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Self-Assembly of Graphene Oxide at Interfaces Jiao-Jing Shao, Wei Lv, and Quan-Hong Yang* energy storage devices, transparent electrodes and electromechanical systems.[14–30] However, the strong π–π attraction usually drives 2D graphene sheets to form stacked structures, and brings great obstacle for assembling graphene sheets into abundant macroforms with various microstructures. In a word, it is a big challenge to directly manipulate graphene sheets into graphenebased macroforms with regular, controllable and ordered microstructures owing to the strong inter-plane interaction. As an oxidized derivative of graphene, graphene oxide (GO) is a type of watersoluble material with atomic thickness, which is obtained through strong acid/ base attack of graphite crystals to introduce oxygen-containing groups in the graphite stacks followed by a complete exfoliation of the oxidized solid into nanosheets.[4,31–33] According to a commonly accepted chemical structure model, GO is composed of planar, graphene-like aromatic patches of random size separated by sp3-hybridized carbons which are decorated by hydroxyl, epoxy and carboxyl groups (Figure 1a).[34–38] The unique structural features endow GO with many intriguing surface chemical properties, such as amphiphilicity, negatively charged nature and multi oxygen-containing groups on its sheets. For example, the hydroxyl groups allow GO to form hydrogen bonds with other hydroxyl-rich molecules;[39] the carboxyl groups located at the edges cause GO to have negative charges owing to the ionization of the –COOH;[40–45] delocalized electrons over sp2-hybridized carbon atom domains introduce π-π interaction with other π-conjugated materials;[46,47] and the chemical composition and molecular structure makes GO behave like a polymer. In addition, the numerous chemical functionalities on the sheets allow GO to be functionalized further,[48,49] so that they are hybridized with other materials including polymers, DNA, metal oxides and inorganic nanoparticles, etc., to form GO-based composite materials.[50–53] The π–π interaction between unfunctionalized graphene patches on GO sheets can also act as bridges to link the sheets together and then lead to macroscopic GO-based materials. Therefore, GO can be viewed as an unconventional 2D soft material (depicted as an amphiphile, colloid, liquid crystal or polymer) and is a building block of diverse GO-based materials with abundant microstructures, such as flexible membranes and porous macroforms.[54–57] The reduction of GO can lead to partial restoration of sp2 carbon network, and hence reducing GO-based materials is a useful approach for preparing graphene-based materials. The microstructure of the

Due to its amphiphilic property, graphene oxide (GO) can achieve a variety of nanostructures with different morphologies (for example membranes, hydrogel, crumpled particles, hollow spheres, sack-cargo particles, Pickering emulsions, and so on) by self-assembly. The self-assembly is mostly derived from the selfconcentration of GO sheets at various interfaces, including liquid-air, liquidliquid and liquid-solid interfaces. This paper gives a comprehensive review of these assembly phenomena of GO at the three types of interfaces, the derived interfacial self-assembly techniques, and the as-obtained assembled materials and their properties. The interfacial self-assembly of GO, enabled by its fantastic features including the amphiphilicity, the negatively charged nature, abundant oxygen-containing groups and two-dimensional flexibility, is highlighted as an easy and well-controlled strategy for the design and preparation of functionalized carbon materials, and the use of self-assembly for uniform hybridization is addressed for preparing hybrid carbon materials with various functions. A number of new exciting and potential applications are also presented for the assembled GO-based materials. This contribution concludes with some personal perspectives on future challenges before interfacial self-assembly may become a major strategy for the application-targeted design and preparation of functionalized carbon materials.

1. Introduction Graphene is a one atom thick and closely packed two-dimensional (2D) sp2-bonded carbon honeycomb lattice and viewed as a basic building unit for various carbon materials.[1–13] Due to its high charge carrier mobility, superior mechanical strength, superb thermal conductivity, unique Klein tunneling, etc., graphene has attracted much attention from the research community and graphene-based materials have shown great potential in various applications, such as superfast transistors, chemical and bio-sensors, J.-J. Shao, Prof. Q.-H. Yang School of Chemical Engineering and Technology Tianjin University Tianjin 300072, P. R. China E-mail: [email protected] Dr. W. Lv, Prof. Q.-H. Yang Engineering Laboratory for Functionalized Carbon Materials Graduate School at Shenzhen Tsinghua University Shenzhen 518055, P. R. China J.-J. Shao, Prof. Q.-H. Yang The Synergistic Innovation Center of Chemistry and Chemical Engineering of Tianjin Tianjin 300072, China

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graphene-based materials can be readily altered by designing and controlling the microstructure of the GO-based materials precursor followed by some appropriate reduction processes. Unlike graphene sheets with the obstruction of strong inter-plane π–π interaction, GO can be readily manipulated in various solvents (such as water, ethylene glycol, dimethylformamide, dimethyl sulfoxide and so on) thanks to the unique surface chemistry and amphiphilicity,[36,58,59] which leads to a large number of self-assembly phenomena at various interfaces and hence producing GO-based materials with a variety of microstructures.[42,46,47,60–64] Huang et al. took Brewster angle microscopy (BAM) technique to confirm the amphiphilic nature and surface activity of GO sheets.[57,65] BAM is a surface-selective imaging technique in which p-polarized light is incident on the water surface at the Brewster angle, the angle at which no reflection is allowed (Figure 1b, top) and hence nothing can be found in the BAM image (Figure 1c). When GO sheets float up to the water-air interface, the surface refractive index changes and thus allow reflection from that region (Figure 1b, bottom), and bright spots corresponding to GO sheets can be seen in the BAM image (Figure 1d). Overall, it is the interfacial activity of GO sheets that results in a wide range of GO-based materials including paper-like membranes,[33,46,62,66–68] three-dimensional hydrogels and porous aerogels,[39,42,69–71] crumpled graphene particles,[72] hollow graphene capsules and other complex macroforms.[60,73,74] The abundant microstructures of these GO-based materials can be simply controlled by designing the corresponding interfaces where the interfacial self-assembly of GO-based materials is carried out, and then these interfacial self-assembled GO-based materials with plentiful microstructures act as rich precursors for graphene-based materials with controllable microstructures. GO is thermally unstable and chemically active, and hence can readily undergo an exothermic disproportionation reaction or be reduced by some reducing reagents to produce chemically modified graphene (CMG, also named as reduced-GO (r-GO)).[75] During the reduction process of GO-based materials into graphene-based materials, the re-stacking problem of GO sheets needs to be considered, because the restoration of sp2 carbon network on GO sheets can again introduce the strong π–π interaction between sheets and then destroy the original microstructures of previous GO-based materials, which is unfavorable for many applications, such as adsorption, ionic transportation, catalysis. The re-stacking can be prevented through weakening the π–π interaction. One solution is introducing some materials as the spacers between GO sheets.[76] Because of the electrostatic force coming from ionization of carboxyl groups, GO sheets can stably disperse in its aqueous suspension, and the highly dispersible ability in aqueous suspension is favorable for synthesizing uniform GO-based materials (hybrids and composites) with other materials. These introduced materials work as the spacers that enlarge the distance between GO sheets and weaken the inter-plane attraction and hence prevent the re-stacking of GO sheets during reduction process. Finally the original microstructures of the GO-based materials can be safely replicated in the resultant graphenebased materials after reduction. The homogeneous GO-based suspension is a prerequisite to uniformly introduce the spacer and then effectively prevent the restacking of graphene sheets

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Jiao-Jing Shao received her B.S degree from the Tianjin University in 2009, and continued her studies as a Ph.D candidate under the supervision of Prof. Quan-Hong Yang in the School of Chemical Engineering and Technology at Tianjin University. Her current research interests are the self-assembly of graphene oxide and the applications of the self-assembled materials in energy-related devices. Wei Lv received his Ph. D. from Tianjin University in 2012 under the supervision of Prof. Quan-Hong Yang. He currently works as a postdoctoral research scholar in the Graduate School at Shenzhen, Tsinghua University. His research interests mainly focus on the preparation and assembly of graphene and graphene-based materials for electrochemical detection and energy storage and his publications have been cited over 700 times.

Quan-Hong Yang joined Tianjin University as a full professor of nanomaterials in 2006. He is now also a “Pengcheng Scholar” professor (short-term) at the Graduate School at Shenzhen, Tsinghua University. He received his Ph. D. (1999) in Carbon Materials from Institute of Coal Chemistry, Chinese Academy of Sciences (CAS) and worked at Institute of Metal Research, CAS, CNRS (France), Tohoku University (Japan) and the University of Southampton (UK). His research career is totally related to novel carbon materials from porous carbons, tubular carbons to sheet-like graphene with their applications in energy storage and environmental protection.

during the reduction process of GO-based materials. Another great idea to prevent the re-stacking of GO sheets is achieved by Huang et al. by changing two-dimensional graphene sheets into three-dimensional crumpled balls,[72] and the interaction between CMG sheets greatly declined due to the crumpled

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morphology. The details of the self-assembly of the crumpled GO balls will be introduced in the liquid-air interfacial self-assembly (Section 2.1). In most cases involving the reduction of GO-based materials into graphene-based materials, the sp2 carbon network is partially restored and it is difficult to completely remove all oxygen-containing groups. The residual functional groups after reduction process behave as the steric hindrance and can also prevent the re-stacking of CMG sheets, and a typical example is the reduction of a lyophilized GO hydrogel,[42] where the porous microstructure of the GO hydrogel was still preserved in the reduced product, and this work will be introduced in details in the liquid-solid interfacial selfassembly (Section 2.3). Overall, initiated by the unique surface chemical activity and amphiphilic nature, the interfacial self-assembly of GO is intrinsically a self-concentrating of GO sheets at interfaces followed by non-covalent intersheet interactions to form well-controlled macrostructures, and therefore a promising way to acquire GO-based or graphene-based (with an appropriate reduction) functional- Figure 1. (a) Schematic of the structure of a graphene oxide sheet. (b) Schematic representation ized materials. Moreover, graphene-based of surface imaging using Brewster angle microscopy (BAM) technique. Reproduced with per[57] materials with desired microstructures can mission. Copyright 2011, De Gruyter/IUPAC. BAM images from a GO-dispersion surface (c) without and (d) with GO on the surface. Reproduced with permission.[65] Copyright 2010, ACS. be readily obtained from the reduction of the GO-based materials, and the microstructures and morphology of these GO-based materials can be realized by a plenty of interfacial self-assembly tech2. Self-Assembly of GO at Interfaces niques. Hence, the interfacial self-assembly of GO sheets is the principal topic in our review. 2.1. Liquid-Air Interfacial Self-Assembly Herein, these interfacial assembly processes mainly involve surface chemical properties of GO sheets. The GO-based mateA liquid-air interface is a good platform for the self-assembly of rials assembled at 2D interfaces are usually of membrane-like GO since such an interface is easily constructed by the ambient morphology, while the self-assembly at three-dimensional (3D) atmosphere as a gas phase together with GO suspension as a space normally leads to 3D GO-based macroforms. Through liquid phase and usually a variety of GO-based membranes can changing and controlling these interfaces, GO-based matebe prepared at such a two-dimensional (2D) platform. Reported rials with diverse microstructures and morphologies can be self-assembly at liquid-air interface are mainly classified into obtained. We will review the ever-reported interfacial selfthree methods: Langmuir-Blodgett (LB) assembly, evaporationassembly of GO at various interfaces, which are constructed by induced assembly at 2D interface and evaporation-induced two different phases and aqueous GO suspension is normally assembly 3D interface. employed as one phase (liquid phase) to interface with another LB assembly can be used to prepare ultrathin GO film, espephase (air, liquid or solid). Hence the involved self-assemblies cially for single-layer GO film. Although GO thin films can be are classified into three kinds of interfacial processes including obtained by various techniques such as spin-coating,[33] dropliquid-air, liquid-liquid and liquid-solid interfacial self-assemcasting, vacuum filtration,[77] spraying[78] and dip-coating,[79] it blies. In each part to discuss a specific type of interfacial is difficult to precisely control the number of the GO layers in assembly, a variety of assembly methods and techniques are the film by these techniques,[57] and moreover, spin-coating and reviewed with detailed discussion on their advantages and feadrop-casting make GO sheets heavily wrinkled and folded.[33,80] tures, which is briefly summarized in Table 1 with representaLayer-by-layer (LbL) assembly by electrostatic interaction is tive images. For each assembly, potential applications of the another effective method to produce thin films, which seems assembled materials are discussed and prospected and in the to be able to precisely control the number of GO layers, while final part, the importance and future development of the intersuch an assembly requires both positively and negatively facial self-assembly of GO are discussed as a potential strategy charged materials, and hence it is difficult to get pure GO for the application-targeted design and preparation of functionthin film. Among these approaches, LB assembly is the only alized carbon materials. technique that can realize controllable deposition of GO in a

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www.MaterialsViews.com Table 1. Typical self-assemblies at interfaces. Types of interfaces

Self-assembly methods

Assembled structures

Liquid-air interface

Langmuir-Blodgett assembly

Single-layer film; thin film

Ref. [63]

Evaporation-induced assembly at 2D interface

Free-standing membrane

Ref. [62]

Evaporation-induced assembly at 3D interface

Crumpled particles

Ref. [72]

Breath Figure (BF) assembly

Polymer/GO hybrid with honeycomb microstructure

Ref. [185]

Assembly at 2D interface

Thin film

Ref. [67]

Assembly at 3D interface in Pickering Emulsions

Hollow or spherical structures

Ref. [65]

Promoted by chemical interaction

Hydrogel

Ref. [42]

Promoted by electrostatic interaction

Hydrogel; Hybrid materials



Assembly at surface of nanoparticles

All-carbon hybrid materials

Ref. [197]

Flow-directed assembly at solid substrate

Freestanding membranes

Ref. [66]

Assembly at dynamic ice-water interface

Porous materials

Ref. [221]

Liquid-liquid interface

Liquid-solid interface

layer-by-layer manner to get a pure GO thin film or even single GO film, and the thickness of the obtained GO films can be accurately controlled upon repeated deposition.[63,81] For comparison, these conventional techniques for thin film preparation are introduced in Section 2.1.1. The evaporation-induced assembly mainly involves the formation of freestanding GO-based membranes and crumpled GO-based particles, which respectively self-assemble at a flat

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Typical images

References

2D and a spherical 3D water-air interface. The liquid-air interface can act as a platform for the self-assembly of GO sheets, which is mainly due to that GO sheets tend to move toward the interface owing to the amphiphilicity, and the movement of GO sheet can reduce the surface tension of the liquid phase. In summary, the amphiphilic nature and surface chemistry activity of GO are believed to be the key factors for the assembly at the liquid-air interfaces.

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Spin Coating: Spin coating has been widely used in the preparation of thin film on a flat substrate (such as silicon wafer).[82–86] In a normal spin coating process, materials with a small amount of suspension is firstly applied on the center of a flat substrate, and then the substrate is accelerated to a desired rotation rate to spread the coating material by centrifugal force, and the excess fluid is ejected off the edge of the substrate, finally a thin film with coating materials is formed on the substrate. The thickness and quality of the as-received film depends on the angular speed of the spinning, the concentration of the suspension, the viscosity of the solvent, etc.[85,87,88] Spin coating has been widely used in the preparation of GObased and graphene-based thin film,[33,88–90] and the potential application of graphene-based thin film in the field of nanoelectronics devices has been extensively reported.[33,83] Usually the graphene-based thin film comes from chemical or thermal reduction of GO-based thin film, since GO suspension with single-layer sheet can be readily realized in a large scale by chemical intercalation and exfoliation of graphite, and amphiphilic GO sheets can be stably and homogenously dispersed in various solvents. Hence GO suspension is a good precursor for preparing graphene-based thin film in a spin coating process. Drop-Casting: Drop-casting is a simple and low-cost method to make thin films, and often used in sample preparation for a variety of characterizations,[77,91–95] such as atomic force microscopy (AFM) and transmission electronic microscopy (TEM) characterization, by simply dropping a suspension of interest onto a silicon wafer and TEM copper grid, respectively. The technique is also applied in making GO- or graphene-based thin films for various applications.[94,96] Dip-Coating: Dip-coating is a conventional technique to produce thin film onto a flat substrate or a cylinder.[97–99] In a typical process, a substrate is firstly immersed into a liquid or liquid-like slurry of coating materials at a constant speed; after being kept in the suspension for a while the substrate is pulled up vertically at a constant speed; the coating materials are then deposited on the substrate during the withdrawing process; a thin film is finally formed on the substrate after the gravitational draining and solvent evaporation. The thickness of the deposited film is dependent on the competition among viscous force, capillary (surface tension) force and gravity and so forth. The faster the substrate is withdrawn, the thicker the film deposits.[97] This technique has also been extensively utilized to obtain GO-based or graphene-based thin films.[100–102] Spraying coating: Spraying technique can be also utilized to produce a thin film of interest by spraying a suspension on various substrates, and many groups have adopted this technique to prepare GO-based or graphene-based films.[91,103] Spraying technique mainly contains electrostatic spray deposition and thermal spraying. For a typical electrostatic spray deposition, a liquid precursor flowing out of a capillary nozzle is forced by a high electric potential at the tip of the nozzle.[104–107] The atomized solution lands on a substrate, and then a solid or porous film is formed there upon evaporation of solvent. The morphology and thickness of the deposited film can be controlled by adjusting the deposition parameters such as the flow rate, applied potential,

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2.1.1. Conventional Techniques for Preparing GO-Based Thin Films

deposition period, and the composition of the precursor solution.[106,107] In a typical thermal spraying, molten materials are sprayed onto a surface. The coating precursor is heated to molten or semi-molten state and accelerated by a heat source (plasma or flame) towards substrates. The precursors for thermal spraying usually include metals, alloys, ceramics, and plastics.[108,109] Thermal spraying can provide thick coatings (micrometers to several millimeters) over a large area at high deposition rate as compared to other coating processes such as electroplating, physical and chemical vapor deposition. Thermal spraying also contains several sub-spraying techniques depending on which kind of heating source is applied. Layer-by-Layer Assembly by Electrostatic Interaction: Layer-bylayer (LbL) assembly typically involves the fabrication of a thin film by depositing alternating layers of oppositely charged materials and has been used in various areas.[110–116] An advantage of LbL is high degree of control over thickness of the obtained films, which arises due to the linear growth of the films with the number of bilayers.[110] While an important feature of the LbL self-assembly is that it usually need oppositely charged materials, and hence the materials applied in the self-assembly technique must be charged or obtain charges through functionalization. GO- or graphene-based membranes can be prepared by the LbL assembly, and additional materials are often used to introduce positive or negative charges on graphene sheets.[114,115] As we know, GO is a negatively charged material, and hence GObased thin films made via the LbL self-assembly process are grown by consecutively alternating adsorption of anionic GO sheets and cationic materials on a substrate. The electrostatic interaction is the driving force for the self-assembly process. In fact, the LbL self-assembly is an ideal way to prepare a GObased hybrid thin film, whereas the LB is an excellent method to obtain a pure GO film. Another positively-charge material is required to initiate the electrostatic interaction with GO sheets in the LbL self-assembly, while GO sheets already satisfy the requirement for amphiphilic precursors in the LB self-assembly. 2.1.2. LB Assembly for the Preparation of Thin Film LB assembly is a classic self-assembly technique for amphiphilic molecules to prepare ultrathin films with controllable interfacial molecular orientation,[81,117–137] and these obtained films can be used in many fields, such as chemical sensors, ultrathin electronic and optical materials, modified electrodes and tissue engineering. In a typical LB process, amphiphilic molecules are first dissolved in a volatile organic solvent, which is dropped onto water, and then a monolayer of molecules is formed at the water-air interface after the evaporation of the volatile solvent.[138,139] GO has been largely viewed as hydrophilic, presumably due to its excellent colloidal stability in water, until Kim et al. reported that GO is an amphiphile with hydrophilic edges and a more hydrophobic basal plane and can act like a surfactant.[65] As an amphiphilic soft molecule, GO sheets can be self-assembled into a single layer film by LB technique.[63] It can easily aggregate on the water surface by dropping a GO dispersion in a water-methanol mixture onto the surface, and an ultrathin GO film is then formed at the interface that can be easily collected

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Figure 2. The morphology of GO sheets in LB assembly can be tuned by changing the pH values: (a,d) fluorescence quenching microscopy (FQM), scale bar: 100 µm; (b,e) scanning electron microscopy (SEM), scale bar: 25 µm and (c,f) atomic force microscopy (AFM), scale bar: 2 µm. Upon compression, the GO sheets tend to wrinkle (a–c) on an acidic subphase and overlap (d–f) on a basic subphase. Reproduced with permission.[147] Copyright 2010, ACS.

by vertical dip-coating.[65] A GO monolayer film, which is free of wrinkling and overlapping edges by adjusting the pH value of the parent GO precursor, can be obtained by LB assembly, which is partially due to the strong edge-to-edge repulsion of GO sheets.[63,64] With an increase of the surface pressure during the LB assembly process, the morphology of the GO monolayer film changes from being closely packed to overlapped. Single-layer graphene film has attracted extensive attention owing to the exceptional properties of individual graphene sheet, such as high transmittance, extraordinary electronic conductivity and superior thermal conductivity, and hence singlelayer graphene film should have many potential electronic applications in energy related devices, for example, as a transparent conductive electrode or field emitter.[138,140–142] The as-produced GO monolayer film from LB assembly is potentially useful in the transparent conducting electrode field after being reduced to a monolayer graphene film. Kim et al. prepared a graphene monolayer film on a glass slide using the LB assembly of GO followed by thermal reduction at 500 °C in an argon atmosphere. The graphene film has an average 95% transmittance in the visible light region and shows a sheet resistance of 4 MΩ ⵧ−1, which is comparable to the reduced GO films reported previously.[64] Low sheet resistance and high transmittance in the optical spectrum are the two most important requirements as transparent conducting electrodes. The electrical resistance of graphene film electrodes mainly comes from the contact resistance at the edges, which would decrease with an increase of the film thickness, and therefore, low sheet resistance is usually accompanied by low transmittance for a graphene film electrode and vice versa. Preparing a graphene film of low contact resistance together with high transmittance in the visible spectrum is the ultimate goal. Extensive efforts have been made to obtain a graphene film electrode with

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remarkable electrical conductivity and high transmittance as an alternative to the traditional transparent conducting electrodes,[79,143–146] such as indium tin oxide (ITO) and fluorine tin oxide (FTO) electrodes that expose some problems including limited availability of the elements, instability in acid or base, the current leakage and brittle nature. The morphology of interacting GO sheets in LB assembly can be precisely manipulated via controlling the surface pressure and pH value (Figure 2) of the parent GO suspension.[63,65,147] The dependence of the morphology of the GO film on the pH value of the parent GO suspension was investigated in details by Huang et al. using fluorescence quenching microscopy (FQM), atomic force microscopy (AFM) and scanning electron microscopy (SEM).[147] Results clearly see that the differences of the film morphology are intimately dependent on the amphiphilic nature of the individual GO sheets, which is changing with the pH values of the parent GO suspension. Under different pH values, the GO parent suspension is characterized by various degrees of ionization, which can be verified by zeta potential measurements. Another study with experimental and molecular dynamics (MD) simulation suggests that the colloidal stability and surface activity of GO aqueous suspensions are sensitive to the pH values,[148] and GO is not a typical surfactant that works in a whole pH range. The results show that the hydrophilicity varies from the edge to the basal plane of a GO sheet, and such a difference is maximized at a higher pH, where the hydrophilic edge (carboxyl groups) strongly interact with surrounding water and GO is dissolved in water like a regular salt. In contrast, the GO sheets aggregate at the lower pH and do not behave like the surfactants to form micelles.[148] This work has been followed up by several other groups.[81,143,149] With the LB assembly technique, Zheng et al. prepared transparent conductive graphene films using very large graphene oxide

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2.1.3. Evaporation-Induced Assembly at 2D Liquid-Air Interface Chen et al. reported an interesting evaporation-induced assembly of GO at a liquid-air interface, which is intrinsically a self-concentrating process of GO sheets at the interface.[46] As the water evaporates, GO sheets move upwards and are captured and stablized by the 2D interface, and finally interlinked into a macroscopic GO membrane (Figure 3a). The assembled GO membrane is freestanding and flexible (Figure 3b), and the process is relatively time-saving compared with the traditional filtration method. SEM image shows the layer-by-layer microstructure of the cross-section of the GO membrane (Figure 3c). Actually, the self-assembly of such a macroscopic GO membrane is inspired by a very similar phenomenon in which a milk skin is formed upon heating milk (Figure 3d). The assembly process of the macroscopic GO membrane is sensitive to various parameters, such as heating temperature, pH value and concentration of the parent GO suspension, and the microstructure of the GO membrane can be finely controlled by tuning these parameters. The as-produced GO membrane is constructed by individual GO sheets through layer-by-layer stacking at the 2D

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sheets,[81] and the surface roughness of the prepared graphene film is much lower than that of the film produced by other techniques including spin-coating, spray-coating, dip-coating and transfer-printing. Hereafter they used LB assembly again to prepare a large-area graphene oxide/carbon nanotube hybrid film.[143] The hybrid film had a conductivity of 400 Ω/sq. at 84% transmittance even without additional doping treatment, which demonstrated an optoelectrical performance comparable to the industrial standard material, ITO. The composition, structure and thickness of the GO/CNT hybrid film self-assembled by the LB technique are readily controlled, which further indicates that the LB self-assembly technique is highly suitable for fabrication of transparent conducting electrode for optoelectronic devices. Recently, Choi et al. took advantage of LB assembly technique to produce a GO/polymer bilayer complex where the star polymer unimicelles well-organized on graphene oxide flakes, and it is claimed that the surface activity of GO is responsible for the adsorption of the amphiphilic star polymer surfactants on the pre-suspended graphene oxide sheets.[149] That is to say, on basis of the amphiphilic nature of GO sheets, LB assembly of GO provides a highly controllable technique for the fabrication of GO-base ultrathin hybrid films and transparent conductive graphene-based electrode.

flat liquid-air interface and its thickness and area can there be easily controlled by tuning the assembly period and the interface area. The GO membranes can be repeatedly obtained by successively heating the GO suspension. The repeatable fabrication of membranes from the same GO suspension indicates a scale-up approach for GO or graphene-based membranes, which can be used for the exploration of their novel properties and the development of new applications. Lv et al. has also found that the transparency of the GO suspension continuously decreases with the decrease of acidity and increase of basicity.[41] When the pH value decreases, the original GO suspension obtaining from modified Hummer method gradually becomes semi-transparent, and its color changes from dark-brown to brown then red-brown. In contrast, the GO suspension totally turns opaque (black in color) when pH value increases to 11 due to the deoxygenation and reduction of the original GO sheets. It is thus suggested that such changes in the transparency of the GO suspension is related to the surface chemistry of the GO sheets, which indicates that the optical properties of the assembled GO membrane at the liquid-air interface can be easily and finely controlled by a pH-mediated strategy.[41] Very recently, Lv et al. realized a full tailoring of the microstructure of the assembled membrane by controlled removal of trapped water among the GO layers.[150] This work was inspired by the phase diagram of pure water, in which, with a reduced pressure, the trapped water boils seriously and then transforms into ice crystal instantaneously around the triple point. This sudden phase change across the triple point provides strong force to change and fix the microstructure of GO membrane. Lv et al. used a two-stage drying process following the phase diagram and turns the tightly layered structure of graphene membrane into an open and graded structure. Such an assembled membrane is a combination of three graded sections: a tightly layered structure (top), a loosely layered structure (middle) and vertically aligned sheets (bottom). The middle loosely layered structure and bottom vertical openings are described as an “open structure”, which guarantee a fast access of foreign molecules to the inside of membrane. Such a membrane with open and graded structure subjected to a reduction possesses high adsorption capability and fast adsorption rate for heavy oil and lithium polysulfide, therefore showing great potentials as an absorbent for environmental protection and a cathode of Li-S battery. In this sense, the trapped water in freshly formed wet GOM actually provides potential forces to tune its microstructure.[150] Note that such a graded structure can only be obtained

Figure 3. (a) Scheme of evaporation-induced assembly at a GO suspension-air interface, (b) a formed GO flexible membrane, and (c) cross-sectional SEM image of a formed GO membrane. Reproduced with permission.[46] Copyright 2009, Wiley-VCH. (d) A formed milk skin, and its formation inspired the work on the self-assembly of GO membrane at the liquid-air interface.

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in a membrane assembled at the liquid/air interface, not in the filtered membrane discussed below where the membrane is totally destroyed by such a drying process owing to the absence of the upper tightly stacked structure, which helps maintain and fix the open structure of this membrane. In summary, the liquid-air interfacial assembly delivers us a power to well control morphology, structure and properties such as membrane size, thickness, transparency and texture, which are very important for the practical application of GO- or graphene-based membrane devices. Due to the incomplete removal of oxygen-containing groups, chemically modified graphene sheets still possess surface chemical activity to some degree, and hence have the potential to be assembled at the liquid-air interfaces. Zhu et al. fabricated a transparent conductive graphene-based films in one step through combining the evaporation-induced assembly with chemical reduction.[89] During this assembly process, chemical reduction was carried out by adding hydrazine monohydrate into the GO suspension, followed by a heating treatment with stirring at 400 rpm, and then a chemically modified graphene thin film is formed at the suspension-air interface. The optical transmittance value and sheet resistance of the obtained thin film can reach 96% at a wavelength of 550 nm and 31.7 KΩ ⵧ−1. It was mentioned in their work that the selfassembly of the thin film was assisted by the stirring, and such a film was not obtained if no stirring while keeping the other conditions the same.[89] As mentioned above, a few amount of oxygen-containing groups on the chemically modified graphene sheets bring some surface chemical activity, which is of course much lower than that of graphene oxide sheets with abundant functional groups, and hence a driving force (such as stirring) is necessary to initiate the self-assembly process at the liquid-air interface. More interestingly, the evaporation-induced assembly at a liquid-air interface suggests a simple approach for the formation of multi-component hybrid membranes, where GO sheets act as the carriers for other components onto the interface to construct a hybrid membrane. That is, using evaporationinduced self-assembly, uniform hybridization of bi- or multicomponents in a thin membrane can be realized as long as such components can be homogeneously mixed and well-dispersed in the parent suspension. The composition of such a hybrid membrane is well-controlled by tuning the fraction of each component in the starting suspension. Lv et al. assembled a GO/graphene hybrid membrane (Figure 4a) at a liquid/air interface by using GO sheets as a sticking component to bind low-temperature exfoliated graphene nanosheets (LGNs),[6,47] and its transmittance (Figure 4b) and conductivity (Figure 4c) can be readily tuned by altering the LGNs fraction in the homogeneous suspension precursor.[47] In fact, the sticking effect of GO sheets in the self-assembly of the hybrid membrane comes from its surfactant function. The surface chemical activity of LGNs can be enhanced when amphiphilic GO sheets stick to them, and then the improved surface chemical activity acts like a driving force to bring the LGNs up to the liquid/air interface together with the GO sheets, and finally a uniform GO/LGNs hybrid membrane forms at the surface of the GO/LGNs suspension. Because of its amphiphilicity, GO is also able to disperse CNT powder to form a stable GO/CNT suspension.[65]

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Inspired by this idea, Shao et al. prepared a homogenous suspension containing CNT-attached GO sheets, and then obtained a GO/CNT hybrid membrane (denoted as CNTGO) at the surface of the suspension by the evaporation-induced self-assembly method (Figure 5a).[62] The CNTGO membrane exhibits extremely high flexibility as shown in Figure 5b and its appearance is similar to a pure GO membrane obtained by the same self-assembly method. The cross-sectional SEM image clearly shows the microstructure of the hybrid membrane (Figure 5c), and the CNTGO membrane combines chemically active GO sheets with highly conductive CNTs. Usually, a redox electrolyte system (Fe(CN)64− /Fe(CN)63−) is chosen to evaluate the rate of electron transfer (ET) in active materials,[151] and herein the electrochemical sensing properties of the CNTGO membrane was investigated using this redox system. The CNTGO membrane was coated onto a glass carbon (GC) electrode by drop-casting method. The electrochemical measurements demonstrated that the two components, CNTs and GO, exhibited a synergistic effect in the sensing performance, and a relatively narrow peak potential separation was observed and the peak current increased with the increasing fraction of CNTs in the hybrid membrane (Figure 5d), and the Randles–Sevcik (R–S) slope curves (Figure 5e) indicated that the redox reaction occurring in the electrolyte system is diffusion- controlled. It was believed that the abundant oxygen-containing groups on GO sheets help accelerate the electron transfer between the redox system and electrochemical active material, and thus a relatively narrow peak potential separation (ΔEp) was observed for the GO-coated GC electrode. The incorporation of highly conductive CNTs enhanced the electronic conductivity of the CNTGO membrane, and hence resulted in high peak currents for the CNTGO membrane modified electrodes. The electrochemical results indicated that the CNTGO membrane combined the advantages of GO and CNTs. In addition to the above mentioned GO-based membranes, extensive efforts have been made to assemble various kinds of graphene-based hybrid membranes by using this evaporation-induced assembly method.[152,153] In an aqueous suspension environment, graphene sheets cannot assemble to form a continuous membrane at the liquid-air interface only under heating treatment, which is due to the lack of abundant functional groups on the sheets. Typically, graphene has a high C/O ratio of 10:1,[6] which means there are only a little amount of oxygen-containing groups attached, and then the surface chemical activity for the graphene sheets is too low to drive these sheets up to the suspension-air interface. Therefore, in order to get a graphene-based membrane at the liquid-air interface by the evaporation-induced assembly, enough surface chemical activity is required. Besides taking advantage of the amphiphilic nature of GO sheets (the aforementioned GO/ LGNs hybrid membrane), another strategy is introducing an amphiphilic polymer to disperse graphene sheets and then obtain a homogeneous graphene/polymer suspension, and the amphiphilic polymer can also provide the graphene with strong enough surface chemical activity to conduct the liquid/ air interfacial assembly only if the polymer has interaction with graphene sheets. On basis of this idea, Wu et al. used polyvinyl alcohol (PVA) as the amphiphilic dispersant to prepare a PVA/ LGN hybrid membrane by the liquid-air interfacial assembly

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REVIEW Figure 4. (a) Scheme of the fabrication of a low-temperature exfoliated graphene nanosheet (LGN)/graphene oxide nanosheet (GON) hybrid membrane, (b) UV–Vis spectra of GON and LGN/GON hybrid membrane (LGN mass fraction: 10%), inset of (b): maximum transmittance relative to LGN fraction, and (c) membrane resistivity relative to LGN. Reproduced with permission.[47] Copyright 2011, RSC.

strategy (Figure 6a).[153] The assembly starts with a uniform suspension containing amphiphilic PVA chains bound to graphene, and the strong interaction between them ensures that the graphene acquires some surface chemical activity coming from the amphiphilic PVA, which enable the modified-graphene sheets to float up to the liquid-air interface and then selfassemble into a hybrid membrane with uniformly distributed graphene sheets in the whole membrane. The obtained hybrid membrane shows improved thermal stability and mechanical strength to some extent as compared to a pure PVA membrane. It was found that the glass transition temperature of the hybrid membrane increases with the graphene incorporation. Due to the high aspect ratio of graphene and strong interaction between graphene sheets and PVA chains, the free motion of the PVA polymer chains is restricted, which increases the glass transition temperature of the hybrid membrane. In addition, the evaporation-induced PVA/LGN hybrid membrane possesses much better mechanical properties (tensile strength and Young’s modulus) than that obtained through vacuum filtration of a PVA/LGN mixture.[153] It is believed that the strong

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interaction between graphene sheets and the PVA polymer matrix is also responsible for this, and the more uniform hybridization of the two components in the assembled PVA/LGNs hybrid membrane induces much higher mechanical improvement as compared to the case derived from the vacuum filtration method. Considering such a hybrid membrane is highly uniform and contains monolayer graphene sheets in a transparent and flexible polymer matrix, its application in optical devices was attempted. Because the unique properties of the single layered graphene can be employed and enough transparence is provided, such a PVA/LGN hybrid membrane was successfully used as a saturable absorption material in a compact Q-switched erbium-doped fiber (EDF) laser (Figure 6b,c).[153] In many cases, graphene-based membranes are more useful than GO-based membranes, especially in energy-storage fields where materials with high electronic conductivity are needed. Energy-related devices mainly include solar cells, fuel cells, supercapacitors and lithium-ion batteries and so forth, and graphene-based materials starting from GO show potentials in these energy storage and conversion devices.[154–156] From this

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reduction approaches (namely the reduction based on borohydride (NaBH4) and hydrazine (N2H4•H2O), etc.) generate substantial amount of gaseous products which destroy the textures and decrease the strength of the assembled materials. Thermal reduction in a confined space or mild chemical reduction approaches has been developed as typical nondestructive reduction methods for GO-based macroform materials. Chen et al. annealed a freestanding GO membrane in a confined space to form a free-standing flexible graphene membrane, which avoids the crack of GO membrane annealed in a free space due to the random volume expansion.[157] Pei et al. developed a more effective hydroiodic (HI) acid reduction method, which can effectively reduce the GO membrane into highly conductive graphene membrane without destroying the original high flexibility and integrity, and even with higher tensile strength than that of the original GO membrane.[158,159] Figure 5. (a) The self-assembly illustration, (b) a photo and (c) cross-sectional SEM image In fact, GO sheets are prone to spontaneof the CNTGO hybrid membrane, (d) representative CV profiles and (e) R–S slope curves of the coated glassy carbon (GC) and uncoated glassy carbon (UGC) electrodes in 0.014 mol/L ously concentrate on the surface of a GO suspension, and this tendency is owing to its ferricyanide solution. Note: the R–S slope curves of UGC and CNTGO-1 electrodes are almost amphiphilic nature,[160] and the acceleration overlapped.Reproduced with permission.[62] Copyright 2012, RSC. of the self-concentration process can be realized by heat treatment or through a flotation process or even point, fast but nondestructive reduction techniques are urgently by adding volatile solvents.[46,64,67,161] A typical example for the required for the self-assembled GO membranes and macroform materials since normal thermal reduction or chemical self-assembly accelerated by heat treatment is the aforementioned evaporation-induced assembly, where the kinetic energy provided by the heat treatment accelerates the movement of GO sheets and hence promotes the self-assembly of GO sheets at the liquid-air interface. Huang et al. reported a flotation-induced acceleration of GO sheets on the water surface, where the selfassembly process was accelerated by blowing gas (nitrogen, air, CO2, etc.) bubbles through the GO suspension (Figure 7a, b),[64,65] and then GO sheets were captured by the rising bubbles and transported to the water surface. In a word, blowing gas bubbles through the GO suspension accelerates the formation of GO membrane on the water-air interface, and Brewster-angle microscopy (BAM) observation (Figure 7c, d) confirms the flotation acceleration process. During the flotation-induced acceleration process, the surface of the GO suspension was initially observed to be free of any scattering points (Figure 7c), indicating that few or no GO sheets were present at the water-air interface. After blowing gas bubbles for a while, the in situ BAM image (Figure 7d) of the water surface shows a large number of Figure 6. (a) Scheme of self-assembly of a (polyvinyl alcohol) PVA/low-temperature exfoliated graphene nanosheet (LGN) membrane, (b) characteristic spectrum of a PVA/LGN Q-switched bright spots, confirming the presence of surEDFL and (c) a typical oscilloscope trace for pulse trains under 68.8 mW pump power. Repro- face-active particles. The amphiphilicity guarantees the stable existence of the GO sheets at duced with permission.[153] Copyright 2012, RSC.

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REVIEW Figure 7. (a,b) Illustration showing the flotation of GO in carbonated water, and (c,d) the in situ Brewster angle microscopy (BAM) images from the GO suspension surface (c) before and (d) after gas bubbles are applied. Reproduced with permission.[65] Copyright 2010, ACS.

the liquid-air interface. Taking advantage of the spontaneous selfassembly of GO sheets on a liquid-air interface, a suspended GO membrane can be formed over a small orifice by drop casting GO aqueous droplets over the orifice. With full evaporation of the water-carrying GO sheets, the assembled membrane at the liquid-air interface (droplet-air interface) is finally transformed into a membrane suspended across the aperture opening. The membrane with the aperture can be used in an environmental electron microscope where the GO membrane is used as an electron-transparent but molecularly impenetrable window. 2.1.4. Evaporation-Induced Assembly at 3D Liquid-Air Interface The above evaporation-induced assembly at flat 2D liquid-air interface promotes the formation of a flat paper-like GO-based material. When the evaporation-induced assembly occurs around some free-standing GO hydrosol droplets, GO sheets in the droplets are isotropically compressed to form near-spherical 3D crumpled particles, which is because these free-standing GO droplets can provide spherical 3D liquid-air interfaces. Luo et al. adopted an ultrasonic atomizer to nebulize GO aqueous suspension to form GO aerosol droplets, which were carried by nitrogen gas to fly through a horizontal tube furnace preheated at a desired temperature, and then the formed crumpled particles from the rapidly evaporating process was collected by a Teflon filter at the exhaust (Figure 8a).[72] The obtained crumpled particles can tightly pack without significantly reducing the area of accessible surface, and possesses superior solution processability, excellent compressive strength, and high surface area. The crumpled structure has been commented as a promising material with many potential applications.[162] Load/displacement curves revealed that the crumple particles are compressionresistant like paper balls, since compressive stress makes them stiffer and harder, and the crumple balls have consistently higher and more stable surface area than regular two-dimensional GO sheets after the same processing histories, such as heat treatment and mechanical compression,[72] and it was found that the crumpled graphene nanoparticles possess superior performance to activated carbon and regular sheet-like graphene in a microbial fuel cell (MFC), where the carbon-based materials were used to modify the anode of MFC, and Luo et al. claimed that better performance of the crumpled graphene nanoparticles in the MFC is due to its high electron conductivity, large accessible surface area, and rapid mass transfer of fuels and ions endowed

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by the open microstructure. When the crumpled graphene particles are used in supercapacitor, it delivered both higher energy and power densities than the cases for flat or wrinkled graphene sheets, and more importantly, the performance of the obtained supercapacitors are less dependent on the electrode mass loading. Moreover, flat graphene sheets can be used as the binder to further enhance the performance of the supercapacitors made with the crumpled graphene particles, in which normally used but less active binder materials are not needed.[163] Inspired by the self-assembly of the crumpled particles, sackcargo GO-based structure was obtained by introducing a second component into the GO hydrosol droplets (Figure 8b).[164] Such self-assembly provides a simple manufacturing process to obtain graphene-wrapped hybrid materials. Normally, the self-assembly mechanism of graphene-wrapped hybrid materials involves electrostatic attraction and covalent cross-linking, which requires oppositely charged and chemically modified surfaces. The evaporation-induced assembly can be also used for the preparation of graphene-wrapped hybrids in which the components possess the same surface charge states, since the capillary compression instead of the electrostatic force results in the formation of the nanostructures, and the 3D spherical interfaces provided by the free-standing GO aerosol droplets can create spherical particles. Molecular dynamics (MD) simulation results show that the GO sheets stick to surface of the droplet and are pulled inward with the drying-induced decrease of the droplet volume, and it is claimed that the strong water-GO interaction introduced by hydrogen bonding is responsible for the assembly. The cell uptake experiments were conducted on the empty graphene nanosacks, and results show that human lung epithelial cells could enter into the nanosacks within 24 h. The nanosacks hold potentials in biomedicine since they demonstrate a biological response comparable to the unfolded GO sheets and have an acute cytotoxicity at doses as low as 5 µg/mL.[164] The sack-cargo GO-based nanostructures assembled by evaporating aerosol droplets exhibit promising applications in controlled delivery and release of drugs due to the sack leakage and biodegradation of the GO. It is found that the sack-cargo nanostructures elicit a biological response similar to unfolded GO and have low acute cytotoxicity at doses below 5 µg /mL. In fact, the strong interaction between water and GO sheets comes from the amphiphilic nature of GO sheets. Following up the work of the self-assembly of the crumpled particles, Sohn et al. prepared hollow graphene capsules by a two-stage process,

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Figure 8. (a) Schematic of the evaporation-induced crumpling process, and the SEM images below show the typical morphologies of the four samples collected along the flying pathway from spots 1 to 4. Reproduced with permission.[72] Copyright 2011, ACS. (b) Conceptual model for the colloidal selfassembly of filled graphene nanosacks. Reproduced with permission.[164] Copyright 2012, ACS.

molding GO sheets against a polystyrene bead template in the rapidly evaporating aerosol droplets and then reducing GO together with the decomposition of the polymer template during ultrasonic spray pyrolysis.[165] It is believed that the structure features of the graphene capsules, such as lightweight and large free volumes, are responsible for the excellent performance of oil absorption (4.5 µL mg−1) and recovery. In the molding process, the soft GO was propelled by the capillary compression to wrap the polystyrene bead template particles, during the pyrolysis the GO sheets were reduced and simultaneously duplicated the spherical surface of the template particles and then formed hollow graphene capsules. In addition, the specific surface recognition between GO and the template is usually required for conventional self-assembly of hollow graphene particles, while the self-assembly process with rapidly evaporating aerosol droplets eliminates the need for the recognition, and hence this should be a general approach for producing hollow graphene particles of various shapes replicating the morphologies of the

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template, such as polyhedral particles, nanorods or nanowires and their aggregates.[165] Different from dye adsorption materials, which usually need both a high specific surface area and abundant functional groups, a high free volume is a critical parameter for effective oil adsorption. The above-discussed graphene capsules, obtained by capillary molding of GO sheets, possess promising oil absorption ability due to their high free volume provided by the capsular microstructure.

2.2. Liquid-Liquid Interfacial Self-Assembly A liquid-liquid interface is another ideal platform for the self-assembly of amphiphilic GO to form GO-based nanostructures.[166] The minimization of free energy is the driving force for the self-assembly at a liquid-liquid interface,[167] which is normally formed between a hydrophobic organic solvent and water. Honeycomb-patterned structures

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2.2.1. Breath Figure (BF) Assembly The BF technique (also referred to as the water-droplet templating method) is a bottom-up self-assembly method to prepare honeycomb structures with controllable size, shape and functionality, and is popularly used in assembling polymerbased nanostructures. The BF technique can be viewed as a liquid-liquid interfacial self-assembly occurring at a volatile solvent-water interface. In a typical BF assembly, polymer dispersion in a water-immiscible volatile solvent is cast onto a substrate in a very humid atmosphere. Hexagonally packed water microdroplets are formed by evaporation cooling on the solution surface and then transferred to the solution front by convection flow or by capillary forces. With the evaporation of the organic solvent, a honeycomb-patterned polymer film forms, with water droplets array as a template filling the space

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are self-assembled by classic breath figure (BF) technique, and paper-like GO membranes are obtained by spontaneous enrichment at 2D interfaces, and spherical GO-based particles are prepared through Pickering emulsions at spherical 3D interfaces. The BF technique helps form honeycomb GO/polymer membranes with a regular morphology. The synthesis of polymergrafted GO is an important precondition for such an assembly, and the abundant oxygen-containing moieties on GO play a critical role in the formation of ordered porous GO/polymer membranes, since the oxygen-containing groups on GO make it possible to graft with polymers. Assembly techniques caused by the 2D interface spontaneous enrichment and 3D interface Pickering emulsions are mainly the results of the amphiphilic nature of GO.

left by the evaporated organics. Finally, further evaporation of the water droplets leaves a porous polymer film.[168–174] The resultant porous films can be further used as the templates to build ordered nanostructures of other materials, and are very promising in the production of microlenses, tissue engineering scaffolds, catalysts, adsorbents, microreactors, cell culture and superhydrophobic materials.[175–179] With BF assembly, honeycomb-pattern films have been prepared from a variety of polymers including polystyrene star or comb polymers, rod-coil block polymers, homo-polymers and amphiphilic copolymers.[172,174–177,180,181] When amphiphilic copolymers are used as precursors, the hydrophilic/hydrophobic balance of the polymer is a key factor in stabilizing water droplets and preventing their coalescence and thereafter controlling the quality and size of the pores in the honeycombpatterned films.[181,182] GO can be viewed as an amphiphilic polymer and can be also grafted onto other polymers or molecule due to its abundant oxygen-containing groups, moreover its amphiphilicity can be precisely tuned by adjusting the size of GO sheets and the pH value of GO suspension, and hence several groups have successfully conducted BF self-assembly technique in the formation of GO-based macrostructures.[183,184] Kim et al. fabricated a polystyrene-grafted GO-based honeycomb-pattern film by using the BF assembly (Figure 9a–b).[185] They firstly prepared polystyrene (PS)-grafted GO sheets by surface-initiated atom transfer radical polymerization, and it was found these PS-grafted GO sheets were able to easily disperse in benzene. As we know, benzene is a water-immiscible volatile solvent, which meets one of the requirements mentioned above for the BF self-assembly. Finally, a mechanically flexible PS-GO honeycomb film on a substrate was created after the complete

Figure 9. (a) Procedure for the self-assembly of the macroporous carbon films, and (b) SEM image of a typical porous film. Reproduced with permission.[185] Copyright 2010, Wiley-VCH. (c) Scheme of the preparation of GO film used in this work. Reproduced with permission.[67] Copyright 2011, Langmuir. (d) The microscopic images of toluene droplets formed in a toluene/GO water mixture, where the concentration of GO was varied, from left to right: 0.95 mg/mL to 0.47, 0.19, 0.095, 0.047, 0.019, and 0.0095 mg/mL. Scale bars = 1 mm. (e-g) fluorescence quenching microscopy (FQM) images of different areas of a deposited GO film on a glass slide. Reproduced with permission.[65] Copyright 2010, ACS. (h) Illustration of hollow graphene oxide sphere (HGOS) synthesis. Reproduced with permission.[60] Copyright 2010, RSC.

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evaporation of water and the solvent during the self-assembly process. The obtained porous film is flexible and with tunable structure, and it was found that the pore size and the number of pore layers of the honeycomb film can be controlled by changing the concentration of the parent solution and the chain length of the grafted polymers. The porous structure produced by controlled self-assembly is an ideal support framework for some metal oxides or noble metal nanoparticles. A porous GObased membrane prepared by BF assembly is a typical example, where the porous microstructure can provide anchoring sites for the nucleation of noble metal nanoparticles. One can control the distribution of the nanoparticles through adjusting the size and shape of the pores, and the microstructure can act as a host to distribute guest nanoparticles as loosely as possible, leading to the improved chemical activity of the nanoparticles due to the increased specific surface area. Because the amphiphilicity of GO influences the hydrophilic-hydrophobic balance of a GO-based precursor, which is a key factor in the BF assembly, the structural adjustment of GO sheets is also crucial in adjusting the structure of the porous film. Luo et al. reported that GO nanocolloids with a smaller size are more hydrophilic than normally-used powderlike GO,[183] and hence it is speculated that utilizing small size GO sheets for the BF assembly of honeycomb GO-based film is worth attempting. By using the BF technique, Yin et al. also synthesized a honeycomb GO-based film.[184] As we mentioned above, a prerequisite to use the BF technique to form honeycomb structures is that the precursor is dispersible in a water-immiscible volatile solvent. While GO is hard to be dispersed in some waterimmiscible solvents, and therefore in this work, a cationic surfactant, dimethyldioctadecylammonium bromide (DODA•Br), was used to form a GO/DODA complex in chloroform by an electrostatic adsorption onto the GO surface with negative charges. In addition, the two long alkyl chains of the DODA hinder the restacking of the adjacent graphene layers when the GO/DODA complex was reduced to DODA-modified graphene sheets, which possess hierarchical structures and a large surface area. In order to manifest the advantages of the hierarchical structure in lithium-ion storage, the electrochemical measurement was conducted on both the DODA-modified graphene honeycomb film and a chemically reduced smooth GO/ DODA film. Results showed that the honeycomb film exhibited higher capacity, higher reversible retention capacity and very good rate capability. Finally, the authors claimed that porosity can significantly influence the capacitance and rate performance of carbon-based electrode materials, since porous microstructure could decrease the diffusion distance of the lithium ions to the active material surface, leading to enhanced electrochemical performance of lithium ion batteries.[184] 2.2.2. Assembly at 2D Interface Besides the aforementioned liquid-air interface that results in GO-based membranes, a liquid-liquid interface can also induce 2D self-assembly of membranes. At a liquid-liquid interface formed between a volatile organic solvent (e.g., toluene, chloroform) and an aqueous GO suspension, GO sheets could be “picked up” from the aqueous dispersion and then moved

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towards the solvent-water interface,[64,67] which is also a way to verify the amphiphilic nature of GO. As we have mentioned in Introduction section, in addition to SEM and AFM, a surface analysis technique (BAM), could be also used to observe this self-assembly process on the basis of the change of the local refractive index which influences the reflection of the incident beam. In a typical example, a few drops of chloroform are firstly spread onto a GO dispersion, forming a thin oil layer, which attracts GO sheets to the vicinity of the oil/water interface, and then BAM is used to verify the existence of GO sheets on the water surface after the chloroform is evaporated.[64] The chloroform significantly increased the concentration of GO sheets on the interface and the resultant GO thin film on the surface was very stable even after the removal of chloroform, and BAM showed that the GO thin film could not be re-dispersed into the water after being left on its surface overnight. Chen et al. claimed that chloroform evaporation provides little driving force for the transfer of GO sheets in water phase towards the chloroform-water interface, and thus results in a dawdling GO film formation process.[67] Moreover, they found that several previous studies have shown some solvents are helpful for the formation of 2D films comprised of carbon nanotubes and nanoparticles. Therefore, in order to speed up the self-assembly process and create controllable upward convection flows to achieve fast formation of GO film at the liquid-liquid interface, ethanol is introduced by Chen et al. to promote the quick formation of a GO thin film at the pentane-water interface (Figure 9c).[67] In addition, the self-assembly method can also be extended to preparing GO-based hybrid films with tunable composition, transmittance and surface resistivity. The work confirms again that the interfacial self-assembly method can easily control the homogeneity of film thickness in compared with other GO thin film formation processes, such as dropcasting, spin-coating, dip-coating, spraying, and vacuum filtration, and hence the advantages of the interfacial self-assembly are manifested herein once more. 2.2.3. Assembly at 3D Interface in Pickering Emulsions Thanks to the amphiphilic nature, GO sheets work as a colloidal surfactant to disperse organic solvent droplets in water and then create stable droplets as the oil phase, where GO is concentrated at the curved oil-water interface. This is a typical characteristic for Pickering emulsion. Kim et al. reported that the toluene droplets can be formed at the toluene-water interface in presence of GO sheets as a surfactant, and the formed droplets are stabilized for months.[65] The diameter of the formed toluene droplets depends on the content of GO sheets in the water phase (Figure 9d), and the toluene droplets stabilized by GO sheets are much larger than those stabilized by other colloidal substances (e. g. silica). These emulsions are relatively stable thanks to the high surface area of GO sheets which is many orders of magnitude larger than the size of the assembled colloidal particle.[65] The emulsion droplets were coated by nearly a monolayer of GO, and the as-received GO film (Figure 9e–g) collected at the liquid (oil)-liquid (water) interface has many multilayer islands (darker domains in Figure 9f, g) with a largely monolayer GO film underneath.[65] The Pickering emulsions can work as a size-sensitive filter for separating GO

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sheets with different sizes, where large GO sheet with size above 5 micron can be selectively extracted from their aqueous suspension, while those with much smaller size (

Self-assembly of graphene oxide at interfaces.

Due to its amphiphilic property, graphene oxide (GO) can achieve a variety of nanostructures with different morphologies (for example membranes, hydro...
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