigation regarding the interac tion between graphene and metal has shown that the differ ence in the Fermi levels of graphene and the metal tends to lead to the charge transfer occurred at the graphene/metal interface. [186] Improved catalytic performance of the NPs/ graphene hybrid could be ascribed to the charge transfer, as demonstrated in various graphenesupported catalysts such as Co 3 O 4 /graphene, [187] Fe 3 O 4 /graphene, [188] Co x S 1−x /gra phene, [189] and MnCo 2 O 4 /graphene [190] for ORR in alkaline or acid media.
The OER and hydrogen evolution reaction (HER) are also crucial electrochemical processes for various energy conver sion systems. Different from substrateassisted electrodes, selfassembled graphene hybrid nanostructures offer new opportunities for fabricating selfstanding electrodes with tun able microstructures and mechanical performance for mass transport and catalytic performance enhancement. Qiao and coworkers presented the first N,Odual doped graphene CNT (NGCNT) hydrogel film by LbL assembly of GO and CNTs followed by Ndoping. The resultant freestanding hydrogel film exhibited prominent OER performance, supe rior to the benchmark noble metal oxide (IrO 2 ) and some reported transitionmetal catalysts. [191] Furthermore, another simple vacuum filtration method has been demonstrated to integrate 2D porous gC 3 N 4 nanolayers and Ndoped gra phene into a freestanding film featuring porous gC 3 N 4 with highly exposed active sites and a desirable porous structure. The porous structure in the electrode not only provides large accessibility to the active centers but also facilitates the mass transport during the reactions. With the above benefits, this selfstanding electrode acted as a remarkably active HER cata lyst, showing a low overpotential (80 mV) at 10 mA cm −2 with a small Tafel slope (49.1 mV dec −1 ) in 0.5 m H 2 SO 4 . [192] Cheng and coworkers. reported the fabrication of CoS 2 /rGOCNT hybrid nanostructures via an efficient hydrothermal pro cess combined with vacuum filtration selfassembly. The asproduced conductive and robust film was evaluated as HER electrocatalysts. The unique CoS 2 /rGOCNT hybrid Adv. Sci. 2018, 5, 1700626 Figure 13. Schematic representation of those three dispersions. Reproduced with permission. [177] Copyright 2017, Wiley.
architectures led to a good HER activity of 142 mV overpo tentials at 10 mA cm −2 and high durability due to synergistic effect between individual components. [193] Other graphene based hybrid nanostructures such as MoS 2 nanoflower/rGO paper and MoS x /NG can also be easily achieved via self assembly, [194,195] showing great potential for highperformance electrocatalysts.

Water Treatment
Water pollution is harmful to the environment and human's health. Nanofiltration, possessing low energy cost and high efficiency, is regarded as a promising technology for the water purification. Due to diverse advantages over conventional mate rials such as availability, environmentally friendliness, and low cost, CMGbased structures will provide superior applications in different water treatment such as dyes adsorption/degrada tion from water.
Singlelayered GO (SLGO) is regarded as a new generation of membrane material for high efficient water purification. To develop highperformance GObased nanofiltration mem brane, it is critical to achieve the tradeoff between selectivity and flux. Yu and coworkers demonstrated that selfassembly of SLGO by simply controlling deposition rate can break the selectivity/flux tradeoff. It was found that selfassembled GO membranes, produced at slow deposition rate, showed dra matically enhanced salt rejection, while counterintuitively pos sessing 2.5-4 folds higher water flux than those of membranes derived at fast deposition rate. This finding is of significance for designing/regulating interlayered structure of ultrathin GO membranes by simply tuning SLGO deposition rate for highly efficient water treatment. [196] By vacuum filtration, Chen and coworkers created MWCNT/rGO hybrid membranes, which exhibited enhanced water permeability and membrane selectivity. When inter calating MWCNTs with the diameter of 10 nm within rGO membranes, the permeability of water was found to reach 52.7 L m −2 h −1 bar −1 . This value is about 4.8 times higher than that obtained for original rGO membrane and 5-10folds higher than those for many commercial membranes. Further more, the membrane achieved almost 100% rejection rate for three various types of organic dyes. It was suggested that the nanotube inner wall could act as spacers, and nanosized rGO sheets exfoliates on the outer walls, which is interconnected with the assimilated rGO sheets to impart superb stability to the membrane. [197] Tang and coworkers produced a ZIF8 metalorganic frame work/rGO hydrogel via selfassembly, in conjunction with the combination effects of crosslinking by metal ions and chem ical reduction. Upon freezedrying, the resulting hydrogel was converted into the ZIF8/rGO aerogel, which exhibited large absorption capacity and high cycling stability for organic sol vents and oil. This was attributed to its super hydrophobic properties and large specific surface areas. Besides, the corresponding hydrogel afforded photocatalytic dye degradation capability, together with superb water purification performance for effective removal of heavy metal ions, toxic dyes, and benzo pollutants. [198] 3D graphene and graphene/CNT sponges were also demon strated as absorbers for a variety of water pollutants due to their large specific surface area and good chemical stability in organic solvents. [78,199] Bi et al. reported an autoclaved reductioninduced selfassembly of rGO sponge with absorp tion capacity for 20-85 times of its own weight. [199] Sun et al. fabricated an rGO-CNT sponge by freezedrying the GO/ CNT solution with subsequent hydrazine vapor reduction. Such sponges showed great elasticity, high hydrophobicity, and ultralow density (a contact angle of 133° and a density of 0.16 mg cm −3 ). These features allowed the sponge to reach a significant absorption capacity of 215-743 g g −1 for various pollutants. [78]

Conclusions and Outlook
Thanks to their unique 2D structures, large specific surface area, and excellent physical and chemical properties, CMGs have been considered as the most appealing building blocks for "bottomup" selfassembly, facilitating hierarchical and anisotropic organization of CMGs at different length scales with other inorganic or organic functional species by virtue of noncovalent forces. Meanwhile, selfassembly of CMGs has been identified as one of the most attractive bottomup strate gies for exploiting a variety of unprecedented carbon materials with tunable microstructure, designed functions and proper ties that are compatible with conventional processing methods. Selfassembly derived processing methods and CMG materials are often featured by a variety of advantages. First, the assem bled architectures have diverse interfacedirected morphologies ranging from 1D fibers, 2D films, to 3D porous network, which are suitable for many applications, such as flexible/compress ible membranelike device. Second, the microstructure and surface/interface chemistry of the formed CMGbased mate rials can be easily tuned, favorable for the controlled func tionalization of materials for the targeted applications. Third, the selfassembly of CMG renders numerous opportunities for hybridization with a wide variety of inorganic and organic components to construct homogeneous hybrid architectures for various applications. More importantly, selfassembled CMGbased materials with welldefined structures have shown great potential in producing advanced functional systems for various technological fields such as electronics, optoelectronics, energy storage and conversion, catalysis, and water treatment.
Despite great advances in this research area of the self assembly of CMGs, at least the following several issues require further studies. First, it still remains a huge challenge to pre pare CMGs with a defined shape and fine control of their sur face chemistry, sizes, compositions, and monodispersity, since the structural variability and complexity of CMGs pose major impediments to afford the uniformity of selfassembled struc tures for their largescale fabrication and realworld applica tions. Meanwhile, it is a demand to develop new exfoliating strategies to achieve novel CMG building blocks with the welldefined shape and desirable functional groups. Second, selfassembly mechanism of CMGs and the structure-prop erty correlation of selfassembled CMG architectures lacks full understanding, yet this aspect is critical for effective control over the selfassembly of CMGs as well as the design and preparation of functionalized CMG materials. Indepth studies on the basic principles and theories of selfassembly are urgently required to render guidance for further improve the performance and broaden the applications for CMGbased structures. Third, it is necessary to explore more functional building blocks and integrate them into CMGbased hybrids with unique struc tures, properties, and functions via selfassembly. Particularly, the homogeneous hybridization is paramount for the prepara tion of CMGbased functional materials. Finally, costeffective and upscalable assembly protocols are needed for the practical applications of the assembled materials. Combining multiple disciplines such as chemistry, physics, and materials science, it is optimistically anticipated that more and more selfassem bled CMGbased architectures, superstructures, and functional devices with unprecedented properties, functions, and appli cations will be realized in the near future for reallife money making applications. By taking into account the developments that have been made involving graphene, it can be said that this 2D "wonder material" and its selfassembled functional archi tectures are heading toward a wonderful future.