Macroscopic Free-Standing Hierarchical 3D Architectures Assembled from Silver Nanowires by Ice Templating

Fabrication of Au aerogels with {110}-rich facets by size-dependent surface reconstruction for enzyme-free glucose detection

Au^6–50 aerogels with {110}-rich facets (35.5%) are fabricated for glucose detection by size-dependent surface reconstruction between two differently-sized gold nanoparticles.

Ultralight Cellulose Porous Composites with Manipulated Porous Structure and Carbon Nanotube Distribution for Promising Electromagnetic Interference Shielding

Lightweight conductive polymer composites based on biomass could be a promising candidate for electromagnetic interference (EMI) shielding application. Herein, tailoring porous microstructure and regulating the distribution of carbon nanotubes (CNTs) in cellulose composites are attempts to achieve highly efficient EMI shielding properties accompanying desired mechanical property and low density. Specifically, aligned porous structure is fabricated by ice-template freeze-drying method; meanwhile, CNT is regulated to decorate inside the cellulose matrix (CNT-matrix/cellulose porous composites) or to directly bind over the cellulose cell walls (CNT-interface/cellulose porous composites). It is found that, owing to the preferential distribution of CNT on the cell walls, the CNT-interface/cellulose porous composites possess a very high electrical conductivity of 38.9 S m^-1 with an extremely low percolation threshold of 0.0083 vol % with regard to CNT-matrix/cellulose porous composites. Therefore, a shielding effectiveness of 40 dB with merely 0.51 vol % CNT under a thickness of 2.5 mm is achieved in CNT-interface/cellulose porous composites, which is attributed to efficient multiple reflections and the accompanying absorption with promoted conductivity and better-defined porous structure. More laudably, the CNT-interface/cellulose porous composites reveal a superior mechanical property with a specific modulus of 279 MPa g^-1 cm³. The value behind the current work is to pave an effective way to fabricate environmentally benign, high-performance EMI shielding materials to practically boost numerous advanced applications of cellulose.

A Hierarchical Silver-Nanowire-Graphene Host Enabling Ultrahigh Rates and Superior Long-Term Cycling of Lithium-Metal Composite Anodes

Metallic lithium (Li) is a promising anode for next-generation high-energy-density batteries, but its applications are still hampered due to the limited charging/discharging rate and poor cycling performance. Here, a hierarchical 3D porous architecture is designed with a binary network of continuous silver nanowires assembled on an interconnected 3D graphene skeleton as the host for Li-metal composite anodes, which offers a significant boost in both charging/discharging rates and long-term cycling performance for Li-metal batteries. This unique hierarchical binary network structure in conjunction with optimized material combination provides ultrafast, continuous, and smooth electron transportation channel and non-nucleation barrier sites to direct and confine Li deposition. It also offers outstanding mechanical strength and toughness to support massive Li deposition and buffer the internal stress fluctuations during long-term repeated Li stripping/plating thereby minimizing fundamental issues of dendrite formation and volume change even under ultrafast charging/discharging rates. As a result, the composite anode using this hierarchical host can work smoothly at an unprecedented high current density of 40 mA cm^-2 over 1000 plating/stripping cycles with low overpotential (<120 mV) in symmetric cells. The as-constructed full cell, paired with LiNi_0.5 Co_0.2 Mn_0.3 O₂ cathode, also exhibits excellent rate capability and high-rate cycling stability.

Nanowire Assemblies for Flexible Electronic Devices: Recent Advances and Perspectives

The fabrication of nanowire (NW)-based flexible electronics including wearable energy storage devices, flexible displays, electrical sensors, and health monitors has received great attention both in fundamental research and market requirements in our daily lives. Other than a disordered state after synthesis, NWs with designed and hierarchical structures would not only optimize the intrinsic performance, but also create new physical and chemical properties, and integration of individual NWs into well-defined structures over large areas is one of the most promising strategies to optimize the performance of NW-based flexible electronics. Here, the recent developments and achievements made in the field of flexible electronics composed of integrated NW structures are presented. The different assembly strategies for the construction of 1D, 2D, and 3D NW assemblies, especially the NW coassembly process for 2D NW assemblies, are comprehensively discussed. The improvements of different NW assemblies on flexible electronics structure and performance are described in detail to elucidate the advantages of well-defined NW assemblies. Finally, a short summary and outlook for future challenges and perspectives in this field are presented.

Visible Light-Responsive Photocatalytic Activity of Boron Nitride Incorporated Composites

Photocatalysts are essential to promote the highly efficient applications of solar energy in water splitting and/or the degradation of organic contaminations. Especially, the visible light-responsive photocatalysts could benefit with the cost-effective splitting or degradation due to the unlimited sunlight and the absence of expensive light emitter. In the photocatalysts, the charge transfer rates as well as the hole-electron recombination rate are two critical factors that determine the photocatalytic activity, which could also be affected by the dimension, defects, doping and morphologies controlled by the synthesis methods. Boron nitride (BN) is an ultrawide-bandgap semiconductor, and the combination of BN with the visible light-responsive photocatalysts has been found to be effective in enhancing the photocatalytic activities. Therefore, it should be meaningful to understand the BN incorporated photocatalytic composites in depth, including the synthetic approaches, the activity improving mechanisms and the versatile applications. In this review, we mainly focused on the assembly method of BN incorporated photocatalysts; the activity enhancing mechanism by introducing the BN in the photocatalytic composites as well as the properties and the applications. In the end, we gave a conclusion and an outlook for the BN incorporated photocatalytic composites.

Biomimetic twisted plywood structural materials

Biomimetic designs based on micro/nanoscale manipulation and scalable fabrication are expected to develop new-style strong, tough structural materials. Although the mimicking of nacre-like ‘brick-and-mortar’ structure is well studied, many highly ordered natural architectures comprising 1D micro/nanoscale building blocks still elude imitation owing to the scarcity of efficient manipulation techniques for micro/nanostructural control in practical bulk counterparts. Herein, inspired by natural twisted plywood structures with fascinating damage tolerance, biomimetic bulk materials that closely resemble natural hierarchical structures and toughening mechanisms are successfully fabricated through a programmed and scalable bottom-up assembly strategy. By accurately engineering the arrangement of 1D mineral micro/nanofibers in biopolymer matrix on the multiscale, the resultant composites display optimal mechanical performance, superior to many natural, biomimetic and engineering materials. The design strategy allows for precise micro/nanostructural control at the macroscopic 3D level and can be easily extended to other materials systems, opening up an avenue for many more micro/nanofiber-based biomimetic designs.

Self-healing and superstretchable conductors from hierarchical nanowire assemblies

It is still a great challenge to improve deformability and fatigue-resistance of stretchable conductors when maintaining their high-level conductivity for practical use. Herein, a high-performance stretchable conductor with hierarchically ternary network and self-healing capability is demonstrated through in situ polymerizing N-isopropylacrylamide (NIPAM) on well-defined sulfur-containing molecule-modified Ag nanowire (AgNW) aerogel framework. Owing to hierarchical architecture from nanoscale to microscale and further to macroscale and strong interactions of polymer chains and AgNWs, the composite exhibits good conductivity of 93 S cm⁻¹, excellent electromechanical stability up to superhigh tensile strain of 800% and strong fatigue-resistant ability through well accommodating the applied deformations and sharing external force in the network. Furthermore, the composite delivers a fast and strong healing capability induced by reversible Ag–S bonds, which enables the healed conductor to hold an impressive electromechanical property. These prominent demonstrations confirm this material as top performer for use as flexible, stretchable electronic devices.

Stretchable conductors are important for further developments in the electronics industry, but improving the deformability when maintaining the high-level conductivity is still challenging. Here the authors demonstrate a ternary self-healing silver nanowire/polymer network as high-performance stretchable conductor.

Ice-Templated Bimodal-Porous Silver Nanowire/PDMS Nanocomposites for Stretchable Conductor

A three-dimensional (3D) bimodal-porous silver nanowire (AgNW) nanostructure with superior electrical properties is fabricated by freeze drying of AgNW aqueous dispersion with macrosized ice spheres for bimodal-porous structure. The ice sphere dispersed AgNW solution yields a 3D AgNW network at the surface of ice sphere and formation of macropores by removal of ice sphere during freeze-drying process. The resulting nanostructures exhibit excellent electrical properties due to their low electrical percolation threshold by the formation of macropores, which results in an efficient and dense 3D AgNW network with a small amount of AgNWs. The highly conductive and stretchable AgNW/poly(dimethylsiloxane) (PDMS) nanocomposites are made by impregnating the 3D porous conductive network with highly stretchable poly(dimethylsiloxane) (PDMS) matrix. The AgNW/PDMS nanocomposites exhibit a high conductivity of 42 S/cm with addition of relatively small amount of 2 wt %. The high conductivity is retained when stretched up to 120% elongation even after 100 stretching-releasing cycles. Due to high electrical conductivity and superior stretchability of AgNW/PDMS nanocomposites, these are expected to be used in stretchable electronic devices.

Synergetic Improvement in Thermal Conductivity and Flame Retardancy of Epoxy/Silver Nanowires Composites by Incorporating "Branch-Like" Flame-Retardant Functionalized Graphene

The significant fire hazards on the polymer-based thermal interface materials (TIM) used in electronic devices are but often neglected. Also, high filler loading with the incident deterioration of mechanical, thermal, and processing properties limits the further application of the traditional polymer-based TIMs. In this work, a ternary TIMs with epoxy resin (EP) matrix, silver nanowires (AgNWs), and a small amount of flame-retardant functionalized graphene (GP-DOPO) were proposed to address the above questions. Briefly, a facile "branch-like" strategy with a polymer as the backbone and flame-retardant molecule as the branch was first used to functionalize reduced graphene oxide (RGO) toward increasing the flame-retardant grafting ratio and RGO's compatibility in matrix, and the resulted GP-DOPO was then in situ introduced into the EP/AgNW composites. As expected, the incorporation of GP-DOPO (2 wt %) can increase the thermal conductivity to 1.413 W/(m K) at a very low AgNW loading (4 vol %), which is 545 and 56% increments compared to pure EP and EP/AgNW, respectively. The prominent improvement in thermal conductivity was put down to the synergetic effect of AgNW and GP-DOPO, i.e., the improving dispersion and bridging effect for AgNWs by adding GP-DOPO. Moreover, the high flame-retardant grafting amount and the excellent compatibility of GP-DOPO resulted in a strong catalytic charring effect on EP matrix, which further formed a robust protective char layer by combining the AgNW and graphene network. Therefore, the flame retardancy of EP/AgNW was significantly improved by introducing GP-DOPO, i.e., the peak heat release rate, total heat release and total smoke production reduced by 27.0, 32.4, and 30.9% reduction compared to EP/AgNW, respectively.

Printable Metal-Polymer Conductors for Highly Stretchable Bio-Devices

Stretchable, biocompatible devices can bridge electronics and biology. However, most stretchable conductors for such devices are toxic, costly, and regularly break/degrade after several large deformations. Here we show printable, highly stretchable, and biocompatible metal-polymer conductors by casting and peeling off polymers from patterned liquid metal particles, forming surface-embedded metal in polymeric hosts. Our printable conductors present good stretchability (2,316 S/cm at a strain of 500%) and repeatability (ΔR/R <3% after 10,000 cycles), which can satisfy most electrical applications in extreme deformations. This strategy not only overcomes large surface tension of liquid metal but also avoids the undesirable sintering of its particles by stress in deformations, such that stretchable conductors can form on various substrates with high resolution (15 μm), high throughput (∼2,000 samples/hour), and low cost (one-quarter price of silver). We use these conductors for stretchable circuits, motion sensors, wearable glove keyboards, and electroporation of live cells.

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A straightforward method for sintering liquid metal particles with high efficiency

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Liquid metal can be patterned on different substrates with high resolution

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Low-cost, high-throughput, stretchable printed conductors can be fabricated

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Conductors are biocompatible and have potentials in implantable electronics

Nanoscaled gold and silver: Simultaneous removal and transformation to functional materials

Based on an "acid-assisted cool welding" technology which was realized by virtue of the freezing process in the presence of acid, nanoscaled gold (Au) and silver (Ag) from wastewater could be removed very facilely and efficiently. The technology was independence of the freezing temperature, size as well as shape of those nanoscaled units. Besides, some functional materials like porous nanostructures with highly and stably catalytic activity could be also obtained during the removal. Our research not only provided a new method to remove nanoscaled Ag or Au from wastewater, but also built up a unique route to transform those nano-units into functional materials simultaneously.

Electrical and thermal properties of silver nanowire fabricated on a flexible substrate by two-beam laser direct writing for designing a thermometer

Accurate knowledge of electrical conductivity and thermal conductivity temperature dependence plays a crucial role in the design of a thermometer. Here, by using a two-beam laser direct writing system, an individual silver nanowire (AgNW) with well-defined dimensions is fabricated on a polyethylene terephthalate (PET) substrate. The temperature dependence of the resistivity of the fabricated AgNW is measured ranging from 10 to 300 K, and fitted with the Bloch–Grüneisen formula. The residual resistivity ((1.62 ± 0.20) × 10⁻⁷ Ω m) of the AgNW is larger than that of the bulk material (1 × 10⁻¹¹ Ω m). The electron–phonon coupling constant of the AgNW is (1.08 ± 0.13) × 10⁻⁷ Ω m, which is larger than that of the bulk silver (5.24 × 10⁻⁸ Ω m). Moreover, the Debye temperature of the AgNW is 199.30 K and is lower than that of the bulk silver (235 K). The Lorenz number of the fabricated AgNW is found to decrease as the temperature increases. Besides, the Lorenz number (2.66 × 10⁻⁷ W Ω K⁻²) is larger than the Sommerfeld value (2.44 × 10⁻⁸ W Ω K⁻²) at room temperature. The measurement results allow one to design a thermometer in the temperature range 40–300 K. The flexibility of the AgNW is also excellent, and the resistance increase of the AgNW is only 2.58% when the AgNW bending about 1000 times with a bending radius of 1 mm.

Investigation of temperature dependence of electrical resistivity, thermal conductivity and Lorenz number of silver nanowire, and design of a thermometer.

Size Controllable, Transparent, and Flexible 2D Silver Meshes Using Recrystallized Ice Crystals as Templates

Ice templates have been widely utilized for the preparation of porous materials due to the obvious advantages, such as environmentally benign and applicable to a wide range of materials. However, it remains a challenge to have controlled pore size as well as dimension of the prepared porous materials with the conventional ice template, since it often employs the kinetically not-stable growing ice crystals as the template. For example, there is no report so far for the preparation of 2D metal meshes with tunable pore size based on the ice template, although facile and eco-friendly prepared metal meshes are highly desirable for wearable electronics. Here, we report the preparation of 2D silver meshes with tunable mesh size employing recrystallized ice crystals as templates. Ice recrystallization is a kinetically stable process; therefore, the grain size of recrystallized ice crystals can be easily tuned, e.g., by adding different salts and changing the annealing temperature. Consequently, the size and line width of silver meshes obtained after freeze-drying can be easily adjusted, which in turn varied the conductivity of the obtained 2D silver film. Moreover, the silver meshes are transparent and display stable conductivity after the repeated stretching and bending. It can be envisioned that this approach for the preparation of 2D conducting films is of practical importance for wearable electronics. Moreover, this study provides a generic approach for the fabrication of 2D meshes with a controllable pore size.

3D Cocontinuous Composites of Hydrophilic and Hydrophobic Soft Materials: High Modulus and Fast Actuation Time

Hydrogels in nature seldom form a single phase, more often forming structured phases with other soft phases, allowing nature to develop responsive and adaptive strategies. Based on knowledge of how hydrogels are utilized in nature, we developed novel 3D cocontinuous composites from soft materials with extremely different properties, a hydrogel and a silicone. These were successfully prepared by infiltrating liquid polydimethylsiloxane (PDMS) into poly(N-isopropylacrylamide) (PNIPAm) frameworks of aligned pores prepared by directional melt crystallization. The composites had outstanding modulus and swelling ratio compared to other mechanically strong hydrogels. More interestingly, the deswelling kinetics were dramatically accelerated (by a factor of 1000), possibly due to the aligned microchannels and the hydrophobic nature of PDMS. As a result, an actuator movement mimicking flowering could be completed in less than 20 s. This novel and versatile cocontinuous composite strategy could overcome the current limitations of soft materials.

3D Printing Hierarchical Silver Nanowire Aerogel with Highly Compressive Resilience and Tensile Elongation through Tunable Poisson's Ratio

Metallic aerogels have attracted intense attention due to their superior properties, such as high electrical conductivity, ultralow densities, and large specific surface area. The preparation of metal aerogels with high efficiency and controllability remains challenge. A 3D freeze assembling printing technique integrated with drop-on-demand inkjet printing and freeze casting are proposed for metallic aerogels preparation. This technique enables tailoring both the macrostructure and microstructure of silver nanowire aerogels (SNWAs) by integrating programmable 3D printing and freeze casting, respectively. The density of the printed SNWAs is controllable, which can be down to 1.3 mg cm^-3 . The ultralight SNWAs reach high electrical conductivity of 1.3 S cm^-1 and exhibit excellent compressive resilience under 50% compressive strain. Remarkably, the printing methodology also enables tuning aerogel architectures with designed Poisson's ratio (from negative to positive). Moreover, these aerogel architechtures with tunable Poisson's ratio present highly electromechanical stability under high compressive and tensile strain (both strain up to 20% with fully recovery).

Microstructure Design of Lightweight, Flexible, and High Electromagnetic Shielding Porous Multiwalled Carbon Nanotube/Polymer Composites

Multiwalled carbon nanotube/polymer composites with aligned and isotropic micropores are constructed by a facile ice-templated freeze-drying method in a wide density range, with controllable types and contents of the nanoscale building blocks, in order to tune the shielding performance together with the considerable mechanical and electrical properties. Under the mutual promotion of the frame and porous structure, the lightweight high-performance shielding is achieved: a 2.3 mm thick sample can reach 46.7 and 21.7 dB in the microwave X-band while the density is merely 32.3 and 9.0 mg cm^-3 , respectively. The lowest density corresponds to a value of shielding effectiveness divided by both the density and thickness up to 10⁴ dB cm² g^-1 , far beyond the conductive polymer composites with other fillers ever reported. The shielding mechanism of the flexible porous materials is further demonstrated by an in situ compression experiment.

Bacteria as Bio-Template for 3D Carbon Nanotube Architectures

It is one of the most important needs to develop renewable, scalable and multifunctional methods for the fabrication of 3D carbon architectures. Even though a lot of methods have been developed to create porous and mechanically stable 3D scaffolds, the fabrication and control over the synthesis of such architectures still remain a challenge. Here, we used Magnetospirillum magneticum (AMB-1) bacteria as a bio-template to fabricate light-weight 3D solid structure of carbon nanotubes (CNTs) with interconnected porosity. The resulting porous scaffold showed good mechanical stability and large surface area because of the excellent pore interconnection and high porosity. Steered molecular dynamics simulations were used to quantify the interactions between nanotubes and AMB-1 via the cell surface protein MSP-1 and flagellin. The 3D CNTs-AMB1 nanocomposite scaffold is further demonstrated as a potential substrate for electrodes in supercapacitor applications.

Ultralight Multifunctional Carbon-Based Aerogels by Combining Graphene Oxide and Bacterial Cellulose

Nanostructured carbon aerogels with outstanding physicochemical properties have exhibited great application potentials in widespread fields and therefore attracted extensive attentions recently. It is still a challenge so far to develop flexible and economical routes to fabricate high-performance nanocarbon aerogels, preferably based on renewable resources. Here, ultralight and multifunctional reduced graphene oxide/carbon nanofiber (RGO/CNF) aerogels are fabricated from graphene oxide and low-cost, industrially produced bacterial cellulose by a three-step process of freeze-casting, freeze-drying, and pyrolysis. The prepared RGO/CNF aerogel possesses a very low apparent density in the range of 0.7-10.2 mg cm^-3 and a high porosity up to 99%, as well as a mechanically robust and electrically conductive 3D network structure, which makes it to be an excellent candidate as absorber for oil clean-up and an ideal platform for constructing flexible and stretchable conductors.

The Conductive Silver Nanowires Fabricated by Two-beam Laser Direct Writing on the Flexible Sheet

Flexible electrically conductive nanowires are now a key component in the fields of flexible devices. The achievement of metal nanowire with good flexibility, conductivity, compact and smooth morphology is recognized as one critical milestone for the flexible devices. In this study, a two-beam laser direct writing system is designed to fabricate AgNW on PET sheet. The minimum width of the AgNW fabricated by this method is 187 ± 34 nm with the height of 84 ± 4 nm. We have investigated the electrical resistance under different voltages and the applicable voltage per meter range is determined to be less than 7.5 × 10³ V/m for the fabricated AgNW. The flexibility of the AgNW is very excellent, since the resistance only increases 6.63% even after the stretched bending of 2000 times at such a small bending radius of 1.0 mm. The proposed two-beam laser direct writing is an efficient method to fabricate AgNW on the flexible sheet, which could be applied in flexible micro/nano devices.

New insights and perspectives into biological materials for flexible electronics

Materials based on biological materials are becoming increasingly competitive and are likely to be critical components in flexible electronic devices.

3D assembly of preformed colloidal nanoparticles into gels and aerogels: function-led design

Nanoparticle-based aerogels combine the properties of traditional aerogels with those of nanoparticles, and hold promise for various applications following a function-led design.

Three-dimensional highly conductive silver nanowires sponges based on cotton-templated porous structures for stretchable conductors

A stretchable conductor was explored by embedding a binary structure fabricated from an interconnected porous structure of cotton as skeleton along with supported 2D AgNWs network into PDMS, which showed excellent electrical and mechanical properties.

Cellulose Nanofiber as a Distinct Structure-Directing Agent for Xylem-like Microhoneycomb Monoliths by Unidirectional Freeze-Drying

Honeycomb structures have been attracting attention from researchers mainly for their high strength-to-weight ratio. As one type of structure, honeycomb monoliths having microscopically dimensioned channels have recently gained many achievements since their emergence. Inspired by the microhoneycomb structure that occurs in natural tree xylems, we have been focusing on the assembly of such a structure by using the major component in tree xylem, cellulose, as the starting material. Through the path that finally led us to the successful reconstruction of tree xylems by the unidirectional freeze-drying (UDF) approach, we verified the function of cellulose nanofibers, toward forming xylem-like monoliths (XMs). The strong tendency of cellulose nanofibers to form XMs through the UDF approach was extensively confirmed with surface grafting or a combination of a variety of second components (or even a third component). The resulting composite XMs were thus imparted with extra properties, which extends the versatility of this kind of material. Particularly, we demonstrated in this paper that XMs containing reduced graphene oxide (denoted as XM/rGO) could be used as strain sensors, taking advantage of their penetrating microchannels and the bulk elasticity property. Our methodology is flexible in its processing and could be utilized to prepare various functional composite XMs.

Highly Flexible and Porous Nanoparticle-Loaded Films for Dye Removal by Graphene Oxide-Fungus Interaction

Highly flexible and porous films with the ability to load various nanoscale adsorbents are of particular importance in the purification field. Herein, we report the sustainable and large-scale fabrication of a porous and flexible hybrid film based on the graphene oxide/hyphae interaction at a relatively low temperature of 130 °C. Under identical conditions, such films cannot be constructed with solely graphene oxide or hyphae. Moreover, through the addition of nanoscale building blocks [e.g., nanoscale poly(m-phenylenediamine) (PmPD) adsorbents] in the interaction process, the nanoparticles can be in situ loaded into the film. According to FTIR and XPS analyses, the film formation mechanisms mainly involve redox and cross-linking reactions between graphene oxide and fungus hyphae. In a proof-of-concept study, a PmPD nanoparticle-loaded hybrid film was used as a superior key component to build a flow-through adsorption device that displayed a promising adsorption performance toward dye pollutants.

Two-Dimensional Alignment of Self-Assembled Organic Nanotubes through Langmuir-Blodgett Technique

A C₃-symmetric molecule was found to form organic nanotubes through supramolecular gel formation in organic solvents. These nanotubes can be dispersed in toluene without destroying the tubular nanostructures. Using the dispersions of these organic nanotubes as "spreading solutions", Langmuir-spreading films of these nanotubes were formed. Through repeated compression and expansion cycles, the nanotubes can be aligned to a certain extent. The formed Langmuir films could be subsequently transferred to a solid substrate, and the well-aligned nanotube films were constructed by Langmuir-Blodgett film deposition technique. Interestingly, many guests including polymers, water-soluble or oil-soluble organic molecules can be encapsulated into the nanotubes and further spread on a water subphase. Through elaborate control, large-scale parallel alignment of self-assembled organic nanotubes encapsulated by guests was also realized. This study implies that 2D hierarchical alignment of one-dimensional organic nanostructures can be realized using a simple method.

Energy Harvesters for Wearable and Stretchable Electronics: From Flexibility to Stretchability

The rapid advancements of wearable electronics have caused a paradigm shift in consumer electronics, and the emerging development of stretchable electronics opens a new spectrum of applications for electronic systems. Playing a critical role as the power sources for independent electronic systems, energy harvesters with high flexibility or stretchability have been the focus of research efforts over the past decade. A large number of the flexible energy harvesters developed can only operate at very low strain level (≈0.1%), and their limited flexibility impedes their application in wearable or stretchable electronics. Here, the development of highly flexible and stretchable (stretchability >15% strain) energy harvesters is reviewed with emphasis on strategies of materials synthesis, device fabrication, and integration schemes for enhanced flexibility and stretchability. Due to their particular potential applications in wearable and stretchable electronics, energy-harvesting devices based on piezoelectricity, triboelectricity, thermoelectricity, and dielectric elastomers have been largely developed and the progress is summarized. The challenges and opportunities of assembly and integration of energy harvesters into stretchable systems are also discussed.

Ultralight Biomass-Derived Carbonaceous Nanofibrous Aerogels with Superelasticity and High Pressure-Sensitivity

Superelastic and pressure-sensitive carbonaceous nanofibrous aerogels with a honeycomb-like structure are fabricated through the combination of sustainable konjac glucomannan biomass and flexible SiO₂ nanofibers. The aerogels can detect dynamic pressure with a wide pressure range and high sensitivity, which enables real pressure signals, such as human blood pulses, to be monitored in real time and in situ.

Organic-Stabilizer-Free Polyol Synthesis of Silver Nanowires for Electrode Applications

The polyol reduction of a Ag precursor in the presence of an organic stabilizer, such as poly(vinylpyrrolidone), is a widely used method for the production of Ag nanowires (NWs). However, organic capping molecules introduce insulating layers around each NW. Herein we demonstrate that Ag NWs can be produced in high yield without any organic stabilizers simply by introducing trace amounts of NaCl and Fe(NO3 )3 during low-temperature polyol synthesis. The heterogeneous nucleation and growth of Ag NWs on initially formed AgCl particles, combined with oxidative etching of unwanted Ag nanoparticles, resulted in the selective formation of long NWs with an average length of about 40 μm in the absence of a capping or stabilizing effect provided by surface-adsorbing molecules. These organic-stabilizer-free Ag NWs were directly used for the fabrication of high-performance transparent or stretchable electrodes without a complicated process for the removal of capping molecules from the NW surface.

Elastic Carbon Aerogels Reconstructed from Electrospun Nanofibers and Graphene as Three-Dimensional Networked Matrix for Efficient Energy Storage/Conversion

Three-dimensional (3D) all-carbon nanofibrous aerogels with good structural stability and elasticity are highly desirable in flexible energy storage/conversion devices. Hence, an efficient surface-induced co-assembly strategy is reported for the novel design and reconstruction of electrospun nanofibers into graphene/carbon nanofiber (CNF) composite aerogels (GCA) with hierarchical structures utilizing graphene flakes as cross-linkers. The as-obtained GCA monoliths possess interconnected macropores and integrated conductive networks, which exhibit high elasticity and great structural robustness. Benefitting from the largely increased surface area and charge-transfer efficiency derived from the multi-form firm interconnections (including pillaring, bridging and jointing) between graphene flakes and CNF ribs, GCA not only reveals prominent capacitive performance as supercapacitor electrode, but also shows excellent hydrogen evolution reaction activity in both acidic and alkaline solutions as a 3D template for decoration of few-layered MoSe2 nanosheets, holding great potentials for energy-related applications.

Consecutive Reaction to Construct Hierarchical Nanocrystalline CuS "Branch" with Tunable Catalysis Properties

New CuS nanocrystals with a 3D hierarchical branched structure are successfully synthesized through in situ consecutive reaction method with copper foam as template. The formation mechanism of the 3D hierarchical branched structure obtained from the secondary reaction is investigated by adjusting the reaction time. The morphology of CuS nanosheet arrays with the 3D hierarchical branched structure is changed through Cu(2+) exchange. In this method, the copper foam reacted completely, and the as-synthesized CuS@Cu9S5 nanocrystals are firmly grown on the surface of the 3D framework. This tunable morphology significantly influence the physical and chemical properties, particularly catalytic performance, of the materials. The as-obtained material of Cu@CuS-2 with the 3D hierarchical branched structure as catalyst for methylene blue degradation exhibits good catalytic performance than that of the material of Cu@CuS with 2D nanosheets in dark environment. Furthermore, the cation exchange between Cu and Cu(2+) indicates that Cu(2+) in wastewater could be absorbed by Cu@CuS-2 with the 3D hierarchical branched structure. The exchanged resultant of CuS@Cu9S5 retains its capability to degrade organic dyes. This in situ consecutive reaction method may have a significant impact on controlling the crystal growth direction of inorganic material.

Cloning Nacre's 3D Interlocking Skeleton in Engineering Composites to Achieve Exceptional Mechanical Properties

Ceramic/polymer composite equipped with 3D interlocking skeleton (3D IL) is developed through a simple freeze-casting method, exhibiting exceptionally light weight, high strength, toughness, and shock resistance. Long-range crack energy dissipation enabled by 3D interlocking structure is considered as the primary reinforcing mechanism for such superior properties. The smart composite design strategy should hold a place in developing future structural engineering materials.

Coupling Microbial Growth with Nanoparticles: A Universal Strategy To Produce Functional Fungal Hyphae Macrospheres

Macroscale assembly of nanoscale building blocks is an intriguing way to translate the unique characteristics of individual nanoparticles into macroscopic materials. However, the lack of the efficient universal assembly strategy seriously hinders the possibility of macroscale architectures in practical applications. Herein, we develop a general, environment-friendly, and scalable microbial growth method for the construction of macroscopic composite assemblies with excellent mechanical strength by in situ integrating various types of nanoparticles into fungal hyphae (FH) macrospheres. Notably, the size of the FH-based composite spheres and the loading amount of the nanoparticles with different dimensions can be well tuned by controlling the cultivation time and the dosage of nanoparticles, respectively. Interestingly, bifunctional FH-based core-shell macrospheres can also be achieved by programmed assembling two different kinds of nanoparticles in the cultivation process. The produced multifunctional FH-based composite spheres exhibit wide potential applications in magnetic actuation, photothermal therapy, and contaminant adsorption, etc.

Hydrogen from Water over Openly-Structured Graphitic Carbon Nitride Polymer through Photocatalysis

Openly-structured g-C3 N4 microspheres (CNMS) are developed through a well-controlled strategy. These materials have unique features of open 3 D structure, ordered hierarchical porosity, and improved optical and electronic properties. Hydrogen evolution from water is performed under a 300 W Xe lamp with a cut-off filter (λ>420 nm) and Pt nanoparticles are used as the co-catalyst (3.0 wt%). The catalyst prepared at 600 °C (CNMS-600) has a hydrogen evolution rate (HER) of 392 μmol h(-1) (apparent quantum yield, AQY=6.3%) at 420 nm. This value is higher than that of g-C3 N4 nanosheets prepared through thermal oxidation, liquid exfoliation, or chemical exfoliation. The HER value is only 27 μmol h(-1) (AQY=0.43%) at 420 nm for bulk g-C3 N4 from melamine. The evolution of openly-structured CNMS was investigated by TEM, FTIR, and XRD. The improved optical and electronic properties were demonstrated through UV/Vis absorption spectra, valence-band X-ray photoelectron spectroscopy, photoluminescence spectroscopy, electron paramagnetic resonance spectroscopy, and electrochemical impedance spectroscopy.

Self-assembly of 2D MnO2 nanosheets into high-purity aerogels with ultralow density

Self-assembling inorganic nanoparticles (NPs) into macroscopic three dimensional (3D) architectures often requires the assistance of organic components, leaving residual organics in the resultant. In this work, organic-free MnO2 aerogels with ultralow density have been achieved by the self-assembly of two dimensional (2D) MnO2 nanosheets via an ice-templating approach. To the authors' best knowledge, it is the first reported case of constructing a high-purity inorganic aerogel from preformed NPs without using any functionalization or stabilization agents. Moreover, it has been demonstrated that an ultralight MnO2 aerogel with a density as low as ∼0.53 mg cm-3, which is the lightest metal oxide aerogel to date, can be well obtained by such an approach. The successful formation of the aerogel can be attributed to the enhanced van der Waals force between the 2D building blocks that have been more orderly arranged by the squeezing of ice crystals during the freezing process. Hence, this work shows a pioneering example of assembling inorganic NPs into aerogels relying only on the weak interactions between NPs (e.g. van der Waals forces). It has also been demonstrated that the obtained MnO2 aerogel can function as an effective absorbent for toxic reducing gas, owing to its strong oxidation ability and high porosity. The strategy presented herein holds good potential to be applied to the fabrication of other high-purity inorganic aerogels, especially those with 2D building blocks readily available.

Reversibly Stretchable, Optically Transparent Radio-Frequency Antennas Based on Wavy Ag Nanowire Networks

We report a facile approach for producing reversibly stretchable, optically transparent radio-frequency antennas based on wavy Ag nanowire (NW) networks. The wavy configuration of Ag NWs is obtained by floating the NW networks on the surface of water, followed by compression. Stretchable antennas are prepared by transferring the compressed NW networks onto elastomeric substrates. The resulting antennas show excellent performance under mechanical deformation due to the wavy configuration, which allows the release of stress applied to the NWs and an increase in the contact area between NWs. The antennas formed from the wavy NW networks exhibit a smaller return loss and a higher radiation efficiency when strained than the antennas formed from the straight NW networks, as well as an improved stability in cyclic deformation tests. Moreover, the wavy NW antennas require a relatively small quantity of NWs, which leads to low production costs and provides an optical transparency. These results demonstrate the potential of these wavy Ag NW antennas in applications of wireless communications for wearable systems.

Fabrication of Highly Stretchable Conductors Based on 3D Printed Porous Poly(dimethylsiloxane) and Conductive Carbon Nanotubes/Graphene Network

The combination of carbon nanomaterial with three-dimensional (3D) porous polymer substrates has been demonstrated to be an effective approach to manufacture high-performance stretchable conductive materials (SCMs). However, it remains a challenge to fabricate 3D-structured SCMs with outstanding electrical conductivity capability under large strain in a facile way. In this work, the 3D printing technique was employed to prepare 3D porous poly(dimethylsiloxane) (O-PDMS) which was then integrated with carbon nanotubes and graphene conductive network and resulted in highly stretchable conductors (OPCG). Two types of OPCG were prepared, and it has been demonstrated that the OPCG with split-level structure exhibited both higher electrical conductivity and superior retention capability under deformations, which was illustrated by using a finite element method. The specially designed split-level OPCG is capable of sustaining both large strain and repeated deformations showing huge potential in the application of next-generation stretchable electronics.

Fabrication of oriented hBN scaffolds for thermal interface materials

Three dimensional scaffolds of hBN microplatelets prepared by ice templating method are used to fabricate hBN/PDMS composites with vertically aligned hBN for thermal interface materials.

Bioinspired large-scale aligned porous materials assembled with dual temperature gradients

Natural materials, such as bone, teeth, shells, and wood, exhibit outstanding properties despite being porous and made of weak constituents. Frequently, they represent a source of inspiration to design strong, tough, and lightweight materials. Although many techniques have been introduced to create such structures, a long-range order of the porosity as well as a precise control of the final architecture remain difficult to achieve. These limitations severely hinder the scale-up fabrication of layered structures aimed for larger applications. We report on a bidirectional freezing technique to successfully assemble ceramic particles into scaffolds with large-scale aligned, lamellar, porous, nacre-like structure and long-range order at the centimeter scale. This is achieved by modifying the cold finger with a polydimethylsiloxane (PDMS) wedge to control the nucleation and growth of ice crystals under dual temperature gradients. Our approach could provide an effective way of manufacturing novel bioinspired structural materials, in particular advanced materials such as composites, where a higher level of control over the structure is required.

Floating compression of Ag nanowire networks for effective strain release of stretchable transparent electrodes

Manipulation of the configuration of Ag nanowire (NW) networks has been pursued to enhance the performance of stretchable transparent electrodes. However, it has remained challenging due to the high Young's modulus and low yield strain of Ag NWs, which lead to their mechanical failure when subjected to structural deformation. We demonstrate that floating a Ag NW network on water and subsequent in-plane compression allows convenient development of a wavy configuration in the Ag NW network, which can release the applied strain. A greatly enhanced electromechanical stability of Ag NW networks can be achieved due to their wavy configuration, while the NW networks maintain the desirable optical and electrical properties. Moreover, the produced NW networks can be transferred to a variety of substrates, offering flexibility for device fabrication. The Ag NW networks with wavy configurations are used as compliant electrodes for dielectric elastomer actuators. The study demonstrates their promising potential to provide improved performance for soft electronic devices.

Converting 2D inorganic-organic ZnSe-DETA hybrid nanosheets into 3D hierarchical nanosheet-based ZnSe microspheres with enhanced visible-light-driven photocatalytic performances

Engineering two-dimensional (2D) nanosheets into three-dimensional (3D) hierarchical structures is one of the great challenges in nanochemistry and materials science. We report a facile and simple chemical conversion route to fabricate 3D hierarchical nanosheet-based ZnSe microspheres by using 2D inorganic-organic hybrid ZnSe-DETA (DETA = diethylenetriamine) nanosheets as the starting precursors. The conversion mechanism involves the controlled depletion of the organic-component (DETA) from the hybrid precursors and the subsequent self-assembly of the remnant inorganic-component (ZnSe). The transformation reaction of ZnSe-DETA nanosheets is mainly influenced by the concentration of DETA in the reaction solution. We demonstrated that this organic-component depletion method could be extended to the synthesis of other hierarchical structures of metal sulfides. In addition, the obtained hierarchical nanosheet-based ZnSe microspheres exhibited outstanding performance in visible light photocatalytic degradation of methyl orange and were highly active for photocatalytic H2 production.

Nanomaterial-enabled stretchable conductors: strategies, materials and devices

Stretchable electronics are attracting intensive attention due to their promising applications in many areas where electronic devices undergo large deformation and/or form intimate contact with curvilinear surfaces. On the other hand, a plethora of nanomaterials with outstanding properties have emerged over the past decades. The understanding of nanoscale phenomena, materials, and devices has progressed to a point where substantial strides in nanomaterial-enabled applications become realistic. This review summarizes recent advances in one such application, nanomaterial-enabled stretchable conductors (one of the most important components for stretchable electronics) and related stretchable devices (e.g., capacitive sensors, supercapacitors and electroactive polymer actuators), over the past five years. Focusing on bottom-up synthesized carbon nanomaterials (e.g., carbon nanotubes and graphene) and metal nanomaterials (e.g., metal nanowires and nanoparticles), this review provides fundamental insights into the strategies for developing nanomaterial-enabled highly conductive and stretchable conductors. Finally, some of the challenges and important directions in the area of nanomaterial-enabled stretchable conductors and devices are discussed.

Aligning 3D nanofibrous networks from self-assembled phenylalanine nanofibers

Self-assembled synthetic materials are typically disordered, and controlling the alignment of such materials at the nanometer scale may be important for a variety of biological applications. In this study, we have applied directional freeze-drying, for the first time, to develop well aligned three dimensional (3D) nanofibrous materials using amino acid like L-phenylalanine (Phe). 3D free-standing Phe nanofibrous monoliths have been successfully prepared using directional freeze-drying, and have presented a unique hierarchical structure with well-aligned nanofibers at the nanometer scale and an ordered compartmental architecture at the micrometer scale. We have found that the physical properties (e.g. nanofiber density and alignment) of the nanofibrous materials could be tuned by controlling the concentration and pH of the Phe solution and the freezing temperature. Moreover, the same strategy (i.e. directional freeze-drying) has been successfully applied to assemble peptide nanofibrous materials using a dipeptide (i.e. diphenylalanine), and to assemble Phe-based nanofibrous composites using polyethylenimine and poly(vinyl alcohol). The tunability of the nanofibrous structures together with the biocompatibility of Phe may make these 3D nanofibrous materials suitable for a variety of applications, including biosensor templates, tissue scaffolds, filtration membranes, and absorbents. The strategy reported here is likely applicable to create aligned nanofibrous structures using other amino acids, peptides, and polymers.