Mussel-Inspired Defect Engineering of Graphene Liquid Crystalline Fibers for Synergistic Enhancement of Mechanical Strength and Electrical Conductivity

Wet-spinning of carbon nanotube fibers: dispersion, processing and properties

Owing to the intrinsic excellent mechanical, electrical, and thermal properties of carbon nanotubes (CNTs), carbon nanotube fibers (CNTFs) have been expected to become promising candidates for the next-generation of high-performance fibers. They have received considerable interest for cutting-edge applications, such as ultra-light electric wire, aerospace craft, military equipment, and space elevators. Wet-spinning is a broadly utilized commercial technique for high-performance fiber manufacturing. Thus, compared with array spinning from drawable CNTs vertical array and direct dry spinning from floating catalyst chemical vapor deposition (FCCVD), the wet-spinning technique is considered to be a promising strategy to realize the production of CNTFs on a large scale. In this tutorial review, we begin with a summative description of CNTFs wet-spinning process. Then, we discuss the high-concentration CNTs wet-spinning dope preparation strategies and corresponding non-covalent adsorption/charge transfer mechanisms. The filament solidification during the coagulation process is another critical procedure for determining the configurations and properties for derived CNTFs. Next, we discuss post-treatment, including continuous drafting and thermal annealing, to further optimize the CNTs orientation and compact configuration. Finally, we summarize the physical property-structure relationship to give insights for further performance promotion in order to satisfy the prerequisite for detailed application. Insights into propelling high-performance CNTFs production from lab-scale to industry-scale are proposed, in anticipation of this novel fiber having an impact on our lives in the near future.

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.

Enhancement of Mechanical Properties of Multilayer Ceramic Capacitors through a BaTiO3/polydopamine Cover Layer

To fabricate multilayer ceramic capacitors (MLCCs) that can withstand external impacts, technologies to achieve excellent adhesion and mechanical strength of the cover layer should be essentially developed. Low adhesion and strength of the cover layer can lead to delamination and cracks in the MLCC, respectively. In this study, we present a method for applying polydopamine (PDA), a mussel-inspired adhesive protein, for as robust cover layer on an MLCC. Barium titanate (BT) particles treated with PDA increase the dispersion stability of the BT/PDA slurry, preventing re-agglomeration of the particles and enhancing the adhesiveness and strength owing to the cohesive properties of PDA. Compared to the BT layer, the adhesion of the BT/PDA layer was significantly enhanced by 217%; consequently, the compression modulus of the BT/PDA cover layer increased by 29.4%. After firing, the N-doped graphitic PDA played an important role in producing an MLCC cover layer with increased hardness and toughness. Furthermore, the N-doped graphitic PDA with a hydrophobic surface forms tortuous moisture paths in the cover layer, preventing the degradation of insulation resistance of the MLCC.

Extremely Robust and Multifunctional Nanocomposite Fibers for Strain-Unperturbed Textile Electronics

Textile electronics are needed that can achieve strain-unaltered performance when they undergo irregular and repeated strain deformation. Such strain-unaltered textile electronics require advanced fibers that simultaneously have high functionalities and extreme robustness as fabric materials. Current synthetic nanocomposite fibers based on inorganic matrix have remarkable functionalities but often suffer from low robustness and poor tolerance against crack formation. Here, we present a design for a high-performance multifunctional nanocomposite fiber that is mechanically and electrically robust, which was realized by crosslinking titanium carbide (MXene) nanosheets with a slide-ring polyrotaxane to form an internal mechanically-interlocked network. This inorganic matrix nanocomposite fiber featured distinct strain-hardening mechanical behavior and exceptional load-bearing capability (toughness approaching 60 MJ m^-3 and ductility over 27%). It retained 100% of its ductility after cyclic strain loading. Moreover, the high electrical conductivity (>1.1 × 10⁵ S m^-1 ) and electrochemical performance (>360 F cm^-3 ) of the nanocomposite fiber can be well retained after subjecting the fiber to extensive (>25% strain) and long-term repeated (10 000 cycles) dimensional changes. Such superior robustness allowed for the fabrication of the nanocomposite fibers into various robust wearable devices, such as textile-based electromechanical sensors with strain-unalterable sensing performance and fiber-shaped supercapacitors with invariant electrochemical performance for 10 000 strain loading cycles.

Bionic structures for optimizing the design of stealth materials

Traditional microwave absorbing materials (MAMs) have exposed more and more problems in multi-spectrum detection and a harsh service environment, which hinder their further application. Bionic materials and structures have attracted more and more attention from researchers in the field of stealth materials due to their excellent properties, such as high strength and high conductivity, along with easy access to scale adjustability and structural design. By introducing the concept of bionics into their structural design and material design, we can obtain highly efficient stealth materials with multiple properties. In addition, the concept of multispectral stealth is furthered by comparing the difference in the principle and methods of achievement between radar stealth and infrared stealth. This paper fundamentally summarizes the research status of bionic structure design ideas in stealth materials, analyzing the structure-activity relationship between the structural size effect and electromagnetic characteristics from low order to high order. Then, the design ideas and universal strategies of typical bionic structures are summarised and an idea for the integrated design of radar absorption compatible with infrared stealth is put forward. This will provide profound insights for the application of biomimetic stealth materials and the future development of intelligent-response and dynamically adjustable materials.

Recent Advances in Graphene-Based Nanocomposites for Ammonia Detection

The increasing demand to mitigate the alarming effects of the emission of ammonia (NH3) on human health and the environment has highlighted the growing attention to the design of reliable and effective sensing technologies using novel materials and unique nanocomposites with tunable functionalities. Among the state-of-the-art ammonia detection materials, graphene-based polymeric nanocomposites have gained significant attention. Despite the ever-increasing number of publications on graphene-based polymeric nanocomposites for ammonia detection, various understandings and information regarding the process, mechanisms, and new material components have not been fully explored. Therefore, this review summarises the recent progress of graphene-based polymeric nanocomposites for ammonia detection. A comprehensive discussion is provided on the various gas sensor designs, including chemiresistive, Quartz Crystal Microbalance (QCM), and Field-Effect Transistor (FET), as well as gas sensors utilising the graphene-based polymer nanocomposites, in addition to highlighting the pros and cons of graphene to enhance the performance of gas sensors. Moreover, the various techniques used to fabricate graphene-based nanocomposites and the numerous polymer electrolytes (e.g., conductive polymeric electrolytes), the ion transport models, and the fabrication and detection mechanisms of ammonia are critically addressed. Finally, a brief outlook on the significant progress, future opportunities, and challenges of graphene-based polymer nanocomposites for the application of ammonia detection are presented.

Human-muscle-inspired single fibre actuator with reversible percolation

Artificial muscles are indispensable components for next-generation robotics capable of mimicking sophisticated movements of living systems. However, an optimal combination of actuation parameters, including strain, stress, energy density and high mechanical strength, is required for their practical applications. Here we report mammalian-skeletal-muscle-inspired single fibres and bundles with large and strong contractive actuation. The use of exfoliated graphene fillers within a uniaxial liquid crystalline matrix enables photothermal actuation with large work capacity and rapid response. Moreover, the reversible percolation of graphene fillers induced by the thermodynamic conformational transition of mesoscale structures can be in situ monitored by electrical switching. Such a dynamic percolation behaviour effectively strengthens the mechanical properties of the actuator fibres, particularly in the contracted actuation state, enabling mammalian-muscle-like reliable reversible actuation. Taking advantage of a mechanically compliant fibre structure, smart actuators are readily integrated into strong bundles as well as high-power soft robotics with light-driven remote control.

Ultrastrong Hybrid Fibers with Tunable Macromolecular Interfaces of Graphene Oxide and Carbon Nanotube for Multifunctional Applications

Individual carbon nanotubes (CNT) and graphene have unique mechanical and electrical properties; however, the properties of their macroscopic assemblies have not met expectations because of limited physical dimensions, the limited degree of dispersion of the components, and various structural defects. Here, a state-of-the-art assembly for a novel type of hybrid fiber possessing the properties required for a wide variety of multifunctional applications is presented. A simple and effective multidimensional nanostructure of CNT and graphene oxide (GO) assembled by solution processing improves the interfacial utilization of the components. Flexible GOs are effectively intercalated between nanotubes along the shape of CNTs, which reduces voids, enhances orientation, and maximizes the contact between elements. The microstructure is finely controlled by the elements content ratio and dimensions, and an optimal balance improves the mechanical properties. The hybrid fibers simultaneously exhibit exceptional strength (6.05 GPa), modulus (422 GPa), toughness (76.8 J g-1 ), electrical conductivity (8.43 MS m-1 ), and knot strength efficiency (92%). Furthermore, surface and electrochemical properties are significantly improved by tuning the GO content, further expanding the scope of applications. These hybrid fibers are expected to offer a strategy for overcoming the limitations of existing fibers in meeting the requirements for applications in the fiber industry.

Carbonene Fibers: Toward Next-Generation Fiber Materials

The development of human society has set unprecedented demands for advanced fiber materials, such as lightweight and high-performance fibers for reinforcement of composite materials in frontier fields and functional and intelligent fibers in wearable electronics. Carbonene materials composed of sp²-hybridized carbon atoms have been demonstrated to be ideal building blocks for advanced fiber materials, which are referred to as carbonene fibers. Carbonene fibers that generally include pristine carbonene fibers, composite carbonene fibers, and carbonene-modified fibers hold great promise in transferring the extraordinary properties of nanoscale carbonene materials to macroscopic applications. Herein, we give a comprehensive discussion on the conception, classification, and design strategies of carbonene fibers and then summarize recent progress regarding the preparations and applications of carbonene fibers. Finally, we provide insights into developing lightweight, high-performance, functional, and intelligent carbonene fibers for next-generation fiber materials in the near future.

Mussel-inspired polydopamine decorated alginate dialdehyde-gelatin 3D printed scaffolds for bone tissue engineering application

This study utilized extrusion-based 3D printing technology to fabricate calcium-cross-linked alginate dialdehyde-gelatin scaffolds for bone regeneration. The surface of polymeric constructs was modified with mussel-derived polydopamine (PDA) in order to induce biomineralization, increase hydrophilicity, and enhance cell interactions. Microscopic observations revealed that the PDA layer homogeneously coated the surface and did not appear to induce any distinct change in the microstructure of the scaffolds. The PDA-functionalized scaffolds were more mechanically stable (compression strength of 0.69 ± 0.02 MPa) and hydrophilic (contact angle of 26) than non-modified scaffolds. PDA-decorated ADA-GEL scaffolds demonstrated greater durability. As result of the 18-days immersion in simulated body fluid solution, the PDA-coated scaffolds showed satisfactory biomineralization. Based on theoretical energy analysis, it was shown that the scaffolds coated with PDA interact spontaneously with osteocalcin and osteomodulin (binding energy values of -35.95 kJ mol-1 and -46.39 kJ mol-1, respectively), resulting in the formation of a protein layer on the surface, suggesting applications in bone repair. PDA-coated ADA-GEL scaffolds are capable of supporting osteosarcoma MG-63 cell adhesion, viability (140.18% after 7 days), and proliferation. In addition to increased alkaline phosphatase secretion, osteoimage intensity also increased, indicating that the scaffolds could potentially induce bone regeneration. As a consequence, the present results confirm that 3D printed PDA-coated scaffolds constitute an intriguing novel approach for bone tissue engineering.

Wide-Range Size Fractionation of Graphene Oxide by Flow Field-Flow Fractionation

Many interesting properties of 2D materials and their assembled structures are strongly dependent on the lateral size and size distribution of 2D materials. Accordingly, effective size separation of polydisperse 2D sheets is critical for desirable applications. Here, we introduce flow field-flow fractionation (FlFFF) for a wide-range size fractionation of graphene oxide (GO) up to 100 μm. Two different separation mechanisms are identified for FlFFF, including normal mode and steric/hyperlayer mode, to size fractionate wide size-distributed GOs while employing a crossflow field for either diffusion or size-controlled migration of GO. Obviously, the 2D GO sheet reveals size separation behavior distinctive from typical spherical particles arising from its innate planar geometry. We also investigate 2D sheet size-dependent mechanical and electrical properties of three different graphene fibers produced from size-fractionated GOs. This FlFFF-based size selection methodology can be used as a generic approach for effective wide-range size separation for 2D materials, including rGO, TMDs, and MXene.

Mechanisms of Strain-Induced Interfacial Strengthening of Wet-Spun Filaments

We investigate the mechanism of binding of dopamine-conjugated carboxymethyl cellulose (DA-CMC) with carbon nanotubes (CNTs) and the strain-induced interfacial strengthening that takes place upon wet drawing and stretching filaments produced by wet-spinning. The filaments are known for their tensile strength (as high as 972 MPa and Young modulus of 84 GPa) and electrical conductivity (241 S cm^-1). The role of axial orientation in the development of interfacial interactions and structural changes, enabling shear load bearing, is studied by molecular dynamics simulation, which further reveals the elasto-plasticity of the system. We propose that the reversible torsion of vicinal molecules and DA-CMC wrapping around CNTs are the main contributions to the interfacial strengthening of the filaments. Such effects play important roles in impacting the properties of filaments, including those related to electrothermal heating and sensing. Our findings contribute to a better understanding of high aspect nanoparticle assembly and alignment to achieve high-performance filaments.

Molecular-Level Lubrication Effect of 0D Nanodiamonds for Highly Bendable Graphene Liquid Crystalline Fibers

Graphene fiber is emerging as a new class of carbon-based fiber with distinctive material properties particularly useful for electroconductive components for wearable devices. Presently, stretchable and bendable graphene fibers are principally employing soft dielectric additives, such as polymers, which can significantly deteriorate the genuine electrical properties of pristine graphene-based structures. We report molecular-level lubricating nanodiamonds as an effective physical property modifier to improve the mechanical flexibility of graphene fibers by relieving the tight interlayer stacking among graphene sheets. Nanoscale-sized NDs effectively increase the tensile strain and bending strain of graphene/nanodiamond composite fibers while maintaining the genuine electrical conductivity of pristine graphene-based fibers. The molecular-level lubricating mechanism is elucidated by friction force microscopy on the nanoscale as well as by shear stress measurement on the macroscopic scale. The resultant highly bendable graphene/nanodiamond composite fiber is successfully weaved into all graphene fiber-based textiles and wearable Joule heaters, proposing the potential for reliable wearable applications.

Microstructure-Controlled Polyacrylonitrile/Graphene Fibers over 1 Gigapascal Strength

Controlling the microstructures in fibers, such as crystalline structures and microvoids, is a crucial challenge for the development of mechanically strong graphene fibers (GFs). To date, although GFs graphitized at high temperatures have exhibited high tensile strength, GFs still have limited the ultimate mechanical strength owing to the presence due to the structural defects, including the imperfect alignment of graphitic crystallites and the presence of microsized voids. In this study, we significantly enhanced the mechanical strength of GF by controlling microstructures of fibers. GF was hybridized by incorporating polyacrylonitrile (PAN) in the graphene oxide (GO) dope solution. In addition, we controlled the orientation of the inner structure by applying a tensile force at 800 °C. The results suggest that PAN can act as a binder for graphene sheets and can facilitate the rearrangement of the fiber's microstructure. PAN was directionally carbonized between graphene sheets due to the catalytic effect of graphene. The resulting hybrid GFs successfully displayed a high strength of 1.10 GPa without undergoing graphitization at extremely high temperatures. We believe that controlling the alignment of nanoassembled structure is an efficient strategy for achieving the inherent performance characteristics of graphene at the level of multidimensional structures including films and fibers.

Universal Alignment of Graphene Oxide in Suspensions and Fibers

Graphene oxide (GO) has become a key component for high-performance carbon-based films or fibers based on its dispersibility and liquid crystallinity in an aqueous suspension. While the superior performance of GO-based fiber relies on their alignment at the submicrometer level, fine control of the microstructure is often hampered, in particular, under dynamic nature of GO-processing involving shear. Here, we systemically studied the structural variation of GO suspensions under shear conditions via in situ rheo-scattering and shear-polarized optical microscope analysis. The evolution of GO alignment under shear is indeed complex. However, we found that the shear-dependent structural equilibrium exists. GO showed a nonlinear structural transition with shear, yet there is a "universal" shear threshold for the best alignment, resulting in graphene fiber achieved an improvement in mechanical properties by ∼54% without any chemical modification. This finding challenges the conventional concept that high shear stress is required for the good alignment of particles and their best performance.

Tuning the photonic properties of graphene oxide suspensions with nanostructured additives

Photonic materials that can selectively reflect light across the visible spectrum are valuable for applications in optical devices, sensors, and decoration. Although two-dimensional (2D) colloids that stack into layers with spacing of hundreds of nanometers are able to selectively diffract light, controlling their separation in solution has proven challenging. In this work, we investigate the role of additives to control the photonic properties of hybrid colloidal suspensions of graphene oxide (GO). We discovered that low concentrations of colloidal additives like cellulose nanocrystals (CNCs) and clay nanoparticles (hectorite) added to GO suspensions lead to dramatic color changes. These hybrid colloidal suspensions demonstrate tunable structural colors and temperature-sensitive properties that likely originate from the entropically driven ejection of guests between the sheets, and from the interactions between colloidal electrical double layers and additional counterions. On the other hand, blending polymeric or molecular additives with GO suspensions either deteriorates or does not impact the photonic properties. These results are helpful to understand the interaction between GO suspensions and additives over different length scales, and open a path to advancing photonic materials based on hybrid colloidal suspensions.

Graphene-Based Helical Micromotors Constructed by "Microscale Liquid Rope-Coil Effect" with Microfluidics

Nature provides diverse inspirations for constructing mobile and functionalized micromachines. For example, artificial helical micro-/nanomotors inspired by bacteria flagella that can be precisely steered for various applications have been constructed by utilizing materials with excellent functions. Graphene-based materials show outstanding properties, and, to date, have not been considered to construct helical micromotors and investigate their potential applications. Here, we propose an interesting "microscale liquid rope-coil effect" strategy to stably and simply fabricate graphene oxide-based helical micromotors (GOFHMs) with high throughput by the capillary microfluidics technique. A range of desirable GOFHMs with different pitch, length, and linear diameter are tailored by smart parameter setting in microfluidic system (flow velocity, concentration, and so on). Afterward, graphene-based helical micromotors (GFHMs) are easily acquired by the reduction of GOFHMs and further drying. Actuated by rotating magnetic field, GFHMs show capability to conduct programmed locomotion in a microchannel. As a proof-of-concept demonstration, GFHMs and Ag modified GFHMs have been successfully applied to water remediation, which exhibits excellent removal efficiency of chemical and biological pollutants. Meanwhile, doxorubicin is modified onto GFHMs for the application of drug delivery. Accordingly, we believe that GFHMs have great potential in a variety of fields by modifying graphene with other nanoparticles or functional molecules.

Sequential Release Platform of Heparin and Urokinase with Dual Physical (NIR-II and Bubbles) Assistance for Deep Venous Thrombosis

Disability and even death from acute thrombosis remain a grave menace to public health. At present, the traditional drugs represented by urokinase (UK) in clinical thrombolysis can cause side effects of bleeding when the dosage is excess. Therefore, a more effective and safer method of thrombolysis is urgently needed. In this paper, a multifunctional dual-drug sequential release thrombolysis platform (UK-UH@PDA@HMSNs) consisting of polydopamine (PDA)-modified hollow mesoporous silicon (HMSNs) loading with UK and unfractionated heparin (UH) was constructed with a double physical assistance (NIR-II and bubbles). With the aid of near infrared-II (NIR-II, 1064 nm, 1.0 W cm^-2) laser, the photothermal effect of PDA could be motivated to facilitate the UH release, thereby accelerating the dissolution of thrombus. Afterward, the local hyperthermia effect could expedite the phase transition of l-menthol in HMSNs to generate bubbles to promote the release of UK, thereby realizing the sequential release of two thrombolytic drugs. Importantly, this method deftly conquered the inherent obstacle that UK and UH cannot be combined directly. In vivo and in vitro experiments proved that the thrombolytic efficiency of UK-UH@PDA@HMSNs stimulated by NIR-II was nearly 3 times than that of UK alone. Collectively, the proposed dual physical assistance and sequential dual-drug delivery system significantly improved the efficiency of thrombolysis under the premise of limiting drug doses; the risk of death from intracranial hemorrhage thus could be decreased radically.

Tungsten nitride-coated graphene fibers for high-performance wearable supercapacitors

Graphene-fiber (GF) supercapacitors have attracted significant research attention in the field of wearable devices. However, there is still a need for active materials with high energy density. Transition Metal Nitrides (TMNs) are promising candidates for this purpose compared with conventional Transition Metal Oxides (TMOs) or conducting polymers (CPs) owing to their higher electrical conductivity, stability and relevant electrochemical properties. We have successfully integrated Tungsten Nitride (WN) with reduced graphene oxide fibers (rGOF) and developed high-performance hybrid fiber (WN-rGOF) supercapacitors. These hybrid supercapacitors attained a high capacitance of 16.29 F cm-3 at 0.05 A cm-3 and an energy density of 1.448 mW h cm-3, which is 7.5 and 1.75 times higher than those of the pure rGOF supercapacitor and the Tungsten Oxide/rGO hybrid fiber (WO3-rGOF) supercapacitor, respectively. The energy density readily increased up to 2.896 mW h cm-3 when three WN-rGOF supercapacitors were connected in series. The WN-rGOF supercapacitor also showed high capacitance retention of 84.7% after 10 000 cycles along with appreciable performance under severe mechanical deformation.

Mussel Inspired Highly Aligned Ti3C2Tx MXene Film with Synergistic Enhancement of Mechanical Strength and Ambient Stability

Two-dimensional (2D) MXene has shown enormous potential in scientific fields, including energy storage and electromagnetic interference (EMI) shielding. Unfortunately, MXene-based material structures generally suffer from mechanical fragility and vulnerability to oxidation. Herein, mussel-inspired dopamine successfully addresses those weaknesses by improving interflake interaction and ordering in MXene assembled films. Dopamine undergoes in situ polymerization and binding at MXene flake surfaces by spontaneous interfacial charge transfer, yielding an ultrathin adhesive layer. Resultant nanocomposites with highly aligned tight layer structures achieve approximately seven times enhanced tensile strength with a simultaneous increase of elongation. Ambient stability of MXene films is also greatly improved by the effective screening of oxygen and moisture. Interestingly, angstrom thick polydopamine further promotes the innate high electrical conductivity and excellent EMI shielding properties of MXene films. This synergistic concurrent enhancement of physical properties proposes MXene/polydopamine hybrids as a general platform for MXene based reliable applications.

Deep-Learning-Based Deconvolution of Mechanical Stimuli with Ti3C2Tx MXene Electromagnetic Shield Architecture via Dual-Mode Wireless Signal Variation Mechanism

Passive component-based soft resonators have been spotlighted in the field of wearable and implantable devices due to their remote operation capability and tunable properties. As the output signal of the resonator-based wireless communication device is given in the form of a vector (i.e., a spectrum of reflection coefficient), multiple information can, in principle, be stored and interpreted. Herein, we introduce a device that can deconvolute mechanical stimuli from a single wireless signal using dual-mode operation, specifically enabled by the use of Ti₃C₂T_x MXene. MXene's strong electromagnetic shielding effect enables the resonator to simultaneously measure pressure and strain without overlapping its output signal, unlike other conductive counterparts that are deficient in shielding ability. Furthermore, convolutional neural-network-based deep learning was implemented to predict the pressure and strain values from unforeseen output wireless signals. Our MXene-integrated wireless device can also be utilized as an on-skin mechanical stimuli sensor for rehabilitation monitoring after orthopedic surgery. The dual-mode signal variation mechanism enabled by integration of MXene allows wireless communication systems to efficiently handle various information simultaneously, through which multistimuli sensing capability can be imparted into passive component-based wearable and implantable electrical devices.

Polydopamine and Its Derivative Surface Chemistry in Material Science: A Focused Review for Studies at KAIST

Polydopamine coating, the first material-independent surface chemistry, and its related methods significantly influence virtually all areas of material science and engineering. Functionalized surfaces of metal oxides, synthetic polymers, noble metals, and carbon materials by polydopamine and its related derivatives exhibit a variety of properties for cell culture, microfluidics, energy storage devices, superwettability, artificial photosynthesis, encapsulation, drug delivery, and numerous others. Unlike other articles, this review particularly focuses on the development of material science utilizing polydopamine and its derivatives coatings at the Korea Advanced Institute of Science and Technology for a decade. Herein, it is demonstrated how material-independent coating methods provide solutions for challenging problems existed in many interdisciplinary areas in bio-, energy-, and nanomaterial science by collaborations and independent research.

Nanoscale Assembly of 2D Materials for Energy and Environmental Applications

Rational design of 2D materials is crucial for the realization of their profound implications in energy and environmental fields. The past decade has witnessed significant developments in 2D material research, yet a number of critical challenges remain for real-world applications. Nanoscale assembly, precise control over the orientational and positional ordering, and complex interfaces among 2D layers are essential for the continued progress of 2D materials, especially for energy storage and conversion and environmental remediation. Herein, recent progress, the status, future prospects, and challenges associated with nanoscopic assembly of 2D materials are highlighted, specifically targeting energy and environmental applications. Geometric dimensional diversity of 2D material assembly is focused on, based on novel assembly mechanisms, including 1D fibers from the colloidal liquid crystalline phase, 2D films by interfacial tension (Marangoni effect), and 3D nanoarchitecture assembly by electrochemical processes. Relevant critical advantages of 2D material assembly are highlighted for application fields, including secondary batteries, supercapacitors, catalysts, gas sensors, desalination, and water decontamination.

Carbon Nanotube Reinforced Strong Carbon Matrix Composites

As an excellent candidate for lightweight structural materials and nonmetal electrical conductors, carbon nanotube reinforced carbon matrix (CNT/C) composites have potential use in technologies employed in aerospace, military, and defense endeavors, where the combinations of light weight, high strength, and excellent conductivity are required. Both polymer infiltration pyrolysis (PIP) and chemical vapor infiltration (CVI) methods have been widely studied for CNT/C composite fabrications with diverse focuses and various modifications. Progress has been reported to optimize the performance of CNT/C composites from broad aspects, including matrix densification, CNT alignment, microstructure control, and interface engineering, etc. Recent approaches, such as using resistance heating for PIP or CVI, contribute to the development of CNT/C composites. To deliver a timely and up-to-date overview of CNT/C composites, we have reviewed the most recent trends in fabrication processes, summarized the mechanical reinforcement mechanism, and discussed the electrical and thermal properties, as well as relevant case studies for high-temperature applications. Conclusions and perspectives addressing future routes for performance optimization are also presented. Hence, this review serves as a rundown of recent advances in CNT/C composites and will be a valuable resource to aid future developments in this field.

Self-Planarization of High-Performance Graphene Liquid Crystalline Fibers by Hydration

Graphene fibers (GFs) are promising elements for flexible conductors and energy storage devices, while translating the extraordinary properties of individual graphene sheets into the macroscopically assembled 1D structures. We report that a small amount of water addition to the graphene oxide (GO) N-methyl-2-pyrrolidone (NMP) dispersion has significant influences on the mesophase structures and physical properties of wet-spun GFs. Notably, 2 wt % of water successfully hydrates GO flakes in NMP dope to form a stable graphene oxide liquid crystal (GOLC) phase. Furthermore, 4 wt % of water addition causes spontaneous planarization of wet-spun GFs. Motivated from these interesting findings, we develop highly electroconductive and mechanically strong flat GFs by introducing highly crystalline electrochemically exfoliated graphene (EG) in the wet-spinning of NMP-based GOLC fibers. The resultant high-performance hybrid GFs can be sewn on cloth, taking advantage of the mechanical robustness and high flexibility.

Graphene-Based Fibers: Recent Advances in Preparation and Application

Graphene-based fibers (GBFs) are macroscopic 1D assemblies formed by using microscopic 2D graphene sheets as building blocks. Their unique structure exhibits the same merits as graphene such as low weight, high specific surface area, excellent mechanical/electrical properties, and ease of functionalization. Furthermore, the fibrous nature of GBFs is intrinsically compatible with existing textile technologies, making them suitable for applications in flexible and wearable electronics. Recently, novel synthetic methods have endowed GBFs with new structures and functions, further improving their mechanical and electrical properties. These improvements have rapidly bridged the gaps between laboratory demonstrations and real-life applications in fiber-shaped batteries, supercapacitors, and electrochemical sensors. Recent advances in the fabrication, optimization, and application of GBFs are systematically reviewed and a perspective on their future development is given.

Conformal 3D Nanopatterning by Block Copolymer Lithography with Vapor-Phase Deposited Neutral Adlayer

Block copolymer (BCP) lithography is an effective nanopatterning methodology exploiting nanoscale self-assembled periodic patterns in BCP thin films. This approach has a critical limitation for nonplanar substrate geometry arising from the reflow and modification of BCP films upon the thermal or solvent annealing process, which is inevitable to induce the mobility of BCP chains for the self-assembly process. Herein, reflow-free, 3D BCP nanopatterning is demonstrated by introducing a conformally grown adlayer by the initiated chemical vapor deposition (iCVD) process. A highly cross-linked poly(divinylbenzene) layer was deposited directly onto the BCP thin film surface by iCVD, which effectively prevented the reflow of BCP thin film during an annealing process. BCP nanopatterns could be stabilized on various substrate geometry, including a nonplanar deformed polymer substrate, a pyramid shape substrate, and a graphene fiber surface. A fiber-type hydrogen evolution reaction (HER) catalyst is suggested by stabilizing lamellar Pt nanopatterns on severely rough graphene fiber surfaces.

Janus Graphene Liquid Crystalline Fiber with Tunable Properties Enabled by Ultrafast Flash Reduction

Flash photothermal treatment via Xenon lamp with a broad wavelength spectrum can effectively remove oxygen functionalities and restore sp² domains at graphitic carbon materials. The chemical composition and relevant structure formation of flash reduced graphene oxide liquid crystal (GOLC) fibers are investigated in accordance with flash irradiation conditions. Owing to the spatial controllability of reduction level via anisotropic flash irradiation, the mechanical properties and electrical conductivity of graphene fibers can be delicately counterbalanced to attain desired properties. High sensitivity humidity sensors can be fabricated from the flash reduced fibers demonstrating notably higher sensitivity over the thermally reduced counterparts. This ultrafast flash reduction holds great promise for multidimensional macroscopic GO based structures, enabling a wide range of potential applications, including textile electronics and wearable sensors.

Mussel-Inspired Graphene Film with Enhanced Durability as a Macroscale Solid Lubricant

Graphene has exhibited massive potential as a macroscale solid lubricant, but its durability is limited due to the weak adhesion between graphene sheets and the substrate. Here, inspired by mussel adhesive protein (MAP), effective reinforcement of the graphene-substrate interaction to attain remarkable enhancement on the durability of the graphene film is presented. The mussel-inspired graphene (mGr) film exhibits a coefficient of friction stabilizing at 0.16 up to 490000 sliding cycles in the friction testing against the silicon nitride ball; in the identical sliding condition, comparatively, the graphene (Gr) film without MAP only lasts 4300 sliding cycles. The analysis of Raman and ATR-FTIR demonstrates that, on the one hand, the MAP film firmly adsorbs onto the substrate via forming metal-catechol coordination bonds with metal atoms; on the other hand, it establishes strong interactions with graphene sheets by hydrogen bonding as well as the π-π overlap. As an interlayer, MAP retains graphene sheets within the contact interface in the form of a compact tribo-layer, which results in an over 2 orders of magnitude enhancement of durability for the mGr film. This strategy of improving the graphene-substrate adhesion via MAP offers an avenue for the development of effective and reliable graphene-based solid lubricants for engineering applications.

Graphene Oxide Liquid Crystal Domains: Quantification and Role in Tailoring Viscoelastic Behavior

Graphene oxide liquid crystals (GOLCs) were exfoliated in a wide variety of solvents (water, ethylene glycol (EG), N-methyl-2-pyrrolidone (NMP), and dimethylformamide (DMF)) by high-speed shearing of graphite oxide. Quantitative polarized light imaging of the equilibrium nematic phases of the lyotropic GOLCs gives insights into the extent of aggregation and quantifiable textural features such as domain size, d. Large nematic domains >100 μm with a high overall degree of order were obtained in water and ethylene glycol, in contrast to ∼5-50 μm domains in NMP and DMF at comparable volume fractions. Comprehensive rheological studies of these GOLCs indicate that larger domains correlate with higher viscosity and higher elasticity, and scaling analysis shows a power-law dependence of the Ericksen number (Er) with domain size (Er ∝ d^3.09). The improved understanding of the relationship between the microstructure and flow properties of GOLCs leads us to an approach of mixed solvent-based GOLCs as a means to tune viscoelastic properties. We demonstrate this approach for the formation of shear-aligned GOLC films for advanced flexible electronic applications such as all-carbon conductive films and thermal heaters.

Perspectives in Liquid-Crystal-Aided Nanotechnology and Nanoscience

The research field of liquid crystals and their applications is recently changing from being largely focused on display applications and optical shutter elements in various fields, to quite novel and diverse applications in the area of nanotechnology and nanoscience. Functional nanoparticles have recently been used to a significant extent to modify the physical properties of liquid crystals by the addition of ferroelectric and magnetic particles of different shapes, such as arbitrary and spherical, rods, wires and discs. Also, particles influencing optical properties are increasingly popular, such as quantum dots, plasmonic, semiconductors and metamaterials. The self-organization of liquid crystals is exploited to order templates and orient nanoparticles. Similarly, nanoparticles such as rods, nanotubes and graphene oxide are shown to form lyotropic liquid crystal phases in the presence of isotropic host solvents. These effects lead to a wealth of novel applications, many of which will be reviewed in this publication.

Polymer nanocomposites having a high filler content: synthesis, structures, properties, and applications

The recent development of nanoscale fillers, such as carbon nanotubes, graphene, and nanocellulose, allows the functionality of polymer nanocomposites to be controlled and enhanced. However, conventional synthesis methods of polymer nanocomposites cannot maximise the reinforcement of these nanofillers at high filler content. Approaches for the synthesis of high content filler polymer nanocomposites are suggested to facilitate future applications. The fabrication methods address the design of the polymer nanocomposite architecture, which encompasses one, two, and three dimensional morphologies. Factors that hamper the reinforcement of nanostructures, such as alignment, dispersion of the filler and interfacial bonding between the filler and polymer, are outlined. Using suitable approaches, maximum potential reinforcement of nanoscale fillers can be anticipated without limitations in orientation, dispersion, and the integrity of the filler particle-matrix interface. High filler content polymer composites containing emerging materials such as 2D transition metal carbides, nitrides, and carbonitrides (MXenes) are expected in the future.

Ultratough Bioinspired Graphene Fiber via Sequential Toughening of Hydrogen and Ionic Bonding

Graphene-based fibers synthesized under ambient temperature have not achieved excellent mechanical properties of high toughness or tensile strength compared with those synthesized by hydrothermal strategy or graphitization and annealing treatment. Inspired by the relationship between organic/inorganic hierarchical structure, interfacial interactions, and moderate growth temperature of natural nacre, we fabricate an ultratough graphene fiber via sequential toughening of hydrogen and ionic bonding through a wet-spinning method under ambient temperature. A slight amount of chitosan is introduced to form hydrogen bonding with graphene oxide nanosheets, and the ionic bonding is formed between graphene oxide nanosheets and divalent calcium ions. The optimized sequential toughening of hydrogen and ionic bonding results in an ultratough graphene fiber with toughness of 26.3 MJ/m³ and ultimate tensile strength of 743.6 MPa. Meanwhile, the electrical conductivity of the resultant graphene fiber is as high as 179.0 S/cm. This kind of multifunctional graphene fiber shows promising applications in photovoltaic wires, flexible supercapacitor electrodes, wearable electronic textiles, fiber motors, etc. Furthermore, the strategy of sequential toughening of hydrogen and ionic bonding interactions also offers an avenue for constructing high-performance graphene-based fibers in the near future.