Scalable Preparation of Multifunctional Fire-Retardant Ultralight Graphene Foams

Highly Aligned Graphene Aerogels for Multifunctional Composites

Stemming from the unique in-plane honeycomb lattice structure and the sp2 hybridized carbon atoms bonded by exceptionally strong carbon-carbon bonds, graphene exhibits remarkable anisotropic electrical, mechanical, and thermal properties. To maximize the utilization of graphene's in-plane properties, pre-constructed and aligned structures, such as oriented aerogels, films, and fibers, have been designed. The unique combination of aligned structure, high surface area, excellent electrical conductivity, mechanical stability, thermal conductivity, and porous nature of highly aligned graphene aerogels allows for tailored and enhanced performance in specific directions, enabling advancements in diverse fields. This review provides a comprehensive overview of recent advances in highly aligned graphene aerogels and their composites. It highlights the fabrication methods of aligned graphene aerogels and the optimization of alignment which can be estimated both qualitatively and quantitatively. The oriented scaffolds endow graphene aerogels and their composites with anisotropic properties, showing enhanced electrical, mechanical, and thermal properties along the alignment at the sacrifice of the perpendicular direction. This review showcases remarkable properties and applications of aligned graphene aerogels and their composites, such as their suitability for electronics, environmental applications, thermal management, and energy storage. Challenges and potential opportunities are proposed to offer new insights into prospects of this material.

A Superior Two-Dimensional Phosphorus Flame Retardant: Few-Layer Black Phosphorus

The usage of flame retardants in flammable polymers has been an effective way to protect both lives and material goods from accidental fires. Phosphorus flame retardants have the potential to be follow-on flame retardants after halogenated variants, because of their low toxicity, high efficiency and compatibility. Recently, the emerging allotrope of phosphorus, two-dimensional black phosphorus, as a flame retardant has been developed. To further understand its performance in flame-retardant efficiency among phosphorus flame retardants, in this work, we built model materials to compare the flame-retardant performances of few-layer black phosphorus, red phosphorus nanoparticles, and triphenyl phosphate as flame-retardant additives in cellulose and polyacrylonitrile. Aside from the superior flame retardancy in polyacrylonitrile, few-layer black phosphorus in cellulose showed the superior flame-retardant efficiency in self-extinguishing, ~1.8 and ~4.4 times that of red phosphorus nanoparticles and triphenyl phosphate with similar lateral size and mass load (2.5~4.8 wt%), respectively. The char layer in cellulose coated with the few-layer black phosphorus after combustion was more continuous and smoother than that with red phosphorus nanoparticles, triphenyl phosphate and blank, and the amount of residues of cellulose coated with the few-layer black phosphorus in thermogravimetric analysis were 10 wt%, 14 wt% and 14 wt% more than that with red phosphorus nanoparticles, triphenyl phosphate and blank, respectively. In addition, although exothermic reactions, the combustion enthalpy changes in the few-layer black phosphorus (-127.1 kJ mol^-1) are one third of that of red phosphorus nanoparticles (-381.3 kJ mol^-1). Based on a joint thermodynamic, spectroscopic, and microscopic analysis, the superior flame retardancy of the few-layer black phosphorus was attributed to superior combustion reaction suppression from the two-dimensional structure and thermal nature of the few-layer black phosphorus.

High-Temperature Fire Resistance and Self-Extinguishing Behavior of Cellular Graphene

The ability to protect materials from fire is vital to many industrial applications and life safety systems. Although various chemical treatments and protective coatings have proven effective as flame retardants, they provide only temporary prevention, as they do not change the inherent flammability of a given material. In this study, we demonstrate that a simple change of the microstructure can significantly boost the fire resistance of an atomically thin material well above its oxidation stability temperature. We show that free-standing graphene layers arranged in a three-dimensional (3D) cellular network exhibit completely different flammability and combustion rates from a graphene layer placed on a substrate. Covalently cross-linked cellular graphene aerogels can resist flames in air up to 1500 °C for a minute without degrading their structure or properties. In contrast, graphene on a substrate ignites immediately above 550 °C and burns down in a few seconds. Raman spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric studies reveal that the exceptional fire-retardant and self-extinguishing properties of cellular graphene originate from the ability to prevent carbonyl defect formation and capture nonflammable carbon dioxide gas in the pores. Our findings provide important information for understanding graphene's fire-retardant mechanism in 3D structures/assemblies, which can be used to enhance flame resistance of carbon-based materials, prevent fires, and limit fire damage.

Bioengineering Tools Applied to Dentistry: Validation Methods for In Vitro and In Silico Analysis

This study aimed to evaluate the use of bioengineering tools, finite element analysis, strain gauge analysis, photoelastic analysis, and digital image correlation, in computational studies with greater validity and reproducibility. A bibliographic search was performed in the main health databases PUBMED and Scholar Google, in which different studies, among them, laboratory studies, case reports, systematic reviews, and literature reviews, which were developed in living individuals, were included. Therefore, articles that did not deal with the use of finite element analysis, strain gauge analysis, photoelastic analysis, and digital image correlation were excluded, as well as their use in computational studies with greater validity and reproducibility. According to the methodological analysis, it is observed that the average publication of articles in the Pubmed database was 2.03 and with a standard deviation of 1.89. While in Google Scholar, the average was 0.78 and the standard deviation was 0.90. Thus, it is possible to verify that there was a significant variation in the number of articles in the two databases. Modern dentistry finds in finite element analysis, strain gauge, photoelastic and digital image correlation a way to analyze the biomechanical behavior in dental materials to obtain results that assist to obtain rehabilitations with favorable prognosis and patient satisfaction.

Fire Intumescent, High-Temperature Resistant, Mechanically Flexible Graphene Oxide Network for Exceptional Fire Shielding and Ultra-Fast Fire Warning

Smart fire alarm sensor (FAS) materials with mechanically robust, excellent flame retardancy as well as ultra-sensitive temperature-responsive capability are highly attractive platforms for fire safety application. However, most reported FAS materials can hardly provide sensitive, continuous and reliable alarm signal output due to their undesirable temperature-responsive, flame-resistant and mechanical performances. To overcome these hurdles, herein, we utilize the multi-amino molecule, named HCPA, that can serve as triple-roles including cross-linker, fire retardant and reducing agent for decorating graphene oxide (GO) sheets and obtaining the GO/HCPA hybrid networks. Benefiting from the formation of multi-interactions in hybrid network, the optimized GO/HCPA network exhibits significant increment in mechanical strength, e.g., tensile strength and toughness increase of ~ 2.3 and ~ 5.7 times, respectively, compared to the control one. More importantly, based on P and N doping and promoting thermal reduction effect on GO network, the excellent flame retardancy (withstanding ~ 1200 °C flame attack), ultra-fast fire alarm response time (~ 0.6 s) and ultra-long alarming period (> 600 s) are obtained, representing the best comprehensive performance of GO-based FAS counterparts. Furthermore, based on GO/HCPA network, the fireproof coating is constructed and applied in polymer foam and exhibited exceptional fire shielding performance. This work provides a new idea for designing and fabricating desirable FAS materials and fireproof coatings.

Graphene-Based Scaffolds: Fundamentals and Applications for Cardiovascular Tissue Engineering

Cardiac tissue engineering requires materials that can faithfully recapitulate and support the native in vivo microenvironment while providing a seamless bioelectronic interface. Current limitations of cell scaffolds include the lack of electrical conductivity and suboptimal mechanical properties. Here we discuss how the incorporation of graphene into cellular scaffolds, either alone or in combination with other materials, can affect morphology, function, and maturation of cardiac cells. We conclude that graphene-based scaffolds hold great promise for cardiac tissue engineering.

Nanocarbon-Based Flame Retardant Polymer Nanocomposites

In recent years, nanocarbon materials have attracted the interest of researchers due to their excellent properties. Nanocarbon-based flame retardant polymer composites have enhanced thermal stability and mechanical properties compared with traditional flame retardant composites. In this article, the unique structural features of nanocarbon-based materials and their use in flame retardant polymeric materials are initially introduced. Afterwards, the flame retardant mechanism of nanocarbon materials is described. The main discussions include material components such as graphene, carbon nanotubes, fullerene (in preparing resins), elastomers, plastics, foams, fabrics, and film-matrix materials. Furthermore, the flame retardant properties of carbon nanomaterials and their modified products are summarized. Carbon nanomaterials not only play the role of a flame retardant in composites, but also play an important role in many aspects such as mechanical reinforcement. Finally, the opportunities and challenges for future development of carbon nanomaterials in flame-retardant polymeric materials are briefly discussed.

A direct foaming approach for carbon nanotube aerogels with ultra-low thermal conductivity and high mechanical stability

Thermally insulating materials (TIMs) with ultra-low thermal conductivity, fire-retardancy, and mechanical stability are demanded to improve energy efficiency in many fields, such as petrochemical plants, energy-saving buildings, and aerospace. However, traditional polymer-based TIMs could not meet these demands. Herein, we propose a direct foaming strategy for obtaining carbon nanotube (CNT) aerogels by the gradual expansion of CNT films with H2O2 as a foaming agent at room temperature. The obtained CNT aerogels have hierarchical cellular structures and possess an ultra-low density (4.6 mg cm-3) and thermal conductivity (16.5 mW m-1 K-1) as well as excellent mechanical robustness and fire-resistance. Our results show that such CNT aerogels have promising applications in the field of thermal insulation and present a facile pathway for the design of thermally insulating, fire-retardant materials based on CNTs.

Ultralight GO-Hybridized CNTs Aerogels with Enhanced Electronic and Mechanical Properties for Piezoresistive Sensors

Extremely low density carbon nanotubes/graphene hybrid aerogels (CNG) are highly potential active materials for fabricating flexible devices, owing to synergistic effects with one (nanotubes) and two (graphene) dimensional characters in a single structure. However, conquering the long-standing dilemma among low electronic conductivity and inferior mechanical properties for CNG remains a challenging task. Here, an ultralight CNG aerogel (1.52 mg cm^-3) with prominent electronic conductivity and mechanical resilience is facilely fabricated through a triple roles design of the sodium dodecyl sulfate (SDS), namely anchoring metal ions, dispersing carbon nanotubes, and inducing self-assembly. It is demonstrated that the Ba²⁺ can be effectively anchored into the GO interlayers by coupling it with the SDS to reinforce the intersheet interactions, thereby achieving remarkable improvement in mechanical properties (Young's moduli up to 18.3 kPa). Density functional theory calculations reveal that the anchored Ba²⁺ acting as molecular bridges can availably reduce the tunneling barrier between the GO sheets and facilitate the multidirectional and fast transport of electronics, inducing the high electrical conductivity of CNG (12.55 S cm^-1). Taking advantage of these features, potential applications in flexible sensing devices have been demonstrated utilizing the remarkable CNG as an active material, giving extraordinary sensing performance including high sensitivity (48.6 kPa^-1), ultralow detection limit (10 Pa), and ultrafast response (18 ms).

Polymer Composites Filled with Metal Derivatives: A Review of Flame Retardants

Polymer composites filled with metal derivatives have been widely used in recent years, particularly as flame retardants, due to their superior characteristics, including high thermal behavior, low environmental degradation, and good fire resistance. The hybridization of metal and polymer composites produces various favorable properties, making them ideal materials for various advanced applications. The fire resistance performance of polymer composites can be enhanced by increasing the combustion capability of composite materials through the inclusion of metallic fireproof materials to protect the composites. The final properties of the metal-filled thermoplastic composites depend on several factors, including pore shape and distribution and morphology of metal particles. For example, fire safety equipment uses polyester thermoplastic and antimony sources with halogenated additives. The use of metals as additives in composites has captured the attention of researchers worldwide due to safety concern in consideration of people's life and public properties. This review establishes the state-of-art flame resistance properties of metals/polymer composites for numerous industrial applications.

Stable electrically conductive, highly flame-retardant foam composites generated from reduced graphene oxide and silicone resin coatings

To acheive flexible polyurethane (PU) foam composites with stable electrical conductivity and high flame retardancy involved first coating of graphene oxide (GO) onto PU foam surfaces and then chemically reducing the GO with hydrazine to form reduced GO (RGO). The RGO-coated PU foam is then dipped into a solution containing silicone resin (SiR) and silica nano-particles and cured. The resulting composites (PU-RGO-SiR) show superior flame retardancy, thermal stability and mechanical stability relative to the PU starting materials or PU coated with either RGO or SiR alone. The electrical conductivity of the PU-RGO-SiR composites (as high as 118 S m^-1 at room temperature) could almost be retained but with small loss of 9.5% of the original value after 150 cyclic compression. When the samples were subjected to a temperature range from -50 to 400 °C, the electrical conductivity could remain constant at -50 °C, 25 °C, 100 °C, 200 °C, and even at 300 °C and 400 °C; the electrical-conductivity exhibited mild vibration but the vibration range was not beyond 5.6%. Flame retardancy tests show that the limiting oxygen index (LOI) increases from 14.7% for the pure foam to 31.5% for PU-RGO-SiR, and the PU-RGO-SiR composites exhibit a 65% reduction in the peak heat release rate (pHRR) and a 30% reduction in total smoke release (TSR). Thus, stable electrically conductive and highly flame-retardant foam composites have potential applications even in a variety of harsh conditions like high temperature, flame, organic solvents, and external compression.

Highly Compressible Polymer Composite Foams with Thermal Heating-Boosted Electromagnetic Wave Absorption Abilities

Polymer composite foams are desirable materials for electromagnetic (EM) energy attenuation. However, a number of challenges limit improvement in the EM energy attenuation properties of foams. In this study, a simple microcellular injection molding method was used to fabricate highly compressible thermoplastic urethane (TPU)/carbon nanotube (CNTs) composite foams, which also had increased conductivity with an increase in CNT content. Compared to unfoamed composites, foamed composites exhibited higher conductivity and EM attenuation properties because of the presence of a microcellular structure. Moreover, the TPU/CNT foam with 4 wt % CNTs (F(4)) demonstrated strong EM dissipation and an optimal reflection loss (RL) value of -30.4 dB. Furthermore, stimulated by thermal heating and cyclic compression, EM attenuation was observed to increase because of the higher conductivity. Note that F(4) foam having a small thickness of 1.3 mm when treated at 333 K had the highest EM dissipation and the lowest RL value of -51.8 dB. Enhanced polarization and ohmic losses and multiscattering were responsible for the increased EM absorption. This behavior is attributed to the movement of CNTs within the TPU elastomer walls via thermal or compression stimulation. For designing stimulation-dependent multifunctional materials, composite foams with response to thermal heating were proved to be an alternative approach.

Advances of 3D graphene and its composites in the field of microwave absorption

The intensive progress of information technology increases the demand for urgent development of practical materials for microwave absorption (MA), meeting the general requirement "thin, wide, light and strong". In the past 6 years, graphene is of great interest for MA performance due to its unique properties such as high specific surface area, high electrical conductivity, strong dielectric loss, and low density. Taking in account that the structure of absorber plays a key role in MA performance, the attempts to produce an efficient microwave absorbing materials (MAMs) have led to 3D graphene - aerogels and foams - due to their extremely high porosity, large specific surface area, excellent mechanical properties with ability of compression and further maintaining the original shape, lightweight, reduced agglomeration of graphene sheets. All listed parameters enhance the impedance matching of MAMs, generate the synergistic loss effects, thereby improving the MA properties. The review describes the bases of MA theory and summarizes the recent achievements in the fabrication of pure 3D graphene networks and their composites with magnetic, ceramic nanoparticles and nanowires, polymers, MXenes, and multicomponent systems, directed to improve the impedance matching and generate loss mechanisms for the overall improvement of their performance as MAMs.

Nitrogen-rich graphitic-carbon@graphene as a metal-free electrocatalyst for oxygen reduction reaction

The metal-free nitrogen-doped graphitic-carbon@graphene (Ng-C@G) is prepared from a composite of polyaniline and graphene by a facile polymerization following by pyrolysis for electrochemical oxygen reduction reaction (ORR). Pyrolysis creates a sponge-like with ant-cave-architecture in the polyaniline derived nitrogenous graphitic-carbon on graphene. The nitrogenous carbon is highly graphitized and most of the nitrogen atoms are in graphitic and pyridinic forms with less oxygenated is found when pyrolyzed at 800 °C. The electrocatalytic activity of Ng-C@G-800 is even better than the benchmarked Pt/C catalyst resulting in the higher half-wave potential (8 mV) and limiting current density (0.74 mA cm^-2) for ORR in alkaline medium. Higher catalytic performance is originated from the special porous structure at microscale level and the abundant graphitic- and pyridinic-N active sites at the nanoscale level on carbon-graphene matrix which are beneficial to the high O₂-mass transportation to those accessible sites. Also, it possesses a higher cycle stability resulting in the negligible potential shift and slight oxidation of pyridinic-N with better tolerance to the methanol.

Lightweight carbon-red mud hybrid foam toward fire-resistant and efficient shield against electromagnetic interference

Lightweight, porous, high-performance electromagnetic interference (EMI) shielding and fire-resistant materials are highly demanded in aerospace and defense applications. Due to the lightweight, open porosity and high surface area, carbon foam has been considered as one of the most promising candidates for EMI shielding applications. In the present investigation, we demonstrate the development of novel carbon-red mud hybrid foams with excellent EMI shielding effectiveness (SE). The carbon-red mud hybrid foams are prepared using phenolic resin as a carbon source and red mud (industrial waste) as filler. We observed that the inclusion of red mud in carbon-red mud hybrid foams significantly enhances their dielectric, magnetic, EMI shielding and thermal properties. The EMI shielding results show that absorption is the main contributor to the total EMI SE. The maximum total EMI shielding effectiveness is achieved to be 51.4 dB in the frequency range of 8.2–12.4 GHz for carbon-red mud hybrid foam having 20 wt. % of red mud. The CF-RM20 also showed excellent fire resistance and high thermal stability at elevated temperatures.

Retarding Ostwald Ripening to Directly Cast 3D Porous Graphene Oxide Bulks at Open Ambient Conditions

Graphene aerogels (GAs) with attractive properties have shown tremendous potentials in energy- and environment-related applications. Unfortunately, current assembly methods for GAs such as sol-gel and freeze-casting processes must be conducted in enclosed spaces with unconventional conditions, thus being literally inoperative for in situ and continuous productions. Herein, a direct slurry-casting method at open ambient conditions is established to arbitrarily prepare three-dimensional (3D) porous graphene oxide (GO) bulks without macroscopic dimension limits on a wide range of solid surfaces by retarding Ostwald ripening of 3D liquid GO foams when being dried in air. A subsequent fast thermal reduction (FTR) of GO foams leads to the formation of graphene aerogels (denoted as FTR-GAs) with hierarchical closed-cellular graphene structures. The FTR-GAs show outstanding high-temperature thermal insulation (70% decrease for 400 °C), as well as superelasticity (>1000 compression-recovery cycles at 50% strain), ultralow density (10-28 mg cm^-3), large specific surface area (BET, 206.8 m² g^-1), and high conductivity (ca. 100 S m^-1). This work provides a viable method to achieve in situ preparations of high-performance GAs as multifunctional structural materials in aircrafts, high-speed trains, or even buildings for the targets of energy efficiency, comfort, and safety.

High-Performance, Long-Life, Rechargeable Li-CO2 Batteries based on a 3D Holey Graphene Cathode Implanted with Single Iron Atoms

A highly efficient cathode catalyst for rechargeable Li-CO₂ batteries is successfully synthesized by implanting single iron atoms into 3D porous carbon architectures, consisting of interconnected N,S-codoped holey graphene (HG) sheets. The unique porous 3D hierarchical architecture of the catalyst with a large surface area and sufficient space within the interconnected HG framework can not only facilitate electron transport and CO₂ /Li⁺ diffusion, but also allow for a high uptake of Li₂ CO₃ to ensure a high capacity. Consequently, the resultant rechargeable Li-CO₂ batteries exhibit a low potential gap of ≈1.17 V at 100 mA g^-1 and can be repeatedly charged and discharged for over 200 cycles with a cut-off capacity of 1000 mAh g^-1 at a high current density of 1 A g^-1 . Density functional theory calculations are performed and the observed appealing catalytic performance is correlated with the hierarchical structure of the carbon catalyst. This work provides an effective approach to the development of highly efficient cathode catalysts for metal-CO₂ batteries and beyond.

Flexible PEBAX/graphene electromagnetic shielding composite films with a negative pressure effect of resistance for pressure sensors applications

In the current work, we fabricated flexible poly(ether-block-amide) (PEBAX)/graphene composite films by a combination of facile melt blending and compression molding technique. The graphene content significantly affects the mechanical properties, electrical conductivity and electromagnetic interference (EMI) shielding performance. An electrically conductive percolation threshold of 1.75 vol% graphene was obtained in the PEBAX/graphene composites. With the introduction of 4.45 vol%, and 8.91 vol% graphene content, the average EMI SE of composite films could reach 16.6 and 30.7 dB, respectively. More interestingly, the PEBAX/graphene composite exhibited a nearly-linear negative pressure coefficient (NPC) effect of resistance with increasing outer pressure stimulation, which was attributed to the formation of more conductive pathways caused by the decreased distance between adjacent graphene. In addition, these composites demonstrated good sensing stability, recoverability and reproducibility after stabilization by cyclic pressure loading. The current study provides guidelines for the large-scale preparation of elastomer NPC sensors and smart EMI shielding devices.

Graphene/PEBAX composite films present high-efficiency EMI shielding properties and good sensitivity as well as sensing stability.

Elastic Aerogels of Cellulose Nanofibers@Metal-Organic Frameworks for Thermal Insulation and Fire Retardancy

Metal-organic frameworks (MOFs) with high microporosity and relatively high thermal stability are potential thermal insulation and flame-retardant materials. However, the difficulties in processing and shaping MOFs have largely hampered their applications in these areas. This study outlines the fabrication of hybrid CNF@MOF aerogels by a stepwise assembly approach involving the coating and cross-linking of cellulose nanofibers (CNFs) with continuous nanolayers of MOFs. The cross-linking gives the aerogels high mechanical strength but superelasticity (80% maximum recoverable strain, high specific compression modulus of ~ 200 MPa cm³ g^-1, and specific stress of ~ 100 MPa cm³ g^-1). The resultant lightweight aerogels have a cellular network structure and hierarchical porosity, which render the aerogels with relatively low thermal conductivity of ~ 40 mW m^-1 K^-1. The hydrophobic, thermally stable MOF nanolayers wrapped around the CNFs result in good moisture resistance and fire retardancy. This study demonstrates that MOFs can be used as efficient thermal insulation and flame-retardant materials. It presents a pathway for the design of thermally insulating, superelastic fire-retardant nanocomposites based on MOFs and nanocellulose.

Nanoreinforcements of Two-Dimensional Nanomaterials for Flame Retardant Polymeric Composites: An Overview

Polymer materials are ubiquitous in daily life. While polymers are often convenient and helpful, their properties often obscure the fire hazards they may pose. Therefore, it is of great significance in terms of safety to study the flame retardant properties of polymers while still maintaining their optimal performance. Current literature shows that although traditional flame retardants can satisfy the requirements of polymer flame retardancy, due to increases in product requirements in industry, including requirements for durability, mechanical properties, and environmental friendliness, it is imperative to develop a new generation of flame retardants. In recent years, the preparation of modified two-dimensional nanomaterials as flame retardants has attracted wide attention in the field. Due to their unique layered structures, two-dimensional nanomaterials can generally improve the mechanical properties of polymers via uniform dispersion, and they can form effective physical barriers in a matrix to improve the thermal stability of polymers. For polymer applications in specialized fields, different two-dimensional nanomaterials have potential conductivity, high thermal conductivity, catalytic activity, and antiultraviolet abilities, which can meet the flame retardant requirements of polymers and allow their use in specific applications. In this review, the current research status of two-dimensional nanomaterials as flame retardants is discussed, as well as a mechanism of how they can be applied for reducing the flammability of polymers.

Improvement of Catalytic Activity of Platinum Nanoparticles Decorated Carbon Graphene Composite on Oxygen Electroreduction for Fuel Cells

High-performance platinum (Pt)-based catalyst development is crucially important for reducing high overpotential of sluggish oxygen reduction reaction (ORR) at Pt-based electrocatalysts, although the high cost and scarcity in nature of Pt are profoundly hampering the practical use of it in fuel cells. Thus, the enhancing activity of Pt-based electrocatalysts with minimal Pt-loading through alloy, core−shell or composite making has been implemented. This article deals with enhancing electrocatalytic activity on ORR of commercially available platinum/carbon (Pt/C) with graphene sheets through a simple composite making. The Pt/C with graphene sheets composite materials (denoted as Pt/Cx:G10−x) have been characterized by several instrumental measurements. It shows that the Pt nanoparticles (NPs) from the Pt/C have been transferred towards the π-conjugated systems of the graphene sheets with better monolayer dispersion. The optimized Pt/C8:G2 composite has higher specific surface area and better degree of graphitization with better dispersion of NPs. As a result, it shows not only stable electrochemical surface area but also enhanced ORR catalytic activity in respect to the onset potential, mass activity and electron transfer kinetics. As shown by the ORR, the Pt/C8:G2 composite is also better resistive to the alcohol crossover effect and more durable than the Pt/C.

Light-Responsive Actuators Based on Graphene

As a typical 2D carbon material, graphene, that possesses outstanding physical/chemical properties, has revealed great potential for developing soft actuators. Especially, the unique properties of graphene, including the excellent light absorption property, softness, and thermal conductivity, play very important roles in the development of light-responsive graphene actuators. At present, various light-driven actuators have been successfully developed based on graphene and its derivatives. In this mini review, we reviewed the recent advances in this field. The unique properties of graphene or graphene-related materials that are of benefit to the development of light-driven actuators have been summarized. Typical smart actuators based on different photothermal/photochemical effects, including photothermal expansion, photothermal desorption, photoisomerization, and photo-triggered shape memory effect, have been introduced. Besides, current challenges, and future perspective have been discussed. The rapid progress of light-responsive actuators based on graphene has greatly stimulated the development of graphene-based soft robotics.

Large-area superelastic graphene aerogels based on a room-temperature reduction self-assembly strategy for sensing and particulate matter (PM2.5 and PM10) capture

Graphene aerogels are emerging low density and superelasticity macroscopic porous materials with various applications. However, it still remains a challenge to develop a versatile strategy under ambient conditions for fabricating large-area, high-performance graphene aerogels, which is crucial for their practical applications. Here, we report a novel room-temperature reduction self-assembly (RTRS) strategy to fabricate large-area graphene aerogels under ambient conditions. The strategy is based on using unique hydrazine hydrates as reducing agents to generate stable microbubbles beneficial for the formation of macroporous graphene hydrogels. Interestingly, the resultant hydrogel followed by a simple pre-freeze treatment can be naturally dried into graphene aerogels without noticeable volume shrinkage or structure cracking. Benefiting from the mild conditions, a large-area graphene aerogel with a diameter of up to 27 cm was prepared as an example. The as-formed aerogels exhibit a stable honeycomb-like coarse-pores structure, a low density of 3.6 mg cm-3 and superelasticity (rapidly recoverable from 95% compression) which are suitable for pressure/strain sensors. Moreover, the aerogel exhibits superior particulate matter adsorption efficiency (PM2.5: 93.7%, PM10: 96.2%) and good recycling ability. Importantly, the preparation process is cost-effective and easily scalable without the need for any special drying techniques and heating processes, which provides an ideal platform for mass production of graphene aerogels toward practical applications.

Lightweight and flexible hybrid film based on delicate design of electrospun nanofibers for high-performance electromagnetic interference shielding

High-performance electromagnetic interference (EMI) shielding materials possess features of light weight, flexibility and excellent EMI shielding effectiveness. However, continuous efforts are still needed to satisfy the urgent demand for electromagnetic pollution shielding. In this study, a lightweight and flexible hybrid film with a multi-scale double-continuous conductive network (TiO2/SiO2@PPy) and sandwich structure (TiO2/SiO2@PPy@rGO) was prepared via a delicate structure design of electrospun TiO2/SiO2 nanofibers. The hybrid film worked as an effective dissipative medium, leading to a high EMI shielding effectiveness of approximately 30 dB in the X band (8-12 GHz) and excellent specific EMI shielding effectiveness (SE) of ∼13 829 dB cm2 g-1. The hybrid film has a tensile strength of 2.71 MPa, while its density is only 0.089 g cm-3. The hybrid films maintained good electrical and EMI shielding properties after repeated bending, indicating their favorable flexibility. The delicate structure-design strategy of the electrospun nanofibers presents a practicable way to prepare lightweight and flexible hybrid films for high-performance EMI shielding materials in flexible electronics, military and healthcare applications.

Dual Functionalities of Few-Layered Boron Nitrides in the Design and Implementation of Ca(OH)2 Nanomaterials toward an Efficient Wall Painting Fireproofing and Consolidation

Preserving ancient wall paintings from damage has become a challenge over the years. Nanosized calcium hydroxide (Ca(OH)₂) has been identified as a promising material to preserve wall paintings. However, the synthesis of nanosized Ca(OH)₂ is extremely difficult. Here, we demonstrate a breakthrough in wall painting protection enabled by boron nitride nanosheets (BNNSs) through strategic synthesis Ca(OH)₂-BNNS nanohybrids using an aqueous method. The BNNS have two significant functionalities in the design and implementation of the Ca(OH)₂ nanomaterials. First, the introduction of BNNS results in the successful synthesis of uniform and nanosized Ca(OH)₂ (∼80 nm) in the nanohybrids, which can be attributed to the supersaturation-induced "etching-stripping" mechanism. More interestingly and importantly, a unique gradient penetration structure is strategically formed when applying Ca(OH)₂-BNNS hybrids on the wall paintings, i.e., the BNNS-rich layer will be at the surface of wall painting, whereas Ca(OH)₂ nanomaterials prefer to penetrate deep in to the wall paintings. This gradient structure will allow the BNNS-rich layer to protect the wall paintings from fire, which is the first report to date among the protection materials for wall paintings; at the same time, nanosized Ca(OH)₂ shows superior wall painting consolidation strength compared to commercial Ca(OH)₂ material. These results endow new applications of the newly emerging two-dimensional nanomaterials for protecting cultural heritage.

Size-Dependent Free Vibration and Buckling of Three-Dimensional Graphene Foam Microshells Based on Modified Couple Stress Theory

In this research, the vibration and buckling of three-dimensional graphene foam (3D-GrF) microshells are investigated for the first time. In the microshells, three-dimensional graphene foams can distribute uniformly or non-uniformly through the thickness direction. Based on Love's thin shell theory and the modified couple stress theory (MCST), size-dependent governing equations and corresponding boundary conditions are established through Hamilton's principle. Then, vibration and axial buckling of 3D-GrF microshells are analyzed by employing the Navier method and Galerkin method. Results show that the graphene foam distribution type, size effect, the foam coefficient, the radius-to-thickness ratio, and the length-to-radius ratio play important roles in the mechanical characteristics of 3D-GrF microshells.

Interfacial polarizations induced by incorporating traditional perovskites into reduced graphene oxide (RGO) for strong microwave response

The as-prepared La_0.7Sr_0.3MnO₃/RGO nanocomposites were synthesized via a simple hydrothermal method to provide excellent microwave absorbing performance resulting from good electrical conductivity and high impedance matching.

A review on graphene-based nanocomposites for electrochemical and fluorescent biosensors

Biosensors with high sensitivity, selectivity and a low limit of detection, reaching nano/picomolar concentrations of biomolecules, are important to the medical sciences and healthcare industry for evaluating physiological and metabolic parameters.

Three-Dimensional Graphene Foams: Synthesis, Properties, Biocompatibility, Biodegradability, and Applications in Tissue Engineering

Presently, clinical nanomedicine and nanobiotechnology have impressively demanded the generation of new organic/inorganic analogues of graphene (as one of the intriguing biomedical research targets) for stem-cell-based tissue engineering. Among different shapes of graphene, three-dimensional (3D) graphene foams (GFs) are highly promising candidates to provide conditions for mimicking in vivo environments, affording effective cell attachment, proliferation,and differentiation due to their unique properties. These include the highest biocompatibility among nanostructures, high surface-to-volume ratio, 3D porous structure (to provide a homogeneous/isotropic growth of tissues), highly favorable mechanical characteristics, and rapid mass and electron transport kinetics (which are required for chemical/physical stimulation of differentiated cells). This review aims to describe recent and rapid advances in the fabrication of 3D GFs, together with their use in tissue engineering and regenerative nanomedicine applications. Moreover, we have summarized a broad range of recent studies about the behaviors, biocompatibility/toxicity,and biodegradability of these materials, both in vitro and in vivo. Finally, the highlights and challenges of these 3D porous structures, compared to the current polymeric scaffold competitors, are discussed.

Reconstruction of Inherent Graphene Oxide Liquid Crystals for Large-Scale Fabrication of Structure-Intact Graphene Aerogel Bulk toward Practical Applications

The inherently formed liquid crystals (LCs) of graphene oxide (GO) in aqueous dispersions severely restrict the fabrication of large-size and structure-intact graphene aerogel bulk by an industry-applicable method. Herein, by developing a surfactant-foaming sol-gel method to effectively disrupt and reconstruct the inherent GO LCs via microbubbles as templates, we achieve the large-size and structure-intact graphene hydrogel bulk (GHB). After simple freezing and air-drying, the resulting graphene aerogel bulk (GAB) with a structure-intact size of about 1 m² exhibits a superelasticity of up to 99% compressive strain, ultralow density of 2.8 mg cm^-3, and quick solar-thermal conversion ability. The modified GAB (GABTP) shows a high decomposition temperature ( T_max) of 735 °C in air and a low heat storage capacity. These excellent performances make the GABs suitable for many practical applications, as proven in this work, including as high compressive force absorbers, high absorption materials for oils or dangerous solvents, superior solar-thermal management materials for rapid heater or controlled shelter, and high-efficiency fire-resistant and thermal insulation materials. The whole preparation process is easily scalable and cost-effective for mass production of structure-intact multifunctional graphene aerogel bulk toward practical applications.

A Microstructured Graphene/Poly(N-isopropylacrylamide) Membrane for Intelligent Solar Water Evaporation

Intelligent solar water evaporation (iSWE) was achieved with a thermally responsive and microstructured graphene/poly(N-isopropylacrylamide) (mG/PNIPAm) membrane. As the solar intensity varies, the water evaporation is tuned through reversible transformations of microstructures reminiscent of the stomatal opening and closing of leaves. Consequently, this mG/PNIPAm membrane displays a high water evaporation rate change (ΔWER) of 1.66 kg m^-2  h^-1 under weak sunlight (intensity<1 sun) and a low ΔWER of 0.24 kg m^-2  h^-1 under intense sunlight (1 sun<intensity<2 sun). Because of the double-layer structure with predictable shape and dynamics, the leaf-like membrane can further autonomously modulate the water evaporation by self-curling under intense solar irradiation in accordance with simulation results. This mG/PNIPAm membrane provides a smart material platform with self-adaptability in response to changing environments.

Highly Compressible, Anisotropic Aerogel with Aligned Cellulose Nanofibers

Aerogels can be used in a broad range of applications such as bioscaffolds, energy storage devices, sensors, pollutant treatment, and thermal insulating materials due to their excellent properties including large surface area, low density, low thermal conductivity, and high porosity. Here we report a facile and effective top-down approach to fabricate an anisotropic wood aerogel directly from natural wood by a simple chemical treatment. The wood aerogel has a layered structure with anisotropic structural properties due to the destruction of cell walls by the removal of lignin and hemicellulose. The layered structure results in the anisotropic wood aerogel having good mechanical compressibility and fragility resistance, demonstrated by a high reversible compression of 60% and stress retention of ∼90% after 10 000 compression cycles. Moreover, the anisotropic structure of the wood aerogel with curved layers stacking layer-by-layer and aligned cellulose nanofibers inside each individual layer enables the wood aerogel to have an anisotropic thermal conductivity with an anisotropy factor of ∼4.3. An extremely low thermal conductivity of 0.028 W/m·K perpendicular to the cellulose alignment direction and a thermal conductivity of 0.12 W/m·K along the cellulose alignment direction can be achieved. The thermal conductivity is not only much lower than that of the natural wood material (by ∼3.6 times) but also lower than most of the commercial thermal insulation materials. The top-down approach is low-cost, scalable, simple, yet effective, representing a promising direction for the fabrication of high-quality aerogel materials.

Coupling interconnected MoO3/WO3 nanosheets with a graphene framework as a highly efficient anode for lithium-ion batteries

A rationally assembled three-dimensional graphene framework coupled with interconnected molybdenum/tungsten oxide nanosheets (MoO₃/WO₃-GF) has been developed via a one-step template-free strategy. With the unique nanostructure, the obtained anode material not only exhibits a high reversible capacity of about 1000 mA h g^-1, approaching the theoretical capacity of MoO₃ and WO₃ materials, but also shows excellent rate capability and cycling performance with negligible capacity attenuation after a long-time test. These features make it a promising candidate material for high-performance commercial lithium-ion batteries in the future.

Green Approach to Improving the Strength and Flame Retardancy of Poly(vinyl alcohol)/Clay Aerogels: Incorporating Biobased Gelatin

Biobased gelatins were used to improve the compressive properties and flammability of poly(vinyl alcohol)/montmorillonite (PVA/MMT) aerogels, fabricated using a simple and environmentally friendly freeze-drying method. Because of the excellent compatibility and strong interfacial adhesion between PVA and gelatin, the compressive moduli of aerogels were enhanced dramatically with the incorporation of gelatin. PVA/MMT/porcine-gelatin aerogels exhibit compressive modulus values as much as 12.4 MPa, nearly 300% that of the control PVA/MMT aerogel. The microstructure of the PVA/MMT/gelatin aerogels shows a three-dimensional co-continuous network. Combustion testing demonstrated that with the addition of gelatin, the self-extinguishing time of the aerogel was cut by half and the limiting-oxygen-index values increased to 28.5%. The peak heat-release rate, obtained from cone calorimetry, also decreased with the incorporation of gelatin. Thermogravimetric analysis demonstrated that the gelatins slowed the sharp decomposition of the PVA matrix polymer and increased the thermal stability of the aerogels at the major decomposition stage of the composite aerogels. These results indicate that as a green, biobased material, gelatin could simultaneously improve the mechanical properties and the properties of flame retardancy.

Deep Eutectic Solvent Functionalized Graphene Composite as an Extremely High Potency Flame Retardant

We report a simple and green approach to develop the deep eutectic solvent functionalized graphene derivative as an effective flame retardant. The deep eutectic solvent functionalized graphene oxide (DESGO) was synthesized by introducing nitrogen-supported phosphorus functional groups on the surface of graphene derivative via a deep eutectic solvent, which is prepared by the treatment of monosodium dihydrogen orthophosphate and choline chloride. Subsequently, the resultant DESGO material is characterized by X-ray photoelectron spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, and scanning electron microscopy. The as prepared DESGO solution coated cloth piece was sustaining its initial shape and size by releasing a little amount of smoke at the early stage without catching fire for more than 540 s (9 min), whereas the pristine cloth is totally burned out within 10 s, leaving small amounts of black mass. This simple method of directly functionalized deep eutectic solvent on a graphene oxide surface can be a common process for the cost-effective bulk production of a nano carbon template for extremely high potency, nontoxic flame retardant applications.

Lightweight and Ultrastrong Polymer Foams with Unusually Superior Flame Retardancy

High-performance flame-retardant materials are urgently needed to address outstanding issues that pertain to safety. Traditional flame retardants are toxic to the environment and/or lack the physical properties required for use in many contemporary applications. Here, we show that isocyanate-based polyimide (PI) foam, a flammable material, can exhibit unusually superior flame retardancy as well as other excellent properties, such as being lightweight and displaying high mechanical strength, by incorporating red phosphorus (RP)-hybridized graphene. The covalent bonds formed between the graphene platelets and the PI matrix provide the resultant PI foam with a specific Young's modulus (83 kNm kg^-1) that is comparable to or even higher than those displayed by state-of-the-art foams, including silica aerogels, polystyrene foams, and polyurethane foams. In addition, even a low content of the RP-hybridized graphene (2.2 wt %) results in an exceptionally higher limiting oxygen index (39.4) than those of traditional flame-retardant polymer-based materials (typically 20-30). The resultant PI foam also exhibits thermal insulation properties that are similar to that of air. Moreover, the RP-hybridized graphene is prepared using a one-step ball milling process in 100% yield, and does not require solvent or produce waste. The preparation of the flame-retardant PI foams can be scaled as the starting materials are commercially available and the techniques employed are industrially compatible.

Built Structure of Ordered Vertically Aligned Codoped Carbon Nanowire Arrays for Supercapacitors

We report an ingenious yet efficient method to fabricate ordered vertically aligned nitrogen- and sulfur-codoped carbon nanowire (NS-CNW) arrays by direct carbonization of the finely designed copolymer. The as-prepared vertically aligned NS-CNWs with unique electronic features and very narrow diameters facilitate ion diffusion to further exhibit ideal electrochemical properties (243.0 F g^-1 at the current density of 0.1 A g^-1) and excellent cycle stability (10 000 cycles) when applied to a supercapacitor electrode. The controllable design and copolymerization of conducting polymers, which can provide doped carbon nanowire array electrodes having high surface area with controllable components and uniform dimensions in a neat way, provide more flexibility to tailor the carbon-based electrodes toward specific applications.

Three-Dimensional Printed Graphene Foams

An automated metal powder three-dimensional (3D) printing method for in situ synthesis of free-standing 3D graphene foams (GFs) was successfully modeled by manually placing a mixture of Ni and sucrose onto a platform and then using a commercial CO₂ laser to convert the Ni/sucrose mixture into 3D GFs. The sucrose acted as the solid carbon source for graphene, and the sintered Ni metal acted as the catalyst and template for graphene growth. This simple and efficient method combines powder metallurgy templating with 3D printing techniques and enables direct in situ 3D printing of GFs with no high-temperature furnace or lengthy growth process required. The 3D printed GFs show high-porosity (∼99.3%), low-density (∼0.015g cm^-3), high-quality, and multilayered graphene features. The GFs have an electrical conductivity of ∼8.7 S cm^-1, a remarkable storage modulus of ∼11 kPa, and a high damping capacity of ∼0.06. These excellent physical properties of 3D printed GFs indicate potential applications in fields requiring rapid design and manufacturing of 3D carbon materials, for example, energy storage devices, damping materials, and sound absorption.

Core-Shell Structured Polyamide 66 Nanofibers with Enhanced Flame Retardancy

We report the preparation of polymer nanofibers with enhanced flame retardancy by coaxial electrospinning polyamide 66 (PA 66) and nanoscale graphene hybridized with red phosphorus (NG-RP). Transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed that the nanofibers contained a NG-RP-based core surrounded by a PA 66 shell. The flame-retardant characteristics of the nanofibers were investigated by thermal gravimetric analysis, micro combustion calorimetry, and a series of vertical flame tests. The encapsulation of the NG-RP not only enhanced the flame-retardant characteristics of the nanofibers, but also improved their mechanical properties while maintaining the color and luster of the polymer, making the resultant nanofibers appropriate for use in a wide range of applications.

Vertically Aligned Graphene Sheets Membrane for Highly Efficient Solar Thermal Generation of Clean Water

Efficient utilization of solar energy for clean water is an attractive, renewable, and environment friendly way to solve the long-standing water crisis. For this task, we prepared the long-range vertically aligned graphene sheets membrane (VA-GSM) as the highly efficient solar thermal converter for generation of clean water. The VA-GSM was prepared by the antifreeze-assisted freezing technique we developed, which possessed the run-through channels facilitating the water transport, high light absorption capacity for excellent photothermal transduction, and the extraordinary stability in rigorous conditions. As a result, VA-GSM has achieved average water evaporation rates of 1.62 and 6.25 kg m^-2 h^-1 under 1 and 4 sun illumination with a superb solar thermal conversion efficiency of up to 86.5% and 94.2%, respectively, better than that of most carbon materials reported previously, which can efficiently produce the clean water from seawater, common wastewater, and even concentrated acid and/or alkali solutions.