Graphene chiral liquid crystals and macroscopic assembled fibres

Robust Production of Ultrahigh Surface Area Carbon Sheets for Energy Storage

Nanostructured carbon materials play essential roles in electrochemical energy storage devices. However, scalable production of high surface area carbon with a cost-effective process while controlling the morphology is challenging. Herein, a one-step procedure to produce carbon sheets with very high specific surface area up to 3411 m² g^-1 by direct pyrolysis of dipotassium ethylene diamine tetraacetate is reported. Unlike that of biomass-pyrolyzed carbons, the surface area of prepared carbon sheets is not sensitive to pyrolysis conditions (e.g., heating temperature and time), which makes the production robust and scalable. Moreover, the pore structure is stable against posttreatments, including solvent washing, which are detrimental to that of graphene-based soft sheet assemblies. When used as supercapacitor electrodes, the ultrahigh surface area carbon sheets (HSACS) show a high specific capacitance of 268 F g^-1 at 5 mV s^-1 , and retain 70% of the capacitance at 100 times higher scan rate in 6 m KOH aqueous electrolyte. Furthermore, the HSACS also exhibit a high specific capacitance of 266 F g^-1 within a 1.6 V symmetric supercapacitor potential window in 2 m Li₂ SO₄ aqueous electrolyte. The symmetric supercapacitor delivers a maximum specific energy of 23.6 W h kg^-1 and high power density of 6.4 kW kg^-1 .

Phase behaviors of colloidal analogs of bent-core liquid crystals

Bent-core liquid crystal (LC) molecules are known to form mesophases with fascinating polar order and supramolecular chirality despite the achiral nature of the mesogens. The assembly of colloidal particles with geometrical similarity to bent-core molecular mesogens not only provides new insights into the physical behaviors of atoms or molecules but also leads to new materials with broad applications. Despite tremendous progress in colloidal synthesis and assembly, there has been a lack of colloidal model systems of bent-core molecular mesogens for LC property discovery and application development. This article describes a systematic study on the phase behaviors of colloidal analogs of bent-core LC mesogens in both experiments and simulations. We demonstrated that bent rods with controlled bending angle (α) and aspect ratio (L/D, with L and D as the length and diameter of each rod arm, respectively) can spontaneously assemble into several typical banana phases including smectic A, smectic C, synclinic tilted antiferroelectric-like smectic, and twist smectic phases, resembling bent-core LC molecules. The formation and transition of these phases were found to be strongly dependent on the geometric parameters of rods. Phase diagrams were developed to illustrate the existence and stability range of all the LC phases in α and L/D space. This work opens the door to the development of novel complex types of molecular or colloidal self-organization and new functional materials with electro-optical or nonlinear optical properties.

Nanocarbon-Based Materials for Flexible All-Solid-State Supercapacitors

Because of the rapid development of flexible electronics, it is important to develop high-performance flexible energy-storage devices, such as supercapacitors and metal-ion batteries. Compared with metal-ion batteries, supercapacitors exhibit higher power density, longer cycling life, and excellent safety, and they can be easily fabricated into all-solid-state devices by using polymer gel electrolytes. All-solid-state supercapacitors (ASSSCs) have the advantages of being lightweight and flexible, thus showing great potential to be used as power sources for flexible portable electronics. Because of their high specific surface area and excellent electrical and mechanical properties, nanocarbon materials (such as carbon nanotubes, graphene, carbon nanofibers, and so on) have been widely used as efficient electrode materials for flexible ASSSCs, and great achievements have been obtained. Here, the recent advances in flexible ASSSCs are summarized, from design strategies to fabrication techniques for nanocarbon electrodes and devices. Current challenges and future perspectives are also discussed.

Conceptually Novel Black Phosphorus/Cellulose Hydrogels as Promising Photothermal Agents for Effective Cancer Therapy

Black phosphorus (BP) has recently emerged as an intriguing photothermal agent in photothermal therapy (PTT) against cancer by virtue of its high photothermal efficiency, biocompatibility, and biodegradability. However, naked BP is intrinsically characterized by easy oxidation (or natural degradation) and sedimentation inside the tumor microenvironment, leading to a short-term therapeutic and inhomogeneous photothermal effect. Development of BP-based nanocomposites for PTT against cancer therefore remains challenging. The present work demonstrates that green and injectable composite hydrogels based on cellulose and BP nanosheets (BPNSs) are of great efficiency for PTT against cancer. The resultant cellulose/BPNS-based hydrogel possesses 3D networks with irregular micrometer-sized pores and thin, strong cellulose-formed walls and exhibits an excellent photothermal response, enhanced stability, and good flexibility. Importantly, this hydrogel nanoplatform is totally harmless and biocompatible both in vivo and in vitro. This work may facilitate the development of BP-polymer-based photothermal agents in the form of hydrogels for biomedical-related clinic applications.

A Bioinspired Interface Design for Improving the Strength and Electrical Conductivity of Graphene-Based Fibers

Graphene-based fibers (GBFs) are attractive for next-generation wearable electronics due to their potentially high mechanical strength, superior flexibility, and excellent electrical and thermal conductivity. Many efforts have been devoted to improving these properties of GBFs in the past few years. However, fabricating GBFs with high strength and electrical conductivity simultaneously remains as a great challenge. Herein, inspired by nacre-like multilevel structural design, an interface-reinforced method is developed to improve both the mechanical property and electrical conductivity of the GBFs by introducing polydopamine-derived N-doped carbon species as resistance enhancers, binding agents, and conductive connection "bridges." Remarkably, both the tensile strength and electrical conductivity of the obtained GBFs are significantly improved to ≈724 MPa and ≈6.6 × 10⁴ S m^-1 , respectively, demonstrating great superiority compared to previously reported similar GBFs. These outstanding integrated performances of the GBFs provide it with great application potential in the fields of flexible and wearable microdevices such as sensors, actuators, supercapacitors, and batteries.

Ultrastrong Graphene-Copper Core-Shell Wires for High-Performance Electrical Cables

Recent development in mobile electronic devices and electric vehicles requires electrical wires with reduced weight as well as enhanced stability. In addition, since electric energy is mostly generated from power plants located far from its consuming places, mechanically stronger and higher electric power transmission cables are strongly demanded. However, there has been no alternative materials that can practically replace copper materials. Here, we report a method to prepare ultrastrong graphene fibers (GFs)-Cu core-shell wires with significantly enhanced electrical and mechanical properties. The core GFs are synthesized by chemical vapor deposition, followed by electroplating of Cu shells, where the large surface area of GFs in contact with Cu maximizes the mechanical toughness of the core-shell wires. At the same time, the unique electrical and thermal characteristics of graphene allow a ∼10 times higher current density limit, providing more efficient and reliable delivery of electrical energies through the GFs-Cu wires. We believe that our results would be useful to overcome the current limit in electrical wires and cables for lightweight, energy-saving, and high-power applications.

Anisotropic Self-Oscillating Reaction in Liquid Crystalline Nanosheet Hydrogels

Anisotropic chemical wave propagation of self-oscillating Belousov-Zhabotinsky (BZ) reaction was demonstrated in the poly( N-isopropylacrylamide) gel films embedded with macroscopically aligned liquid crystalline inorganic nanosheets. Although the average propagation rate of chemical wave v̅ was 3.56 mm min^-1 in the gels without nanosheets, the propagation was retarded in the gels with 1 wt % of nanosheets: [Formula: see text] = 1.89 mm min^-1 and [Formula: see text] = 1.33 mm min^-1 along the direction parallel and perpendicular to the nanosheet planes, respectively. Thus, the wave propagation is anisotropic with the anisotropy ratio [Formula: see text] = 1.42 in these gels and the periodic patterns formed by the BZ reaction were concentric ellipses, different from circles seen in isotropic gels. Furthermore, the propagation rate and degree of anisotropy were controllable by nanosheet concentration. These phenomena can be explained that the diffusion of molecules inside the gel is effectively hindered along the direction perpendicular to the nanosheet planes due to the very large aspect ratio of the aligned nanosheets. The present systems will be applicable for anisotropic self-oscillating soft actuators with one-dimensional motions as well as for ideal model system of BZ reactions.

Highly confined stacks of graphene oxide sheets in water

Since the discovery of graphene oxide (GO), the most accessible of the precursors of graphene, this material has been widely studied for applications in science and technology. In this work, we describe a procedure to obtain GO dispersions in water at high concentrations, these highly dehydrated dispersions being in addition fully redispersible by dilution. With the availability of such concentrated samples, it was possible to investigate the structure of hydrated GO sheets in a previously unexplored range of concentrations, and to evidence a structural phase transition. Tentatively applying models designed for describing the small-angle scattering curve in the Smectic A (or [Formula: see text]) phase of lyotropic systems, it was possible to extract elastic parameters characterising the system on the dilute side of the transition, thereby evidencing the relevance of both electrostatic and steric (Helfrich) interactions in stabilising aqueous lamellar stacks of GO sheets.

Water treatment by new-generation graphene materials: hope for bright future

Water is the most important and essential component of earth's ecosystem playing a vital role in the proper functioning of flora and fauna. But, our water resources are contaminating continuously. The whole world may be in great water scarcity after few decades. Graphene, a single-atom thick carbon nanosheet, and graphene nanomaterials have bright future in water treatment technologies due to their extraordinary properties. Only few papers describe the use of these materials in water treatment by adsorption, filtration, and photodegradation methods. This article presents a critical evaluation of the contribution of graphene nanomaterials in water treatment. Attempts have been made to discuss the future perspectives of these materials in water treatment. Besides, the efforts are made to discuss the nanotoxicity and hazards of graphene-based materials. The suggestions are given to explore the full potential of these materials along with precautions of nanotoxicity and its hazards. It was concluded that the future of graphene-based materials is quite bright.

Nitrogen-Doped Single Graphene Fiber with Platinum Water Dissociation Catalyst for Wearable Humidity Sensor

Humidity sensors are essential components in wearable electronics for monitoring of environmental condition and physical state. In this work, a unique humidity sensing layer composed of nitrogen-doped reduced graphene oxide (nRGO) fiber on colorless polyimide film is proposed. Ultralong graphene oxide (GO) fibers are synthesized by solution assembly of large GO sheets assisted by lyotropic liquid crystal behavior. Chemical modification by nitrogen-doping is carried out under thermal annealing in H₂ (4%)/N₂ (96%) ambient to obtain highly conductive nRGO fiber. Very small (≈2 nm) Pt nanoparticles are tightly anchored on the surface of the nRGO fiber as water dissociation catalysts by an optical sintering process. As a result, nRGO fiber can effectively detect wide humidity levels in the range of 6.1-66.4% relative humidity (RH). Furthermore, a 1.36-fold higher sensitivity (4.51%) at 66.4% RH is achieved using a Pt functionalized nRGO fiber (i.e., Pt-nRGO fiber) compared with the sensitivity (3.53% at 66.4% RH) of pure nRGO fiber. Real-time and portable humidity sensing characteristics are successfully demonstrated toward exhaled breath using Pt-nRGO fiber integrated on a portable sensing module. The Pt-nRGO fiber with high sensitivity and wide range of humidity detection levels offers a new sensing platform for wearable humidity sensors.

Self-Assembled Graphene-Based Architectures and Their Applications

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

Green synthesis of graphene oxide by seconds timescale water electrolytic oxidation

Graphene oxide is highly desired for printing electronics, catalysis, energy storage, separation membranes, biomedicine, and composites. However, the present synthesis methods depend on the reactions of graphite with mixed strong oxidants, which suffer from explosion risk, serious environmental pollution, and long-reaction time up to hundreds of hours. Here, we report a scalable, safe and green method to synthesize graphene oxide with a high yield based on water electrolytic oxidation of graphite. The graphite lattice is fully oxidized within a few seconds in our electrochemical oxidation reaction, and the graphene oxide obtained is similar to those achieved by the present methods. We also discuss the synthesis mechanism and demonstrate continuous and controlled synthesis of graphene oxide and its use for transparent conductive films, strong papers, and ultra-light elastic aerogels.

Graphene oxide is a graphene derivative showing wide applications, but it suffers from harsh synthetic conditions and long reaction time. Pei et al. show a green electrochemical method to fully oxidize the graphite lattice in a few seconds, which is over 100 times faster than existing methods.

Intercalation Polymerization Approach for Preparing Graphene/Polymer Composites

The rapid development of society has promoted increasing demand for various polymer materials. A large variety of efforts have been applied in order for graphene strengthened polymer composites to satisfy different requirements. Graphene/polymer composites synthesized by traditional strategies display some striking defects, like weak interfacial interaction and agglomeration of graphene, leading to poor improvement in performance. Furthermore, the creation of pre-prepared graphene while being necessary always involves troublesome processes. Among the various preparation strategies, an appealing approach relies on intercalation and polymerization in the interlayer of graphite and has attracted researchers' attention due to its reliable, fast and simple synthesis. In this review, we introduce an intercalation polymerization strategy to graphene/polymer composites by the intercalation of molecules/ions into graphite interlayers, as well as subsequent polymerization. The key point for regulating intercalation polymerization is tuning the structure of graphite and intercalants for better interaction. Potential applications of the resulting graphene/polymer composites, including electrical conductivity, electromagnetic absorption, mechanical properties and thermal conductivity, are also reviewed. Furthermore, the shortcomings, challenges and prospects of intercalation polymerization are discussed, which will be helpful to researchers working in related fields.

Porous Graphene Microflowers for High-Performance Microwave Absorption

Graphene has shown great potential in microwave absorption (MA) owing to its high surface area, low density, tunable electrical conductivity and good chemical stability. To fully realize graphene's MA ability, the microstructure of graphene should be carefully addressed. Here we prepared graphene microflowers (Gmfs) with highly porous structure for high-performance MA filler material. The efficient absorption bandwidth (reflection loss ≤ -10 dB) reaches 5.59 GHz and the minimum reflection loss is up to -42.9 dB, showing significant increment compared with stacked graphene. Such performance is higher than most graphene-based materials in the literature. Besides, the low filling content (10 wt%) and low density (40-50 mg cm^-3) are beneficial for the practical applications. Without compounding with magnetic materials or conductive polymers, Gmfs show outstanding MA performance with the aid of rational microstructure design. Furthermore, Gmfs exhibit advantages in facile processibility and large-scale production compared with other porous graphene materials including aerogels and foams.

Wet-spun graphene filaments: effect of temperature of coagulation bath and type of reducing agents on mechanical & electrical properties

Although many factors are considered to improve the properties of graphene filaments, there is no report in the existing literature on the effect of the temperature of the coagulation bath to the mechanical properties of graphene oxide filaments obtained in the wet-spinning process and also to the mechanical and electrical properties of the resulting graphene filaments after reduction. In this study, the effect of the temperature of the isopropanol coagulation bath during wet-spinning of graphene filaments on their final properties after formation was investigated and it was found that the decrease of the coagulation bath temperature resulted in more compact filaments having better mechanical properties for both graphene oxide and corresponding reduced graphene filaments. The best tensile strength and Young's modulus values were obtained in isopropanol coagulation bath which was kept at 15 °C. On the other hand, the types of the chemical reduction agents which can provide better electrical conductivity to graphene filaments after reduction were also investigated and it was determined that the use of hydriodic acid/acetic acid mixture resulted in graphene filaments having the best electrical conductivity (1.28 × 10⁴ S m⁻¹) and also tensile strength (234 ± 26 MPa) values. The addition of acetic acid into hydriodic acid increased the tensile strength 26% when compared with the plain HI treatment. Both electrical conductivity and tensile strength results were higher than most of the previously reported values of the wet-spun neat graphene filaments in the literature.

Cold isopropanol coagulation bath and use of acetic acid/hydriodic acid reduction resulted in better tensile strength for wet-spun graphene filaments.

Sulfur–oleyl amine platelet derivatives with liquid crystalline behavior

A novel sulfur-based platelet derivative was synthesized by reacting elemental sulfur with oleyl amine. The sulfur–oleyl amine (S–OA) derivative has an ionic salt form, layered morphology and forms a highly lamellar structure. Polarized optical microscopy (POM) clearly shows the birefringent lyotropic liquid crystalline behavior of the S–OA platelets dispersions.

A novel sulfur-based platelet derivative was synthesized by reacting elemental sulfur with oleyl amine.

Bioinspired Nanocomposite Hydrogels with Highly Ordered Structures

In the human body, many soft tissues with hierarchically ordered composite structures, such as cartilage, skeletal muscle, the corneas, and blood vessels, exhibit highly anisotropic mechanical strength and functionality to adapt to complex environments. In artificial soft materials, hydrogels are analogous to these biological soft tissues due to their "soft and wet" properties, their biocompatibility, and their elastic performance. However, conventional hydrogel materials with unordered homogeneous structures inevitably lack high mechanical properties and anisotropic functional performances; thus, their further application is limited. Inspired by biological soft tissues with well-ordered structures, researchers have increasingly investigated highly ordered nanocomposite hydrogels as functional biological engineering soft materials with unique mechanical, optical, and biological properties. These hydrogels incorporate long-range ordered nanocomposite structures within hydrogel network matrixes. Here, the critical design criteria and the state-of-the-art fabrication strategies of nanocomposite hydrogels with highly ordered structures are systemically reviewed. Then, recent progress in applications in the fields of soft actuators, tissue engineering, and sensors is highlighted. The future development and prospective application of highly ordered nanocomposite hydrogels are also discussed.

Three-Dimensional Printed Thermal Regulation Textiles

Space cooling is a predominant part of energy consumption in people's daily life. Although cooling the whole building is an effective way to provide personal comfort in hot weather, it is energy-consuming and high-cost. Personal cooling technology, being able to provide personal thermal comfort by directing local heat to the thermally regulated environment, has been regarded as one of the most promising technologies for cooling energy and cost savings. Here, we demonstrate a personal thermal regulated textile using thermally conductive and highly aligned boron nitride (BN)/poly(vinyl alcohol) (PVA) composite (denoted as a-BN/PVA) fibers to improve the thermal transport properties of textiles for personal cooling. The a-BN/PVA composite fibers are fabricated through a fast and scalable three-dimensional (3D) printing method. Uniform dispersion and high alignment of BN nanosheets (BNNSs) can be achieved during the processing of fiber fabrication, leading to a combination of high mechanical strength (355 MPa) and favorable heat dispersion. Due to the improved thermal transport property imparted by the thermally conductive and highly aligned BNNSs, better cooling effect (55% improvement over the commercial cotton fiber) can be realized in the a-BN/PVA textile. The wearable a-BN/PVA textiles containing the 3D-printed a-BN/PVA fibers offer a promising selection for meeting the personal cooling requirement, which can significantly reduce the energy consumption and cost for cooling the whole building.

Hydrothermally Activated Graphene Fiber Fabrics for Textile Electrodes of Supercapacitors

Carbon textiles are promising electrode materials for wearable energy storage devices owing to their conductive, flexible, and lightweight features. However, there still lacks a perfect choice for high-performance carbon textile electrodes with sufficient electrochemical activity. Graphene fiber fabrics (GFFs) are newly discovered carbon textiles, exhibiting various attractive properties, especially a large variability on the microstructure. Here we report the fabrication of hierarchical GFFs with significantly enlarged specific surface area using a hydrothermal activation strategy. By carefully optimize the activation process, the hydrothermally activated graphene fiber fabrics (HAGFFs) could achieve an areal capacitance of 1060 mF cm^-2 in a very thin thickness (150 μm) and the capacitance is easily magnified by overlaying several layers of HAGFFs, even up to a record value of 7398 mF cm^-2. Meanwhile, a good rate capability and a long cycle life are also attained. As compared with other carbon textiles, including the commercial carbon fiber cloths, our HAGFFs present much better capacitive performance. Therefore, the mechanically stable, flexible, conductive, and highly active HAGFFs have provided an option for high-performance textile electrodes.

Lyotropic Liquid Crystal Phases from Anisotropic Nanomaterials

Liquid crystals are an integral part of a mature display technology, also establishing themselves in other applications, such as spatial light modulators, telecommunication technology, photonics, or sensors, just to name a few of the non-display applications. In recent years, there has been an increasing trend to add various nanomaterials to liquid crystals, which is motivated by several aspects of materials development. (i) addition of nanomaterials can change and thus tune the properties of the liquid crystal; (ii) novel functionalities can be added to the liquid crystal; and (iii) the self-organization of the liquid crystalline state can be exploited to template ordered structures or to transfer order onto dispersed nanomaterials. Much of the research effort has been concentrated on thermotropic systems, which change order as a function of temperature. Here we review the other side of the medal, the formation and properties of ordered, anisotropic fluid phases, liquid crystals, by addition of shape-anisotropic nanomaterials to isotropic liquids. Several classes of materials will be discussed, inorganic and mineral liquid crystals, viruses, nanotubes and nanorods, as well as graphene oxide.

Controlled Gelation of Graphene Towards Unprecedented Superstructures

Graphene exhibits a range of exceptional physical properties and holds great promise for development of novel bulk materials for widespread applications. Properly engineering of assembled structures of graphene at multiple length scales is essential to realize its full potential in bulk forms. In this Concept article, we highlight the unique colloidal and gelation behavior of a commonly used precursor for graphene, graphene oxide (GO), and discuss how the colloidal chemistry of GO and reduced GO can enable new scalable and cost-effective approaches to construct graphene-based soft superstructures with excellent properties.

Preparation and application of N-doped carbon nanotube arrays on graphene fibers

A new kind of carbon hybrid material with a unique structure and outstanding mechanical and functional properties is reported in this article. Nitrogen-doped carbon nanotube (CNT) arrays with inside located Ni particles are in situ grown on the surface of phenolic carbon modified graphene fibers during their conversion from graphene oxide fibers. The carbon hybrid fibers exhibit not only high tensile strength and elongation at the break, but also excellent flexibility since the CNT arrays cover all the surface of the highly strong graphene fiber. This well-constructed carbon material would be suitable for catalysts, polymer composites, hydrogen storage, oxygen reduction reaction etc.

Graphite Oxide Improves Adhesion and Water Resistance of Canola Protein-Graphite Oxide Hybrid Adhesive

Protein derived adhesives are extensively explored as a replacement for synthetic ones, but suffers from weak adhesion and water resistance. Graphite oxide (GO) has been extensively used in nanocomposites, but not in adhesives applications. The objectives of this study were to prepare functionally improved protein adhesive by exfoliating GO with different oxidation levels, and to determine the effect of GO on adhesion mechanism. GO were prepared by oxidizing graphite for 0.5, 2, and 4 h (GO-A, GO-B and GO-C, respectively). Increasing oxidation time decreased C/O ratio; while the relative proportion of C-OH, and C = O groups initially increased up to 2 h of oxidation, but reduced upon further oxidation. Canola protein-GO hybrid adhesive (CPA-GO) was prepared by exfoliating GO at a level of 1% (w/w). GO significantly increased (p < 0.05) adhesion; where GO-B addition showed the highest dry, and wet strength of 11.67 ± 1.00, and 4.85 ± 0.61 MPa, respectively. The improvements in adhesion was due to the improved exfoliation of GO, improved adhesive and cohesive interactions, increased hydrogen bonding, increased hydrophobic interactions and thermal stability of CPA-GO. GO, as we proposed for the first time is easier to process and cost-effective in preparing protein-based adhesives with significantly improved functionalities.

Dry spinning approach to continuous graphene fibers with high toughness

Graphene fiber (GF) has emerged as a new carbonaceous fiber species since graphene-based liquid crystals were discovered. The growing performances of GFs in terms of their mechanical performance and their functionalities have assured their extensive applications in structural materials and functional textiles. To date, many spinning strategies utilizing coagulation baths have been applied in GF, which necessitates a complicated washing process. Dry spinning is a more convenient and green method for use with fibers in the chemical fiber industry, and should be a good option for GFs; however, this technique has never been used in a system of GF. In this research, first the dry spinning technique was used to fabricate continuous GFs and the dry spun GFs showed good toughness and flexibility. The dry spinnability of graphene oxide liquid crystals was achieved by choosing dispersive solvents with low surface tension and high volatility. The dry spun neat GFs possessed high toughness up to 19.12 MJ m^-3, outperforming the wet spun neat GFs. This dry spinning methodology facilitates the green fabrication of fibers of graphene and graphene-beyond two-dimensional nanomaterials, and it may also be extended to other printing technologies for complex graphene architectures.

Wrinkle-stabilized metal-graphene hybrid fibers with zero temperature coefficient of resistance

The interfacial adhesion between graphene and metals is poor, as metals tend to generate superlubricity on smooth graphene surface. This problem renders the free assembly of graphene and metals to be a big challenge, and therefore, some desired conducting properties (e.g., stable metal-like conductivities in air, lightweight yet flexible conductors, and ultralow temperature coefficient of resistance, TCR) likely being realized by integrating the merits of graphene and metals remains at a theoretical level. This work proposes a wrinkle-stabilized approach to address the poor adhesion between graphene surface and metals. Cyclic voltammetry (CV) tests and theoretical analysis by Scharifker-Hills models demonstrate that multiscale wrinkles effectively induce nucleation of metal particles, locking in metal nuclei and guiding the continuous growth of metal islands in an instantaneous model on rough graphene surface. The universality and practicability of the wrinkle-stabilized approach is verified by our investigation through the electrodeposition of nine kinds of metals on graphene fibers (GF). The strong interface bonding permits metal-graphene hybrid fibers to show metal-level conductivities (up to 2.2 × 10⁷ S m^-1, a record high value for GF in air), reliable weatherability and favorable flexibility. Due to the negative TCR of graphene and positive TCR of metals, the TCR of Cu- and Au-coated GFs reaches zero at a wide temperature range (15 K-300 K). For this layered model, the quantitative analysis by classical theories demonstrates the suitable thickness ratio of graphene layer and metal layer to achieve zero TCR to be 0.2, agreeing well with our experimental results. This wrinkle-stabilized approach and our theoretical analysis of zero-TCR behavior of the graphene-metal system are conducive to the design of high-performance conducting materials based on graphene and metals.

Sheet Collapsing Approach for Rubber-like Graphene Papers

Understanding and modulating the conformation of graphene are pivotal in designing graphene macroscopic materials. Here, we revealed the sheet collapsing behavior of graphene oxide (GO) sheets by poor solvents in an analogy with linear macromolecules. Triggered by poor solvents, extended GO sheets in good solvents can collapse to hierarchically wrinkled conformations. The collapsing behavior of GO enabled the fabrication of amorphous self-standing GO and graphene papers with rich hierarchical wrinkles and folds over mutliple size scales. The collapsed GO and graphene papers had a rubber-like mechanical behavior with viscoelasticity. By our collapsing method, GO and graphene self-standing papers were designed to be stiff with high modulus or to become soft with low modulus of 100 MPa at a remarkably large breakage elongation up to 23%. Our philosophy of treating graphene as a 2D polymer enables the efficient control of molecular conformations of graphene and other 2D polymers and the design of macroscopic materials of 2D nanomaterials as in the polymer industry.

Highly conductive and environmentally stable gold/graphene yarns for flexible and wearable electronics

Here, we fabricated high-performance gold/graphene yarns through a facile method by the electroless deposition of gold nanoparticles onto the surface of graphene yarns. The gold/graphene yarns are fabricated using a completely solution-based process that can be scaled up for practical applications. They possess high electrical conductivity (2.86 × 10² S cm^-1) and good gravimetric specific conductivity (6.81 × 10² S cm² g^-1) as well as good reliability under 1000 bending tests with a maximum bending angle of 170° and 10 washing tests with laundry detergents. These stable conducting yarns could also be integrated into textiles and clothes in various forms to create smart fabrics and wearable devices. In addition, this facile approach is easily applicable to various graphene films and devices on soft substrates that are presently used in flexible/wearable electronics.

In situ twisting for stabilizing and toughening conductive graphene yarns

Graphene fibres are of great interest for diverse applications such as their use in supercapacitors, batteries, stimulus-responsive actuators and smart textiles. However, previously reported graphene-based fibres commonly have very low toughness and poor stretchability, which can limit many of their possible applications. We demonstrate an efficient approach for preparing super-tough yarns of reduced graphene oxides via the in situ twisting of multiplied graphene oxide fibres during fibre extrusion and coagulation, followed by chemical reduction and drying. These twisted multiplied yarns have a toughness of 46.3 J m^-3, much higher than previous results for graphene-based fibres and films, and benefit from the retention of high strength and a significant increase in the failure strain due to the stably twisted hollow structure of the yarns. These yarns also show very stable electrical resistance when subjected to cyclic stretching at a large tensile strain. We demonstrate the utilization of such yarns for stretchable conducting electronic wires.

A Mini Review on Nanocarbon-Based 1D Macroscopic Fibers: Assembly Strategies and Mechanical Properties

Nanocarbon-based materials, such as carbon nanotubes (CNTs) and graphene have been attached much attention by scientific and industrial community. As two representative nanocarbon materials, one-dimensional CNTs and two-dimensional graphene both possess remarkable mechanical properties. In the past years, a large amount of work have been done by using CNTs or graphene as building blocks for constructing novel, macroscopic, mechanically strong fibrous materials. In this review, we summarize the assembly approaches of CNT-based fibers and graphene-based fibers in chronological order, respectively. The mechanical performances of these fibrous materials are compared, and the critical influences on the mechanical properties are discussed. Personal perspectives on the fabrication methods of CNT- and graphene-based fibers are further presented.

Graphene-Fiber-Based Supercapacitors Favor N-Methyl-2-pyrrolidone/Ethyl Acetate as the Spinning Solvent/Coagulant Combination

One-dimensional flexible fiber supercapacitors (FSCs) have attracted great interest as promising energy-storage units that can be seamlessly incorporated into textiles via weaving, knitting, or braiding. The major challenges in this field are to develop tougher and more efficient FSCs with a relatively easy and scalable process. Here, we demonstrate a wet-spinning process to produce graphene oxide (GO) fibers from GO dispersions in N-methyl-2-pyrrolidone (NMP), with ethyl acetate as the coagulant. Upon chemical reduction of GO, the resulting NMP-based reduced GO (rGO) fibers (rGO@NMP-Fs) are twice as high in the surface area and toughness but comparable in tensile strength and conductivity as that of the water-based rGO fibers (rGO@H₂O-Fs). When assembled into parallel FSCs, rGO@NMP-F-based supercapacitors (rGO@NMP-FSCs) offered a specific capacitance of 196.7 F cm^-3 (147.5 mF cm^-2), five times higher than that of rGO@H₂O-F-based supercapacitors (rGO@H₂O-FSCs) and also higher than most existing wet-spun rGO-FSCs, as well as those FSCs built with metal wires, graphene/carbon nanotube (CNT) fibers, or even pseudocapacitive materials. In addition, our rGO@NMP-FSCs can provide good bending and cycling stability. The energy density of our rGO@NMP-FSCs reaches ca. 6.8 mWh cm^-3, comparable to that of a Li thin-film battery (4 V/500 μAh).

Optimizing Interfacial Cross-Linking in Graphene-Derived Materials, Which Balances Intralayer and Interlayer Load Transfer

Graphene-derived layer-by-layer (LbL) assemblies in the form of films or fibers have recently attracted particular interests owing to their low cost, facile fabrication, and outstanding mechanical properties, which could be further tuned by surface functionalization that cross-links graphene sheets in the assembly. However, this interfacial engineering approach has not yet been finely utilized considering the dual roles of cross-links in modifying the intrinsic properties of graphene sheets and their interlayer interactions. In this work, combining first-principles calculations and continuum-mechanics-based model analysis, we find that the functionalization weakens the intrinsic mechanical resistance of graphene, whereas it enhances interlayer load transfer through interlayer cross-linking. There are optimum cross-linking densities or concentrations of the surface functional groups that maximize the overall tensile stiffness, tensile strength and strain to failure of graphene-derived LbL assemblies, arising from the competition between intralayer and interlayer load-bearing mechanisms, as defined by the type of functionalization and size of graphene sheets. Our work quantifies the ultimate mechanical performance of graphene-derived LbL assemblies, on the condition that their microstructures and functionalization could be adequately controlled in the fabrication process.

Supercapacitor Electrodes with Remarkable Specific Capacitance Converted from Hybrid Graphene Oxide/NaCl/Urea Films

A novel approach to improve the specific capacitance of reduced graphene oxide (rGO) films is reported. We combine the aqueous dispersion of liquid-crystalline GO incorporating salt and urea with a blade-coating technique to make hybrid films. After drying, stacked GO sheets mediated by solidified NaCl and urea are hydrothermally reduced, resulting in a nanoporous film consisting of rumpled N-doped rGO sheets. As a supercapacitor electrode, the film exhibits a high gravimetric specific capacitance of 425 F g^-1 and a record volumetric specific capacitance of 693 F cm^-3 at 1 A g^-1 in 1 M H₂SO₄ aqueous electrolyte when integrated into a symmetric cell. When using Li₂SO₄ aqueous electrolyte, which can extend the potential window to 1.6 V, the device exhibits high energy densities up to 35 Wh kg^-1, and high power densities up to 10⁴ W kg^-1. This novel strategy to intercalate solidified chemicals into stacked GO sheets to functionalize them and prevent them from restacking provides a promising route toward supercapacitors with high specific capacitance and energy density.

Flexible Lithium-Ion Fiber Battery by the Regular Stacking of Two-Dimensional Titanium Oxide Nanosheets Hybridized with Reduced Graphene Oxide

Increasing interest has recently been devoted to developing small, rapid, and portable electronic devices; thus, it is becoming critically important to provide matching light and flexible energy-storage systems to power them. To this end, compared with the inevitable drawbacks of being bulky, heavy, and rigid for traditional planar sandwiched structures, linear fiber-shaped lithium-ion batteries (LIB) have become increasingly important owing to their combined superiorities of miniaturization, adaptability, and weavability, the progress of which being heavily dependent on the development of new fiber-shaped electrodes. Here, we report a novel fiber battery electrode based on the most widely used LIB material, titanium oxide, which is processed into two-dimensional nanosheets and assembled into a macroscopic fiber by a scalable wet-spinning process. The titania sheets are regularly stacked and conformally hybridized in situ with reduced graphene oxide (rGO), thereby serving as efficient current collectors, which endows the novel fiber electrode with excellent integrated mechanical properties combined with superior battery performances in terms of linear densities, rate capabilities, and cyclic behaviors. The present study clearly demonstrates a new material-design paradigm toward novel fiber electrodes by assembling metal oxide nanosheets into an ordered macroscopic structure, which would represent the most-promising solution to advanced flexible energy-storage systems.

Hierarchical architecture for flexible energy storage

The introduction of hierarchy and chirality into structure is of great interest, and can result in new optical and electronic properties due to the synergistic effect of helical and anisotropic structures. Herein, we demonstrate a simple and straightforward route toward the fabrication of hierarchical chiral materials based on the assembly of two-dimensional graphene oxide nanosheets (GO) and one-dimensional cellulose nanocrystals (CNCs). The unique layered structure of CNC/GO could be preserved in the solid state, allowing electrode active SnO₂ to be loaded for potential applications in energy storage. The resultant SnO₂/CNC/reduced GO (SnO₂/CNC/rGO) composite could be processed into film, fiber, and textile with an extremely high tensile strength of 100 MPa. The free-standing SnO₂/CNC/rGO electrodes exhibit highly improved energy storage performance, with a reversible capacity of ∼500 mA h g^-1 maintained for 1500 cycles in the film and ∼800 mA h g^-1 maintained for 150 cycles in the textile at a current density of 500 mA g^-1. This is attributed to the prepared hierarchical chiral structures. The presented technique provides an effective approach to producing hierarchical functional materials from nanoparticles as building blocks, which might open an avenue for the creation of new flexible energy storage devices.

Superconducting Continuous Graphene Fibers via Calcium Intercalation

Superconductors are important materials in the field of low-temperature magnet applications and long-distance electrical power transmission systems. Besides metal-based superconducting materials, carbon-based superconductors have attracted considerable attention in recent years. Up to now, five allotropes of carbon, including diamond, graphite, C₆₀, CNTs, and graphene, have been reported to show superconducting behavior. However, most of the carbon-based superconductors are limited to small size and discontinuous phases, which inevitably hinders further application in macroscopic form. Therefore, it raises a question of whether continuously carbon-based superconducting wires could be accessed, which is of vital importance from viewpoints of fundamental research and practical application. Here, inspired by superconducting graphene, we successfully fabricated flexible graphene-based superconducting fibers via a well-established calcium (Ca) intercalation strategy. The resultant Ca-intercalated graphene fiber (Ca-GF) shows a superconducting transition at ∼11 K, which is almost 2 orders of magnitude higher than that of early reported alkali metal intercalated graphite and comparable to that of commercial superconducting NbTi wire. The combination of lightness and easy scalability makes Ca-GF highly promising as a lightweight superconducting wire.

Graphene and Other 2D Colloids: Liquid Crystals and Macroscopic Fibers

Two-dimensional colloidal nanomaterials are running into renaissance after the enlightening researches of graphene. Macroscopic one-dimensional fiber is an optimal ordered structural form to express the in-plane merits of 2D nanomaterials, and the formation of liquid crystals (LCs) allows the creation of continuous fibers. In the correlated system from LCs to fibers, understanding their macroscopic organizing behavior and transforming them into new solid fibers is greatly significant for applications. Herein, we retrospect the history of 2D colloids and discuss about the concept of 2D nanomaterial fibers in the context of LCs, elaborating the motivation, principle and possible strategies of fabrication. Then we highlight the creation, development and typical applications of graphene fibers. Additionally, the latest advances of other 2D nanomaterial fibers are also summarized. Finally, conclusions, challenges and perspectives are provided to show great expectations of better and more fibrous materials of 2D nanomaterials. This review gives a comprehensive retrospect of the past century-long effort about the whole development of 2D colloids, and plots a clear roadmap - "lamellar solid - LCs - macroscopic fibers - flexible devices", which will certainly open a new era of structural-multifunctional application for the conventional 2D colloids.

Self-Assembling Halloysite Nanotubes into Concentric Ring Patterns in a Sphere-on-Flat Geometry

Highly ordered and concentric ring patterns consisting of halloysite nanotubes (HNTs) with hierarchical cholesteric architectures are prepared by evaporation-induced self-assembly in a sphere-on-flat geometry. The structure and properties of HNTs are investigated. HNTs show a perfect tubular morphology on the nanoscale with high dispersion stability in water. Upon drying the HNTs aqueous suspension in a sphere-on-flat confined space, regular concentric HNTs rings are formed on the substrate via a self-assembly process. The widths of the inner and outer rings and the spacing between the adjacent rings increase with an increase in the concentration of the HNTs suspension. The highly ordered and concentric HNTs rings show a pronounced Maltese cross-like pattern under crossed polarizers, which suggests the formation of hierarchical cholesteric architectures. Scanning electron microscopy and atomic force microscopy observations show a disclination alignment of HNTs in the ring strips, especially with a high concentration of the HNTs suspension. The patterned rough surfaces of the HNTs show low cytotoxicity and can be used as a cell-supporting scaffold. The HNTs rings can guide the growth and orientation of C2C12 myoblast cells perpendicular to the rings. This work provides a simple, repeatable, mild, and high-efficiency method for obtaining HNTs with hierarchical architectures, which show potential for a large variety of applications, for example, in vascular grafts and skin regeneration.

Graphene Oxide Involved Air-Controlled Electrospray for Uniform, Fast, Instantly Dry, and Binder-Free Electrode Fabrication

We report a facile air-controlled electrospray method to directly deposit binder-free active materials/graphene oxide (GO) onto current collectors. This method is inspired from an electrospinning process, and possesses all the advantages that electrospinning has such as low cost, easy scaling up, and simultaneous solvent evaporation during the spraying process. Moreover, the spray slurry is only a simple mixture of active materials and GO suspension in water, no binder polymer, organic solvent, and conductive carbon required. In our research, high-capacity Si nanoparticles (Si NP, 70-100 nm) and SiO microparticles (SiO MP, 3-10 μm) were selected to demonstrate the capability of this method to accommodate particles with different sizes. Their mixture with GO was sprayed onto a collector and then thermally annealed in an inert gas to obtain Si NP or SiO MP/reduced graphene oxide (RGO) binder-free electrodes. We are also able to directly deposit fairly large electrode sheets (e.g., 12 × 21 in.) upon the application requirement. To the best of our knowledge, this is the simplest approach to produce Si-related materials/RGO layered structures directly on current collector with controllable area and loading. Si and SiO MP/RGO are evaluated in both half and full lithium cells, showing good electrochemical performance. Prelithiation is also studied and gives a high first cycle Coulombic efficiency. In addition to Si-related materials, other materials with different shapes and sizes (e.g., MoO₃ nanobelts, Sn/carbon nanofibers, and commercial sulfur particles) can also be sprayed. Beyond the preparation of battery electrodes, this approach can also be applied for other types of electrode preparation such as that of a supercapacitor, fuel cell, and solar cell.

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocatalysis, batteries, supercapacitors, solar cells, photocatalysis, and sensing platforms. Finally, the challenges and outlooks in this promising field are featured on the basis of its current development.

A facile way to produce epoxy nanocomposites having excellent thermal conductivity with low contents of reduced graphene oxide

A well-dispersed phase of exfoliated graphene oxide (GO) nanosheets was initially prepared in water. This was concentrated by centrifugation and was mixed with a liquid epoxy resin. The remaining water was removed by evaporation, leaving a GO dispersion in epoxy resin. A stoichiometric amount of an anhydride curing agent was added to this epoxy-resin mixture containing the GO nanosheets, which was then cured at 90 °C for 1 h followed by 160 °C for 2 h. A second thermal treatment step of 200 °C for 30 min was then undertaken to reduce further the GO in situ in the epoxy nanocomposite. An examination of the morphology of such nanocomposites containing reduced graphene oxide (rGO) revealed that a very good dispersion of rGO was achieved throughout the epoxy polymer. Various thermal and mechanical properties of the epoxy nanocomposites were measured, and the most noteworthy finding was a remarkable increase in the thermal conductivity when relatively very low contents of rGO were present. For example, a value of 0.25 W/mK was measured at 30 °C for the nanocomposite with merely 0.06 weight percentage (wt%) of rGO present, which represents an increase of ~40% compared with that of the unmodified epoxy polymer. This value represents one of the largest increases in the thermal conductivity per wt% of added rGO yet reported. These observations have been attributed to the excellent dispersion of rGO achieved in these nanocomposites made via this facile production method. The present results show that it is now possible to tune the properties of an epoxy polymer with a simple and viable method of GO addition.

Continuous Draw Spinning of Extra-Long Silver Submicron Fibers with Micrometer Patterning Capability

Ultrathin metal fibers can serve as highly conducting and flexible current and heat transport channels, which are essential for numerous applications ranging from flexible electronics to energy conversion. Although industrial production of metal fibers with diameters of down to 2 μm is feasible, continuous production of high-quality and low-cost nanoscale metal wires is still challenging. Herein, we report the continuous draw spinning of highly conductive silver submicron fibers with the minimum diameter of ∼200 nm and length of more than kilometers. We obtained individual AgNO₃/polymer fibers by continuous drawing from an aqueous solution at a speed of up to 8 m/s. With subsequent heat treatment, freestanding Ag submicron fibers with high mechanical flexibility and electric conductivity have been obtained. Woven mats of aligned Ag submicron fibers were used as transparent electrodes with high flexibility and high performance with sheet resistance of 7 Ω sq^-1 at a transparency of 96%. Continuous draw spinning opened new avenues for scalable, flexible, and ultralow-cost fabrication of extra-long conductive ultrathin metal fibers.