Enhanced Self-Assembly and Spontaneous Separation for Ultrathin, Air‒Floating Graphene Macrofilms and their Application in Ultrasensitive In-Site Growth Sensors

Self-assembled graphene films at air-liquid interface have great potential for multiple applications. However, the large size and ultrathin thickness for graphene-based films are hardly achieved simultaneously because of poor assembly ability of graphene oxide (GO) and cautious film transfer process (FTP). Herein, the ethanol-based binary solution with supramolecular clusters and weak polarity is introduced to enhance the assembly performance of GO. In this regime, GO film is formed within 52 s, and its formation temperature can be as low as -20 °C. More importantly, the attractive force between the formed GO film and this binary solution is 62% lower than that between GO film and traditional aqueous solution. On this basis, for the first time a spontaneous separation of the self-assembled GO film from solution is reported, and the air‒floating graphene films (AGFs) with nanoscale thickness and centimeter-scale diameter are prepared without FTP. By means of this separation behavior, the in situ growth AGFs sensors are fabricated, and they express fast response and ultrahigh sensitivity to imperceptible change in temperature and disturbances that are hardly recognized by common sensors. Therefore, a new accessible strategy is demonstrated to prepare ultrathin graphene macrofilms, which can be the excellent candidates for multifunctional, ultrasensitive sensors.

Making 2D Materials Sparkle in Energy Storage via Assembly

ConspectusTwo-dimensional (2D) materials such as graphene and MXenes offer appealing opportunities in electrochemical energy storage due to their large surface area, tunable surface chemistry, and unique electronic properties. One of the primary challenges in utilizing these materials for practical electrodes, especially those with industrial-level thickness, is developing a highly interconnected and porous conductive network. This network is crucial for supporting continuous electron transport, rapid ion diffusion, and effective participation of all active materials in electrochemical reactions. Moreover, the demand for efficient energy storage in advanced electronic devices and electric vehicles has led to the need for not only thicker but also denser electrodes to achieve compact energy storage. Traditional densification methods often compromise between volumetric capacitance and ion-accessible surface area, which can diminish rate performance. As versatile building blocks, 2D materials can overcome these limitations through the assembly into complex superstructures such as 1D fibers, 2D thin films, and 3D porous networks, a capability less attainable by other nanomaterials.This Account explores the pathways from exfoliated 2D nanosheets to densely packed, yet porous assemblies tailored for compact energy storage. Focusing on graphene and MXenes, we delve into the intricate relationships between surface structure, assembly behaviors, and electrochemical performance. We emphasize the crucial role of surface chemistry and interfacial interactions in forming stable colloidal dispersions and subsequent macroscopic structures. Furthermore, we highlight how solvents, acting as spacers, are instrumental in microstructure formation and how capillary force-driven densification is essential for creating compact assemblies. With precise control over shrinkage, the customized dense assemblies can strike a balance between high packing density and sufficient porosity, ensuring efficient ion transport, mechanical stability, and high volumetric performance across various electrochemical energy storage technologies.Furthermore, we highlight the importance of understanding and manipulating the surface chemistry of 2D materials at the atomic level to optimize their assembly and enhance electrochemical behaviors. Advanced in situ characterizations with high temporal and spatial resolution are necessary to gain deeper insights into the complex assembly process. Moreover, the integration of machine learning and computational chemistry emerges as a promising method to predict and design new materials and assembly strategies, potentially accelerating the development of next-generation energy storage systems. Our insights into the assembly and densification of 2D materials provide a comprehensive foundation for future research and practical applications in compact, high-performance energy storage devices. This exploration sets the stage for a transformative approach to overcoming the challenges of current energy storage technologies, promising significant advancements in 2D materials in the field.

Covalent Organic Framework/Graphene Hybrids: Synthesis, Properties, and Applications

To address current energy crises and environmental concerns, it is imperative to develop and design versatile porous materials ideal for water purification and energy storage. The advent of covalent organic frameworks (COFs), a revolutionary terrain of porous materials, is underscored by their superlative features such as divinable structure, adjustable aperture, and high specific surface area. However, issues like inferior electric conductivity, inaccessible active sites impede mass transfer and poor processability of bulky COFs restrict their wider application. As a herculean stride forward, COF/graphene hybrids amalgamate the strengths of their constituent components and have in consequence, enticed significant scientific intrigue. Herein, the current progress on the structure and properties of graphene-based materials and COFs are systematically outlined. Then, synthetic strategies for preparing COF/graphene hybrids, including one-pot synthesis, ex situ synthesis, and in situ growth, are comprehensively reviewed. Afterward, the pivotal attributes of COF/graphene hybrids are dissected in conjunction with their multifaceted applications spanning adsorption, separation, catalysis, sensing, and energy storage. Finally, this review is concluded by elucidating prevailing challenges and gesturing toward prospective strides within the realm of COF/graphene hybrids research.

Effect of Surface Treatment on Performance and Internal Stacking Mode of Electrohydrodynamic Printed Graphene and Its Microsupercapacitor

Microelectronic devices are developing rapidly in portability, wearability, and implantability. This puts forward an urgent requirement for the delicate deposition process of materials. Electrohydrodynamic printing has attracted academic and industrial attention in preparing ultrahigh-density microelectronic devices as a new noncontact, direct graphic, and low-loss thin film deposition process. In this work, a printed graphene with narrow line width is realized by combining the electrohydrodynamic printing and surface treatment. The line width of printed graphene on the hydrophobic treatment surface reduced from 80 to 28 μm. The resistivity decreased from 0.949 to 0.263 Ω·mm. Unexpectedly, hydrophobic treatment can effectively induce random stacking of electrohydrodynamic printed graphene, which avoids parallel stacking and agglomeration of graphene sheets. The performance of printed graphene is thus effectively improved. After optimization, a graphene planar supercapacitor with a printed line width of 28 μm is successfully obtained. Its capacitance can reach 5.39 mF/cm² at 50 mV/s, which is twice higher than that of the untreated devices. The device maintains 84.7% capacitance after 5000 cycles. This work provides a reference for preparing microelectronic devices by ultrahigh precision printing and a new direction for optimizing two-dimensional material properties through stacking adjustment.

Graphene oxide film guided skeletal muscle differentiation

Engineered muscle tissues can be used for the regeneration or substitution of irreversibly damaged or diseased muscles. Recently, graphene oxide (GO) has been shown to improve the adsorption of biomolecules through its biocompatibility and intrinsic π-π interactions. The possibility of producing various GO modifications may also provide additional functionality as substrates for cell culture. In particular, substrates fabricated from pristine GO have been shown to improve cellular functions and influence stem cell differentiation. In this study, we fabricated tunable GO substrates with various physical and chemical properties and demonstrated the ability of the substrate to support myogenic differentiation. Higher cellular adhesion affinity with unique microfilament anchorage was observed for GO substrates with increased GO concentrations. In addition, amino acid (AA)-conjugated GO (GO-AA) substrates were fabricated to modify GO chemical properties and study the effects of chemically modified GO substrates on myogenic differentiation. Our findings demonstrate that minor tuning of GO significantly influences myogenic differentiation.

Preparation of 2D material dispersions and their applications

A comprehensive review on the exfoliation of layer materials into 2D materials, their assembly, and applications in electronics and energy.

Imaging and Analysis of Encapsulated Objects through Self-Assembled Electron and Optically Transparent Graphene Oxide Membranes

We demonstrate a technique for facile encapsulation and adhesion of micro- and nano objects on arbitrary substrates, stencils, and micro structured surfaces by ultrathin graphene oxide membranes via a simple drop casting of graphene oxide solution. A self-assembled encapsulating membrane forms during the drying process at the liquid-air and liquid-solid interfaces and consists of a water-permeable quasi-2D network of overlapping graphene oxide flakes. Upon drying and interlocking between the flakes, the encapsulating coating around the object becomes mechanically robust, chemically protective, and yet highly transparent to electrons and photons in a wide energy range, enabling microscopic and spectroscopic access to encapsulated objects. The characteristic encapsulation scenarios were demonstrated on a set of representative inorganic and organic micro and nano-objects and microstructured surfaces. Different coating regimes can be achieved by controlling the pH of the supporting solution, and the hydrophobicity and morphology of interfaces. Several specific phenomena such as compression of encased objects by contracting membranes as well as hierarchical encapsulations were observed. Finally, electron as well as optical microscopy and analysis of encapsulated objects along with the membrane effect on the image contrast formation, and signal attenuation are discussed.

Graphene Emerges as a Versatile Template for Materials Preparation

Graphene and its derivatives are emerging as a class of novel but versatile templates for the controlled preparation and functionalization of materials. In this paper a conceptual review on graphene-based templates is given, highlighting their versatile roles in materials preparation. Graphene is capable of acting as a low-dimensional hard template, where its two-dimensional morphology directs the formation of novel nanostructures. Graphene oxide and other functionalized graphenes are amphiphilic and may be seen as soft templates for formatting the growth or inducing the controlled assembly of nanostructures. In addition, nanospaces in restacked graphene can be used for confining the growth of sheet-like nanostructures, and assemblies of interlinked graphenes can behave either as skeletons for the formation of composite materials or as sacrificial templates for novel materials with a controlled network structure. In summary, flexible graphene and its derivatives together with an increasing number of assembled structures show great potentials as templates for materials production. Many challenges remain, for example precise structural control of such novel templates and the removal of the non-functional remaining templates.

Assembling carbon quantum dots to a layered carbon for high-density supercapacitor electrodes

It is found that carbon quantum dots (CQDs) self-assemble to a layer structure at ice crystals-water interface with freeze- drying. Such layers interconnect with each other, forming a free-standing CQD assembly, which has an interlayer distance of about 0.366 nm, due to the existence of curved carbon rings other than hexagons in the assembly. CQDs are fabricated by rupturing C₆₀ by KOH activation with a production yield of ~15 wt.%. The CQDs obtained have an average height of 1.14 nm and an average lateral size of 7.48 nm, and are highly soluble in water. By packaging annealed CQD assembly to high density (1.23 g cm⁻³) electrodes in supercapacitors, a high volumetric capacitance of 157.4 F cm⁻³ and a high areal capacitance of 0.66 F cm⁻² (normalized to the loading area of electrodes) are demonstrated in 6 M KOH aqueous electrolyte with a good rate capability.

Versatile Wafer-Scale Technique for the Formation of Ultrasmooth and Thickness-Controlled Graphene Oxide Films Based on Very Large Flakes

We present a new strategy to form thickness-adjusted and ultrasmooth films of very large and unwrinkled graphene oxide (GO) flakes through the transfer of both hemispherical and vertical water films stabilized by surfactants. With its versatility in terms of substrate type (including flexible organic substrates) and in terms of flake density (from isolated flakes to continuous and multilayer films), this wafer-scale assembly technique is adapted to a broad range of experiments involving GO and rGO (reduced graphene oxide). We illustrate its use through the evaluation of transparent rGO electrodes.

Advanced Graphene-Based Binder-Free Electrodes for High-Performance Energy Storage

The increasing demand for energy has triggered tremendous research effort for the development of high-performance and durable energy-storage devices. Advanced graphene-based electrodes with high electrical conductivity and ion accessibility can exhibit superior electrochemical performance in energy-storage devices. Among them, binder-free configurations can enhance the electron conductivity of the electrode, which leads to a higher capacity by avoiding the addition of non-conductive and inactive binders. Graphene, a 2D material, can be fabricated into a porous and flexible structure with an interconnected conductive network. Such a conductive structure is favorable for both electron and ion transport to the entire electrode surface. In this review, the main processes used to prepare binder-free graphene-based hybrids with high porosity and well-designed electron conductive networks are summarized. Then, the applications of free-standing binder-free graphene-based electrodes in energy-storage devices are discussed. Future research aspects with regard to overcoming the technological bottlenecks are also proposed.

Low interfacial contact resistance of Al-graphene composites via interface engineering

Al-based composites incorporating multilayered graphene sheets were developed via a facile approach. The multilayered graphene sheets were fabricated from the expanded graphite via a simple mechanical exfoliation process. The facile extrusion molding process with Al powder and graphene sheets exfoliated from expended graphite afforded Al-based graphene composite rods. These composites showed enhanced thermal conductivity compared to the pristine Al rods. Moreover, the Al-based multilayered graphene sheet composites exhibited lower interfacial contact resistance between graphene-based electrodes than the pristine Al. With increasing degrees of dispersion, the number of exposed graphene sheets increases, thereby significantly decreasing the interfacial contact resistance between the composite and external graphite electrode.

Recent advances in the fabrication and structure-specific applications of graphene-based inorganic hybrid membranes

The preparation and applications of graphene (G)-based materials are attracting increasing interests due to their unique electronic, optical, magnetic, thermal, and mechanical properties. Compared to G-based hybrid and composite materials, G-based inorganic hybrid membrane (GIHM) offers enormous advantages ascribed to their facile synthesis, planar two-dimensional multilayer structure, high specific surface area, and mechanical stability, as well as their unique optical and mechanical properties. In this review, we report the recent advances in the technical fabrication and structure-specific applications of GIHMs with desirable thickness and compositions. In addition, the advantages and disadvantages of the methods utilized for creating GIHMs are discussed in detail. Finally, the potential applications and key challenges of GIHMs for future technical applications are mentioned.

Synthesis of lithium iron phosphate/carbon microspheres by using polyacrylic acid coated iron phosphate nanoparticles derived from iron(III) acrylate

Lithium iron phosphate/carbon (LiFePO4 /C) microspheres with high rate and cycling performance are synthesized from iron phosphate/polyacrylic acid (FePO4 /PAA) nanoparticles. Iron(III) acrylate is used as a precursor for both the iron and carbon sources. FePO4 nanoparticles are first produced by a coprecipitation reaction. The byproduct, acrylic acid ions, is polymerized in situ to form a uniform PAA layer on the surface of the FePO4 nanoparticles. The as-prepared LiFePO4 /C microspheres are composed of primary nanoparticles with sizes of 40-50 nm. The nanoparticles are fully coated with a thin, uniform carbon layer derived from the decomposition of the PAA layer. The uniform carbon-coating layer cooperates with interstitial and boundary carbon derived from sucrose successfully to construct an excellent interconnecting conductive network in the microspheres. As a result of the unique structure, the as-prepared LiFePO4 /C microspheres display both high electronic and ionic conductivities, which contribute to their high rate performance (162.9 mAh g(-1) at 0.1C and 126.1 mAh g(-1) at 5C) and excellent cycling stability (97.1% of capacity retention after 500 cycles at 5C/5C).

Self-Assembled 3D Graphene Monolith from Solution

Three-dimensional (3D) graphene-assembled monoliths (GAs), especially ones prepared by self-assembly in the liquid phase, represent promising forms to realize the practical applications of graphene due to their high surface utilization and operability. However, the understanding of the assembly process and structure control of 3D GAs, as a new class of carbon materials, is quite inadequate. In this Perspective, we give a demonstration of the assembly process and discuss the key factors involved in the structure control of 3D GAs to pave the way for their future applications. It is shown that the assembly process starts with the phase separation, which is responsible for the formation of the 3D networked structure and liquid phase as the spacers avoid the parallel overlap of graphene layers and help form an interlinked pore system. Well-tailored graphene sheets and selected assembly media must be a precondition for a well-controlled assembly process and microstructure of a 3D GA. The potential applications in energy storage featuring high rate and high volumetric energy density demonstrate advantages of 3D GAs in real applications.

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.

Structural diversity of bulky graphene materials

The unique two-dimensional (2D) structure and chemical properties of graphene and its derivatives make it a distinctive nanoscale building block for constructing novel bulky architectures with different dimensions, such as 1D fibers, 2D films and 3D architectures. These bulky graphene materials, depending on the manner in which graphene sheets are assembled, show a variety of fascinating features that cannot be achieved from individual graphene sheet or conventional materials. Thus, over the past several years, considerable effort has been expended in fabricating various structures of bulky graphene materials and developing their corresponding applications. Here, we present a broad and comprehensive overview of the recent developments in expanding the structural diversity of bulky graphene materials and their applications in energy storage and conversion, composites, environmental remediation, etc. Finally, prospects and further developments in this exciting field of bulky graphene materials are also suggested.

Hierarchical porous graphene/polyaniline composite film with superior rate performance for flexible supercapacitors

A highly flexible graphene free-standing film with hierarchical structure is prepared by a facile template method. With a porous structure, the film can be easily bent and cut, and forms a composite with another material as a scaffold. The 3D graphene film exhibits excellent rate capability and its capacitance is further improved by forming a composite with polyaniline nanowire arrays. The flexible hierarchical composite proves to be an excellent electrode material for flexible supercapacitors.

Highly wrinkled cross-linked graphene oxide membranes for biological and charge-storage applications

Inspired by the amphiphilicity of graphene oxide (GO), the surface of water is used as a template for the assembly of a GO film. Methacrylate-functionalized GO sheets can be cross-linked instantaneously at the water-air interface to form a highly wrinkled membrane spreading over an extended area. The multiple covalent linkages amongst the GO sheets enhances the in-plane stiffness of the film compared to noncovalently bonded GO films. The highly convoluted GO membrane can be used in two applications: the promoting of spontaneous stem-cell differentiation towards bone cell lineage without any chemical inducers, and for supercapacitor electrodes. Due to reduced van der Waals restacking, capacitance values up to 211 F g(-1) can be obtained. The scalable and inexpensive nature of this assembly route enables the engineering of membranes for applications in regenerative medicine and energy-storage devices where secondary structures like nanotopography and porosity are important performance enhancers.