Strong and Tough MXene Bridging-induced Conductive Nacre.

Nacre is a classic model, providing an inspiration for fabricating high-performance bulk nanocomposites with the two-dimensional platelets. However, the "brick" of nacre, aragonite platelet, is an ideal building block for making high-performance bulk nanocomposites. Herein, we demonstrated a strong and tough conductive nacre through reassembling aragonite platelets with bridged by MXene nanosheets and hydrogen bonding, not only providing high mechanical properties but also excellent electrical conductivity. The flexural strength and fracture toughness of the obtained conductive nacre reach ~282 MPa and ~6.3 MPa m1/2, which is 1.6 and 1.6 times higher than that of natural nacre, respectively. These properties are attributed to densification and high orientation degree of the conductive nacre, which is effectively induced by the combined interactions of hydrogen bonding and MXene nanosheets bridging. The crack propagations in conductive nacre are effectively inhibited through crack deflection with hydrogen bonding, and MXene nanosheets bridging between aragonite platelets. In addition, our conductive nacre also provides a self-monitoring function for structural damage and offers exceptional electromagnetic interference shielding performance. Our strategy of reassembling the aragonite platelets exfoliated from waste nacre into high-performance artificial nacre, provides an avenue for fabricating high-performance bulk nanocomposites through the sustainable reutilization of shell resources.

Improving strength and toughness of graphene film through metal ion bridging.

The fangs, jaws, and mandibles of marine invertebrates such as Chiton and Glycera show excellent mechanical properties, which are mostly contributed to the interactions between metal (Fe, Cu, Zn, etc.) and oxygen-containing functional groups in proteins. Inspired by these load-bearing skeletal biomaterials, we improved tensile strength and toughness of graphene films through bridging graphene oxide (GO) nanosheets by metal ions. By optimizing the metal coordination form and density of cross-linking network. We revealed the relationship between mechanical properties and the unique spatial geometry of the GO nanosheets bridged by different valence metal ions. The results demonstrated that the divalent metal ions form tetrahedral geometry with carboxylate groups on the edges of the GO nanosheets, and the bond energy is relatively low, which is helpful for improving the toughness of resultant graphene films. While the trivalent metal ions are easily to form octahedral geometry with the GO nanosheets with higher bond energy, which is better for enhancing the tensile strength of graphene films. After reduction, the reduced GO (rGO) film bridged by divalent metal ions shows 43% improvement in toughness, while the rGO film bridged by trivalent metal ions shows 64% improvement in tensile strength. Our work reveals the mechanism of metal coordination bond energy and spatial geometry to improve the mechanical properties of graphene films, which lays a theoretical foundation for improving the tensile strength and toughness of resultant graphene films, and provides an avenue for fabricating high-performance graphene films and other two-dimensional nanocomposites.

Nonclassical crystallization of goethite nanorods in limpet teeth by self-assembly of silica-rich nanoparticles reveals structure-mechanical property relations.

The intricate organization of goethite nanorods within a silica-rich matrix makes limpet teeth the strongest known natural material. However, the mineralization pathway of goethite in organisms under ambient conditions remains elusive. Here, by investigating the multi-level structure of limpet teeth at different growth stages, it is revealed that the growth of goethite crystals proceeds by the attachment of amorphous nanoparticles, a nonclassical crystallization pathway widely observed during the formation of calcium-based biominerals. Importantly, these nanoparticles contain a high amount of silica, which is gradually expelled during the growth of goethite. Moreover, in mature teeth of limpet, the content of silica correlates with the size of goethite crystals, where smaller goethite crystals are densely packed in the leading part with higher content of silica. Correspondingly, the leading part exhibits higher hardness and elastic modulus. Thus, this study not only reveals the nonclassical crystallization pathway of goethite nanorods in limpet teeth, but also highlights the critical roles of silica in controlling the hierarchical structure and the mechanical properties of limpet teeth, thus providing inspirations for fabricating biomimetic materials with excellent properties.

Water-induced strong isotropic MXene-bridged graphene sheets for electrochemical energy storage.

Graphene and two-dimensional transition metal carbides and/or nitrides (MXenes) are important materials for making flexible energy storage devices because of their electrical and mechanical properties. It remains a challenge to assemble nanoplatelets of these materials at room temperature into in-plane isotropic, free-standing sheets. Using nanoconfined water-induced basal-plane alignment and covalent and π-π interplatelet bridging, we fabricated Ti3C2Tx MXene-bridged graphene sheets at room temperature with isotropic in-plane tensile strength of 1.87 gigapascals and moduli of 98.7 gigapascals. The in-plane room temperature electrical conductivity reached 1423 siemens per centimeter, and volumetric specific capacity reached 828 coulombs per cubic centimeter. This nanoconfined water-induced alignment likely provides an important approach for making other aligned macroscopic assemblies of two-dimensional nanoplatelets.

Sustainable liquid metal-induced conductive nacre.

Nacre has inspired research to fabricate tough bulk composites for practical applications using inorganic nanomaterials as building blocks. However, with the considerable pressure to reduce global carbon emissions, preparing nacre-inspired composites remains a significant challenge using more economical and environmentally friendly building blocks. Here we demonstrate tough and conductive nacre by assembling aragonite platelets exfoliated from natural nacre, with liquid metal and sodium alginate used as the "mortar". The formation of GaOC coordination bonding between the gallium ions and sodium alginate molecules reduces the voids and improves compactness. The resultant conductive nacre exhibits much higher mechanical properties than natural nacre. It also shows excellent impact resistance attributed to the synergistic strengthening and toughening fracture mechanisms induced by liquid metal and sodium alginate. Furthermore, our conductive nacre exhibits exceptional self-monitoring sensitivity for maintaining structural integrity. The proposed strategy provides a novel avenue for turning natural nacre into a valuable green composite.

Interlocking-Governed Ultra-Strong and Highly Conductive MXene Fibers Through Fluidics-Assisted Thermal Drawing.

High-performance MXene fibers are always of significant interest for flexible textile-based devices. However, achieving high mechanical property and electrical conductivity remains challenging due to the uncontrolled loose microstructures of MXene (Ti3 C2 Tx and Ti3 CNTx ) nanosheets. Herein, high-performance MXene fibers directly obtained through fluidics-assisted thermal drawing are demonstrated. Tablet interlocks are formed at the interface layer between the outer cyclic olefin copolymer and inner MXene nanosheets due to the thermal drawing induced stresses, resulting in thousands of meters long macroscopic compact MXene fibers with ultra-high tensile strength, toughness, and outstanding electrical conductivity. Further, large-scale woven textiles constructed by these fibers offer exceptional electromagnetic interference shielding performance with excellent durability and stability. Such an effective and sustainable approach can be applied to produce functional fibers for applications in both daily life and aerospace.

Tough and Conductive Nacre-inspired MXene/Epoxy Layered Bulk Nanocomposites.

A long-standing quest in materials science has been the development of tough epoxy resin nanocomposites for use in numerous applications. Inspired by nacre, here we report tough and conductive MXene/epoxy layered bulk nanocomposites. The orientation of MXene lamellar scaffolds is enhanced by annealing treatment. The improved interfacial interactions between MXene lamellar scaffold and epoxy through surface chemical modification resulted in a synergistic effect. Tailoring the interlayer spacing of MXene nanosheets to a critical distance resulted in a fracture toughness about eight times higher than that of pure epoxy, surpassing other epoxy nanocomposites. Our nacre-inspired MXene/epoxy layered bulk nanocomposites also show high electrical conductivity that provides self-monitoring capability for structural integrity and exhibits an excellent electromagnetic interference shielding efficiency. Our proposed strategy provides an avenue for fabricating high-performance epoxy nanocomposites.

Ginkgo seed shell provides a unique model for bioinspired design.

Natural structural materials typically feature complex hierarchical anisotropic architectures, resulting in excellent damage tolerance. Such highly anisotropic structures, however, also provide an easy path for crack propagation, often leading to catastrophic fracture as evidenced, for example, by wood splitting. Here, we describe the weakly anisotropic structure of Ginkgo biloba (ginkgo) seed shell, which has excellent crack resistance in different directions. Ginkgo seed shell is composed of tightly packed polygonal sclereids with cell walls in which the cellulose microfibrils are oriented in a helicoidal pattern. We found that the sclereids contain distinct pits, special fine tubes like a "screw fastener," that interlock the helicoidal cell walls together. As a result, ginkgo seed shell demonstrates crack resistance in all directions, exhibiting specific fracture toughness that can rival other highly anisotropic natural materials, such as wood, bone, insect cuticle, and nacre. In situ characterization reveals ginkgo's unique toughening mechanism: pit-guided crack propagation. This mechanism forces the crack to depart from the weak compound middle lamella and enter into the sclereid, where the helicoidal cell wall significantly inhibits crack growth by the cleavage and breakage of the fibril-based cell walls. Ginkgo's toughening mechanism could provide guidelines for a new bioinspired strategy for the design of high-performance bulk materials.

Ultrastrong MXene films via the synergy of intercalating small flakes and interfacial bridging.

Titanium carbide MXene combines high mechanical and electrical properties and low infrared emissivity, making it of interest for flexible electromagnetic interference (EMI) shielding and thermal camouflage film materials. Conventional wisdom holds that large MXene is the preferable building block to assemble high-performance films. However, the voids in the films comprising large MXene degrade their properties. Although traditional crosslinking strategies can diminish the voids, the electron transport between MXene flakes is usually disrupted by the insulating polymer bonding agents, reducing the electrical conductivity. Here we demonstrate a sequential densification strategy to synergistically remove the voids between MXene flakes while strengthening the interlayer electron transport. Small MXene flakes were first intercalated to fill the voids between multilayer large flakes, followed by interfacial bridging of calcium ions and borate ions to eliminate the remaining voids, including those between monolayer flakes. The obtained MXene films are compact and exhibit high tensile strength (739 MPa), Young's modulus (72.4 GPa), electrical conductivity (10,336 S cm-1), and EMI shielding capacity (71,801 dB cm2 g-1), as well as excellent oxidation resistance and thermal camouflage performance. The presented strategy provides an avenue for the high-performance assembly of other two-dimensional flakes.

MXene-Regulated Perovskite Vertical Growth for High-Performance Solar Cells.

Defects at the interfaces of perovskite (PVK) thin films are the main factors responsible for instability and low photoelectric conversion efficiency (PCE) of PVK solar cells (PSCs). Here, a SnO2 -MXene composite electron transport layer (ETL) is used in PSCs to improve interfacial contact and passivate defects at the SnO2 /perovskite interface. The introduced MXene regulates SnO2 dispersion and induces a vertical growth of PVK. The lattice matching of MXene and perovskite suppresses the concentration of interfacial stress, thereby obtaining a perovskite film with low defects. Compared with SnO2 -based device, the PCE of SnO2 -MXene-based device is improved by 15 % and its short-circuit current is up to 25.07 mA cm-2 . Furthermore, unencapsulated device maintained about 90 % of its initial efficiency even after 500 h of storage at 30-40 % relative humidity in ambient air. The composite ETL strategy provides a route to engineer interfacial passivation between metal halide perovskites and ETLs.

Ultra-compact MXene fibers by continuous and controllable synergy of interfacial interactions and thermal drawing-induced stresses.

Recent advances in MXene (Ti3C2Tx) fibers, prepared from electrically conductive and mechanically strong MXene nanosheets, address the increasing demand of emerging yet promising electrode materials for the development of textile-based devices and beyond. However, to reveal the full potential of MXene fibers, reaching a balance between electrical conductivity and mechanical property is still the fundamental challenge, mainly due to the difficulties to further compact the loose MXene nanosheets. In this work, we demonstrate a continuous and controllable route to fabricate ultra-compact MXene fibers with an in-situ generated protective layer via the synergy of interfacial interactions and thermal drawing-induced stresses. The resulting ultra-compact MXene fibers with high orientation and low porosity exhibit not only excellent tensile strength and ultra-high toughness, but also high electrical conductivity. Then, we construct meter-scale MXene textiles using these ultra-compact fibers to achieve high-performance electromagnetic interference shielding and personal thermal management, accompanied by the high mechanical durability and stability even after multiple washing cycles. The demonstrated generic strategy can be applied to a broad range of nanostructured materials to construct functional fibers for large-scale applications in both space and daily lives.

Large-Area Ultrastrong and Stiff Layered MXene Nanocomposites by Shear-Flow-Induced Alignment of Nanosheets.

To shield increasingly severe radiation pollution, ultrathin MXene-based electromagnetic interference (EMI) shielding materials with excellent mechanical properties are urgently demanded in wearable electrical devices or aerospace fields. However, it is still a challenge to fabricate ultrastrong and stiff MXene-based nanocomposites with excellent EMI shielding capacity in a universal and scalable manner. Here, inspired by the natural nacre structure, we propose an efficient superspreading strategy to construct a highly oriented layered "brick-and-mortar" structure using shear-flow-induced alignment of MXene nanosheets at an immiscible hydrogel/oil interface. A continuous and large-area MXene nanocomposite film has been fabricated through a homemade industrial-scale continuous fabrication setup. The prepared MXene nanocomposite films exhibit a tensile strength of 647.6 ± 56 MPa and a Young's modulus of 59.8 ± 6.1 GPa, respectively. These outstanding mechanical properties are attributed to the continuous interphase layer that formed between the well-aligned MXene nanosheets. Moreover, the obtained MXene nanocomposites also show great EMI shielding effectiveness (51.6 dB). We consider that our MXene-based nanocomposite films may be potentially applied as electrical or aerospace devices with superior mechanical properties and high EMI shielding capacity.

MXene based nanocomposite films.

Since the first report in 2011, two-dimensional transition metal carbide/nitride MXenes have aroused widespread attention owing to the particular structure and physiochemical properties. In the last few years, MXene-based nanocomposite films have been widely investigated, showing promising applications in many fields. However, poor mechanical properties and thermal/electrical conductivities of MXene-based nanocomposite films still limited their practical applications. Herein, we summarize the fabrication approach of MXene-based nanocomposite films and discuss the mechanical properties and other applications, including electromagnetic interference shielding, thermal conductivity, and supercapacitors. Then, several vital factors for fabricating high performance MXene based nanocomposite films have been refined. To further fabricate high performance MXene-based nanocomposite films, some effective sequential bridging strategies are also discussed. Lastly, a conclusion of the challenges and opportunities of MXene-based nanocomposite films is provided to facilitate their development and application for various purposes in the future of scientific research.

High-strength scalable MXene films through bridging-induced densification.

MXenes are a growing family of two-dimensional transition metal carbides and/or nitrides that are densely stacked into macroscopically layered films and have been considered for applications such as flexible electromagnetic interference (EMI) shielding materials. However, the mechanical and electrical reliabilities of titanium carbide MXene films are affected by voids in their structure. We applied sequential bridging of hydrogen and covalent bonding agents to induce the densification of MXene films and removal of the voids, leading to highly compact MXene films. The obtained MXene films show high tensile strength, in combination with high toughness, electrical conductivity, and EMI shielding capability. Our high-performance MXene films are scalable, providing an avenue for assembling other two-dimensional platelets into high-performance films.

Stiff and tough PDMS-MMT layered nanocomposites visualized by AIE luminogens.

Polydimethylsiloxane (PDMS) is a widely used soft material that exhibits excellent stability and transparency. But the difficulty of fine-tuning its Young's modulus and its low toughness significantly hinder its application in fields such as tissue engineering and flexible devices. Inspired by nacre, here we report on the development of PDMS-montmorillonite layered (PDMS-MMT-L) nanocomposites via the ice-templating technique, resulting in 23 and 12 times improvement in Young's modulus and toughness as compared with pure PDMS. Confocal fluorescence microscopy assisted by aggregation-induced emission (AIE) luminogens reveals three-dimensional reconstruction and in situ crack tracing of the nacre-inspired PDMS-MMT-L nanocomposite. The PDMS-MMT-L nanocomposite is toughened with mechanisms such as crack deflection and bridging. The AIE-assisted visualization of the crack propagation for nacre-inspired layered nanocomposites provides an advanced and universal characterization technique for organic-inorganic nanocomposites.

Bioinspired Color Switchable Photonic Crystal Silicone Elastomer Kirigami.

Bioinspired dynamic structural color has great potential for use in dynamic displays, sensors, cryptography, and camouflage. However, it is quite rare for artificial structural color devices to withstand thousands of cycles. Male hummingbird's crowns and gorgets are brightly colored, demonstrating frequent color switching that is induced by regulating the orientation of the feathers through movement of skin or joints. Inspired by this unique structural color modulation, we demonstrate a flexible, mechanically triggered color switchable sheet based on a photonic crystal (PhC)-coated polydimethylsiloxane (PDMS) kirigami (PhC-PDMS kirigami) made by laser cutting. Finite element modeling (FEM) simulation reveals that the thickness of PDMS kirigami and the chamfer at the incision induced by laser cutting both dominate the out-of-plane deformation through in-plane stretching. The bioinspired PhC-PDMS kirigami shows precisely programmable structural color and keeps the color very well after recycling over 10 000 times. This bioinspired PhC-PDMS kirigami also shows excellent viewability even in bright sunlight, high readability, robust functionality, technical flexibility, and mechanical durability, which are readily exploitable for applications, such as chromic mechanical monitors for the sports industry or for medical applications, wearable camouflage, and security systems.

Chemical Strategies for Making Strong Graphene Materials.

Graphene materials have been widely applied in various fields because of their remarkable mechanical and electrical properties. However, two obstacles arise during the assembly of graphene platelets into macroscale graphene materials and composites that impair the performance of the resultant graphene materials: 1) the voids between the graphene platelets, and 2) the wrinkling of the graphene platelets. In the past decade, several strategies have been developed to eliminate these obstacles. These strategies result in strong macroscale graphene materials, such as graphene fibers with tensile strengths of over 3.4 GPa and sheets with tensile strengths of over 1.5 GPa, which have many practical applications. This Minireview summarizes the effective strategies for assembling graphene materials and compares their advantages and drawbacks. The preparation processes as well as the resulting fundamental mechanical properties and wide spectrum of electrical and magnetic properties are also discussed. Finally, our outlook for the future of this field is presented.

High-strength scalable graphene sheets by freezing stretch-induced alignment.

ABSTRACCT: Efforts to obtain high-strength graphene sheets by near-room-temperature assembly have been frustrated by the misalignment of graphene layers, which degrades mechanical properties. While in-plane stretching can decrease this misalignment, it reappears when releasing the stretch. Here we use covalent and π-π inter-platelet bridging to permanently freeze stretch-induced alignment of graphene sheets, and thereby increase isotropic in-plane sheet strength to 1.55 GPa, in combination with a high Young's modulus, electrical conductivity and weight-normalized shielding efficiency. Moreover, the stretch-bridged graphene sheets are scalable and can be easily bonded together using a commercial resin without appreciably decreasing the performance, which establishes the potential for practical applications.

Moiré-Potential-Induced Band Structure Engineering in Graphene and Silicene.

A moiré pattern results from the projection of one periodic pattern to another with relative lattice constant or misalignment and provides great periodic potential to modify the electronic properties of pristine materials. In this Review, recent research on the effect of the moiré superlattice on the electronic structures of graphene and silicene, both of which possess a honeycomb lattice, is focused on. The moiré periodic potential is introduced by the interlayer interaction to realize abundant phenomena, including new generation of Dirac cones, emergence of Van Hove singularities (vHs) at the cross point of two sets of Dirac cones, Mott-like insulating behavior at half-filling state, unconventional superconductivity, and electronic Kagome lattice and flat band with nontrivial edge state. The role of interlayer coupling strength, which is determined by twist angle and buckling degree, in these exotic properties is discussed in terms of both the theoretical prediction and experimental measurement, and finally, the challenges and outlook for this field are discussed.

Near-Infrared-Driven Photocatalysts: Design, Construction, and Applications.

Photocatalysts, which utilize solar energy to catalyze the oxidation or reduction half reactions, have attracted tremendous interest due to their great potential in addressing increasingly severe global energy and environmental issues. Solar energy utilization plays an important role in determining photocatalytic efficiencies. In the past few decades, many studies have been done to promote photocatalytic efficiencies via extending the absorption of solar energy into near-infrared (NIR) light. This Review comprehensively summarizes the recent progress in NIR-driven photocatalysts, including the strategies to harvest NIR photons and corresponding photocatalytic applications such as the degradation of organic pollutants, water disinfection, water splitting for H2 and O2 evolution, CO2 reduction, etc. The application of NIR-active photocatalysts employed as electrocatalysts is also presented. The subject matter of this Review is designed to present the relationship between material structure and material optical properties as well as the advantage of material modification in photocatalytic reactions. It paves the way for future material design in solar energy-related fields and other energy conversion and storage fields.