Graphene Oxide: Surface Activity and Two‐Dimensional Assembly

Graphene oxide (GO) is a promising precursor for preparing graphene‐based composites and electronics applications. Like graphene, GO is essentially one‐atom thick but can be as wide as tens of micrometers, resulting in a unique type of material building block, characterized by two very different length scales. Due to this highly anisotropic structure, the collective material properties are highly dependent on how these sheets are assembled. Therefore, understanding and controlling the assembly behavior of GO has become an important subject of research. In this Research News article the surface activity of GO and how it can be employed to create two‐dimensional assemblies over large areas is discussed.     

Self‐Assembly of Thiourea‐Crosslinked Graphene Oxide Framework Membranes toward Separation of Small Molecules

The poor mechanical strength of graphene oxide (GO) membranes, caused by the weak interlamellar interactions, poses a critical challenge for any practical application. In addition, intrinsic but large‐sized 2D channels of stacked GO membranes lead to low selectivity for small molecules. To address the mechanical strength and 2D channel size control, thiourea covalent‐linked graphene oxide framework (TU‐GOF) membranes on porous ceramics are developed through a facile hydrothermal self‐assembly synthesis. With this strategy, thiourea‐bridged GO laminates periodically through the dehydration condensation reactions via ? NH₂ and/or ? SH with ? O?C? OH as well as the nucleophilic addition reactions of ? NH₂ to C? O? C, leading to narrowed and structurally well‐defined 2D channels due to the small dimension of the covalent TU‐link and the deoxygenated processes. The resultant TU‐GOF/ceramic composite membranes feature excellent sieving capabilities for small species, leading to high hydrogen permselectivities and nearly complete rejections for methanol and small ions in gas, solvent, and saline water separations. Moreover, the covalent bonding formed at the GO/support and GO/GO interfaces endows the composite membrane with significantly enhanced stability.     

A Cut‐and‐Paste Approach to 3D Graphene‐Oxide‐Based Architectures

Properly cut sheets can be converted into complex 3D structures by three basic operations including folding, bending, and pasting to render new functions. Folding and bending are extensively employed in crumpling, origami, and pop‐up fabrications for 3D structures. Pasting joins different parts of a material together, and can create new geometries that are fundamentally unattainable by folding and bending. However, it has been much less explored, likely due to limited choice of weldable thin film materials and residue‐free glues. Here it is shown that graphene oxide (GO) paper is one such suitable material. Stacked GO sheets can be readily loosened up and even redispersed in water, which upon drying, restack to form solid structures. Therefore, water can be utilized to heal local damage, glue separated pieces, and release internal stress in bent GO papers to fix their shapes. Complex and dynamic 3D GO architectures can thus be fabricated by a cut‐and‐paste approach, which is also applicable to GO‐based hybrid with carbon nanotubes or clay sheets.     

Laser‐Ignited Relay‐Domino‐Like Reactions in Graphene Oxide/CL‐20 Films for High‐Temperature Pulse Preparation of Bi‐Layered Photothermal Membranes

Light‐ignited combustions have been proposed for a variety of industrial and scientific applications. They suffer, however, from ultrahigh light ignition thresholds and poor self‐propagating combustion of typical high‐energy density materials, e.g., 2,4,6,8,10,12‐(hexanitrohexaaza)cyclododecane (CL‐20). Here, reported is that both light ignition and combustion performance of CL‐20 are greatly enhanced by embedding ε‐CL‐20 particles in a graphene oxide (GO) matrix. The GO matrix yields a drastic temperature rise that is sufficient to trigger the combustion of GO/CL‐20 under low laser irradiation (35.6 mJ) with only 6 wt% of GO. The domino‐like reduction‐combustion of the GO matrix can serve as a relay and deliver the decomposition‐combustion of CL‐20 to its neighbor sites, forming a relay‐domino‐like reaction. In particular, a synergistic reaction between GO and CL‐20 occurrs, facilitating more energy release of the GO/CL‐20 composite. The novel relay‐domino‐like reaction coupled with the synergistic reaction of CL‐20 and GO results in a deflagration of the material, which generates a high‐temperature pulse (HTP) that can be guided to produce advanced functional materials. As a proof of concept, a bi‐layered photothermal membrane is prepared by HTP treatment in an extremely simple and fast way, which can serve as a model architecture for efficient interfacial water evaporation.     

Solution Adsorption Formation of a π‐Conjugated Polymer/Graphene Composite for High‐Performance Field‐Effect Transistors

Semiconducting polymers with π‐conjugated electronic structures have potential application in the large‐scale printable fabrication of high‐performance electronic and optoelectronic devices. However, owing to their poor environmental stability and high‐cost synthesis, polymer semiconductors possess limited device implementation. Here, an approach for constructing a π‐conjugated polymer/graphene composite material to circumvent these limitations is provided, and then this material is patterned into 1D arrays. Driven by the π–π interaction, several‐layer polymers can be adsorbed onto the graphene planes. The low consumption of the high‐cost semiconductor polymers and the mass production of graphene contribute to the low‐cost fabrication of the π‐conjugated polymer/graphene composite materials. Based on the π‐conjugated system, a reduced π–π stacking distance between graphene and the polymer can be achieved, yielding enhanced charge‐transport properties. Owing to the incorporation of graphene, the composite material shows improved thermal stability. More generally, it is believed that the construction of the π‐conjugated composite shows clear possibility of integrating organic molecules and 2D materials into microstructure arrays for property‐by‐design fabrication of functional devices with large area, low cost, and high efficiency.     

Wetting‐Induced Climbing for Transferring Interfacially Assembled Large‐Area Ultrathin Pristine Graphene Film

Owing to inherent 2D structure, marvelous mechanical, electrical, and thermal properties, graphene has great potential as a macroscopic thin film for surface coating, composite, flexible electrode, and sensor. Nevertheless, the production of large‐area graphene‐based thin film from pristine graphene dispersion is severely impeded by its poor solution processability. In this study, a robust wetting‐induced climbing strategy is reported for transferring the interfacially assembled large‐area ultrathin pristine graphene film. This strategy can quickly convert solvent‐exfoliated pristine graphene dispersion into ultrathin graphene film on various substrates with different materials (glass, metal, plastics, and cloth), shapes (film, fiber, and bulk), and hydrophobic/hydrophilic patterns. It is also applicable to nanoparticles, nanofibers, and other exfoliated 2D nanomaterials for fabricating large‐area ultrathin films. Alternate climbing of different ultrathin nanomaterial films allows a layer‐by‐layer transfer, forming a well‐ordered layered composite film with the integration of multiple pristine nanomaterials at nanometer scale. This powerful strategy would greatly promote the development of solvent‐exfoliated pristine nanomaterials from dispersions to macroscopic thin film materials.     

Microwave Energy Drives “On–Off–On” Spin‐Switch Behavior in Nitrogen‐Doped Graphene

The established application of graphene in organic/inorganic spin‐valve spintronic assemblies is as a spin‐transport channel for spin‐polarized electrons injected from ferromagnetic substrates. To generate and control spin injection without such substrates, the graphene backbone must be imprinted with spin‐polarized states and itinerant‐like spins. Computations suggest that such states should emerge in graphene derivatives incorporating pyridinic nitrogen. The synthesis and electronic properties of nitrogen‐doped graphene (N content: 9.8%), featuring both localized spin centers and spin‐containing sites with itinerant electron properties, are reported. This material exhibits spin‐switch behavior (on–off–on) controlled by microwave irradiation at X‐band frequency. This phenomenon may enable the creation of novel types of switches, filters, and spintronic devices using sp²‐only 2D systems.     

Light‐Driven Active Proton Transport through Photoacid‐ and Photobase‐Doped Janus Graphene Oxide Membranes

Biological electrogenic systems use protein‐based ionic pumps to move salt ions uphill across a cell membrane to accumulate an ion concentration gradient from the equilibrium physiological environment. Toward high‐performance and robust artificial electric organs, attaining an antigradient ion transport mode by fully abiotic materials remains a great challenge. Herein, a light‐driven proton pump transport phenomenon through a Janus graphene oxide membrane (JGOM) is reported. The JGOM is fabricated by sequential deposition of graphene oxide (GO) nanosheets modified with photobase (BOH) and photoacid (HA) molecules. Upon ultraviolet light illumination, the generation of a net protonic photocurrent through the JGOM, from the HA‐GO to the BOH‐GO side, is observed. The directional proton flow can thus establish a transmembrane proton concentration gradient of up to 0.8 pH units mm^?2 membrane area at a proton transport rate of 3.0 mol h^?1 m^?2. Against a concentration gradient, antigradient proton transport can be achieved. The working principle is explained in terms of asymmetric surface charge polarization on HA‐GO and BOH‐GO multilayers triggered by photoisomerization reactions, and the consequent intramembrane proton concentration gradient. The implementation of membrane‐scale light‐harvesting 2D nanofluidic system that mimics the charge process of the bioelectric organs makes a straightforward step toward artificial electrogenic and photosynthetic applications.     

Quantum‐Confined‐Superfluidics‐Enabled Moisture Actuation Based on Unilaterally Structured Graphene Oxide Papers

The strong interaction between graphene oxides (GO) and water molecules has trigged enormous research interest in developing GO‐based separation films, sensors, and actuators. However, sophisticated control over the ultrafast water transmission among the GO sheets and the consequent deformation of the entire GO film is still challenging. Inspired from the natural “quantum‐tunneling‐fluidics‐effect,” here quantum‐confined‐superfluidics‐enabled moisture actuation of GO paper by introducing periodic gratings unilaterally is reported. The folded GO nanosheets that act as quantum‐confined‐superfluidics channels can significantly promote water adsorption, enabling controllable and sensitive moisture actuation. Water‐adsorption‐induced expansion along and against the normal direction of a GO paper is investigated both theoretically and experimentally. Featuring state‐of‐the‐art of ultrafast response (1.24 cm^?1 s^?1), large deformation degree, and complex and predictable deformation, the smart GO papers are used for biomimetic mini‐robots including a creeping centipede and a smart leaf that can catch a living ladybug. The reported method is simple and universal for 2D materials, revealing great potential for developing graphene‐based smart robots.     

A Dynamic Graphene Oxide Network Enables Spray Printing of Colloidal Gels for High‐Performance Micro‐Supercapacitors

Properly controlling the rheological properties of nanoparticle inks is crucial to their printability. Here, it is reported that colloidal gels containing a dynamic network of graphene oxide (GO) sheets can display unusual rheological properties after high‐rate shearing. When mixed with polyaniline nanofiber dispersions, the GO network not only facilitates the gelation process but also serves as an effective energy‐transmission network to allow fast structural recovery after the gel is deformed by high‐rate shearing. This extraordinary fast recovery phenomenon has made it possible to use the conventional air‐brush spray technique to print the gel with high‐throughput and high fidelity on nonplanar flexible surfaces. The as‐printed micro‐supercapacitors exhibit an areal capacitance 4–6 times higher than traditionally spray‐printed ones. This work highlights the hidden potential of 2D materials as functional yet highly efficient rheological enhancers to facilitate industrial processing of nanomaterial‐based devices.     

Lithiophilic 3D Nanoporous Nitrogen‐Doped Graphene for Dendrite‐Free and Ultrahigh‐Rate Lithium‐Metal Anodes

The key bottlenecks hindering the practical implementations of lithium‐metal anodes in high‐energy‐density rechargeable batteries are the uncontrolled dendrite growth and infinite volume changes during charging and discharging, which lead to short lifespan and catastrophic safety hazards. In principle, these problems can be mitigated or even solved by loading lithium into a high‐surface‐area, conductive, and lithiophilic porous scaffold. However, a suitable material that can synchronously host a large loading amount of lithium and endure a large current density has not been achieved. Here, a lithiophilic 3D nanoporous nitrogen‐doped graphene as the sought‐after scaffold material for lithium anodes is reported. The high surface area, large porosity, and high conductivity of the nanoporous graphene concede not only dendrite‐free stripping/plating but also abundant open space accommodating volume fluctuations of lithium. This ingenious scaffold endows the lithium composite anode with a long‐term cycling stability and ultrahigh rate capability, significantly improving the charge storage performance of high‐energy‐density rechargeable lithium batteries.     

A Top‐Down Strategy toward SnSb In‐Plane Nanoconfined 3D N‐Doped Porous Graphene Composite Microspheres for High Performance Na‐Ion Battery Anode

Engineering of 3D graphene/metal composites with ultrasmall sized metal and robust metal–graphene interfacial interaction for energy storage application is still a challenge and rarely reported. In this work, a facile top‐down strategy is developed for the preparation of SnSb‐in‐plane nanoconfined 3D N‐doped porous graphene networks for sodium ion battery anodes, which are composed of several tens of interconnected empty N‐graphene boxes in‐plane firmly embedded with ultrasmall SnSb nanocrystals. The all‐around encapsulation (plane‐to‐plane contact) architecture that provides a large interface between N‐graphene and SnSb nanocrystal not only effectively enhances the electron conductivity and structural integrity of the overall electrode, but also offers excess interfacial sodium storage, thus leading to much enhanced high‐rate sodium storage capacity and stability, which has been proven by both experimental results and first‐principles simulations. Moreover, this top‐down strategy can enable new paths to the low‐cost and high‐yield synthesis of 3D graphene/metal composites for applications in energy‐related fields and beyond.     

Healable and Mechanically Super‐Strong Polymeric Composites Derived from Hydrogen‐Bonded Polymeric Complexes

It is challenging to fabricate mechanically super‐strong polymer composites with excellent healing capacity because of the significantly limited mobility of polymer chains. The fabrication of mechanically super‐strong polymer composites with excellent healing capacity by complexing polyacrylic acid (PAA) with polyvinylpyrrolidone (PVPON) in aqueous solution followed by molding into desired shapes is presented. The coiled PVPON can complex with PAA in water via hydrogen‐bonding interactions to produce transparent PAA–PVPON composites homogenously dispersed with nanoparticles of PAA–PVPON complexes. As healable materials, the PAA–PVPON composite materials with a glass transition temperature of ≈107.9 °C exhibit a super‐high mechanical strength, with a tensile strength of ≈81 MPa and a Young's modulus of ≈4.5 GPa. The PAA–PVPON composites are stable in water because of the hydrophobic interactions among pyrrolidone groups. The super‐high mechanical strength of the PAA–PVPON composite materials originates from the highly dense hydrogen bonds between PAA and PVPON and the reinforcement of in situ formed PAA–PVPON nanoparticles. The reversibility of the relatively weak but dense hydrogen bonds enables convenient healing of the mechanically strong PAA–PVPON composite materials from physical damage to restore their original mechanical strength.     

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, hydrogel, crumpled particles, hollow spheres, sack‐cargo particles, Pickering emulsions, and so on) by self‐assembly. The self‐assembly is mostly derived from the self‐concentration of GO sheets at various interfaces, including liquid‐air, liquid‐liquid 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.     

Laminated Object Manufacturing of 3D‐Printed Laser‐Induced Graphene Foams

Laser‐induced graphene (LIG), a graphene structure synthesized by a one‐step process through laser treatment of commercial polyimide (PI) film in an ambient atmosphere, has been shown to be a versatile material in applications ranging from energy storage to water treatment. However, the process as developed produces only a 2D product on the PI substrate. Here, a 3D LIG foam printing process is developed on the basis of laminated object manufacturing, a widely used additive‐manufacturing technique. A subtractive laser‐milling process to yield further refinements to the 3D structures is also developed and shown here. By combining both techniques, various 3D graphene objects are printed. The LIG foams show good electrical conductivity and mechanical strength, as well as viability in various energy storage and flexible electronic sensor applications.     

Micrometer‐Thick Graphene Oxide–Layered Double Hydroxide Nacre‐Inspired Coatings and Their Properties

Robust, functional, and flame retardant coatings are attractive in various fields such as building construction, food packaging, electronics encapsulation, and so on. Here, strong, colorful, and fire‐retardant micrometer‐thick hybrid coatings are reported, which can be constructed via an enhanced layer‐by‐layer assembly of graphene oxide (GO) nanosheets and layered double hydroxide (LDH) nanoplatelets. The fabricated GO–LDH hybrid coatings show uniform nacre‐like layered structures that endow them good mechanic properties with Young's modulus of ≈18 GPa and hardness of ≈0.68 GPa. In addition, the GO–LDH hybrid coatings exhibit nacre‐like iridescence and attractive flame retardancy as well due to their well‐defined 2D microstructures. This kind of nacre‐inspired GO–LDH hybrid thick coatings will be applied in various fields in future due to their high strength and multifunctionalities.     

Interface Chemistry Engineering of Protein‐Directed SnO2 Nanocrystal‐Based Anode for Lithium‐Ion Batteries with Improved Performance

A novel uniform amorphous carbon‐coated SnO₂ nanocrystal (NCs) for use in lithium‐ion batteries is formed by utilizing bovine serum albumin (BSA) as both the ligand and carbon source. The SnO₂–carbon composite is then coated by a controlled thickness of polydopamine (PD) layer through in situ polymerization of dopamine. The PD‐coated SnO₂–carbon composite is finally mixed with polyacrylic acid (PAA) which is used as binder to accomplish a whole anode system. A crosslink reaction is built between PAA and PD through formation of amide bonds to produce a robust network in the anode system. As a result, the designed electrode exhibits improved reversible capacity of 648 mAh/g at a current density of 100 mA/g after 100 cycles, and an enhanced rate performance of 875, 745, 639, and 523 mAh/g at current densities of 50, 100, 250, and 500 mA/g, respectively. FTIR spectra confirm the formation of crosslink reaction and the stability of the robust network during long‐term cycling. The great improvement of capacity and rate performance achieved in this anode system is attributed to two stable interfaces built between the active material (SnO₂–carbon composite) and the buffer layer (PD) and between the buffer layer and the binder (PAA), which effectively diminish the volume change of SnO₂ during charge/discharge process and provide a stable matrix for active materials.     

Edge‐Functionalized Graphene Nanoplatelets as Metal‐Free Electrocatalysts for Dye‐Sensitized Solar Cells

A scalable and low‐cost production of graphene nanoplatelets (GnPs) is one of the most important challenges for their commercialization. A simple mechanochemical reaction has been developed and applied to prepare various edge‐functionalized GnPs (EFGnPs). EFGnPs can be produced in a simple and ecofriendly manner by ball milling of graphite with target substances (X = nonmetals, halogens, semimetals, or metalloids). The unique feature of this method is its use of kinetic energy, which can generate active carbon species by unzipping of graphitic C? C bonds in dry conditions (no solvent). The active carbon species efficiently pick up X substance(s), leading to the formation of graphitic C? X bonds along the broken edges and the delamination of graphitic layers into EFGnPs. Unlike graphene oxide (GO) and reduced GO (rGO), the preparation of EFGnPs does not involve toxic chemicals, such as corrosive acids and toxic reducing agents. Furthermore, the prepared EFGnPs preserve high crystallinity in the basal area due to their edge‐selective functionalization. Considering the available edge X groups that can be selectively employed, the potential applications of EFGnPs are unlimited. In this context, the synthesis, characterizations, and applications of EFGnPs, specifically, as metal‐free carbon‐based electrocatalysts for dye‐sensitized solar cells (DSSCs) in both cobalt and iodine electrolytes are reviewed.     

Bottom‐Up Single‐Electron Transistors

As the downscaling of conventional semiconductor electronics becomes more and more challenging, the interest in alternative material systems and fabrication methods is growing. A novel bottom‐up approach for the fabrication of high‐quality single‐electron transistors (SETs) that can easily be contacted electrically in a controllable manner is developed. This approach employs the self‐assembly of Au nanoparticles forming the SETs, and Au nanorods forming the leads to macroscopic electrodes, thus bridging the gap between the nano‐ and microscale. Low‐temperature electron‐transport measurements reveal exemplary single‐electron tunneling characteristics. SET behavior can be significantly changed, post‐fabrication, using molecular exchange of the tunnel barriers, demonstrating the tunability of the assemblies. These results form a promising proof of principle for the versatility of bottom‐up nanoelectronics, and toward controlled fabrication of nanoelectronic devices.     

Boosting the Heat Dissipation Performance of Graphene/Polyimide Flexible Carbon Film via Enhanced Through‐Plane Conductivity of 3D Hybridized Structure

The development of materials with efficient heat dissipation capability has become essential for next‐generation integrated electronics and flexible smart devices. Here, a 3D hybridized carbon film with graphene nanowrinkles and microhinge structures by a simple solution dip‐coating technique using graphene oxide (GO) on polyimide (PI) skeletons, followed by high‐temperature annealing, is constructed. Such a design provides this graphitized GO/PI (g‐GO/PI) film with superflexibility and ultrahigh thermal conductivity in the through‐plane (150 ± 7 W m^‐1 K^‐1) and in‐plane (1428 ± 64 W m^‐1 K^‐1) directions. Its superior thermal management capability compared with aluminum foil is also revealed by proving its benefit as a thermal interface material. More importantly, by coupling the hypermetallic thermal conductivity in two directions, a novel type of carbon film origami heat sink is proposed and demonstrated, outperforming copper foil in terms of heat extraction and heat transfer for high‐power devices. The hypermetallic heat dissipation performance of g‐GO/PI carbon film not only shows its promising application as an emerging thermal management material, but also provides a facile and feasible route for the design of next‐generation heat dissipation components for high‐power flexible smart devices.